Optical Coherence Correlation Spectroscopy on Gold Nanoparticles for Point-of-Care Immunoassays

The interest in very sensitive diagnostic techniques pushes towards the development of always new detection methods. Here we present a technique to perform immunoassays based on the detection of gold nanoparticles (NPs) via analysis of their scattered light. This interferometric method, called Optical Coherence Correlation Spectroscopy (OCCS), allows both the analysis of several sampling volumes simultaneously, such as Optical Coherence Tomography (OCT), and correlation measurements similar to Fluorescence Correlation Spectroscopy (FCS). The aim of this project was to give a proof of concept of OCCS to perform immunoassays: this technique was used to measure several values of the diffusion coefficient of gold nanoparticles in the presence of different concentrations of either specific or non-specific antibodies. We studied immunoassays with OCCS in solution in well-plates, and verified the coherence of some results with FCS. We worked with streptavidin-coated gold NPs in the presence of anti-streptavidin or anti-β-lactoglobulin. We found that the coating does not affect the diffusion of the NPs with respect to their naked form. Conversely, the presence of anti-streptavidin in solution has major consequences on their motion. A small interaction is suspected to exist between coated-NPs and the non-specific antibody, but statistical analysis seems to infirm this fact. Still, a putative non-specific interaction has been found between non-coated NPs and anti-streptavidin, which could result from adsorption of the body of the antibody onto the NP surface. Optimization of the size and the concentration of the gold NPs for OCCS measurements could result in a sensitive manner to detect antibodies in in blood sample (e.g. to diagnose allergies). Furthermore, as in OCCS the sampling volume is no more limited to a small confocal volume, novel solutions for sensing in microfluidic channels would be interesting to investigate.