Sensing Bisphenol A by Means of Surface-Enhanced Raman Spectroscopy and DFT Calculations to Elucidate the Enhancement Mechanism That Dominates the Spectrum

Surface-Enhanced Raman Spectroscopy (SERS) was employed as a spectroscopic tool to detect Bisphenol A (BPA), a building block in polycarbonate and epoxy resins or an additive in other polymer plastics like PVC, which has an endocrine disruptor effect. Silver nanoparticles (AgNPs) synthesized by using different reducing agents such as hydroxylamine (Ag@HX), citrate (Ag@Cit), borohydride (Ag@BH), and beta-cyclodextrin (Ag@beta CD) were employed, aiming to select the best standard SERS substrate. The lowest limit of quantification was reached at a concentration of 0.01 mM (2.3 mu g/mL) of a sonicated aqueous solution by using Ag@Cit NPs and identifying two enhanced bands recorded at about 350 and 460 cm(-1). In order to gain insight into the nature of the enhanced bands, and therefore into which mechanism governs the SERS signal, electrochemical spectra recorded at different electrode potentials were acquired and TD-DFT calculations were applied to a neutral silver complex of BPA, Ag-2-BPA, and to its monohydroxylated chemical specie, Ag-2-BPA(OH), which is present in sonicated solution. The calculated electronic structure and the resonance Raman spectra point out that a surface plasmon-like resonance inside the silver cluster dominates the SERS spectrum corresponding to the physisorbed BPA(OH) species, a charge transfer enhancement mechanism or an intramolecular resonance transition localized in the phenolic framework was then discarded.

Automated summary

Surface-Enhanced Raman Spectroscopy (SERS) was used to detect Bisphenol A (BPA), an endocrine disruptor commonly found in plastic products. Silver nanoparticles (AgNPs) synthesized using different reducing agents were tested, and Ag@Cit NPs were identified as the best SERS substrate. The lowest limit of quantification for BPA was 0.01 mM (2.3 μg/mL) using Ag@Cit NPs, with enhanced bands observed at 350 cm(-1) and 460 cm(-1). Electrochemical spectra and TD-DFT calculations provided insights into the SERS signal mechanism and confirmed a surface plasmon-like resonance within the silver cluster.