Redox-Responsive Fluorescent Probes with Different Design Strategies

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.

Switchable Dual-Emissive DNA-Stabilized Silver Nanoclusters

We investigated an ss-DNA sequence that can stabilize a red- and a green-emissive silver nanocluster (DNA-AgNC). These two emitters can convert between each other in a reversible way. The change from red- to green-emitting DNA-AgNCs can be triggered by the addition of H₂O₂, while the opposite conversion can be achieved by the addition of NaBH₄. Besides demonstrating the switching between red- and green-emissive DNA-AgNCs and determining the recoverability, we fully characterized the photophysical properties, such as steady-state emission, quantum yield, fluorescence lifetime, and anisotropy of the two emissive species. Understanding the mechanism behind the remarkable conversion between the two emitters could lead to the development of a new range of DNA-AgNC-based ratiometric sensors.

Crosstalk between Oxidative Stress and Tauopathy

Tauopathy is a collective term for neurodegenerative diseases associated with pathological modifications of tau protein. Tau modifications are mediated by many factors. Recently, reactive oxygen species (ROS) have attracted attention due to their upstream and downstream effects on tauopathy. In physiological conditions, healthy cells generate a moderate level of ROS for self-defense against foreign invaders. Imbalances between ROS and the anti-oxidation pathway cause an accumulation of excessive ROS. There is clear evidence that ROS directly promotes tau modifications in tauopathy. ROS is also highly upregulated in the patients’ brain of tauopathies, and anti-oxidants are currently prescribed as potential therapeutic agents for tauopathy. Thus, there is a clear connection between oxidative stress (OS) and tauopathies that needs to be studied in more detail. In this review, we will describe the chemical nature of ROS and their roles in tauopathy.

New perspective on the fluorescence and sensing mechanism of TNP chemosensor 2-(4,5-bis(4-chlorophenyl)-1H-imidazol-2-yl)-4-chlorolphenol

For TNP chemosensor 2-(4,5-Bis(4-Chlorophenyl)-1H-Imidazol-2-yl)-4-Chlorolphenol (HPICI), previous thought with no theoretical basis was that excited-state intramolecular proton transfer (ESIPT) process and the ground-state HPICI-TNP complex are mainly responsible for its fluorescence emission and the detection of TNP. However, this interpretation has been proved to be wrong by the present theoretical DFT/TDDFT explorations. Actually, the strong fluorescence of HPICI is mainly induced by the local excitation of the enol form HPICI(E) without ESIPT, and the fluorescence quenching by TNP is due to the photo-induced electron transfer (PET) process together with the cooperative effect of hydrogen-bonding interaction and π-π stacking interaction coexisting in the HPICI-TNP complex. The strengthened excited-state hydrogen bond promotes the PET process, thus facilitates the fluorescence quenching. This mechanism is proposed on the basis of the theoretical analyses on molecule geometry, binding energy, Gibbs free energy, electronic transitions, and frontier molecular orbitals (FMOs).

In Vivo High-resolution Ratiometric Fluorescence Imaging of Inflammation Using NIR-II Nanoprobes with 1550 nm Emission

Quantitatively imaging the spatiotemporal distribution of biological events in living organisms is essential to understand fundamental biological processes. Self-calibrating ratiometric fluorescent probes enable accurate and reliable imaging and sensing, but conventional probes using wavelength of 400-900 nm suffer from extremely low resolution for in vivo application due to the disastrous photon scattering and tissue autofluorescence background. Here, we develop a NIR-IIb (1500-1700 nm) emissive nanoprobe for high-resolution ratiometric fluorescence imaging in vivo. The obtained nanoprobe shows fast ratiometric response to hypochlorous acid (HOCl) with a detection limit down to 500 nM, through an absorption competition-induced emission (ACIE) bioimaging system between lanthanide-based downconversion nanoparticles and Cy7.5 fluorophores. Additionally, we demonstrate the superior spatial resolution of 1550 nm to a penetration depth of 3.5 mm in a scattering tissue phantom, which is 7.1-fold and 2.1-fold higher than that of 1064 and 1344 nm, respectively. With this nanoprobe, clear anatomical structures of lymphatic inflammation in ratiometric channel are observed with a precise resolution of ∼477 μm. This study will motivate the further research on the development of NIR-II probes for high-resolution biosensing in vivo.

Anion Sensing by Novel Triarylboranes Containing Boraanthracene: DFT Functional Assessment, Selective Interactions, and Mechanism Demonstration

Analytical methods often involve expensive instrumentation and tedious sample pretreatment for an analyte detection. Being toxic and detrimental to human health, sensing of cyanide (CN-), fluoride (F-), chloride (Cl-), bromide (Br-), nitrate (NO3 -), acetate (CH3COO-), and bisulfate (HSO4 -) is performed by a boron-based molecular receptor, N,N,N,3,5-pentamethyl-4-{2-thia-9-boratricyclo[8.4.0.03,8]tetradeca-1(10),3(8),4,6,11,13-hexaen-9-yl}anili-nium (1), and the three newly designed receptors from it. Thermodynamics, electronic structure, and photophysical properties are computed by employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to explore selective sensing of these anions and its mechanism. Free-energy changes (ΔG) and binding energies (ΔE) suggest that among these anions, only binding of CN- and F- is thermodynamically feasible with a very strong binding affinity with the receptors. Boron atoms containing positive natural charges act as the electrophilic centers to bind the anions involving a 2p-2p orbital overlap resulting in charge transfer. In the receptor-analyte complexes with CN- and F-, fluorescence is quenched due to the intramolecular charge transfer transitions (π-π* transitions in the case of the receptors lead to fluorescence), internal conversion, and associated configurational changes. Among the six tested functionals, CAM-B3LYP/6-31G(d) is found to be the most accurate one. The designed receptors are better fluorescent probes for F- and CN-, demonstrating their importance for the practical utility.

Bioinspired Small-Molecule Tools for the Imaging of Redox Biology

The availability of electrons to biological systems underpins the mitochondrial electron transport chain (ETC) that powers living cells. It is little wonder, therefore, that the sufficiency of electron supply is critical to cellular health. Considering mitochondrial redox activity alone, a lack of oxygen (hypoxia) leads to impaired production of adenosine triphosphate (ATP), the major energy currency of the cell, whereas excess oxygen (hyperoxia) is associated with elevated production of reactive oxygen species (ROS) from the interaction of oxygen with electrons that have leaked from the ETC. Furthermore, the redox proteome, which describes the reversible and irreversible redox modifications of proteins, controls many aspects of biological structure and function. Indeed, many major diseases, including cancer and diabetes, are now termed "redox diseases", spurring much interest in the measurement and monitoring of redox states and redox-active species within biological systems. In this Account, we describe recent efforts to develop magnetic resonance (MR) and fluorescence imaging probes for studying redox biology. These two classes of molecular imaging tools have proved to be invaluable in supplementing the structural information that is traditionally provided by MRI and fluorescence microscopy, respectively, with highly sensitive chemical information. Importantly, the study of biological redox processes requires sensors that operate at biologically relevant reduction potentials, which can be achieved by the use of bioinspired redox-sensitive groups. Since oxidation-reduction reactions are so crucial to modulating cellular function and yet also have the potential to damage cellular structures, biological systems have developed highly sophisticated ways to regulate and sense redox changes. There is therefore a plethora of diverse chemical structures in cells with biologically relevant reduction potentials, from transition metals to organic molecules to proteins. These chemical groups can be harnessed in the development of exogenous molecular imaging agents that are well-tuned to biological redox events. To date, small-molecule redox-sensitive tools for oxidative stress and hypoxia have been inspired from four classes of cellular regulators. The redox-sensitive groups found in redox cofactors, such as flavins and nicotinamides, can be used as reversible switches in both fluorescent and MR probes. Enzyme substrates that undergo redox processing within the cell can be modified to provide fluorescence or MR readout while maintaining their selectivity. Redox-active first-row transition metals are central to biological homeostasis, and their marked electronic and magnetic changes upon oxidation/reduction have been used to develop MR sensors. Finally, redox-sensitive amino acids, particularly cysteine, can be utilized in both fluorescent and MR sensors.

TDDFT study on aluminum and fluoride dual-sensing mechanism of a Schiff-Base sensor

The aluminum and fluoride dual-sensing mechanism of a previously reported sensor with a Schiff-base moiety (Spectrochim. Acta A, 2017, 183, 267-274) has been investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The present calculations reproduce the photoproperties of the sensor as well as its aluminum and fluoride complexes, which illustrates that DFT and TDDFT constitute a reliable tool for uncovering detailed fluorescence-based sensing mechanisms in diverse electronic states. Theoretical results indicate that there are two OH⋯N hydrogen bonds in the sensor and two OH⋯F hydrogen bonds in its F¯ complex. Different degrees of coplanarity caused by these hydrogen bonds are responsible for their distinct absorption wavelengths. However, excited-state geometry optimization and a scan of the potential-energy surface show that there is twisted intramolecular charge transfer about the CN bond in the sensor molecule and an excited-state proton-transfer process from the fluoride anion to the neighboring N atom in the fluoride-sensor complex, whereby the fluorescence is quenched. A chelation-enhanced fluorescence effect associated with the aluminum-sensor complex shows a different excited-state process. The local excitation and emission occur exclusively within the planar fluorophore, and negligible structural change upon excitation of the aluminum-sensor complex leads to its strong fluorescence. Therefore, it is theoretically explained why the sensor may be successfully used to analyze the fluoride anion by absorption spectroscopy and the aluminum cation by emission spectroscopy.

A novel profluorescent paramagnetic diaza-crown ether: synthesis, characterization and alkaline metal-ion complexation

Starting from Kryptofix 22 two different branches were covalently attached through the nitrogen atoms, one containing a fluorescent moiety and the other the stable free radical TEMPO. The novel derivative exhibits fluorescence and paramagnetic properties, while the diaza-crown part ensures the affinity for alkaline metal-ions.

Starting from Kryptofix 22 two different branches were covalently attached through the nitrogen atoms, one containing a fluorescent moiety and the other the stable free radical TEMPO.

A novel porphyrin-based near-infrared fluorescent probe for hypochlorite detection and its application in vitro and in vivo

Reactive oxygen species (ROS), especially HOCl/ClO-, have been demonstrated to play essential roles in both physiological and pathological processes, and an abnormal level of HOCl/ClO- is related to some diseases. In this work, a very fast responsive (within 30 seconds) porphyrin-based fluorescent probe, TPP-TCF, for ClO- with a NIR emissive wavelength was prepared. This probe exhibited excellent selectivity towards ClO- and would not be interfered with by other ROS and typical nucleophiles. The limit of detection (LOD) for ClO- was evaluated to be 0.29 μM, indicating high sensitivity towards ClO-. In further bioimaging experiments, TPP-TCF displayed low-cytotoxicity and good cell penetrability for recognizing exogenous ClO- in HeLa cells. Moreover, this probe was successfully applied in imaging endogenous ClO- in living animals.

Advance of Seriniquinone Analogues as Melanoma Agents

Seriniquinone, a marine natural product, displayed potent cytotoxicity and selectivity against melanoma cancer cells. This selectivity, combined with a novel mode of action (MOA), prompted studies to translate a pharmacologically relevant lead. Herein, we report on structure-activity relationships (SARs), and provide a strategy to prepare analogues that retain activity and offer an improved water solubility and isomeric purity. From intermediates made on a gram-scale, derivatives were prepared and evaluated for their antiproliferation activity and melanoma selectivity. Overall these studies provide methods to install side chain motifs that demonstrate a common, and yet unique, biological profile.

Enhanced fluorescence sensing of hypochlorous acid using serum albumin as a signal amplifier

A mitochondria-targeting fluorescent probe for ClO⁻ was developed and a signal amplifier BSA was utilized to promote the fluorescent signal.

Straightforward synthesis of photoactive chalcogen functionalized benzimidazo[1,2-a]quinolines

A series of new organochalcogen derivatives of benzimidazo[1,2-a]quinolines were synthesized. Both sulfur and selenium derivatives presented similar photophysical properties with absorption in the UV region and fluorescence emission in the violet-blue.

Novel fluorescent probe with a bridged Si–O–Si bond for the reversible detection of hypochlorous acid and biothiol amino acids in live cells and zebrafish

Herein, we report the design of a novel fluorescent probe consisting of a naphthalimide fluorophore and a silicone small molecule for the reversible detection of hypochlorous acid and biothiol amino acids.

A reversible and turn-on type fluorescence behaviour of hydrogen sulfide via a redox cycle between selenoxide and selenide

Upon treatment with H₂S in MeCN–PBS, the fluorescence dormant selenoxides of dibenzobarrelene- and benzobarrelene-based 1-seleno-1,3-butadiene derivatives are rapidly converted to strongly fluorescent selenides.

Dual-site functionalized NIR fluorescent material for a discriminative concentration-dependent response to SO2 in cells and mice

Sulfur dioxide (SO₂), as an important anti-oxidant and gaseous signaling molecule, plays fundamental roles in the regulation of intracellular signaling and cell death cellular bioenergetics.

A novel near-infrared fluorescent probe for detecting intracellular alkaline phosphatase and imaging of living cells

Design, synthesis and application of a fluorescent probe with a novel near-infrared fluorophore for in vivo imaging of alkaline phosphatase.

Two-isophorone fluorophore-based design of a ratiometric fluorescent probe and its application in the sensing of biothiols

A newly designed ratiometric fluorescent probe is applied in the sensing of biothiols.

A reversible fluorescent probe based on C[double bond, length as m-dash]N isomerization for the selective detection of formaldehyde in living cells and in vivo

Formaldehyde (FA) is an endogenously produced reactive carbonyl species (RCS) through biological metabolic processes whose concentration is closely related to human health and disease. Noninvasive and real-time detection of FA concentration in organisms is very important for revealing the physiological and pathological functions of FA. Herein, we design and synthesize a reversible fluorescent probe BOD-NH₂ for the detection of FA in living cells and in vivo. The probe is composed of two moieties: the BODIPY fluorophore and the primary amino group response unit. The probe undergoes an intracellular aldimine condensation reaction with FA and forms imine (C[double bond, length as m-dash]N) which will result in C[double bond, length as m-dash]N isomerization and rotation to turn-off the fluorescence of the probe. It is important that the probe can show a reversible response to FA. The probe BOD-NH₂ has been successfully applied for detecting and imaging FA in the cytoplasm of living cells. BOD-NH₂ is capable of detecting fluctuations in the levels of endogenous and exogenous FA in different types of living cells. The probe can be used to visualize the FA concentration in fresh hippocampus and the probe can further qualitatively evaluate the FA concentrations in ex vivo-dissected organs. Moreover, BOD-NH₂ can also be used for imaging in mice. The above applications make our new probe a potential chemical tool for the study of physiological and pathological functions of FA in cells and in vivo.

Small-molecule luminescent probes for the detection of cellular oxidizing and nitrating species

Reactive oxygen species (ROS) have been implicated in both pathogenic cellular damage events and physiological cellular redox signaling and regulation. To unravel the biological role of ROS, it is very important to be able to detect and identify the species involved. In this review, we introduce the reader to the methods of detection of ROS using luminescent (fluorescent, chemiluminescent, and bioluminescent) probes and discuss typical limitations of those probes. We review the most widely used probes, state-of-the-art assays, and the new, promising approaches for rigorous detection and identification of superoxide radical anion, hydrogen peroxide, and peroxynitrite. The combination of real-time monitoring of the dynamics of ROS in cells and the identification of the specific products formed from the probes will reveal the role of specific types of ROS in cellular function and dysfunction. Understanding the molecular mechanisms involving ROS may help with the development of new therapeutics for several diseases involving dysregulated cellular redox status.

A ratiometric fluorescent probe for bioimaging and biosensing of HBrO in mitochondria upon oxidative stress

A novel ratiometric fluorescent probe with high selectivity and sensitivity was designed and developed for bioimaging and biosensing of HBrO in mitochondria. Using this useful tool with low cytotoxicity and good biocompatibility, it was found that O2˙--induced oxidative stress triggered the burst of HBrO in mitochondria.

Exploring molecular mechanism of allosteric inhibitor to relieve drug resistance of multiple mutations in HIV-1 protease by enhanced conformational sampling

Recently, allosteric regulations of HIV-1 protease (PR) are suggested as a promising approach to relieve drug resistance of mutations toward inhibitors targeting the active site of PR. Replica-exchange molecular dynamics (REMD) simulations and normal mode analysis (NMA) are integrated to enhance conformational sampling of PR. Molecular mechanics generalized Born surface area (MM-GBSA) method was applied to calculate binding free energies of three inhibitors APV, DRV, and NIT to the wild-type (WT) and multidrug resistance (MDR) PRs. The results suggest that binding free energies of APV and DRV are decreased in the MDR PR relative to the WT PR, suggesting drug resistance of mutations on these two inhibitors. However, the binding ability of the allosteric inhibitor NIT is not impaired in the MDR PR. In addition, internal dynamics analysis based on REMD simulations proves that mutations hardly produce obvious effect on the conformation of the MDR PR in comparison to the WT PR. Scanning of hydrophobic contacts and hydrogen bond contacts of inhibitors with residues of PRs on the concatenated trajectories of REMD demonstrates that mutations change the symmetric interaction networks of APV and DRV with PR, but do not generate obvious influence on the asymmetric interaction network of NIT with PR. In summary, allosteric inhibitor NIT can adapt the MDR PR better than those inhibitors toward the active site of PR, thus allosteric inhibitors of PR may be a possible channel to overcome drug resistance of PR.

Tetrathiafulvalene (TTF)-Annulated Calix[4]pyrroles: Chemically Switchable Systems with Encodable Allosteric Recognition and Logic Gate Functions

Molecular and supramolecular systems capable of switching between two or more states as the result of an applied chemical stimulus are attracting ever-increasing attention. They have seen wide application in the development of functional materials including, but not limited to, molecular and supramolecular switches, chemosensors, electronics, optoelectronics, and logic gates. A wide range of chemical stimuli have been used to control the switching within bi- and multiple state systems made up from either singular molecular entities or supramolecular ensembles. In general, chemically triggered switching systems contain at least two major functional components that provide for molecular recognition and signal transduction, respectively. These components can be connected to one another via either covalent or noncovalent linkages. Of particular interest are switchable systems displaying cooperative or allosteric features. Such advanced control over function is ubiquitous in nature and, in the case of synthetic systems, may allow the capture and release of a targeted chemical entity or permit the transduction of binding information from one recognition site to another. Allosterically controlled complexation and decomplexation could also permit the amplification or deamplification of analyte-specific binding affinity, lead to nonlinear binding characteristics, or permit a magnification of output signals. Our own efforts to develop chemically driven supramolecular switches, advanced logic gates, and multifunction cascade systems have focused on the use of tetrathiafulvalene (TTF) annulated calix[4]pyrroles (C4Ps). These systems, TTF-C4Ps, combine several orthogonal binding motifs within what are conformationally switchable receptor frameworks. Their basic structure and host-guest recognition functions can be controlled via application of an appropriate chemical stimulus. Homotropic or heterotropic allosteric molecular recognition behavior is often seen. This has allowed us to (1) produce self-assembled structures, (2) control switching between bi- and multistate constructs, (3) generate chemical logic gates performing chemical-based Boolean logic operations, (4) create ionically controlled three-state logic systems that release different chemical messengers and activate disparate downstream reactions, and (5) encode a variety advanced functional operations into what are relatively simple molecular-scale devices. Looking to the future, we believe that exploiting allosteric control will expand opportunities for supramolecular chemists and allow some of the complexity seen in biology to be reproduced in simple constructs. Of particular appeal would be a capacity to release chemical messengers at will, perhaps after a prior capture and chemical modification step, that then encode for further downstream functions as seen in the case of the small molecules, such as neurotransmitters and pheromones, used by nature for the purpose of intraentity communication. Molecular scale logic devices with allosteric functions are thus the potential vanguard of a new area of study involving interactions between multiple discrete components with an emphasis on functional outcomes.

A highly selective fluorescent probe for the detection of hypochlorous acid in tap water and living cells

A turn-on fluorescent probe (DAME) for sensing hypochlorous acid (HClO) with excellent selectivity was presented. The fluorescent probe was composed of coumarin derivative as the fluorophore and dimethylcarbamothioic chloride group with a sulfide moiety as modulator. Additionally, the sulfide moiety would be oxidized by HClO, and then free dye of coumarin derivate was released and exhibited significant fluorescence. In addition, the probe could respond to HClO in solutions within 60 s and the limit of detection was down to 34.75 nM. Moreover, the probe was used for the detection of HClO in tap water through the home-made test paper. And confocal images confirmed that probe DAME could be used for recognizing HClO in living cells.

Selective Monitoring and Imaging of Eosinophil Peroxidase Activity with a J-Aggregating Probe

The specific detection of eosinophil peroxidase (EPO) activity requires the difficult distinction between hypobromous acid generated by EPO and hypochlorous acid generated by other haloperoxidases. Here we report a fluorogenic probe that is halogenated with high kinetic selectivity (≥1200:1) for HOBr over HOCl. Heavy-atom effects do not quench the dibrominated product because of its self-assembly into emissive J-aggregates that provide a turn-on signal. Applications of this fluorogen to EPO activity assays, dipstick sensors, fluorescence imaging of EPO activity, assays of oxidative stress in cancer cells, and immune response detection in live mice are reported.

Challenges and Opportunities for Small-Molecule Fluorescent Probes in Redox Biology Applications

SIGNIFICANCE

The concentrations of reactive oxygen/nitrogen species (ROS/RNS) are critical to various biochemical processes. Small-molecule fluorescent probes have been widely used to detect and/or quantify ROS/RNS in many redox biology studies and serve as an important complementary to protein-based sensors with unique applications. Recent Advances: New sensing reactions have emerged in probe development, allowing more selective and quantitative detection of ROS/RNS, especially in live cells. Improvements have been made in sensing reactions, fluorophores, and bioavailability of probe molecules.

CRITICAL ISSUES

In this review, we will not only summarize redox-related small-molecule fluorescent probes but also lay out the challenges of designing probes to help redox biologists independently evaluate the quality of reported small-molecule fluorescent probes, especially in the chemistry literature. We specifically highlight the advantages of reversibility in sensing reactions and its applications in ratiometric probe design for quantitative measurements in living cells. In addition, we compare the advantages and disadvantages of small-molecule probes and protein-based probes.

FUTURE DIRECTIONS

The low physiological relevant concentrations of most ROS/RNS call for new sensing reactions with better selectivity, kinetics, and reversibility; fluorophores with high quantum yield, wide wavelength coverage, and Stokes shifts; and structural design with good aqueous solubility, membrane permeability, low protein interference, and organelle specificity. Antioxid. Redox Signal. 29, 518-540.

Transgenic Organisms Meet Redox Bioimaging: One Step Closer to Physiology

SIGNIFICANCE

Redox signaling is a common mechanism in the cellular response toward a variety of stimuli. For analyzing redox-dependent specific alterations in a cell, genetically encoded biosensors were highly instrumental in the past. To advance the knowledge about the importance of this signaling mechanism in vivo, models that are as close as possible to physiology are needed. Recent Advances: The development of transgenic (tg) redox biosensor animal models has enhanced the knowledge of redox signaling under patho(physio)logical conditions. So far, commonly used small animal models, that is, Caenorhabditis elegans, Drosophila melanogaster, and Danio rerio, and genetically modified mice were employed for redox biosensor transgenesis. However, especially the available mouse models are still limited.

CRITICAL ISSUES

The analysis of redox biosensor responses in vivo at the tissue level, especially for internal organs, is hampered by the detection limit of the available redox biosensors and microscopy techniques. Recent technical developments such as redox histology and the analysis of cell-type-specific biosensor responses need to be further refined and followed up in a systematic manner.

FUTURE DIRECTIONS

The usage of tg animal models in the field of redox signaling has helped to answer open questions. Application of the already established models and consequent development of more defined tg models will enable this research area to define the role of redox signaling in (patho)physiology in further depth. Antioxid. Redox Signal. 29, 603-612.

Small-molecule fluorescent probes and their design

Small-molecule fluorescent probes have become powerful tools for using light to advance the study of cell biology, discover new drugs, detect environmental contaminants, and further the detection of cancer. These applications correlate with the expansion of the fluorescent probe research community - small in the late 20th century, now a collection of more than a hundred research groups world-wide. This expansion required the entry of adventurous scientists from many other fields. This tutorial review introduces some important concepts related to fluorescent probe development. It is hoped that it will facilitate further expansion of the field by demystifying it.

Rapid Response Fluorescence Probe Enabled In Vivo Diagnosis and Assessing Treatment Response of Hypochlorous Acid-Mediated Rheumatoid Arthritis

Diagnosis and early assessment of the treatment response of rheumatoid arthritis (RA) necessitates a reliable bioanalytical method for rapid, sensitive, and specific detection of the hypochlorous acid (HOCl) biomarker in inflammatory diseases. Herein, two fluorescence probes, Probe-1 and Probe-2 are developed for quantitative monitoring and visualization of inflammatory response-related HOCl levels in vitro and in vivo. In the presence of HOCl, fluorescence "OFF-ON" response is obtained for both the probes as a result of specific HOCl-triggered C=N bond cleavage reaction. Probe-1 and Probe-2 feature rapid response (<4 s), a high degree of sensitivity and selectivity toward HOCl, which allow them to be used for quantification of HOCl in a simulated physiological condition. Using Probe-2 as the probe, fluorescence imaging and flow cytometry analysis of HOCl levels in lysosome of inflammatory mimic cells, visualization of HOCl generation in endotoxin-induced inflammation of adult zebrafish and RA of mice are possible. Probe-2 exhibits high effectiveness for early assessment of the treatment response of HOCl-mediated RA in mice with an antiarthritic drug, methotrexate (MTX). The results demonstrate that Probe-2 is a powerful tool for future studies on diagnosis and monitoring treatment efficiency in a broad range of inflammatory diseases, including RA.

The photoinduced isomerization mechanism of the 2-(1-(methylimino)methyl)-6-chlorophenol (SMAC): Nonadiabatic surface hopping dynamics simulations

The photophysical properties of the Schiff base family are crucial for their applications such as molecular switches and molecular memories. However, it was found that the photophysical behavior is not uniform for all Schiff base molecules, which shows a significant substituent dependent property. In this article, we studied the photoisomerization mechanism of one Schiff base chlorosubstituted derivative 2-(1-(methylimino)methyl)-6-chlorophenol by employing geometrical optimization, energy profiles scanning, and on-the-fly dynamical simulations. Three types of minimum energy conical intersections were located on the S₁/S₀ crossing seam, with two characterized by twisting motion of the C=N bond and one featured with the excited state intramolecular proton transfer process and then twisting motion around the C=C bond [excited-state intramolecular proton transfer process (ESIPT)-then-twisting]. By a combination of the dynamics simulation results with the energy profiles scanned along with the ESIPT coordinate, it was found that the photophysical property of the targeted molecule is different from that of most Schiff base members, which prefer to decay by a twisting motion around the C=N bridge bond rather than the ESIPT-then-twisting channel. The minor ESIPT channel is probably governed by a tunneling mechanism. The proposed deactivation mechanism can provide a reasonable explanation for the observations in the experiment and would provide fundamental indications for further design of new and efficient photochromic products.

Fluorescent in vivo imaging of reactive oxygen species and redox potential in plants

Reactive oxygen species (ROS) are by-products of aerobic metabolism, and excessive production can result in oxidative stress and cell damage. In addition, ROS function as cellular messengers, working as redox regulators in a multitude of biological processes. Understanding ROS signalling and stress responses requires methods for precise imaging and quantification to monitor local, subcellular and global ROS dynamics with high selectivity, sensitivity and spatiotemporal resolution. In this review, we summarize the present knowledge for in vivo plant ROS imaging and detection, using both chemical probes and fluorescent protein-based biosensors. Certain characteristics of plant tissues, for example high background autofluorescence in photosynthetic organs and the multitude of endogenous antioxidants, can interfere with ROS and redox potential detection, making imaging extra challenging. Novel methods and techniques to measure in vivo plant ROS and redox changes with better selectivity, accuracy, and spatiotemporal resolution are therefore desirable to fully acknowledge the remarkably complex plant ROS signalling networks.

Easy-to-Use Colorimetric Cyanine Probe for the Detection of Cu2+ in Wilson's Disease

Copper(II) is one of the essential metal elements in human body, which can accumulate in many organs and finally excrete in urine. Excessive load of Cu²⁺ can cause liver cirrhosis, kidney dysfunction, and many neurological symptoms in the case of Wilson's disease (WD). Therefore, the selective and efficient detection of Cu²⁺ is of great importance. Although various fluorescent probes have been reported for the detection of Cu²⁺, an efficient and capable probe is still rare for patients' self-use on a routine basis. In this study, we developed an easy-to-use probe CY1 based on UV-vis-near-infrared absorption changes with excellent sensitivity and selectivity for Cu²⁺. The mechanism of oxidation of CY1 by Cu²⁺ was first explored. We demonstrated the role of the probe in the quantitative detection of Cu²⁺ in urine from WD patients and showed that it has great potential for clinical applications.

A near-infrared fluorescent probe based on chloroacetate modified naphthofluorescein for selectively detecting cysteine/homocysteine and its application in living cells

We have prepared a near-infrared (NIR) turn-on fluorescent probe (NFC) based on chloroacetate modified naphthofluorescein for specific detection of cysteine (Cys) and homocysteine (Hcy) over glutathione (GSH) and other amino acids (AAs) with the detection limits of 0.30 μM and 0.42 μM, respectively. The fluorescence intensity of the naphthofluorescein (NF) chromophore is modulated by an internal charge transfer (ICT) process. The probe NFC is readily available and weakly fluorescent, but of observably enhanced fluorescence after reacting with Cys or Hcy. We assumed and then demonstrated that the fluorescence off-on process involves a conjugate nucleophilic substitution/cyclization sequence. Furthermore, the probe has been successfully applied for detecting the total content of Cys and Hcy in human plasma and imaging in living cells with low toxicity.

Fluoride anion sensing mechanism of a BODIPY-linked hydrogen-bonding probe

The sensing mechanism of a fluoride-anion probe BODIPY-amidothiourea (1c) has been elucidated through the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The theoretical study indicates that in the DMSO/water mixtures the fluorescent sensing has been regulated by the fluoride complex that formed between the probe 1c/two water molecules and the fluoride anion, and the excited-state intermolecular hydrogen bond (H-B) plays an important role in the fluoride sensing mechanism. In the first excited state, the H-Bs of the fluoride complex 1cFH₂ are overall strengthened, which induces the weak fluorescence emission. In addition, molecular orbital analysis demonstrates that 1cFH₂ has more obvious intramolecular charge transfer (ICT) character in the S₁ state than 1cH₂ formed between the probe 1c and two water molecules, which also gives reason to the weaker fluorescence intensity of 1cFH₂ . Further, our calculated UV-vis absorbance and fluorescence spectra are in accordance with the experimental measurements. © 2018 Wiley Periodicals, Inc.