A quantitative immunoassay method based on the extinction spectra of colloidal-gold bioconjugates

In this paper, we report on the optical properties of aggregates formed by biospecific interactions like antigen/antibody, with one or both reaction components immobilized on gold particles. In the case of biospecific aggregation, the temporal changes in the absorption spectra differed from those recorded during rapid and slow salt aggregations. As in the case of rapid salt aggregation, the absorption peak decreased and shifted to the red part of the spectrum with simultaneous broadening. However, we did not observe the second red peak of the optical density. According to the transmission electron microscopy data, the slow, rapid, and biospecific aggregations resulted in small clusters with compact structures, branching aggregates of fractal type, and aggregates without direct conductive contacts of primary particles, respectively. It is supposed that the recorded differences in absorption spectra can be explained by the corresponding differences in aggregate structures. We have found a direct correlation between the amount of the second added protein initiating aggregation on the one hand, and the rate of spectral changes on the other. Using these spectral changes, we have plotted a calibration curve for a sufficiently rapid and technically simple quantitative test like sol-particle immunoassay.