Label-free colorimetric and quantitative detection of cancer marker protein using noncrosslinking aggregation of Au/Ag nanoparticles induced by target-specific peptide probe

Abstract

Assays for non-enzyme protein based on peptide–protein interaction are few due to the fact that most of peptide–protein bindings do not produce easily measurable output signals. Here we report a homogenous assay for colorimetric and quantitative detection of a cancer marker and promising antitumor target, cyclin A 2, using noncrosslinking aggregation of unmodified AuNPs/AgNPs by utilizing the difference of coagulating ability of a cationic peptide probe (P1) and its binding form toward naked AuNPs/AgNPs. In the absence of cyclin A 2, P1 coagulates particles immediately, whereas cyclin A 2 binding prevents the interaction of P1 with metal particles surface, significantly reducing the magnitude of aggregation. The extent of aggregation is dependent on the concentration of the target protein cyclin A 2 and the difference in color can readily be distinguished by spectrometer and naked eyes. The assay is sensitive and selective. Cyclin A 2 assay using AuNPs as colorimetric indicator is more easily monitored by naked eyes owing to the distinct color change, and 40 nM cyclin A 2 can be detected without the aid of any instruments. Using inexpensive desktop spectrometer, cyclin A 2 assay using AgNPs as colorimetric indicator can detect as low as 30 nM cyclin A 2, which is 20 fold lower than that of cyclin A 2 assay using terbium-chelating peptide as the probe reported recently ( Pazos et al., 2008, 130, 9652–9653). This strategy will shed light on developing of unlabeled peptide-based protein biosensors.