Potentiometric RuO2–Ta2O5 pH sensors fabricated using thick film and LTCC technologies
Graphical abstract
Introduction
Miniaturized, highly sensitive and accurate electrochemical pH sensors which contain both sensitive and reference electrodes in a single platform have become of great importance due to their remarkable predicted application scope in the next generation of the electrochemical sensors. For the fabrication of miniaturized analytical devices, LTCC (low temperature cofired ceramics) technology is offering numerous advantages, including integration of sensing layer and electronic circuits for the wireless sensor part in one module [1], [2]. LTCC technology enables development of the complex three dimensional structure of a device. The electrochemical solid state pH sensors made in microsystem technology are especially useful for the application in wireless sensor networks. Such sensors have been already successfully used in water pollution monitoring and remote sensing applications [3]. Furthermore, solid state pH sensors have been recognized for several decades for bio-medical-chemical applications, food processing, agricultural and industrial applications, etc. [4], [5], [6].
RuO2 is a versatile and attractive solid material for developing electrochemical sensors, biosensors and supercapacitors [3], [7], [8], [9], [10]. RuO2 based pH sensors have been found to exhibit outstanding sensing performance over wide pH ranges. Their significant advantages over the traditional glass pH electrode and other metal oxide based pH electrodes include high sensitivity and stability, long lifetime, very fast response, small hysteresis effect, low sensitivity to interferences caused by different ions and very good repeatability [3], [8], [11]. In our previous papers, we reported a thick film potentiometric pH sensor based on RuO2 with a Nernstian response close to the theoretical value [12] and impedance spectroscopic analysis of RuO2 based pH conductimetric sensor [13]. However, since RuO2 is an expensive component, for reducing the cost, as well as to enhance the stability and sensitivity, RuO2 as the pH sensing electrode material can be mixed with other oxides [11], [14], [15]. Recently, there are several studies reporting on the development of low cost pH sensors by using binary metal oxides. The advantage of such sensors is low cost, stability, long lifetime and good electrochemical activity. For the sensing electrodes, the RuO2- TiO2 system is the most frequently used one [11], [15], [16].
As a pH sensitive material, Ta2O5 has received significant attention for the fabrication of thin film pH sensors, especially of ISFET (Ion Sensitive Field Effect Transistor) type [17], and EIOS (electrolyte-ion sensitive membrane-oxide-semiconductor) type [18]. In our previous work, we developed a thick film Ta2O5 based pH sensor which exhibited a sub-Nernstian potentiometric response [19].
So far, major studies devoted to RuO2–Ta2O5 system were focused on supercapacitors [20] and DSA-type electrodes (Dimensionally Stable Anodes) [21]. It was found that the introduction of Ta2O5 which is a proton conducting solid electrolyte enhances the rate of proton intercalation at the surface of RuO2 grains [20]. DSA electrodes are prepared in the form of thin film coatings on Ti-substrate and employed as electrochemical devices for oxidation of organic compounds, in chlorine and oxygen production [21], [22]. For this application, good electrochemical properties, high corrosion resistance, and long lifetime of RuO2–Ta2O5 thin films on Ti substrate were reported by Ribeiro et al. [22], [23].
This article presents the studies of thick film potentiometric pH sensors based on RuO2–Ta2O5 binary composition. The nanostructured and porous morphology of the applied thick film RuO2–Ta2O5 electrode provides a high surface to volume ratio and hence large number of adsorption centers on the surface of the sensitive film. This improves sensing performance, contributing to high sensitivity, fast response and low power consumption [24], [25]. The structural properties and composition of the films were characterized by using X-ray diffractometry (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX). In this work, the use of the binary RuO2–Ta2O5 composition and screen printing method allowed for fabrication of a thick film pH sensor characterized by low cost, ease of miniaturization and excellent sensing performance. Furthermore, a potentiometric sensor for wireless application was fabricated using LTCC technology and its sensing performance was discussed. The inbuilt reference and sensitive electrodes and the possibility of integration of passive components for the electrical circuit of the wireless system are significant advantages of the LTCC pH sensor. To the best of our knowledge, this article reports for the first time the application of RuO2-Ta2O5 as the sensing material in potentiometric thick film pH sensors.
Section snippets
Experimental
Two binary RuO2–Ta2O5 mixtures were prepared by ball milling. An active oxide RuO2 (99.9%, Aldrich) was mixed with Ta2O5 (99.8%, Aldrich) in a suitable ratio to obtain RuO2–Ta2O5 (70:30) wt% and RuO2–Ta2O5 (30:70) wt% compositions destined for sensitive electrodes of thick film pH sensors. After weighing, the starting materials were wet-ball milled in isopropyl alcohol for 5 h in a planetary ball mill (Fritsch Pulverisette 5, Germany) and dried at 70 °C. As a result of ball milling, fine powders
Microstructural analysis
The XRD pattern of RuO2–Ta2O5 (70:30%) thick film sintered at 850 °C on an alumina substrate is presented in Fig. 3a. The observed distinct diffraction peaks can be assigned to the rutile structure of RuO2 and to the orthorhombic structure of Ta2O5 [23] and no additional phases were found. The intensities of RuO2 peaks are higher than those of Ta2O5, due to higher amount of RuO2 in the composition. The average crystallite sizes calculated from the major peaks width on the basis of Scherrer
Conclusions
In this work, new RuO2–Ta2O5 based thick film pastes were prepared and applied for pH sensing electrodes. Potentiometric pH sensors were fabricated on alumina substrates by thick film technology. Furthermore, LTCC technology was successfully utilized for development of a miniaturized potentiometric pH sensor for wireless solution monitoring. The XRD pattern reveals the binary phase composition of the film with nanometric crystal sizes of RuO2 and Ta2O5. Raman studies confirm the presence of RuO2
Acknowledgment
The authors gratefully acknowledged the European Commission in the framework of the FP 7 project SENSEIVER, Grant no. 289481 for financially supporting the work. The authors are grateful to Elvira Djurdjic, Department of Physics, for Raman experiment at Faculty of Science, University of Novi Sad, Serbia.
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