Ag@SiO₂-entrapped hydrogel microarray: A new platform for a metal-enhanced fluorescence-based protein assay

We developed a novel protein-based bioassay platform utilizing metal-enhanced fluorescence (MEF), which is a hydrogel microarray entrapping silica-coated silver nanoparticles (Ag@SiO₂). As a model system, different concentrations of glucose were detected using a fluorescence method by sequential bienzymatic reaction of hydrogel-entrapped glucose oxidase (GOX) and peroxidase (POD) inside a hydrogel microarray. Microarrays based on poly(ethylene glycol)(PEG) hydrogels were prepared by photopatterning a solution containing PEG diacrylate (PEG-DA), photoinitiator, enzymes, and Ag@SiO₂. The resulting hydrogel microarrays were able to entrap both enzymes and Ag@SiO₂ without leaching and deactivation problems. The presence of Ag@SiO₂ within the hydrogel microarray enhanced the fluorescence signal, and the extent of the enhancement was dependent on the thickness of silica shells and the amount of Ag@SiO₂. Optimal MEF effects were achieved when the thickness of the silica shell was 17.5 nm, and 0.5 mg mL^-1 of Ag@SiO₂ was incorporated into the assay systems. Compared with the standard hydrogel microarray-based assay performed without Ag@SiO₂, more than a 4-fold fluorescence enhancement was observed in a glucose concentration range between 10^-3 mM and 10.0 mM using hydrogel microarray entrapping Ag@SiO₂, which led to significant improvements in the sensitivity and the limit of detection (LOD). The hydrogel microarray system presented in this study could be successfully combined with a microfluidic device as an initial step to create an MEF-based micro-total-analysis-system (μ-TAS).