Electrochemical determination of superoxide and nitric oxide generated from biological samples

Development of a nanozyme-based electrochemical catalyst for real-time biomarker sensing of superoxide and nitric oxide anions released from living cells and exogenous donors

Developing quantitative biosensors of superoxide (O2•-) and nitric oxide (NO) anion is crucial for pathological research. As of today, the main challenge for electrochemical detection is to develop high-selectivity nano-mimetic materials to replace natural enzymes. In this study, the dendritic-like morphological structure of silver organic framework (Ag-MOF) was successfully synthesized via a solvothermal strategy. Owing to the introduction of polymeric composites results in improved electrical conductivity and catalytic activity, which promotes mass transfer and leads to faster electron efficiency. For monitoring the electrochemical signals of O2•- and NO, the Ag-MOF electrode substrate was produced by drop-coating, and composites were designed by cyclic voltammetric potential cycles. The designed electrode substrates demonstrate high sensitivity, wide linear concentrations of 1 nM-1000 μM and 1 nM-850 μM, and low detection limits of 0.27 nM and 0.34 nM (S/N = 3) against O2•- and NO. Aside from that, the sensor successfully monitored the cellular release of O2•-, and NO from HepG2 and RAW 264.7 living cells and has the potential to monitor exogenous NO release from donors of Diethylamine (DEA)-NONOate and sodium nitroprusside (SNP). Additionally, the developed system was applied to the analysis of O2•- and NO in real biological fluid samples, and the results were good satisfactory (94.10-99.57 ± 1.23%). The designed system provides a novel approach to obtaining a good electrochemical biosensor platform that is highly selective, stable, and flexible. Finally, the proposed method provides a quantitative way to follow the dynamic changes in O2•- and NO in biological systems.

Environmental implications of superoxide radicals: From natural processes to engineering applications

The roles of superoxide radical (O2•-) in the domains of physiological, physical, and material chemistry are becoming increasingly recognized. Although extensive efforts have been directed to understand O2•- functions in diverse aquatic systems, there is a lack of systematic and in-depth review for its kinetics and mechanisms in various environmental scenarios. This review aims to bridge this gap through discussion of O2•- generation pathways under both natural and controlled conditions. The merits and limitations of the generation and detection methods under various conditions are compared, with emphasis on different approaches for the determination of O2•--triggered reaction kinetics. We summarize the reaction rate constants of O2•- with organic contaminants covering a wide diversity of structures and reactivity. The comparison indicates that O2•- exhibits weak reactivity with most contaminants and lacks selectivity towards compounds with different functional groups, except with quinones which exhibit higher reactivity compared to non-quinones. Further, the reaction mechanisms, namely single electron transfer, nucleophilic substitution, hydrogen atom abstraction, and radical-adduct formation, are critically evaluated. Various environmental implications of O2•- are highlighted including maintenance of biogeochemical iron cycle, synthesis of nanoparticles for antibacterial purposes, desorption of contaminants from heterogeneous interfaces, and synergetic degradation of contaminants.

In vitro detection of superoxide anions released from cancer cells based on potassium-doped carbon nanotubes-ionic liquid composite gels

A newly developed electrochemical biosensor for the determination of superoxide anions (O(2)˙(-)) released from cancer cells using potassium-doped multi-walled carbon nanotubes (KMWNTs)-1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF(6)) ionic liquid composite gels is demonstrated. The KMWNTs-[BMIM]PF(6) can electrocatalyze oxygen reduction to generate a strong current signal in neutral solution. Compared with KMWNTs without [BMIM]PF(6) or MWNTs-[BMIM]PF(6) composites, the KMWNTs-[BMIM]PF(6) can enhance the oxygen reduction peak current by 6.2-fold and 2.8-fold, which greatly increases the detection sensitivity of oxygen. Then, O(2)˙(-) biosensors are fabricated by mixing superoxide dismutase (SOD) in the KMWNTs-[BMIM]PF(6) gels via monitoring oxygen produced by an enzymic reaction between SOD/O(2)˙(-) without the help of electron mediators. The resulting biosensors show a linear range from 0.04 to 38 μM with a high sensitivity of 98.2 μA mM(-1), and a lower detection limit of 0.024 μM. The common interferents such as hydrogen peroxide (H(2)O(2)), ascorbic acid (AA), uric acid (UA), and metabolites of neurotransmitters, do not interfere with the detection of O(2)˙(-). The proposed biosensor is tested to determine O(2)˙(-) in vitro and from liver cancer and leukemia cells and shows good application potential in biological electrochemistry.

Determination of the antioxidants' ability to scavenge free radicals using biosensors

Free radicals are highly reactive molecules generated during cellular metabolism. However, their overproduction results in oxidative stress, a deleterious process that can damage cell structures, including lipids and membranes, proteins and DNA. Antioxidants respond to this problem, scavenging free radicals. This chapter critically reviews the electrochemical biosensors developed for the evaluation of the antioxidant capacity of specific compounds. Due to the ability of these devices to perform simple, fast and reliable analysis, they are promising biotools for the assessment ofantioxidant properties.

Redox reactions of reactive oxygen species in aqueous solutions as the probe for scanning electrochemical microscopy of single live T24 cells

The redox reactions of two main components of reactive oxygen species (ROS), superoxide and hydrogen peroxide, along with oxygen in aqueous solutions were investigated using a conventional electrochemical technique, differential pulse voltammetry (DPV). Superoxide undergoes oxidation at a Pt working electrode biased at 0.055 V versus Ag/AgCl, while hydrogen peroxide can be oxidized and reduced at 0.817 and –0.745 V, respectively. Oxygen in the solutions is reduced at the electrode with an applied potential of –0.455 V. Based on these results, hydrogen peroxide and superoxide released from live cells can be successfully monitored, identified, and mapped using scanning electrochemical microscopy (SECM) at different potentials. Single human bladder (T24) cells were imaged using a 5 μm diameter SECM probe biased at –0.400, –0.600, and –0.800 V. Oxygen reduction that seems an interference can be discriminated from that of hydrogen peroxide by means of SECM.

Comparison study of live cells by atomic force microscopy, confocal microscopy, and scanning electrochemical microscopy

In this report, three kinds of scanning probe microscopy techniques, atomic force microscopy (AFM), confocal microscopy (CM), and scanning electrochemical microscopy (SECM), were used to study live cells in the physiological environment. Two model cell lines, CV-1 and COS-7, were studied. Time-lapse images were obtained with both contact and tapping mode AFM techniques. Cells were more easily scratched or moved by contact mode AFM than by tapping mode AFM. Detailed surface structures such as filamentous structures on the cell membrane can be obtained and easily discerned with tapping mode AFM. The toxicity of ferrocenemethanol (Fc) on live cells was studied by CM in reflection mode by recording the time-lapse images of controlled live cells and live cells with different Fc concentrations. No significant change in the morphology of cells was caused by Fc. Cells were imaged by SECM with Fc as the mediator at a biased potential of 0.35 V (vs. Ag/AgCl with a saturated KCl solution). Cells did not change visibly within 1 h, which indicated that SECM was a noninvasive technique and thus has a unique advantage for the study of soft cells, since the electrode scanned above the cells instead of in contact with them. Reactive oxygen species (ROS) generated by the cells were detected and images based on these chemical species were obtained. It is demonstrated that SECM can provide not only the topographical images but also the images related to the chemical or biochemical species released by the live cells.Key words: live cells, atomic force microscopy, confocal microscopy, scanning electrochemical microscopy.

An electrochemical sensor array system for the direct, simultaneous in vitro monitoring of nitric oxide and superoxide production by cultured cells

A new approach for an amperometric array sensor platform employing arrays of sensors in a 24-well cell culture plate format has been developed for simultaneous in vitro determination of nitric oxide (NO) and superoxide free radicals (O(2)(-)) produced by stimulated cells. The work reported focuses on the direct, real-time monitoring of extracellular production of these two analytes, as well as the effects of their interaction. The sensor platform was manufactured by a combination of sputtering gold electrodes and screen-printing carbon electrodes. The O(2)(-) sensor uses covalent immobilization of cytochrome c via a binder, DTSSP (3,3'-dithio-bis(sulphosuccinimidylpropionate) onto the surface of the Au electrodes, whereas the NO sensor system involves an NiTSPc (nickel tetrasulfonated phthalocyanine) film electrodeposited onto the surface of the carbon electrodes and subsequently covered with an external layer of Nafion. For in vitro demonstration of the platforms as a potential drug-screening system, A172 glioblastoma cells were cultured and transferred into the 24-well arrays. Simultaneous and direct monitoring of NO and O(2)(-) production as a response to chemicals of biomedical relevance was carried out. The results obtained demonstrated that it would be possible to envisage a drug screening platform for compounds designed to be inhibitors of nitric oxide synthase or to have an inhibitory effect on superoxide free radical production. By suitable modification of the electrodes employed it would also be possible to extend the platform to measure alternative species.

A carbon fiber microelectrode-based third-generation biosensor for superoxide anion

Implantable and miniature carbon fiber microelectrode (CFME)-based third-generation biosensor for superoxide anion (O(2)(-)) was fabricated for the first time. The CFME-based biosensor was constructed by electro-deposition of Au nanoparticles on the CFMEs and then modification of the Au nanoparticles by cysteine followed by immobilization of superoxide dismutase (SOD) on the electrodes. The direct electrochemistry of the SOD immobilized on the CFME-based electrodes was efficiently realized by electron transfer promoter - cysteine molecules confined on the Au nanoparticles deposited on the CFMEs. The CFME-based biosensors were demonstrated to possess striking analytical properties for O(2)(-) determination, such as optional operation potentials, high selectivity and sensitivity as well as good stability. Along with the implantable capacity inherent in the CFMEs, these striking analytical properties of the CFME-based biosensors substantially make them potential for in vivo determination of O(2)(-).

Electrochemical nitric oxide sensor preparation: a comparison of two electrochemical methods of electrode surface modification

Platinum electrodes modified with Mn(II) 5-(N-(8-pyrrole-yl-3,6-dioxa-1-aminooctane)phenylamide-10,15,20-trimethoxyphenylporphyrin (Mn(II)triOMeTCPPyP) using multi-sweep cyclic voltammetry and differential pulse amperometry were evaluated as electrocatalytic surfaces for the oxidation of nitric oxide. The electrodes modified using the pulse amperometric approach were more sensitive towards the detection of nitric oxide. The increased sensitivity led to the attainment of a wider linear dynamic range for the quantification of nitric oxide.

Third-generation superoxide anion sensor based on superoxide dismutase directly immobilized by sol–gel thin film on gold electrode

A third-generation biosensor for superoxide anion (O(2)*-) was developed based on superoxide dismutase (SOD) immobilized by thin silica-PVA sol-gel film on gold electrode surface. A rapid and direct electron transfer of SOD in the thin sol-gel film at the gold electrode was realized without any mediators or promoters. The characterization of the SOD electrodes showed a quasi-reversible electrochemical redox behavior with a formal potential of 80 + 5 mV (versus SCE) in 50 mmol l(-1) phosphate buffer solution (PBS), pH 7.0. The heterogeneous electron transfer rate constant was evaluated to be about 2.1s(-1). The anodic and cathodic transfer coefficients are 0.6 and 0.4, respectively. Based on biomolecular recognition for specific reactivity of SOD toward O(2)*- the SOD electrode was applied to a sensitive and selective measurement of O(2)*- with the low operation potential (-0.15 V versus SCE) in phosphate buffer solution, pH 7.0. The amperometric response was proportional to O(2)*- concentration in the range of 0.2-1.6 micromol l(-1) and the detection limit was 0.1 micromol l(-1) at a signal-to-noise ration of 3. The preparation of SOD electrode is easy and simple. The uniform porous structure of the silica-PVA sol-gel matrix results in a fast response rate of immobilized SOD and is very efficient for stabilizing the enzyme activity.

Superoxide dismutase-based third-generation biosensor for superoxide anion

A third-generation biosensor for superoxide anion (O2-) was developed by immobilizing superoxide dismutase (SOD) on a self-assembled monolayer of cysteine on gold electrode; i.e., a SOD/cysteine-modified gold electrode (SOD/Cys/Au) was fabricated. A rapid and direct electron transfer of SOD was realized at the gold electrode by using the cysteine molecule as an electron-transfer promoter. The promoted direct electron transfer of SOD and biomolecular recognition by the exploitation of specific and significant enzyme-substrate reactivity of SOD toward O2- combined with the low operating potential enabled a sensitive measurement of O2-. At SOD/Cys/Au, O2- could be specifically oxidized and reduced to O2 and hydrogen peroxide, respectively, through the inherent catalytic reaction of SOD. This allowed us to measure O2- by polarizing the electrode both anodically and cathodically. We could successfully measure O2- by suitably polarizing the electrode, typically at 300 and -200 mV versus Ag/AgCl without the virtual interference from physiological levels of H2O2, ascorbic acid, uric acid, and metabolites of neurotransmitters. The response mechanism of SOD/Cys/Au to O2- and its sensor characteristics are also presented and discussed.

Superoxide release from interleukin-1B-stimulated human vascular cells: in situ electrochemical measurement

Release of superoxide anion by cultured vascular cells was investigated with the use of selective microelectrodes. Local concentration of superoxide anion (O2*-) was followed by differential pulse amperometry on a carbon microfiber at 0.1 V/SCE. The oxidation current allows O2*- detection in the 10(-8) M concentration range without interference of the other major oxygen species. Interleukin-1beta-stimulated O2*- release that progressively increased to reach local concentrations at the cell membrane level of 76 +/- 11 nm 40-60 min after stimulation in human cord vein endothelial cells, and 131 +/- 18 nm 1-2 h after stimulation in internal mammary artery smooth muscle cells. In the two types of cells, the O2*- oxidation signal was suppressed in the presence of superoxide dismutase. Spontaneous O2*-release from unstimulated cells was undetectable. These results demonstrate that selective microelectrodes allow direct and real-time monitoring of local O2*- released from vascular endothelial as well as from smooth muscle cells submitted to an inflammatory stimulus.