Synergistic enhancement of EM and CM in Ag/AZO thin films for high-performance SERS detection of R6G and methylparaben

Abstract

In Surface-Enhanced Raman Scattering (SERS) analysis, enhancing the Raman signal is crucial and is primarily governed by two mechanisms: electromagnetic mechanism (EM) and chemical mechanism (CM). While the EM is well understood, gaining insight into the CM and its synergy with EM is crucial for optimizing SERS performance. In this study, CM effects were investigated through molecular adsorption and charge transfer (CT) processes using SERS substrates composed of silver nanoparticles (AgNPs) integrated with aluminium-doped zinc oxide (AZO) semiconductor thin films. AgNPs were synthesized via microwave irradiation and centrifugation, followed by incubation with Rhodamine 6G (R6G) and Methylparaben (MP) to enhance analyte adsorption. Adsorption behaviours were analysed using the Langmuir and Freundlich isotherm models. The analyte-loaded AgNPs were then combined with AZO thin films prepared by sputtering. These hybrid substrates not only generated electromagnetic “hot spots” to boost Raman signals via the EM but also facilitated CT, thereby enhancing signals through CM. A proposed CT mechanism between the substrate and analytes is also presented. The results demonstrate that the Ag/AZO SERS substrates exhibit a strong synergistic EM–CM effect, enabling the sensitive detection of R6G and MP at low concentrations, achieving enhancement factors (EFs) of 8.67 × 10¹⁰ (at 10⁻¹³ M) and 1.45 × 10⁶ (at 0.001 ppm or 6.58 × 10⁻⁹ M), respectively.