Detection of Hydrogen Peroxide with Fluorescent Dyes

A novel electrochemical strategy to detect hydrogen peroxide by utilizing peroxidase-mimicking activity of cerium oxide/graphene oxide nanocomposites

We herein describe a novel electrochemical strategy to detect hydrogen peroxide (H2O2) by utilizing the peroxidase-mimicking activity of cerium oxide nanoparticles (CeO2 NP) and reduced graphene oxide (rGO). Particularly, CeO2 NP/rGO nanocomposites were deposited on the commercial electrode by a very convenient and direct electrochemical reduction of graphene oxide. Due to the peroxidase-mimicking activity of CeO2 NP and the outstanding electrochemical properties of reduced graphene oxide, the reduction current of H2O2 was greatly enhanced. Based on this strategy, we reliably determined H2O2 down to 1.67 μM with excellent specificity and further validated its practical capabilities by robustly detecting H2O2 present in heterogeneous human serum samples. We believe that this work could serve as a new facile platform for H2O2 detection.

Excited-state dynamics of 4-hydroxyisoindoline-1,3-dione and its derivative as fluorescent probes

Fluorescent probes have become promising tools for monitoring the concentration of peroxynitrite, which is linked to many diseases. However, despite focusing on developing numerous peroxynitrite based fluorescent probes, limited emphasis is placed on their sensing mechanism. Here, we investigated the sensing mechanism of a peroxynitrite fluorescent probe, named BHID-Bpin, with a focus on the relevant excited state dynamics. The photoexcited BHID-Bpin relaxes to its ground state via an efficient nonradiative process (∼300 ps) due to the presence of a minimum energy conical intersection between its first excited state and ground state. However, upon reacting with peroxynitrite, the Bpin moiety is cleaved from BHID-Bpin and BHID is formed. The formed BHID exhibits strong dual band fluorescence which is caused by an ultrafast excited-state intramolecular proton transfer process (∼1 ps).

Direct electrochemical evidence suggests that aqueous microdroplets spontaneously produce hydrogen peroxide

Recent reports have detailed the striking observation that electroactive molecules, such as hydrogen peroxide (H2O2) and radical water species (H2O.+/H2O.-), are spontaneously produced in aqueous microdroplets. Stochastic electrochemistry allows one to study reactions in real-time occurring inside subfemtoliter droplets, one droplet at a time, when a microdroplet irreversibly adsorbs to an ultramicroelectrode surface (radius ~ 5 µm). Here, we use stochastic electrochemistry to probe the formation of hydrogen peroxide (H2O2) in single aqueous microdroplets suspended in 1,2-dichloroethane. The oxidation of H2O2 at alkaline pH (11.5) differs from near-neutral conditions (6.4), allowing us to create a digital, turn-off sensing modality for the presence of H2O2. Further, we show that the stochastic electrochemical signal is highest at the mass transfer limitation of the H2O2 couple and is dampened when the potential nears the formal potential. We validate these results by showing that the addition of a H2O2 selective probe, luminol, decreases the stochastic electrochemical response at alkaline pH (11.5). Our results support the observation that H2O2 is generated in water microdroplets at concentrations of ~100 s of µM.

Pt-Black-Modified (Hemi)spherical AFM Sensors: In Situ Imaging of Light-Driven Hydrogen Peroxide Evolution

In this work, we present (hemi)spherical atomic force microscopy (AFM) sensors for the detection of hydrogen peroxide. Platinum-black (Pt-B) was electrodeposited onto conductive colloidal AFM probes or directly at recessed microelectrodes located at the end of a tipless cantilever, resulting in electrocatalytically active cantilever-based sensors that have a small geometric area but, due to the porosity of the films, exhibit a large electroactive surface area. Focused ion beam-scanning electron microscopy tomography revealed the porous 3D structure of the deposited Pt-B. Given the accurate positioning capability of AFM, these probes are suitable for local in situ sensing of hydrogen peroxide and at the same time can be used for (electrochemical) force spectroscopy measurements. Detection limits for hydrogen peroxide in the nanomolar range (LOD = 68 ± 7 nM) were obtained. Stability test and first in situ proof-of-principle experiments to achieve the electrochemical imaging of hydrogen peroxide generated at a microelectrode and at photocatalytically active structured poly(heptazine imide) films are demonstrated. Force spectroscopic data of the photocatalyst films were recorded in ambient conditions, in solution, and by applying a potential, which demonstrates the versatility of these novel Pt-B-modified spherical AFM probes.

A sequential one-pot approach for rapid and convenient characterization of putative restriction-modification systems

IMPORTANCE

The elucidation of the molecular basis of virus-host coevolutionary interactions is boosted with state-of-the-art sequencing technologies. However, the sequence-only information is often insufficient to output a conclusive argument without biochemical characterizations. We proposed a 1-day and one-pot approach to confirm the exact function of putative restriction-modification (R-M) genes that presumably mediate microbial coevolution. The experiments mainly focused on a series of putative R-M enzymes from a deep-sea virus and its host bacterium. The results quickly unveiled unambiguous substrate specificities, superior catalytic performance, and unique sequence preferences for two new restriction enzymes (capable of cleaving DNA) and two new methyltransferases (capable of modifying DNA with methyl groups). The reality of the functional R-M system reinforced a model of mutually beneficial interactions with the virus in the deep-sea microbial ecosystem. The cell culture-independent approach also holds great potential for exploring novel and biotechnologically significant R-M enzymes from microbial dark matter.

Health position paper and redox perspectives on reactive oxygen species as signals and targets of cardioprotection

The present review summarizes the beneficial and detrimental roles of reactive oxygen species in myocardial ischemia/reperfusion injury and cardioprotection. In the first part, the continued need for cardioprotection beyond that by rapid reperfusion of acute myocardial infarction is emphasized. Then, pathomechanisms of myocardial ischemia/reperfusion to the myocardium and the coronary circulation and the different modes of cell death in myocardial infarction are characterized. Different mechanical and pharmacological interventions to protect the ischemic/reperfused myocardium in elective percutaneous coronary interventions and coronary artery bypass grafting, in acute myocardial infarction and in cardiotoxicity from cancer therapy are detailed. The second part keeps the focus on ROS providing a comprehensive overview of molecular and cellular mechanisms involved in ischemia/reperfusion injury. Starting from mitochondria as the main sources and targets of ROS in ischemic/reperfused myocardium, a complex network of cellular and extracellular processes is discussed, including relationships with Ca2+ homeostasis, thiol group redox balance, hydrogen sulfide modulation, cross-talk with NAPDH oxidases, exosomes, cytokines and growth factors. While mechanistic insights are needed to improve our current therapeutic approaches, advancements in knowledge of ROS-mediated processes indicate that detrimental facets of oxidative stress are opposed by ROS requirement for physiological and protective reactions. This inevitable contrast is likely to underlie unsuccessful clinical trials and limits the development of novel cardioprotective interventions simply based upon ROS removal.

Hydrogen Peroxide-Triggered Disassembly of Boronic Ester-Cross-Linked Brush-Arm Star Polymers

The concentrations of reactive oxygen species (ROS), e.g., H2O2, are often elevated in diseased tissue microenvironments. Therefore, the selective detection of ROS could enable new diagnostic methods or tools for chemical biology. Here, we report the synthesis of boronic ester-bis-norbornene core-cross-linked brush-arm star polymers (BASPs) with polyethylene glycol (PEG) or PEG-branch-spirocyclohexyl nitroxide (chex) shells. Size exclusion chromatography (SEC) and dynamic light scattering (DLS) showed that these BASPs have narrowly dispersed molar masses and average hydrodynamic diameters of 23 ± 2 nm, respectively. Moreover, due to their core-shell structures, these BASPs disassemble into bottlebrush fragments with improved selectivity for H2O2 over ROS such as peroxynitrite (ONOO-) and hypochlorite (-OCl). Finally, H2O2 induced disassembly of chex-containing BASPs induces a change in transverse magnetic relaxivity that can be detected via magnetic resonance imaging (MRI). Chex-BASPs may represent a valuable new diagnostic tool for H2O2 sensing.

Delivery of Lipid Nanoparticles with ROS Probes for Improved Visualization of Hepatocellular Carcinoma

Reactive oxygen species (ROS) are highly reactive products of the cell metabolism derived from oxygen molecules, and their abundant level is observed in many diseases, particularly tumors, such as hepatocellular carcinoma (HCC). In vivo imaging of ROS is a necessary tool in preclinical research to evaluate the efficacy of drugs with antioxidant activity and for diagnosis and monitoring of diseases. However, most known sensors cannot be used for in vivo experiments due to low stability in the blood and rapid elimination from the body. In this work, we focused on the development of an effective delivery system of fluorescent probes for intravital ROS visualization using the HCC model. We have synthesized various lipid nanoparticles (LNPs) loaded with ROS-inducible hydrocyanine pro-fluorescent dye or plasmid DNA (pDNA) with genetically encoded protein sensors of hydrogen peroxide (HyPer7). LNP with an average diameter of 110 ± 12 nm, characterized by increased stability and pDNA loading efficiency (64 ± 7%), demonstrated preferable accumulation in the liver compared to 170 nm LNPs. We evaluated cytotoxicity and demonstrated the efficacy of hydrocyanine-5 and HyPer7 formulated in LNP for ROS visualization in mouse hepatocytes (AML12 cells) and in the mouse xenograft model of HCC. Our results demonstrate that obtained LNP could be a valuable tool in preclinical research for visualization ROS in liver diseases.

Phosphate-driven H2O2 decomposition on DNA-bound bio-inspired activated carbon-based sensing platform for biological and food samples

Hydrogen peroxide (H₂O₂) is one of the most important reactive oxygen species (ROS). Increased endogenous H₂O₂ levels indicate oxidative stress and could be a potential marker of many diseases, including Alzheimer's, cardiovascular diseases, and diabetes. However, consuming H₂O_2-incorporated food has adverse effects on humans and is a serious health concern. We used salmon testes DNA with bio-inspired activated carbon (AC) as an electrocatalyst for developing a novel H₂O₂ sensor. The phosphate backbone of DNA contains negatively charged oxygen groups that specifically attract protons from H₂O₂ reduction. We observed a linearity range of 0.01-250.0 μM in the H₂O₂ reduction peak current with a detection limit of 2.5 and 45.7 nM for chronoamperometric and differential pulse voltammetric studies. High biocompatibility of the sensor was achieved by the DNA, facilitating endogenous H₂O₂ detection. Moreover, this non-enzymatic sensor could also help in the rapid screening of H₂O₂-contaminated foods.

Real Time and Spatiotemporal Quantification of pH and H2O2 Imbalances with a Multiplex Surface-Enhanced Raman Spectroscopy Nanosensor

Oxidative stress is involved in many aging-related pathological disorders and is the result of defective cellular management of redox reactions. Particularly, hydrogen peroxide (H2O2), is a major byproduct and a common oxidative stress biomarker. Monitoring its dynamics and a direct correlation to diseases remains a challenge due to the complexity of redox reactions. Sensitivity and specificity are major drawbacks for H2O2 sensors regardless of their readout. Luminiscent boronate-based probes such as 3-mercaptophenylboronic acid (3-MPBA) are emerging as the most effective quantitation tool due to their specificity and sensitivity. Problems associated with these probes are limited intracellular sensing, water solubility, selectivity, and quenching. We have synthesized a boronate-based nanosensor with a surface-enhanced Raman spectroscopy (SERS) readout to solve these challenges. Furthermore, we found out that environmental pH gradients, as found in biological samples, affect the sensitivity of boronate-based sensors. When the sensor is in an alkaline environment, the oxidation of 3-MPBA by H2O2 is more favored than in an acidic environment. This leads to different H2O2 measurements depending on pH. To solve this issue, we synthesized a multiplex nanosensor capable of concomitantly quantifying pH and H2O2. Our nanosensor first measures the local pH and based on this value, provides the amount of H2O2. It seems that this pH-dependent sensitivity effect applies to all boronic acid based probes. We tested the multiplexing ability by quantitatively measuring intra- and extracellular pH and H2O2 dynamics under physiological and pathological conditions on healthy cells and cells in which H+ and/or H2O2 homeostasis has been altered.

A turn-on NIR fluorescent probe for risk-assessing oxidative stress in cabbage roots under abiotic stress

Oxidative stress is closely related to the crop health status under stress conditions. H₂O₂ is an important signaling molecule in plants under stress. Therefore, monitoring H₂O₂ fluctuations is of great significance when risk-assessing oxidative stress. However, few fluorescent probes have been reported for the in situ tracking of H₂O₂ fluctuations in crops. Herein, we designed a "turn-on" NIR fluorescent probe (DRP-B) to detect and in situ-image H₂O₂ in living cells and crops. DRP-B exhibited good detection performance for H₂O₂ and could image endogenous H₂O₂ in living cells. More importantly, it could semi-quantitatively visualize H₂O₂ in cabbage roots under abiotic stress. Visualization of H₂O₂ in cabbage roots revealed H₂O₂ upregulation in response to adverse environments (metals, flood, and drought). This study provides a new method for risk-assessing oxidative stress in plants under abiotic stress and is expected to provide guidance for the development of new antioxidant defense strategies to enhance plant resistance and crop productivity.

A novel enzyme-free long-lasting chemiluminescence system based on a luminol functionalized β-cyclodextrin hydrogel for sensitive detection of H2O2 in urine and cells

A novel long-lasting chemiluminescent (CL) hydrogel (β-CD@luminol-Co²⁺) was synthesized by embedding luminol and cobalt ions (Co²⁺) into β-cyclodextrin (β-CD) through non-covalent interactions. Due to its porous structure and viscosity, the synthesized β-CD@luminol-Co²⁺ hydrogel exhibited long-lasting CL properties and can emit light for 12 h under both alkaline and neutral conditions. In addition, the CL intensities of β-CD@luminol-Co²⁺ were linear with the logarithm of the hydrogen peroxide (H₂O₂) concentration in the range of 1.0 × 10^-11-1.0 × 10^-7 M, and the limit of detection (LOD) was 0.63 × 10^-11 M and 0.85 × 10^-11 M under alkaline and neutral conditions, respectively. On this basis, an enzyme-free CL sensor based on β-CD@luminol-Co²⁺ was fabricated for the sensitive detection of H₂O₂ in human urine samples under alkaline conditions, and showed good accuracy and recovery. Since β-CD@luminol-Co²⁺ showed good CL properties under neutral conditions, it can be applied to detect H₂O₂ in cells. In order to prolong the emission wavelength of β-CD@luminol-Co²⁺ for better cell imaging, β-CD@luminol-FL-Co²⁺ was prepared by adding fluorescein (FL) to β-CD@luminol-Co²⁺. The as-prepared β-CD@luminol-FL-Co²⁺ also displayed long-lasting CL properties and showed a linear relationship with H₂O₂ concentrations. In addition, the maximum emission wavelength of β-CD@luminol-FL-Co²⁺ was 520 nm, which was red-shifted by 95 nm compared with β-CD@luminol-Co²⁺. The methyl thiazolyl tetrazolium (MTT) assay results and confocal microscopy images illustrated that β-CD@luminol-FL-Co²⁺ had low toxicity and can be taken up by A549 cells. Finally, β-CD@luminol-FL-Co²⁺ was successfully applied for CL imaging and detection of intracellular H₂O₂ in A549 cells under neutral conditions. This enzyme-free long-lasting CL system with high sensitivity can also be extended to real-time monitoring of H₂O₂in vivo.

Defining roles of specific reactive oxygen species (ROS) in cell biology and physiology

'Reactive oxygen species' (ROS) is a generic term that defines a wide variety of oxidant molecules with vastly different properties and biological functions that range from signalling to causing cell damage. Consequently, the description of oxidants needs to be chemically precise to translate research on their biological effects into therapeutic benefit in redox medicine. This Expert Recommendation article pinpoints key issues associated with identifying the physiological roles of oxidants, focusing on H₂O₂ and O₂^.-. The generic term ROS should not be used to describe specific molecular agents. We also advocate for greater precision in measurement of H₂O₂, O₂^.- and other oxidants, along with more specific identification of their signalling targets. Future work should also consider inter-organellar communication and the interactions of redox-sensitive signalling targets within organs and whole organisms, including the contribution of environmental exposures. To achieve these goals, development of tools that enable site-specific and real-time detection and quantification of individual oxidants in cells and model organisms are needed. We also stress that physiological O₂ levels should be maintained in cell culture to better mimic in vivo redox reactions associated with specific cell types. Use of precise definitions and analytical tools will help harmonize research among the many scientific disciplines working on the common goal of understanding redox biology.

A non-enzymatic electrochemical hydrogen peroxide sensor based on copper oxide nanostructures

This article describes the synthesis of nanostructured copper oxide on copper wires and its application for the detection of hydrogen peroxide. Copper oxide petal nanostructures were obtained by a one-step hydrothermal oxidation method. The resulting coating is uniform and dense and shows good adhesion to the wire surface. Structure, surface, and composition of the obtained samples were studied using field-emission scanning electron microscopy along with energy-dispersive spectroscopy and X-ray diffractometry. The resulting nanostructured samples were used for electrochemical determination of the H₂O₂ content in a 0.1 M NaOH buffer solution using cyclic voltammetry, differential pulse voltammetry, and i-t measurements. A good linear relationship between the peak current and the concentration of H₂O₂ in the range from 10 to 1800 μM was obtained. The sensitivity of the obtained CuO electrode is 439.19 μA·mM^-1. The calculated limit of detection is 1.34 μM, assuming a signal-to-noise ratio of 3. The investigation of the system for sensitivity to interference showed that the most common interfering substances, that is, ascorbic acid, uric acid, dopamine, NaCl, glucose, and acetaminophen, do not affect the electrochemical response. The real milk sample test showed a high recovery rate (more than 95%). According to the obtained results, this sensor is suitable for practical use for the qualitative detection of H₂O₂ in real samples, as well as for the quantitative determination of its concentration.

SERS and Indicator Paper Sensing of Hydrogen Peroxide Using Au@Ag Nanorods

The detection of hydrogen peroxide and the control of its concentration are important tasks in the biological and chemical sciences. In this paper, we developed a simple and quantitative method for the non-enzymatic detection of H₂O₂ based on the selective etching of Au@Ag nanorods with embedded Raman active molecules. The transfer of electrons between silver atoms and hydrogen peroxide enhances the oxidation reaction, and the Ag shell around the Au nanorod gradually dissolves. This leads to a change in the color of the nanoparticle colloid, a shift in LSPR, and a decrease in the SERS response from molecules embedded between the Au core and Ag shell. In our study, we compared the sensitivity of these readouts for nanoparticles with different Ag shell morphology. We found that triangle core-shell nanoparticles exhibited the highest sensitivity, with a detection limit of 10^-4 M, and the SERS detection range of 1 × 10^-4 to 2 × 10^-2 M. In addition, a colorimetric strategy was applied to fabricate a simple indicator paper sensor for fast detection of hydrogen peroxide in liquids. In this case, the concentration of hydrogen peroxide was qualitatively determined by the change in the color of the nanoparticles deposited on the nitrocellulose membrane.

Factors affecting liver mitochondrial hydrogen peroxide emission

Mitochondria are key cellular sources of reactive oxygen species (ROS) and contain at least 12 known sites on multiple enzymes that convert molecular oxygen to superoxide and hydrogen peroxide (H₂O₂). Quantitation of site-specific ROS emission is critical to understand the relative contribution of different sites and the pathophysiologic importance of mitochondrial ROS. However, factors that affect mitochondrial ROS emission are not well understood. We characterized and optimized conditions for maximal total and site-specific H₂O₂ emission during oxidation of standard substrates and probed the source of the high H₂O₂ emission in unenergized rainbow trout liver mitochondria. We found that mitochondrial H₂O₂ emission capacity depended on the substrate being oxidized, mitochondrial protein concentration, and composition of the ROS detection system. Contrary to our expectation, addition of exogenous superoxide dismutase reduced H₂O₂ emission. Titration of conventional mitochondrial electron transfer system (ETS) inhibitors over a range of conditions revealed that one size does not fit all; inhibitor concentrations evoking maximal responses varied with substrate and were moderated by the presence of other inhibitors. Moreover, the efficacy of suppressors of electron leak (S1QEL1.1 and S3QEL2) was low and depended on the substrate being oxidized. We found that H₂O₂ emission in unenergized rainbow trout liver mitochondria was suppressed by GKT136901 suggesting that it is associated with NADPH oxidase activity. We conclude that optimization of assay conditions is critical for quantitation of maximal H₂O₂ emission and would facilitate more valid comparisons of mitochondrial total and site-specific H₂O₂ emission capacities between studies, tissues, and species.

Exponential amplification by redox cross-catalysis and unmasking of doubly protected molecular probes

The strength of autocatalytic reactions lies in their ability to provide a powerful means of molecular amplification, which can be very useful for improving the analytical performances of a multitude of analytical and bioanalytical methods. However, one of the major difficulties in designing an efficient autocatalytic amplification system is the requirement for reactants that are both highly reactive and chemically stable in order to avoid limitations imposed by undesirable background amplifications. In the present work, we devised a reaction network based on a redox cross-catalysis principle, in which two catalytic loops activate each other. The first loop, catalyzed by H₂O₂, involves the oxidative deprotection of a naphthylboronate ester probe into a redox-active naphthohydroquinone, which in turn catalyzes the production of H₂O₂ by redox cycling in the presence of a reducing enzyme/substrate couple. We present here a set of new molecular probes with improved reactivity and stability, resulting in particularly steep sigmoidal kinetic traces and enhanced discrimination between specific and nonspecific responses. This translates into the sensitive detection of H₂O₂ down to a few nM in less than 10 minutes or a redox cycling compound such as the 2-amino-3-chloro-1,4-naphthoquinone down to 50 pM in less than 30 minutes. The critical reason leading to these remarkably good performances is the extended stability stemming from the double masking of the naphthohydroquinone core by two boronate groups, a counterintuitive strategy if we consider the need for two equivalents of H₂O₂ for full deprotection. An in-depth study of the mechanism and dynamics of this complex reaction network is conducted in order to better understand, predict and optimize its functioning. From this investigation, the time response as well as detection limit are found to be highly dependent on pH, nature of the buffer, and concentration of the reducing enzyme.

Reduction of the non-specific background in autocatalytic molecular amplifications by a double masking strategy.

Quantitative detection of hydrogen peroxide in rain, air, exhaled breath, and biological fluids by NMR spectroscopy

Hydrogen peroxide (H₂O₂) plays a key role in environmental chemistry, biology, and medicine. H₂O₂ concentrations typically are 6 to 10 orders of magnitude lower than that of water, making its quantitative detection challenging. We demonstrate that optimized NMR spectroscopy allows direct, interference-free, quantitative measurements of H₂O₂ down to submicromolar levels in a wide range of fluids, ranging from exhaled breath and air condensate to rain, blood, urine, and saliva. NMR measurements confirm the previously reported spontaneous generation of H₂O₂ in microdroplets that form when condensing water vapor on a hydrophobic surface, which can interfere with atmospheric H₂O₂ measurements. Its antimicrobial activity and strong seasonal variation speculatively could be linked to the seasonality of respiratory viral diseases.

Targeted contrast agents and activatable probes for photoacoustic imaging of cancer

Photoacoustic (PA) imaging has emerged as a powerful technique for the high resolution visualization of biological processes within deep tissue. Through the development and application of exogenous targeted contrast agents and activatable probes that can respond to a given cancer biomarker, researchers can image molecular events in vivo during cancer progression. This information can provide valuable details that can facilitate cancer diagnosis and therapy monitoring. In this tutorial review, we provide a step-by-step guide to select a cancer biomarker and subsequent approaches to design imaging agents for in vivo use. We envision this information will be a useful summary to those in the field, new members to the community, and graduate students taking advanced imaging coursework. We also highlight notable examples from the recent literature, with emphasis on the molecular designs and their in vivo PA imaging performance. To conclude, we provide our outlook and future perspective in this exciting field.

Bio-Inspired Casein-Derived Antioxidant Peptides Exhibiting a Dual Direct/Indirect Mode of Action

Antioxidant compounds are chemicals of primary importance, especially for their applications in nutrition and healthcare, thanks to their abilities to prevent oxidation processes and to limit and/or rebalance the oxidative stress, well-known for its impact on a wide variety of diseases. While several biomolecules are well-known for their antioxidant properties (e.g., ascorbic acid, carotenoids, phenolic derivatives), bio-sourced antioxidants have drawn considerable attention in the last decades, especially bioactive peptides, mainly obtained by the hydrolysis process. Antioxidant peptide sequences are mainly identified a posteriori, thanks to fastidious and time-consuming approaches and techniques, limiting the discovery of new efficient peptides. In this context and taking inspiration from nature, we report herein on a new series of three bio-inspired antioxidant peptides derived from the milk protein casein. These phosphopeptides, designed to chelate the redox-active iron(III) and forming highly soluble complexes up to pH 9, act both as indirect (i.e., inhibition of the metal redox activity) and direct (i.e., radical scavenging) antioxidants.

NMR characterization of H2O2 hydrogen exchange

Quantification of H2O2 concentration in aqueous solutions is of interest in many fields. It usually is based on indirect methods that rely on oxidation reactions that turn on/off fluorescent probes. Such methods can suffer from reaction incompleteness and interfering chemical species. We describe optimization of NMR detection that enables direct quantification of H2O2 down to the nanomolar range. Taking advantage of fast hydrogen exchange (HX) between H2O2 and water permits the use of very short interscan delays, greatly increasing sensitivity. The specific acid-, base- and water-catalyzed HX rates at 2 °C were measured to be 2.1 × 107, 6.1 × 109, and 1.4 × 10-1 M-1s-1, respectively, which result in a minimum HX rate at pH 6.2. Furthermore, the exchange is accelerated by general acid/base catalysis. MES and phosphate buffers catalyze HX strongest in their unprotonated forms. For imidazole, only the unprotonated form catalyzes HX, which contrasts with acetic acid where only the protonated state catalyzes exchange. Inorganic salts such as sodium chloride and azide have negligible effect on HX. We present optimal conditions for accurate measurement of H2O2 concentrations as low as 40 nM in aqueous samples in a few hours.

A Novel Fluorescence Probe for the Reversible Detection of Bisulfite and Hydrogen Peroxide Pair in Vitro and in Vivo

The detection of changes in the reactive oxygen species (ROS)/reactive sulfur species (RSS) couple is important for studying the cellular redox state. Herein, we developed a 1,8-naphthalimide-based fluorescence probe (NI) for the reversible detection of bisulfite (HSO₃ ^- ) and hydrogen peroxide (H₂ O₂ ) in vitro and in vivo. NI has been designed with a reactive ethylene unit which specifically reacts with HSO₃ ^- by a Michael addition reaction mechanism, resulting in the quenching of yellow fluorescence at 580 nm and the appearing of green fluorescence at 510 nm upon excitation at 500 nm and 430 nm, respectively. The addition product (NI-HSO₃ ) could be specifically oxidized to form the original C=C bond of NI, recovering the fluorescence emission and color. The detection limits of NI for HSO₃ ^- and NI-HSO₃ for H₂ O₂ were calculated to be 2.05 μM and 4.23 μM, respectively. The reversible fluorescence response of NI towards HSO₃ ^- /H₂ O₂ couple can be repeated for at least five times. NI is reliable at a broad pH range (pH 3.0-11.5) and features outstanding selectivity, which enabled its practical applications in biological and food samples. Monitoring the reversible and dynamic inter-conversion between HSO₃ ^- and H₂ O₂ in vitro and in vivo has been verified by fluorescence imaging in live HeLa cells, adult zebrafish and nude mice. Moreover, NI has been successfully applied to detect of HSO₃ ^- levels in food samples.

An update on methods and approaches for interrogating mitochondrial reactive oxygen species production

The chief ROS formed by mitochondria are superoxide (O2·−) and hydrogen peroxide (H₂O₂). Superoxide is converted rapidly to H₂O₂ and therefore the latter is the chief ROS emitted by mitochondria into the cell. Once considered an unavoidable by-product of aerobic respiration, H₂O₂ is now regarded as a central mitokine used in mitochondrial redox signaling. However, it has been postulated that O2·− can also serve as a signal in mammalian cells. Progress in understanding the role of mitochondrial H₂O₂ in signaling is due to significant advances in the development of methods and technologies for its detection. Unfortunately, the development of techniques to selectively measure basal O2·− changes has been met with more significant hurdles due to its short half-life and the lack of specific probes. The development of sensitive techniques for the selective and real time measure of O2·− and H₂O₂ has come on two fronts: development of genetically encoded fluorescent proteins and small molecule reporters. In 2015, I published a detailed comprehensive review on the state of knowledge for mitochondrial ROS production and how it is controlled, which included an in-depth discussion of the up-to-date methods utilized for the detection of both superoxide (O2·−) and H₂O₂. In the article, I presented the challenges associated with utilizing these probes and their significance in advancing our collective understanding of ROS signaling. Since then, many other authors in the field of Redox Biology have published articles on the challenges and developments detecting O2·− and H₂O₂ in various organisms [[1], [2], [3]]. There has been significant advances in this state of knowledge, including the development of novel genetically encoded fluorescent H₂O₂ probes, several O2·− sensors, and the establishment of a toolkit of inhibitors and substrates for the interrogation of mitochondrial H₂O₂ production and the antioxidant defenses utilized to maintain the cellular H₂O₂ steady-state. Here, I provide an update on these methods and their implementation in furthering our understanding of how mitochondria serve as cell ROS stabilizing devices for H₂O₂ signaling.

•

Details on the toolkit for interrogating the 12 sites for mitochondrial ROS production.

•

Approaches to assess mitochondrial ROS clearance.

•

Novel genetically encoded H₂O₂ sensors.

•

Small chemical probes for sensitive detection of O2·−.

Photooxidation-induced fluorescence amplification system for an ultra-sensitive enzyme-linked immunosorbent assay (ELISA)

This report suggests a method of enhancing the sensitivity of chemifluorescence-based ELISA, using photooxidation-induced fluorescence amplification (PIFA). The PIFA utilized autocatalytic photooxidation of the chemifluorescent substrate, 10-acetyl 3,7-dihydroxyphenoxazine (ADHP, Amplex Red) to amplify the fluorescent product resorufin, initially oxidized by horse radish peroxidase (HRP). As the amplification rate is proportional to the initial level of resorufin, the level of antigen labeled by HRP is quantified by analyzing the profile of fluorescence intensity. The normalized profile was interpolated into an autocatalysis model, and the rate of increase at half-maximum time was quantified by the use of an amplification index (AI). The lower limit of detection, for resorufin or HRP, was less than one-tenth that of the plate reader. It requires only slight modification of the fluorescence reader and is fully compatible with conventional or commercial ELISA. When it is applied to a commercial ELISA kit for the detection of amyloid beta, it is verified that the PIFA assay enhanced the detection sensitivity by more than a factor of 10 and was compatible with a conventional 96-well ELISA assay kit. We anticipate this PIFA assay to be used in research for the detection of low levels of proteins and for the early diagnosis of various diseases with rare protein biomarkers, at ultra-low (pg/mL) concentrations.

Detection of Vascular Reactive Oxygen Species in Experimental Atherosclerosis by High-Resolution Near-Infrared Fluorescence Imaging Using VCAM-1-Targeted Liposomes Entrapping a Fluorogenic Redox-Sensitive Probe

Excessive production of reactive oxygen species (ROS) and the ensuing oxidative stress are instrumental in all phases of atherosclerosis. Despite the major achievements in understanding the regulatory pathways and molecular sources of ROS in the vasculature, the specific detection and quantification of ROS in experimental models of disease remain a challenge. We aimed to develop a reliable and straightforward imaging procedure to interrogate the ROS overproduction in the vasculature and in various organs/tissues in atherosclerosis. To this purpose, the cell-impermeant ROS Brite™ 700 (RB700) probe that produces bright near-infrared fluorescence upon ROS oxidation was encapsulated into VCAM-1-targeted, sterically stabilized liposomes (VLp). Cultured human endothelial cells (EC) and macrophages (Mac) were used for in vitro experiments. C57BL6/J and ApoE-/- mice were randomized to receive normal or high-fat, cholesterol-rich diet for 10 or 32 weeks. The mice received a retroorbital injection with fluorescent tagged VLp incorporating RB700 (VLp-RB700). After two hours, the specific signals of the oxidized RB700 and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (NBD-DSPE), inserted into liposome bilayers, were measured ex vivo in the mouse aorta and various organs by high-resolution fluorescent imaging. VLp-RB700 was efficiently taken up by cultured human EC and Mac, as confirmed by fluorescence microscopy and spectrofluorimetry. After systemic administration in atherosclerotic ApoE-/- mice, VLp-RB700 were efficiently concentrated at the sites of aortic lesions, as indicated by the augmented NBD fluorescence. Significant increases in oxidized RB700 signal were detected in the aorta and in the liver and kidney of atherosclerotic ApoE-/- mice. RB700 encapsulation into sterically stabilized VCAM-1-sensitive Lp could be a novel strategy for the qualitative and quantitative detection of ROS in the vasculature and various organs and tissues in animal models of disease. The accurate and precise detection of ROS in experimental models of disease could ease the translation of the results to human pathologies.

Direct and rapid measurement of hydrogen peroxide in human blood using a microfluidic device

The levels of hydrogen peroxide (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2) in human blood is of great relevance as it has emerged as an important signalling molecule in a variety of disease states. Fast and reliable measurement of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2 levels in the blood, however, continues to remain a challenge. Herein we report an automated method employing a microfluidic device for direct and rapid measurement of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2 in human blood based on laser-induced fluorescence measurement. Our study delineates the critical factors that affect measurement accuracy—we found blood cells and soluble proteins significantly alter the native \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2 levels in the time interval between sample withdrawal and detection. We show that separation of blood cells and subsequent dilution of the plasma with a buffer at a ratio of 1:6 inhibits the above effect, leading to reliable measurements. We demonstrate rapid measurement of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2 in plasma in the concentration range of 0–49 µM, offering a limit of detection of 0.05 µM, a sensitivity of 0.60 µM⁻¹, and detection time of 15 min; the device is amenable to the real-time measurement of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2 in the patient’s blood. Using the linear correlation obtained with known quantities of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2, the endogenous \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2 concentration in the blood of healthy individuals is found to be in the range of 0.8–6 µM. The availability of this device at the point of care will have relevance in understanding the role of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}H2O2 in health and disease.

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.

Cross-Talk between NADPH Oxidase and Mitochondria: Role in ROS Signaling and Angiogenesis

Angiogenesis, a new vessel formation from the pre-existing ones, is essential for embryonic development, wound repair and treatment of ischemic heart and limb diseases. However, dysregulated angiogenesis contributes to various pathologies such as diabetic retinopathy, atherosclerosis and cancer. Reactive oxygen species (ROS) derived from NADPH oxidase (NOX) as well as mitochondria play an important role in promoting the angiogenic switch from quiescent endothelial cells (ECs). However, how highly diffusible ROS produced from different sources and location can communicate with each other to regulate angiogenesis remains unclear. To detect a localized ROS signal in distinct subcellular compartments in real time in situ, compartment-specific genetically encoded redox-sensitive fluorescence biosensors have been developed. Recently, the intercellular communication, “cross-talk”, between ROS derived from NOX and mitochondria, termed “ROS-induced ROS release”, has been proposed as a mechanism for ROS amplification at distinct subcellular compartments, which are essential for activation of redox signaling. This “ROS-induced ROS release” may represent a feed-forward mechanism of localized ROS production to maintain sustained signaling, which can be targeted under pathological conditions with oxidative stress or enhanced to promote therapeutic angiogenesis. In this review, we summarize the recent knowledge regarding the role of the cross-talk between NOX and mitochondria organizing the sustained ROS signaling involved in VEGF signaling, neovascularization and tissue repair.

A Highly Selective and Sensitive Chemiluminescent Probe for Real-Time Monitoring of Hydrogen Peroxide in Cells and Animals

Selective and sensitive molecular probes for hydrogen peroxide (H₂ O₂ ), which plays diverse roles in oxidative stress and redox signaling, are urgently needed to investigate the physiological and pathological effects of H₂ O₂ . A lack of reliable tools for in vivo imaging has hampered the development of H₂ O₂ mediated therapeutics. By combining a specific tandem Payne/Dakin reaction with a chemiluminescent scaffold, H₂ O₂ -CL-510 was developed as a highly selective and sensitive probe for detection of H₂ O₂ both in vitro and in vivo. A rapid 430-fold enhancement of chemiluminescence was triggered directly by H₂ O₂ without any laser excitation. Arsenic trioxide induced oxidative damage in leukemia was successfully detected. In particular, cerebral ischemia-reperfusion injury-induced H₂ O₂ fluxes were visualized in rat brains using H₂ O₂ -CL-510, providing a new chemical tool for real-time monitoring of H₂ O₂ dynamics in living animals.

Hydrogen Peroxide Sensors Based on Fluorescence Quenching of the 2-AminobenzimidazoleFluorophore

The fluorescence emission of the parent 2-aminobenzimidazole (ABZ, 1), the mono- and disubstituted derivatives (2, 3), 2-aminonaphthoimidazole (4), and 4-amino dinaphthodiazepine 5 (λ_em = 315-400 nm) is strongly quenched in the presence of aqueous hydrogen peroxide. The quenching process is dual: for diazepine 5, quenching is dynamic at lower H₂O₂ concentrations with linear reduction of the fluorescence lifetime from 4.3 to 2.6 ns. At higher H₂O₂ concentrations, a second species appears in the absorption and emission spectra with fluorescence lifetimes of 1.3 ns, indicating the formation of a new (ground-state) hydrogen-bonded ABZ-H₂O₂ complex (static quenching). Sensors 1 and 2 show also dual quenching that fits with a static 1:1 and 1:2 model with K_1:1 = 8(11) M^-1 and K_1:2 = 21(147) M^-1 for 1(2). The formation of a 1:2 complex (1:(H₂O₂)₂) is also supported by density functional theory (DFT) calculations and spectra simulations.

Metallothionein Protects the Heart Against Myocardial Infarction via the mTORC2/FoxO3a/Bim Pathway

Aims: Cardiac-specific overexpression of metallothionein (MT) has been shown to be beneficial in ischemic heart disease, but the detailed mechanisms through which MT protects against myocardial infarction (MI) remain unknown. This study assessed the involvement of the mTORC2/FoxO3a/Bim pathway in the cardioprotective effects of MT. Results: MI was induced in wild-type (FVB) mice and in cardiac-specific MT-overexpressing transgenic (MT-TG) mice by ligation of the left anterior descending (LAD) coronary artery. Cardiac function was better; infarct size and cardiomyocyte apoptosis were lower in MT-TG mice than in FVB mice after MI. Moreover, MT-TG mice exhibited better phenotypes after LAD ligation than FVB mice treated with Mn(III)tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP; a reactive oxygen species [ROS] scavenger) and cardiac-specific catalase-overexpressing transgenic (CAT-TG) mice, which showed the same ROS levels as MT-TG mice after MI. Activation of mechanistic target of rapamycin complex 2 (mTORC2) was essential for the cardioprotective effects of MT against MI. In addition, MT attenuated the downregulation of phospho-FoxO3a after MI, inhibiting the expression of the apoptosis-associated gene Bim, located downstream of FoxO3a, and reducing the level of apoptosis after MI. To mimic ischemic-injured FVB and MT-TG mice in vitro, H9c2 and MT-overexpressing H9c2 (H9c2MT7) cardiomyocytes were subjected to oxygen and glucose deprivation, with the results being consistent with those obtained in vivo. Innovation and Conclusion: The cardioprotective effects of MT against MI are not entirely dependent upon its ability to eliminate ROS. Rather, MT overexpression mostly protects against MI through the mTORC2-FoxO3a-Bim pathway.

DNA-SWCNT Biosensors Allow Real-Time Monitoring of Therapeutic Responses in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a highly desmoplastic cancer with limited treatment options. There is an urgent need for tools that monitor therapeutic responses in real time. Drugs such as gemcitabine and irinotecan elicit their therapeutic effect in cancer cells by producing hydrogen peroxide (H₂O₂). In this study, specific DNA-wrapped single-walled carbon nanotubes (SWCNT), which precisely monitor H₂O₂, were used to determine the therapeutic response of PDAC cells in vitro and tumors in vivo. Drug therapeutic efficacy was evaluated in vitro by monitoring H₂O₂ differences in situ using reversible alteration of Raman G-bands from the nanotubes. Implantation of the DNA-SWCNT probe inside the PDAC tumor resulted in approximately 50% reduction of Raman G-band intensity when treated with gemcitabine versus the pretreated tumor; the Raman G-band intensity reversed to its pretreatment level upon treatment withdrawal. In summary, using highly specific and sensitive DNA-SWCNT nanosensors, which can determine dynamic alteration of hydrogen peroxide in tumor, can evaluate the effectiveness of chemotherapeutics. SIGNIFICANCE: A novel biosensor is used to detect intratumoral hydrogen peroxide, allowing real-time monitoring of responses to chemotherapeutic drugs.

Optogenetic Monitoring of the Glutathione Redox State in Engineered Human Myocardium

Redox signaling affects all aspects of cardiac function and homeostasis. With the development of genetically encoded fluorescent redox sensors, novel tools for the optogenetic investigation of redox signaling have emerged. Here, we sought to develop a human heart muscle model for in-tissue imaging of redox alterations. For this, we made use of (1) the genetically-encoded Grx1-roGFP2 sensor, which reports changes in cellular glutathione redox status (GSH/GSSG), (2) human embryonic stem cells (HES2), and (3) the engineered heart muscle (EHM) technology. We first generated HES2 lines expressing Grx1-roGFP2 in cytosol or mitochondria compartments by TALEN-guided genomic integration. Grx1-roGFP2 sensor localization and function was verified by fluorescence imaging. Grx1-roGFP2 HES2 were then subjected to directed differentiation to obtain high purity cardiomyocyte populations. Despite being able to report glutathione redox potential from cytosol and mitochondria, we observed dysfunctional sarcomerogenesis in Grx1-roGFP2 expressing cardiomyocytes. Conversely, lentiviral transduction of Grx1-roGFP2 in already differentiated HES2-cardiomyocytes and human foreskin fibroblast was possible, without compromising cell function as determined in EHM from defined Grx1-roGFP2-expressing cardiomyocyte and fibroblast populations. Finally, cell-type specific GSH/GSSG imaging was demonstrated in EHM. Collectively, our observations suggests a crucial role for redox signaling in cardiomyocyte differentiation and provide a solution as to how this apparent limitation can be overcome to enable cell-type specific GSH/GSSG imaging in a human heart muscle context.

The fluorescent protein sensor roGFP2-Orp1 monitors in vivo H2 O2 and thiol redox integration and elucidates intracellular H2 O2 dynamics during elicitor-induced oxidative burst in Arabidopsis

Hydrogen peroxide (H₂ O₂ ) is ubiquitous in cells and at the centre of developmental programmes and environmental responses. Its chemistry in cells makes H₂ O₂ notoriously hard to detect dynamically, specifically and at high resolution. Genetically encoded sensors overcome persistent shortcomings, but pH sensitivity, silencing of expression and a limited concept of sensor behaviour in vivo have hampered any meaningful H₂ O₂ sensing in living plants. We established H₂ O₂ monitoring in the cytosol and the mitochondria of Arabidopsis with the fusion protein roGFP2-Orp1 using confocal microscopy and multiwell fluorimetry. We confirmed sensor oxidation by H₂ O₂ , show insensitivity to physiological pH changes, and demonstrated that glutathione dominates sensor reduction in vivo. We showed the responsiveness of the sensor to exogenous H₂ O₂ , pharmacologically-induced H₂ O₂ release, and genetic interference with the antioxidant machinery in living Arabidopsis tissues. Monitoring intracellular H₂ O₂ dynamics in response to elicitor exposure reveals the late and prolonged impact of the oxidative burst in the cytosol that is modified in redox mutants. We provided a well defined toolkit for H₂ O₂ monitoring in planta and showed that intracellular H₂ O₂ measurements only carry meaning in the context of the endogenous thiol redox systems. This opens new possibilities to dissect plant H₂ O₂ dynamics and redox regulation, including intracellular NADPH oxidase-mediated ROS signalling.

Measuring Calcium and ROS by Genetically Encoded Protein Sensors and Fluorescent Dyes

Oxidative modifications of cellular building blocks such as proteins, lipids, and DNA have a major impact on cell behavior, fate, and clinical outcome. Reactive oxygen species (ROS) are important factors that influence these redox processes. Calcium ion (Ca²⁺) dynamics and signals are also essential regulators of key cellular processes. Therefore, the combined and precise monitoring of ROS and Ca²⁺ in single cells, with a high spatial and temporal resolution and in physiological environments, is essential to better understand their functional impact. Here, we describe protocols to detect one of the most prominent ROS (hydrogen peroxide, H₂O₂) using genetically encoded protein sensors and fluorescent dyes. We also provide guidelines on how to simultaneously detect Ca²⁺ and H₂O₂ and how to examine the influence of Ca²⁺ signals on cellular ROS production and vice versa.