9.1: Interferometry
The Application of Vertical Scanning Interferometry to the Study of Crystal Surface Processes
The processes which occur at the surfaces of crystals depend on many external and internal factors such as crystal structure and composition, conditions of a medium where the crystal surface exists and others. The appearance of a crystal surface is the result of complexity of interactions between the crystal surface and the environment. The mechanisms of surface processes such as dissolution or growth are studied by the physical chemistry of surfaces. There are a lot of computational techniques which allows us to predict the changing of surface morphology of different minerals which are influenced by different conditions such as temperature, pressure, pH and chemical composition of solution reacting with the surface. For example, Monte Carlo method is widely used to simulate the dissolution or growth of crystals. However, the theoretical models of surface processes need to be verified by natural observations. We can extract a lot of useful information about the surface processes through studying the changing of crystal surface structure under influence of environmental conditions. The changes in surface structure can be studied through the observation of crystal surface topography. The topography can be directly observed macroscopically or by using microscopic techniques. Microscopic observation allows us to study even very small changes and estimate the rate of processes by observing changing the crystal surface topography in time.
Much laboratory worked under the reconstruction of surface changes and interpretation of dissolution and precipitation kinetics of crystals. Invention of AFM made possible to monitor changes of surface structure during dissolution or growth. However, to detect and quantify the results of dissolution processes or growth it is necessary to determine surface area changes over a significantly larger field of view than AFM can provide. More recently, vertical scanning interferometry (VSI) has been developed as new tool to distinguish and trace the reactive parts of crystal surfaces. VSI and AFM are complementary techniques and practically well suited to detect surface changes.
VSI technique provides a method for quantification of surface topography at the angstrom to nanometer level. Time-dependent VSI measurements can be used to study the surface-normal retreat across crystal and other solid surfaces during dissolution process. Therefore, VSI can be used to directly and nondirectly measure mineral dissolution rates with high precision. Analogically, VSI can be used to study kinetics of crystal growth.
Physical Principles of Optical Interferometry
Optical interferometry allows us to make extremely accurate measurements and has been used as a laboratory technique for almost a hundred years. Thomas Young observed interference of light and measured the wavelength of light in an experiment, performed around 1801. This experiment gave an evidence of Young's arguments for the wave model for light. The discovery of interference gave a basis to development of interferomertry techniques widely successfully used as in microscopic investigations, as in astronomic investigations.
The physical principles of optical interferometry exploit the wave properties of light. Light can be thought as electromagnetic wave propagating through space. If we assume that we are dealing with a linearly polarized wave propagating in a vacuum in z direction, electric field E can be represented by a sinusoidal function of distance and time.
\[ E(x,y,z,t)\ =\ a \cos[2\pi (vt\ -\ z/\lambda )] \label{1} \]
Where a is the amplitude of the light wave, v is the frequency, and λ is its wavelength. The term within the square brackets is called the phase of the wave. Let’s rewrite this equation in more compact form,
\[ E(x,y,z,t)\ =\ a \cos(\omega t\ -\ kz) \label{2} \]
where ω=2πv is the circular frequency, and k=2π/λ is the propagation constant. Let’s also transform this second equation into a complex exponential form,
\[ E(x,y,z,t)\ =\ Re(a\ e^{i(\psi + \omega t)})\ =\ Re(a\ e^{i\omega t}) \label{3} \]
where ϕ=2πz/λ and A=e −iϕ is known as the complex amplitude. If n is a refractive index of a medium where the light propagates, the light wave traverses a distance d in such a medium. The equivalent optical path in this case is
\[ p\ =\ n\ \cdot \ d \label{4} \]
When two light waves are superposed, the result intensity at any point depends on whether reinforce or cancel each other (Figure \(\PageIndex{1}\)). This is well known phenomenon of interference. We will assume that two waves are propagating in the same direction and are polarized with their field vectors in the same plane. We will also assume that they have the same frequency. The complex amplitude at any point in the interference pattern is then the sum of the complex amplitudes of the two waves, so that we can write,
\[ A\ =\ A_{1}\ +\ A_{2} \label{5} \]
where A1=a1exp(−iϕ 1 ) and A2=a2exp(−iϕ 2 ) are the complex amplitudes of two waves. The resultant intensity is, therefore,
\[ I\ =\ |A|^{2}\ =\ I_{1}\ +\ I_{2}\ +\ 2(I_{1}I_{2})^{1/2} cos (\Delta \psi) \label{6} \]
where I 1 and I 2 are the intensities of two waves acting separately, and Δϕ=ϕ 1 −ϕ 2 is the phase difference between them. If the two waves are derived from a common source, the phase difference corresponds to an optical path difference,
\[ \Delta p\ =\ (\lambda /2 \pi) \Delta \psi \label{7} \]
If Δϕ, the phase difference between the beams, varies linearly across the field of view, the intensity varies cosinusoidally, giving rise to alternating light and dark bands or fringes (Figure \(\PageIndex{1}\)). The intensity of an interference pattern has its maximum value:
\[ I_{max}\ =\ I_{1}\ +\ I_{2}\ +\ 2(I_{1}I_{2})^{1/2} \label{8} \]
when Δϕ=2mπ, where m is an integer and its minimum value i determined by:
\[ I_{min}\ =\ I_{1}\ +\ I_{2}\ -\ 2(I_{1}I_{2})^{1/2} \label{9} \]
when Δϕ=(2m+1)π The principle of interferometry is widely used to develop many types of interferometric set ups. One of the earliest set ups is Michelson interferometry. The idea of this interferometry is quite simple: interference fringes are produced by splitting a beam of monochromatic light so that one beam strikes a fixed mirror and the other a movable mirror. An interference pattern results when the reflected beams are brought back together. The Michelson interferometric scheme is shown in Figure \(\PageIndex{2}\).
The difference of path lengths between two beams is 2x because beams traverse the designated distances twice. The interference occurs when the path difference is equal to integer numbers of wavelengths,
\[ \Delta p\ =\ 2x\ m\lambda (m= 0, \pm 1, \pm 2 ... ) \label{10} \]
Modern interferometric systems are more complicated. Using special phase-measurement techniques they capable to perform much more accurate height measurements than can be obtained just by directly looking at the interference fringes and measuring how they depart from being straight and equally spaced. Typically interferometric system consist of lights source, beamsplitter, objective system, system of registration of signals and transformation into digital format and computer which process data. Vertical scanning interferometry is contains all these parts. Figure \(\PageIndex{3}\) shows a configuration of a VSI interferometric system.
Many of modern interferometric systems use Mirau objective in their constructions. Mireau objective is based on a Michelson interferometer. This objective consists of a lens, a reference mirror and a beamsplitter. The idea of getting interfering beams is simple: two beams (red lines) travel along the optical axis. Then they are reflected from the reference surface and the sample surface respectively (blue lines). After this these beams are recombined to interfere with each other. An illumination or light source system is used to direct light onto a sample surface through a cube beam splitter and the Mireau objective. The sample surface within the field of view of the objective is uniformly illuminated by those beams with different incidence angles. Any point on the sample surface can reflect those incident beams in the form of divergent cone. Similarly, the point on the reference symmetrical with that on the sample surface also reflects those illuminated beams in the same form.
The Mireau objective directs the beams reflected of the reference and the sample surface onto a CCD (charge-coupled device) sensor through a tube lens. The CCD sensor is an analog shift register that enables the transportation of analog signals (electric charges) through successive stages (capacitors), controlled by a clock signal. The resulting interference fringe pattern is detected by CCD sensor and the corresponding signal is digitized by a frame grabber for further processing with a computer.
The distance between a minimum and a maximum of the interferogram produced by two beams reflected from the reference and sample surface is known. That is, exactly half the wavelength of the light source. Therefore, with a simple interferogram the vertical resolution of the technique would be also limited to λ/2. If we will use a laser light as a light source with a wavelength of 300 nm the resolution would be only 150 nm. This resolution is not good enough for a detailed near-atomic scale investigation of crystal surfaces. Fortunately, the vertical resolution of the technique can be improved significantly by moving either the reference or the sample by a fraction of the wavelength of the light. In this way, several interferograms are produced. Then they are all overlayed, and their phase shifts compared by the computer software Figure. This method is widely known as phase shift interferometry (PSI).
Most optical testing interferometers now use phase-shifting techniques not only because of high resolution but also because phase-shifting is a high accuracy rapid way of getting the interferogram information into the computer. Also usage of this technique makes the inherent noise in the data taking process very low. As the result in a good environment angstrom or sub-angstrom surface height measurements can be performed. As it was said above, in phase-shifting interferometry the phase difference between the interfering beams is changed at a constant rate as the detector is read out. Once the phase is determined across the interference field, the corresponding height distribution on the sample surface can be determined. The phase distribution \(φ(x, y)\) is recorded by using the CCD camera.
Let’s assign \(A(x, y)\), \(B(x, y)\), \(C(x, y)\) and \(D(x, y)\) to the resulting interference light intensities which are corresponded to phase-shifting steps of 0, π/2, π and 3π/2. These intensities can be obtained by moving the reference mirror through displacements of λ/8, λ/4 and 3λ/8, respectively. The equations for the resulting intensities would be:
\[ A(x,y)\ =\ I_{1}(x,y)\ +\ I_{2}(x,y) \cos(\alpha (x,y)) \label{11} \]
\[ B(x,y)\ =\ I_{1}(x,y)\ -\ I_{2}(x,y) \sin(\alpha (x,y)) \label{12} \]
\[ C(x,y)\ =\ I_{1}(x,y)\ -\ I_{2}(x,y) \cos(\alpha (x,y)) \label{13} \]
\[ D(x,y)\ =\ I_{1}(x,y)\ +\ I_{2}(x,y) \sin(\alpha (x,y)) \label{14} \]
where \(I_1(x,y)\) and \(I_2(x,y)\) are two overlapping beams from two symmetric points on the test surface and the reference respectively. Solving Equations \ref{11} - \ref{14}, the phase map \(φ(x, y)\) of a sample surface will be given by the relation:
\[ \psi (x,y)\ =\ \frac{B(x,y)\ -\ D(x,y)}{A(x,y)\ -\ C(x,y)} \label{15} \]
Once the phase is determined across the interference field pixel by pixel on a two-dimensional CCD array, the local height distribution/contour, \(h(x, y)\), on the test surface is given by
\[ h(x,y)\ =\ \frac{\lambda}{4\pi } \psi (x,y) \label{16} \]
Normally the resulted fringe can be in the form of a linear fringe pattern by adjusting the relative position between the reference mirror and sample surfaces. Hence any distorted interference fringe would indicate a local profile/contour of the test surface.
It is important to note that the Mireau objective is mounted on a capacitive closed-loop controlled PZT (piezoelectric actuator) as to enable phase shifting to be accurately implemented. The PZT is based on piezoelectric effect referred to the electric potential generated by applying pressure to piezoelectric material. This type of materials is used to convert electrical energy to mechanical energy and vice-versa. The precise motion that results when an electric potential is applied to a piezoelectric material has an importance for nanopositioning. Actuators using the piezo effect have been commercially available for 35 years and in that time have transformed the world of precision positioning and motion control.
Vertical scanning interferometer also has another name; white-light interferometry (WLI) because of using the white light as a source of light. With this type of source a separate fringe system is produced for each wavelength, and the resultant intensity at any point of examined surface is obtained by summing these individual patterns. Due to the broad bandwidth of the source the coherent length L of the source is short:
\[ L\ =\ \frac{\lambda ^{2}}{n \Delta \lambda} \label{17} \]
where λ is the center wavelength, n is the refractive index of the medium, ∆λ is the spectral width of the source. In this way good contrast fringes can be obtained only when the lengths of interfering beams pathways are closed to each other. If we will vary the length of a pathway of a beam reflected from sample, the height of a sample can be determined by looking at the position for which a fringe contrast is a maximum. In this case interference pattern exist only over a very shallow depth of the surface. When we vary a pathway of sample-reflected beam we also move the sample in a vertical direction in order to get the phase at which maximum intensity of fringes will be achieved. This phase will be converted in height of a point at the sample surface.
The combination of phase shift technology with white-light source provides a very powerful tool to measure the topography of quite rough surfaces with the amplitude in heights about and the precision up to 1-2 nm. Through a developed software package for quantitatively evaluating the resulting interferogram, the proposed system can retrieve the surface profile and topography of the sample objects Figure \(\PageIndex{5}\).
A Comparison of Common Methods to Determine Surface Topography: SEM, AFM and VSI
Except the interferometric methods described above, there are a several other microscopic techniques for studying crystal surface topography. The most common are scanning electron microscopy (SEM) and atomic force microscopy (AFM). All these techniques are used to obtain information about the surface structure. However they differ from each other by the physical principles on which they based.
Scanning Electron Microscopy
SEM allows us to obtain images of surface topography with the resolution much higher than the conventional light microscopes do. Also it is able to provide information about other surface characteristics such as chemical composition, electrical conductivity etc, see Figure \(\PageIndex{6}\). All types of data are generated by the reflecting of accelerated electron beams from the sample surface. When electrons strike the sample surface, they lose their energy by repeated random scattering and adsorption within an outer layer into the depth varying from 100 nm to 5 microns.
The thickness of this outer layer also knows as interactive layer depends on energy of electrons in the beam, composition and density of a sample. Result of the interaction between electron beam and the surface provides several types of signals. The main type is secondary or inelastic scattered electrons. They are produced as a result of interaction between the beam of electrons and weakly bound electrons in the conduction band of the sample. Secondary electrons are ejected from the k-orbitals of atoms within the surface layer of thickness about a few nanometers. This is because secondary electrons are low energy electrons (<50 eV), so only those formed within the first few nanometers of the sample surface have enough energy to escape and be detected. Secondary backscattered electrons provide the most common signal to investigate surface topography with lateral resolution up to 0.4 - 0.7 nm.
High energy beam electrons are elastic scattered back from the surface. This type of signal gives information about chemical composition of the surface because the energy of backscattered electrons depends on the weight of atoms within the interaction layer. Also this type of electrons can form secondary electrons and escape from the surface or travel father into the sample than the secondary. The SEM image formed is the result of the intensity of the secondary electron emission from the sample at each x,y data point during the scanning of the surface.
Atomic Force Microscopy
AFM is a very popular tool to study surface dissolution. AFM set up consists of scanning a sharp tip on the end of a flexible cantilever which moves across a sample surface. The tips typically have an end radius of 2 to 20 nm, depending on tip type. When the tip touch the surface the forces of these interactions leads to deflection of a cantilever. The interaction between tip and sample surface involve mechanical contact forces, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces etc. The deflection of a cantilever is usually measured by reflecting a laser beam off the back of the cantilever into a split photodiode detector. A schematic drawing of AFM can be seen in Figure \(\PageIndex{7}\). The two most commonly used modes of operation are contact mode AFM and tapping mode AFM, which are conducted in air or liquid environments.
Working under the contact mode AFM scans the sample while monitoring the change in cantilever deflection with the split photodiode detector. Loop maintains a constant cantilever reflection by vertically moving the scanner to get a constant signal. The distance which the scanner goes by moving vertically at each x,y data point is stored by the computer to form the topographic image of the sample surface. Working under the tapping mode AFM oscillates the cantilever at its resonance frequency (typically~300 kHz) and lightly “taps” the tip on the surface during scanning. The electrostatic forces increase when tip gets close to the sample surface, therefore the amplitude of the oscillation decreases. The laser deflection method is used to detect the amplitude of cantilever oscillation. Similar to the contact mode, feedback loop maintains a constant oscillation amplitude by moving the scanner vertically at every x,y data point. Recording this movement forms the topographical image. The advantage of tapping mode over contact mode is that it eliminates the lateral, shear forces present in contact mode. This enables tapping mode to image soft, fragile, and adhesive surfaces without damaging them while work under contact mode allows the damage to occur.
Comparison of Techniques
All techniques described above are widely used in studying of surface nano- and micromorphology. However, each method has its own limitations and the proper choice of analytical technique depends on features of analyzed surface and primary goals of research.
All these techniques are capable to obtain an image of a sample surface with quite good resolution. The lateral resolution of VSI is much less, then for other techniques: 150 nm for VSI and 0.5 nm for AFM and SEM. Vertical resolution of AFM (0.5 Ǻ) is better then for VSI (1 - 2 nm), however VSI is capable to measure a high vertical range of heights (1 mm) which makes possible to study even very rough surfaces. On the contrary, AFM allows us to measure only quite smooth surfaces because of its relatively small vertical scan range (7 µm). SEM has less resolution, than AFM because it requires coating of a conductive material with the thickness within several nm.
The significant advantage of VSI is that it can provide a large field of view (845 × 630 µm for 10x objective) of tested surfaces. Recent studies of surface roughness characteristics showed that the surface roughness parameters increase with the increasing field of view until a critical size of 250,000 µm is reached. This value is larger then the maximum field of view produced by AFM (100 × 100 µm) but can be easily obtained by VSI. SEM is also capable to produce images with large field of view. However, SEM is able to provide only 2D images from one scan while AFM and VSI let us to obtain 3D images. It makes quantitative analysis of surface topography more complicated, for example, topography of membranes is studied by cross section and top view images.
| VSI | AFM | SEM | |
|---|---|---|---|
| Lateral resolution | 0.5 - 1.2µm | 0.5 nm | 0.5 - 1 nm |
| Vertical Resolution | 2 nm | 0.5 Å | Only 2D images |
| Field of View | 845 x 630 µm (10x objective) | 100 x 100 µm | 1 - 2 mm |
| Vertical Range of Scan | 1 mm | 10 µm | - |
| Preparation of Sample | - | - | Required coating of a conducted material |
| Required environment | Air | Air, liquid | Vacuum |
The Experimental Studying of Surface Processes Using Microscopic Techniques
The limitations of each technique described above are critically important to choose appropriate technique for studying surface processes. Let’s explore application of these techniques to study dissolution of crystals.
When crystalline matter dissolves the changes of the crystal surface topography can be observed by using microscopic techniques. If we will apply an unreactive mask (silicon for example) on crystal surface and place a crystalline sample into the experiment reactor then we get two types of surfaces: dissolving and remaining the same or unreacted. After some period of time the crystal surface starts to dissolve and change its z-level. In order to study these changes ex situ we can pull out a sample from the reaction cell then remove a mask and measure the average height difference Δh bar between the unreacted and dissolved areas. The average heights of dissolved and unreacted areas are obtained through digital processing of data obtained by microscopes. The velocity of normal surface retreat v SNR during the time interval ∆t is defined by \ref{18}
\[ \nu _{SNR}\ =\ \frac{\Delta \hbar}{\Delta t} \label{18} \]
Dividing this velocity by the molar volume (cm 3 /mol) gives a global dissolution rate in the familiar units of moles per unit area per unit time:
\[ R\ =\ \frac{\nu _{SNR}}{\bar{V}} \label{19} \]
This method allows us to obtain experimental values of dissolution rates just by precise measuring of average surface heights. Moreover, using this method we can measure local dissolution rates at etch pits by monitoring changes in the volume and density of etch pits across the surface over time. VSI technique is capable to perform these measurements because of large vertical range of scanning. In order to get precise values of rates which are not depend on observing place of crystal surface we need to measure enough large areas. VSI technique provides data from areas which are large enough to study surfaces with heterogeneous dissolution dynamics and obtain average dissolution rates. Therefore, VSI makes possible to measure rates of normal surface retreat during the dissolution and observe formation, growth and distribution of etch pits on the surface.
However, if the mechanism of dissolution is controlled by dynamics of atomic steps and kink sites within a smooth atomic surface area, the observation of the dissolution process need to use a more precise technique. AFM is capable to provide information about changes in step morphology in situ when the dissolution occurs. For example, immediate response of the dissolved surface to the changing of environmental conditions (concentrations of ions in the solution, pH etc.) can be studied by using AFM.
SEM is also used to examine micro and nanotexture of solid surfaces and study dissolution processes. This method allows us to observe large areas of crystal surface with high resolution which makes possible to measure a high variety of surfaces. The significant disadvantage of this method is the requirement to cover examine sample by conductive substance which limits the resolution of SEM. The other disadvantage of SEM is that the analysis is conducted in vacuum. Recent technique, environmental SEM or ESEM overcomes these requirements and makes possible even examine liquids and biological materials. The third disadvantage of this technique is that it produces only 2D images. This creates some difficulties to measure Δh bar within the dissolving area. One of advantages of this technique is that it is able to measure not only surface topography but also chemical composition and other surface characteristics of the surface. This fact is used to monitor changing in chemical composition during the dissolution.
Dual Polarization Interferometry for Thin Film Characterization
Overview
As research interests begin to focus on progressively smaller dimensions, the need for nanoscale characterization techniques has seen a steep rise in demand. In addition, the wide scope of nanotechnology across all fields of science has perpetuated the application of characterization techniques to a multitude of disciplines. Dual polarization interferometry (DPI) is an example of a technique developed to solve a specific problem, but was expanded and utilized to characterize fields ranging surface science, protein studies, and crystallography. With a simple optical instrument, DPI can perform label-free sensing of refractive index and layer thickness in real time, which provides vital information about a system on the nanoscale, including the elucidation of structure-function relationships.
History
DPI was conceived in 1996 by Dr. Neville Freeman and Dr. Graham Cross (Figure \(\PageIndex{8}\)) when they recognized a need to measure refractive index and adlayer thickness simultaneously in protein membranes to gain a true understanding of the dynamics of the system. They patented the technique in 1998, and the instrument was commercialized by Farfield Group Ltd. in 2000.
Freeman and Cross unveiled the first full publication describing the technique in 2003, where they chose to measure well-known protein systems and compare their data to X-ray crystallography and neutron reflection data. The first system they studied was sulpho-NHS-LC-biotin coated with streptavidin and a biotinylated peptide capture antibody, and the second system was BS 3 coated with anti-HSA. Molecular structures are shown in Figure \(\PageIndex{9}\). Their results showed good agreement with known layer thicknesses, and the method showed clear advantages over neutron reflection and surface plasmon resonance. A schematic and picture of the instrument used by Freeman and Cross in this publication is shown in Figure \(\PageIndex{10}\) and Figure \(\PageIndex{11}\), respectively.
Instrumentation
Theory
The optical power of DPS comes from the ability to measure two different interference fringe patterns simultaneously in real time. Phase changes in these fringe patterns result from changes in refractive index and layer thickness that can be detected by the waveguide interferometer, and resolving these interference patterns provides refractive index and layer thickness values.
Optics
A representation of the interferometer is shown in Figure \(\PageIndex{12}\). The interferometer is composed of a simplified slab waveguide, which guides a wave of light in one transverse direction without scattering. A broad laser light is shone on the side facet of stacked waveguides separated with a cladding layer, where the waveguides act as a sensing layer and a reference layer that produce an interference pattern in a decaying (evanescent) electric field.
A full representation of DPI theory and instrumentation is shown in Figure \(\PageIndex{13}\) and Figure \(\PageIndex{14}\), respectively. The layer thickness and refractive index measurements are determined by measuring two phase changes in the system simultaneously because both transverse-electric and transverse-magnetic polarizations are allowed through the waveguides. Phase changes in each polarization of the light wave are lateral shifts of the wave peak from a given reference peak. The phase shifts are caused by changes in refractive index and layer thickness that result from molecular fluctuations in the sample. Switching between transverse-electric and transverse-magnetic polarizations happens very rapidly at 2 ms, where the switching mechanism is performed by a liquid crystal wave plate. This enables real-time measurements of the parameters to be obtained simultaneously.
Comparison of DPI with Other Techniques
Initial DPI Evaluations
The first techniques rigorously compared to DPI were neutron reflection (NR) and X-ray diffraction. These studies demonstrated that DPI had a very high precision of 40 pm, which is comparable to NR and superior to X-ray diffraction. Additionally, DPI can provide real time information and conditions similar to an in-vivo environment, and the instrumental requirements are far simpler than those for NR. However, NR and X-ray diffraction are able to provide structural information that DPI cannot determine.
DPI Comparison with orthogonal Analytical Techniques
Comparisons between DPI and alternative techniques have been performed since the initial evaluations, with techniques including surface plasmon resonance (SPR), atomic force microscopy (AFM), and quartz crystal microbalance with dissipation monitoring (QCM-D).
SPR is well-established for characterizing protein adsorption and has been used before DPI was developed. These techniques are very similar in that they both use an optical element based on an evanescent field, but they differ greatly in the method of calculating the mass of adsorbed protein. Rigorous testing showed that both tests give very accurate results, but their strengths differ. Because SPR uses spot-testing with an area of 0.26 mm2 while DPI uses the average measurements over the length of the entire 15 mm chip, SPR is recommended for use in kinetic studies where diffusion in involved. However, DPI shows much greater accuracy than SPR when measuring refractive index and layer thickness.
Atomic Force Microscopy is a very different analytical technique than DPI because it is a type of microscopy used for high-resolution surface characterization. Hence, AFM and DPI are well-suited to be used in conjunction because AFM can provide accurate molecular structures and surface mapping while DPI provides layer thickness that AFM cannot determine.
QCM-D is a technique that can be used with DPI to provide complementary data. QCM-D differs from DPI by calculating both mass of the solvent and the mass of the adsorbed protein layer. These techniques can be used together to determine the amount of hydration in the adsorbed layer. QCM-D can also quantify the supramolecular conformation of the adlayer using energy dissipation calculations, while DPI can detect these conformational changes using birefringence, thus making these techniques orthogonal. One way that DPI is superior to QCM-D is that the latter techniques loses accuracy as the film becomes very thin, while DPI retains accuracy throughout the angstrom scale.
A tabulated representation of these techniques and their ability to measure structural detail, in-vivoconditions, and real time data is shown in Table \(\PageIndex{2}\).
| Technique | Real Time | Close to In-vivo | Structural Details |
|---|---|---|---|
| QCM-D | Yes | Yes | Medium |
| SPR | Yes | Yes | Low |
| X-ray | No | No | Very high |
| AFM | No | No | High |
| NR | No | Yes | High |
| DPI | Yes | Yes | Medium |
Applications of DPI
Protein Studies
DPI has been most heavily applied to protein studies. It has been used to elucidate membrane crystallization, protein orientation in a membrane, and conformational changes. It has also been used to study protein-protein interactions, protein-antibody interactions, and the stoichiometry of binding events.
Thin Film Studies
Since its establishment using protein interaction studies, DPI has seen its applications expanded to include thin film studies. DPI was compared to ellipsometry and QCM-D studies to indicate that it can be applied to heterogeneous thin films by applying revised analytical formulas to estimate the thickness, refractive index, and extinction coefficient of heterogeneous films that absorb light. A non-uniform density distribution model was developed and tested on polyethylenimine deposited onto silica and compared to QCD-M measurements. Additionally, this revised model was able to calculate the mass of multiple species of molecules in composite films, even if the molecules absorbed different amounts of light. This information is valuable for providing surface composition. The structure of polyethylenimine used to form an adsorbing film is shown in Figure \(\PageIndex{15}\).
A challenge of measuring layer thickness in thin films such as polyethylenimine is that DPI’s evanescent field will create inaccurate measurements in inhomogeneous films as the film thickness increases. An error of approximately 5% was seen when layer thickness was increased to 90 nm. Data from this study determining the densities throughout the polyethylenimine film are shown in Figure \(\PageIndex{16}\).
Thin Layer Adsorption Studies
Similar to thin film characterization studies, thin layers of adsorbed polymers have also been elucidated using DPI. It has been demonstrated that two different adsorption conformations of polyacrylamide form on resin, which provides useful information about adsorption behaviors of the polymer. This information is industrially important because polyacrylamide is widely used through the oil industry, and the adsorption of polyacrylamide onto resin is known to affect the oil/water interfacial stability.
Initial adsorption kinetics and conformations were also illuminated using DPI on bottlebrush polyelectrolytes. Bottlebrush polyelectrolytes are show in Figure \(\PageIndex{17}\). It was shown that polyelectrolytes with high charge density initially adsorbed in layers that were parallel to the surface, but as polyelectrolytes were replaced with low charge density species, alignment changed to prefer perpendicular arrangement to the surface.
Hg 2+ Biosensing Studies
In 2009, it was shown by Wang et al. that DPI could be used for small molecule sensing. In their first study describing this use of DPI, they used single stranded DNA that was rich in thymine to complex Hg2+ ions. When DNA complexed with Hg2+, the DNA transformed from a random coil structure to a hairpin structure. This change in structure could be detected by DPI at Hg2+ concentrations smaller than the threshold concentration allowed in drinking water, indicating the sensitivity of this label-free method for Hg2+ detection. High selectivity was indicated when the authors did not observe similar structural changes for Mg2+, Ca2+, Mn2+, Fe3+, Cd2+, Co2+, Ni2+, Zn2+ or Pb2+ ions. A graphical description of this experiment is shown in Figure. Wang et al. later demonstrated that biosensing of small molecules and other metal cations can be achieved using other forms of functionalized DNA that specifically bind the desired analytes. Examples of molecules detected in this manner are shown in Figure \(\PageIndex{18}\).


13.1: The Embryologic Perspective
Learning Objectives
- Describe the growth and differentiation of the neural tube
- Relate the different stages of development to the adult structures of the central nervous system
- Explain the expansion of the ventricular system of the adult brain from the central canal of the neural tube
- Describe the connections of the diencephalon and cerebellum on the basis of patterns of embryonic development
The brain is a complex organ composed of gray parts and white matter, which can be hard to distinguish. Starting from an embryologic perspective allows you to understand more easily how the parts relate to each other. The embryonic nervous system begins as a very simple structure—essentially just a straight line, which then gets increasingly complex. Looking at the development of the nervous system with a couple of early snapshots makes it easier to understand the whole complex system.
Many structures that appear to be adjacent in the adult brain are not connected, and the connections that exist may seem arbitrary. But there is an underlying order to the system that comes from how different parts develop. By following the developmental pattern, it is possible to learn what the major regions of the nervous system are.
The Neural Tube
To begin, a sperm cell and an egg cell fuse to become a fertilized egg. The fertilized egg cell, or zygote, starts dividing to generate the cells that make up an entire organism. Sixteen days after fertilization, the developing embryo’s cells belong to one of three germ layers that give rise to the different tissues in the body. The endoderm, or inner tissue, is responsible for generating the lining tissues of various spaces within the body, such as the mucosae of the digestive and respiratory systems. The mesoderm, or middle tissue, gives rise to most of the muscle and connective tissues. Finally the ectoderm, or outer tissue, develops into the integumentary system (the skin) and the nervous system. It is probably not difficult to see that the outer tissue of the embryo becomes the outer covering of the body. But how is it responsible for the nervous system?
As the embryo develops, a portion of the ectoderm differentiates into a specialized region of neuroectoderm, which is the precursor for the tissue of the nervous system. Molecular signals induce cells in this region to differentiate into the neuroepithelium, forming a neural plate . The cells then begin to change shape, causing the tissue to buckle and fold inward (Figure \(\PageIndex{1}\)). A neural groove forms, visible as a line along the dorsal surface of the embryo. The ridge-like edge on either side of the neural groove is referred as the neural fold . As the neural folds come together and converge, the underlying structure forms into a tube just beneath the ectoderm called the neural tube . Cells from the neural folds then separate from the ectoderm to form a cluster of cells referred to as the neural crest , which runs lateral to the neural tube. The neural crest migrates away from the nascent, or embryonic, central nervous system (CNS) that will form along the neural groove and develops into several parts of the peripheral nervous system (PNS), including the enteric nervous tissue. Many tissues that are not part of the nervous system also arise from the neural crest, such as craniofacial cartilage and bone, and melanocytes.
At this point, the early nervous system is a simple, hollow tube. It runs from the anterior end of the embryo to the posterior end. Beginning at 25 days, the anterior end develops into the brain, and the posterior portion becomes the spinal cord. This is the most basic arrangement of tissue in the nervous system, and it gives rise to the more complex structures by the fourth week of development.
Primary Vesicles
As the anterior end of the neural tube starts to develop into the brain, it undergoes a couple of enlargements; the result is the production of sac-like vesicles. Similar to a child’s balloon animal, the long, straight neural tube begins to take on a new shape. Three vesicles form at the first stage, which are called primary vesicles . These vesicles are given names that are based on Greek words, the main root word being enkephalon , which means “brain” (en- = “inside”; kephalon = “head”). The prefix to each generally corresponds to its position along the length of the developing nervous system.
The prosencephalon (pros- = “in front”) is the forward-most vesicle, and the term can be loosely translated to mean forebrain . The mesencephalon (mes- = “middle”) is the next vesicle, which can be called the midbrain . The third vesicle at this stage is the rhombencephalon . The first part of this word is also the root of the word rhombus, which is a geometrical figure with four sides of equal length (a square is a rhombus with 90° angles). Whereas prosencephalon and mesencephalon translate into the English words forebrain and midbrain, there is not a word for “four-sided-figure-brain.” However, the third vesicle can be called the hindbrain . One way of thinking about how the brain is arranged is to use these three regions—forebrain, midbrain, and hindbrain—which are based on the primary vesicle stage of development (Figure13.1.2.a).
Secondary Vesicles
The brain continues to develop, and the vesicles differentiate further (see Figure13.1.2.b). The three primary vesicles become five secondary vesicles . The prosencephalon enlarges into two new vesicles called the telencephalon and the diencephalon . The telecephalon will become the cerebrum. The diencephalon gives rise to several adult structures; two that will be important are the thalamus and the hypothalamus. In the embryonic diencephalon, a structure known as the eye cup develops, which will eventually become the retina, the nervous tissue of the eye called the retina. This is a rare example of nervous tissue developing as part of the CNS structures in the embryo, but becoming a peripheral structure in the fully formed nervous system.
The mesencephalon does not differentiate into any finer divisions. The midbrain is an established region of the brain at the primary vesicle stage of development and remains that way. The rest of the brain develops around it and constitutes a large percentage of the mass of the brain. Dividing the brain into forebrain, midbrain, and hindbrain is useful in considering its developmental pattern, but the midbrain is a small proportion of the entire brain, relatively speaking.
The rhombencephalon develops into the metencephalon and myelencephalon . The metencephalon corresponds to the adult structure known as the pons and also gives rise to the cerebellum. The cerebellum (from the Latin meaning “little brain”) accounts for about 10 percent of the mass of the brain and is an important structure in itself. The most significant connection between the cerebellum and the rest of the brain is at the pons, because the pons and cerebellum develop out of the same vesicle. The myelencephalon corresponds to the adult structure known as the medulla oblongata. The structures that come from the mesencephalon and rhombencephalon, except for the cerebellum, are collectively considered the brain stem , which specifically includes the midbrain, pons, and medulla.
Spinal Cord Development
While the brain is developing from the anterior neural tube, the spinal cord is developing from the posterior neural tube. However, its structure does not differ from the basic layout of the neural tube. It is a long, straight cord with a small, hollow space down the center. The neural tube is defined in terms of its anterior versus posterior portions, but it also has a dorsal–ventral dimension. As the neural tube separates from the rest of the ectoderm, the side closest to the surface is dorsal, and the deeper side is ventral.
As the spinal cord develops, the cells making up the wall of the neural tube proliferate and differentiate into the neurons and glia of the spinal cord. The dorsal tissues will be associated with sensory functions, and the ventral tissues will be associated with motor functions.
Relating Embryonic Development to the Adult Brain
Embryonic development can help in understanding the structure of the adult brain because it establishes a framework on which more complex structures can be built. First, the neural tube establishes the anterior–posterior dimension of the nervous system, which is called the neuraxis . The embryonic nervous system in mammals can be said to have a standard arrangement. Humans (and other primates, to some degree) make this complicated by standing up and walking on two legs. The anterior–posterior dimension of the neuraxis overlays the superior–inferior dimension of the body. However, there is a major curve between the brain stem and forebrain, which is called the cephalic flexure . Because of this, the neuraxis starts in an inferior position—the end of the spinal cord—and ends in an anterior position, the front of the cerebrum. If this is confusing, just imagine a four-legged animal standing up on two legs. Without the flexure in the brain stem, and at the top of the neck, that animal would be looking straight up instead of straight in front (Figure \(\PageIndex{3}\)).
In summary, the primary vesicles help to establish the basic regions of the nervous system: forebrain, midbrain, and hindbrain. These divisions are useful in certain situations, but they are not equivalent regions. The midbrain is small compared with the hindbrain and particularly the forebrain. The secondary vesicles go on to establish the major regions of the adult nervous system that will be followed in this text. The telencephalon is the cerebrum, which is the major portion of the human brain. The diencephalon continues to be referred to by this Greek name, because there is no better term for it (dia- = “through”). The diencephalon is between the cerebrum and the rest of the nervous system and can be described as the region through which all projections have to pass between the cerebrum and everything else. The brain stem includes the midbrain, pons, and medulla, which correspond to the mesencephalon, metencephalon, and myelencephalon. The cerebellum, being a large portion of the brain, is considered a separate region. Table connects the different stages of development to the adult structures of the CNS.
One other benefit of considering embryonic development is that certain connections are more obvious because of how these adult structures are related. The retina, which began as part of the diencephalon, is primarily connected to the diencephalon. The eyes are just inferior to the anterior-most part of the cerebrum, but the optic nerve extends back to the thalamus as the optic tract, with branches into a region of the hypothalamus. There is also a connection of the optic tract to the midbrain, but the mesencephalon is adjacent to the diencephalon, so that is not difficult to imagine. The cerebellum originates out of the metencephalon, and its largest white matter connection is to the pons, also from the metencephalon. There are connections between the cerebellum and both the medulla and midbrain, which are adjacent structures in the secondary vesicle stage of development. In the adult brain, the cerebellum seems close to the cerebrum, but there is no direct connection between them.
Another aspect of the adult CNS structures that relates to embryonic development is the ventricles—open spaces within the CNS where cerebrospinal fluid circulates. They are the remnant of the hollow center of the neural tube. The four ventricles and the tubular spaces associated with them can be linked back to the hollow center of the embryonic brain (see Table \(\PageIndex{1}\)).
Table \(\PageIndex{1}\)
| Stages of Embryonic Development | ||||
|---|---|---|---|---|
| Neural tube | Primary vesicle stage | Secondary vesicle stage | Adult structures | Ventricles |
| Anterior neural tube | Prosencephalon | Telencephalon | Cerebrum | Lateral ventricles |
| Anterior neural tube | Prosencephalon | Diencephalon | Diencephalon | Third ventricle |
| Anterior neural tube | Mesencephalon | Mesencephalon | Midbrain | Cerebral aqueduct |
| Anterior neural tube | Rhombencephalon | Metencephalon | Pons cerebellum | Fourth ventricle |
| Anterior neural tube | Rhombencephalon | Myelencephalon | Medulla | Fourth ventricle |
| Posterior neural tube | Spinal cord | Central canal |
DISORDERS OF THE...
Nervous System
Early formation of the nervous system depends on the formation of the neural tube. A groove forms along the dorsal surface of the embryo, which becomes deeper until its edges meet and close off to form the tube. If this fails to happen, especially in the posterior region where the spinal cord forms, a developmental defect called spina bifida occurs. The closing of the neural tube is important for more than just the proper formation of the nervous system. The surrounding tissues are dependent on the correct development of the tube. The connective tissues surrounding the CNS can be involved as well.
There are three classes of this disorder: occulta, meningocele, and myelomeningocele (Figure \(\PageIndex{4}\)). The first type, spina bifida occulta, is the mildest because the vertebral bones do not fully surround the spinal cord, but the spinal cord itself is not affected. No functional differences may be noticed, which is what the word occulta means; it is hidden spina bifida. The other two types both involve the formation of a cyst—a fluid-filled sac of the connective tissues that cover the spinal cord called the meninges. “Meningocele” means that the meninges protrude through the spinal column but nerves may not be involved and few symptoms are present, though complications may arise later in life. “Myelomeningocele” means that the meninges protrude and spinal nerves are involved, and therefore severe neurological symptoms can be present.
Often surgery to close the opening or to remove the cyst is necessary. The earlier that surgery can be performed, the better the chances of controlling or limiting further damage or infection at the opening. For many children with meningocele, surgery will alleviate the pain, although they may experience some functional loss. Because the myelomeningocele form of spina bifida involves more extensive damage to the nervous tissue, neurological damage may persist, but symptoms can often be handled. Complications of the spinal cord may present later in life, but overall life expectancy is not reduced.
Chapter Review
The development of the nervous system starts early in embryonic development. The outer layer of the embryo, the ectoderm, gives rise to the skin and the nervous system. A specialized region of this layer, the neuroectoderm, becomes a groove that folds in and becomes the neural tube beneath the dorsal surface of the embryo. The anterior end of the neural tube develops into the brain, and the posterior region becomes the spinal cord. Tissues at the edges of the neural groove, when it closes off, are called the neural crest and migrate through the embryo to give rise to PNS structures as well as some non-nervous tissues.
The brain develops from this early tube structure and gives rise to specific regions of the adult brain. As the neural tube grows and differentiates, it enlarges into three vesicles that correspond to the forebrain, midbrain, and hindbrain regions of the adult brain. Later in development, two of these three vesicles differentiate further, resulting in five vesicles. Those five vesicles can be aligned with the four major regions of the adult brain. The cerebrum is formed directly from the telencephalon. The diencephalon is the only region that keeps its embryonic name. The mesencephalon, metencephalon, and myelencephalon become the brain stem. The cerebellum also develops from the metencephalon and is a separate region of the adult brain.
The spinal cord develops out of the rest of the neural tube and retains the tube structure, with the nervous tissue thickening and the hollow center becoming a very small central canal through the cord. The rest of the hollow center of the neural tube corresponds to open spaces within the brain called the ventricles, where cerebrospinal fluid is found.
Interactive Link Questions
Watch this animation to examine the development of the brain, starting with the neural tube. As the anterior end of the neural tube develops, it enlarges into the primary vesicles that establish the forebrain, midbrain, and hindbrain. Those structures continue to develop throughout the rest of embryonic development and into adolescence. They are the basis of the structure of the fully developed adult brain. How would you describe the difference in the relative sizes of the three regions of the brain when comparing the early (25th embryonic day) brain and the adult brain?
Answer: The three regions (forebrain, midbrain, and hindbrain) appear to be approximately equal in size when they are first established, but the midbrain in the adult is much smaller than the others—suggesting that it does not increase in size nearly as much as the forebrain or hindbrain.
Watch this video to learn about the white matter in the cerebrum that develops during childhood and adolescence.  As the color changes to blue, the ratio of gray matter to white matter changes. The caption for the video describes it as “less gray matter,” which is another way of saying “more white matter.” If the brain does not finish developing until approximately 20 years of age, can teenagers be held responsible for behaving badly?
Answer: This is really a matter of opinion, but there are ethical issues to consider when a teenager’s behavior results in legal trouble.
Multiple Choice
Q. Aside from the nervous system, which other organ system develops out of the ectoderm?
A. digestive
B. respiratory
C. integumentary
D. urinary
Answer: C
Q. Which primary vesicle of the embryonic nervous system does not differentiate into more vesicles at the secondary stage?
A. prosencephalon
B. mesencephalon
C. diencephalon
D. rhombencephalon
Answer: B
Q. Which adult structure(s) arises from the diencephalon?
A. thalamus, hypothalamus, retina
B. midbrain, pons, medulla
C. pons and cerebellum
D. cerebrum
Answer: A
Q. Which non-nervous tissue develops from the neuroectoderm?
A. respiratory mucosa
B. vertebral bone
C. digestive lining
D. craniofacial bone
Answer: D
Q. Which structure is associated with the embryologic development of the peripheral nervous system?
A. neural crest
B. neuraxis
C. rhombencephalon
D. neural tube
Answer: A
Critical Thinking Questions
Q. Studying the embryonic development of the nervous system makes it easier to understand the complexity of the adult nervous system. Give one example of how development in the embryonic nervous system explains a more complex structure in the adult nervous system.
A. The retina, a PNS structure in the adult, grows from the diencephalon in the embryonic nervous system. The mature connections from the retina through the optic nerve/tract are to the hypothalamus and thalamus of the diencephalon, and to the midbrain, which developed directly adjacent to the diencephalon as the mesencephalon in the embryo.
Q. What happens in development that suggests that there is a special relationship between the skeletal structure of the head and the nervous system?
A. The neural crest gives rise to PNS structures (such as ganglia) and also to cartilage and bone of the face and cranium.
Glossary
- brain stem
- region of the adult brain that includes the midbrain, pons, and medulla oblongata and develops from the mesencephalon, metencephalon, and myelencephalon of the embryonic brain
- cephalic flexure
- curve in midbrain of the embryo that positions the forebrain ventrally
- diencephalon
- region of the adult brain that retains its name from embryonic development and includes the thalamus and hypothalamus
- forebrain
- anterior region of the adult brain that develops from the prosencephalon and includes the cerebrum and diencephalon
- hindbrain
- posterior region of the adult brain that develops from the rhombencephalon and includes the pons, medulla oblongata, and cerebellum
- mesencephalon
- primary vesicle of the embryonic brain that does not significantly change through the rest of embryonic development and becomes the midbrain
- metencephalon
- secondary vesicle of the embryonic brain that develops into the pons and the cerebellum
- midbrain
- middle region of the adult brain that develops from the mesencephalon
- myelencephalon
- secondary vesicle of the embryonic brain that develops into the medulla
- neural crest
- tissue that detaches from the edges of the neural groove and migrates through the embryo to develop into peripheral structures of both nervous and non-nervous tissues
- neural fold
- elevated edge of the neural groove
- neural groove
- region of the neural plate that folds into the dorsal surface of the embryo and closes off to become the neural tube
- neural plate
- thickened layer of neuroepithelium that runs longitudinally along the dorsal surface of an embryo and gives rise to nervous system tissue
- neural tube
- precursor to structures of the central nervous system, formed by the invagination and separation of neuroepithelium
- neuraxis
- central axis to the nervous system, from the posterior to anterior ends of the neural tube; the inferior tip of the spinal cord to the anterior surface of the cerebrum
- primary vesicle
- initial enlargements of the anterior neural tube during embryonic development that develop into the forebrain, midbrain, and hindbrain
- prosencephalon
- primary vesicle of the embryonic brain that develops into the forebrain, which includes the cerebrum and diencephalon
- rhombencephalon
- primary vesicle of the embryonic brain that develops into the hindbrain, which includes the pons, cerebellum, and medulla
- secondary vesicle
- five vesicles that develop from primary vesicles, continuing the process of differentiation of the embryonic brain
- telencephalon
- secondary vesicle of the embryonic brain that develops into the cerebrum


12.3: Teamwork in the Workplace
- What is a team, and what makes a team effective?
Teamwork has never been more important in organizations than it is today. Whether you work in a manufacturing environment and utilize self-directed work teams, or if you work in the “knowledge economy” and derive benefits from collaboration within a team structure, you are harnessing the power of a team.
A team, according to Katzenbach and Smith in their Harvard Business Review (HBR) article “The Discipline of Teams,” is defined as “people organized to function cooperatively as a group” (Katzenbach & Smith, 2023). The five elements that make teams function are:
- Common commitment and purpose
- Specific performance goals
- Complementary skills
- Commitment to how the work gets done
- Mutual accountability
A team has a specific purpose that it delivers on, has shared leadership roles, and has both individual and mutual accountabilities. Teams discuss, make decisions, and perform real work together, and they measure their performance by assessing their collective work products. Wisdom of Teams reference. This is very different from the classic working group in an organization (usually organized by functional area) in which there is a focused leader, individual accountabilities and work products, and a group purpose that is the same as the broader organizational mission. Think of the finance organization or a particular business unit in your company—these are, in effect, larger working groups that take on a piece of the broader organizational mission. They are organized under a leader, and their effectiveness is measured by the unit's influence on others within the business (e.g., financial performance of the business.)
So, what makes a team truly effective? According to Katzenbach and Smith’s “Discipline of Teams,” the authors have observed several practices in successful teams:
Establish urgency, demanding performance standards, and direction. Teams work best when they have a compelling reason for being, and it is thus more likely that the teams will be successful and live up to performance expectations. We’ve all seen the teams that are brought together to address an “important initiative” for the company, but without clear direction and a truly compelling reason to exist, the team will lose momentum and wither.
Select members for their skill and skill potential, not for their personality. This is not always as easy as it sounds for several reasons. First, most people would prefer to have those with good personalities and positive attitudes on their team to promote a pleasant work environment. This is fine, but make sure that those individuals have the skill sets needed (or the potential to acquire/learn) for their piece of the project. The second caveat here is that you don’t always know what skills you need on a project until you really dig in and see what’s going on. Spend some time upfront thinking about the purpose of the project and the anticipated deliverables you will be producing, and think through the specific types of skills you’ll need on the team.
Pay particular attention to first meetings and actions. This is one way of saying that first impressions mean a lot—and it is just as important for teams as for individuals. Teams will interact with everyone from functional subject-matter experts all the way to senior leadership, and the team must look competent and be perceived as competent. Keeping an eye on your team’s level of emotional intelligence is very important and will enhance your team’s reputation and ability to navigate stakeholders within the organization.
Set some clear rules of behavior. I have been through many meetings and team situations in which we have rushed through “ground rules” because they seemed obvious—and everyone always came up with the same list. It is critical that the team takes the time upfront to capture their own rules of the road to keep the team in check. Rules that address areas such as attendance, discussion, confidentiality, project approach, and conflict are key to keeping team members aligned and engaged appropriately.
Set and seize upon a few immediate performance-oriented tasks and goals. What does this mean? Have some quick wins that make the team feel they’re accomplishing something and working together well. This is very important to the team’s confidence, as well as just getting into the practices of working as a team. Success in the larger tasks will come soon enough, as the larger tasks are really just a group of smaller tasks that fit together to produce a larger deliverable.
Challenge the group regularly with fresh facts and information. That is, continue to research and gather information to confirm or challenge what you know about your project. Don’t assume that all the facts are static and that you received them at the beginning of the project.  I think that the pace of change is so great in the world today that new information is always presenting itself and must be considered in the overall context of the project.
Spend lots of time together. Here’s an obvious one that is often overlooked. People are so busy that they forget that an important part of the team process is spending time together, thinking together, and bonding. Time in person, time on the phone, time in meetings—all counts and helps build camaraderie and trust.
Exploit the power of positive feedback, recognition, and reward. Positive reinforcement is a motivator that will help the team members feel more comfortable contributing. It will also reinforce the behaviors and expectations that you’re driving within the team. Although many extrinsic rewards can serve as motivators, a successful team begins to feel that its own success and performance is the most rewarding.
Collaboration is another key concept and method by which teams can work together successfully. Bringing together a team of experts from across the business would be a best practice in any situation. However, Gratton and Erickson (2007), in their article Eight Ways to Build Collaborative Teams, found that collaboration seems to decrease sharply when a team is working on complex project initiatives. Their study examined 55 larger teams and identified those with strong collaboration skills despite the level of complexity. There were eight success factors for having strong collaboration skills:
- “Signature” relationship practices
- Role models of collaboration among executives
- Establishment of a “gift” culture, in which managers mentor employees
- Training in relationship skills
- A sense of community
- Ambidextrous leaders—good at task and people leadership
- Good use of heritage relationships
- Role clarity and talk ambiguity
As teams grow in size and complexity, the standard practices that worked well with small teams in the past don’t work anymore. Organizations need to think about how to make collaboration work and leverage the above best practices to build relationships and trust.
- What is the definition of a team?
- Name some practices that can make a team more successful.


14.8: Interview Questions
- Discuss various question types common in interviews
For most job candidates, the burning question is “What will I be asked?” There’s no way to anticipate every single question that may arise during an interview. It’s possible that, no matter how well prepared you are, you may get a question you just didn’t expect. But that’s okay. Do as much preparation as you can—which will build your confidence—and trust that the answers will come.
As you respond to the questions, try to remind yourself, that this is not so much “about you” as about the interviewer finding the right fit for this opening. The questions are establishing whether your skills and experiences will meet the needs of this company. That is where your research comes in. You can work to explain your background relative to this new environment. If the interviewer says, “Tell me about you,” that is not a cue to start with your earliest memory. Instead, focus on the specific knowledge and skills you possess as related to what you know about this position.
The simplest place to start is to have a list of about four to six examples of workplace actions that you are proud of. Think of times you excelled. Then think about how this same story might fit several situations. One story might show initiative, leading others, decision making, and more. With these stories in mind, when a question comes, pull out the best fit and reword it to match the specific question. Try creating that list now, then use the question banks below to see what fits and what other situations you might need to have mentally ready.
There is no substitute for going through as many questions as you can prior to the interview. As you practice on your own, do not just read these questions and think.  This is the practice that will set you up for adapting to various interview situations.
To help you reach that point of sureness and confidence, take time to review common interview questions. Think about your answers. Make notes if that helps. Then conduct a practice interview with a friend, a family member, or a colleague. Speak your answers out loud. Below is a list of resources that contain common interview questions and good explanations/answers you might want to adopt.
If you can use the databases below to find questions to practice with, record yourself. Then watch the recording and score each response against the STAR technique discussed earlier. From these databases, look at the broad categories of questions so that you may prepare some responses and examples for each category. Some categories may be:
| Type | Example | Considerations |
|---|---|---|
| Goodwill, Greetings and Get Acquainted | Tell me about yourself. | No more than two minutes. List the highlights of your resume with a brief example, if possible. |
| Gauging Your Interest | Why are you interested in this position? | Make this position tops on your interest list, without ever alluding to any other search. Avoid sounding like this might be any other than a first choice (For example,”When I happened to see your ad” makes the job posting sound trivial to you.) |
| Your Experience and Accomplishments | How has your education prepared you for this position? | Be confident. Everyone knows you have not done this job yet, but you must sound like you are ready for this job. Avoid the natural hesitation you may feel (“I think I’ll be great” versus “With these skills, I can…”) |
| The Future | What would you most like to accomplish if you get this position? | There’s no need to over promise or worry, but do demonstrate you have a plan for this job or for life versus just hoping things will work out. Offer some realistic career goals based on some practical skill or education you have. |
| Challenging | What type of people do you have no patience for? | We all have weaknesses. Being aware of them is a great skill. Turning them to our advantage is even better. “While I get along well with most people, those who complain rather than try to find a solution can be hard on my patience.” |
| Situational | If you were aware a co-worker was falsifying data, what would you do? | The employer probably wants to see how you handle difficulties on your own and what logical process you may use to solve problems. Remember to focus on the company’s outcome and expense while not compromising your own standards. |
| Behavioral | Describe a time you worked as part of a team. | While all interview responses work well with the STAR technique, this is the type of question best suited to it. |
Why Should We Hire You
From the Ohio State University Fisher College of Business Career Management Office, here is a video featuring representatives from recruiting companies offering advice for answering the question “Why should we hire you?” As you watch, make mental notes about how you would answer the question in an interview for a job you really want.
In closing, we suggest reading the essay “It’s Like Online Dating,” by Jackie Vetrano. In this essay, the writer compares job hunting—including résumé creation and cover-letter writing—to online dating. In this last section, she concludes with a look at the job interview and compares it to a first date.
Contributors and Attributions
- Practice question. Authored by : Susan Kendall. Provided by : Lumen Learning. License : CC BY: Attribution
- College Success. Authored by : Linda Bruce. Provided by : Lumen Learning. Located at : https://courses.lumenlearning.com/waymaker-collegesuccess/ . License : CC BY: Attribution
- Why Should We Hire You? How to Answer this Interview Question. Authored by : Fisher OSU. Located at : https://youtu.be/5NVYg2HNAdA . License : CC BY: Attribution


Page 3.1: Matter
Atoms, Molecules, & Compounds
At its most fundamental level, life is made of matter . Matter is something that occupies space and has mass. All matter is composed of elements , substances that cannot be broken down or transformed chemically into other substances. Each element is made of atoms, each with a constant number of protons and unique properties. A total of 118 elements have been defined; however, only 92 occur naturally and fewer than 30 are found in living cells. The remaining 26 elements are unstable and therefore do not exist for very long or are theoretical and have yet to be detected. Each element is designated by its chemical symbol (such as H, N, O, C, and Na), and possesses unique properties. These unique properties allow elements to combine and to bond with each other in specific ways.
An atom is the smallest component of an element that retains all of the chemical properties of that element. For example, one hydrogen atom has all of the properties of the element hydrogen, such as it exists as a gas at room temperature and it bonds with oxygen to create a water molecule. Hydrogen atoms cannot be broken down into anything smaller while still retaining the properties of hydrogen. If a hydrogen atom were broken down into subatomic particles, it would no longer have the properties of hydrogen. At the most basic level, all organisms are made of a combination of elements. They contain atoms that combine together to form molecules. In multicellular organisms, such as animals, molecules can interact to form cells that combine to form tissues, which make up organs. These combinations continue until entire multicellular organisms are formed.
All matter, whether it be a rock or an organism, is made of atoms. Often, these atoms combine to form molecules . Molecule are chemicals made from two or more atoms bonded together. Some molecules are very simple, like O2, which is comprised of just two oxygen atoms. Some molecules used by organisms, such as DNA, are made of many millions of atoms. All atoms contain protons, electrons, and neutrons (Figure \(\PageIndex{1}\) below). The only exception is hydrogen (H), which is made of one proton and one electron. A proton is a positively charged particle that resides in the nucleus (the core of the atom) of an atom and has a mass of 1 and a charge of +1. An electron is a negatively charged particle that travels in the space around the nucleus. In other words, it resides outside of the nucleus. It has a negligible mass and has a charge of –1. Neutrons , like protons, reside in the nucleus of an atom. They have a mass of 1 and no charge. The positive (protons) and negative (electrons) charges balance each other in a neutral atom, which has a net zero charge.
Figure \(\PageIndex{1}\). Atoms are comprised of protons and neutrons located within the nucleus, and electrons surrounding the nucleus. The nucleus of an atom should not be confused with the nucleus of a cell.Each element contains a different number of protons and neutrons, giving it its own atomic number and mass number. The atomic number of an element is equal to the number of protons that element contains. The mass number is the number of protons plus the number of neutrons of that element. Therefore, it is possible to determine the number of neutrons by subtracting the atomic number from the mass number.
Isotopes are different forms of the same element that have the same number of protons, but a different number of neutrons. Some elements, such as carbon, potassium, and uranium, have naturally occurring isotopes. Carbon 12, the most common isotope of carbon, contains six protons and six neutrons. Therefore, it has a mass number of 12 (six protons and six neutrons) and an atomic number of 6 (which makes it carbon). Carbon 14 contains six protons and eight neutrons. Therefore, it has a mass number of 14 (six protons and eight neutrons) and an atomic number of 6, meaning it is still the element carbon. These two alternate forms of carbon are isotopes. Some isotopes are unstable and will lose protons, other subatomic particles, or energy to form more stable elements. These are called radioactive isotopes or radioisotopes.
EVOLUTION IN ACTION: Carbon dating
Carbon-14 (14C) is a naturally occurring radioisotope that is created in the atmosphere by cosmic rays. This is a continuous process, so more 14C is always being created. As a living organism develops, the relative level of 14C in its body is equal to the concentration of 14C in the atmosphere. When an organism dies, it is no longer ingesting 14C, so the ratio will decline. 14C decays to 14N by a process called beta decay; it gives off energy in this slow process. After approximately 5,730 years, only one-half of the starting concentration of 14C will have been converted to 14N. The time it takes for half of the original concentration of an isotope to decay to its more stable form is called its half-life.
Because the half-life of 14C is long, it is used to age formerly living objects, such as fossils. Using the ratio of the 14C concentration found in an object to the amount of 14C detected in the atmosphere, the amount of the isotope that has not yet decayed can be determined. Based on this amount, the age of the fossil can be calculated to about 50,000 years (Figure \(\PageIndex{2}\) below). Isotopes with longer half-lives, such as potassium-40, are used to calculate the ages of older fossils. Through the use of carbon dating, scientists can reconstruct the ecology and biogeography of organisms living within the past 50,000 years.
Figure \(\PageIndex{2}\). The age of remains that contain carbon and are less than about 50,000 years old, such as this pygmy mammoth, can be determined using carbon dating. (credit: Bill Faulkner/ NPS)Chemical Bonds
How elements interact with one another depends on the number of electrons and how they are arranged. When an atom does not contain equal numbers of protons and electrons it is called an ion . Because the number of electrons does not equal the number of protons, each ion has a net charge . For example, if sodium loses an electron, it now has 11 protons and only 10 electrons, leaving it with an overall charge of +1. Positive ions are formed by losing electrons and are called cations . Negative ions are formed by gaining electrons and are called anions . Elemental anionic names are changed to end in -ide. As an example, when chlorine becomes an ion it is referred to as chloride.
Ionic and covalent bondsare strong bonds formed between two atoms. These bonds hold atoms together in a relatively stable state. Ionic bonds are formed between two oppositely charged ions (an anion and a cation). Because positive and negative charges attract, these ions are held together much like two oppositely charged magnets would stick together. Covalent bonds form when electrons are shared between two atoms. Each atom shares one of their electrons, which then orbits the nuclei of both atoms, holding the two atoms together. Covalent bonds are the strongest and most common form of chemical bond in organisms. Unlike most ionic bonds, covalent bonds do not dissociate in water.
Covalent bonds come in two varieties: polar and non-polar. A non-polar covalent bond occurs when electrons are shared equally between the two atoms. Polar covalent bonds form when the electrons are shared unequally. Why does this occur? Each element has a known electronegativity : a measure of their affinity for electrons. Some elements, such as oxygen, are very electronegative because they strongly attract electrons from other atoms. Hydrogen, meanwhile, has low electronegativity and thus weakly attracts electrons, in comparison. Polar covalent bonds form when the two atoms involved have significantly different electronegativities. In biological systems, this occurs when oxygen bonds with hydrogen and when nitrogen (also quite electronegative) bonds with hydrogen.
When oxygen and hydrogen bond, for example, the shared electrons are pulled more strongly toward oxygen and thus farther away from hydrogen’s nucleus. Because the electrons move farther away from hydrogen, it becomes slightly positively charged (δ+). The oxygen becomes slightly negatively charged as the electrons become closer to it (δ–). If two molecules with polar covalent bonds approach one another, they can interact due to the attraction of opposite electrical charges. For example, the slight positive charge of hydrogen in a water molecule can be attracted to the slight negative charge of oxygen in a different water molecule (Figure \(\PageIndex{3}\)). This interaction between two polar molecules is called a hydrogen bond . This type of bond is very common in organisms. Notably, hydrogen bonds give water the unique properties that sustain life. If it were not for hydrogen bonding, water would be a gas rather than a liquid at room temperature.
Figure \(\PageIndex{3}\). Hydrogen bonds form between slightly positive (δ+) and slightly negative (δ–) charges of polar covalent molecules, such as water.WATER IS CRUCIAL TO MAINTAINING LIFE
Do you ever wonder why scientists spend time looking for water on other planets? It is because water is essential to life; even minute traces of it on another planet can indicate that life could or did exist on that planet. Water is one of the more abundant molecules in living cells and the one most critical to life as we know it.  Without it, life simply would not exist.
- Water is polar. The hydrogen and oxygen atoms within water molecules form polar covalent bonds. The shared electrons spend more time associated with the oxygen atom than they do with hydrogen atoms. There is no overall charge to a water molecule, but there is a slight positive charge on each hydrogen atom and a slight negative charge on the oxygen atom. Because of these charges, the slightly positive hydrogen atoms repel each other and form the unique shape. Each water molecule attracts other water molecules because of the positive and negative charges in the different parts of the molecule. Water also attracts other polar molecules (such as sugars) that can dissolve in water and are referred to as hydrophilic (“water-loving”).
- Water stabilizes temperature. The hydrogen bonds in water allow it to absorb and release heat energy more slowly than many other substances. Temperature is a measure of the motion (kinetic energy) of molecules. As the motion increases, energy is higher and thus temperature is higher. Water absorbs a great deal of energy before its temperature rises. Increased energy disrupts the hydrogen bonds between water molecules. Because these bonds can be created and disrupted rapidly, water absorbs an increase in energy and temperature changes only minimally. This means that water moderates temperature changes within organisms and in their environments.
- Water is an excellent solvent. Because water is polar, with slight positive and negative charges, ionic compounds and polar molecules can readily dissolve in it. Water is, therefore, what is referred to as a solvent—a substance capable of dissolving another substance. The charged particles will form hydrogen bonds with a surrounding layer of water molecules.
- Water is cohesive. Have you ever filled up a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water actually forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-air (gas) interface, although there is no more room in the glass. Cohesion gives rise to surface tension, the capacity of a substance to withstand rupture when placed under tension or stress. When you drop a small scrap of paper onto a droplet of water, the paper floats on top of the water droplet, although the object is denser (heavier) than the water. This occurs because of the surface tension that is created by the water molecules. Cohesion and surface tension keep the water molecules intact and the item floating on the top. It is even possible to “float” a steel needle on top of a glass of water if you place it gently, without breaking the surface tension. These cohesive forces are also related to the water’s property of adhesion, or the attraction between water molecules and other molecules. This is observed when water “climbs” up a straw placed in a glass of water. You will notice that the water appears to be higher on the sides of the straw than in the middle. This is because the water molecules are attracted to the straw and therefore adhere to it. Cohesive and adhesive forces are important for sustaining life. For example, because of these forces, water can flow up from the roots to the tops of plants to feed the plant.
Buffers, pH, Acids, and Bases
The pH of a solution is a measure of its acidity or alkalinity . The pH scale ranges from 0 to 14. A change of one unit on the pH scale represents a change in the concentration of hydrogen ions by a factor of 10, a change in two units represents a change in the concentration of hydrogen ions by a factor of 100. Thus, small changes in pH represent large changes in the concentrations of hydrogen ions. Pure water is neutral. It is neither acidic nor basic and has a pH of 7.0. Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. The blood in your veins is slightly alkaline (pH = 7.4). The environment in your stomach is highly acidic (pH = 1 to 2). Orange juice is mildly acidic (pH = approximately 3.5), whereas baking soda is basic (pH = 9.0).
Acids are substances that provide hydrogen ions (H+) and lower pH, whereas bases provide hydroxide ions (OH–) and raise pH. The stronger the acid, the more readily it donates H+. For example, hydrochloric acid and lemon juice are very acidic and readily give up H+ when added to water. Conversely, bases are those substances that readily donate OH–. The OH– ions combine with H+ to produce water, which raises a substance’s pH. Sodium hydroxide and many household cleaners are very alkaline and give up OH– rapidly when placed in water, thereby raising the pH.
Figure \(\PageIndex{4}\). The pH scale measures the amount of hydrogen ions (H+) in a substance. (credit: modification of work by Edward Stevens)How is it that we can ingest or inhale acidic or basic substances and not die? Buffers are the key. Buffers readily absorb excess H+ or OH–, keeping the pH of the body carefully maintained in the aforementioned narrow range. Carbon dioxide is part of a prominent buffer system in the human body; it keeps the pH within the proper range. This buffer system involves carbonic acid (H 2 CO 3 ) and bicarbonate (HCO 3 –) anion. If too much H+ enters the body, bicarbonate will combine with the H+ to create carbonic acid and limit the decrease in pH. Likewise, if too much OH– is introduced into the system, carbonic acid will combine with it to create bicarbonate and limit the increase in pH. While carbonic acid is an important product in this reaction, its presence is fleeting because the carbonic acid is released from the body as carbon dioxide gas each time we breathe. Without this buffer system, the pH in our bodies would fluctuate too much and we would fail to survive.
Biological Molecules
Besides water, the molecules necessary for life are organic. Organic molecules are those that contain carbon covalently bonded to hydrogen. In addition, they may contain oxygen, nitrogen, phosphorus, sulfur, and additional elements.There are four major classes of organic molecules: carbohydrates , lipids , proteins , and nucleic acids. Each is an important component of the cell and performs a wide array of functions.
It is often said that life is “carbon-based.” This means that carbon atoms, bonded to other carbon atoms or other elements, form the fundamental components of many of the molecules found uniquely in living things. Other elements play important roles in biological molecules, but carbon certainly qualifies as the “foundation” element for molecules in living things. It is the bonding properties of carbon atoms that are responsible for its important role.
Carbon can form four covalent bonds with other atoms or molecules. The simplest organic carbon molecule is methane (CH4), in which four hydrogen atoms bind to a carbon atom (Figure \(\PageIndex{5}\)). Carbohydrates include what are commonly referred to as simple sugars, like glucose, and complex carbohydrates such as starch. While many types of carbohydrates are used for energy, some are used for structure by most organisms, including plants and animals. For example, cellulose is a complex carbohydrate that adds rigidity and strength to the cell walls of plants. The suffix “-ose” denotes a carbohydrate, but note that not all carbohydrates were given that suffix when names (e.g., starch).
Figure \(\PageIndex{5}\). Carbon can form four covalent bonds to create an organic molecule. The simplest carbon molecule is methane (CH4), depicted here.Lipids include a diverse group of compounds that are united by a common feature. Lipids are hydrophobic (“water-fearing”), or insoluble in water, because they are non-polar molecules (molecules that contain non-polar covalent bonds) . Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of lipids called fats. Lipids also provide insulation from the environment for plants and animals. For example, they help keep aquatic birds and mammals dry because of their water-repelling nature. Lipids are also the building blocks of many hormones and are an important constituent of cellular membranes. Lipids include fats, oils, waxes, phospholipids, and steroids.
Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. They are all polymers of amino acids. The functions of proteins are very diverse because there are 20 different chemically distinct amino acids that form long chains, and the amino acids can be in any order. Proteins can function as enzymes, hormones, contractile fibers, cytoskeleton rods, and much more. Enzymes are vital to life because they act as catalyst in biochemical reactions (like digestion). Each enzyme is specific for the substrate (a reactant that binds to an enzyme) upon which it acts. Enzymes can function to break molecular bonds, to rearrange bonds, or to form new bonds.
Figure \(\PageIndex{6}\). The double-helix model shows DNA as two parallel strands of intertwining molecules.Nucleic acids are very large molecules that are important to the continuity of life. They carry the genetic blueprint of a cell and thus the instructions for its functionality. The two main types of nucleic acids are deoxyribonucleic acid ( DNA ) and ribonucleic acid (RNA). DNA is the genetic material found in all organisms, ranging from single-celled bacteria to multicellular mammals. The other type of nucleic acid, RNA , is mostly involved in protein synthesis. The DNA molecules never leave the nucleus, but instead use an RNA intermediary to communicate with the rest of the cell. Other types of RNA are also involved in protein synthesis and its regulation. DNA and RNA are made up of small building blocks known as nucleotides . The nucleotides combine with each other to form a polynucleotide: DNA or RNA.  DNA has a beautiful double-helical structure (Figure \(\PageIndex{6}\)).
Additional Resources:
Figure \(\PageIndex{8}\). A video that provides some wicked-awesome information about atoms.Attribution
- Concepts of Biology by OpenStax is licensed under CC BY 4.0. Modified from original by Matthew R. Fisher.


33.4: Animal Form and Function - Limiting Effects of Diffusion on Size and Development
- Describe how diffusion limits cell size and development
Limiting Effects of Diffusion on Size and Development
The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective at completing all of these tasks. The center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.
An important concept in understanding the efficiency of diffusion as a transportation mechanism is the surface-to-volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr 2 , and a volume of (4/3)πr 3 . The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface-to-volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.
The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become less efficient at completing all tasks since they are now more efficient at doing fewer tasks. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Surface-to-volume ratio also applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons or in the relationship between muscle mass and the generation of dissipation of heat.
Key Points
- Diffusion is effective over a specific distance, so it’s more efficient in small, single-celled microorganisms.
- Diffusion becomes less efficient as the surface-to-volume ratio decreases, so diffusion is less effective in larger animals.
- To overcome the limitations of diffusion, multicellular organisms have developed specialized tissues and systems that are responsible for completing a limited number of nutrient and waste tasks.
Key Terms
- surface-to-volume ratio : the amount of surface area per unit volume of an object or collection of objects; decreases as volume increases