A New Nitric Oxide Gas Sensor Based on Reticulated Vitreous Carbon/Nafion and Its Applications

Full Paper

A New Nitric Oxide Gas Sensor Based on Reticulated VitreousCarbon/Nafion and Its Applications

Jie Sun,a Peter C. Hauser,b Valentine Zhelyaskov,a Jie Lin,a Mark Broderick,a Harry Fein,a Xueji Zhang*a

a Department of Chemistry, World Precision Instruments, Inc., 175 Sarasota Center Boulevard, International Trade Center,Sarasota, FL 34240-9258, USA

*e-mail: [email protected]; [email protected] Department of Chemistry, University of Basel, Spitalstrasse 51, 4004 Basel, Switzerland

Received: November 19, 2003Final version: December 16, 2003

AbstractReticulated vitreous carbon (RVC), and Nafion membrane are used to fabricate a composition electrode to measurenitric oxide (NO) concentration amperometrically in the gas phase. Limit of detection was found to be 6 ppb at anapplied voltage of 0.66 V (vs. mercury sulfate reference electrode) with average response time of less than 30 seconds.The response of the sensor was linearly dependent on the concentration over the whole tested range from 19 ppb-50 ppm of NO. Simplicity in electrode fabrication and consistent performance between individual sensors make RVCand Nafion attractive materials for detecting very low levels of nitric oxide gas in routine analysis.

Keywords: Nitric oxide, NO, Gas sensor, Electrode, Amperometry, Reticulated vitreous carbon

1. Introduction

Recently discovered physiological importance of nitricoxide has lead to an explosion of research in the past twodecades [1, 2]. Nitric oxide is produced by mammalian cellsand can be detected in the exhaled air at ppb level [3]. Thelevel of exhaled nitric oxide is elevated in a number ofdiseases related to airway inflammation due to induction ofiNOS gene [4, 5]. Therefore, measurement of exhaled nitricoxide can be an easy, non-invasive procedure that helpsdiagnosing disease conditions such as asthma [6, 7].Anotherparticular useful application of exhaled nitric oxide meas-urement is to monitor and to guide treatment of asthma[8, 9].Nitric oxide is a well-known vasodilator. Potential ther-

apeutic use of inhaled nitric oxide as a selective pulmonaryvasodilator has been extensively explored in recent years[10]. Inhaled nitric oxide therapy is typically administratedusing mechanical ventilators. The levels of delivered NOand side product NO2 must be continuously monitored toensure safety/effectiveness of the NO therapy and toprevent potential toxic effect of NO2 [11].Nitric oxide has also been long recognized as a significant

air pollutant. Environmental Protection Agency (US) hasestablished National Ambient Air Quality Standards thatrequires oxides of nitrogen (nitric oxide and nitrogendioxide) level to be below 0.053 ppm (annual average).Extended inhalation of high concentration of nitric oxidecan lead to hypotension, sepsis, hemorrhage and otheradverse conditions. In addition, nitric oxide gas can beoxidized in the air into nitrogen dioxide, which can be toxicto lung tissue and is a major contributor for ground-levelozone formation. Continued measurement of nitric oxide

may also be necessary to make sure air nitric oxide level isbelow the limit set by United State Occupational Safetyand Health Administration (25 ppm/8 hours) in high riskareas.Taken together, there has been an increasing demand for

accurate, stable and long-lasting measurement devices fornitric oxide gas at and below ppm level. Currently, chem-iluminescence technique in which nitric oxide reacts withozone to produce a light-emitting nitrogen dioxide in anexcited state is widely used for determining NO concen-tration [12, 13]. Electrochemical methods have also beenused for themeasurement ofNO.AmodifiedClark-type gassensor has been described to determine NO gas concen-tration [14]. Blurton and Sedlak explored Teflon-bond goldelectrode to detect nitric oxide [15 – 17]. In recent years,solid polymer electrolyte (SPE) such as Nafion, has beenbecoming popular in amperometric sensors [18] for nitricoxide in gas phase [19 – 22] and in solution [23, 24] Theperformance of such sensors have been improving, forexample, Hauser and coworkers described a gold/Nafionsensor capable of detecting 1.2 ppb NO in gas [20]. Thesensitivity of HauserFs sensor approaches that of thechemiluminescence method. Additionally, amperometricsensors aremuchmore economical and do not involve usingtoxic ozone gas. In this paper, we will describe a NO sensorfabricatedwith aNafionmembrane and reticulated vitreouscarbon (RVC).Reticulated vitreous carbon is a porous foammaterial with high surface area [25]. NO gas is detecteddirectly at the three-phase boundary formed by RVC,Nafion and gas, where it is oxidized into nitrosonium ion,with output signal linearly proportional to NO concentra-tion. Previouswork has shown that the perimeter (length) ofthe three-phase boundary but not the area of the sensor

1723

Electroanalysis 2004, 16, No. 20 G 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200302987

determines sensitivity of solid polymer electrolyte gassensors [20]. Therefore, fine mesh materials such as RVCare ideal to fabricate highly sensitive sensors with solidpolymer electrolyte.

2. Experimental

Reticulated vitreous carbon was obtained from EnergyResearch and Generation, Inc. (Oakland, CA) in the formof porous carbon sheets (ca. 5 mm in thickness). It was useddirectly and fabricated into round disks of 1.27 cm2 size.

Nafion 117 solid electrolyte membrane was purchased fromAldrich (0.18 mm in thickness). It was cut into disks withmatching size. The Nafion disks were cleaned with boilingnitric acid and deionized water. A Nafion membrane diskwas then placed on a clean flat surface. A RVC disk wasimmersed into a 5% Nafion 117 solution (Fluka ChemicalCorp., Milwaukee,WI) for 30 seconds and was immediatelyplacedon topof theNafionmembranedisk.Aweight of ca. 5pounds with clean surface was used to press RVC andNafion membrane disks together over night. Intimatecontact between the Nafion membrane and RVC wascreatedwhen a newNafion filmwas formed on the interfacewhen the solvents evaporated. The RVC-Nafion electrodethus fabricated was used as working electrode and washoused in Teflon blocks shown below (Figure 1). The RVCsidewas directly exposed to gases to bemeasured andwas incontact with a gold wire as a current collector. The gold wirewas connected to a POT-500 Potentiostat (World PrecisionInstruments, Inc., Sarasota, FL). The Nafion side was incontact with 0.5 M sulfuric acid (or other electrolytes whenstated) and served as a barrier between working electrodecompartment and reference/counter electrode compart-ment. The reference electrode was an Hg/HgSO4 referenceelectrode (CH Instruments, Inc., Austin, TX) with apotential of þ 680 mV (versus SHE). For counter electrode,we selected a platinum wire with 1 mm diameter (WorldPrecision Instruments, Inc., Sarasota, FL). Both referenceelectrode and counter electrode were in contact with theelectrolyte and were connected to a POT500 Potentiostat.Several RVC-Nafion sensors were built using differentdensities of RVC ranging from 100 pores/inch (PPI) to 300PPI.Figure 2 shows the set-up for measuring NO in gas with

the RVC-Nafion gas sensor. Certified NO gas standards(1.04 ppm and 447.9 ppm balanced with N2 gas) werepurchased from Airgas (Radnor, PA). The certified gaseswere further diluted with a high purity N2 gas (Terry SupplyCo., Tampa, FL) using a commercial gas diluter/mixer(Dundee Scientific, UK). The outflow from the gas mixerranged from zero gas (pure N2) to NO/N2mixture at variousconcentrations (ca. 5X – 100X dilution). The outflow rate

Fig. 1. RVC/Nafion composite gas sensor assembly. Teflonblocks were used to house electrodes and electrolyte. The pre-fabricated RVC/Nafion disk was placed tightly against a gold ringthat was connected to a gold wire. The working electrode blockwas then joined with the electrolyte block and the blocks wereheld together tight with screws (not shown). Luer fittings wereused as inlet and outlet for the gas chamber. Dimensions are notproportional to actual sizes and are for illustration purpose.

Fig. 2. Experimental set-up for measuring nitric oxide in the gas phase.

1724 J. Sun et al.

Electroanalysis 2004, 16, No. 20 G 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

was constant regardless of NO concentration, and wasadjusted according to flow rate set in the vacuum pump.Before the inlet of the RVC-Nafion gas sensor, a T-tube wasused to maintain the gas pressure close to 1 atmosphere byopening one end of the T-tube to air. The gas to bemeasuredwas drawn into the sensor at a constant flow rate by aminiature diaphragm pump (Virtual Industries, Inc., Colo-rado Springs, CO). The flow rate of the pump was adjustedto be always slower than the output flow rate from the gasmixer so that no room air was drawn into the sensor. Theflow rate at the outlet of the pump was measured by aGilmont flowmeter (Barrington, IL) and was recorded.

3. Results and Discussion

3.1. Cyclic Voltammetry

Figure 3 shows a cyclic voltammogram of a RVC/Nafion gassensor when it was purged with pure N2 gas. It is noted thatthe RVC is very resistant to oxidation itself. The anodiccurrent (upper branch) is almost constant in the regionaround 600 mV (vs. MSE). The useful range of appliedpotential for NO detection is thus very wide, up to ca.900 mV (vs.MSE) (1580 mV vs. SHE) applied potential canbe used.When a very low applied potential is used, little NOis oxidized at the working electrode and therefore littlecurrent (response signal) is generated. At a high potential,NO2 can also be oxidized at the electrode to produceinterference for the measurement. Therefore, for measure-ment ofNO ingasmixture containing bothNOandNO2, it isimportant to select a potential with good response to NOand with minimum interference from NO2. To operate at ahigh applied voltage, other techniques may be used toeliminate NO2 interference. For example, gas mixtures maybe allowed to pass through a NO2 scrubber column beforedrawn into the gas sensor.

3.2. Response to NO

To measure NO concentration, the gas sensor was condi-tioned by polarizing at the selected potential (e.g., 0.66 V vs.MSE) for several hours under a N2 stream. When thepurging gas was switched into a NO/N2 mixture, an increaseof current can be observed. The magnitude of the currentresponse is directly proportional to NO concentration, andis dependent on the density of vitreous carbon and otherfactors. Under identical conditions, higher density vitreouscarbon gives higher response.NO-responding signal of a 300PPI RVC sensor was about 20% higher that that generatedby a 100 PPI RVC-Nafion sensor. Figure 4 shows a typicalresponse curve of RVC-Nafion gas sensor to NO/N2

mixture. In response to NO, the current rose quickly andreached an almost constant value. The response time asdefined as 90%of plateau value seemed to vary from ca. 10 sto ca. 40 s depending on the concentration of NO andindividual sensors. The current change is linearly propor-tional to NO concentration over a wide range from low ppblevel to ca. 50 ppmandprobably beyond. Figure 5 showsoneset ofmeasurementwithin a single experiment.Only a singlemeasurement was made for high concentrations of NO(>1900 ppb), whereas five replicate measurements weremade for NO at 19 ppb, 100 ppb and 128 ppb (not shown).The current response had good repeatability even at lowNOconcentrations. The linear regression result was excellent:y¼ 0.0009522 xþ 0.3254 (R2¼ 0.999), where y is currentchange in mA and x is NO concentration in ppb. Themagnitude of response of 0.0009522 mA/ppb or 0.9522 mA/ppm is typical for a 300 PPI RVC/Nafion sensor althoughsmall variation exists for individual sensorsmade from samematerial. The range of linearity was maintained for all theRVC-Nafion sensors tested. The lower detection limit wasestimated to be between 6 – 12 ppbNO (2S/N) for ourRVC/Nafion sensors at 0.66 V (vs. MSE) and 40 mL/min gas flowrate, depending on the density of RVC. The detection limitachieved by the RVC/Nafion electrode is not as good as

Fig. 3. Cyclic voltammetry of a 100 PPI RVC/Nafion sensorunder a stream of N2 gas at 40 mL/min flow rate. 50 mV/s scanrate.

Fig. 4. Typical response curve of a 300 PPI RVC/Nafion sensor.The sensor was polarized under a N2 gas stream for several hoursbefore the experiment. Gas flow rate was 40 mL/min and appliedvoltage was 0.66 V (vs. MSE).

1725Nitric Oxide Gas Sensor Based on Reticulated Vitreous Carbon/Nafion


HauserFs gold/Nafion electrode [20]. However, fabricationof gold/Nafion electrode is complicated. FollowingHauserFsprocedure, we made gold/Nafion electrodes with a widerange of sensitivity (detection limit from 2 ppb to 20 ppb)under the same exact conditions.WithRVC, industrial scaleproduction makes material to be of uniform quality. Thefabrication of RVC/Nafion electrode is simple and itproduces consistent quality in measuring NO.

3.3. Effects of Changing Applied Anodic Potential and ofChanging Gas Flow Rate

The current response is dependent on the applied voltagebetween the reference electrode and the working electrode.Under a constant gas flow rate (40 mL/min), the currentincreased about 70% when the applied potential rose by0.1 V (Figure 6). Under the constant applied voltage, gasflow rate affected the sensor response. From Figure 6, it canbenoted a smallerNOsignalwas obtainedwhenahigher gasflow ratewas used. This is opposite to the behavior exhibitedby agold-Nafion sensor [20]. The real reason for this unusualphenomenon is not clear. It is probably attributed to thesluggish electron transfer of nitric oxide at the RVCelectrode. Additionally the magnitude of signal changedue to flow rate variation is very small for RVC-Nafionsensors compared with that of the gold-Nafion sensor.

3.4. Effect of Changing Electrolytes

We typically used 0.5 M sulfuric acid as electrolyte. Previouswork has shown that this electrolyte provided high NOresponse for sensors based on Nafion membrane [18, 19]. Inthis work, we investigated the effect of concentrationchange of sulfuric acid. We also used three other commonelectrolytes (Figure 7). The results indicate that sulfuric acidis the better electrolyte forNOdetection usingRVC/Nafionsensor since Na2SO4 and K2SO4 yielded lower NO response.The concentration effect is minimal at low sulfuric acidconcentration. But very strong sulfuric acid does seem to

Fig. 5. Current response to different concentrations of NO/N2 gas obtained with a 300 PPI RVC/Nafion sensor. A linear regressioncurve of results is also shown. A) sequential exposure to different levels of NO/N2 gas, the sensor was in a stream of N2 gas before andafter each NO exposure; B) consecutive measurement of 100 ppb and 19 ppb NO/N2 gas, the sensor was in a stream of N2 gas before andafter each NO exposure. Gas flow rate was 40 mL/min and applied voltage was 0.66 V (vs. MSE).

Fig. 6. Current response of a 100 PPI RVC/Nafion sensor to NOexposure at various levels of applied voltages and gas flow rates.Several levels of NO gas were used for each appliedd voltage/flowrate to obtain a regression value of current response. Theconnected lines show effect of flow rate at a constant appliedvoltage.

1726 J. Sun et al.


inhibit the sensor response. Sulfuric acid solution withconcentration between 0.05 M and 1.0 M may be used forNO detection.

3.5. Effect of Temperature

To study the change of NO response as a function oftemperature, we incorporated a heated brass tube of ca.70 cm in length in front of the T-tube (see Figure 1). A1.9 ppm NO/N2 gas mixture was heated up in a brass tubeand the gas temperature was monitored at the T-tube with acommercial thermo-coupler. The distance between the T-tube outlet and sensor inlet was kept minimum. We wereable to elevate temperature up to 28 8C from room temper-ature (23 8C). Our study shows that temperature effect isvery limited, having only a ca. 0.1% increase in response per8C (data not shown).

3.6. Effect of Humidity

Usually, test gases such as exhaled air and stack exhaustioncontain moisture. It would be interesting to know if RVC/Nafion sensor response signal would be affected by mois-ture. As an experimental control, we used dry NO/N2 gasmeasurement under regular conditions. For gases with highhumidity, we bubbled dry NO/N2 gas through 10 mL ofdistilled water in a 20 mL sealed vial installed in front of theT-tube (see Figure 1). First, distilled water was placed in thevial. After sealed with a rubber stopper, the water wasshaken to fill the headspacewith saturated water vapor. DryNO/N2 gas mixture was delivered through an epidermal

needle into the bottomof the vial and exited into the T-tube/sensor through another epidermal needle placed abovewater surface.Thehumidity in the gaswasnotmeasured, butit should be close to saturation at room temperature (23 8C).Throughout concentration range studied (0 – 54.4 ppm), aconsistent and significant signal increase (from 0.6 – 2.3 mA)was noted when the gas stream was switched from a by-passloop into the water-bubbling loop (Figure 8). The increasewas smaller for lower NO concentration, however, percent-age change with respect to NO response was moresignificant at lower concentrations (>30% for NO concen-tration of 1.9 ppm or less). Our results show that humidityeffect is negligible (<5%) when NOmeasured is more than40 ppm. If the measured NO is below that level andespecially below 1.9 ppm, consistent humidity has to bemaintained for the standards and samples. For example,humidity in a sample such as exhaled airmay be removed byusing a moisture-removing column before the gas is drawninto RVC-Nafion sensors.

3.7. Interference from other Gases

One of the drawbacks of electrochemical method for theNOmeasurement is that NO2 and SO2 gases sometimes alsoproduce a signal response because their oxidation potentialsare close to that of NO. For some sensors, one needs toremoveNO2 and/or SO2 through scrubber column(s) beforeNO measurement. As expected, this interference effectwould be dependent on the applied voltage. We have testedNO2, SO2 and CO with a 300 PPI RVC-Nafion sensor at0.66 V (vs. MSE) and 40 mL/min flow rate. We observed0.02 mA/ppm signal response to NO2, compared with typicalca. 1 mA/ppm response to NO. At 1 :1 ratio, NO2 wouldintroduce ca. þ 2% error in the measurement. Its removalwith a sample preconditioning loop is not necessary unless

Fig. 7. Current response of RVC/Nafion sensors to NO exposurewith various electrolyte solutions. A) Effect of concentration ofsulfuric acid; B) Effect of cations of 0.5 M electrolytes. Twodifferent sensors both fabricated with 300 PPI RVC were used forthese two sets of experiment. Gas flow rate was 40 mL/min andapplied voltage was 0.66 V (vs. MSE) in both experiments.

Fig. 8. Effect of humidity on current response of a 300 PPI RVC/Nafion sensor to NO exposure. Gas flow rate was 40 mL/min andapplied voltage was 0.66 V (vs. MSE).



NO2 :NO ratio is very high. However, inference from SO2

was fairly significant with a 0.3 mA/ppm response. Previouswork has shown that this interference effect can be reducedusing higher concentration of sulfuric acid as electrolyte.But this will be at the expense of reduced sensitivity to NOfor RVC-Nafion sensor. When SO2 concentration is signifi-cant in a sample, accurateNOmeasurement canbe achievedby passing the sample through a SO2 scrubber column. COvirtually did not elicit any response from the RVC-Nafionsensor (ca. 9� 10�6 mA/ppm).

3.8. Applications

NO produced in biological systems is short lived and ismetabolized into nitrate and nitrite. Nitrite or total nitrite/nitrate concentration is widely used as an index of NOproduction. One standard application of a chemilumines-cence NO sensor is to measure nitrite concentration inaqueous solutions. Nitrite is first converted into NO (byreduction of KI/Hþ) that reacts with ozone to produce achemiluminescent NO2* species. As little as 10 nMof nitritein solution can be measured. With our gas sensor, nitrite isconverted into NO by a similar method as that in thechemiluminescence method, but the resultant NO is meas-urement directly. Figure 9 shows our experimental set-up.NO is generated by injecting 10 mL of sample into the vialcontaining 4 mL of KI/Hþ, and is carried into a 300 PPIRVC-gas sensor by the carrier gas (N2). The results are

shown in Figure 10. So far, as little as 25 nM of nitrite insample can be determined. With improvement of thequalities of reaction vessel and connection tubing, we canlower the detection limit further.

4. Conclusions

We have demonstrated that RVC is a useful material todetect NO in the gas phase. The RVC-Nafion sensors builtwith our method have a lower detection limit of 6 – 12 ppband produces ca. 1 mA/ppm response signal at 0.66 V (vs.MSE) and gas flow rate of 40 mL/min.The response toNO islinear up to ca. 54 ppm. Under such conditions, interferencefromNO2 andCO is negligible, however, SO2 elicits a strongsignal of 0.3 mA/ppm.Accuratemeasurement of NO at PPBlevel probably requires a NO2/SO2 scrubber column(s) andmoisture trap to condition the samples. For the detection ofNObelow 10 ppb in samples devoid of SO2 and high amountof NO2, higher applied voltage may be used to enhance theperformance of RVC-Nafion sensor. This sensor has beensuccessfully used to measure nitrite as low as 25 nM.

5. Acknowledgements

This research was funded through NIH research grants(1R43GM62077-01, 2R44GM62077-02, 5R44GM62077-03)and a WPI R&D Research Funds to XZ.

Fig. 9. Experimental setup for measuring nitrite in solution usingRVC/Nafion NO gas sensor. The flow rate from nitrogen tank wasslightly slower than that set in the vacuum pump so that a slightnegative pressure was maintained in the systems to allow efficientdelivery of generated NO to the gas sensor. The first vialcontained freshly made KI/acetic acid solution and was stirredusing a magnetic stirrer (not shown). Nitrite solution wasintroduced into this vial through a rubber septa with a syringe.The second vial contained KOH solution to trap acetic acidcarried over by N2 gas stream. The RVC sensor was connected to aPOT 500 potentialstat and a A/D converter. Data was collectedwith a PC computer.

Fig. 10. Current response of a 300 PPI RVC/Nafion sensor toinjection of 10 mL of nitrite solutions into a 4 mL KI/acetic acidsolution. Before each injection, solution(s) in the first vial wasreplaced with a fresh KI/acetic acid solution and recording wasrestarted after a stable flat baseline was re-established. Gas flowrate was 40 mL/min and applied voltage was 0.66 V (vs. MSE).

1728 J. Sun et al.


6. References

[1] R. F. Furchgott, J. V. Zawadzki, Nature 1980, 288.[2] S. Moncada, R. J. M. Palmer, E. A. Higgs, Pharmacol. Rev.

1991, 43, 109.[3] L. E. Gustafsson, A. M. Leone, M. G. Persson, N. P. Wiklund,

S. Moncada, Biochem. Biophys. Res. Commun. 1991, 181,852.

[4] S. Cuzzocrea, E. Mazzon, G. Calabro, et al., Am. J. Respir.Crit. Care Med. 2000, 162, 1859.

[5] F. H. Guo, S. A. A. Comhair, S. Zheng, et al., J. Immunol.2000, 164, 5970.

[6] A. H. Henriksen, T. Lingaas-Holmen, M. Sue-Chu, L.Bjermer, Eur. Respir. J. 2000, 15, 849.

[7] L. J. Dupont, M. G. Demedts, G. M. Verleden, Am. J. Respir.Crit. Care Med. 1999, 159, A861.

[8] E. L. J. van Rensen, K. C. M. Straathof, M. A.Veselic-Char-vat, A. H. Zwinderman, E. H. Bel, P. J. Sterkj, Thorax 1999,54, 403.

[9] M. J. Lanz, D. Y. M. Leung, D. R. McCormick, R. Harbeck,S. J. Szefler, C. W. White, Pediatr. Pulmonol. 1997, 24, 305.

[10] W. E. Hurford, Respir. Care 1999, 44, 360.[11] R. D. Branson, D. R. Hess, R. S. Campbell, J. A. Johannig-

man, Respir. Care 1999, 44 , 281

[12] A. Fontijn, A. J. Sabadell, R. J. Ronco, Anal. Chem. 1970, 42,575.

[13] Y. Maeda, K. Aoki, M. Munemori, Anal. Chem. 1980, 52,307.

[14] T. K. Gibbs, D. Pletcher, Electrochim. Acta 1980, 25, 1105.[15] K. F. Blurton, J. M. Sedlak, US Patent 4,052,268, 1977.[16] J. M. Sedlak, K. F. Blurton, J. Electrochim. Soc. 1976, 123,

1476.[17] J. M. Sedlak, K. F. Blurton, Talanta 1976, 23, 811.[18] K.-C. Ho, W.-T. Huang, J.-C. Yang, Sensors 2003, 3, 290[19] J.-S. Do, K.-J. Wu, Sens. Actuators 2000, B67, 209.[20] P. Jacquinot, A. W. E. Hodgson, P. C. Hauser, Anal. Chim.

Acta 2001, 443, 53.[21] F. Opekar, K. Stulik, Anal. Chim. Acta 1999, 385, 151.[22] F. Maseeh, M. J. Tierney, W. S. Chu, J. Joseph, H.-O. L. Kim,

T. Otagawa, Transducers 591 Int. Conf. Solid State Sens.Actuators, San Francisco 1991, p. 359.

[23] X. Zhang, Y. Kislyak, J. Lin, A. Dickson, L. Cardosa, M.Broderick, H. Fein, Electrochem. Commun. 2002, 4, 11.

[24] M. Pontie, F. Lecture, F. Bedioui, Sens. Actuators 1999, B56,1.

[25] F. C. Cowlard, J. C. Lewis, J. Mater. Sci. 1967, 2, 507.



Documents

A New Nitric Oxide Gas Sensor Based on Reticulated Vitreous Carbon/Nafion and Its Applications