In Situ Drift Monitoring and Calibration of Field-Deployed Potentiometric Sensors Using Temperature Supervision.

Potentiometric ion-selective electrodes (ISEs) have broad applications in personalized healthcare, smart agriculture, oil/gas exploration, and environmental monitoring. However, high-precision potentiometric sensing is difficult with field-deployed sensors due to time-dependent voltage drift and the need for frequent calibration. In the laboratory setting, these issues are resolved by repeated calibration by measuring the voltage response at multiple standard solutions at a constant temperature. For field-deployed sensors, it is difficult to frequently interrupt operation and recalibrate with standard solutions. Moreover, the constant surrounding temperature constraint imposed by the traditional calibration process makes it unsuitable for temperature-varying field use. To address the challenges of traditional calibration for field-deployed sensors, in this study, we propose a novel in situ calibration approach in which we use natural/external temperature variation in the field to obtain the time-varying calibration parameters, without having to relocate the sensors or use any complex system. We also develop a temperature-supervised monitoring method to detect the drift of the sensor during operation. Collectively, the temperature-based drift monitoring and in situ calibration methods allow us to monitor the drift of sensors and correct them periodically to achieve high-precision sensing. We demonstrate our approach in three testbeds: (1) under controlled temperature variation in the lab, (2) under natural temperature variation in a greenhouse, and (3) in the field to monitor nitrate activity of an agricultural site. In the laboratory study, we validate that the calibration parameters of printed nitrate ISEs can be reproduced by our proposed calibration process; therefore, it can serve as an alternative to traditional calibration processes. In the greenhouse, we show the use of natural temperature variation to calibrate the sensors and detect the drift in a fixed concentration nitrate solution. Finally, we demonstrate the use of the method to monitor the nitrate activity of an agricultural field within 10% of laboratory-based measurements (i.e., a sensitivity of 0.03 mM) for a period of 22 days. The findings highlight the prospect of temperature-based calibration and drift monitoring for high-precision sensing with field-deployed ISEs.

A new paradigm of reliable sensing with field-deployed electrochemical sensors integrating data redundancy and source credibility.

For a continuous healthcare or environmental monitoring system, it is essential to reliably sense the analyte concentration reported by electrochemical sensors. However, environmental perturbation, sensor drift, and power-constraint make reliable sensing with wearable and implantable sensors difficult. While most studies focus on improving sensor stability and precision by increasing the system's complexity and cost, we aim to address this challenge using low-cost sensors. To obtain the desired accuracy from low-cost sensors, we borrow two fundamental concepts from communication theory and computer science. First, inspired by reliable data transmission over a noisy communication channel by incorporating redundancy, we propose to measure the same quantity (i.e., analyte concentration) with multiple sensors. Second, we estimate the true signal by aggregating the output of the sensors based on their credibility, a technique originally developed for "truth discovery" in social sensing applications. We use the Maximum Likelihood Estimation to estimate the true signal and the credibility index of the sensors over time. Using the estimated signal, we develop an on-the-fly drift-correction method to make unreliable sensors reliable by correcting any systematic drifts during operation. Our approach can determine solution pH within 0.09 pH for more than three months by detecting and correcting the gradual drift of pH sensors as a function of gamma-ray irradiation. In the field study, we validate our method by measuring nitrate levels in an agricultural field onsite over 22 days within 0.06 mM of a high-precision laboratory-based sensor. We theoretically demonstrate and numerically validate that our approach can estimate the true signal even when the majority (~ 80%) of the sensors are unreliable. Moreover, by restricting wireless transmission to high-credible sensors, we achieve near-perfect information transfer at a fraction of the energy cost. The high-precision sensing with low-cost sensors at reduced transmission cost will pave the way for pervasive in-field sensing with electrochemical sensors. The approach is general and can improve the accuracy of any field-deployed sensors undergoing drift and degradation during operation.

Temperature Self-Calibration of Always-On, Field-Deployed Ion-Selective Electrodes Based on Differential Voltage Measurement.

Originally developed for use in controlled laboratory settings, potentiometric ion-selective electrode (ISE) sensors have recently been deployed for continuous, in situ measurement of analyte concentration in agricultural (e.g., nitrate), environmental (e.g., ocean acidification), industrial (e.g., wastewater), and health-care sectors (e.g., sweat sensors). However, due to uncontrolled temperature and lack of frequent calibration in these field applications, it has been difficult to achieve accuracy comparable to the laboratory setting. In this paper, we propose a novel temperature self-calibration method where the ISE sensors can serve as their own thermometer and therefore precisely measure the analyte concentration in the field condition by compensating for the temperature variations. We validate the method with controlled experiments using pH and nitrate ISEs, which use the Nernst principle for electrochemical sensing. We show that, using temperature self-calibration, pH and nitrate can be measured within 0.3% and 5% of the true concentration, respectively, under varying concentrations and temperature conditions. Moreover, we perform a field study to continuously monitor the nitrate concentration of an agricultural field over a period of 6 days. Our temperature self-calibration approach determines the nitrate concentration within 4% of the ground truth measured by laboratory-based high-precision nitrate sensors. Our approach is general and would allow battery-free temperature-corrected analyte measurement for all Nernst principle-based sensors being deployed as wearable or implantable sensors.

Steady-State and Transient Performance of Ion-Sensitive Electrodes Suitable for Wearable and Implantable Electro-Chemical Sensing.

Traditional Potentiometric Ion-selective Electrodes (ISE) are widely used in industrial and clinical settings. The simplicity and small footprint of ISE have encouraged their recent adoption as wearable/implantable sensors for personalized healthcare and precision agriculture, creating a new set of unique challenges absent in traditional ISE. In this paper, we develop a fundamental physics-based model to describe both steady-state and transient responses of ISE relevant for wearable/implantable sensors. The model is encapsulated in a "generalized Nernst formula" that explicitly accounts for the analyte density, time-dynamics of signal transduction, ion-selective membrane thickness, and other sensor parameters. The formula is validated numerically by self-consistent modeling of multispecies ion-transport and experimentally by interpreting the time dynamics and thickness dependence of thin-film solid-contact and graphene-based ISE sensors for measuring soil nitrate concentration. These fundamental results will support the accelerated development of ISE for wearable/implantable applications.

Roll-to-Roll Manufactured Sensors for Nitroaromatic Organophosphorus Pesticides Detection.

A fully roll-to-roll manufactured electrochemical sensor with high sensing and manufacturing reproducibility has been developed for the detection of nitroaromatic organophosphorus pesticides (NOPPs). This sensor is based on a flexible, screen-printed silver electrode modified with a graphene nanoplatelet (GNP) coating and a zirconia (ZrO2) coating. The combination of the metal oxide and the 2-D material provided advantageous electrocatalytic activity toward NOPPs. Manufacturing, scanning electron microscopy-scanning transmission electron microscopy image analysis, electrochemical surface characterization, and detection studies illustrated high sensitivity, selectivity, and stability (∼89% current signal retention after 30 days) of the platform. The enzymeless sensor enabled rapid response time (10 min) and noncomplex detection of NOPPs through voltammetry methods. Furthermore, the proposed platform was highly group-sensitive toward NOPPs (e.g., methyl parathion (MP) and fenitrothion) with a detection limit as low as 1 µM (0.2 ppm). The sensor exhibited a linear correlation between MP concentration and current response in a range from 1 µM (0.2 ppm) to 20 µM (4.2 ppm) and from 20 to 50 µM with an R2 of 0.992 and 0.991, respectively. Broadly, this work showcases the first application of GNPs/ZrO2 complex on flexible silver screen-printed electrodes fabricated by entirely roll-to-roll manufacturing for the detection of NOPPs.

Strongly enhanced static friction using a film-terminated fibrillar interface.

We examine the behavior under shear of a bio-inspired fibrillar interface that consists of poly(dimethlysiloxane) micro-posts terminated by a thin film. These structures demonstrate significantly enhanced adhesion due to a crack trapping mechanism. We study the response of this structure to shear displacement relative to a spherical indenter placed on its surface under a fixed normal force. The shear force required to initiate sliding between the indenter and the sample, its static friction, is strongly enhanced compared to a flat control, and increases with inter-fibril spacing. Examination of the contact region reveals that its area changes with applied shear and that static friction is controlled by a mechanical instability. The shear force resisting steady sliding, surprisingly, is independent of fibril spacing and is nearly the same as for the flat unstructured control samples. We interpret dynamic friction to result from the action of Schallamach-like waves. Our results show that the film-terminated architecture can be used to design an interface with significantly enhanced static friction without altering its sliding frictional resistance.

Biologically inspired crack trapping for enhanced adhesion.

We present a synthetic adaptation of the fibrillar adhesion surfaces found in nature. The structure consists of protruding fibrils topped by a thin plate and shows an experimentally measured enhancement in adhesion energy of up to a factor of 9 over a flat control. Additionally, this structure solves the robustness problems of previous mimic structures and has preferred contact properties (i.e., a large surface area and a highly compliant structure). We show that this geometry enhances adhesion because of its ability to trap interfacial cracks in highly compliant contact regimes between successive fibril detachments. This results in the requirement that the externally supplied energy release rate for interfacial separation be greater than the intrinsic work of adhesion, in a manner analogous to lattice trapping of cracks in crystalline solids.

Adhesion enhancement in a biomimetic fibrillar interface.

Two important putative functions of the fibrillar contact interfaces commonly found in lizards and insects are to provide contact compliance and enhanced adhesion. To explore the question of whether a fibrillar architecture inherently enhances adhesion, we constructed model structures consisting of thin sheets of poly(vinyl butyral) (PVB) bonded on one of their thin sides to glass plates. The PVB samples had two flat, unstructured regions interrupted by a central fibrillar region along the bonded interface. The effect of the fibrillar geometry on the performance of the adhesive bond was tested using a tensile pull-off test, in which failure occurred by interfacial crack propagation, starting at an end where a crack initiator was introduced. We observed that fibrils in all samples fail consistently at the same critical stress, which is consistent with a previous theoretical result we have determined for flaw insensitivity during fibrillar pull-off. In addition, we measured the energy release rate required to fail the interface in the fibrillar region to be about an order of magnitude greater than that in the non-fibrillar region. We present experimental evidence demonstrating that this increase results partly from dissipation of strain energy stored in the fibrils.

Can a fibrillar interface be stronger and tougher than a non-fibrillar one?

Elasticity analysis and finite element simulations are carried out to study the strength of an elastic fibrillar interface. The fibrils are assumed to be in perfect contact with a rigid substrate. The adhesive interaction between the fibrils and the substrate is modelled by the Dugdale-Barenblatt model (DB). The condition for a fibrillar interface to be stronger than a non-fibrillar one is obtained for two regimes: (i) small fibril or flaw insensitive regime; (ii) large fibril or flaw sensitive regime. The transition between the two regimes is characterized by a dimensionless parameter that incorporates the material constants of the elastic fibrils and interfacial properties. The condition for a fibrillar interface to be tougher is also given. Lateral collapse is found to be detrimental to the strength and toughness of a fibrillar interface.

Effect of stamp deformation on the quality of microcontact printing: theory and experiment.

Microcontact printing (microCP) is an effective way to generate micrometer- or submicrometer-sized patterns on a variety of substrates. However, the fidelity of the final pattern depends critically on the coupled phenomena of stamp deformation, fluid transfer between surfaces, and the ability of the ink to self-assemble on the substrate. In particular, stamp deformation can produce undesirable effects that limit the practice and precision of microCP. Experimental observations and comparison with theoretical predictions are presented here for three of the most undesirable consequences of stamp deformation: (1) roof collapse of low aspect ratio recesses, (2) buckling of high aspect ratio plates, and (3) lateral sticking of high aspect ratio plates. Stamp behavior was observed visually with an inverted optical microscope while load-displacement data were collected during compression and retraction of stamps. Additionally, a "robotic stamper" was used to deliver ink patterns in precise locations on substrates. These monomolecular ink patterns were then observed in high contrast using the surface potential scanning mode of an atomic force microscope. Theoretical models based on continuum mechanics were used to accurately predict both physical deformation of the stamp and the resultant inking patterns. The close agreement between these models and the experimental data presented clearly demonstrates the essential considerations one must weigh when designing stamp geometry, material, and loading conditions for optimal pattern fidelity.