In situ visualization of peroxisomal viscosity in the liver of mice with non-alcoholic fatty liver disease by near-infrared fluorescence and photoacoustic imaging

A dual-emission fluorescent probe with independent polarity and viscosity responses: The synthesis, spectroscopy and bio-imaging applications

Viscosity and polarity are essential parameters that play critical roles in various physiological processes. Thus, dual-emission fluorescent probes that respond to both polarity and viscosity are highly sought-after tools for studying these processes. In addressing this need, a novel fluorescent probe (L), with dual emissions centered at 460 nm and 780 nm, which can sensitively respond to polarity and viscosity respectively, has been developed. Probe (L) is constructed through rational molecular design, utilizing two conjugated synthons connected by a π-bond to form a D-π-A system. The twisted intramolecular charge transfer (TICT) state is dominant in low-viscosity environments, resulting in weak near-infrared (NIR) fluorescence. Conversely, the intramolecular charge transfer (ICT) state is expected to prevail in high-viscosity environments, leading to strong NIR fluorescence. The polarity-sensitive fluorescence centered at 460 nm can be attributed to the emission of the coumarin unit. Moreover, probe (L) exhibits low cytotoxicity and primarily targets mitochondria. By leveraging the dual-emission properties of probe (L), real-time imaging of polarity and viscosity fluctuations within cells has been achieved. Additionally, probe (L) can be used for in situ and in vivo imaging of rheumatoid arthritis (RA) with good imaging resolution.

Cancer-Targeting and Viscosity-Activatable Near-Infrared Fluorescent Probe for Precise Cancer Cell Imaging

Small-molecule fluorescent probes have emerged as potential tools for cancer cell imaging-based diagnostic and therapeutic applications, but their limited selectivity and poor imaging contrast hinder their broad applications. To address these problems, we present the design and construction of a novel near-infrared (NIR) biotin-conjugated and viscosity-activatable fluorescent probe, named as QL-VB, for selective recognition and imaging of cancer cells. The designed probe exhibited a NIR emission at 680 nm, with a substantial Stokes shift of 100 nm and remarkably sensitive responses toward viscosity changes in solution. Importantly, QL-VB provided an evidently enhanced signal-to-noise ratio (SNR: 6.2) for the discrimination of cancer cells/normal cells, as compared with the control probe without biotin conjugation (SNR: 1.8). Moreover, we validated the capability of QL-VB for dynamic monitoring of stimulated viscosity changes within cancer cells and employed QL-VB for distinguishing breast cancer tissues from normal tissues in live mice with improved accuracy (SNR: 2.5) in comparison with the control probe (SNR: 1.8). All these findings indicated that the cancer-targeting and viscosity-activatable NIR fluorescent probe not only enables the mechanistic investigations of mitochondrial viscosity alterations within cancer cells but also holds the potential as a robust tool for cancer cell imaging-based applications.

Near-infrared fluorescent probe for the imaging of viscosity in fatty liver mice and valuation of drug efficacy

Fatty liver disease affects at least 25 percent of the population worldwide and is a severe metabolic syndrome. Viscosity is closely related to fatty liver disease, so it is urgent to develop an effective tool for monitoring viscosity. Herein, a NIR fluorescent probe called MBC-V is developed for imaging viscosity, consisting of dimethylaniline and malonitrile-benzopyran. MBC-V is non-fluorescent in low viscosity solutions due to intramolecular rotation. In high viscosity solution, the intramolecular rotation of MBC-V is suppressed and the fluorescence is triggered. MBC-V has long emission wavelength at 720 nm and large Stokes shift about 160 nm. Moreover, MBC-V can detect changes in cell viscosity in fatty liver cells, and can image the therapeutic effects of drug in fatty liver cells. By taking advantage of NIR emission, MBC-V can be used as an imaging tool for fatty liver disease and a way to evaluate the therapeutic effect of drug for fatty liver disease.

Fluorescent probes for targeting the Golgi apparatus: design strategies and applications

The Golgi apparatus is an essential organelle constructed by the stacking of flattened vesicles, that is widely distributed in eukaryotic cells and is dynamically regulated during cell cycles. It is a central station which is responsible for collecting, processing, sorting, transporting, and secreting some important proteins/enzymes from the endoplasmic reticulum to intra- and extra-cellular destinations. Golgi-specific fluorescent probes provide powerful non-invasive tools for the real-time and in situ visualization of the temporal and spatial fluctuations of bioactive species. Over recent years, more and more Golgi-targeting probes have been developed, which are essential for the evaluation of diseases including cancer. However, when compared with systems that target other important organelles (e.g. lysosomes and mitochondria), Golgi-targeting strategies are still in their infancy, therefore it is important to develop more Golgi-targeting probes. This review systematically summarizes the currently reported Golgi-specific fluorescent probes, and highlights the design strategies, mechanisms, and biological uses of these probes, we have structured the review based on the different targeting groups. In addition, we highlight the future challenges and opportunities in the development of Golgi-specific imaging agents and therapeutic systems.

Dietary composition and its association with metabolic dysfunction-associated fatty liver disease among Chinese adults: A cross-sectional study

BACKGROUND AND STUDY AIMS

Metabolic dysfunction-associated fatty liver disease (MAFLD) has become the most common cause of chronic liver disease worldwide. Diet plays a critical role in the prevention and treatment of MAFLD. Our hypothesis was that the intake of some macronutrients, vitamins, or mineral elements is associated with MAFLD.

PATIENTS AND METHODS

Patients with MAFLD can be diagnosed based on the evidence of hepatic steatosis and if they meet any of the three additional criteria of overweight/obesity, diabetes mellitus, or metabolic dysregulation. Diets were recorded using photographs and diaries of meals for seven consecutive days. The consumed dietary composition was compared with the recommended intake according to the China Food Composition Tables (Standard Edition) version 2019 and the Chinese Dietary Reference Intakes version 2013, and its association with MAFLD was assessed by logistical regression analyses.

RESULTS

A total of 229 MAFLD patients and 148 healthy controls were included in this study. MAFLD patients, compared with that by non-MAFLD participants, consumed more polyunsaturated fatty acids (PUFAs) (p < 0.001), vitamin E (p < 0.001), and iron (p = 0.008). The intake of PUFAs (OR = 1.070, 95 % CI: 1.017-1.127, p = 0.009) and vitamin E (OR = 1.100, 95 % CI: 1.018-1.190, p = 0.016) was positively associated with MAFLD. In addition, the percentages of individuals who consumed PUFAs (p = 0.006), vitamin E (p < 0.001), or iron (p = 0.046) above the recommended intake were higher among the individuals with MAFLD. Daily intake of PUFAs > 11 % (OR = 2.328, 95 % CI: 1.290-4.201, p = 0.005) and vitamin E > 14 mg (OR = 2.189, 95 % CI: 1.153-4.158, p = 0.017) was positively correlated with MAFLD.

CONCLUSIONS

Patients with MAFLD consumed more PUFAs, vitamin E, and iron in their daily diet. Excessive consumption of PUFAs and vitamin E might be independent risk factors for the incidence of MAFLD.

A novel pathway for ratiometric hydrazine sensing in environmental samples and the detection of intracellular viscosity by a mitochondria-targeted fluorescent sensor

Mass and signal transfer, dispersion of reactive metabolites in living cells, and interactions between biomacromolecules are greatly affected by viscosity inside the cells. It is crucial to accurately determine viscosity for reliable results because of the complexities of live cells. Herein, we introduce a new fluorescence probe based on the cyanobiphenyl and benzothiazolium units. This probe not only responds to intracellular viscosity but also detects hydrazine, a widely used chemical that poses significant environmental and toxic risks to organisms. The proposed sensing mechanism provides a new pathway that includes intramolecular cyclization with hydrazine, which differs from other sensing mechanisms. A weak emission (at 590 nm) of the probe under excitation at 365 nm resulted in 25-fold higher emission at 488 nm after the addition of N2H4. The quantum yield of the probe (Φ = 0.089) increased to Φ = 0.199 with the addition of N2H4. In addition, the probe demonstrated 45-fold emission enhancement at 560 nm in viscous media, with a color change from non-fluorescence to yellow fluorescence. Good hydrazine sensing features with high adaptability, selectivity, sensitivity, ratiometric and fast response (90 s), low cytotoxicity (more than 90% of cell viability), low detection limit (86.0 nM), good linearity in the range of 0-35.0 μM, and high signal-to-noise ratio sensing capability were achieved. The hydrazine-sensing capability of the mitochondria-targetable probe in living cells makes it a strong candidate for various biological and environmental applications, including intracellular tracking and imaging. These results suggest that the present probe shows significant potential for the effective fluorescence detection of hydrazine.

A near-infrared fluorescent probe for viscosity: Differentiating cancer cells from normal cells and dual-modal imaging in tumor mice

As a basic parameter of the intracellular microenvironment, viscosity is closely related to the development of cancer. Thus, it is necessary to utilize a sensitive tool to visualize the viscosity in tumor cells and mice, which is helpful for the diagnosis of cancer. Herein, a novel dual-modal probe (IX-V) that has a near-infrared fluorescence (NIRF) and photoacoustic (PA) response to viscosity is synthesized. In low viscosity media, the probe has no fluorescence. With the increase of viscosity, the fluorescence is produced in the near-infrared region due to the inhibition of the TICT process. At the same time, the probe shows different photoacoustic (PA) signals in different viscosity media. Most notably, the viscosity in tumor cells has been imaged successfully by the application of IX-V, and the probe can effectively distinguish cancer cells from normal cells co-cultured in one dish by the difference of fluorescence intensity. In addition, the probe has been used for dual-modal imaging (NIRF and PA) of viscosity in tumor mice, which provides a tool for exploring the relationship between viscosity and diseases. That is to say, IX-V can achieve complementary imaging effects and has great application prospects in the tumor diagnosis.

Carbon Monoxide: A Second Biomarker to Couple with Viscosity for the Construction of "Dual-Locked" Near-Infrared Fluorescent Probes for Accurately Diagnosing Non-Alcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) can progress to cirrhosis and liver cancer if left untreated. Therefore, it is of great importance to develop useful tools for the noninvasive and accurate diagnosis of NAFLD. Increased microenvironmental viscosity was considered as a biomarker of NAFLD, but the occurrence of increased viscosity in other liver diseases highly reduces the diagnosis accuracy of NAFLD by a single detection of viscosity. Hence, it is very necessary to seek a second biomarker of NAFLD. It has been innovatively proposed that the overexpressed heme oxygenase-1 enzyme in NAFLD would produce abnormally high concentrations of CO in hepatocytes and that CO could serve as a potential biomarker. In this work, we screened nine lactam Changsha dyes (HCO-1-HCO-9) with delicate structures to obtain near-infrared (NIR), metal-free, and "dual-locked" fluorescent probes for the simultaneous detection of CO and viscosity. Changsha dyes with a 2-pyridinyl hydrazone substituent could sense CO, and the 5-position substituents on the 2-pyridinyl moiety had a great electron effect on the reaction rate. The double bond in these dyes served as the sensing group for viscosity. Probe HCO-9 was utilized for precise diagnosis of NAFLD by simultaneous detection of CO and viscosity. Upon reacting with CO in a high-viscosity microenvironment, strong fluorescence at 745 nm of probe HCO-9 was turned on with NIR excitation at 700 nm. Probe HCO-9 was proven to be an effective tool for imaging CO and viscosity. Due to the advantages of NIR absorption and low toxicity, probe HCO-9 was successfully applied to image NAFLD in a mouse model.

γ-Glutamyltranspeptidase-Activated Near-Infrared fluorescent probe for visualization of Drug-Induced liver injury

Drug-induced liver injury (DILI), induced by overdose or chronic administration of drugs, has become the leading cause of acute liver failure. Therefore, an accurate diagnostic method for DILI is critical to improve treatment efficiency. The production of γ-glutamyltranspeptidase (GGT) is closely related to the progression of drug-induced hepatotoxicity. KL-Glu exhibits a prominent GGT-activated NIR fluorescence (734 nm) with a large Stokes shift (137 nm) and good sensitivity/selectivity, making it favorable for real-time detection of endogenous GGT activity. Using this probe, we evaluated the GGT up-regulation under the acetaminophen-induced liver injury model. Moreover, KL-Glu was successfully used to assess liver injury induced by the natural active ingredient triptolide and the effective amelioration upon treatment with N-acetyl cysteine (NAC) or Glutathione (GSH) in cells and in vivo by fluorescent trapping the fluctuation of GGT for the first time. Therefore, the fluorescent probe KL-Glu can be used as a potential tool to explore the function of GGT in the progression of DILI and for the early diagnosis and prognostic evaluation of DILI.

Design, Synthesis and Evaluation of Fluorescent Properties of Benzothiazole Derivatives

Fluorescence imaging is conducive to establish a bridge between molecular biology and clinical medicine, and provides new tools for disease process research, early diagnosis, and efficacy evaluation, because of the advantages of rapid imaging and nondestructive detection. Herein, a series of fluorescent molecules with thiadiazole, or thiazole, or benzothiazole cores were designed and synthesized to develop more excellent fluorescent molecules in bio-imaging. According to theoretical and experimental methods, we found that benzothiazole derivative 14 B with conjugate expansion by (4-aminophenyl) ethynyl group was the most excellent fluorescent molecule among all the investigated compounds and exhibited low cytotoxicity and strong blue and green fluorescence by confocal cell imaging.

Viscosity-responsive NIR-II fluorescent probe with aggregation-induced emission features for early diagnosis of liver injury

As the primary organ for drug metabolism and detoxification, the liver is susceptible to damage and seriously impaired function. In situ diagnosing and real-time monitoring of liver damage are thus of great significance but remain limited owing to the lack of reliable in vivo visualization protocols with minimal invasion. Herein, we reported for the first time an aggregation-induced emission (AIE) probe, namely DPXBI, emitting light in the second near-infrared window (NIR-II) for early diagnosis liver injury. DPXBI featured by strong intramolecular rotations, excellent aqueous solubility and robust chemical stability, is powerfully sensitive to viscosity alteration affording rapid response and high selectivity, through NIR-Ⅱ fluorescence intensity changes. The prominent viscosity-responsive performance enables DPXBI to accurately monitor both drug-induced liver injury (DILI) and hepatic ischemia-reperfusion injury (HIRI) with excellent image contrast to the background. By using the presented strategy, the detection of liver injury in mouse model can be achieved at least several hours earlier than typical clinical assays. Moreover, DPXBI is able to dynamically track the liver improvement process in vivo in the case of DILI when the hepatotoxicity is alleviated by using hepatoprotective medication. All these results demonstrate that DPXBI is a promising probe for investigating viscosity-associated pathological and physiological processes.

Rational design of water-soluble mitochondrial-targeting near-infrared fluorescent probes with large Stokes shift for distinguishing cancerous cells and bioimaging

In the paper, two new near-infrared fluorescent probes (TTHPs) with D-π-A structure were successfully synthesized. TTHPs exhibited polarity and viscosity sensitivity and mitochondrial targeting under physiological conditions. The emission spectra of TTHPs showed strong polarity/viscosity dependence with more than a large Stokes shift of 200 nm. Based on their unique merits, TTHPs were used to distinguish cancerous and normal cells, which could be new tools for cancer diagnosis. Moreover, TTHPs were the first to achieve biological imaging of Caenorhabditis elegans, which could be labeling probes to apply in multicellular organisms.

A viscosity-sensitivity probe for cross-platform multimodal imaging from mitochondria to animal

Viscosity in biological systems is a critical factor for various physiological process, including signal transduction and metabolisms of substance and energy. Abnormal viscosity has been proven as a key feature of many diseases, thereby real-time monitoring of viscosities in cells and in vivo is of great significance for the diagnosis and therapy of related diseases. Up to date, it is still challenging to monitor viscosity cross-platform from organelles to cells to animals with a single probe. Here, we report a benzothiazolium-xanthene probe with rotatable bonds that switch on the optical signals in high viscosity environment. The enhancements of absorption, fluorescence intensity and lifetime signals allow to dynamically monitoring the viscosity change in mitochondria and cells, while near infrared absorption and emission facilitate imaging the viscosity with both fluorescence and photoacoustic imaging in animals. The cross-platform strategy is capable of monitoring the microenvironment with multifunctional imaging across various levels.

Dual-Channel Fluorescent Probe for Detecting Viscosity and ONOO- without Signal Crosstalk in Nonalcoholic Fatty Liver

Nonalcoholic fatty liver disease (NAFLD) is a global health issue. Peroxynitrite and liver viscosity have recently been found to be potential biomarkers of NAFLD. Therefore, it is of great significance to develop dual-response fluorescent probes for simultaneous detecting peroxynitrite and viscosity. We report herein a new probe (CQ) that can simultaneously detect peroxynitrite and viscosity at two independent fluorescent channels without signal crosstalk. CQ shows high selectivity, rapid response, good water solubility, low cytotoxicity, and mitochondrial localization properties. In particular, CQ responds sensitively to viscosity and peroxynitrite with off-on fluorescence changes at 710 and 505 nm, respectively. The wavelength gap between these two channels is more than 200 nm, ensuring that there is no signal crosstalk during detection. With this property, the probe was applied to simultaneously detect mitochondrial viscosity and peroxynitrite and image the changes of liver viscosity and peroxynitrite concentration during the pathogenesis of NAFLD. All results show that the CQ probe is a powerful tool for simultaneous detection of viscosity and peroxynitrite and provides a potential new diagnostic method for NAFLD.

In vivo quantitative photoacoustic evaluation of the liver and kidney pathology in tyrosinemia

Hereditary tyrosinemia type Ⅰ (HT1) is a severe autosomal recessive inherited metabolic disease, which can result in severe damage of liver and kidney. Photoacoustic imaging (PAI) uses pulsed laser light to induce ultrasonic signals to facilitate the visualization of lesions that are strongly related to disease progression. In this study, the structural and functional changes of liver and kidney in HT1 was investigated by cross-scale PAI. The results showed that the hepatic lobule and renal tubule were severely damaged during HT1 progression. The hemoglobin content, vessel density, and liver function reserve were decreased. The metabolic half-life of indocyanine green declined from 59.8 s in health to 262.6 s in the advanced stage. Blood oxygen saturation was much lower than that in health. This study highlights the potential of PAI for in vivo evaluation of the liver and kidney lesions in HT1.

Bichromatic Imaging with Hemicyanine Fluorophores Enables Simultaneous Visualization of Non-alcoholic Fatty Liver Disease and Metastatic Intestinal Cancer

Simultaneous detection of different diseases via a single fluorophore is challenging. We herein report a bichromatic fluorophore named Cy-914 for the simultaneous diagnosis of non-alcoholic fatty liver disease (NAFLD) and metastatic intestinal cancer by leveraging its NIR-I/NIR-II dual-color imaging capability. Cy-914 with a pK_a of 6.98 exhibits high sensitivity to pH and viscosity, showing turn-on NIR-I fluorescence at 795 nm in an acidic tumor microenvironment, meanwhile displaying intense NIR-II fluorescence at 914/1030 nm under neutral to slightly basic viscous conditions. Notably, Cy-914 could sensitively and noninvasively monitor viscosity variations in the progression of NAFLD. More importantly, it was able to simultaneously visualize NAFLD (ex/em = 808/1000-1700 nm) and intestinal metastases (ex/em = 570/810-875 nm) in two independent channels without spectral cross interference after topical spraying, further improving fluorescence-guided surgery of tiny metastases less than 3 mm. This strategy may provide an understanding for developing multi-color fluorophores for multi-disease diagnosis.

Biomedical Photoacoustic Imaging for Molecular Detection and Disease Diagnosis: "Always-On" and "Turn-On" Probes

Photoacoustic (PA) imaging is a nonionizing, noninvasive imaging technique that combines optical and ultrasonic imaging modalities to provide images with excellent contrast, spatial resolution, and penetration depth. Exogenous PA contrast agents are created to increase the sensitivity and specificity of PA imaging and to offer diagnostic information for illnesses. The existing PA contrast agents are categorized into two groups in this review: "always-on" and "turn-on," based on their ability to be triggered by target molecules. The present state of these probes, their merits and limitations, and their future development, is explored.

Natural flavylium-inspired far-red to NIR-II dyes and their applications as fluorescent probes for biomedical sensing

Fluorescent probes that emit in the far-red (600-700 nm), first near-infrared (NIR-I, 700-900 nm), and second NIR (NIR-II, 900-1700 nm) regions possess unique advantages, including low photodamage and deep penetration into biological samples. Notably, NIR-II optical imaging can achieve tissue penetration as deep as 5-20 mm, which is critical for biomedical sensing and clinical applications. Much research has focused on developing far-red to NIR-II dyes to meet the needs of modern biomedicine. Flavylium compounds are natural colorants found in many flowers and fruits. Flavylium-inspired dyes are ideal platforms for constructing fluorescent probes because of their far-red to NIR emissions, high quantum yields, high molar extinction coefficients, and good water solubilities. The synthetic and structural diversities of flavylium dyes also enable NIR-II probe development, which markedly advance the field of NIR-II in vivo imaging. In the last decade, there have been huge developments in flavylium-inspired dyes and their applications as far-red to NIR fluorescent probes for biomedical applications. In this review, we highlight the optical properties of representative flavylium dyes, design strategies, sensing mechanisms, and applications as fluorescent probes for detecting and visualizing important biomedical species and events. This review will prompt further research not only on flavylium dyes, but also into all far-red to NIR fluorophores and fluorescent probes. Moreover, this interest will hopefully spillover into applications related to complex biological systems and clinical treatments, ranging in focus from the sub-organelle to whole-animal levels.

Advances in Optical Imaging of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD), emerging as one of the most common chronic liver diseases including simple steatosis and non-alcoholic steatohepatitis (NASH), is likely to progress to liver fibrosis and hepatic carcinoma if not treated in time. Therefore, early diagnosis and treatment of NAFLD are necessary. Currently, liver biopsy, as the gold standard for clinical diagnosis of NAFLD, is not widely accepted by patients due to its invasiveness. However, other non-invasive methods that had been reported for NAFLD (such as magnetic resonance imaging, positron emission tomography, and ultrasound) still suffer from low resolution and sensitivity, which are available as a guide for liver biopsy sometimes. As a non-invasive modality with high spatiotemporal resolution and superior sensitivity, optical imaging methods have been widely favored in recent years, mainly including fluorescence imaging, photoacoustic imaging, and bioluminescence imaging. With these optical imaging approaches, a series of optical probes based on optical and molecular-specific design have been developed for the biomarker diagnosis and research of diseases. In this review, we summarize the existing non-invasive optical imaging probes for the detection of biomarkers in NAFLD, including microenvironment (viscosity, polarity), ROS, RSS, ions, proteins, and nucleic acids. Design strategies for optical imaging probes and their applications in NAFLD bioimaging are discussed and focused on. We also highlight the potential challenges and prospects of designing new generations of optical imaging probes in NAFLD studies, which will further enhance the diversity, practicality, and clinical feasibility of NAFLD research.

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.

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.

D-π-A structure fluorophore: NIR emission, response to viscosity, detection cyanide and bioimaging of lipid droplets

Intracellular viscosity, an important microenvironment factor, is closely involved in various cell processes as well as diseases. On the other hand, cyanide is one of the most hazardous chemicals for human health and environments. However, a NIR fluorescent probe for both response to viscosity and detection of cyanide remains vacant. Herein, we reported a D-π-A structure fluorophore (named CTR) which exhibited NIR emission and fluorescent enhancement response to viscosity via the molecular rotor strategy. Furthermore, CTR displayed fluorescent and colorimetric response to cyanide. Notably, test strips stained with CTR were fabricated, which could serve as an efficient and suitable cyanide test kit. Moreover, CTR could selectively accumulate in lipid droplets and visualize the metabolism of lipid droplets in live cells. These findings would provide new avenue to design fluorescent probe for effective response to viscosity, detection of cyanide, and bioimaging of lipid droplets in live cells.

p-Aminostyryl thiazole orange derivatives for monitoring mitochondrial viscosity in live cells

Viscosity of cell microenvironment plays a significant role in maintaining the normal life activities of cells. Particularly, the abnormal viscosity in mitochondria is closely associated with lots of diseases and cellular dysfunctions. Herein, we developed a group of p-aminostyryl thiazole orange derivatives with different amino side chains. These probes showed good fluorescence response to viscosity with twisted intramolecular charge transfer mechanism, among them, the probes with diethylamino (TOB), dibutylamino (TOC) and pyrrolidin (TOE) side chains showed better response to the viscosity with 78-fold, 55-fold, and 88-fold fluorescence enhancement in 95% glycerol solution respectively. TOB, TOC, and TOE could enter live cells and mainly located in mitochondria. Treatment HeLa cells with nystatin, lipopolysaccharide or oleic acid caused significant fluorescence enhancement of these probes, suggesting the good potential for monitoring the variation of mitochondrial viscosity, as well as for investigating the related physiological process of inflammation and lipid metabolism.

A coumarin based fluorescent probe for NIR imaging guided photodynamic therapy against S. aureus-induced infection in mice models

A coumarin based and viscosity responsive fluorescent probe (HZAU800) was designed and synthesized. The probe, containing a strong electron-donating and rigid group on 7-position of coumarin, and a rhodamine derivative...

N-acetylcysteine alleviates high fat diet-induced hepatic steatosis and liver injury via regulating intestinal microecology in mice

N-acetylcysteine (NAC), a well-accepted antioxidant, has been shown to protect against high fat diet (HFD)-induced obesity-associated non-alcoholic fatty liver disease (NAFLD) in mice. However, the underlying mechanism(s) of the beneficial...

A simple dual-response fluorescent probe for imaging of viscosity and ONOO- through different fluorescence signals in living cells and zebrafish

Cellular viscosity is a prominent micro-environmental parameter and peroxynitrite is an essential reactive oxygen special, both of which are involved in various pathological and physiological processes. When the intracellular viscosity is abnormal or the ONOO^- concentration is irregular, the normal function of cells will be disturbed. Herein, we rationally designed and synthesized a novel multichannel fluorescent probe (probe 1) for multichannel imaging of viscosity and peroxynitrite. Probe 1 displayed about 108-fold enhancement as the viscosity increased from 1.005 cP to 1090 cP. Moreover, the fluorescence intensity at 540 nm was quickly increased after adding ONOO^-. It should be noted that probe 1 has high sensitivity, selectivity and low cytotoxicity, which can be successfully employed for the visualization of exogenous and endogenous ONOO^- and imaging viscosity changes in HeLa cells by different fluorescent signals. Furthermore, probe 1 could monitor the change of ONOO^- induced by LPS (lipopolysaccharide) and IFN-γ (interferon-γ) in zebrafish. This result reveals that probe 1 may inspire more diagnostic and therapeutic programs for viscosity-peroxynitrite related diseases shortly.

A Solvatochromic Fluorescent Probe Reveals Polarity Heterogeneity upon Protein Aggregation in Cells

We report a crystallization-induced emission fluorophore to quantitatively interrogate the polarity of aggregated proteins. This solvatochromic probe, namely "AggRetina" probe, inherently binds to aggregated proteins and exhibits both a polarity-dependent fluorescence emission wavelength shift and a viscosity-dependent fluorescence intensity increase. Regulation of its polarity sensitivity was achieved by extending the conjugation length. Different proteins bear diverse polarity upon aggregation, leading to different resistance to proteolysis. Polarity primarily decreases during protein misfolding but viscosity mainly increases upon the formation of insoluble aggregates. We quantified the polarity of aggregated protein-of-interest in live cells via HaloTag bioorthogonal labeling, revealing polarity heterogeneity within cellular aggregates. The enriched micro-environment details inside misfolded and aggregated proteins may correlate to their bio-chemical properties and pathogenicity.

A "double-locked" probe for the detection of hydrogen sulfide in a viscous system

A novel "double-locked" probe, DCO-H2S-V, was prepared for detecting hydrogen sulfide in a highly viscous system. Experiments demonstrated that only when H2S and a high viscosity environment coexist in a cell, can the probe be activated effectively and emit fluorescence. This has been successfully used for detecting the changes in viscosity and H2S in a Parkinson's disease model, PC-12 cells treated with glutamate.

Enzyme-activatable fluorescent probes for β-galactosidase: from design to biological applications

β-Galactosidase (β-gal), a typical hydrolytic enzyme, is a vital biomarker for cell senescence and primary ovarian cancers. Developing precise and rapid methods to monitor β-gal activity is crucial for early cancer diagnoses and biological research. Over the past decade, activatable optical probes have become a powerful tool for real-time tracking and in vivo visualization with high sensitivity and specificity. In this review, we summarize the latest advances in the design of β-gal-activatable probes via spectral characteristics and responsiveness regulation for biological applications, and particularly focus on the molecular design strategy from turn-on mode to ratiometric mode, from aggregation-caused quenching (ACQ) probes to aggregation-induced emission (AIE)-active probes, from near-infrared-I (NIR-I) imaging to NIR-II imaging, and from one-mode to dual-mode of chemo-fluoro-luminescence sensing β-gal activity.

Recent progress in near-infrared photoacoustic imaging

The emergence of the photoacoustic imaging (PAI) expands the application of biomolecules bioimaging in cells, various tissues, and living body to monitor multiple physiological processes in complex internal environments. The PAI possesses intriguing properties such as non-invasive, highly selective excitation, and weak signal attenuation. Especially, the near-infrared (NIR) PAI displays low optical absorption and scattering, good temporal or spatial resolution and deep penetration, holds great potential in biomedical applications. We briefly compare different imaging modalities to provide a comprehensive understanding of their characteristics and related applications, highlighting the feature of the PAI. The principle of PAI is then delineated and the emerging NIR-PAI is discussed. We then focus on elaboration of the recent achievement of typical NIR-PAI contrast and their biomedical applications, especially the strategies used to improve contrast rational design and PAI performance are summarized. The PAI-related multimodal imaging approaches for improving imaging accuracy are also covered in the review. Finally, the challenges and prospective are pointed out for attracting more researchers to accelerate the development of PAI.

Fluorescence imaging of pathophysiological microenvironments

Abnormal microenvironments (viscosity, polarity, pH, etc.) have been verified to be closely associated with numerous pathophysiological processes such as inflammation, neurodegenerative diseases, and cancer. As a result, deep insights into these pathophysiological microenvironments are particularly beneficial for clinical diagnosis and treatment. However, the monitoring of pathophysiological microenvironments is unattainable by the traditional clinical diagnostic techniques such as magnetic resonance imaging, computed tomography, and positron emission tomography. Recently, fluorescence imaging has shown tremendous advantages and potential in the tracing of pathophysiological microenvironment variations. In this context, a general discussion is provided on the state-of-the-art progress of fluorescent probes for visualizing pathophysiological microenvironments (viscosity, pH, and polarity), since 2016, as well as the future perspectives in this challenging field.

Exploring the Changes of Peroxisomal Polarity in the Liver of Mice with Nonalcoholic Fatty Liver Disease

Peroxisome proliferator-activated receptor alpha (PPAR-a) is a crucial nuclear transcription regulator of lipid metabolism, which is closely associated with the initiation and development of nonalcoholic fatty liver disease (NAFLD). Because PPAR-a can directly decide the level of peroxisomal metabolic enzymes, its changes might directly cause variations in peroxisomal polarity. Therefore, we developed a new two-photon fluorescence imaging probe, PX-P, in which the triphenylamine and cyanide moieties can real-time sense peroxisomal polarity changes. Using PX-P, we observed a prominent decrease in the peroxisomal polarity in the liver of mice with NAFLD for the first time. More importantly, we discovered that intracellular excessive peroxynitrite (ONOO^-) and hydrogen peroxide (H₂O₂) underwent nitrification and oxidation, respectively, with various sites of PPAR-a. Interestingly, the key site of PPAR-a was nitrated by a low concentration of ONOO^- rather than being oxidized by the high level of H₂O₂. These drastically reduced the activity of PPAR-a, accelerating the occurrence of NAFLD. Moreover, through activating PPARs with pioglitazone, peroxisomal polarity markedly increased compared with that of NAFLD. Altogether, our work presents a new approach for the early diagnosis of NAFLD and identifies potential therapeutic targets.

A Mitochondrial-Targeting Near-Infrared Fluorescent Probe for Revealing the Effects of Hydrogen Peroxide And Heavy Metal Ions on Viscosity

As an important cell organelle, the mitochondrion has special viscosities, while abnormal mitochondrial viscosity is closely related to many diseases. Hydrogen peroxide (H₂O₂) is an active molecule related to the cell microenvironment, and its influence on mitochondrial viscosity is still not clear, so further investigation is needed. In addition, since excessive accumulation of heavy metal ions would lead to cells' dysfunction, the study of effect of excessive heavy metal ions on mitochondrial viscosity has not been reported. Herein, we designed and synthesized a mitochondrial-targeting near-infrared fluorescent probe (Mito-NV) for real-time in situ imaging and analysis of mitochondrial viscosity. Furthermore, the probe revealed that H₂O₂ can raise mitochondrial viscosity, while heavy metal ions reduce the viscosity. This work is of great significance for understanding the execution of mitochondrial functions and the occurrence and development of related diseases.

Fluorophore-Dapagliflozin Dyad for Detecting Diabetic Liver/Kidney Damages via Fluorescent Imaging and Treating Diabetes via Inhibiting SGLT2

Type II diabetes is a prevalent disease; if left untreated, it could cause serious complications including liver and kidney damages. Hence, early diagnosis for these damages and effective treatment of diabetes are of high importance. Herein, a fluorophore-dapagliflozin dyad (DX-B-DA) has been developed as a theranostic system that can be triggered by intrahepatic/intrarenal reactive oxygen species (ROS) to concomitantly release a near-infrared (NIR) fluorescent dye (DX) and a SGLT2 inhibitor dapagliflozin (DA). In this dyad (DX-B-DA), the NIR fluorophore (DX) and the drug DA were covalently linked through a boronate ester bond which serves as the fluorescence quencher as well as the ROS-responsive moiety that can be cleaved by pathological levels of ROS in diabetics. The in vitro experiments indicate that, in the absence of hydrogen peroxide, the dyad is weakly emissive and keeps its drug moiety in an inactive state, while upon responding to hydrogen peroxide, the dyad simultaneously releases the NIR dye and the drug DA, suggesting that it can serve as an activatable probe for detecting and imaging diabetic liver/kidney damages as well as a prodrug for diabetes treatment upon being triggered by ROS. The dyad was then injected in mouse model of type II diabetes, and it is found that the dyad can not only offer visualized diagnosis for diabetes-induced liver/kidney damages but also exhibit high efficacy in treating type II diabetes and consequently ameliorating diabetic liver/kidney damages.

A bifunctional mitochondrial targeting AIE-active fluorescent probe with high sensitivity to hydrogen peroxide and viscosity for fatty liver diagnosis

A mitochondrial targeting AIE-active fluorescent probe with highly sensitive dual-detection of hydrogen peroxide and viscosity for fatty liver diagnosis.