The fluorescence of QDs is generated when the excited electron emits photon and returns to the ground-state. The lifetime is a delay between the moment of absorbtion a photon from the light sorce and moment of emitted light (**Figure 5**). The lifetime of larger dots have more closely spaced energy levels in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer [10]. Long lifetime provides difference of QD fluorescence signal from background fluorescence. For example, see [17]. The life time of QDs and fluorophores is shown in **Figure 6**.

**Brightness and photostability**

**Figure 5.** Fluorescence lifetime [2].

Minimizing overlap between total excitation and emission bands enhances the clarity and brightness of the fluorescing QD by avoiding re-absorption of emitted light into nearby quantum dots—characteristics that display manufacturers and end-users find highly desirable. Broad absorption spectra make it possible to excite all QDs simultaneously with a single light source and minimize sample autofluorescence by choosing an appropriate excitation wavelength. QDs

larger than that of organic dyes. QDs are able to absorb 10–50 times more photons than organic

, about 10–50 times

Applicability of Quantum Dots in Biomedical Science http://dx.doi.org/10.5772/intechopen.71428 27

have very large molar excitation coefficient in the order of 0.5–5 × 10 6 M −1

**Figure 6.** Fluorescence lifetime quantum dots and organic fluorophore [19].

In order to extend the lifetime of the QDs, rare earths can be incorporated into the QDs which create local quantum states.

**Figure 4.** Color of light depends on size quantum dot.

**Figure 5.** Fluorescence lifetime [2].

**Fluorescence intensity and lifetime**

26 Ionizing Radiation Effects and Applications

**Figure 3.** Broad absorption and narrow emission spectrum [15].

shown in **Figure 6**.

create local quantum states.

**Figure 4.** Color of light depends on size quantum dot.

The fluorescence of QDs is generated when the excited electron emits photon and returns to the ground-state. The lifetime is a delay between the moment of absorbtion a photon from the light sorce and moment of emitted light (**Figure 5**). The lifetime of larger dots have more closely spaced energy levels in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer [10]. Long lifetime provides difference of QD fluorescence signal from background fluorescence. For example, see [17]. The life time of QDs and fluorophores is

In order to extend the lifetime of the QDs, rare earths can be incorporated into the QDs which

#### **Brightness and photostability**

Minimizing overlap between total excitation and emission bands enhances the clarity and brightness of the fluorescing QD by avoiding re-absorption of emitted light into nearby quantum dots—characteristics that display manufacturers and end-users find highly desirable. Broad absorption spectra make it possible to excite all QDs simultaneously with a single light source and minimize sample autofluorescence by choosing an appropriate excitation wavelength. QDs have very large molar excitation coefficient in the order of 0.5–5 × 10 6 M −1 , about 10–50 times larger than that of organic dyes. QDs are able to absorb 10–50 times more photons than organic

**Figure 6.** Fluorescence lifetime quantum dots and organic fluorophore [19].

dyes at the same excitation photon flux, leading to a significant improvement in the probe brightness. Owing to their inorganic nature, QDs have minimal interaction with the surrounding environment which contributes to their photostability. Inorganic QDs are more photostable under ultraviolet excitation than organic molecules, and their fluorescence is more saturated [18].

#### **Quantum yield**

Quantum yield is a measure of the "brightness" of a fluorophore and is defined as the ratio of the number of photons emitted to the number of photons absorbed. Some organic dyes have quantum yields approaching 100%, but conjugates (from biological affinity molecules) made from these generally have a significantly lower quantum yield. QDs are relatively efficient with regards to conversion of the excitation light into emission, where the quantum yield is generally over 50%. QDs retain their high quantum yield even after conjugation to biological affinity molecules [14]. The fluorescence quantum yield gives the efficiency of the fluorescence process.

#### **Photobleaching**

Photobleaching (fading) is the photochemical alteration of a dye or a fluorophore molecule, such that it is permanently unable to fluoresce. This is caused by cleaving of covalent bonds or non-specific reactions between the fluorophore and surrounding molecules. QDs are several thousand times more stable against photobleaching than organic dye molecules, and are thus ideal probes for fluorescent spectroscopy and bioimaging applications [20].

outmost surface of these nanoparticles is passivated with an organic ligand.1

try, called core-shell QDs, are the core, shell, and ligand (**Figure 8**).

logical studies [26].

**4.1. Surface coatings and water-solubility**

**Figure 7.** Band gap energy core/shell/ligand structure [12].

and phase transfer method [29].

*4.1.1. The ligand exchange method*

a covalent bond with the central atom.

TOPO/TOP: Typically trioctylphosphine oxide/trioctylphosphine.

1

2

water soluble by surface modifying them with various caps to ensure aqueous solubility and enhancing biocompatibility. The three important features of QDs used in the display indus-

When these three features of QDs are tuned to how we need them, we develop exciting new applications. In order to utilize in a biological environment, they need to be made hydrophilic. Thus, they are necessarily made water soluble by surface modifying them with various bifunctional surface ligands or caps to promote aqueous solubility and enhancing biocompatibility [25]. QDs must be conjugated with molecules which have the capabilities of recognizing the target. These surface modifications can also help prevent aggregation, reduce the nonspecific binding, and are critical in achieving specific target imaging in bio-

The key to develop QDs as a tool in biological systems is to achieve water solubility, biocompatibility, and photostability [27]. QDs, single or core/shell structures, do not exhibit aqueous solubility. After synthesis, hydrophobic QD must be covered with an organic layer or incorporated within the organic shell to make them water-soluble and biocompatible [28]. Some of the techniques used to achieve solubilization include ligand exchange, surface silanization,

Native cap exchange is a strategy in which the native hydrophobic layer (TOPO/TOP)2

replaced with a bifunctional moiety that can bind QDs from one side while exposing hydrophilic

As organic ligands, molecules and ions, containing O, S, N, and P, are bonded so that their electronic pair can produce

QDs are made

29

Applicability of Quantum Dots in Biomedical Science http://dx.doi.org/10.5772/intechopen.71428

cap is

## **4. Core/shell structure quantum dot**

Quantum dots are used as bare core or as core/shell structures. Although these "core" QDs determine the optical properties of the conjugate, they are by themselves unsuitable for biological probes owing to their poor stability and quantum yield [21]. In fact, the quantum yield of QD cores has been reported to be very sensitive to the presence of particular ions in solution [22]. A bare nanocrystal core is highly reactive and toxic, resulting in a very unstable structure that is prone to photochemical degradation. The core is highly reactive due to their large surface area/volume ratio, resulting in a very unstable structure which is particularly prone to photochemical degradation. Capping the core with a semiconductor material of a higher band gap not only increases the stability and quantum yield, but also passivates the toxicity of the core by shielding reactive ions from being exposed to photo-oxidative environments, e.g., exposure to UV and air (**Figure 7**). Traditionally, the typical QDs consist of a II–IV, IV–VI, or III–V semiconductor core (CdTe, CdSe, Pb, Se, GaAs, GaN, InP, and InAs), which is surrounded by a covering of wide band gap semiconductor shell as zinc sulfide ZnS or cadmium sulfide CdS is generally used (approx. 1.5 nm thick) [23]. Capping core nanocrystals with ZnS has shown to increase stability and performance, producing QDs with improved photophysical and chemical properties at room temperature. However, ZnS capping alone is not sufficient to stabilize the core, particularly in biological solutions [24]. Quantum dots do not exhibit aqueous solubility as they are generally synthesized in organic solution. The

**Figure 7.** Band gap energy core/shell/ligand structure [12].

dyes at the same excitation photon flux, leading to a significant improvement in the probe brightness. Owing to their inorganic nature, QDs have minimal interaction with the surrounding environment which contributes to their photostability. Inorganic QDs are more photostable under ultraviolet excitation than organic molecules, and their fluorescence is more saturated [18].

Quantum yield is a measure of the "brightness" of a fluorophore and is defined as the ratio of the number of photons emitted to the number of photons absorbed. Some organic dyes have quantum yields approaching 100%, but conjugates (from biological affinity molecules) made from these generally have a significantly lower quantum yield. QDs are relatively efficient with regards to conversion of the excitation light into emission, where the quantum yield is generally over 50%. QDs retain their high quantum yield even after conjugation to biological affinity molecules [14]. The fluorescence quantum yield gives the efficiency of the fluorescence process.

Photobleaching (fading) is the photochemical alteration of a dye or a fluorophore molecule, such that it is permanently unable to fluoresce. This is caused by cleaving of covalent bonds or non-specific reactions between the fluorophore and surrounding molecules. QDs are several thousand times more stable against photobleaching than organic dye molecules, and are thus

Quantum dots are used as bare core or as core/shell structures. Although these "core" QDs determine the optical properties of the conjugate, they are by themselves unsuitable for biological probes owing to their poor stability and quantum yield [21]. In fact, the quantum yield of QD cores has been reported to be very sensitive to the presence of particular ions in solution [22]. A bare nanocrystal core is highly reactive and toxic, resulting in a very unstable structure that is prone to photochemical degradation. The core is highly reactive due to their large surface area/volume ratio, resulting in a very unstable structure which is particularly prone to photochemical degradation. Capping the core with a semiconductor material of a higher band gap not only increases the stability and quantum yield, but also passivates the toxicity of the core by shielding reactive ions from being exposed to photo-oxidative environments, e.g., exposure to UV and air (**Figure 7**). Traditionally, the typical QDs consist of a II–IV, IV–VI, or III–V semiconductor core (CdTe, CdSe, Pb, Se, GaAs, GaN, InP, and InAs), which is surrounded by a covering of wide band gap semiconductor shell as zinc sulfide ZnS or cadmium sulfide CdS is generally used (approx. 1.5 nm thick) [23]. Capping core nanocrystals with ZnS has shown to increase stability and performance, producing QDs with improved photophysical and chemical properties at room temperature. However, ZnS capping alone is not sufficient to stabilize the core, particularly in biological solutions [24]. Quantum dots do not exhibit aqueous solubility as they are generally synthesized in organic solution. The

ideal probes for fluorescent spectroscopy and bioimaging applications [20].

**4. Core/shell structure quantum dot**

**Quantum yield**

28 Ionizing Radiation Effects and Applications

**Photobleaching**

outmost surface of these nanoparticles is passivated with an organic ligand.1 QDs are made water soluble by surface modifying them with various caps to ensure aqueous solubility and enhancing biocompatibility. The three important features of QDs used in the display industry, called core-shell QDs, are the core, shell, and ligand (**Figure 8**).

When these three features of QDs are tuned to how we need them, we develop exciting new applications. In order to utilize in a biological environment, they need to be made hydrophilic. Thus, they are necessarily made water soluble by surface modifying them with various bifunctional surface ligands or caps to promote aqueous solubility and enhancing biocompatibility [25]. QDs must be conjugated with molecules which have the capabilities of recognizing the target. These surface modifications can also help prevent aggregation, reduce the nonspecific binding, and are critical in achieving specific target imaging in biological studies [26].

#### **4.1. Surface coatings and water-solubility**

The key to develop QDs as a tool in biological systems is to achieve water solubility, biocompatibility, and photostability [27]. QDs, single or core/shell structures, do not exhibit aqueous solubility. After synthesis, hydrophobic QD must be covered with an organic layer or incorporated within the organic shell to make them water-soluble and biocompatible [28]. Some of the techniques used to achieve solubilization include ligand exchange, surface silanization, and phase transfer method [29].

#### *4.1.1. The ligand exchange method*

Native cap exchange is a strategy in which the native hydrophobic layer (TOPO/TOP)2 cap is replaced with a bifunctional moiety that can bind QDs from one side while exposing hydrophilic

1 As organic ligands, molecules and ions, containing O, S, N, and P, are bonded so that their electronic pair can produce a covalent bond with the central atom.

2 TOPO/TOP: Typically trioctylphosphine oxide/trioctylphosphine.

**Figure 8.** Core/shell/ligand structure of hydrophobic quantum dot [12].

groups on the surface to achieve optimal dispersion [30]. The ligand exchange method includes the exchange of the hydrophobic surfactant molecules with bifunctional molecules, which are hydrophilic on one side and hydrophobic on the other, to bind to the ZnS shell on the QD [23] (**Figure 9**). These biocompatible polymers usually have functional anchor groups, such as thiol, amine, and carboxyl, which can passivate QDs more strongly than the original ligand [31]. Most often thiols (−SH) as functional groups are used to bind to the ZnS and carboxyl (−COOH) groups are used as hydrophilic ends. The resulting QDs are soluble in both aqueous and polar solvents [29]. Biomolecules, such as proteins, peptides, and DNA, were also conjugated to the free carboxyl groups by crosslinking to the reactive amine. This process did not affect the optical characters of the QDs compared with the original QDs.

may reduce emission and limits their use biological applications [35]. The aqueous coating can then be tagged with various biomolecules of interest. Biomolecules, such as proteins, peptides, and antibodies, were conjugated to the free functional groups by crosslinking to the reactive amine. This process did not affect the optical characters of the QDs compared with

Applicability of Quantum Dots in Biomedical Science http://dx.doi.org/10.5772/intechopen.71428 31

Once solubilization has been achieved, QDs can be functionalized by conjugation to a number of biological molecules. QDs are adapted to the desired biological application by conjugation to biological molecules without disturbing the function of these molecules. QDs must be conjugated with molecules which have the capabilities of recognizing the target [36]. Biomolecules include antibodies, peptides, avidin, biotin, oligonucleotides, albumin, or by coating with streptavidin (**Figure 10**) [6]. Owing to large surface area-to-volume ratio, several biomolecules of varying types can be attached to a single QD. Each of these biomolecules provides a desired function which affords multi-functionality [37]. Various surface modification techniques were developed to ensure the specific bioconjugation: covalent linkage, electrostatic attraction, adsorption, and mercapto (−SH) exchange. The choice depends on the features of the biomolecule of interest

the original hydrophobic surfactant layer.

**5. Functionalization of quantum dots**

**Figure 9.** Modification from hydrophobic to hydrophilic quantum dots [12].

#### *4.1.2. Surface silanization*

Surface silanization involves the growth of a silica shell around the nanocrystal. As silica shells are highly cross-linked, they are very stable [32]. Silica coating enhances the mechanical stability of colloidal QDs and protects them against oxidation and agglomeration. The adventure of silica encapsulation is QDs chemical stability over a much broader pH range compared to carboxy-terminated ligands [29]. The chemistry of glass surfaces can be readily extended to silanized QDs, providing more flexibility for bioconjugation (**Figure 9**).

#### *4.1.3. Polymer coating (phase transfer)*

Polymer coating (phase transfer) method uses amphiphilic polymers to coat the surface [33, 34]. The hydrophobic alkyl chains of the polymer interdigitate with the alkyl groups on the QDs surface, while the hydrophilic groups orientate outwards to attain water solubility. However, coating with a polymer may increase the overall diameter of the QDs, and this

**Figure 9.** Modification from hydrophobic to hydrophilic quantum dots [12].

may reduce emission and limits their use biological applications [35]. The aqueous coating can then be tagged with various biomolecules of interest. Biomolecules, such as proteins, peptides, and antibodies, were conjugated to the free functional groups by crosslinking to the reactive amine. This process did not affect the optical characters of the QDs compared with the original hydrophobic surfactant layer.

## **5. Functionalization of quantum dots**

groups on the surface to achieve optimal dispersion [30]. The ligand exchange method includes the exchange of the hydrophobic surfactant molecules with bifunctional molecules, which are hydrophilic on one side and hydrophobic on the other, to bind to the ZnS shell on the QD [23] (**Figure 9**). These biocompatible polymers usually have functional anchor groups, such as thiol, amine, and carboxyl, which can passivate QDs more strongly than the original ligand [31]. Most often thiols (−SH) as functional groups are used to bind to the ZnS and carboxyl (−COOH) groups are used as hydrophilic ends. The resulting QDs are soluble in both aqueous and polar solvents [29]. Biomolecules, such as proteins, peptides, and DNA, were also conjugated to the free carboxyl groups by crosslinking to the reactive amine. This process did not affect the optical

Surface silanization involves the growth of a silica shell around the nanocrystal. As silica shells are highly cross-linked, they are very stable [32]. Silica coating enhances the mechanical stability of colloidal QDs and protects them against oxidation and agglomeration. The adventure of silica encapsulation is QDs chemical stability over a much broader pH range compared to carboxy-terminated ligands [29]. The chemistry of glass surfaces can be readily extended to

Polymer coating (phase transfer) method uses amphiphilic polymers to coat the surface [33, 34]. The hydrophobic alkyl chains of the polymer interdigitate with the alkyl groups on the QDs surface, while the hydrophilic groups orientate outwards to attain water solubility. However, coating with a polymer may increase the overall diameter of the QDs, and this

characters of the QDs compared with the original QDs.

**Figure 8.** Core/shell/ligand structure of hydrophobic quantum dot [12].

silanized QDs, providing more flexibility for bioconjugation (**Figure 9**).

*4.1.2. Surface silanization*

30 Ionizing Radiation Effects and Applications

*4.1.3. Polymer coating (phase transfer)*

Once solubilization has been achieved, QDs can be functionalized by conjugation to a number of biological molecules. QDs are adapted to the desired biological application by conjugation to biological molecules without disturbing the function of these molecules. QDs must be conjugated with molecules which have the capabilities of recognizing the target [36]. Biomolecules include antibodies, peptides, avidin, biotin, oligonucleotides, albumin, or by coating with streptavidin (**Figure 10**) [6]. Owing to large surface area-to-volume ratio, several biomolecules of varying types can be attached to a single QD. Each of these biomolecules provides a desired function which affords multi-functionality [37]. Various surface modification techniques were developed to ensure the specific bioconjugation: covalent linkage, electrostatic attraction, adsorption, and mercapto (−SH) exchange. The choice depends on the features of the biomolecule of interest

**5.3. Non-specific adsorption**

**5.4. Mercapto exchange**

with living tissue.

**6. Toxicity of quantum dots**

Simple small molecules such as oligonucleotides and various serum albumins were found to readily adsorb to the surface of water-soluble QDs [6]. This adsorption is nonspecific and

Applicability of Quantum Dots in Biomedical Science http://dx.doi.org/10.5772/intechopen.71428 33

Thiol-containing biomolecules can be conjugated to QDs via mercapto exchange. Thiols (SH) bond of mercapto group bind to the surface of the most often used semiconductor materials (CdSe, CdS, CdTe, and ZnS), and therefore QDs can be conjugated to biological molecules bearing mercapto (SH) groups in this way [43]. ZnS-thiol bond is not very strong. The conju-

Surface of QDs are decorated with different biomolecules: proteins, peptides, nucleic acids, ligands, or other biomolecules to achieve bioconjugation that mediate specific interactions

Most of the current studies were designed to ask questions concerning the physicochemical properties of novel QD products such as fluorescence, detectability, stability, and cell labeling efficacy, and not QD toxicity *per se*. Several studies reveal that the toxicity of QDs depends on many factors which can be summarized as inherent physicochemical properties and environmental conditions. Understanding the potential toxicity of QDs requires a fundamental grasp of QD physicochemical properties. Each QD type will need to be characterized individually to its potential toxicity. Each individual type of QD possesses its own unique physicochemical properties, which in turn determines its potential toxicity or lack thereof. QD sizes, charges, concentrations, outer coating materials and functional groups, oxidation, and mechanical stability all have been implicated as contributing factors to toxicity [44]. Most QDs consist of heavy metal which may be potentially toxic. Possibly the most important aspect of QD toxicity is their stability, both *in vivo* and during synthesis and storage. QDs with an outer ZnS shell, referred to as core-shell QDs, are generally more chemically stable. Degradation of the QD coating may also result in reaction of the QD in undesirable/unanticipated ways *in vivo*. Further, some QD coating materials have themselves been found to be cytotoxic. Cytotoxicity of QDs has been observed in a large number of *in vitro* studies [45], affecting cell growth and viability [22]. Several studies suggest QD cytotoxicity to be due to photolysis or oxidation. Under oxidative and photolytic conditions, QD core-shell coatings have been found to be labile, degrading, and thus exposing potentially toxic "capping" material or intact core metalloid complexes or resulting in dissolution of the core complex to QD core metal components, e.g., Cd and Se [46]. The toxicity is associated with the cadmium, lead, or arsenic containing QDs. For *in vivo* studies, the main concern is the robustness of the surface coating.

depends on ionic strength, pH, temperature, and surface charge of the molecule [5].

gated biomolecules may break off and the QDs may precipitate out of solution [6].

**Figure 10.** Functionalization of hydrophilic quantum dots [16].

[38]. Surface modifications can also help prevent aggregation, reduce the nonspecific binding, and are critical to achieving specific target imaging in biomedical studies [3]. QDs retain excellent stability of optical properties upon conjugation to biomolecules, and can be simultaneously excited by a single light source.

#### **5.1. Covalent linkage**

The first bioconjugation method offers a most stable covalent linkage of QDs to biomolecules and is the most commonly used approach for making biofunctionalized QDs for *in vivo* applications. Most water solubilization methods result in QDs covered with carboxylic acid, amino or thiol groups. Under these situations, it is easy to link QDs to biological molecules which also have these functional groups [39]. Using these methods, there have been numerous reports of conjugating QDs with various biological molecules, including proteins, antibodies, peptides, oligonucleotides, and albumin [5].

#### **5.2. Electrostatic interaction**

In the second procedure, an electrostatic interaction between QDs and charged adapter molecules or proteins with incorporated charged domains was employed [40, 41]. In this case, the protein of interest can be fused to a positively charged domain that will in turn bind electrostatically to the negatively charged surface of the QDs [6]. In the electrostatic interaction approach, the binding energies are highly dependent on both the chemical environment and the ambient temperature and QDs size can strongly affect the interaction efficiency. Electrostatic interactions generally are not sufficiently specific, however, given the complexity of biological milieu [5]. Therefore, conjugates made by this way are not suitable for *in vivo* or *ex vivo* cell labeling due to the possible interference with positively charged proteins [42].

#### **5.3. Non-specific adsorption**

Simple small molecules such as oligonucleotides and various serum albumins were found to readily adsorb to the surface of water-soluble QDs [6]. This adsorption is nonspecific and depends on ionic strength, pH, temperature, and surface charge of the molecule [5].

#### **5.4. Mercapto exchange**

[38]. Surface modifications can also help prevent aggregation, reduce the nonspecific binding, and are critical to achieving specific target imaging in biomedical studies [3]. QDs retain excellent stability of optical properties upon conjugation to biomolecules, and can be simultaneously

The first bioconjugation method offers a most stable covalent linkage of QDs to biomolecules and is the most commonly used approach for making biofunctionalized QDs for *in vivo* applications. Most water solubilization methods result in QDs covered with carboxylic acid, amino or thiol groups. Under these situations, it is easy to link QDs to biological molecules which also have these functional groups [39]. Using these methods, there have been numerous reports of conjugating QDs with various biological molecules, including proteins, antibodies,

In the second procedure, an electrostatic interaction between QDs and charged adapter molecules or proteins with incorporated charged domains was employed [40, 41]. In this case, the protein of interest can be fused to a positively charged domain that will in turn bind electrostatically to the negatively charged surface of the QDs [6]. In the electrostatic interaction approach, the binding energies are highly dependent on both the chemical environment and the ambient temperature and QDs size can strongly affect the interaction efficiency. Electrostatic interactions generally are not sufficiently specific, however, given the complexity of biological milieu [5]. Therefore, conjugates made by this way are not suitable for *in vivo* or *ex vivo* cell labeling due to the possible interference with positively charged

excited by a single light source.

32 Ionizing Radiation Effects and Applications

**5.2. Electrostatic interaction**

proteins [42].

peptides, oligonucleotides, and albumin [5].

**Figure 10.** Functionalization of hydrophilic quantum dots [16].

**5.1. Covalent linkage**

Thiol-containing biomolecules can be conjugated to QDs via mercapto exchange. Thiols (SH) bond of mercapto group bind to the surface of the most often used semiconductor materials (CdSe, CdS, CdTe, and ZnS), and therefore QDs can be conjugated to biological molecules bearing mercapto (SH) groups in this way [43]. ZnS-thiol bond is not very strong. The conjugated biomolecules may break off and the QDs may precipitate out of solution [6].

Surface of QDs are decorated with different biomolecules: proteins, peptides, nucleic acids, ligands, or other biomolecules to achieve bioconjugation that mediate specific interactions with living tissue.

## **6. Toxicity of quantum dots**

Most of the current studies were designed to ask questions concerning the physicochemical properties of novel QD products such as fluorescence, detectability, stability, and cell labeling efficacy, and not QD toxicity *per se*. Several studies reveal that the toxicity of QDs depends on many factors which can be summarized as inherent physicochemical properties and environmental conditions. Understanding the potential toxicity of QDs requires a fundamental grasp of QD physicochemical properties. Each QD type will need to be characterized individually to its potential toxicity. Each individual type of QD possesses its own unique physicochemical properties, which in turn determines its potential toxicity or lack thereof. QD sizes, charges, concentrations, outer coating materials and functional groups, oxidation, and mechanical stability all have been implicated as contributing factors to toxicity [44]. Most QDs consist of heavy metal which may be potentially toxic. Possibly the most important aspect of QD toxicity is their stability, both *in vivo* and during synthesis and storage. QDs with an outer ZnS shell, referred to as core-shell QDs, are generally more chemically stable. Degradation of the QD coating may also result in reaction of the QD in undesirable/unanticipated ways *in vivo*. Further, some QD coating materials have themselves been found to be cytotoxic. Cytotoxicity of QDs has been observed in a large number of *in vitro* studies [45], affecting cell growth and viability [22]. Several studies suggest QD cytotoxicity to be due to photolysis or oxidation. Under oxidative and photolytic conditions, QD core-shell coatings have been found to be labile, degrading, and thus exposing potentially toxic "capping" material or intact core metalloid complexes or resulting in dissolution of the core complex to QD core metal components, e.g., Cd and Se [46]. The toxicity is associated with the cadmium, lead, or arsenic containing QDs. For *in vivo* studies, the main concern is the robustness of the surface coating. An unstable surface coating could expose the core of the QD to UV damage or air oxidation. The toxic heavy metal ions can be easily leaked out into biological systems, if the surfaces are not properly covered by the shells or protected by ligands [44]. The stability of QDs and their resistance to metabolic degradation in live cells would allow long-term imaging studies, and several studies have indicated lack of cytotoxicity for period up to 4 months [47]. Although such QDs should not be acutely toxic as long as their polymer coating is stable enough to restrain the release of cadmium, both short- and long-term safety of QDs will need to be established in toxicological studies in clinically relevant animal models. Studies in cell lines have shown that QDs do not affect cell growth under normal media conditions and shortterm administration of QDs into animals seems not to affect the metabolism and behavior of the animals [48]. Xiao and coworkers showed that the cytotoxicity of CdSe quantum was proportional to QDs concentration in the tested range. There was a negligible cytotoxicity at concentration of 20 nM, which is twice of the applied concentration of fluorescent cell imaging test. On the other side, no size-dependent cytotoxicity was observed. For time-dependence, cytotoxicity increased along with prolongation of incubation period, and tended to be stable after 12 h of incubation. Therefore, cytotoxicity can be neglected in a typical fluorescent cell imaging procedure (10 nM/L, 12 h), and the labeling effect to microtubule can be guaranteed satisfactorily [49]. There are also concerns about toxicity related to the molecules used for surface functionalization of the QDs [6]. However, toxicity concerns are still valid for longer periods and *in vivo* applications. Given the highly intrinsic toxicity of cadmium, the biological applications of Cd-QDs have been limited. The limitation of heavy metal containing QDs stimulates extensive research interests in exploring alternative strategies for the design of fluorescent nanocrystals with high biocompatibility. In this case, the practical strategy is to develop highly fluorescent nanoparticles based on nontoxic elements [3].

and environment-friendly nature, Si QDs are used as fluorescent probes for bioimaging. Si QDs are produced in nonpolar solvents with hydrophobic ligands on their surface in other to protect the Si core. Therefore, it is a common problem that silicon QDs show poor solubility and unstable photoluminescence in aqueous solutions. Thus, fabricating good water-disperse Si QDs with stable optical characters is vital to their applications in bioimaging studies. In the demand of using biocompatible and nontoxic QDs as nanoprobes, rare-earth (RE) elements

Applicability of Quantum Dots in Biomedical Science http://dx.doi.org/10.5772/intechopen.71428 35

The ability to study molecular and cellular events by using fluorescent probes has broadly impacted many areas in biomedical research. QDs offer several adventures over traditional fluorophores. Traditional fluorophores have certain properties that limit their biomedical applications: narrow absorption and broad emission spectra, low luminescence and quantum yield, rapid photobleaching, and low brightness signal. A limitation of traditional small-molecule fluorescent dyes is in the labeling of other small molecules, drugs, transporters, and small-molecule probes to cell-surface receptors. Conjugates of dyes to these small molecules often lack sensitivity or specificity in the detection of the desired targets. Conjugates of small molecules to QDs produce conjugates with much greater light output per binding event, owing to the increased absorbance and emission of the QD [14]. The fact that several QDs can be excited by the same wavelength of light opens up several multiplexing potentials, including high-throughput screening of biological samples [9]. Size-tunable absorption and emission property of QDs is an extremely valuable property for biological imaging as they can be tuned all the way from the UV to the near-infrared of the spectrum. QDs exhibit better photophysical properties than conventional fluorophores under appropriate conditions. A limitation of traditional small-molecule fluorescent dyes is in the labeling of other small molecules, drugs, transporters, and small-molecule probes to cell-surface receptors. Conjugates of dyes to these small molecules often lack sensitivity or specificity in the detection of the desired targets. Conjugates of small molecules to QDs produce conjugates with much greater light output binding event, owing to the increased absorbance and emission of the QD. Furthermore, there is the possibility of improved avidity compared of a dye conjugate, owing to the combined effect of many molecules of the binding ligand on the surface of the quantum dot [14]. Biocompatible QDs represent a powerful tool for the direct readout of information down to single molecule level [28]. For example, by conjugating an antibody to the QD, targeting to specific cells or tissues can be affected *in vivo* or in fluorescent antibody immunoassays. QDs have shown great potential to provide spatial, temporal, and structural information for biological systems: *in vivo* cell labeling, *in vitro* cell labeling, detection of tumor marker, *in situ* tissue diagnostic, noninvasive tumor imaging, tracking of local cancer grown and its distant dissemination, detection and therapy of various disease, identifying of various types of biomarkers, more effective and early diagnosis of cancer,

are used to fabricate a new type of QDs, such as Gd-doped ZnO QDs [44].

**7. Adventures of quantum dots compared to conventional**

**fluorophores**

imaging of live tissue, etc.

#### **6.1. Nontoxic quantum dots**

The last few years of research are focused on QDs that do not contain Cd. QDs made up of III–V semiconductors such as InP. Groups III–V QDs may provide a more stable alternative to groups II–VI (CdSe, Cd, Te, etc.) [50]. QDs due to the presence of a covalent, rather than an ionic bond, and have been reported to have lower cytotoxicity. However these QDs are difficult to prepare on a competitive time scale, and tend to have much lower quantum efficiencies, meaning uptake has been slow [51]. The most highlighted candidates for Cd-free QDs are the InP-based QDs. InP QDs a very attractive candidate for replacing cadmium-based QDs for biological studies. However, till date, there are very little studies reporting the use of InP QDs in bioimaging applications, as they are difficult to prepare because of the sensitivity of precursors and surfactants toward the reaction environment in obtaining good quality InP QDs [52]. Therefore, progresses in the synthesis of InP-based QDs occurred much slowly compared to that of CdSe QDs [53]. InP/ZnS QDs combining the brightness, photostability, and biocompatibility characteristics will serve as a new generation of targeted optical probes for several biomedical applications, including early detection of cancer, replacing the cadmium-based [54]. Silicon nanoparticle is another type of QDs which possess unique properties when their sizes are reduces to below 10 nm. Similar to their predecessors, silicon QDs also have many advantages over traditional fluorescent organic dye. Moreover, owing to silicon's nontoxic and environment-friendly nature, Si QDs are used as fluorescent probes for bioimaging. Si QDs are produced in nonpolar solvents with hydrophobic ligands on their surface in other to protect the Si core. Therefore, it is a common problem that silicon QDs show poor solubility and unstable photoluminescence in aqueous solutions. Thus, fabricating good water-disperse Si QDs with stable optical characters is vital to their applications in bioimaging studies. In the demand of using biocompatible and nontoxic QDs as nanoprobes, rare-earth (RE) elements are used to fabricate a new type of QDs, such as Gd-doped ZnO QDs [44].

## **7. Adventures of quantum dots compared to conventional fluorophores**

An unstable surface coating could expose the core of the QD to UV damage or air oxidation. The toxic heavy metal ions can be easily leaked out into biological systems, if the surfaces are not properly covered by the shells or protected by ligands [44]. The stability of QDs and their resistance to metabolic degradation in live cells would allow long-term imaging studies, and several studies have indicated lack of cytotoxicity for period up to 4 months [47]. Although such QDs should not be acutely toxic as long as their polymer coating is stable enough to restrain the release of cadmium, both short- and long-term safety of QDs will need to be established in toxicological studies in clinically relevant animal models. Studies in cell lines have shown that QDs do not affect cell growth under normal media conditions and shortterm administration of QDs into animals seems not to affect the metabolism and behavior of the animals [48]. Xiao and coworkers showed that the cytotoxicity of CdSe quantum was proportional to QDs concentration in the tested range. There was a negligible cytotoxicity at concentration of 20 nM, which is twice of the applied concentration of fluorescent cell imaging test. On the other side, no size-dependent cytotoxicity was observed. For time-dependence, cytotoxicity increased along with prolongation of incubation period, and tended to be stable after 12 h of incubation. Therefore, cytotoxicity can be neglected in a typical fluorescent cell imaging procedure (10 nM/L, 12 h), and the labeling effect to microtubule can be guaranteed satisfactorily [49]. There are also concerns about toxicity related to the molecules used for surface functionalization of the QDs [6]. However, toxicity concerns are still valid for longer periods and *in vivo* applications. Given the highly intrinsic toxicity of cadmium, the biological applications of Cd-QDs have been limited. The limitation of heavy metal containing QDs stimulates extensive research interests in exploring alternative strategies for the design of fluorescent nanocrystals with high biocompatibility. In this case, the practical strategy is to

develop highly fluorescent nanoparticles based on nontoxic elements [3].

The last few years of research are focused on QDs that do not contain Cd. QDs made up of III–V semiconductors such as InP. Groups III–V QDs may provide a more stable alternative to groups II–VI (CdSe, Cd, Te, etc.) [50]. QDs due to the presence of a covalent, rather than an ionic bond, and have been reported to have lower cytotoxicity. However these QDs are difficult to prepare on a competitive time scale, and tend to have much lower quantum efficiencies, meaning uptake has been slow [51]. The most highlighted candidates for Cd-free QDs are the InP-based QDs. InP QDs a very attractive candidate for replacing cadmium-based QDs for biological studies. However, till date, there are very little studies reporting the use of InP QDs in bioimaging applications, as they are difficult to prepare because of the sensitivity of precursors and surfactants toward the reaction environment in obtaining good quality InP QDs [52]. Therefore, progresses in the synthesis of InP-based QDs occurred much slowly compared to that of CdSe QDs [53]. InP/ZnS QDs combining the brightness, photostability, and biocompatibility characteristics will serve as a new generation of targeted optical probes for several biomedical applications, including early detection of cancer, replacing the cadmium-based [54]. Silicon nanoparticle is another type of QDs which possess unique properties when their sizes are reduces to below 10 nm. Similar to their predecessors, silicon QDs also have many advantages over traditional fluorescent organic dye. Moreover, owing to silicon's nontoxic

**6.1. Nontoxic quantum dots**

34 Ionizing Radiation Effects and Applications

The ability to study molecular and cellular events by using fluorescent probes has broadly impacted many areas in biomedical research. QDs offer several adventures over traditional fluorophores. Traditional fluorophores have certain properties that limit their biomedical applications: narrow absorption and broad emission spectra, low luminescence and quantum yield, rapid photobleaching, and low brightness signal. A limitation of traditional small-molecule fluorescent dyes is in the labeling of other small molecules, drugs, transporters, and small-molecule probes to cell-surface receptors. Conjugates of dyes to these small molecules often lack sensitivity or specificity in the detection of the desired targets. Conjugates of small molecules to QDs produce conjugates with much greater light output per binding event, owing to the increased absorbance and emission of the QD [14]. The fact that several QDs can be excited by the same wavelength of light opens up several multiplexing potentials, including high-throughput screening of biological samples [9]. Size-tunable absorption and emission property of QDs is an extremely valuable property for biological imaging as they can be tuned all the way from the UV to the near-infrared of the spectrum. QDs exhibit better photophysical properties than conventional fluorophores under appropriate conditions. A limitation of traditional small-molecule fluorescent dyes is in the labeling of other small molecules, drugs, transporters, and small-molecule probes to cell-surface receptors. Conjugates of dyes to these small molecules often lack sensitivity or specificity in the detection of the desired targets. Conjugates of small molecules to QDs produce conjugates with much greater light output binding event, owing to the increased absorbance and emission of the QD. Furthermore, there is the possibility of improved avidity compared of a dye conjugate, owing to the combined effect of many molecules of the binding ligand on the surface of the quantum dot [14]. Biocompatible QDs represent a powerful tool for the direct readout of information down to single molecule level [28]. For example, by conjugating an antibody to the QD, targeting to specific cells or tissues can be affected *in vivo* or in fluorescent antibody immunoassays. QDs have shown great potential to provide spatial, temporal, and structural information for biological systems: *in vivo* cell labeling, *in vitro* cell labeling, detection of tumor marker, *in situ* tissue diagnostic, noninvasive tumor imaging, tracking of local cancer grown and its distant dissemination, detection and therapy of various disease, identifying of various types of biomarkers, more effective and early diagnosis of cancer, imaging of live tissue, etc.

## **8. Conclusion**

Quantum dots have emerged as a new promising class of fluorescent probes for biomolecular and cellular imaging. In comparison with organic dyes and fluorescent proteins, QDs have unique optical and electronic properties. QDs do not exhibit aqueous solubility as they are generally synthesized in organic solution and are surface-stabilized with hydrophobic organic ligands. To make them useful for biomedical applications, QDs need to be conjugated to biological molecules without disturbing the biological function of these molecules. Various surface modification techniques were developed to ensure the specific bioconjugation. This is usually achieved by decorating QDs with proteins, peptides, nucleic acids, streptavidin, or other biomolecules that mediate specific interactions with living systems. Biocompatible quantum dots represent a powerful tool for the direct readout of information down to single molecule level. The limiting factor of application QDs *in vivo* is their toxicity. Although QD technology is still not in much use due to their hydrophobicity, toxicity, and many issues need to be solved in order to apply them safely in biomedical applications.

[7] Dey S, Jain YS. On the wave mechanics of a particle in two different impenetrable spheri-

Applicability of Quantum Dots in Biomedical Science http://dx.doi.org/10.5772/intechopen.71428 37

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## **Author details**

Slavica Brkić

Address all correspondence to<EMAIL_ADDRESS>
Faculty of Science and Education, University of Mostar, Mostar, Bosnia and Herzegovina

## **References**

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**8. Conclusion**

36 Ionizing Radiation Effects and Applications

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**Author details**

Address all correspondence to<EMAIL_ADDRESS>
Quantum dots have emerged as a new promising class of fluorescent probes for biomolecular and cellular imaging. In comparison with organic dyes and fluorescent proteins, QDs have unique optical and electronic properties. QDs do not exhibit aqueous solubility as they are generally synthesized in organic solution and are surface-stabilized with hydrophobic organic ligands. To make them useful for biomedical applications, QDs need to be conjugated to biological molecules without disturbing the biological function of these molecules. Various surface modification techniques were developed to ensure the specific bioconjugation. This is usually achieved by decorating QDs with proteins, peptides, nucleic acids, streptavidin, or other biomolecules that mediate specific interactions with living systems. Biocompatible quantum dots represent a powerful tool for the direct readout of information down to single molecule level. The limiting factor of application QDs *in vivo* is their toxicity. Although QD technology is still not in much use due to their hydrophobicity, toxicity, and many issues need to be solved in order to apply them safely in biomedical

Faculty of Science and Education, University of Mostar, Mostar, Bosnia and Herzegovina

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38 Ionizing Radiation Effects and Applications

pp. 16-17

**Chapter 3**

**Provisional chapter**

**Hydrogen Gas Detection by Mini-Raman Lidar**

**Hydrogen Gas Detection by Mini-Raman Lidar**

DOI: 10.5772/intechopen.74630

Now, Hydrogen gas is of particular interest as new energy source and dangerous material in nuclear facility. Fuel cell is started to use in home power generation system in 2008 and fuel cell vehicle (FCV) is commercialized from 2014 in Japan. On contrary, the Great East Japan Earthquake revealed the fear of hydrogen explosion on Fukushima Nuclear Power plant in 2011. Contact type hydrogen sensors induce changes on the gas flow, and the actual concentration cannot be known. It is hard to get the gas concentration distribution in hydrogen leakage area. We focused on optical remote sensing for the hydrogen detection. Raman scattering detection was accomplished for the hydrogen gas with a compact Diode Pumped Solid State (DPSS) laser-based Raman lidar. The quantitative measurement was conducted on the hydrogen gas concentration of 1 - 100% and the detectable distance of <50 m. Next, a LED-based mini Raman lidar was developed with the same optical design as the former one in viewpoints of its robust operation and usability in the nuclear facility. The high-speed photon counter was developed to follow the high repetition frequency of LED pulse of >500 kHz, and the quantitative measurements of hydrogen concentration were conducted on lab-experiment and at outdoor.

Nowadays, there is a strong demand for remote sensing in near range. Needless to say, the fear of terrorism, the human daily lives whether at urban or at countryside have the threat of nonsafety. The urban polluted air, smoke on chemical factory, dust from construction site pose a threat to human social activities. Certain dangerous gas detections in volcanic zone, construction site, and water inspection, air quality check in factory, hall, park, and so on, are desired for safe life. Certain gas/dust monitoring in construction site, and air quality estimation in daily

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Keywords:** hydrogen, lidar, Raman, scattering, LED

http://dx.doi.org/10.5772/intechopen.74630

Tatsuo Shiina

**Abstract**

**1. Introduction**

Tatsuo Shiina

#### **Hydrogen Gas Detection by Mini-Raman Lidar Hydrogen Gas Detection by Mini-Raman Lidar**

DOI: 10.5772/intechopen.74630

#### Tatsuo Shiina Tatsuo Shiina

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74630

#### **Abstract**

Now, Hydrogen gas is of particular interest as new energy source and dangerous material in nuclear facility. Fuel cell is started to use in home power generation system in 2008 and fuel cell vehicle (FCV) is commercialized from 2014 in Japan. On contrary, the Great East Japan Earthquake revealed the fear of hydrogen explosion on Fukushima Nuclear Power plant in 2011. Contact type hydrogen sensors induce changes on the gas flow, and the actual concentration cannot be known. It is hard to get the gas concentration distribution in hydrogen leakage area. We focused on optical remote sensing for the hydrogen detection. Raman scattering detection was accomplished for the hydrogen gas with a compact Diode Pumped Solid State (DPSS) laser-based Raman lidar. The quantitative measurement was conducted on the hydrogen gas concentration of 1 - 100% and the detectable distance of <50 m. Next, a LED-based mini Raman lidar was developed with the same optical design as the former one in viewpoints of its robust operation and usability in the nuclear facility. The high-speed photon counter was developed to follow the high repetition frequency of LED pulse of >500 kHz, and the quantitative measurements of hydrogen concentration were conducted on lab-experiment and at outdoor.

**Keywords:** hydrogen, lidar, Raman, scattering, LED

## **1. Introduction**

Nowadays, there is a strong demand for remote sensing in near range. Needless to say, the fear of terrorism, the human daily lives whether at urban or at countryside have the threat of nonsafety. The urban polluted air, smoke on chemical factory, dust from construction site pose a threat to human social activities. Certain dangerous gas detections in volcanic zone, construction site, and water inspection, air quality check in factory, hall, park, and so on, are desired for safe life. Certain gas/dust monitoring in construction site, and air quality estimation in daily

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

life circumference are included in such demands for the improvement of working condition in factory and facility.

is explained. Some results of quantitative measurements are shown with actual observation

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 43

Utilization of hydrogen energy is undergoing, which is led mainly by the Agency for Natural Resources and Energy in the Ministry of Economy, Trade and Industry, Japan to promote the

Fuel cell generates electricity by chemical reaction between hydrogen and oxygen. It causes only water vapor and its power generation efficiency exceeds 30%. Hydrogen gas is stored in a high-pressure tank at 35 MPa in hydrogen station and FCV. 96 hydrogen stations were built in 2016 in Japan, and it will increase 160 stations in 2020. FCV were installed 2000 cars in 2016, and bus and forklift truck types were installed into on-site, too. The agency aims to expand it up to 40,000 cars in 2020. Now, FCV seems to conceal itself under electric vehicle (EV). FCV, however, can drive for a long travel distance of 700–850 km at one-time insufflations of hydrogen of 3 min [11]. For that reason, EV is good for small automobile for short distance, while

Home-use fuel cell system is established its technologies in complex with cogeneration. Hydrogen is generated from urban gas or LP gas, and chemically reacts with oxygen, then generates the electricity. Exhaust heat on electricity generation is utilized to the other purpose (for example hot water supply). 200,000 systems have been already installed up to 2016.

Hydrogen gas on the nuclear reactor is generated by the different process against the above. In general, the nuclear fuel rod boils water with its high temperature in nuclear reaction. At the Great East Japan Earthquake, it is considered that the tragic event was caused by the following process. The nuclear reactor was stopped automatically, while the fuel cell rod continued heating. Because the cooling system was fallen down, the water level was lowered and the rod was naked. Although the nuclear fuel rod was covered with zirconium, it reacted with water on the high temperature. As a result, zirconium took oxygen away from the water, the

Under the controlled condition to generate the electricity or the unforeseen circumstances,

Hydrogen is flammable gas. It explodes at the concentration of >4% under the oxygen circumference [12]. The Fire Defense Agency defined that the gas sensor must have the minimal detection of the one fourth of the explosion limit of the target gas. It is essential to detect such flammable gas before the risk of its explosion. Methane, Carbon oxide, and so on have

hydrogen gas monitoring is essential and should prevent the accident.

**2. Hydrogen generation – controlled and unprepared**

spread of fuel cell car (FCV) and home-use fuel cell system.

FCV has a merit for large-size automobile for long distance.

Reducing the price of the system is undergoing.

**3. Optical sensing for hydrogen gas**

hydrogen gas was generated.

data.

In the viewpoint of safety management, the working condition focus on the dust and leaked gas detections under construction site at outdoor. The mass media concerns to the atmospheric suspended materials represented as SPM (Suspended Particle Material), PM10/PM2.5, yellow sand, pollens, and also industrial dusts as dioxin, photochemical smog, and asbestos. The dust flow information, including the nuclear material, is strongly desired after the Great East Japan Earthquake. On indoor application, chemical gas flow in a factory, dust density measurement and dangerous gas monitoring such as CO, CO2 , CH4 , and H2 S in a tunnel construction site in mountain and under the sea are essential for such safety managements.

At the atmospheric monitoring indoor and outdoor, the "contact" sensors are utilized. These sensors are compact, cheap, and easy-to-use, while they need a lot to cover the whole area. A lot of sensors should be scattered in a certain area to get the target distribution and behavior. Low concentration gas and shooting out gas is difficult to perform the quantitative measurement [1, 2].

The word "LIDAR (Light Detection And Ranging)" is sometimes written as LiDAR for hard target detection and lidar for soft target detection. The former is utilized as a range finder to detect the distance up to the target. The latter is for monitoring the target condition and behavior at a certain distance. It is an ideal technique to monitor the suspended materials such as aerosol, dust, gas, and atmosphere remotely and safely [3–5]. Unlike a transmissometer which consists of an separated pair of transmitter and receiver, lidar can fix at a point and scan the observing direction. Then it can cover a certain area remotely. Lidar can obtain the distribution and change of the target in monitoring site by scanning the observation direction. Its observation range is enough long of a few to a hundred kilometers. In contrary, recently it applied for near range monitoring within a few hundred meters. Its application is industrial use.

Lidar technique utilizes various kinds of scattering. Mie scattering, Rayleigh scattering, Raman scattering, and so on. Mie scattering is generated by larger materials such as aerosols and dusts, while the Rayleigh scattering is caused by smaller materials such as molecules configuring the atmosphere. Raman scattering is brought about every material but it shifts wavelength of scattering light from the incident light one. It is convenient for target gas detection.

In this study, we developed the compact Raman lidar for hydrogen gas detection. To prevent the explosion accident of hydrogen gas, the sensor should detect the target gas of the low concentration of 1/4 of the explosion limit. In the case of hydrogen gas, its concentration is 1% [6–8]. The optical intensity of the lidar transmitter is designed to be safe for human eye and skin. This study has been started to apply for leaked gas detection in a hydrogen gas station [9, 10]. After that it has expanded to adapt to the nuclear facility. This lidar should be compact and easy-to-use. It is also applied to monitor the air qualities of working environment including hydrogen gas, atmospheric nitrogen gas and water vapor.

This report states the developments of the Diode Pumped Solid State (DPSS) laser-based compact Raman lidar and the LED-based mini-Raman lidar. At first Raman scattering and lidar principle are briefly mentioned, and after that, the development of concrete Raman lidars is explained. Some results of quantitative measurements are shown with actual observation data.

## **2. Hydrogen generation – controlled and unprepared**

life circumference are included in such demands for the improvement of working condition

In the viewpoint of safety management, the working condition focus on the dust and leaked gas detections under construction site at outdoor. The mass media concerns to the atmospheric suspended materials represented as SPM (Suspended Particle Material), PM10/PM2.5, yellow sand, pollens, and also industrial dusts as dioxin, photochemical smog, and asbestos. The dust flow information, including the nuclear material, is strongly desired after the Great East Japan Earthquake. On indoor application, chemical gas flow in a factory, dust density

struction site in mountain and under the sea are essential for such safety managements.

At the atmospheric monitoring indoor and outdoor, the "contact" sensors are utilized. These sensors are compact, cheap, and easy-to-use, while they need a lot to cover the whole area. A lot of sensors should be scattered in a certain area to get the target distribution and behavior. Low concentration gas and shooting out gas is difficult to perform the quantitative measurement [1, 2].

The word "LIDAR (Light Detection And Ranging)" is sometimes written as LiDAR for hard target detection and lidar for soft target detection. The former is utilized as a range finder to detect the distance up to the target. The latter is for monitoring the target condition and behavior at a certain distance. It is an ideal technique to monitor the suspended materials such as aerosol, dust, gas, and atmosphere remotely and safely [3–5]. Unlike a transmissometer which consists of an separated pair of transmitter and receiver, lidar can fix at a point and scan the observing direction. Then it can cover a certain area remotely. Lidar can obtain the distribution and change of the target in monitoring site by scanning the observation direction. Its observation range is enough long of a few to a hundred kilometers. In contrary, recently it applied for near range monitoring within a few hundred meters. Its application is

Lidar technique utilizes various kinds of scattering. Mie scattering, Rayleigh scattering, Raman scattering, and so on. Mie scattering is generated by larger materials such as aerosols and dusts, while the Rayleigh scattering is caused by smaller materials such as molecules configuring the atmosphere. Raman scattering is brought about every material but it shifts wavelength of scattering light from the incident light one. It is convenient for target gas detection. In this study, we developed the compact Raman lidar for hydrogen gas detection. To prevent the explosion accident of hydrogen gas, the sensor should detect the target gas of the low concentration of 1/4 of the explosion limit. In the case of hydrogen gas, its concentration is 1% [6–8]. The optical intensity of the lidar transmitter is designed to be safe for human eye and skin. This study has been started to apply for leaked gas detection in a hydrogen gas station [9, 10]. After that it has expanded to adapt to the nuclear facility. This lidar should be compact and easy-to-use. It is also applied to monitor the air qualities of working environment includ-

This report states the developments of the Diode Pumped Solid State (DPSS) laser-based compact Raman lidar and the LED-based mini-Raman lidar. At first Raman scattering and lidar principle are briefly mentioned, and after that, the development of concrete Raman lidars

, CH4

, and H2

S in a tunnel con-

measurement and dangerous gas monitoring such as CO, CO2

ing hydrogen gas, atmospheric nitrogen gas and water vapor.

in factory and facility.

42 Ionizing Radiation Effects and Applications

industrial use.

Utilization of hydrogen energy is undergoing, which is led mainly by the Agency for Natural Resources and Energy in the Ministry of Economy, Trade and Industry, Japan to promote the spread of fuel cell car (FCV) and home-use fuel cell system.

Fuel cell generates electricity by chemical reaction between hydrogen and oxygen. It causes only water vapor and its power generation efficiency exceeds 30%. Hydrogen gas is stored in a high-pressure tank at 35 MPa in hydrogen station and FCV. 96 hydrogen stations were built in 2016 in Japan, and it will increase 160 stations in 2020. FCV were installed 2000 cars in 2016, and bus and forklift truck types were installed into on-site, too. The agency aims to expand it up to 40,000 cars in 2020. Now, FCV seems to conceal itself under electric vehicle (EV). FCV, however, can drive for a long travel distance of 700–850 km at one-time insufflations of hydrogen of 3 min [11]. For that reason, EV is good for small automobile for short distance, while FCV has a merit for large-size automobile for long distance.

Home-use fuel cell system is established its technologies in complex with cogeneration. Hydrogen is generated from urban gas or LP gas, and chemically reacts with oxygen, then generates the electricity. Exhaust heat on electricity generation is utilized to the other purpose (for example hot water supply). 200,000 systems have been already installed up to 2016. Reducing the price of the system is undergoing.

Hydrogen gas on the nuclear reactor is generated by the different process against the above. In general, the nuclear fuel rod boils water with its high temperature in nuclear reaction. At the Great East Japan Earthquake, it is considered that the tragic event was caused by the following process. The nuclear reactor was stopped automatically, while the fuel cell rod continued heating. Because the cooling system was fallen down, the water level was lowered and the rod was naked. Although the nuclear fuel rod was covered with zirconium, it reacted with water on the high temperature. As a result, zirconium took oxygen away from the water, the hydrogen gas was generated.

Under the controlled condition to generate the electricity or the unforeseen circumstances, hydrogen gas monitoring is essential and should prevent the accident.

## **3. Optical sensing for hydrogen gas**

Hydrogen is flammable gas. It explodes at the concentration of >4% under the oxygen circumference [12]. The Fire Defense Agency defined that the gas sensor must have the minimal detection of the one fourth of the explosion limit of the target gas. It is essential to detect such flammable gas before the risk of its explosion. Methane, Carbon oxide, and so on have absorption lines in the near infrared spectrum, and the differential absorption technique can be adapted for these gas detections. The hydrogen gas, however, has its absorption line in < l10 nm and does not have any clear absorption lines in ultra violet, visible – near infrared spectra [13]. The hydrogen gas detection is only conducted by contact chemical sensors such as combustible material, catalyst materials (Pt or Pd), and semiconductor type (SnO2 ) [1, 2]. These sensors are high-sensitive to detect its concentration of a few tens ppm, while it is hard to get the gas distribution in whole of monitoring site. Furthermore, the contact sensor disturbs the gas flow, and its sensing direction influences to the measurement value. Its response time is slow, too.

> To estimate the quantitative concentration of hydrogen gas, Raman scattering intensities of atmospheric nitrogen is utilized. When the concentration of hydrogen gas is negligible amount than that of atmospheric nitrogen, the ratio between the Raman scattering intensities of hydrogen gas and atmospheric nitrogen can indicate the concentration of the hydrogen gas quantitatively. The ratio is calibrated by the relative Raman scattering cross sections of hydrogen and nitrogen, which is about 3:1 [5]. **Table 1** shows the concrete Raman shifts and

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 45

**Target Raman shift (cm−1) Wavelength (nm)**

Nitrogen 2331 380 Water vapor 3652 400 Oxygen 1556 369 Hydrogen 4160 408

**Table 1.** Raman shift and its wavelength against 349 nm light source.

The principle of lidar is illustrated in **Figure 2**. The pulsed laser beam was collimated and fired into the atmosphere with almost parallel to the receiver's optical axis. The optical receiver is adjusted its field of view (FOV) to detect the lidar echo from the target with an adequate signal-to-noise ratio. The receiver's FOV decides the detecting range and the blind area, which the transmitting beam cannot enter within a receiver's FOV, especially in near range. It also affects the detecting intensity of the background light. As a traditional lidar has individual optics (biaxial optics) for a laser transmitter and a receiver, the system has a blind area. The blind area also causes in the case of coaxial optics that the beam is transmitted within a receiver's area. As FOV is wider, or the beam divergence is wider, the blind area will be shortened, while the background light enters a

lot into the receiver. As a result, the signal-to-noise ratio of the lidar echo will be worse.

hard to detect an analogue signal, photon counting method is selectable.

The lidar echo is detected by high sensitive detectors such as photomultiplier and Avalanche Photo Diode (APD), and stored in PC via digitizer. When the lidar echo becomes weak and

The compact lidar is suitable to the near range observation of less than a few hundred meters, while the traditional large lidar is good at the long-range observation from a few kilometers to a hundred kilometers. The compact lidar, however, is not a down-size one of the large lidar. **Table 2** shows the spatial and time scales in atmospheric phenomena. The large phenomena such as HP/LP have a large spatial and time scales, while the small phenomena like tornado takes small spatial and time scales. That is, the small phenomenon becomes small structure and quick motion. To detect and visualize such small phenomena, the compact lidar should follow to its quick motion with high resolution. Furthermore, near range detection is often a sensing in human living space. In that meaning, the transmitting optical power should be

their shifted wavelengths against the 349 nm laser light source.

**4. Compact Raman lidar**

Hydrogen gas has strong Raman effect, which depends on material. Raman scattering shifts the wavelength of the scattering light from that of the incident light. In general, the intensity of Raman scattering is 3 digits smaller than that of the Mie scattering [14, 15]. When electrons in atmospheric molecules (or certain gas, atom, and so on) are resonant with photons, most photons are elastically scattered (it is Rayleigh scattering), such that the scattered photons have the same optical frequency and wavelength as the incident light (photons). A small fraction of such photons is scattered with a frequency different from the original frequency of the incident photons. The frequency shift is usually lower than and partially higher than the original; Stokes scattering as longer wavelength and anti-Stokes scattering as shorter wavelength. The interaction of light with a material in a linear region allows absorption and emission of a photon precisely matching the difference in energy levels of the interacting electron. The energy absorption of the molecules causes Stokes scattering, while the energy loss of the molecules generates the anti-Stokes scattering. Stokes scattering shifts the wavelength of the incident light to the longer side. Anti-Stokes scattering shifts that to the shorter side as shown in **Figure 1**. Intensity of Anti-Stokes scattering is smaller than that of Stokes scattering. In general, this trend is remarkably strong when the Raman shift becomes large. In this study, Stokes scattering of hydrogen gas is selected for the lidar detection target under these backgrounds.

**Figure 1.** Raman scattering.

**Table 1.** Raman shift and its wavelength against 349 nm light source.

To estimate the quantitative concentration of hydrogen gas, Raman scattering intensities of atmospheric nitrogen is utilized. When the concentration of hydrogen gas is negligible amount than that of atmospheric nitrogen, the ratio between the Raman scattering intensities of hydrogen gas and atmospheric nitrogen can indicate the concentration of the hydrogen gas quantitatively. The ratio is calibrated by the relative Raman scattering cross sections of hydrogen and nitrogen, which is about 3:1 [5]. **Table 1** shows the concrete Raman shifts and their shifted wavelengths against the 349 nm laser light source.

## **4. Compact Raman lidar**

absorption lines in the near infrared spectrum, and the differential absorption technique can be adapted for these gas detections. The hydrogen gas, however, has its absorption line in < l10 nm and does not have any clear absorption lines in ultra violet, visible – near infrared spectra [13]. The hydrogen gas detection is only conducted by contact chemical sensors such

These sensors are high-sensitive to detect its concentration of a few tens ppm, while it is hard to get the gas distribution in whole of monitoring site. Furthermore, the contact sensor disturbs the gas flow, and its sensing direction influences to the measurement value. Its response

Hydrogen gas has strong Raman effect, which depends on material. Raman scattering shifts the wavelength of the scattering light from that of the incident light. In general, the intensity of Raman scattering is 3 digits smaller than that of the Mie scattering [14, 15]. When electrons in atmospheric molecules (or certain gas, atom, and so on) are resonant with photons, most photons are elastically scattered (it is Rayleigh scattering), such that the scattered photons have the same optical frequency and wavelength as the incident light (photons). A small fraction of such photons is scattered with a frequency different from the original frequency of the incident photons. The frequency shift is usually lower than and partially higher than the original; Stokes scattering as longer wavelength and anti-Stokes scattering as shorter wavelength. The interaction of light with a material in a linear region allows absorption and emission of a photon precisely matching the difference in energy levels of the interacting electron. The energy absorption of the molecules causes Stokes scattering, while the energy loss of the molecules generates the anti-Stokes scattering. Stokes scattering shifts the wavelength of the incident light to the longer side. Anti-Stokes scattering shifts that to the shorter side as shown in **Figure 1**. Intensity of Anti-Stokes scattering is smaller than that of Stokes scattering. In general, this trend is remarkably strong when the Raman shift becomes large. In this study, Stokes scattering of hydrogen gas is selected for the lidar

) [1, 2].

as combustible material, catalyst materials (Pt or Pd), and semiconductor type (SnO2

time is slow, too.

44 Ionizing Radiation Effects and Applications

detection target under these backgrounds.

**Figure 1.** Raman scattering.

The principle of lidar is illustrated in **Figure 2**. The pulsed laser beam was collimated and fired into the atmosphere with almost parallel to the receiver's optical axis. The optical receiver is adjusted its field of view (FOV) to detect the lidar echo from the target with an adequate signal-to-noise ratio. The receiver's FOV decides the detecting range and the blind area, which the transmitting beam cannot enter within a receiver's FOV, especially in near range. It also affects the detecting intensity of the background light. As a traditional lidar has individual optics (biaxial optics) for a laser transmitter and a receiver, the system has a blind area. The blind area also causes in the case of coaxial optics that the beam is transmitted within a receiver's area. As FOV is wider, or the beam divergence is wider, the blind area will be shortened, while the background light enters a lot into the receiver. As a result, the signal-to-noise ratio of the lidar echo will be worse.

The lidar echo is detected by high sensitive detectors such as photomultiplier and Avalanche Photo Diode (APD), and stored in PC via digitizer. When the lidar echo becomes weak and hard to detect an analogue signal, photon counting method is selectable.

The compact lidar is suitable to the near range observation of less than a few hundred meters, while the traditional large lidar is good at the long-range observation from a few kilometers to a hundred kilometers. The compact lidar, however, is not a down-size one of the large lidar. **Table 2** shows the spatial and time scales in atmospheric phenomena. The large phenomena such as HP/LP have a large spatial and time scales, while the small phenomena like tornado takes small spatial and time scales. That is, the small phenomenon becomes small structure and quick motion. To detect and visualize such small phenomena, the compact lidar should follow to its quick motion with high resolution. Furthermore, near range detection is often a sensing in human living space. In that meaning, the transmitting optical power should be

**Figure 2.** Lidar principle.

lower for eye-safety. It causes the limit of transmitting power and the selection of optical wavelength.

The Raman scattering intensity is estimated by Eq. (1).

$$\mathbf{P}\_{\mathrm{R}} = \frac{\lambda\_{\mathrm{o}}}{\lambda\_{\mathrm{R}}} \mathbf{P}\_{\mathrm{o}} \,\chi\mathrm{p}\sigma\_{\mathrm{g}} \,\mathrm{L}\,\eta\_{\mathrm{D}} \tag{1}$$

measurement is calculated by the intensity ratio of nitrogen echo and hydrogen echo. The lidar echo is inversely proportional to the distance to the power of square. But the above intensity

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 47

The concept of the hydrogen remote sensing is to install the "remote sensor" into the area monitoring in a hydrogen station. The sensor is put on a small car and monitors the inside area of the station on the street. In that meaning, the remote sensor should be compact. The

**Figure 3.** Optics of compact Raman lidar. (a) Compact Raman lidar—optical setup. (b) Outlook of compact Raman lidar.

ratio cancels the intensity decrease [6].

eye-safety should be considered, too.

where P0 , PR are wavelength λ0 , λR of laser light source and Raman scattering intensity, respectively. χ is molar function of the target, ρ is number density of the molecules, σR is Raman scattering cross-section, L is pulse length and ηD is receiver's optical efficiency. The quantitative

**Table 2.** Spatial and time scales of atmosphere.

measurement is calculated by the intensity ratio of nitrogen echo and hydrogen echo. The lidar echo is inversely proportional to the distance to the power of square. But the above intensity ratio cancels the intensity decrease [6].

The concept of the hydrogen remote sensing is to install the "remote sensor" into the area monitoring in a hydrogen station. The sensor is put on a small car and monitors the inside area of the station on the street. In that meaning, the remote sensor should be compact. The eye-safety should be considered, too.

lower for eye-safety. It causes the limit of transmitting power and the selection of optical

P0 χρσR L ηD (1)

, λR of laser light source and Raman scattering intensity, respec-

λR

**Phenomena Spatial scale Time scale** HP/LP 1000 km 10 h Typhoon 100 km 3 h Convection 50 km 2 h Thunder clouds 10 km 1 h Cumulus 2 km 10 min Down burst 600 m 7 min Tornado 200 m 5 min Boundary layer 60 m 10 s

tively. χ is molar function of the target, ρ is number density of the molecules, σR is Raman scattering cross-section, L is pulse length and ηD is receiver's optical efficiency. The quantitative

The Raman scattering intensity is estimated by Eq. (1).

PR = λ\_\_0

, PR are wavelength λ0

**Table 2.** Spatial and time scales of atmosphere.

wavelength.

**Figure 2.** Lidar principle.

46 Ionizing Radiation Effects and Applications

where P0

**Figure 3.** Optics of compact Raman lidar. (a) Compact Raman lidar—optical setup. (b) Outlook of compact Raman lidar.

In this study, we have developed the "in-line" type compact Raman lidar [16–21]. The in-line type optics has no blind area in principle. The in-line lidar has common optics for a transmitter and a receiver. An optical divider, here dichroic mirror, to separate the receiving echo and transmitting beam is installed. We designed and developed the compact Raman lidar to detect leaked hydrogen gas as minimum setup. It means that the system should be compact, light-weight and low electric power consumption. Our goal is the detection of the hydrogen gas concentration of 1% within the observation range of 0–50 m. The lidar optics is illustrated in **Figure 3**. The DPSS laser is utilized for lidar transmitting beam. Its power is 120 μJ and the wavelength is 349 nm. The receiver's aperture is 50 mmϕ. The transmitting beam is expanded to 10 mmϕ to be safe for human eye even in front of the system. The wavelength of the incident beam is decided to obtain the strong Raman signal. Shorter wavelength causes the larger Raman signal than the longer wavelength. The receiver's FOV was 2 mrad. The Raman shifted echoes passes through the first dichroic mirror (1), which is a divider, and echoes never returns to the light source. The Raman echoes are divided by the second dichroic mirror (2) depending on the wavelength for each Raman detection. The detectors are PMTs; one is for hydrogen gas (408 nm) and the other is for atmospheric nitrogen gas (380 nm). The interference filters of the wavelength widths of 3 nm are fixed on their PMTs. The nitrogen signal is used to estimate the quantitative concentration of hydrogen gas. The system size is 58 × 36 × 23 cm. It can put on the small desk.

A quantitative measurement of the Raman scattering echo intensity from hydrogen gas was conducted. The hydrogen gas was filled in a gas cell of the size of 10 cm cube, of which window is quartz. The cell was positioned at a distance of 7–8 m from the lidar system and put with a small tilt against the optical axis to reduce the reflection and fluorescent lights to return the lidar. Although the reflection and fluorescence lights appear on the hydrogen and nitrogen echoes, they can be subtracted out. Furthermore, the Raman echo from hydrogen gas fluctuates in actual gas leaks, and it can be separated from the reflection and fluorescence lights from a hard target, which are constant in time. The observation distance of 8 m is defined as the safe distance for the explosion protection of hydrogen gas. The lidar echo was measured by an

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 49

**Figure 5.** Estimation of hydrogen gas concentration. (a) High concentration of about 10–100% and (b) low concentration

of 0–15%.

oscilloscope with a photomultiplier tube (PMT) operating in analogue mode.

**Figure 4** shows the detected Raman echo of atmospheric nitrogen. Its signal was obtained from 1.5 to 50 m. The nearest echo in 0–1.5 m in **Figure 4(a)** was spoiled because of the misalignment and the aberration of the lenses. **Figure 4(b)** shows the range-corrected nitrogen Raman echo, which is corrected by the square of the distance and the overlap function between the transmitter and the receiver. The corrected echo intensity becomes flat and the fluctuation is minimum at any point within the observation range. The constant intensity indicates stable concentration of atmospheric nitrogen. This nitrogen echo is utilized to estimate the concentration of hydrogen gas in the observation range by taking the ratio between them.

**Figure 4.** Lidar echo of atmospheric nitrogen gas. (a) A-scope signal. (b) Range-corrected data.

A quantitative measurement of the Raman scattering echo intensity from hydrogen gas was conducted. The hydrogen gas was filled in a gas cell of the size of 10 cm cube, of which window is quartz. The cell was positioned at a distance of 7–8 m from the lidar system and put with a small tilt against the optical axis to reduce the reflection and fluorescent lights to return the lidar. Although the reflection and fluorescence lights appear on the hydrogen and nitrogen echoes, they can be subtracted out. Furthermore, the Raman echo from hydrogen gas fluctuates in actual gas leaks, and it can be separated from the reflection and fluorescence lights from a hard target, which are constant in time. The observation distance of 8 m is defined as the safe distance for the explosion protection of hydrogen gas. The lidar echo was measured by an oscilloscope with a photomultiplier tube (PMT) operating in analogue mode.

In this study, we have developed the "in-line" type compact Raman lidar [16–21]. The in-line type optics has no blind area in principle. The in-line lidar has common optics for a transmitter and a receiver. An optical divider, here dichroic mirror, to separate the receiving echo and transmitting beam is installed. We designed and developed the compact Raman lidar to detect leaked hydrogen gas as minimum setup. It means that the system should be compact, light-weight and low electric power consumption. Our goal is the detection of the hydrogen gas concentration of 1% within the observation range of 0–50 m. The lidar optics is illustrated in **Figure 3**. The DPSS laser is utilized for lidar transmitting beam. Its power is 120 μJ and the wavelength is 349 nm. The receiver's aperture is 50 mmϕ. The transmitting beam is expanded to 10 mmϕ to be safe for human eye even in front of the system. The wavelength of the incident beam is decided to obtain the strong Raman signal. Shorter wavelength causes the larger Raman signal than the longer wavelength. The receiver's FOV was 2 mrad. The Raman shifted echoes passes through the first dichroic mirror (1), which is a divider, and echoes never returns to the light source. The Raman echoes are divided by the second dichroic mirror (2) depending on the wavelength for each Raman detection. The detectors are PMTs; one is for hydrogen gas (408 nm) and the other is for atmospheric nitrogen gas (380 nm). The interference filters of the wavelength widths of 3 nm are fixed on their PMTs. The nitrogen signal is used to estimate the quantitative concentration of hydrogen gas. The system size is 58 × 36 ×

**Figure 4** shows the detected Raman echo of atmospheric nitrogen. Its signal was obtained from 1.5 to 50 m. The nearest echo in 0–1.5 m in **Figure 4(a)** was spoiled because of the misalignment and the aberration of the lenses. **Figure 4(b)** shows the range-corrected nitrogen Raman echo, which is corrected by the square of the distance and the overlap function between the transmitter and the receiver. The corrected echo intensity becomes flat and the fluctuation is minimum at any point within the observation range. The constant intensity indicates stable concentration of atmospheric nitrogen. This nitrogen echo is utilized to estimate the concen-

tration of hydrogen gas in the observation range by taking the ratio between them.

**Figure 4.** Lidar echo of atmospheric nitrogen gas. (a) A-scope signal. (b) Range-corrected data.

23 cm. It can put on the small desk.

48 Ionizing Radiation Effects and Applications

**Figure 5.** Estimation of hydrogen gas concentration. (a) High concentration of about 10–100% and (b) low concentration of 0–15%.

The echo intensity was obtained by the developed Raman lidar system. The concentration of hydrogen gas was controlled by adjusting the gas pressure in the cell. The results are shown in **Figure 5**, for (a) high concentration of about 10–100% and (b) enlarge graph of low concentration of 0–15%. These results were calibrated by subtracting the reference signal of the empty gas cell. The summation was about 30,000 shots. It is equal to a measurement time of about 30 s. The echo intensity ratio is linearly proportional to the hydrogen gas concentration. The result indicates the successful estimation down to the low concentration of 1%. The estimation error becomes less than ±1%. The fluctuation in the echo intensity was mainly due to the background light, unstable power of the laser beam and thermal noise of the detector. The actual leaked gas fluctuates more drastically.

laser power was gradually decreased up to the Raman detection limit, which is estimated 1 W (=10 nJ/10 ns) by the numerical analysis. The reflection light by the glass plate was utilized to decrease the laser intensity efficiently and stably. The experimental result is shown in **Figure 7**. It shows that the Raman scattering echo was successfully detected by the estimated optical

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 51

Considering the above results, we developed the LED-based mini-Raman lidar. The lidar optics is biaxial type and the gas detection method is as same as the laser based compact Raman lidar as shown in **Figure 3**. The LED lamp module of the optical power of 1 W was used as the lidar light source. The wavelength of LED light is 365 nm, and the wavelengths of Raman scattering echoes become 400 nm for atmospheric nitrogen, 420 nm for water vapor and 430 nm for hydrogen gas with the same Raman shift shown in **Table 1**. The LED mini-Raman lidar optics is shown with photo in **Figure 8**. The optical setup is the same as the DPSSlaser based compact Raman lidar. The LED-based Raman lidar added the water vapor port by changing the optical interference filter with a filter wheel. All of the wavelength widths of the

Although LED beam is hard to collimate, it never focuses after the transmission. It gives advantage in the closed space in the viewpoint of human safety concern. In this study, the transmitting beam was enlarged to 50 mmϕ. Its divergence was about 10 mrad. The LED pulsed power was <1 W (=10 nJ/10 ns), while the pulse repetition frequency was 500 kHz. The receiving telescope was Cassegrain type of the aperture of 127 mmϕ. The receiver's FOV was 3 mrad. The transmitting power is so weak, and the echo detection is photon counting with

The system is so compact and handy that one can use it in any observation direction. The system power consumption is about 2 W and can operate with a small PC battery in 8–10 h. The observation range is aimed up to 20–30 m for hydrogen Raman signal. This observation range design comes from the facility space of a nuclear reactor to detect the hydrogen gas leakage.

interference filters were 10 nm. It is the same width the used LED spectrum.

power of the transmitting beam.

photomultipliers.

The specifications are summarized in **Table 3**.

**Figure 7.** Raman scattering echo detection with lowered incident power.

## **5. LED-based mini-Raman lidar**

The developed DPSS-based compact Raman lidar is designed to be eye-safe, while it is hard to install it into a certain monitoring site. The laser beam is possible to focus on a point, and it is not acceptable especially in dangerous gas environment because of its ignition. It is also hard to use it at outdoor with no AC power supply. In such demands, we started to develop the LED-based mini-Raman lidar with low optical power.

At first, we had the experiment to obtain the minimum power to detect the Raman echo of hydrogen gas as shown in **Figure 6**. The laser beam is entered into the quartz-windows cell filled with hydrogen gas. The hydrogen gas concentration was 100%. The Raman scattering echo was detected at the vertical direction. It is because the Raman intensity becomes maximum at the perpendicular direction against the optical axis of the transmitting beam. The

**Figure 6.** Verification of Raman scattering echo measurement with the minimum light intensity.

laser power was gradually decreased up to the Raman detection limit, which is estimated 1 W (=10 nJ/10 ns) by the numerical analysis. The reflection light by the glass plate was utilized to decrease the laser intensity efficiently and stably. The experimental result is shown in **Figure 7**. It shows that the Raman scattering echo was successfully detected by the estimated optical power of the transmitting beam.

The echo intensity was obtained by the developed Raman lidar system. The concentration of hydrogen gas was controlled by adjusting the gas pressure in the cell. The results are shown in **Figure 5**, for (a) high concentration of about 10–100% and (b) enlarge graph of low concentration of 0–15%. These results were calibrated by subtracting the reference signal of the empty gas cell. The summation was about 30,000 shots. It is equal to a measurement time of about 30 s. The echo intensity ratio is linearly proportional to the hydrogen gas concentration. The result indicates the successful estimation down to the low concentration of 1%. The estimation error becomes less than ±1%. The fluctuation in the echo intensity was mainly due to the background light, unstable power of the laser beam and thermal noise of the detector.

The developed DPSS-based compact Raman lidar is designed to be eye-safe, while it is hard to install it into a certain monitoring site. The laser beam is possible to focus on a point, and it is not acceptable especially in dangerous gas environment because of its ignition. It is also hard to use it at outdoor with no AC power supply. In such demands, we started to develop

At first, we had the experiment to obtain the minimum power to detect the Raman echo of hydrogen gas as shown in **Figure 6**. The laser beam is entered into the quartz-windows cell filled with hydrogen gas. The hydrogen gas concentration was 100%. The Raman scattering echo was detected at the vertical direction. It is because the Raman intensity becomes maximum at the perpendicular direction against the optical axis of the transmitting beam. The

The actual leaked gas fluctuates more drastically.

the LED-based mini-Raman lidar with low optical power.

**Figure 6.** Verification of Raman scattering echo measurement with the minimum light intensity.

**5. LED-based mini-Raman lidar**

50 Ionizing Radiation Effects and Applications

Considering the above results, we developed the LED-based mini-Raman lidar. The lidar optics is biaxial type and the gas detection method is as same as the laser based compact Raman lidar as shown in **Figure 3**. The LED lamp module of the optical power of 1 W was used as the lidar light source. The wavelength of LED light is 365 nm, and the wavelengths of Raman scattering echoes become 400 nm for atmospheric nitrogen, 420 nm for water vapor and 430 nm for hydrogen gas with the same Raman shift shown in **Table 1**. The LED mini-Raman lidar optics is shown with photo in **Figure 8**. The optical setup is the same as the DPSSlaser based compact Raman lidar. The LED-based Raman lidar added the water vapor port by changing the optical interference filter with a filter wheel. All of the wavelength widths of the interference filters were 10 nm. It is the same width the used LED spectrum.

Although LED beam is hard to collimate, it never focuses after the transmission. It gives advantage in the closed space in the viewpoint of human safety concern. In this study, the transmitting beam was enlarged to 50 mmϕ. Its divergence was about 10 mrad. The LED pulsed power was <1 W (=10 nJ/10 ns), while the pulse repetition frequency was 500 kHz. The receiving telescope was Cassegrain type of the aperture of 127 mmϕ. The receiver's FOV was 3 mrad. The transmitting power is so weak, and the echo detection is photon counting with photomultipliers.

The system is so compact and handy that one can use it in any observation direction. The system power consumption is about 2 W and can operate with a small PC battery in 8–10 h. The observation range is aimed up to 20–30 m for hydrogen Raman signal. This observation range design comes from the facility space of a nuclear reactor to detect the hydrogen gas leakage. The specifications are summarized in **Table 3**.

**Figure 7.** Raman scattering echo detection with lowered incident power.

obtained up to 70 m (**Figure 9(a)**). After that the fluorescence echo of the tree was also detected at 80 m. The atmospheric nitrogen is stable and its echo waveform becomes smooth. On the other hand, the water vapor echo had fluctuation because of low counts (**Figure 9(b)**). It was drastically changed by depending on the humidity. In the figure, the humidity was 25% on Feb. 7 in fine day, while it was 90% on Feb. 9 in rain day. The observation range of the water vapor was 70 m, which is equal to that of the nitrogen echo. The fluorescence echoes of trees

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 53

The quantitative measurement of hydrogen gas was conducted with a large gas chamber as shown in **Figure 10**. The hydrogen gas chamber has an aperture of 200 mm and a length of 500 mm. Pyrex glass of 10 mm thickness was used for light path windows to restrict the fluorescent light. It was set 5 m ahead from the LED mini-Raman lidar. The transmitting beam was adjusted its optical path against the receiver's field of view to cross both of optical paths

The hydrogen gas concentration was adjusted by vacuuming the air inside the chamber of 0.5 Pa and inserting the 100% hydrogen. This procedure repeated twice, and the hydrogen gas concentration of 75% was prepared. We also confirmed the air and hydrogen gases were well mixed in the chamber. After the first measurement, hydrogen gas concentration set to half, that is, 37.5% by vacuuming the inside gas of 0.5 Pa and inserting the air. Repeating these procedures, hydrogen gas concentration was adjusted as 18.8%, 9.4%,

Experimental results are shown in **Figure 11**. The accumulation time was 3.5 min. The hydrogen Raman echo was appeared on the fluorescent light from the Pyrex glass. **Figure 11** shows enlargement of the peak change of the echo signals. Depending on the hydrogen gas concentration, the peak counts were changed. The echo counts of hydrogen gas Raman signal were subtracted from the reference echo (0% hydrogen gas). The results were summarized in **Figure 12**. In **Figure 12(a)**, hydrogen Raman echo counts at each detected distance were

were detected, too.

**Table 3.** Specifications of LED mini-Raman lidar.

at the chamber.

4.7%, and 2.4%.

**Figure 8.** LED mini-Raman lidar. (a) Optical structure. (b) Lidar setup.

The high-speed photon counter was developed, too. It can follow the pulse repetition frequency of >500 kHz. Its time resolution is 1 ns. It is equivalent to the spatial (distance) resolution of 0.15 m. It has 2 channels, and their simultaneous measurements are possible with a single input or output trigger signal.

**Figure 9** shows the observed Raman echo of the atmospheric nitrogen gas and the water vapor. The observation direction is nearly horizontal. The experiment was conducted in night time. Trees were captured at 80 m ahead. Accumulation was 3.5 min. Nitrogen echo was

**Table 3.** Specifications of LED mini-Raman lidar.

The high-speed photon counter was developed, too. It can follow the pulse repetition frequency of >500 kHz. Its time resolution is 1 ns. It is equivalent to the spatial (distance) resolution of 0.15 m. It has 2 channels, and their simultaneous measurements are possible with a

**Figure 9** shows the observed Raman echo of the atmospheric nitrogen gas and the water vapor. The observation direction is nearly horizontal. The experiment was conducted in night time. Trees were captured at 80 m ahead. Accumulation was 3.5 min. Nitrogen echo was

single input or output trigger signal.

52 Ionizing Radiation Effects and Applications

**Figure 8.** LED mini-Raman lidar. (a) Optical structure. (b) Lidar setup.

obtained up to 70 m (**Figure 9(a)**). After that the fluorescence echo of the tree was also detected at 80 m. The atmospheric nitrogen is stable and its echo waveform becomes smooth. On the other hand, the water vapor echo had fluctuation because of low counts (**Figure 9(b)**). It was drastically changed by depending on the humidity. In the figure, the humidity was 25% on Feb. 7 in fine day, while it was 90% on Feb. 9 in rain day. The observation range of the water vapor was 70 m, which is equal to that of the nitrogen echo. The fluorescence echoes of trees were detected, too.

The quantitative measurement of hydrogen gas was conducted with a large gas chamber as shown in **Figure 10**. The hydrogen gas chamber has an aperture of 200 mm and a length of 500 mm. Pyrex glass of 10 mm thickness was used for light path windows to restrict the fluorescent light. It was set 5 m ahead from the LED mini-Raman lidar. The transmitting beam was adjusted its optical path against the receiver's field of view to cross both of optical paths at the chamber.

The hydrogen gas concentration was adjusted by vacuuming the air inside the chamber of 0.5 Pa and inserting the 100% hydrogen. This procedure repeated twice, and the hydrogen gas concentration of 75% was prepared. We also confirmed the air and hydrogen gases were well mixed in the chamber. After the first measurement, hydrogen gas concentration set to half, that is, 37.5% by vacuuming the inside gas of 0.5 Pa and inserting the air. Repeating these procedures, hydrogen gas concentration was adjusted as 18.8%, 9.4%, 4.7%, and 2.4%.

Experimental results are shown in **Figure 11**. The accumulation time was 3.5 min. The hydrogen Raman echo was appeared on the fluorescent light from the Pyrex glass. **Figure 11** shows enlargement of the peak change of the echo signals. Depending on the hydrogen gas concentration, the peak counts were changed. The echo counts of hydrogen gas Raman signal were subtracted from the reference echo (0% hydrogen gas). The results were summarized in **Figure 12**. In **Figure 12(a)**, hydrogen Raman echo counts at each detected distance were

**Figure 10.** Hydrogen gas quantitative measurement. (a) Quantitative measurement of hydrogen gas. (b) System setup

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 55

**Figure 11.** Peak signal changes depending on hydrogen gas concentration.

and arrangement.

**Figure 9.** Atmospheric measurement by LED mini-Raman lidar. (a) Nitrogen Raman echo (b) Water Vapor echo.

graphed. The Raman echo counts at 5.25 and 5.4 m had the higher slant, while the firster (initial distance arising echo) and the later parts (main part of fluorescent light) of the peak signals showed lower slant. It means that the Raman scattering appears firster than the fluorescent light. The lidar counts at each distance in **Figure 12(a)** was summed as shown in **Figure 12(b)**. The errors were large, which came from the inexact concentration control of the hydrogen gas and the fluctuation of the fluorescent light, while the linear change was obtained down to the lowest concentration of a few percent. The Raman echo counts were remained at the 0% hydrogen gas concentration. We regarded hydrogen concentration after the procedure of vacuuming the content gas of 0.5 Pa and inserting the air two times as equal to 0%. It caused the remained echo counts at 0% concentration. And the fluctuation of the fluorescent light peak echo will cause it, too.

**Figure 10.** Hydrogen gas quantitative measurement. (a) Quantitative measurement of hydrogen gas. (b) System setup and arrangement.

**Figure 11.** Peak signal changes depending on hydrogen gas concentration.

graphed. The Raman echo counts at 5.25 and 5.4 m had the higher slant, while the firster (initial distance arising echo) and the later parts (main part of fluorescent light) of the peak signals showed lower slant. It means that the Raman scattering appears firster than the fluorescent light. The lidar counts at each distance in **Figure 12(a)** was summed as shown in **Figure 12(b)**. The errors were large, which came from the inexact concentration control of the hydrogen gas and the fluctuation of the fluorescent light, while the linear change was obtained down to the lowest concentration of a few percent. The Raman echo counts were remained at the 0% hydrogen gas concentration. We regarded hydrogen concentration after the procedure of vacuuming the content gas of 0.5 Pa and inserting the air two times as equal to 0%. It caused the remained echo counts at 0% concentration. And the fluctuation of the

**Figure 9.** Atmospheric measurement by LED mini-Raman lidar. (a) Nitrogen Raman echo (b) Water Vapor echo.

fluorescent light peak echo will cause it, too.

54 Ionizing Radiation Effects and Applications

**Figure 12.** Change of hydrogen Raman echo counts depending on hydrogen gas concentration. (a) Echo changes at each detected distance. (b) Count sum at each concentration.

The hydrogen leakage experiment was conducted at outdoor in night time as shown in **Figure 13**. The hydrogen leakage nozzle shown in **Figure 13(b)** was fixed at 2.5 m ahead from the mini-lidar. The optical alignment between the transmitter and the receiver was adjusted to overlap fully. The leaked hydrogen gas was 50 l/min. The result is summarized in **Figure 14**. The accumulation time was 3.5 min. It is the result of 3 average times. The obvious difference between the hydrogen gas leakage and leakage stop conditions was obtained. The detected counts on the leakage stop condition were caused by the fluorescent light from the materials near the hydrogen nozzle. As compared with the lab experiment in **Figure 12**, the hydrogen Raman counts was low. The leakage gas volume, especially gas flow width against the optical axis was shorter than the hydrogen gas chamber or the optical pulse length. As a result, the

**Figure 14.** Measurement result of hydrogen leakage gas outside.

**Figure 13.** Hydrogen leakage measurement experiment outside. (a) LED mini-Raman lidar setup. (b) Hydrogen nozzle.

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 57

**Figure 13.** Hydrogen leakage measurement experiment outside. (a) LED mini-Raman lidar setup. (b) Hydrogen nozzle.

**Figure 14.** Measurement result of hydrogen leakage gas outside.

**Figure 12.** Change of hydrogen Raman echo counts depending on hydrogen gas concentration. (a) Echo changes at each

detected distance. (b) Count sum at each concentration.

56 Ionizing Radiation Effects and Applications

The hydrogen leakage experiment was conducted at outdoor in night time as shown in **Figure 13**. The hydrogen leakage nozzle shown in **Figure 13(b)** was fixed at 2.5 m ahead from the mini-lidar. The optical alignment between the transmitter and the receiver was adjusted to overlap fully. The leaked hydrogen gas was 50 l/min. The result is summarized in **Figure 14**. The accumulation time was 3.5 min. It is the result of 3 average times. The obvious difference between the hydrogen gas leakage and leakage stop conditions was obtained. The detected counts on the leakage stop condition were caused by the fluorescent light from the materials near the hydrogen nozzle. As compared with the lab experiment in **Figure 12**, the hydrogen Raman counts was low. The leakage gas volume, especially gas flow width against the optical axis was shorter than the hydrogen gas chamber or the optical pulse length. As a result, the counts rate outdoor was smaller than the lab-experiment. After that, the authors repeated the quantitative measurement with more large chamber at the outdoor field. The same result in the lab-experiment was obtained.

The Raman signal is weak. It is 1/1000 of the Mie scattering signals. Our current experiments were limited in nighttime. When the DPSS laser-based lidar is fixed in a sunshade area, it will act with enough accuracy. At indoor, the artificial light was no effect for Raman signal detecting by excluding for its direct insertion to the detector even if in the case of the LED-based

Hydrogen Gas Detection by Mini-Raman Lidar http://dx.doi.org/10.5772/intechopen.74630 59

The hydrogen has a potential to become major energy resource. It will be popular in daily life near future. On contrary, its dangerousness is important to recognize once more. Remote sensing device for leaked hydrogen gas is essential. Compact and mini-Raman lidars will be

This study was granted by Chubu Electric Power Co., Inc. from 2013 to 2016. Shikoku research institution Inc. was cooperated on the outdoor experiment, too. The author really appreciates

Graduate School of Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba-shi, Japan

[1] Gu H, Wang Z, Hu Y. Hydrogen gas sensors based on semiconductor oxide nanostruc-

[2] Zeng XQ, Latimer ML, Xiao ZL, Panuganti S, Welp U, Kwok WK, Xu T. Hydrogen gas sensing with networks of ultrasmall palladium nanowires formed on filtration mem-

[3] Measures RM. Laser Remote Sensing: Fundamentals and Applications. New York: John

[5] Weitkamp C, editor. Lidar: Range-Resolved Optical Remote Sensing of the Atmosphere.

[6] Ninomiya H, Yaeshima S, Ichikawa K, Fukuchi T. Raman lidar system for hydrogen gas

[4] Fujii T, Fukuchi T, editors. Laser Remote Sensing. Taylor & Francis; 2005

lidar.

one of the solutions.

to these companies.

**Author details**

Address all correspondence to<EMAIL_ADDRESS>
tures. Sensors. 2012;**12**:5517-5550

Wiley & Sons; 1984

Springer; 2005

branes. Nano Letters. 2011;**11**:262-268

detection. Optical Engineering. 2007;**46**:094301

Tatsuo Shiina

**References**

**Acknowledgements**

## **6. Discussions**

In the lab-experiment, that is, the hydrogen cell/chamber used experiments at indoor, the linear change of the lidar counts was obtained depending on the hydrogen concentration up to 1%. The accuracy is enough to evaluate the concentration quantitatively. The DPSS laser-based lidar can perform it within a few seconds by adequate setup. The LED-based lidar needs more time to evaluate the concentration. The evaluation error is remained on the current setup. At outdoor experiment, the obvious hydrogen Raman echo by the LED Raman lidar was detected at the gas leakage situation, while it is not easy to verify the concrete concentration. It is not only the system task, but also the gas flow fluctuation. As the leakage area was narrower than the pulsed beam length, the counts rate became lower. The accumulation time, beam size, and transmitting power intensity should be balanced to satisfy the demands.

The hydrogen gas detection is not only to estimate the precise concentration, but also on the easy-to-use and easy-to-install at on-site. The LED-based mini-Raman lidar is designed under this consideration. Its gas detection optics is as same as the laser-based Raman lidar, while the LED module is installed into the system. The observation range becomes short, while the system becomes so small, light-weight, low power consumption, and safe. The power consumption is enough small to operate with a small buttery. The LED-based mini-Raman lidar needs the accumulation time, and it is adequate to install it to the regular observation fixed on a certain position and monitor the stored gas on a ceiling. When Ultra Violet/Near Ultra Violet Laser Diode (UV/NUV-LD) is possible to utilize, the beam collimation will be improved and it can increase signal-to-noise ratio with the same optical pulse power. It will shorten the accumulation time, and become one of the solutions to detect the leakage hydrogen gas in the emergency case under no electrical power.

## **7. Conclusions**

We developed the compact and the mini-Raman lidars for quantitative measurement of hydrogen gas. The current minimal detection limit of hydrogen gas concentration is 1% at the observation range of 0–50 m with DPSS-based compact lidar and 0–20 m with LED-based mini-lidar. The accumulation time is 30 s for the DPSS laser-based compact Raman lidar and 3.5 min for the LED-based mini-Raman lidar.

Now, the DPSS laser-based compact Raman lidar was improved to detect the ionization effect by nuclear materials. On contrary, the LED-based mini-Raman lidar expanded the target to the environmental measurement. The Raman scattering signal is a proof of a target gas existence. Each gas has its own Raman shift, and the optical sensing method of Raman lidar becomes one of solutions in the actual atmosphere, which contains complex materials.

The Raman signal is weak. It is 1/1000 of the Mie scattering signals. Our current experiments were limited in nighttime. When the DPSS laser-based lidar is fixed in a sunshade area, it will act with enough accuracy. At indoor, the artificial light was no effect for Raman signal detecting by excluding for its direct insertion to the detector even if in the case of the LED-based lidar.

The hydrogen has a potential to become major energy resource. It will be popular in daily life near future. On contrary, its dangerousness is important to recognize once more. Remote sensing device for leaked hydrogen gas is essential. Compact and mini-Raman lidars will be one of the solutions.

## **Acknowledgements**

counts rate outdoor was smaller than the lab-experiment. After that, the authors repeated the quantitative measurement with more large chamber at the outdoor field. The same result in

In the lab-experiment, that is, the hydrogen cell/chamber used experiments at indoor, the linear change of the lidar counts was obtained depending on the hydrogen concentration up to 1%. The accuracy is enough to evaluate the concentration quantitatively. The DPSS laser-based lidar can perform it within a few seconds by adequate setup. The LED-based lidar needs more time to evaluate the concentration. The evaluation error is remained on the current setup. At outdoor experiment, the obvious hydrogen Raman echo by the LED Raman lidar was detected at the gas leakage situation, while it is not easy to verify the concrete concentration. It is not only the system task, but also the gas flow fluctuation. As the leakage area was narrower than the pulsed beam length, the counts rate became lower. The accumulation time, beam size, and transmitting power intensity should be balanced to satisfy the demands. The hydrogen gas detection is not only to estimate the precise concentration, but also on the easy-to-use and easy-to-install at on-site. The LED-based mini-Raman lidar is designed under this consideration. Its gas detection optics is as same as the laser-based Raman lidar, while the LED module is installed into the system. The observation range becomes short, while the system becomes so small, light-weight, low power consumption, and safe. The power consumption is enough small to operate with a small buttery. The LED-based mini-Raman lidar needs the accumulation time, and it is adequate to install it to the regular observation fixed on a certain position and monitor the stored gas on a ceiling. When Ultra Violet/Near Ultra Violet Laser Diode (UV/NUV-LD) is possible to utilize, the beam collimation will be improved and it can increase signal-to-noise ratio with the same optical pulse power. It will shorten the accumulation time, and become one of the solutions to detect the leakage hydrogen gas in the

We developed the compact and the mini-Raman lidars for quantitative measurement of hydrogen gas. The current minimal detection limit of hydrogen gas concentration is 1% at the observation range of 0–50 m with DPSS-based compact lidar and 0–20 m with LED-based mini-lidar. The accumulation time is 30 s for the DPSS laser-based compact Raman lidar and

Now, the DPSS laser-based compact Raman lidar was improved to detect the ionization effect by nuclear materials. On contrary, the LED-based mini-Raman lidar expanded the target to the environmental measurement. The Raman scattering signal is a proof of a target gas existence. Each gas has its own Raman shift, and the optical sensing method of Raman lidar

becomes one of solutions in the actual atmosphere, which contains complex materials.

the lab-experiment was obtained.

58 Ionizing Radiation Effects and Applications

emergency case under no electrical power.

3.5 min for the LED-based mini-Raman lidar.

**7. Conclusions**

**6. Discussions**

This study was granted by Chubu Electric Power Co., Inc. from 2013 to 2016. Shikoku research institution Inc. was cooperated on the outdoor experiment, too. The author really appreciates to these companies.

## **Author details**

Tatsuo Shiina

Address all correspondence to<EMAIL_ADDRESS>
Graduate School of Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba-shi, Japan

## **References**

[7] Ninomiya H, Asahi I, Sugimoto S, Shimamoto Y. Development of remote sensing technology for hydrogen gas concentration measurement using Raman scattering effect. IEEJ Transactions on Electronics, Information and Systems. 2009;**129**:1181-1185

**Chapter 4**

Provisional chapter

. The Bohr's

**Investigating the Nature of Insulator-Metal Transition**

DOI: 10.5772/intechopen.72241