Photoacoustic Signal Enhancement Research Articles

Copper-Catalyzed Covalent Dimerization of Near-Infrared Fluorescent Cyanine Dyes: Synergistic Enhancement of Photoacoustic Signals for Molecular Imaging of Tumors.

Photoacoustic (PA) imaging relies on the absorption of light by chromophores to generate acoustic waves used to delineate tissue structures and physiology. Here, we demonstrate that Cu(II) efficiently catalyzes the dimerization of diverse near-infrared (NIR) cyanine molecules, including a peptide conjugate. NMR spectroscopy revealed a C-C covalent bond along the heptamethine chains, creating stable molecules under conditions such as a wide range of solvents and pH mediums. Dimerization achieved >90% fluorescence quenching, enhanced photostability, and increased PA signals by a factor of about 4 at equimolar concentrations compared to the monomers. In vivo study with a mouse cancer model revealed that dimerization enhanced tumor retention and PA signal, allowing cancer detection at doses where the monomers are less effective. While the dye dimers highlighted peritumoral blood vessels, the PA signal for dimeric tumor-targeting dye-peptide conjugate, LS301, was diffuse throughout the entire tumor mass. A combination of the ease of synthesis, diversity of molecules that are amenable to Cu(II)-catalyzed dimerization, and the high acoustic wave amplification by these stable dimeric small molecules ushers a new strategy to develop clinically translatable PA molecular amplifiers for the emerging field of molecular photoacoustic imaging.

Lipid-stabilized hydrophobically modified silica-coated gold nanorods generate enhanced photoacoustic signal from cavitation with nonlinear dependence on laser fluence

In this work, we report that gold nanorods coated with hydrophobically modified mesoporous silica shells not only enhance photoacoustic (PA) signal by more than 10-fold over unmodified mesoporous silica coated gold nanorods but also 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 air in the mesopores: the air has a larger expansion coefficient than silica that enhances conversion to acoustic energy, and the air undergoes thermal size oscillations that increase with laser fluence, leading to a nonlinear trend 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.

Humidity enhanced N2O photoacoustic sensor with a 4.53 μm quantum cascade laser and Kalman filter

A high-sensitivity N2O photoacoustic sensor using a 4.53 μm quantum cascade laser was developed. Sharply enhancement of photoacoustic signal of N2O with the increasing of humidity was investigated experimentally. Finally, 2.3 % water vapor was added to the analyzed sample to improve the vibrational-translational (VT) relaxation rate of N2O molecule transition, and therefore enhance the N2O photoacoustic signal. High performance with a minimum detection limit of 28 ppbv in 1 s and a measurement precision of 34 ppbv have been achieved, respectively. Kalman adaptive filtering was used to remove the shot-to-shot variability related to the real-time noise in the measurement data and further improve the measurement precision. Without sacrificing the time resolution of the system, the Kalman adaptive filtering improves the measurement precision of the system by 2.3 times. The ability of the N2O photoacoustic sensor was demonstrated by continuous measurement of atmospheric N2O concentration for a period of 7 h.

An activatable water-soluble photoacoustic probe for real–time imaging of endogenous cysteine in the mouse tumor model

Cysteine (Cys), one of the most common cellular biothiols, is closely related with many diseases such as Alzheimer's disease, Parkinson's disease and cancer. However, photoacoustic (PA) probes for detecting Cys-associated complicated diseases are still rare, especially for deep imaging of Cys in tumor. Therefore, the development of a reliable approach for noninvasive imaging Cys in tumor to fully understand the physiological functions of Cys has great significance. Herein, a novel activatable water-soluble photoacoustic probe (NCTy) for accurately monitoring of Cys was developed. The probe NCTy could specifically respond to Cys and release a near-infrared absorbing dye, causing the PA signals at 695 nm to be turned on. Furthermore, the Cys-activated probe exhibited a 5.3-fold PA signal enhancement, while high-resolution PA images were obtained. In light of these outstanding features, the PA probe NCTy was utilized for PA imaging of exogenous and endogenous Cys in living cells and mice. Notably, the probe NCTy had been employed for real-time monitoring Cys level in vivo tumor through PA imaging technology, for the first time. We expect that the probe NCTy would provide a new method for the research and diagnosis of tumor-related pathological processes.

Activatable Photoacoustic Probe for In Situ Imaging of Endogenous Carbon Monoxide in the Murine Inflammation Model.

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that combines the advantages of optical and ultrasound imaging. Carbon monoxide (CO), which is a vital endogenous cell-signaling molecule in the human body, exerts critical physiological functions such as anti-inflammatory, antiapoptotic, and antiproliferative. The imbalance of CO homeostasis is also associated with numerous diseases. Therefore, it is critically important to noninvasively monitor the steady-state changes of CO in vivo. However, the activatable photoacoustic (PA) probes for detecting CO-associated complicated diseases have not yet developed. In this work, we developed the first turn-on PA probe (MTR-CO) to visualize the CO level in the lipopolysaccharide (LPS)-induced acute inflammation murine model through PA imaging technology. MTR-CO is composed of a near-infrared absorption cyanine-like dye (MTR-OH) and allyl formate, showing a 10.2-fold PA signal enhancement at 690 nm upon activation by CO. Furthermore, the results revealed that MTR-CO has high sensitivity, excellent specificity, and good biocompatibility for CO in vivo. MTR-CO was then applied for PA imaging of CO in cells and for monitoring the development of acute inflammation in the murine model by tracking the changes of the CO level. These findings provide a promising strategy for accurately detecting the steady-state changes of CO in living organisms.

Axial accuracy and signal enhancement in acoustic-resolution photoacoustic microscopy by laser jitter effect correction and pulse energy compensation.

In recent years, photoacoustic imaging has found vast applications in biomedical imaging. Photoacoustic imaging has high optical contrast and high ultrasound resolution allowing deep tissue non-invasive imaging beyond the optical diffusion limit. Q-switched lasers are extensively used in photoacoustic imaging due to the availability of high energy and short laser pulses, which are essential for high-resolution photoacoustic imaging. In most cases, this type of light source suffers from pulse peak-power energy variations and timing jitter noise, resulting in uncertainty in the output power and arrival time of the laser pulses. These problems cause intensity degradation and temporal displacement of generated photoacoustic signals which in turn deteriorate the quality of the acquired photoacoustic images. In this study, we used a high-speed data acquisition system in combination with a fast photodetector and a software-based approach to capture laser pulses precisely in order to reduce the effect of timing jitter and normalization of the photoacoustic signals based on pulse peak-powers simultaneously. In the experiments, maximum axial accuracy enhancement of 14 µm was achieved in maximum-amplitude projected images on XZ and YZ planes with ±13.5 ns laser timing jitter. Furthermore, photoacoustic signal enhancement of 77% was obtained for 75% laser pulses peak-power stability.

Quartz-Enhanced Photoacoustic Spectroscopy for Four-Component Gas Detection Based on Two Off-Beam Acoustic Microresonators

A compact and portable quartz-enhanced photoacoustic spectroscopy gas sensor was developed for four-component gas detection using two off-beam acoustic microresonators (AMRs). The two AMRs were placed in parallel on opposing sides of a quartz tuning fork (QTF) for photoacoustic signal enhancement. Four distributed feedback (DFB) lasers were connected to the four ends of the two off-beam AMRs using a fiber collimator for photoacoustic signal excitation. Four-component gas sensing was achieved via time-division multiplexing of the DFB laser driver currents. The four-component gas sensing scheme was used to detect acetylene (C2H2) at 1532.83 nm, methane (CH4) at 1653.722 nm, water vapor (H2O) at 1368.597 nm and carbon dioxide (CO2) at 1577.787 nm for feasibility testing. Minimum detection limits of 3.6 ppmv for C2H2, 34.71 ppmv for CH4, 1.09 ppmv for H2O, and 341.18 ppmv for CO2 were obtained, and the linear responses reached 0.9982, 0.9969, 0.99843 and 0.99591 for C2H2, CH4, H2O and CO2, respectively, at 1.5 s intervals.

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.

Photoacoustic Imaging Quantifies Drug Release from Nanocarriers via Redox Chemistry of Dye‐Labeled Cargo

AbstractWe report a new approach to monitor drug release from nanocarriers via a paclitaxel–methylene blue conjugate (PTX‐MB) with redox activity. This construct is in a photoacoustically silent reduced state inside poly(lactic‐co‐glycolic acid) (PLGA) nanoparticles (PTX‐MB@PLGA NPs). During release, PTX‐MB is spontaneously oxidized to produce a concentration‐dependent photoacoustic signal. An in vitro drug‐release study showed an initial burst release (25 %) between 0–24 h and a sustained release between 24–120 h with a cumulative release of 40.6 % and a 670‐fold increase in photoacoustic signal. An in vivo murine drug release showed a photoacoustic signal enhancement of up to 649 % after 10 hours. PTX‐MB@PLGA NPs showed an IC50 of 78 μg mL−1 and 44.7±4.8 % decrease of tumor burden in an orthotopic model of colon cancer via luciferase‐positive CT26 cells.

In vivo assessment of inflammation in carotid atherosclerosis by noninvasive photoacoustic imaging.

Objectives: The objective of this study was to demonstrate the feasibility of using noninvasive photoacoustic imaging technology along with novel semiconducting polymer nanoparticles for in vivo identifying inflammatory components in carotid atherosclerosis and assessing the severity of inflammation using mouse models.Methods and Results: Healthy carotid arteries and atherosclerotic carotid arteries were imaged in vivo by the noninvasive photoacoustic imaging system. Molecular probes PBD-CD36 were used to label the inflammatory cells to show the inflammation information by photoacoustic imaging. In in vivo imaging experiments, we observed the maximum photoacoustic signal enhancement of 4.3, 5.2, 8 and 16.3 times between 24 h post probe injection and that before probe injection in four carotid arteries belonging to three atherosclerotic mice models. In the corresponding carotid arteries stained with CD36, the ratio of 0.043, 0.061, 0.082 and 0.113 was found between CD36 positive (CD36(+)) expression area and intima-media area (P < 0.05). For the CD36(+) expression less than 0.008 in eight arteries, no photoacoustic signal enhancement was found due to the limited system sensitivity. The photoacoustic signal reflects CD36(+) expression in plaques, which shows the feasibility of using photoacoustic imaging for in vivo assessment of carotid atherosclerosis.Conclusion: This research demonstrates a semiconducting polymer nanoparticle along with photoacoustic technology for noninvasive imaging and assessment of inflammation of carotid atherosclerotic plaques in vivo.

Perfluorocarbon bubbles as photoacoustic signal amplifiers for cancer theranostics

Phase change nanoemulsions have been proposed as theranostic agents, using light to induce vaporization into bubbles (also called optical droplet vaporization). The current work uses perfluorohexane nanoemulsions (PFH-NEs) stabilized by a highly biocompatible and optically absorbing fluorosurfactant shell. Once vaporized, the bubbles can be used for contrast enhanced ultrasound (CEUS) imaging but also to enhance photoacoustic (PA) signals due to the presence of bubbles and optical absorbing shell material. The formation and expansion of these gas filled bubbles leads to increasing photoacoustic signals for imaging. Compared to other contrast agents which may not give stable signals due to photo-degradation, these contrast agents are shown to be stable up to 24 hours. The source of PA signal enhancement is through the presence of long lasting perfluorohexane (PFH) bubbles resulting from the optical vaporization. These bubbles generated from the PFH-NEs directly generate photoacoustic signals due to the optical absorption from the fluorosurfactant shell, but also secondary signals from the subsequent scattering of the photoacoustic waves from the PFH bubbles. In addition, the pressures generated from vaporization of NEs and ability to load chemotherapeutic agents enable these nanoparticles to also be used for cancer therapy by contributing to drug delivery and transport.

Pulsed photoacoustic and photothermal response of gold nanoparticles

Gold nanospheres and nanorods are promising agents for photoacoustic imaging and photothermal therapy. In this work, seed-mediated methods were optimized for the synthesis of gold nanosphere and nanorod collides of different sizes. Nanosecond pulse photoacoustic and photothermal analysis of these nanoparticles was carried out and compared with finite element simulation. The simulations were performed to quantify the size dependent photoacoustic signal enhancement for nanospheres and nanorods. The non-sphericity contribution of nanospheres was found to enhance the photoacoustic signal. Nanosecond pulse photoacoustic studies of nanorods of different aspect ratio were carried out. Nanorods of aspect ratio ∼4.8 were found to be the most efficient photoacoustic signal generators. Photoacoustic studies of nanorods at varying laser fluence were performed and threshold fluence of 5 mJ cm−2 was observed. Nanorods exposed to nanosecond laser pulses underwent size and shape variations which were confirmed by optical absorbance and transmission electron microscopy measurements. Simulations of nanorods of different aspect ratio and diameter were performed to investigate the photoacoustic signal enhancement and photothermal stability. The miniature size nanorod with a diameter of 10 nm and aspect ratio of 5 was found to be most appropriate in terms of photoacoustic signal generation and photothermal stability.

Nitric Oxide-Activated "Dual-Key-One-Lock" Nanoprobe for in Vivo Molecular Imaging and High-Specificity Cancer Therapy.

Cancer treatments are confounded by severe toxic effects toward patients. To address these issues, activatable nanoprobes have been designed for specific imaging and destruction of cancer cells under the stimulation of specific cancer-associated biomarkers. Most activatable nanoprobes were usually activated by some single-factor stimulation, but this restricts therapeutic specificity between diseased and normal tissue; therefore, multifactor activation is highly desired. To this end, we herein develop a novel dual-stimuli responsive theranostic nanoprobe for simultaneously activatable cancer imaging and photothermal therapy under the coactivation of "dual-key" stimulation of "nitric oxide (NO)/acidity", so as to further improve the therapeutic specificity. Specifically, we have integrated a weak electron acceptor (benzo[c][1,2,5]thiadiazole-5,6-diamine) into a donor-π-acceptor-π-donor type chromophore. When the weak acceptor was oxidized by NO in acidic conditions to form a stronger acceptor (5H-[1,2,3]triazolo[4,5-f]-2,1,3-benzothiadiazole), the molecule absorption was significantly increased in the near-infrared region, based on the intramolecular charge transfer (ICT) mechanism. Under the dual-key stimulation of NO/acidity within the tumor associated with inflammation, the nanoprobe can correspondingly output dual signals for ratiometric photoacoustic and photothermal imaging of cancer in vivo and do so with enhanced accuracy and specificity. Our novel nanoprobe exhibited higher photoacoustic signal enhancement under dual-factor activation at 9.8 times that of NO and 132 times that of acidity alone, respectively. Moreover, through such dual activation of NO/acidity, the nanoprobe produces more differentiation of hyperthermia between tumor and normal tissues, to afford satisfactory photothermal therapy with minimal toxic side effects. Thus, our work presents a promising strategy for significantly improving the precision and specificity of cancer imaging and therapy.

Quartz tuning fork embedded off-beam quartz-enhanced photoacoustic spectroscopy.

In order to achieve a high acoustic coupling strength and detection sensitivity and to simplify the assembly and alignment process in quartz-enhanced photoacoustic spectroscopy (QEPAS) technique, a novel quartz tuning fork (QTF) embedded off-beam QEPAS (E-OB-QEPAS) spectrophone was proposed. The structural parameters of the acoustic micro-resonator of the E-OB-QEPAS spectrophone were optimized for enhancing the signal-to-noise ratio gain based on experimental investigation. Compared with the on-beam configuration using a bare QTF, a detection sensitivity enhancement by a factor of ∼25 was achieved by embedding the QTF in one resonant tube. By using two resonant tubes simultaneously embedded with a QTF, dual-channel detection and a two-fold photoacoustic signal enhancement were realized and a detection sensitivity enhancement by a factor of ∼20 and ∼40 were achieved for the single-tube-enhanced and dual-tube-enhanced E-OB-QEPAS spectrophone, respectively.

Erythrocyte-cancer hybrid membrane-camouflaged melanin nanoparticles for enhancing photothermal therapy efficacy in tumors

Cell membrane coating has emerged as an intriguing biomimetic strategy to endow nanomaterials with functions and properties inherent to source cells for various biomedical applications. Hybrid membrane of different types of cells could be coated onto nanoparticle surface to achieve additional functions. Herein, we fused red blood cell (RBC) membrane together with MCF-7 cell membrane and fabricated an erythrocyte-cancer (RBC-M) hybrid membrane-camouflaged melanin nanoparticle (Melanin@RBC-M) platform for enhancing therapeutic efficacy of photothermal therapy (PTT). The fused RBC-M hybrid membrane vesicles retained both RBC and MCF-7 cell membrane proteins and the resultant Melanin@RBC-M exhibited prolonged blood circulation and homotypic targeting to source MCF-7 cells simultaneously. Interestingly, increasing MCF-7 membrane components in RBC-M significantly enhanced the homotypic targeting function of Melanin@RBC-M while increasing RBC membrane components in RBC-M effectively reduced the cellular uptake of Melanin@RBC-M by macrophages and improved their circulation time in the blood. After intravenous injection into MCF-7 tumor-bearing athymic nude mice, Melanin@RBC-M with 1:1 membrane protein weight ratio of RBC to MCF-7 exhibited significantly higher tumor accumulation and better PTT efficacy compared with other Melanin@RBC-M with different membrane protein weight ratios as well as pristine melanin nanoparticles, due to the optimal balance between prolonged blood circulation and homotypic targeting. In addition, in vitro photoacoustic results revealed that Melanin@RBC-M had a photoacoustic signal enhancement with the increase of nanoparticle size (64 → 148 nm) and the photoacoustic amplitudes increased linearly with nanoparticle concentration at the excitation wavelength ranged from 680 nm to 800 nm, which could be used for quantification of Melanin@RBC-M in vivo. Looking forward, coating hybrid membrane onto nanoparticles could add flexibility and controllability in enhancing nanoparticles functionality and offer new opportunities for biomedical applications.

Random Multiple Scattering Enhanced Photoacoustic Gas Spectroscopy with Disordered Porous Ceramics.

Light-gas interaction can be enhanced by using disordered porous materials because multiple random scattering increases light intensity near the surface of the material. Here we report signal enhancement of photoacoustic gas spectroscopy with disordered porous ceramics. The amplitude and frequency characteristics of photoacoustic signal due to gas absorption in disordered materials are modeled theoretically. Experiment with a porous Al2O3 sample demonstrates photoacoustic signal enhancement of ∼4 times at 5 kHz.

Aggregation induced photoacoustic detection of mercury (Ⅱ) ions using quaternary ammonium group-capped gold nanorods

Photoacoustic imaging is an emerging detection tool of metal ions. Mercury is a typical heavy metal pollutant that can cause severe water and soil pollution. Therefore, it is particularly meaningful to develop a photoacoustic probe with excellent selectivity and sensitivity for the detection of mercury ions. In this study, gold nanorods modified with (11-mercapto-undecyl)-trimethylammonium (MTA) molecules containing sulfydryl groups were designed and synthesized for the photoacoustic detection of mercury ions (Hg2+). In the present of Hg2+, MTA molecules would be displaced quickly from the surface of gold nanorods due to the high affinity of Hg2+ toward sulfydryl group. Then gold nanorods were aggregated due to the loss of ligand protection on their surface, resulting in the enhancement of photoacoustic signals. In addition, this work showed a good linear relationship between the change of photoacoustic signal at 780 nm and the concentrations of Hg2+ (0–10 µM). More importantly, the devised photoacoustic detection system performs good stability and anti-interference compared to the traditional colorimetric detection system, making it possible to apply to the actual Hg2+-contaminated water samples.

Strong Photoacoustic Signal Enhancement by Coating Gold Nanoparticles with Melanin for Biomedical Imaging

AbstractPhotoacoustics is a powerful biomedical imaging and detection technique, because it is a noninvasive, nonionizing, and low‐cost method facilitating deep tissue penetration. However, suitable contrast agents need to be developed to increase the contrast for in vivo imaging. Gold nanoparticles are often discussed as potential sonophores due to their large absorption cross‐section and their tunable plasmon resonance. However, disadvantages such as toxicity and low photoacoustic efficiency in the tissue transparency window prevail, preventing their clinical application. As a result, there remains a strong need to develop colloidal photoacoustic contrast agents which absorb in the tissue transparency window, exhibit high photoacoustic signal, and are biocompatible. Here, a facile synthetic approach is presented to produce melanin shells around various gold nanoparticle geometries, from spheres to stars and rods. These hybrid particles show excellent dispersability, better biocompatibility, and augmented photoacoustic responses over the pure melanin or pristine gold particles, with a rod‐shape geometry leading to the highest performance. These experimental results are corroborated using numerical calculations and explain the improved photoacoustic performance with a thermal confinement effect. The applicability of melanin coated gold nanorods as gastrointestinal imaging probes in mouse intestine is showcased.

Photoacoustic signal enhancements from gold nano-colloidal suspensions excited by a pair of time-delayed femtosecond pulses.

Photoacoustic signal enhancements were observed with a pair of time-delayed femtosecond pulses upon excitation of gold nanosphere colloidal suspension. A systematic experimental investigation of photoacoustic intensity within the delay time, Δt = 0 to 15 ns, was carried out. The results revealed a significant enhancement factor of ∼2 when the pre-pulse energy is 20-30% of the total energy. Pre-pulse and main pulse energy ratios, Ep(1):Es(2), were varied to determine the optimal ratio that yields to maximum photoacoustic signal enhancement. This enhancement was ascribed to the initial stage of thermalization and bubble generation in the nanosecond time scale. Pre-pulse scattering intensity measurements and numerical finite-difference time-domain calculations were performed to reveal dynamics and light field enchancement, respectively.

Enhanced Photoacoustic Signal of Passion Fruit-Like Nanoarchitectures in a Biological Environment

Photoacoustic imaging (PAI) is a promising noninvasive technique for molecular and cellular characterization of cancer. Cancer detection by PAI is an area of active research, and the recent advent of contrast agents based on near-infrared (NIR)-absorbing dyes or organic/inorganic nanoparticles tremendously improved the specificity and the signal contrast. Here we report the photoacoustic (PA) signal enhancement produced by biodegradable passion fruit-like nanoarchitectures in phantoms and ex vivo. By this approach, the synergistic interaction between commercial NIR-fluorophores and ultrasmall metal nanoparticles is exploited to produce a PA signal enhancement in the biological window. Moreover, the degradation of the nanoarchitectures is investigated in physiological environment as reference matrix.