A Smart Fluorescent Probe for NO Detection and Application in Myocardial Fibrosis Imaging

Construction of a Near-Infrared Fluorescent Probe for Dynamic Monitoring and Early Diagnosis of Heart Failure

Cardiac hypertrophy characterized by abnormal cardiomyocyte viscosity is a typical sign of heart failure (HF) with vital importance for early diagnosis. However, current biochemical and imaging diagnostic methods are unable to detect this subclinical manifestation. In this work, we developed a series of NIR-I fluorescence probes for detecting myocardial viscosity based on the pyridazinone scaffold. The probes showed weak fluorescence due to free intramolecular rotation under low-viscosity conditions, while they displayed strong fluorescence with limited intramolecular rotation in response to a high-viscosity environment. Among them, CarVis2 exhibited higher stability and photobleaching resistance than commercial dyes. Its specific response to viscosity was not influenced by the pH and biological species. Furthermore, CarVis2 showed rapid and accurate responses to the viscosity of isoproterenol (ISO)-treated H9C2 cardiomyocytes with good biocompatibility. More importantly, CarVis2 demonstrated excellent sensitivity in monitoring myocardial viscosity variation in HF mice in vivo, potentially enabling earlier noninvasive identification of myocardial abnormalities compared to traditional clinical imaging and biomarkers. These findings revealed that CarVis2 can serve as a powerful tool to monitor myocardial viscosity, providing the potential to advance insights into a pathophysiological mechanism and offering a new reference strategy for early visual diagnosis of HF.

Fluorescent Probes for Disease Diagnosis

The identification and detection of disease-related biomarkers is essential for early clinical diagnosis, evaluating disease progression, and for the development of therapeutics. Possessing the advantages of high sensitivity and selectivity, fluorescent probes have become effective tools for monitoring disease-related active molecules at the cellular level and in vivo. In this review, we describe current fluorescent probes designed for the detection and quantification of key bioactive molecules associated with common diseases, such as organ damage, inflammation, cancers, cardiovascular diseases, and brain disorders. We emphasize the strategies behind the design of fluorescent probes capable of disease biomarker detection and diagnosis and cover some aspects of combined diagnostic/therapeutic strategies based on regulating disease-related molecules. This review concludes with a discussion of the challenges and outlook for fluorescent probes, highlighting future avenues of research that should enable these probes to achieve accurate detection and identification of disease-related biomarkers for biomedical research and clinical applications.

Comprehensive insights into fluorescent probes for the determination nitric oxide for diseases diagnosis

Nitric oxide (NO) plays an important role in multiple physiological processes of the body involved in regulation, such as cardiovascular relaxation, neural homeostasis, and immune regulation, etc. The real-time monitoring of NO is of great significance in the investigation of related disease mechanisms and the evaluation of pharmacodynamics. Fluorescent probes are considered as a highly promising approach for pharmaceutical analysis and bioimaging due to their non-invasive character, real-time detection, and high sensitivity. However, there are still some challenges in the determination of biological nitric oxide with fluorescent probes, such as low anti-interference ability, poor function modifiability, and low organ specificity. Therefore, it would be beneficial to develop a new generation of fluorescent probes for real-time bioimaging of NO in vivo after this systematic summary.

A Microenvironment-responsive small-molecule probe and application in quick acute myocardial infarction imaging

Acute myocardial infarction (AMI) patients are at an elevated risk for life-threatening myocardial ischemia/reperfusion injury. Early-stage nonradioactive and noninvasive diagnosis of AMI is imperative for the subsequent disease treatment, yet it presents substantial challenges. After AMI, the myocardium typically exhibits elevated levels of peroxynitrite (ONOO-), constituting a distinct microenvironmental feature. In this context, the near-infrared imaging probe (BBEB) is employed to precisely delineate the boundaries of AMI lesions with a high level of sensitivity and specificity by monitoring endogenous ONOO-. This probe allows for the early detection of myocardial damage at cellular and animal levels, providing exceptional temporal and spatial resolution. Notably, BBEB enables visualization of ONOO- level alterations during AMI treatment incorporating antioxidant drugs. Overall, BBEB can rapidly and accurately visualize myocardial injury, particularly in the early stages, and can further facilitate antioxidant drug screening.

Mitochondrial-Targeted Ratiometric Near-Infrared Fluorescence Probe for Monitoring Nitric Oxide in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a destructive autoimmune disease, where nitric oxide (NO) is closely implicated in the inflammatory processes of RA. Therefore, direct visualization of NO is essential to assess the pathological changes in RA. Herein, a mitochondrial-targeted near-infrared ratiometric fluorescent probe (NFL-NH2), based on the intramolecular charge transfer effect, was synthesized and applied to monitor the changes of NO content in early RA. Specially, probe NFL-NH2 showed a 44-fold fluorescent intensity ratio (I705/I780) response toward NO with a detection limit of 0.536 nM, enabling qualitative and quantitative analysis of NO. Additionally, NFL-NH2 can accurately target mitochondria and sensitively detect exogenous and endogenous NO in RAW 264.7 cells. Notably, in vivo RA monitoring assays demonstrated that NFL-NH2 can rapidly detect NO levels associated with the inflammatory damage degree in RA mice models by ratiometric fluorescence imaging. These results validate that NFL-NH2 holds significant potential for diagnosing NO-mediated RA diseases.

A simple 'turn-on' fluorescence chemosensor for Al(iii) detection in aqueous solution and solid matrix

A simple fluorescence chemosensor of FHS-OH based on salicylaldehyde Schiff base was developed via a one-step reaction, which achieved a fast and highly selective response for Al(iii). Mechanism studies showed that when FHS-OH was exposed to Al(iii) with 1 : 2 binding stoichiometry in an aqueous solution at neutral pH, C[double bond, length as m-dash]N isomerization and PET processes were limited, resulting in a 'turn-on' fluorescence response with a low detection limit of 63 nmol L-1 and a satisfying linear range of 0.0-20.0 μmol L-1. Compared to traditional detection methods for Al(iii), fluorometry using FHS-OH has several advantages, including simplicity, quick response, and capability of real-time detection. More importantly, the detection of Al(iii) on a solid matrix (test paper) was successfully achieved. After the addition of Al(iii), a significant emission colour change from green to bright blue was observed by the naked eye owing to the intrinsic aggregation-induced emission (AIE) characteristic of FHS-OH.

Chemical Strategies Toward Prodrugs and Fluorescent Probes for Gasotransmitters

Three gaseous molecules are widely accepted as important gasotransmitters in mammalian cells, namely NO, CO and H2S. Due to the pharmacological effects observed in preclinical studies, these three gasotransmitters represent promising drug candidates for clinical translation. Fluorescent probes of the gasotransmitters are also in high demand; however, the mechanisms of actions or the roles played by gasotransmitters under both physiological and pathological conditions remain to be answered. In order to bring these challenges to the attention of both chemists and biologists working in this field, we herein summarize the chemical strategies used for the design of both probes and prodrugs of these three gasotransmitters.

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.

Theoretical characterization of two-photon fluorescent probes for nitric oxide detection: sensing mechanism, photophysical properties and protonation effects

Nitric oxide (NO) is an important signal molecule in biological systems and is correlated with many physiological processes and pathological diseases. To date, numerous fluorescent probes based on o-diamino aromatics have been designed and synthesized for NO detection utilizing the principle of photoinduced electron transfer (PET). However, the underlying PET mechanism has rarely been validated, and a systematic computational study on the photophysical properties is urgently desired. In this study, we used a theoretical protocol to comparatively investigate the sensing mechanism, photophysical properties and protonation effects of two emblematic probes NINO and PYSNO in aqueous solution, which combines a polarizable continuum model (PCM), time-dependent density functional theory (TD-DFT) and thermal vibration correlation function formalism (TVCF). Our findings reveal that the weak emission of NINO is due to activated PET with negative driving energy and blocked fluorescence with significant charge separation. In contrast, the poor luminescence of PYSNO is caused by the facilitated non-radiative dissipation, even though the fluorescence emission remains unobstructed. Although NINO has been successfully used in two-photon microscopy for detecting NO, we suggest that PYSNO possesses a superior two-photon absorption (TPA) cross section in the near-infrared region. The protonation effects suggest that both probes can function effectively in practical acidic lysosomal environments. Our study opens a new avenue for understanding the mechanism and predicting the properties of two-photon fluorescent probes for NO detection, thus aiding the rational design of efficient fluorescent sensors.

"Iridium Signature" Mass Spectrometric Probes: New Tools Integrated in a Liquid Chromatography-Mass Spectrometry Workflow for Routine Profiling of Nitric Oxide and Metabolic Fingerprints in Cells

Nitric oxide (NO) is a highly reactive signaling molecule involved in diverse biological processes. Simultaneous profiling of NO and associated metabolic fingerprints in a single assay allows more accurate assessments of cell states and offers the possibility to better understand its exact biological roles. Herein, a multiplexing LC-MS workflow was established for simultaneous detection of intracellular NO and various metabolites based on a novel "iridium signature" mass spectrometric probe (Ir-MSP841). This Ir-MSP841 can convert highly liable NO to a stable permanently charged triazole product (Ir-TP852), enabling direct MS detection of NO. This ^191/193Ir-signature mass spectrometric probe-based approach is endowed with overwhelming advantages of interference-free, high quantitative accuracy, and great sensitivity (limit of detection down to 0.14 nM). It also reveals good linearity over a wide concentration range 12.5-500 nM and has been successfully employed for exploring the release behaviors of three representative NO donors in cells. Meanwhile, metabolic profiling results reveal that varying the concentrations of NO has distinct effects on various cellular metabolites. This study provides a robust, sensitive, and versatile method for simultaneous detection of NO and numerous metabolites in a single LC-MS run and expands its applications in biomedical research.

Recent Development of Advanced Fluorescent Molecular Probes for Organelle-Targeted Cell Imaging

Fluorescent molecular probes are very powerful tools that have been generally applied in cell imaging in the research fields of biology, pathology, pharmacology, biochemistry, and medical science. In the last couple of decades, numerous molecular probes endowed with high specificity to particular organelles have been designed to illustrate intracellular images in more detail at the subcellular level. Nowadays, the development of cell biology has enabled the investigation process to go deeply into cells, even at the molecular level. Therefore, probes that can sketch a particular organelle's location while responding to certain parameters to evaluate intracellular bioprocesses are under urgent demand. It is significant to understand the basic ideas of organelle properties, as well as the vital substances related to each unique organelle, for the design of probes with high specificity and efficiency. In this review, we summarize representative multifunctional fluorescent molecular probes developed in the last decade. We focus on probes that can specially target nuclei, mitochondria, endoplasmic reticulums, and lysosomes. In each section, we first briefly introduce the significance and properties of different organelles. We then discuss how probes are designed to make them highly organelle-specific. Finally, we also consider how probes are constructed to endow them with additional functions to recognize particular physical/chemical signals of targeted organelles. Moreover, a perspective on the challenges in future applications of highly specific molecular probes in cell imaging is also proposed. We hope that this review can provide researchers with additional conceptual information about developing probes for cell imaging, assisting scientists interested in molecular biology, cell biology, and biochemistry to accelerate their scientific studies.

Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

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.

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.

A fluorescent probe based on POSS for facilitating the visualization of HClO and NO in living cells and zebrafish

The main production area of HClO and NO is the mitochondria and has modulatory effects on multiple human diseases. Simultaneous detection of signaling molecules such as HClO and NO is an important approach for exploring the complex relationship between HClO and NO in mitochondria. However, most probes can detect only one species or are unable to complete the monitoring of HClO and NO in the NIR channel. There are only few reports on reasonable tools that can simultaneously monitor the presence of HClO and NO in the NIR channel. In this work, to solve this difficulty, a POSS-assisted NIR fluorescent probe with dual-response was rationally devised and developed. The probe Mito-Cy possessed high specificity and responsiveness to HClO and NO in spectral experiments. Notably, the probe exhibited excellent responsiveness and sensitivity to HClO and NO in living cells and the zebrafish model.

Detection of subcellular nitric oxide in mitochondria using a pyrylium probe: assays in cell cultures and peripheral blood

Fluorescent probes for the detection of intracellular nitric oxide (NO) are abundant, but those targeted to the mitochondria are scarce. Among those molecules targeting mitochondrial NO (mNO), the majority use a triphenylphosphonium (TPP) cation as a vector to reach such organelles. Here we describe a simple molecule (mtNOpy) based on the pyrylium structure, made in a few synthetic steps, capable of detecting selectively NO (aerated medium) over other reactive species. The calculated detection limit for mtNOpy is 88 nM. The main novelty of this probe is that it has a simple molecular architecture and can act both as a fluorogenic and as a mitochondriotropic agent, without using TPP. mtNOpy has been tested in two different scenarios: (a) in a controlled environment of cell line cultures (human colon carcinoma HT-29 cells and mouse macrophage RAW 264.7 cells), using confocal laser scanning microscopy, and (b) on a much more complex sample of peripheral blood, using flow cytometry. In the first context, mtNOpy has been found to be responsive (turn-on fluorescence) to exogenous and endogenous NO stimuli (via SNAP donor and LPS stimulation, respectively). In the second area, mtNOpy has been able to discriminate between NO-generating phagocytes (neutrophils and monocytes) from other leukocytes (NK, B and T cells).

Small-molecule fluorescence-based probes for interrogating major organ diseases

Chemical tools that allow the real-time monitoring of organ function and the visualisation of organ-related processes at the cellular level are of great importance in biological research. The upregulation/downregulation of specific biomarkers is often associated with the development of organ related diseases. Small-molecule fluorescent probes have the potential to create advances in our understanding of these disorders. Viable probes should be endowed with a number of key features that include high biomarker sensitivity, low limit of detection, fast response times and appropriate in vitro and in vivo biocompatibility. In this tutorial review, we discuss the development of probes that allow the targeting of organ related processes in vitro and in vivo. We highlight the design strategy that underlies the preparation of various promising probes, their optical response to key biomarkers, and proof-of-concept biological studies. The inherent drawbacks and limitations are discussed as are the current challenges and opportunities in the field. The hope is that this tutorial review will inspire the further development of small-molecule fluorescent probes that could aid the study of pathogenic conditions that contribute to organ-related diseases.

Early Diagnosis of Cerebral Ischemia Reperfusion Injury and Revelation of Its Regional Development by a H3R Receptor-Directed Probe

In vivo imaging of cerebral hydrogen peroxide (H2O2) may facilitate early diagnosis of cerebral ischemia reperfusion injury (CIRI) and a revelation of its pathological progression. In this study, we report our rational design of a brain-targeting fluorescent probe using the basis of a pyridazinone scaffold. A structure-activity relationship study reveals that PCAB is the best candidate (K_i = 15.8 nM) for a histamine H3 receptor (H3R), which is highly expressed in neurons of the central nervous system. As a two-photon fluorescent probe, PCAB exhibits a fast, selective reaction toward both extra- and intracellular H2O2 in SH-SY5Y cells under oxygen glucose deprivation and resupply. In vivo fluorescent imaging of a middle cerebral artery occlusion mouse confirms that PCAB is an ultrasensitive probe with potent blood-brain barrier penetration, precise brain targeting, and fast detection of CIRI.

Highly Sensitive D-A-D-Type Near-Infrared Fluorescent Probe for Nitric Oxide Real-Time Imaging in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a common gastrointestinal inflammatory disease, affecting a huge number of people worldwide with increasing morbidity each year. Although the etiology of IBD has not been fully elucidated, it is understood to be closely related to upregulation of the production of NO. Herein, we first report a donor-acceptor-donor (D-A-D)-type near-infrared (NIR) fluorescent probe LS-NO for real-time detection of NO in IBD by harnessing the enhanced intramolecular charge transfer mechanism. LS-NO exhibited good water solubility, high photostability, and excellent NIR absorbance and emission at 700 and 750/800 nm, respectively. Moreover, it was able to sensitively and specifically detect exogenous and endogenous NO in the lysosomes of living cells. Notably, LS-NO enabled to noninvasively visualize NO generation in a lipopolysaccharide-induced IBD mouse model for 30 h, showing a two- to threefold higher NIR fluorescence intensity in the intestines and feces of IBD mice than normal mice. This work demonstrates that LS-NO is promising as a diagnosis agent for real-time detection of NO in IBD and may promote inflammatory stool examination simultaneously.

Visualizing the Underlying Signaling Pathway Related to Nitric Oxide and Glutathione in Cardiovascular Disease Therapy by a Sequentially Activated Fluorescent Probe

Clarifying the signaling pathway associated with nitric oxide (NO) and glutathione (GSH) in cardiovascular disease therapy is important for understanding its physiological and pathological processes but is challenging due to the lack of efficient analytical techniques. Herein, we report a BODIPY-based fluorescent probe for recognition of NO and GSH in sequence with high sensitivity and selectivity. The probe exhibits turn-on fluorescence triggered by NO, followed by red-shifted emission in the presence of GSH. The sequentially activated mechanism allows the visualization of NO-induced GSH upregulation in drug-treated endothelial cells and zebrafish for the first time, revealing a signal pathway during the therapy. We hope that it can be used as a convenient and efficient tool for the study of the interplay between NO and GSH and for the screening of effective drugs for cardiovascular disease therapy.

A coumarin embedded highly sensitive nitric oxide fluorescent sensor: kinetic assay and bio-imaging applications

Fluorescence spectroscopy is a significant bio-analytical technique for specific detection of nitric oxide (NO) and for broadcasting the in vitro and in vivo biological activities of this gasotransmitter. Herein, a benzo-coumarin embedded smart molecular probe (BCM) is employed for NO sensing through detailed fluorescence studies in purely aqueous medium. All the spectroscopic analysis and literature reports clearly validate the mechanistic insight of this sensing strategy i.e., the initial formation of 1,2,3,4-oxatriazole on treatment of the probe with NO which finally converted to its carboxylic acid derivative. This oxatriazole formation results in a drastic enhancement in fluoroscence intensity due to the photoinduced electron transfer (PET) effect. The kinetic investigation unveils the second and first-order dependency on [NO] and [BCM] respectively. The very low detection limit (16 nM), high fluorescence enhancement (123 fold) in aqueous medium and good formation constant (K_f = (4.33 ± 0.48) × 10⁴ M^-1) along with pH invariability, non-cytotoxicity, biocompatibility and cell permeability make this probe a very effective one for tracking NO intracellularly.

A highly specific probe for the imaging of inflammation-induced endogenous nitric oxide produced during the stroke process

In this study, a turn-on two-photon fluorescent probe (Lyso-TP-NO) for nitric oxide (NO) was developed. It was synthesized using 4-ethylamino-1,8-naphthalimide as the two-photon fluorophore and N-methylaniline moiety as the reaction site. The probe and fluorophore were tested under one- and two-photon modes. The fluorescence intensity of the system was enhanced 23.1-fold after reacting with NO in the one-photon mode. However, the maximal two-photon action cross-section value of 200 GM was obtained under excitation at 840 nm. The probe exhibits high selectivity and sensitivity over other reactive oxygen species (ROS) and reactive nitrogen species (RNS), with a detection limit as low as 3.3 nM. The two-photon fluorescence imaging of living cells and mouse brain tissues can capture inflammation-induced endogenous NO production in lysosomes during stroke occurrence.