Surface renewable nano-iridium oxide polymeric composite pH electrodes

Sensors and Actuators B 204 (2014) 197–202

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Surface renewable nano-iridium oxide polymeric composite pH electrodes Jongman Park ∗ , Moonhee Kim, Shinseon Kim Microanalytical system laboratory, Department of Chemistry, Konkuk University, 120 Neungdongro, Gwangjingu, Seoul 143-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 9 May 2014 Received in revised form 16 July 2014 Accepted 23 July 2014 Available online 1 August 2014 Keywords: pH electrode Iridium oxide Composite electrode

a b s t r a c t A simple but effective preparation method of a surface renewable nano-iridium oxide polymeric composite electrode showing highly improved pH response characteristics was developed. Nano-iridium oxide was incorporated in polymethylmethacrylate (PMMA) matrix as an electrical conductor and active sensing material for hydrogen ion. It was prepared by the hydrolysis of (NH4 )2 IrCl6 and were then simply dispersed in acetone containing PMMA. The suspension of the nano-iridium oxide and PMMA was precipitated rapidly into water and then pressure-molded to disk-type nano-iridium oxide composite electrode material. The electrode showed excellent linear response between pH 3 and 11 with a slope of −59.6 mV/pH (R = 0.9999) and fast response time of  90 < 2 s. Steady response with an average slope of −58.9 ± 0.3 mV/pH and Eo  intercept of 653.4 ± 2.9 mV vs. 3 M Ag/AgCl reference electrode was observed for a month without appreciable deactivation. Less porous and hydrophobic surface of the polymeric composite electrode minimized equilibration time resulting in the fast response. The electrode can be stored in deionized water or in air after uses. The lifetime of the electrode was extended greatly because the surface can be renewed reproducibly by simple polishing whenever contaminated or deactivated. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Despite the distinctive performance of the glass membranebased pH electrode, it has drawbacks of fragility, difficulties in maintenance, and easy fouling in harsh environmental measurements. The polymeric membrane-based hydrogen ion-selective electrodes are not fragile but can tear easily. In order to improve the ruggedness of the pH electrodes, metal oxides such as TiO2 , RuO2 , RhO2 , SnO2 , Ta2 O5 , OsO2 , PtO2 , or IrO2 have been used as hydrogen ion sensing materials [1–21]. Among these oxides, iridium oxide has been used widely because of its excellent response to the hydrogen ion concentration. The electrodes have been prepared by thermal decomposition, sputtering or electrochemical deposition in the form of oxide films on the surface of their mother metals or conducting inert metals. They are mostly inert and rugged in harsh applications, and easy to be miniaturized. However, serious drift of the electrode potential due to the pores or cracks formed during decomposition and the lack of surface uniformity of the oxide films have been major drawbacks [1,9,13,20]. Such problems were overcome by a thick film IrO2 -based pH electrode formed on the iridium wire by high temperature carbonate-melt

∗ Corresponding author. Tel.: +82 2 450 3438; fax: +82 2 3436 5382. E-mail address: [email protected] (J. Park). http://dx.doi.org/10.1016/j.snb.2014.07.104 0925-4005/© 2014 Elsevier B.V. All rights reserved.

oxidation technique [13]. Excellent pH response characteristics and a durability of 2.5 years were achieved. Such improved characteristics have been attributed to the uniform and dense iridium oxide film on the iridium wire. Nevertheless, surface renewable pH electrodes are still desired for harsh environmental applications such as food processing, soil analysis, sludge analysis, or on-line monitoring systems where antifouling and deactivation of the sensing elements are critical. Recently we have reported two types of surface renewable iridium oxide-based composite pH electrodes. The electrode matrix was either a polymeric carbon composite or a glass composite [20,21]. Both electrodes contain micro-fine IrO2 particles throughout the electrode matrix. Therefore, the active surface could be regenerated reproducibly (0.7% in relative standard deviation, RSD) by simple grinding with 2000 grit SiC emery paper whenever it was deactivated or fouled. The electrodes could be stored in deionized water or even in air. The IrO2 -modified carbon polymeric composite pH electrodes showed about −55.7 mV/pH response with fast response time ( 90 = 2 s), while the IrO2 –glass composite electrode showed −58.6 mV/pH but slower response time ( 90 = 7 s). The carbon black in the polymeric composite electrode matrix was added as an electrical conductor to support the electrical conductivity of the electrode. The exposure of the carbon black to the test solution could not be ignored. Any substantial existence of electroactive species might generate a potential and would affect

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the measured electrode potential. We have found that the electrical conduction by the iridium oxide itself was enough for the potentiometric pH measurements through the IrO2 –glass composite electrode work. A better pH response close to the theoretical value (−59.2 mV/pH) was obtained in the glass–based IrO2 composite electrode work by omitting the carbon black. However, the response time characteristics became worse because of the porosity of the IrO2 –glass composite electrode prepared by high temperature sintering at ambient pressure. More time was required for the surface equilibration until the electrode response stabilized. Large drift of the electrode potential was also observed in prolonged measurements. From the results of both of our previous reports [20,21] and the thick film IrO2 electrode [13] we have reached the conclusion that the porosity or defects on the electrode surface have a critical effect on the potentiometric equilibration. Further, the pH measurement based on the redox reaction of Ir(III)/Ir(IV) couple can be performed without the aid of another conductor such as carbon black or metals. Since the uniformity of the surface with less porosity is a critical requirement for fast potentiometric equilibration resulting in a stabilized response in a short time, we selected polymeric matrix for the compactness of the electrode matrix and nano-iridium oxide particles for better dispersion of the sensing material. Random dispersion of the nano-iridium oxide particles in the polymer solution ensured better performance of the nano-IrO2 polymeric composite pH electrodes with very short response time and excellent response slope close to the theoretical value of −59.2 mV/pH. The polymeric composite electrode material can be pressure-molded easily at moderate temperatures in various sizes and shapes according to system requirements. The electrode is surface renewable, allowing extended lifetime, and can be stored in air conveniently without losing its activity. Here, we report the preparation method and characteristics of the nano-IrO2 polymeric composite pH electrodes. 2. Materials and methods 2.1. Reagents and instruments All chemicals were reagent grade and used as received unless otherwise mentioned. Ammonium hexachloroiridate(IV) was obtained from Alfa Aesar. Polymethylmethacrylate (PMMA, MW 120,000) was from Sigma–Aldrich. Aqueous solutions were prepared with deionized water (18 M). In most cases, commercial pH buffer solutions (pH 4, 7, and 10) were used for the calibration of the pH meter. For stepwise pH measurement, the pH of 0.01 M H3 PO4 –H3 BO3 –CH3 COOH–KCl universal buffer solution was adjusted appropriately by adding 0.1 M NaOH or 0.1 M HCl. A combination pH glass membrane electrode (Accumet 13-670287) with an Accumet 50 pH meter (Fisher Scientific) was used for pH monitoring. Electrode potential was measured vs. Ag/AgCl reference electrode (3 M KCl) at 25 ◦ C. The iridium oxide content in the composite electrode material was analyzed using a thermal gravimetric analyzer (TGA 7, Perkin Elmer) at 600 ◦ C under air atmosphere. A transmission electron microscope (TEM, Tecnai G2 20 s from FEI) was used for particle size observation of the iridium oxide nanoparticles. 2.2. Electrode preparation Nano-iridium oxide (possibly IrO2 ·xH2 O) was prepared by the hydrolysis of hexachloroiridate at elevated temperature [22]. Typically 0.40 g of (NH4 )2 IrCl6 was dissolved in 120 mL of deionized water. The pH of the resulting reddish brown solution was adjusted to about 8.2 with 0.25 M NaOH. This was followed by heating to

95 ◦ C with constant stirring for 1 h under reflux setting in an oil bath. The solution turned into a deep blue suspension. It was cooled down to room temperature, and then, the pH was adjusted again to pH 8.2 with NaOH solution. The process of pH adjustment and heating was repeated until the pH stabilized to near pH 8.2 for the completion of the reaction. The nano-iridium oxide was collected and washed with deionized water thoroughly using a centrifuge. The washing solvent was changed to acetone finally. After dispersing the nanoiridium oxide particles in 7 mL acetone thoroughly 0.21 g of PMMA powder for 37 wt% IrO2 electrode was added and dissolved completely. The syrupy mixture was injected into a large amount of deionized water using a syringe with vigorous to extract the acetone out of the nano-IrO2 /PMMA mixture. After washing the solid composite mixture of nano-IrO2 /PMMA thoroughly with deionized water it was dried at 60 ◦ C overnight and was then ground well using a ceramic mortar and pestle. The mixtures were analyzed using the TGA under air stream at 600 ◦ C to estimate the actual iridium oxide contents in the composite materials. Because of the loss in the synthesis and mixture preparation ∼10% of discrepancy in iridium oxide contents between the calculated and analyzed values was found. The analyzed iridium oxide contents are reported in this work. The mixture was pressure molded at 8 ton/cm2 and 150 ◦ C into nano-IrO2 polymeric composite electrode materials (3 mm in length, 4 mm in diameter). They were fabricated into rod-type electrodes in a plastic housing with silver epoxy connection to copper lead wire. The surface of the electrode was ground using 2000 grit SiC emery paper and then polished using 0.5 ␮m alumina. The electrodes were soaked in deionized water for a day prior to use. The electrodes were stored in deionized water between daily tests.

3. Results and discussion 3.1. General characteristics of the electrodes The surface of the polished composite electrode was smooth and shiny with less defects compared to that of the IrO2 /glass composite electrodes or IrO2 /C-black polymeric composite electrodes as shown in Fig. 1A [20,21]. The electrical resistance was higher than 40 M, so it was not possible to measure it with an ordinary digital multimeter. Random noise was observed on the raw potential signals sampled in every second due to the high resistance of the composite electrode material. Therefore, ten consecutive signals were averaged manually for the estimation of the electrode potentials and response slopes in this work. Such noise can be reduced if an instrument is used having data averaging function with higher sampling rate. Nevertheless, they showed potentiometric response depending on the hydrogen ion concentration if the iridium oxide content was between 30 and 40 wt%. Sometimes surface defects (Fig. 1B) were found on the electrodes, which showed low response slope or delayed stabilization of the potential. The defects were mainly due to the micro-lumps of the iridium oxide aggregates formed by incomplete dispersion in the mixture. Since a minimum equilibration layer on the defect-free electrode surface with random distribution of the iridium oxide nanoparticles was desired for quick equilibration to enable a fast and stable electrode response, we had deep concerns on the particle size distribution and dispersion of the nano-iridium oxide particles in the polymeric matrix. TEM image of the iridium oxide nanoparticles prepared in this work is shown in Fig. 1C. The nanoparticles were about 2 nm and quite uniform in size. Rapid injection of the syrupy suspension of the nanno-IrO2 /PMMA mixture in acetone to water was effective to solidify the mixture without losing good dispersion of the iridium oxide in the PMMA matrix. This composite mixture was analyzed using TGA for fine-tuning the iridium oxide content to get optimum response characteristics discussed below. High pressure

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Fig. 1. (A) Partial optical microscopic image of the electrode surface. a1 : Composite electrode surface, a2 : housing, (B) defect on the electrode surface (in dotted circle), (C) TEM image of iridium oxide nanoparticles. Sonicated iridium nanoparticles in acetone were spotted on a stage, and then dried.

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thermosetting of the composite electrode at 8 ton/cm2 , 150 ◦ C was also effective to prepare compact polymeric composite electrode materials with minimized voids. Good response slopes ranging from −57 to −60 mV/pH were obtained between 35 and 37 wt% of iridium oxide. However, detailed response characteristics were different as it shown in Fig. 2. The electrode potential with 40 wt% iridium oxide (curve A) drifted continuously for the stabilization, and did not stabilize in 10 min. It was thought that the active surface layer was too thick to be soaked completely and caused delayed equilibration.

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The electrode potential with 35 wt% of iridium oxide (curve B) drifted slower compared to that of 40 wt% iridium oxide but it did not stabilize in 10 min eventually. The electrode with 37 wt% of iridium oxide showed best response characteristics among them

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Fig. 3A shows pH response characteristics of the composite electrode with 37 wt% of iridium oxide between pH 3 and 11. The electrode potential was stabilized quickly at the pH below 9, but it became slow gradually at higher pH. The pH response slope of the nano-iridium polymeric composite pH electrode was very close to the theoretical value in the pH range of 3–11. It was −59.6 mV/pH with an Eo intercept of 662.7 mV (R = 0.9999). Only 1.7 mV of hysteresis was observed when the pH was changed abruptly from 11 to 3. Such linearity can be extended to pH 1 and 13, but the slope was slightly leveled off (−57.7 mM/pH, R = 0.9994). Moreover, the electrode response became slow severely at high pH near 13 (Fig. 2B). Once the electrode was exposed to such high pH solution, the electrode response became sluggish thereafter in alkaline solution. Large hysteresis between pH 1 and 13 was observed also. At this moment, such electrode behavior in strong alkaline solution is thought due to deepen penetration of hydroxide ions to the electrode matrix and gradual hydrolysis of the iridium oxide to its

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hydroxides. Detailed studies are on the way for better understanding of such observation. 3.3. Response time Fig. 4 shows the response time characteristics of the electrode. The pH of a universal buffer solution was monitored with abrupt pH changes from 3 to 6 and then back to 3 by adding a premeasured solution of 1.5 M NaOH or 1.5 M HCl with vigorous stirring. As it can be seen in the inset A and B of Fig. 3, the electrode potential was changed and stabilized in less than four seconds as soon as the base or acid was added. Considering the time required for homogenous mixing upon the addition of acid or base the response time ( 90 ) was less than 2 s. Less porous surface with random distribution of the iridium oxide nanoparticles in the PMMA matrix was beneficial for quick equilibration for fast response. The electrode surface was somewhat hydrophobic because of the PMMA matrix. Such hydrophobicity would limit continuous wetting of the active iridium oxide nanoparticles resulting in a thin equilibration layer for quick stabilization of the electrode potential. When the iridium oxide content was increased to 40 wt%, slower stabilization was observed possibly because more active surface needed to be wetted and equilibrated with the solution (not shown). The importance of the surface tightness was mentioned the work of the IrO2 film-based pH electrode formed by carbonate-melt oxidation technique [13]. Most of the IrO2 film-based pH electrodes were formed by thermal decomposition or sputtering techniques, in which formation of pores or cracks was not avoidable. Owing to the porous surface, more equilibration time was required upon sample changes. Severe drift of the signal during aging was another problem due to the same reason. Using the carbonate-melt oxidation technique, they were able to prepare tight and thick IrO2 film on an iridium wire substrate showing excellent pH response characteristics in the pH slope, response time, and low hysteresis as well as long-term stability. In this work, it was possible to show similar pH electrode characteristics with iridium oxide nanoparticles aided by the polymeric matrix as shown in Fig. 3. 3.4. Long term stability Fig. 5 shows the long-term stability of the electrode. The electrode response slope was checked daily for a month and is shown in Fig. 4A. The electrode was not deactivated appreciably during

Fig. 5. Long-term stability test. (A) pH response slope variation for a month and (B) response characteristics after the whole test.

the test period, but a slight decrease in the slope toward the average value of −58.9 ± 0.3 mV/pH with Eo intercept of 653.4 ± 2.9 mV vs. Ag/AgCl was observed during the initial week. Fig. 4B shows the electrode response behavior for 5 h after the daily response tests were run for a month. The electrode was not deactivated and still showed a reproducible response with an average slope of −58.8 ± 0.2 mV/pH and Eo intercept of 657.8 ± 1.3 mV vs. Ag/AgCl reference electrode in the sixteen measurements taken at a pH of 4.01, 7.00, and 10.00. However, a somewhat slower stabilization was observed at pH 10 as time elapsed. 3.5. Surface renewability The nano-IrO2 polymeric composite electrode has the additional advantage of surface regeneration. Since the active iridium oxide nanoparticles are dispersed and interconnected throughout the composite matrix, the electrode surface can be renewed easily by simple polishing or grinding whenever the surface is contaminated or deactivated. Therefore, the lifetime of the pH electrode can be extended greatly. Indeed reproducible electrode responses were observed with an average slope of −58.2 mV/pH (0.5% of RSD, n = 8) in the renewability test. In addition, the electrode could be stored in air without appreciable changes in the response if the electrode was soaked in deionized water for several hours before use. 3.6. Interferences The hydrogen ion sensing mechanism of the IrO2 -based pH electrodes is based on pH dependent redox reaction of Ir(III)/Ir(IV) rather than the boundary potential developed across the H+ selective membrane. Consequently IrO2 -based pH electrodes are relatively free from the interference of electrochemically inactive ions such as alkali or alkaline earth metal cations, halides, nitrates, and sulfates. Fig. 6 shows the interference characteristics of typical cations and anions on the electrode response. For the interference test, the concentration of each test ion was adjusted to 0.0100 M in universal buffer solutions. Then, the pH dependence was monitored by adding HNO3 or NaOH. As shown, the response slope for pH did not change significantly in most cases, except for the ferricyanide. Inactive ions do not affect the electrode response but electrochemically active ferricyanide altered the response slope greatly. The poisoned electrode was not recovered by washing but simple polishing. Similar results were obtained for the chlorite or hydrogen

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(AOAC official method 994.16) [23]. The potential measurements was repeated three cycles for each sample. A combination glass membrane pH electrode was immersed in same time for the comparison. As shown in Fig. 7 the electrode potentials became stable within 30 s for the most samples and the standard buffers except the samples having heterogeneous matrix such as the culture soil and blueberry grinder samples. Especially the blueberry grinder sample took more time for the stabilization of the potential and showed larger discrepancy (0.09 in pH) to the result with a glass membrane pH electrode compared to others (<0.04 in pH). Interestingly some drift of the electrode potentials was observed between the measurement cycles for the dairy product. However the pH response slope was not changed (∼60 mV/pH unit), and resulting pH value was reproducible within ±0.01 pH unit. It is not clear, but it is likely due to the nonspecific adsorption of the species like fatty acids or proteins on the electrode surface. Nevertheless all the pH values of the samples measured with the composite electrode were reproducible within ±0.01 pH unit, and not erratic compared to those of the glass membrane pH electrode (<0.1 pH unit discrepancy). 4. Conclusions

peroxide. Some potential shifts of the calibration curves are thought to occur mainly due to the ionic strength change of the solution. As known already, the utility of the IrO2 -based pH electrodes is limited by the existence of electrochemically active compounds [3,12,21]. However, the electrodes can be used without problems for various samples in the areas of biochemistry, physiology, food or soil chemistry, environmental chemistry, and so on. 3.7. Real sample analysis The possibility of the application of the nano-IrO2 composite pH electrode in real sample analysis was tested using some agricultural and food samples such as culture soil, Hyponex solution, whole milk, flavored yogurt, and blueberry grinder. For the calibration the electrode potentials in the standard buffer solutions of pH 4.00 and 7.01 were recorded for about 5 min, and then for the sample. The electrode with a Ag/AgCl reference electrode was immersed directly to the sample matrix except the culture soil sample. The culture soil sample was tested with an addition of distilled water in 1:1 ratio according to the recommended procedure

A surface renewable IrO2 -based polymeric composite pH electrode showing excellent pH response characteristics was developed successfully by dispersing iridium oxide nanoparticles in PMMA matrix. The composite electrode has simple composition and easy to be prepared. However, the pH response slope was close to the theoretical value of −59.2 mV/pH, and the response time,  90 was less than 2 s. Fast stabilization of the potential was attributed to the fast equilibration of the thin and compact active layer exposed on the surface. Its long-term stability and surface renewability will extend the lifetime and utility in harsh environmental applications. The possibility of atmospheric storage may allow minimum care of the pH electrode to the users, who are not familiar with the care of the glass membrane-based pH electrodes. Acknowledgement This work was financially supported by Konkuk University, Republic of Korea in 2010. References

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Biographies Jongman Park is a full professor of the department of chemistry at Konkuk University in Korea. His research interest focuses on the development of the electrochemical sensors and detectors based on metal oxides in composite matrix for various analytical applications. Recently he expanded his research span to capillary electrochromatographic microchips having unique microchannel packed with various colloidal spheres by self-assembly behavior. He received PhD degree in analytical chemistry from the University of Connecticut, USA in 1991. After finishing postdoctoral training at the University of New Mexico, USA he joined to Konkuk University in 1993. Moonhee Kim received both BS in chemistry and history at Konkuk University in 2011. She got MS degree in analytical chemistry at Konkuk University in 2014. She is a research scientist at COSMAX in Korea. Shinseon Kim received BS in chemistry at Ewha Womans University in 1983. She is a research scientist at the microanalytical system laboratory at Konkuk University since 2009.