Great influences of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of the analyte and their alignments compared to the Fermi level of the substrate on the charge transfer (CT) process, and consequently, on the surface-enhanced Raman scattering (SERS) phenomenon have been described via theoretical calculations. To provide experimental evidence, in this study, two antibiotics, chloramphenicol (CAP) and amoxicillin (AMX), were investigated as analytes in SERS sensors based on electrochemically synthesized colloidal silver nanoparticles (e-AgNPs) as the substrate. Despite the same experimental condition, similarities in analyte structure, and in the ability of absorbing onto e-AgNPs, the detection of the two antibiotics showed obvious distinction. While CAP was able to be detected using e-AgNP-based SERS sensors at concentrations down to 1.2 × 10 −9 M, there were no characteristic peaks observed in the SERS spectra of AMX even at a high concentration of 10 −3 M. The LUMO and HOMO energy levels of the two analytes were measured using electrochemical cyclic voltammetry. The obtained results showed that the LUMO levels of both analytes were higher than the Fermi level of Ag, and the LUMO level of AMX was higher than that of CAP. The larger gap between the LUMO level of AMX and the Fermi level of Ag might have prevented the metal-to-molecule CT process, which is related to the Raman signal enhancement in both chemical and electromagnetic mechanisms. In contrast, the smaller energy gap in the case of CAP might have allowed the transfer of hot electrons from the Fermi level of the e-AgNPs to the LUMO level of the analyte. Therefore, CAP could experience an SERS effect on the e-AgNPs under the excitation of a 785 nm laser source, while AMX could not. The hypothesis was then confirmed using three other organic compounds, including furazolidone, 4-nitrophenol, and tricyclazole. The results revealed a clear correlation between the LUMO level of the analytes and their SERS signals.

The preparation of core/shell Ag@Fe3O4 nanoparticles (NPs) and its potential application toward highly sensitive electrochemical detection of furazolidone (FZD) have been reported. UV–visible spectroscopy, X-ray diffraction, scanning electron microscopy, and Zeta sizer were systematically carried out to confirm the formation, size distribution, and the composition of Ag@Fe3O4 NPs. Based on the electrochemical characteristic parameters such as electrochemically active surface area (ECSA), electron-transfer resistance (Rct), standard heterogeneous rate constant (k0), adsorption capacity (Γ), and electron transfer rate constant (ks), the Ag@Fe3O4-modified electrode possessed remarkably enhanced electrochemical sensing performance for FZD determination compared to the unmodified screen-printed electrode (SPE). This enhancement of electrochemical activity can be attributed to the fast electron transfer kinetics and great adsorption capacity that arise from the synergistic coupling between good electrical conductivity of the core AgNPs and porosity of the protective Fe3O4 shell. Under optimum conditions, the Ag@Fe3O4-based electrochemical nanosensor exhibited not only high sensitivity toward FZD detection of 1.36 µA µM−1 cm−2 in the linear ranges from 0.5-15 µM and 15-100 µM, and low detection limit of 0.24 µM but also long-term stability, repeatability, and anti-interference ability. The applicability of the proposed sensing platform in honey and milk samples was also investigated.

Surface-enhanced Raman scattering (SERS) is a powerful analysis technique that allows both the identification and detection of analytes at trace levels. However, the low rate of charge transfer (CT) between noble-metal nanoparticles and several analytes prevents them from being effectively detected by SERS-based sensors. They are regarded as low Raman crosssection molecules. In this study, we enhanced the performance of the silver nanoparticles (AgNPs)-based SERS sensing platform for a low Raman cross-section molecule, urea, focusing on improving the rate of CT. First, a set of Ag/titanium dioxide (TiO 2 ) nanocomposites were synthesized. The presence of TiO 2 improved the intensity of the SERS signal of urea, in comparison to the use of bare AgNPs. Second, a photoinduced enhanced Raman spectroscopy (PIERS) technique was employed to further elevate the Raman signal of urea. Thanks to the step of preirradiation using UV light at λ = 365 nm, with the use of the substrates containing 25%, 33%, and 50% TiO 2 content, enhancements of 1.93, 3.42, and 7.45 times were achieved, respectively, compared to the use of Ag/TiO 2 composites without UV irradiation. Through modification of the substrate, combined with application of the PIERS technique, the SERS system for urea detection using Ag/3TiO 2 (50% TiO 2 ) achieved a competitive detection limit of 4.6 × 10 −6 M. It also allowed the detection of urea in milk at concentrations down to 10 −5 M. This substrate modification and PIERS technique are promising for improvement of the sensing performance of other low-cross-section molecules.