Synthesis, Stabilization, and Functionalization of Silver Nanoplates for Biosensor Applications

Abstract

Silver nanoplates (NPTs) were prepared by a seed-mediated growth method with different diameters (d = 25, 32, 53, and 100 nm) and with thicknesses of approximately 10 nm in all cases. As the concentration of silver seeds increased, the diameter of the nanoplates increased, resulting in an overall shift in the localized surface plasmon resonance (LSPR) band maximum from 570 to 900 nm, thus providing a novel method to tune the plasmon resonance. The LSPR was calculated from theory for both triangular and circular nanoplate geometries. In agreement with transmission electron micrographs, the model results confirmed the shape of nanoplates as being truncated prisms, intermediate between that of a prism and a disk. Because of the toxicity of the surfactant hexadecyltrimethylammonium bromide (CTAB), the stabilizing CTAB bilayer surrounding the NPT was replaced by a nontoxic alkanethiol with surfactant properties. This enabled the extraction of metal nanoparticles into deionized water or buffer for bioconjugation without aggregation. Silver nanoplates were also coated with polyelectrolyte layers using the standard layer-by-layer (LbL) method. The LSPR was found to be very sensitive to the addition of polyelectrolyte layers, with a plasmon band shift from 728 to 740 nm after adding only one monolayer (thickness ∼1.5 nm). Bioconjugation of these nanoplates was achieved with the addition of a mercaptolinker containing a carboxyl group. The carboxyl groups were activated with 1-ethyl-3-(3-dimethylaminopropyl) hydrochloride (EDC)/N-hydroxysuccinimide (NHS) and conjugated to green fluorescent protein (GFP) in order to validate the potential of the NPTs for enhancement of bioassays. The fluorescence of the conjugated NPTs was 5.6-fold brighter than that of NPTs added to GFP without activation.