Stable rGO/Ag/CeO2 Nanostructured Substrates for Superior SERS Detection of Trace Analytes

Abstract

Surface-enhanced Raman spectroscopy (SERS) is a potent analytical technique that has been widely applied in the trace detection of important analyte molecules such as pesticides, drugs, reaction intermediates, and biomarkers. Although the sensitivity of noble metals has reached its pinnacle, they suffer from poor reusability, stability, and high cost that are crucial for commercialization. We have designed a layered SERS substrate, rGO/Ag/CeO₂ (RAC) substrate, comprising reduced graphene oxide (rGO) film, 20 nm Ag nanoparticle film, and cerium oxide overlayer, bearing 2–3 nm ultrasmall CeO₂ nanoparticles, due to its excellent oxidation protection and redox behavior and demonstrated the superior performance employing 4-mercaptobenzoic acid (MBA) as the standard analyte. The rGO layer aids in fluorescence quenching, while the combined effect of surface plasmon resonance by silver nanoparticles and prominent charge transfer by a nanocrystalline CeO₂ layer plays a significant role in providing a superior SERS substrate with an enhancement factor of 10⁸ and a detection limit of 10^–8 M for MBA. The oxygen deficiencies in CeO₂ are responsible for the higher degree of charge transfer observed for the RAC substrate than the rGO/Ag substrate. The SERS substrate has retained its stable performance under high relative humidity and temperature conditions, reproducing uniform signals of analytes across the substrate, attributed to the oxidation protection nature of CeO₂. RAC substrate is also shown to be highly sensitive to the trace detection of a range of nitroaromatic and nitramine explosives down to nanomolar level. The substrate is fabricated by physical deposition routes and is scalable. This study demonstrates the enormous potential of the distinctive RAC films to realize a scalable SERS substrate with sensitivity, reliability, and stability that are crucial in SERS-based applications.