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Pulsed laser irradiated gold nanoparticles can mediate cell membrane permeabilization, cell elimination and protein
inactivation by mechanical effects of nanocavitation. Besides therapeutic applications irradiated gold nanoparticles are
investigated as contrast agent in photoacoustic imaging. Especially for imaging of structures in deeper tissue the use of
irradiation wavelengths in the range between 700 and 1100 nm is necessary, because of lower tissue absorption and
scattering. Depending on their shape the nanorods absorption band can be shifted into this near infrared range. Thus, it
can be expected to mediate stronger effects with nanorods irradiated at this band. In contrast to spherical particles
nanorods irradiated with nanosecond laser pulses tuned to the wavelength of their maximum absorption are not suitable
to cause expected effects. We found that an expanding vapor bubble causes a rapid change in refractive index of the
surrounding medium and results changes of the nanorods optical properties. These changes remain transient for a stable
particle shape and transcent into permanent change, when melting occurs. Thus, for the purpose of cell killing or
enhanced contrast in photoacoustic imaging higher photothermal stability is required. We show here by means of
calculations and experiments, that a porous silica coating stabilizes the wavelength position of the longitudinal plasmon
resonance of irradiated nanorods. These silica shelled gold-nanorods retained their optical properties and showed
increased photothermal stability under nanosecond pulsed laser irradiation.
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