Solid phase single-molecule counting of antibody binding to supported protein layers surface with low nonspecific adsorption

Abstract

On the basis of the supported protein layers (SPLs) substrate, the study presented an ultrasensitive and highly specific platform for single-molecule fluorescence detection of antibody using quantum dots (QDs) as probes. To construct the SPLs surface platform for antibody immobilization, bovine serum albumin (BSA), anti-BSA, and protein G were firstly attached to carboxyl-terminated substrate surfaces by turns. Then nonspecific adsorption of single antibody molecules on SPLs surfaces was investigated. Through the irreversible interaction between streptavidin and biotin, streptavidin-QD conjugates were employed to conjugate with biotinylated antibody, producing QD-antibody conjugates for generating fluorescent signals in fluorescent imaging. Epi-fluorescence microscopy equipped with an electron multiplying charge-coupled device was chosen as the tool for single-molecule fluorescence detection here. The concentration of antibody is quantified based on the direct counting of individual fluorescent spots, one by one. The generated fluorescent signals increased with the increasing concentration of immobilized antibody and were found to be proportional to antibody concentrations. The better brightness and photostability of QDs, and slower increase in the number of counted molecules make a large linear dynamic range of 1.0 × 10 −14 to 3.0 × 10 −12 mol L −1 between the number of single molecules and antibody concentrations, which is comparable to the previously reported surface-based SMD analysis.