A rapid and sensitive method for hydroxyl radical detection on a microfluidic chip using an N-doped porous carbon nanofiber modified pencil graphite electrode

Dimethyl Sulfoxide: An Ideal Electrochemical Probe for Hydroxyl Radical Detection

In situ and real-time determination of hydroxyl radicals (•OH) in physiological and pathological processes is a great challenge due to their ultrashort lifetime. Herein, an electrochemical method was developed by using dimethyl sulfoxide (DMSO) as a trapping probe for rapid determination of •OH in aqueous solution. When DMSO reacted with •OH, an intermediate product methane sulfinic acid (MSIA) was formed, which can be electrochemically oxidized to methanesulfonic acid (MSA) on the glassy carbon electrode (GCE), resulting in a distinct voltammetric signal that is directly proportional to the concentration of •OH. Other commonly encountered reactive oxygen species (ROS), including hypochlorite anions (ClO-), superoxide anions (O2•-), sulfate radicals (SO4•-), and singlet oxygen (1O2), have showed no interference for •OH determination. Thus, an electrochemical method was developed for the determination of •OH, which exhibits a wide linear range (0.4-5120 μM) and a low limit detection of 0.13 μM (S/N = 3) and was successfully applied for the quantification of •OH in aqueous extracts of cigarette tar (ACT). Alternatively, the same reaction mechanism is also applicable for the determination of DMSO, in which a linear range of 40-320 μM and a detection limit 13.3 μM (S/N = 3) was achieved. The method was used for the evaluation of DMSO content in cell cryopreservation medium. This work demonstrated that DMSO can serve as an electrochemical probe and has valuable application potential in radical study, biological research, and environmental monitoring.

Ratiometric electrochemical determination of hydroxyl radical based on graphite paper modified with metal-organic frameworks and impregnated with salicylic acid

Hydroxyl radical (•OH) detection is pivotal in medicine, biochemistry and environmental chemistry. Yet, electrochemical method-specific detection is challenging because of hydroxyl radicals' high reactivity and short half-life. In this study, we aimed to modify the electrode surface with a specific recognition probe for •OH. To achieve this, we conducted a one-step hydrothermal process to fabricate a CoZnMOF bimetallic organic framework directly onto conductive graphite paper (Gp). Subsequently, we introduced salicylic acid (SA) and methylene blue (MB), which easily penetrated the pores of CoZnMOF. By selectively capturing •OH by SA and leveraging the electrochemical signal generated by the reaction product, we successfully developed an electrochemical sensor Gp/CoZnMOF/SA + MB. The prepared sensor exhibited a good linear relationship with •OH concentrations ranging from 1.25 to 1200 nM, with a detection limit of 0.2 nM. Additionally, the sensor demonstrated excellent reproducibility and accuracy due to the incorporation of an internal reference. It exhibited remarkable selectivity for •OH detection, unaffected by other electrochemically active substances. The establishment of this sensor provides a way to construct MOF-modified sensors for the selective detection of other reactive oxygen species (ROS), offering a valuable experimental basis for ROS-related disease research and environmental safety investigations.

Reduced Glutathione-Modified Electrode for the Detection of Hydroxyl Free Radicals

Hydroxyl radicals (•OH) are known as essential chemicals for cells to maintain their normal functions and defensive responses. However, a high concentration of •OH may cause oxidative stress-related diseases, such as cancer, inflammation, and cardiovascular disorders. Therefore, •OH can be used as a biomarker to detect the onset of these disorders at an early stage. Reduced glutathione (GSH), a well-known tripeptide for its antioxidant capacity against reactive oxygen species (ROS), was immobilized on a screen-printed carbon electrode (SPCE) to develop a real-time detection sensor with a high selectivity towards •OH. The signals produced by the interaction of the GSH-modified sensor and •OH were characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV curve of the GSH-modified sensor in the Fenton reagent exhibited a pair of well-defined peaks, demonstrating the redox reaction of the electrochemical sensor and •OH. The sensor showed a linear relationship between the redox response and the concentration of •OH with a limit of detection (LOD) of 49 µM. Furthermore, using EIS studies, the proposed sensor demonstrated the capability of differentiating •OH from hydrogen peroxide (H2O2), a similar oxidizing chemical. After being immersed in the Fenton solution for 1 hr, redox peaks in the CV curve of the GSH-modified electrode disappeared, revealing that the immobilized GSH on the electrode was oxidized and turned to glutathione disulfide (GSSG). However, it was demonstrated that the oxidized GSH surface could be reversed back to the reduced state by reacting with a solution of glutathione reductase (GR) and nicotinamide adenine dinucleotide phosphate (NADPH), and possibly reused for •OH detection.

Supersensitive CeOx-based nanocomposite sensor for the electrochemical detection of hydroxyl free radicals

It is well known that an excess of hydroxyl radicals (˙OH) in the human body is responsible for oxidative stress-related diseases. An understanding of the relationship between the concentration of ˙OH and those diseases could contribute to better diagnosis and prevention. Here we present a supersensitive nanosensor integrated with an electrochemical method to measure the concentration of ˙OH in vitro. The electrochemical sensor consists of a composite comprised of ultrasmall cerium oxide nanoclusters (<2 nm) grafted to a highly conductive carbon deposited on a screen-printed carbon electrode (SPCE). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to analyze the interaction between cerium oxide nanoclusters and ˙OH. The CV results demonstrated that this electrochemical sensor had the capacity of detecting ˙OH with a high degree of accuracy and selectivity, achieving a consistent performance. Additionally, EIS results confirmed that our electrochemical sensor was able to differentiate ˙OH from hydrogen peroxide (H2O2), which is another common reactive oxygen species (ROS) found in the human body. The limit of detection (LOD) observed with this electrochemical sensor was of 0.6 μM. Furthermore, this nanosized cerium oxide-based electrochemical sensor successfully detected in vitro the presence of ˙OH in preosteoblast cells from newborn mouse bone tissue. The supersensitive electrochemical sensor is expected to be beneficially used in multiple applications, including medical diagnosis, fuel-cell technology, and food and cosmetic industries.

Detection Technologies for Reactive Oxygen Species: Fluorescence and Electrochemical Methods and Their Applications

Reactive oxygen species (ROS) have been found in plants, mammals, and natural environmental processes. The presence of ROS in mammals has been linked to the development of severe diseases, such as diabetes, cancer, tumors, and several neurodegenerative conditions. The most common ROS involved in human health are superoxide (O2•-), hydrogen peroxide (H2O2), and hydroxyl radicals (•OH). Organic and inorganic molecules have been integrated with various methods to detect and monitor ROS for understanding the effect of their presence and concentration on diseases caused by oxidative stress. Among several techniques, fluorescence and electrochemical methods have been recently developed and employed for the detection of ROS. This literature review intends to critically discuss the development of these techniques to date, as well as their application for in vitro and in vivo ROS detection regarding free-radical-related diseases. Moreover, important insights into and further steps for using fluorescence and electrochemical methods in the detection of ROS are presented.

Detection of Hydroxyl Radicals Using Cerium Oxide/Graphene Oxide Composite on Prussian Blue

A composite sensor consisting of two separate inorganic layers of Prussian blue (PB) and a composite of cerium oxide nanoparticles (CeNPs) and graphene oxide (GO), is tested with •OH radicals. The signals from the interaction between the composite layers and •OH radicals are characterized using cyclic voltammetry (CV). The degradation of PB in the presence of H₂O₂ and •OH radicals is observed and its impact on the sensor efficiency is investigated. The results show that the composite sensor differentiates between the solutions with and without •OH radicals by the increase of electrochemical redox current in the presence of •OH radicals. The redox response shows a linear relation with the concentration of •OH radicals where the limit of detection, LOD, is found at 60 µM (100 µM without the PB layer). When additional composite layers are applied on the composite sensor to prevent the degradation of PB layer, the PB layer is still observed to be degraded. Furthermore, the sensor conductivity is found to decrease with the additional layers of composite. Although the CeNP/GO/PB composite sensor demonstrates high sensitivity with •OH radicals at low concentrations, it can only be used once due to the degradation of PB.

A novel electrochemical sensor for determination of hydroxyl radicals in living cells by coupling nanoporous gold layer with self-assembled 6-(Ferrocenyl) hexanethiol

The detection of hydroxyl radicals (•OH) in live cells is significant to study its physiological and pathological roles, while it is full of challenge due to the extremely low concentration and short lifetime of •OH. Herein, we have developed a novel electrochemical sensor based on 6-(Ferrocenyl) hexanethiol (6-FcHT) self-assembled nanoporous gold layer (NPGL) modified GE (6-FcHT/NPGL/GE), which can detect the release of •OH from living cells with high sensitivity and selectivity. The superior sensitivity can stem from the unique porous architecture of NPGL, which enlarged electrode surface area and expedited electron transportation during electrochemical reactions. Additionally, NPGL provides more active binding sites for the assembly of capture agent (6-FcHT) of •OH, thus ensuring high selectivity. For comparison, 6-FcHT/GE was applied to detect •OH, and the obtained sensitivity was 0.0305 mA nM^-1 and detection limit was 0.133 nM in the linear range of 0.4 nM-70 nM. After modification of NPGL, the sensitivity of 6-FcHT/NPGL/GE to the •OH response was increased to 0.1364 mA nM^-1, detection limit was reduced to 0.316 pM and the linear range was extended from 1 pM to 100 nM. It is worth mentioning that a plenty of extra merits has also been validated like reproducibility, repeatability and stability, enabling to direct electrochemical detection of •OH in HepG2 cells.

Pencil Lead as a Material for Microfluidic 3D-Electrode Assemblies

We present an electrochemical, microfluidic system with a working electrode based on an ordered 3D array of pencil leads. The electrode array was integrated into a plexiglass/PDMS channel. We tested the setup using a simple redox probe and compared the results with computer simulations. As a proof of concept application of the device we showed that the setup can be used for determination of dopamine concentration in physiological pH and ultrasensitive, although only qualitative, detection of p-nitrophenol with a limit of detection below 1 nmol L⁻¹. The observed limit of detection for p-nitrophenol is not only much lower than achieved with similar methods but also sufficient for evaluation of exposure to pesticides such as methyl parathion through urinalysis. This low cost setup can be fabricated without the need for clean room facilities and in the future, due to the ordered structure of the electrode could be used to better understand the process of electroanalysis and electrode functionalization. To the best of our knowledge it is the first application of pencil leads as 3D electrochemical sensor in a microfluidic channel.

Pencil Graphite Electrodes: A Versatile Tool in Electroanalysis

Due to their electrochemical and economical characteristics, pencil graphite electrodes (PGEs) gained in recent years a large applicability to the analysis of various types of inorganic and organic compounds from very different matrices. The electrode material of this type of working electrodes is constituted by the well-known and easy commercially available graphite pencil leads. Thus, PGEs are cheap and user-friendly and can be employed as disposable electrodes avoiding the time-consuming step of solid electrodes surface cleaning between measurements. When compared to other working electrodes PGEs present lower background currents, higher sensitivity, good reproducibility, and an adjustable electroactive surface area, permitting the analysis of low concentrations and small sample volumes without any deposition/preconcentration step. Therefore, this paper presents a detailed overview of the PGEs characteristics, designs and applications of bare, and electrochemically pretreated and chemically modified PGEs along with the corresponding performance characteristics like linear range and detection limit. Techniques used for bare or modified PGEs surface characterization are also reviewed.

Dual-emissive fluorescence measurements of hydroxyl radicals using a coumarin-activated silica nanohybrid probe

This work reports a novel dual-emissive fluorescent probe based on dye hybrid silica nanoparticles for ratiometric measurement of the hydroxyl radical (˙OH). In the probe sensing system, the blue emission of coumarin dye (coumarin-3-carboxylic acid, CCA) immobilized on the nanoparticle surface is selectively enhanced by ˙OH due to the formation of a coumarin hydroxylation product with strong fluorescence, whereas the emission of red fluorescent dye encapsulated in the silica nanoparticle is insensitive to ˙OH as a self-referencing signal, and so the probe provides a good quantitative analysis based on ratiometric fluorescence measurement with a detection limit of 1.65 μM. Moreover, the probe also shows high selectivity for ˙OH determination against metal ions, other reactive oxygen species and biological species. More importantly, it exhibits low cytotoxicity and high biocompatibility in living cells, and has been successfully used for cellular imaging of ˙OH, showing its promising application for monitoring of intracellular ˙OH signaling events.

A microfluidic cigarette smoke collecting platform for simultaneous sample extraction and multiplex analysis

In this work, we report a novel microfluidic gas collecting platform aiming at simultaneous sample extraction and multiplex mass spectrometry (MS) analysis. An alveolar-mimicking elastic polydimethylsiloxane (PDMS) structures was designed to move dynamically driven by external pressure. The movement was well tuned both by its amplitude and rhythm following the natural process of human respiration. By integrating the alveolar units into arrays and assembling them to gas channels, a cyclic contraction/expansion system for gas inhale and exhale was successfully constructed. Upon equipping this system with a droplet array on the alveolar array surface, we were able to get information of inhaled smoke in a new strategy. Here, with cigarette smoke as an example, analysis of accumulation for target molecules during passive smoking is taken. Relationships between the breathing times, distances away from smokers and inhaled content of nicotine are clarified. Further, by applying different types of extraction solvent droplets on different locations of the droplet array, simultaneous extraction of nicotine, formaldehyde and caproic acid in sidestream smoke (SS) are realized. Since the extract droplets are spatially separated, they can be directly analyzed by MS which is fast and can rid us of all complex sample separation and purification steps. Combining all these merits, this small, cheap and portable platform might find wide application in inhaled air pollutant analysis both in and outdoors.