A selective, cell-permeable optical probe for hydrogen peroxide in living cells

Small-molecule fluorescent probes based on covalent assembly strategy for chemoselective bioimaging

In this review, we comprehensively summarize the recent progress in the development of small molecular fluorescent probes based on the covalent assembly principle. The challenges and perspective in this field are also presented.

Expeditious base-free solid-state reaction between phenyl boronates and hydrogen peroxide on silica gel

The expeditious base-free reaction between a phenyl boronate film and H2O2 vapor can be realized on a silica gel surface, playing an important role in sensor manufacturing applications and chemical production.

Dual-Locked Near-Infrared Fluorescent Probes for Precise Detection of Melanoma via Hydrogen Peroxide-Tyrosinase Cascade Activation

Activity-based near-infrared (NIR) fluorescent probes provide powerful tools for diagnosis of diseases. However, most of these probes suffer from low specificity due to "off-target" reaction. The dual-locked strategy, which utilizes two biomarkers as triggers, can increase the specificity and precision of diagnosis. Here, we report a dual-locked NIR probe, MB-m-borate, which releases fluorophore methylene blue (MB) after hydrogen peroxide-tyrosinase (H₂O₂-TYR) cascade activation. Both MB-m-borate and its intermediate MB-m-phenol (the product after H₂O₂ activation) show almost nondetectable fluorescence. MB-m-borate exhibits "turn on" fluorescence upon H₂O₂-TYR cascade activation. The further live cell bioimaging results indicate that MB-m-borate only responds to melanoma cells, providing it as a robust probe for precise detection of melanoma. Finally, the probe is applied for the diagnosis of melanoma in vivo with a xenogeneic mouse model.

Responsive fluorescence enhancement for in vivo Cu(II) monitoring in zebrafish larvae

Several neurodegenerative diseases are ascribed to disorders caused by the secretion of Cu ions. However, a majority of the current techniques for copper ion detection are restricted to in vivo monitoring and nonspecific interactions. Their methods are limited to the systematic analysis of Cu ions in living organisms. Thus, a synthetic molecular fluorophore, 5-amino 2,3-dihydroquinolinimine (NDQI), has been developed and successfully utilized in in vivo monitoring of the distribution of Cu(II) in zebrafish larvae. The reversible formation of the NDQI-Cu complex allows its use with high metal concentrations and in oxidative stress conditions. The NDQI-directed strategy developed here can quantitatively differentiate cells with different Cu(II) concentrations. Remarkably, dynamic distribution of Cu(II) in the intestine and liver can be observed.

Evaluation of borinic acids as new, fast hydrogen peroxide-responsive triggers

Hydrogen peroxide (H2O2) is responsible for numerous damages when overproduced, and its detection is crucial for a better understanding of H2O2-mediated signaling in physiological and pathological processes. For this purpose, various "off-on" small fluorescent probes relying on a boronate trigger have been prepared, and this design has also been involved in the development of H2O2-activated prodrugs or theranostic tools. However, this design suffers from slow kinetics, preventing activation by H2O2 with a short response time. Therefore, faster H2O2-reactive groups are awaited. To address this issue, we have successfully developed and characterized a prototypic borinic-based fluorescent probe containing a coumarin scaffold. We determined its in vitro kinetic constants toward H2O2-promoted oxidation. We measured 1.9 × 104 m-1⋅s-1 as a second-order rate constant, which is 10,000-fold faster than its well-established boronic counterpart (1.8 m-1⋅s-1). This improved reactivity was also effective in a cellular context, rendering borinic acids an advantageous trigger for H2O2-mediated release of effectors such as fluorescent moieties.

Microalgae-Derived Health Supplements to Therapeutic Shifts: Redox-Based Study Opportunities with AIE-Based Technologies

Reactive oxygen species (ROS) are highly reactive molecules, serve the normal signaling in different cell types. Targeting ROS as the chemical signals, different stress based strategies have been developed to synthesis different anti-inflammatory molecules in microalgae. These molecules could be utilized as health supplements in human. To provoke the ROS-mediated defence systems, their connotation with the associated conditions must be well understood, therefore, proper tools for studying ROS in natural state are essential. The in vivo detection of ROS with phosphorescent probes offers promising opportunities to study these molecules in a non-invasive manner. Most of the common problems in the traditional fluorescent probes are lower photostability, excitation intensity, slow responsiveness, and the microenvironment that challenge their performance. Some ROS-specific aggregationinduced emission luminogens (AIEgens) with pronounced spatial and temporal resolution have recently demonstrated high selectivity, rapid responsiveness, and efficacies to resolve the aggregation-caused quenching issues. The nanocomposites of some AIE-photosensitizers can also improve the ROS-mediated photodynamic therapy. These AIEgens could be used to induce bioactive components in microalgae through altering the ROS signaling, therefore are more auspicious for biomedical research. This study reviews the prospects of AIEgen-based technologies to understand the ROS mediated bio-physiological processes in microalgae for better healthcare benefits.

Lectin-Triggered Aggregation of Glyco-Gold Nanoprobes for Activity-based Sensing of Hydrogen Peroxide by the Naked Eye

The purpose of this study was to develop a colorimetric assay for detecting hydrogen peroxide (H₂ O₂ ) through a combination of using an aryl boronate (AB) derivative and gold nanoparticles (AuNPs). The unique optical property of AuNPs is applied to design a detection probe. The aggregation of AuNPs could be directly observed as a color change by the naked eye. A mannoside-boronate-sulfide (MBS) ligand was designed that contains an arylboronate (AB), a mannoside, and a thiol group. The thiol group bonds covalently with the surface of AuNPs to obtain MBS@AuNPs. The mannoside moiety recognizes concanavalin A (Con A), a lectin with four carbohydrate recognition sites that can specifically recognize the non-reducing end of an α-D-mannoside or α-D-glucoside structure. The AB structure on MBS first reacts with H₂ O₂ and then inserts an oxygen atom in the B-H bond, which triggers intramolecular electron rearrangement to cleave the covalent bond, resulting in a MBSt mixture. The MBS or MBSt is then modified to citrate-coated AuNPs (c-AuNPs) to have MBS@AuNPs or MBSt@AuNPs. When the MBS@AuNPs are incubated with Con A, the Con A recognizes multiple mannosides on the surface of the MBS@AuNPs. Subsequently, the MBS@AuNPs aggregate and the solution's color changes from red to purple, but this color change does not occur in the case of MBSt@AuNPs. The phenomenon can be observed by the naked eye.

Rational design of a selective and sensitive "turn-on" fluorescent probe for monitoring and imaging hydrogen peroxide in living cells

As one type of reactive oxygen species (ROS), hydrogen peroxide (H2O2) plays a key role in regulating a variety of cellular functions. Herein, a fluorescent probe N-Py-BO was well designed and synthesized and its ability for detecting H2O2 by fluorescence intensity was evaluated. In the design, the arylboronate ester group was acted as a reaction site for H2O2. Upon reaction with H2O2 under physiological conditions, the boronate moiety in the probe was oxidized, followed by detachment from the probe and as a result, a "turn-on" fluorescence response for H2O2 was acquired. Due to the D-A structure formation between N,N'-dimethylaminobenzene and the -CN group and the linkage by thiophene and C[double bond, length as m-dash]C bonds to increase the conjugate length, this probe showed a remarkable red shift of emission wavelength (650 nm) as well as a large Stokes shift (214 nm). An excellent linear relation with concentrations of H2O2 ranging from 2.0 to 200 μM and a good selectivity over other biological species were obtained. Importantly, taking advantage of the low toxicity and good biocompatibility, the developed probe was successfully applied to monitoring and imaging H2O2 and its level fluctuation in living cells, which provided a powerful tool for evaluation of cellular oxidative stress and understanding the pathophysiological process of H2O2-related diseases.

Redox and catalase-like activities of four widely used carbon monoxide releasing molecules (CO-RMs)

The pathophysiological roles of the endogenous signaling molecule, carbon monoxide (CO), have been extensively studied and validated in cell culture and animal models. Further, evidence supporting the therapeutic effects of CO in various human diseases has been mounting over the last two decades. Along this line, there has been intensive interest in developing various delivery forms including CO gas, CO in solution, metal–carbonyl complexes widely known as CO-releasing molecules (CO-RMs), and organic CO prodrugs. Among them, two ruthenium-based carbonyl complexes, CORM-2 and -3, occupy a very special place because they have been used in over 500 published studies. One of the mechanisms for CO's actions is known to be through attenuation of oxidative stress and regulation of production of reactive oxygen species (ROS). For this reason, it is important that CO delivery forms do not have intrinsic chemical redox properties. Herein, we describe our findings of catalase-like activities of CORM-2 and -3 in a CO-independent fashion, leading to the rapid degradation of hydrogen peroxide (H₂O₂) in PBS buffer (pH = 7.4) and in cell culture media. Further, we have found that CORM-2 and CORM-3 possess potent radical scavenging abilities. We have also studied two other widely used CO donors: CORM-401 and CORM-A1. Both showed chemical reactivity with ROS, but to a lesser degree than CORM-2 and -3. Because of the central role of ROS in some of the proposed mechanisms of actions for CO biology, the discovery of intrinsic chemical redox properties for these CO-RMs means that additional attention in designing proper controls is needed in future biological experiments using these CO-RMs for their CO-donating functions. Further, much more work is needed to understand the true implications of the chemical reactivity of these CO-RMs in cell-culture and animal-model studies of CO biology.

Four CO-releasing molecules are found to degrade H₂O₂ and free radicals either catalytically (CORM-2 and -3) or through direct reactions (CORM-401 and -A1) in solution under near-physiological conditions.

Recent Advances in Hydrogen Peroxide Responsive Organoborons for Biological and Biomedical Applications

Hydrogen peroxide is the most stable reactive oxygen species generated endogenously, participating in numerous physiological processes and abnormal pathological conditions. Mounting evidence suggests that a higher level of H₂ O₂ exists in various disease conditions. Thus, H₂ O₂ functions as an ideal target for site-specific bioimaging and therapeutic targeting. The unique reactivity of organoborons with H₂ O₂ provides a method for developing chemoselective molecules for biological and biomedical applications. This review highlights the design and application of boron-derived molecules for H₂ O₂ detection, and the utility of boron moieties toward masking reactive compounds leading to the development of metal prochelators and prodrugs for selectively delivering an active species at the target sites with elevated H₂ O₂ levels. Additionally, the emergence of H₂ O₂ -responsive theranostic agents consisting of both therapeutic and diagnostic moieties in one integrated system are discussed. The purpose of this review is to provide a better understanding of the role of boron-derived molecules toward biological and pharmacological applications.

Detection of hydrogen peroxide and glucose with a novel fluorescent probe by the enzymatic reaction of amino functionalized MOF nanosheets

Amino-functionalized two-dimensional (2D) MOFs have great potential in biosensors due to their excellent water solubility, high fluorescence, large specific surface area, good adsorption properties and good ability to enrich the target analytes. Fluorescence detection of hydrogen peroxide and glucose mostly relies on monitoring the single fluorescence intensity changes in a single excitation wavelength. Here, a ratiometric fluorescence sensor based on NH₂-MIL-53(Al) nanosheets to sensitively detect H₂O₂ and glucose through enzymatic reactions was developed. o-Phenylenediamine (OPD) was oxidized by H₂O₂ in the presence of horseradish peroxidase (HRP). Then, the oxidation product could be self-assembled on NH₂-MIL-53(Al) nanosheets by hydrogen bonding and π-π stacking. The orbital interaction or the fluorescence resonance energy transfer (FRET) between the nanosheets and the oxidation product could effectively quench the fluorescence of the nanosheets at 433 nm. At the same time, the oxidation product provided a new emission peak at 564 nm. The fluorescence ratio signal changes generated by this oxidation process were used to stably and sensitively detect H₂O₂ and glucose. Structural and mechanistic analysis was carried out by calculation methods such as AICD and ORCA to explore the π electron structure characteristics, the hole/electron orbitals and the quenching phenomenon. The detection limit was 26.9 nM for H₂O₂ and 0.041 μM for glucose. The detection of glucose in human serum has a satisfactory recovery of 97.4-102.8%. It is clear that the sensor has a good application prospect in real sample analysis.

Induced fit activity-based sensing: a mechanistic study of pyrophosphate detection with a "flexible" Fe-salen complex

Activity-based sensing of biological targets is attracting increasing attention. In this work, we report detailed UV-Vis and fluorescence mechanistic studies on an Fe-salen based probe, [Fe^III{salenMeCl₂(SO₃)₂}OH₂]⁻ for pyrophosphate (PPi) detection. In the presence of PPi as an analyte, the probe disassembles into its molecular subunits and releases a fluorescent signal. Our studies illustrate that the aqua form of the complex (1-OH2) is the active species and that upon substitution of Fe-coordinated H₂O and an initial end-on coordination of HP₂O₇³⁻, the “trapped” pyrophosphate species switches from a monodentate to a bidentate coordination mode (i.e. linkage isomerism) via a probable equilibrium process. The elusive intermediate is further stabilized by a hydrogen bonding interaction that activates the probe for the subsequent final irreversible rate-limiting step, and allows selective discrimination between the other pyrophosphate (H₂P₂O₇²⁻ and P₂O₇⁴⁻) species in favour of the HP₂O₇³⁻. The flexible mode of molecular recognition and binding of HP₂O₇³⁻ by the tetradentate probe 1-OH2 is unexpected and most effective at physiological pH, and has precedence in enzymatic catalysis (i.e. induced fit principle). These binding properties explain the previously observed outstanding selectivity of 1-OH2 for pyrophosphate over other (poly)oxophosphates and potentially competing analytes.

A detailed mechanistic study of pyrophosphate (PPi) detection with a fluorometric Fe-salen based probe unravels the key structural switch of the Fe-bound PPi (“induced fit principle”) explaining the novel selectivity over other competing analytes.

An oxidation responsive nano-radiosensitizer increases radiotherapy efficacy by remolding tumor vasculature

As an excellent candidate material for nano-sensitizers, gold nanostructures have shown great potential in radiotherapy. Nevertheless, severe hypoxia and low accumulation of nanomedicine caused by poor perfusion at the tumor site have significantly reduced radiotherapy efficacy. Vascular normalization has gained attention owing to its ability to relieve hypoxia and increase perfusion. The synergistic therapy of tumor vascular normalization and radiotherapy has become a new option to increase anti-cancer efficacy. However, the commonly used strategy of suppressing a single growth factor to induce vascular normalization is limited by tumor compensatory effects. In this work, we developed a strategy to inhibit oxidative stress in tumors by generating chelating agents in response to hydrogen peroxide, thereby inhibiting multi-angiogenic factors simultaneously to normalize blood vessels. Concretely, sodium alginate (SA) reacted with 8-quinoline boric acid (QBA) to form SA-QBA. Then gold nanoparticles (Au NPs) were modified with SA-QBA to obtain Au@SA-QBA. The system was simple in structure and could generate 8HQ in response to H₂O₂in vitro to inhibit oxidative stress and reduce the expression of VEGF, bFGF, and Ang-2. In vivo, the perfusion unit (PU) increased by 78% after Au@SA-QBA treatment, and the coverage of pericytes increased by 32%, which in turn induced vascular normalization. In addition, blood routine and blood biochemical tests confirmed its good biocompatibility and 8HQ was not detected in the supernatant after homogenization of major organs. More importantly, after the synergistic treatment of vascular normalization and radiotherapy (4 Gy), the tumor growth inhibition rate was increased by 38.6% compared to the Au@SA-treated group with negligible side effects to normal tissues.

Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number, charge, oxidation states and stability of manganese complexes throughout redox cycling

Reactivity assays previously suggested that two quinol-containing MRI contrast agent sensors for H₂O₂, [Mn(H2qp1)(MeCN)]²⁺ and [Mn(H4qp2)Br₂], could also catalytically degrade superoxide. Subsequently, [Zn(H2qp1)(OTf)]⁺ was found to use the redox activity of the H2qp1 ligand to catalyze the conversion of O₂˙⁻ to O₂ and H₂O₂, raising the possibility that the organic ligand, rather than the metal, could serve as the redox partner for O₂˙⁻ in the manganese chemistry. Here, we use stopped-flow kinetics and cryospray-ionization mass spectrometry (CSI-MS) analysis of the direct reactions between the manganese-containing contrast agents and O₂˙⁻ to confirm the activity and elucidate the catalytic mechanism. The obtained data are consistent with the operation of multiple parallel catalytic cycles, with both the quinol groups and manganese cycling through different oxidation states during the reactions with superoxide. The choice of ligand impacts the overall charges of the intermediates and allows us to visualize complementary sets of intermediates within the catalytic cycles using CSI-MS. With the diquinolic H4qp2, we detect Mn(iii)-superoxo intermediates with both reduced and oxidized forms of the ligand, a Mn(iii)-hydroperoxo compound, and what is formally a Mn(iv)-oxo species with the monoquinolate/mono-para-quinone form of H4qp2. With the monoquinolic H2qp1, we observe a Mn(ii)-superoxo ↔ Mn(iii)-peroxo intermediate with the oxidized para-quinone form of the ligand. The observation of these species suggests inner-sphere mechanisms for O₂˙⁻ oxidation and reduction that include both the ligand and manganese as redox partners. The higher positive charges of the complexes with the reduced and oxidized forms of H2qp1 compared to those with related forms of H4qp2 result in higher catalytic activity (k_cat ∼ 10⁸ M⁻¹ s⁻¹ at pH 7.4) that rivals those of the most active superoxide dismutase (SOD) mimics. The manganese complex with H2qp1 is markedly more stable in water than other highly active non-porphyrin-based and even some Mn(ii) porphyrin-based SOD mimics.

Manganese complexes with polydentate quinol-containing ligands are found to catalyze the degradation of superoxide through inner-sphere mechanisms. The redox activity of the ligand stabilizes higher-valent manganese species.

A tandem activity-based sensing and labeling strategy enables imaging of transcellular hydrogen peroxide signaling

Reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂) are transient species that have broad actions in signaling and stress, but spatioanatomical understanding of their biology remains insufficient. Here, we report a tandem activity-based sensing and labeling strategy for H₂O₂ imaging that enables capture and permanent recording of localized H₂O₂ fluxes. Peroxy Green-1 Fluoromethyl (PG1-FM) is a diffusible small-molecule probe that senses H₂O₂ by a boronate oxidation reaction to trigger dual release and covalent labeling of a fluorescent product, thus preserving spatial information on local H₂O₂ changes. This unique reagent enables visualization of transcellular redox signaling in a microglia-neuron coculture cell model, where selective activation of microglia for ROS production increases H₂O₂ in nearby neurons. In addition to identifying ROS-mediated cell-to-cell communication, this work provides a starting point for the design of chemical probes that can achieve high spatial fidelity by combining activity-based sensing and labeling strategies.

Selective targeting of cancer cells using a hydrogen peroxide-activated Hsp90 inhibitor

Heat shock protein 90 (Hsp90) plays an important role in cancer cell proliferation, survival, and migration by regulating the maturation and stabilization of numerous oncoproteins. Despite significant efforts in developing Hsp90 inhibitors, none of these have been approved for clinical use, mostly due to toxicity, such as liver, cardiac, and retinal toxicity. To avoid undesirable toxicity, we herein report a hydrogen peroxide-activated Hsp90 inhibitor, Boro-BZide (3), which is capable of selectively targeting cancer cells over normal cells. Boro-BZide (3) can be activated by high levels of hydrogen peroxide, releasing its parent active Hsp90 inhibitor. The mechanism of action was determined by a series of experiments including fluorescence polarization assay, cell viability assay, western blotting, high-pressure liquid chromatography (HPLC), and fluorescence-activated cell sorting (FACS) analysis. These efforts ultimately led to the identification of a novel hydrogen peroxide-activated Hsp90 prodrug with improved therapeutic index, which was less prone to furnish unwanted adverse effects. This hydrogen peroxide-responsive prodrug strategy will be beneficial for overcoming the toxicity hurdles of Hsp90 inhibitors for clinical application.

A microtubule-localizing activity-based sensing fluorescent probe for imaging hydrogen peroxide in living cells

Hydrogen peroxide (H₂O₂) is a major reactive oxygen species (ROS) in living systems with broad roles spanning both oxidative stress and redox signaling. Indeed, owing to its potent redox activity, regulating local sites of H₂O₂ generation and trafficking is critical to determining downstream physiological and/or pathological consequences. We now report the design, synthesis, and biological evaluation of Microtubule Peroxy Yellow 1 (MT-PY1), an activity-based sensing fluorescent probe bearing a microtubule-targeting moiety for detection of H₂O₂ in living cells. MT-PY1 utilizes a boronate trigger to show a selective and robust turn-on response to H₂O₂ in aqueous solution and in living cells. Live-cell microscopy experiments establish that the probe co-localizes with microtubules and retains its localization after responding to changes in levels of H₂O₂, including detection of endogenous H₂O₂ fluxes produced upon growth factor stimulation. This work adds to the arsenal of activity-based sensing probes for biological analytes that enable selective molecular imaging with subcellular resolution.

Glucose oxidase kinetics using MnO2 nanosheets: confirming Michaelis-Menten kinetics and quantifying decreasing enzyme performance with increasing buffer concentration

MnO₂ nanosheets and ultraviolet-visible (UV-Vis) absorbance spectroscopy are used to study glucose oxidase (GOx) kinetics. Glucose oxidation by GOx produces H₂O₂, which rapidly decomposes the nanosheets and reduces their absorption. This direct approach for monitoring glucose oxidation enables simpler, real time kinetics analysis compared to methods that employ additional enzymes. Using this approach, the present study confirms that GOx kinetics is consistent with the Michaelis-Menten (MM) model, and reveals that the MM constant increases by an order of magnitude with increasing buffer concentration. Since larger MM constants imply higher enzyme substrate concentrations are required to achieve the same rate of product formation, increasing MM constants imply decreasing enzyme performance. These results demonstrate the facility of using MnO₂ nanosheets to study GOx kinetics and, given the widespread applications of enzymes with buffers, the important sensitivity of enzyme-buffer systems on buffer concentration.

Nanohoop Rotaxane Design to Enhance the Selectivity of Reaction-Based Probes: A Proof-of-Principle Study

Mechanical interlocking of a nanohoop fluorophore and a reactive thread couples the benefits of a reaction-based probe with a sterically congested active site for enhanced selectivity. Advantageously, the thread design uses dual function stoppers that act as both a quencher and a trigger for sensing. In progress toward expanding this approach to biologically relevant analytes, this system is used to demonstrate steric differentiation and provide a selective turn-on fluorescent response with size selectivity for HS^- rather than larger thiolates.

Hyaluronic acid-decorated carborane-TAT conjugation nanomicelles: A potential boron agent with enhanced selectivity of tumor cellular uptake

Boron neutron capture therapy (BNCT) has received widespread attention as a new type of radiation therapy. The main problem encountered in BNCT is insufficient tumor cellular uptake of boron agents. In this study, cell-penetrating peptide TAT-conjugated o-carborane was synthesized. The conjugation can self-assemble to form positively charged carborane-TAT micelles, and then adsorb negatively charged hyaluronic acid (HA) to give core-shell structured carborane-TAT@HA micelles. Carborane-TAT@HA micelles exhibits a large amount of boron uptake at the tumor tissue through the enhanced permeability and retention (EPR) effect and the ability of HA to bind to CD44 receptors. Carborane-TAT@HA was wrapped by the HA shell during systemic circulation to avoid non-specific uptake of TAT with normal cells, while tumor microenvironment-responsive shedding of HA shell could expose Carborane-TAT to penetrate the cell membrane into tumor cells. Experiments have proved the enhanced selectivity of tumor cellular uptake of the boron drug, displayed excellent drug delivery potential, and can meet the basic requirements of BNCT.

Norcyanine-Carbamates Are Versatile Near-Infrared Fluorogenic Probes

Fluorogenic probes in the near-infrared (NIR) region have the potential to provide stimuli-dependent information in living organisms. Here, we describe a new class of fluorogenic probes based on the heptamethine cyanine scaffold, the most broadly used NIR chromophore. These compounds result from modification of heptamethine norcyanines with stimuli-responsive carbamate linkers. The resulting cyanine carbamates (CyBams) exhibit exceptional turn-ON ratios (∼170×) due to dual requirements for NIR emission: carbamate cleavage through 1,6-elimination and chromophore protonation. Illustrating their utility in complex in vivo settings, a γ-glutamate substituted CyBam was applied to imaging γ-glutamyl transpeptidase (GGT) activity in a metastatic model of ovarian cancer. Overall, CyBams have significant potential to extend the reach of fluorogenic strategies to intact tissue and live animal imaging applications.

Early Diagnosis of Cerebral Ischemia Reperfusion Injury and Revelation of Its Regional Development by a H3R Receptor-Directed Probe

In vivo imaging of cerebral hydrogen peroxide (H2O2) may facilitate early diagnosis of cerebral ischemia reperfusion injury (CIRI) and a revelation of its pathological progression. In this study, we report our rational design of a brain-targeting fluorescent probe using the basis of a pyridazinone scaffold. A structure-activity relationship study reveals that PCAB is the best candidate (K_i = 15.8 nM) for a histamine H3 receptor (H3R), which is highly expressed in neurons of the central nervous system. As a two-photon fluorescent probe, PCAB exhibits a fast, selective reaction toward both extra- and intracellular H2O2 in SH-SY5Y cells under oxygen glucose deprivation and resupply. In vivo fluorescent imaging of a middle cerebral artery occlusion mouse confirms that PCAB is an ultrasensitive probe with potent blood-brain barrier penetration, precise brain targeting, and fast detection of CIRI.

Designing an ESIPT-based fluorescent probe for imaging of hydrogen peroxide during the ferroptosis process

Hydrogen peroxide (H₂O₂), depending on its levels, plays a crucial role in either modulating various biological processes as a signal molecule, or mediating oxidative damage as a toxin. Therefore, monitoring intracellular H₂O₂ levels is pivotal for exploring its physiological and pathological roles. Using a modified 2-(2'-hydroxyphenyl) benzothiazole (HBT) as the fluorophore, and a pinacol phenylborate ester as the responsive group, herein we developed an excited-state intramolecular proton transfer (ESIPT)-based probe BTFMB. The probe exhibited turn-on fluorescence response, large Stokes shift (162 nm) and low detection limit (109 nM) toward H₂O₂, and was successfully applied for monitoring exogenous and endogenous production of H₂O₂, and identifying accumulation of H₂O₂ during the ferroptosis process.

Integrating amino acid oxidase with photoresponsive probe: A fast quantitative readout platform of amino acid enantiomers

Low-cost, high-throughput, broadly useful photoresponsive enantiomeric excess (ee) sensing of amino acids remains challenging to date. Herein, based on the selective oxidation reaction of amino acid oxidase (AAO) to amino acid enantiomers (D/L-AA) and the oxidation reaction of substrate (H₂O₂) with aromatic boronic ester, we put forward a photoresponsive strategy for the determination of D/L-AA at a certain concentration. In this scheme, the substrate H₂O₂ produced by the enzyme-catalyzed reaction was determined by sensitive fluorescent and colorimetric response of ethyl-3-(3-(benzothiazol-2-yl)-5-methyl-2-((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)phenyl)-2-cyanoacrylate (HBT-PB) to reflect the enantiomeric content at a certain concentration. The photoresponsive probe HBT-PB was readily available and inexpensive with sensitive long-wavelength red fluorescence and colorimetric light response to H₂O₂, the detection limit (LOD) was estimated as 2.91 μM. The operation of the sensing method was simple and data collection and processing are straightforward. The practicability of the scheme was favorably confirmed by accurate and scientific analysis of methionine and Dopa samples. As a result, the scheme was not only suitable for high-throughput screening but also adaptable to low-cost and sensitive RGB colorimetric analysis platform (LOD of methionine and Dopa was calculated as 9.23 μM and 8.34 μM respectively) with modern plate readers, and possessed extremely high enantioselectivity and wide applicability which benefited from the specificity and efficiency of enzyme catalytic reaction.

Design, synthesis and application of a dual-functional fluorescent probe for reactive oxygen species and viscosity

A fluorescence probe based on cyanine fluorophore was designed and synthesized in this work, which can be used to determine viscosity and reactive oxygen species (e.g., OCl^-, ONOO^-) at different wavelengths. Under a low viscosity medium, the fluorescence quantum yield of the probe is very low; however, with the increase of the medium viscosity, the probe's emission at 571 nm is enhanced by nearly 25-fold due to the inhibition of intramolecular rotations. On the other hand, the probe shows a rapid and linear fluorescence response at 710 nm to OCl^- or ONOO^- within 1 min. The different spectral response regions of the probe permit the selective detection of both viscosity and reactive oxygen species. Furthermore, the probe is demonstrated to be cell permeable and capable of detecting the viscosity and the total amount of OCl^-/ONOO^- in living cells with the help of confocal microscope fluorescence imaging.

Highly Sensitive Near-Infrared Imaging of Peroxynitrite Fluxes in Inflammation Progress

Inflammation is an important protection reaction in living organisms associated with many diseases. Since peroxynitrite (ONOO^-) is engaged in the inflammatory processes, illustrating the key nexus between ONOO^- and inflammation is significant. Due to the lack of sensitive ONOO^- in vivo detection methods, the research still remains at its infancy. Herein, a highly sensitive NIR fluorescence probe DDAO-PN for in vivo detection of ONOO^- in inflammation progress was reported. The probe responded to ONOO^- with significant NIR fluorescence enhancement at 657 nm (84-fold) within 30 s in solution. Intracellular imaging of exogenous ONOO^- with the probe demonstrated a 68-fold fluorescence increase (F/F₀). Impressively, the probe can in vivo detect ONOO^- fluxes in LPS-induced rear leg inflammation with a 4.0-fold fluorescence increase and LPS-induced peritonitis with an 8.0-fold fluorescence increase The remarkable fluorescence enhancement and quick response enabled real-time tracking of in vivo ONOO^- with a large signal-to-noise (S/N) ratio. These results clearly denoted that DDAO-PN was able to be a NIR fluorescence probe for in vivo detection and high-fidelity imaging of ONOO^- with high sensitivity and will boost the research of inflammation-related diseases.

Design of ratiometric monoaromatic fluorescence probe via modulating intramolecular hydrogen bonding: A case study of alkaline phosphatase sensing

Monoaromatic molecules are a category of molecules containing a single aromatic ring which generally emit light in the ultraviolet (UV) region. Despite their facile preparation, the UV emission greatly limits their application as organic probes. In this study, we developed a general method to red shift the emission of monoaromatic molecules. Significant fluorescence red-shift (∼100 nm per intramolecular hydrogen bonding) can be achieved by introducing intramolecular hydrogen bonding units to benzene, a typical monoaromatic molecule. Upon increasing the number of hydrogen bonding units on the benzene ring, UV, blue, and green emissions are screened, which are switchable by simply breaking/restoration the intramolecular hydrogen bonding. As a demonstration, with the breaking of one intramolecular H-bonding, the green emission (λ_em^max = 533 nm) of 2,5-dihydroxyterephthalic acid (DHTA) changed to cyan (λ_em^max = 463 nm) upon the formation of its phosphorylated form (denoted as PDHTA), which, in the presence of alkaline phosphatase (ALP), hydrolyzed and recovered the green emission. By taking advantage of the switchable emission colors, ratiometric in vitro and endogenous ALP sensing was achieved. This general approach offers a great promise to develop organic probes with tunable emissions for fluorescence analysis and imaging by different intramolecular hydrogen bonding.

Advances in Imaging Reactive Oxygen Species

Reactive oxygen species (ROS) play a pivotal role in many cellular processes and can be either beneficial or harmful. The design of ROS-sensitive fluorophores has allowed for imaging of specific activity and has helped elucidate mechanisms of action for ROS. Understanding the oxidative role of ROS in the many roles it plays allows us to understand the human body. This review provides a concise overview of modern advances in the field of ROS imaging. Indeed, much has been learned about the role of ROS throughout the years; however, it has recently been shown that using nanoparticles, rather than individual small organic fluorophores, for ROS imaging can further our understanding of ROS.

Imaging Mitochondrial Hydrogen Peroxide in Living Cells

Hydrogen peroxide (H₂O₂) produced from mitochondria is intimately involved in human health and disease, but is challenging to selectively monitor inside living systems. The fluorescent probe MitoPY1 provides a practical tool for imaging mitochondrial H₂O₂ and has been demonstrated to function in a variety of diverse cell types. In this chapter, we describe the synthetic preparation of the small molecule probe MitoPY1 , methods for validating this probe in vitro and in live cells, and an example procedure for measuring mitochondrial H₂O₂ in a cell culture model of Parkinson's disease.

Reactive Oxygen Species Link Gene Regulatory Networks During Arabidopsis Root Development

Plant development under altered nutritional status and environmental conditions and during attack from invaders is highly regulated by plant hormones at the molecular level by various signaling pathways. Previously, reactive oxygen species (ROS) were believed to be harmful as they cause oxidative damage to cells; however, in the last decade, the essential role of ROS as signaling molecules regulating plant growth has been revealed. Plant roots accumulate relatively high levels of ROS, and thus, maintaining ROS homeostasis, which has been shown to regulate the balance between cell proliferation and differentiation at the root tip, is important for proper root growth. However, when the balance is disturbed, plants are unable to respond to the changes in the surrounding conditions and cannot grow and survive. Moreover, ROS control cell expansion and cell differentiation processes such as root hair formation and lateral root development. In these processes, the transcription factor-mediated gene expression network is important downstream of ROS. Although ROS can independently regulate root growth to some extent, a complex crosstalk occurs between ROS and other signaling molecules. Hormone signals are known to regulate root growth, and ROS are thought to merge with these signals. In fact, the crosstalk between ROS and these hormones has been elucidated, and the central transcription factors that act as a hub between these signals have been identified. In addition, ROS are known to act as important signaling factors in plant immune responses; however, how they also regulate plant growth is not clear. Recent studies have strongly indicated that ROS link these two events. In this review, we describe and discuss the role of ROS signaling in root development, with a particular focus on transcriptional regulation. We also summarize the crosstalk with other signals and discuss the importance of ROS as signaling molecules for plant root development.

A periodic development of BPA and BSH based derivatives in boron neutron capture therapy (BNCT)

A schematic representation of various judicious approaches for the synthesis of BPA and BSH modified compounds for effective BNCT.

Four xanthene–fluorene based probes for the detection of Hg2+ ions and their application in strip tests and biological cells

Four new fluorescent probes based on the xanthene structure to detect mercury ions with different colors of fluorescence have been reported.

Reactive Oxygen Species (ROS)-Responsive Prodrugs, Probes, and Theranostic Prodrugs: Applications in the ROS-Related Diseases

Elevated levels of reactive oxygen species (ROS) have commonly been implicated in a variety of diseases, including cancer, inflammation, and neurodegenerative diseases. In light of significant differences in ROS levels between the nonpathogenic and pathological tissues, an increasing number of ROS-responsive prodrugs, probes, and theranostic prodrugs have been developed for the targeted treatment and precise diagnosis of ROS-related diseases. This review will summarize and provide insight into recent advances in ROS-responsive prodrugs, fluorescent probes, and theranostic prodrugs, with applications to different ROS-related diseases and various subcellular organelle-targetable and disease-targetable features. The ROS-responsive moieties, the self-immolative linkers, and the typical activation mechanism for the ROS-responsive release are also summarized and discussed.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Ultrasensitive, Specific, and Rapid Fluorescence Turn-On Nitrite Sensor Enabled by Precisely Modulated Fluorophore Binding

The precise regulation of fluorophore binding sites in an organic probe is of great significance toward the design of fluorescent sensing materials with specific functions. In this study, a probe with specific fluorescence properties and nitrite detection ability is designed by precisely modulating benzothiazole binding sites. Only the fluorophore bond at the ortho-position of the aniline moiety can specifically recognize nitrite, which ensures that the reaction products displays a robust green emission. The unique 2-(2-amino-4-carboxyphenyl) benzothiazole (ortho-BT) shows superior nitrite detection performance, including a low detection limit (2.2 fg), rapid detection time (<5 s), and excellent specificity even in the presence of >40 types of strong redox active, colored substances, nitro compounds, and metal ions. Moreover, the probe is highly applicable for the rapid on-site and semiquantitative measurement of nitrite. The proposed probe design strategy is expected to start a new frontier for the exploration of probe design methodology.

Quantitative Imaging of Biochemistry in Situ and at the Nanoscale

Biochemical reactions in eukaryotic cells occur in subcellular, membrane-bound compartments called organelles. Each kind of organelle is characterized by a unique lumenal chemical composition whose stringent regulation is vital to proper organelle function. Disruption of the lumenal ionic content of organelles is inextricably linked to disease. Despite their vital roles in cellular homeostasis, there are large gaps in our knowledge of organellar chemical composition largely from a lack of suitable probes. In this Outlook, we describe how, using organelle-targeted ratiometric probes, one can quantitatively image the lumenal chemical composition and biochemical activity inside organelles. We discuss how excellent fluorescent detection chemistries applied largely to the cytosol may be expanded to study organelles by chemical imaging at subcellular resolution in live cells. DNA-based reporters are a new and versatile platform to enable such approaches because the resultant probes have precise ratiometry and accurate subcellular targeting and are able to map multiple chemicals simultaneously. Quantitatively mapping lumenal ions and biochemical activity can drive the discovery of new biology and biomedical applications.

Fluorescent probes for in vitro and in vivo quantification of hydrogen peroxide

Hydrogen peroxide (H2O2) plays essential roles in redox signaling and oxidative stress, and its dynamic concentration is critical to human health and diseases. Here we report the design, syntheses, and biological applications of HKPerox-Red and HKPerox-Ratio for quantitative measurement of H2O2. Both probes were successfully applied to detect endogenous H2O2 fluxes in living cells or zebrafish, and biological effects of multiple stress inducers including rotenone, arsenic trioxide, and starvation were investigated. As H2O2 is a common by-product for oxidase oxidation, a general assay was developed for ultrasensitive detection of various metabolites (glucose, uric acid, and sarcosine). Moreover, cellular H2O2 measurements were achieved for the first time by combining flow cytometry with live cell calibration. This study provides a pair of unique molecular tools for advanced H2O2 bio-imaging and assay development.

Boronate-Based Probes for Biological Oxidants: A Novel Class of Molecular Tools for Redox Biology

Boronate-based molecular probes are emerging as one of the most effective tools for detection and quantitation of peroxynitrite and hydroperoxides. This review discusses the chemical reactivity of boronate compounds in the context of their use for detection of biological oxidants, and presents examples of the practical use of those probes in selected chemical, enzymatic, and biological systems. The particular reactivity of boronates toward nucleophilic oxidants makes them a distinct class of probes for redox biology studies. We focus on the recent progress in the design and application of boronate-based probes in redox studies and perspectives for further developments.