Ratiometric Surface-Enhanced Raman Spectroscopy Quantitative Detection Based on Interfacial Competitive Adsorption

Surface-enhanced Raman spectroscopy (SERS) is widely recognized for its high sensitivity and molecular fingerprinting capabilities, but its practical quantitative applications are hindered by its inherent strong intensity fluctuation. Herein, we present an internal-standard free ratiometric SERS strategy for the sensitive and accurate quantitation of cationic analyte (CV⁺) in aquatic environments. The method leverages the competitive adsorption between CV⁺ and Hg²⁺ on an I⁻-functionalized silver substrate. The Hg²⁺-triggered signal attenuation yields a ratiometric response (IR = I/I₀) that exhibits a robust linear relationship with the logarithm of CV concentration across a wide dynamic range (1 × 10⁻¹² to 1 × 10⁻⁷ M). The assay achieves a detection limit of ∼10⁻¹² M and demonstrates excellent quantitation accuracy, with a recovery rate up to 98% in spiked sample tests. A microscopic Langmuir adsorption model is proposed to elucidate the quantitation mechanism, linking the ratiometric response to the surface coverage of CV⁺ and its competitive displacement. This work extends the ratiometric concept to the quantitation of Raman-active molecules and provides a strategy for indirect sensing via interfacial competition. Integrated with microfluidic technology, the proposed approach holds great potential for the analysis of cationic analytes in environmental monitoring, food safety, and biomedical fields.