4-Aminophenol was selected as a model hapten for thermal-lens detection in a heterogeneous indirect immunoassay for small organic compounds. The assay is based on the competition of the free hapten and 4-aminophenol labeled with tetramethyl rhodamine isothiocyanate (TRITC) for rabbit anti-4-aminophenol antibodies (4-AP-GA-BSA) immobilized on a transparent poly(ethylene terephthalate) plate. The amount of the colored analyte was directly measured by thermal lensing at the plate surface. The developed method is specific for 4-aminophenol. The limit of detection is 2 × 10<sup>−7</sup> M, which corresponds to the absolute amount of 3 × 10<sup>−11</sup> mol of 4-aminophenol. The sensitivities of the developed thermal-lens and fluorescence-based measurements of the immunoassay under the same conditions are in good agreement with the expected and discussed theoretical sensitivities of photothermal and fluorescence measurements.
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