An ultra-sensitive SERS approach by electrostatic enrichment of charged analytes on Ag/laponite nanocomposites

An electrostatic adsorption-based, surface-enhanced Raman scattering (SERS) assay for the detection of charged analytes was developed, which utilized Ag-deposited laponite nanoparticles (Ag@Lap NPs) as both a SERS substrate and negatively charged absorber for the capture of positively charged analytes, such as methylene blue (MB) and rhodamine B (RB). The strong electrostatic interaction between the MB analyte and the Ag@Lap NPs resulted in an effective concentration of MB on the Ag content of Ag@Lap and provided a limit of detection (LOD) sensitivity of 10⁻¹¹ M, which was more sensitive by a factor of 25,000 over the LOD of MB (0.25 × 10⁻⁶ M) in a similar SERS assay using bare Ag NPs with low charges. Additionally, a 100-fold improvement in LOD was also observed for RB analyte on Ag@Lap NPs over that on bare Ag NPs. Unlike positively charged, the negatively charged analytes, such as direct black-168 (DB-168) and methyl orange (MO), exhibited a weaker adsorption on the same negatively charged surface of Ag@Lap NPs due to electrostatic repulsion, leading to a lower SERS sensitivity. These results indicated that the electrostatic attraction between the analytes and SERS substrate played an important role in enriching the number of analytes on the active substrate to significantly improve the SERS intensity, suggesting a new strategy, which could be easily adapted to environmental analysis and biosensor applications by concentrating and detecting charged analytes, such as pollutants and dye molecules in nanotags.