Surface-enhanced Raman optical activity (SEROA) is a new technique combining the sensitivity of the surface-enhanced Raman scattering (SERS) with the detailed information about molecular structure provided by the chiral spectroscopies. So far, experimental SEROA spectra have been reported in several studies, but the interpretation and theoretical background are rather limited. In this work, general expressions for the electromagnetic contribution to SEROA are derived using the matrix polarization theory and used to investigate the enhancement in model systems. The results not only reveal a strong dependence of the enhancement on the distance between the molecule and a metal part but also the dependence of the ratio of ROA and Raman intensities (circular intensity difference, CID) on the distance and rotational averaging. For a ribose model, an optimal molecule-colloid distance was predicted which provided the highest CIDs. However, the CID maximum disappeared after a rotational averaging. For cysteine zwitterion, the simulated SEROA and SERS spectra provided a qualitative agreement with previous experiments.
p-Aminobenzenethiol (ABT) is a popular molecule for surfaceenhanced Raman scattering experiments (SERS), providing large signal enhancements on a range of metal surfaces. However, SERS intensities vary very much according to experimental conditions, and the interplay between ABT protonation, polymer state, and electronic structure/Raman cross section is still not completely clear. To understand main factors affecting Raman intensities, density functional theory (DFT) and matrix polarization theory (MPT) models were used to generate the spectra and compare to the experiment. The simulations showed that ABT protonation as well as its binding to the metal surface shift the absorption threshold, which invokes resonance or preresonance conditions favorable to the signal enhancement. The MPT approximation enabled modeling of the effect of the metal bulk and orientation of the dye on the metal surface on the enhancement and relative band intensities. The simulations can be done relatively easily and reveal chemical changes and system geometry important in rational design of SERS molecular sensors.
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