Field Effect Transistor pH Sensor Research Articles

Preparation of Copper Oxide Films by Electrochemical Deposition Technique for Extended Gate Field Effect Transistor pH Sensor

In this work, copper oxide (CuO) film was prepared by electrochemical deposition technique on commercial indium-tin oxide/glass substrate with two electrode configuration. The precursor solution was the 0.1 molar aqueous of copper sulfate. The films were prepared at different times of 30 to 120 second. Then, the deposited films were annealed at 450 °C to achieve CuO film. The influence of electro-deposition voltage and deposition times on structural property, morphological features had been studied. The CuO films were used to fabricate the pH sensor based on extended-gate field effect transistor (EGFET). The sensitivity and linearity of prepared device was performed in the standard buffer solutions with pH range of 2-12. It was found that the device demonstrated the sensitivity and linearity of 28.5 mV/pH and 90.1 %. It can be seen that the prepared CuO films can be used as pH sensing application with EGFET device.

Structural properties and sensing performance of Ni–Ti alloy films for extended-gate field-effect transistor pH sensors

This study presents a facile Ni–Ti alloyed film grown on the n+-type Si through a DC sputtering technique for electrochemical pH sensing and its comprehensive structural, morphological, compositional, profiling, microstructural, and elemental characterization. In addition to pH sensing measurements, the properties of the sensing layer were analyzed using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy analysis. The impact of rapid thermal annealing (RTA) treatment (500–700 °C) on the alloyed Ni–Ti sensitive films was extensively studied. The structural feature of the alloyed Ni–Ti sensitive films is more closely related to their relative sensing performances. The performance of the alloyed Ni–Ti sensitive film at the RTA temperature of 600 °C showed a higher pH sensitivity of 60.73 mV/pH in a wide pH range of 2–12 in comparison with other RTA temperatures. Moreover, the stability of this 600 °C-annealed EGFET sensor exhibited a small drift rate of 0.21 mV/h in pH 7 and a low hysteresis voltage of 1 mV in a loop of pH 7 → 4→7 → 10→7. The alloyed Ni–Ti sensitive film is fully compatible with a standard CMOS process and demonstrates better sensing performance than other metal nitride sensitive films (e.g. RuN, TiN), suggesting it may be a strong candidate for future sensor and biosensor applications.

Impact of rapid thermal annealing on structural and sensing properties of Ni–Ti sensitive films for extended-gate field-effect transistor pH sensors

In the present work, a facile Ni–Ti film deposited on n+-type Si substrate is introduced as a sensitive film for the development of a solid-state extended-gate field-effect transistor (EGFET) pH sensor. Influence of rapid thermal annealing (RTA) treatment (500–700 °C) on structural properties of the Ni–Ti sensitive films was extensively investigated. The film structures, surface morphologies, element profiles, film microstructures, and chemical compositions of the Ni–Ti sensitive films were characterized by a combination of X-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy, respectively. The structural features of the Ni–Ti sensitive films are most closely related to their sensing characteristics. The experimental results indicated that the Ni–Ti sensitive film annealed at the RTA temperature of 600 °C exhibited better sensing performances (a higher pH sensitivity of 71.01 mV/pH, a lower hysteresis voltage of 1 mV and a smaller drift rate of 0.15 mV/h) than other RTA temperatures. The proposed Ni–Ti sensitive film is expected to be effectively useful for various long-term pH sensor and biosensor applications, since it satisfies the typically required sensing performances such as sensitivity and stability.

Carbon nanotube field-effect transistor based pH sensors

Field-effect transistor (FET) biosensor has received widespread attention due to their unique high-sensitivity properties. In conventional FET biosensors, the complicated interaction between the sample solution and the semiconductor channel always results in poor stability. Especially for FET-based pH sensors, the interface failure caused by the harsh acid-base working environment makes accurate, continuous, and low hysteresis pH measurement quite challenging. Here, we report a carbon nanotube (CNT)-FET pH sensor with enhanced environmental stability by introducing HfO2 film into the gate insulator. The proposed devices show desired immunity to environmental interference including various chemical and physical factors. More importantly, we verified the feasibility of this CNT-FET device for continuous pH sensing, which demonstrates a sensitivity of 67.62 mV/pH, a wide pH range of 1.34–12.68, and low hysteresis of 500 pA with pH switching loops of 4.22–10.24. This work not only realizes a high-performance pH sensor but also paves the way for the development of FET biosensors with high environmental stability, which is of great significance for FET biosensors to maintain normal working performance in complex systems.

All-solid-state AlGaN/GaN-based ion-sensitive heterostructure field effect transistor pH sensor microprobe encapsulated in medical needle

It is very important to monitor pH values. In this paper, we fabricated an all-solid-state AlGaN/GaN-based ion-sensitive heterostructure field effect transistor (ISHFET) pH sensor microprobe. The microprobe could facilitate measurement of pH values. The microprobe was fabricated by packaging the pH sensor microprobe chip into a 21 G medical needle. The performance of the integrated pH sensor microprobe was evaluated in pH range of 4–9.18. A sensitivity of 143.57 µA/pH can be achieved using the pH sensor microprobe with a width to length ratio of 4.2 at a drain–source voltage of 0.5 V. The pH sensor microprobe keeps a similar current when the pH values of solutions are the same. A response time of 1 s can be achieved when no neutralization reaction occurs. The pH sensor microprobe can accurately measure pH of solutions under 0.1M interfering ions. This probe-type AlGaN/GaN-based ISHFET pH sensor may be used in medicine or biology, especially in the detection of pH of biological fluid, blood, and some precious biological samples.

Net community production in the northwestern Mediterranean Sea from glider and buoy measurements

Abstract. The Mediterranean Sea comprises just 0.8 % of the global oceanic surface, yet considering its size, it is regarded as a disproportionately large sink for anthropogenic carbon due to its physical and biogeochemical characteristics. An underwater glider mission was carried out in March–April 2016 close to the BOUSSOLE and DyFAMed time series moorings in the northwestern Mediterranean Sea. The glider deployment served as a test of a prototype ion-sensitive field-effect transistor pH sensor. Dissolved oxygen (O2) concentrations and optical backscatter were also observed by the glider and increased between 19 March and 1 April, along with pH. These changes indicated the start of a phytoplankton spring bloom, following a period of intense mixing. Concurrent measurements of CO2 fugacity and O2 concentrations at the BOUSSOLE mooring buoy showed fluctuations, in qualitative agreement with the pattern of glider measurements. Mean net community production rates (N) were estimated from glider and buoy measurements of dissolved O2 and inorganic carbon (DIC) concentrations, based on their mass budgets. Glider and buoy DIC concentrations were derived from a salinity-based total alkalinity parameterisation, glider pH and buoy CO2 fugacity. The spatial coverage of glider data allowed the calculation of advective O2 and DIC fluxes. Mean N estimates for the euphotic zone between 10 March and 3 April were (-17±36) for glider O2, (44±94) for glider DIC, (17±37) for buoy O2 and (49±86) mmolm-2d-1 for buoy DIC, all indicating net metabolic balance over these 25 d. However, these 25 d were actually split into a period of net DIC increase and O2 decrease between 10 and 19 March and a period of net DIC decrease and O2 increase between 19 March and 3 April. The latter period is interpreted as the onset of the spring bloom. The regression coefficients between O2 and DIC-based N estimates were 0.25 ± 0.08 for the glider data and 0.54 ± 0.06 for the buoy, significantly lower than the canonical metabolic quotient of 1.45±0.15. This study shows the added value of co-locating a profiling glider with moored time series buoys, but also demonstrates the difficulty in estimating N, and the limitations in achievable precision.

Structural properties and sensing performance of TaOx/Ta stacked sensing films for extended-gate field-effect transistor pH sensors

In this article, the TaOx/Ta stacked sensing films as sensing membranes were deposited onto n+-type Si substrate by a radio-frequency (RF) magnetron sputtering technique for solid-state extended-gate field-effect transistor (EGFET) pH sensors. The effect of rapid thermal annealing (RTA) treatment (300–700 °C) on structural properties of the TaOx/Ta stacked sensing films was investigated in detail. The chemical compositions, element profiles, crystallographic structures, surface morphologies, and film microstructures of the TaOx/Ta stacked sensing films were characterized by X-ray photoelectron spectroscopy, Auger electron spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy, respectively. The sensing performance of the TaOx/Ta stacked sensing films is most closely correlated with their relative structural features. Among these RTA temperatures, the best sensing performance including pH sensitivity, hysteresis and drift was achieved at the RTA temperature of 500 °C. The obtained experimental results showed that the TaOx/Ta stacked sensing film is considered a feasible alternative for pH sensor and biosensor applications.

Thermal evaporation based V2O5 thin film for extended gate field effect transistor pH sensor

In this work, vanadium pentoxide (V2O5) thin film was synthesized on the corning glass substrate by using thermal evaporation method. For this purpose, 99.9% pure V2O5 powder was used that was evaporated from a tungsten boat. Subsequently, the prepared film was used as pH sensor in the extended gate field-effect transistor (EGFET) system. Structural properties, morphological features, sensing performance and linearity measurements of synthesized film were studied. The structural properties indicated that the V2O5 thin film has the preferred orientation along (001) plane. The pH-EGFET sensor based on V2O5 thin film was fabricated. The sensitivity and linearity of the device were determined in the pH range of 2–12 of the buffer solution. The sensitivity and linearity values were found to be 41.7 mV pH−1 and 83% respectively in the linear region. The results of present study show that the V2O5 can act as promising material for the EGFET PH sensor.

Rapid detection of Klebsiella pneumoniae, Klebsiella oxytoca, Raoultella ornithinolytica and other related bacteria in food by lateral-flow test strip immunoassays

Bacteria of the genera Klebsiella and Raoultella, which are present in foods and the natural environment, are associated with health hazards in humans. In the present study, two types of strips-based methods were developed to detect these bacteria simply and quickly. One method used lateral-flow test strips (LFTS) in combination with anti-Klebsiella antibodies labeled with palladium nanoparticles that bind to target bacteria, allowing their visualization. In the other, the antibodies were immobilized on nitrocellulose membranes, and urease activity was measured using an ion sensitive field effect transistor pH sensor. Testing of a combination of these two methods on 72 cultured strains successfully identified all 25 strains of Klebsiella pneumoniae, Klebsiella oxytoca and Raoultella ornithinolytica. This approach also accurately identified 76 of 77 (99%) strains isolated from meats and pastries. When combined with preculture, this method was accurate in identifying 19 of 26 (73%) target bacteria in food. These results suggest that the novel combination of strip-based assays may be effective for the on-site monitoring of food production plants, and thereby enhance food safety.

Zinc Oxide Nanorods Grown on Printed Circuit Board for Extended-Gate Field-Effect Transistor pH Sensor

Zinc oxide (ZnO) nanorods (NRs) were grown directly on printed circuit boards with a 35-μm-thick copper layer using a seedless galvanic-cell hydrothermal process. The hexagonal structure of the synthesized ZnO NRs was observed by scanning electron microscopy. The microstructural characteristics of the as-grown ZnO NRs were investigated by x-ray diffraction analysis, revealing preferred (002) growth direction. Raman and photoluminescence spectra confirmed the high crystalline quality of the ZnO NRs. As-grown ZnO NRs were then grown for 7 h using the galvanic effect for use as the pH membrane of an extended-gate field-effect transistor pH sensor (pH-EGFET). The current–voltage characteristics showed sensitivity of 15.4 mV/pH and 0.26 (μA)1/2/pH in the linear and saturated region, respectively. Due to their cost effectiveness, low-temperature processing, and ease of fabrication, such devices are potential candidates for use as flexible, low-cost, disposable biosensors.

The effects of pressure on pH of Tris buffer in synthetic seawater

Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (ΔV) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The ΔV of Tris in seawater media of salinity 35 (ΔVTris⁎) was determined between 10 and 30°C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode directly exposed to solution. The effects of pressure on the potentiometric cell were quantified in aqueous HCl solution prior to measurements in Tris buffer. The experimentally determined ΔVTris⁎ were fitted to the equation ΔVTris⁎=4.528+0.04912t where t is temperature in Celsius; the resultant fit agreed to experimental data within uncertainty of the measurements, which was estimated to be 0.9cm−3mol−1. Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200bar, and can be expressed as:∆pHTris=−4.528+0.04912tPln10RT.where T is temperature in Kelvin, R is the universal gas constant (83.145cm3barK−1mol−1), and P is gauge pressure in bar. On average, pH of Tris buffer changes by approximately −0.02 at 200bar.

Deep-Sea DuraFET: A Pressure Tolerant pH Sensor Designed for Global Sensor Networks.

Increasing atmospheric carbon dioxide is driving a long-term decrease in ocean pH which is superimposed on daily to seasonal variability. These changes impact ecosystem processes, and they serve as a record of ecosystem metabolism. However, the temporal variability in pH is observed at only a few locations in the ocean because a ship is required to support pH observations of sufficient precision and accuracy. This paper describes a pressure tolerant Ion Sensitive Field Effect Transistor pH sensor that is based on the Honeywell Durafet ISFET die. When combined with a AgCl pseudoreference sensor that is immersed directly in seawater, the system is capable of operating for years at a time on platforms that cycle from depths of several km to the surface. The paper also describes the calibration scheme developed to allow calibrated pH measurements to be derived from the activity of HCl reported by the sensor system over the range of ocean pressure and temperature. Deployments on vertical profiling platforms enable self-calibration in deep waters where pH values are stable. Measurements with the sensor indicate that it is capable of reporting pH with an accuracy of 0.01 or better on the total proton scale and a precision over multiyear periods of 0.005. This system enables a global ocean observing system for ocean pH.

Investigation of extended-gate field-effect transistor pH sensors based on different-temperature-annealed bi-layer MWCNTs-In2O3 films.

In this paper, indium (In) films were deposited on glass substrates using DC sputtering method. Multiwalled carbon nanotubes (MWCNTs) and dispersant were dissolved in alcohol, and the mixed solution was deposited on the In films using the spray method. The bi-layer MWCNTs-In2O3 films were annealed at different temperatures (from room temperature to 500°C) in O2 atmosphere. The influences of annealing temperature on the characteristics of the bi-layer MWCNTs-In2O3 films were investigated by scanning electron microscopy, X-ray diffraction pattern, Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. A separative extended-gate field-effect transistor (EGFET) device combined with a bi-layer MWCNTs-In2O3 film was constructed as a pH sensor. The influences of different annealing temperatures on the performances of the EGFET-based pH sensors were investigated. We would show that the pH sensitivity was dependent on the thermal oxygenation temperature of the bi-layer MWCNTs-In2O3 films.

Graphene Channel Liquid Container Field Effect Transistor as pH Sensor

Graphene channel liquid container field effect transistor pH sensor with interdigital microtrench for liquid ion testing is presented. Growth morphology and pH sensing property of continuous few‐layer graphene (FLG) and quasi‐continuous monolayer graphene (MG) channels are compared. The experiment results show that the source‐to‐drain current of the graphene channel FET has a significant and fast response after adsorption of the measured molecule and ion at the room temperature; at the same time, the FLG response time is less than 4 s. The resolution of MG (0.01) on pH value is one order of magnitude higher than that of FLG (0.1). The reason is that with fewer defects, the MG is more likely to adsorb measured molecule and ion, and the molecules and ions can make the transport property change. The output sensitivities of MG are from 34.5% to 57.4% when the pH value is between 7 and 8, while sensitivity of FLG is 4.75% when the pH = 7. The sensor fabrication combines traditional silicon technique and flexible electronic technology and provides an easy way to develop graphene‐based electrolyte gas sensor or even biological sensors.

Hydroquinone monosulfonate-doped polypyrrole electrodeposited on very low cost commercial junction field effect transistors as a novel ion sensitive field effect transistor pH sensor

In this study, for the first time, we introduce a simple and low cost pH sensor based on a commercial junction field effect transistor. The transistor was mechanically treated, isolated and used as an ion sensitive field effect transistor for pH detection, after electrodeposition of the sensing membrane on its surface. Polypyrrole, as a suitable sensing membrane, was electrodeposited on the copper metal gate of transistor from a solution containing pyrrole monomer, hydroquinone monosulfonate as a proper ligand and sodium salicylate for avoiding the substrate oxidation, under a two-step deposition conditions. The prepared sensor showed a near-Nernstian response of 52.3 mV pH−1 over a linear pH range of 2.75–12.20, an ultra low hysteresis of 0.56 mV, a very low drift of 0.14 mV h−1 and a low response time of less than 8 s.

Fabrication of High-Sensitivity Polycrystalline Silicon Nanowire Field-Effect Transistor pH Sensor Using Conventional Complementary Metal–Oxide–Semiconductor Technology

High-sensitivity polycrystalline silicon (poly-Si) nanowire field-effect transistor (NW FET) pH sensors using top-down and self-aligned fabrication approaches involving the conventional complementary metal–oxide–semiconductor (CMOS) process are reported. For the top-down NW FET, the shrinkage due to reoxidation enables the nanowire width to be scaled to 40 nm without requiring the use of extra lithography equipment, and this improves the electrical uniformity and the performance of the sensors. The surface-ionic coupling operation of this buried-channel field-effect sensor exhibits superior pH sensitivity (threshold voltage shift > 100 mV/pH) as compared to the surface-channel ion-sensitive FET (ISFET). In addition, we report a novel method for fabricating self-aligned, vertical-channel, poly-Si nanowire sensors. The resulting 65-nm self-aligned vertical-channel poly-Si device was found to be feasible for independent-gate bias control, thus enabling its possible integration in very-large-scale integration (VLSI) circuits. Both the abovementioned approaches enable the manufacture of nanowire devices on a large-scale integrated (LSI) circuit using only CMOS manufacturing processes; this provides a high sensitivity, compact and cost-efficient biosensor systems-on-a-chip application.

Fabrication of High-Sensitivity Polycrystalline Silicon Nanowire Field-Effect Transistor pH Sensor Using Conventional Complementary Metal–Oxide–Semiconductor Technology

High-sensitivity polycrystalline silicon (poly-Si) nanowire field-effect transistor (NW FET) pH sensors using top-down and self-aligned fabrication approaches involving the conventional complementary metal–oxide–semiconductor (CMOS) process are reported. For the top-down NW FET, the shrinkage due to reoxidation enables the nanowire width to be scaled to 40 nm without requiring the use of extra lithography equipment, and this improves the electrical uniformity and the performance of the sensors. The surface-ionic coupling operation of this buried-channel field-effect sensor exhibits superior pH sensitivity (threshold voltage shift > 100 mV/pH) as compared to the surface-channel ion-sensitive FET (ISFET). In addition, we report a novel method for fabricating self-aligned, vertical-channel, poly-Si nanowire sensors. The resulting 65-nm self-aligned vertical-channel poly-Si device was found to be feasible for independent-gate bias control, thus enabling its possible integration in very-large-scale integration (VLSI) circuits. Both the abovementioned approaches enable the manufacture of nanowire devices on a large-scale integrated (LSI) circuit using only CMOS manufacturing processes; this provides a high sensitivity, compact and cost-efficient biosensor systems-on-a-chip application.

Enhanced sensitivity of an organic field-effect transistor pH sensor using a fatty acid Langmuir–Blodgett film

An ion-sensitive organic field-effect transistor (ISOFET) has been fabricated by using poly(3-hexylthiophene) as the semiconductor and polymethylmethacrylate as the gate insulator. With the gate metallization replaced by an Ag/AgCl reference electrode, and following suitable encapsulation, the ISOFETs exhibit transistor behavior in an aqueous environment. Moreover, the devices are shown to respond to changes in pH of the solution. A significant increase in the sensitivity of the ISOFET to H + ions was found by depositing a Langmuir–Blodgett film of arachidic acid on top of the gate insulator.

Application of Rapid, Electrochemical Flash Titration™ to Total Acidity and Alkalinity Determinations in Buffers, Foods, and Beverages

A new technique (Flash Titration) for the determination of total acidity and total alkalinity was applied to food and beverage analysis. Requiring no liquid titrants, Flash Titration is a technique made possible through microfabrication of electrochemical components on a silicon chip. Acidic or basic titrant was generated electrochemically at a noble-metal electrode that intimately surrounded an ion-selective, field-effect transistor pH sensor. As acid or base was generated through electrolysis, sample alkalinity or acidity, respectively, was neutralized in the immediate vicinity of the electrode. Through diffusion, a zone comprising a gradient of partially to totally neutralized sample expanded from the generating electrode into the volume element sensed by the nearby pH sensor. An analysis of the pH signal versus time revealed an end point inflection at an elapsed time related to the total alkalinity or acidity of the sample. End point times were typically a few seconds. In this paper, Flash Titration was applied to the analysis of a variety of samples, including juices, soft drinks, wines, and food products. The differences between the results obtained by the Flash Titration method and a commonly used conventional volumetric method were less than 2% in most food products tested. Analysis costs were reduced both through time saving and reduction or elimination of hazardous liquid titrant disposal, by the use of the Flash Titration method.

Measurement of gastric pH during digestion of a solid meal in dogs.

The behavior of gastric pH during digestion of a solid meal in beagle dogs was determined by use of an ion-selective field effect transistor pH sensor. The pH in the stomach was 3.9 +/- 0.4 (mean +/- S.D., n = 6) at 0.5 h after meals. It was maintained at a mildly acidic (about pH 3) level for long periods.