SERS-active microgels designed by simultaneous photoreduction and photocrosslinking for direct molecular detection

Abstract

Surface-enhanced Raman scattering(SERS) refers to the phenomenon where Raman scattering by molecules is enhanced when the molecules are in the vicinity of the surface of metal nanostructure. SERS is one of the practical detection methods in environmental monitoring, food safety, and biomedical diagnosis. However, metal surfaces are vulnerable to contamination by adhesives such as protein, restricting its uses in complex mixtures. Encapsulation of metal nanoparticles with protection layers is employed to prevent the adsorption of proteins on the surface of the metal. Here, we design microgels containing highly concentrated gold nanoparticles through the simultaneous formation of hydrogel and gold nanoparticles in emulsion droplets. Monodisperse water-in-oil droplets containing a gold precursor, hydrogel precursor, and photoinitiator are prepared using microfluidics. Once the drop is irradiated by UV, gold nanoparticles are synthesized through photoreduction while hydrogel is formed through photocrosslinking. During thermal treatment, sodium citrate, which is a reducing agent induces the growth of gold nanoparticles formed in the previous step. This method enables the uniform distribution of gold nanoparticles at very high concentrations without aggregation in microgels. Using the microgels, a high Raman signal of small molecules can be obtained from the solution containing protein. As an application of the microgels, we demonstrate the direct detection of pyocyanin, a biomarker for Pseudomonas infection spiked in unpurified saliva.