A ratiometric fluorescent probe based on a coumarin-hemicyanine scaffold for sensitive and selective detection of endogenous peroxynitrite

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

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

Construction of a dual "off-on" near-infrared fluorescent probe for bioimaging of HClO in rheumatoid arthritis

Hypochlorous acid (HClO) serves as a critical biomarker in inflammatory diseases such as rheumatoid arthritis (RA), and its real-time imaging is essential for understanding its biological functions. In this study, we designed and synthesized a novel probe, RHMB, which ingeniously integrates rhodamine B and methylene blue fluorophores with HClO-specific responsive moieties into a single molecular framework. Upon exposure to HClO, RHMB exhibited significant dual-channel fluorescence enhancement characterized by high sensitivity (LODs of 2.55 nM and 14.08 nM), excellent selectivity, and rapid response time (within 5 s). Notably, RHMB enabled reliable imaging of both exogenous and endogenous HClO in living cells and in zebrafish, employing a unique duplex-imaging turn-on approach that highlighted its adaptability across various biological contexts. Furthermore, RHMB effectively monitored HClO fluctuations in an RA mouse model and assessed the therapeutic efficacy of diclofenac (Dic) in alleviating RA symptoms. These findings underscore the potential of RHMB as an invaluable tool for elucidating the biological roles of HClO in various diseases.

Super-resolution imaging for in situ monitoring sub-cellular micro-dynamics of small molecule drug

Small molecule drugs play a pivotal role in the arsenal of anticancer pharmacological agents. Nonetheless, their small size poses a challenge when directly visualizing their localization, distribution, mechanism of action (MOA), and target engagement at the subcellular level in real time. We propose a strategy for developing triple-functioning drug beacons that seamlessly integrate therapeutically relevant bioactivity, precise subcellular localization, and direct visualization capabilities within a single molecular entity. As a proof of concept, we have meticulously designed and constructed a boronic acid fluorescence drug beacon using coumarin-hemicyanine (CHB). Our CHB design includes three pivotal features: a boronic acid moiety that binds both adenosine triphosphate (ATP) and adenosine diphosphate (ADP), thus depleting their levels and disrupting the energy supply within mitochondria; a positively charged component that targets the drug beacon to mitochondria; and a sizeable conjugated luminophore that emits fluorescence, facilitating the application of structured illumination microscopy (SIM). Our study indicates the exceptional responsiveness of our proof-of-concept drug beacon to ADP and ATP, its efficacy in inhibiting tumor growth, and its ability to facilitate the tracking of ADP and ATP distribution around the mitochondrial cristae. Furthermore, our investigation reveals that the micro-dynamics of CHB induce mitochondrial dysfunction by causing damage to the mitochondrial cristae and mitochondrial DNA. Altogether, our findings highlight the potential of SIM in conjunction with visual drug design as a potent tool for monitoring the in situ MOA of small molecule anticancer compounds. This approach represents a crucial advancement in addressing a current challenge within the field of small molecule drug discovery and validation.

Mechanistic analysis of viscosity-sensitive fluorescent probes for applications in diabetes detection

Diabetes is one of the most detrimental diseases affecting the human life because it can initiate several other afflictions such as liver damage, kidney malfunctioning, and cardiac inflammation. The primary method for diabetes diagnosis involves the analysis of blood samples to quantify the level of glucose, while secondary diagnostic methods involve the qualitative analysis of obesity, fatigue, etc. However, all these symptoms start showing up only when the patient has been suffering from diabetes for a certain period of time. In order to avoid such delay in diagnosis, the development of specific fluorescent probes has attracted considerable attention. Prominent biomarkers for diabetes include abundance of certain analytes in blood serum, e.g., glucose, methylglyoxal, albumin, and reactive oxygen species; high intracellular viscosity; alteration of enzyme functionality, etc. Among these, high viscosity can greatly affect the fluorescence properties of various chromophores owing to the environment sensitivity of fluorescence spectra. In this review article, we have illustrated the application of some prominent fluorophores such as coumarin, BODIPY, xanthene, and rhodamine in the development of viscosity-dependent fluorescent probes. Detailed mechanistic aspects determining the influence of viscosity on the fluorescent properties of the probes have also been elaborated. Fluorescence mechanisms that are directly affected by the high-viscosity heterogeneous microenvironment are based on intramolecular rotations like twisted intramolecular charge transfer (TICT), aggregation-induced emission (AIE), and through-bond energy transfer (TBET). In this regard, this review article will be highly useful for researchers working in the field of diabetes treatment and fluorescent probes. It also provides a platform for the planning of futuristic clinical translation of fluorescent probes for the early-stage diagnosis and therapy of diabetes.

Near-infrared Fluorescent Probes with Long-acting Cyclic Monitoring and Effectively Eliminating Peroxynitrite

Two through-bond energy transfer fluorescent probes with a dihydroxyl naphthyl-pyrenyl conjugated system were synthesized for long-acting cyclic monitoring and eliminating peroxynitrite (ONOO- ). The probes exhibit large Stokes shifts (230 or 280 nm) and the fluorescence at 620 or 652 nm rapidly change in response to continuously variable concentrations of ONOO- under physiological conditions. The probes show good reversibility and can rapidly monitor the concentration changes of ONOO- in real time. In addition, with the additions of the probes, the decomposition of ONOO- is greatly accelerated. Therefore, the probes can effectively eliminate the excess ONOO- as well as sensing it. The biological studies showed that the probes can effectively and reversibly eliminate both exogenous and endogenous ONOO- in-situ as well as sensing its changes in cells, which can help to maintain the normal physiological concentration of ONOO- in organisms. This is the first system that a probe achieves multifunction including real-time detection, long-acting cyclic monitoring and in-situ elimination, thereby maintaining a normal physiological balance for ONOO- .

Construction of HPQ-based activatable fluorescent probe for peroxynitrite and its application in ferroptosis and mice model of LPS-induced inflammation

As one of the important members of reactive oxygen species, ONOO- plays a crucial role in signal transduction, immune response, and other physiological activities. Aberrant changes in ONOO- levels in the living organism are usually associated with many diseases. Therefore, it is important to establish a highly selective and sensitive method for the determination of ONOO- in vivo. Herein, we designed a novel ratio near-infrared fluorescent probe for ONOO- by directly conjugating dicyanoisophorone (DCI) to hydroxyphenyl-quinazolinone (HPQ). Surprisingly, HPQD was unaffected by environmental viscosity and responded rapidly to ONOO- within 40 s. The linear range of ONOO- detection was from 0 μM to 35 μM. Impressively, HPQD did not react with reactive oxygen species and was sensitive to exogenous/endogenous ONOO- in live cells. We also investigated the relationship between ONOO- and ferroptosis and achieved in vivo diagnosis and efficacy evaluation of mice model of LPS-induced inflammation, which showed promising prospects of HPQD in ONOO--related studies.

A mitochondria-targeted dual-response sensor for monitoring viscosity and peroxynitrite in living cells with distinct fluorescence signals

Viscosity and peroxynitrite (ONOO-) are two significant indicators to affect and evaluate the mitochondrial functional status, which are nearly relational with pathophysiological process in many diseases. Developing suitable analytical methods for monitoring mitochondrial viscosity changes and ONOO- is thus of great importance. In this research, a new mitochondria-targeted sensor DCVP-NO2 for the dual determination of viscosity and ONOO- was exploited based on the coumarin skeleton. DCVP-NO2 displayed a red fluorescence "turn-on" response toward viscosity along with about 30-fold intensity increase. Meanwhile, it could be used as ratiometric probe for detection of ONOO- with excellent sensitivity and extraordinary selectivity for ONOO- over other chemical and biological species. Moreover, thanks to its good photostability, low cytotoxicity and ideal mitochondrion-targeting capability, DCVP-NO2 was successfully utilized for fluorescence imaging of viscosity variations and ONOO- in mitochondria of living cells through different channels. In addition, the results of cell imaging revealed that ONOO- would lead to the increase of viscosity. Taken together, this work provides a potential molecular tool for researching biological functions and interactions of viscosity and ONOO- in mitochondria.

Recent Progress of Spectroscopic Probes for Peroxynitrite and Their Potential Medical Diagnostic Applications

Peroxynitrite (ONOO-) is a crucial reactive oxygen species that plays a vital role in cellular signal transduction and homeostatic regulation. Determining and visualizing peroxynitrite accurately in biological systems is important for understanding its roles in physiological and pathological activity. Among the various detection methods, fluorescent probe-based spectroscopic detection offers real-time and minimally invasive detection, high sensitivity and selectivity, and easy structural and property modification. This review categorizes fluorescent probes by their fluorophore structures, highlighting their chemical structures, recognition mechanisms, and response behaviors in detail. We hope that this review could help trigger novel ideas for potential medical diagnostic applications of peroxynitrite-related molecular diseases.

Affinity Studies of Hemicyanine Derived Water Soluble Colorimetric Probes with Reactive Oxygen/Nitrogen/Sulfur Species

Peroxynitrite (ONOO^- ) is an essential endogenous reactive oxygen species (ROS) generated in mitochondria under various pathological and physiological conditions. An increase in its level in mitochondria is related to numerous diseases. Herein, we report a series of hemicyanine-derived water-soluble colorimetric probes (1-4) and the reactivity of which was studied with various reactive oxygen, nitrogen, and sulfur species. Probes 1-4 are formed by conjugating 1,2,3,3-tetramethyl-3H-indolium iodide and 4-hydroxybenzaldehyde or its derivatives through an alkene linkage formed by the Knoevenagel reaction. Oxidative cleavage of the electron-rich double bond of the conjugated hemicyanine dye revealed a discerning affinity of probe 3 towards peroxynitrite among all reactive oxygen species. The rapid change in color of 3 provides a sensitive and selective method for detecting peroxynitrite with a low detection limit of 180 nM. Notably, the water solubility of the probe displays excellent performance for the selective detection of peroxynitrite among ROS and reactive nitrogen (RNS)/sulfur species (RSS). UV-vis, ¹ H NMR, and ¹³ C NMR spectroscopic data and results from theoretical calculations provide further information on the interaction of peroxynitrite with probe 3.

pH-Switched Near-Infrared Fluorescent Strategy for Ratiometric Detection of ONOO- in Lysosomes and Precise Imaging of Oxidative Stress in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is well-known as a kind of autoimmune disease, which brings unbearable pain to the patients by multiple organ complications besides arthritis. To date, RA can be hardly cured, but early diagnosis and standard treatment can relieve symptoms and pain. Therefore, an effective tool to assist the early diagnosis of RA deserves considerable attention. On account of the overexpressed ONOO^- during the early stage of RA, a near-infrared (NIR) receptor, Lyso-Cy, is proposed in this work by linker chemistry to expand the conjugated rhodamine framework by cyanine groups. Contributed by the pH-sensitive spiral ring in rhodamine, receptor Lyso-Cy has been found to be workable in lysosomes specifically, which was confirmed by the pH-dependent spectra with a narrow responding region and a well-calculated pK_a value of 5.81. We presented an excellent ratiometric sensing protocol for ONOO^- in an acidic environment, which was also available for targeting ONOO^- in lysosomes selectively. This innovative dual-targeting responsive design is expected to be promising for assisting RA diagnosis at an early stage with respect to the joint inflammatory model established in this work at the organism level.

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.

A ratiometric fluorescent nanoprobe for ultrafast imaging of peroxynitrite in living cells

For ratiometrically imaging peroxynitrite (ONOO^-) in living cells, we devised and fabricated a novel fluorescent nanoprobe, NC-NP_530/460, in this study. To achieve ratiometric fluorescence response towards ONOO^-, NC-NP_530/460 used 3-(2-benzothiazolyl) coumarin (Cou-Bz) as the internal reference and 1,8-naphthimide derivative (Naph-PN) as a fluorescent ONOO^- probe. These compounds were incorporated into an amphiphilic block polymer called Pluronic F-127. In addition to an ultrafast response to ONOO^-, NC-NP_530/460 also showed great selectivity and sensitive detection (detection limit was 4.51 μM). It was important to note that NC-NP_530/460 demonstrated solid performance for ONOO^- fluorescence ratio imaging in living cells, highlighting its potential for ONOO^--related chemical biology research.

Unraveling the Chemosensing Mechanism by the 7-(Diethylamino)coumarin-hemicyanine Hybrid: A Ratiometric Fluorescent Probe for Hydrogen Peroxide

The hemicyanine hybrid containing the 7-(diethylamino)coumarin (ACou) donor attached to the cationic indolenium (Ind) acceptor through a vinyl linkage (ACou-Ind) represents a classic ratiometric fluorescent probe for detecting nucleophilic analytes, such as cyanide and reactive sulfur species (RSS), through addition reactions that disrupt dye conjugation to turn off red internal charge transfer (ICT) fluorescence and turn on blue coumarin emission. The chemosensing mechanism for RSS detection by ACou-Ind suggested in the literature has now been revised. Our studies demonstrate that thiolates react with ACou-Ind through conjugate addition to afford C4-SR adducts that lack coumarin fluorescence due to photoinduced electron transfer quenching by the electron-rich enamine intermediate. Thus, ACou-Ind serves as a turn-off probe through loss of red ICT fluorescence upon RSS addition. The literature also suggests that blue coumarin emission of thiolate adducts is enhanced in the presence of reactive oxygen species (ROS) due to ROS-mediated cellular changes. Our studies predict that such a scenario is unlikely and that thiolate adducts undergo oxidative deconjugation in the presence of H₂O₂, the pervasive ROS. Under basic conditions, H₂O₂ also reacts directly with ACou-Ind to generate intense coumarin fluorescence through an epoxidation process. The relevance of our chemosensing mechanism for ACou-Ind was assessed within live zebrafish, and implications for the utility of ACou-Ind for unraveling the interplay between RSS and ROS are discussed.

Photocontrollable Fluorescence Imaging of Mitochondrial Peroxynitrite during Ferroptosis with High Fidelity

Ferroptosis, a new regulatory cell death modality, underlies the pathogenesis of a broad range of disorders. Although much efforts have been made to uncover the molecular mechanisms, some mechanistic details of ferroptosis still remain poorly understood. Particularly, the functional relevance of mitochondrial reactive oxygen species (ROS) in ferroptosis is still highly controversial, which is partially due to the fact that it still remains puzzled how the mitochondrial ROS level varies during ferroptosis. The conventional mitochondria-targeted probes may react with cytosolic ROS and show fluorescence variation before entering mitochondria, thus probably giving a false result on the mitochondrial ROS level and leading to the misjudgment on its biofunction. To circumvent this issue, we rationally designed a photocontrollable and mitochondria-targeted fluorescent probe to in situ visualize the mitochondrial peroxynitrite (ONOO^-), which is the ROS member and mediator of ferroptosis. The photoactivated probe was endowed with a highly specific and sensitive fluorescence response to ONOO^-. Notably, the response activity could be artificially regulated with light irradiation, which ensured that all the probe molecules passed through the cytosol in the locked status and were then photoactivated after reaching mitochondria. This photocontrolled fluorescence imaging strategy eliminated the interference of ONOO^- outside the mitochondria, thus potentially afforded improved fidelity for mitochondrial ONOO^- bioimaging in live cells and animal models. With this probe, for the first time, we revealed the mitochondrial ONOO^- flux and its probable biological source during erastin-induced ferroptosis. These results suggest a tight correlation between mitochondrial ONOO^-/ROS and ferroptotic progression, which will further facilitate the comprehensive exploration and manipulation of ferroptosis.

A coumarin-based reversible two-photon fluorescence probe for imaging glutathione near N-methyl-D-aspartate (NMDA) receptors

Glutathione (GSH) is known to play a key role in the modulation of the redox environment in N-methyl-d-aspartate (NMDA) receptors. Coumarin derivative 1 bearing cyanoacrylamide and ifenprodil moieties was synthesized and reported to monitor GSH near NMDA receptors. The cyanoacrylamide moiety allows probe 1 to monitor GSH reversibly at pH 7.4 and the ifenprodil group acts as a directing group for NMDA receptors. Two-photon fluorescence microscopy allows probe 1 to successfully sense endogenous GSH in neuronal cells and hippocampal tissues with excitation at 750 nm. Furthermore, the addition of H₂O₂ and GSH induced a decrease and an increase in fluorescence emission. Probe 1 can serve as a potential practical imaging tool to get important information on GSH in the brain.

Resorufin-based fluorescent probe with elevated water solubility for visualizing fluctuant peroxynitrite in progression of inflammation

Inflammation is a significant protective response in biological systems and associated with various diseases. Peroxynitrite (ONOO^-) as a highly active oxidant participates in the inflammatory process of organisms. Thus, it is necessary to construct novel fluorescent probes for exploring inflammation-related diseases through detecting endogenous ONOO^-. Resorufin-based fluorescent probes for testing ONOO^- were rare and suffered from poor water solubility. In this work, we elaborately designed three resorufin-based incorporating isatin derivatives probes RF-ITs and successfully obtained two highly selective probes RF-IT-OC and RF-IT-EG for ONOO^-. Comparing the other two probes, RF-IT-EG containing triethylene glycol monomethyl ether on isatin moiety displayed better water solubility (3.2 mg/L), faster response rate (60 s), larger signal-to-noise ratio (103-fold) and lower detection limit (87 nM) for monitoring ONOO^-. The cells imaging results manifested that probe RF-IT-EG could be applied to trace endogenous ONOO^- with inappreciable cytotoxicity. Moreover, the RF-IT-EG was capable of tracking the fluctuation of endogenous ONOO^- in LPS-stimulated inflamed mouse leg models. This work will provide a faithful and promising probe for illustrating the roles of ONOO^- in various inflammation-related diseases.

Visualization of peroxynitrite in cyclophosphamide-induced oxidative stress by an activatable probe

Oxidative stress is considered to be one of the main contributors of cyclophosphamide (CP)-induced toxicity, and the generation of free radicals will cause the interruption of multiple signal transduction pathways. Peroxynitrite (ONOO^-) has strong oxidation and nitrification ability and is considered as an indirect indicator of oxidative stress. Therefore, it is necessary to design a fluorescent probe that can detect ONOO^- and monitor CP-induced oxidative stress during chemotherapy. Herein, we synthesized a lipid droplet targeting fluorescent probe SX-1 based on triphenylamine-benzoindocyanine. When ONOO^- is added to the probe SX-1, the CC bond in the probe is broken, thereby releasing fluorescence. The good spectral response characteristics enable SX-1 to successfully track the fluctuations of ONOO^- in living cells. Most importantly, we provided the first visual evidence that the level of ONOO^- in HeLa cells was up-regulated under CP induction. All results indicated that SX-1 has great potential in detecting drug-induced ONOO^-, and provided a new detection tool for a deeper understanding of drug-induced organism injury.

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...

A novel fast-response and highly selective AIEgen fluorescent probe for visualizing peroxynitrite in living cells, C. elegans and inflammatory mice

Most of the ONOO^- fluorescent probes have restricted applications because of their aggregation-caused quenching (ACQ) effect, long response time and low fluorescence enhancement. Herein, we developed a novel AIEgen fluorescent probe (PE-XY) based on a benzothiazole and quinolin scaffold with high sensitivity and selectivity for imaging of ONOO^-. The results indicated that probe PE-XY exhibited fast response towards ONOO^- with 2000-fold enhancement of fluorescence intensity ratio in vitro. Moreover, PE-XY exhibited a relatively high sensitivity (limit of detection: 8.58 nM), rapid response (<50 s), high fluorescence quantum yield (δ = 0.81) and excellent selectivity over other analytes towards ONOO^-in vitro. Furthermore, PE-XY was successfully applied to detect endogenous ONOO^- levels in living HeLa cells, C. elegans and inflammatory mice with low cytotoxicity. Overall, this work provided a novel fast-response and highly selective AIEgen fluorescent probe for real-time monitoring ONOO^- fluctuations in living systems.

Rational Design of Meso-Phosphino-Substituted BODIPY Probes for Imaging Hypochlorite in Living Cells and Mice

Meso-phosphino-substituted BODIPY probes were developed for concise and rapid detection of hypochlorite (ClO^-). Interestingly, the probe BP gave a turn-on fluorescence response by shutting the photoinduced electron transfer (PET) effect and extending the coplanar conjugated π-system. In contrast, the probe TMBP showed a colorimetric response toward ClO^-. The key role of the steric repulsions was revealed to be for altering the electronic distribution of the BODIPY core, resulting in these obviously different responses. Finally, the probe BP, with high selectivity and sensitivity toward ClO^- (LOD = 1.9 nM; response time, <15 s), was further employed in imaging the variations of exogenous and endogenous hypochlorite (ClO^-) in living RAW 264.7 cells and mouse inflammation models. If wisely utilized, this strategy with meso-phosphino BODIPY dyes may serve as a powerful platform for the preparation of novel chemosensors.

Indication of Dynamic Peroxynitrite Fluctuations in the Rat Epilepsy Model with a Near-Infrared Two-Photon Fluorescent Probe

Epilepsy is a chronic neurodegenerative disease that has seriously threatened human health. Accumulating evidence reveals that the pathological progression of epilepsy is closely related to peroxynitrite (ONOO^-). Unfortunately, understanding the physiological roles of ONOO^- in epilepsy is still challenging due to the lack of powerful imaging probes for the determination of the level of fluctuations of ONOO^- in the epileptic brain. Herein, a near-infrared (NIR) two-photon (TP) fluorescent probe [dicyanomethylene-4H-pyran (DCM)-ONOO] is presented to trace ONOO^- in living cells and in kainate (KA)-induced rat epilepsy models with satisfactory sensitivity and selectivity. The probe is composed of a NIR TP DCM fluorophore and a recognition moiety diphenylphosphinamide. The phosphoramide bond of the probe is interrupted after reacting with ONOO^- for 10 min, and then, the released amino groups emit strong fluorescence due to the restoration of the intramolecular charge transfer process. The probe can effectively detect the changes of endogenous ONOO^- with excellent temporal and spatial resolution in living cells and in rat epileptic brain. The imaging results demonstrate that the increasing level of ONOO^- is closely associated with epilepsy and severe neuronal damage in the brain under KA stimulation. In addition, the low-dose resveratrol can effectively inhibit ONOO^- overexpression and further relieve neuronal damage. With the assistance of TP fluorescence imaging in the epileptic brain tissue, we hypothesize that the abnormal levels of ONOO^- may serve as a potential indicator for the diagnosis of epilepsy. The TP fluorescence imaging based on DCM-ONOO provides a great potential approach for understanding the epilepsy pathology and diagnosis.

A highly selective AIEgen fluorescent probe for visualizing Cys in living cells and C. elegans

As essential biological thiols in organisms, Cys, Hcy, and GSH are closely related to each other, and they can be involved in various pathological processes if expression levels are abnormal.

ICT-based fluorescent ratiometric probe for monitoring mitochondrial peroxynitrite in living cells

Mitochondria-targeted near-infrared fluorescent probe for the detection of peroxynitrite and the bioimaging of peroxynitrite in cells.

"Loading-type" Plasmonic Nanoparticles for Detection of Peroxynitrite in Living Cells

To date, plasmon resonance energy transfer (PRET)-based analytical approaches still inevitably suffer from limitations, such as lack of appropriate acceptor-donor pairs and the extra requirements of active groups of acceptors, which place great obstacles in extending the application of such methods, especially in the area of living cell studies. Herein, we design and fabricate a kind of "loading-type" plasmonic nanomaterials constituting gold nanoparticles as donors of PRET coated with mesoporous silicon, in which organic small molecules (CHCN) as acceptors of PRET were loaded (Au@MSN-CHCN). This "loading-type" strategy could conveniently integrate acceptor-donor pairs into one nanoparticle, so as to achieve the goal of sensitive detection of biomolecules in a complex physiological microenvironment. Based on the change of PRET efficiency of Au@MSN-CHCN induced by the specific reaction between CHCN and peroxynitrite (ONOO^-), ONOO^-, which plays an irreplaceable role in a series of physiological and pathological processes, is sensitively and selectively detected. Furthermore, in situ imaging of exogenous and endogenous ONOO^- in living cells was achieved even at a single nanoparticle level. This work provides a general approach to construct PRET probes for visualizing various biomolecules in living cells.

A turn-on red-emitting fluorescent probe for determination of copper(II) ions in food samples and living zebrafish

Herein, we developed a turn-on red-emitting fluorescent probe for the sensitive and selective detection of copper ions (Cu²⁺) in food samples and living zebrafish. The probe employs a hemicyanine scaffold as the fluorophore and a 2-pyridinecarbonyl group as the recognition receptor and quenching moiety. The 2-pyridinecarbonyl moiety can be specifically cleaved by Cu²⁺ and results in an approximately 18-fold fluorescence enhancement of the probe, thereby providing a fluorescence turn-on assay for Cu²⁺. Additionally, the probe exhibited excellent selectivity, high sensitivity, a broad linear relationship (0.020 to 8.0 μM), and a low limit of detection (4.0 nM, S/N = 3) for Cu²⁺. Concomitantly, the probe exhibited satisfactory analytical performance when used with actual food samples. Moreover, the probe could be used for in situ determination of Cu²⁺ in both living plant tissues and in living zebrafish.

A caveat to common hemicyanine dye components and their resolution

Near-infrared-emitting hemicyanine dyes are widely used in activatable fluorescent probes for various biological analytes; however, they are chemically unstable and show photoinstability, as shown here with naphthalene-based hemicyanines containing a typical hemicyanine moiety, 2-indolium. These issues can be resolved with a 4-pyridinium derivative, which also has good two-photon imaging capability.

Fluorescent detection of peroxynitrite during antibody-dependent cellular phagocytosis

Peroxynitrite (PNT) is a highly reactive oxidant that plays a key role in the destruction of foreign pathogens by specific phagocytic immune cells such as macrophages. However, when its production is dysregulated, this oxidant can contribute to cardiovascular disease, neurological diseases, and cancer. To facilitate the detection of PNT in living cells, we designed and synthesized a fluorescent sensor termed PS3 that accumulates in membranes of the endoplasmic reticulum (ER). This subcellular targeting enhances the proximity of PS3 to the phagosome of macrophages where PNT is generated. When PS3-treated macrophages are stimulated with 10 µm opsonized tentagel microspheres, antibody-dependent cellular phagocytosis (ADCP) of these particles results in production of endogenous PNT, oxidative cleavage of the fluorescence-quenching phenolic side chain of PS3, and increased fluorescence that can be detected by confocal laser scanning microscopy, flow cytometry, and other assays. We describe methods for the synthesis of PS3 and evaluation of its photophysical properties, selectivity, and reactivity. We further report differential production of PNT during ADCP by the phagocytic cell lines RAW 264.7, J774A.1, and THP-1, as detected by confocal microscopy and changes in fluorescence intensity on 96-well plates. This approach may be useful for identification of modulators of PNT and related studies of ADCP.

A Silyl Ether Based Fluorescent Probe for Rapid Monitoring of Endogenous Peroxynitrite Concentration and Imaging in Living Cells through Multicolor Emission

The peroxynitrite ion (ONOO^- ) has important roles in many biological processes. We have developed a multicolor ONOO^- -sensing probe (SiONNOH) that undergoes deprotonation and desilylation processes, which result in several changes in the emission wavelengths. In response to different concentrations of ONOO^- , the probe exhibits fluorescence changes from pink (595 nm at 2 eq. ONOO^- ) to green (540 nm at 6 eq. ONOO^- ) via orange (3 eq. ONOO^- ) and yellow (4 eq. ONOO^- ) under physiological conditions until no fluorescence signal is observed after ONOO^- is completely eliminated by lipoic acid. The probe shows the high selectivity for ONOO^- and the limit of detection is calculated to be 1.27 μM. Moreover, the probe shows the capacity to monitor the concentration ranges of ONOO^- through multicolor fluorescence in living cells, which will greatly facilitate the rapid detection of ONOO^- concentration ranges by the naked eye under a UV light without any precision instrumentation.

Activatable fluorescent probes for hydrolase enzymes based on coumarin–hemicyanine hybrid fluorophores with large Stokes shifts

We show that the equilibrium of intramolecular spirocyclization of coumarin–hemicyanine hybrid fluorophores can be finely tuned by means of chemical modifications to develop activatable fluorescence probes for hydrolases with large Stokes shift.

Rational modulation of coumarin–hemicyanine platform based on OH substitution for higher selective detection of hypochlorite

A NIR and ratiometric fluorescent probe was developed to quantitate arthritis-dependent HOCl production in vivo with high selectivity and sensitivity.

A new long-wavelength fluorescent probe for tracking peroxynitrite in live cells and inflammatory sites of zebrafish

Design of a long-wavelength fluorescent probe for tracking peroxynitrite in live cells and inflammatory sites of zebrafish.

Simultaneous Detection of Intracellular Nitric Oxide and Peroxynitrite by a Surface-Enhanced Raman Scattering Nanosensor with Dual Reactivity

A functional surface-enhanced Raman scattering (SERS) nanosensor which can simultaneously detect nitric oxide (NO) and peroxynitrite (ONOO^-) in living cells is explored. The SERS nanosensor is fabricated through modifying gold nanoparticles (AuNPs) with newly synthesized 3,4-diaminophenylboronic acid pinacol ester (DAPBAP), which has two reactive groups. The simultaneous detection achieved in this work is not only because of the SERS spectral changes of the nanosensor resulting from the dual reactivity of DAPBAP on AuNPs with NO and ONOO^- but also by the narrow SERS bands suitable for multiplex detection. Owing to the combination of SERS fingerprinting information and chemical reaction specificity, the nanosensor has great selectivity for NO and ONOO^-, respectively. In addition, the nanosensor has a wide linearity range from 0 to 1.0 × 10^-4 M with a submicromolar sensitivity. More importantly, simultaneous monitoring of NO and ONOO^- in the Raw264.7 cells has been fulfilled by this functional nanosensor, which shows that the SERS strategy will be promising in comprehension of the physiological issues related with NO and ONOO^-.

A specific AIE and ESIPT fluorescent probe for peroxynitrite detection and imaging in living cells

Developing specific and sensitive method for endogenous peroxynitrite (ONOO^-) in living systems is valuable to understand its various pathological events. In this work, a simple and novel fluorescent probe (HPP) based on aggregation induced emission (AIE) as well as excited state intramolecular proton transfer (ESIPT) processes for endogenous ONOO^- detection was constructed containing a diphenylphosphinate as the reactive site and a salicylaldehyde azine as the fluorophore. The probe showed specific fluorescence turn-on response toward ONOO^- owing to the diphenylphosphinate group of HPP reacted with ONOO^-, releasing the salicylaldehyde azine with both AIE and ESIPT characteristics. In addition, the probe exhibited a large Stokes shift, an excellent light-up ratio, high selectivity and excellent sensitivity with a low detection limit of 8 × 10^-8 M for endogenous ONOO^-. Moreover, the probe could be applied to bioimaging of endogenous ONOO^- in live cell, which demonstrated the probe can be used as effective tool for investigation of ONOO^- in biological systems.

A fluorescent sensor for folic acid based on crown ether-bridged bis-tetraphenylethylene

Aggregation-induced emission (AIE) provides a new strategy for preparing fluorescent sensors in aggregated state. In this paper, a series of crown ether-bridged bis-tetraphenylethylene compounds were synthesized in 78-84% yield by a simple procedure. The molecules exhibited excellent AIE properties in THF/H2O solutions and solid films. The investigation on sensing abilities for various biomolecules and metal ions suggested that Bis-TPE-1 possessed obvious response to folic acid, with fluorescence enhancement and blue shift of maximum emission wavelength from 380 nm to 365 nm. The detection limit for folic acid was 6.36 × 10-7 M, and the sensor's selectivity for folic acid was little interfered by the other species. The sensor mechanism was studied by FT-IR, 1H NMR, MS spectra and fluorescence Jobs' plot. The selective sensor for folic acid was applied in test paper and the analyses of real samples of mung bean and spinach. The superior bioimaging performance of Bis-TPE-1 for sensing folic acid was confirmed by the live cell imaging experiments, which indicated its good practical application potential for detecting folic acid.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

A red-emitting styrylnaphthalimide-based fluorescent probe providing a ratiometric signal change for the precise and quantitative detection of H2O2

A red-emitting and ratiometric fluorescence probe 1 for detecting H₂O₂, based on a styrylnaphthalimide-boronate ester was developed. Upon a H₂O₂-mediated hydrolysis of boronate ester, probe 1 was transformed to 2 with a ratiometric fluorescence change, decrease at 535 and increase at 640 nm. It was also found that the fluorescent reaction of 1 with H₂O₂ in solution could be completed within 10 min and the detection limit was estimated to be 0.30 μM. Moreover, this ratiometric change was highly selective for H₂O₂ over other redox species, metal ions, and anions. Also, this system was found to be capable of detecting H₂O₂ in the pH range of 6-9. Furthermore, probe 1 was preferentially accumulated into the endoplasmic reticulum (ER) in the live HeLa cells, and an increased H₂O₂ level in the presence of an ER stress inducer, thapsigargin was revealed.

An ultralow concentration of two-photon fluorescent probe for rapid and selective detection of lysosomal cysteine in living cells

Herein we reported a two-photon (TP) fluorescence "turn-on" probe MNPO, exhibiting high selectivity and sensitivity towards intracellular cysteine (Cys) with excellent lysosomal localization. The probe displayed fast response towards Cys over homocysteine (Hcy), glutathione (GSH), and other various analytes under physiological conditions. Low cytotoxicity made it successful for TP imaging of Cys in HeLa cells with an ultralow probe concentration of 250 nM, and a rapid response of only 10 min. Simultaneously, colocalization experiments in lysosome demonstrated its ability for specific in situ detection of lysosomal Cys in living cells, which shed light on its potential applications in biomedical applications. Beyond that MNPO was successfully applied for TP imaging of Cys in mice organ tissues such as heart, liver, and spleen, and the penetration depth of mice heart tissue was up to 184 μm, which disclosed the predominant TP characteristic. We believe that this study will provide some useful information toward diagnosis and treatment of pathogenesis associated with Cys or lysosomes in future.

Research progress in the development of organic small molecule fluorescent probes for detecting H2O2

Hydrogen peroxide (H2O2), as an important signaling molecule during biological metabolism, is a key member of the reactive oxygen species (ROS) family. The excess of H2O2 will lead to oxidative stress, which is a crucial factor in the production of various ROS-related diseases. In order to study the diverse biological roles of H2O2 in cells and animal tissues, many methods have been developed to detect H2O2. Recently, fluorescence imaging has attracted more and more attention because of its high sensitivity, simple operation, experimental feasibility, and real-time online monitoring. Based on the response group, this study will review the research progress on hydrogen peroxide and summarizes the mechanisms, actualities and prospects of fluorescent probes for H2O2.