Enhancing the sensitivity of a monoclonal antibody-based heterologous coating strategy for the detection of 6-benzyladenine through the LSPR effect of AuNRs

6-Benzyladenine (6-BA), a toxic, synthetic plant growth regulator, is widely used in agriculture despite its harmful properties. However, the detection of 6-BA remains a challenge due to the insufficient sensitivity of current monoclonal antibodies. In this study, the synthetic pathways of 6-BA hapten were optimized based on structural analogues by introducing a carboxyl spacer arm into the benzene ring and combining with a heterologous coating strategy. Through molecular simulation and experiments, a monoclonal antibody with extremely high sensitivity (IC₅₀ = 0.13 ng mL^-1) and specificity was developed. The characteristic structure of the distal purine of the binding protein was easily recognized by the immune system, thereby producing a high-sensitivity antibody by linking the carboxy spacer arm to the benzene ring. Based on this antibody, a visual colorimetric sensor based on TMB²⁺-mediated etching of gold nanorods (AuNRs) was further constructed. A linear relationship was established between Δ<i>λ</i> of the LSPR peak and the concentration of 6-BA in the range of 0.04 to 0.63 ng mL^-1, with a low detection limit of 0.04 ng mL^-1. In addition, the accuracy and precision of the immunosensor were further confirmed by recovery tests (81.0-92.0%) and HPLC validation, which showed the potential application of this immunosensor in the rapid detection of 6-BA.