Modern biosensors play a critical role in healthcare and have a quickly growing commercial market. Compared to traditional optical-based sensing, electrochemical biosensors are attractive due to superior performance in response time, cost, complexity and potential for miniaturization. To address the shortcomings of traditional benchtop electrochemical instruments, in recent years, many complementary metal oxide semiconductor (CMOS) instrumentation circuits have been reported for electrochemical biosensors. This paper provides a review and analysis of CMOS electrochemical instrumentation circuits. First, important concepts in electrochemical sensing are presented from an instrumentation point of view. Then, electrochemical instrumentation circuits are organized into functional classes, and reported CMOS circuits are reviewed and analyzed to illuminate design options and performance tradeoffs. Finally, recent trends and challenges toward on-CMOS sensor integration that could enable highly miniaturized electrochemical biosensor microsystems are discussed. The information in the paper can guide next generation electrochemical sensor design.

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This paper introduces a robust electrochemical gas sensor featuring room temperature ionic liquid (RTIL) as electrolyte and porous polytetrafluoroethylene as a flexible substrate. Using a planar-electrodes-on-permeable-membrane structure, a flexible RTIL sensor with 9.6 mm 2 sensing area is microfabricated. The sensor's response to oxygen is measured, achieving sensitivity of 0.48 µA/% O 2 , a linearity of 0.997 R 2 in range of 0%-21% O 2 , and limit of detection of 0.08% O 2 . The reported sensor structure and fabrication process enable realization of sensor arrays for multi-gas monitoring in a low power, miniaturized, and wearable system platform.Index Terms-Gas sensor, flexible sensor, electrochemical sensor, room temperature ionic liquid.

We present a method to identify and quantify methane using a hydrophobic ionic liquid (IL)-electrified metal electrode interface by electrochemical impedance spectroscopy. We investigated the mechanisms of the responses of the IL-electrified electrode interface to the exposure of methane and other interfering gases (H 2 , C 6 H 12 , SO 2 , NO, NO 2 , CO 2 , O 2 , H 2 O). Our results show that at low frequency the IL-electrified electrode interface shows a predominantly capacitive response. The IL-electrode double layer (EDL) was found to be the primary response layer while the transition zone and bulk region of the IL-electrode interface contribute little to the overall signal change. For recognition and quantification of methane using the Langmuir adsorption model and measurement of differential capacitance change, an optimum EDL interface structure was found to form at a specific DC bias potential. The cumulative results shown in this work suggest that an ideal IL-electrode interface can be formed by varying IL structure and applied DC bias electrode potential for a specific analyte and that the semi-ordered structure of IL-electrified interface can act as a recognition element for the sensitive and selective adsorption and detection of gaseous molecules.Due to their unique properties and increasing availability, room temperature ionic liquids (ILs) have received great interests in electrochemistry, catalysis, electronics, and energy conversion as well as interdisciplinary investigations on both fundamental and practical applications. 1-5 For example, ILs as solvent free and ion-coupled material exhibit strong benefits as non-aqueous electrolytes for enhancing the safety and robustness for sensor and transistor devices. 6-12 However, the low intrinsic conductivity of the ILs correlates with their high viscosity, which limits the response time and sensitivity of detection methods such as those based on amperometry and potentiometry. [13][14][15][16][17] Methane, which has been considered as a clean energy source and one of the most important greenhouse gases, has attracted significant interest to the characterization of its adsorption on surfaces and its quantification in atmosphere as well. Since methane is relative chemically and electrochemically inert, current methods for methane detection are either relatively high-cost (e.g. optical), which prevents widespread deployment, or lack the selectivity (e.g. catalytic bead) 18 demanded by various applications. Many methane sensors also need improvement regarding their size, power consumption and the ease of use. [19][20][21] Current results show that the potential-dependent interface of an IL and metal electrode is very sensitive to surface conditions on the electrode, such as the proton adsorption on an oxide electric interface 22 and the adsorption of CO on a metal electrode 23 . As shown in Figure 1, the molecular selectivity of an IL-electrode interface comes from the ordering of the electric double layer (EDL) as well as molecular interactions between the IL...

We have developed an ultrasensitive gas-detection method based on the measurement of a differential capacitance of electrified ionic liquid (IL) electrode interfaces in the presence and absence of adsorbed gas molecules. The observed change of differential capacitance has a local maximum at a certain potential that is unique for each type of gas, and its amplitude is related to the concentration of the gas molecules. We establish and validate this gas-sensing method by characterizing SO2 detection at ppb levels with less than 1.8% signal from other interfering species (i.e., CO2, O2, NO2, NO, SO2, H2O, H2, and cyclohexane, tested at the same concentration as SO2). This study opens a new avenue of utilizing tunable electrified IL-electrode interfaces for selective sensing of molecules with a kinetic size resolution of 0.1 Å.

Miniaturized detector arrays are critical to reducing size and maintaining measurement quality of integrated microgas chromatographs (μGC) used for the analysis of complex vapor mixtures. This paper presents an array of chemiresistors (CRs) with monolayer-protected gold nanoparticle films formed on the surface of a complementary-metal-oxide semiconductor (CMOS) readout chip, featuring high-resolution resistance measurement with adaptive cancellation of baseline resistance. The 8-channel readout circuit occupies 2.2 × 2.2 mm 2 in 0.5 μm CMOS and consumes 66 μW per channel from a 3.3-V power supply. It achieves a worst-case resolution of 125 ppm over a broad baseline resistance range of 60 k to 10 M , equivalent to 122 dB dynamic range. Implementation of the CMOS monolithic detector array is discussed, and preliminary measurement results using chamber exposures to several vapors are presented. Eventual integration into a μGC is discussed.Index Terms-Chemiresistor, complementary-metal-oxide semiconductor (CMOS) monolithic sensor array, gas chromatography (GC) detector, μGC.

The growing demand for personal healthcare monitoring requires a challenging combination of performance, size, power, and cost that is difficult to achieve with existing gas sensor technologies. This paper presents a new CMOS monolithic gas sensor microsystem that meets these requirements through a unique combination of electrochemical readout circuits, post-CMOS planar electrodes, and room temperature ionic liquid (RTIL) sensing materials. The architecture and design of the CMOS-RTIL-based monolithic gas sensor are described. The monolithic device occupies less than 0.5mm2 per sensing channel and incorporates electrochemical biasing and readout functions with only 1.4mW of power consumption. Oxygen was tested as an example gas, and results show that the microsystem demonstrates a highly linear response (R2 = 0.995) over a 0 – 21% oxygen concentration range, with a limit of detection of 0.06% and a 1 second response time. Monolithic integration reduces manufacturing cost and is demonstrated to improve limits of detection by a factor of five compared to a hybrid implementation. The combined characteristics of this device offer an ideal platform for portable/wearable gas sensing in applications such as air pollutant monitoring.

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