Preparation of Fe₃O₄@Ag for sensitive detection of organic arsonic acids via surface-enhanced Raman spectroscopy

Surface enhanced Raman spectroscopy (SERS) can detect organic arsonic acid contaminants in the environment, but its detection mechanism is less studied. In this study, Ag NPs were modified on Fe₃O₄ to prepare Fe₃O₄@Ag substrates, and the Raman scattering activities of 3-nitro-4-hydroxyphenylarsonic acid (Rox), dimethylarsinic acid (DMA), and phenylarsinic acid (PhAs) were investigated on this substrate, respectively. The experimental results demonstrated that the substrate was capable of markedly enhancing the Raman peak intensities of Rox, DMA, and PhAs, with enhancement factors of 7.31 × 10⁸, 6.04 × 10⁷, and 5.1 × 10⁶, respectively. The limits of detection were 3.40, 2.83 and 1.94 µg L^-1 for Rox, DMA and PhAs, respectively, and the linear range was from 5 µg L^-1-3000 µg L^-1 with a precision of 6.18%. To verify the performance of the method for practical applications, organic arsonic acid standard solutions were added to actual water samples for testing. The molecular structure of the substrate was investigated using density functional theory (DFT) calculations, and the theoretically calculated predicted spectra of prismatic Ag4 showed the best agreement with the measured Raman spectra. Based on the molecular structure of prismatic Ag4, the charge interaction, bond length variation, and minimum adsorption energy were calculated to elucidate its Raman enhancement mechanism.