Hybrid nanostructures that couple plasmon and exciton resonances generate hybridized energy states, called plexcitons, which may result in unusual light-matter interactions. We report the formation of a transparency dip in the visible spectra of colloidal suspensions containing silver nanoplatelets and a cyanine dye, 1,1′-diethyl-2,2′-cyanine iodide (PIC). PIC was electrostatically adsorbed onto the surface of silver nanoplatelet core particles, forming an outer J-aggregate shell. This core−shell architecture provided a framework for coupling the plasmon resonance of the silver nanoplatelet core with the exciton resonance of the J-aggregate shell. The sizes and aspect ratios of the silver nanoplatelets were controlled to ensure the overlap of the plasmon and exciton resonances. As a measure of the plasmon-exciton coupling strength in the system, the experimentally observed transparency dips correspond to a Rabi splitting energy of 207 meV, among the highest reported for colloidal nanoparticles. The optical properties of the silver platelet-J-aggregate nanocomposites were supported numerically and analytically by the boundary-element method and temporal coupled-mode theory, respectively. Our theoretical predictions and experimental results confirm the presence of a transparency dip for the silver nanoplatelet core Jaggregate shell structures. Additionally, the numerical and analytical calculations indicate that the observed transparencies are dominated by the coupling of absorptive resonances, as opposed to the coupling of scattering resonances. Hence, we describe the suppressed extinction in this study as an induced transparency rather than a Fano resonance.

Thiophenol is commonly used as a model system for surface-enhanced Raman scattering (SERS) of molecules due to the strong affinity of the −SH group toward noble metal surfaces. By performing time-, temperature-, and pH-dependent measurements of thiophenol adsorption on commercial nanostructured gold SERS substrates, we have observed both physisorption and chemisorption processes. These two distinct adsorption regimes were found dependent on the pH which controlled the ionization state of thiophenol in an aqueous medium. At low pH the sulfhydryl proton remains bound, and the kinetic adsorption profile obtained from the SERS intensity follows a sigmoid-shaped curve with an initially slow adsorption rate that deviates from a Langmuir profile. In addition, from temperature-dependent measurements, a near zero value for the activation energy is obtained, indicating that physisorption is the rate-limiting step. At high pH, where the sulfhydryl proton becomes detached, the kinetic adsorption profile follows a classical Langmuir profile, and the activation energy is significantly higher than at low pH, indicating that chemisorption is the rate-limiting step.

Initial results demonstrating the ability to classify surface-enhanced Raman (SERS) spectra of chemical and biological warfare agent simulants are presented. The spectra of two endospores (B. subtilis and B. atrophaeus), two chemical agent simulants (dimethyl methylphosphonate (DMMP) and diethyl methylphosphonate (DEMP)), and two toxin simulants (ovalbumin and horseradish peroxidase) were studied on multiple substrates fabricated from colloidal gold adsorbed onto a silanized quartz surface. The use of principal component analysis (PCA) and hierarchical clustering were used to evaluate the efficacy of identifying potential threat agents from their spectra collected on a single substrate. The use of partial least squares-discriminate analysis (PLS-DA) and soft independent modeling of class analogies (SIMCA) on a compilation of data from separate substrates, fabricated under identical conditions, demonstrates both the feasibility and the limitations of this technique for the identification of known but previously unclassified spectra.

Surface-enhanced Raman spectroscopy (SERS) is a useful technique for probing analyte-noble metal interactions and determining thermodynamic properties such as their surface reaction equilibrium constants and binding energies. In this study, we measure the binding equilibrium constants and Gibbs free energy of binding for a series of nitrogen-containing aromatic molecules adsorbed on Klarite substrates. A dual Langmuir dependence of the SERS intensity on concentration was observed for the six species studied, indicating the presence of at least two different binding energies. We relate the measured binding energies to the previously described SERS enhancement value (SEV) and show that the SEV is proportional to the traditional SERS enhancement factor G, with a constant of proportionality that is critically dependent on the adsorption equilibrium constant determined from the dual Langmuir isotherm. We believe the approach described is generally applicable to many SERS substrates, both as a prescriptive approach to determining their relative performance and as a probe of the substrate's affinity for a target adsorbate.

Resonance Raman cross sections of common explosives have been measured by use of excitation wavelengths in the deep-UV from 229 to 262 nm. These measurements were performed both in solution and in the native solid state for comparison. While measurements of UV Raman cross sections in solution with an internal standard are straightforward and commonly found in the literature, measurements on the solid phase are rare. This is due to the difficulty in preparing a solid sample in which the molecules of the internal standard and absorbing analyte/explosive experience the same laser intensity. This requires producing solid samples that are mixtures of strongly absorbing explosives and an internal standard transparent at the UV wavelengths used. For the solid-state measurements, it is necessary to use nanostructured mixtures of the explosive and the internal standard in order to avoid this bias due to the strong UV absorption of the explosive. In this study we used a facile spray-drying technique where the analyte of interest was codeposited with the nonresonant standard onto an aluminum-coated microscope slide. The generated resonance enhancement profiles and quantitative UV-vis absorption spectra were then used to plot the relative Raman return as a function of excitation wavelength and particle size.