Influence of nanoscale temperature rises on photoacoustic generation: Discrimination between optical absorbers based on thermal nonlinearity at high frequency

Abstract

In this work, we experimentally investigate thermal-based nonlinear photoacoustic generation as a mean to discriminate between different types of absorbing particles. The photoacoustic generation from solutions of dye molecules and gold nanospheres (same optical densities) was detected using a high frequency ultrasound transducer (20 MHz). Photoacoustic emission was observed with gold nanospheres at low fluence for an equilibrium temperature around 4 °C, where the linear photoacoustic effect in water vanishes, highlighting the nonlinear emission from the solution of nanospheres. The photoacoustic amplitude was also studied as a function of the equilibrium temperature from 2 °C to 20 °C. While the photoacoustic amplitude from the dye molecules vanished around 4 °C, the photoacoustic amplitude from the gold nanospheres remained significant over the whole temperature range. Our preliminary results suggest that in the context of high frequency photoacoustic imaging, nanoparticles may be discriminated from molecular absorbers based on nanoscale temperature rises.

Photoacoustic generation by a gold nanosphere: From linear to nonlinear thermoelastics in the long-pulse illumination regime

“…Because the thermal nonlinearity was shown to be significant only at high frequencies, it is expected that it may not be observable at low frequencies. In a recent experimental study 22 with a detection frequency of 20 MHz and 40-nm diameter gold nanospheres suspended in 100 µm diameter tube, a linear relationship was observed between the peak-to-peak amplitude and the fluence, up to a fluence of Φ = 7 mJ/cm 2 , yet close to the corresponding critical fluence predicted here Φ c = 6.5 mJ/cm 2 . However, as discussed above, the critical fluences predicted here are only valid for a single nanoparticle, and the prediction for ensembles of nanoparticles, beyond the scope of this work, requires taking into account the nanoparticles spatial distribution.…”

“…More generally, nonlinear photoacoustic phenomena provide a means of selectively detecting contrast agents from an absorption background that behaves linearly 14 , similarly to what is done in the field of ultrasound imaging. Several phenomena may induce nonlinear relationships between the photoacoustic signal amplitude and the energy of the incident light in addition to bubble formation, such as optical saturation 13,15 , photochemical reaction 16 and temperature-dependent thermodynamic parameters 3,10, [17][18][19][20][21][22] . The latter phenomenon is related to the temperature dependence of the thermal expansion coefficient, and is at the core of this paper, in which we investigate theoretically the photoacoustic generation by a gold nanosphere in the thermoelastic regime (absence of bubble formation).…”

Nonlinear optoacoustic readings from diffusive media at near‐infrared wavelengths

“…Typically, studies in optoacoustic imaging assume that the detected signal amplitude varies linearly with the light fluence delivered on the sample. However, recent reports suggested nonlinear optoacoustic intensity readings in response to increasing light fluence on microscopic specimens . Nonlinear signals have been collected from exogenous dyes, nanoparticles and chromophores , and different mechanisms have been proposed responsible for the nonlinear behavior, including the formation of nanobubbles , changes of thermophysical parameters, such as thermal expansion coefficient, due to local temperature enhancement around particles at high laser fluence , saturation of the absorption coefficient or evaporation of fluids surrounding heated particles (250°C‐355°C for water) .…”

Nonlinear photoacoustic spectroscopy of hemoglobin

“…[3][4][5] Generally, the amplitude of the PA signal is assumed to be linearly proportional to the excitation pulse fluence. However, as the excitation laser intensity increases, both the saturation of the optical absorption 6,7 and the temperature dependence of thermal expansion [8][9][10] result in a measurable nonlinear dependence of the PA signal on the excitation pulse fluence. PA nonlinearity has recently been used in several applications such as quantifying picosecond absorption relaxation times with a nanosecond laser, 6 differentiating optical absorbers, 10 measuring oxygen saturation in vivo, 7 and performing label-free PA nanoscopy of biological structures having undetectable fluorescence.…”

“…However, as the excitation laser intensity increases, both the saturation of the optical absorption 6,7 and the temperature dependence of thermal expansion [8][9][10] result in a measurable nonlinear dependence of the PA signal on the excitation pulse fluence. PA nonlinearity has recently been used in several applications such as quantifying picosecond absorption relaxation times with a nanosecond laser, 6 differentiating optical absorbers, 10 measuring oxygen saturation in vivo, 7 and performing label-free PA nanoscopy of biological structures having undetectable fluorescence. 11 In the presence of nonlinearity, quantitative PA measurements require a detailed analysis of the intensity-dependence of the PA signal, particularly for hemoglobin, the major intrinsic absorber in tissue for PA imaging.…”