A naphthalene-based two-photon fluorescent probe for selective and sensitive detection of endogenous hypochlorous acid

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.

A ratiometric small-molecule fluorescent probe for the selective detection of hypochlorite by an oxidative cyclization reaction: application to commercial disinfectants and live cells

A highly selective thiophene-thioimidazole hydrazine-based ratiometric chemodosimeter (TPBN) was designed and synthesized to detect hypochlorite (ClO-). The probe showed yellow fluorescence and exhibited ultra sensitivity towards hypochlorite (detection limit 8.74 nM) through the oxidative intramolecular cyclization process to give a blue fluorescent triazole product (TPBN-P). Additionally, the as-designed sensor displayed a fast response (80 s) to hypochlorite with excellent selectivity over other competing analytes. DFT calculations, ESI-MS, and 1H NMR titration experiments supported the detection mechanism. The probe was a valuable and practical ratiometric sensor for test strips, commercial disinfectants, and water samples. The probe was successfully used in the bio-imaging of hypochlorite in human breast cancer cells due to its noteworthy photophysical characteristics and good cell permeability.

Quinoline-based fluorescent probe for the detection and monitoring of hypochlorous acid in a rheumatoid arthritis model

The development of effective bioanalytical methods for the visualization of hypochlorous acid (HOCl) in situ in rheumatoid arthritis (RA) directly contributes to better understanding the roles of HOCl in this disease. In this work, a new quinoline-based fluorescence probe (HQ) has been developed for the detection and visualization of a HOCl-mediated inflammatory response in a RA model. HQ possesses a donor–π–acceptor (D–π–A) structure that was designed by conjugating p-hydroxybenzaldehyde (electron donor) and 1-ethyl-4-methylquinolinium iodide (electron acceptor) through a C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C double bond. In the presence of HOCl, oxidation of phenol to benzoquinone led to the red-shift (93 nm) of the adsorption and intense quenching of the fluorescence emission. The proposed response reaction mechanism was verified by high performance liquid chromatography (HPLC) and high-resolution mass spectroscopy (HRMS) titration analysis. The remarkable color changes of the HQ solution from pale yellow to pink enabled the application of HQ-stained chromatography plates for the “naked-eye” detection of HOCl in real-world water samples. HQ featured high selectivity and sensitivity (6.5 nM), fast response time (<25 s) to HOCl, reliability at different pH (3.0 to 11.5) and low cytotoxicity. HQ's application in biological systems was then demonstrated by the monitoring of HOCl-mediated treatment response to RA. This work thus provided a new tool for the detection and imaging of HOCl in inflammatory disorders.

A quinoline-based fluorescent probe (HQ) has been designed and synthesized for the monitoring of HOCl-mediated treatment response of a rheumatoid arthritis (RA) model and “naked-eye” detection of HOCl in real water samples.

Two-photon fluorescent probe for cellular peroxynitrite: Fluorescence detection, imaging, and identification of peroxynitrite-specific products

A new naphthalene-based boronate probe, NAB-BE, for the fluorescence-based detection of inflammatory oxidants, including peroxynitrite, hypochlorous acid, and hydrogen peroxide, is reported. The chemical reactivity and fluorescence properties of the probe and the products are described. The major, phenolic oxidation product, NAB-OH, is formed in case of all three oxidants tested. This product shows green fluorescence, with a maximum at 512 nm, and can be excited either at 340 nm or in the near infrared region (745 nm) for two-photon fluorescence imaging. Peroxynitrite is the fastest of the oxidants tested and, in addition to the phenolic product, leads to the formation of a nitrated product, NAB-NO₂, which can serve as a fingerprint for peroxynitrite. The probe was applied to detect peroxynitrite in activated macrophages using fluorimetry and two-photon fluorescence microscopy, and both NAB-OH and NAB-NO₂ products were detected in cell extracts by liquid chromatography-mass spectrometry. The combined use of fluorometric high-throughput analyses, fluorescence imaging, and liquid chromatography-mass spectrometry-based product identification and quantitation is proposed for most comprehensive and rigorous characterization of oxidants in biological systems.

Recent advances in two-photon absorbing probes based on a functionalized dipolar naphthalene platform

Two-photon microscopy (TPM) techniques have been highlighted over the past two decades throughout various fields, including physics, chemistry, biology, and medicine. In particular, the two-photon near-infrared excitation of fluorophores or molecular probes emitting fluorescence have ushered in a new biomedical era, specifically in the deep-tissue imaging of biologically relevant species. Non-linear two-photon optics enables the development of 3D fluorescence images via focal point excitation of biological samples with low photo-damage and photo-bleaching. Many studies have disclosed the relationship between the chemical structure of fluorophores and their two-photon absorbing properties. In this review, we have summarized the recent advances in two-photon absorbing probes based on a functionalized electron donor (D)-acceptor (A) type dipolar naphthalene platform (FDNP) that was previously reported between 2015 and 2019. Our systematic outline of the synthesis, photophysical properties, and examples of two-photon imaging applications will provide useful context for the future development of new naphthalene backbone-based two-photon probes.

Fast detecting hypochlorous acid based on electron-withdrawing group promoted oxidation and its biological applications in cells and root tips of plants

Hypochlorous acid, a type of reactive oxygen species, has been shown to play an important role in organisms. Nowadays, there are many kinds of fluorescence detecting mechanisms to detect hypochlorous acid in vivo. Due to the high selectivity, the mechanism of using the strong oxidation of hypochlorous acid to break carbon‑carbon double bonds has been favored by many scientists. However, the reported probes of breaking carbon‑carbon double bonds still had drawback such as slow response. Based on this, we introduced electron-withdrawing group malonitrile to accelerate the oxidation of hypochlorous acid, resulting in reaction time less than 150 s. Meanwhile, the probe exhibited excellent selectivity, optical stability, high sensitivity and the detection limit as low as 0.19 μM. More importantly, we also successfully proved the potential application of the probe for the detection of intracellular ClO^- living cells and Arabidopsis root tip by fluorescence imaging.

A highly selective fluorescent probe for hydrogen polysulfides in living cells based on a naphthalene derivative

Hydrogen polysulfides (H₂S_n, n > 1) are members of reactive sulfur species (RSS) and signaling molecules derived from hydrogen sulfide (H₂S). Recently, the functions of H₂S_n in physiological and pathological processes have been increasingly recognized. However, their biological effects and detailed mechanisms of action are still little known. Therefore, there is an urgent need to develop highly selective and sensitive techniques for monitoring hydrogen polysulfides (H₂S_n) in living cells. In this study, we designed and synthesized a fluorescent probe based on a naphthalene derivative for the detection of hydrogen polysulfides. A naphthalene derivative was applied as the fluorescent main structure and the 2-fluoro-5-nitrobenzoate group was chosen as the recognition unit. In the absence of hydrogen polysulfides, the fluorescent probe displayed almost no fluorescence. In the presence of hydrogen polysulfides, the fluorescent probe exhibited strong fluorescence. The sensing mechanism was based on H₂S_n-mediated aromatic substitution-cyclization reactions. The linear range of the response concentration of the probe to hydrogen polysulfide was acquired in a concentration range of H₂S_n from 7.5 × 10^-7 to 2.5 × 10^-5 mol L^-1. The detection limit was evaluated to be 5.0 × 10^-7 mol L^-1 for H₂S_n. The fluorescent probe can applied in a wide pH range including physiological condition pH. The fluorescent probe showed high specificity for H₂S_n over other reactive sulfur species (RSS). Moreover, the fluorescent probe has been successfully applied to confocal imaging of hydrogen polysulfides in HepG2 cells without cell cytotoxicity. All of such good qualities indicated that it could be used to detect H₂S_n in living cells.

A mitochondria-targeted fluorescent probe for hypochlorite sensing and its application in bioimaging

A coumarin-diaminomaleonitrile derivative was prepared and used for detecting OCl⁻ in living cells and zebrafish. Its high selectivity, sensitivity and low toxicity indicate that it is an ideal tool for biological applications.

A new "off-on" NIR fluorescence probe for determination and bio-imaging of mitochondrial hypochlorite in living cells and zebrafish

Hypochlorite anion (ClO^-) has been recognized as host defense destructing incursive bacteria and pathogens, a signal molecule inducing occurrence of apoptosis and a noxious agent when it is overproduced. It is significant to detect ClO^- in mitochondria for getting meaningful physiological and pathological information. Compared with the fluorescence probes of emission wavelength in ultraviolet or visible region, those with near-infrared (NIR) fluorescence signal are advantageous due to the deeper tissue penetrability and less photo-bleaching effect. In this work, a new "off-on" NIR ClO^--specific fluorescence probe (Mito-NClO) especially located in mitochondria was designed and synthesized by condensation of diaminomaleonitrile with a new fluorophore (Mito-NCHO). A marked "turn-on" NIR fluorescence signal was observed on account of the oxidation of the imine bond by NaClO. Moreover, in the range from 0 to 20 μM, this probe had the capability to quantitatively detect ClO^- with a detection limit as low as 90.2 nM. Additionally, the probe exerted other excellent properties, including larger stokes shift (117 nm), better aqueous solubility, high selectivity toward ClO^-, rapid response and selective mitochondrial location. Furthermore, the bio-imaging experiments clearly demonstrated that Mito-NClO facilitated the visualization of exogenous and endogenous ClO^- in living HeLa cells and zebrafish model. Therefore, we speculate that the probe Mito-NClO can be served as an ideal tool for the monitoring of ClO^- in biosystems.

A mitochondria-targetable two-photon fluorescent probe with a far-red to near-infrared emission for sensing hypochlorite in biosystems

Hypochlorite (ClO^-), one of reactive oxygen species (ROS), is closely related with many physiological and pathological processes. Especially as one of cellular reactive oxygen species in mitochondria, ClO^- can induce mitochondrial permeability, which leads to apoptosis. Thus, developing an effective method which is able to sense ClO^- in mitochondria is important. Although fluorescent probe has become a powerful tool for imaging ClO^- in mitochondria, most of them suffered from phototoxicity to biosamples, autofluorescence, and photobleaching phenomenon due to their short-wavelength excitations and emissions. Based on advantages of two-photon fluorescent probe and far-red to NIR fluorescent probe, a mitochondria-targetable two-photon fluorescent probe with a turn-on signal in far-red to NIR region, Mito-TP-ClO, was developed for ClO^- in this paper. Mito-TP-ClO is consisted of a triphenylphosphonium cations as a mitochondria-targetable unit and a structure of dibenzoylhydrazine as a response unit to ClO^-. Mito-TP-ClO exhibited a high sensitivity and a high selectivity to ClO^-, with a linear range from 6.0 × 10^-8 to 1.0 × 10^-5 M and a detection limit of 2.5 × 10^-8 M. Due to its large two-photon cross section (267 GM) and far-red to NIR emission, Mito-TP-ClO exhibits excellent performances including low autofluorescence, photostable fluorescence signal, and deep tissue penetration (230 μM). Moreover, Mito-TP-ClO was successfully used to detect endogenous ClO^- in bacteria-infected cells and inflammatory mouse model, which confirmed that Mito-TP-ClO is a powerful tool to monitor ClO^- in mitochondria and study on effects of hypochlorite on mitochondria.

A novel coumarin-based fluorescent probe with fine selectivity and sensitivity for hypochlorite and its application in cell imaging

It is necessary to develop simple and highly sensitive methods for the detection of hypochlorous acid (HOCl)/hypochlorite (ClO^-) to ravel its relationship with disease. In this paper, a novel bio-compatible fluorescent (Z)-8-(hydrazonomethyl)-7-hydroxy-4-methyl-2H-chromen-2-one (probe 1b) based on coumarin was synthesized and used for detecting ClO^- in PBS buffer (pH = 7.2, 10 mM, 60% C₂H₅OH). Probe 1b showed a remarkable fluorescence change from yellow to blue with the limit of detection as low as 2.4 × 10^-9 M^-1 when ClO^- was added. The coexisted anions, metal ions and reactive oxygen species (•OH, ¹O₂, H₂O₂, KO₂) showed any competitiveness towards ClO^-. In response to ClO^-, the fluorescence emission intensity of probe 1b was obviously enhanced within 20 s. The mechanism was confirmed by the ESI - MS and density functional theory calculations (DFT) to reveal that the coumarin lactone bonds C - O was cleavaged by oxidation of ClO^-. What's more, probe 1b could be used in practical water samples and showed well recovery. Additionally, the cell imaging experiment was demonstrated that probe 1b could be effectively exploited to imaging exogenous ClO^- in vitro.

A novel "turn-on" mitochondria-targeting near-infrared fluorescent probe for determination and bioimaging cellular hydrogen sulfide

Hydrogen sulfide (H₂S) has been regarded as an important gas transmitter playing vital role in cytoprotective processes and redox signaling. It is very meaningful to monitor and analyze it in biosystem for obtaining important physiological and pathological information. Despite numerous fluorescent probes for cellular H₂S have been reported in past decades, only a few have capability to detect mitochondrial H₂S with near-infrared (NIR) emission. Therefore, a new mitochondria-targeting NIR fluorescent probe (Mito-NSH) for detection of cellular H₂S was developed by introducing 2,4-dinitrophenyl ether into a novel dye (Mito-NOH). A large "turn-on" NIR fluorescence response was obtained due to thiolysis of ether to hydroxyl group when Mito-NSH was treated with NaHS. Moreover, Mito-NSH could quantitatively detect H₂S at concentration ranging from 0 to 30 μM with a detection limit of 68.2 nM, and it exerts some superior optical properties, such as large stokes shift (107 nm), highly selectively mitochondria location, fast response and high selectivity to H₂S. More impressively, it was successfully applied to imaging exogenous and endogenously generated H₂S in living HeLa cells via confocal fluorescence microscopy.

Mitochondria and lysosome-targetable fluorescent probes for HOCl: recent advances and perspectives

Hypochlorous acid (HOCl), as one of the reactive oxygen species (ROS), plays an important role in the destruction of pathogens in the immune system. However, abnormal concentration of biogenic HOCl can also damage host tissues, and it has been shown to be associated with many diseases. Accordingly, detection of HOCl at the subcellular level is important for understanding inflammation and cellular apoptosis. Toward this end, in the past few years, a wide variety of fluorescent HOCl probes have been engineered and applied for imaging of HOCl in subcellular organelles. In this review, we highlight the representative cases of the fluorescent HOCl probes with mitochondria and lysosome-targetable ability. The discussion includes their design strategies, sensing mechanisms, and applications in bio-imaging of HOCl in mitochondria and lysosomes.

A novel acidic pH fluorescent probe based on a benzothiazole derivative

A novel acidic pH fluorescent probe 1 based on a benzothiazole derivative has been designed, synthesized and developed. The linear response range covers the acidic pH range from 3.44 to 6.46, which is valuable for pH researches in acidic environment. The evaluated pKa value of the probe 1 is 4.23. The fluorescence enhancement of the studied probe 1 with an increase in hydrogen ions concentration is based on the hindering of enhanced photo-induced electron transfer (PET) process. Moreover, the pH sensor possesses a highly selective response to H⁺ in the presence of metal ions, anions and other bioactive small molecules which would be interfere with its fluorescent pH response. Furthermore, the probe 1 responds to acidic pH with short response time that was less than 1min. The probe 1 has been successfully applied to confocal fluorescence imaging in live HeLa cells and can selectively stain lysosomes. All of such good properties prove it can be used to monitoring pH fluctuations in acidic environment with high sensitivity, pH dependence and short response time.

A New Two-Photon Ratiometric Fluorescent Probe for Detecting Alkaline Phosphatase in Living Cells

Alkaline phosphatase (ALP) is an important diagnostic indicator of many human diseases. To quantitatively track ALP in biosystems, herein, for the first time, we report an efficient two-photon ratiometric fluorescent probe, termed probe 1 and based on classic naphthalene derivatives with a donor-π-acceptor (D-π-A) structure and deprotection of the phosphoric acid moiety by ALP. The presence of ALP causes the cleave of the phosphate group from naphthalene derivatives and the phosphate group changes the ability of the intramolecular charge transfer (ICT) and remarkably alters the probe's photophysical properties, thus an obvious ratiometric signal with an isoemissive point is observed. The fluorescence intensity ratio displayed a linear relationship against the concentration of ALP in the concentration range from 20 to 180 U/L with the limit of detection of 2.3 U/L. Additionally, the probe 1 is further used for fluorescence imaging of ALP in living cells under one-photon excitation (405 nm) or two-photon excitation (720 nm), which showed a high resolution imaging, thus demonstrating its practical application in biological systems.

A mitochondria-targeted near-infrared probe for colorimetric and ratiometric fluorescence detection of hypochlorite in living cells

A mitochondria-targeted near-infrared probe for rapid, sensitive and specific detection of hypochlorite with colorimetric and ratiometric fluorescence dual responses.