Enhanced solid-phase immunoassay using gold nanoshells: effect of nanoparticle opticalproperties

Abstract

Plasmon-resonant nanoparticle-labeled immunoassays provide a simple, low-cost andeffective way of detecting target molecules in solutions. The optical mechanisms behindtheir efficiency, however, have not been addressed until now. We present the firsttheoretical description of nanoparticle-labeled dot immunoassay and its experimentalverification with functionalized 15 nm colloidal gold nanospheres and silica/gold nanoshells(GNs). Three types of GNs, with silica core diameters of 100, 140 and 180 nmand a gold shell thickness of about 15 nm, were studied in our experiments. Thefabricated markers were characterized by electron and atomic-force microscopy,UV–vis spectroscopy and dynamic light scattering. A normal rabbit serum (thetarget IgG molecules) and sheep antirabbit antibodies (the probing molecules)were used as a biospecific model. The minimal detection limit for IgG targetmolecules was about 15 ng in the case of a standard dot-assay protocol based on15 nm colloidal gold particles conjugated with probing molecules. In contrast tothis observation, a simple replacement of 15 nm gold labels by GN conjugatesresulted in a drastic increase in detection sensitivity of up to 0.25 ng in the case of180/15 nm GNs and of up to 0.5–1 ng for 100/15 and 140/15 GNs. By using thetheory developed, we explained the dependences of the low detection limit, themaximal-color intensity and the probe-load saturation limit on the particle parameters.