Design and Synthesis of an Activatable Photoacoustic Probe for Hypochlorous Acid

Hypochlorous Acid-Activatable NIR Fluorescence/Photoacoustic Dual-Modal Probe with High Signal-to-Background Ratios for Imaging of Liver Injury and Plasma Diagnosis of Sepsis

Hypochlorous acid can be employed as a biomarker for blood infection (such as sepsis) and tissue damage (such as drug-induced liver injury, DILI), and the diagnosis of tissue damage or blood infection can be achieved through the detection of hypochlorous acid in relevant biological samples. Considering the complex environment and the diverse interferences in living organisms and blood plasma, developing new detection methods for HClO with high signal-to-background ratios is particularly important, and it can improve the accuracy of detection and quality of imaging based on a higher contrast, which makes the detection of HClO clearer and more accurate. Here, based on the advantages of the NIR fluorescence/photoacoustic dual-modal probe, we reported a hypochlorous acid-activatable NIR fluorescence/photoacoustic dual-modal probe (NIRF-PA-HClO) based on the spirolactam ring-opening strategy in this paper. NIRF-PA-HClO showed excellent NIRF/PA dual-modal responses with high SBRs for HClO in solution, cells, and mice. Moreover, NIRF-PA-HClO was successfully applied for high-contrast imaging of DILI. Finally, NIRF-PA-HClO was employed for the blood plasma diagnosis of sepsis with satisfactory results. In summary, the above results proved that NIRF-PA-HClO would be a potentially useful tool for the study of physiological and pathological roles of HClO, the investigation of the pathology and therapeutic mechanisms of hepatotoxicity, and the diagnosis of blood infection. Also, the development of NIRF-PA-HClO provides new design mentality for constructing other analyte-activatable NIRF/PA dual-modal probes with high SBRs.

Development of Novel Fluorescent Probes for Specific Detection of Hypochlorous Acid

Hypochlorous acid (HClO) is widely used in everyday life for bleaching and disinfecting tap water, and also in human metabolism, where it plays an important role in destroying foreign bacterial invaders and pathogens as well as immune defense and cellular functioning maintenance. Abnormal levels of hypochlorous acid have the potential to cause joint inflammation, neuronal degeneration, and even life-threatening cancer. Specific identification and effective detection of hypochlorous acid are important for monitoring human health and the environment. In recent years, organic fluorescent probes have attracted much attention because of their simple synthesis, easy operation, high sensitivity, and high specificity, and a variety of hypochlorous acid fluorescent probes based on low-cost, easy-to-operate, and rapid identification have been developed. In this paper, we review the fluorescent probes that have been developed in the past five years for the specific recognition of hypochlorous acid based on different fluorophores, such as triphenylamine, coumarin, 1,8-naphthalize, etc., as well as recognition units, such as N-N dimethyl thiosemicarbazone, and describe how the probes and hypochlorous acid interact for identification in the same manner as other fluorescent probes. In addition, the reaction mechanism between the probe and hypochlorous acid, the fluorescence change of the probe, and the detection limit are described to illustrate the progress in the detection of hypochlorous acid in recent years and to provide ideas for the development of hypochlorous acid fluorescent probes in the future.

Electrochemiluminescence enhanced by molecular engineering linear π-conjugated polymer: An ingenious ECL emitter for the construction of exosome sensing platform

Linear π-conjugated polymers (LCPs) with π-electron conjugation system have many remarkable optical characteristics such as fluorescence and electrochemiluminescence (ECL). However, the extremely strong interchain interaction and π-π stacking limit the luminescence efficiency. In this work, 1H-1,2,4-triazole-3,5-diamine was chosen as the polymer monomer and reacted with terephthalaldehyde via simple Schiff base condensation to synthesize LCPs. Subsequently, molecular engineering strategy was adopted to construct zirconium-based LCPs (MLCPs), which not only prevented π-π stacking but also ensured that extended π-coupling was maintained in the LCPs, thus effectively promoting charge transport and achieving strong luminescence. Second, the coreactant polyethyleneimine (PEI) was assembled onto the MLCPs (MLCPs@PEI) to further promote the emission of ECL. To further explore the potential of the obtained MLCPs@PEI as emerging ECL emitter, colorectal cancer exosome was chosen as model biomarker, and an innovative ECL ratiometric system based on MLCPs@PEI and luminol was designed to improve the validity and accuracy of the sensors. This research provides a fresh nanoplatform for exosome detection and broadens the application of LCPs in ECL immunoassay.

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Chemigenetic Far-Red Labels and Ca2+ Indicators Optimized for Photoacoustic Imaging

Photoacoustic imaging is an emerging modality with significant promise for biomedical applications such as neuroimaging, owing to its capability to capture large fields of view deep inside complex scattering tissue. However, widespread adoption of this technique has been hindered by a lack of suitable molecular reporters for this modality. In this work, we introduce chemigenetic labels and calcium sensors specifically tailored for photoacoustic imaging, using a combination of synthetic dyes and HaloTag-based self-labeling proteins. We rationally design and engineer far-red "acoustogenic" dyes, showing high photoacoustic turn-ons upon binding to HaloTag, and develop a suite of tunable calcium indicators based on these scaffolds. These first-generation photoacoustic reporters show excellent performance in tissue-mimicking phantoms, with the best variants outperforming existing sensors in terms of signal intensity, sensitivity, and photostability. We demonstrate the application of these ligands for labeling HaloTag-expressing neurons in mouse brain tissue, producing strong, specifically targeted photoacoustic signal, and provide a first example of in vivo labeling with these chemigenetic photoacoustic probes. Together, this work establishes a new approach for the design of photoacoustic reporters, paving the way toward deep tissue functional imaging.

Merged Molecular Switches Excel as Optoacoustic Dyes: Azobenzene-Cyanines Are Loud and Photostable NIR Imaging Agents

Optoacoustic (or photoacoustic) imaging promises micron-resolution noninvasive bioimaging with much deeper penetration (>cm) than fluorescence. However, optoacoustic imaging of enzyme activity would require loud, photostable, NIR-absorbing molecular contrast agents, which remain unknown. Most organic molecular contrast agents are repurposed fluorophores, with severe shortcomings of photoinstability or phototoxicity under optoacoustic imaging, as consequences of their slow S1→S0 electronic relaxation. We now report that known fluorophores can be rationally modified to reach ultrafast S1→S0 rates, without much extra molecular complexity, simply by merging them with molecular switches. Here, we merge azobenzene switches with cyanine dyes to give ultrafast relaxation (<10 ps, >100-fold faster). Without even adapting instrument settings, these azohemicyanines display outstanding improvements in signal longevity (>1000-fold increase of photostability) and signal loudness (>3-fold even at time zero). We show why this simple but unexplored design strategy can still offer stronger performance in the future, and can also increase the spatial resolution and the quantitative linearity of photoacoustic response over extended longitudinal imaging. By bringing the world of molecular switches and rotors to bear on problems facing optoacoustic agents, this practical strategy will help to unleash the full potential of optoacoustic imaging in fundamental studies and translational uses.

A Tumor-Targeting Dual-Modal imaging probe for nitroreductase in vivo

Nitroreductase (NTR) overexpression often occurs in tumors, highlighting the significance of effective NTR detection. Despite the utilization of various optical methods for this purpose, the absence of an efficient tumor-targeting optical probe for NTR detection remains a challenge. In this research, a novel tumor-targeting probe (Cy-Bio-NO2) is developed to perform dual-modal NTR detection using near-infrared fluorescence and photoacoustic techniques. This probe exhibits exceptional sensitivity and selectivity to NTR. Upon the reaction with NTR, Cy-Bio-NO2 demonstrates a distinct fluorescence "off-on" response at 800 nm, with an impressive detection limit of 12 ng/mL. Furthermore, the probe shows on-off photoacoustic signal with NTR. Cy-Bio-NO2 has been successfully employed for dual-modal NTR detection in living cells, specifically targeting biotin receptor-positive cancer cells for imaging purposes. Notably, this probe effectively detects tumor hypoxia through dual-modal imaging in tumor-bearing mice. The strategy of biotin incorporation markedly enhances the probe's tumor-targeting capability, facilitating its engagement in dual-modal imaging at tumor sites. This imaging capacity holds substantial promise as an accurate tool for cancer diagnosis.

Xanthene-Based Dyes for Photoacoustic Imaging and their Use as Analyte-Responsive Probes

Developing imaging tools that can report on the presence of disease-relevant analytes in multicellular organisms can provide insight into fundamental disease mechanisms as well as provide diagnostic tools for the clinic. Photoacoustic imaging (PAI) is a light-in, sound-out imaging technique that allows for high resolution, deep-tissue imaging with applications in pre-clinical and point-of-care settings. The continued development of near-infrared (NIR) absorbing small-molecule dyes promises to improve the capabilities of this emerging imaging modality. For example, new dye scaffolds bearing chemoselective functionalities are enabling the detection and quantification of disease-relevant analytes through activity-based sensing (ABS) approaches. Recently described strategies to engineer NIR absorbing xanthenes have enabled development of analyte-responsive PAI probes using this classic dye scaffold. Herein, we present current strategies for red-shifting the spectral properties of xanthenes via bridging heteroatom or auxochrome modifications. Additionally, we explore how these strategies, coupled with chemoselective spiroring-opening approaches, have been employed to create ABS probes for in vivo detection of hypochlorous acid, nitric oxide, copper (II), human NAD(P)H: quinone oxidoreductase isozyme 1, and carbon monoxide. Given the versatility of the xanthene scaffold, we anticipate continued growth and development of analyte-responsive PAI imaging probes based on this dye class.

A novel fluorescent probe for cascade detection of hydrogen sulfide and hypochlorous acid and its application in bioimaging

Herein we developed a cascade detection mode for the detection of HS- and ClO- by the novel probe NM-Cl bearing a conjugating naphthalene-dicyanoisophorone unit. The probe displayed sensitive and remarkable fluorescent enhancement in response to HS-, but not to other analytes. The mixture of probe and HS- constructed a specific sensing system for ClO- by fluorescent quenching response. The mechanism studies indicated that the successive reacting of HS- substitution Cl atom in probe and then addition of ClO- facilitation a thiofuran ring-forming induced differentiated fluorescence emission. This study provides a novel mechanism for the detection of HS- and ClO-, the imaging of cell and living animal further indicating the good application prospects of the probe in biosensing and bioimaging.

A general fluorescence off/on strategy for fluorogenic probes: Steric repulsion-induced twisted intramolecular charge transfer (sr-TICT)

Various control strategies are available for building fluorogenic probes to visualize biological events in terms of a fluorescence change. Here, we performed the time-dependent density functional theory (TD-DFT) computational analysis of the twisted intramolecular charge transfer (TICT) process in rhodamine dyes. On the basis of the results, we designed and synthesized a series of rhodamine dyes and established a fluorescence quenching strategy that we call steric repulsion-induced TICT (sr-TICT), in which the fluorescence quenching process is greatly accelerated by simple intramolecular twisting. As proof of concept of this design strategy, we used it to develop a fluorogenic probe, 2-Me PeER (pentyloxyethylrhodamine), for the N-dealkylation activity of CYP3A4. We applied 2-Me PeER for CYP3A4 activity-based fluorescence-activated cell sorting (FACS), providing access to homogeneous, highly functional human-induced pluripotent stem cell (hiPSC)-derived hepatocytes and intestinal epithelial cells. Our results suggest that sr-TICT represents a general fluorescence control method for fluorogenic probes.

Single Atom Stabilization of Phosphinate Ester-Containing Rhodamines Yields Cell Permeable Probes for Turn-On Photoacoustic Imaging

Photoacoustic imaging (PAI) is an emerging imaging technique that uses pulsed laser excitation with near-infrared (NIR) light to elicit local temperature increases through non-radiative relaxation events, ultimately leading to the production of ultrasound waves. The classical xanthene dye scaffold has found numerous applications in fluorescence imaging, however, xanthenes are rarely utilized for PAI since they do not typically display NIR absorbance. Herein, we report the ability of Nebraska Red (NR) xanthene dyes to produce photoacoustic (PA) signal and provide a rational design approach to reduce the hydrolysis rate of ester containing dyes, affording cell permeable probes. To demonstrate the utility of this approach, we construct the first cell permeable rhodamine-based, turn-on PAI imaging probe for hypochlorous acid (HOCl) with maximal absorbance within the range of commercial PA instrumentation. This probe, termed SNR700 -HOCl, is capable of detecting exogenous HOCl in mice. This work provides a new set of rhodamine-based PAI agents as well as a rational design approach to stabilize esterified versions of NR dyes with desirable properties for PAI. In the long term, the reagents described herein could be utilized to enable non-invasive imaging of HOCl in disease-relevant model systems.

Targeted Molecular Imaging and Therapy Based on Nuclear and Optical Technologies

Both nuclear and optical imaging are used for in vivo molecular imaging. Nuclear imaging displays superior quantitativity, and it permits imaging in deep tissues. Thus, this method is widely used clinically. Conversely, because of the low permeability of visible to near-IR light in living animals, it is difficult to visualize deep tissues via optical imaging. However, the light at these wavelengths has no ionizing effect, and it can be used without any restrictions in terms of location. Furthermore, optical signals can be controlled in vivo to accomplish target-specific imaging. Nuclear medicine and phototherapy have also evolved to permit targeted-specific imaging. In targeted nuclear therapy, beta emitters are conventionally used, but alpha emitters have received significant attention recently. Concerning phototherapy, photoimmunotherapy with near-IR light was approved in Japan in 2020. In this article, target-specific imaging and molecular targeted therapy utilizing nuclear medicine and optical technologies are discussed.

Reaction-Activated Disassembly of the NIR-II Probe Enables Fast Detection and Ratiometric Photoacoustic Imaging of Glutathione In Vivo

Glutathione (GSH), the most abundant nonprotein biothiol, is a significant endogenous molecule that plays a key role in redox equilibrium in vivo and is regarded as a critical biomarker of cancer. Currently, various fluorescent probes have been designed and synthesized for imaging GSH at the cellular level in the visible range and the first near-infrared window (NIR-I, 750-900 nm). However, the application of these fluorescent probes for bioimaging and biosensing in vivo has been extremely hindered by the high biobackground and low tissue penetration. Herein, based on the self-assembly and disassembly of J-aggregation, we designed and synthesized a GSH-activatable probe MC-PSE for second near-infrared window (NIR-II) fluorescence and ratiometric photoacoustic imaging of GSH in vivo. The anionic cyanine-based MC-PSE tends to form stable J-aggregates in an aqueous solution. Upon the reaction with GSH, the J-aggregates of MC-PSE disassembled, the emission peak intensity of MC-PSE at 940 nm significantly increased by about 20 times, and the PA900/PA980 ratio increased by 4 times within 15 min in vitro. Notably, we used MC-PSE to visualize GSH in tumor-bearing mice and to distinguish normal and tumor areas successfully by virtue of NIR-II FL and PA dual-modal imaging. The design strategy of MC-PSE provides a novel method for ratiometric photoacoustic imaging, and MC-PSE is expected to be a powerful tool for the accurate detection of GSH in cancer diagnosis.

Fluorescent dyes based on rhodamine derivatives for bioimaging and therapeutics: recent progress, challenges, and prospects

Since their inception, rhodamine dyes have been extensively applied in biotechnology as fluorescent markers or for the detection of biomolecules owing to their good optical physical properties. Accordingly, they have emerged as a powerful tool for the visualization of living systems. In addition to fluorescence bioimaging, the molecular design of rhodamine derivatives with disease therapeutic functions (e.g., cancer and bacterial infection) has recently attracted increased research attention, which is significantly important for the construction of molecular libraries for diagnostic and therapeutic integration. However, reviews focusing on integrated design strategies for rhodamine dye-based diagnosis and treatment and their wide application in disease treatment are extremely rare. In this review, first, a brief history of the development of rhodamine fluorescent dyes, the transformation of rhodamine fluorescent dyes from bioimaging to disease therapy, and the concept of optics-based diagnosis and treatment integration and its significance to human development are presented. Next, a systematic review of several excellent rhodamine-based derivatives for bioimaging, as well as for disease diagnosis and treatment, is presented. Finally, the challenges in practical integration of rhodamine-based diagnostic and treatment dyes and the future outlook of clinical translation are also discussed.

Methyl position affect the fluorescence performance of HBT derivatives for the detection of hypochlorite under alkaline condition

Five fluorescent derivatives of hydroxyphenyl-benzothiazole (HBT) with different methyl positions at the hydroxyphenyl group were synthesized with good yield. Their reactivity and fluorescent response to hypochlorite were carefully studied. It was found that the HBT derivatives with meta-methyl (3-HBT or 5-HBT) showed the highest reactivity to hypochlorite under basic conditions, accompanied by the most efficient fluorescence quenching, whereas HBT derivatives with ortho or para methyl exhibited the least reactivity to hypochlorite. The LUMO and HOMO of 3-HBT were further verified to explain the fluorescence behavior by density functional theory (DFT) calculation. The excellent selectivity of 3-HBT toward hypochlorite against other reactive oxygen species (ROS) was also evaluated under the same conditions. The compounds emit bright green fluorescence in a solid-state, which is convenient for designing sensing devices for hypochlorite in water samples. Thus, the HBT derivatives with meta methyl (3-HBT) were successfully applied to fabricate paper sensors for the quantification of hypochlorite in tap water. Hence, the fluorescent 3-HBT exhibits great promise as a selective and sensitive hypochlorite probe in chemical and biological applications.

A long-wavelength xanthene dye for photoacoustic imaging

Photoacoustic (PA) imaging is a powerful biomedical imaging modality. We designed KeTMR and KeJuR, two xanthene-based dyes that were readily obtained through a 2-step synthetic route. KeJuR has low molecular weight, good aqueous solubility, and superior chemical stability compared to KeTMR. KeJuR shows a robust PA signal under 860 nm excitation and can be paired with traditional PA dyes for multiplex imaging in blood samples under a tissue-mimicking environment.

A New Phenylazo-Based Fluorescent Probe for Sensitive Detection of Hypochlorous Acid in Aqueous Solution

In this paper, we designed and synthesized a novel phenylazo-based fluorescent probe (RHN) for the sensing and imaging of hypochlorous acid (HClO) in mitochondria in living cells. In this process, HClO promoted the oxidation of the phenylazo group to generate a free Rhodol fluorophore moiety, which in turn restored strong fluorescence and realized the detection of HClO. As expected, RHN exhibited high selectivity, high sensitivity and rapid response, with detection limits as low as 22 nM (1.155 ng/mL). Importantly, the results of the cell imaging experiments indicated that RHN has the ability to image and sense HClO in mitochondria, which is of great significance for exploration of the specific role of HClO in both the immune system and diseases.

Chemical Design of Activatable Photoacoustic Probes for Precise Biomedical Applications

Photoacoustic (PA) imaging technology, a three-dimensional hybrid imaging modality that integrates the advantage of optical and acoustic imaging, has great application prospects in molecular imaging due to its high imaging depth and resolution. To endow PA imaging with the ability for real-time molecular visualization and precise biomedical diagnosis, numerous activatable molecular PA probes which can specifically alter their PA intensities upon reacting with the targets or biological events of interest have been developed. This review highlights the recent developments of activatable PA probes for precise biomedical applications including molecular detection of the biotargets and imaging of the biological events. First, the generation mechanism of PA signals will be given, followed by a brief introduction to contrast agents used for PA probe design. Then we will particularly summarize the general design principles for the alteration of PA signals and activatable strategies for developing precise PA probes. Furthermore, we will give a detailed discussion of activatable PA probes in molecular detection and biomedical imaging applications in living systems. At last, the current challenges and outlooks of future PA probes will be discussed. We hope that this review will stimulate new ideas to explore the potentials of activatable PA probes for precise biomedical applications in the future.

Heteroatom-Substituted Xanthene Fluorophores Enter the Shortwave-Infrared Region

This article is a highlight of the paper by Ivanic and Schnermann et al. in this issue of Photochemistry and Photobiology (Daly et al. Photochem. Photobiol. 2022). The collaborative team utilized computational approaches to investigate the influence of electron-withdrawing groups at the 10' position of tetramethylrhodamine (TMR). Leveraging this information, the team was able to extend the emission of the TMR scaffold into the shortwave-infrared region (SWIR, 1000-2500 nm) by incorporation of a ketone functional group at the 10' position (Daly et al. Photochem. Photobiol. 2022). This work provides the first example of a TMR derivative with peak SWIR emission (λ_abs : 862 nm, λ_em : 1058 nm). The authors utilize the ketone rhodamine scaffold to generate fluorogenic, pH-responsive reporters. This work demonstrates the potential of the classic xanthene scaffold for use as a SWIR reporter, an important step in the ultimate expansion of the repertoire of small-molecule organic fluorophore scaffolds available for deep-tissue imaging applications.

Precise in Vivo Inflammation Imaging in the NIR-II Window Using 1065 nm Photoacoustic Probe for in Situ Visual Monitoring of Pathological Processes Related to Hepatitis

The interaction between light and biological tissues in the second near-infrared (NIR-II) window is weak, which can effectively reduce the scattering and absorption of incoming light by biological tissues and enhance the resolution and sensing ability of in vivo photoacoustic (PA) imaging. In particular, tissues that carry blood and water produce the lowest PA background in the wavelength range of 1050 to 1150 nm. However, the development of the NIR-II PA probe for the above window faces great challenges. To tackle this challenge, the reduction-reoxidation of an organic dye was used to develop a PA imaging probe (Hydro-1048) as the first NIR-II PA probe of a hydroxy radical (·OH) for molecular imaging in deep tissue. The ·OH oxidized the C-N single bond in Hydro-1048 to double bonds, which formed Et-1065. This conversion extended the conjugate system of the molecule and shifted the absorption peak from 520 to 1065 nm, which resulted in a strong PA signal after irradiation with a 1065 nm laser. At a detection limit of 0.6 nM, a good linear relationship within the range of 5-1000 nM was obtained for the PA signal intensity versus the concentration of ·OH. The developed NIR-II PA probe can be used for the noninvasive high-resolution imaging of ·OH in deep tissue, and the PA imaging of ·OH can also be used to visually monitor in situ pathological processes related to hepatitis.

Luminescence Nanoprobe in the Near-Infrared-II Window for Ultrasensitive Detection of Hypochlorite

Sensitive and selective detection of hypochlorite is in great demand for food safety, especially in fresh cold chain products. However, the detection limit of traditional visible emission-based strategies cannot satisfy the requirement of ultrasensitive analysis in practical applications. In this work, we explored a novel luminescent nanoprobe in the near-infrared-II (NIR-II) window to greatly improve the hypochlorite detection limit for analysis of real milk samples, which was based on the fluorescence resonance energy-transfer process between the hypochlorite-responsive dye (FD1080) and the lanthanide-doped downconverted nanoparticles. Specifically, the NIR-II luminescence from Yb ions was first suppressed by FD1080 due to the energy-transfer mechanism. In the presence of hypochlorite, FD1080 was bleached to recover the luminescence. As a proof-of-concept, the optimal nanoprobe exhibited a linear luminescence recovery in the range of 0.1-1 nM with the detection limit of 0.0295 nM for hypochlorite. Real milk sample detection experiments showed that the probe had good accuracy and precision.

Thienylpiperidine Donor NIR Xanthene-Based Dye for Photoacoustic Imaging

Few xanthene-based near-infrared (NIR) photoacoustic (PA) dyes with absorbance >800 nm exist. As accessibility to these dyes requires long and tedious synthetic steps, we designed a NIR dye (XanthCR-880) with thienylpiperidine donors and a xanthene acceptor that is accessible in 3-4 synthetic steps. The dye boasts a strong PA signal at 880 nm with good biological compatibility and photostability, yields multiplexed imaging with an aza-BODIPY reference dye, and is detected at a depth of 4 cm.

A General Approach to Convert Hemicyanine Dyes into Highly Optimized Photoacoustic Scaffolds for Analyte Sensing*

Most photoacoustic (PA) imaging agents are based on the repurposing of existing fluorescent dye platforms that exhibit non-optimal properties for PA applications. Herein, we introduce PA-HD, a new dye scaffold optimized for PA probe development that features a 4.8-fold increase in sensitivity and a red-shift of the λabs from 690 nm to 745 nm to enable ratiometric imaging. Computational modeling was used to elucidate the origin of these enhanced properties. To demonstrate the generalizability of our remodeling efforts, we developed three probes for β-galactosidase activity (PA-HD-Gal), nitroreductase activity (PA-HD-NTR), and H2 O2 (PA-HD-H2 O2 ). We generated two cancer models to evaluate PA-HD-Gal and PA-HD-NTR. We employed a murine model of Alzheimer's disease to test PA-HD-H2 O2 . There, we observed a PA signal increase at 735 nm of 1.79±0.20-fold relative to background, indicating the presence of oxidative stress. These results were confirmed via ratiometric calibration, which was not possible using the parent HD platform.

Development of a small-molecule-based activatable photoacoustic probe

Photoacoustic (PA) imaging is an emerging imaging modality that combines the advantages of optical imaging and ultrasound imaging. In particular, activatable PA probes, which visualize the presence or the activity of target molecules in terms of a change of the PA signal, are useful tools for functional imaging. In this chapter, we describe the development of small-molecule-based activatable PA probes, focusing on the design and synthesis of PA-MMSiNQ, our recently developed activatable PA probe for HOCl. We also describe the protocols used for evaluation of PA-MMSiNQ with a UV-vis spectrometer and a PA imaging microscope.

Near-infrared II photoacoustic probes for nitric oxide sensing

In this chapter, we introduce a two-phase tuning approach for developing highly sensitive photoacoustic probes for imaging nitric oxide (NO) in the near-infrared (NIR)-II window. Due to the synthetically challenging nature of current NIR-II dye platforms, our two-phase tuning approach circumvents this issue by first allowing one to tune the reactivity using a synthetically accessible dye. We have used a physical organic workflow to understand the reaction kinetics and identify the most reactive sensing component. The selected reactive trigger is then introduced to phase two where it is appended to a range of well-established NIR-II dyes. This strategy is used to select the ideal photoacoustic probe for NIR-II imaging in vivo. Here, we have detailed procedures for synthesis, in vitro studies, and in vivo imaging.

Development of a new water-soluble fluorescence probe for hypochlorous acid detection in drinking water

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A responsive small-molecule fluorescence probe is developed for hypochlorous acid (HOCl) detection.

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The new probe has good solubility and stability in water, can rapidly and selectively respond to HOCl in pure aqueous solution.

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HOCl-triggered specific reaction leads to a blue shift of UV–vis absorption and enhancement in fluorescence that are recorded for HOCl detection.

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The application of this probe for HOCl detection in drinking water samples is demonstrated.

Responsive small-molecule fluorescence probe specific for target analyte detection is an emerging technology for food safety and quality analysis. In this work, we report a new water soluble small-molecule fluorescence probe (PG) for the detection of hypochlorous acid (HOCl) in drinking water samples. Probe PG was developed by coupling of a glucosamine into 10-methyl-10H-phenothiazine fluorophore with a HOCl-responsive C=N bond. The thioether is another recognition site that can be oxidized to be sulfoxide in water. Due to the specific reactions triggered by HOCl, probe PG’s absorption band is blue shifted from 388 to 340 nm, and fluorescence at 488 nm is more than 55-fold enhanced. Probe PG features high fluorescence stability in PBS buffer with varied pH, fast response and high selectivity to HOCl. The application of the probe PG for HOCl detection in real-world samples is demonstrated by HOCl detection in drinking water, including tap water, purified water, and spring water samples. The recoveries of this method for HOCl detection in drinking water are in the range of 99.17–102.3%. This work thus provides a new method for HOCl detection in drinking water with high precision and accuracy.

Recent development of near-infrared photoacoustic probes based on small-molecule organic dye

Photoacoustic imaging (PAI), which integrates the higher spatial resolution of optical imaging and the deeper penetration depth of ultrasound imaging, has attracted great attention. Various photoacoustic probes including inorganic and organic agents have been well fabricated in last decades. Among them, small-molecule based agents are most promising candidates for preclinical/clinical applications due to their favorite in vivo features and facile functionalization. In recent years, PAI, in the near-infrared region (NIR, 700-1700 nm) has developed rapidly and has made remarkable achievements in the biomedical field. Compared with the visible light region (400-700 nm), it can significantly reduce light scattering and meanwhile provide deeper tissue penetration. In this review, we discuss the recent developments of near-infrared photoacoustic probes based on small molecule dyes, which focus on their "always on" and "activatable" form in biomedicine. Further, we also suggest current challenges and perspectives.

Development of NIR-II Photoacoustic Probes Tailored for Deep-Tissue Sensing of Nitric Oxide

Photoacoustic (PA) imaging has emerged as a reliable in vivo technique for diverse biomedical applications ranging from disease screening to analyte sensing. Most contemporary PA imaging agents employ NIR-I light (650-900 nm) to generate an ultrasound signal; however, there is significant interference from endogenous biomolecules such as hemoglobin that are PA active in this window. Transitioning to longer excitation wavelengths (i.e., NIR-II) reduces the background and facilitates the detection of low abundance targets (e.g., nitric oxide, NO). In this study, we employed a two-phase tuning approach to develop APNO-1080, a NIR-II NO-responsive probe for deep-tissue PA imaging. First, we performed Hammett and Brønsted analyses to identify a highly reactive and selective aniline-based trigger that reacts with NO via N-nitrosation chemistry. Next, we screened a panel of NIR-II platforms to identify chemical structures that have a low propensity to aggregate since this can diminish the PA signal. In a head-to-head comparison with a NIR-I analogue, APNO-1080 was 17.7-fold more sensitive in an in vitro tissue phantom assay. To evaluate the deep-tissue imaging capabilities of APNO-1080 in vivo, we performed PA imaging in an orthotopic breast cancer model and a heterotopic lung cancer model. Relative to control mice not bearing tumors, the normalized turn-on response was 1.3 ± 0.12 and 1.65 ± 0.07, respectively.

Design Strategies of Photoacoustic Molecular Probes

Photoacoustic (PA) probes have been developed very quickly and applied in broad areas in recent years. Most of them are constructed based on organic dyes with intrinsic near-infrared (NIR) absorption properties. To increase PA contrast and improve imaging resolution and the sensitivity of detection, various methods for the design of PA probes have been developed. This minireview mainly focuses on the development and design strategies of activatable small-molecule PA probes in four aspects: reaction-cleavage, metal ion chelation, photoswitch, and protonation-deprotonation. It highlights some key points of designing PA probes corresponding to their properties and applications. The challenges and perspectives for small-molecule PA probes are also discussed.

A novel fluorescent probe with extremely low background fluorescence for sensing hypochlorite in zebrafish

Development of an efficient fluorescent probe for sensing hypochlorite in water samples and biological samples is highly demanded. However, the currently reported fluorescent probes for hypochlorite frequently suffered from the problem of high background fluorescence. Herein, based on the combined effect of two different fluorescence quenching groups, we rationally developed a novel fluorescent probe for hypochlorite with extremely low background fluorescence. Notably, due to the doubly quenching groups, the probe could even keep low background fluorescence in a solution with high viscosity. Furthermore, the probe displayed highly sensitive and selective response to hypochlorite, with the detection limits calculated to be 10.5 nM. Practical application demonstrated that the probe was able to quantitatively detect hypochlorite in various water samples with good recovery. Significantly, the probe showed extremely low background fluorescence in living cells and was capable of detecting minor variation of endogenous hypochlorite in RAW 264.7 cells. Moreover, the fluorescence imaging different concentration of hypochlorite in zebrafish has been successfully conducted. The probe developed herein will be widely used as a reliable tool to accurately monitor the variation of hypochlorite in living organism.

A portable photoacoustic device for facile and sensitive detection of serum alkaline phosphatase activity

It is still a high challenge to develop a simple, sensitive and portable approach for bioassay in strong scattering medium. Herein, a photoacoustic (PA) device is developed for the detection of alkaline phosphatase (ALP) in serum with silver nanoparticles (AgNPs) as signal probe, without any requirements for expensive equipment, professional operation and pre-processing of real samples. ALP as an important disease marker could catalyze the breakdown of sodium L-ascorbyl-2-phosphate (AAP) into ascorbic acid (AA), thereby reducing Ag⁺ to AgNPs. AgNPs could generate strong PA signal under the irradiation of modulated 638-nm laser due to their localized plasmon resonance, and detected by the self-made portable PA device. Under the optimized experimental conditions, the present PA device exhibits excellent photostability and reproducibility with the relative standard deviation (RSD) of 2.2% at the concentration of 25 U L^-1 ALP. Linear calibration graph is obtained within 5-70 U L^-1 for ALP, along with a detection limit of 1.1 U L^-1. This portable PA device is applied to detect ALP in serum samples, providing satisfactory spiking recoveries and competitive analytical performances with the current techniques. The PA-based analytical strategy obviously opens up a new avenue to the detection of disease-correlated biomarker in practice.

A mitochondria-targeted fluorescent probe for hypochlorite sensing and its application in bioimaging

A coumarin-diaminomaleonitrile derivative was prepared and used for detecting OCl⁻ in living cells and zebrafish. Its high selectivity, sensitivity and low toxicity indicate that it is an ideal tool for biological applications.

Engineering an NIR rhodol derivative with spirocyclic ring-opening activation for high-contrast photoacoustic imaging

A novel NIR rhodol derivative is engineered for the development of high-contrast activatable photoacoustic imaging based on generally applicable ring-opening responsiveness.

Molecular probes that enable high-contrast photoacoustic (PA) imaging of cellular processes are valuable tools for in vivo studies. Design of activatable PA probes with high contrast remains elusive. We develop a new NIR rhodol derivative, Rhodol-NIR, with a large extinction coefficient, low quantum yield and structural switching from a ‘ring-open’ form to a ‘closed’ spirolactone upon esterification. This structural transition, together with the ideal photophysical properties, enables the development of activatable probes for high-contrast PA imaging via a target-specific de-esterification reaction. This strategy is demonstrated using a PA probe designed for a tumor biomarker, human NAD(P)H: quinone oxidoreductase isozyme 1 (hNQO1), which affords high contrast and excellent sensitivity for PA detection and imaging of hNQO1 in living cells and animals. The strategy can provide a new paradigm for engineering activatable PA probes for high-contrast imaging.