Description of ionophore-doped membranes with a blocked interface
Introduction
Ionophore-doped membranes are widely used for functionalisation in ion sensing technologies such as ion-selective electrodes (ISEs). The earliest work based on electrically neutral ionophores comes from the observation by Moore and Pressman that the antibiotic valinomycin is highly selective towards K+ ions [1]. This led to tremendous advances from the 1960s and onwards with the development of a range of sensors with natural or synthetic ionophores for a plethora of different types of ions [2], [3], [4].
The desire to further miniaturise sensors, and improve their versatility and ruggedness for application outside the lab environment, has led to the development of solid-contact sensors such as coated-wire electrodes (CWEs) and ion-selective field effect transistors (ISFETs). In the 1970s, the CWE was introduced in which an ion-selective membrane was directly contacted with a metal wire [5], [6]. Concurrently, the ISFET concept was also introduced in which a membrane was deposited on a Si-based transistor in place of the metal gate [7], [8]. However, these sensors suffered from poor stability and potential drift, as well as poor membrane adhesion [9]. In the late 1980s and early 1990s, these issues were partially resolved by introducing conductive polymers and a hydrogel as inter-layers for CWEs and Si-based ISFETs respectively [10], [11].
Recently, a new type of ISFET has been introduced based on AlGaN/GaN transistors, which appears to be a promising alternative to Si-based transistors, having a higher device transconductance and improved chemical stability [12], [13]. A reference electrode free configuration has also been demonstrated to work well for AlGaN/GaN sensors, where changes in gate charge are sensed through measuring the changes in the conductivity of the electron channel [14], [15].
As early as 1937, the very first theoretical description of ion-selective membranes was given by Nikolskii for glass ISEs [16]. This led to the well-known semi-empirical equation: the Nikolskii-Eisenman (N-E) equation, which relates the concentration of the ion of interest to the equilibrium potential response. A critical feature of Nikolskii's earlier work and the later N-E description is that it provided a formal definition of the selectivity of ISEs towards different ions. However, it is based on equilibrium assumptions and cannot provide insight into fundamental ion charging mechanisms [17].
Presently, the most generalised description of ISEs is the Nernst-Planck-Poisson (NPP) model which has been developed by various researchers [18], [19], [20], based on the pioneering work of Brumleve and Buck [21]. This model contains the least number of assumptions and can be used to provide greater insight into the underlying ion charging mechanisms in the membrane. Recent developments led to the incorporation of reaction terms in the NPP equations [22] that allowed for the description of ion-ionophore complexation, which is vital for greater discrimination between ions. However, the application of the NPP model was mainly in the context of traditional ISEs containing an inner filling solution and a suspended membrane. There is earlier work in which the model was applied to a CWE [23], but it was applied to the specific context where a conductive polymer was used as an inter-layer [22] and ion-ionophore complexation was not considered [22].
This work aims to clarify the understanding of how the membrane itself affects the response of solid-contact sensors, using the NPP model. This is achieved by making the assumption that the solid-contact completely blocks ion transfer (i.e. blocked interface) and does not interact electrostatically. Ion-ionophore complexation rates are included in the model [22], providing an understanding of their role. The ion charging mechanisms in the membrane and the boundary layer behaviour exhibited at the membrane|solution interface are studied, and the fundamental sensing principles elucidated. Simulations were conducted to make comparisons between the response of a sensor with a perfectly blocked interface and a traditional ISE. For the first time, we use the NPP model to analyse experimental results for AlGaN/GaN ISFETs with ionophore-doped membranes, achieving good qualitative agreement and model validation.
Section snippets
Context
The primary focus of this paper is the solution|membrane|solid-contact system illustrated in Fig. 1(a). This is in contrast to the ISE setup with inner filling solution and a suspended membrane investigated by Jasielec et al. [22], and depicted in Fig. 1(b).
Ions transfer into the membrane phase through solvation processes, described by the heterogenous rates of phase transfer (see Fig. 1). The mobile ionic sites, R−, provide for neutrality in the bulk membrane and thus suppress interference of
Comparison between ISE (with inner filling solution) and a membrane with a blocked interface
This subsection addresses the differences between ISEs with an inner filling solution (see Fig. 1(b)) and solid-contact sensors with a blocked interface, which are the primary focus of this work (see Fig. 1(a)). For the simulations in this subsection no interfering ions, I+, are present in the external solution. However, 0.1 M I+ is present in the inner filling solution of the ISE, as in other studies [20], [22], [28], [29]. Throughout this paper a 15 μm thick membrane with absolute permittivity ϵ
Conclusion
Using the NPP model, we have successfully elucidated the influence of ionophore-doped membranes on the response of solid-contact sensors, by assuming an ideal blocked interface. For the first time, a comparison is made between blocked interface sensors and traditional ISEs using the NPP model, which is in alignment with experimental findings from the literature. The role of ion-ionophore complexation in influencing the lower detection limit is in alignment with previous work by Jasielec et al.
Acknowledgements
The experimental work was partially funded by the Australian Research Council, grant number DP140100827, and performed at the Western Australian Node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia's researchers. The authors also gratefully acknowledge the help and support provided over email by Bartosz Grysakowski and Marek Danielewski from
Tarun Sanders obtained his B.E. in Electrical and Electronic Engineering in 2017. He will commence a Ph.D. in electrical and electronic engineering in 2017. He has experience in III-V nitride devices, analysing x-ray photoelectron spectroscopy data, numerical modelling as well as optical fibres for high quality imaging. He is also currently a Research Assistant for the department of Mechanical & Chemical Engineering at the University of Western Australia. His current interests are in developing
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Tarun Sanders obtained his B.E. in Electrical and Electronic Engineering in 2017. He will commence a Ph.D. in electrical and electronic engineering in 2017. He has experience in III-V nitride devices, analysing x-ray photoelectron spectroscopy data, numerical modelling as well as optical fibres for high quality imaging. He is also currently a Research Assistant for the department of Mechanical & Chemical Engineering at the University of Western Australia. His current interests are in developing novel chemical sensors and modelling of ionic charging mechanisms in polymer based membranes.
Matthew Myers obtained his B.S. in Chemical Engineering from the California Institute of Technology in 2003 and his Ph.D. in organic chemistry and materials science with Prof. Colin Nuckolls at the Columbia University (New York) in 2008, for work on curved polycyclic aromatic hydrocarbons and their applications in organic electronics. He is currently interested in nanomaterials, polymers and chemical sensors.
Mohsen Asadnia is a lecturer in the Engineering Department, Macquarie University. He received his Ph.D. in Mechanical Engineering from Nanyang Technological University-Singapore, and undertook his postdoctoral trainings at Singapore-MIT Alliance for Research and Technology (SMART)-Singapore, MIT-Boston and the University of Western Australia (UWA). His PhD research was focused on development of bio-inspired sensory systems using MEMS techniques for underwater sensing. At MIT, he expanded his research interest towards developing artificial inner ear haircell sensors. His research interests include, chemical sensors, soft-polymer materials, Microfluidics and BioMEMS.
Gilberto A. Umana-Membreno received his PhD in Electrical and Electronic Engineering from the University of Western Australia (UWA) in 2007, where he currently holds a Principal Research Fellow appointment with the Microelectronics Research Group (MRG). His research interests include characterization and modelling of electronic transport and defect-related effects in semiconductor materials, and optimisation and reliability of electronic and optoelectronic semiconductor devices. At the MRG, his current research activities are in the development and optimisation of novel sensor technologies in HgCdTe and III-nitrides, as well as in the investigation of electronic processes in nanostructured devices, high-k dielectrics and in semiconductor-based micro-electro-mechanical systems. He is a Senior Industry Support Engineer for the Australian National Fabrication Facility (ANFF).
Murray Baker obtained his Ph.D. in organic chemistry with Prof. Les Field at the University of Sydney in 1988, for work on C-H activation by iron phosphine complexes. After a year in the Biologicals Group at ICI Australia's research group at Ascot Vale, Melbourne, and two years learning surface and materials chemistry with Prof. George M. Whitesides at Harvard University, he joined The University of Western Australia, where he is Professor of Chemistry. He has research interests in organometallic chemistry (especially applications of organometallic compounds in medicine and catalysis), biomaterials, and surface chemistry.
Neville D. Fowkes is a mathematical modeller who works on continuum models arising out of engineering, and physical and biological science. He received his B.S. and Ph.D. in applied mathematics at the University of Queensland (1970) and subsequently worked at The Division of Engineering and Applied Physics at Harvard University and at the University of Western Australia, Perth (1980). He also frequently works with the industrial maths group at Oxford University. He has attended more than 60 MISG (maths in industry study group meetings) throughout the world (SE Asia, China, UK, Canada, Australia, Portugal, South Africa) and has been instrumental in initiating these activities, whose aim is to use mathematics as a tool for understanding industrial problems.
Giacinta Parish received the B.S. degree in Chemistry in 1995, and the B.E. and M.Eng.Sc degrees in electronic engineering in 1995 and 1997, respectively, all from The University of Western Australia, Perth, and the Ph.D. degree in electrical engineering in 2001, from the University of California, Santa Barbara. She joined The University of Western Australia as an Australian Postdoctoral Fellow in 2001, and is now an Associate Professor at the same institution. Her main research interests are III-V nitride materials and devices, and more recently porous silicon. Specific interests within these areas currently include development of processing technology, MEMS and novel chemo- and bio-sensors.
Brett D. Nener received his B.E. degree in 1977 and Ph.D. degree in 1987 from The University of Western Australia, and the M.E.Sc. (1980) from the University of Tokyo. He is Professor in the Electrical, Electronic and Computer Engineering department at The University of Western Australia. His current areas of research interest are Biosensors; III-N devices; Modelling of atmospheric effects such as refraction, scintillation and aerosol scattering; Electro-optic systems; Infrared and UV photodetector devices and models; and the measurement of semiconductor deep-level traps.