Protein A-conjugated luminescent gold nanodots as a label-free assay for immunoglobulin G in plasma.

We have employed protein A-modified gold nanodots (PA-Au NDs) as a luminescence sensor for the detection of human immunoglobulin G (hIgG) in homogeneous solutions. The luminescent PA-Au NDs were prepared simply by mixing protein A with the luminescent Au NDs (average diameter: ca. 1.8 nm). The specific interactions that occur between protein A and hIgG allowed us to use the PA-Au NDs to detect hIgG selectively. Under optimal conditions [10 nM PA-Au NDs (two protein A molecules per Au ND), 5.0 mM phosphate buffer solution, pH 7.4], the PA-Au ND probe detected hIgG with high sensitivity (limit of detection = 10 nM) and remarkable selectivity (>50-fold) over other proteins. In an assay that took advantage of the competition between protein G and the PA-Au NDs for IgG, we detected protein G at concentrations as low as 85 nM. This PA-Au ND probe allowed determination of the hIgG concentration in plasma samples without any need for sample pretreatment. Our results exhibited a good linear correlation (R(2)=0.97) with those obtained using an enzyme-linked immunosorbent assay. Our simple, sensitive, and selective approach appears to hold practical potential for use in the clinical diagnosis of immune diseases associated with changes in hIgG levels. This journal is © The Royal Society of Chemistry 2011