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Enhancing the Selectivity of Amperometric Nitric Oxide Sensor over Ammonia and Nitrite by Modifying Gas-Permeable Membrane with Teflon AF ®

Authors:
Wansik Cha at Korea Atomic Energy Research Institute (KAERI)
Wansik Cha
Mark E Meyerhoff
Mark E Meyerhoff
Mark E Meyerhoff
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Abstract and Figures

A planar amperometric nitric oxide (NO(g)) sensor based on a platinized platinum (pPt) working electrode (as anode) is one of the most sensitive NO detection methods reported to date with sub-nmol L-1 detection limits. The use of an outer gas permeable membrane (porous polytetrafluoroethylene (PTFE) membrane) in this sensor design has been shown to impart superior NO selectivity over common interfering species present in biological samples, such as nitrite and ascorbate. Recently, however, it has been recognized that ammonia (NH3(g)) present in biological samples, e.g., cell culture medium or blood, can interfere with NO detection using this sensor configuration owing to the concomitant oxidation capability of ammonia at the surface of the inner platinized platinum electrode. Herein, the selectivity of such an amperometric NO sensor is investigated in detail over both ammonia and nitrite and these results are compared to experimental data obtained with other types of amperometric NO sensors (including commercial WPI, Inc. device). Further, it is demonstrated that the NO selectivity of the planar-type NO sensor can be enhanced significantly by treating the porous PTFE gas permeable outer membrane with a Teflon AF® solution. By filling the pores of the outer membrane with Teflon AF®, the flux of ammonia and nitrite to the internal working electrode is greatly reduced, while maintaining good permeability toward NO(g).
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Content uploaded by Wansik Cha
Author content
All content in this area was uploaded by Wansik Cha
Content may be subject to copyright.
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... Teflon AFs, highly permeable and poor solvating materials, when coated on electrodes, have been found to exhibit superior performance to improve the selectivity of electrochemical sensors for non-polar neutral gases [66,67]. Meyerhoff and coworkers have been devoted to the real time measurement of NO in biological samples. ...
... Meyerhoff and coworkers have been devoted to the real time measurement of NO in biological samples. They discovered that integrating an outer gas permeable membrane (porous polytetrafluoroethylene, PTFE) with a platinized platinum electrode can decrease significantly the ionic interferences (e.g., nitrite and ascorbate) [66]. However, NH 3 might be a potential interfering species because around 1% of the total ammonia (NH 3 + NH 4 + ) is in the neutral form at physiological pH. ...
... However, NH 3 might be a potential interfering species because around 1% of the total ammonia (NH 3 + NH 4 + ) is in the neutral form at physiological pH. The oxidation of NH 3 , yielding N 2 , N 2 O, or NO, is potential dependent, and will interfere with the signal from the NO oxidation [66]. By implanting Teflon AF in the pores of the porous PTFE, the flux of NH 3 to the internal working electrode decreased significantly, while the permeability of NO remains high [66]. ...
Teflon AF materials
Article
Full-text available
  • Sep 2011
  • TOP CURR CHEM
The unique combination of chemical, thermal, and mechanical stability, high fractional free volume, low refractive index, low surface energy, and wide optical transparency has led to growing interest in Teflon Amorphous Fluoropolymers (AFs) for a wide spectrum of applications ranging from chemical separations and sensors to bioassay platforms. New opportunities arise from the incorporation of nanoscale materials in Teflon AFs. In this chapter, we highlight fractional free volume - the most important property of Teflon AFs - with the aim of clarifying the unique transport behavior through Teflon AF membranes. We then review state-of-the-art developments based on Teflon AF platforms by focusing on the chemistry behind the applications.
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... Furthermore, the PTFE membrane resulted in complete selectivity over nitrite up to 10 mM NO 2 − concentrations (132). Unfortunately, ammonia (NH 3 ) was found to interfere with sensor response (134). To overcome this interference, the PTFE-based sensor was modified by applying a Teflon AF ® (i.e., 2,2bis(trifluoroethylene)-4,5-difluoro-1,3-dioxole) coating over the PTFE membrane. ...
... To overcome this interference, the PTFE-based sensor was modified by applying a Teflon AF ® (i.e., 2,2bis(trifluoroethylene)-4,5-difluoro-1,3-dioxole) coating over the PTFE membrane. The Teflon AF ® layer improved the sensor selectivity for NO over ammonia by ~1000-fold (134). ...
... To simplify the sensor fabrication protocol, Shin et al. (135) replaced the aminoalkoxysilane precursor with fluoroalkoxysilane, a "PTFE-like" polymer precursor. In contrast to the previously-developed PTFE-based sensors (132,134), the fluorinated xerogels provided a more straightforward and reproducible method for coating the working electrode. Indeed, a benefit of sol-gel chemistry is the ability to dipcoat thin layers of the xerogel onto electrodes. ...
Analytical Chemistry of Nitric Oxide
Article
  • Jul 2009
  • Annu Rev Anal Chem
Nitric oxide (NO) is the focus of intense research primarily because of its wide-ranging biological and physiological actions. To understand its origin, activity, and regulation, accurate and precise measurement techniques are needed. Unfortunately, analytical assays for monitoring NO are challenged by NO's unique chemical and physical properties, including its reactivity, rapid diffusion, and short half-life. Moreover, NO concentrations may span the picomolar-to-micromolar range in physiological milieus, requiring techniques with wide dynamic response ranges. Despite such challenges, many analytical techniques have emerged for the detection of NO. Herein, we review the most common spectroscopic and electrochemical methods, with a focus on the underlying mechanism of each technique and on approaches that have been coupled with modern analytical measurement tools to create novel NO sensors.
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... According to researchers' results, pH zpc of this carbon catalyst is 7.7 [26]. It should be mentioned that the acid and base constant (pK a ) of ammonium is 9.25 [27]. In other words, in higher pHs ammonium will change to ammonium anion (ammonia), and the best pH for catalyst adsorption is in the range of pH zpc to pK a , other words in the range of 7.7-9.25 which averages to the pH of 8. ...
... With due attention to this note that the ammonium concentration related to the pH value (Eqs. (2) and (3)) and properties of surface of adsorbent, the optimum pH for adsorption of ammonium by zeolite is in the range of pH zpc to pK a , or in the range of 5.3-9.25 [27], in other word, averaging in pH = 8. Since, ammonium is neutral or positive zeolite, has negative potential, so they attract each other extremely. ...
A new Method for the Removal of Ammonium from Drinking Water Using Hybrid Method of Modified Zeolites / Catalytic Ozonation
Article
Full-text available
  • Jul 2021
Introduction: Ammonia in form of ammonium ions is toxic and could decrease the dissolved oxygen in water and endanger the aquatic life. The aim of this study is the removal of ammonium using oxidation and adsorption by catalytic ozonation and clinoptilolite zeolite, respectively. Methods: The research method is Experimental. First, optimal pH of ammonium adsorption on carbon catalyst (5 g/L), Garmsar and Firoozkooh zeolites and oxidation were determine. Then, in catalytic ozonation process, the effect of other variables on ammonium removal efficiency such as the concentration of carbonic catalyst (0.5- 50 g/L) and the reaction time were investigated. Then the effect of retention time and adsorbent concentration on adsorption of the remaining ammonium and nitrate produced by the oxidation process using zeolites and their modifications were determined. Resuts: The results showed that optimum pH for the ammonium adsorption process by carbon catalyst, catalytic ozonation and zeolite was 8, 9 and 8, respectively. However, the optimum pH 4 was determined for nitrate removal. The highest ammonium absorption capacity was related to natural Firoozookh zeolite and 18.5 mg/g, and the effect of ligand and acid modification decreased 12 and 14% of absorbed capacity, respectively. It is also, the highest nitrate removal efficiency was related to Garmsar ligand modified zeolite (98%) and an absorption capacity of 11.2 mg/g. In the COP/absorption process the concentration of ammonium was decreased to 0.6 m /L. Conclusion: This method effectively eliminates ammonium, and the modification of zeolite with cationic surfactant increases the efficiency of nitrate removal and the concentrates of all pollutants are brought below standards.
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... Membrane-based electrochemical systems are rapidly developing for the fabrication of sensors for a variety of analytical applications [1][2][3][4][5][6][7]. In particular, porous membranes deposited onto conducting substrates are of considerable interest for the construction of devices that combine the intrinsic properties of the membrane-chemical separation, based on specific interactions, and filtration, based on size-exclusion to particular redox processes occurring at the electrode/solution interface [8][9][10][11][12][13][14][15][16][17]. As a whole, channels and pinholes of the membranes create systems working as ensembles of recessed nanoelectrodes with advantageous properties, such as lower charging currents, high current densities and low effects due to ohmic drop [18]. ...
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A New Approach in Detection of Biomarker 2-propanol with PTFE-Coated TiO2 Nanostructured Films
Chapter
  • Sep 2023
Certain molecules act as biomarkers in exhaled breath or outgassing vapors of biological systems. Metal oxide gas sensors are of great interest to detect these molecules. However, often they are not selective enough to identify the specific molecules. In addition, they typically lose their excellent performance at high humidity levels. In this study nanoscale polytetrafluoroethylene (PTFE) thin films deposited via solvent-free initiated chemical vapor deposition (iCVD) were investigated as a possible pathway to tune the selectivity of metal oxide gas sensors as well as hydrophobic surface functionalization. The gas-sensing properties of two types of PTFE-coated gas-sensing structures are measured for this purpose at several operating temperatures. The first structure is a thermally annealed TiO2 film while the second structure is a thermally annealed TiO2 film with an additional CuO film. After the deposition of the iCVD PTFE thin films the structures exhibit a high response and excellent selectivity to 2-propanol vapor. The experimental data presented here, promote the use of such PTFE-coated gas sensing structures as reliable, accurate and selective sensor structures for the tracking of gases at low concentrations. This enables new possibilities in application fields like biomedical diagnosis, biosensors, and the development of non-invasive technology.
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Nitric Oxide Permselectivity in Electropolymerized Films for Sensing Applications
Article
  • Nov 2016
The presence of biological interferents in physiological media necessitates chemical modification of the working electrode to facilitate accurate electrochemical measurement of nitric oxide (NO). In this study, we evaluated a series of self-terminating electropolymerized films prepared from one of three isomers of phenylenediamine (PD), phenol, eugenol, or 5-amino-1-naphthol (5A1N) to improve the NO selectivity of a platinum working electrode. The electrodeposition procedure for each monomer was individually optimized using cyclic voltammetry (CV) or constant potential amperometry (CPA). Cyclic voltammetry deposition parameters favoring slower film formation generally yielded films with improved selectivity for NO over nitrite and l-ascorbate. Nitric oxide sensors were fabricated and compared using the optimized deposition procedure for each monomer. Sensors prepared using poly-phenol and poly-5A1N film-modified platinum working electrodes demonstrated the most ideal analytical performance, with the former demonstrating the best selectivity. In simulated wound fluid, platinum electrodes modified with poly-5A1N films proved superior with respect to the NO sensitivity and detection limit.
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Enhanced flow injection analysis for measurements of S-nitrosothiols species in biological samples using highly selective amperometric nitric oxide sensor
Article
  • Feb 2012
  • CHINESE CHEM LETT
A highly selective nitric oxide (NO) sensor is fabricated and applied to devise an enhanced flow injection analysis (FIA) system for S-nitrosothiols (RSNOs) measurement in biological samples. The NO sensor is prepared using a polytetrafluoroethylene (PTFE) gas-permeable membrane loaded with Teflon AF® solution, a copolymer of tetrafluoroethylene and 2,2-bis(trifluoroethylene)-4,5-difluoro-1,3-dioxole, to improve selectivity. This method is much simpler and possesses good performance over a wide range of RSNOs concentrations. Standard deviation for three parallel measurements of blood plasma is 4.0%. The use of the gas sensing configuration as the detector enhances selectivity of the FIA measurement vs. using less selective electrochemical detectors that do not use PTFE/Teflon type outer membranes.
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Amperometric Nitrosothiol Sensor Using Immobilized Organoditelluride Species as Selective Catalytic Layer Dedicated to Professor Ernç Pretsch on the Occasion of His Retirement from ETH Zürich
Article
  • Feb 2008
  • ELECTROANAL
A new amperometric sensor capable of responding to various biological S-nitrosothiol species (RSNOs) is described. The sensor is prepared using an organoditelluride-tethered poly(allyamine hydrochloride) (PAH) polymer crosslinked within a dialysis membrane support mounted at the distal surface of an amperometric NO probe. The surface immobilized organoditelluride layer serves as a selective catalyst to decompose various RSNO species to NO in the presence of a thiol reducing agent added to the sample. The proposed sensor responds directly and reversibly to various low molecular weight (LMW) RSNOs in the range of 0.1 mM to 10 mM with nearly equal sensitivity. The main advantage of this sensor over previously reported Cu(II/I) and organodiselenium-based RSNO sensors is its long operational life-time (at least one month). A discussion regarding solution phase transnitrosation reactions potentially allowing the measurement of higher molecular weight S-nitrosoproteins is provided, along with data showing preliminary results in this direction. Further, the direct detection of endogenous RSNO species in diluted fresh whole sheep blood is also demonstrated using this new sensor.
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Modeling the Effect of Oxygen on the Amperometric Response of Immobilized Organoselenium-Based S -Nitrosothiol Sensors
Article
  • Jan 2011
  • ANAL CHEM
Amperometric detection of S-nitrosothiols (RSNOs) at submicromolar levels in blood samples is of potential importance for monitoring endothelial function and other disease states that involve changes in physiological nitric oxide (NO) production. It is shown here that the elimination of dissolved oxygen from samples is critical when covalently attached diselenocystamine-based amperometric RSNO sensors are used for practical RSNO measurements. The newest generation of RSNO sensors utilizes an amperometric NO gas sensor with a thin organoselenium modified dialysis membrane mounted at the distal sensing tip. Sample RSNOs are catalytically reduced to NO within the dialysis membrane by the immobilized organoselenium species. In the presence of oxygen, the sensitivity of these sensors for measuring low levels of RSNOs (<μM) is greatly reduced. It is demonstrated that the main scavenger of the generated nitric oxide is not the dissolved oxygen but rather superoxide anion radical generated from the reaction of the reduced organoselenium species (the reactive species in the catalytic redox cycle) and dissolved oxygen. Computer simulations of the response of the RSNO sensor using rate constants and diffusion coefficients for the reactions involved, known from the literature or estimated from fitting to the observed amperometric response curves, as well as the specific geometric dimensions of the RSNO sensor, further support that nitric oxide and superoxide anion radical quickly react resulting in near zero sensor sensitivity toward RSNO concentrations in the submicromolar concentration range. Elimination of oxygen from samples helps improve sensor detection limits to ca. 10 nM levels of RSNOs.
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Patterned Electrode-Based Amperometric Gas Sensor for Direct Nitric Oxide Detection within Microfluidic Devices
Article
  • Mar 2010
  • ANAL CHEM
This article describes a thin amperometric nitric oxide (NO) sensor that can be microchannel embedded to enable direct real-time detection of NO produced by cells cultured within the microdevice. A key for achieving the thin ( approximately 1 mm) planar sensor configuration required for sensor-channel integration is the use of gold/indium-tin oxide patterned electrode directly on a porous polymer membrane (pAu/ITO) as the base working electrode. The electrochemically deposited Au-hexacyanoferrate layer on pAu/ITO is used to catalyze NO oxidation to nitrite at lower applied potentials (0.65-0.75 V vs Ag/AgCl) and stabilize current output. Furthermore, use of a gas-permeable membrane to separate internal sensor compartments from the sample phase imparts excellent NO selectivity over common interfering agents (e.g., nitrite, ascorbate, ammonia, etc.) present in culture media and biological fluids. The optimized sensor design reversibly detects NO down to the approximately 1 nM level in stirred buffer and <10 nM in flowing buffer when integrated within a polymeric microfluidic device. We demonstrate utility of the channel-embedded sensor by monitoring NO generation from macrophages cultured within non-gas-permeable microchannels, as they are stimulated with endotoxin.
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The Chemical Biology of Nitric Oxide
Chapter
  • Dec 2000
Publisher Summary The multiple effects of nitric oxide (NO) in biological systems have resulted in intense investigation into the mechanisms of NO-mediated events. The chemistry of NO is the primary determinant of its biological properties. However, not all the reactions of NO that can be performed in test tube are pertinent in vivo. This chapter provides a guide through the diverse reactions of NO in biological systems. The scheme of the chemical biology of NO divides the reactions into the two categories of direct and indirect effects. Direct effects are defined as those reactions that are fast enough to occur between NO and specific biological targets. Indirect effects do not involve NO, but rather, are mediated by reactive NO species formed from the reaction of NO with either oxygen or superoxide. These species can mediate either nitrosative or oxidative stress. Aspects of the chemical biology of NO relating to biological molecules such as guanylate cyclase, cytochrome P-450, nitric oxide synthase, catalase, and DNA are reviewed and the possible roles NO performs in different biological situations are explored.
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Gas Sorption, Diffusion, and Permeation in Poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole- c o -tetrafluoroethylene)
Article
  • Dec 1999
  • MACROMOLECULES
The solubility and permeability of H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8 in TFE/BDD87, a random copolymer prepared from 87 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole [BDD] and 13 mol % tetrafluoroethylene [TFE], are reported as a function of temperature and pressure. Sorption isotherms of all penetrants except hydrogen are concave to the pressure axis and are well-described by the dual-mode model. Hydrogen exhibits linear sorption isotherms. In contrast to previous results in hydrocarbon-rich polymers, the solubility of perfluorocarbon penetrants is higher in TFE/BDD87 than that of their hydrocarbon analogues. The solubility of all penetrants in TFE/BDD87 decreases with increasing temperature. Enthalpies of sorption become more negative as penetrant size increases. Fluorocarbon enthalpies of sorption at infinite dilution are significantly more exothermic than those of their hydrocarbon analogues, suggesting more favorable interactions between fluorocarbon penetrants and perfluorinated TFE/BDD87 than between hydrocarbon penetrants and this polymer. Perfluorocarbon permeability coefficients are nearly an order of magnitude lower than those of their hydrocarbon analogues due to the larger size of the fluorocarbons and their subsequently lower diffusivities. The permeability of TFE/BDD87 increases with increasing temperature, indicating that activation energies of permeation (Ep) are positive. Ep values in TFE/BDD87 are smaller than those of conventional glassy polymers. Diffusion coefficients of the lower sorbing gases (O2, N2, CO2, CH4, CF4) exhibit a concentration dependence that is consistent with dual-mode transport in unplasticized glassy polymers. For more strongly sorbing C2H6, C3H8, C2F6, and C3F8, diffusion coefficients increase exponentially with increasing penetrant concentration, suggesting plasticization. Activation energies of diffusion in TFE/BDD87 are positive and increase linearly with penetrant diameter squared. Relative to conventional glassy polymers, ED values in TFE/BDD87 are low. However, |ED| is larger than |ΔHS|. TFE/BDD87 is easily plasticized by the larger, more soluble penetrants and is susceptible to penetrant-induced conditioning. The level of conditioning is highest for the largest, most soluble penetrant examined (C3F8), and the conditioned state gradually relaxes toward that of the as-cast state.
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Solubility of nitric oxide in aqueous and nonaqueous solvents
Article
  • Jan 1977
  • Faraday Trans 1
The solubility of NO in various solvents has been measured over a range of temperatures. The results are compared with the values predicted by various current theories of gas solubility.
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An Integrated Nitric Oxide Sensor Based on Carbon Fiber Coated with Selective Membranes
Article
  • Oct 2000
  • ELECTROANAL
In vivo measurement of nitric oxide (NO) in a biological matrix is very difficult because of its assumed low stability and fugacity, in addition to the complexity of such matrix, limited space and volume of biological samples. Among different NO detection strategies, electrochemical NO sensors are still widely used by NO researchers. Though many kinds of NO sensors are commercial available from World Precision Instruments, Inc. and other companies, the small NO sensors still are needed for the NO detection, especially in single cell levels. In this article a NO-selective ultramicrosensor was developed as an easily applicable tool for real time nitric oxide (NO) detection. The sensor consists of a 7 µm carbon fiber working electrode coated with cation exchanger (Nafion), then covered with NO-selective gas permeable polymeric membranes, and Ag/AgCl micro-reference/counter electrode. Compared with other reported NO sensors, the sensor described herein offers several advantages: i) high selectivity against ascorbate (>104:1), dopamine (>103:1) and nitrite (104:1); ii) detection limit to low nanomolar concentration; iii) rapid, inexpensive and reproducible fabrication; iv) wide linear calibration range from 10 nM to 5 µM with R2=0.995; v) integrated ultramicrosensor eliminating the need of an external reference electrode, accordingly, experiments in small volume are possible with an integrated ultramicrosensor, even at single cell levels.
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A Novel Microchip Nitric Oxide Sensor with sub‐nM Detection Limit
Article
  • May 2002
  • ELECTROANAL
Since the discovery of nitric oxide (NO) as a vasodilatory messenger, in particular after its identification as an endothelial-derived relaxing factor (EDRF), there has been a mushroom effect in the research of NO in biological systems. Monitoring of NO in biological samples in vivo and in real time is desirable in many fields of NO research. Although several techniques are available for measurement of NO, the electrochemical method is most advantageous because of its speed and sensitivity. Since the first commercially available electrochemical NO detection system, many NO electrodes have been developed with dimensions from μm to cm, and with detection limits in the low nanomolar range. However, there is still a continuing demand for new NO sensors with lower detection limits. An electrochemical sensor to detect nitric oxide gas dissolved in solution as well as in gas phase is described as well as a fabrication method for the electrode. The sensor is based on the selective oxidation of nitric oxide by a multielectrode array of microelectrodes created on activated carbon, which has been deposited on a silicon chip substrate. The array, in turn, is further modified with several layers of cationic ion exchanger then the subsequent addition of NO selective membranes. The microchip NO sensor, is characterized by a linear response to concentrations of NO up to 100 μM, a response time of a few seconds, and a detection limit of less than 0.3 nM. In biological samples the sensor discriminates against substances such as nitrite, dopamine and ascorbic acid. Moreover, compared with present NO sensors, this sensor is significantly less temperature sensitive, resulting in significantly improved sensor detectivity as compared with present electrochemical sensors. Major applications for measurement of NO concentration are in chemical media, biological tissue, cell cultures, or in blood.
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DEMS-Cyclic Voltammetry Investigation of the Electrochemistry of Nitrogen Compounds in 0.5 M Potassium Hydroxide
Article
  • Jan 1994
  • ChemInform
The electro-oxidation and -reduction of 0.05 M ammonia, 0.01 M hydroxylamine, 0.01 M sodium nitrite and 0.05 M sodium nitrate in 0.5 M KOH at Pt-black electrodes has been investigated using a combination of cyclic voltammetry with on-line MS analysis of volatile products (DEMS).All compounds investigated in this study form adsorbates with very similar properties. These adsorbates can be reduced to ammonia and oxidised to nitrogen. Ammonia is adsorbed anodically, while nitrite and nitrate are adsorbed cathodically. In contrast to the adsorbates of the other compounds, adsorbed nitrite can also be reduced to nitrogen oxides in addition to ammonia. The combination of cyclic voltammetry and on-line MS proves that the adsorbates do not consist solely of triple bonded nitrogen.Ammonia and hydroxylamine also form nitrogen oxides in bulk oxidation processes.For the first time, a reaction mechanism which takes into account the interconversion of nitrogen compounds and adsorbate processes is discussed.
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Measurements of nitric oxide in biological materials using a porphyrinic microsensor
Article
  • Jul 1993
  • ANAL CHIM ACTA
The application and optimization of a microsensor for in situ measurements of nitric oxide (NO) is biological systems are described. The sensor (diameter 0.5–0.8 μm), exhibiting a response time better than 10 ms and a detection limit of 10 nM, consists of several layers of p-type semiconducting polymeric porphyrin and cation exchanger (Nafion) deposited on a thermally-sharpened carbon fiber. The sensor has been applied to studies of NO release from a single endothelial cell in a pulmonary artery, as well as for the determination of NO in blood.
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Electrochemical oxidation of ammonia in alkaline solutions: Its application to an amperometric sensor
Article
  • Dec 1998
  • ELECTROCHIM ACTA
The electrochemical oxidation of ammonia was studied at room temperature on a black platinum electrode, iridium black, a mixture of both and PtIr electrodeposits in alkaline solutions of potassium hydroxide. The current density for the ammonia discharge was proportional to the ammonia concentration in the range from 50 to 200 ppm of the functioning of the sensor. It was found that the current density on the platinum-iridium mixture is higher than on platinum black and iridium black.On the other hand, PtIr electrodeposits on Au and Pt were prepared and characterized by X-ray analysis. The diffractograms evidenced the presence of a continual platinum-iridium solid solution where platinum is replaced by iridium atoms in the fcc structure. The PtIr electrodeposits showed a good response to the ammonia gas.
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Electrochemical Nitric Oxide Sensors for Biological Samples – Principle, Selected Examples and Applications
Article
  • Jan 2003
  • ELECTROANAL
The discoveries made in the 1980s that NO could be synthesized by mammalian cells and could act as physiological messenger and cytotoxic agent had elevated the importance of its detection. The numerous properties of NO, that enable it to carry out its diverse functions, also present considerable problems when attempting its detection and quantification in biological systems. Indeed, its total free concentration in physiological conditions has been established to be in nanomolar range. Thus, detection of nitric oxide remains a challenge, pointing out the difficult dual requirements for specificity and sensitivity. Exception made for the electrochemical techniques, most of the approaches (namely UV-visible spectroscopy, fluorescence, electron paramagnetic resonance spectroscopy) use indirect methods for estimating endogenous NO, relying on measurements of secondary species such as nitrite and nitrate or NO-adducts. They also suffer from allowing only ex situ measurements. So, the only strategies that allow a direct and in vivo detection of NO are those based on the use of ultramicroelectrodes. The reality is that surface electrode modification is needed to make the ultramicroelectrode material selective for NO. Therefore, the design of modified electrode surfaces using organized layers is very attractive and provides the ideal strategy. This review addresses a global description of the various approaches that have involved chemically modified microelectrodes specially designed for the electrochemical detection of NO in biological media. Selected significant examples of applications in biological tissues are also reported in order to highlight the importance of this approach in having new insights into the modulatory role of NO in physiology and pathophysiology.
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