Amperometric Nitric Oxide Sensors: a Comparative Study

Catalytic selectivity of metallophthalocyanines for electrochemical nitric oxide sensing

The catalytic properties of metallophthalocyanine (MPc) complexes have long been applied to electrochemical sensing of nitric oxide (NO) to amplify sensitivity and reduce the substantial overpotential required for NO oxidation. The latter point has significant ramifications for in situ amperometric detection, as large working potentials oxidize biological interferents (e.g., nitrite, L-ascorbate, and carbon monoxide). Herein, we sought to isolate and quantify, for the first time, the selectivity benefits of MPc modification of glassy carbon electrodes. A series of the most catalytically active MPc complexes towards NO, including Fe(II)Pc, Co(II)Pc, Ni(II)Pc, and Zn(II)Pc, was selected and probed for NO sensing ability under both differential pulse voltammetry (DPV) and constant potential amperometry (CPA). Data from DPV measurements provided information with respect to MPc signal sensitivity amplification (~1.5×) and peak shifting (100-200 mV). Iron-Pc exerted the most specific catalytic activity towards NO over nitrite. Catalyst-enabled reduction of the working potential under CPA was found to improve selectivity for NO over high potential interferents, regardless of MPc. However, impaired selectivity against low potential interferents was also noted.

Fluorinated xerogel-derived microelectrodes for amperometric nitric oxide sensing

An amperometric fluorinated xerogel-derived nitric oxide (NO) microelectrode is described. A range of fluorine-modified xerogel polymers were synthesized via the cohydrolysis and condensation of alkylalkoxy- and fluoroalkoxysilanes. Such polymers were evaluated as NO sensor membranes to identify the optimum composition for maximizing NO permeability while providing sufficient selectivity for NO in the presence of common interfering species. By taking advantage of both the versatility of sol-gel chemistry and the "poly(tetrafluoroethylene)-like" high NO permselective properties of the xerogels, the performance of the fluorinated xerogel-derived sensors was excellent, surpassing all miniaturized NO sensors reported to date. In contrast to previous electrochemical NO sensor designs, xerogel-based NO microsensors were fabricated using a simple, reliable dip-coating procedure. An optimal permselective membrane was achieved by synthesizing xerogels of methyltrimethoxysilane (MTMOS) and 20% (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trimethoxysilane (17FTMS, balance MTMOS) under acid-catalyzed conditions. The resulting NO microelectrode had a conical tip of approximately 20 microm in diameter and approximately 55 microm in length and exhibited sensitivities of 7.91 pA x nM (-1) from 0.2 to 3.0 nM (R (2) = 0.9947) and 7.60 nA x microM (-1) from 0.5 to 4.0 microM ( R (2) = 0.9999), detection limit of 83 pM (S/ N = 3), response time ( t 95%) of <3 s, and selectivity (log K NO, j (amp)) of -5.74, <-6, <-6, <-6, <-6, -5.84, and -1.33 for j = nitrite, ascorbic acid, uric acid, acetaminophen, dopamine, ammonia/ammonium, and carbon monoxide. In addition, the sensor proved functional up to 20 d, maintaining >or=90% of the sensor's initial sensitivity without serious deterioration in selectivity.

Sol-gel derived amperometric nitric oxide microsensor

An amperometric sol-gel derived nitric oxide microsensor is described. Several silicon-based xerogel membranes are evaluated to identify the optimum composition for maximizing NO permeability while providing sufficient selectivity for NO in the presence of common interfering species. Xerogel permeability and selectivity are further manipulated as a function of reaction/processing conditions. In addition, the effects of incorporating Nafion into the xerogel matrix on sensor performance and the stability of the ensuing xerogel/Nafion hybrid film are evaluated. The optimal permselective membrane is achieved by catalyzing polycondensation of the xerogel composed of methyltrimethoxysilane and (aminoethylaminomethyl)phenethyltrimethoxysilane and Nafion with NO gas. The resulting NO microsensor exhibits a sensitivity of 0.17 +/-0.02 pA/nM (from 25 to 800 nM, r = 0.9991), detection limit of 25 nM (S/N = 3), response time of 9 s (t(95%), a NO concentration change from 400 to 500 nM), selectivity (log K(NOj) amp) of -5.8, <-6, <-6, and <-6 for j = nitrite, ascorbic acid, uric acid, and acetaminophen, and a lifetime of 8 d (82% of initial sensitivity without serious deterioration in selectivity).

Calibration of NO sensors for in-vivo voltammetry: laboratory synthesis of NO and the use of UV?visible spectroscopy for determining stock concentrations

The increasing scientific interest in nitric oxide (NO) necessitates the development of novel and simple methods of synthesising NO on a laboratory scale. In this study we have refined and developed a method of NO synthesis, using the neutral Griess reagent, which is inexpensive, simple to perform, and provides a reliable method of generating NO gas for in-vivo sensor calibration. The concentration of the generated NO stock solution was determined using UV-visible spectroscopy to be 0.28+/-0.01 mmol L(-1). The level of NO(2) (-) contaminant, also determined using spectroscopy, was found to be 0.67+/-0.21 mmol L(-1). However, this is not sufficient to cause any considerable increase in oxidation current when the NO stock solution is used for electrochemical sensor calibration over physiologically relevant concentrations; the NO sensitivity of bare Pt-disk electrodes operating at +900 mV (vs. SCE) was 1.08 nA micromol(-1) L, while that for NO(2) (-) was 5.9 x 10(-3) nA micromol(-1) L. The stability of the NO stock solution was also monitored for up to 2 h after synthesis and 30 min was found to be the time limit within which calibrations should be performed.

Microelectrode for in vivo real-time detection of NO

Nitric oxide (NO) is gaining importance with its diverse spectrum of clinic effects. However, there is still a need for an ideal sensor to monitor its concentration in tissue. An ideal sensor should not interfere with the ongoing physiological process, while making fast, reliable, and repeatable measurements. We have designed a microelectrode for electrochemical NO measurement from tissue with relatively low interference and reliable results upon calibration. Details of electrode preparation and calibration procedure are explained along with an experiment to monitor effects of photodynamic therapy.

Improved planar amperometric nitric oxide sensor based on platinized platinum anode. 1. Experimental results and theory when applied for monitoring NO release from diazeniumdiolate-doped polymeric films

An improved miniature amperometric nitric oxide sensor design with a planar sensing tip (ranging from 150 microm to 2 mm in diameter) is reported. The sensor is fabricated using a platinized platinum anode and a Ag/AgCl cathode housed behind a microporous poly(tetrafluoroethylene) (PTFE; Gore-tex) gas-permeable membrane. Platinization of the working platinum electrode surface dramatically improves the analytical performance of the sensor by providing approximately 10-fold higher sensitivity (0.8-1.3 pA/nM), approximately 10-fold lower detection limit (< or =1 nM), and extended (at least 3-fold) stability (>3 d) compared to sensors prepared with bare Pt electrodes. These improvements in performance arise from increasing the kinetics and lowering the required potential for the 3-electron oxidation of NO to nitrate, relative to that observed using a nonplatinized working electrode. The outer porous PTFE membrane provides complete selectivity for NO over nitrite ions (up to 10 mM nitrite). The new sensor is applied for surface measurements of NO released from diazeniumdiolate-loaded silicone rubber films (SR-DACA-6/N(2)O(2)). The effects of sensor size (for sensor dimensions of 0.15-, 1-, and 2-mm o.d.) and the distance of the sensor from the surface of the NO-emitting polymer film are investigated via experiments as well as theoretical calculations. A significant analyte trapping effect is demonstrated, the degree of which depends on the sensor size and its distance from the surface. It is further demonstrated that surface NO concentrations for fresh SR-DACA-6/N(2)O(2) loaded films are also influenced by the polymer film thickness, with thicker films generating higher surface concentrations of NO.

Use of an electrochemical nitric oxide sensor to detect neuronal nitric oxide production in conscious, unrestrained rats

INTRODUCTION

Amperometric sensors that directly measure nitric oxide (NO) are readily employed in pharmacologic research. While several of these sensors have been developed, none has been investigated for use in conscious, freely moving animals. An approach was developed and validated for real-time quantitation of neuronal NO production in rats without restricting locomotor activity or other potentially useful behavioral endpoints.

METHODS

Male Sprague-Dawley rats were equipped with a femoral vein or intraperitoneal cannula. A guide cannula and an amperometric NO sensor were placed in the left and right hippocampus, respectively. Following recovery, rats received a 6-h intravenous infusion of saline, L-arginine (an NO precursor; 250 or 500 mg/kg/h), or incremental intraperitoneal 7-nitroindazole (an NO synthase inhibitor; 200-mg/kg loading dose and 100 mg/kg every 2 h). The sensor recorded NO production continuously and microdialysis samples were collected incrementally throughout the experiment. Griess assay analysis of microdialysate samples was compared to sensor readings in vivo. In vitro degradation of an NO donor also was used to validate sensor performance.

RESULTS

Exogenous administration of L-arginine resulted in incremental increases in the neuronal NO signal. A reduction in NO production was observed during administration of 7-nitroindazole, a selective neuronal NO synthase inhibitor. A significant correlation was observed in vitro between the Griess assay analysis, an indirect analytical approach, and the NO sensor readings. The lack of a strong correlation between these measures in vivo is consistent with the indirect nature of the Griess assay.

DISCUSSION

The current approach allows real-time determination of neuronal NO production in unrestrained rats. This model will be invaluable in evaluating pharmacologic issues regarding brain tissue NO synthesis, assessing brain NO synthase as a molecular target, and establishing the effects of pharmacologic agents on neuronal NO production.

Detection of ATP-induced nitric oxide in a biomimetic circulatory vessel containing an immobilized endothelium

Conditions for the adhesion of bovine pulmonary artery endothelial cells (bPAECs) in microbore tubing of 250-microm i.d. are described. When immobilized to the lumen of microbore tubing, these cells represent a mimic of a circulatory vessel's endothelium. The microbore tubing is coated with 100 microg mL(-1) fibronectin in order to promote bPAEC adhesion to the lumen of the tubing. A series of micrographs of the cells inside of the tubing indicates that approximately 3.5 h is necessary for cell adhesion. In this study, adenosine triphosphate (ATP) is used to induce the release of nitric oxide from the endothelium mimic. The endothelium-derived NO is detected amperometrically at a parallel flow cell containing a glassy carbon working electrode modified with Nafion. Results indicate that detectable amounts of NO are only produced by the endothelium mimic when ATP is present in the buffer. The typical concentration of NO produced by the endothelium mimic upon the introduction of 100 microM ATP is approximately 0.80 microM. Based on the injection volume of ATP and the estimated number of cells on the tubing lumen, this value corresponds to approximately 1 amol of NO/cell. Moreover, shear stress alone does not provide the agonistic effect required for NO production in the submicromolar range.

Nitric oxide determination by amperometric carbon fiber microelectrode

Nitric oxide (NO) amperometric microsensor was prepared by the modification of bare carbon fiber electrode by Nafion and cellulose acetate (CA). Detection limit, response time, reproducibility and influence of some possible interferences (nitrite, nitrate, arginine) were tested and evaluated. This sensor was used for in vitro determination of NO release from fresh porcine aorta induced by calcium ionophore A23187 (CI).