Ion-selective Field Effect Transistor Research Articles

Concept and Realisation of ISFET-Based Measurement Modules for Infield Soil Nutrient Analysis and Hydroponic Systems

Ion-selective field-effect transistors (ISFETs) offer potential as micro-sensors for in situ monitoring of complex target variables in real-time closed loop actions. This article presents the concept and realisation of application-specific ISFET-based measurement systems for two different agricultural domains: infield soil measurements and hydroponic systems. Commercially available ISFETs were integrated as multi-sensor modules as well as single-sensor units for the measurement of plant-available nutrients, such as H2PO4−, NO3−, K+ or NH4+, and pH-values. Moreover, application-relevant pH values as well as temperatures for calibration purposes were measured. ISFETs were selected according to the relevant measurement dynamics for the applications. For the development and testing procedures, a laboratory setup was built up. Supported by reference materials, the outputs of the ISFETs were evaluated with respect to stability under the influence of disturbance variables, reproducibility and settling time. The results were used to develop new readout electronics. Next to stability, conditioning and calibration processes were relevant. The micro-sensors were integrated in new application-specific mechatronic handling systems and process flows. The realisation and tests are presented as well as first measurements in outdoor fields and indoor hydroponic environments.

PHYSICAL MODEL OF TEMPORARY CURRENT INSTABILITY IN ION- SELECTIVE FIELD-EFFECT TRANSISTORS WITH SILICON OXIDE/NITRIDE DIELECTRIC LAYERS

The work is devoted to the study of the instability (drift) of the threshold voltage of p-channel ion selective field- effect transistors (ISFET) with an induced channel, caused by the long-term effect of a negative voltage applied to the channel section of the transistor through a double- layer dielectric SiO2/Si3N4. Based on the obtained experimental data, a physical model of the observed instability is proposed, according to which it is caused by the process of accumulation of an unbalanced positive charge in the near-surface layer of silicon nitride. The mechanisms of Pool–Frenkel superbarrier ionization and multiphonon ionization of deep centers are involved in the quantitative calculation of model parameters. Design and technological improvements are outlined that can improve the stability of ISFET sensors for long-term continuous measurements in solutions.

Graphene-Based Ion-Selective Field-Effect Transistor for Sodium Sensing.

Field-effect transistors have attracted significant attention in chemical sensing and clinical diagnosis, due to their high sensitivity and label-free operation. Through a scalable photolithographic process in this study, we fabricated graphene-based ion-sensitive field-effect transistor (ISFET) arrays that can continuously monitor sodium ions in real-time. As the sodium ion concentration increased, the current–gate voltage characteristic curves shifted towards the negative direction, showing that sodium ions were captured and could be detected over a wide concentration range, from 10−8 to 10−1 M, with a sensitivity of 152.4 mV/dec. Time-dependent measurements and interfering experiments were conducted to validate the real-time measurements and the highly specific detection capability of our sensor. Our graphene ISFETs (G-ISFET) not only showed a fast response, but also exhibited remarkable selectivity against interference ions, including Ca2+, K+, Mg2+ and NH4+. The scalability, high sensitivity and selectivity synergistically make our G-ISFET a promising platform for sodium sensing in health monitoring.

A Review on the Development of Spectroscopic Sensors for the Detection of Creatinine in Human Blood Serum

Creatinine measurement is the key parameter in detecting renal, muscular and thyroid dysfunction. The accurate detection of creatinine level may be informative regarding the functional processes of these systems and help in early detection of acute diseases.There are lots of techniques available for detecting creatinine in human blood serum, most of them are of mainly based on spectroscopic (spectrophotometry, colorimetry and fluorimetric). Other techniques are based on electrochemical, impedometrical, Ion Selective Field-Effect Transistor (ISFET) and chromatography techniques. Each method has its own advantages and few limitations(limitation would be better word) regarding selectivity, sensitivity, reproducibility, cost effective, point-of-care level detection etc. Few methods based on electrochemical techniques are recently promising in detecting creatinine at the point-of-care level with adequate sensitivity and selectivity. On the other hand some biosensors based on spectroscopic techniques are recognized as the most promising substitute in recent years. As creatinine levels in the blood serum offer better information about patient status, here in this review it is thoroughly discussed over other biological samples such as urine, saliva.

All-electrical antibiotic susceptibility testing within 30 min using silicon nano transistors

Rapid and reliable antibiotic susceptibility testing (AST) platform is highly desired to select the right antibiotics to treat infectious disease at early stage. Here, we demonstrate rapid ASTs using nanoscale silicon ion-selective field-effect transistor sensors. Our sensors profile bacterial metabolic kinetics by monitoring the metabolism induced acidification in the growth media with the absence and the presence of different antibiotics. Rapid AST results could be determined from the metabolic profiles with a total assay time less than 30 min for different bacterial strains. In addition, the sensors could also distinguish the bactericidal mechanisms for antibiotics with different modes of actions. Furthermore, the initial bacterial concentration in an unknown sample, a key parameter to determine its clinic relevance, could be estimated based on the metabolic profiles. Our demonstrated AST method is all-electrical, label-free and silicon technology compatible, and holds great promise for the development of a high-throughput and low-cost point-of-care device.

Sequential Injection System for Analysis of Degree Brix, Orthophosphate and pH in Raw Sugarcane Juice Applicable to Sugar Industry.

This work presents, for the first time, a new sequential injection analysis (SIA) method to simultaneously analyze degree Brix, orthophosphate and pH in raw cane juice. These key parameters relate to price of harvested sugarcane and quality of cane juice for sugar production. The SIA system employed two detectors: the first detector is a diode-array spectrophotometer, equipped with a regular flow cell, for measurements of degree Brix and orthophosphate. Quantitative of degree Brix (°Bx; ca. % (w/w) sucrose) was based on manipulation of the schlieren effect at the interface between plugs of sample and water. Orthophosphate analysis was carried out based on the molybdenum blue method with significant reduction in consumption of the reagents. Compensation of the schlieren effect from sucrose for determination of orthophosphate was achieved by using a dual-wavelength spectrometric detection. Second detector is a pH-sensing device, called ion-selective field-effect transistors (ISFET). The ISFET is based on the current through the ISFET arising according to the H+ concentration in solution. Our developed SIA system provides linear calibration graphs fitting for purpose in analysis of sugarcane juice (pH: 0–14, °Bx: 1.0–7.0 and P2O5: 20–200 mg L−1). Simultaneous analysis of sugarcane juice for pH, °Bx and P2O5 is carried out within 5 min (12 sample per h). Precision of SIA system is acceptable (RSD < 3%). Our SIA system gave quantitative results insignificantly different, as compared with conventional methods for analysis of pH, °Bx and P2O5 in sugarcane juice.

Electrical and Label-Free Detection of T-Cell Activation Against COVID-19: A Method Towards Point-of-Care Monitoring of Immune System

Despite great progress in biotechnology, COVID19 has revealed the inadequacy of current healthcare technologies to contain an infectious outbreak, specifically in rapid infection recognition to facilitate containment. Wide deployment of testing methods such as viral RNA (antigen) PCR and antibody detection have provided some relief in this regard; however, current technologies still require many hours to provide results. Thus, there remains a critical need for fast, sensitive, low-cost point-of-care (POC) biosensing assays to report on the infection status and immunity of patients. Emerging data on the importance of T-cell responses in generating long-term immunity against COVID19 has highlighted T-cell activation detection as a potential way for real-time monitoring native and acquired immunity. However, current approaches for T-cell stimulation monitoring are complex, laborious, time-consuming, and costly.Here we provide proof-of-principle experiments which indicate that an electrical and label-free detection method based on CMOS-compatible silicon nanowire ion-selective field-effect transistors (SNW-ISFET) can act as a fast, sensitive, reliable, and low-cost approach to monitor T-cell response to pathogens such COVID-19. This technology enables direct detection of COVID-19-specific T-cells directly from patient blood, with potential utility as a POC diagnostic device. Detection of antigen-specific T-cell activation from clonal human T-cells lines utilizing CD8-restricted peptide-MHC complexes has been demonstrated using SNW-ISFET devices. The cell concentration was varied to achieve a clear understanding about the device sensitivity, detection limit, and reliability (Figure 1). SNW-ISFET structures were applied to detect T-cell activation in clonal T-cell lines and whole peripheral blood mononuclear cell (PBMC) with multiple cell concentrations (Figure 2). Reference experiments were applied to ensure the device reliability and sensing mechanism (Figure 3). These findings show that SNW-FETs platform are promising candidates for antigen-specific T-cell activation detection from whole blood. Figure 1

Highly sensitive FET sensors for cadmium detection in one drop of human serum with a hand-held device and investigation of the sensing mechanism.

As the heavy metal contamination is becoming worse, monitoring the heavy metal content in water or human body gets more and more important. In this research, a cadmium ion-selective field effect transistor (Cd-ISFET) for rapidly detecting cadmium ions has been developed and the mechanism of the sensor is also investigated in depth. Our Cd-ISFET sensor exhibits high sensitivity beyond the ideal Nernst sensitivity, wide dynamic range, low detection limit (∼10-11M), which is comparable with inductively coupled plasma mass spectrometry, and easy operation enabling people to detect cadmium ion by themselves. From the analysis of electrical measurement results, this Cd-ISFET is preferred to operate at the bias with the maximum transconductance of the FET to enhance the sensor signal. The AC impedance measurement is carried out to directly investigate the mechanism of an ion-selective membrane (ISM). From impedance results, the real part of the total impedance, which is the resistance, was shown to dominate the sensor signal. The potential drop across the ISM is caused by the heavy metal ion in the membrane, which is employed to the gate of the FET via an extended gate electrode. Cadmium ion detection in one drop of human serum with this sensor was demonstrated. This cost-effective and highly sensitive sensor is promising and can be used by anyone and anywhere to prevent people from cadmium poisoning.

Measuring pH in low ionic strength glacial meltwaters using ion selective field effect transistor (ISFET) technology

AbstractMeasuring pH in glacial meltwaters is challenging, because they are cold, remote, subject to freeze‐thaw cycles and have low ionic strength. Traditional methods often perform poorly there; glass electrodes have high drift and long response times, and spectrophotometric techniques are unpractical in cold, remote environments. Ion selective field effect transistor (ISFET) sensors are a promising alternative, proven in marine and industrial applications. We assess the suitability of two models of ISFET, the Honeywell Durafet and Campbell Scientific Sentron, for use in glacial melt through a series of lab and field experiments. The sensors have excellent tolerance of freeze‐thaw and minimal long‐term drift, with the Durafet experiencing less drift than the Sentron model. They have predictable response to temperature, although the Durafet housing causes some lag during rapid cycling, and the impact of stirring is an order of magnitude less than that of glass electrodes. At low ionic strength (&lt; 1 mmol L−1), there is measurable error, but this is quantifiable, and less than glass electrodes. Field tests demonstrated low battery consumption, excellent longevity and resistance to extreme conditions, and revealed biogeochemical processes that were unlikely to be recorded by standard methods. Meltwater pH in two glacial catchments in Greenland remained &gt; 7 with consistent diurnal cycles from the very first meltwater flows. We recommend that ISFET sensors are used to assess the pH of glacial meltwater, since their tolerance is significantly better than alternative methods: the Durafet is accurate to ± 0.2 pH when waters are &gt; 1 mmol L−1 ionic strength, and ± 0.3 pH at &lt; 1 mmol L−1.

Microstructural properties and sensitive performance of TbTaxOy sensing membranes for electrolyte-insulator-semiconductor pH sensors

The sensitivity and stability of sensing film material are mainly factors to impede the practical applications of ion-selective field effect transistor (ISFET) or electrolyte-insulator-semiconductor (EIS) pH sensors. Metal oxide films have demonstrated great advances on achieving high-performance pH sensors. However, some metal oxide films with low sensitivity and poor reliability still hinder the biochemical, environmental and medical applications. Therefore, choosing a proper sensing film material plays a very important role for the fabrication of highly sensitive and reliable sensors. In this article, the novel TbTaxOy sensing membrane demonstrating a super-Nernstian pH response and excellent stabililty was developed for a solid-state pH sensor. We investigated the relationship between sensing performance and microstructural property of the TbTaxOy sensing membranes deposited onto Si substrates through a reactive co-sputtering system and subsequently post-deposition annealing (PDA) at four temperatures (from 600°C to 900°C). To check the influence of PDA treatment on the microstructural properties of the TbTaxOy films, atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were employed to manifest the morphological, microstructural, and chemical features, respectively, of these films. Among these PDA temperatures, the TbTaxOy EIS device after annealing at 900 °C exhibited the best sensing performances including, the super-Nernstian sensitivity (67.04 mV/pH), the lowest drift rate (0.12 mV/h), and the smallest hysteresis voltage (1 mV). Probably, this PDA temperature can enhance the surface roughness, form the stoichiometry of TbTaO4 film, and reduce the crystal defects. The super-Nernstian response may be contributed to a variation in the oxidation state of Tb ion changing from Tb3+ to Tb2+, thereby generating two H+ ions and one electron transferred in the chemical reaction. Moreover, this TbTaxOy sensing film demonstrates better sensing performance compared to previously fabricated sensing materials available in the literature.

Internal Electrical Noises of BioFET Sensors Based on Various Architectures

The results of a comparative literature analysis of internal electrical noises and signal-to-noise ratio for nanoscale BioFET (biological field-effect transistor) and DNA (deoxyribonucleic acid) sensors based on different architectures MIS (metal-insulator-semiconductor), EIS (electrolyte-insulator-semi-conductor) and ISFET (ion-selective field-effect transistor) are presented. Main types, models and mechanisms of internal noises of bio- & chemical field-effect based sensors are analyzed, summarized and presented. For the first time, corresponding detail electrical equivalent circuits were built to calculate the spectral densities of noises generated in the active part of a solid (semiconductor, dielectric) and in an aqueous solution for MIS, EIS and ISFET structures based sensors. Complete expressions are obtained for the rms (root mean square) value of the noise current (or voltage), as well as the noise spectral densities for the architectures under study. The miniaturization of biosensors leads to a decrease in the level of the useful signal-current. For successful operation of the sensor, it is necessary to ensure a high value of the SNR (signal-to-noise ratio). In case of weak useful signals, it is necessary to reduce the level of internal electrical noise. This work is devoted to a detailed study of the types and mechanisms of internal electrical noises in specific biosensor architectures.

Квазісинхронна термокомпенсація в іонометрії із застосуванням ІСПТ. Частина 1. Теорія та моделювання

Solid-state ion selective transducers, as an alternative to the traditional liquid electrolyte-filled glass electrodes, are known for over four decades now, and find their use in various areas of industry and applied science, such as in vivo analysis of the ions activity in biological and medical research, monitoring of toxic and aggressive environments, and biosensors design. However, along with potential advantages — short response time, small size, chemical inertness and durability — solid-state devices also possess certain inherent drawbacks — namely intrinsic noise, drift and instability of sensing properties, and cross-sensitivity to various interfering environmental conditions — that inhibit their widespread acceptance. Further improvement of the fabrication technology and methodology of application of these devices is thus still an important practical task even today. This paper is a first part of the two-part work dedicated to the problem of compensating the temperature dependence of a solid-state ion selective transducer output. Specifically, presented work considers the possibility of using ion-selective field-effect transistors (ISFET) that serve as primary transducers in an ionometric device, as temperature sensors. This allows compensating the temperature dependence of ionometric signal without substantial complication of the ionometer structure, and eliminates the need to include a separate thermometric channel as part of the instrument. Ionometric and thermometric channels are combined into a unified measuring path, with the sensor functions separated in time. The ISFET operation modes are switched by changing polarity of the bias voltage, and thus direction of the current flowing through the sensor. The authors propose a corresponding secondary transducer structure and simplified schematic illustrating the implementation of its key components. The concept’s applicability is supported by the circuit simulation results. Some aspects of the practical implementation of the proposed concept will be presented further in the upcoming second part of the paper.

Induced bioresistance via BNP detection for machine learning-based risk assessment

Machine Learning (ML) is a powerful tool for big data analysis that shows substantial potential in the field of healthcare. Individual patient data can be inundative, but its value can be extracted by ML's predictive power and ability to find trends. A great area of interest is early diagnosis and disease management strategies for cardiovascular disease (CVD), the leading cause of death in the world. Treatment is often inhibited by analysis delays, but rapid testing and determination can help improve frequency for real time monitoring. In this research, an ML algorithm was developed in conjunction with a flexible BNP sensor to create a quick diagnostic tool. The sensor was fabricated as an ion-selective field effect transistor (ISFET) in order to be able to quickly gather large amounts of electrical data from a sample. Artifical samples were tested to characterize the sensors using linear sweep voltammetry, and the resulting data was utilized as the initial training set for the ML algorithm, an implementation of quadratic discriminant analysis (QDA) written in MATLAB. Human blood serum samples from 30 University of Pittsburgh Medical Center (UPMC) patients were tested to evaluate the effective sorting power of the algorithm, yielding 95% power in addition to ultra fast data collection and determination.

Current-Mode Signal Enhancement in the Ion-Selective Field Effect Transistor (ISFET) in the Presence of Drift and Hysteresis

The accuracy of the ion-selctive field effect transistor (ISFET) is limited by errors ascribed to drift and hysteresis. In this work operation of the ISFET as a stand-alone pH sensor operating in the current mode is demonstrated, the dependence of drift on pH is characterized and modeled, and the relationship between drift and hysteresis is investigated. Also, a post-processing method for extraction of the ISFET current signal in the presence of drift and hysteresis is formally developed. This method, which is based on sampling the drain current over relatively short time intervals, is verified experimentally in the presence of drift and hysteresis by monitoring step changes in pH using a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -gate ISFET biased in the triode region with the pH of the solution cycled up and down over a seven-unit range. The corrective algorithm was also demonstrated by extracting the measuring signal using an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -gate ISFET operating in the current mode to monitor pH variations ranging from 4 to 10. The theoretical basis of the proposed method is validated by developing the concept of signal-to-drift ratio as a figure of merit for the resolution of pH-sensitive ISFETs. The signal-to-drift ratio is shown to increase in proportion to the ISFET transconductance (g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) suggesting that the optimization of g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> can improve the resolution of pH readings. SPICE simulations based on the measured drift data for the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -gate ISFET indicated an approximately 0.1pH unit improvement in the accuracy of the ISFET for a two-fold increase in the aspect ratio of the channel (W/L).

One-step immunoassay without washing steps for influenza A virus detection using ISFET

A one-step immunoassay for influenza A virus detection was developed using two different microbeads and a filter-inserted bottle. Two bead types with diameters of 15 (capture bead) and 3 (detection bead) μm were prepared to specifically detect influenza A virus. Anti-influenza A virus antibodies were coated on both bead types, whereas urease was immobilized only on the detection bead. An influenza A-positive sample could form a sandwich complex with the capture and detection beads; this complex would not pass through the filter, which had a controlled pore size. As the detection bead was used at a limiting concentration, it would be prevented from crossing the filter; thus, it would further react with the substrate urea and consequently increase the pH. An influenza A-negative sample would fail to form the sandwich complex in the presence of the capture and detection beads. Accordingly, the detection bead would pass through the filter into the urea buffer and increase the pH. The pH change in the urease reaction could be quantitatively measured by an indicator such as phenol red or using ion-selective field-effect transistor (ISFET). This one-step immunoassay was used for the detection of influenza A virus in real samples. The receiver operating characteristic (ROC) plot analysis showed an area under the curve (AUC) value of 0.931; the sensitivity and specificity of the assay was 80% and 90%, respectively, at a cutoff value of 0.9986. These results demonstrate that the one-step immunoassay could increase the sensitivity of influenza A virus detection in real samples.

Perspective of Printed Solid‐State Ion Sensors toward High Sensitivity and Selectivity

Ion‐selective field‐effect transistors (ISFETs) are integrated device systems with transistors and ion‐selective electrodes. The printed ISFET's sensing mechanisms are discussed. Then, different sensitive membranes linked with ISFETs are highlighted. To achieve high sensitivity and selectivity of ISFETs, various printing methods have been introduced recently. The real‐time monitoring sensing applications, such as ion detection in health, food safety, and environmental sectors, are discussed with the highlight on the biological applications for the cost‐efficient detection of DNA molecules. Finally, the challenges of the ISFETs are considered and illustrated with the proposed perspectives.

Effect of amino-, mercapto-silane coupling as a molecular bridge of polyvinyl chloride ion-selective membrane on silicon nitride for nitrate ISFET sensors

This paper presents a modification of a Si3N4 Polyvinyl chloride (PVC) membrane on a surface of an ion selective field effect transistor (ISFET) using silane with two different functional groups for comparison of nitrate sensing characteristics. Effects of amino (NH2) and thiol groups (SH) toward structures and characteristics of nitrate sensors were analyzed using the following techniques: ellipsometry, scanning electron microscope (SEM), infrared spectroscopy (FTIR), zeta potential and electrochemical impedance spectroscopy (EIS). After surface modification, the thickness of the silane layers for both cases were similar with uniform surface topology. Absolute zeta potentials of PVC ion-selective membrane (7.65 mV) and Si3N4 surface modification with MPTMS (−51.46 mV) reflected the surface functional group and showed that membranes could be bonded to the PVC ion-selective membrane. Since APTMS produced higher EIS than MPTMS, the resulting nitrate sensors modified by MPTMS had higher sensitivity than the sensors with APTMS. Finally, the modified sensors had a useful life time of 3 months.

Modification of polyvinyl chloride ion-selective membrane for nitrate ISFET sensors

In this work, a Polyvinyl Chloride (PVC) ion-selective membrane was modified to detect nitrate based on Ion Selective Field Effect Transistor (ISFET) sensing technology in order to eliminate chloride interference. A modification was done with ethylenediamine solution to achieve animated PVC, which was then oxidized with sodium tungstate to form nitrone coated PVC membrane. The modified PVC was characterized using FTIR, 1H NMR, GPC, FE-SEM and EIS. The FTIR spectrum of animate PVC demonstrated -NH2 bending at 1580 cm−1, NO2 asymmetric stretching at 1650 cm−1 and NO out-of-plane deformation vibration at 837 cm−1. GPC analysis of the modified PVC showed that the PVC molecular weight was shifted to a high molecular weight. The PVC ion-selective membrane was immobilized on the ISFET to create nitrate sensors with the following characteristics. The Nitrate-Nitrogen detection limit is 1.20 ppm with linear range from 3−20 ppm at sensitivity of 56 ± 2 mV/dec. The sensor is useful for detecting small amount of nitrate in a mixed solution, which usually is the case in most situations. This sensor could be applied in many applications, such as agricultural industry, water reserve management and medical field as well.

Highly Sensitive Lead Ion Detection in One Drop of Human Whole Blood Using Impedance-Modulated Field-Effect Transistors and a Portable Measurement Device

This study reports the development of a rapid heavy metal ion screening tool for whole blood samples. The test system consists of an ion selective membrane-based impedance modulated field effect transistor (FET) sensor and a portable sensor measurement unit. This study focuses on the direct detection of lead ions in whole blood, without extensive sample pre-treatments. The sensing methodology is based on impedance changes in the membrane due to the specific ionophore-target metal ion interaction in the presence of a strong electric field. The changes in impedance caused by heavy metal ions are amplified by the FET to obtain a highly sensitive lead ion detection, with very low detection limit (near 10−11 ∼ 10−10 M Pb2+). To reduce the complexity and enhance sensor performance, the whole blood samples are fractionated on chip without any external actuation/automation using simple gravitational blood cell separation. The test results in PBS, human serum and whole blood samples demonstrate high sensitivity and wide dynamic range of lead ion detection. The miniaturized extended gate FET sensor system is ideal for point of care and home-care diagnostics for heavy metal ion in blood.

Estimating Detection Limits of Potentiometric DNA Sensors Using Surface Plasmon Resonance Analyses.

As the signals of potentiometric-based DNA ion-selective field effect transistor (ISFET) sensors differ largely from report to report, a systematic revisit to this method is needed. Herein, the hybridization of the target and the probe DNA on the sensor surface and its dependence on the surface probe DNA coverage and the ionic strength were systematically investigated by surface plasmon resonance (SPR). The maximum potentiometric DNA hybridization signal that could be registered by an ISFET sensor was estimated based on the SPR measurements, without considering buffering effects from any side interaction on the sensing electrode. We found that under physiological solutions (200 to 300 mM ionic strength), the ISFET sensor could not register the DNA hybridization events on the sensor surface due to Debye screening. Lowering the salt concentration to enlarge the Debye length would at the same time reduce the surface hybridization efficiency, thus suppressing the signal. This adverse effect of low salt concentration on the hybridization efficiency was also found to be more significant on the surface with higher probe coverage due to steric hindrance. With the method of diluting buffer, the maximum potentiometric signal generated by the DNA hybridization was estimated to be only around 120 mV with the lowest detection limit of 30 nM, occurring on a surface with optimized probe coverage and in the tris buffer with 10 mM NaCl. An alternative method would be to achieve high-efficiency hybridization in the buffer with high salt concentration (1 M NaCl) and then to perform potentiometric measurements in the buffer with low salt concentration (1 mM NaCl). Based on the characterization of the stability of the hybridized DNA duplexes on the sensor surface in low salt concentration buffer solutions, the estimated maximum potentiometric signal could be significantly higher using the alternative method. The lowest detection limit for this alternative method was estimated to be around 0.6 nM. This work can serve as an important quantitative reference for potentiometric DNA sensors.