Homogeneous silver-coated nanoparticle substrates for enhanced fluorescence detection

A simple method has been developed for the deposition of uniform silver-coated nanoparticles on glass substrates, with a homogeneous distribution shown by scanning electron microscopy (SEM). UV-visible spectroscopy and energy-dispersive X-ray analysis (EDX) have been used to characterize both the optical density and elemental content of the deposited nanoparticles. The fluorescence enhancement was investigated using a monolayer of FITC-conjugated human serum albumin (FITC-HSA) and tested using laser scanning microscopy at 488 nm excitation wavelength. The enhancement factor was calculated from individual spectra recorded with a Fluorolog-Tau-3 spectrofluorometer. We identified the nanoparticle growth regime which led to fluorescence enhancement. Such enhancement is detectable when Au core-Ag shell nanoparticles increased their size to 47 nm, in agreement with theoretical estimates. The origin of this enhancement for appropriate size nanoparticles is attributed to the effect of an increased excitation rate from the local field enhanced by the interaction of incident light with the nanoparticles and/or higher quantum yield from an increase of the intrinsic decay rate of the fluorophore. We thus demonstrated that the Au core-Ag shell nanostructures on glass surfaces are promising substrates for fluorescence enhancement with outstanding macroscopic homogeneity. This important feature will pave the way for the application of our substrates in biotechnology and life sciences such as imaging and sensing of biomolecules in proteomics.