Nonlinear Photoacoustic Research Articles

NIR-II femtosecond laser-induced nonlinear absorption facilitates foreground-extraction photoacoustic imaging by monolayer WS2-PVP nanosheets

Exogenous contrast agents have been extensively applied in photoacoustic (PA) molecular imaging for disease diagnosis, benefiting from their advantageous optical, thermal, and internal delivery properties. However, their in vivo performance was inevitably interfered by background tissue optical absorption, resulting in low imaging contrast and sensitivity. Here, we report a NIR-II femtosecond laser-induced nonlinearly enhanced PA imaging technique based on two-photon absorption of monolayer WS2-PVP nanosheets (NSs), which facilitates foreground extraction of the targeting region with the background signal being significantly suppressed. The optical nonlinearity of the monolayer WS2-PVP NSs was first demonstrated by a Z-scan system under the irradiation of a femtosecond laser to be 0.38, with an antithesis of virtually zero for the tissue-mimicking sample. Experiments on tissue-mimicking phantoms and in vitro chicken breast showed that the nonlinear PA enhancement of monolayer WS2-PVP NSs can facilitate foreground-extraction imaging at deep-seated position up to 4 mm. In addition, the in vivo foreground-extraction imaging ability by using monolayer WS2-PVP NSs was further demonstrated by mouse tumor models, where the tumor regions were specifically extracted with high imaging contrast. This work proposed a nonlinearly enhanced contrast mechanism of PA nanoprobes, prefiguring great potential for high-contrast and high-specificity PA molecular imaging.

Dependence of the Nonlinear Photoacoustic Response of Gold Nanoparticles on the Heat-Transfer Process.

Photoacoustic (PA) imaging using the nonlinear PA response of gold nanoparticles (GNPs) can effectively attenuate the interference from background noise caused by biomolecules (e.g., hemoglobin), thus offering a highly potential noninvasive biomedical imaging method. However, the mechanism of the nonlinear PA response of GNPs based on the thermal expansion mechanism, especially the effect of heat-transfer ability, still lacks quantitative investigation. Therefore, this work investigated the effect of heat-transfer ability on the nonlinear PA response of GNPs using the critical energy and fluence concept, taking into account the Au@SiO2 core–shell nanoparticles (weakened heat transfer) and gold nanochains (enhanced heat transfer). The results showed that the stronger the heat transferability, the smaller the critical energy, indicating that the nonlinear PA response of different nanoparticles cannot be contrasted directly through the critical energy. Moreover, the critical fluence can directly contrast the proportion of nonlinear components in the PA response of different GNPs as governed by the combined effect of heat transferability and photothermal conversion ability.

Hydrophobically Modified Silica-Coated Gold Nanorods for Generating Nonlinear Photoacoustic Signals.

In this work, we report that gold nanorods coated with hydrophobically-modified mesoporous silica shells not only enhance photoacoustic (PA) signal over unmodified mesoporous silica coated gold nanorods, but that 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 solvent vapor in the mesopores: the vapor has both a larger expansion coefficient and thermal resistance than silica that enhances conversion to acoustic energy, and the hydrophobic porous surface is able to promote phase transition at the surface, leading to a nonlinear PA response 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.

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.

Nonlinear photoacoustic imaging dedicated to thermal-nonlinearity characterization

We proposed a nonlinear photoacoustic (PA) technique as a new imaging contrast mechanism for tissue thermal-nonlinearity characterization. When a sine-modulated Gaussian temperature field is introduced by a laser beam, in view of the temperature dependence of the thermal diffusivity, the nonlinear PA effect occurs, which leads to the production of second-harmonic PA (SHPA) signals. By extracting the fundamental frequency PA and SHPA signal amplitudes of samples through the lock-in technique, a parameter that only reflects nonlinear thermal-diffusivity characteristics of the sample then can be obtained. The feasibility of the technique for thermal-nonlinearity characterization has been studied on phantom samples. In vitro biological tissues have been studied by this method to demonstrate its medical imaging capability, prefiguring great potential of this new method in medical imaging applications.

Manipulating the Dynamics of Dark Excited States in Organic Materials for Phototheranostics.

Manipulating the dynamics of dark excited states (DESs), such as higher excited singlet or excited triplet states with no or small radiative decay, are of both fundamental and practical interests, an important application being photoactivated diagnosis and therapy (phototheranostics), which include photoacoustic (PA) imaging, photodynamic therapy (PDT), and photothermal therapy (PTT). However, the current understanding of DESs in organic structures is rather limited, thus making any rational manipulation of DES in organic materials very challenging.A DES decays primarily by radiationless transition through two pathways: (i) singlet-to-triplet intersystem crossing (ISC) and (ii) internal conversion (IC) relaxation. The deactivation of a DES via ISC can generate cytotoxic reactive oxygen species (ROS) for PDT, while IC could convert photons into heat for PA imaging and PTT. In this Account, we highlight our research on developing a fundamental understanding of structure-property relationships for manipulation of DESs in organic materials in relation to phototheranostic applications. We describe the application of femtosecond transient absorption (fs-TA) spectroscopy for obtaining valuable insights into the DES dynamics. Afterward, we present our work on DESs in nonrigid molecules that revealed greatly enhanced ISC through geometry twisting, which leads to an innovative pathway to develop organic materials exhibiting external stimuli-responsive reversible switching of ISC. We introduce the concept of smart PDT where highly efficient ISC imparted by geometry twisting in the acidic environment specific to tumors leads to very efficient and highly localized PDT, thus leaving surrounding healthy tissues at a different pH unaffected. This insightful understanding of ISC can lead to the development of more advanced photosensitizers for PDT. Two other emergent concepts from our work presented here are (1) significantly enhanced IC producing strong local heating by combining two-photon absorption with excited state absorption for cumulative multiphoton absorption, thus greatly increasing the strength of the PA signal for nonlinear PA imaging, and (2) shown by an example of an organic molecule, BODIPY, nanoscale charge-transfer state mediated strong IC in aggregate nanoparticles resulting in exceptionally high photothermal conversion efficiency of 61% for both PA and PTT. Some in vivo results of the phototheranostic studies using BODIPY are presented, providing an elegant example of nanoscale manipulation of the excited state dynamics.This Account concludes with a summary and discussion of future perspectives. We hope this Account will deepen the understanding of molecular and nanoscale control of excited state dynamics in organic materials, hopefully enticing a broad range of scientists within different disciplinary areas.

Time delay effect in a microchip pulse laser for the nonlinear photoacoustic signal enhancement.

The Grüneisen relaxation effect has been successfully employed to improve the photoacoustic (PA) imaging contrast. However, complex system design and cost hinder the progress from benchside to bedside, since an additional pre-heating laser source needs to be coupled into the original light path and synchronized with other equipment for conducting the nonlinear effect. To overcome the limitation, we propose a time delay heating PA imaging (TDH-PAI) method based on the time delay effect in a passively Q-switched laser. Experimentally, only one single microchip pulse laser is built and utilized for the nonlinear PA signal enhancement without additional components. The 808 nm pump pulse of the laser diode and the excited 1064 nm pulse are respectively used for pre-heating and acquiring PA signals. The heating effect is optimized by adjusting the input parameters and an enhancement of more than 30% in PA signals is achieved. TDH-PAI reduces the cost and complexity of the nonlinear PA system, which provides an efficient way for achieving a high-contrast PA imaging.

Dual-Contrast Nonlinear Photoacoustic Sensing and Imaging Based on Single High-Repetition- Rate Pulsed Laser

Photoacoustic imaging (PAI) is a fast-developing non-invasive imaging technique, which provides optical contrast in deep scattering tissue with high spatial resolution. Conventional photoacoustic (PA) imaging has been widely studied based on linear PA effect, which is induced by single short laser pulse. Recently, nonlinear photoacoustic effect has been proposed by using two consecutive laser pulses with two laser sources. In this paper, we report a dual-contrast nonlinear photoacoustic (dNPA) sensing and imaging technique by single high-repetition-rate laser induced nonlinear PA effect. Using the high-repetition-rate pulsed laser, two different kinds of imaging contrasts can be extracted: optical and thermal contrasts. Unlike conventional PA imaging that exhibits optical absorption only, we could extract thermal contrast that is reflected by the rising slope/trend of temperature and consequent PA signals. In this paper, the high-repetition-rate nonlinear PA effect and dual-contrast mechanism were proposed in theory with detailed mathematical modeling. The dual-contrast PA sensing and imaging were demonstrated by experimental results with both in vitro phantom and ex vivo porcine tissues. The results indicate both optical absorption and thermal accumulation properties of different samples, which could enhance the image contrast significantly.

Photoacoustic and fluorescent effects in multilayer plasmon-dye interfaces.

Progress in understanding the cell biology and diseases depends on advanced imaging and labeling techniques. Here, we address this demand by exploring novel multilayered nanocomposites (MNCs) with plasmonic nanoparticles and absorbing dyes in thin nonabsorbing shells as supercontrast multimodal photoacoustic (PA) and fluorescent agents in the near-infrared range. The proof of concept was performed with gold nanorods (GNRs) and indocyanine green (ICG) dispersed in a matrix of biodegradable polymers. We demonstrated synergetic PA effects in MNCs with the gold-ICG interface that could not be achieved with ICG and GNRs alone. We also observed ultrasharp PA and emission peaks that could be associated with nonlinear PA and spaser effects, respectively. Low-toxicity multimodal MNCs with unique plasmonic, thermal and acoustic properties have the potential to make a breakthrough in PA flow cytometry and near-infrared spasers in vivo by using the synergetic interaction of plasmonic modes with a nearby absorbing medium.

Correlation of Photophysical Properties with the Photoacoustic Emission for a Selection of Established Chromophores

An archetypal study is presented to correlate both the optical and the photoacoustic (PA) properties for a diverse selection of dyes whose structural properties range across organic and inorganic, symmetric and asymmetric, and neutral and cationic systems. Three distinct classes of molecular PA contrast agents have been identified and classified according to their optical-PA response as either (i) linear absorbers linear PA emitter, (ii) saturable absorbers weak PA emitter, or (iii) reverse saturable absorbers nonlinear PA emitter. The molecular characteristics instrumental in determining the nature of the dyes optical absorption properties, i.e., ground state molar extinction coefficient (eg), excited state molar extinction coefficient (ee), and excited state lifetime (τ), are discussed to aid in the interpretation of a molecule’s optical vs PA response. An excellent linear PA emitter is established in crystal violet, which exhibits the strongest possible PA signal under low laser fluence conditions in b...

Nonlinear photoacoustic spectroscopy of hemoglobin

As light intensity increases in photoacoustic imaging, the saturation of optical absorption and the temperature dependence of the thermal expansion coefficient result in a measurable nonlinear dependence of the photoacoustic (PA) signal on the excitation pulse fluence. Here, under controlled conditions, we investigate the intensity-dependent photoacoustic signals from oxygenated and deoxygenated hemoglobin at varied optical wavelengths and molecular concentrations. The wavelength and concentration dependencies of the nonlinear PA spectrum are found to be significantly greater in oxygenated hemoglobin than in deoxygenated hemoglobin. These effects are further influenced by the hemoglobin concentration. These nonlinear phenomena provide insights into applications of photoacoustics, such as measurements of average inter-molecular distances on a nm scale or with a tuned selection of wavelengths, a more accurate quantitative PA tomography.

Photoacoustically guided wavefront shaping for enhanced optical focusing in scattering media.

Non-invasively focusing light into strongly scattering media, such as biological tissue, is highly desirable but challenging. Recently, ultrasonically guided wavefront shaping technologies have been developed to address this limitation. So far, the focusing resolution of most implementations has been limited by acoustic diffraction. Here, we introduce nonlinear photoacoustically guided wavefront shaping (PAWS), which achieves optical diffraction-limited focusing in scattering media. We develop an efficient dual-pulse excitation approach to generate strong nonlinear photoacoustic (PA) signals based on the Grueneisen relaxation effect. These nonlinear PA signals are used as feedback to guide iterative wavefront optimization. As a result, light is effectively focused to a single optical speckle grain on the scale of 5–7 µm, which is ~10 times smaller than the acoustic focus with an enhancement factor of ~6,000 in peak fluence. This technology has the potential to benefit many applications that desire highly confined strong optical focus in tissue.

Nonlinear photoacoustic signal amplification from single targets in absorption background

Photoacoustic (PA) detection of single absorbing targets such as nanoparticles or cells can be limited by absorption background. We show here that this problem can be overcome by using the nonlinear photoacoustics based on the differences in PA signal dependences on the laser energy from targets and background. Among different nonlinear phenomena, we focused on laser generation of nanobubbles as more efficient PA signal amplifiers from strongly absorbing, highly localized targets in the presence of spatially homogenous absorption background generating linear signals only. This approach was demonstrated by using nonlinear PA flow cytometry platform for label-free detection of circulating melanoma cells in blood background in vitro and in vivo. Nonlinearly amplified PA signals from overheated melanin nanoclusters in melanoma cells became detectable above still linear blood background. Nonlinear nanobubble-based photoacoustics provide new opportunities to significantly (5–20-fold) increase PA contrast of single nanoparticles, cells, viruses and bacteria in complex biological environments.

In vivo magnetic enrichment, photoacoustic diagnosis, and photothermal purging of infected blood using multifunctional gold and magnetic nanoparticles.

Bacterial infections are a primary cause of morbidity and mortality worldwide. Bacteremia is a particular concern owing to the possibility of septic shock and the development of metastatic infections. Treatment of bacteremia is increasingly compromised by the emergence of antibiotic resistant strains, creating an urgent need for alternative therapy. Here, we introduce a method for in vivo photoacoustic (PA) detection and photothermal (PT) eradication of Staphylococcus aureus in tissue and blood. We show that this method could be applicable for label-free diagnosis and treatment of in the bloodstream using intrinsic near-infrared absorption of endogenous carotenoids with nonlinear PA and PT contrast enhancement. To improve sensitivity and specificity for detection of circulating bacteria cells (CBCs), two-color gold and multilayer magnetic nanoparticles with giant amplifications of PA and PT contrasts were functionalized with an antibody cocktail for molecular targeting of S. aureus surface-associated markers such as protein A and lipoprotein. With a murine model, the utility of this approach was demonstrated for ultrasensitive detection of CBCs with threshold sensitivity as low as 0.5 CBCs/mL, in vivo magnetic enrichment of CBCs, PT eradication of CBCs, and real-time monitoring of therapeutic efficacy by CBC counting. Our PA-PT nano-theranostic platform, which integrates in vivo multiplex targeting, magnetic enrichment, signal amplification, multicolor recognition, and feedback control, could be used as a biological tool to gain insights on dissemination pathways of CBCs, infection progression by bacteria re-seeding, and sepsis development and treatment, and could potentially be feasible in humans, especially using bypass schematic.

Photothermal and photoacoustic Raman cytometry in vitro and in vivo

An integrated Raman-based cytometry was developed with photothermal (PT) and photoacoustic (PA) detection of Raman-induced thermal and acoustic signals in biological samples with Raman-active vibrational modes. The two-frequency, spatially and temporally overlapping pump–Stokes excitation in counterpropagating geometry was provided by a nanosecond tunable (420–2300 nm) optical parametric oscillator and a Raman shifter (639 nm) pumped by a double-pulsed Q-switched Nd:YAG laser using microscopic and fiberoptic delivery of laser radiation. The PA and PT Raman detection and imaging technique was tested in vitro with benzene, acetone, olive oil, carbon nanotubes, chylomicron phantom, and cancer cells, and in vivo in single adipocytes in mouse mesentery model. The integration of linear and nonlinear PA and PT Raman scanning and flow cytometry has the potential to enhance its chemical specificity and sensitivity including nanobubble-based amplification (up to 10- fold) for detection of absorbing and nonabsorbing targets that are important for both basic and clinically relevant studies of lymph and blood biochemistry, cancer, and fat distribution at the single-cell level.

Application of nonlinear laser photoacoustic technique to crack detection

Nonlinear acoustics is a promising method for non-destructive testing (NDT) as it allows to improve the sensitivity and contrast of defects detection. The application of contactless laser photoacoustic technique for the tasks of nonlinear acoustics looks very promising. Meanwhile, real examples of industrial systems using nonlinear photoacoustics are still absent. In this work nonlinear photoacoustic response of artificially prepared cracks has been studied. Acoustic vibrations have been excited through the loASAHGRcal heating caused by the absorption of a focused beam of visible or near-infrared laser radiation. Several methods have been applied for the detection of surface displacement including piezo-transducers, air-coupled transducers and optical laser interferometry. It has been shown, that nonlinear acoustic response increases drastically when both pump and probe points are localized near the crack. For this reason, the last two methods allowing local probing look very promising. Two dimensional scans of generation and detection points on the sample surface could be used to obtain the images of crack at new spectral components induced by crack nonlinearity. The goal of this work is to find optimal methods of the excitation and detection of nonlinear photoacoustic response of the crack. It should make the technique attractive for the industrial applications.