A sensitive, universal and homogeneous method for determination of biomarkers in biofluids by resonance light scattering correlation spectroscopy (RLSCS)

In this paper, we reported a sensitive, universal and homogeneous method for assay of biomarkers by combining resonance light scattering correlation spectroscopy (RLSCS) with silver nanoparticles (SNPs) as labeling probes. In the homogeneous assay, the competitive immunoreaction mode was used, and antibody and antigen (or hapten) were labeled with SNPs with strong plasmonic scattering property, respectively. The antibody-labeled SNPs were firstly mixed with a sample containing antigens, and the part of antibody-labeled SNPs was bound to antigens of interest in the sample. And then, the antigen-labeled SNPs were added into the mixed solution above, and they were bound to free antibody-labeled SNPs (excess) to form dimers (or oligomers), which led to the significant increase in the characteristic diffusion time of SNPs in the tiny detection volume (about 0.5 fL). In the competitive mode, the characteristic diffusion time of SNPs decreased with an increase of antigen concentration. The RLSCS is a novel single particle method and can sensitively detect the changes in the characteristic diffusion time of SNPs before and after the immunoreactions. In order to demonstrate the universality of this new method, small biomolecules, 17-β estradiol (E2), and biomacromolecules, liver cancer antigen alpha-fetoprotein (AFP), were used as assay models. In the optimal conditions, the linear ranges of this method were from 10 pM to 10 nM for E2 and 100 pM to 10 nM for AFP, respectively, and the detection limits were 10 pM for E2 and 100 pM for AFP, respectively. The presented method was successfully used to the determination of E2 levels in human urine and AFP levels in human sera, and the results obtained were in good agreement with conventional ELISA assays.