We present a combined experimental and theoretical study dedicated to analyzing the surface-enhanced Raman spectra of solutions of citrate-covered silver nanoparticles (NPs) in the presence of acetil-neuraminic acid (Neu5Ac). The Raman signals from Neu5Ac (particularly the bands located at 1002 and 1237 cm-1) can easily be detected for concentrations as low as 1 mg/dl, providing outstanding molecular sensing properties for our synthesized Ag-NPs. When compared to its solid phase, Neu5Ac adsorption on citrate-covered Ag particles leads to enhanced Raman intensities; many vibrational frequencies are shifted; and relative intensities undergo significant changes. These variations in the spectra complicate molecular identification, especially in mixed overlayers as the ones considered in this work. Consequently, experimental results are discussed on the basis of extensive density functional theory (DFT) calculations on model citrate-covered silver clusters with coadsorbed Neu5Ac species. Several citrate-Neu5Ac-cluster complexes are optimized, and the Raman spectra of every stable structure are calculated. The theoretical data reveal a complex interplay between the chemical nature and distribution of the adsorbates. Citrate molecules prefer to aggregate on the silver surface, leaving thus sizable regions of the Ag cluster exposed for Neu5Ac adsorption. The simulated Raman spectra of citrate-Neu5AcAg complexes show acceptable similarities with experimental SERS measurements, allowing for a clear identification of the different vibrational bands. By changing the functional groups attached to the Ag cluster it is possible to modulate the adsorption properties and Raman response of Neu5Ac. DFT calculations reveal that propanethiol adsorption on model Ag clusters allows the formation of a more uniform molecular overlayer, effectively protecting the silver surface from the surrounding media. We predict that Ag-NPs functionalized with propanethiol species are not efficient substrates for Raman detection of Neu5Ac. The previous predictions are corroborated by performing SERS measurements on propanethiol-covered Ag-NPs exposed to different concentrations of Neu5Ac from which the absence of Neu5Ac spectral features is observed. The here-used DFT scheme provides an adequate theoretical frame to predict and understand the physical origin of the Raman response of adsorbed molecules in Ag-NPs, and we expect that this predictability may extend to other metallic NPs.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films