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Structural characterization of biologically formed materials is essential for understanding biological phenomena and their environment, and generating new bio-inspired engineering concepts. For example, nacre -- formed by mollusks in the ocean -- encodes local environmental conditions throughout its formation and has exceptional strength due to its nanoscale brick-and-mortar structure. This layered structure, comprising transparent aragonite tablets bonded with an ultra-thin organic polymer, also results in stunning interference colors. Existing methods of structural characterization of nacre rely on some form of cross-sectional analysis, such as scanning electron microscopy or polarization-dependent imaging contrast (PIC) mapping. However, these techniques are destructive and too time- and resource-intensive to analyze large sample areas. Here we present an all-optical, rapid, and non-destructive imaging technique -- hyperspectral interference tomography (HIT) -- to spatially map the structural parameters of nacre and other disordered layered materials. We combined hyperspectral imaging with optical-interference modeling to infer the mean tablet thickness and disordering of nacre layers across entire mollusk shells at various stages of development, observing a previously unknown relationship between the growth of the mollusk and tablet thickness. Our rapid, inexpensive, and nondestructive method can be readily applied to in-field studies., Main text + supplementary

Experimental data for waveguide-coupled surface-plasmon-polariton (SPP) cones generated from dielectric waveguides is presented. The results demonstrate a simpler route to collect plasmon waveguide resonance (i.e., PWR) data. In the reverse-Kretschmann configuration (illumination from the sample side) and Kretschmann configuration (illumination from the prism side), all the waveguide modes are excited simultaneously with p- or s-polarized incident light, which permits rapid acquisition of PWR data without the need to scan the incident angle or wavelength, in the former configuration. The concentric SPP cone properties depend on the thickness and index of refraction of the waveguide. The angular intensity pattern of the cone is well-matched to simulation results in the reverse-Kretschmann configuration, and is found to be dependent on the polarization of the incident light and the polarization of the waveguide mode. In the Kretschmann geometry, all waveguide-coupled SPP cones are measured at incident angles that produce attenuated light reflectivity. In addition, the enhanced electric field produced under total internal reflection allows high signal-to-noise ratio multimodal spectroscopies (e.g., Raman scattering, luminescence) to measure the chemical content of the waveguide film, which traditionally is not measured with PWR.

Directional-surface-plasmon-coupled Raman scattering (directional RS) has the combined benefits of surface plasmon resonance and Raman spectroscopy, and provides the ability to measure adsorption and monolayer-sensitive chemical information. Directional RS is performed by optically coupling a 50 nm gold film to a Weierstrass prism in the Kretschmann configuration and scanning the angle of the incident laser under total internal reflection. The collected parameters on the prism side of the interface include a full surface-plasmon-polariton cone and the full Raman signal radiating from the cone as a function of incident angle. An instrument for performing directional RS and a quantitative study of the instrumental parameters are herein reported. To test the sensitivity and quantify the instrument parameters, self-assembled monolayers and 10 to 100 nm polymer films are studied. The signals are found to be well-modeled by two calculated angle-dependent parameters: three-dimensional finite-difference time-domain calculations of the electric field generated in the sample layer and projected to the far-field, and Fresnel calculations of the reflected light intensity. This is the first report of the quantitative study of the full surface-plasmon-polariton cone intensity, cone diameter, and directional Raman signal as a function of incident angle. We propose that directional RS is a viable alternative to surface plasmon resonance when added chemical information is beneficial.

Fourier transform (FT)-plasmon waveguide resonance (PWR) spectroscopy measures light reflectivity at a waveguide interface as the incident frequency and angle are scanned. Under conditions of total internal reflection, the reflected light intensity is attenuated when the incident frequency and angle satisfy conditions for exciting surface plasmon modes in the metal as well as guided modes within the waveguide. Expanding upon the concept of two-frequency surface plasmon resonance developed by Peterlinz and Georgiadis (Opt. Commun. 1996, 130, 260), the apparent index of refraction and the thickness of a waveguide can be measured precisely and simultaneously by FT-PWR with an average percent relative error of 0.4%. Measuring reflectivity for a range of frequencies extends the analysis to a wide variety of sample compositions and thicknesses since frequencies with the maximum attenuation can be selected to optimize the analysis. Additionally, the ability to measure reflectivity curves with both p- and s-polarized light provides anisotropic indices of refraction. FT-PWR is demonstrated using polystyrene waveguides of varying thickness, and the validity of FT-PWR measurements are verified by comparing the results to data from profilometry and atomic force microscopy (AFM).

The application of surface plasmon resonance (SPR) measurements to the study of ultrathin organic films adsorbed onto gold surfaces utilizing near-infrared (NIR) excitation from a Fourier transform (FT) spectrometer is described. The FT-SPR experiment measures the NIR reflectivity spectrum from a prism/gold film/water assembly at a fixed angle of incidence approximately 1−2° greater than the critical angle. A strong reflectivity minimum is observed in the FT-SPR spectrum; this minimum can be shifted from 12 000 to 6000 cm-1 by tuning the angle of incidence. Upon adsorption of a thin biopolymer film from solution, a shift in the minimum is observed that can be correlated to a film thickness using Fresnel calculations. From experiments on the adsorption of electrostatically bound poly(lysine)/poly(glutamic acid) multilayers, an ∼60-cm-1 shift per 10-A change in film thickness was measured. Frequency shifts of 2 cm-1 (corresponding to a thickness change of the polymer layer of ∼0.3 A) can be easily measured ...

Surface plasmon resonance (SPR) imaging affords label-free monitoring of biomolecule interactions in an array format. A surface plasmon conducting metal thin film is required for SPR measurements. Gold thin films are traditionally used in SPR experiments as they are readily functionalized with thiol-containing molecules through formation of a gold-sulfur bond. The lability of this gold-thiol linkage upon exposure to oxidizing conditions and ultraviolet light renders these surfaces incompatible with light-directed synthetic methods for fabricating DNA arrays. It is shown here that applying a thin carbon overlayer to the gold surface yields a chemically robust substrate that permits light-directed synthesis and also supports surface plasmons. DNA arrays fabricated on these carbon-metal substrates are used to analyze two classes of biomolecular interactions: DNA-DNA and DNA-protein. This new strategy allows the combinatorial study of binding interactions directly from native, unmodified biomolecules of interest and offers the possibility of discovering new ligands in complex mixtures such as cell lysates.

Organic films electrodeposited on a copper electrode from basic solutions containing imidazole were investigated by two different in situ infrared spectroscopic techniques: real-time polarization modulation and static linear polarization reflection absorption spectroscopy. The spectra indicate that as the electrode potential is made more positive, the amount of imidazole bound to the electrode surface increases. The coating on the electrode is found to consist mainly of copper(I) imidazolate with either a random or perpendicular orientation to the polycrystalline surface. No change in orientation of the imidazole ring is seen with the variation in potential. The infrared data from both reflection techniques indicates that before the Cu(0)/Cu(I) onset the film is formed at the solution metal oxide/hydroxide interface and at potentials above this, ≥−0.54 V, a more compact film forms closer to the metal surface.

A new methodology for the real-time in situ monitoring of atmospheric corrosion processes is presented. The mid-infrared spectra of surface films formed on polished copper substrates exposed to humid air containing sub-ppm levels of sulfur dioxide, nitric oxide, and hydrogen chloride were obtained by using a novel surface-sensitive infrared technique: real-time polarization modulation grazing angle reflection-absorption spectroscopy. The new methodology demonstrated improved signal-to-noise ratios, by a factor of 2.5, as compared to conventional Fourier transform infrared reflection absorption spectroscopy. Molecular spectroscopic data for water- and hydroxyl-containing species present at the metal surface were obtained with no interference from water vapor. Over the spectral region 4000–800 cm−1, bands were identified and assigned to nitro and nitrito adsorbates, to sulfite ions, and to bound water on the copper surface. Analysis of the time evolution and the profile composition of these bands is possible because of the sensitivity of this new optical sampling technique.

Both conventional static linear polarization selection and a new real-time sampling technique for polarization modulation were used for in situ FT-IR spectroelectrochemical studies of copper electrodes. Three aqueous probe species were investigated: thiocyanate ion, imidazole, and glucose. As the data were obtained under identical electrochemical conditions for the systems studied, a direct comparison of the optical methodologies was possible. The modulation technique provided excellent rejection of signals due to atmospheric gases and increases in overall sensitivity relative to the static method. Both techniques provided equivalent information with respect to surface species, but differed in the extent to which the electrolyte solution was sampled. Thus, by using both techniques to study a given electrochemical process, a means to discriminate spectral features due to adsorbed species from those due to solution-phase components is established.

The sections in this article are Introduction to Fourier Transform Surface Plasmon Resonance Surface Plasmon Resonance in the Near-Infrared Experimental Fourier Transform Surface Plasmon Resonance Applications of Fourier Transform Surface Plasmon Resonance Measurements Summary

The sections in this article are Introduction to Polarization-Modulation Reflection–Absorption Spectroscopy Experimental Considerations for the PM-IRRAS Measurement Experimental Setup The PEM The PM-IRRAS Measurement PM-IRRAS Spectral Normalization Examples and Applications of PM-IRRAS Measurement Metal Surfaces – Ex Situ Self-Assembled Monolayers in the CH Stretching Region SAMs in the Fingerprint Region Metal Surfaces – Liquid Phase In Situ Structural Studies of SAMs Electrochemical Studies Metal Surfaces – Gas Phase In Situ Corrosion Studies on Copper Carbon Monoxide on Co(0001) Organic Vapors on Silicon Dioxide Nanoparticle Thin Films on Gold Air/Water Interface Summary