Silicon nanowire ion sensitive field effect transistor with integrated Ag/AgCl electrode: pH sensing and noise characteristics

We have fabricated Si nanowire (SiNW) based ion-sensitive field effect transistors (ISFETs) for biosensing applications. The ability to prepare a large number of sensors on a wafer, the use of standard silicon microfabrication techniques resulting in cost savings, and potential high sensitivity are significant advantages in favor of nanoscale SiNW ISFETs. The SiNW ISFETs with embedded Ag/AgCl reference electrode were fabricated on a standard silicon-on-insulator wafer using electron-beam lithography and conventional semiconductor processing technology. The current-voltage characteristics show an n-type FET behavior with a relatively high on/off current ratio, reasonable sub-threshold swing value, and low gate-leakage current. The pH responses of the ISFETs with different pH solutions were characterized at room temperature which showed a clear lateral shift of the drain current vs. gate voltage curve with a change in the pH value of the solution and a sensitivity of 40 mV pH(-1). The low frequency noise characteristics were investigated to evaluate the signal to noise ratio and sensing limit of the devices.     

New membrane materials for potassium-selective ion-sensitive field-effect transistors

Several polymeric materials were studied as membrane materials for potassium-selective ion-sensitive field-effect transistors (ISFETs) to overcome the problems related with the use of conventional plasticized poly(vinyl chloride) membranes casted on ISFET gate surfaces. Several acrylate materials, such as ACE, Epocryl and derivatives, showed no reproducible results. Three room-temperature vulcanizing (RTV)-type silicone rubbers were tested. The addition-type RTV-2 silicone rubber was not suitable as a membrane material, but the condensation-type RTV-1 and especially the RTV-2 silicone rubber showed good results. ISFETs with a Silopren membrane showed a durability of at least 2 months.     

Photocurable ISFET for anionic surfactants. Monitoring of photodegradation processes

The preparation of a new ion-selective field-effect transistor (ISFET) based on a photocurable membrane sensitive to anionic surfactants is described. The membrane is formed by an urethane-acrylate matrix with 2-cyanophenyl octyl ether as the plasticiser. When compared to conventional ion-selective electrodes, the prepared ISFETs do not show significant differences in sensitivity and reproducibility (P=0.05). When calibrating with dodecylbenzenesulfonate (DBS(-)) the prepared ISFETs show a nernstian behaviour, with a slope of 57.5 mV per decade. The linear working range is 1.0x10(-3) to 3.0x10(-6) M DBS(-) and the detection limit is 1.2x10(-6) M. The response times were below 0.7 min in all cases (95% of the step change). As the application, photodegradation processes using titanium dioxide dispersions, were monitored for two common anionic surfactants: DBS(-), being aromatic, and the more alkylic dodecylsulfate, DS(-). The determination of surfactant concentration was performed following a standard addition methodology, using ISFETs as the sensors, and without any previous separation stages. The degradation kinetics in both cases are first-order processes, with half-life times (t(0.5)) of 31.5 min for DBS(-) and 52.0 min for DS(-).     

Selective sensing of triazine herbicides in imprinted membranes using ion-sensitive field-effect transistors and microgravimetric quartz crystal microbalance measurements

A series of triazine herbicides consisting of the chlorotriazine atranex (atrazine), (1), prozinex, (2), tyllanex, (3), simanex, (4) and the methylthiolated triazines ametrex, (5), prometrex, (6) and terbutex, (7), were imprinted in an acrylamide-methacrylate copolymer. The polymer was deposited on the gate surface of ion-sensitive field-effect transistors (ISFETs) and piezoelectric Au-quartz crystals. Selective sensing of the imprinted substrates was accomplished by the imprinted polymer membrane associated with the ISFETs and Au-quartz crystals. Binding of the substrates onto the imprinted polymer associated with the gate of the ISFET alters the electrical charge and potential of the gate interface, thus allowing the potentiometric transduction of the binding events. The association of the substrates with the imprinted membrane linked to the Au-quartz crystal results in the membrane swelling, thus enabling the microgravimetric quartz crystal microbalance assay of the substrate binding events. The specificity of the imprinted recognition sites is attributed to complementary H-bond and electrostatic interactions between the substrates and the acrylamide-methacrylic acid copolymer.     

ISFETs with sputtered sodium alumino-silicate glass membranes

Sodium ion sensitive field-effect transistors (ISFETs) based on reactively sputtered sodium aluminosilicate (NAS) glass membranes are investigated. Using an inverted cylindrical magnetron, NAS films with defined and reproducible composition are obtained. Radiation damage in the gate dielectric is largely eliminated by annealing at 450 degrees C. The Na(+) sensitivity, the selectivity with respect to H(+) and K(+) ions, the response time, the conditioning period, the long-term drift, and the lifetime are evaluated. The ISFETs exhibit a near-Nernstian Na(+) response down to 10(-4) mol/l in solutions of pH>/=7. The NAS films are physically very robust and have excellent chemical durability, e.g., a sensor with a 110 nm thick membrane lasts for about 8 months. The overall sensor properties compare well with those of a commercial Na(+) selective glass electrode.     

New solid-state ion-selective electrodes — Sensors for chemical analysis of solutions

Summary From the historic point of view one can consider chalcogenide glass ion-selective electrodes (CGISEs) and ion-selective field effect transistors (ISFETs) as new solid-state sensors for chemical analysis. The paper describes the development of the sensors, the methods of investigation, analytical properties and the sensing mechanism of CGISEs and ISFETs.

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Neue ionen-selektive Festkörperelektroden — Sensoren für die chemische Analyse von Lösungen

     

Analysis of dopamine and tyrosinase activity on ion-sensitive field-effect transistor (ISFET) devices

Dopamine (1) and tyrosinase (TR) activities were analyzed by using chemically modified ion-sensitive field-effect transistor (ISFET) devices. In one configuration, a phenylboronic acid functionalized ISFET was used to analyze 1 or TR. The formation of the boronate-1 complex on the surface of the gate altered the electrical potential associated with the gate, and thus enabled 1 to be analyzed with a detection limit of 7x10(-5) M. Similarly, the TR-induced formation of 1, and its association with the boronic acid ligand allowed a quantitative assay of TR to be performed. In another configuration, the surface of the ISFET gate was modified with tyramine or 1 to form functional surfaces for analyzing TR activities. The TR-induced oxidation of the tyramine- or 1-functionalized ISFETs resulted in the formation of the redox-active dopaquinone units. The control of the gate potential by the redox-active dopaquinone units allowed a quantitative assay of TR to be performed. The dopaquinone-functionalized ISFETs could be regenerated to give the 1-modified sensing devices by treatment with ascorbic acid.     

Ion-selective field effect transistors with heterogeneous membranes

Polyfluorinated polyphosphazene can be used as an elastomer for the preparation of heterogeneous membranes which can be applied to the ISFETs by solvent casting. The performance characteristics of such devices prepared with AgCl, AgCl-Ag₂S and AgI-Ag₂S membranes are very similar to those found for heterogeneous membranes based on silicone rubber, which have been used in the corresponding ion-selective electrodes. The sensor is suitable for direct determinations of chloride in sweat.     

Simultaneous evaluation of electroactive membranes on a four-function isfet by a constant dilution method

Multifunctional ISFETs are used in testing the performance of several nitrate-sensitive polymeric membranes simultaneously. By mounting the ISFET in a miniature flow-through cell, a continuous dilution technique can be used to obtain calibration curves and selectivity coefficient data for several membranes under identical conditions.     

Membrane-oxide semiconductor field-effect transistor (MOSFET) sensors

MOSFET-sensors can be considered as membrane-oxide-semiconductor field-effect transistors. MOSFETs can be divided into ISFETs, GasFETs, BioFETs, these sensors being sensitive to ion, gas, biomolecules respectively and there is also a possibility to fabricate MOSFET reference electrode (RefFET). The development and theoretical treatment of various type of MOSFET-sensors are presented in this paper.     

Potentiometric Biosensors Based on ISFETs and Immobilized Cholinesterases

This paper describes the authors' achievements in the development, investigation, and application of cholinesterase biosensors based on ISFETs. Various biosensors for determination of concentrations of different toxic substances (organophosphorous and carbamate pesticides, hypochlorite, glycoalkaloids) were designed on the basis of reversible and/or irreversible enzyme inhibition effects. The main analytical characteristics of the biosensors developed have been studied under different conditions and optimal experimental protocols for toxic substances determination have been proposed. Most of these biosensors show a high reproducibility and a good operational and storage stability. A quite good correlation with results obtained through routinely used standard methods as HPLC has been shown.     

Grafting of phosphonate groups on the silica surface for the elaboration of ion-sensitive field-effect transistors

Ion-sensitive field-effect transistors (ISFETs) sensitive to Ca(2+) ions could be elaborated by means of a new grafting process of the phosphonate group at the surface of the silica gate of FETs. A grafting process involving only one chemical reaction step at the surface afforded a significant improvement of the ISFET properties. The sensitivity of the ISFET towards Ca(2+) ions at pH 10 was quasi-linear in the concentration range from 10(-1) to 10(-3) M, and the slope was 10 mV pCa(-1). The site-binding model works well in predicting the experimental data, giving the complexation constant of 10(2.7) and a low value of the grafting density. The origin of the poor response of ISFETs sensitized by means of a multistep grafting process was investigated on silica powders of high specific area: the cleavage of the organic grafts at the SiOSi bonds occurring at each step could be disclosed by means of elemental analyses, infrared, and cross-polarization and magic angle spinning nuclear magnetic resonance of the grafts.     

Microelectrodes for the measurement of cellular metabolism

The study of metabolism of cellular cultures is of great interest. Although some papers reports the monitoring of parameters such as pH, potassium, dissolved oxygen and temperature in cellular cultures using different kind of sensors, no definitive solution has been found to get reproducible and feasible results. The advantage of using sensors fabricated with microelectronic technology is focused on their small size, rapid response and the possibility of having an integrated array of microelectrodes, which favors the measurement of small volumes and fast cell metabolism changes. In this work, the viability of using a multisensor platform of microelectrodes, such as potassium and pH ISFETs, amperometric microelectrodes for dissolved O₂ and pseudo-reference microelectrodes for the study of metabolism of cellular cultures is described. Preliminary results of pH ISFETS’ response in neuronal cultures is reported.     

Reference field effect transistors based on chemically modified ISFETs

Different hydrophobic polymers were used for chemical modification of ion-sensitive field effect transistors (ISFETs) in order to prepare a reference FET (REFET). Chemical attachment of the polymer to the ISFET gate results in a long lifetime of the device. Properties of polyacrylate (polyACE) REFETs are described in detail. The polyACE-REFET is superior to other polymer modified REFETs, showing an excellent pH insensitivity (?1 mV pH^?1), a long lifetime and an electrically identical behaviour as an unmodified pH ISFET or a cation-selective PVC-MEMFET (membrane FET). The cation permeselectivity of the polymer can be significantly reduced by addition of immobile cations. The applicability of a polyACE-REFET in differential measurements with a pH ISFET and a K⁺ MEMFET is demonstrated.     

ISFETs with sputtered sodium alumino-silicate glass membranes

Sodium ion sensitive field-effect transistors (ISFETs) based on reactively sputtered sodium aluminosilicate (NAS) glass membranes are investigated. Using an inverted cylindrical magnetron, NAS films with defined and reproducible composition are obtained. Radiation damage in the gate dielectric is largely eliminated by annealing at 450°C. The Na⁺ sensitivity, the selectivity with respect to H⁺ and K⁺ ions, the response time, the conditioning period, the long-term drift, and the lifetime are evaluated. The ISFETs exhibit a near-Nernstian Na⁺ response down to 10^-4 mol/l in solutions of pH7. The NAS films are physically very robust and have excellent chemical durability, e.g., a sensor with a 110 nm thick membrane lasts for about 8 months. The overall sensor properties compare well with those of a commercial Na⁺ selective glass electrode.     

Theory and applications of ion-selective electrodes. Part 8

This review on ion-selective electrodes (ISEs) is arranged in the same way as earlier reviews of this series. The whole subject continues to grow. As in the preceding review, the most discussed field is that of analysis for body-fluid cations, particularly with automatic devices. Almost 800 papers published between Spring 1987 and Spring 1989 are mentioned, including some omitted in the preceding review. They cover the theory of membrane processes, technology (including ISFETs and electrolysis at liquid/liquid interfaces), fixed-site electrodes, potentiometric biosensors and miscellaneous systems.     

Investigation of chloride sensitive ISFETs with different membrane compositions suitable for medical applications

Cl⁻-ion sensitive ISFETs with photocured polyurethane-based polymer membranes with six different ionophores (ETH 9033, ETH 9009, MnTPPCl, organotin compounds and traditional ion-exchanger TDMACl) have been studied in pure NaCl solutions and in background solutions containing anions in concentrations normally found in a whole blood and serum. PVC or silicon resin were used to form the membranes in cases when it was not possible to use the photocurable polymer composition. Experimental results on determination of chloride ions in serum samples are presented. Performed tests showed that all chloride-selective ionophores commercially proposed up to now, both neutral and charged carriers, do not provide better selectivity and stability of chloride ion sensors for clinical application than the traditional anionic ion-exchanger TDMACl.     

Integrated multi-sensor chip with photocured polymer membranes containing copolymerised plasticizer for direct pH, potassium, sodium and chloride ions determination in blood serum

An analytical system based on a sensor array with ion-selective field effect transistors (ISFETs) monolithically integrated in one chip covered with photocured polymer membranes containing copolymerised plasticizer and a sequential injection analysis (SIA) is shown to offer an automation of the analysis of blood serum components. For sequential injection system a custom made dual channel flow cell for the sensor array was developed. Optimisation of ion-sensitive membrane characteristics and calibration solution compositions were carried out. The system was used to analyze sodium, potassium, chloride ion contents in blood serum samples. The precision of the ion determination in samples was typical for potentiometic method with standard deviation of about 3-5%.     

Borazon-gate pH-sensitive field effect transistor

A borazon-gate ISFET is used as a pH sensor. Boron nitride was deposited by the reactive-pulse plasma method and electron diffraction served for membrane identification. The borazon-gate sensors responded linearly to pH in the range 1.8–10; the slope was about 52 mV pH^?1. Selectivity for H⁺ ions over K⁺, Na⁺ or Ca²⁺ ions was better than that of silicon nitride-gate ISFETs.     

Reducing the interference from CO2 or organic acids in ion-selective polymer membrane sensors having a field-effect transistor as internal reference element

Changes in sample concentrations of CO₂ or organic acids cause potential instabilities when polymer membranes are directly applied to the surface of ion-selective field-effect transistors (ISFETs). Currently used designs avoid this well-documented effect by placing a layer of aqueous buffer between polymeric membrane and ISFET serving as internal reference element. Here, we propose another solution to the problem. In order to compensate for the effect of pH changes on the ISFET threshold voltage, a double membrane is applied whose inner layer is pH-sensitive, while the outer layer exposed to the sample is a conventional ion-selective membrane. It is shown that this approach strongly reduces the earlier-mentioned interference effects.