Stabilization of the Longitudinal Plasmon Resonance Absorption of Nanorods by Porous Silica Shell Layer

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.