We experimentally measured the photoacoustic responses from monodispersed suspensions of spherical nanoparticles (NPs) of metal (gold, diameter = 9.4, 19.9, 40.2, 49.3, and 59.9 nm) and dye-containing polymer (polystyrene, diameter = 63.1, 73.0, 85.9, and 93.7 nm) in water at different NP concentrations and temperatures (4, 20, and 37 degrees C), where the samples were irradiated with 5-ns laser pulses at low laser fluence. The obtained experimental results were explained by a physical model, where the acoustic pressure pulse was represented by a sum of two contributions from the NPs and the surrounding liquid medium. These results revealed that the photoacoustic responses from a laser-irradiated NP suspension are influenced by the physicochemical properties of the NPs and medium. We also proposed a strategy on how to design NPs for photoacoustic contrast imaging on the basis of obtained results. (C) 2015 Elsevier B.V. All rights reserved.