Nanogold probe enhanced Surface Plasmon Resonance immunosensor for improved detection of antibiotic residues

An exhaustive study is reported on the effect that antibody nanogold probes produce on the performance of a Surface Plasmon Resonance (SPR) immunosensor. The paper studies the improvement that different nanogold probes prepared at different antibody:gold nanoparticle (IgG:AuNP) ratios and AuNP sizes produce on the maximum signal and detectability of a simple SPR immunosensor developed to analyze fluoroquinolone (FQ) antibiotic residues (SPReeta system). The investigation compares the features of sensor enhanced formats using both, secondary and primary nanogold probes (anti-IgG and IgG coupled to AuNP, on double and single-antibody immunochemical assay steps, respectively), in respect to the unenhanced format. For this purpose, a reproducible bioconjugation procedure for preparing gold biohybrid nanoparticles has been established, involving the formation of a mixed self-assembled monolayer (m-SAM) with PEGylated cross-linkers around the AuNP followed by the covalent attachment of the antibodies. The procedure allows controlling the IgG:AuNP ratio of the nanogold probes on a reproducible manner and the functionalized NPs have been found to be stable during assay and storage. Both formats, using secondary and primary nanogold probes, are excellent strategies to improve immunosensor detectability. Thus, using anti-IgG-AuNP, the detectability could be improved by a factor of 14 (LOD 0.07±0.01 μg L(-1) vs. 0.98±0.38 μg L(-1)) reducing at the same time the amount of primary antibody used (30,000 vs. 1000 dilution factor). Likewise, the format using IgG-AuNP also allows improving detectability (LOD 0.11±0.01 μg L(-1)), but reducing the number of needed steps.