Silver Nanoparticles-Based SERS Platform towards Detecting Chloramphenicol and Amoxicillin: An Experimental Insight into the Role of HOMO–LUMO Energy Levels of the Analyte in the SERS Signal and Charge Transfer Process

Abstract

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.

Photoinduced Enhanced Raman Spectroscopy for the Ultrasensitive Detection of a Low-Cross-Section Chemical, Urea, Using Silver–Titanium Dioxide Nanostructures

“…To confirm the potential of the PIERS technique as a promising tool to sense low Raman cross-section molecules, herein, we provide another example of 4-NP. In our previous study, 4-NP was proven to be a low cross-section molecule with a large gap between its LUMO level and the Fermi level of Ag (0.71 eV) and low SERS signal . To improve the performance of the 4-NP sensor, we employed Ag/3TiO 2 as the substrate.…”

“…The LUMO level of urea forms a relatively large gap (0.76 eV) with the Fermi level of Ag (−4.26 eV). This gap is even larger than that of 4-NP (0.71 eV), which was reported to have a low SERS signal on the Ag substrate . This large gap suggested a low rate of CT between AuNPs and urea in SERS experiments, which limits the cross section of the molecule.…”

“…Unfortunately, such a powerful analytical tool is not effective at detecting every organic analyte. In a recent study, we reported on the role of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the analyte on the SERS signal and CT process . Our study proved that a large gap between the LUMO energy of the analyte and the Fermi level of the substrate could prevent the CT process.…”

“…Under light excitation, the collective oscillation of conductive electrons of the plasmonic materials creates a strong magnetic field on the materials surface, coupling with the vibrational modes of the analytes and, therefore, enhancing their Raman signals. , It is also the principle of the most important contribution of Raman enhancement in the SERS effect, the electromagnetic mechanism. In addition, despite a smaller contribution, the formation of a chemical complex between the analytes and metal surface, also known as the chemical mechanism (CM), cannot be ignored because charge-transfer (CT) transitions between the substrate and the molecule are also essential for the SERS phenomenon to occur. − These two mechanisms generate a giant enhancement in the Raman signal, allowing many kinds of analytes, including pesticides, food additives, biomarkers, etc., to be detected at trace levels. − …”

“…However, improving the SERS enhancement of the molecules with low CT levels is not impossible. In the report mentioned above, we suggested modification of the nanomaterial to increase the Fermi level . In a 2018 study, Tao et al were successful in improving the rate of CT and enlarging the cross section of the analyte by modifying the substrate with WTe 2 , instead of employing the initial substrate of graphene .…”