Environment-dependent generation of photoacoustic waves from plasmonic nanoparticles

Abstract

Nanoparticle-augmented photoacoustics is an emerging technique for molecular imaging. This study investigates the fundamental process of the photoacoustic signal generation by plasmonic nanoparticles suspended in a weakly absorbing fluid. The photoacoustic signal of gold nanospheres with varying silica shell thicknesses is shown to be dominated by the heat transfer between the nanoparticles and the surrounding environment.

Figure 1

(a) Schematic representation of the photoacoustic response generated from a sphere (gold) and the surrounding liquid (blue). (b) Generalized temperature profile without interfacial resistance as a function of time and distance, summarizing the major assumptions made for the analytical pressure amplitude estimation.

Figure 2

(a) Transmission electron microscopy images of silica coated gold nanospheres (core radius: 13 ± 3 nm) with 6 ± 0.8 nm, 20 ± 2.8 nm, 38 ± 3.9 nm of silica shell. (b) Photographic image of organic solvent soluble gold nanospheres (left) and of aqueous soluble gold nanospheres (right). (c) Schematic illustration of combined photoacoustic/ultrasound system used in this study, where M1–3 represent mirror 1 to 3; L1–2 are lens 1–2; ND represent neutral density filters; BS represent beam splitter; PM represent power meter, and UT represent ultrasound transducer. (d) ) Close-up schematic illustration of the sample irradiated by a pulsed laser beam while photoacoustic transients were measured using the ultrasound transducer.

Figure 3

Photoacoustic signal amplitude of gold nanospheres with varying silica shell thickness as a function of temperature in different environments: (a) water (b) silicon oil (c) toluene. The silica thickness is: 0 nm (circle), 6 nm (triangle), 18 nm (square), and 38 nm (rhombus). (d) Temperature dependence of the Gruneisen parameter (left axis) for water (open black circle) and toluene (open black triangle), and of the measured photoacoustic signal (right axis) of water (solid orange circle) and silicon oil (solid orange square). The error bar represents the standard error of 16 signals of eight pulses each.