Gold nanoparticles hold great promise in the fi elds of diagnostics, drug delivery, and light-triggered therapies, due to their properties related to localized surface plasmon resonance (LSPR) and their chemical inertness.[ 1 ] Especially, long gold nanorods (AuNRs) are known to exhibit strong longitudinal LSPR, leading to very effi cient energy conversion under near-IR excitation. [ 1,2 ] Here, we show that upon near-IR ultrashort-pulsed irradiation single AuNRs induce the production of reactive oxygen species (ROS) over a few micrometers in their aqueous environment, particularly hydroxyl radicals (• OH). The variation of the excitation wave polarization angle relative to the AuNR orientation reveals a nonlinear polarization-controlled action combined with a three-photon luminescence of the AuNRs. The longitudinal LSPR enhances the local electromagnetic fi eld, and therefore controls both luminescence and ROS production through multiphoton excitation of electrons in gold and water followed by hot plasma formation in water. These results offer new insights into the potential use of AuNRs as a replacement for photosensitizers in photodynamic cancer therapy or in combinations of therapeutic and imaging methods.The LSPR phenomenon in gold nanoparticles is associated with (i) an effective light absorption around the resonant wavelength and (ii) a high enhancement of the electromagnetic energy in the near-fi eld. These properties, along with biocompatibility, [ 1,3 ] favor a variety of biomedical uses in imaging techniques [ 4 ] and light-triggered therapies. [ 5,6 ] AuNRs exhibit two LSPR modes known as transverse and longitudinal. The spectral location of the latter is particularly sensitive to the AuNR aspect ratio and can be tuned to the near-IR 'therapeutic window' (650-1350 nm) where light
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