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[en] The authors describe the combination of headspace single drop microextraction with microchip-photothermal lens microscopy (HS-SDME/MC-PTLM) as a new method for highly sensitive determination of the angiotensin-converting enzyme inhibitor captopril. A single drop of a colloidal solution of gold nanoparticles is applied to the headspace. This then acts as both acceptor phase and labeling agent for the thiol groups of captopril. Following extraction of captopril, the drop is injected into a glass microchip and detected by PTLM using a 50-mW diode solid-state laser with an emission line of 532 nm. The interaction of captopril and GNPs reduces the surface plasmon resonance of GNPs and as a consequence the PTLM signal of GNPs decreases. At the optimum condition, the PTLM signal increases linearly in the 0.5–70.0 nM captopril concentration range, and the detection limit (at an S/N ratio of 3) is 0.31 nM. This is much better than those of previously reported methods. The method was successfully applied to the determination of captopril in (spiked) human serum and in pharmaceutical samples. < Image>.