Au nanoparticle-encapsulated hydrogel substrates for robust and reproducible SERS measurement

Abstract

A reliable and reproducible surface-enhanced Raman scattering (SERS) measurement utilizing Au nanoparticle-encapsulated hydrogels as a substrate has been demonstrated. A hydrogel matrix was adopted to: (i) take advantage of its excellent water uptake capacity for facile access of an analyte into the substrate and (ii) securely hold Au nanoparticles. Silica-coated Au (Au@SiO₂) nanoparticles were initially prepared and uniquely used as an initiator as well as a cross-linker for the polymerization of acrylic acid to synthesize Au nanoparticle-encapsulated hydrogels. Then, the outer silica layer of the Au nanoparticles in the hydrogel was etched out using hydrofluoric acid (HF) to make it possible for an analyte to approach the surface of the Au nanoparticles for generation of the SERS signal. In parallel, locally occurring SERS signals over the hydrogel were integrated using a wide area illumination scheme capable of covering a large area to improve quantitative representation of analyte concentration. To evaluate reproducibility of the proposed method, 6 independent hydrogels were prepared every two months over one year and then Raman spectra of 2-naphthalenethiol (2-NAT) captured hydrogels were collected. The resulting SERS intensities of 2-NAT acquired at each concentration were reproducible and clearly increased according to the elevation of 2-NAT concentration.