Photoacoustic Generation Research Articles

Picosecond photoacoustic generation of ultrasounds with composites of graphene-decorated gold nanoparticles

Photoacoustic or light-to-pressure transducer materials exhibit tremendous potential across various disciplines and applications, embracing theragnostic ultrasounds and permeabilization of biological barriers for drug delivery or for gene transfection. Here, we report the photoacoustic response in the picosecond excitation regime of graphene and gold nanoparticles decorated graphene dispersed in a polydimethylsiloxane polymer matrix. A novel decoration method was developed to nucleate Au nanoparticles of 25 nm on the graphene surface without reducing agent. We observed that the picosecond excitation enforced by the stress confinement generates high-frequency waves, with bandwidths of −6 dB and −20 dB of around 70 MHz and 130 MHz respectively, and peak pressure > 5 MPa. Further, the presence of gold surface decoration leads to a better dispersibility of the flake into the polymer matrix and to an increasing bandwidth at −6 dB up to 85 MHz. The decrease in source thickness leads to an increase in ultrasound frequency, which retains its value even when the laser fluence is increased up to 150 mJ cm –2. This enables the generation of high pressures while keeping the same bandwidth. We experimentally show that the photoacoustic waveform can be tailored by changing the temporal duration of the laser light, the geometry of the acoustic source, and the nature of the graphene absorber. Flexible decoration and fabrication methods of thin films, along with the generation of high-frequency and high-pressure ultrasound waves, offer new perspectives for the use of physical methods to permeabilize biological barriers and develop high-resolution photoacoustic imaging systems.

Effect of gas turbulence in quartz-enhanced photoacoustic spectroscopy: A comprehensive flow field analysis

Here we present a computational and experimental fluid dynamics study for the characterization of the flow field within the gas chamber of a Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) sensor, at different flow rates at the inlet of the chamber. The transition from laminar to turbulent regime is ruled both by the inlet flow conditions and dimension of the gas chamber. The study shows how the distribution of the flow field in the chamber can influence the QEPAS sensor sensitivity, at different operating pressures. When turbulences and eddies are generated within the gas chamber, the efficiency of photoacoustic generation is significantly altered.

Fine Tuning of the Photoacoustic Generation Efficiency by Aggregation-Caused Quenching and Excitation Energy Transfer in Bodipy-Labeled Polylactide Nanoparticles

Fine Tuning of the Photoacoustic Generation Efficiency by Aggregation-Caused Quenching and Excitation Energy Transfer in Bodipy-Labeled Polylactide Nanoparticles

Acoustic Wave Generation in Nanofluids: Effect of the Kapitza Resistance on the Thermophone to Mechanophone Generation Mechanism Transition

High frequency, short wavelength acoustic wave generation in fluids, by means of immersed nanotransducers, is key to applications ranging from the biomedical to the ICT sector, all the way to investigating fundamental aspects of the mechanics of fluids. In this context, the commonly adopted generation mechanism is the thermophone, where the nanotransducer serves as a nanoheater for the surrounding fluid and the acoustic wave is launched by water thermal expansion. Its performance, however, is severely degraded when reaching up to hypersonic frequencies. We analytically investigate the thermoacoustic response in the frequency domain for the case of a gold nanofilm transducer in contact with water. We prove that another generation mechanism, the mechanophone, sets in this critical frequency range, widening the acoustic generation bandwidth. In the mechanophone, it is the nanotransducer thermal expansion that launches pressure waves in water. We find a threshold frequency discriminating between these two regimes, which can be tuned by acting on the Kapitza resistance at the solid–liquid interface. We then show how the mechanophone mechanism can be activated in the frame of photoacoustic generation with pulsed laser sources. The unveiled physics bears generality beyond the specific system and explains the acoustic generation mechanisms in nanofluids.

Mid-infrared light-induced photoacoustic wave in water and its application

We demonstrate photoacoustic (PA) wave generation at an air–water interface using a mid-infrared (MIR) laser pulse and observe its propagation in liquid water by shadowgraph imaging. The PA wave reaches a depth of more than 4 mm, which is over 100 times deeper than the penetration depth of MIR light in water. As one of the applications of the PA wave, we quantitatively analyze the ethanol mole fraction in a water–ethanol mixed solution. We achieve the generation of PA waves induced by a compact MIR laser system, which provides a new tool for imaging and inspecting the object in water.

Characterization of H2S QEPAS detection in methane-based gas leaks dispersed into environment

The increase in fatal accidents and chronic illnesses caused by hydrogen sulfide (H2S) exposure occurring in various workplaces is pushing the development of sensing systems for continuous and in-field monitoring of this hazardous gas. We report here on the design and realization of a Near-IR quartz-enhanced photoacoustic sensor (QEPAS) for H2S leaks detection. H2S QEPAS signal was measured in matrixes containing up to 1 % of methane (CH4) and nitrogen (N2) which were chosen as the laboratory model environment for leakages from oil and gas wells or various industrial processes where H2S and CH4 can leak simultaneously. An investigation of the influence of CH4 on H2S relaxation and photoacoustic generation was proposed in this work and the sensor performances were carefully assessed with respect to CH4 content in the mixture. We demonstrated the high selectivity, with no cross talk between H2S, H2O and CH4 absorption lines, high sensitivity, and fast response time of the developed sensor, achieving a minimum detection limit (MDL) of 2.5 ppm for H2S with 2 s lock-in integration time. The employed 2.6 µm laser allowed us to employ the sensor also for CH4 detection, achieving an MDL of 85 ppm. The realized QEPAS sensor lends itself to the development of a portable and compact device for industrial monitoring.

Surface-enhanced photoacoustic wave generation from light absorbers located in the gap of high-refractive-index dielectric nanoparticles

Plasmonic nanoparticles, such as Au nanoparticles, have been used as effective exogeneous contrast agents in photoacoustic imaging. However, using these particles to enhance photoacoustic wave generation from other light absorbers is difficult because the signal is overwhelmed by the signal from plasmonic nanoparticles themselves. In this study, we realized the surface-enhanced photoacoustic effect by using high-refractive-index dielectric nanoparticles in their ultralow light absorption Ohmic-loss wavelength band. We performed finite element method simulation on a model with gallium phosphide nanorod dimmer and light absorbers in the dimmer gap. The Au nanowire, carbon nanotube, and nanotube filled with melanin molecules were sequentially considered as the light absorber. It is found that a photoacoustic wave surface enhancement factor of approximately 10 was achieved and the enhanced photoacoustic wave ensures the photoacoustic spectral signature of the light absorber. The potential of optimization for boosting enhancement factors was further analyzed. These results can provide considerable insight into molecular photoacoustic sensing and imaging.

Understanding the near-field photoacoustic spatiotemporal profile from nanostructures

Understanding the mechanism of photoacoustic generation at the nanoscale is key to developing more efficient photoacoustic devices and agents. Unlike the far-field photoacoustic effect that has been well employed in imaging, the near-field profile leads to a complex wave-tissue interaction but is understudied. Here we show that the spatiotemporal profile of the near-field photoacoustic waves can be shaped by laser pulses, anisotropy, and the spatial arrangement of nanostructure(s). Using a gold nanorod as an example, we discovered that the near-field photoacoustic amplitude in the short axis is ∼75 % stronger than the long axis, and the anisotropic spatial distribution converges to an isotropic spherical wave at ∼50 nm away from the nanorod's surface. We further extend the model to asymmetric gold nanostructures by arranging isotropic nanoparticles anisotropically with broken symmetry to achieve a precisely controlled near-field photoacoustic "focus" largely within an acoustic wavelength.

Techniques to enhance the photoacoustic signal for trace gas sensing: A review

Photoacoustic spectroscopy (PAS), which relies on the detection of absorption-induced acoustic waves, is widely used for gas sensing. This paper summarizes and discusses most of the recent techniques for photoacoustic enhancement throughout the overall process from laser-molecule interaction to acoustic wave detection. Considering the theoretical principle and system composition of PAS gas sensor, we classify photoacoustic enhancing techniques into three aspects. The first one is to enhance the photoacoustic excitation by building up the laser power in terms of the positive proportional relation between the optical power and photoacoustic signal. The second one is to amplify the acoustic strength with various kinds of resonators after the photoacoustic generation. The third approach to further improve the photoacoustic detection is to develop more efficient acoustic transducers, for example, custom tuning fork, cantilever and fiber-optic microphone. Finally, the advantages and limitations of these different techniques are analyzed.

Biocompatible and Photostable Photoacoustic Contrast Agents as Nanoparticles Based on Bodipy Scaffold and Polylactide Polymers: Synthesis, Formulation, and In Vivo Evaluation.

We have designed a new Bodipy scaffold for efficient in vivo photoacoustic (PA) imaging of nanoparticles commonly used as drug nanovectors. The new dye has an optimized absorption band in the near-infrared window in biological tissue and a low fluorescence quantum yield that leads to a good photoacoustic generation efficiency. After Bodipy-initiated ring-opening polymerization of lactide, the polylactide-Bodipy was formulated into PEGylated nanoparticles (NPs) by mixing with PLA-PEG at different concentrations. Formulated NPs around 100 nm exhibit excellent PA properties: an absorption band at 760 nm and a molar absorption coefficient in between that of molecular PA absorbers and gold NPs. Highly improved photostability compared to cyanine-labeled PLA NPs as well as innocuity in cultured macrophages were demonstrated. After intravenous injection in healthy animals, NPs were easily detected using a commercial PA imaging system and spectral unmixing, opening the way to their use as theranostic agents.

Modeling photoacoustic pressure generation in colloidal suspensions at different volume fractions based on a multi-scale approach

Further development of quantitative photoacoustic tomography requires understanding the photoacoustic pressure generation by modeling the generation process. This study modeled the initial photoacoustic pressure in colloidal suspensions, used as tissue phantoms, at different volume fractions on a multi-scale approach. We modeled the thermodynamic and light scattering properties on a microscopic scale with/without treating the hard-sphere interaction between colloidal particles. Meanwhile, we did the light energy density on a macroscopic scale. We showed that the hard-sphere interaction significantly influences the initial pressure and related quantities at a high volume fraction except for the thermodynamic properties. We also showed the initial pressure at the absorber inside the medium logarithmically decreases with increasing the volume fractions. This result is mainly due to the decay of the light energy density with light scattering. Our numerical results suggest that modeling light scattering and propagation is crucial over modeling thermal expansion.

Optimization of the synthesis conditions of gold nanoparticle–polydimethylsiloxane composites for ultrasound generation

We show that using alcohol as solvents for the synthesis of Au–PDMS photoacoustic generation composites offers superior photoacoustic pulse amplitude at the expense of pulse decay time. Isopropanol the best of all tested solvents.

Morphology-dependent resonance enhanced nonlinear photoacoustic effect in nanoparticle suspension: a temporal-spatial model.

The morphology-dependent resonances (MDRs) hotspot, ubiquity formed between the pairs of nanoparticles in close vicinity, has garnered considerable recent attention. By extending this phenomenon to pulse-laser irradiated nanoparticle suspension, we demonstrate that such collective optical/thermal enhancement can give rise to the nonlinear photoacoustic (PA) generation. In this study, a temporal-spatial analytical expression is derived to quantitatively describe the nonlinear PA signal generation from nanoparticles, incorporating the Grüneisen increase at the microscopic individual particle level and MRDs enhancement at the macroscopic suspension level. The dependence of PA nonlinearity on the critical contributors, including the laser pulse width, the particle size, and the statistical interparticle spacing, is quantitatively discussed. The theory is well validated with the finite element method (FEM) and experimentally proved with semiconducting polymer nanoparticles (SPN) suspension. This work may pave a new direction towards effective MDR based nonlinear PA contract agent design.

Numerical Simulation of Enhanced Photoacoustic Generation and Wavefront Shaping by a Distributed Laser Array

In photoacoustic imaging, the use of arrayed laser sources brings several advantages. Acoustic waves can be generated with flexible control of wavefronts, bringing functionality such as ultrasonic beam steering and focusing. The use of arrays reduces the optical intensity while increasing the strength of the ultrasonic wave, bringing the advantages of improved signal-to-noise ratio (SNR) while avoiding laser-induced damage. In this paper, we report a numerical model for studying the generation and shaping of acoustic wavefronts with laser arrays. The propagation of mechanical waves, photoacoustically generated by thermal expansion, is simulated and discussed in detail. In addition, a partially delayed distributed array is studied both theoretically and quantitatively. The developed model for wavefront control through time-delayed laser pulses is shown to be highly suited for the optimization of laser array generation schemes.

Broadband All-Optical Plane-Wave Ultrasound Imaging System Based on a Fabry-Perot Scanner.

A broadband all-optical plane-wave ultrasound imaging system for high-resolution 3-D imaging of biological tissues is presented. The system is based on a planar Fabry-Perot (FP) scanner for ultrasound detection and the photoacoustic generation of ultrasound in a carbon-nanotube-polydimethylsiloxane (CNT-PDMS) composite film. The FP sensor head was coated with the CNT-PDMS film which acts as an ultrasound transmitting layer for pulse-echo imaging. Exciting the CNT-PDMS coating with nanosecond laser pulses generated monopolar plane-wave ultrasound pulses with MPa-range peak pressures and a -6-dB bandwidth of 22 MHz, which were transmitted into the target. The resulting scattered acoustic field was detected across a 15 mm ×15 mm scan area with a step size of 100 [Formula: see text] and an optically defined element size of [Formula: see text]. The -3-dB bandwidth of the sensor was 30 MHz. A 3-D image of the scatterer distribution was then recovered using a k -space reconstruction algorithm. To obtain a measure of spatial resolution, the instrument line-spread function (LSF) was measured as a function of position. At the center of the scan area, the depth-dependent lateral LSF ranged from 46 to 65 [Formula: see text] for depths between 1 and 12 mm. The vertical LSF was independent of position and measured to be [Formula: see text] over the entire field of view. To demonstrate the ability of the system to provide high-resolution 3-D images, phantoms with well-defined scattering structures of arbitrary geometry were imaged. To demonstrate its suitability for imaging biological tissues, phantoms with similar impedance mismatches, sound speed and scattering properties to those present in the tissue, and ex vivo tissue samples were imaged. Compared with conventional piezoelectric-based ultrasound scanners, this approach offers the potential for improved image quality and higher resolution for superficial tissue imaging. Since the FP scanner is capable of high-resolution 3-D photoacoustic imaging of in vivo biological tissues, the system could ultimately be developed into an instrument for dual-mode all-optical ultrasound and photoacoustic imaging.

Optimizing light delivery in scanning photoacoustic imaging for prostate

The purpose of this study was to optimize the light delivery method in a prostate photoacoustic imaging system. A three dimensional (3D) optical model of the human prostate was developed, and the optical energy distribution in the prostate was estimated via three-dimensional Monte Carlo simulation. Then, the feasibility of prostate photoacoustic imaging (PAI) using two endoscopic light delivery methods was studied. Photoacoustic pressure generation and the corresponding photoacoustic images had been obtained and the comparisons were made between each other. Also, the results of cylinder diffusing light source with different lengths were compared. After that, phantom experiment was carried out to validate the simulation results. Our results would be significant in the optimizing photoacoustic imaging system for an accurate diagnosis of prostate cancer.

Neural Stimulation In Vitro and In Vivo by Photoacoustic Nanotransducers

Summary Neuromodulation is an invaluable approach for the study of neural circuits and clinical treatment of neurological diseases. Here, we report semiconducting polymer nanoparticles based photoacoustic nanotransducers (PANs) for neural stimulation in vitro and in vivo. Our PANs strongly absorb the nanosecond pulsed laser in the near-infrared second window (NIR-II) and generate localized acoustic waves. PANs are shown to be surface modified and selectively bind onto neurons. PAN-mediated activation of primary neurons in vitro is achieved with ten 3-ns laser pulses at 1,030 nm over a 3-ms duration. In vivo neural modulation of mouse brain activities and motor activities is demonstrated by PANs directly injected into brain cortex. With submillimeter spatial resolution and negligible heat deposition, PAN stimulation is a new non-genetic method for precise control of neuronal activities, opening up potentials in non-invasive brain modulation.

Plane photoacoustic wave generation in liquid water using irradiation of terahertz pulses

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. The THz light can effectively generate the photoacoustic wave in water because of strong absorption via a stretching vibration mode of the hydrogen bonding network. The excitation of a large-area water surface irradiated by loosely focused THz light produces a plane photoacoustic wave. This is in contrast with conventional methods using absorbers or plasma generation using near-infrared laser light. The photoacoustic wave generation and plane wave propagation are observed using a system with a THz free-electron laser and shadowgraph imaging. The plane photoacoustic wave is generated by incident THz light with a small radiant exposure of < 1 mJ/cm2 and delivered 600 times deeper than the penetration depth of THz light for water. The THz-light-induced plane photoacoustic wave offers great advantages to non-invasive operations for industrial and biological applications as demonstrated in our previous report (Yamazaki et al. in Sci Rep 10:9008, 2020).

Photoacoustic fluctuation imaging: theory and application to blood flow imaging

Photoacoustic fluctuation imaging, which exploits randomness in photoacoustic generation, provides enhanced images in terms of resolution and visibility, as compared to conventional photoacoustic images. While a few experimental demonstrations of photoacoustic fluctuation imaging have been reported, it has to date not been described theoretically. In the first part of this work, we propose a theory relevant to fluctuations induced either by random illumination patterns or by random distributions of absorbing particles. The theoretical predictions are validated by Monte Carlo finite-difference time-domain simulations of photoacoustic generation in random particle media. We provide a physical insight into why visibility artefacts are absent from second-order fluctuation images. In the second part, we demonstrate experimentally that harnessing randomness induced by the flow of red blood cells produce photoacoustic fluctuation images free of visibility artefacts. As a first proof of concept, we obtain two-dimensional images of blood vessel phantoms. Photoacoustic fluctuation imaging is finally applied in vivo to obtain 3D images of the vascularization in a chicken embryo.

Ultra-short photoacoustic pulse generation through hot electron pressure in two-dimensional electron gas.

Launching ultrashort femtosecond photoacoustic pulses with multi-terahertz bandwidth will find broad applications from fundamental acoustics in 2D materials and THz-acoustic and phonon spectroscopy to nondestructive detection in opaque materials with a sub-nanometer resolution. Here we report the generation of ultra-short 344 fs photoacoustic pulses with a 2.1 THz bandwidth from interfacial two-dimensional electron gas using optical femtosecond excitation. A comparison with simulation supports the dominant contribution of hot electron pressure and the ultrafast electron relaxation to produce pulsewidth shorter than the acoustic transit time across the electron wavefunction. Our simulation further indicates the possibility to generate <200 fs photoacoustic pulse.