Ion-selective Field Effect Transistor Research Articles

Verification of a Stability of Polymer Matrix for the Salivary-Nitrate-ISFETS

[Introduction] We have studied the stress measurement by making use of salivary nitrate, which can be a candidate of stress markers. Ion-selective Field-Effect Transistors (ISFETs) are suitable for the on-site single-drop analysis of salivary nitrate within 10 seconds. The ISFETs are based on field-effect transistors (FET) combined with an ion-selective membrane (ISM). The ISFETs are characterized by the ISM which consists of ionophore, plasticizer and polymer matrix. An ionophore and a plasticizer of the ISM are optimized to determine salivary nitrate from our researches. However, the performance of the polymer matrix is not enough to do so. Hence, the ISFETs have a problem for salivary nitrate which is called the initial drift. The initial drift makes it difficult to determine salivary nitrate. Fogt, et al. considered that the cause of the initial drift is the aqueous layer on between the ISM and the ISFETs’ gate (gate)-interface generating the pH change. This pH change is concerned with CO2 gas diffusion which approaches the aqueous layer through the ISM. The gate is made of pH-sensitive material (Ta2O5, SiO2, and Si3N4), hereinafter pH-sensitive ISFETs called pH-ISFETs. Other previous researches attempted to insert the inner layer: pHEMA-gel saturated buffer, Ag/AgCl and H+ selective membrane in the aqueous layer. On the other hand, Abramova, et al. considered that the cause of initial drift is the protein adsorption to the ISM surface. Photocurable polyurethane as a matrix membrane is effective to measure the electrolyte of blood electrolyte. However, there is no report that aims to suppress drift in saliva. In this research, we verified polymer stability of ISM and introduce the novel polyurethane matrix. Actually we used three polyurethanes as polymer matrix: a novel polyurethane A and a conventional B and C. The polyurethane is widely used as a catheter in the medical field. Hence, polyurethane is expected to suppress the adsorption of glycoproteins in saliva. Furthermore, the ISM based on the novel polyurethane A is adhesiveness and it can be closely adhered to the gate to restrict the appearance of the aqueous layer. [Method] Nitrate selective ISFETs (NO3-ISFETs) are prepared by the following procedure. We dissolve completely and make the THF solution which forms the nitrate-selective membrane, containing 5 wt% of the nitrate ionophore ([Cu(bcp)2]NO3), 65 wt % of the plasticizer (NPDDE) and 30 wt% of the polymer matrixes. We cast the THF solution onto the gate part of the pH-ISFETs. After this casting, THF solution on the gate part is evaporated in a clean space, and this manner repeats several times. After all, the nitrate-selective membrane is completely dried half a day. The thickness of the nitrate selective membrane is ca. 0.2 mm. The NO3-ISFETs was conditioned in 1 mM (mol/L) KNO3 solution (standard solution) for 3 hours. Its calibration curve and selectivity coefficients are determined using the Nicolsky-Eisenman’s equation. The evaluation of initial drift is performed by looking at the output values of ISFETs immersed in various solution about 8 hours. Main measurement target is human saliva. Two types of human saliva are prepared for the sampling method. The one is obtained from the saliva-collection kit (Salivette®) and another is obtained from a metal spoon. For comparison of the initial drift, we use four kinds of solutions: standard solution, artificial saliva, the standard solution with 0.5 wt% of Mucin (mucin solution), and standard solution with CO2 bubbling (CO2 solution). Artificial saliva is the aqueous solution reproducing human’s salivary electrolytes. All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the saliva sample experiments were approved by the Ethics Committee of National Institute of Advanced Industrial Science and Technology (AIST). [Results and Discussion] The calibration curves and selectivity the coefficients show that potential determinants are not polymer matrix but ionophore and plasticizer. The drift characteristics of ISFETs based on the novel polyurethane A is more stable than other ISFETs based on the conventional polyurethanes B and C. These results may be due to the adhesiveness of the novel polyurethane A. On the other hand, we hypothesize that the urethane bond captures H+ and OH- in the ISM. However, this hypothesis is not applied for the initial drift in conventional polyurethane membrane. Therefore, this hypothesis is under investigation and we would like to introduce and discuss the details on the day of my presentation.

Investigation of Polyurethane Matrix Membranes for Salivary Nitrate ISFETs to Prevent the Drift.

We have investigated human-stress monitoring by making use of salivary nitrate, which can be a candidate for stress markers, with ion-selective field-effect transistors (ISFETs). ISFETs are suitable for on-site single-drop analysis of salivary nitrate within 10 s. However, when ISFETs are used for salivary nitrate, ISFETs have a problem that is called the initial drift. The initial drift makes accurate nitrate monitoring difficult. Thus, the purpose of this study is to prevent the initial drift and to search for a new, simple polymer to possess a better performance of sensor responses than conventional matrix membranes, such as PVC. In this research, we investigated ISFETs using specific matrix membranes, for example KP-13, Pellethane®­­, and P7281-PU. The initial drift was evaluated from the fluctuations of the response values generated by the ISFETs when immersed in saliva or aqueous solution. As a result, P7281-PU showed a prevention effect on the initial drift, both in the whole saliva and in various solutions. Furthermore, the cause of drift may be H+ diffusion, and the drift prevention effect of P7281-PU may be affected by urethane bond capturing H+ in the ion-selective membrane. This result suggests that a continuous nitrate monitoring is feasible and may be applied to wearable sensors.

High Field Modulated Field Effect Transistors Coated By Ion Selective Membrane for Cadmium Ion Detection in Water

Cadmium is one among the most hazardous heavy metals. Heavy metal cannot be metabolized or digested by human body, therefore, heavy metal accumulates in our body through time. Cadmium in human body causes lung cancer, osteomalacia and even proteinuria. Mostly cadmium ions get in to human body by ingestion, so detecting cadmium ion in drinking water, tap water and liquid form of food is very important. However, traditional laboratory detection methods of cadmium ion are complex and time consuming and requires heavy and expensive machinery. Though the electrochemical ion selective electrode is easy to use, its poor stability and larger detection limit are huge disadvantages. In this research, we devised a small size, high sensitivity, low detection limit, short response time and easy to fabricate cadmium ion selective membrane field effect transistor (ISMFET). This device is easy to use and can measure the cadmium concentration in water quickly with the combination of ion-selective membrane and field effect transistor. The sensing mechanism depends on electrical double layer (EDL) gating coupled with high electric field to enhance the sensitivity of ISHEMT beyond the limit of ideal Nernst sensitivity. World Health Orgnization (WHO) suggests that cadmium ion in drinking water should be lower than 3 ppb, equal to 2.68 10-8 M. This device can detect cadmium ion concentration form 10-7 M to 10-11 M, covering the wide dynamic range according to WHO’s suggestion. Such high sensitivity and low detection limits devices are highly desirable for consumers who intend to regularly monitor the water quality, for food and environmental safety. In contrast, testing cadmium ion in water by ICP-MS takes several hours and the instrument occupies larger space. To improve the yield and sensitivity of our sensor, we are studying on different condition of ISM. With this cadmium, ISMFET, we can determine the cadmium concentration in water in 15 minutes and even higher sensitivity and lower detection limit, in comparison with conventional laboratory based techniques. Not only Cadmium, but also other heavy metals, for example lead and mercury, can use similar methodology to get better detection time and detection limit. This design makes quality monitoring of water we consume in daily life become much easier and convenient.

High Field Modulated Field Effect Transistors Coated by Ion Selective Membrane for Cadmium Ion Detection in Water

Cadmium is one among the most hazardous heavy metals. Heavy metal cannot be metabolized or digested by human body, therefore, heavy metal accumulates in our body through time. Cadmium in human body causes lung cancer, osteomalacia and even proteinuria. Mostly cadmium ions get in to human body by ingestion, so detecting cadmium ion in drinking water, tap water and liquid form of food is very important. However, traditional laboratory detection methods of cadmium ion are complex and time consuming and requires heavy and expensive machinery. In this research, we devised a small size, high sensitivity, low detection limit, short response time and easy to fabricate cadmium ion selective membrane field effect transistor (ISMFET). This device is easy to use and can measure the cadmium concentration in water quickly with the combination of ion-selective membrane and field effect transistor. The sensing mechanism depends on electrical double layer (EDL) gating coupled with high electric field to enhance the sensitivity of ISHEMT beyond the limit of ideal Nernst sensitivity. World Health Orgnization (WHO) suggests that cadmium ion in drinking water should be lower than 3 ppb, equal to 2.68*10-8 M. This device can detect cadmium ion concentration form 10-7 M to 10-11 M, covering the wide dynamic range according to WHO’s suggestion. Such high sensitivity and low detection limits devices are highly desirable for consumers who intend to regularly monitor the water quality, for food and environmental safety.

Continuous Monitoring of pH and Blood Gases Using Ion-Sensitive and Gas-Sensitive Field Effect Transistors Operating in the Amperometric Mode in Presence of Drift.

Accurate and cost-effective integrated sensor systems for continuous monitoring of pH and blood gases continue to be in high demand. The capacity of ion-selective and Gas-sensitive field effect transistors (FETs) to serve as low-power sensors for accurate continuous monitoring of pH and blood gases is evaluated in the amperometric or current mode of operation. A stand-alone current-mode topology is employed in which a constant bias is applied to the gate with the drain current serving as the measuring signal. Compared with voltage-mode operation (e.g., in the feedback mode in ion-selective FETs), current-mode topologies offer the advantages of small size and low power consumption. However, the ion-selective FET (ISFET) and the Gas-sensitive FET (GasFET) exhibit a similar drift behavior, imposing a serious limitation on the accuracy of these sensors for continuous monitoring applications irrespective of the mode of operation. Given the slow temporal variation associated with the drift characteristics in both devices, a common post-processing technique that involves monitoring the variation of the drain current over short intervals of time can potentially allow extraction of the measuring signal in presence of drift in both sensor types. Furthermore, in the amperometric mode the static sensitivity of a FET-based sensor, given by the product of the FET transconductance and the sensitivity of the device threshold voltage to the measurand concentration, can be increased by adjusting the device design parameters. Increasing the sensitivity, while of interest in its own right, also enhances the accuracy of the proposed method. Rigorous analytical validation of the method is presented for GasFET operation in the amperometric mode. Moreover, the correction algorithm is verified experimentally using a Si3N4-gate ISFET operating in the amperometric mode to monitor pH variations ranging from 3.5 to 10.

Improved pH Sensitivity and Reliability for Extended Gate Field-Effect Transistor Sensors Using High-k Sensing Membranes.

In this study, we fabricated extended-gate (EG) field-effect transistor (FET) pH sensors with dualgate (DG) structures, using a range of dielectric sensing membranes (SiO₂, Si₃N₄, HfO₂ and Ta₂O5) to vary their sensitivity. The fabricated EGFETs consisted of a silicon-on-insulator (SOI)-based metal-oxide semiconductor field-effect transistor (MOSFET) transducer and an EG sensor. We amplified the sensitivity of the device far beyond the Nernst limit (59 mV/pH), which is the theoretical maximum of conventional ion-selective FET (ISFET) sensing, by applying capacitive coupling. Among the evaluated dielectric sensing membranes, we obtained the highest sensitivity (478 mV/pH), low hysteresis (100.2 mV) and drift rate (24.6 mV/h) from the pH sensor with a Ta₂O5 membrane. Hence, we expect DG FET configurations using Ta₂O5 films as EG sensing membranes to be useful for high performance biosensor applications, as they satisfy the requirements for sensitivity, stability and reliability.

Toward a highly selective artificial saliva sensor using printed hybrid field effect transistors

Ion selective field effect transistors (ISFETs) have been widely used in the biological and environmental sensing applications. However, the rigid feature of common ISFETs blocked the real-time monitoring applications. Here, we report a feasible 3D printing method to fabricate flexible ISFETs by hybridization of printed organic ion selective electrodes and inorganic transistors. The fabricated ISFETs demonstrated high sensitivity, ranged about 100 mV/decade, towards ions such as ammonium (NH4+), potassium (K+), and calcium (Ca2+). Furthermore, the fabricated hybrid ISFETs showed their distinct selectivity from the mixture of varied interference ions as an artificial saliva. This capability provides an integrated platform for a comprehensive ion analysis by closely monitoring interrelated saliva parameters as wearable healthcare solutions manufactured by 3D printing methods.

High-field modulated ion-selective field-effect-transistor (FET) sensors with sensitivity higher than the ideal Nernst sensitivity

Lead ion selective membrane (Pb-ISM) coated AlGaN/GaN high electron mobility transistors (HEMT) was used to demonstrate a whole new methodology for ion-selective FET sensors, which can create ultra-high sensitivity (−36 mV/log [Pb2+]) surpassing the limit of ideal sensitivity (−29.58 mV/log [Pb2+]) in a typical Nernst equation for lead ion. The largely improved sensitivity has tremendously reduced the detection limit (10−10 M) for several orders of magnitude of lead ion concentration compared to typical ion-selective electrode (ISE) (10−7 M). The high sensitivity was obtained by creating a strong filed between the gate electrode and the HEMT channel. Systematical investigation was done by measuring different design of the sensor and gate bias, indicating ultra-high sensitivity and ultra-low detection limit obtained only in sufficiently strong field. Theoretical study in the sensitivity consistently agrees with the experimental finding and predicts the maximum and minimum sensitivity. The detection limit of our sensor is comparable to that of Inductively-Coupled-Plasma Mass Spectrum (ICP-MS), which also has detection limit near 10−10 M.

Theoretical study of the influence of surface effects on GaN-based chemical sensors

This paper presents the results of the theoretical study of various factors affecting the charging mechanism and characteristics of the Ga2O3/GaN/AlGaN based ion sensitive field effect transistor (ISFET). The relaxed atomic coordinates of electrolytes and water molecules at the α-gallium oxide (0 0 1) surface are used to calculate the interfacial properties of gallium nitride based ion selective field effect transistors. The hydration radius of electrolytes along with the distance of the first and the second layer of water on the surface are used to derive the equilibrium reaction rates of the protonation/deprotonation of surface sites and adsorption/desorption of ions to the second layer. The Triple Layer Model (TLM) is used to obtain the redistribution of electrolytes when they are exposed to the surface of the solid. Single site binding, TLM, Poisson-Boltzmann and Schrodinger equations are solved self-consistently to obtain the potential distribution across the device and electrolyte, accumulated charge at the surface and the density of electrons in the channel. Sensitivity of the device is affected by the thickness of the surface oxide on the GaN-cap layer. These results indicate that the inclusion of a thin layer of oxide at the surface of the GaN-based chemical sensor accounts for the surface chemistry, equilibrium reaction rates, and the relative surface conduction-band offset with the redox levels of electrolyte. The calculated values for the surface acidity, oxide/GaN-cap barrier height and the corresponding electron density in the channel as a function of the thickness of the AlGaN barrier, the GaN-cap and the surface oxide layer, distribution of electric fields within the device, and the Nernstian-slope of various structures, were compared with the literature for validation and were in close proximity of the experimental value. This model can be used to further refine GaN- based sensors for a range of applications.

Counteracting Threshold-Voltage Drift in Ion-Selective Field Effect Transistors (ISFETs) Using Threshold-Setting Ion Implantation

Ion-selective field effect transistors (ISFETs) exhibit instability, commonly known as drift, in the form of a slow, monotonic, temporal increase in the threshold voltage of the device. A method for counteracting instability inspired by a physical model for threshold voltage drift in pH-sensitive ISFETs is presented. This method involves adjusting the charge density at the insulator-semiconductor interface using threshold-setting ion implantation such that the net charge induced in the semiconductor at the operating point of the device is minimized. The proposed method is analytically validated based on characterization and modeling of drift in an Al2O3-gate pH-sensitive ISFET. Counteraction of ISFET drift by ion implantation is also demonstrated using TCAD simulations. The optimum donor-type implant dose of $3.25 \times 10^{11}$ cm−2 determined based on ATLAS simulations is in good agreement with the corresponding dose of $6.58 \times 10^{11}$ cm−2 obtained analytically.

An interdigitated ISFET-type sensor based on LPCVD grown graphene for ultrasensitive detection of carbaryl

Consider advances are being made to develop new technologies capable of fast in-situ tracing of agricultural toxins. In this work, we reported a simple carbaryl sensor with very high sensitivity using graphene interdigitated ion selective field effect transistor (ISFET). The graphene films were first prepared on polycrystalline copper foil by a low-pressure chemical vapor deposition method, and then transferred onto the interdigitated electrodes of ISFETs by a chemical etching technique. The biorecognition is based on the enzymatic inhibition of carbaryl towards urease. As expected, the weaker enzymatic activity of urease in addition of carbaryl would result in the weaker current response. It was demonstrated that the interdigitated ISFET sensor could achieve high sensitivity to detect carbaryl as low as 10−8 μg mL−1 and the current response of the device showed a good linearity against the logarithm of carbaryl concentration Ccarbaryl with a regression equation ΔIds = −0.4 − 0.6 logCcarbaryl (mA) (R2 = 0.99) in the concentration range from 2.58 × 10−7 to 2.58 × 10−2 μg mL−1. In conlusion, such convenient graphene-based ISFET configuration could be advantageously extended for on-line screening of other pesticide and herbicide agents.

Ca2+ detection utilising AlGaN/GaN transistors with ion-selective polymer membranes

We demonstrate highly selective and sensitive potentiometric ion sensors for calcium ion detection, operated without the use of a reference electrode. The sensors consist of AlGaN/GaN heterostructure-based transistor devices with chemical functionalisation of the gate area using poly (vinylchloride)-based (PVC) membranes having high selectivity towards calcium ions, Ca2+. The sensors exhibited stable and rapid responses when introduced to various concentrations of Ca2+. In both 0.01 M KCl and 0.01 M NaCl ionic strength buffer solutions, the sensors exhibited near Nernstian responses with detection limits of less than 10−7 M, and a linear response range between 10−7–10−2 M. Also, detection limits of less than 10−6 M were achieved for the sensors in both 0.01 M MgCl2 and 0.01 M LiCl buffer solutions. AlGaN/GaN-based devices for Ca2+ detection demonstrate excellent selectivity and response range for a wide variety of applications. This work represents an important step towards multi-ion sensing using arrays of ion-selective field effect transistor (ISFET) devices.

Field-effect transition sensor for KI detection based on self-assembled calixtube monolayers

A series of novel calixarene-based tubes comprising different numbers of silatrane anchoring groups was synthesized. For the first time, a self-assembled monolayer (SAM) derived from calixtubes was formed on a SiO2 surface. The formation of the SAM was confirmed by X-ray photoelectron spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis, and contact angle measurements. Modification of the sensitive surface of a conventional ion-selective field effect transistor (ISFET) with the afforded SAM resulted in the production of a KI-sensitive sensor. This sensor selectively determined KI compare to different alkali metal iodides: NaI, RbI, CsI; also investigation of different potassium salts (acetate, iodide, nitrate, chloride, dihydrophosphate, perchlorate) showed the highest response to KI. This sensor was successfully employed to determine the presence of KI in artificial saliva with a limit of detection of ~3 × 10−8 М. In addition, it was found that the detection limit of the sensor could be increased by combining the sensor with a microfluidic system. Due to the obtained sensor sensitivity and its ability to detect KI in artificial saliva, we could conclude that this sensor shows great potential for application in the determination of KI in different media, such as the human body and in biological liquids, such as saliva or urine.

Description of ionophore-doped membranes with a blocked interface

Ion sensors with blocked interfaces, such as coated-wire electrodes (CWEs) and ion-selective field effect transistors (ISFETs), are increasingly being used as alternatives to traditional ion-selective electrodes (ISEs). This work provides insights into the behaviour of ionophore-doped membranes in solution, where one side completely blocks ion transfer, using the Nernst-Planck-Poisson (NPP) model, including reaction terms. Extensive investigations were conducted, demonstrating that the NPP model provides reasonable predictions, as well as insight into the interplay between interfacial kinetics, complexation, diffusion and electrical processes. Model validation is achieved through its application to and analysis of experimental results obtained for AlGaN/GaN ISFETs with ionophore-doped membranes.

Kinetics of the hydration reaction at the electrolyte–insulator interface

The instability of the dc operating point in the pH-sensitive ion-selective field effect transistors (ISFETs) has been ascribed to a chemical ageing at the electrolyte–insulator. This instability, commonly referred to as a drift, is believed to involve formation of a chemically-modified insulator surface layer as a result of hydration of the insulator material. A kinetic model for hydration of the amorphous insulator material is presented. The kinetics of hydration is limited by the hopping and/or trap-limited transport mechanism known as dispersive transport, the key characteristic of which is a power-law time dependence of the diffusion coefficient. The power-law time dependence of the diffusion coefficient will be shown to lead to a stretchedexponential decay in the form exp[–(t/τ)β] for the excess density of sites or traps occupied by the hydrating chemical species undergoing dispersive diffusion, where τ is the time constant associated with a structural relaxation and β is the dispersion parameter satisfying 0 < β < 1. The kinetics associated with a hydration reaction limited by the dispersive diffusion has been shown to lead to a hydrated layer thickness exhibiting a time dependence in the form {1–exp[–(t/τ)β]}. The first order rate equation describing the kinetics of the hydration reaction is characterized by the time-dependent rate coefficient.

Portable Electronic Tongue Based on Microsensors for the Analysis of Cava Wines.

Cava is a quality sparkling wine produced in Spain. As a product with a designation of origin, Cava wine has to meet certain quality requirements throughout its production process; therefore, the analysis of several parameters is of great interest. In this work, a portable electronic tongue for the analysis of Cava wine is described. The system is comprised of compact and low-power-consumption electronic equipment and an array of microsensors formed by six ion-selective field effect transistors sensitive to pH, Na+, K+, Ca2+, Cl−, and CO32−, one conductivity sensor, one redox potential sensor, and two amperometric gold microelectrodes. This system, combined with chemometric tools, has been applied to the analysis of 78 Cava wine samples. Results demonstrate that the electronic tongue is able to classify the samples according to the aging time, with a percentage of correct prediction between 80% and 96%, by using linear discriminant analysis, as well as to quantify the total acidity, pH, volumetric alcoholic degree, potassium, conductivity, glycerol, and methanol parameters, with mean relative errors between 2.3% and 6.0%, by using partial least squares regressions.

(Invited) Chemical Sensors Based on AlGaN/GaN Transistors

Reliable, economically accessible technology for in situ monitoring of contaminants in water has the power to transform health, industry, and society. Applications of such monitoring range from process control monitoring and optimisation for industry, to water supply quality and wastewater monitoring, to environmental monitoring for resource extraction, and beyond. Semiconductor-based technology offers high performance for in situ, real-time contaminant monitoring and can also be mass produced at low-cost with flexible functionalisation allowing for a variety of analytes. Furthermore, it offers the ability to integrate multiple sensors into one chip, along with wireless communication technology for maximum benefit of the in situ monitoring capability. AlGaN/GaN transistors have the unique combined benefits of high sensitivity to surface charge, fast response, and excellent chemical and thermal stability. AlGaN and GaN are members of the III-nitride alloy family (AlN, GaN and InN), which has become extremely technologically important for a variety of applications such as wireless base stations, solid-state lighting, automotive electronics, power conditioning and DVD information storage. The properties of AlGaN/GaN transistors are also very attractive for sensors; however, this application niche has not yet been widely explored. The strained AlGaN layer induces a region of high sheet carrier concentration, electron mobility and high saturation velocity induced by piezoelectric polarization known as a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The 2DEG responds to changes in surface charge, so ion selective field effect transistors (ISFETS) can be developed into robust and sensitive ion sensors. Importantly, in contrast to many other potentiometric sensors, including Si-based ISFETS, high sensitivity can be obtained without the need for a reference electrode. This is a significant advantage over electrochemical cell techniques, for which a reference electrode is a major impediment to in situ and robust operation. This paper will report on development of various AlGaN/GaN ISFETs for chemical sensing – in particular pH sensors, nitrate ion, mercury (II) ion and calcium ion sensors. The ability to fabricate different sensors from the same underlying technology paves the way for array-based multi-analyte sensing solutions. The ion sensors were all obtained by functionalising the transistors with a plasticised PVC membrane containing a dissolved ionophore selective to the ion of interest. Using transistor theory, we show that the apparent gate response for these sensors is near-Nenrstian under a variety of testing conditions. In each case, devices without a PVC membrane coating were tested under identical conditions and exhibited negligible response to ion exposure. Some other key results are summarised below. pH sensors AlGaN/GaN transistor-based pH sensors have been investigated by several research groups. When operating without a reference electrode, researchers have reported two types of pH sensor response: linear (related to pH selectivity) and U-shape (related to anion selectivity). Previously, this distinct difference in behaviour had not been well explained. We have compared the pH response of a reference electrode-free AlGaN/GaN device with and without a GaN capping layer. From our investigations, in the absence of a reference electrode, a linear response towards pH requires a GaN capping layer. Apparently the difference in surface chemistry for GaN versus AlGaN surface affects pH/anion selectivity when there is no reference electrode controlling the surface potential. Nitrate ion sensors In a 0.1 M KH2PO4 ion buffer, a detection limit of less than 10− 6M and a linear response range of 10-6-10− 3M were achieved. The detection limit remains consistently low over multiple runs/days. In addition, there was minimal change in sensor response upon addition of KOH increasing the pH from approximately 4–11 [1]. Mercury (II) ion sensors At pH 2.8 in a 10-2 M KNO3 ion buffer, a detection limit below 10−8 M in and a linear response range between 10-8 M-10−4 M were achieved. X-ray photoelectron spectroscopy (XPS) experiments confirmed that the sensing membrane is reversible after being exposed to Hg2+ solution and rinsed with deionised water. Calcium ion sensors In 0.1M KOH ion, a buffer detection limit of less than 10-8 M and a linear response range between 10-8M - 10-3M were achieved. In addition, there was negligible change in sensor response upon the addition of KOH, which increased pH level from approximately 4 to 11. [1] M. Myers, F.L.M. Khir, A. Podolska, G.A. Umana-Membreno, B. Nener, M. Baker, G. Parish, “Nitrate ion detection using AlGaN/GaN heterostructure-based devices without a reference electrode”, Sens. Actuators, B 181 301-5 (2013).

Chemical sensor network for pH monitoring

Monitoring of water sources is a major concern worldwide. Wireless sensor networks (WSN) may be used for this monitoring. However, current systems employ mainly physical sensors for variables such as temperature, pressure, humidity and light. Wireless chemical sensors networks (WCSNs) for environmental monitoring are scarce due to the lack of autonomy of conventional sensors. This paper presents results of a WCSN for monitoring pH based on ion selective field effect transistors (ISFETs). Sensing nodes employ a human interface required for in situ calibration of chemical sensors. Unlike most studies, our work evaluates the network employing chemical measurements and wireless network metrics. Results show zero packet losses by using a time division multiple access (TDMA) protocol. The network allows wireless communication within 300m including attenuation from buildings and trees. Therefore, the system presented in this paper is suitable for long range applications with unobstructed line of sight. pH measurements present a standard deviation below 1%, showing high repeatability. When compared to a commercial pH meter, difference in measurements is below 5%. As a consequence, accuracy is adequate for the application. Measurements also presented high stability during 3 h of continuous measurement.

Development of a novel enzymatic biosensor based on an ion-selective field effect transistor for the detection of explosives

A novel ion-selective field effect transistor (ISFET)-based biosensor was developed for the detection of explosives. Escherichia coli nitroreductase (NTR) was used as the recognizing element of the biosensor to afford high specificity and sensitivity toward nitroaromatic compounds. The ISFET-based device demonstrated an analytical response toward the NTR-catalyzed reduction of several nitroaromatic substances, conjugated to the oxidation of the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) to NADP+. Catalytic properties of nitroreductase were characterized by spectrophotometric analysis. Nitroreductase showed maximum activity toward tetryl and trinitrotoluene, and the Michaelis constants were determined to be 70 and 91μM, respectively. The enzyme was covalently attached to the surface of ISFET using a self-assembly method. Two SiO2-surface immobilization strategies using an asymmetric spacer, 3-maleimidobenzoic acid N-hydroxysuccinimide ester, and a symmetric spacer, glutaric dialdehyde, were compared. The use of the symmetric spacer resulted in higher enzymatic activity of the immobilized nitroreductase. An NTR-functionalized complementary metal–oxide-semiconductor (CMOS)-compatible ISFET chip was combined with a microfluidic system for analyte delivery. The response of the system to several nitroaromatic compounds was examined. The device demonstrated a low detection limit and may be applied for the rapid detection of explosives in various samples.

Oxygen metabolism and pH in coastal ecosystems: Eddy Covariance Hydrogen ion and Oxygen Exchange System (ECHOES)

An aquatic eddy covariance (EC) system was developed to measure the exchange of oxygen (O2) and hydrogen ions (H+) across the sediment-water interface. The system uses O2 optodes and a newly developed micro-flow cell H+ ion selective field effect transistor; these sensors displayed sufficient precision and rapid enough response times to measure concentration changes associated with turbulent exchange. Discrete samples of total alkalinity and dissolved inorganic carbon were used to determine the background carbonate chemistry of the water column and relate the O2 and H+ fluxes to benthic processes. The ECHOES system was deployed in a eutrophic estuary (Waquoit Bay), and revealed that the benthos was a sink for acidity during the day and a source of acidity during the night, with H+ and O2 fluxes of ± 0.0001 and ± 10 mmol m−2 h−1, respectively. H+ and O2 fluxes were also determined using benthic flux chambers, for comparison with the EC rates. Chamber fluxes determined in 0.25 h intervals co-varied with EC fluxes but were ∼ 4 times lower in magnitude. This difference was likely due to suppressed pore-water advection in the chambers and changes in the chemistry of the enclosed chamber overlying water. The individual H+ and O2 fluxes were highly correlated in each dataset (EC and chambers), and both methods yielded H+ fluxes that could not be explained by O2 metabolism alone. The ECHOES system provides a new tool for determining the influence of benthic biogeochemical cycling on coastal ocean acidification and carbon cycling.