Photoacoustic Signal Generation in Gold Nanospheres in Aqueous Solution: Signal Generation Enhancement and Particle Diameter Effects

Abstract

Gold nanoparticles can be used as an exogenous contrast agent for biomedical photoacoustic (PA) imaging. The generation of PA signals in monodispersed gold nanosphere suspensions (diameters 20–150 nm) from pulsed-laser excitation (5 ns pulse width, wavelength 532 nm) was investigated experimentally and compared to signals measured in solutions of a homogeneous molecular absorber. The PA signal amplitude was found to increase linearly with excitation fluence for the homogeneous absorber and the nanospheres up to 80 nm in diameter. By contrast, the signal amplitude was found to increase quadratically with respect to fluence for larger nanospheres. In the linear regime, the PA signal amplitude in gold nanosphere suspensions was found to be on average 26% higher than that in the homogeneous absorber with identical absorption coefficient, which were measured using an integrating sphere. Furthermore, in suspensions with identical absorption coefficient, no dependence of the PA signal amplitude on nanosphere diameter was found in the linear regime, entailing that suspensions with identical extinction coefficient display a decreasing trend in PA signal amplitude with increasing nanosphere diameter due to increasing contribution of scattering. This study presents experimental evidence of some of the physical phenomena governing the photoacoustic signal generation in gold nanosphere suspensions, which may inform on approaches to molecular biomedical PA imaging.

Exploiting Nonlinear Photoacoustic Signal Generation in Gold Nanospheres for Selective Detection in Serial 3D PA Tomography

“…The gold nanospheres were found to sediment at the bottom of the tube, which differs from the experiments reported in Pang et al [13] on nanospheres suspended in water. To avoid any temporal changes in the local concentration of the gold nanospheres, imaging acquisitions were started at least 30 min after the preparation of the phantom to allow the particles to settle.…”

“…Additional non-linear functions were examined to fit the nonlinear PA signal in the gold nanospheres, including a third-order polynomial (R 2 3p = 0.98) and an exponential function (R 2 exp = 0.99). However, because neither of these alternatives provided a significantly better agreement with the data, and because the nonlinearity is expected to follow a quadratic function [13], the second-order polynomial was chosen for subsequent analysis. Figure 3 shows a schematic of the steps involved in the image analysis process of Method I as described in Section 2.4.…”

“…Early studies investigating this effect had only been able to observe the nonlinearity at a temperature of 4 • C (where the Grüneisen parameter for water is zero) for gold nanospheres of 40 nm in diameter [12]. Recently, Pang et al [13] presented experimental evidence that the predicted nonlinearity of the PA signal amplitude with respect to fluence can be observed at room temperature in suspensions of large gold nanospheres with a diameter of up to 150 nm. This nonlinearity leads to a greater PA signal amplitude in gold nanospheres compared to that measured in a homogeneous dye solution of identical absorption coefficient and linear fluence dependence of the PA signal amplitude [1].…”

Sulfates as chromophores for multiwavelength photoacoustic imaging phantoms

“…24 Gold nanoparticles can also exhibit changes in absorption after prolonged exposure, 9 with the added difficulty that their photoacoustic efficiency is not straightforward to model and characterize. 25,26 Inorganic absorbers such as Prussian blue and chloride salts such as CuCl 2 :2H 2 O and NiCl 2 :6H 2 O are highly photostable and have suitable spectra. 13 However, we have observed that the specific absorption spectra of the two chloride salts are affected both by their own concentrations and also by the presence of each other when dissolved in a mixture.…”

Molecular Hyperthermia: Spatiotemporal Protein Unfolding and Inactivation by Nanosecond Plasmonic Heating

“…The laser energy density values used in this study are considered below the vaporization threshold based on previous measurements (400 mJ cm −2 for 250 nm gold nanoshells and 10 ns laser pulse, and dramatically increased vaporization threshold for smaller GNP) . To further confirm the nanoscale GNP heating, we measured the photoacoustic signal (Figure S1, Supporting Information) corresponding to thermoelastic expansion of the GNP and water, which is a result of the nanoscale heating. Furthermore, gold nanocrystals have vortexes and edges (Figure S5, Supporting Information) and lead to higher heat generation and altered temperature distribution of water around the GNP.…”