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.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Self-Assembled Activatable Probes to Monitor Interactive Dynamics of Intracellular Nitric Oxide and Hydrogen Sulfide

The increasing understanding of the intricate relationship between two crucial gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) in biological actions has generated significant interest. However, comprehensive monitoring of the dynamic fluctuations of endogenous NO and H2S remains a challenge. In this study, we have designed an innovative aggregation-induced reporter SAB-NH-SC with enhanced responsiveness to H2S for visualizing the fluctuations of intracellular NO and H2S. This probe leverages the hydrophilic properties of the pyridinium salt derivative, which can rapidly self-assemble into positively charged nanoparticles under physiological conditions, avoiding the introduction of organic solvents or tedious preparations. Notably, the reporter can repeatedly cycle S-nitrosation and SNO-transnitrosation reactions when successively treated with NO and H2S. Consequently, fluorescence alternation at 751 (H2S) and 639 nm (NO) facilitates the dynamic visualization of the alternating presence of H2S and NO within cells. This dynamic and reversible probe holds immense potential for unraveling the intricate interactions between NO and H2S in a complex network of biological applications.

Progress in the Development of Imidazopyridine-Based Fluorescent Probes for Diverse Applications

Different classes of Imidazopyridine i.e., Imidazo[1,2-a]pyridine, Imidazo[1,5-a] pyridine, Imidazo[4,5-b]pyridine, have shown versatile applications in various fields. In this review, we have concisely presented the usefulness of the fluorescent property of imidazopyridine in different fields such as imaging tools, optoelectronics, metal ion detection, etc. Fluorescence mechanisms such as excited state intramolecular proton transfer, photoinduced electron transfer, fluorescence resonance energy transfer, intramolecular charge transfer, etc. are incorporated in the designed fluorophore to make it for fluorescent applications. It has been widely employed for metal ion detection, where selective metal ion detection is possible with triazole-attached imidazopyridine, β-carboline imidazopyridine hybrid, quinoline conjugated imidazopyridine, and many more. Also, other popular applications involve organic light emitting diodes and cell imaging. This review shed a light on recent development in this area especially focusing on the optical properties of the molecules with their usage which would be helpful in designing application-based new imidazopyridine derivatives.

Naphthalimide derivatives as fluorescent probes for imaging endogenous gasotransmitters

Gasotransmitters have gained significant recognition attributed to their evident biological impacts, and is accepted as a promising and less-explored area with immense research scope. The three-member family comprising of nitric oxide, carbon monoxide and hydrogen sulphide as endogenous gaseous signaling molecules have been found to elicit a plethora of crucial biological functions, spawning a new research area. The sensing of these small molecules is vital to gain deeper insights into their functions, as they can act both as a friend or a foe in mammalian systems. The initial sections of the review present the physiological and pathophysiological roles of these endogenous gas transmitters and their synergistic interactions. Further, various detection approaches, especially the usage of fascinating features of 1,8-naphthalimide as fluorescent probe in the detection and monitoring of these small signaling molecules are highlighted. The current limitations and the future scope of improving the sensing of the three gasotransmitters are also discussed.

A therapeutic keypad lock decoded in drug resistant cancer cells

A molecular keypad lock that displays photodynamic activity when exposed to glutathione (GSH), esterase and light in the given order, is fabricated and its efficacy in drug resistant MCF7 cancer cells is investigated. The first two inputs are common drug resistant tumor markers. GSH reacts with the agent and shifts the absorption wavelength. Esterase separates the quencher from the structure, further activating the agent. After these sequential exposures, the molecular keypad lock is exposed to light and produces cytotoxic singlet oxygen. Among many possible combinations, only one 'key' can activate the agent, and initiate a photodynamic response. Paclitaxel resistant MCF7 cells are selectively killed. This work presents the first ever biological application of small molecular keypad locks.

Dual-locked spectroscopic probes for sensing and therapy

Optical imaging probes allow us to detect and uncover the physiological and pathological functions of an analyte of interest at the molecular level in a non-invasive, longitudinal manner. By virtue of simplicity, low cost, high sensitivity, adaptation to automated analysis, capacity for spatially resolved imaging and diverse signal output modes, optical imaging probes have been widely applied in biology, physiology, pharmacology and medicine. To build a reliable and practically/clinically relevant probe, the design process often encompasses multidisciplinary themes, including chemistry, biology and medicine. Within the repertoire of probes, dual-locked systems are particularly interesting as a result of their ability to offer enhanced specificity and multiplex detection. In addition, chemiluminescence is a low-background, excitation-free optical modality and, thus, can be integrated into dual-locked systems, permitting crosstalk-free fluorescent and chemiluminescent detection of two distinct biomarkers. For many researchers, these dual-locked systems remain a 'black box'. Therefore, this Review aims to offer a 'beginner's guide' to such dual-locked systems, providing simple explanations on how they work, what they can do and where they have been applied, in order to help readers develop a deeper understanding of this rich area of research.

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.

Dual functional luminescent nanoprobes for monitoring oxygen and chloride concentration changes in cells

A dual functional nanoprobe Pd-Q+@PDMS was proposed to simultaneously monitor Cl- and O2, leading to the determination of an average Cl- concentration of 85.7 ± 5.5 mM in lysosomes of HeLa cells. Mimicking ischemic conditions, the cells exhibited a luminescence change corresponding to a decreasing subcellular Cl- concentration.

Förster resonance energy transfer (FRET)-based small-molecule sensors and imaging agents

In this tutorial review, we will explore recent advances in the construction and application of Förster resonance energy transfer (FRET)-based small-molecule fluorescent probes. The advantages of FRET-based fluorescent probes include: a large Stokes shift, ratiometric sensing and dual/multi-analyte responsive systems. We discuss the underlying energy donor-acceptor dye combinations and emphasise their applications for the detection or imaging of cations, anions, small neutral molecules, biomacromolecules, cellular microenvionments and dual/multi-analyte responsive systems.

Luminescent probes for hypochlorous acid in vitro and in vivo

HClO/ClO^- is the most effective antibacterial active oxygen in neutrophils. However, its excessive existence often leads to the destruction of human physiological mechanisms. In recent years, the developed luminescent probes for the detection of HClO/ClO^- are not only conducive to improve the sensitivity and selectivity of HClO/ClO^- detection, but also play a crucial role in understanding the biological functions of HClO/ClO^-. In addition, luminescent probe-based biological imaging for HClO/ClO^- at sub-cellular resolution has become a powerful tool for biopathology and medical diagnostic research. This article reviews a variety of luminescent probes for the detection of HClO/ClO^-in vitro and in vivo with different design principles and mechanisms, including fluorescence, phosphorescence, and chemiluminescence. The photophysical/chemical properties and biological applications of these luminescent probes were outlined. Finally, we summarized the merits and demerits of the developed luminescent probes and discussed their challenges and future development trends. It is hoped that this review can provide some inspiration for the development of luminescent probe-based strategies and to promote the further research of biomedical luminescent probes for HClO/ClO^-.

Azide functionalized porphyrin based dendritic polymers for in vivo monitoring of Hg2+ ions in living cells

A porphyrin cored azide functionalised dendritic polymer was developed as a selective sensor for in vivo monitoring of mercuric ions in living (normal and cancer) cells and in an aqueous medium. The developed sensor could sense mercuric ions even at a nanomolar concentration with a limit of detection value of 0.9 nM. This probe can be used to monitor mercuric ions in living cells due to its low cytotoxicity and high cell permeability. The hydrophilic nature of the polymer makes it a promising candidate for sensing mercuric ions in real water samples. Moreover, the reversibility of this sensing strategy helps in constructing a logic gate, which is particularly useful in smart sensor design.

Experimental and Theoretical Studies on a Simple S-S-Bridged Dimeric Schiff Base: Selective Chromo-Fluorogenic Chemosensor for Nanomolar Detection of Fe2+ & Al3+ Ions and Its Varied Applications

A simple S-S (disulfide)-bridged dimeric Schiff base probe, L, has been designed, synthesized, and successfully characterized for the specific recognition of Al3+ and Fe2+ ions as fluorometric and colorimetric "turn-on" responses in a dimethylformamide (DMF)-H2O solvent mixture, respectively. The probe L and each metal ion bind through a 1:1 complex stoichiometry, and the plausible sensing mechanism is proposed based on the inhibition of the photoinduced electron transfer process (PET). The reversible chemosensor L showed high sensitivity toward Al3+ and Fe2+ ions, which was analyzed by fluorescence and UV-vis spectroscopy techniques up to nanomolar detection limits, 38.26 × 10-9 and 17.54 × 10-9 M, respectively. These experimental details were advocated by density functional theory (DFT) calculations. The practical utility of the chemosensor L was further demonstrated in electrochemical sensing, in vitro antimicrobial activity, molecular logic gate function, and quantification of the trace amount of Al3+ and Fe2+ ions in real water samples.

A ratiometric fluorescent probe based on AIEgen for detecting HClO in living cells

A high performance ratiometric fluorescent probe, namely TPE-RNS, has been developed for detecting exogenous and endogenous HClO/ClO⁻ in living cells and discriminating cancer cells from normal cells.

Zn(ii) phthalocyanines tetra substituted by aryl and alkyl azides: design, synthesis and optical detection of H2S

Experimental examination of two novel Zn(ii)-phthalocyanines having aryl and alkyl azide functional groups at the peripheral positions that have been designed/synthesized for hydrogen sulfide (H₂S) sensing purposes.

Naphthalimides in fluorescent imaging of tumor hypoxia - An up-to-date review

Hypoxia is a distinctive characteristic of advanced solid malignancies that results from a disparity between oxygen supply and its consumption. The degree of hypoxia is believed to have adverse prognostic significance. Therefore detecting cellular hypoxia can potentially offer insights into the grade of tumour as well as its evolution towards a progressive malignant phenotype, which clinically translates to greater metastatic potential and treatment resistance. Fluorescence imaging to visualize hypoxia in biological systems is a minimally-invasive method. Recently there are several reports on interdisciplinary research that aims at developing functional probes that can be efficiently used for non-invasive imaging of hypoxic tumours. Upregulated levels of nitroreductase (NTR) is detected in hypoxic solid malignancies, and this characteristic feature is increasingly utilized in the development of NTR-targeted fluorescent molecules to selectively sense hypoxia in vivo. The present review summarizes various reports published on the design concepts of nitro naphthalimide-based bio-reductive fluorescent sensors that can be applied noninvasively to image hypoxia in cancer.

Fluorometric determination and intracellular imaging of cysteine by using glutathione capped gold nanoclusters and cerium(III) induced aggregation

A turn-on fluorometric method is described for selective and sensitive detection of cysteine (Cys). Gold nanoclusters (Au NCs) capped with glutathione (GSH) are used as a fluorescent probe. If Ce³⁺ ion are present, they will bind to the carboxy groups of the GSH-capped Au NC. This results in aggregation-induced emission enhancement (AIEE), best measured at excitation/emission wavelengths of 360/575 nm. On addition of Cys, which has less steric hindrance compared with GSH and higher affinity for Ce³⁺, it will bind to Ce³⁺ through the carboxyl group and link with Au NCs via Au-S bond. Hence, the AIEE is increased and Cys can be quantified via this effect with a linear response in the 0.4-120 μmol L^-1 Cys concentration range and a detection limit of 0.15 μmol L^-1. Graphical abstract Schematic presentation of cysteine detection via the Ce³⁺-triggered aggregation of glutathione capped gold nanoclusters which leads to increased yellow fluorescence.

Bimodal fluorogenic sensing of matrix proteolytic signatures in lung cancer

Optical biosensing based on the activation of fluorescent reporters offers a powerful methodology for the real-time molecular interrogation of pathology. Here we report a first-in-class, bimodal fluorescent reporter strategy for the simultaneous and highly specific detection of two independent proteases (thrombin and matrix metalloproteases (MMPs)) pivotal in the fibroproliferative process surrounding lung cancer, based on a dual, multiplexing, peptide FRET system. This sophisticated synthetic smartprobe, with a molecular weight of 6 kDa, contains two independent fluorophores and quenchers that generate photonic signatures at two specific wavelengths upon activation by target enzymes within human lung cancer tissue.

A multiple target chemosensor for the sequential fluorescence detection of Zn2+ and S2− and the colorimetric detection of Fe3+/2+ in aqueous media and living cells

A novel multiple target sensor DHIC was developed for the fluorescence detection of Zn²⁺ and S²⁻ and colorimetric detection of Fe^3+/2+. Moreover, DHIC could image sequentially Zn²⁺ and S²⁻ in living cells.

A colorimetric and fluorescent chemosensor for Hg2+ based on a photochromic diarylethene with a quinoline unit

A new colorimetric and fluorescent 'on-off' chemosensor, 1O, based on a photochromic diarylethene with a quinoline unit was designed and synthesized. The chemosensor 1O demonstrated selective and sensitive detection of Hg2+ ions in the presence of other competitive metal ions in acetonitrile. The stoichiometric ratio of the sensor 1O for Hg2+ was determined to be 1 : 1, and the limit of detection of the probe 1O was calculated to be 56.3 nM for Hg2+. In addition, a molecular logic circuit with four inputs and one output was successfully constructed with UV/vis light and metal-responsive behavior. ESI-MS spectroscopy, Job's plot analysis, and 1H NMR titration experiments confirm the binding behavior between 1O and Hg2+.

Consumption of H2S from Our Daily Diet: Determination by a Simple Chemosensing Method

A unique method has been developed for comparative analysis of H₂S produced from food samples from our daily diet, both qualitatively and quantitatively. The selective detection of H₂S has been executed by introducing a simple chemodosimeter (PN-N ₃ ) that gives response on the basis of intramolecular charge transfer. UV-vis, fluorimetric, and NMR titrations were performed to demonstrate the sensing mechanism and electronic environment of PN-N ₃ in the presence of H₂S. Density functional theory calculations were performed to validate the mechanism of azide (PN-N ₃ ) reduction to amine (PN-NH ₂ ) by the strong reducing power of H₂S. The potentiality of this chemosensing method is that it could be treated as a simple, less-time-consuming, and cost-effective method for determining H₂S in biological samples in the nanomolar range.

Two colorimetric and ratiometric fluorescence probes for hydrogen sulfide based on AIE strategy of α-cyanostilbenes

Aggregation-induced emission (AIE) active fluorescent probes have attracted great potential in biological sensors. In this paper two cyanostilbene based fluorescence chemoprobe Cya-NO₂ (1) and Cya-N₃ (2) were developed and evaluated for the selective and sensitive detection of hydrogen sulfide (H₂S). Both of these probes behave aggression-induced emission (AIE) activity which fluoresces in the red region with a large Stokes shift. They exhibit rapid response to H₂S with enormous colorimetric and ratiometric fluorescent changes. They are readily employed for assessing intracellular H₂S levels.

A dual-function fluorescent probe for monitoring the degrees of hypoxia in living cells via the imaging of nitroreductase and adenosine triphosphate

A new dual-function fluorescent probe is developed for detecting nitroreductase (NTR) and adenosine triphosphate (ATP) with different responses. Imaging application of the probe reveals that intracellular NTR and ATP display an adverse changing trend during a hypoxic process and ATP can serve as a new sign for cell hypoxia.

Evaluating Drug-Induced Liver Injury and Its Remission via Discrimination and Imaging of HClO and H2S with a Two-Photon Fluorescent Probe

Drug-induced liver injury (DILI) has aroused wide concern. Finding new markers or indicators as well as detoxification molecules for DILI is of great significance and good application prospect, which can help develop effective preclinical screening methodology and corresponding treatment protocols. Herein, in this article, DILI caused by antidepressant drugs of duloxetine and fluoxetine and its remission were evaluated by a two-photon fluorescent probe, RPC-1, through discriminating and imaging HClO and H₂S simultaneously. By being applied both in vitro and in vivo, RPC-1 revealed slight up-regulation of HClO and negligible liver damage after administration of either of the two drugs. In contrast, an apparent up-regulation of HClO and obvious liver damage was observed after combined administration of the drugs. Meanwhile, the pretreatment of N-acetyl cysteine (NAC) resulted in the increasing of endogenous H₂S level, which contributed to the remission of DILI. The histological analysis and serological test both gave good consistency with the imaging results. These findings demonstrate that HClO may be an appropriate indicator of DILI, and H₂S plays an important role in the antidotal effect of NAC. We envision that RPC-1 can be used as a powerful tool to predict clinical DILI and study the effect of antidote, as well as explore the molecular mechanisms involved.

In vivo imaging of hepatocellular nitric oxide using a hepatocyte-targeting fluorescent sensor

A hepatocyte-targeting fluorescent NO sensor has been fabricated with good water solubility, excellent selectivity, and high sensitivity (∼1.62 nM).

Fluorescent probes for the simultaneous detection of multiple analytes in biology

This review identifies and discusses fluorescent sensors that are capable of simultaneously reporting on the presence of two analytes for biological application.

An H2S-sensing/CO-releasing Flavonol that Operates via Logic Gates

Signaling molecules, including H2S and CO, are involved in a complex interplay within cells to maintain cellular homeostasis. In order to investigate the intracellular interplay of different gasotransmitters, new molecular tools are needed that combine sensing and release capabilities of different small molecules in a single, multifunctional, and highly-regulated molecular platform. This report gives the first example of a combined H2S sensor/CO-releasing molecule. This flavonol-based molecular tool operates intracellularly via a highly regulated AND logic gated input-output sequence and provides fluorescent feedback for the H2S detection and CO release steps. This linear sequence can be combined with a fluorescent molecular sensor for CO detection via a third distinct emission. Overall, this is the first molecular framework that can combine the sensing of H2S with the controlled release of CO, two gaseous molecules that are known to exhibit reciprocal regulation and have overlapping targets in cellular environments.

Fluorescent and cooperative ion pair receptor based on tolan for Na+ (or Li+) and HSO4-: logic AND gate

A tolan derivative was synthesized as a fluorescent and cooperative ion pair receptor. As both Na⁺ and HSO₄^- ions were complexed to the receptor, only substantial fluorescence was quenched. Thus, it also acts as a logic AND gate.

One-pot fabrication of FRET-based fluorescent probe for detecting copper ion and sulfide anion in 100% aqueous media

The design of effective tools for detecting copper ion (Cu²⁺) and sulfide anion (S^2-) is of great importance due to the abnormal level of Cu²⁺ and S^2- has been associated with an increase in risk of many diseases. Herein, we report on the fabrication of fluorescence resonance energy transfer (FRET) based fluorescent probe PF (PEI-FITC) for detecting Cu²⁺ and S^2- in 100% aqueous media via a facile one-pot method by covalent linking fluorescein isothiocyanate (FITC) with branched-polyethylenimine (b-PEI). PF could selectively coordinate with Cu²⁺ among 10 metal ions to form PF-Cu²⁺ complex, resulting in fluorescence quenching through FRET mechanism. Furthermore, the in situ generated PF-Cu²⁺ complex can be used to selectively detect S^2- based on the displacement approach, resulting in an off-on type sensing. There is no obvious interference from other anions, such as Cl^-, NO₃^-, ClO₄^-, SO₄^2-, HCO₃^-, CO₃^2-, Br^-, HPO₄^2-, F^- and S₂O₃^2-. In addition, PF was successfully used to determine Cu²⁺ and S^2- in human serum and tap water samples. Therefore, the FRET-based probe PF may provide a new method for selective detection of multifarious analysts in biological and environmental applications, and even hold promise for application in more complicated systems.

Ratiometric imaging of lysosomal hypochlorous acid enabled by FRET-based polymer dots

FRET-based fluorescent polymer dots (FPD) with good membrane permeability have been developed for ratiometric imaging of lysosomal HClO in living cells.

Selective visualization of endogenous hydrogen sulfide in lysosomes using aggregation induced emission dots

The AIE dots (AIED) with superior sensor properties have been developed for selective imaging of lysosomal H₂S in living cells.