Surface-enhanced Raman scattering (SERS) enhancements and excitation profiles for 3,5-pyridinedicarboxylate and dabsyl aspartate on colloidal gold

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Abstract
Surface-enhanced Raman scattering (SERS) of 3,5-pyridinedicarboxylate on colloidal gold in aqueous media and of the azo dye, dabsyl(N-4-dimethylaminoazobenzene-4′-sulphonyl) aspartate (DABS-ASP), on colloidal gold in ethanol was observed. Enhanced Raman spectra and excitation profiles over wavelengths 457.9–676.4 nm were measured. SERS enhancements of the non-chromophoric adsorbate reached a maximum of 1.4 × 105 at 676.4 nm while those for the azo dye were in the 102–103 range and peaked near 590 nm. In each case the peak in the SERS excitation profile was matched by a band in the absorption spectra of the gold colloids that was assigned to small aggregates of gold particles. The influence of gold salt (HAuCl4), protecting polymer [poly(vinylpyrrolidone)], and additional base (sodium ethoxide) concentrations and reflux time periods on the stability of the resulting gold particles in ethanol were determined from the absorption spectra of the gold organosols. The stoichiometric requirement of the base:gold molar ratio was found to be 4.5 and was used to formulate a redox equation for the reduction of gold(III) by ethanol. The separation of surface resonance Raman scattering (SRRS) from SERS of DABS-ASP on colloidal gold in ethanol was achieved since single particles of gold, which absorb characteristically at 525 nm, did not give additional enhancement over the SRRS already present. The influence of DABS-ASP surface coverage on SERS enhancements was also determined after obtaining adsorption isotherms for DABS-ASP on colloidal gold by two methods:(1) from enhanced Raman spectra;(2) from ultracentrifugation of the sols and spectrophotometric determination of DABS-ASP in the supernatant. The data from these two isotherms were further used to establish an upper limit on the fraction of SERS active sites for DABS-ASP on colloidal gold and, therefore, a lower bound on the SERS enhancement.

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