Facile synthesis of Au nanoparticle-coated Fe3O4 magnetic composite nanospheres and their application in SERS detection of malachite green.

A facile method for synthesizing Au nanoparticle-coated Fe3O4 magnetic composite nanospheres (Fe3O4@Au MCS) via seed-mediated growth and iterative reduction is reported. The nanospheres were then successfully used to detect malachite green (MG) residues in water bodies via surface-enhanced Raman scattering (SERS) technique. Fe3O4@Au MCS has excellent optical properties and superparamagnetism; it can be dispersed into the solution to fully adsorb target molecules and then collected with a magnet to increase the molecular density and the number of SERS hot spots. Magnetic enrichment was superior to conventional detection method. The limit of detection for MG was 10-7 M and the enhancement factor was 1.1 × 105. The logarithm of the SERS intensity of the characteristic peak at 1618 cm-1 exhibited a linear relationship with the logarithm of the MG concentration over the range of 10-3- 10-7 M, with a correlation coefficient of 0.966. The Fe3O4@Au MCS had good uniformity of SERS signals, with a 18.59% relative standard deviation for the SERS intensity. MG detection in aquaculture water conformed with the established quantitative regulations. The SERS spectrum calculated with density function theory for MG adsorbed on Fe3O4@Au MCS was very close to the experimental spectrum, which verified enhancement by the substrate. Overall, Fe3O4@Au MCS enabled ultrasensitive, quantitative SERS detection of MG.