Aromatic secondary amine-functionalized fluorescent NO probes: improved detection sensitivity for NO and potential applications in cancer immunotherapy studies

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.

Nanomaterial-Based Repurposing of Macrophage Metabolism and Its Applications

Macrophage immunotherapy represents an emerging therapeutic approach aimed at modulating the immune response to alleviate disease symptoms. Nanomaterials (NMs) have been engineered to monitor macrophage metabolism, enabling the evaluation of disease progression and the replication of intricate physiological signal patterns. They achieve this either directly or by delivering regulatory signals, thereby mapping phenotype to effector functions through metabolic repurposing to customize macrophage fate for therapy. However, a comprehensive summary regarding NM-mediated macrophage visualization and coordinated metabolic rewiring to maintain phenotypic equilibrium is currently lacking. This review aims to address this gap by outlining recent advancements in NM-based metabolic immunotherapy. We initially explore the relationship between metabolism, polarization, and disease, before delving into recent NM innovations that visualize macrophage activity to elucidate disease onset and fine-tune its fate through metabolic remodeling for macrophage-centered immunotherapy. Finally, we discuss the prospects and challenges of NM-mediated metabolic immunotherapy, aiming to accelerate clinical translation. We anticipate that this review will serve as a valuable reference for researchers seeking to leverage novel metabolic intervention-matched immunomodulators in macrophages or other fields of immune engineering.

A fluorescent probe with an ultra-rapid response to nitric oxide

Nitric oxide (NO) is a diatomic inorganic free radical ubiquitous in mammalian tissues and cells that plays a multifaceted role in a variety of physiological and pathophysiological processes. The strict dependence of the biological effects of NO on its concentration makes its real-time monitoring crucial. In view of the reactivity of NO with multiple bio-targets, the development of NO sensors that associate a fast response rate with selectivity and sensitivity is very challenging. Herein we report a fluorescent NO probe based on a BODIPY fluorogenic unit covalently linked to a trimethoxy aniline derivative through a flexible spacer. NO leads to effective nitrosation of the highly electron-rich amino active site of the probe through the secondary oxide N2O3, resulting in an increase of BODIPY fluorescence quantum yield from Φf = 0.06 to Φf = 0.55, accompanied by significant changes in the relative amplitude of the fluorescence lifetimes. In situ generation of NO, achieved by a tailored light-activatable NO releaser, allows the real-time detection of NO as a function of its concentration and permits demonstrating that the probe exhibits a very fast response time, being ≤0.1 s. This remarkable data combines with the high sensitivity of the probe to NO (LOD = 35 nM), responsiveness also to ONOO-, the other important secondary oxide of NO, independence from the fluorescence response within a wide pH range, good selectivity towards different analytes and small interference by typical physiological concentrations of glutathione. Validation of this probe in melanoma cell lines is also reported.

A Tandem-Locked Chemiluminescent Probe for Imaging of Tumor-Associated Macrophage Polarization

Tumor-associated macrophages (TAMs) play a role in both pro- and anti-tumor functions; and the targeted polarization from M2 to M1 TAMs has become an effective therapy option. Although detection of M1 TAMs is imperative to assess cancer immunotherapeutic efficacy, existing optical probes suffer from shallow tissue penetration depth and poor specificity toward M1 TAMs. Herein, we report a tandem-locked NIR chemiluminescent (CL) probe for specific detection of M1 TAMs. Through a combined molecular engineering approach via both atomic alternation and introduction of electron-withdrawing groups, near-infrared (NIR) chemiluminophores are screened out to possess record-long emission (over 800 nm), record-high CL quantum yield (2.7 % einstein/mol), and prolonged half-life (7.7 h). Based on an ideal chemiluminophore, the tandem-locked probe (DPDGN) is developed to only activate CL signal in the presence of both tumour (γ-glutamyl transpeptidase) and M1 macrophage biomarkers (nitric oxide). Such a tandem-lock design ensures its high specificity towards M1 macrophages in the tumor microenvironment over those in normal tissues or peripheral blood. Thus, DPDGN permits noninvasive imaging and tracking of M1 TAM in the tumor of living mice during R837 treatment, showing a good correlation with ex vivo methods. This study not only reports a new molecular approach towards highly efficient chemiluminophores but also reveals the first tandem-locked CL probes for enhanced imaging specificity.

Unique Synthetic Strategy for Probing in Situ Lysosomal NO for Screening Neuroinflammatory Phenotypes against SARS-CoV-2 RNA in Phagocytotic Microglia

In the pathogenesis of microglia, brain immune cells promote nitrergic stress by overproducing nitric oxide (NO), leading to neuroinflammation. Furthermore, NO has been linked to COVID-19 progression, which has caused significant morbidity and mortality. SARS-CoV-2 infection activates inflammation by releasing excess NO and causing cell death in human microglial clone 3 (HMC3). In addition, NO regulates lysosomal functions and complex machinery to neutralize pathogens through phagocytosis. Therefore, developing lysosome-specific NO probes to monitor phagocytosis in microglia during the COVID-19 infection would be a significant study. Herein, a unique synthetic strategy was adopted to develop a NO selective fluorescent probe, PDM-NO, which can discriminate activated microglia from their resting state. The nonfluorescent PDM-NO exhibits a turn-on response toward NO only at lysosomal pH (4.5-5.5). Quantum chemical calculations (DFT/TD-DFT/PCM) and photophysical study revealed that the photoinduced electron transfer (PET) process is pivotal in tuning optical properties. PDM-NO demonstrated good biocompatibility and lysosomal specificity in activated HMC3 cells. Moreover, it can effectively map the dynamics of lysosomal NO against SARS-CoV-2 RNA-induced neuroinflammation in HMC3. Thus, PDM-NO is a potential fluorescent marker for detecting RNA virus infection and monitoring phagocytosis in HMC3.

Mitochondria-targeted BODIPY dyes for small molecule recognition, bio-imaging and photodynamic therapy

Mitochondria are essential for a diverse array of biological functions. There is increasing research focus on developing efficient tools for mitochondria-targeted detection and treatment. BODIPY dyes, known for their structural versatility and excellent spectroscopic properties, are being actively explored in this context. Numerous studies have focused on developing innovative BODIPYs that utilize optical signals for imaging mitochondria. This review presents a comprehensive overview of the progress made in this field, aiming to investigate mitochondria-related biological events. It covers key factors such as design strategies, spectroscopic properties, and cytotoxicity, as well as mechanism to facilitate their future application in organelle imaging and targeted therapy. This work is anticipated to provide valuable insights for guiding future development and facilitating further investigation into mitochondria-related biological sensing and phototherapy.

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.

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

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

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.

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.

Thermodynamic stability of cisplatin-loaded polymeric micelles and the phenotypic switching of the tumor-associated macrophages induced by combination of cisplatin-loaded micelles and Anti-PD-L1 antibody

Chemotherapy is an effective anti-tumor treatment. Some anticancer chemotherapeutic drugs can not only induce cell death, but can also elicit antitumor immune responses. Here, the stability of cisplatin-loaded polymeric micelles (CDDP-PMs), pharmacokinetic drug-drug interactions of CDDP and anti-PD-L1 antibody (aPD-L1) in vivo and the alteration of the tumor microenvironment by combination of CDDP-PMs and aPD-L1 were evaluated. CDDP-PMs were fabricated by coordinated complexation and self-assembly method for tumor targeting. CDDP-PMs with higher mass ratio of copolymer have higher thermodynamic stability. The pharmacokinetic study showed that the CDDP and aPD-L1 were metabolized and cleared by two different pathways, suggesting that there is almost no risk of potential drug interactions between CDDP and aPD-L1 and the combination of aPD-L1 and CDDP- PMs may not alter the tissue distribution of CDDP. In vivo antitumor test showed that the tumor growth inhibition rates of CDDP-PMs combined with medium-dose aPD-L1 and CDDP-PMs combined with high-dose PD-L1 were 89.41% and 93.16%, respectively and therapeutic efficacy can be further increased by increasing the dose of aPD-L1 in co-administration group. This therapeutic system by combining chemotherapy and immunotherapy further increases the link between them and holds great potential to offer better safety and antitumor efficacy profiles.

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.

Tumor associated macrophage and microbe: The potential targets of tumor vaccine delivery

The occurrence and development of tumors depend on the tumor microenvironment (TME), which is made of various immune cells, activated fibroblasts, basement membrane, capillaries, and extracellular matrix. Tumor associated macrophages (TAMs) and microbes are important components in TME. Tumor cells can recruit and educate TAMs and microbes, and the hijacked TAMs and microbes can promote the progression of tumor reciprocally. Tumor vaccine delivery remodeling TME by targeting TAM and microbes can not only enhance the specificity and immunogenicity of antigens, but also contribute to the regulation of TME. Tumor vaccine design benefits from nanotechnology which is a suitable platform for antigen and adjuvant delivery to catalyze new candidate vaccines applying to clinical therapy at unparalleled speed. In view of the characteristics and mechanisms of TME development, vaccine delivery targeting and breaking the malignant interactions among tumor cells, TAMs, and microbes may serve as a novel strategy for tumor therapy.

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).

A highly selective ratiometric molecular probe for imaging peroxynitrite during drug-induced acute liver injury

Drug-induced acute liver injury (DIALI) is a common liver disease, affecting a number of people worldwide with increasing morbidity each year. Thus, it is vital to develop new tools for intervention and diagnosis. Peroxynitrite (ONOO^-), a highly reactive species, plays an important role in the DIALI process. Thus, in situ molecular imaging of endogenous ONOO^- levels is considerably significant for detecting ONOO^-. In this work, we present two destroyed-type ratiometric fluorescent probes, AHC and AHMC, for ONOO^- detection by using a molecular hybridization strategy. The probe AHMC was developed by introducing the ester structure to AHC directly to enhance its membrane penetrability for living cell imaging. Probe AHC exhibited good analytical performance toward ONOO^- compared to other reactive species, with a low detection limit (≈1.8 nM) and a strong ratiometric fluorescence response (134-fold). In cell imaging experiments, AHMC showed outstanding selectivity, favourable biocompatibility and mitochondria-targeting ability, which not only was used to detect endogenous and exogenous ONOO^- changes, but also enabled noninvasive visualization of ONOO^- generation in a different drug-induced DIALI model. We hope that these ratiometric probes can be useful chemical tools for the in-depth research of drug-induced acute hepatotoxicity.

Polymer nanoparticles regulate macrophage repolarization for antitumor treatment

We demonstrate an intrinsic antitumor effect of polymer nanoparticles (P-NPs), which could re-program tumor-associated macrophages to pro-inflammatory phenotype. The intrinsic effect of P-NPs on macrophage repolarization and its combination with other therapies provide new ideas for drug delivery, macrophage regulation and immunotherapy in cancer treatment.

Naphthalimide-4-(4-nitrophenyl)thiosemicarbazide: A Fluorescent Probe for Simultaneous Monitoring of Viscosity and Nitric Oxide in Living Cells

Intracellular viscosity is a physicochemical factor that determines the outcomes of various biological processes, while nitric oxide (NO) is an essential signaling molecule that controls many cellular processes, including oxidative stress. Anticipating that both may be interrelated with a variety of pathologies, their simultaneous measurement would be highly valuable for the investigation of the pathological condition of cells. However, the development of a sensor for such simultaneous detection has not been attempted yet. Herein, we present the synthesis of naphthalimide-4-(4-nitrophenyl)thiosemicarbazide, probe 1, and its application to living cells under conditions of lipopolysaccharide or nystatin treatment, adopted as oxidative stress and altered intracellular viscosity models, respectively. The probe showed increased fluorescence in response to elevation of viscosity and NO levels at 470 and 550 nm, respectively, in the solution studies. When the probe was used for a confocal microscopic study of HeLa cells under stressed conditions, simultaneous monitoring of viscosity and NO level elevations was possible through fluorescence imaging using band-pass filters of 420-475 and 505-600 nm, respectively, upon excitation at a wavelength of 405 nm. Interestingly, both the cellular viscosity and NO levels increased together under lipopolysaccharide or nystatin treatment. Therefore, we suggest that probe 1 is a fluorescent chemical probe that enables the monitoring of alterations in intracellular viscosity and NO levels in living cells, which would be valuable in studies of various cellular damage models.

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.

Highly selective, colorimetric probes for cyanide ion based on β-formylBODIPY dyes by an unprecedented nucleophilic addition reaction

Two β-formylBODIPYs with strong daylight excitable fluorescence and highly selective visual and colorimetric responses to cyanide anion (CN^-) have been prepared. NMR titration experiments have been performed to study the sensing mechanism for these two dyes. Surprisingly, cyanide anion is nucleophilic addition to the α-position of BODIPY core (the azafulvene framework) in aqueous system instead of the expected classical nucleophilic addition to the formyl moiety of the probes. This nucleophilic addition of cyanide anion to the azafulvene framework causes the interruption of the π-conjugation of the BODIPY system, which brings a significant blue-shift (up to 104 nm) of the absorption maxima. A broadened and decreased absorption as well as ratiometrical fluorescence response (with maxima shifts from 523 to 670 nm) were observed with the titration of cyanide anion to probe 1b.

Facilitation of molecular motion to develop turn-on photoacoustic bioprobe for detecting nitric oxide in encephalitis

Nitric oxide (NO) is an important signaling molecule overexpressed in many diseases, thus the development of NO-activatable probes is of vital significance for monitoring related diseases. However, sensitive photoacoustic (PA) probes for detecting NO-associated complicated diseases (e.g., encephalitis), have yet to be developed. Herein, we report a NO-activated PA probe for in vivo detection of encephalitis by tuning the molecular geometry and energy transformation processes. A strong donor-acceptor structure with increased conjugation can be obtained after NO treatment, along with the active intramolecular motion, significantly boosting "turn-on" near-infrared PA property. The molecular probe exhibits high specificity and sensitivity towards NO over interfering reactive species. The probe is capable of detecting and differentiating encephalitis in different severities with high spatiotemporal resolution. This work will inspire more insights into the development of high-performing activatable PA probes for advanced diagnosis by making full use of intramolecular motion and energy transformation processes.

Fluorescent small organic probes for biosensing

Small-molecule based fluorescent probes are increasingly important for the detection and imaging of biological signaling molecules due to their simplicity, high selectivity and sensitivity, whilst being non-invasive, and suitable for real-time analysis of living systems. With this perspective we highlight sensing mechanisms including Förster resonance energy transfer (FRET), intramolecular charge transfer (ICT), photoinduced electron transfer (PeT), excited state intramolecular proton transfer (ESIPT), aggregation induced emission (AIE) and multiple modality fluorescence approaches including dual/triple sensing mechanisms (DSM or TSM). Throughout the perspective we highlight the remaining challenges and suggest potential directions for development towards improved small-molecule fluorescent probes suitable for biosensing.

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 (Kf = (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.

Recent progress on the organic and metal complex-based fluorescent probes for monitoring nitric oxide in living biological systems

Nitric oxide (NO) is an important gaseous signaling molecule related to various human diseases. To investigate the biological functions of NO, many strategies have been developed for real-time monitoring the NO levels in biological systems. Among these strategies, fluorescent probes are considered to be one of the most efficient and applicable methods owing to their excellent sensitivity and selectivity, high spatiotemporal resolution, noninvasiveness, and experimental convenience. Therefore, great efforts have been paid to the design, synthesis, and fluorescence investigation of novel NO fluorescent probes in the past several years. However, few of them exhibit practical applications owing to the low concentration, short half-life, and rapid diffusion characteristics of NO in biological systems. Rational design of NO fluorescent probes with excellent selectivity and sensitivity, low cytotoxicity, long-lived fluorescent emission, and low background interference is still a challenge for scientists all over the word. To provide spatial-temporal information, this article focuses on the progress made in the organic and metal complex-based NO fluorescent probes during the past five years. The key structural elements and sensing mechanisms of NO fluorescent probes are discussed. Some novel ratiometric, luminescence, and photoacoustic probes with low background interference and deep tissue penetrating ability are mentioned. All these probes have been used for imaging exogenous and endogenous NO in cells and animal models. More importantly, this article also describes the development of multi-functional NO fluorescent probes, such as organelle targeting probes, dual-analysis probes, and probe-drug conjugates, which will inspire the design of various functional fluorescent probes.

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.

Dihydropyridine-coumarin-based fluorescent probe for imaging nitric oxide in living cells

Nitric oxide (NO) is a messenger molecule in organisms, participating in the regulation of many biological processes. The abnormal expression of NO is often observed in a variety of diseases, including cerebral ischemia, atherosclerosis, and cancer. However, a suitable tool that can directly and sensitively detect NO in vitro and in vivo is important for understanding its various biological functions. In this report, a new fluorescent probe for nitric oxide, DHP-4, was prepared, based on dihydropyridine-coumarin. DHP-4 was able to greatly enhance the fluorescence of NO, but did not affect the fluorescence emissions of other reactive oxygen species and nitrogen species, demonstrating its highly selective and sensitive response to NO. The probe generated stable optical signals in a buffer solution at pH values ranging from 3 to 10. In addition, DHP-4 could detect NO directly, showed low cellular toxicity, and was successfully applied to determine NO in Raw 264.7 cells, indicating its great potential as a tool for investigating the biological roles of NO in vivo.

High-Selectivity Fluorescent Reporter toward Peroxynitrite in a Coexisting Nonalcoholic Fatty Liver and Drug-Induced Liver Diseases Model

Peroxynitrite (ONOO^-), a highly reactive species, is profoundly involved in many physiological and pathological processes. Change of the ONOO^- level usually indicates an abnormal body function. Thus, it is desired to develop a highly reliable ONOO^- assay to elucidate its roles in a related disease environment. In this work, we have constructed a ratiometric molecule fluorescent probe RTFP toward ONOO^- with high specificity by the combination strategy of probe screening and a rational design method. RTFP displayed excellent detection sensitivity (detection limit: 4.1 nM) and produced a highly ratiometric emission signal (130-fold). Leveraging this probe, we showed the change of ONOO^- content in the free-fatty-acid-induced nonalcoholic fatty liver disease (NAFLD) and acetaminophen-induced drug-induced liver injury (DILI) cellular model and for the first time disclosed the involved mechanism of cytochrome P450 2E1 (CYP2E1) enzyme in NAFLD with a DILI pathological environment. Furthermore, RTFP also was utilized to visualize ONOO^- fluctuation of living liver tissues in a high-fat-diet-caused NAFLD model. We expected that this probe may help the study of liver injury in the exploration of mechanism and signal path.

Small-molecule fluorescent probes for imaging gaseous signaling molecules: current progress and future implications

Endogenous gaseous signaling molecules including nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) have been demonstrated to perform significant physiological and pharmacological functions and are associated with various diseases in biological systems. In order to obtain a deeper insight into their roles and mechanisms of action, it is desirable to develop novel techniques for effectively detecting gaseous signaling molecules. Small-molecule fluorescent probes have been proven to be a powerful approach for the detection and imaging of biological messengers by virtue of their non-invasiveness, high selectivity, and real-time in situ detection capability. Based on the intrinsic properties of gaseous signaling molecules, numerous fluorescent probes have been constructed to satisfy various demands. In this perspective, we summarize the recent advances in the field of fluorescent probes for the detection of NO, CO and H2S and illustrate the design strategies and application examples of these probes. Moreover, we also emphasize the challenges and development directions of gasotransmitter-responsive fluorescent probes, hoping to provide a general implication for future research.

A FRET-based supramolecular nanoprobe with switch on red fluorescence to detect SO2 derivatives in living cells

A fluorescent nanoprobe for detection of SO₂, an important gasotransmitter, is reported.

Rapid and sensitive detection of nitric oxide by a BODIPY-based fluorescent probe in live cells: glutathione effects

Glutathione effects on the sensing reaction toward nitric oxide in live cells.

Photostable Fluorescent Tracker for Imaging Mitochondria with Super Resolution

The power factories in cells, mitochondria, play important roles in all physiological processes. It is reported that progressive mitochondrial swelling and outer mitochondrial membrane rupture could be induced by a wide variety of apoptotic and necrotic stimuli. Regrettably, although a variety of mitochondrial probes have been developed, most of them are based on the detection of active species in mitochondria. Probes that can monitor the status and distribution of mitochondria for a long time are still urgently needed. In this study, a fluorescent sensor with excellent properties, EtNBEn, is described. Outstanding performance allows it to be observed not only in cells but also in living Daphnia and zebrafish under confocal microscopy for a long time. Moreover, the swelling process of mitochondria under light stimulation is also visualized under super-resolution (SR) microscopy. All these results suggest that EtNBEn could be employed for tagging mitochondria in various physiological processes, which makes a great contribution to the cure of diseases.

BODIPY-based hydrazine as a fluorescent probe for sensitive and selective detection of nitric oxide: a new strategy

A novel fluorescent probe 8-HB was developed with a BODIPY as a fluorophore and 8-substituted hydrazine as a reactive site for sensitive and selective detection of nitric oxide (NO), generating major fluorescent dehydrazinated BODIPY and minor non-fluorescent azide BODIPY.