Ion-sensitive Field-effect Transistor Arrays Research Articles

Robust chemical analysis with graphene chemosensors and machine learning.

Ion-sensitive field-effect transistors (ISFETs) have emerged as indispensable tools in chemosensing applications1-4. ISFETs operate by converting changes in the composition of chemical solutions into electrical signals, making them ideal for environmental monitoring5,6, healthcare diagnostics7 and industrial process control8. Recent advancements in ISFET technology, including functionalized multiplexed arrays and advanced data analytics, have improved their performance9,10. Here we illustrate the advantages of incorporating machine learning algorithms to construct predictive models using the extensive datasets generated by ISFET sensors for both classification and quantification tasks. This integration also sheds new light on the working of ISFETs beyond what can be derived solely from human expertise. Furthermore, it mitigates practical challenges associated with cycle-to-cycle, sensor-to-sensor and chip-to-chip variations, paving the way for the broader adoption of ISFETs in commercial applications. Specifically, we use data generated bynon-functionalized graphene-based ISFET arrays to train artificial neural networks that possess a remarkable ability to discern instances of food fraud, food spoilage and food safety concerns. We anticipate that the fusion of compact, energy-efficient and reusable graphene-based ISFET technology with robust machine learning algorithms holds the potential to revolutionize the detection of subtle chemical and environmental changes, offering swift, data-driven insights applicable across a wide spectrum of applications.

Improved Stability of Graphene-Coated CMOS ISFETs for Biosensing.

A polymer-assisted graphene transfer method is used to transfer sheets of monolayer and multilayer graphene onto the passivation layer of ion-sensitive field effect transistor arrays. The arrays are fabricated using commercial 0.35 μm complementary metal-oxide-semiconductor (CMOS) technology and contain 3874 pixels sensitive to pH changes on the top silicon nitride surface. By inhibiting dispersive ion transport and hydration of this underlying nitride layer, the transferred graphene sheets help address non-idealities in the sensor response while retaining some pH sensitivity due to the presence of ion adsorption sites. Improvements in hydrophilicity and electrical conductivity of the sensing surface after graphene transfer, as well as in-plane molecular diffusion along the graphene-nitride interface, also greatly improve spatial consistency across an array, allowing for ∼20% more pixels to remain within operating range and enhancing sensor reliability. Multilayer graphene offers a better performance trade-off than monolayer graphene, reducing drift rate by ∼25% and drift amplitude by ∼59% with minimal reduction in pH sensitivity. Monolayer graphene offers slightly better temporal and spatial uniformity in performance of a sensing array, which is associated with the consistency in layer thickness and a lower defect density.

A 640 × 640 ISFET array for detecting cell metabolism

Ion sensitive field effect transistor (ISFET) devices are highly accurate, convenient, fast and low-cost in the detection of ions and biological macromolecules, such as DNA molecules, antibodies, enzymatic substrates and cellular metabolites. For high-throughput cell metabolism detection, we successfully designed a very large-scale biomedical sensing application specific integrated circuit (ASIC) with a 640 × 640 ISFET array. The circuit design is highly integrated by compressing the size of a pixel to 7.4 × 7.4 μm2 and arranging the layout of even and odd columns in an interdigital pattern to maximize the utilization of space. The chip can operate at a speed of 2.083M pixels/s and the dynamic process of the fluid flow on the surface of the array was monitored through ion imaging. The pH sensitivity is 33 ± 4 mV/pH and the drift rate is 0.06 mV/min after 5 h, indicating the stability and robustness of the chip. Moreover, the chip was applied to monitor pH changes in CaSki cells metabolism, with pH shifting from 8.04 to 7.40 on average. This platform has the potential for continuous and parallel monitoring of cell metabolism in single-cell culture arrays.

Discrimination of bacterial and viral infection using host-RNA signatures integrated in a lab-on-chip platform

The unmet clinical need for accurate point-of-care (POC) diagnostic tests able to discriminate bacterial from viral infection demands a solution that can be used both within healthcare settings and in the field, and that can also stem the tide of antimicrobial resistance. Our approach to solve this problem combine the use of host gene signatures with our Lab-on-a-Chip (LoC) technology enabling low-cost POC expression analysis to detect Infectious Disease. Transcriptomics have been extensively investigated as a potential tool to be implemented in the diagnosis of infectious disease. On the other hand, LoC technologies using ion-sensitive field-effect transistor (ISFET), in conjunction with isothermal chemistries, are offering a promising alternative to conventional amplification instruments, owing to their portable and affordable nature. Currently, the data analysis of ISFET arrays are restricted to established methods by averaging the output of every sensor to give a single time-series. This simple approach makes unrealistic assumptions, leading to insufficient performance for applications that require accurate quantification such as Host-Transcriptomics. In order to reliably quantify transcripts on our LoC platform enabling the classification of infectious disease on-chip, we propose a novel data-driven algorithm for extracting time-to-positive values from ISFET arrays. The algorithm proposed correctly outputs a time-to-positive for all the reactions, with a high correlation to RT-qLAMP (0.85, R2 = 0.98, p < 0.01), resulting in a classification accuracy of 100% (CI, 95–100%). This work aims to bridge the gap between translating assays from microarray analysis to ISFET arrays providing benefits on tackling infectious disease and diagnostic testing in hard-to-reach areas of the world.

CMOS ISFET Arrays for Integrated Electrochemical Sensing and Imaging Applications: A Tutorial

The ion-sensitive field-effect transistor (ISFET) is a type of electrochemical sensor with a wide range of applications. They offer advantages of being compatible with standard CMOS technology, a miniaturised form-factor, robustness, scalability and low power requirements to name a few. There are now many architectures and design strategies to construct ISFET-based systems in CMOS, and the appropriate choice of these depends heavily on the specifications of the intended application. This tutorial aims to give a designer the knowledge needed to make the best decisions for the required specifications by providing a background in theory and an overview of design trade-offs and existing approaches. Example designs which maximise performance in particular applications are discussed and practical considerations for simulation, layout and implementation are presented.

A comprehensive review of FET‐based pH sensors: materials, fabrication technologies, and modeling

AbstractThe demand for miniaturized point‐of‐care chemical/biochemical sensors has driven the development of field‐effect transistors (FETs) based pH sensors over the last 50 years. This paper aims to review the fabrication technologies, device structures, sensing film materials, and modeling techniques utilized for FET‐based pH sensors. We present the governing principles of potentiometric sensors, with major focus on the working principles of ion‐sensitive FETs (ISFETs). We extensively review different sensing film materials deposited by various techniques, which is critical to the sensing performance of ISFETs. The popular fabrication technologies have been presented, with special emphasis on state‐of‐the‐art silicon‐on‐insulator based technology, which can achieve high sensitivity by utilizing the dual‐gate effect. Furthermore, recent advancements in nano‐ISFETs has been elucidated. We also discuss the adoption of unmodified complementary metal‐oxide semiconductor (CMOS) ISFETs using standard CMOS processes, which has enabled the fabrication of integrated ISFET arrays, which are especially suited for ion‐imaging applications. Moreover, recent developments in extended‐gate FETs has been discussed, which have gained lot of attention due to their design flexibility and ease of fabrication, which is desirable for wearable sensing applications. In addition, recently there have been efforts to utilize nonsilicon channel materials for pH‐sensing application to obtain superior performance and various channel materials have been reviewed. Finally, we have extensively reviewed the ISFET device modeling and simulation techniques using various computer‐aided design tools, which aid in sensor design and characterization.

(Invited) Recent Advances in Ion Sensitive Graphene Field Effect Transistors

I will summarize recent advances [1-3] in large-area graphene based ion sensitive field effect transistors (ISFETs), including the achievement of record potentiometric ion concentration resolution. Sensor resolution, the smallest measurable change in analyte concentration, is a critical parameter in the assessment of sensor performance. Notably, the absence of a bandgap in graphene is frequently perceived as prohibiting competitive graphene FET performance. A graphene FET cannot be driven into saturation, limiting intrinsic gain and high-frequency performance. The graphene FET cannot be turned off, severely limiting the on/off current ratio for switching applications. In contrast, in the case of graphene ISFETs, the graphene FET functions as an analog pre-amplifier directly integrated with a sensing layer, enabling record sensor resolution to be achieved.Large-area graphene ISFETs combine high mobility charge transport, low flicker noise, facile integration with a variety of ion sensitive layers. The advent of wafer-scale graphene processing methods facilitates reproducible production of graphene ISFETs. Field effect mobilities in graphene ISFETs can reach μ = 5000 cm2 V-1 s-1, contributing to high gain gm = ∂Id /∂Vg ∝ μVDS. In particular, high mobility allows for the reduction of bias voltage VDS , which improves potentiometric uniformity across the sensor channel. We further find that low-frequency (1/f) noise, which ultimately determines sensor resolution, decreases approximately inversely with graphene FET channel area, in accord with 1/f scaling in Si MOSFETs. In other words, large transistors are quiet transistors. Scaling graphene FETs up to a 5 mm x 5 mm channel area, we observe a normalized 1/f noise parameter K = f <Δv 2>/v0 2 = 5×10-13, the lowest reported to date for a graphene FET at room temperature. Facile integration of graphene FETs with ion sensitive layers, such as metal oxides and ionophore polymer membranes, enables sensitization to a wide range of ions including H+, K+, Na+, NH4 +, Cl-, NO3 -, HPO4 2- and SO4 2-. Combining the Nernstian limited sensitivity with low 1/f noise enables record ion concentration resolution as low as r ~ 0.003 log M.Beyond sensor resolution, specificity is another important sensor parameter. Ionophores suffer cross-sensitivity to ionic species other than their target ion. The combined stability and resolution of graphene ISEFTs allows simultaneous, real-time measurement of multiple ionic analytes through the use of an array of graphene ISFETs, with each graphene ISFET element sensitized to a particular ion. Measurement of the cross-sensitivity within the array allows the application of Nikolskii-Eisenmann analysis to quantitatively account for cross-sensitivity, and achieve selective response with the graphene ISFET array that would otherwise not be possible with graphene ISFETs operated in isolation. I will conclude with a discussion of challenges and prospects for the future development of graphene FET based sensors. REFERENCES [1] Tran et al., Nanotechnology 32, 045502 (2020).[2] Fakih et al., Nature Communications 11, 3226, (2020).[3] Fakih et al., Sensors and Actuators B 291 89 (2019).

A CMOS Test Chip With Simple Post-Processing Steps for Dry Characterization of ISFET Arrays

ISFETs (ion sensitive field effect transistors) are ion solid-state sensors based on MOSFETs (metal oxide semiconductor field effect transistors) applied in many areas including ion imaging, detection of viruses and bacteria and DNA sequencing. ISFETs implemented in standard CMOS technology adopt the inherent Si3N4 passivation as the sensing layer, and benefit from a high integration level with no need of post-processing steps. However, this implementation is associated with non-idealities such as output temporal drift, capacitive attenuation of the input signal and random offsets in the threshold voltage due to trapped charges. Most of these non-idealities, which require better understanding, are characterized through biasing a reference electrode immersed in a solution in contact with the passivation layer. To remove electrochemical effects from the measurements, this paper proposes a dry test where the wet test setup is replaced with a sputtered gold thin film that provides electrical contact with the ISFETs. Dry tests were performed in arrays of n-ISFETs and p-ISFETs from chips fabricated in a standard 180 nm high voltage CMOS technology. The intra-die threshold voltages of the n-ISFETs and p-ISFETs presented, on average, similar offsets, but their die-to-die fluctuation was high. Wet tests on an n-ISFET and a p-ISFET of two non-metalized chips indicated sensitivities of 38 mV/pH and 46 mV/pH, respectively. The dry test introduced herein allows the electrical parameters of ISFETs to be determined without the interference of chemical contributions as well as the charges trapped in the passivation layer to be estimated.

Complementary Metal-Oxide-Semiconductor Potentiometric Field-Effect Transistor Array Platform Using Sensor Learning for Multi-ion Imaging.

This work describes an array of 1024 ion-sensitive field-effect transistors (ISFETs) using sensor-learning techniques to perform multi-ion imaging for concurrent detection of potassium, sodium, calcium, and hydrogen. Analyte-specific ionophore membranes are deposited on the surface of the ISFET array chip, yielding pixels with quasi-Nernstian sensitivity to K+, Na+, or Ca2+. Uncoated pixels display pH sensitivity from the standard Si3N4 passivation layer. The platform is then trained by inducing a change in single-ion concentration and measuring the responses of all pixels. Sensor learning relies on offline training algorithms including k-means clustering and density-based spatial clustering of applications with noise to yield membrane mapping and sensitivity of each pixel to target electrolytes. We demonstrate multi-ion imaging with an average error of 3.7% (K+), 4.6% (Na+), and 1.8% (pH) for each ion, respectively, while Ca2+ incurs a larger error of 24.2% and hence is included to demonstrate versatility. We validate the platform with a brain dialysate fluid sample and demonstrate reading by comparing with a gold-standard spectrometry technique.

A High Offset Distribution Tolerance High Resolution ISFET Array With Auto-Compensation for Long-Term Bacterial Metabolism Monitoring.

This paper presents a CMOS ion-sensitive-field-effect-transistor (ISFET) array with superior offset distribution tolerance, resolution and linearity for long-term bacterial metabolism monitoring. A floating gate ISFET is adopted as the sensing front end to maximize ion sensitivity and support ultra-long-term measurement. To solve the DC offset issue induced by trapped chargers and drifts in each ISFET sensor, a complementary readout scheme with column offset compensation is proposed. P-type and N-type source followers are combined to cover a wide range of input DC offsets while maintaining small area and high linearity. The DC offset is digitally compensated during signal readout to facilitate global amplification and quantization. Fabricated in 0.18μm standard CMOS process, the ISFET array can tolerate an offset distribution beyond power supply with a linear pH-to-output response. Due to high ion sensitivity and low circuit noise, the whole system achieves a high resolution of 0.017pH. The proposed ISFET system has successfully demonstrated an accurate pH monitoring of normal Escherichis coli growth for 11 hours and its response to antibiotics, showing long-term bacterial metabolism monitoring capability.

An Ion-Sensitive Field-Effect Transistor with Three-Dimensional Extended-Gate Architecture

The ion-sensitive field-effect transistor (ISFET) is crucial to the field of chemical and biological sensing. Although ISFET with extended gate configuration is the current trend for application due to compatibility with standard CMOS process and convenience for array arrangement, the electric signal is attenuated significantly because an additional passivation capacitance is introduced, leading to a trade-off between pixel-resolution and sensitivity. Therefore, an ISFET array sensor with high pixel-resolution but at the same time with high sensitivity is a great desire. In this study, an attenuation-ameliorated extended gate-ISFET (EG-ISFET) with specially designed three-dimensional (3D) extended gate structure is proposed and realized based on the standard 0.35 μm CMOS technology. The transconductance of EG-ISFET is improved at most three times by the 3D structure as compared with the conventional EG-ISFET occupied the exactly same wafer area, achieving the goal of better sensitivity without sacrificing pixel-resolution.

A high-speed small-area pixel 16 × 16 ISFET array design using 0.35-μm CMOS process

This paper reports the design of a 16 × 16 ion-sensitive field-effect transistor (ISFET) chip with tightly arranged small pixels that can work in relatively higher speeds. During the chip design, we thoroughly considered the pixel area, speed, power consumption, and extensibility. The chip was fabricated using a conventional 4-metal 0.35-μm CMOS process. As a result, we obtained a chip's pixel area of 7.4 μm × 7.4 μm, power consumption of 3.3 V × 4 mA, and scanning rate reaching 6.25 M pixels/s. The passivation layer, mainly consisting of Si3N4, was directly treated as the sensing film. By arranging the overall layout tightly, this chip could be regarded as a module to expand the scale of ISFET arrays and can be used in applications having larger scale arrays. Initially, we obtained an average sensitivity of 20 mV/pH ±2 mV/pH and modified it up to 35 mV/pH ±2 mV/pH by rinsing it in BOE (Buffered Oxide Etch) with a volumn ratio of 1:4:4 (HF:H2O:NH4F) for 15 min.

Robust label-free microRNA detection using one million ISFET array.

Detection of nucleic acid molecules is one of the most pervasive assays in biology, medicine, and agriculture applications. Currently, most comely used DNA/RNA detection platforms use fluorescence labeling and require lab-scale setting for performing the assay. There is a need for developing less expensive, label-free, and rapid detection of biomolecules with minimal utilization of resources. Use of electrical approaches for detection of biomolecules by utilizing their inherent charge is a promising direction for biosensing assays. Here, we report a 1024 × 1024 array of Ion Sensitive Field Effect Transistors (ISFET) as label free sensors for detection of nucleic acid molecules. Using PNA probe functionalized on these ISFET array, we robustly detected miRNA Let-7b by measuring changes in drain current after hybridization of target molecules with concentration as low as 1nM. We demonstrate that mismatched or non-complementary target molecules resulted in statistically smaller changes. Most importantly, the high-density sensor array shows unprecedented reliability and robustness with P values <0.0001 for all experiments. Practical implementation of this platform could have a wide range of applications in high-throughput nucleic acid genotyping, detection of amplified pathogenic nucleic acid, detection of cell-free DNA, and electrical readouts for current hybridization-based DNA biomolecular assays.

A Thermally Powered ISFET Array for On-Body pH Measurement.

Recent advances in electronics and electrochemical sensors have led to an emerging class of next generation wearables, detecting analytes in biofluids such as perspiration. Most of these devices utilize ion-selective electrodes (ISEs) as a detection method; however, ion-sensitive field-effect transistors (ISFETs) offer a solution with improved integration and a low power consumption. This work presents a wearable, thermoelectrically powered system composed of an application-specific integrated circuit (ASIC), two commercial power management integrated circuits and a network of commercial thermoelectric generators (TEGs). The ASIC is fabricated in 0.35 m CMOS and contains an ISFET array designed to read pH as a current, a processing module which averages the signal to reduce noise and encodes it into a frequency, and a transmitter. The output frequency has a measured sensitivity of 6 to 8 kHz/pH for a pH range of 7-5. It is shown that the sensing array and processing module has a power consumption 6 W and, therefore, can be entirely powered by body heat using a TEG. Array averaging is shown to reduce noise at these low power levels to 104 V (input referred integrated noise), reducing the minimum detectable limit of the ASIC to 0.008 pH units. The work forms the foundation and proves the feasibility of battery-less, on-body electrochemical for perspiration analysis in sports science and healthcare applications.

An Automatic Gain Control System for ISFET Array Compensation

This paper presents an automatic gain calibration system for ion-sensitive field effect transistors (ISFETs) sensing arrays which compensates for mismatch in the gain of the pixels. The system utilizes the high frequency spectrum of the sensed signal to superimpose a reference sine wave through the reference electrode, which allows identification and calibration of the local gain. The complete system has been fabricated in a 0.35 μm CMOS process and has been designed to achieve very high accuracy and a small settling time to allow fast pixel switching. Measured results confirm good performance with only a 1.15% deviation in gain caused by noise and harmonic distortion and a rapid settling time of 70.5 μs which is suitable for CMOS-based ISFET arrays.

ISFETs in CMOS and Emergent Trends in Instrumentation: A Review

Over the past decade, ion-sensitive field-effect transistors (ISFETs) have played a major role in enabling the fabrication of fully integrated CMOS-based chemical sensing systems. This has allowed several new application areas, with the most promising being the fields of ion imaging and full genome sequencing. This paper reviews the new trends in front-end topologies toward the design of ISFET sensing arrays in CMOS for these new applications. More than a decade after the review of the ISFET by Bergveld which summarized the state of the art in terms of device and early readout circuity, we describe the evolution in terms of device macromodel and identify the main sensor challenges for current designers. We analyze the techniques that have been reported for both ISFET instrumentation and compensation, and conclude that topologies are focusing on device adaptation for offset and drift cancellation, as opposed to system compensation which are often not as robust. Guidelines are provided to build a tailored CMOS ISFET array, emphasizing that the needs in terms of applications are the keys to selecting the right pixel architecture. Over the next few years, the race for the largest and densest array is likely to be put on hold to allow the research to focus on new pixel topologies, ultimately leading to the development of reliable and scalable arrays. A wide range of new applications are expected to motivate this paper for at least another decade.

An Integrated Circuit for Chip-Based Analysis of Enzyme Kinetics and Metabolite Quantification.

We have created a novel chip-based diagnostic tools based upon quantification of metabolites using enzymes specific for their chemical conversion. Using this device we show for the first time that a solid-state circuit can be used to measure enzyme kinetics and calculate the Michaelis-Menten constant. Substrate concentration dependency of enzyme reaction rates is central to this aim. Ion-sensitive field effect transistors (ISFET) are excellent transducers for biosensing applications that are reliant upon enzyme assays, especially since they can be fabricated using mainstream microelectronics technology to ensure low unit cost, mass-manufacture, scaling to make many sensors and straightforward miniaturisation for use in point-of-care devices. Here, we describe an integrated ISFET array comprising 2(16) sensors. The device was fabricated with a complementary metal oxide semiconductor (CMOS) process. Unlike traditional CMOS ISFET sensors that use the Si3N4 passivation of the foundry for ion detection, the device reported here was processed with a layer of Ta2O5 that increased the detection sensitivity to 45 mV/pH unit at the sensor readout. The drift was reduced to 0.8 mV/hour with a linear pH response between pH 2-12. A high-speed instrumentation system capable of acquiring nearly 500 fps was developed to stream out the data. The device was then used to measure glucose concentration through the activity of hexokinase in the range of 0.05 mM-231 mM, encompassing glucose's physiological range in blood. Localised and temporal enzyme kinetics of hexokinase was studied in detail. These results present a roadmap towards a viable personal metabolome machine.

Handheld readout system for field‐effect transistor biosensor arrays for label‐free detection of biomolecules

Sensors for label‐free biomolecule detection using ion‐sensitive field‐effect transistor (ISFET) arrays working in a liquid environment offer great opportunities for fast diagnostics of diseases and other point‐of‐care applications. In the last decades, ISFET‐based sensors with progressively improved sensitivity, robustness, and diversity of detected biomolecules were described. However, a reliable, miniaturized electronic readout for such sensors is one of the main demands in the field. In this work, we developed a handheld readout system for biomolecule sensing with ISFET arrays, which can measure four channels simultaneously and can be used for a wide range of ISFET devices from microsized ISFETs to silicon nanowire transistor arrays. The hardware was built around a 32‐bit PIC microcontroller. A user‐friendly software written in LabVIEW communicates with the hardware to record and to display the measurement results. After a simple calibration, the error of the handheld system was below 5% in comparison with a standard semiconductor parameter analyzer. For the first applications, pH sensing and deoxyribonucleic acid (DNA) immobilization and hybridization events were successfully measured with our ISFET sensors. Our readout system in combination with our top‐down fabricated sensor devices has the potential to bring the ISFET sensors closer to the point‐of‐care testing in near future.

Improved inverter‐based read‐out scheme for low‐power ISFET sensing array

The digital read-out scheme in an ion-sensitive field effect transistor (ISFET) sensing array is more advantageous when compared with the analogue counterpart because of its lower power consumption, smaller area and less susceptibility to environmental noise and parasitics. An improved read-out scheme is proposed in which each ISFET is stacked with a CMOS inverter to form a pH-to-time converter. The pH level of the solution regulates the strength of the ISFET, which in turn modulates the delay of the stacked inverter and hence the pulse width of the output signal. Simulation results using the 0.18 µm/2.5 V CMOS process show that the modulated pulse width changes linearly over a wide range of pH. The design achieves five orders of magnitude smaller leakage and 40% lower dynamic power consumption, while it requires only 50% of silicon area when compared with the conventional design. It is therefore more suitable for large ISFET arrays implemented in nano-scale CMOS technologies.

High-Speed Imaging of 2-D Ionic Diffusion Using a 16$\,\times\,$16 Pixel CMOS ISFET Array on the Microfluidic Scale

We present an electrochemical high-speed measurement for direct observation of the mixing of ions in solution over a sensor surface. The method is applied to the determination of time-varying mixing and diffusion rates in microfluidic systems. The system generates high-speed videos of the analytes under investigation as they mix, without the need for any preliminary staining or labeling and has a pixel-to-pixel pitch of 14 μm. We demonstrate ion displacement phenomena on the surface of a Si3N4-passivated complementary metal oxide semiconductor ion-sensitive field effect transistor array device and also monitor the pH change induced by the addition of sulfuric acid to normal saline. These chemical videos are recorded with a frame rate of up to 333 frames/s. By evaluating the time-varying change in ionic concentrations across the surface of the chip, we calculate time-varying estimates of diffusivity coefficient values for the mixing analytes.