Combining photonic excitation and acoustic detection, photoacoustic imaging (PAI) presents one of the most promising noninvasive biomedical diagnostic modalities, but this technique still lacks efficient nano-sized contrast agents absorbing light in the region of relative tissue transparency (630–900 nm). Here, we explore the use of titanium nitride (TiN) nanoparticles (NPs) fabricated by methods of pulsed laser ablation in liquids as a contrast agent in PAI. When prepared in acetone, the NPs are spherical, have an average size of 25 nm, and exhibit a broad plasmonic absorption peak around 700 nm. We show that solutions of these NPs render possible a strong nonlinear photoacoustic response and the generation of photoacoustic images with 67  μm resolution within the biological transparency window. The observed effect is explained by a plasmonically enhanced two-photon absorption process in TiN NPs. Combined with earlier demonstrated capability of generating photothermal therapeutic effect, relative chemical purity, and excellent biocompatibility, laser-synthesized TiN NPs promise attractive applications in biomedical theranostics involving imaging modalities based on photoacoustics microscopy or tomography.

Er3+-Yb3+ co-doped KGd3F10 cores, prepared by an ethylenediaminetetraacetic acid (EDTA)-assisted hydrothermal synthesis, and core-shell Er3+-Yb3+ KGd3F10@SiO2 particles, prepared through Stöber method have been morphologically and optically characterized. Besides being employed in nanothermometry, both core and core-shell have also been exploited as contrast agents for photoacoustic images (PAI). Results of PAI upon excitation at 520 nm and 980 nm in the Er3+-Yb3+ core and core-shell nanoparticles in ethanol suspension are demonstrated and its biomedical applications discussed.