Abstract Photoacoustic tomography as an optical-ultrasound hybrid imaging modality provides rich optical contrast over the extended penetration depth of biological tissues, enabling multiscale multicontrast structural and functional imaging. However, inherent limitations in the state-of-the-art piezoelectric transducer arrays of the photoacoustic tomography, including size-dependent sensitivity, narrow bandwidth, and high material rigidity, compromise the resolution, penetration depth, and functional assessment precision. Here, an arc-shaped fiber ultrasound transducer array with a sheet-like ultrasound focus is demonstrated for photoacoustic computed tomography. At the ultrasound focus, a low detection limit of ~ 5.2 Pa and a dual-frequency response spanning several octaves are achieved. Whole mouse brain imaging with a depth up to ~ 1.2 cm and a spatial resolution of ~ 70 μm in the cerebral cortex region is showcased. The blood oxygen saturation within the entire mouse brain and the brain tumors is visualized, and the assessment precision is improved by leveraging the dual-frequency response of the transducer array. The centimeter-scale imaging depth, fine resolution of the cerebral vessels, and improved precision in the blood oxygenation evaluation make the fiber-array photoacoustic tomography a competitive candidate to the sought-after magnetic resonance imaging and ultrasound localization microscopy for brain functionality study and disease diagnosis.

In situ and continuous monitoring of the gas concentration can provide valuable insights into the cell viability and allow accurate concentration control of signaling gas molecule for disease therapy. However, current techniques are mostly benchtop instrument for off-line dissolved gas analysis or require the cell labelling disturbing the cell normal activities. To achieve in-situ, fast and sensitive dissolved gas detection, a miniature fiber photoacoustic spectrometer integrating the gas cell, gas-liquid separator and analyzer into a single micro-gas-cavity at the fiber tip is developed here. The gas-liquid interface directly separates at the gas from the liquid and simultaneously acts as an acoustic sensitive element that detects the locally-generated photoacoustic waves from excited gas molecules in the micro-gas-cavity. The flexible gas-liquid interface forms a Fabry-Perot cavity with the fiber tip end fact and reads out the acoustic wave induced deformation with high-resolution fiber-optic interferometry. A noise-equivalent pressure level of similar to 270 mu Pa/Hz(1/2) is achieved, which allows sensitive dissolved acetylene with a concentration down to 500 ppb and a response time similar to 10 s. Temporal evolutions of the carbon dioxide concentration for the normal and cancer cells at different glucose levels and the algae cells at different photosynthesis conditions are recorded in situ and continuously. The proposed photoacoustic spectrometer with microscale size, high-sensitivity, and fast response provides an all-optical technique for in-situ and continuous dissolved gas monitoring in diverse areas ranging from biological science to medical treatment.

Optical fiber acoustic sensors with high sensitivity and compact footprints have advanced the development of miniaturized photoacoustic spectroscopy, rendering them attractive for in situ and real-time trace gas detection. However, the limited fabrication accuracy and scalability prevent the rational design of miniature fiber photoacoustic spectroscopy for multi-gas sensing at the sub-ppm detection limit. In this study, fiber-tip Fabry-P & eacute;rot microcavities with ultrathin and scalable optomechanical membranes are prepared using high-precision direct laser writing for miniaturized multigas photoacoustic spectroscopy. This fabrication technique facilitates the flexible and precise preparation of trampoline-shaped submicron-thick membranes. By adjusting the size of the membranes, their size-dependent resonant frequencies could be tuned for multi-gas photoacoustic sensing based on a frequency-multiplexing scheme. As a proof of concept, a photoacoustic microprobe (similar to 300 mu m in diameter) is assembled from three microcavities for simultaneous detection of a gas mixture comprising acetylene (C2H2), carbon dioxide (CO2), and water vapor (H2O). For C2H2, a high detection sensitivity of 258 ppb and a short response time of similar to 28 ms are achieved by amplifying the photoacoustic signals by leveraging the mechanical resonance of the membrane. This compact fiber-tip structure with the capability of in situ and fast multicomponent gas detection is promising for broad applications such as explosive gas monitoring and medical diagnosis.

Significance Biomedical detection holds irreplaceable importance in the medical field,providing robust support for disease diagnosis and treatment,driving progress in medical research,and enhancing public health standards,Compared to conventional blood tests,which can reveal signs of infection,inflammation,or metabolic abnormalities,laser spectroscopies based on Raman,direct absorption,and photoacoustic effects have garnered widespread attention for biomedical detection owing to their high sensitivity,specificity,and speed,and their label-free nature.In particular,photoacoustic spectroscopy relies on the detection of optically excited acoustic waves,a hybrid integration of light and sound.The signal strength depends on the amount of light energy absorbed,making it less influenced by reflected or scattered light.Consequently,background signal interference is precluded,which is suitable for detecting samples in solid,liquid,and gaseous forms.This characteristic,combined with the low scattering of acoustic waves compared to light,enables the detection of biochemical molecules with weak absorption or biological tissues with strong scattering given that the signal-to-noise ratio of photoacoustic spectroscopy can be easily enhanced by increasing the pump light power.From a technical perspective,photoacoustic spectroscopy typically uses microphones to detect acoustic signals rather than directly measuring changes in light intensity.This enables the use of low-cost,easy-to-maintain microphones to detect signals excited by light across the visible to mid-infrared range,eliminating the need for expensive mid-infrared photodetectors that require cooling devices.From an application perspective,photoacoustic signals are generated by the non-radiative relaxation process of excited molecules transitioning to lower energy states.This complements traditional stimulated emission and photochemical processes,making it useful for studying fluorescence,photochemical phenomena,and quantum efficiency of relaxation and radiation processes.These advantages make photoacoustic spectroscopy attractive to biomedical detection for biological research and clinical diagnosis. Although photoacoustic spectroscopy technology has been developed since the 1970s with commercial instruments and equipment already available on the market,its primary applications have been in environmental detection,industrial process control,energy development,and other fields.In recent years,research in the biomedical detection areas such as blood glucose and oxygen detection,disease biomarker detection,and respiratory gas analysis has grown rapidly.Therefore,summarizing existing research on photoacoustic spectroscopy techniques relevant to biomedical detection is both important and necessary for guiding future developments in this field toward practical clinical applications. Progress First,the basic working principle of photoacoustic spectroscopy is introduced,and the photoacoustic equations for biomedical detection in both aqueous and gaseous environments are derived.These equations serve as a guide for designing photoacoustic spectroscopy systems for various targets.Models describing the photoacoustic signals in both non-resonant and resonant gas cells are also provided.Next,the core components of the photoacoustic system,including the pump light source and acoustic detector,are reviewed.In particular,the spectrum from ultraviolet to mid-infrared covered by different types of lasers is summarized,along with the absorption fingerprints of various biomedical molecules,offering intuitive guidance for system construction.This is followed by an introduction to acoustic detectors,including electrical and optical types,which may vary in sensitivity,operating frequency,and bandwidth.Promising techniques for achieving high sensitivity are also discussed,including pump power enhancement through a multi-pass gas cell or a high-quality-factor resonant cavity,and acoustic wave amplification using rationally designed resonant photoacoustic cells.Subsequently,the applications of photoacoustic spectroscopy for detecting biomolecules and trace gases are reviewed,with emphasis on blood glucose detection and breath gas analysis.Other biomolecules such as hemoglobin,uric acid,DNA,and percutaneously released carbon dioxide/oxygen gases are also included.The current status and challenges faced by photoacoustic spectroscopy in detecting blood glucose and oxygen,biological disease markers,and breathing/releasing gases are also discussed.Finally,recent research efforts aimed at improving signal stability,increasing response speed,and reducing device footprint are introduced.The prospects and suggested future directions for advancing photoacoustic spectroscopy in biomedical applications are also outlined. Conclusions and Prospects Photoacoustic spectroscopy,with its advantages of high sensitivity,large penetration depth,low background noise,and compact structure,is suitable for detecting samples in solid,liquid,and gas forms.It holds significant potential for the detection of biochemical components,including blood glucose and oxygen,lipids,biological disease markers,pathogens and human exhaled/released gas.With the continuous progress in key performance parameters such as sensitivity,response speed,size,and cost,along with rapid developments in laser technology,acoustic detection technology,and artificial intelligence algorithms,photoacoustic spectroscopy is expected to be a promising solution in the biomedical field.Potential applications include home health monitoring,bedside point-of-care diagnostics,and early disease screening.

Drug-loaded hydrogels with excellent adhesion, moisture, and biocompatibility are promising wound dressing and particularly appealing for chronic wound therapy due to their controllable and sustained drug release. However, the treatment effect of the hydrogel dressing is regularly assessed by ex vivo histology, hampering in vivo longitudinal monitoring of neovascularization for precise therapeutic efficiency evaluation. Herein, optical-resolution photoacoustic microscopy (OR-PAM) is adopted for longitudinal microvasculature imaging of the murine ear wound treated with a redox-responsive S-nitrosoglutathione-loaded guanosine-borate hydrogel. The hydrogel engineered via borate ester-mediated guanosine tetramerization features sustained nitric oxide release for more than 100 h. High-resolution OR-PAM images reveal vessel remodeling including initial vessel pruning, endothelial sprouting, and hierarchical branching over a two-week period. The treated wounds exhibit increased change rates of 63% in the vascular area density and 49% in the total vessel length compared with the control group, due to the high biocompatibility, resistance to the skin tensile strain, and sustained drug release of the hydrogel dressing. Integration of the responsive drug-releasing hydrogel dressing with the high-resolution OR-PAM for longitudinal feedback of the treatment outcome provides a promising theranostic platform to develop hydrogel dressing for chronic wound therapy such as diabetic ulcers.

We report a single fiber-tip photoacoustic spectroscopy probe prepared by direct laser writing technology. The probe demonstrates ppb-level gas detection and multi-gas detection of acetylene (C2H2), carbon dioxide (CO2), and water vapor (H2O) based on frequency multiplexing.

Photoacoustic imaging with large penetration depth and high spatial resolution requires broadband detection of ultrasound from biological tissues. However, conventional piezoelectrical ultrasound transducers typically have limited bandwidth less than one octave band of the central frequency, making it challenging to retrieve both the low and high frequency signals. Here, a dual-band fiber ultrasound transducer array is developed for photoacoustic computed tomography (PACT) system to simultaneously measure both low-frequency signals (similar to 2.5 MHz) from deep tissues and high- frequency signals (similar to 20 MHz) from fine structures of the mouse brain. The dual-band frequency response is realized by controllable ultrasound coupling between the silica fiber and the polymer coating layer. An arc-shape PACT array system with 150 degrees angular coverage is constructed by eight dual-band fiber ultrasound transducers and applied for whole-brain imaging of mice with depths up to similar to 1 cm. This system is further utilized to visualize oxygen saturation of hemoglobin within the entire mouse brain and glioblastoma-grown brain regions, demonstrating its potential for brain functionality study and disease diagnosis.

Noninvasive high-resolution deep-brain imaging is essential to fundamental cognitive process study and neuroprotective drugs development. Although optical microscopes can resolve fine biological structures with good contrast without exposure to ionizing radiation or a strong magnetic field, the optical scattering limits the penetration depth and hinders its capability for deep-brain imaging. Here, in vivo high-resolution imaging of the whole mouse brain is demonstrated by using a photoacoustic computed tomography system with a negatively focused fiber-laser ultrasound transducer. By leveraging the high flexibility and low bending loss of the optical fiber, a rationally designed negatively focused fiber laser cavity exhibits a low detection limit down to 5.4 Pa and a broad view angle of similar to 120 deg, enabling mouse brain imaging with a penetration larger than 7 mm and a nearly isotropic spatial resolution of similar to 130 mu m . In addition, the negative curvature of the fiber laser reduces the working distance, which facilitates the development of a compact and portable linear scanning imaging system. In vivo imaging of a mouse model with intracerebral hemorrhage is also showcased to demonstrate its capability for potential biomedical and clinical applications. With high spatial resolution and large tissue penetration, the system may provide a noninvasive, user-friendly, and high-performance imaging solution for biomedical research and preclinical/clinical diagnosis. (c) 2024 Chinese Laser Press

With high atomic utilization and remarkable catalytic activity, Cu-N-C type catalysts display great potential for electro-catalysis in CO2 reduction. However, the relationship between the active moiety and catalytic activity of generating high-value C2 products is still unclear, and the explicit screening criteria is scarcity. Herein, based on the first-principle simulation, the structure-performance relationship on Cu-N-C type catalysts has been investigated by modulating the CO2 reduction process as the number of Cu atom (Cu1, Cu2, Cu3) and the ligand environment (B, C, N, O, P, S) changed. We find the adsorption strength of intermediate *CO strongly affect the possibility of C-C coupling, which can be determined by Bader charge on Cu atom, mainly depending on the number of loaded atomic Cu on Cu-N-C catalysts. Furthermore, the Bader charge can be refined by adjusting the coordination atom of Cu, thus optimizing catalytic activity for the CO2 to ethanol. The moderate Bader charge value, between +0.35 and +0.45, enables the catalyst to behave as a potentially excellent activity with low limiting potential for generating ethanol. More importantly, an intrinsic descriptor, composed of the radius, electronegativity, and number of valence electrons of coordination atoms (φ=∑χ∑r⁎∑np), was established to characterize the catalytic activity of Cu-N(X)-C catalysts for producing ethanol. Two excellent catalysts, Cu3-N2O2 (-0.51V) and Cu3-N3S (-0.64V), are screened out for the CO2RR to generate ethanol. This work discloses theoretical basis for catalytic selectivity of C2 products on Cu-N-C catalysts and provides a regulating and screening principle for high performance catalysts to ethanol.

Hydrogen (H-2) is a renewable energy gas and an important industrial raw material playing important roles in many fields. However, H-2 is colorless, odorless and highly flammable within a wide concentration range. So H-2 detection is very important for leakage monitoring especially at low concentration. Here we propose an ultrahigh sensitivity H-2 sensor with a perforated palladium (Pd) film on the tip of an optical fiber. The suspended Pd nanohole film forms a Fabry-P e'rot (FP) interferometer with a silica capillary fused onto the fiber. Upon H2 adsorption, the Pd nanohole film bends inwards leading to spectral shift of the FP cavity. With the film perforated, its Young's modulus becomes effectively smaller giving rise to larger spectral shift and higher H-2 sensitivity. We experimentally studied the effect of the structural parameters of the holey film on the H-2 sensing performance and achieved a H-2 detection sensitivity of 7.1 pm/ppm and a detection limit of 1.7 ppm. The sensor also shows good repeatability and gas selectivity and is expected to find applications as optical H-2 sensors working at low concentrations.

Optical microscopy is indispensable to biomedical research and clinical investigations. As all molecules absorb light, optical-resolution photoacoustic microscopy (PAM) is an important tool to image molecules at high resolution without labeling. However, due to tissue-induced optical aberration, the imaging quality degrades with increasing imaging depth. To mitigate this effect, we develop an imaging method, called acoustic-feedback wavefront-adapted PAM (AWA-PAM), to dynamically compensate for tissue-induced aberration at depths. In contrast to most existing adaptive optics assisted optical microscopy, AWA-PAM employs acoustic signals rather than optical signals to indirectly determine the optimized wavefront. To demonstrate this technique, we imaged zebrafish embryos and mouse ears in vivo. Experimental results show that compensating for tissue-induced aberration in live tissue effectively improves both signal strength and lateral resolution. With this capability, AWA-PAM reveals fine structures, such as spinal cords and microvessels, that were otherwise unidentifiable using conventional PAM. We anticipate that AWA-PAM will benefit the in vivo imaging community and become an important tool for label-free optical imaging in the quasi-ballistic regime.

Miniaturized laser spectroscopy capable of in situ and real-time ppb-level trace gas sensing is of fundamental importance for numerous applications, including environment monitoring, industry process control, and biomedical diagnosis. Benchtop laser spectroscopy systems based on direct absorption, photoacoustic, and Raman effects exhibit high sensitivity but face challenges for in situ and real-time gas sensing due to their bulky size, slow response, and offline sampling. We demonstrate a microscale high-performance all-fiber photoacoustic spectrometer integrating the key components, i.e., the photoacoustic gas cell and the optical microphone, into a single optical fiber tip with a diameter of 125 mu m. Without a long optical path to enhance the light-gas interaction, the fiber-tip gas cell with acoustic-hard boundary tightly confines and amplifies the local photoacoustic wave, compensating for the sensitivity loss during miniaturization. This localized acoustic wave is demodulated by high-sensitivity fiber-optic interferometry, enabling a similar to 9 ppb detection limit for acetylene gas approaching the benchtop system. The microscale fiber spectrometer also exhibits a short response time of similar to 18 ms and a subnanoliter sample volume, not only suitable for routine real-time in situ trace gas measurement but also inspiring new applications such as two-dimensional gas flow concentration mapping and in vivo intravascular blood gas monitoring as showcased.

SignificancePhotoacoustic (PA) imaging is an emerging biomedical imaging modality that can map optical absorption contrast in biological tissues by detecting ultrasound signal. Piezoelectric transducers are commonly used in PA imaging to detect the ultrasound signals. However, piezoelectric transducers suffer from low sensitivity when the dimensions are reduced and are easily influenced by electromagnetic interference. To avoid these limitations, various optical ultrasound sensors have been developed and shown their great potential in PA imaging.AimOur study aims to summarize recent progress in optical ultrasound sensor technologies and their applications in PA imaging.ApproachThe commonly used optical ultrasound sensing techniques and their applications in PA systems are reviewed. The technical advances of different optical ultrasound sensors are summarized.ResultsOptical ultrasound sensors can provide wide bandwidth and improved sensitivity with miniatured size, which enables their applications in PA imaging.ConclusionsThe optical ultrasound sensors are promising transducers in PA imaging to provide higher-resolution images and can be used in new applications with their unique advantages.

Dopamine (DA) is a biomarker of many psychiatric diseases, such as schizophrenia and Parkinson's disease. Most current DA detection research adopts electrochemical method, which suffers interferences caused by many analogs with similar structures or redox potentials. To realize highly sensitive and selective DA sensing, herein we propose an optofluidic-laser-based DA sensor. Via the competitive adsorption mechanism, rhodamine 6G (Rh6G) is fixed on graphene oxide (GO) and replaced by DA in the sample, with the re-excited Rh6G serving as the gain medium of the optofluidic laser. As the DA concentration increases, more Rh6G is released into the sample, which induces an increase in the relative slope efficiency of the optofluidic laser and a reduction in the laser threshold. Both the relative slope efficiency and threshold attain a suitable linear relationship with the DA concentration within the 5-800 & mu;M range. Under ideal circumstances, the detection limit of the biosensor is 0.3 & mu;M. Moreover, the optofluidic laser-based biosensor achieves favorable selectivity for DA detection, proving the advantages of using optical laser sensors for DA detection in complex samples.

Optical fiber ultrasonic transducers have exhibited excellent performance in medical diagnosis, material characterization, and structural health monitoring. The functional integration of transmission and reception of ultrasound through the side walls of optical fibers has been little studied and the corresponding devices have barely been explored. Here, we demonstrate a novel approach based on side-viewing ultrasonic transceiver fabricated in single-mode optical fiber. The transceiver consists of a tilted fiber Bragg grating (TFBG) whose cladding is coated with a composite functionalized carbon nanotube layer. The abundance of cladding modes in optical fibers enables the generation of ultrasound signals using any cladding mode whose wavelength matches the wavelength of light injected from a nanosecond pulse laser. We describe a core mode-based ultrasonic detection function that was able to detect a 5-MHz ultrasound signal with a signal-to-noise ratio (SNR) of similar to 51 dB. Using this device, we performed a side-viewing ultrasound transceiving experiment to successfully capture the variation in pulse-echo signals with target distance for the first time. This type of fiber-optic ultrasound transducer exhibits higher mechanical strength than other fiber-optic ultrasound transducers, making it a potential tool for quasi-distributed nondestructive monitoring in confined spaces and extreme environments.

Optical fiber acoustic sensors with miniature size and high sensitivity are attractive to develop compact photoacoustic spectroscopy. Here, a compact photoacoustic gas sensor was demonstrated by utilizing a diaphragm-based fiber-optic Fabry-Perot cavity as both the acoustic sensor and the multipass cell. A nanoscale graphite film was used as the flexible diaphragm to increase the acoustic sensitivity of the Fabry-Perot cavity and the cavity inner surface was coated with highly-reflective Au film to form a multipass cell for amplification of the photoacoustic signal. With a laser power of 20 mW at 1532.8 nm, the sensor demonstrated a low detection limit of ∼ 50 ppb for C2H2 gas with an integration time of ∼ 100 s. The optical fiber photoacoustic gas sensor with a millimeter-scale diameter and ppb-level detection limit is promising for trace gas sensing in various areas including industrial process and environmental monitoring.

Hydrogen (H2) sensors are critical to various applications such as the situation where H2 is used as the clean energy for industry or the indicator for human disease diagnosis. Palladium (Pd) is widely used as the hydrogen sensing material in different types of sensors. Optical fiber H2 sensors are particularly promising due to their compactness and spark-free operation. Here, we report a Fabry–Pérot (FP)-cavity-based H2 sensor that is formed with a freestanding Pd membrane and integrated on a conventional single-mode optical fiber end. The freestanding Pd membrane acts both as the active hydrogen sensing material and as one of the reflective mirrors of the cavity. When the Pd film absorbs H2 to form PdHx, it will be stretched, resulting in a change of the cavity length and thus a shift of the interference spectrum. The H2 concentration can be derived from the amplitude of the wavelength shift. Experimental results showed that H2 sensors based on suspended Pd membranes can achieve a detection sensitivity of about 3.6 pm/ppm and a detection limit of about 3.3 ppm. This highly sensitive detection scheme is expected to find applications for sensing low-concentration H2.

Photoacoustic tomography emerged as a promising tool for noninvasive biomedical imaging and diseases diagnosis. However, most of the current piezoelectric ultrasound transducers suffer optical opacity and tissue-mismatched acoustic impedance, hindering the miniaturization and integration of the system for multiscale and multimodal imaging. Here, a transparent polydimethylsiloxane (PDMS) encapsulated optical microfiber ultrasound sensor was demonstrated for photoacoustic imaging with scalable spatial resolution and penetration depth. The sensor comprised a microfiber loop sandwiched by a pair of in-line Bragg gratings, which formed an ultrasound-sensitive Fabry-Perot cavity allowing free delivery of ultrasound/light beams and unique needle-shaped ultrasound focusing along the penetration depth. The sensor with a detection limit of ∼ 700 Pa and a bandwidth of ∼ 10 MHz was applied for multiscale photoacoustic imaging of mouse ear and brain vasculatures. With advantages of flexibility, optical transparence and focusing capability, the sensor offers new opportunities for developing photoacoustic/ultrasound imaging devices for biomedical and clinic applications.

Capacitive deionization in environmental decontamination has been widely studied and now requires intensive development to support large-scale deployment. Porous nanomaterials have been demonstrated to play pivotal roles in determining decontamination efficiency and manipulating nanomaterials to form functional architecture has been one of the most exciting challenges. Such nanostructure engineering and environmental applications highlight the importance of observing, recording, and studying basically electrical-assisted charge/ion/particle adsorption and assembly behaviors localized at charged interfaces. In addition, it is generally desirable to increase the sorption capacity and reduce the energy cost, which increase the requirement for recording collective dynamic and performance properties that stem from nanoscale deionization dynamics. Herein, we show how a single optical fiber can serve as an in situ and multifunctional opto-electrochemical platform for addressing these issues. The surface plasmon resonance signals allow the in situ spectral observation of nanoscale dynamic behaviors at the electrode-electrolyte interface. The parallel and complementary optical-electrical sensing signals enable the single probe but multifunctional recording of electrokinetic phenomena and electrosorption processes. As a proof of concept, we experimentally decipher the interfacial adsorption and assembly behaviors of anisotropic metal-organic framework nanoparticles at a charged surface and decouple the interfacial capacitive deionization within an assembled metal-organic framework nanocoating by visualizing its dynamic and energy consumption properties, including the adsorptive capacity, removal efficiency, kinetic properties, charge, specific energy consumption, and charge efficiency. This simple "all-in-fiber" opto-electrochemical platform offers intriguing opportunities to provide in situ and multidimensional insights into interfacial adsorption, assembly, and deionization dynamics information, which may contribute to understanding the underlying assembly rules and the exploring structure-deionization performance correlations for the development of tailor-made nanohybrid electrode coatings for deionization applications.