Surface-enhanced infrared absorption (SEIRA) spectroscopy has been developed for the nondestructive analysis of trace molecules. Herein, we found that ethylene glycol (EG) decorated TiO2 nanosheet exhibits a selective SEIRA effect for molecules with nucleophilic groups, such as -NH2 and -OH. The SEIRA effect was attributed to the chemical mechanism originating from the interactions between the surface EG and the analytes. The enhancement factor was negatively correlated with the electrophilicity index of the analytes (p = 0.004), and the noncovalent bond dominates the interactions between the analytes and EG. The charge distribution analysis revealed that the -CH2 groups of EG exposed on the TiO2 surface are positively charged, attracting the electron-rich groups of the analyte. This attraction concentrates the analyte, redistributes its charge, defines its molecular dipole moment, and thereby enhances the SEIRA effect. The insights gained from this study shed light on developing new SEIRA substrates and emphasized the critical role of surface ligands in SEIRA applications.
Keywords: SEIRA; TiO2 nanosheet; chemical mechanisms; nucleophilicity; surface ligand.