In situ homogeneous formation of Au@AgNPs for the rapid determination of formaldehyde residues by surface-enhanced Raman spectroscopy coupled with microhydrodistillation.

The reaction of formaldehyde (HCHO) with the solution containing Au nanoparticles and [Ag(NH3)2]+ through Tollens' reaction is described to form Au@Ag nanoparticles (Au@AgNPs), which are used as a SERS enhancement material. A minimized hydrodistillation device with air condense is designed to obtain a clean aqueous background for SERS analysis and simplified pretreatment process. A good linear relationship is found between the SERS signals and the concentration of HCHO in the range 0.3 to 55 mg L-1 when p-aminothiophenol (p-ATP) is selected as the Raman signal molecule, and the detection limit is 0.08 mg L-1. Stable SERS signals could be achieved due to both homogeneous formation of Au@AgNPs and clean aqueous background. The relative standard deviation (RSD) of 15 batches samples is found to be 7.1%. The proposed approach has successfully been applied to the determination of HCHO residues in beer samples with low interferences. The recoveries of spiked samples are in the range 96 to 117% with RSDs lower than 5.9%. Graphical Abstract.