Abstract
In this work, we report that gold nanorods coated with hydrophobically modified mesoporous silica shells not only enhance photoacoustic (PA) signal by more than 10-fold over unmodified mesoporous silica coated gold nanorods but also the relationship between PA amplitude and input laser fluence is strongly nonlinear. Mesoporous silica shells of ∼14 nm thickness and with ∼3 nm pores were grown on gold nanorods showing near infrared absorption. The silica was rendered hydrophobic with addition of dodecyltrichlorosilane, then re-suspended in aqueous media with a lipid monolayer. Analysis of the PA signal revealed not only an enhancement of PA signal compared to mesoporous silica coated gold nanorods at lower laser fluences but also a nonlinear relationship between PA signal and laser fluence. We attribute each effect to the entrapment of air in the mesopores: the air has a larger expansion coefficient than silica that enhances conversion to acoustic energy, and the air undergoes thermal size oscillations that increase with laser fluence, leading to a nonlinear trend 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.
Published Version
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