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Optical control of gene expression has many potential applications, but the wavelengths of light required by existing methods are outside the effective optical window for biological tissue. Carbon-based nanoparticles are efficient photothermal energy converters with good absorption inside the optical window, but the metal catalysts used in their production present biocompatibility concerns. Synthesis of carbon nanohorns (CNHs), however, does not use metal catalysts. Miyako et al. show that bovine serum albumin (BSA)-coated CNHs are well-dispersed in solution and that the CNHs efficiently convert near-infrared (NIR) light—which falls inside the optical window—into heat, and with less cytotoxicity than other nanocarbon particles. Injection of suspensions of BSA-CNH and cells expressing a luciferase under a heat-shock promoter into mice, followed by NIR irradiation and imaging, showed a sevenfold increase in luminescence over non-irradiated controls and no evidence of toxicity.