Compared to glass, the surface of polymers has usually higher roughness, but depending on the specific polymer, the roughness can vary a lot. Due to this fact, the morphology of film sputtered on polymers is very dependent on the surface structure of the polymer. Surface structure of substrates is formed during the preparation of the polymer foil (e.g., extrusion, casting of a solution or melted polymer, rolling, and blowing). Chemical and physical properties often govern the method that can be used to prepare a sheet the polymer—for example, sheets of polytetrafluoroethylene are prepared by cutting a block of the polymer, because it cannot be mechanically formed after its polymerization [32]. The character of functional groups on the polymer chain can also have a significant impact on the surface morphology, since it determines the polymer crystallinity [33]. This effect can be observed during annealing of the polymer. When exceeding temperatures of secondary phase transitions of the surface morphology can be significantly altered—at temperatures over the glass transition point (*Tg* ), polymer chain segments are enabled to shift which leads to surface recrystallization. With exceeding the melting temperature (*Tm*), the strain in the bulk of the polymer relaxes and a completely new surface morphology are formed [32]. The surface structure of different polymers therefore varies significantly and cannot be manipulated as precisely as the surface structure of crystalline materials.

An example of polymer substrates with very different surface morphologies are polyimide (PI) and polyetheretherketone (PEEK). The polymers were used as substrates for thin Au layer deposition and subsequently annealed. In the case of the PEEK substrate, the *Tg* was exceeded during annealing and the structure of the substrate surface was significantly disrupted leading to cleavage of the overlaying Au layer and changes of its optical properties (**Figure 3**).

The PI substrate is more thermally stable and preserves its surface structure during annealing at 300°C (**Figure 4**). Compared to the abovementioned case of glass substrate, the solid state dewetting was not observed on the polymer substrate, probably due to much better adhesion of the Au film [34]. Another thermally stable polymer substrate is polytetrafluoroethylene, which has a very rough surface structure. However, its surface structure can be manipulated, for example, by ion exposition in plasma discharge [35]. An intriguing substrate to support Au layers is poly-l-lactic acid. Samples with thin films show evolution of an interesting lamellar nanostructure during annealing at 60°C, which is caused by exposing of the polymer crystallites [36].

**4. Sputtering of noble metals onto liquid substrates**

In this part of the chapter, synthesis, properties, stability, and applications of noble metals sputtered directly into liquid substrates will be discussed. Metal nanoparticles (MNPs) have broad spectrum of applications, from electronics and catalysis to drug carriers and bio-imaging. Gold NPs (Au NPs), in particular, are widely used because of their unique properties such as size, shape, optical properties, and low toxicity [37]. NPs can be prepared by either chemical (Mie, Turkevich, Brust-Schiffrin, etc., methods) [38–40] or physical route (physical vapor deposition, laser ablation, and γ irradiation) [3, 41, 42]. The size, shape, and geometry of the NPs are strongly dependent on the employed method. Chemical synthesis offers advantages in simplicity of NP size and shape control, which enables preparation of NPs with required properties for specific application. Downside of this method is generation of harmful products and NPs with limited purity. On the other hand, the physical methods are very clean and the purity of the NPs synthesized is the same as the bulk material. In 1996 radiofrequency, sputtering of Ag NPs into a liquid substrate (silicone oil) was performed for the first time [3, 43]. However, silicone oil proved to be a suboptimal capping agent and

**Figure 4.** The DSC curves of the PI and PEEK substrates show PI is stable up to 400°C, where PEEK undergoes glass

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167

transition at ca. 144°C, then recrystallization takes place at around 170°C and it melts at about 320°C [34].

**Figure 3.** AFM images of the PI and PEEK substrates show significant differences of the surface structure of the polymers. While annealing of PI leads to no visible surface structure transformation, the surface of PEEK changes dramatically after annealing due to the recrystallization process. Values of surface roughness (*Ra* ) are stated for each sample [34].

**Figure 4.** The DSC curves of the PI and PEEK substrates show PI is stable up to 400°C, where PEEK undergoes glass transition at ca. 144°C, then recrystallization takes place at around 170°C and it melts at about 320°C [34].

#### **4. Sputtering of noble metals onto liquid substrates**

(*Tg*

(**Figure 3**).

tallites [36].

structure of crystalline materials.

166 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

), polymer chain segments are enabled to shift which leads to surface recrystallization. With exceeding the melting temperature (*Tm*), the strain in the bulk of the polymer relaxes and a completely new surface morphology are formed [32]. The surface structure of different polymers therefore varies significantly and cannot be manipulated as precisely as the surface

An example of polymer substrates with very different surface morphologies are polyimide (PI) and polyetheretherketone (PEEK). The polymers were used as substrates for thin Au layer deposition and subsequently annealed. In the case of the PEEK substrate, the *Tg*

exceeded during annealing and the structure of the substrate surface was significantly disrupted leading to cleavage of the overlaying Au layer and changes of its optical properties

The PI substrate is more thermally stable and preserves its surface structure during annealing at 300°C (**Figure 4**). Compared to the abovementioned case of glass substrate, the solid state dewetting was not observed on the polymer substrate, probably due to much better adhesion of the Au film [34]. Another thermally stable polymer substrate is polytetrafluoroethylene, which has a very rough surface structure. However, its surface structure can be manipulated, for example, by ion exposition in plasma discharge [35]. An intriguing substrate to support Au layers is poly-l-lactic acid. Samples with thin films show evolution of an interesting lamellar nanostructure during annealing at 60°C, which is caused by exposing of the polymer crys-

**Figure 3.** AFM images of the PI and PEEK substrates show significant differences of the surface structure of the polymers. While annealing of PI leads to no visible surface structure transformation, the surface of PEEK changes dramatically

) are stated for each sample [34].

after annealing due to the recrystallization process. Values of surface roughness (*Ra*

was

In this part of the chapter, synthesis, properties, stability, and applications of noble metals sputtered directly into liquid substrates will be discussed. Metal nanoparticles (MNPs) have broad spectrum of applications, from electronics and catalysis to drug carriers and bio-imaging. Gold NPs (Au NPs), in particular, are widely used because of their unique properties such as size, shape, optical properties, and low toxicity [37]. NPs can be prepared by either chemical (Mie, Turkevich, Brust-Schiffrin, etc., methods) [38–40] or physical route (physical vapor deposition, laser ablation, and γ irradiation) [3, 41, 42]. The size, shape, and geometry of the NPs are strongly dependent on the employed method. Chemical synthesis offers advantages in simplicity of NP size and shape control, which enables preparation of NPs with required properties for specific application. Downside of this method is generation of harmful products and NPs with limited purity. On the other hand, the physical methods are very clean and the purity of the NPs synthesized is the same as the bulk material. In 1996 radiofrequency, sputtering of Ag NPs into a liquid substrate (silicone oil) was performed for the first time [3, 43]. However, silicone oil proved to be a suboptimal capping agent and thus the synthesized NPs were not sufficiently stable. After 10 years of experiments, in 2006, researchers started to use ionic liquids (ILs) as liquid substrates for preparation of noble metal nanoparticles. Apart from the ILs, there are other liquid substrates, which are suitable for preparation of NPs by physical vapor deposition (i.e., sputtering). The main condition for selecting a suitable liquid substrate is sufficiently low vapor pressure in vacuum [44]. Vegetable oils [45], liquid polyethylene glycol [46, 47], molten 6-mercaptohexyltrimethylammonium bromide [48], and glycerol were used for preparation of MNPs [49]. For example, Siegel et al. demonstrated that by sputtering of various noble metals spherical NPs of different sizes can be prepared: 6.1 ± 1.0 nm for Au, 4.2 ± 0.9 nm for Ag, 2.5 ± 0.6 nm for Pd, and 1.8 ± 0.4 nm for Pt [50]. Polyethylene glycol (PEG) is one of the most commonly used stabilizing agents. It is a neutral amphiphilic polymer that gives "stealth" character to nanostructures. PEGylation of NPs is a process that introduces PEG ligands onto NPs surface. This method was introduced by Ballou et al. for the first time [51]. PEGylation generally increases circulation of NPs in blood, decreases immunity response, and slows the cleansing by the reticuloendothelial system [52]. PEG is versatile, inexpensive, and it has been approved by Food and Drug Administration (FDA) [53]. Properties of PEGylated MNPs also depend on molecular weight of the liquid PEG. Slepicka et al. prepared Au and Ag NPs by sputtering into PEG with molecular weight of 200, 400, and 600 g mol−1. Since PEG-200 and PEG-600 showed lower stability during aging, only PEG-400 was used for further experiments. The prepared Au and Ag NPs were spherical with size of 7.97 ± 2.92 nm and 30–50 nm, respectively. Localized surface plasmon resonance absorption bands lied between 513 and 560 nm for Au NPs and 401–421 nm for Ag NPs [46].

#### **4.1. Synthesis and properties of PEGylated gold nanoparticles**

In this segment, results of our research for the preparation, properties, and examination of temperature and aging stability of gold nanoparticles (Au NPs) are presented. In many research papers, NPs synthesis is a multistep process consisting of many purification processes. In the first step, NPs are synthesized using a surfactant (e.g., citrate) as a stabilizing agent and, in the next step, the capping agent is exchanged with PEG molecules [54–56]. We present a simple, reproducible, environmentally friendly, low cost, and easily applicable approach of Au NPs preparation by direct sputtering of Au into a mixture of polyethylene glycol (PEG) and thiolated PEG (see **Figure 5A**). NPs consist of two components: the core and the corona, which interacts with the surrounding media (**Figure 5B**). The ─OH groups of PEG create hydrogen bridges with water, which leads to formation of a solvation shell and stabilization of the NPs, preventing aggregation during dissolution. A stronger stabilization is achieved with thiol groups that have good affinity to Au. In the case of thiolated PEGs, the bond between Au and the thiol group is very strong (197.4 kJ/mol) and minimizes desorption of PEG from NP surface (**Figure 5C**) [57].

were: 400 g mol−1 for PEG, 800 g mol−1 for PEG-SH, and 1500 g mol−1 for PEG-S2

apparent difference in size, distribution, and concentration was detected.

+ PEG-SH or PEG + PEG-S2

H2

which reacts with the surrounding solvent [47].

tendency to aggregate. During the stabilization of Au NPs in PEG-S2

dency to agglomerate (**Figure 6D**). In the case of Au sputtering into PEG-S2

tively. The TEM images in **Figure 6** show that Au NPs were successfully prepared in all types of the capturing media. Despite the fact that the shape of Au NPs was spherical in all cases, an

**Figure 5.** Schema of (A) preparation of PEGylated Au NPs by direct sputtering into liquid media of (i) PEG; (ii) PEG

respectively; (B) stabilization of Au NP by PEG, indicating metal core with immobilized PEG chains; (C) monomers of ethylene glycol are polymerized into PEG. PEG contains two end groups R1 linkage group and R2 terminus group,

, and (iii) PEG and subsequent mixing with either PEG: PEG-SH or PEG: PEG-S2

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The size and size distribution of individual PEGylated Au NP solutions are summarized in **Table 1** and in insets in **Figure 6**. The direct sputtering of Au atoms into pure PEG for 300 s and 900 s led to spherical NPs with an average diameter of 5.6 and 5.9 nm. TEM images show that the nanoparticles prepared under such conditions have the worst size distribution with inclination to create agglomerates (see **Figure 6A** and **B**). The Au NPs prepared in the solution of PEG-SH had average diameter of 2.9 nm. The small size of the nanoparticles can be caused by low interfacial tension of PEG-SH and high nucleation rates or by PEG-SH slowing down the growth of the Au NPs (**Figure 6C**) due to high stabilization effect of thiolated PEG [3, 58]. On top of that, these NPs have a narrower size distribution and a lower

bridges are created. Due to interaction of thiol groups, the nanoparticles have higher ten-

the tendency to aggregate was less apparent and the size distribution was narrower (i.e., 3.6 ± 0.6 nm, **Figure 6E**). Post-deposition addition of PEG-SH improves the homogeneity of the nanoparticles in terms of size, shape, and aggregation (**Figure 6F** and **G**) and leads

H2

solution, disulfide

for 300 s,

H2

H2

, respec-

H2 , 169

Our research group prepared Au NPs by direct sputtering for 300, 900, and 1800 s into PEG (300 s—A; 900 s—B), PEG: PEG-SH (1800 s—C), PEG: PEG-S2 H2 (900 s—D; 300 s—E), PEG admixed with PEG-SH (300 s—F; 900 s—G) and PEG admixed with PEG-S2 H2 (300 s—H; 900 s—I). Immediately after sputtering, the PEG with Au NPs was mixed with distilled water in the volume ratio of 1:9 (PEG: water). Molecular weights of selected polyethylene glycols

thus the synthesized NPs were not sufficiently stable. After 10 years of experiments, in 2006, researchers started to use ionic liquids (ILs) as liquid substrates for preparation of noble metal nanoparticles. Apart from the ILs, there are other liquid substrates, which are suitable for preparation of NPs by physical vapor deposition (i.e., sputtering). The main condition for selecting a suitable liquid substrate is sufficiently low vapor pressure in vacuum [44]. Vegetable oils [45], liquid polyethylene glycol [46, 47], molten 6-mercaptohexyltrimethylammonium bromide [48], and glycerol were used for preparation of MNPs [49]. For example, Siegel et al. demonstrated that by sputtering of various noble metals spherical NPs of different sizes can be prepared: 6.1 ± 1.0 nm for Au, 4.2 ± 0.9 nm for Ag, 2.5 ± 0.6 nm for Pd, and 1.8 ± 0.4 nm for Pt [50]. Polyethylene glycol (PEG) is one of the most commonly used stabilizing agents. It is a neutral amphiphilic polymer that gives "stealth" character to nanostructures. PEGylation of NPs is a process that introduces PEG ligands onto NPs surface. This method was introduced by Ballou et al. for the first time [51]. PEGylation generally increases circulation of NPs in blood, decreases immunity response, and slows the cleansing by the reticuloendothelial system [52]. PEG is versatile, inexpensive, and it has been approved by Food and Drug Administration (FDA) [53]. Properties of PEGylated MNPs also depend on molecular weight of the liquid PEG. Slepicka et al. prepared Au and Ag NPs by sputtering into PEG with molecular weight of 200, 400, and 600 g mol−1. Since PEG-200 and PEG-600 showed lower stability during aging, only PEG-400 was used for further experiments. The prepared Au and Ag NPs were spherical with size of 7.97 ± 2.92 nm and 30–50 nm, respectively. Localized surface plasmon resonance absorption bands lied between 513 and 560 nm

In this segment, results of our research for the preparation, properties, and examination of temperature and aging stability of gold nanoparticles (Au NPs) are presented. In many research papers, NPs synthesis is a multistep process consisting of many purification processes. In the first step, NPs are synthesized using a surfactant (e.g., citrate) as a stabilizing agent and, in the next step, the capping agent is exchanged with PEG molecules [54–56]. We present a simple, reproducible, environmentally friendly, low cost, and easily applicable approach of Au NPs preparation by direct sputtering of Au into a mixture of polyethylene glycol (PEG) and thiolated PEG (see **Figure 5A**). NPs consist of two components: the core and the corona, which interacts with the surrounding media (**Figure 5B**). The ─OH groups of PEG create hydrogen bridges with water, which leads to formation of a solvation shell and stabilization of the NPs, preventing aggregation during dissolution. A stronger stabilization is achieved with thiol groups that have good affinity to Au. In the case of thiolated PEGs, the bond between Au and the thiol group is very strong (197.4 kJ/mol) and minimizes desorption

Our research group prepared Au NPs by direct sputtering for 300, 900, and 1800 s into PEG

900 s—I). Immediately after sputtering, the PEG with Au NPs was mixed with distilled water in the volume ratio of 1:9 (PEG: water). Molecular weights of selected polyethylene glycols

H2

(900 s—D; 300 s—E), PEG

H2

(300 s—H;

for Au NPs and 401–421 nm for Ag NPs [46].

168 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

of PEG from NP surface (**Figure 5C**) [57].

**4.1. Synthesis and properties of PEGylated gold nanoparticles**

(300 s—A; 900 s—B), PEG: PEG-SH (1800 s—C), PEG: PEG-S2

admixed with PEG-SH (300 s—F; 900 s—G) and PEG admixed with PEG-S2

**Figure 5.** Schema of (A) preparation of PEGylated Au NPs by direct sputtering into liquid media of (i) PEG; (ii) PEG + PEG-SH or PEG + PEG-S2 H2 , and (iii) PEG and subsequent mixing with either PEG: PEG-SH or PEG: PEG-S2 H2 , respectively; (B) stabilization of Au NP by PEG, indicating metal core with immobilized PEG chains; (C) monomers of ethylene glycol are polymerized into PEG. PEG contains two end groups R1 linkage group and R2 terminus group, which reacts with the surrounding solvent [47].

were: 400 g mol−1 for PEG, 800 g mol−1 for PEG-SH, and 1500 g mol−1 for PEG-S2 H2 , respectively. The TEM images in **Figure 6** show that Au NPs were successfully prepared in all types of the capturing media. Despite the fact that the shape of Au NPs was spherical in all cases, an apparent difference in size, distribution, and concentration was detected.

The size and size distribution of individual PEGylated Au NP solutions are summarized in **Table 1** and in insets in **Figure 6**. The direct sputtering of Au atoms into pure PEG for 300 s and 900 s led to spherical NPs with an average diameter of 5.6 and 5.9 nm. TEM images show that the nanoparticles prepared under such conditions have the worst size distribution with inclination to create agglomerates (see **Figure 6A** and **B**). The Au NPs prepared in the solution of PEG-SH had average diameter of 2.9 nm. The small size of the nanoparticles can be caused by low interfacial tension of PEG-SH and high nucleation rates or by PEG-SH slowing down the growth of the Au NPs (**Figure 6C**) due to high stabilization effect of thiolated PEG [3, 58]. On top of that, these NPs have a narrower size distribution and a lower tendency to aggregate. During the stabilization of Au NPs in PEG-S2 H2 solution, disulfide bridges are created. Due to interaction of thiol groups, the nanoparticles have higher tendency to agglomerate (**Figure 6D**). In the case of Au sputtering into PEG-S2 H2 for 300 s, the tendency to aggregate was less apparent and the size distribution was narrower (i.e., 3.6 ± 0.6 nm, **Figure 6E**). Post-deposition addition of PEG-SH improves the homogeneity of the nanoparticles in terms of size, shape, and aggregation (**Figure 6F** and **G**) and leads

to a small decrease of average NP size too (see **Table 1**). In the case of the post-deposition

the energy of the system after addition of PEG-SH, and thus, the growth slowdown is more pronounced [59]. Another reason could be the different molecule weights of the used PEG-

To confirm measured dimensions of NPs obtained from TEM, dynamic light scattering (DLS) analysis was conducted. The values of nanoparticle sizes together with polydispersity index (quantifying the degree of the size dispersion) are listed in **Table 1**. In the case of DLS, the statistical significance of designated values of particle size is much higher because all particles in a particular sample are used for the calculation (determination). The prepared colloidal solutions have mono-nodal distribution of sizes (see **Table 1**), that means, they do not contain nanoparticle aggregates, which are in good agreement with TEM results (**Figure 6**). We believe that this discrepancy is caused by the coalescence of NPs during preparation of TEM sample for the analysis. The phenomenon probably occurs during evaporation of solution on the copper grid. To assess the monodispersity of the prepared PEGylated Au NPs, we present the results of DLS analysis in the form of intensity-esteemed distribution curve of the sizes. This curve is most sensitive to presence of agglomerates, which would not be visible in the case of transformation to other types of distribution curve (volume-esteemed or numberesteemed). Another reason for discrepancy between the DLS and the TEM results is that the DLS method is not based only on hydrodynamic average of the particles themselves, but also includes the solvation shel.l [60]. Since the nanoparticles are stabilized with relatively long macromolecular chains, the detected NP sizes may be influenced by PEG corona. This is the

 and PEG-SH. Sputtering of Au into thiolated PEG or its addition leads to change of the color of the solution from dark/light red to dark/light brown (depending on concentration

decrease of energy of the system after post-deposition addition of PEG-S2

**Sample Au NP size (nm) PDI TEM DLS** A 5.6 ± 1.8 9.2 ± 1.5 0.429 B 5.9 ± 0.8 8.8 ± 2.2 0.201 C 2.9 ± 0.4 3.0 ± 0.6 0.026 D 4.0 ± 1.2 4.9 ± 1.0 0.066 E 3.6 ± 0.6 4.3 ± 1.2 0.107 F 4.2 ± 0.8 5.2 ± 1.5 0.517 G 5.7 ± 1.3 7.0 ± 1.9 0.311 H 2.1 ± 0.7 4.3 ± 1.1 0.112 I 4.1 ± 1.3 6.5 ± 1.3 0.214

index (PDI) of colloidal systems was determined by DLS analysis [47].

**Table 1.** Size of PEGylated Au NPs (in nm; with relative error) determined by TEM and DLS methods. Polydispersity

H2 addition (**Figure 6H**, **I** and **Table 1**) the AuNPs size decreased as well, but the agglomeration was more noticable (**Figure 6I**). This behavior might be connected to the

Substrate Effects of Noble Metal Nanostructures Prepared by Sputtering

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compared to

171

PEG-S2

S2 H2

of the Au NPs in the solution) [47].

**Figure 6.** TEM images of Au NPs prepared by direct sputtering into PEG (300 s—A; 900 s—B), PEG: PEG-SH (1800 s—C), PEG: PEG-S2 H2 (900 s—D; 300 s—E), PEG admixed with PEG-SH (300 s—F; 900 s—G) and PEG admixed with PEG-S2 H2 (300 s—H; 900 s—I). Histograms of Au NPs size distribution (from TEM analysis) are shown in the inset on the left. Color of prepared PEGylated Au NPs is represented by cuvette images in insets on the right of each image [47].

to a small decrease of average NP size too (see **Table 1**). In the case of the post-deposition PEG-S2 H2 addition (**Figure 6H**, **I** and **Table 1**) the AuNPs size decreased as well, but the agglomeration was more noticable (**Figure 6I**). This behavior might be connected to the decrease of energy of the system after post-deposition addition of PEG-S2 H2 compared to the energy of the system after addition of PEG-SH, and thus, the growth slowdown is more pronounced [59]. Another reason could be the different molecule weights of the used PEG-S2 H2 and PEG-SH. Sputtering of Au into thiolated PEG or its addition leads to change of the color of the solution from dark/light red to dark/light brown (depending on concentration of the Au NPs in the solution) [47].

To confirm measured dimensions of NPs obtained from TEM, dynamic light scattering (DLS) analysis was conducted. The values of nanoparticle sizes together with polydispersity index (quantifying the degree of the size dispersion) are listed in **Table 1**. In the case of DLS, the statistical significance of designated values of particle size is much higher because all particles in a particular sample are used for the calculation (determination). The prepared colloidal solutions have mono-nodal distribution of sizes (see **Table 1**), that means, they do not contain nanoparticle aggregates, which are in good agreement with TEM results (**Figure 6**). We believe that this discrepancy is caused by the coalescence of NPs during preparation of TEM sample for the analysis. The phenomenon probably occurs during evaporation of solution on the copper grid. To assess the monodispersity of the prepared PEGylated Au NPs, we present the results of DLS analysis in the form of intensity-esteemed distribution curve of the sizes. This curve is most sensitive to presence of agglomerates, which would not be visible in the case of transformation to other types of distribution curve (volume-esteemed or numberesteemed). Another reason for discrepancy between the DLS and the TEM results is that the DLS method is not based only on hydrodynamic average of the particles themselves, but also includes the solvation shel.l [60]. Since the nanoparticles are stabilized with relatively long macromolecular chains, the detected NP sizes may be influenced by PEG corona. This is the

**Table 1.** Size of PEGylated Au NPs (in nm; with relative error) determined by TEM and DLS methods. Polydispersity index (PDI) of colloidal systems was determined by DLS analysis [47].

**Figure 6.** TEM images of Au NPs prepared by direct sputtering into PEG (300 s—A; 900 s—B), PEG: PEG-SH (1800 s—C),

(300 s—H; 900 s—I). Histograms of Au NPs size distribution (from TEM analysis) are shown in the inset on the left. Color

of prepared PEGylated Au NPs is represented by cuvette images in insets on the right of each image [47].

170 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

(900 s—D; 300 s—E), PEG admixed with PEG-SH (300 s—F; 900 s—G) and PEG admixed with PEG-S2

H2

PEG: PEG-S2

H2

reason for not using number-esteemed averages for DLS, because the calculation for this type of results may alter the output significantly. Despite some weaknesses mentioned above, the results presented on the basis of DLS correspond well with published studies [61].

of SPR band was noticed for Au NPs obtained by sputtering for 300 s into PEG-S2

into PEG and the PEG-S2

band is visible in the spectrum.

H2

solutions such as color and UV-Vis absorption (see **Figures 6** and **7**).

**4.2. Aging and temperature stability of PEGylated gold nanoparticles**

PEG and subsequently mixed with PEG-S2

of direct Au sputtering into PEG-SH and PEG-S2

**Sample cAu (mg l−1)** A 30.0 B 84.2 C 163.0 D 77.2 E 25.5 F 22.0 G 80.9 H 24.8 I 65.6

for sample C, because their size was close to 3 nm (see **Table 1**). Admixing thiolated PEG in both cases of 300 s and 900 s sputtering time leads to the disappearance of SPR bands. In the literature, the SPR absorption decrease is associated with binding of stabilizing agents containing thiol groups onto NP surface [54, 63]. Higher UV-Vis absorption of sample G is caused by the higher concentration of metal in the colloid solution. If the gold is sputtered for 300 s

diameter which induce the loss of the SPR band. Another possible cause is the connection of thiol groups to the Au NPs surface [54]. In the case of the gold sputtered for 900 s into pure

time of 900 s seems to be sufficient for the growth of bigger nanoparticles and, thus, the SPR

**Table 2** summarizes Au concentration in the solution measured by atomic absorption spectroscopy (AAS). Obtained data are in good agreement with results published by Slepicka et al.; in that, Au concentration linearly increases with the increasing sputtering time [46]. AAS confirmed results (see above) that dithiol-stabilized Au NPs are less stable aggregates and thus lower Au concentration is observed. Since these results were examined in the case of post-deposition addition of dithiols as well, we can infer that the lower concentration of Au is not connected to different liquid substrates used in the Au sputtering. With this knowledge, we are able to prepare colloids of desired NPs' size and concentration by managing the preparation conditions. Gold concentration in colloid solutions influences optical properties of the

For aging and temperature stability study, we have chosen samples C and D as representatives

ity of Au colloids was studied by UV-Vis spectroscopy (see **Figures 8** and **9**). Aging stability

**Table 2.** Concentration of Au (cAu, mg l−1) in water-PEG solutions (by volume 1:9) determined by AAS spectroscopy [47].

H2

is added subsequently, the prepared nanoparticles are 2.1 nm in

Substrate Effects of Noble Metal Nanostructures Prepared by Sputtering

H2, bigger nanoparticles were observed. Sputtering

, respectively. Aging and temperature stabil-

H2

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as well as

173

**Figure 7** shows UV-Vis spectra of Au NPs sputtered into solutions of different polyethylene glycols. Comparing absorbance of each sample, significant differences in the spectra of the PEGylated Au NPs are observed. The occurrence of a singular SPR absorption peak confirms that spherical NPs are present in all colloidal PEG solutions with size ranging between 4 and 50 nm. PEGylated Au NPs prepared by sputtering for 300 s into pure PEG exhibit a narrow SPR band (at 519 nm) with the highest UV-Vis absorption. With increasing sputtering time, the SPR band tends to get wider, which is caused by the poor size distribution of NPs. The absorption also decreases, which suggests formation of sedimentation aggregates (see **Figure 6B**). In contrast, SPR bands are not observed in the spectra of Au NPs sputtered for 1800 s into PEG-SH (**Figure 7**—line C). According to literature, the peak position and absorption of the plasmon band depend on Au NPs' size and shape [59]. When the diameter of spherical Au NPs is lower than 3 nm, the plasmon band disappears [62]. Weak and broad SPR band observed for NPs stabilized with PEG-S2 H2 (**Figure 7**—line D) might be connected to the creation of disulfide bridges, aggregation of the nanoparticles, their sedimentation and the subsequent decrease of metal concentration in the colloidal solution. The disappearance

**Figure 7.** UV-Vis spectra of Au NPs prepared by different time of sputtering of Au into: PEG (300 s—A; 900 s—B), PEG: PEG-SH (1800 s—C), PEG: PEG-S2 H2 (900 s—D; 300 s—E), PEG admixed with PEG-SH (300 s—F; 900 s—G) and PEG admixed with PEG-S2 H2 (300 s—H; 900 s—I) [47].

of SPR band was noticed for Au NPs obtained by sputtering for 300 s into PEG-S2 H2 as well as for sample C, because their size was close to 3 nm (see **Table 1**). Admixing thiolated PEG in both cases of 300 s and 900 s sputtering time leads to the disappearance of SPR bands. In the literature, the SPR absorption decrease is associated with binding of stabilizing agents containing thiol groups onto NP surface [54, 63]. Higher UV-Vis absorption of sample G is caused by the higher concentration of metal in the colloid solution. If the gold is sputtered for 300 s into PEG and the PEG-S2 H2 is added subsequently, the prepared nanoparticles are 2.1 nm in diameter which induce the loss of the SPR band. Another possible cause is the connection of thiol groups to the Au NPs surface [54]. In the case of the gold sputtered for 900 s into pure PEG and subsequently mixed with PEG-S2 H2, bigger nanoparticles were observed. Sputtering time of 900 s seems to be sufficient for the growth of bigger nanoparticles and, thus, the SPR band is visible in the spectrum.

**Table 2** summarizes Au concentration in the solution measured by atomic absorption spectroscopy (AAS). Obtained data are in good agreement with results published by Slepicka et al.; in that, Au concentration linearly increases with the increasing sputtering time [46]. AAS confirmed results (see above) that dithiol-stabilized Au NPs are less stable aggregates and thus lower Au concentration is observed. Since these results were examined in the case of post-deposition addition of dithiols as well, we can infer that the lower concentration of Au is not connected to different liquid substrates used in the Au sputtering. With this knowledge, we are able to prepare colloids of desired NPs' size and concentration by managing the preparation conditions. Gold concentration in colloid solutions influences optical properties of the solutions such as color and UV-Vis absorption (see **Figures 6** and **7**).

#### **4.2. Aging and temperature stability of PEGylated gold nanoparticles**

reason for not using number-esteemed averages for DLS, because the calculation for this type of results may alter the output significantly. Despite some weaknesses mentioned above, the

**Figure 7** shows UV-Vis spectra of Au NPs sputtered into solutions of different polyethylene glycols. Comparing absorbance of each sample, significant differences in the spectra of the PEGylated Au NPs are observed. The occurrence of a singular SPR absorption peak confirms that spherical NPs are present in all colloidal PEG solutions with size ranging between 4 and 50 nm. PEGylated Au NPs prepared by sputtering for 300 s into pure PEG exhibit a narrow SPR band (at 519 nm) with the highest UV-Vis absorption. With increasing sputtering time, the SPR band tends to get wider, which is caused by the poor size distribution of NPs. The absorption also decreases, which suggests formation of sedimentation aggregates (see **Figure 6B**). In contrast, SPR bands are not observed in the spectra of Au NPs sputtered for 1800 s into PEG-SH (**Figure 7**—line C). According to literature, the peak position and absorption of the plasmon band depend on Au NPs' size and shape [59]. When the diameter of spherical Au NPs is lower than 3 nm, the plasmon band disappears [62]. Weak and broad

H2

to the creation of disulfide bridges, aggregation of the nanoparticles, their sedimentation and the subsequent decrease of metal concentration in the colloidal solution. The disappearance

**Figure 7.** UV-Vis spectra of Au NPs prepared by different time of sputtering of Au into: PEG (300 s—A; 900 s—B), PEG:

(900 s—D; 300 s—E), PEG admixed with PEG-SH (300 s—F; 900 s—G) and PEG

(**Figure 7**—line D) might be connected

results presented on the basis of DLS correspond well with published studies [61].

SPR band observed for NPs stabilized with PEG-S2

172 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

PEG-SH (1800 s—C), PEG: PEG-S2

H2

admixed with PEG-S2

H2

(300 s—H; 900 s—I) [47].

For aging and temperature stability study, we have chosen samples C and D as representatives of direct Au sputtering into PEG-SH and PEG-S2 H2 , respectively. Aging and temperature stability of Au colloids was studied by UV-Vis spectroscopy (see **Figures 8** and **9**). Aging stability

**Table 2.** Concentration of Au (cAu, mg l−1) in water-PEG solutions (by volume 1:9) determined by AAS spectroscopy [47].

**Figure 8.** Aging stability (from 0 to 14 days) of Au NPs prepared by sputtering into PEG: PEG-SH (sample C) and PEG: PEG-S2 H2 (sample D) examined by UV-Vis spectroscopy [47].

was examined in the period of 0–14 days (see **Figure 8**). **Figure 8** shows a decrease of UV-Vis absorption, which means the PEGylated Au NPs tend to slightly aggregate with aging time. Au NPs protected by PEG-SH appear to be more stable than the NPs protected by PEG-S2 H2 , since there are no significant changes in the UV-Vis absorption spectrum of those after 14 days of aging.

The progress in temperature stability of samples C and D is well documented by spectra in **Figure 9**. Immediately after the preparation of Au NPs, the samples were heated for one hour at 35, 60, 75–100°C, respectively. UV-Vis spectra measured at room temperature (RT) were used as reference. **Figure 9** shows that the post-deposition heating of colloidal solution C (Au NPs stabilized by PEG-SH) at temperature 35°C induces only a mild decrease of UV-Vis absorption, which did not change with further temperature increase. The post-sputtering heating of sample D (Au NPs protected by PEG-S2 H2 ) caused a gradual decrease of UV-Vis absorption with increasing temperature. The sample D turns out to have lower temperature stability with the tendency for aggregation compared to the sample C. The results of aging and temperature stability of Au NPs with core shielded by PEG-SH molecules (sample C) are in good agreement with data published by Hatakeyama et al. [41].

Hatakeyama et al. investigated the effect of temperature on the structure and growth of Au NPs prepared by sputtering method by small-angle X-ray scattering. Ionic liquid 1-buthyl-3-methylimidazolium tetrafluoroborate was used as the liquid substrate. The

**Figure 9.** Temperature stability (from RT to 100°C) of Au NPs prepared by sputtering into PEG: PEG-SH (Sample C) and

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175

H2 (Sample D) examined by UV-Vis spectroscopy [47].

PEG: PEG-S2

Substrate Effects of Noble Metal Nanostructures Prepared by Sputtering http://dx.doi.org/10.5772/intechopen.71340 175

was examined in the period of 0–14 days (see **Figure 8**). **Figure 8** shows a decrease of UV-Vis absorption, which means the PEGylated Au NPs tend to slightly aggregate with aging time. Au NPs protected by PEG-SH appear to be more stable than the NPs protected by PEG-S2

**Figure 8.** Aging stability (from 0 to 14 days) of Au NPs prepared by sputtering into PEG: PEG-SH (sample C) and PEG:

since there are no significant changes in the UV-Vis absorption spectrum of those after 14 days

The progress in temperature stability of samples C and D is well documented by spectra in **Figure 9**. Immediately after the preparation of Au NPs, the samples were heated for one hour at 35, 60, 75–100°C, respectively. UV-Vis spectra measured at room temperature (RT) were used as reference. **Figure 9** shows that the post-deposition heating of colloidal solution C (Au NPs stabilized by PEG-SH) at temperature 35°C induces only a mild decrease of UV-Vis absorption, which did not change with further temperature increase. The post-sputtering

absorption with increasing temperature. The sample D turns out to have lower temperature stability with the tendency for aggregation compared to the sample C. The results of aging and temperature stability of Au NPs with core shielded by PEG-SH molecules (sample C) are

H2

) caused a gradual decrease of UV-Vis

heating of sample D (Au NPs protected by PEG-S2

(sample D) examined by UV-Vis spectroscopy [47].

174 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

in good agreement with data published by Hatakeyama et al. [41].

of aging.

PEG-S2 H2

> H2 ,

**Figure 9.** Temperature stability (from RT to 100°C) of Au NPs prepared by sputtering into PEG: PEG-SH (Sample C) and PEG: PEG-S2 H2 (Sample D) examined by UV-Vis spectroscopy [47].

Hatakeyama et al. investigated the effect of temperature on the structure and growth of Au NPs prepared by sputtering method by small-angle X-ray scattering. Ionic liquid 1-buthyl-3-methylimidazolium tetrafluoroborate was used as the liquid substrate. The liquid was heated from 20 to 80°C before sputtering. It was found that the Au NP sizes (i) are uniform at constant temperature, (ii) increase as the temperature rises, and (iii) values of *W*FWHM (full width at half maxima of distribution curves) increase with the rising temperature. At lower temperatures (20 and 30°C), *W*FWHM values were almost the same as *d*peak (diameter at the peak-top of distribution curves) values. The values gradually increase and reach 40–70°C, and then increases dramatically with temperature [64].

stabilized by PEG-SH and used them as CT contrast agents for *in vivo* experiments on mice. They have reported that the surface immobilization together with the small hydrodynamic diameters (3.8 nm) of the custom tailored Au NPs allowed their efficient renal cleansing, pro-

Substrate Effects of Noble Metal Nanostructures Prepared by Sputtering

http://dx.doi.org/10.5772/intechopen.71340

177

LSPR band of Au NPs leads to strong light absorption and efficient conversion of the energy into localized heat, which can be used for selective ablation of cancer cells [74]. This procedure is called plasmonic photothermal therapy (PPT). Liu et al. have shown that the multidentate PEG ligand functionalized Au NRs can be used as an effective NIR photothermal agent for tumor ablation *in vivo*. PEGylated Au NRs showed that low cytotoxicity on most mice organs *in vivo* had high efficacy of cancer cells ablation *in vitro*, accumulated in tumors preferentially and showed good stability in serum [75]. Li et al. prepared PEGylated Cu nanowires (Cu NWs) with efficient photothermal conversion (12.5%) by NIR irradiation at 808 nm and studied selective killing of cancer cells *in vitro* and effective photothermal ablation of cancer *in vivo*. The authors showed the intratumoral injection of PEGylated Cu-NWs into colon

raised the local temperature to >50°C, induced necrosis, and suppressed tumor growth [76]. Ruff et al. studied the effect of multivalency on the stability of NIR-absorbing Au NRs (CTAB stabilized) immobilized with mono-, bi- and tridentate PEG thiol ligands. Their PEGylation approach leads to Au NPs and Au NRs with CTAB concentration below the detection limit. Furthermore, high biocompatibility after PEGylation of both Au NPs and Au NRs was found and no significant difference in cytotoxicity against human cervical cancer cells comparing the mono-, bi- and tridentate PEGylated species was observed, which makes them suitable as

Besides the PPT, PEGylated gold nanoparticles are very promising material for photodynamic therapy (PDT) drug delivery [78, 79]. NPs can be stabilized by steric repulsion to inhibit colloid aggregation in physiological conditions by using water-soluble PEG [80]. PEGylated Au NPs show remarkable resistance in protein adsorption due to the high degree of hydration of the randomly coiled PEG molecules [81]. According to this property, drugs carried by NPs could be shielded from being cleared/removed by the reticuloendothelial system (RES) [82]. Therefore, the blood circulation time of NPs can be prolonged. Drug distribution *in vivo* can be regulated too. Such drug carriers can preferentially accumulate in tumor sites through the leaky tumor neovasculature and do not re-enter the blood stream, which is called "enhanced permeability and retention" (EPR) effect [82]. Cheng et al. prepared PEGylated Au NPs conjugated with silicone phthalocyanine with a reversible PDT drug adsorption. It was found that the delivery mode of this system greatly improves the transport of the drug to the tumor compared to conventional drug administration [83]. Li et al. demonstrated that the 75 nm latex particles remained in rat circulation 40 times longer (from 20 min to 13 h) when immobilized with PEG larger than 5000 kDa [84]. Klibanov and Huang found that PEGylation leads to advantageous results in other drug delivery systems as well. PEGylation of liposomes improved their circulation half-life from 30 min to 5 h without increasing leakage of the liposome interior [85]. In the mid-1990s, Doxil® (liposomal delivery vehicle for doxorubicin) and Oncospar (PEG-L-asparaginase) became the first FDA

) for 6 min significantly

longed blood circulation, and stealth capability [73].

biocompatible contrast agents [77].

approved NP therapeutics [86, 87].

tumor-bearing mice and ensuing NIR irradiation (808 nm, 1.5 W/cm2

#### **4.3. Applications of PEGylated nanoparticles**

Polyethylene glycol is one of the most used NP stabilizing polymer and the gold standard for "stealth" polymers in the emerging area of polymer-based drug carriers, gene vectors, imaging, therapeutic agents, bio-sensing, and tissue regeneration [65, 66].

Localized surface plasmon resonance (LSPR) is specific property of Au NPs that depends on particle size and distance between the particles. In the case of Au NPs, the absorption maximum connected to LSPR is between 500 and 550 nm. LSPR phenomenon is associated with oscillation of the electron gas due to excitation by incident light beam. This property puts the Au NPs to the forefront of interest in the area of biosensors technology. Creation of Au NPs aggregates invokes interparticle surface plasmon coupling, which leads to a dramatic change of color of the colloidal solution from red to blue. It also causes broadening of the surface plasmon resonance peak. Significant color change of colloids can be easily seen by the naked eye, without usage of any analytical method. Aggregation of Au NPs is extensively used in colorimetric detection of DNA, metal ions, proteins, toxins, and enzyme activity too [67, 68]. Kim et al. have found a simple procedure that can identify differences between normal and increased calcium ion (Ca2+) levels in serum using calsequestrin (CSQ)-immobilized Au NPs [69, 70]. Zhang et al. successfully grafted PEG-SH onto CTAB shielded gold nanorods (Au NR) and prepared uniform monolayer of Au NRs on silicon by gradual evaporation of the residual water. This approach brings a new opportunity to prepare uniform plasmonic sensing platforms due to stability of PEGylated Au NRs in the serum [71].

Molecular imaging is associated with processes ongoing in living organisms at cellular and molecular levels. Diseases in early stage can be identified and monitored by specific molecular-imaging agents, which are very powerful method enabling a rapid and targeted treatment. Nowadays, MRI, CT, ultrasound, optical imaging, single-photon emission computed tomography (SPECT), and positron-emission tomography (PET) are the most commonly used imaging methods [72]. In contrast to other imaging agents, Au NPs offer better optical imaging properties because: (i) their light scattering effect is very strong, (ii) they are much brighter than chemical fluorophores, (iii) they resist photo-bleaching, and (iv) they can be easily spotted at low concentrations (10−16 M). For Au NPs-based contrast agents, the most promising imaging technique is Raman spectroscopy. Au NPs immobilized with thiolated PEG and conjugated with antibody specifically bind to the epidermal growth factor. These SERS nanoparticles for active targeting of both cancer cells and tumor xenografts were more than 200 times brighter than near-infrared-emitting quantum dots, and allowed spectroscopic detection of small tumors at a penetration depth of 1–2 cm [70]. Silvestri et al. have synthesized Au NPs stabilized by PEG-SH and used them as CT contrast agents for *in vivo* experiments on mice. They have reported that the surface immobilization together with the small hydrodynamic diameters (3.8 nm) of the custom tailored Au NPs allowed their efficient renal cleansing, prolonged blood circulation, and stealth capability [73].

liquid was heated from 20 to 80°C before sputtering. It was found that the Au NP sizes (i) are uniform at constant temperature, (ii) increase as the temperature rises, and (iii) values of *W*FWHM (full width at half maxima of distribution curves) increase with the rising temperature. At lower temperatures (20 and 30°C), *W*FWHM values were almost the same as *d*peak (diameter at the peak-top of distribution curves) values. The values gradually increase

Polyethylene glycol is one of the most used NP stabilizing polymer and the gold standard for "stealth" polymers in the emerging area of polymer-based drug carriers, gene vectors, imag-

Localized surface plasmon resonance (LSPR) is specific property of Au NPs that depends on particle size and distance between the particles. In the case of Au NPs, the absorption maximum connected to LSPR is between 500 and 550 nm. LSPR phenomenon is associated with oscillation of the electron gas due to excitation by incident light beam. This property puts the Au NPs to the forefront of interest in the area of biosensors technology. Creation of Au NPs aggregates invokes interparticle surface plasmon coupling, which leads to a dramatic change of color of the colloidal solution from red to blue. It also causes broadening of the surface plasmon resonance peak. Significant color change of colloids can be easily seen by the naked eye, without usage of any analytical method. Aggregation of Au NPs is extensively used in colorimetric detection of DNA, metal ions, proteins, toxins, and enzyme activity too [67, 68]. Kim et al. have found a simple procedure that can identify differences between normal and increased calcium ion (Ca2+) levels in serum using calsequestrin (CSQ)-immobilized Au NPs [69, 70]. Zhang et al. successfully grafted PEG-SH onto CTAB shielded gold nanorods (Au NR) and prepared uniform monolayer of Au NRs on silicon by gradual evaporation of the residual water. This approach brings a new opportunity to prepare uniform plasmonic sensing platforms due to stability of PEGylated Au

Molecular imaging is associated with processes ongoing in living organisms at cellular and molecular levels. Diseases in early stage can be identified and monitored by specific molecular-imaging agents, which are very powerful method enabling a rapid and targeted treatment. Nowadays, MRI, CT, ultrasound, optical imaging, single-photon emission computed tomography (SPECT), and positron-emission tomography (PET) are the most commonly used imaging methods [72]. In contrast to other imaging agents, Au NPs offer better optical imaging properties because: (i) their light scattering effect is very strong, (ii) they are much brighter than chemical fluorophores, (iii) they resist photo-bleaching, and (iv) they can be easily spotted at low concentrations (10−16 M). For Au NPs-based contrast agents, the most promising imaging technique is Raman spectroscopy. Au NPs immobilized with thiolated PEG and conjugated with antibody specifically bind to the epidermal growth factor. These SERS nanoparticles for active targeting of both cancer cells and tumor xenografts were more than 200 times brighter than near-infrared-emitting quantum dots, and allowed spectroscopic detection of small tumors at a penetration depth of 1–2 cm [70]. Silvestri et al. have synthesized Au NPs

and reach 40–70°C, and then increases dramatically with temperature [64].

ing, therapeutic agents, bio-sensing, and tissue regeneration [65, 66].

**4.3. Applications of PEGylated nanoparticles**

176 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

NRs in the serum [71].

LSPR band of Au NPs leads to strong light absorption and efficient conversion of the energy into localized heat, which can be used for selective ablation of cancer cells [74]. This procedure is called plasmonic photothermal therapy (PPT). Liu et al. have shown that the multidentate PEG ligand functionalized Au NRs can be used as an effective NIR photothermal agent for tumor ablation *in vivo*. PEGylated Au NRs showed that low cytotoxicity on most mice organs *in vivo* had high efficacy of cancer cells ablation *in vitro*, accumulated in tumors preferentially and showed good stability in serum [75]. Li et al. prepared PEGylated Cu nanowires (Cu NWs) with efficient photothermal conversion (12.5%) by NIR irradiation at 808 nm and studied selective killing of cancer cells *in vitro* and effective photothermal ablation of cancer *in vivo*. The authors showed the intratumoral injection of PEGylated Cu-NWs into colon tumor-bearing mice and ensuing NIR irradiation (808 nm, 1.5 W/cm2 ) for 6 min significantly raised the local temperature to >50°C, induced necrosis, and suppressed tumor growth [76]. Ruff et al. studied the effect of multivalency on the stability of NIR-absorbing Au NRs (CTAB stabilized) immobilized with mono-, bi- and tridentate PEG thiol ligands. Their PEGylation approach leads to Au NPs and Au NRs with CTAB concentration below the detection limit. Furthermore, high biocompatibility after PEGylation of both Au NPs and Au NRs was found and no significant difference in cytotoxicity against human cervical cancer cells comparing the mono-, bi- and tridentate PEGylated species was observed, which makes them suitable as biocompatible contrast agents [77].

Besides the PPT, PEGylated gold nanoparticles are very promising material for photodynamic therapy (PDT) drug delivery [78, 79]. NPs can be stabilized by steric repulsion to inhibit colloid aggregation in physiological conditions by using water-soluble PEG [80]. PEGylated Au NPs show remarkable resistance in protein adsorption due to the high degree of hydration of the randomly coiled PEG molecules [81]. According to this property, drugs carried by NPs could be shielded from being cleared/removed by the reticuloendothelial system (RES) [82]. Therefore, the blood circulation time of NPs can be prolonged. Drug distribution *in vivo* can be regulated too. Such drug carriers can preferentially accumulate in tumor sites through the leaky tumor neovasculature and do not re-enter the blood stream, which is called "enhanced permeability and retention" (EPR) effect [82]. Cheng et al. prepared PEGylated Au NPs conjugated with silicone phthalocyanine with a reversible PDT drug adsorption. It was found that the delivery mode of this system greatly improves the transport of the drug to the tumor compared to conventional drug administration [83]. Li et al. demonstrated that the 75 nm latex particles remained in rat circulation 40 times longer (from 20 min to 13 h) when immobilized with PEG larger than 5000 kDa [84]. Klibanov and Huang found that PEGylation leads to advantageous results in other drug delivery systems as well. PEGylation of liposomes improved their circulation half-life from 30 min to 5 h without increasing leakage of the liposome interior [85]. In the mid-1990s, Doxil® (liposomal delivery vehicle for doxorubicin) and Oncospar (PEG-L-asparaginase) became the first FDA approved NP therapeutics [86, 87].

#### **5. Conclusions**

Cathode sputtering is a well-established technique for the preparation of metal nanostructures. The manipulation of the deposition parameters enable formation of precisely controlled material. For the properties of the resulting material, the proper selection of substrate is necessary. For the traditional application of the thin metal films in electronics, insulating substrates are usually necessary, of which the glass and polymer substrates are the most important. In this chapter, we have discussed the problems of glass substrates in adhesion of the metal layers and possibilities of dewetting. The polymer substrates on the other hand provide better adhesion to the metals, but surface morphology effects are more pronounced. A new and perspective branch of substrates for sputtering deposition are liquids. Therefore, this chapter provides a deeper insight into the problems of direct sputtering of noble metals into liquid substrate, i.e., preparation of NPs. Basic characteristics, technology of preparation, and applications of those materials are explained. The results of the authors' team support the general overview. Direct sputtering of Au atoms onto PEG and thiol terminated PEG has led to successful formation of spherical NPs. The diameters of the prepared Au NPs ranged from 2.1 to 5.9 nm depending on the capturing medium. NPs smaller than 3 nm in diameter exhibited loss of SPR band because of the small NP size and/or attachment of thiolated PEG on the Au NPs' surface. Solution of Au NPs stabilized by PEG-SH (sample C) exhibited higher time and temperature stability compared to the solution where Au NPs were stabilized by PEG-S2 H2 (sample D). This colloidal solution showed the best time and temperature stability with the size of Au NPs of 2.9 nm, which can have a positive effect in biomedical use and for catalysis of chemical reactions. PEGylated NPs have huge potential in biomedical applications (drug delivery, therapy, imaging, and biosensing).

[2] Frey H, Khan HR, editors. Handbook of Thin Film Technology. Berlin: Springer Verlag;

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#### **Acknowledgements**

Financial support of this work under the GACR projects 17-00939S and P108/12/G108 is gratefully acknowledged.

#### **Author details**

Alena Reznickova\*, Ondrej Kvitek, Dominik Fajstavr, Nikola Slavikova and Vaclav Svorcik

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Solid State Engineering, University of Chemistry and Technology Prague, Prague, Czech Republic

#### **References**

[1] Wasa K, Kanno I, Kotera H, editors. Handbook of Sputter Deposition Technology: Fundamentals and Applications for Functional Thin Films, Nano-Materials and MEMS. 2nd ed. Oxford: Elsevier; 2012 660 p

[2] Frey H, Khan HR, editors. Handbook of Thin Film Technology. Berlin: Springer Verlag; 2015. 380 p. DOI: 10.1007/978-3-642-05430-3

**5. Conclusions**

solution where Au NPs were stabilized by PEG-S2

178 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

\*Address all correspondence to<EMAIL_ADDRESS>
2nd ed. Oxford: Elsevier; 2012 660 p

**Acknowledgements**

fully acknowledged.

**Author details**

Prague, Czech Republic

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Cathode sputtering is a well-established technique for the preparation of metal nanostructures. The manipulation of the deposition parameters enable formation of precisely controlled material. For the properties of the resulting material, the proper selection of substrate is necessary. For the traditional application of the thin metal films in electronics, insulating substrates are usually necessary, of which the glass and polymer substrates are the most important. In this chapter, we have discussed the problems of glass substrates in adhesion of the metal layers and possibilities of dewetting. The polymer substrates on the other hand provide better adhesion to the metals, but surface morphology effects are more pronounced. A new and perspective branch of substrates for sputtering deposition are liquids. Therefore, this chapter provides a deeper insight into the problems of direct sputtering of noble metals into liquid substrate, i.e., preparation of NPs. Basic characteristics, technology of preparation, and applications of those materials are explained. The results of the authors' team support the general overview. Direct sputtering of Au atoms onto PEG and thiol terminated PEG has led to successful formation of spherical NPs. The diameters of the prepared Au NPs ranged from 2.1 to 5.9 nm depending on the capturing medium. NPs smaller than 3 nm in diameter exhibited loss of SPR band because of the small NP size and/or attachment of thiolated PEG on the Au NPs' surface. Solution of Au NPs stabilized by PEG-SH (sample C) exhibited higher time and temperature stability compared to the

H2

the best time and temperature stability with the size of Au NPs of 2.9 nm, which can have a positive effect in biomedical use and for catalysis of chemical reactions. PEGylated NPs have huge

Financial support of this work under the GACR projects 17-00939S and P108/12/G108 is grate-

Alena Reznickova\*, Ondrej Kvitek, Dominik Fajstavr, Nikola Slavikova and Vaclav Svorcik

Department of Solid State Engineering, University of Chemistry and Technology Prague,

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**Chapter 9**

**Provisional chapter**

**Silver Nanoparticles Fabricated Using Chemical Vapor**

**Deposition and Atomic Layer Deposition Techniques:**

**Silver Nanoparticles Fabricated Using Chemical Vapor**

DOI: 10.5772/intechopen.71571

**Deposition and Atomic Layer Deposition Techniques:**

Silver nanoparticles with unique physicochemical properties and high biocide activity attract great interest in the design and in the manufacture of the new generation materials intended for biomedical technologies. This review aims to provide assessment of the bioactivity and usefulness of Ag-based materials in biomedical technologies, which are produced with the use of chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods. The use of CVD and ALD technologies in the fabrication of silver layers, nanoparticles, and nanocomposites are discussed in the first part of this chapter. The second half of this review deals with the topics associated with the release of silver ions from nanoparticles or nanolayers and discusses the antimicrobial activity, bio-integration

**Keywords:** silver, silver-based materials, CVD, ALD, bioactivity, biomedical materials

The rapid development of nanotechnologies is a result of the numerous applications of nanomaterials in various fields of our life. The metallic nanograins or metallic nanolayers, of either simple or composite nature, are important materials in the improvement of such traditional areas as electronics, optics, and catalysis. However, in last few years we have observed a great interest in the application of these nanomaterials in biological, biomedical, pharmaceutical, cosmetic, food, and textile technologies [1–5]. The use of different methods offers the possibility of the fabrication of nano-sized materials (materials in which at least one dimension is less than 100 nm) of different structure, surface morphology,

> © 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,

© 2018 The Author(s). Licensee IntechOpen. 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.

and reproduction in any medium, provided the original work is properly cited.

**Properties, Applications and Perspectives: Review**

**Properties, Applications and Perspectives: Review**

Piotr Piszczek and Aleksandra Radtke

Piotr Piszczek and Aleksandra Radtke

http://dx.doi.org/10.5772/intechopen.71571

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

properties and toxicity of these materials.

Additional information is available at the end of the chapter

**Provisional chapter**

#### **Silver Nanoparticles Fabricated Using Chemical Vapor Deposition and Atomic Layer Deposition Techniques: Properties, Applications and Perspectives: Review Deposition and Atomic Layer Deposition Techniques: Properties, Applications and Perspectives: Review**

**Silver Nanoparticles Fabricated Using Chemical Vapor**

DOI: 10.5772/intechopen.71571

Piotr Piszczek and Aleksandra Radtke Additional information is available at the end of the chapter

Piotr Piszczek and Aleksandra Radtke

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71571

#### **Abstract**

Silver nanoparticles with unique physicochemical properties and high biocide activity attract great interest in the design and in the manufacture of the new generation materials intended for biomedical technologies. This review aims to provide assessment of the bioactivity and usefulness of Ag-based materials in biomedical technologies, which are produced with the use of chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods. The use of CVD and ALD technologies in the fabrication of silver layers, nanoparticles, and nanocomposites are discussed in the first part of this chapter. The second half of this review deals with the topics associated with the release of silver ions from nanoparticles or nanolayers and discusses the antimicrobial activity, bio-integration properties and toxicity of these materials.

**Keywords:** silver, silver-based materials, CVD, ALD, bioactivity, biomedical materials

#### **1. Introduction**

The rapid development of nanotechnologies is a result of the numerous applications of nanomaterials in various fields of our life. The metallic nanograins or metallic nanolayers, of either simple or composite nature, are important materials in the improvement of such traditional areas as electronics, optics, and catalysis. However, in last few years we have observed a great interest in the application of these nanomaterials in biological, biomedical, pharmaceutical, cosmetic, food, and textile technologies [1–5]. The use of different methods offers the possibility of the fabrication of nano-sized materials (materials in which at least one dimension is less than 100 nm) of different structure, surface morphology,

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 IntechOpen. 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.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

physicochemical properties, and bioactivity [6–9]. From the medical point of view, it is interesting that silver as an antimicrobial agent has accompanied human civilization since ancient times. The use of silver vessels or putting a silver coin/spoon into tanks in order to preserve drinking water from spoilage by bacteria and algae, was practiced by ancient Egyptians and Greeks, as well as by sailors in times of great geographic discoveries [10]. In modern times, silver (usually used as colloidal silver) is applied to treatment water and its disinfection. The system working on the board of International Space Station is a good example [9, 11]. Moreover, the fabrication of this metal coating on the surface of medical devices or pieces of furniture, and also incorporation of Ag nanoparticles to the building and textile materials, influences the reduction of the hospital-related infections [12–14]. Silver nanoparticles (AgNPs) are also component of many health and cosmetic products due to their antimicrobial activity and deodorizing properties [15, 16]. Based on previous results, it is noted that the above-mentioned nanoparticles exhibit antimicrobial activity against 650 strains of pathogenic microorganisms such as bacteria, fungi, viruses, molds, and yeasts [17, 18]. A number of mechanisms have been proposed to explain the antimicrobial activity of AgNPs: (1) blocking the active respiratory chains of organisms; (2) disrupting the cellular membrane leading to leakage of cellular contents; (3) binding to the functional groups of microbial proteins that lead to protein denaturation and DNA malfunctions; and (4) blocking of nutrient transportation enzymes across the cell membrane [18]. The above-mentioned biological properties of the Ag nanoparticles and nanolayers depend strongly on their structure, shape, size, surface morphology, and the substrate used in the synthesis.

as the procedure allowing for obtaining nontoxic antimicrobial agent [26, 28]. The need to the fabrication of silver coatings, both continuous as well as composed of dispersed metallic grains on the surface of metallic, ceramic, or polymeric substrates, requires the use of other methods. Often used photochemical synthesis of AgNPs on the surface of titania nanotube (TNT) coatings is a good example [29–31]. In this method, TNT substrate is soaked in

drying, is irradiated using UV light at room temperature. Silver particles with diameters

AgNPs on the surface of different titania substrates may be also prepared using electrodeposition and electrochemical methods [32–34]. Above-mentioned techniques enabled the deposition of dispersed Ag particles of diameters ca. 2–12 nm on the surface of titania par-

Chemical vapor deposition (CVD) and atomic layer deposition (ALD) belong to the strategy based on the fabrication of materials from vapor phase. Analysis of literature reports revealed that both methods are not widely used in the production of biomaterials, although both methods allow exact control of the nucleation and growth of metallic single grains or layers. It may be associated with necessity for the use of vacuum and high temperatures as in the case of CVD processes, which in turn translates into high price of the final product. However, the rapid development of both above-mentioned methods, related to the introduction of new precursors, improvement of the equipment, and optimization of the deposition conditions, has led to increased interest of their application in the production of materials, with strictly defined structure, morphology, physicochemical and biological properties, and also characterized by high purity. Therefore, in this chapter we have focused on a more detailed discussion on the use of CVD and ALD methods for the production of silver-based materials, their

Chemical vapor deposition (CVD) is a technique which allows the synthesis of nanometric layers of inorganic materials on the surface of 3D substrates. The deposition process occurs in three successive stages: (i) introduction of the volatile precursor by carrier gas to the reactor chamber; (ii) adsorption of precursor vapors on the substrate surface and the formation of intermediate products; and (iii) decomposition of these products on the heated substrate followed by nucleation and growth of the solid layer/grains, and the formation of volatile by-products and their removal from the chamber by the carrier gas [35]. Among the main factors that influence the deposition process and the quality of produced nanomaterials, the following ones can be highlighted: the precursor delivery method, total pressure in the reactor chamber (*p*), carrier gas and its chemical properties, substrate temperature (*T*D), substrate properties, flow rate of precursor vapors, deposition time (*t*), and deposition rate. Among these factors, precursors as a source of formed material, seem to be the most important factor. The CVD precursor

10–70 nm were deposited on the top edges and walls of ca. 50–180 nm TiO2

solutions of different concentration, and after cleaning with deionized water and

Silver Nanoparticles Fabricated Using Chemical Vapor Deposition and Atomic Layer Deposition…

tubes [30, 31].

189

http://dx.doi.org/10.5772/intechopen.71571

AgNO3

ticles or nanotubes.

properties and biological activity.

**2. Chemical vapor deposition**

Silver may form continuous nanolayers and/or dispersed nanoparticles on the substrate surface or it can be incorporated within the matrix (ceramic, glass, and polymer). Generally, two strategies based on wet chemistry and vapor phase deposition are used for the preparation of Ag-containing nanomaterials. Analysis of the literature reports showed that the reduction processes of AgNO3 are the main methods, which are used in the fabrication of AgNPs both on the substrate surface as well as in the form of the dispersed particles in colloidal systems, glasses, or polymers [12, 15]. According to these reports, the wet chemical techniques are mainly used for the fabrication of silver nanoparticles [9]. In these methods, the colloidal silver is obtained by the use of three components: (a) silver precursor, predominantly AgNO3 ; (b) reducing agent – for example, NaBH4 , citric acid or its sodium salt, gallic acid, ascorbic acid [9, 19–22]; and (c) stabilizing agents—for example, polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, ethylene diamine tetra-acetic acid [23, 24]. The nucleation of nanoparticles and their growth in defined conditions (temperature, pH, reduction agents, and stabilizing agents) leads to the formation of similar size and shape particles [9, 23]. Dried Ag nanopowders, which can be added to, for example, polymeric, glass, or ceramic matrix, were obtained by freeze-drying of the colloidal solution [20]. Biosynthesis of AgNPs is a promising technique which has been intensively studied recently [25–27]. It involves the reduction of silver salt (e.g. silver nitrate) by such reducing agents as plants, bacteria, and fungi [27, 28]. The great interest of the above-mentioned method is due to its potential use in medicine (especially in cancer therapy and diagnostic) as the procedure allowing for obtaining nontoxic antimicrobial agent [26, 28]. The need to the fabrication of silver coatings, both continuous as well as composed of dispersed metallic grains on the surface of metallic, ceramic, or polymeric substrates, requires the use of other methods. Often used photochemical synthesis of AgNPs on the surface of titania nanotube (TNT) coatings is a good example [29–31]. In this method, TNT substrate is soaked in AgNO3 solutions of different concentration, and after cleaning with deionized water and drying, is irradiated using UV light at room temperature. Silver particles with diameters 10–70 nm were deposited on the top edges and walls of ca. 50–180 nm TiO2 tubes [30, 31]. AgNPs on the surface of different titania substrates may be also prepared using electrodeposition and electrochemical methods [32–34]. Above-mentioned techniques enabled the deposition of dispersed Ag particles of diameters ca. 2–12 nm on the surface of titania particles or nanotubes.

Chemical vapor deposition (CVD) and atomic layer deposition (ALD) belong to the strategy based on the fabrication of materials from vapor phase. Analysis of literature reports revealed that both methods are not widely used in the production of biomaterials, although both methods allow exact control of the nucleation and growth of metallic single grains or layers. It may be associated with necessity for the use of vacuum and high temperatures as in the case of CVD processes, which in turn translates into high price of the final product. However, the rapid development of both above-mentioned methods, related to the introduction of new precursors, improvement of the equipment, and optimization of the deposition conditions, has led to increased interest of their application in the production of materials, with strictly defined structure, morphology, physicochemical and biological properties, and also characterized by high purity. Therefore, in this chapter we have focused on a more detailed discussion on the use of CVD and ALD methods for the production of silver-based materials, their properties and biological activity.

#### **2. Chemical vapor deposition**

physicochemical properties, and bioactivity [6–9]. From the medical point of view, it is interesting that silver as an antimicrobial agent has accompanied human civilization since ancient times. The use of silver vessels or putting a silver coin/spoon into tanks in order to preserve drinking water from spoilage by bacteria and algae, was practiced by ancient Egyptians and Greeks, as well as by sailors in times of great geographic discoveries [10]. In modern times, silver (usually used as colloidal silver) is applied to treatment water and its disinfection. The system working on the board of International Space Station is a good example [9, 11]. Moreover, the fabrication of this metal coating on the surface of medical devices or pieces of furniture, and also incorporation of Ag nanoparticles to the building and textile materials, influences the reduction of the hospital-related infections [12–14]. Silver nanoparticles (AgNPs) are also component of many health and cosmetic products due to their antimicrobial activity and deodorizing properties [15, 16]. Based on previous results, it is noted that the above-mentioned nanoparticles exhibit antimicrobial activity against 650 strains of pathogenic microorganisms such as bacteria, fungi, viruses, molds, and yeasts [17, 18]. A number of mechanisms have been proposed to explain the antimicrobial activity of AgNPs: (1) blocking the active respiratory chains of organisms; (2) disrupting the cellular membrane leading to leakage of cellular contents; (3) binding to the functional groups of microbial proteins that lead to protein denaturation and DNA malfunctions; and (4) blocking of nutrient transportation enzymes across the cell membrane [18]. The above-mentioned biological properties of the Ag nanoparticles and nanolayers depend strongly on their structure, shape, size, surface morphology, and the substrate

188 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

Silver may form continuous nanolayers and/or dispersed nanoparticles on the substrate surface or it can be incorporated within the matrix (ceramic, glass, and polymer). Generally, two strategies based on wet chemistry and vapor phase deposition are used for the preparation of Ag-containing nanomaterials. Analysis of the literature reports showed that the

AgNPs both on the substrate surface as well as in the form of the dispersed particles in colloidal systems, glasses, or polymers [12, 15]. According to these reports, the wet chemical techniques are mainly used for the fabrication of silver nanoparticles [9]. In these methods, the colloidal silver is obtained by the use of three components: (a) silver precursor,

; (b) reducing agent – for example, NaBH4

salt, gallic acid, ascorbic acid [9, 19–22]; and (c) stabilizing agents—for example, polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, ethylene diamine tetra-acetic acid [23, 24]. The nucleation of nanoparticles and their growth in defined conditions (temperature, pH, reduction agents, and stabilizing agents) leads to the formation of similar size and shape particles [9, 23]. Dried Ag nanopowders, which can be added to, for example, polymeric, glass, or ceramic matrix, were obtained by freeze-drying of the colloidal solution [20]. Biosynthesis of AgNPs is a promising technique which has been intensively studied recently [25–27]. It involves the reduction of silver salt (e.g. silver nitrate) by such reducing agents as plants, bacteria, and fungi [27, 28]. The great interest of the above-mentioned method is due to its potential use in medicine (especially in cancer therapy and diagnostic)

are the main methods, which are used in the fabrication of

, citric acid or its sodium

used in the synthesis.

reduction processes of AgNO3

predominantly AgNO3

Chemical vapor deposition (CVD) is a technique which allows the synthesis of nanometric layers of inorganic materials on the surface of 3D substrates. The deposition process occurs in three successive stages: (i) introduction of the volatile precursor by carrier gas to the reactor chamber; (ii) adsorption of precursor vapors on the substrate surface and the formation of intermediate products; and (iii) decomposition of these products on the heated substrate followed by nucleation and growth of the solid layer/grains, and the formation of volatile by-products and their removal from the chamber by the carrier gas [35]. Among the main factors that influence the deposition process and the quality of produced nanomaterials, the following ones can be highlighted: the precursor delivery method, total pressure in the reactor chamber (*p*), carrier gas and its chemical properties, substrate temperature (*T*D), substrate properties, flow rate of precursor vapors, deposition time (*t*), and deposition rate. Among these factors, precursors as a source of formed material, seem to be the most important factor. The CVD precursor should be characterized by: (a) the appropriate volatility to achieve the highest possible concentration of the precursor in vapors; (b) the thermal stability of the compound which avoids its premature degradation during the transportation of vapors by carrier gas; (c) its ability for thermal decomposition on the substrate surface leading to the deposition of desirable materials; (d) inexpensive and simple synthesis; and (e) low toxicity. Considering these reports, we find that for CVD, two types of compounds are usually used as silver precursors, namely inorganic silver salts and volatile silver (I) complexes. Silver nitrate (AgNO3 ) is the widely used precursor in such techniques as the flameassisted CVD (FACVD) and the atmospheric pressure CVD (APCVD) [36–38]. The first of these method enables the deposition of metallic silver layers of thickness 60–90 nm [36] or 60–250 nm [37] on the substrate surface heated to the temperature TD = 573 K at atmospheric pressure. The advantage of this method is the ability to the formation of Ag-metal oxide nanocomposite coatings (e.g. TiO2 [36], SiO2 [39]) in one deposition process on large substrate areas. The nanocomposite coatings on the textile surface, composed of silicon dioxide with in situ incorporated Ag nanoparticles were produced by APCVD technique [38]. In the classical CVD techniques (metal-organic CVD (MOCVD), aerosolassistant CVD (AACVD), plasma-enhanced CVD (PECVD)), a variety of volatile silver(I) compounds are used as precursors [35, 40]. This group includes the silver(I) compounds with β-diketonates and carboxylates (mainly perfluorinated carboxylates), as well as their complexes [Ag(A)(L)] (A = carboxylate or β-diketonate groups, L = trimethyl phosphine (PMe3 ), trimethyl phosphine (PEt3 ), vinyltriethylsilane (VTES), bis(trimethylsilyl)acetylene (BTMSA), bis(trimethylsilyl)ethyne (BTMSE)) [35, 40, 41]. The detailed data concerning silver(I) complexes, which are mainly used as CVD precursors are listed in **Table 1**. Liquid Ag(I) β-diketonates and their complexes are precursors usually used in the fabrication of metallic nanofilms of appropriate electrical conductivity [35]. Results from our investigations showed that silver(I) carboxylates (solids) and their complexes with tertiary phosphines (oils) are the group of precursors, which may be applied to the deposition of coatings composed of dispersed metallic nanoparticles. Because of the weak volatility of Ag(I) acetate and Ag(I) tri-fluoro-acetate, these compounds cannot be used in conventional CVD methods, although pure silver continuous films were deposited from these compounds using laser-inducted CVD (LICVD) technique [35, 42]. The low volatile Ag(I) carboxylates and their complexes with tri-phenyl-phosphines were also introduced into the CVD reactor chamber as a solution or a suspension in AACVD method [43]. This enabled the deposition of metallic silver layers at temperatures below 573 K, but resulting materials were contaminated with carbon, oxygen, and phosphorus.

Considering results of these investigations, it is noted that silver(I) penta-fluoro-propionate (Ag(OOCC2 F5 )) exhibits particularly good properties as Ag CVD precursor [44, 45] since layers consisting of dispersed AgNPs of diameters *d*Ag = 20–60 nm were obtained after 5 min of CVD process at 563 K on the surface of Si(111) substrate. After 10 min, particles already began to coalescence and after 30 min, the continuous metallic films, of thickness ca. 2.50 μm, were formed [44]. Under the same conditions, with titanium or Ti6Al4V alloy substrates, the layer composed of dispersed Ag grains of *d*Ag = 35–40 nm were deposited (**Figure 1**). Using titania nanotube (TNT) coatings as a substrate, our studies show clear correlations between the tube diameters and the grain size, their dispersion and the position on the substrate surface (**Figure 1**). The silver particles of *d*Ag = 35–45 nm were deposited on the surface of layers composed of dense packed tubes of diameter *d*TNT = 20–30 nm. The increase of tube diameter to 30–60 nm caused metallic grains to be mostly located in their insides and thus it determines the direction of their growth. The rapid decrease of Ag grains diameter below 15 nm and their location on the tubes top edges and walls surfaces was observed for TNT coatings composed of separated tubes of *d*TNT = 80–150 nm (**Figure 1**) [46]. The use of complexes [Ag(OOCR')(L)]

acac: acetylacetonato; fod: 2,2-dimethyl-6,67,78,8,8-heptafluoro-3,5-octanedionato; hfac: 1,1,1,5,5,5-hexafluoroacetylacetonato; tfpz: trifluoroacetylacetonate; VTES: vinyltriethylsilane; BTMSA: bis(trimethylsilyl)acetylene; BTMSE: bis(trimethylsilyl)

**Table 1.** The selected CVD metal-organic precursors used in the deposition of silver-based nanomaterials (the presented

**Precursor Deposition temperature TD (K) Reactor pressure (hPa) Carrier gas**

)] 503–573 0.13 H2

Silver Nanoparticles Fabricated Using Chemical Vapor Deposition and Atomic Layer Deposition…

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191

)] 503–533 0.13 H2

)] 523–723 0.07 —

)] 523–623 0.07 —

)] 873 No data —

)] 543–563 4 Ar

)] 483–563 4 Ar

)] 463–513 4 Ar

] 573–623 1–50 N2

)] 573–623 1–50 N2

)] 583 2–3 N2

)] 453–493 1.5–3 Ar, N2

] 453–553 2 Ar, N2

] 473–553 2 Ar, N2

[Ag(acac)] 473–523 — H2

[Ag(hfac)(VTES)] 433–553 0.13 — [Ag(hfac)(BTMSA)] 423–523 0.13–13.0 — [Ag(hfac)(BTMSE)] 373–473 13.0 — [Ag(tfpz)]3 523–623 0.67 Ar, H2

, PEt3

group decided the morphology of deposited metallic layers as well as the way of the grain

) revealed that the type of carboxylate

Et, L = PMe3

(R' = C2

[Ag(fod)(PMe3

[Ag(fod)(PEt3

[Ag(hfac)(PMe3

[Ag(hfac)(PEt3

[Ag(OOCCF3

[Ag(OOCC2

[Ag(OOCC2

[Ag(OOCC2

[Ag(OOCCF3

[Ag(OOCC2

[Ag(OOCt

[Ag(OOCt

ethyne.

[Ag(OOCCH2

[Ag(OOCCMe2

F5

F5 )(PMe3

F5 )(PEt3

F5 )(PnBu3

Bu)(PMe3

Bu)(PEt3

t Bu)(PEt3 )2

> Et)(PEt3 )2

data are based on Ref. [35, 40–49]).

)(PnBu3 )2

F5 , nBu, t

Bu, CH2 t

Bu, CMe2

packing on the substrate surface (**Figure 2**) [47–49].

Silver Nanoparticles Fabricated Using Chemical Vapor Deposition and Atomic Layer Deposition… http://dx.doi.org/10.5772/intechopen.71571 191

should be characterized by: (a) the appropriate volatility to achieve the highest possible concentration of the precursor in vapors; (b) the thermal stability of the compound which avoids its premature degradation during the transportation of vapors by carrier gas; (c) its ability for thermal decomposition on the substrate surface leading to the deposition of desirable materials; (d) inexpensive and simple synthesis; and (e) low toxicity. Considering these reports, we find that for CVD, two types of compounds are usually used as silver precursors, namely inorganic silver salts and volatile silver (I) complexes.

assisted CVD (FACVD) and the atmospheric pressure CVD (APCVD) [36–38]. The first of these method enables the deposition of metallic silver layers of thickness 60–90 nm [36] or 60–250 nm [37] on the substrate surface heated to the temperature TD = 573 K at atmospheric pressure. The advantage of this method is the ability to the formation of Ag-metal

[36], SiO2

large substrate areas. The nanocomposite coatings on the textile surface, composed of silicon dioxide with in situ incorporated Ag nanoparticles were produced by APCVD technique [38]. In the classical CVD techniques (metal-organic CVD (MOCVD), aerosolassistant CVD (AACVD), plasma-enhanced CVD (PECVD)), a variety of volatile silver(I) compounds are used as precursors [35, 40]. This group includes the silver(I) compounds with β-diketonates and carboxylates (mainly perfluorinated carboxylates), as well as their complexes [Ag(A)(L)] (A = carboxylate or β-diketonate groups, L = trimethyl phosphine

lene (BTMSA), bis(trimethylsilyl)ethyne (BTMSE)) [35, 40, 41]. The detailed data concerning silver(I) complexes, which are mainly used as CVD precursors are listed in **Table 1**. Liquid Ag(I) β-diketonates and their complexes are precursors usually used in the fabrication of metallic nanofilms of appropriate electrical conductivity [35]. Results from our investigations showed that silver(I) carboxylates (solids) and their complexes with tertiary phosphines (oils) are the group of precursors, which may be applied to the deposition of coatings composed of dispersed metallic nanoparticles. Because of the weak volatility of Ag(I) acetate and Ag(I) tri-fluoro-acetate, these compounds cannot be used in conventional CVD methods, although pure silver continuous films were deposited from these compounds using laser-inducted CVD (LICVD) technique [35, 42]. The low volatile Ag(I) carboxylates and their complexes with tri-phenyl-phosphines were also introduced into the CVD reactor chamber as a solution or a suspension in AACVD method [43]. This enabled the deposition of metallic silver layers at temperatures below 573 K, but resulting

Considering results of these investigations, it is noted that silver(I) penta-fluoro-propionate

ers consisting of dispersed AgNPs of diameters *d*Ag = 20–60 nm were obtained after 5 min of CVD process at 563 K on the surface of Si(111) substrate. After 10 min, particles already began to coalescence and after 30 min, the continuous metallic films, of thickness ca. 2.50 μm, were formed [44]. Under the same conditions, with titanium or Ti6Al4V alloy substrates, the layer composed of dispersed Ag grains of *d*Ag = 35–40 nm were deposited (**Figure 1**). Using titania

)) exhibits particularly good properties as Ag CVD precursor [44, 45] since lay-

materials were contaminated with carbon, oxygen, and phosphorus.

) is the widely used precursor in such techniques as the flame-

[39]) in one deposition process on

), vinyltriethylsilane (VTES), bis(trimethylsilyl)acety-

Silver nitrate (AgNO3

(PMe3

(Ag(OOCC2

F5

oxide nanocomposite coatings (e.g. TiO2

190 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

), trimethyl phosphine (PEt3

acac: acetylacetonato; fod: 2,2-dimethyl-6,67,78,8,8-heptafluoro-3,5-octanedionato; hfac: 1,1,1,5,5,5-hexafluoroacetylacetonato; tfpz: trifluoroacetylacetonate; VTES: vinyltriethylsilane; BTMSA: bis(trimethylsilyl)acetylene; BTMSE: bis(trimethylsilyl) ethyne.

**Table 1.** The selected CVD metal-organic precursors used in the deposition of silver-based nanomaterials (the presented data are based on Ref. [35, 40–49]).

nanotube (TNT) coatings as a substrate, our studies show clear correlations between the tube diameters and the grain size, their dispersion and the position on the substrate surface (**Figure 1**). The silver particles of *d*Ag = 35–45 nm were deposited on the surface of layers composed of dense packed tubes of diameter *d*TNT = 20–30 nm. The increase of tube diameter to 30–60 nm caused metallic grains to be mostly located in their insides and thus it determines the direction of their growth. The rapid decrease of Ag grains diameter below 15 nm and their location on the tubes top edges and walls surfaces was observed for TNT coatings composed of separated tubes of *d*TNT = 80–150 nm (**Figure 1**) [46]. The use of complexes [Ag(OOCR')(L)] (R' = C2 F5 , nBu, t Bu, CH2 t Bu, CMe2 Et, L = PMe3 , PEt3 ) revealed that the type of carboxylate group decided the morphology of deposited metallic layers as well as the way of the grain packing on the substrate surface (**Figure 2**) [47–49].

**Figure 1.** Scanning electron microscopy (SEM) images of titania nanotube coatings enriches with silver particles deposited by CVD method ((a) reference sample, (b) *d*TNT = 23–30 nm, (c) *d*TNT = 30–45 nm, (d) *d*TNT = 80–150 nm) [46].

**3. Atomic layer deposition**

the atomic level and the layer-by-layer film deposition.

activation energies (CVD technique, precursor [Ag(OOCR')(PEt3

Generally, the atomic layer deposition (ALD) may be defined as a technique for the deposition of uniform conformal nanolayers by alternating exposures of a substrate surface to vapors of two precursors interacting in self-limiting reactions [53, 54]. Such reactions allow the same amount of material to be deposited on the whole surface. It provides excellent film uniformity even on large substrates. ALD technique enables controlling the film thickness at

**Figure 2.** Influence of the carboxyl are groups on the morphology of silver layers, the deposition rate, and the value of

)]; R' = −t

Silver Nanoparticles Fabricated Using Chemical Vapor Deposition and Atomic Layer Deposition…

Bu (a), ─CH2

t

Bu (b), and ─C(Me2

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193

)et (c)).

The interest in the use of ALD methods to obtain metallic silver materials (e.g. monolayers) is motivated in part by their use as catalysts in the oxidation of methanol to formaldehyde and in the epoxidation of ethylene [55]. The application of silver as an effective and relatively cheap antimicrobial agent in medical, cosmetic, and environmental technologies also led to the use of ALD techniques in these areas. According to previous reports, the following ALD techniques are used: (a) thermal ALD [56, 57], (b) plasma-enhanced ALD (PEALD [58–60]),

Summarizing this section, it can be stated that the CVD methods offer wide range of possibilities for the fabrication of silver-based materials with different morphology and structure. Moreover, the CVD method allows the formation of nanocomposite materials, in which the organic or inorganic matrix is enriched with silver nanoparticles. In this regard, the flame assistant technique, that is, FACVD, is especially useful [50]. However, TiO2 /Ag nanocomposities may also be fabricated in a one-step process, that is, direct-liquid injection metal-organic CVD (DLI-MOCVD) [51]. TiO2 /Ag materials may be also synthesized in the two-step classical CVD technique in which the titania nanocrystalline matrix is enriched with silver nanograins [52]. The above-mentioned methods allow the control of structure as well as the morphology of the TiO2 matrix and the size of the synthesized silver particles.

Silver Nanoparticles Fabricated Using Chemical Vapor Deposition and Atomic Layer Deposition… http://dx.doi.org/10.5772/intechopen.71571 193

**Figure 2.** Influence of the carboxyl are groups on the morphology of silver layers, the deposition rate, and the value of activation energies (CVD technique, precursor [Ag(OOCR')(PEt3 )]; R' = −t Bu (a), ─CH2 t Bu (b), and ─C(Me2 )et (c)).

#### **3. Atomic layer deposition**

Summarizing this section, it can be stated that the CVD methods offer wide range of possibilities for the fabrication of silver-based materials with different morphology and structure. Moreover, the CVD method allows the formation of nanocomposite materials, in which the organic or inorganic matrix is enriched with silver nanoparticles. In this regard, the flame

**Figure 1.** Scanning electron microscopy (SEM) images of titania nanotube coatings enriches with silver particles deposited by CVD method ((a) reference sample, (b) *d*TNT = 23–30 nm, (c) *d*TNT = 30–45 nm, (d) *d*TNT = 80–150 nm) [46].

ities may also be fabricated in a one-step process, that is, direct-liquid injection metal-organic

CVD technique in which the titania nanocrystalline matrix is enriched with silver nanograins [52]. The above-mentioned methods allow the control of structure as well as the morphology

/Ag materials may be also synthesized in the two-step classical

/Ag nanocompos-

assistant technique, that is, FACVD, is especially useful [50]. However, TiO2

192 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

matrix and the size of the synthesized silver particles.

CVD (DLI-MOCVD) [51]. TiO2

of the TiO2

Generally, the atomic layer deposition (ALD) may be defined as a technique for the deposition of uniform conformal nanolayers by alternating exposures of a substrate surface to vapors of two precursors interacting in self-limiting reactions [53, 54]. Such reactions allow the same amount of material to be deposited on the whole surface. It provides excellent film uniformity even on large substrates. ALD technique enables controlling the film thickness at the atomic level and the layer-by-layer film deposition.

The interest in the use of ALD methods to obtain metallic silver materials (e.g. monolayers) is motivated in part by their use as catalysts in the oxidation of methanol to formaldehyde and in the epoxidation of ethylene [55]. The application of silver as an effective and relatively cheap antimicrobial agent in medical, cosmetic, and environmental technologies also led to the use of ALD techniques in these areas. According to previous reports, the following ALD techniques are used: (a) thermal ALD [56, 57], (b) plasma-enhanced ALD (PEALD [58–60]), and (c) direct-liquid injection ALD (DLI ALD [61–63]). As in the CVD method, precursors are also the most important factor possessing a direct impact on the type of the produced material and the choice of deposition method. Considering the specificity of the discussed method, it should be noted that beside the chemical compound, being the source of the produced material (silver in this case), reducing agent must also be used. Such nature of ALD processes requires that precursors used in ALD techniques must exhibit greater chemical reactivity and lower deposition rate in comparison to CVD precursors. The silver precursors which are commonly used in ALD techniques are listed in **Table 2**.

Survey of literature reports shows that most of noble metals tend towards nucleation and grow as islands (Volmer-Weber growth mode) and many ALD cycles are required to obtain a continuous coating [64, 65]. This likely occurs because of the stronger interactions between metal atoms as compared to metal-substrate interactions leading to the formation of islands of metal nanoparticles on the substrate surface [66]. The good example is the deposition of silver film on the surface of the titania nanotubes (TNT) coating of tube diameters ca. *d*TNT 30–65 nm (**Figure 3**) [58]. The dispersed AgNPs were deposited on the top edges of titania tubes after 25 ALD cycles. With increasing ALD cycles, the growth and the coalescence of particles were observed up to the formation of continuous metallic silver layer (after 150 ALD cycles). Moreover, results of our works revealed that the size of deposited AgNPs and their location on the surface of TNT layers significantly depend on tubes diameters [58]. The dependency between the sizes of silver particles deposited on the TNT layers of different tubes diameter confirms it (**Figure 4**). Obtained results revealed that the size of AgNPs, which were deposited on the surface of TNT coatings composed of dense packed tubes of the diameter *d*TNT > 15–30 nm (*d*Ag = 8–13 nm), was clearly larger in comparison to particles located on the surface of larger tubs (*d*TNT = 30–80 nm, *d*Ag = 6–10 nm). A similar effect has been observed for AgNPs deposited on TNT substrates using CVD technique [46]. The exchange of the substrate from titania nanotubes to titania nano-needles led to the necessity of increasing ALD cycles number from 25 up to 50, in order to obtain a layer composed of dispersed AgNPs [58]. It showed that the substrate-type also has an impact on the nucleation and the growth of nanoparticles.

**Figure 3.** Silver nanoparticles deposited on the surface of TNT coatings after 25, 50, 100, 150, and 200 ALD cycles

Silver Nanoparticles Fabricated Using Chemical Vapor Deposition and Atomic Layer Deposition…

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195

10 30 50 70 90 **TNT diam e te rs (nm )**

**Figure 4.** Effect of TNT surface morphology on the diameter of AgNPs after 100 ALD cycles on the surface of Ti6Al4V/

(PEALD; [Ag(fod)(PEt3)], H2; TD = 393 K; substrate Ti6Al4V/TNT; *d*TNT = 30–65 nm).

6

TNT substrates.

8

10

**AgNPs**

**diametrs (nm)**

12

14

16

Considering the results noted above, it can be stated that atomic layer deposition (ALD) is an excellent technique for the fabrication of both uniform silver nanolayers as well as dispersed

fod: 2,2-dimethyl-6,67,78,8,8-heptafluoro-3,5-octanedionato; hfac: 1,1,1,5,5,5-hexafluoroacetylacetonato; 1,5-COD: 1,5-cyclooktadiene.

**Table 2.** Metallic silver precursors, which are commonly used in ALD techniques.

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and (c) direct-liquid injection ALD (DLI ALD [61–63]). As in the CVD method, precursors are also the most important factor possessing a direct impact on the type of the produced material and the choice of deposition method. Considering the specificity of the discussed method, it should be noted that beside the chemical compound, being the source of the produced material (silver in this case), reducing agent must also be used. Such nature of ALD processes requires that precursors used in ALD techniques must exhibit greater chemical reactivity and lower deposition rate in comparison to CVD precursors. The silver precursors which are com-

Survey of literature reports shows that most of noble metals tend towards nucleation and grow as islands (Volmer-Weber growth mode) and many ALD cycles are required to obtain a continuous coating [64, 65]. This likely occurs because of the stronger interactions between metal atoms as compared to metal-substrate interactions leading to the formation of islands of metal nanoparticles on the substrate surface [66]. The good example is the deposition of silver film on the surface of the titania nanotubes (TNT) coating of tube diameters ca. *d*TNT 30–65 nm (**Figure 3**) [58]. The dispersed AgNPs were deposited on the top edges of titania tubes after 25 ALD cycles. With increasing ALD cycles, the growth and the coalescence of particles were observed up to the formation of continuous metallic silver layer (after 150 ALD cycles). Moreover, results of our works revealed that the size of deposited AgNPs and their location on the surface of TNT layers significantly depend on tubes diameters [58]. The dependency between the sizes of silver particles deposited on the TNT layers of different tubes diameter confirms it (**Figure 4**). Obtained results revealed that the size of AgNPs, which were deposited on the surface of TNT coatings composed of dense packed tubes of the diameter *d*TNT > 15–30 nm (*d*Ag = 8–13 nm), was clearly larger in comparison to particles located on the surface of larger tubs (*d*TNT = 30–80 nm, *d*Ag = 6–10 nm). A similar effect has been observed for AgNPs deposited on TNT substrates using CVD technique [46]. The exchange of the substrate from titania nanotubes to titania nano-needles led to the necessity of increasing ALD cycles number from 25 up to 50, in order to obtain a layer composed of dispersed AgNPs [58]. It showed that the substrate-type also has an impact on the nucleation and the growth

Considering the results noted above, it can be stated that atomic layer deposition (ALD) is an excellent technique for the fabrication of both uniform silver nanolayers as well as dispersed

) 383 ALD [56]

**Precursor ALD co-reactant Deposition temperature TD (K) Technique Refs.**

[Ag(hfac)(1,5-COD)] Propan-1-ol 383–423 DLI ALD [61–63]

fod: 2,2-dimethyl-6,67,78,8,8-heptafluoro-3,5-octanedionato; hfac: 1,1,1,5,5,5-hexafluoroacetylacetonato; 1,5-COD:

)] H2 413 ALD [57]

monly used in ALD techniques are listed in **Table 2**.

194 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

of nanoparticles.

[Ag(fod)(PEt3

[Ag(fod)(PEt3

[Ag(OOCBut

1,5-cyclooktadiene.

)(PEt3

)] H2

)] BH3

; N2

(NHMe2

**Table 2.** Metallic silver precursors, which are commonly used in ALD techniques.

**Figure 3.** Silver nanoparticles deposited on the surface of TNT coatings after 25, 50, 100, 150, and 200 ALD cycles (PEALD; [Ag(fod)(PEt3)], H2; TD = 393 K; substrate Ti6Al4V/TNT; *d*TNT = 30–65 nm).

**Figure 4.** Effect of TNT surface morphology on the diameter of AgNPs after 100 ALD cycles on the surface of Ti6Al4V/ TNT substrates.

silver nanoparticles on the surface of different substrates. The type of the precursor and the number of ALD cycles significantly affect the type of produced nanomaterial. The number of cycles allows controlling the nucleation and growth processes of nanoparticles on the substrate surface, which are important for the production of coatings composed of dispersed particles of the similar shape and size.

For TNT/Ag coatings, in which silver grains were localized inside of TiO2

Ag+

face of TiO2

tration of Ag+

the Ag-TiO2

the interior part of the layer.

silver for human [30, 76].

surface) immersed in the PBS solution. Ag+