A Ratiometric Acoustogenic Probe for in Vivo Imaging of Endogenous Nitric Oxide

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.

Ratiometric fluorescence imaging of lysosomal NO in living cells and mice brains with Alzheimer's disease

We report an integrated ratiometric lysosomal nitric oxide (NO) nanoprobe based on engineered semiconducting polymer dots (Pdots), LyNO-Pdots, which consist of a newly designed NO-responsive dye, a fluorescent conjugated polymer and two functional polymers. The developed probe LyNO-Pdots exhibit high specificity and stability, good photostability and favorable blood-brain barrier (BBB) penetration ability. The LyNO-Pdots are successfully applied to ratiometric imaging of lysosomal NO variations in brain-derived endothelial cells, brain tissues and mice brains with Alzheimer's disease (AD). The results demonstrate that the NO content in the brains of AD mice is considerably higher than that in normal mice.

Nitric Oxide-Activated Bioorthogonal Codelivery Nanoassembly for In Situ Synthesis of Photothermal Agent for Precise and Safe Anticancer Treatment

The development of bioorthogonal activation in drug release represents a promising avenue for precise and safe anticancer treatment. However, two significant limitations currently hinder their clinical application: i) the necessity for separate administration of the drug precursor and its corresponding activator, leading to poor drug accumulation and potential side effects; ii) the reliance on exogenous metal or organic activators for triggering bioorthogonal activation, which often exhibit low efficiency and systemic toxicity when extending to living animals. To overcome these limitations, a nitric oxide (NO)-mediated bioorthogonal codelivery nanoassembly, termed TTB-NH2@PArg, which comprises a precursor molecular (TTB-NH2) and amphipathic polyarginine (PArg) is developed. In TTB-NH2@PArg, PArg serves as both self-assembled nanocarrier for TTB-NH2 and a NO generator. In tumor microenvironment (TME), the TME-specific generation of NO acts as a gas activator, triggering in situ bioorthogonal bond formation that transforms TTB-NH2 into TTB-AZO. This tumor-specific generation of TTB-AZO not only serves as a potential photothermal agent for effective tumor inhibition but also induces fluorescence change that enables real-time monitoring of bioorthogonal activation. This study presents a drug codelivery approach that enables precise and safe control of bioorthogonal activation for anticancer treatment, improving cancer therapy efficacy while minimizing side effects.

A Macrophage Membrane-Coated Cu-WO3-x-Hydro820 Nanoreactor for Treatment and Photoacoustic/Fluorescence Dual-Mode Imaging of Inflamed Liver Tissue

A disease-targeting nanoplatform that integrates imaging with therapeutic activity would facilitate early diagnosis, treatment, and therapeutic monitoring. To this end, a macrophage membrane-coated Cu-WO3-x-Hydro820 (CWHM) nanoreactor was prepared. This reactor was shown to target inflammatory tissues. The reactive oxygen species (ROS) such as H2O2 and ·OH in inflammatory tissues can react with Hydro820 in the reactor to form the NIR fluorophore IR820. This process allowed photoacoustic/fluorescence dual-mode imaging of H2O2 and ·OH, and it is expected to permit visual diagnosis of inflammatory diseases. The Cu-WO3-x nanoparticles within the nanoreactor shown catalase and superoxide enzyme mimetic activity, allowing the nanoreactor to catalyze the decomposition of H2O2 and ·O2- in inflammatory cells of hepatic tissues in a mouse model of liver injury, thus alleviating the oxidative stress of damaged liver tissue. This nanoreactor illustrates a new strategy for the diagnosis and treatment of hepatitis and inflammatory liver injury.

An activatable fluorescent-photoacoustic dual-modal probe for highly sensitive imaging of nitroxyl in vivo

Nitroxyl (HNO) plays a vital role in various biological functions and pharmacological activities, so the development of an excellent near-infrared fluorescent (NIRF) and photoacoustic (PA) dual-modality probe is crucial for understanding HNO-related physiological and pathological progression. Herein, we proposed and synthesized a novel NIRF/PA dual probe (QL-HNO) by substituting an indole with quinolinium in hemicyanine for the sensitive detection of exogenous and endogenous HNO in vivo. The designed probe showed the highest sensitivity in NIRF mode and a desirable PA signal-to-noise ratio for HNO detection in vitro and was further applied for NIRF/PA dual-modal imaging of HNO with high contrast in living cells and tumor-bearing animals. Based on the excellent performance of QL-HNO, we believe that this study provides a promising molecular tool for further understanding of HNO-related physiological and pathological progression.

A novel ratiometric fluorescent probe with high selectivity for lysosomal nitric oxide imaging

Nitric oxide (NO) plays critical roles in both physiology and pathology, serving as a significant signaling molecule. Recent investigations have uncovered the pivotal role of lysosome as a critical organelle where intracellular NO exists and takes function. In this study, we developed a novel ratiometric fluorescent probe called XL-NO and modified it with a morpholine unit, which followed the intramolecular charge transfer (ICT) mechanism. The probe could detect lysosomal nitric oxide with high selectivity and sensitivity. The probe XL-NO contained a secondary amine moiety that could readily react with NO in lysosomes, leading to the formation of the N-nitrosation product. The N-nitroso structure enhanced the capability in push-pull electron, which obviously led to the change of fluorescence from 621 nm to 521 nm. In addition, XL-NO was discovered to have some evident advantages, such as significant ratiometric signal (I521/I621) change, strong anti-interference ability, good biocompatibility, and a low detection limit (LOD = 44.3 nM), which were crucial for the detection of lysosomal NO. To evaluate the practical application of XL-NO, NO imaging experiments were performed in both living cells and zebrafish. The results from these experiments confirmed the feasibility and reliability of XL-NO for exogenous/endogenous NO imaging and lysosome targeting.

Single Side-Chain-Modulatory of Hemicyanine for Optimized Fluorescence and Photoacoustic Dual-Modality Imaging of H2S In Vivo

Near-infrared fluorescence (NIRF)/photoacoustic (PA) dual-modality imaging integrated high-sensitivity fluorescence imaging with deep-penetration PA imaging has been recognized as a reliable tool for disease detection and diagnosis. However, it remains an immense challenge for a molecule probe to achieve the optimal NIRF and PA imaging by adjusting the energy allocation between radiative transition and nonradiative transition. Herein, a simple but effective strategy is reported to engineer a NIRF/PA dual-modality probe (Cl-HDN3) based on the near-infrared hemicyanine scaffold to optimize the energy allocation between radiative and nonradiative transition. Upon activation by H2S, the Cl-HDN3 shows a 3.6-fold enhancement in the PA signal and a 4.3-fold enhancement in the fluorescence signal. To achieve the sensitive and selective detection of H2S in vivo, the Cl-HDN3 is encapsulated within an amphiphilic lipid (DSPE-PEG2000) to form the Cl-HDN3-LP, which can successfully map the changes of H2S in a tumor-bearing mouse model with the NIRF/PA dual-modality imaging. This work presents a promising strategy for optimizing fluorescence and PA effects in a molecule probe, which may be extended to the NIRF/PA dual-modality imaging of other disease-relevant biomarkers.

A Self-Sustaining Near-Infrared Afterglow Chemiluminophore for High-Contrast Activatable Imaging

Afterglow imaging holds great promise for ultrasensitive bioimaging due to its elimination of autofluorescence. Self-sustaining afterglow molecules (SAMs), which enable all-in-one photon sensitization, chemical defect formation and afterglow generation, possess a simplified, reproducible, and efficient superiority over commonly used multi-component systems. However, there is a lack of SAMs, particularly those with much brighter near-infrared (NIR) emission and structural flexibility for building high-contrast activatable imaging probes. To address these issues, this study for the first time reports a methylene blue derivative-based self-sustaining afterglow agent (SAN-M) with brighter NIR afterglow chemiluminescence peaking at 710 nm. By leveraging the structural flexibility and tunability, an activatable nanoprobe (SAN-MO) is customized for simultaneously activatable fluoro-photoacoustic and afterglow imaging of peroxynitrite (ONOO- ), notably with a superior activation ratio of 4523 in the afterglow mode, which is at least an order of magnitude higher than other reported activatable afterglow systems. By virtue of the elimination of autofluorescence and ultrahigh activation contrast, SAN-MO enables early monitoring of the LPS-induced acute inflammatory response within 30 min upon LPS stimulation and precise image-guided resection of tiny metastatic tumors, which is unattainable for fluorescence imaging.

Activity-Based Nitric Oxide-Responsive Porphyrin for Site-Selective and Nascent Cancer Ablation

Nitric oxide (NO) generated within the tumor microenvironment is an established driver of cancer progression and metastasis. Recent efforts have focused on leveraging this feature to target cancer through the development of diagnostic imaging agents and activatable chemotherapeutics. In this context, porphyrins represent an extraordinarily promising class of molecules, owing to their demonstrated use within both modalities. However, the remodeling of a standard porphyrin to afford a responsive chemical that can distinguish elevated NO from physiological levels has remained a significant research challenge. In this study, we employed a photoinduced electron transfer strategy to develop a panel of NO-activatable porphyrin photosensitizers (NOxPorfins) augmented with real-time fluorescence monitoring capabilities. The lead compound, NOxPorfin-1, features an o-phenylenediamine trigger that can effectively capture NO (via N2O3) to yield a triazole product that exhibits a 7.5-fold enhancement and a 70-fold turn-on response in the singlet oxygen quantum yield and fluorescence signal, respectively. Beyond demonstrating excellent in vitro responsiveness and selectivity toward NO, we showcase the potent photodynamic therapy (PDT) effect of NOxPorfin-1 in murine breast cancer and human non-small cellular lung cancer cells. Further, to highlight the in vivo efficacy, two key studies were executed. First, we utilized NOxPorfin-1 to ablate murine breast tumors in a site-selective manner without causing substantial collateral damage to healthy tissue. Second, we established a nascent human lung cancer model to demonstrate the unprecedented ability of NOxPorfin-1 to halt tumor growth and progression completely. The results of the latter study have tremendous implications for applying PDT to target metastatic lesions.

Engineering organelle-specific activatable molecules for ultra-fast and reliable in situ mapping of subcellular nitric oxide

Nitric oxide (NO), a ubiquitous gaseous transmitter in living systems, is closely associated with physiopathological processes in the endoplasmic reticulum and lysosomes. This free radical gas is very widely but very heterogeneously distributed in the biological microenvironment, which poses a great challenge to specifically detect its localized levels in certain subcellular regions. In this study, we proposed six subcellular targeting probes by rational molecular engineering and selected two probes with optimal performance for the precise spatiotemporal identification of endoplasmic reticulum (ER) and lysosomal NO fluctuations. The probes could rapidly undergo a N-nitrosation reaction with NO at a riveted subcellular location, blocking the initial photoinduced electron transfer (PET) process and generating bright fluorescence for precise mapping of NO in the ER and lysosomes. The screened probes have ultra-sensitive reactivity and ultra-low detection limits for NO, realizing the precise depiction of exogenous and endogenous NO in the corresponding subcellular area. Fluctuations in the subcellular levels of NO during inflammation were also successfully mapped by the probes. Our work will contribute to the accurate study of the physiological and pathological consequences of subcellular NO in various biological events.

Activatable Probes for Ratiometric Imaging of Endogenous Biomarkers In Vivo

Dynamic variations in the concentration and abnormal distribution of endogenous biomarkers are strongly associated with multiple physiological and pathological states. Therefore, it is crucial to design imaging systems capable of real-time detection of dynamic changes in biomarkers for the accurate diagnosis and effective treatment of diseases. Recently, ratiometric imaging has emerged as a widely used technique for sensing and imaging of biomarkers due to its advantage of circumventing the limitations inherent to conventional intensity-dependent signal readout methods while also providing built-in self-calibration for signal correction. Here, the recent progress of ratiometric probes and their applications in sensing and imaging of biomarkers are outlined. Ratiometric probes are classified according to their imaging mechanisms, and ratiometric photoacoustic imaging, ratiometric optical imaging including photoluminescence imaging and self-luminescence imaging, ratiometric magnetic resonance imaging, and dual-modal ratiometric imaging are discussed. The applications of ratiometric probes in the sensing and imaging of biomarkers such as pH, reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione (GSH), gas molecules, enzymes, metal ions, and hypoxia are discussed in detail. Additionally, this Review presents an overview of challenges faced in this field along with future research directions.

Evaluation of a biomarker (NO) dynamics in inflammatory zebrafish and periodontitis saliva samples via a fast-response and sensitive fluorescent probe

Many pathological processes include nitric oxide (NO), a signaling transduction molecule. Tumors, cardiovascular, cerebrovascular, neurodegenerative, and other illnesses are linked to abnormal NO levels. Thus, evaluating NO levels in vitro and in vivo is crucial for studying chemical biology process of associated disorders. This work devised and manufactured a coumarin-based fluorescent probe ZPS-NO to detect nitric oxide (NO). The reaction between ZPS-NO and NO produced a highly selective and sensitive optical response that caused a powerful fluorescence "turn-on" effect with a ultra-low NO detection limit of 14.5 nM. Furthermore, the probe was applied to sense and image NO in living cells and inflammatory model of zebrafish, as well as to detect NO in periodontitis patients' saliva samples. We anticipate that probe ZPS-NO will serve as a practical and effective tool for assessing the interactions and evaluation of periodontitis development.

Visualization diagnosis of acute cerebral ischemia via sulfane sulfur-activated photoacoustic imaging

A photoacoustic (PA) imaging probe, HCy-SH, was designed and synthesized. This probe can react rapidly and specifically with sulfane sulfur to produce a strong PA signal. This probe also exhibited low cytotoxicity and biotoxicity. Thus, HCy-SH has been used for visual diagnosis of acute cerebral ischemia.

Inflammation-Responsive Nanoagents for Activatable Photoacoustic Molecular Imaging and Tandem Therapies in Rheumatoid Arthritis

Rheumatoid arthritis (RA) severely lowers the life quality by progressively destructing joint functions and eventually causing permanent disability, representing a pressing public health concern. The pathogenesis of RA includes the excessive production of proinflammatory cytokines and harmful oxygen-derived free radicals, such as nitric oxide (NO), which constitute vital targets for precise diagnosis and effective treatment of RA. In this study, we introduce an advanced nanoagent that integrates the RA microenvironment-activatable photoacoustic (PA) imaging with multitarget synergistic treatment for RA. A highly sensitive organic probe with NO-tunable energy transformation and molecular geometry is developed, which enables strong near-infrared absorption with a turn-on PA signal, and the active intramolecular motion could further boost PA conversion. The probe is coassembled with an inflammation-responsive prodrug to construct the theranostic nanoagent, on which a macrophage-derived cell membrane with natural tropism to the inflammatory sites is camouflaged to improve the targeting ability to inflamed joints. The nanoagent could not only sensitively detect RA and differentiate the severity but also efficiently alleviate RA symptoms and improve joint function. The combination of activatable probe-mediated NO scavenging and on-demand activation of anti-inflammatory prodrug significantly inhibits the proinflammatory factors and promotes macrophage repolarization from M1 to M2 phenotype. This meticulously designed nanoagent ingeniously integrates RA-specific PA molecular imaging with synergistic multitarget therapy, rendering tremendous promise for precise intervention of RA-related diseases.

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.

An Activated Structure Transformable Ratiometric Photoacoustic Nanoprobe for Real-Time Dynamic Monitoring of H2S In Vivo

H2S has emerged as a promising biomarker for many diseases such as colon cancer and metformin-induced hepatotoxicity. Real-time monitoring of H2S levels in vivo is significant for early accurate diagnosis of these diseases. Herein, a new accurate and reliable nanoprobe (Au NRs@Ag) was designed for real-time dynamic ratiometric photoacoustic (PA) imaging of H2S in vivo based on the endogenous H2S-triggered local surface plasmon resonance (LSPR) red-shift. The Au NRs@Ag nanoprobe can be readily converted into Au NRs@Ag2S via the endogenous H2S-activated in situ sulfurative reaction, subsequently leading to a significant red-shift of the LSPR wavelength from 808 to 980 nm and enabling accurate ratiometric PA (PA980/PA808) imaging of H2S. Moreover, dynamic ratiometric PA imaging of metformin-induced hepatotoxicity was also successfully achieved by the designed PA imaging strategy. These findings provide the possibility of designing a new ratiometric PA imaging strategy for dynamic in situ monitoring of H2S-related diseases.

A Mitochondria-Localized Iridium(III) Complex for Simultaneous Two-Photon Phosphorescence Lifetime Imaging of Downstream Products N2O3 and ONOO- of Endogenous Nitric Oxide

Nitric oxide (NO) serves as a ubiquitous and fundamental signaling molecule involved in intricate effects on both physiological and pathological processes. NO, biosynthesized by nitric oxide synthase (NOS) or generated from nitrite, can form nitrosation reagent N2O3 (4NO + O2 = 2N2O3) through its oxidation or quickly produce peroxynitrite anion ONOO- (NO + •O2- = ONOO-) by reacting with superoxide anion (•O2-). However, most of the existing luminescent probes for NO just focus on specificity and utilize only a single signal to distinguish products N2O3 or ONOO-. In most of the present work, they differentiate one product from another simply by fluorescence signal or fluorescence intensity, which is not enough to distinguish accurately the behavior of NO in living cells. Herein, a new mitochondria-targeted and two-photon near-infrared (NIR) phosphorescent iridium(III) complex, known as Ir-NBD, has been designed for accurate detection and simultaneous imaging of two downstream products of endogenous NO, i.e., N2O3 and ONOO-. Ir-NBD exhibits a rapid response to N2O3 and ONOO- in enhanced phosphorescence intensity, increased phosphorescence lifetime, and an exceptionally high two-photon cross-section, reaching values of 78 and 85 GM, respectively, after the reaction. Furthermore, we employed multiple imaging methods, phosphorescence intensity imaging, and phosphorescence lifetime imaging together to image even distinguish N2O3 and ONOO- by probe Ir-NBD. Thus, coupled with its excellent photometrics, Ir-NBD enabled the detection of the basal level of intracellular NO accurately by responding to N2O3 and ONOO- in the lipopolysaccharide-stimulated macrophage model in virtue of fluorescence signal and phosphorescence lifetime imaging, revealing precisely the endogenous mitochondrial NO distribution during inflammation in a cell environment.

In Situ Noninvasive Observation of Nitric Oxide Fluctuation in SARS-CoV-2 Infection In Vivo by Organic Near-Infrared-II Fluorescent Molecular Nanoprobes

The pathogenesis understanding of SARS-CoV-2 infection is crucial to prevent the rampant spread of COVID-19 and its contribution to deterioration in health, even death. Nitric oxide (NO), a crucial molecule involved in signal transduction and cytotoxicity, is a possible key regulator in the occurrence and development of COVID-19. However, understanding the pathogenesis of NO in SARS-CoV-2 infection is still in its infancy due to the lack of suitable in situ monitoring probes of NO fluctuation in the complex SARS-CoV-2 infection environment in deep lung tissues. Herein, we developed an activatable near-infrared-II fluorescent molecular nanoprobe (OSNP) that uncages high-resolution and deep-tissue-penetrating near-infrared-II fluorescence signal in specific response to NO for in situ and noninvasive visualization of NO fluctuation in a SARS-CoV-2 infection mouse model in lung tissues. In vivo visualization revealed that the NO level is a positive relationship with SARS-CoV-2 infection progress. With the assistance of immuno-histochemical analyses, we uncovered the NO-involved pathological mechanism, that being the improved NO level is associated with an increase in inducible NO synthase rather than endothelial NO synthase. Our study not only provides the example of a near-infrared-II fluorescent imaging of NO in SARS-CoV-2 infection but also provides opportunities to uncover tunderlying pathomechanism of NO for SARS-Cov-2 infections.

N-Nitrosation Based Fluorescence Turn-On Nitric Oxide Probe: Kinetic and Cell Imaging Studies

Nitric oxide (NO) is a ubiquitous messenger molecule playing a key role in various physiological and pathological processes. However, producing a selective turn-on fluorescence response to NO is a challenging task due to (a) the very short half-life of NO (typically in the range of 0.1-10 s) in the biological milieu and (b) false positive responses to reactive carbonyl species (RCS) (e.g., dehydroascorbic acid and methylglyoxal etc.) and some other reactive oxygen/nitrogen species (ROS/RNS), especially with o-phenylenediamine (OPD) based fluorosensors. To avoid these limitations, NO sensors should be designed in such a way that they react spontaneously with NO to give turn-on response within the time frame of t1/2 (typically in the range of 0.1-10 s) of NO and λem in the visible wavelength along with good cell permeability to achieve biocompatibility. With these views in mind, a N-nitrosation based fluorescent sensor, NDAQ, has been developed that is highly selective to NO with ∼27-fold fluorescence enhancement at λem = 542 nm with high sensitivity (LOD = 7 ± 0.4 nM) and shorter response time, eliminating the interference of other reactive species (RCS/ROS/RNS). Furthermore, all the photophysical studies with NDAQ have been performed in 98% aqueous medium at physiological pH, indicating its good stability under physiological conditions. The kinetic assay illustrates the second-order dependency with respect to NO concentration and first-order dependency with respect to NDAQ concentration. The biological studies reveal the successful application of the probe to track both endogenous and exogenous NO in living organisms.

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.

Detection and Imaging of Active Substances in Early Atherosclerotic Lesions Using Fluorescent Probes

Atherosclerosis (AS) is a vascular disease caused by chronic inflammation and lipids that is the main cause of myocardial infarction, stroke and other cardiovascular diseases. Atherosclerosis is often difficult to detect in its early stages due to the absence of clinically significant vascular stenosis. This is not conducive to early intervention or treatment of the disease. Over the past decade, researchers have developed various imaging methods for the detection and imaging of atherosclerosis. At the same time, more and more biomarkers are being found that can be used as targets for detecting atherosclerosis. Therefore, the development of a variety of imaging methods and a variety of targeted imaging probes is an important project to achieve early assessment and treatment of atherosclerosis. This paper provides a comprehensive review of the optical probes used to detect and target atherosclerosis imaging in recent years, and describes the current challenges and future development directions.

Perylenediimides with Enhanced Autophagy Inhibition for a Dual-Light Activatable Photothermal Gas Therapy

Photothermal therapy (PTT) has emerged as a promising strategy for the treatment of tumors. However, the intrinsic self-repair mechanism of cells and the nonspecific photothermal effect of photothermal agents can result in poor treatment outcomes and normal tissue injury. To address this issue, we developed a dual light activatable perylenediimide derivative (P-NO) for nitric oxide-enhanced PTT. P-NO can self-assemble into nanoparticles in aqueous solutions. The P-NO nanoparticles are capable of releasing both NO and a photothermal molecule (P-NH) upon green light irradiation. The simultaneous release of NO and P-NH activates the photothermal effect and inhibits cell protection autophagy, thereby improving the therapeutic efficacy of PTT under near-infrared (NIR) light. Moreover, the switch on of NIR fluorescence allows real-time monitoring of the release of P-NH. Remarkably, in a mouse subcutaneous tumor model, significant tumor ablation can be achieved following dual light activated photothermal gas therapy. This work offers a promising and straightforward approach to constructing activatable perylenediimide-based photothermal agents for enhancing the effectiveness of photothermal gas therapy.

Activity-Based Photoacoustic Probes for Detection of Disease Biomarkers beyond Oncology

The earliest activity-based photoacoustic (PA) probes were developed as diagnostic agents for cancer. Since this seminal work over a decade ago that specifically targeted matrix metalloproteinase-2, PA instrumentation, dye platforms, and probe designs have advanced considerably, allowing for the detection of an impressive list of cancer types. However, beyond imaging for oncology purposes, the ability to selectively visualize a given disease biomarker, which can range from aberrant enzymatic activity to the overproduction of reactive small molecules, is also being exploited to study a myriad of noncancerous disease states. In this review, we have assembled a collection of recent papers to highlight the design principles that enable activity-based sensing via PA imaging with respect to biomarker identification and strategies to trigger probe activation under specific conditions.

Vision for Ratiometric Nanoprobes: In Vivo Noninvasive Visualization and Readout of Physiological Hallmarks

Lesion areas are distinguished from normal tissues surrounding them by distinct physiological characteristics. These features serve as biological hallmarks with which targeted biomedical imaging of the lesion sites can be achieved. Although tremendous efforts have been devoted to providing smart imaging probes with the capability of visualizing the physiological hallmarks at the molecular level, the majority of them are merely able to derive anatomical information from the tissues of interest, and thus are not suitable for taking part in in vivo quantification of the biomarkers. Recent advances in chemical construction of advanced ratiometric nanoprobes (RNPs) have enabled a horizon for quantitatively monitoring the biological abnormalities in vivo. In contrast to the conventional probes whose dependency of output on single-signal profiles restricts them from taking part in quantitative practices, RNPs are designed to provide information in two channels, affording a self-calibration opportunity to exclude the analyte-independent factors from the outputs and address the issue. Most of the conventional RNPs have encountered several challenges regarding the reliability and sufficiency of the obtained data for high-performance imaging. In this Review, we have summarized the recent progresses in developing highly advanced RNPs with the capabilities of deriving maximized information from the lesion areas of interest as well as adapting themselves to the complex biological systems in order to minimize microenvironmental-induced falsified signals. To provide a better outlook on the current advanced RNPs, nanoprobes based on optical, photoacoustic, and magnetic resonance imaging modalities for visualizing a wide range of analytes such as pH, reactive species, and different derivations of amino acids have been included. Furthermore, the physicochemical properties of the RNPs, the major constituents of the nanosystems and the analyte recognition mechanisms have been introduced. Moreover, the alterations in the values of the ratiometric signal in response to the analyte of interest as well as the time at which the highest value is achieved, have been included for most of RNPs discussed in this Review. Finally, the challenges as well as future perspectives in the field are discussed.

Biobased Agents for Single-Particle Detection with Optoacoustics

Optoacoustic (OA, photoacoustic) imaging synergistically combines rich optical contrast with the resolution of ultrasound within light-scattering biological tissues. Contrast agents have become essential to boost deep-tissue OA sensitivity and fully exploit the capabilities of state-of-the-art OA imaging systems, thus facilitating the clinical translation of this modality. Inorganic particles with sizes of several microns can also be individually localized and tracked, thus enabling new applications in drug delivery, microrobotics, or super-resolution imaging. However, significant concerns have been raised regarding the low bio-degradability and potential toxic effects of inorganic particles. Bio-based, biodegradable nano- and microcapsules consisting of an aqueous core with clinically-approved indocyanine green (ICG) and a cross-linked casein shell obtained in an inverse emulsion approach are introduced. The feasibility to provide contrast-enhanced in vivo OA imaging with nanocapsules as well as localizing and tracking individual larger microcapsules of 4-5 µm is demonstrated. All components of the developed capsules are safe for human use and the inverse emulsion approach is known to be compatible with a variety of shell materials and payloads. Hence, the enhanced OA imaging performance can be exploited in multiple biomedical studies and can open a route to clinical approval of agents detectable at a single-particle level.

Blood-Brain Barrier Permeable Photoacoustic Probe for High-Resolution Imaging of Nitric Oxide in the Living Mouse Brain

Alternations in the brain nitric oxide (NO) homeostasis are associated with a variety of neurodegeneration diseases; therefore, high-resolution imaging of NO in the brain is essential for understanding pathophysiological processes. However, currently available NO probes are unsuitable for this purpose due to their poor ability to cross the blood-brain barrier (BBB) or to image in deep tissues with spatial resolution. Herein, we developed a photoacoustic (PA) probe with BBB crossing ability to overcome this obstacle. The probe shows a highly selective ratiometric response toward NO, which enables the probe to image NO with micron resolution in the whole brain of living mice. Using three-dimensional PA imaging, we demonstrated that the probe could be used to visualize the detailed NO distribution in varying depth cross-sections (0-8 mm) of the living Parkinson's disease (PD) mouse brain. We also investigated the therapeutic properties of natural polyphenols in the PD mouse brain using the probe as an imaging agent and suggested the potential of the probe for screening therapeutic agents. This study provides a promising imaging agent for imaging of NO in the mouse brain with high resolution. We anticipate that these findings may open up new possibilities for understanding the biological functions of NO in the brain and the development of new imaging agents for the diagnosis and treatment of brain diseases.

A nitric oxide responsive AIE probe for detecting the progression of osteoarthritis

Nitric oxide (NO) is reported to be elevated in osteoarthritis (OA) both in vitro and in vivo and may be adopted to develop fluorescent probes for detecting the progression of OA. Here we report a nitric oxide responsive aggregation induced emission (AIE) probe TPE-2NHCOCH₂CH₂-(PEG)₂₄-NH-Diacerein, which is derived from tetraphenylethene (TPE) modified with the hydrophilic group long poly(ethylene glycol) chain and an anti-inflammatory drug diacerein. o-Phenylenediamine within its structure can react with NO to form benzotriazole and emit fluorescence. The results show that the NO-responsive AIE probe can smartly monitor the progression of OA with the change of fluorescence intensity in vitro and in vivo. This study may provide a new development direction for early OA monitoring in clinics.

A ratiometric near-infrared fluorescence/photoacoustic dual-modal probe with strong donor dithienopyrrole for in vivo nitric oxide detection

Integrating the imaging techniques of near-infrared fluorescence (NIRF) and photoacoustic (PA) can make up for each other and provide more useful medical information. Ratiometric imaging activated by disease-associated biomarkers can further augment imaging specificity. However, very few studies have employed the NIRF/PA dual-modal ratiometric imaging to improve the accuracy and specificity of disease diagnosis to date. In this paper, we present the synthesis of a nitric oxide (NO)-activated ratiometric NIRF/PA dual-modal nanoprobe RAPNP for in vivo NO imaging. The ratiometric imaging function was achieved jointly by a NO/acidity-responsive molecule DTP-BTDA and a nonresponsive fluorophore DTP-BBTD. In these fluorophores, the dithienopyrrole (DTP) moiety had strong electron-donating ability and imparted strong intramolecular charge transfer and relatively long emission wavelengths. The BTDA moiety in DTP-BTDA could be rapidly oxidized by NO under weak acidic environments, achieving the NIRF and PA signal activation. By using RAPNP as a contrast agent, we achieved the ratiometric detection of the endogenous NO in inflammatory bowel disease by NIRF/PA dual-modal imaging. This work provides the first case of the NIRF/PA dual-signal ratiometric probe for the real-time detection of NO in vivo.

Intracerebral Fluorescence-Photoacoustic Dual-Mode Imaging for Precise Diagnosis and Drug Intervention Tracing in Depression

Unravelling the pathophysiology of depression is a unique challenge. Depression is closely associated with reduced norepinephrine (NE) levels; therefore, developing bioimaging probes to visualize NE levels in the brain is a key to elucidating the pathophysiological process of depression. However, because NE is similar in structure and chemical properties to two other catecholamine neurotransmitters, epinephrine and dopamine, designing an NE-specific multimodal bioimaging probe is a difficult task. In this work, we designed and synthesized the first near-infrared fluorescent-photoacoustic (PA) dual-modality imaging probe for NE (FPNE). The β-hydroxyethylamine of NE was shown to react via nucleophilic substitution and intramolecular nucleophilic cyclization, resulting in the cleavage of a carbonic ester bond in the probe molecule and release of a merocyanine molecule (IR-720). This process changed the color of the reaction solution from blue-purple to green, and the absorption peak was red-shifted from 585 to 720 nm. Under light excitation at 720 nm, linear relationships between the concentration of NE and both the PA response and the fluorescence signal intensity were observed. Thus, the use of intracerebral in situ visualization for diagnosis of depression and monitoring of drug interventions was achieved in a mouse model by fluorescence and PA imaging of brain regions after administration of FPNE by tail-vein injection.

Triphenylamine-embedded copper(II) complex as a "turn-on" fluorescent probe for the detection of nitric oxide in living animals

Nitric oxide (NO) is one of three major signaling molecules, which is involved in a large amount of physiological and pathological processes in biological systems. Furthermore, more and more evidence indicates that NO levels are closely associated with several aspects of human health. Accordingly, it is of great significance to develop a convenient and reliable detection method for NO in biological systems. In this work, a novel triphenylamine-embedded copper(II) complex (NZ-Cu²⁺) has been developed to be used as a fluorescence probe for the detection of NO in living animals. The proposed sensing mechanism of NZ-Cu²⁺ towards NO has been confirmed by high-resolution mass spectrometry, spectroscopic titration and density functional theory calculation. NO induced the conversion of paramagnetic Cu²⁺ to diamagnetic Cu⁺, which blocked the photoinduced electron transfer process of NZ-Cu²⁺, resulting in a remarkable enhancement of the emission spectra. The NZ-Cu²⁺ probe possesses several advantages including high selectivity, low detection limit (12.9 nM), long emission wavelength (640 nm), large Stokes shift (201 nm), fast response time (60 s) and low cytotoxicity. More importantly, NZ-Cu²⁺ has been successfully applied to detect NO in vivo by fluorescence imaging.

Activity-based Photoacoustic Probes Reveal Elevated Intestinal MGL and FAAH Activity in a Murine Model of Obesity

Obesity is a chronic health condition characterized by the accumulation of excessive body fat which can lead to and exacerbate cardiovascular disease, type-II diabetes, high blood pressure, and cancer through systemic inflammation. Unfortunately, visualizing key mediators of the inflammatory response, such as monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH), in a selective manner is a profound challenge owing to an overlapping substrate scope that involves arachidonic acid (AA). Specifically, these enzymes work in concert to generate AA, which in the context of obesity, has been implicated to control appetite and energy metabolism. In this study, we developed the first selective activity-based sensing probes to detect MGL (PA-HD-MGL) and FAAH (PA-HD-FAAH) activity via photoacoustic imaging. Activation of PA-HD-MGL and PA-HD-FAAH by their target enzymes resulted in 1.74-fold and 1.59-fold signal enhancements, respectively. Due to their exceptional selectivity profiles and deep-tissue photoacoustic imaging capabilities, these probes were employed to measure MGL and FAAH activity in a murine model of obesity. Contrary to conflicting reports suggesting levels of MGL can be attenuated or elevated, our results support the latter. Indeed, we discovered a marked increase of both targets in the gastrointestinal tract. These key findings set the stage to uncover the role of the endocannabinoid pathway in obesity-mediated inflammation.

Design, synthesis and evaluation of 15N- and 13C-labeled molecular probes as hyperpolarized nitric oxide sensors

Nitric oxide (NO) is an important signaling molecule involved in a wide range of biological processes. Development of non-invasive, real-time detection of NO is greatly desired yet remains challenging. Here we report the design and development of novel 15N- and 13C-labeled NO-sensing probes for hyperpolarized nuclear magnetic resonance (HP-NMR) studies. These probes undergo selective and rapid reaction with NO to generate in situ AZO-products that can be monitored with distinguishable NMR signals as a read-out. This study also allows for a direct comparison of the 15N and 13C nuclei performances in hyperpolarized reaction-based probes. The simple and general SABRE-SHEATH hyperpolarization method works on the 15N- and 13C-NO-sensing probes. Measured long spin-lattice relaxation (T1) values, especially for 15N-NO probes, will allow for real-time reaction-based imaging of NO.

A novel dual-channel fluorescent probe for selectively and sensitively imaging endogenous nitric oxide in living cells and zebrafish

Nitric oxide (NO) plays various physiological and pathological roles in lots of biological processes. It is crucial to detect NO sensitively and selectively in vivo and in vitro as homeostasis of NO is closely related to various diseases. Herein, a novel dual-channel fluorescent dye (ENNH₂) based on dicarboxyimide anthracene was developed as a highly sensitive and selective probe to detect NO in living systems using the dual-channel fluorescence. ENNH₂ can emit bright red fluorescence due to the intramolecular charge transfer (ICT) from the amino group at the 6-position of 1,2-dicarboxyimide anthracene to the conjugated aromatic ring, and the ICT is effectively inhibited by the reductive deamination of the amino in the presence of NO to obtain the remarkable strong green emission with the excellent sensitivity (5.52 nM). Promisingly, ENNH₂ exhibits an excellent performance in endogenous NO dual-channel fluorescence imaging of RAW 264.7 cells and zebrafish.

Nitric-Oxide-Releasing aza-BODIPY: A New Near-Infrared J-Aggregate with Multiple Antibacterial Modalities

The development of near-infrared (NIR) J-aggregates has received increasing attention due to their broad applications. Here, we report the nitrosation of an amine-containing aza-BODIPY precursor (BDP-NH₂ ), affording the first nitric oxide (NO)-releasing NIR J-aggregate (BDP-NO). The introduction of N-nitrosamine moieties efficiently inhibits the aromatic interactions of BDP-NH₂ , which instead promotes the formation of J-aggregates within micellar nanoparticles with a remarkable bathochromic shift of ≈109 nm to the NIR window (820 nm). Interestingly, the NO release and photothermal conversion efficiency (PTCE) can be delicately tuned by the loading contents of BDP-NO within micellar nanoparticles, thereby enabling multiple antibacterial modalities by exploring either NO release, photothermal therapy (PTT), or both. We demonstrate the combination of NO and PTT can elevate antibacterial activity while attenuating PTT-associated inflammation for the in vivo treatment of MRSA infection.

Contrast Agents for Photoacoustic Imaging: A Review Focusing on the Wavelength Range

Photoacoustic imaging using endogenous chromophores as a contrast has been widely applied in biomedical studies owing to its functional imaging capability at the molecular level. Various exogenous contrast agents have also been investigated for use in contrast-enhanced imaging and functional analyses. This review focuses on contrast agents, particularly in the wavelength range, for use in photoacoustic imaging. The basic principles of photoacoustic imaging regarding light absorption and acoustic release are introduced, and the optical characteristics of tissues are summarized according to the wavelength region. Various types of contrast agents, including organic dyes, semiconducting polymeric nanoparticles, gold nanoparticles, and other inorganic nanoparticles, are explored in terms of their light absorption range in the near-infrared region. An overview of the contrast-enhancing capacity and other functional characteristics of each agent is provided to help researchers gain insights into the development of contrast agents in photoacoustic imaging.

Biological Sensing of Nitric Oxide in Macrophages and Atherosclerosis Using a Ruthenium-Based Sensor

Macrophage-derived nitric oxide (NO) plays a critical role in atherosclerosis and presents as a potential biomarker. We assessed the uptake, distribution, and NO detection capacity of an irreversible, ruthenium-based, fluorescent NO sensor (Ru-NO) in macrophages, plasma, and atherosclerotic plaques. In vitro, incubation of Ru-NO with human THP1 monocytes and THP1-PMA macrophages caused robust uptake, detected by Ru-NO fluorescence using mass-cytometry, confocal microscopy, and flow cytometry. THP1-PMA macrophages had higher Ru-NO uptake (+13%, p < 0.05) than THP1 monocytes with increased Ru-NO fluorescence following lipopolysaccharide stimulation (+14%, p < 0.05). In mice, intraperitoneal infusion of Ru-NO found Ru-NO uptake was greater in peritoneal CD11b+F4/80+ macrophages (+61%, p < 0.01) than CD11b+F4/80− monocytes. Infusion of Ru-NO into Apoe−/− mice fed high-cholesterol diet (HCD) revealed Ru-NO fluorescence co-localised with atherosclerotic plaque macrophages. When Ru-NO was added ex vivo to aortic cell suspensions from Apoe−/− mice, macrophage-specific uptake of Ru-NO was demonstrated. Ru-NO was added ex vivo to tail-vein blood samples collected monthly from Apoe−/− mice on HCD or chow. The plasma Ru-NO fluorescence signal was higher in HCD than chow-fed mice after 12 weeks (37.9%, p < 0.05). Finally, Ru-NO was added to plasma from patients (N = 50) following clinically-indicated angiograms. There was lower Ru-NO fluorescence from plasma from patients with myocardial infarction (−30.7%, p < 0.01) than those with stable coronary atherosclerosis. In conclusion, Ru-NO is internalised by macrophages in vitro, ex vivo, and in vivo, can be detected in atherosclerotic plaques, and generates measurable changes in fluorescence in murine and human plasma. Ru-NO displays promising utility as a sensor of atherosclerosis.

Ratiometric Detection of H2S in Liver Injury by Activated Two-Wavelength Photoacoustic Imaging

Metformin is commonly used for clinical treatment of type-2 diabetes, but long-term or overdose intake of metformin usually causes selective upregulation of H₂S level in the liver, resulting in liver injury. Therefore, tracking the changes of H₂S content in the liver would contribute to the prevention and diagnosis of liver injury. However, in the literature, there are few reports on ratiometric PA molecular probes for H₂S detection in drug-induced liver injury (DILI). Accordingly, here we developed a H₂S-activated ratiometric PA probe, namely BDP-H₂S, based Aza-BODIPY dye for detecting the H₂S upregulation of metformin-induced liver injury. Due to the intramolecular charge transfer (ICT) effect, BDP-H₂S exhibited a strong PA signal at 770 nm. Following the response to H₂S, its ICT effect was recovered which showed a decrement of PA₇₇₀ and an enhancement of PA₈₄₀. The ratiometric PA signal (PA₈₄₀/PA₇₇₀) showed excellent H₂S selectivity response with a low limit of detection (0.59 μM). Bioimaging experiments demonstrated that the probe has been successfully used for ratiometric PA imaging of H₂S in cells and metformin-induced liver injury in mice. Overall, the designed probe emerges as a powerful tool for noninvasive and accurate imaging of H₂S level and tracking its distribution and variation in liver in-real time.

Rational Design of a Theranostic Agent Triggered by Endogenous Nitric Oxide in a Cellular Model of Alzheimer's Disease

Oxidative damage caused by upregulated nitric oxide (NO) plays an important role in the pathogenesis of Alzheimer's disease (AD). Currently, stimulus-triggered theranostic agents have received much attention due to benefits on disease imaging and targeted therapeutic effects. However, the development of a theranostic agent triggered by NO for AD remains unexplored. Herein, through the mechanism analysis of the reaction between a fluorophore of 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) and NO, which we occasionally found and thereafter structure optimization of DPAC, a theranostic agent DPAC-(peg)₄-memantine was fabricated. In an AD cellular model, DPAC-(peg)₄-memantine exhibits NO sensing ability for AD imaging. Meanwhile, DPAC-(peg)₄-memantine shows improved therapeutic by targeted drug release triggered by NO and sustained therapeutic effects owing to the synergetic antioxidative abilities via the anti-AD drug and NO scavenging. This work provides an unprecedented avenue for the studies on not only AD but also other diseases with NO upregulation.

Activity-Based NIR Bioluminescence Probe Enables Discovery of Diet-Induced Modulation of the Tumor Microenvironment via Nitric Oxide

Nitric oxide (NO) plays a critical role in acute and chronic inflammation. NO's contributions to cancer are of particular interest due to its context-dependent bioactivities. For example, immune cells initially produce cytotoxic quantities of NO in response to the nascent tumor. However, it is believed that this fades over time and reaches a concentration that supports the tumor microenvironment (TME). These complex dynamics are further complicated by other factors, such as diet and oxygenation, making it challenging to establish a complete picture of NO's impact on tumor progression. Although many activity-based sensing (ABS) probes for NO have been developed, only a small fraction have been employed in vivo, and fewer yet are practical in cancer models where the NO concentration is <200 nM. To overcome this outstanding challenge, we have developed BL₆₆₀-NO, the first ABS probe for NIR bioluminescence imaging of NO in cancer. Owing to the low intrinsic background, high sensitivity, and deep tissue imaging capabilities of our design, BL₆₆₀-NO was successfully employed to visualize endogenous NO in cellular systems, a human liver metastasis model, and a murine breast cancer model. Importantly, its exceptional performance facilitated two dietary studies which examine the impact of fat intake on NO and the TME. BL₆₆₀-NO provides the first direct molecular evidence that intratumoral NO becomes elevated in mice fed a high-fat diet, which became obese with larger tumors, compared to control animals on a low-fat diet. These results indicate that an inflammatory diet can increase NO production via recruitment of macrophages and overexpression of inducible nitric oxide synthase which in turn can drive tumor progression.

Near-Infrared Photoacoustic Probe for Reversible Imaging of the ClO-/GSH Redox Cycle In Vivo

Homeostasis of the cellular redox status plays an indispensable role in diverse physiological and pathological processes. Hypochlorite anion (ClO^-) and glutathione (GSH) represent an important redox couple to reflect the redox status in living cells. The current cellular redox probes that detect either ClO^- or GSH alone are not accurate enough to monitor the real redox status. In this work, a reversible photoacoustic (PA) probe, DiOH-BDP, has been synthesized and applied for PA imaging to monitor the ClO^-/GSH couple redox state in an acute liver injury (ALI) model. The near-infrared PA probe DiOH-BDP features significant changes in absorption between 648 and 795 nm during the selective oxidation by ClO^- and the reductive recovery of GSH, which exhibits excellent selectivity and sensitivity toward ClO^- and GSH with the limits of detection of 77.7 nM and 7.2 μM, respectively. Additionally, using PA₇₇₀ as a detection signal allows for the in situ monitoring of the ClO^-/GSH couple, which realizes mapping of the localized redox status of the ALI by the virtue of a PA imaging system. Therefore, the probe provides a potentially technical tool to understand redox imbalance-related pathological formation processes.

Structural derivatization strategies of natural phenols by semi-synthesis and total-synthesis

Structural derivatization of natural products has been a continuing and irreplaceable source of novel drug leads. Natural phenols are a broad category of natural products with wide pharmacological activity and have offered plenty of clinical drugs. However, the structural complexity and wide variety of natural phenols leads to the difficulty of structural derivatization. Skeleton analysis indicated most types of natural phenols can be structured by the combination and extension of three common fragments containing phenol, phenylpropanoid and benzoyl. Based on these fragments, the derivatization strategies of natural phenols were unified and comprehensively analyzed in this review. In addition to classical methods, advanced strategies with high selectivity, efficiency and practicality were emphasized. Total synthesis strategies of typical fragments such as stilbenes, chalcones and flavonoids were also covered and analyzed as the supplementary for supporting the diversity-oriented derivatization of natural phenols.

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.

Functional dissection of neural circuitry using a genetic reporter for fMRI

The complex connectivity of the mammalian brain underlies its function, but understanding how interconnected brain regions interact in neural processing remains a formidable challenge. Here we address this problem by introducing a genetic probe that permits selective functional imaging of distributed neural populations defined by viral labeling techniques. The probe is an engineered enzyme that transduces cytosolic calcium dynamics of probe-expressing cells into localized hemodynamic responses that can be specifically visualized by functional magnetic resonance imaging. Using a viral vector that undergoes retrograde transport, we apply the probe to characterize a brain-wide network of presynaptic inputs to the striatum activated in a deep brain stimulation paradigm in rats. The results reveal engagement of surprisingly diverse projection sources and inform an integrated model of striatal function relevant to reward behavior and therapeutic neurostimulation approaches. Our work thus establishes a strategy for mechanistic analysis of multiregional neural systems in the mammalian brain.

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.

Targeted contrast agents and activatable probes for photoacoustic imaging of cancer

Photoacoustic (PA) imaging has emerged as a powerful technique for the high resolution visualization of biological processes within deep tissue. Through the development and application of exogenous targeted contrast agents and activatable probes that can respond to a given cancer biomarker, researchers can image molecular events in vivo during cancer progression. This information can provide valuable details that can facilitate cancer diagnosis and therapy monitoring. In this tutorial review, we provide a step-by-step guide to select a cancer biomarker and subsequent approaches to design imaging agents for in vivo use. We envision this information will be a useful summary to those in the field, new members to the community, and graduate students taking advanced imaging coursework. We also highlight notable examples from the recent literature, with emphasis on the molecular designs and their in vivo PA imaging performance. To conclude, we provide our outlook and future perspective in this exciting field.