Homogeneous immunoassays by using photon burst counting technique of single gold nanoparticles.

In this paper, we reported a sensitive single particle method by combining the photon burst counting technique with gold nanoparticles (GNPs) as labeling probes. The photon bursting of single GNPs will be generated in a highly focused laser beam (less than 1 fL) due to the plasmon resonance scattering and Brownian motion of GNPs. We observed an excellent linear relationship between the photon burst counts and the number of particles in GNPs solution. We investigated the statistical behaviors of background noise and photon burst signal of GNPs, and proposed the data processing method based on Gaussian distribution of the background noise. A new homogeneous sandwich immunoassay was developed by using this single particle method. We evaluated the performance of this method by using prostate-specific antigen (PSA) as a model. The linear range of PSA was 1-1000 pmol/L and the detection limit was 0.8 pmol/L. This novel method was successfully used for the direct detection of cancer biomarker PSA in human serum samples. Our results were in good agreement with conventional ELISA assays.