Spectroscopic detection of chiral compounds is often hampered by a low sensitivity. For Raman optical activity (ROA), the signal can be dramatically increased in surface-enhanced experiments. So far, however, reproducible surface-enhanced ROA (SEROA) spectra were obtained for a reporter molecule only via induced chirality, and the intensities were just proportional to the Raman scattering. In the present study, we show that the signal can be substantially increased if colloidal silver nanoparticles are prepared already in the presence of a chiral analyte. In this case, both the analyte's and reporter's bands are visible. In addition, some experiments provided bisignate SEROA patterns, thus significantly enhancing information about the molecular structure provided by this spectroscopic method. Increased electronic circular dichroism (ECD) of the capped aggregated colloids suggests that ECD and polarized Raman scattering (ECD-Raman) contribute to the monosignate SEROA intensities, while well-dispersed nonaggregating colloids are important for observation of true (bisignate) molecular vibrational SEROA.

• Keywords
• chiral analyte capped colloid, chirality, electronic circular dichroism, silver nanoparticles, surface-enhanced Raman optical activity,
• Publication type
• Journal Article MeSH

Resonance Raman optical activity (RROA) possesses all aspects of a sensitive tool for molecular detection, but its measurement remains challenging. We demonstrate that reliable recording of RROA of chiral colorful compounds is possible, but only after considering the effect of the electronic circular dichroism (ECD) on the ROA spectra induced by the dissolved chiral compound. We show RROA for a number of model vitamin B12 derivatives that are chemically similar but exhibit distinctively different spectroscopic behavior. The ECD/ROA effect is proportional to the concentration and dependent on the optical pathlength of the light propagating through the sample. It can severely alter relative band intensities and signs in the natural RROA spectra. The spectra analyses are supported by computational modeling based on density functional theory. Neglecting the ECD effect during ROA measurement can lead to misinterpretation of the recorded spectra and erroneous conclusions about the molecular structure.

• Keywords
• Raman scattering, chirality, electronic circular dichroism, resonance Raman optical activity, vitamin B12,
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH