Ž .Sensors and Actuators B 66 2000 184–186www.elsevier.nlrlocatersensorb

Gold-solid polymer electrolyte sensor for detecting dissolved oxygen inwater

T.C. Chou), K.M. Ng, S.H. WangDepartment of Chemical Engineering, National Cheng Kung UniÕersity, Tainan 700, Taiwan

Received 30 July 1998; received in revised form 1 February 1999; accepted 29 January 2000

Abstract

Ž . Ž .The gold-solid polymer electrolyte Au-SPE sensor for detecting dissolved oxygen DO in water was studied. The results indicatedthat the prepared Au-SPE sensor has a good performance in sensing DO in water. The preparation of electrode was optimized to obtainthe best sensitivity. The potential windows of DO reduction on AurNafionw electrode in aqueous solutions were determined. Theinterference of NHq, NOy and Cly were also examined. The Au-SPE sensor shows a high selectivity for DO detection. q 2000 Elsevier4 3

Science S.A. All rights reserved.

Keywords: Gold-solid polymer electrolyte; Sensor; Dissolved oxygen; Water

1. Introduction

Ž .Dissolved oxygen DO sensor has a great deal ofinterest due to its importance in analyzing DO in manyareas, such as the production of food and pharmaceutics,the biochemical and biological studies, and the treatmentof waste water. Many methods of sensing DO in water

w xwere reported 1–4 . The electrochemical sensor, such asw xClark electrode 5 , is the most popular sensor for analyz-

ing DO in water. The disadvantages of the conventionalDO sensor are low sensitivity and its difficult miniaturew x Ž .5 . The novel gold-solid polymer electrolyte Au-SPEsensor for detecting DO in water is developed.

2. Experimental

The AurNafionw electrode was prepared by the Take-Ž . w xnaka-Torikai T-T method 6 . Conditions for preparing

electrode was 0.02 M HAuCl , 0.1 M NaBH , pH 14 and4 4

at 458C for a 3-h run. The sensing cell was constructed bya divided cell with sensing chamber and anodic chamber.

Ž .The electrolyte 0.5 M H SO filled in the anodic cham-2 4

) Corresponding author. Tel.: q886-6-2757575, ext. 62639; fax:q886-6-2366836.

Ž .E-mail address: tcchou@mail.ncku.edu.tw T.C. Chou

ber and the deionized water filled in the sensing chamberor cathode. The Pt wire anode is the counter electrode andthe reference electrode is AgrAgCl. The inlet testing gasor oxygen was passed through the sensing chamber whenthe testing run was carried out. Before any testing run,nitrogen gas was passed through the sensing chamber andpurged out the DO in the sensing chamber. When thepurge was finished, usually about 20 min, a blank test wascarried out and recorded the background current of thissystem at a fixed applied potential. Then, the desiredcomposition of oxygen gas or air was passed through thesensing chamber or working electrode and the responsecurrent was recorded at a fixed applied potential, whichwas similar to that of the blank test. In general, a back-ground current was the current reached at steady state inthe blank test. When the background current was obtained,an oxygen stream was introduced into the sensing chamberand the measured current was obtained. The differencebetween the measured current and background current wasthe response current.

3. Results and discussion

3.1. Response current of the AurNafionw electrode

Fig. 1 was a typical response current sensed by using aAurNafionw electrode, which was prepared at 0.02 M

0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0925-4005 00 00349-X

( )T.C. Chou et al.rSensors and Actuators B 66 2000 184–186 185

Fig. 1. Response patterns of AurNafionw electrode for different DOconcentration in water. Supporting electrolyte: 0.5 M H SO ; tempera-2 4

Ž .ture: 258C; applied voltage: y0.1 V vs. AgrAgCl .

HAuCl , 0.1 M NaBH , pH 14, temperature 458C and4 4w xlasted 3 h by the T-T method 6 . For a 40-ppm DO in

water, the response current of a 3 cm2 electrode was 1400mA at the sensing operating conditions: 0.5 M H SO2 4

supporting electrolyte, temperature 258C and y0.1 V vs.AgrAgCl applied voltage. The response current increasedwhen the DO in water increased as shown in Fig. 1. Theresponse current was plotted against the DO concentrationresulting in a straight line as shown in Fig. 2. Increasingthe concentration of DO from 5.5 to 39.9 ppm increasedthe response current from 105 to 1400 mA. The sensitivityis 38.4 mArppm. The lowest sensing concentration is 3.8ppm in this case.

Fig. 2. Effect of DO concentration on response current in water. Support-ing electrolyte: 0.5 M H SO ; temperature: 258C; applied voltage: y0.12 4

Ž .V vs. AgrAgCl .

Fig. 3. Response pattern of AurNafionw for DO in water. Supportingelectrolyte: 0.5 M H SO ; temperature: 258C; DO concentration: 44 ppm;2 4

Ž .applied voltage: y0.1 V vs. AgrAgCl .

3.2. Stability of the AurNafionw electrode

Fig. 3 shows the results of the response currents byalternative through the changing of the inlet nitrogen gasand oxygen gas stream. Based on the six cycles of thealternative change of the nitrogen and oxygen streams,both the background and the response currents did notchange and were kept at 386 and 2182 mA, respectively,for the 3-h continuous run. The stability of this DO sensoris very good. Furthermore, the stability test was carried at40-ppm DO at y0.1 V vs. AgrAgCl applied potential.The results indicated that the drift of this DO sensor wasinsignificant for a period of 30 h.

Fig. 4. I – E curve of AurNafionw electrode for DO in different aqueoussolutions. Supporting electrolyte: 0.5 M H SO ; temperature: 258C;2 4

temperature: 458C.

( )T.C. Chou et al.rSensors and Actuators B 66 2000 184–186186

Fig. 5. Effect of oxygen partial pressure on response current in differentaqueous solutions. Supporting electrolyte: 0.5 M H SO ; temperature:2 4

Ž .258C; applied voltage: y0.1 V vs. AgrAgCl .

3.3. Interference of AurNafionw sensor

The DO sensor may be applied in the pool for raisingfishes in Taiwan or in the world. The decomposition of thefeed for fish might generate ammonia. Ammonium chlo-ride, NH Cl, and ammonium nitrate, NH NO , were cho-4 4 3

sen to explore the effect of other species on the perfor-mance of this sensor. The results of the I–E curve ofAurNafionw electrode for the saturated DO in differentaqueous solutions were shown in Figs. 4 and 5. The effectof the ammonium salt for the testing water on the responsecurrent was significant although the limiting response cur-

Table 1Sensitivity and the lowest limit of DO in different aqueous solutions.Supporting electrolyte: 0.5 M H SO ; temperature: 258C2 4

Aqueous solution Saturated Sensitivity LowestŽ .DO mArppm limit

Ž .concentration ppmŽ .ppm

H O 39.9 38.4 3.82

1 M NH NO 30.0 47.8 1.84 3

1 M NH Cl 28.6 41.1 2.54

rent in pure water and 1 M NH NO aqueous solution was4 3

the same. However, the saturated DO for these aqueoussolutions was different. The results indicated that the rela-tionship between response current and the DO concentra-tion of this sensor for different testing solutions was astraight line. The sensitivities for sensing the pure water, 1M NH NO and 1 M NH Cl aqueous solutions are 38.4,4 3 4

47.8 and 41.1 mArppm, respectively, as shown in Table 1.The sensitivity in the 1 M NH NO aqueous solution is4 3

the best one.

4. Conclusions

The prepared Au-SPE sensor is a very good sensor fordetecting the DO in water. The potential window of DOreduction on the AurNafionw electrode was in the rangeof q0.40 to y0.20 V vs. AgrAgCl with acid as thesupporting electrolyte. The reaction was mass transfercontrolled when the potential was more negative than

Ž .y0.05 V vs. AgrAgCl . The sensitivity of DO was 38.4mArppm. The lowest limit of DO was 3.8 ppm. Thestability of this sensor is excellent.

Acknowledgements

The support of the National Science Council of theŽ .Republic of China NSC 87-2214-E-006-018 and National

Cheng Kung University is acknowledged.

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