Visual colorimetric immunosensor for sensitive detection of 4-Chlorophenoxyacetic based on TMB²⁺-mediated etching of Au NRs

4-Chlorophenoxyacetic acid (4-CPA), a synthetic plant regulator, has been banned due to its cumulative toxicity to humans. However, unqualified sampling remains common in the market. To address the poor sensitivity of 4-CPA antibodies reported previously, a highly sensitive monoclonal antibody specific to 4-CPA was produced by redesigning and synthesizing a novel hapten in this study. Additionally, a visual colorimetric immunosensor based on TMB²⁺ mediated etching of gold nanorods (Au NRs) was developed. The ∆λ of the localized surface plasmon resonance (LSPR) peak exhibited a linear dependence on the 4-CPA concentration in the range of 0.2–6.25 ng mL⁻¹, with a low limit of detection (LOD) of 0.2 ng mL⁻¹. Recovery tests (85.0% to 108%) and HPLC validation demonstrated the immunosensor’s accuracy and precision. This visual colorimetric immunosensor illustrates significant potential for rapid detection of 4-CPA in biological environments.