Rapid analysis of herbicide diquat in apple juice with surface enhanced Raman spectroscopy: Effects of particle size and the ratio of gold to silver with gold and gold-silver core-shell bimetallic nanoparticles as substrates

Abstract

Twelve different Au nanoparticles (NPs; diameter, 19–87 nm) and Au–Ag core-shell NPs (diameters of core and particles: 19 nm, 35–91 nm; 43 nm, 66–127 nm) varying in size and the ratio of gold to silver were employed as surface enhanced Raman scattering (SERS) substrates for rapid analysis of herbicide diquat cation in apple juice. The minimum detectible concentrations for diquat cation standards with three selected optimal SERS substrates were 0.005 mg/L (43 nm Au NPs), 0.05 mg/L (91 nm Au–Ag NPs, 19 nm core), and 0.025 mg/L (78 nm Au–Ag NPs, 43 nm core), respectively. However, only the 78 nm Au–Ag NPs led to acceptable SERS enhancement effect for diquat cation in apple juice with minimum sample preparation, allowing its detection at as low as 0.025 mg/L. In addition to providing a rapid analysis approach for diquat in fruit juices, this study indicates that the influences of particle size and composition of NPs on SERS enhancement effects for an analyte in standard solutions could be quite different from that in a real food sample, while the optimal ratio of gold to silver of the bimetallic NPs depended on the Au seeds and sample matrix.