Hydrophobically Modified Silica-Coated Gold Nanorods for Generating Nonlinear Photoacoustic Signals

In this work, we report that not only do gold nanorods coated with hydrophobically modified mesoporous silica shells enhance the photoacoustic (PA) signal over unmodified mesoporous silica-coated gold nanorods but also that the relationship between the PA amplitude and the input laser fluence is strongly nonlinear. Mesoporous silica shells of ∼14 nm thickness with ∼3 nm pores were grown on gold nanorods showing near-infrared absorption. The silica shell was rendered hydrophobic with addition of dodecyltrichlorosilane and then resuspended in aqueous media with a lipid monolayer. Analysis of the PA signal revealed not only an enhancement of the PA signal compared to mesoporous silica-coated gold nanorods at lower laser fluences but also a nonlinear relationship between the PA signal and the laser fluence. We attribute each effect to the entrapment of solvent vapor in the mesopores: the vapor has both a larger expansion coefficient and a larger thermal resistance than silica that enhances conversion to acoustic energy, and the hydrophobic porous surface is able to promote a phase transition at the surface, leading to a nonlinear PA response even at fluences as low as 5 mJ cm–2. At 21 mJ cm–2, the highest laser fluence tested, the PA enhancement was >12-fold over mesoporous silica-coated gold nanorods.