Fluorometric competitive immunoassay for chlorpyrifos using rhodamine-modified gold nanoparticles as a label

A new kind of labelled antibody was synthesized by modifying gold nanoparticles (AuNPs) with the fluorophore rhodamine B isothiocyanate (RBITC) and a secondary antibody (IgG). The conjugate thus obtained was used in a competitive sandwich assay with a turn-on signal change. It was designed to detect the organophosphorus pesticide chlorpyrifos. The fluorescence of the RBITC-labeled gold immunoprobe with emission at 575 nm and excitation at 556 nm is almost completely quenched. If, however, cysteamine is added, the fluorophore is released from the labeled secondary antibody and fluorescence increases in accordance with the quantity of secondary antibody bound to the sandwich. This assay was applied to determine chlorpyrifos in dried tangerine peels. The detection results were also independently confirmed by LC-MS/MS. The method allows the concentrations of chlorpyrifos to be quantified down to 4.9 ng·mL^-1, which is equivalent to 61 μg·kg^-1 in dried tangerine peels. In our perception, this approach has a wide potential to be applied in the determination of numerous analytes for which antibodies are available. Graphical abstract A new kind of labelled antibody was synthesized by modifying gold nanoparticles with fluorophore rhodamine B isothiocyanate (RBITC) and a secondary antibody (IgG). The conjugate was used in fluoroimmunoassay with a turn-on signal change. The method was designed to detect the organophosphorus pesticide chlorpyrifos.