<p>Matrix interference is an obvious barrier for visible-light-based point-of-care testing (POCT), primarily attributed to the complicated sample pretreatment required to reduce color interference from complex sample matrices. Recently, photothermal sensors have been explored as an effective strategy for food safety monitoring, owing to near-infrared (NIR) light with strong penetration capability and inherent resistance to matrix interference. In this study, the thickness of the polydopamine coating layer on the surface of gold nanorods was precisely regulated to structurally engineer gold nanorod@mesoporous polydopamine composites (Au NRs@mPDA-X), resulting in a pronounced red shift of the absorption spectrum from the visible region to the NIR region. Upon irradiation with an 808&#xa0;nm NIR laser, Au NRs@mPDA-3 exhibited a photothermal conversion efficiency as high as 34.2%. Based on an indirect competitive immunoassay, a photothermal-colorimetric immunosensor was designed, which utilized the temperature signal generated by Au NRs@mPDA-3 to induce a chromogenic response in thermochromic polydiacetylene films-from the initial blue to purple, and ultimately to red, thereby converting a photothermal signal into a visually detectable colorimetric signal. Furthermore, a smartphone-based colorimetric analysis was established for the detection of rosiglitazone (RSG) with a detection limit as low as 6.3×10^− 5 µg/mL. This photothermal immunosensor based a signal transduction mechanism significantly simplifies photothermal detection for POCT, facilitating the application of photothermal immunosensors in food safety, disease surveillance and environmental monitoring.</p> Graphical abstract <p></p>