Cation Bonding on Functional Groups for Greatly Enhanced Dipole Moments and Excellent SERS Sensing as a Universal Approach

The recently developed dipole-based surface-enhanced Raman scattering (SERS) technique has ushered in a new era of opportunities for crafting cutting-edge sensing platforms, boasting a host of advantages, including the ability to identify molecules, reduced costs, and heightened sensitivity. While significant efforts have been dedicated to augmenting the molecular dipole density on the surface to enhance sensing sensitivity, the constrained dipole moments have limited further advancement and application possibilities. In the work, we present a universal method known as cation bonding to enhance the dipole moments of oxygen-containing functional groups (OFGs) with the goal of achieving high-sensitivity trace sensing through Surface-Enhanced Raman Scattering (SERS).<br/>Utilizing Cu²⁺ as a representative model, we have demonstrated substantial enhancements in dipole moments through density functional theory-based calculations, which have been further validated through SERS measurements. These enhancements have been compared with typical OFGs and SERS platforms based on graphene oxide (GO). The analysis results from Transmission Electron Microscopy (TEM) show no formation of copper particles, while Energy Dispersive X-ray Spectroscopy (EDS) clearly indicates the presence of copper signals, which indicates the formation of copper ions on the surface and rule out common electromagnetic mechanisms. A comprehensive analysis of the electronic structures of analytes and SERS substrates has been conducted to exclude conventional chemical mechanisms. The results obtained from Infrared Spectroscopy (IR), X-ray Photoelectron Spectroscopy (XPS), and Electrostatic Force Microscopy (EFM) confirm the formation of C-O-Cu bonds and the reversal of dipole moments through copper bonding. The Cu-OFG SERS platforms thus prepared not only exhibit remarkable sensitivity in detecting Rhodamine6G (at concentrations as low as 10^-10 M) but also prove their utility in detecting trace amounts of nitrate (at 10^-10 M), a harmful contaminant in drinking water, and thiabendazole (at 0.01 ppm), a parasiticide used in fruit preservation. This cation bonding strategy is highly versatile, applicable to various cations, and opens the door to creating exceptional SERS platforms for a wide array of sensing applications.