[en] We study the photothermal response of water solution of 10-, 50-, and 100-nm gold nanoparticles as a function of concentration under irradiation of a continuous wave 6-W 445-nm diode laser. The method allows determining concentrations as low 8 × 10⁻⁷ mg/ml, which corresponds to the detection of a single nanoparticle of 100- and 50-nm diameter or about 1000 nanoparticles of 10-nm diameter. The results demonstrate the application for the study of the optical response of a single nanoparticle in water suspension. We discuss the potential applications of the finding for detection of low concentration of biomolecules using a single nanoparticle.