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Schematic of p-pPt electrode based amperometric NO sensor examined in detail in this work. The outer PTFE gas permeable membrane can be further impregnated with varying amounts of Teflon AF ® to enhance NO selectivity
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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...
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... a planar pPt (p-pPt) disk (250-µm O.D.) sealed in a glass wall tubing and a Ag/AgCl wire as the reference/counter electrode were employed to create the electrochemical cell. These two electrodes were incorporated behind a PTFEGPM as illustrated in Figure 1. To enhance NO selectivity primarily against ammonia, the PTFEGPM (0.12 cm 2 ) was coated with 0.5 µL of Teflon AF ® solution (1%, used as received, Dupont Fluoroproducts, Wilmington, DE) and then dried before sensor fabrication. ...
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... based planar gas sensor with Teflon AF ® treatment on PTFEGPM As shown in Figure 2, the addition of ammonium chloride to the test buffer solu- tion (PBS, pH 7.3) leads to an elevation in the amperometric current levels of a planar NO sensor (platinized Pt (pPt) working electrode and reference electrode behind microporous PTFEGPM (Fig. 1). The calculated logarithm of the amperometric selectivity coefficient of this conventional planar-pPt (p-pPt) based gas sensor is 3.1 for ammonium chloride or 1.1 for ammonia (log(k NO,j ), j = NH 4 Cl (aq) or NH 3(aq) , respec- tively (Tab. 1). These values are calculated based on the amperometric responses shown in Figure 2a and ...
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... difference in the pK a value of ammonia (9.25) and the test buffer pH value (7.3). At a higher pH of the test solution, a larger amperometric current change is observed since the equilibrium shifts to increase the amount of dissolved free ammonia gas, NH 3(aq) (data not shown). Figure 2. Amperometric response of p-pPt electrode based NO sensor (Fig. 1) NO added to 10 nmol L ...
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... that was found to be quite effective is to modify the nature of the porous PTFEGPM using a solution of Tef- lon AF ® (an amorphous fluoro-copolymer with large free volume and high gas permea- bility [27]). As shown in Figure 2b, when the same sensor configuration is prepared using the PTFEGPM that has been impregnated with the Teflon AF ® (Fig. 1), NO selectivity is dramatically changed. After the membrane modification, the logarithm of the selectivity coefficient of the p-pPt based sensor improves to 6.2 for ammo- nium ion at pH 7.3 (4.2 for NH 3(aq) ), which is about a thousand-fold improvement compared to the conventional p-pPt based sensor (data shown in Fig. 2a). Further, ...
Citations
... 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. ...
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.
... 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]. ...
... 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]. ...
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.
... 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. ...
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.
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.
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.
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.
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.
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.
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.
