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pH sensor based on deposited film of lead oxide on aluminum substrate electrode
Sensors and Actuators B 88 (2003) 234±238
pH sensor based on deposited ®lm of lead oxide on aluminum substrate electrode Ali Eftekhari* Department of Chemistry, KN Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran Received 30 May 2002; received in revised form 25 July 2002; accepted 1 September 2002
Abstract Two modi®ed electrodes were fabricated by deposition of thin ®lms of a-PbO2 and b-PbO2 on aluminum substrate electrodes. Both electrodes exhibit potentiometric response to the hydrogen ion concentration and, consequently, can be used as pH sensors. The potentiometric responses of the modi®ed electrodes were close to the theoretical slope of Nerustian response and comparable with glass electrodes. The electrochemical properties and stability of the constructed electrode were investigated. The possibility of the pH sensor for using in ¯ow injection analysis (FIA) was also studied, and the optimum conditions were obtained. # 2002 Published by Elsevier Science B.V. Keywords: pH sensor; Nerustian slope; Lead oxide; Solid ®lm; Modi®ed electrode; Al electrode
1. Introduction Measurement of acidity (pH) of an aqueous solution is very important for various applications. The classical method for this purpose is related to glass electrodes. Due to many disadvantages of the glass electrodes such as high-cost, high-resistance, temperature instability, large size, limited shape, mechanical fragility, etc., a numerous studies have devoted to the development of new pH sensors based on different material during the past two decades [1±5]. These studies are mainly related to two classes of materials named metal oxides [6±11] and conductive polymers [12,13]. However, among different materials, metal oxides have many advantages for this purpose including mechanical strength, stability, construction technology, etc. Metal oxides are very interesting semiconductors and the most interesting class of electrocatalysts [14,15]. The electrochemical properties of lead oxide are well known. It also has been used as a component of solid membrane for construction of pH sensors. The most of these works are related to use of powder of the metal oxide for the preparation of mixture membrane. In the present study, we wish to report possibility of the modi®ed electrode based on the direct formation of lead oxide on the electrode surface to
* Tel.: 98-21-204-2549; fax: 98-21-205-7621. E-mail address: [email protected] (A. Eftekhari).
0925-4005/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 3 2 1 - 0
form a thin ®lm of the electroactive ®lm. For this purpose, aluminum was used as a substrate electrode as it can improve growth and stability of the electroactive ®lm. More recently, we have shown the possibility of aluminum electrode as substrate electrode for the deposition of different electroactive ®lms such as transition metal hexacyanoferrates [16], conducting polymers [17], enzymes for biosensors application [18], etc. This is due to the well-known behavior of aluminum in aqueous media, named fast passivation, to form a passive layer on the substrate electrode surface. The formed layer is very thin and protects the electrode against corrosion. In addition, the generated surface has more suitable sites for the deposition of electroactive ®lms in comparison with bare substrate electrodes. 2. Experimental All chemicals were of analytical grade without further puri®cation. Solutions were prepared from the reagents and doubly distilled water. An aluminum rod was used as the substrate electrode. A thin ®lm of lead was deposited onto the substrate electrode by dipping the Al electrode into a solution of 0.1 M Pb(NO3)2 containing 0.1 M KCl as supporting electrolyte. Then the deposited metallic ®lm was transformed to a-PbO2 and b-PbO2 by its oxidation at alkaline and acidic solutions, respectively. A detail study of two different forms of lead oxide and their preparation can
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
be found in [19]. Potentiometric measurements were carried out using a Multimeter (Sanua, PC 200). All potential were referenced to a saturated calomel electrode (SCE). The temperature was kept constant during the experiments at 25 0:5 8C. 3. Results and discussion It is assumed that lead oxide can react with hydrogen ion according to the following reaction: PbO2 H e ! PbOOH
(1)
In this case, the Nernst equation at 25 8C can be written as: E E0
0:0592 log aPbO2 aPbOOH
0:0592 pH
(2)
Therefore, its pH sensitivity is 59.2 mV/pH. Fig. 1 shows the potentiometric response of both the a- and b-PbO2-modi®ed electrodes to the solution pH. The measurements were carried out by varying the pH of the buffer solution by drop-wise addition of potassium hydroxide or hydrochloric acid. As seen, linear range for both modi®ed electrodes are located between pH range 1.0 and 12.0. The calibration slopes for the a-PbO2-modi®ed electrode and b-PbO2-modi®ed electrode are 57.96 and 57.80 mV/pH, respectively. The correlation coef®cient for both electrodes was 0.9998, indicating a very linear pHpotential response. Moreover, both electrodes have pH sensitivity close to the theoretical slope. The modi®ed electrodes have fast potentiometric response to the solution pH in about a few seconds. After reaching the response potential, there is a tiny drift in the electrode potential in a long time. The relative standard deviation (R.S.D.) of the pH sensors, for seven measurements in pH 3, was 2.7%,
235
indicating good repeatability. Similar repeatability and R.S.D. can also be obtained from different measurements at different pHs. Due to simplicity of the modi®cation process (fabrication procedure), different prepared electrodes are similar. The obtained results from experimental studies of different modi®ed electrode constructed by the proposed procedure show that they have a similar response to pH, indicating good reproducibility. The stability of the pH sensors was investigated during the time of usage. The variation of the electrode responses in a relatively long period of usage is presented in Fig. 2. It indicates that the pH sensors have high stability during longterm usage. This long-term stability of the pH sensors is mainly related to some parameters such as electrochemical properties of metal oxides, deposition of the lead oxide on the substrate electrode, etc. As a part of our investigation, possibility of using the modi®ed electrodes for the measurement of pH during ¯ow injection analysis (FIA) was also investigated. To this aim, the optimization of the modi®ed electrodes as pH sensors for this purpose was studied. A carrier solution of low buffer capacity (pH 7) was utilized to avoid zone acidity sample change [20]. The in¯uence of the sample volume on the potentiometric responses of the pH sensors was investigated in the volume range from 10 to 500 ml at typical pH 3.0 (Fig. 3). It is presented that the potentiometric response of the pH sensor increases by increasing the sample volume. Then, it reaches its maximum values at the sample volume of 300 ml. Therefore, it can be concluded that the sample volume of 300 ml is the optimum value and it was used for further analysis. Effect of ¯ow rate on the potentiometric response of the pH sensors was also investigated to ®nd the optimum condition of the constructed electrode for FIA. Fig. 4 shows the typical ¯ow rate dependency of the electrode responses at pH 3.0. The curves shows maximum response at ¯ow rates
Fig. 1. Potentiometric response of the a-PbO2-modified electrode (&) and b-PbO2-modified electrode (*) to the pH.
236
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
Fig. 2. Long-term stability of the pH sensors based on a-PbO2 (&) and b-PbO2 (*).
about 4.0 ml min 1. Thus, ¯ow rate of 4.0 ml min 1 was selected as the optimum ¯ow rate for the system under investigated. Effect of some possible interferences such as Li, Na, Mg2, Ca2, NH4, NO3 and HCO3 , phosphate, citrate, etc., was examined on the electrode responses. The interfering effects were investigated by mixed solution method in hydrogen ion concentration of 1:0 10 7 (pH 7) in the presence of the 1:0 10 3 mol l 1 of the interferences. No interference was observed from the cations but anions make a tiny error on the potentiometric responses of the electrode, which are not noticeable. The obtained results
from examination of changes in the sensor response showed that relative error made by each interference at the concentration of 1 mM and pH 7 was less than 5%. At basic pHs, the alkali cations such as NH4 can made high interfering effect on the electrode responses. It also should be noted that similar to the electrochemical behavior of metal oxides, a negative error may causes in the sensor response in the presence of some anions and agents such as hydrogen peroxide. To complete the study of the introduced modi®ed electrodes and their possibility for using as pH sensor, they were also used for measurement of pH of some real samples. For this purpose, some soft drinks such as Coca-cola and Fanta
Fig. 3. Dependence of the injection sample volume on the potentiometric response of the a-PbO2-modified electrode (&) and b-PbO2-modified electrode (*).
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
237
Fig. 4. Dependence of the flow rate on the potentiometric response of the a-PbO2-modified electrode (&) and b-PbO2-modified electrode (*). For injection volume of 300 ml.
were selected as usual samples. As known, the main interfering effects in soft drinks are related to phosphate in Cocacola and citrates in Fanta. Similar to our preliminary study of interferences, phosphate and citrates do not make noticeable errors in the electrode response even in real samples. Consequently, the proposed pH sensors were successfully used for the measurement of pH in soft drinks. All the obtained results were checked by using a glass electrode. We also examined the sensor for the measurement of pH in fruit juices. The results were also satisfactory for different fruit juices. Due to the general purpose of this research on feasible study of the modi®ed electrodes, detailed studies of interfering effects are not listed here. However, it can be concluded that the sensors are not suitable for the measurement of pH in very complex samples. It is due to the ionexchange properties of lead oxide and different properties of biological compounds. For example, absorption of fat or protein in milk samples hinders the proposed mechanism of the modi®ed electrodes for the response to hydrogen ion concentration. This behavior is usual for pH sensors based on metal oxide-modi®ed electrodes [9]. The advantages of the modi®ed electrodes and novelty of the present study for the fabrication of pH sensors can be summarized as follows. Using aluminum substrate electrode for the deposition of the electroactive ®lms (lead oxide). During the modi®cation, a thin ®lm of passive oxide ®lm forms on the Al electrode due to natural properties of aluminum in aqueous media. The formation of this passive ®lm improves the deposition of lead oxide as stronger connection occurs between lead oxide and the passive ®lm of the Al electrode in comparison with that deposited on bare substrate electrode. In addition, this strong connection greatly improves the stability of the electroactive ®lm deposited on the electrode surface during long-term usage.
On the other hand, the mentioned procedure for the preparation of lead oxide-modi®ed electrode is very simple. In addition to the simplicity, this procedure can be used for the construction of microelectrodes. This is similar to those reported for Ag/AgCl-modi®ed electrode [21] and Ag/Ag2Smodi®ed electrode [22]. As the electrode is also suitable for FIA, it is very interesting to use pH sensors based on microelectrodes. It allows designing FIA systems in micro-scale. In addition, ease of preparation made the pH sensor suitable for specialized cases such as microsystems. Indeed, it is very useful as can be easily added to a designed system. On the other hand, good analytical performance of this pH sensor makes it a desirable candidate to substitute with sensitive ISFET-based pH sensors, where a simple sensor is needed. 4. Conclusion Two modi®ed electrodes were fabricated based on deposition of two different types of lead oxides named a-PbO2 and b-PbO2 on aluminum substrate electrode. Using Al substrate electrode greatly improves the stability of the modi®ed electrodes. It makes them suitable for long-term usage. Both modi®ed electrodes exhibit potentiometric responses to pH from 1.0 to 12.0. The constructed electrodes have many preferences in comparison with glass electrodes. The fabricated modi®ed electrode can be used as pH sensors with some advantages such as simplicity, high-stability, fast response, low potential drift, repeatability, reproducibility, etc. In addition, possibility of the modi®ed electrodes was examined for ¯ow injection analysis (FIA), which the estimated optimum conditions for FIA, shows possibility of the fabricated sensor for this purpose. It was accompanied
238
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
with possibility of using the sensors for measurements of pH in real samples. The simplicity of the proposed procedure is also suitable for preparation of microelectrodes. References [1] L.D. Burke, J.K. Mulcahy, D.P. Whelan, Preparation of an oxidized iridium electrode and the variation of its potential with pH, J. Electroanal. Chem. 163 (1984) 117±128. [2] G. Cheek, C.P. Wales, R.J. Nowak, pH response of platinum and vitreous carbon electrode modified by electropolymerized films, Anal. Chem. 55 (1983) 380. [3] M.J. Natan, M.S. Wrighton, Progress in Inorganic Chemistry, vol. 37, Wiley, New York, 1989, p. 391. [4] F. Tedjar, L. Zerroual, All solid pH sensor, Sens. Actuators B 2 (1990) 215. [5] L. Zerroual, L. Telli, Application of a proton-conducting electrolyte for a pH sensor, Sens. Actuators B 24/25 (1995) 741±743. [6] K. Pasztor, A. Sekiguchi, N. Shimo, H. Masuhara, Iridium oxidebased microelectrochemical transistors for pH sensing, Sens. Actuators B 12 (1993) 225±230. [7] R. Koncki, M. Mascini, Screen-printed ruthenium dioxide electrodes for pH measurements, Anal. Chim. Acta 351 (1997) 143±149. [8] A. Fog, R.P. Buck, Electronic semiconductive oxides as pH sensors, Sens. Actuators 5 (1984) 137. [9] L. Qingwen, W. Yiming, L. Guoan, pH-response of nanosized MnO2 prepared with solid state reaction route at room temperature, Sens. Actuators B 59 (1999) 42±47. [10] J.A. Mihell, J.K. Atkinson, Planar thick-film pH electrodes based on ruthenium dioxide hydrate, Sens. Actuators B 48 (1998) 505±511. [11] L. Telli, B. Brahimi, A. Hammouche, Study of a pH sensor with MnO2 and montmorillonite-based solid-state internal reference, Solid State Ionics 128 (2000) 255±259.
[12] F. Yue, T.S. Ngin, Novel paper pH sensor based on polypyrrole, Sens. Actuators B 32 (1998) 33±39. [13] T. Afshad, Conducting polymer based pH detector: a new out-look to pH sensing technology, Polymer 38 (1997) 1145±1150. [14] S. Trasatti (Ed.), Electrodes of Conductive Metal Oxides, Parts A/B, Elsevier, Amsterdam, 1980/1981. [15] S. Trasatti (Ed.), Progress in Electrocatalysis: Theory and Practice (special issue), Electrochim. Acta. 39 (11/12) (1994). [16] A. Eftekhari, Aluminum electrode modified with manganese hexacyanoferrate as a chemical sensor for hydrogen peroxide, Talanta 55 (2001) 395±402. [17] A. Eftekhari, Glycerol biosensor based on glycerol dehydrogenase incorporated into polyaniline modified aluminum electrode using hexacyanoferrate as mediator, Sens. Actuators B 80 (2001) 283±289. [18] A. Eftekhari, Aluminum as a suitable substrate for the deposition of conducting polymers: application to polyaniline and enzymemodified electrode, Synth. Met. 125 (2001) 295±300. [19] E. Voss, J. Freundlich, in: D.H. Collins (Ed.), Batteries, Pergamon Press, Oxford, 1963, p. 73. [20] H. Hongbo, E.H. Hansen, J. Ruzicka, Anal. Chim. Acta 169 (1985) 209. [21] N.J. Gray, P.R. Unwin, Simple procedure for the fabrication of silver/ silver chloride potentiometric electrodes with micrometer and smaller dimensions: application to scanning electrochemical microscopy, Analyst 125 (2000) 889±893. [22] A. Eftekhari, Chemical sensor based on silver/silver sulfide microelectrode, Anal. Lett. 34 (2001) 1087±1095.
Biography Ali Eftekhari, PhD (Electrochemistry), the President of Electrochemical Research Center, is a researcher in various fields of electrochemistry. As a part of his activities in electroanalytical chemistry subjects, he is currently working on chemically-modified electrodes based on thin solid film for sensor applications.
pH sensor based on deposited ®lm of lead oxide on aluminum substrate electrode Ali Eftekhari* Department of Chemistry, KN Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran Received 30 May 2002; received in revised form 25 July 2002; accepted 1 September 2002
Abstract Two modi®ed electrodes were fabricated by deposition of thin ®lms of a-PbO2 and b-PbO2 on aluminum substrate electrodes. Both electrodes exhibit potentiometric response to the hydrogen ion concentration and, consequently, can be used as pH sensors. The potentiometric responses of the modi®ed electrodes were close to the theoretical slope of Nerustian response and comparable with glass electrodes. The electrochemical properties and stability of the constructed electrode were investigated. The possibility of the pH sensor for using in ¯ow injection analysis (FIA) was also studied, and the optimum conditions were obtained. # 2002 Published by Elsevier Science B.V. Keywords: pH sensor; Nerustian slope; Lead oxide; Solid ®lm; Modi®ed electrode; Al electrode
1. Introduction Measurement of acidity (pH) of an aqueous solution is very important for various applications. The classical method for this purpose is related to glass electrodes. Due to many disadvantages of the glass electrodes such as high-cost, high-resistance, temperature instability, large size, limited shape, mechanical fragility, etc., a numerous studies have devoted to the development of new pH sensors based on different material during the past two decades [1±5]. These studies are mainly related to two classes of materials named metal oxides [6±11] and conductive polymers [12,13]. However, among different materials, metal oxides have many advantages for this purpose including mechanical strength, stability, construction technology, etc. Metal oxides are very interesting semiconductors and the most interesting class of electrocatalysts [14,15]. The electrochemical properties of lead oxide are well known. It also has been used as a component of solid membrane for construction of pH sensors. The most of these works are related to use of powder of the metal oxide for the preparation of mixture membrane. In the present study, we wish to report possibility of the modi®ed electrode based on the direct formation of lead oxide on the electrode surface to
* Tel.: 98-21-204-2549; fax: 98-21-205-7621. E-mail address: [email protected] (A. Eftekhari).
0925-4005/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 3 2 1 - 0
form a thin ®lm of the electroactive ®lm. For this purpose, aluminum was used as a substrate electrode as it can improve growth and stability of the electroactive ®lm. More recently, we have shown the possibility of aluminum electrode as substrate electrode for the deposition of different electroactive ®lms such as transition metal hexacyanoferrates [16], conducting polymers [17], enzymes for biosensors application [18], etc. This is due to the well-known behavior of aluminum in aqueous media, named fast passivation, to form a passive layer on the substrate electrode surface. The formed layer is very thin and protects the electrode against corrosion. In addition, the generated surface has more suitable sites for the deposition of electroactive ®lms in comparison with bare substrate electrodes. 2. Experimental All chemicals were of analytical grade without further puri®cation. Solutions were prepared from the reagents and doubly distilled water. An aluminum rod was used as the substrate electrode. A thin ®lm of lead was deposited onto the substrate electrode by dipping the Al electrode into a solution of 0.1 M Pb(NO3)2 containing 0.1 M KCl as supporting electrolyte. Then the deposited metallic ®lm was transformed to a-PbO2 and b-PbO2 by its oxidation at alkaline and acidic solutions, respectively. A detail study of two different forms of lead oxide and their preparation can
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
be found in [19]. Potentiometric measurements were carried out using a Multimeter (Sanua, PC 200). All potential were referenced to a saturated calomel electrode (SCE). The temperature was kept constant during the experiments at 25 0:5 8C. 3. Results and discussion It is assumed that lead oxide can react with hydrogen ion according to the following reaction: PbO2 H e ! PbOOH
(1)
In this case, the Nernst equation at 25 8C can be written as: E E0
0:0592 log aPbO2 aPbOOH
0:0592 pH
(2)
Therefore, its pH sensitivity is 59.2 mV/pH. Fig. 1 shows the potentiometric response of both the a- and b-PbO2-modi®ed electrodes to the solution pH. The measurements were carried out by varying the pH of the buffer solution by drop-wise addition of potassium hydroxide or hydrochloric acid. As seen, linear range for both modi®ed electrodes are located between pH range 1.0 and 12.0. The calibration slopes for the a-PbO2-modi®ed electrode and b-PbO2-modi®ed electrode are 57.96 and 57.80 mV/pH, respectively. The correlation coef®cient for both electrodes was 0.9998, indicating a very linear pHpotential response. Moreover, both electrodes have pH sensitivity close to the theoretical slope. The modi®ed electrodes have fast potentiometric response to the solution pH in about a few seconds. After reaching the response potential, there is a tiny drift in the electrode potential in a long time. The relative standard deviation (R.S.D.) of the pH sensors, for seven measurements in pH 3, was 2.7%,
235
indicating good repeatability. Similar repeatability and R.S.D. can also be obtained from different measurements at different pHs. Due to simplicity of the modi®cation process (fabrication procedure), different prepared electrodes are similar. The obtained results from experimental studies of different modi®ed electrode constructed by the proposed procedure show that they have a similar response to pH, indicating good reproducibility. The stability of the pH sensors was investigated during the time of usage. The variation of the electrode responses in a relatively long period of usage is presented in Fig. 2. It indicates that the pH sensors have high stability during longterm usage. This long-term stability of the pH sensors is mainly related to some parameters such as electrochemical properties of metal oxides, deposition of the lead oxide on the substrate electrode, etc. As a part of our investigation, possibility of using the modi®ed electrodes for the measurement of pH during ¯ow injection analysis (FIA) was also investigated. To this aim, the optimization of the modi®ed electrodes as pH sensors for this purpose was studied. A carrier solution of low buffer capacity (pH 7) was utilized to avoid zone acidity sample change [20]. The in¯uence of the sample volume on the potentiometric responses of the pH sensors was investigated in the volume range from 10 to 500 ml at typical pH 3.0 (Fig. 3). It is presented that the potentiometric response of the pH sensor increases by increasing the sample volume. Then, it reaches its maximum values at the sample volume of 300 ml. Therefore, it can be concluded that the sample volume of 300 ml is the optimum value and it was used for further analysis. Effect of ¯ow rate on the potentiometric response of the pH sensors was also investigated to ®nd the optimum condition of the constructed electrode for FIA. Fig. 4 shows the typical ¯ow rate dependency of the electrode responses at pH 3.0. The curves shows maximum response at ¯ow rates
Fig. 1. Potentiometric response of the a-PbO2-modified electrode (&) and b-PbO2-modified electrode (*) to the pH.
236
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
Fig. 2. Long-term stability of the pH sensors based on a-PbO2 (&) and b-PbO2 (*).
about 4.0 ml min 1. Thus, ¯ow rate of 4.0 ml min 1 was selected as the optimum ¯ow rate for the system under investigated. Effect of some possible interferences such as Li, Na, Mg2, Ca2, NH4, NO3 and HCO3 , phosphate, citrate, etc., was examined on the electrode responses. The interfering effects were investigated by mixed solution method in hydrogen ion concentration of 1:0 10 7 (pH 7) in the presence of the 1:0 10 3 mol l 1 of the interferences. No interference was observed from the cations but anions make a tiny error on the potentiometric responses of the electrode, which are not noticeable. The obtained results
from examination of changes in the sensor response showed that relative error made by each interference at the concentration of 1 mM and pH 7 was less than 5%. At basic pHs, the alkali cations such as NH4 can made high interfering effect on the electrode responses. It also should be noted that similar to the electrochemical behavior of metal oxides, a negative error may causes in the sensor response in the presence of some anions and agents such as hydrogen peroxide. To complete the study of the introduced modi®ed electrodes and their possibility for using as pH sensor, they were also used for measurement of pH of some real samples. For this purpose, some soft drinks such as Coca-cola and Fanta
Fig. 3. Dependence of the injection sample volume on the potentiometric response of the a-PbO2-modified electrode (&) and b-PbO2-modified electrode (*).
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
237
Fig. 4. Dependence of the flow rate on the potentiometric response of the a-PbO2-modified electrode (&) and b-PbO2-modified electrode (*). For injection volume of 300 ml.
were selected as usual samples. As known, the main interfering effects in soft drinks are related to phosphate in Cocacola and citrates in Fanta. Similar to our preliminary study of interferences, phosphate and citrates do not make noticeable errors in the electrode response even in real samples. Consequently, the proposed pH sensors were successfully used for the measurement of pH in soft drinks. All the obtained results were checked by using a glass electrode. We also examined the sensor for the measurement of pH in fruit juices. The results were also satisfactory for different fruit juices. Due to the general purpose of this research on feasible study of the modi®ed electrodes, detailed studies of interfering effects are not listed here. However, it can be concluded that the sensors are not suitable for the measurement of pH in very complex samples. It is due to the ionexchange properties of lead oxide and different properties of biological compounds. For example, absorption of fat or protein in milk samples hinders the proposed mechanism of the modi®ed electrodes for the response to hydrogen ion concentration. This behavior is usual for pH sensors based on metal oxide-modi®ed electrodes [9]. The advantages of the modi®ed electrodes and novelty of the present study for the fabrication of pH sensors can be summarized as follows. Using aluminum substrate electrode for the deposition of the electroactive ®lms (lead oxide). During the modi®cation, a thin ®lm of passive oxide ®lm forms on the Al electrode due to natural properties of aluminum in aqueous media. The formation of this passive ®lm improves the deposition of lead oxide as stronger connection occurs between lead oxide and the passive ®lm of the Al electrode in comparison with that deposited on bare substrate electrode. In addition, this strong connection greatly improves the stability of the electroactive ®lm deposited on the electrode surface during long-term usage.
On the other hand, the mentioned procedure for the preparation of lead oxide-modi®ed electrode is very simple. In addition to the simplicity, this procedure can be used for the construction of microelectrodes. This is similar to those reported for Ag/AgCl-modi®ed electrode [21] and Ag/Ag2Smodi®ed electrode [22]. As the electrode is also suitable for FIA, it is very interesting to use pH sensors based on microelectrodes. It allows designing FIA systems in micro-scale. In addition, ease of preparation made the pH sensor suitable for specialized cases such as microsystems. Indeed, it is very useful as can be easily added to a designed system. On the other hand, good analytical performance of this pH sensor makes it a desirable candidate to substitute with sensitive ISFET-based pH sensors, where a simple sensor is needed. 4. Conclusion Two modi®ed electrodes were fabricated based on deposition of two different types of lead oxides named a-PbO2 and b-PbO2 on aluminum substrate electrode. Using Al substrate electrode greatly improves the stability of the modi®ed electrodes. It makes them suitable for long-term usage. Both modi®ed electrodes exhibit potentiometric responses to pH from 1.0 to 12.0. The constructed electrodes have many preferences in comparison with glass electrodes. The fabricated modi®ed electrode can be used as pH sensors with some advantages such as simplicity, high-stability, fast response, low potential drift, repeatability, reproducibility, etc. In addition, possibility of the modi®ed electrodes was examined for ¯ow injection analysis (FIA), which the estimated optimum conditions for FIA, shows possibility of the fabricated sensor for this purpose. It was accompanied
238
A. Eftekhari / Sensors and Actuators B 88 (2003) 234±238
with possibility of using the sensors for measurements of pH in real samples. The simplicity of the proposed procedure is also suitable for preparation of microelectrodes. References [1] L.D. Burke, J.K. Mulcahy, D.P. Whelan, Preparation of an oxidized iridium electrode and the variation of its potential with pH, J. Electroanal. Chem. 163 (1984) 117±128. [2] G. Cheek, C.P. Wales, R.J. Nowak, pH response of platinum and vitreous carbon electrode modified by electropolymerized films, Anal. Chem. 55 (1983) 380. [3] M.J. Natan, M.S. Wrighton, Progress in Inorganic Chemistry, vol. 37, Wiley, New York, 1989, p. 391. [4] F. Tedjar, L. Zerroual, All solid pH sensor, Sens. Actuators B 2 (1990) 215. [5] L. Zerroual, L. Telli, Application of a proton-conducting electrolyte for a pH sensor, Sens. Actuators B 24/25 (1995) 741±743. [6] K. Pasztor, A. Sekiguchi, N. Shimo, H. Masuhara, Iridium oxidebased microelectrochemical transistors for pH sensing, Sens. Actuators B 12 (1993) 225±230. [7] R. Koncki, M. Mascini, Screen-printed ruthenium dioxide electrodes for pH measurements, Anal. Chim. Acta 351 (1997) 143±149. [8] A. Fog, R.P. Buck, Electronic semiconductive oxides as pH sensors, Sens. Actuators 5 (1984) 137. [9] L. Qingwen, W. Yiming, L. Guoan, pH-response of nanosized MnO2 prepared with solid state reaction route at room temperature, Sens. Actuators B 59 (1999) 42±47. [10] J.A. Mihell, J.K. Atkinson, Planar thick-film pH electrodes based on ruthenium dioxide hydrate, Sens. Actuators B 48 (1998) 505±511. [11] L. Telli, B. Brahimi, A. Hammouche, Study of a pH sensor with MnO2 and montmorillonite-based solid-state internal reference, Solid State Ionics 128 (2000) 255±259.
[12] F. Yue, T.S. Ngin, Novel paper pH sensor based on polypyrrole, Sens. Actuators B 32 (1998) 33±39. [13] T. Afshad, Conducting polymer based pH detector: a new out-look to pH sensing technology, Polymer 38 (1997) 1145±1150. [14] S. Trasatti (Ed.), Electrodes of Conductive Metal Oxides, Parts A/B, Elsevier, Amsterdam, 1980/1981. [15] S. Trasatti (Ed.), Progress in Electrocatalysis: Theory and Practice (special issue), Electrochim. Acta. 39 (11/12) (1994). [16] A. Eftekhari, Aluminum electrode modified with manganese hexacyanoferrate as a chemical sensor for hydrogen peroxide, Talanta 55 (2001) 395±402. [17] A. Eftekhari, Glycerol biosensor based on glycerol dehydrogenase incorporated into polyaniline modified aluminum electrode using hexacyanoferrate as mediator, Sens. Actuators B 80 (2001) 283±289. [18] A. Eftekhari, Aluminum as a suitable substrate for the deposition of conducting polymers: application to polyaniline and enzymemodified electrode, Synth. Met. 125 (2001) 295±300. [19] E. Voss, J. Freundlich, in: D.H. Collins (Ed.), Batteries, Pergamon Press, Oxford, 1963, p. 73. [20] H. Hongbo, E.H. Hansen, J. Ruzicka, Anal. Chim. Acta 169 (1985) 209. [21] N.J. Gray, P.R. Unwin, Simple procedure for the fabrication of silver/ silver chloride potentiometric electrodes with micrometer and smaller dimensions: application to scanning electrochemical microscopy, Analyst 125 (2000) 889±893. [22] A. Eftekhari, Chemical sensor based on silver/silver sulfide microelectrode, Anal. Lett. 34 (2001) 1087±1095.
Biography Ali Eftekhari, PhD (Electrochemistry), the President of Electrochemical Research Center, is a researcher in various fields of electrochemistry. As a part of his activities in electroanalytical chemistry subjects, he is currently working on chemically-modified electrodes based on thin solid film for sensor applications.