Effects of Surface Epitope Coverage on the Sensitivity of Displacement Assays that Employ Modified Nanoparticles: Using Bisphenol A as a Model Analyte.

With the ever-increasing use of nanoparticles in immunosensors, a fundamental study on the effect of epitope density is presented herein, with a small molecule epitope, on the performance of the displacement assay format in an enzyme-linked immunosorbent assay (ELISA). Thiolated bisphenol A (BPA) functionalized gold nanoparticles (cysBPAv-AuNPs) and specific anti-BPA antibodies are employed for this purpose. It is shown that the displacement of cysBPAv-AuNPs bound to the immobilized antibodies was influenced by both the avidity of bound cysBPAv-AuNPs and the concentration of free BPA to displace it. The importance of surface epitope density was that it changed the number of epitopes in close proximity to the antibody-binding site. This then influenced the avidity of cysBPAv-AuNPs bound to the immobilized antibody. Furthermore, the molar epitope concentration in an assay appears to affect the degree of antibody binding site saturation. Controlling surface epitope density of the functionalized nanoparticles and molar epitope concentration in an assay leads to a decrease of the concentration of free BPA required to displace the bound cysBPAv-AuNP, and hence better assay performance with regards to the D50 value and dynamic range in the displacement assay.

Development of sensitive direct and indirect enzyme-linked immunosorbent assays (ELISAs) for monitoring bisphenol-A in canned foods and beverages.

Enzyme-linked immunosorbent assays (ELISAs) are investigated in this work for the detection of bisphenol-A (BPA), a plastic monomer and a critical contaminant in food and environment. A series of polyclonal antibodies generated in vivo using BPA-butyrate-protein conjugate and BPA-valerate-protein conjugate were evaluated on direct and indirect competitive assay formats with five competing haptens (BPA-butyrate, BPA-valerate, BPA-crotonate, BPA-acetate, and BPA-2-valerate). Two indirect ELISAs and one direct ELISA exhibiting high sensitivity and specificity for BPA were developed. The 50 % inhibition of antibody binding (IC(50)) values were 0.78 ± 0.01-1.20 ± 0.26 µg L(-1), and the limits of detection as measured by the IC(20) values were 0.10 ± 0.03-0.20 ± 0.04 µg L(-1). The assays were highly specific to BPA, only displaying low cross-reactivity (3-8 % for the indirect assays and 26 % for the direct assay) for 4-cumylphenol (4-CP), at pH 7.2. The degree of cross-reaction of 4-CP was influenced by the antibody/hapten conjugate combination, assay conditions, and the assay format. The assays were optimized for the analysis of BPA in canned vegetables, bottled water and carbonated drinks. The limits of quantification for these three evaluated sample types, based on the spike and recovery data, were 0.5, 2.5, and 100 µg L(-1), respectively.