Poly(styrene-co-acrylic acid) core and silver nanoparticle/silica shell composite microspheres as high performance surface-enhanced Raman spectroscopy (SERS) substrate and molecular barcode label

Abstract

A new type of polymer core and silver nanoparticle/silica shell multifunctional composite microsphere is reported. The monodisperse polymer core of poly(styrene-co-acrylic acid), prepared by surfactant-free emulsion polymerization, serves as the supporting template to facilitate the synthesis and handling of the highly functionalized substrates to achieve consistent mechanical and surface chemical properties. The shell of silver nanoparticles, formed in situ by controlled interfacial reduction of AgNO₃ with polyvinylpyrrolidone (PVP), serves as the optimal metal enhancer for surface enhanced Raman spectroscopy (SERS). The Ag nanoparticle shell is stabilized with a silica layer using a sol–gel process, which is also functionalizable with reactive epoxides for further bio-conjugation using epoxy ring-opening methods. Preliminary results indicated that the microspheres are easily prepared with exceptional chemical and physical uniformity. SERS experiments with 4-aminobenzenethiol (4-ABT) as the indicator showed that the resulting multifunctional microspheres allowed the production of highly consistent enhancement of the Raman signals down to nM concentrations of 4-ABT, making them highly desirable candidates as the enhancers for high performance SERS analysis and as SERS optical labels in biomedical imaging. The microspheres are robust and the high density of the SERS active Ag nanoparticles on the microspheres provided unique signal averaging for the method to be potentially quantitative which is an urgent goal for SERS substrate design and SERS analysis.