Photoacoustic imaging of fluorophores using pump-probe excitation

Effect of Molecular Dynamics and Internal Water Contact on the Photophysical Properties of Red pH-Sensitive Proteins

The pH dependence of the absorption and (time-resolved) fluorescence of two red-shifted fluorescent proteins, mCardinal and mNeptune, was investigated. Decay-associated spectra were measured following fluorescence excitation at 470 nm in PBS buffer with a pH that ranged from 5.5 to 8.0. The fluorescence of both proteins shows two different decay components. mCardinal exhibits an increase in the long-lived fluorescence component with acidification from 1.34 ns at pH 8.0 to 1.62 ns at pH 5.5. An additional fast decay component with 0.64 ns at pH 8.0 up to 1.1 ns at pH 5.5 was found to be blue-shifted compared to the long-lived component. The fluorescence lifetime of mNeptune is insensitive to pH. DAS of mCardinal were simulated assuming a coupled two-level system to describe the 1S state of the chromophore within two different conformations of the protein. MD simulations were conducted to correlate the experimentally observed pH-induced change in the lifetime in mCardinal with its molecular properties. While the chromophores of both protein variants are stabilized by the same number of hydrogen bonds, it was found that the chromophore in mCardinal exhibits more water contacts compared to mNeptune. In mCardinal, interaction between the chromophore and Glu-145 is reduced as compared to mNeptune, but interaction with Thr-147 which is Ser-147 in mNeptune is stronger in mCardinal. Therefore, the dynamics of the excited-state proton transfer (ESPT) might be different in mCardinal and mNeptune. The pH dependency of ESPT is suggested as a key mechanism for pH sensitivity.

Characterizing a photoacoustic and fluorescence imaging platform for preclinical murine longitudinal studies

Significance

To effectively study preclinical animal models, medical imaging technology must be developed with a high enough resolution and sensitivity to perform anatomical, functional, and molecular assessments. Photoacoustic (PA) tomography provides high resolution and specificity, and fluorescence (FL) molecular tomography provides high sensitivity; the combination of these imaging modes will enable a wide range of research applications to be studied in small animals.

Aim

We introduce and characterize a dual-modality PA and FL imaging platform using in vivo and phantom experiments.

Approach

The imaging platform's detection limits were characterized through phantom studies that determined the PA spatial resolution, PA sensitivity, optical spatial resolution, and FL sensitivity.

Results

The system characterization yielded a PA spatial resolution of 173 ± 17    μ m in the transverse plane and 640 ± 120    μ m in the longitudinal axis, a PA sensitivity detection limit not less than that of a sample with absorption coefficient μ a = 0.258    cm - 1 , an optical spatial resolution of 70    μ m in the vertical axis and 112    μ m in the horizontal axis, and a FL sensitivity detection limit not < 0.9    μ M concentration of IR-800. The scanned animals displayed in three-dimensional renders showed high-resolution anatomical detail of organs.

Conclusions

The combined PA and FL imaging system has been characterized and has demonstrated its ability to image mice in vivo, proving its suitability for biomedical imaging research applications.

Tuning the Internal Compartmentation of Single-Chain Nanoparticles as Fluorescent Contrast Agents

Controlling the internal structures of single-chain nanoparticles (SCNPs) is an important factor for their targeted chemical design and synthesis, especially in view of nanosized compartments presenting different local environments as a main feature to control functionality. We here design SCNPs bearing near-infrared fluorescent dyes embedded in hydrophobic compartments for use as contrast agents in pump-probe photoacoustic (PA) imaging, displaying improved properties by the location of the dye in the hydrophobic particle core. Compartment formation is controlled via single-chain collapse and subsequent crosslinking of an amphiphilic polymer using external crosslinkers in reaction media of adjustable polarity. Different SCNPs with hydrodynamic diameters of 6-12 nm bearing adjustable label densities are synthesized. It is found that the specific conditions for single-chain collapse have a major impact on the formation of the desired core-shell structure, in turn adjusting the internal nanocompartments together with the formation of excitonic dye couples, which in turn increase their fluorescence lifetime and PA signal generation. SCNPs with the dye molecules accumulate at the core also show a nonlinear PA response as a function of pulse energy-a property that can be exploited as a contrast mechanism in molecular PA tomography.

Arylphosphonate-Tethered Porphyrins: Fluorescence Silencing Speaks a Metal Language in Living Enterocytes*

We report the application of a highly versatile and engineerable novel sensor platform to monitor biologically significant and toxic metal ions in live human Caco-2 enterocytes. The extended conjugation between the fluorescent porphyrin core and metal ions through aromatic phenylphosphonic acid tethers generates a unique turn off and turn on fluorescence and, in addition, shifts in absorption and emission spectra for zinc, cobalt, cadmium and mercury. The reported fluorescent probes p-H8 TPPA and m-H8 TPPA can monitor a wide range of metal ion concentrations via fluorescence titration and also via fluorescence decay curves. Cu- and Zn-induced turn off fluorescence can be differentially reversed by the addition of common chelators. Both p-H8 TPPA and m-H8 TPPA readily pass the mammalian cellular membrane due to their amphipathic character as confirmed by confocal microscopic imaging of living enterocytes.

Synchronously Amplified Photoacoustic Image Recovery (SAPhIRe)

In molecular and cellular photoacoustic imaging with exogenous contrast agents, image contrast is plagued by background resulting from endogenous absorbers in tissue. By using optically modulatable nanoparticles, we develop ultra-sensitive photoacoustic imaging by rejecting endogenous background signals and drastically improving signal contrast through time-delayed pump-probe pulsed laser illumination. Gated by prior pump excitation, modulatable photoacoustic (mPA) signals are recovered from unmodulatable background through simple, real-time image processing to yield background-free photoacoustic signal recovery within tissue mimicking phantoms and from ex-vivo tissues. Inherently multimodal, the fluorescence and mPA sensitivity improvements demonstrate the promise of Synchronously Amplified Photoacoustic Image Recovery (SAPhIRe) for PA imaging in diagnosis and therapy.

Spontaneous loss versus stimulation gain in pump-probe microscopy: a proof of concept demonstration

SIGNIFICANCE

The large background, narrow dynamic range, and detector saturation have been the common limiting factors in stimulated emission (SE)-based pump-probe microscopy, attributed to the very small signal overriding the very intense laser probe beam. To better differentiate the signal of interest from the background, lock-in detection is used to measure the fluorescence quenching, which is termed spontaneous loss (SL). The advantages are manifold. The spontaneous fluorescence signal can be well separated from both the pump and the probe beams with filters, thus eliminating the background, enlarging the dynamic range, and avoiding the saturation of the detector.

AIM

We propose and demonstrate an integrated pump-probe microscopy technique based on lock-in detection for background removal and dynamic range enhancement through SL detection.

APPROACH

The experimental setup is configured with a pulsed diode laser at a wavelength λpu  =  635  nm, acting as a pump (excitation) and a mode-locked Ti:sapphire laser at a central wavelength λpr  =  780  nm, serving as the probe beam (stimulation). Both pulse trains are temporally synchronized through high precision delay control by adjusting the length of the triggering cables. The pump and probe beams are alternatively modulated at different frequencies f1 and f2 to extract the stimulated gain (SG) and SL signal.

RESULTS

SG signal shows saturation due to the irradiation of the intense probe beam onto the photodetector. However, the detector saturation does not occur at high probe beam power for SL detection. The fluorescence lifetime images are acquired with reduced background. The theoretical signal-to-noise ratios for SG and SL are also estimated by photon statistics.

CONCLUSION

We have confirmed that the detection of SL allows the elimination of the large background without photodetector saturation, which commonly exists in SG configuration. This modality would allow unprecedented manipulation and investigation of fluorophores in fluorescence imaging.

Technology Advancements in Blood Coagulation Measurements for Point-of-Care Diagnostic Testing

In recent years, blood coagulation monitoring has become crucial to diagnosing causes of hemorrhages, developing anticoagulant drugs, assessing bleeding risk in extensive surgery procedures and dialysis, and investigating the efficacy of hemostatic therapies. In this regard, advanced technologies such as microfluidics, fluorescent microscopy, electrochemical sensing, photoacoustic detection, and micro/nano electromechanical systems (MEMS/NEMS) have been employed to develop highly accurate, robust, and cost-effective point of care (POC) devices. These devices measure electrochemical, optical, and mechanical parameters of clotting blood. Which can be correlated to light transmission/scattering, electrical impedance, and viscoelastic properties. In this regard, this paper discusses the working principles of blood coagulation monitoring, physical and sensing parameters in different technologies. In addition, we discussed the recent progress in developing nanomaterials for blood coagulation detection and treatments which opens up new area of controlling and monitoring of coagulation at the same time in the future. Moreover, commercial products, future trends/challenges in blood coagulation monitoring including novel anticoagulant therapies, multiplexed sensing platforms, and the application of artificial intelligence in diagnosis and monitoring have been included.

Bioengineered bacterial vesicles as biological nano-heaters for optoacoustic imaging

Advances in genetic engineering have enabled the use of bacterial outer membrane vesicles (OMVs) to deliver vaccines, drugs and immunotherapy agents, as a strategy to circumvent biocompatibility and large-scale production issues associated with synthetic nanomaterials. We investigate bioengineered OMVs for contrast enhancement in optoacoustic (photoacoustic) imaging. We produce OMVs encapsulating biopolymer-melanin (OMVMel) using a bacterial strain expressing a tyrosinase transgene. Our results show that upon near-infrared light irradiation, OMVMel generates strong optoacoustic signals appropriate for imaging applications. In addition, we show that OMVMel builds up intense heat from the absorbed laser energy and mediates photothermal effects both in vitro and in vivo. Using multispectral optoacoustic tomography, we noninvasively monitor the spatio-temporal, tumour-associated OMVMel distribution in vivo. This work points to the use of bioengineered vesicles as potent alternatives to synthetic particles more commonly employed for optoacoustic imaging, with the potential to enable both image enhancement and photothermal applications.

Fluorescence quantum yield and excited state lifetime determination by phase sensitive photoacoustics: concept and theory

In this Letter, we theoretically describe photoacoustic signal generation of molecules, for which triplet relaxation can be neglected, by considering the excited state lifetime, the fluorescence quantum yield, and the fast vibrational relaxation. We show that the phase response of the photoacoustic signal can be exploited to determine the excited state lifetime of dark molecules. For fluorescent molecules, the phase response can be used to determine the fluorescence quantum yield directly without the need of reference samples.

Advanced optoacoustic methods for multiscale imaging of in vivo dynamics

Visualization of dynamic functional and molecular events in an unperturbed in vivo environment is essential for understanding the complex biology of living organisms and of disease state and progression. To this end, optoacoustic (photoacoustic) sensing and imaging have demonstrated the exclusive capacity to maintain excellent optical contrast and high resolution in deep-tissue observations, far beyond the penetration limits of modern microscopy. Yet, the time domain is paramount for the observation and study of complex biological interactions that may be invisible in single snapshots of living systems. This review focuses on the recent advances in optoacoustic imaging assisted by smart molecular labeling and dynamic contrast enhancement approaches that enable new types of multiscale dynamic observations not attainable with other bio-imaging modalities. A wealth of investigated new research topics and clinical applications is further discussed, including imaging of large-scale brain activity patterns, volumetric visualization of moving organs and contrast agent kinetics, molecular imaging using targeted and genetically expressed labels, as well as three-dimensional handheld diagnostics of human subjects.

Photoacoustic pump-probe tomography of fluorophores in vivo using interleaved image acquisition for motion suppression

In fluorophores, the excited state lifetime can be modulated using pump-probe excitation. By generating photoacoustic (PA) signals using simultaneous and time-delayed pump and probe excitation pulses at fluences below the maximum permissible exposure, a modulation of the signal amplitude is observed in fluorophores but not in endogenous chromophores. This provides a highly specific contrast mechanism that can be used to recover the location of the fluorophore using difference imaging. The practical challenges in applying this method to in vivo PA tomography include the typically low concentrations of fluorescent contrast agents, and tissue motion. The former results in smaller PA signal amplitudes compared to those measured in blood, while the latter gives rise to difference image artefacts that compromise the unambiguous and potentially noise-limited detection of fluorescent contrast agents. To address this limitation, a method based on interleaved pump-probe image acquisition was developed. It relies on fast switching between simultaneous and time-delayed pump-probe excitation to acquire PA difference signals in quick succession, and to minimise the effects of tissue motion. The feasibility of this method is demonstrated in tissue phantoms and in initial experiments in vivo.

Characterisation of a phantom for multiwavelength quantitative photoacoustic imaging

Quantitative photoacoustic imaging (qPAI) has the potential to provide high- resolution in vivo images of chromophore concentration, which may be indicative of tissue function and pathology. Many strategies have been proposed recently for extracting quantitative information, but many have not been experimentally verified. Experimental phantom-based validation studies can be used to test the robustness and accuracy of such algorithms in order to ensure reliable in vivo application is possible. The phantoms used in such studies must have well-characterised optical and acoustic properties similar to tissue, and be versatile and stable. Polyvinyl chloride plastisol (PVCP) has been suggested as a phantom for quality control and system evaluation. By characterising its multiwavelength optical properties, broadband acoustic properties and thermoelastic behaviour, this paper examines its potential as a phantom for qPAI studies too. PVCP's acoustic properties were assessed for various formulations, as well as its intrinsic optical absorption, and scattering with added TiO2, over a range of wavelengths from 400-2000 nm. To change the absorption coefficient, pigment-based chromophores that are stable during the phantom fabrication process, were used. These yielded unique spectra analogous to tissue chromophores and linear with concentration. At the high peak powers typically used in photoacoustic imaging, nonlinear optical absorption was observed. The Grüneisen parameter was measured to be [Formula: see text]  =  1.01  ±  0.05, larger than typically found in tissue, though useful for increased PA signal. Single and multiwavelength 3D PA imaging of various fabricated PVCP phantoms were demonstrated.

Emerging concepts in functional and molecular photoacoustic imaging

Providing the specific imaging contrast of optical absorption and excellent spatial scalability across the optical and ultrasonic dimensions, photoacoustic imaging has been rapidly emerging and expanding in the past two decades. In this review, I focus on a few latest advances in this enabling technology that hold the potential to transform in vivo functional and molecular imaging at multiple length scales. Specifically, multi-parametric photoacoustic microscopy enables simultaneous high-resolution mapping of hemoglobin concentration, oxygen saturation and blood flow-opening up the possibility of quantifying the metabolic rate of oxygen at the microscopic level. The pump-probe approach harnesses a variety of photoinduced transient optical absorption as novel contrast mechanisms for high-specificity molecular imaging at depth and as nonlinear excitation strategies for high-resolution volumetric microscopy beyond the conventional limit. Novel magneto-optical and photochromic probes lead to contrast-enhanced molecular photoacoustic imaging through differential detection.