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We demonstrate high contrast optical-resolution photoacoustic microscopy (OR-PAM) imaging by using biocompatible, indirect band gap silicon nanoparticles (SiNPs) as contrast agents. The SiNPs were synthesized by high energy ball milling of crystalline silicon wafers, followed by hydrosilylation with bifunctional olefinic ligands to enable water solubility and bioconjugation. When uniformly dispersed in water, the SiNPs with an average particle size about 114 nm showed remarkably high extinction coefficients, more than 2 × 1010 M−1 cm−1, from the visible to near-infrared spectral range. For the first OR-PAM imaging demonstration, the SiNPs were microinjected to live zebrafish embryonic cells. As the embryo developed, the distribution of the cells carrying the SiNPs can be clearly traced by the spread of the photoacoustic signals. Secondly, photoacoustic-fluorescence dual-modality imaging was demonstrated on live zebrafish larvae which were microinjected with a mixture of the SiNPs and dextran-rhodamine dye...