Extended gate field effect transistor using V2O5 xerogel sensing membrane by sol–gel method
Graphical abstract
Introduction
Research and development of sensors are advancing rapidly because of the demands of a number of different areas such as clinical chemistry and industrial processes [1]. In view of the fact that there is a high demand for specific sensors, many different types have been developed, such as those based on field effect devices. Furthermore, the determination of pH values is one of the most important tasks in analytical chemistry [2]. A recent development in pH measurement was the introduction of the ion-sensitive field effect transistor (ISFET) technology as an alternative to the glass electrode [3], [4].
Ion-selective field effect transistors or ISFETs were first developed in the 1970s as an alternative to the fragile glass electrode in pH measurement and ion concentrations (Na+, K+, Cl−, NH4+, Ca2+, etc.) [5]. However, the ISFET has several disadvantages like device instability and low current sensitivity. Those disadvantages can be overcome by using the structure of the extended gate field effect transistor (EGFET) (Fig. 1). The flexible shape of the extended gate structure is another advantage of the EGFET whose better long-term stability is due to the fact that ions from the chemical environment are excluded from any region close to the FET gate insulator [6]. The sensitive layer structure of the EGFET is fabricated on the end of the signal line extended from the FET gate electrode [7]. Protonation/deprotonation of the gate material is influenced by the pH at the gate area, which controls the surface potential. According to the theory, the response of the sensor should reach the maximum value of 59.2 mV/pH, following a Nernstian behavior [8]. The response of an ion-selective electrode is given bywhere E is the measured potential (in volts), E0 is a characteristic constant for the ion-selective/external electrode system, R is the gas constant, T is the absolute temperature (K), z is the signed ionic charge, F is the Faraday constant, and [i] is the molar concentration of the free non-complexed ionic species [8]. The development and manufacture of sensitive, specific, miniature and cheap sensors will undoubtedly cause important changes in the nature and methods of information gathering with respect to objects and media in medicine, biotechnology, agriculture and environmental monitoring [9].
In addition, as in the case of ISFET sensors, there are several kinds of ion-sensing membranes that can be applied in the pH sensing dielectric layers of pH-EGFET, such as ruthenium oxide [10], carbon nanotube [11], SnO2 [12], [13], and ZnO2 [14]. Among the alternatives of oxide ion-sensing membranes used in pH sensors, there is the V2O5 thin film obtained by the sol–gel route. The electrical and optical properties of V2O5 thin film, [15], [16] make them suitable for a variety of applications such as catalyst, gas sensor, cathode for solid-state batteries, window for solar cells, not to mention that they can be used in electrochromic devices as well as in electronic and optical switches [17].
The EGFET consists of two parts as shown in Fig. 1. The sensitive part (on the left) is composed of a V2O5/Al, structure and the system is completed with a commercial MOSFET CD4007UB (on the right). This structure is easily constructed and has a lot of advantages compared to the ISFET since it does not require the fabrication of the MOSFET [12].
The aim of this work was to fabricate a pH-EGFET sensor based on a vanadium pentoxide xerogel obtained by a sol–gel route and to try to explore the interaction of the ion-sensitive membrane with charges in solution, to find out the sensor sensitivity, and to verify whether it is suitable as a pH sensor. In addition to the electrical response as pH sensor, the V2O5·nH2O films were also characterized by various techniques.
Section snippets
Experimental
The vanadium pentoxide gel, V2O5·nH2O, was prepared from sodium metavanadate (NaVO3, 99%, Fluka) by the ion-exchange method (ion-exchange resin Dowex-50X8), as described in the literature [18], [19]. A decavanadic acid was obtained by percolating 0.10 M of a NaVO3 aqueous solution through a cationic ion-exchange resin. Upon standing at room temperature (24 °C) for 2 weeks, the fresh HVO3 solution was polymerized, leading to a viscous red V2O5 gel. The V2O5 gel was dropped on a glassy carbon
Results and discussion
Fig. 2 shows the XRD pattern for the vanadium pentoxide xerogel. The most intense diffraction peak (00l) is characteristic of the one-dimensional stacking of the vanadium pentoxide xerogel ribbons perpendicular to the substrate [20]. The vanadium–oxygen layers are formed by tangled fibres and connected by water molecules [21]. Therefore, the typical diffraction peaks in the XRD pattern indicate the presence of a layered framework, showing that the lamellar structure of the V2O5 xerogel is
Conclusion
The V2O5 xerogel presents a lamellar character and the surface of the film showed the presence of agglomerates, fibrils, and ribbons that are interconnected, forming a network of chains. As a membrane on a pH-EGFET sensor, this xerogel demonstrated a linear behavior and a high sensitivity of 58.1 mV/pH for the pH range of 2–10, a value close to the theoretical limit (59.2 mV/pH). Further, applications using this EGFET sensor will focus on urea and glucose detection.
Acknowledgements
This work was supported by FAPESP and CAPES. We thank Prof. Herenilton P. Oliveira for allocation of his laboratory resources and P.D. Batista for experimental help.
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