AC Electrothermal Research Articles

Low-cost portable sensor for rapid and sensitive detection of Pb2+ ions using capacitance sensing integrated with microfluidic enrichment.

Lead ion (Pb2+) pollution is a critical global issue due to its ability to accumulate in the human body, resulting in severe health problems. Despite extensive research efforts devoted to the detection of Pb2+ contamination, practical, rapid, and field-deployable sensors for Pb2+ is yet to be developed to effectively safeguard the environment and public health. Herein, a label-free affinity-based sensing device is developed based on printed circuit board (PCB) for low-cost, easy-to-use, and real-time on-site detection of Pb2+ ions. The sensors are prepared by forming a self-assembled monolayer of glutathione (GSH) on the surface of gold-plated PCB electrodes, which serves as a molecular probe to recognize Pb2+. Rapid and sensitive detection is achieved by using capacitance sensing integrated with microfluidic enrichment. The sensor's interfacial capacitance is used to indicate specific binding, while the capacitance reading process simultaneously induces alternating current electrothermal (ACET) acceleration of analyte's travel towards the probes. Thus, the enrichment and detection are integrated into a single step, making pre-concentration unnecessary and shortening the assay time to 30s. This Pb2+ sensor has demonstrated one of the lowest limits of detection reported so far (1.85 fM) with a linear range of 0.01-10pM. To evaluate the sensor's specificity, non-target metal ions are tested, all showing negligible responses. Testing of tap water sample also yields reasonable results, validating the sensor's robustness. The above-mentioned features, together with a commercial portable readout, make this sensor well-suited for point-of-use Pb2+ detection at low cost.

A disposable microfluidic aptasensor for one-step and real-time detection of sub-femtomolar-level aflatoxin B1 in food

Aflatoxin B1 (AFB1) is a highly toxic substance found in food, necessitating rapid and sensitive detection methods. Combining interfacial capacitive sensing with AC electrothermal (ACET) enrichment, an aptasensor based on a PCB electrode array is developed for real-time detection of trace AFB1. Owing to the sensitive solid–liquid capacitance with a pF-level resolution, AFB1 detection at sub-femtomolar level is achieved. The induced ACET flows enrich AFB1 molecules towards the electrode surface during capacitance measurement, enabling a one-step detection containing target enrichment, with an overall time of 20 s without any extra concentrating devices or processes. This sensor has a low detection limit of 620 aM, a selectivity of 2262:1 against interferences, and a wide semi-log linear range from 1 fM to 10 pM. It is with a cost below 1 US dollar for disposable detection and a convenient operation for on-site application. The detection of AFB1 in three types of quality control samples demonstrates a good feasibility for food safety inspection.

Ultra-sensitive and rapid detection of perfluorooctanesulfonic acid by a capacitive molecularly-imprinted-polymer sensor integrated with AC electrokinetic acceleration

The development of point-of-use sensors capable of rapidly detecting per- and polyfluoroalkyl substances (PFAS) is crucial for real-time monitoring and effective management of PFAS contamination in the aquatic environment. Addressing current limitations in existing techniques for sensitive in-field analysis, this work employs a multifaceted approach to develop a rapid, ultra-sensitive, and single-step sensing platform for perfluorooctanesulfonic acid (PFOS). The approach integrates self-referencing interfacial capacitance sensing with microfluidic enrichment by AC electrothermal (ACET) effect. A molecular imprinted polymer (MIP) on gold interdigitated microelectrode chip is employed as a robust recognition element for the affinity-based capture of PFOS. A predetermined AC signal applied to the sensor will not only accelerate PFOS molecules towards the sensing surface for binding, but also track the interfacial capacitance change during binding, enabling real-time detection with extremely high sensitivity. The resulting single-step and rapid assay demonstrates detection as low as 0.5 fg/L (5×10−7 ppt) in 10 s, a linear range of 0.5–500 fg/L (5×10−7–5×10−4 ppt), and high selectivity (105:1) in Phosphate-buffered saline (PBS) media against other PFAS compounds including perfluorooctanoic acid (PFOA). The success of this work will potentially provide a routine PFOS monitoring test for drinking water.

AC electrothermal trapping for efficient nanoparticles enrichment in a microchannel

Trapping nanoparticles in a microfluidic device holds promise for enriching low-concentration species in samples. This article explores AC electrothermal (ACET) flow as an effective mechanism for nanoparticle trapping within a microfluidic system. Employing a precisely designed microfluidic device with coplanar symmetric electrodes, we systematically analyze the trapping performance of 100-nm polystyrene nanoparticles (PsNPs) dispersed in deionized water. Through fluorescence microscopy, we observe the dynamic behavior of PsNPs under the influence of an AC electric field. The concentration factor (CF) is introduced as a metric for evaluating trapping efficiency, revealing a remarkable up to 30-fold increase in concentration within the electrode gap. The quadratic relationship between electrothermal forces and electric field strength is investigated, highlighting the direct impact on trapping enhancement. By adjusting the applied voltage to vary the electric field strength, CF is found to significantly improve, reaching a peak value of 16.3 at 3.5 MV m−1. Furthermore, we explore the influence of frequency on ACET velocity, emphasizing the importance of aligning frequency and electric field strength for effective nanoparticle trapping. This study contributes valuable insights into the optimization of ACET for precise and controlled nanoparticle manipulation, holding promise for diverse applications in fields requiring nanoparticle enrichment.

Boosting the thermal stability of paralleled GaAs HBTs through temperature-dependent ballasting resistors: A proof-of-concept study

This paper explores the efficacy of base ballasting strategies in enhancing the electrothermal (ET) stability of paralleled InGaP/GaAs heterojunction bipolar transistors (HBTs) for radiofrequency (RF) applications. More specifically, technologically-feasible temperature-dependent base ballasting resistors are proposed as a solution to shift the collapse of current gain to higher power densities. A three-cell test device fabricated through a state-of-the-art process is considered as a simple case study. Extremely accurate DC and AC ET simulations are performed with SPICE to quantify the advantages of the proposed strategy with respect to traditional approaches.

Numerical Simulations of Combined Dielectrophoresis and Alternating Current Electrothermal Flow for High-Efficient Separation of (Bio)Microparticles.

High-efficient separation of (bio)microparticles has important applications in chemical analysis, environmental monitoring, drug screening, and disease diagnosis and treatment. As a label-free and high-precision separation scheme, dielectrophoresis (DEP) has become a research hotspot in microparticle separation, especially for biological cells. When processing cells with DEP, relatively high electric conductivities of suspending media are sometimes required to maintain the biological activities of the biosample, which results in high temperature rises within the system caused by Joule heating. The induced temperature gradient generates a localized alternating current electrothermal (ACET) flow disturbance, which seriously impacts the DEP manipulation of cells. Based on this, we propose a novel design of the (bio)microparticle separator by combining DEP with ACET flow to intensify the separation process. A coupling model that incorporates electric, fluid flow, and temperature fields as well as particle tracking is established to predict (bio)microparticle trajectories within the separator. Numerical simulations reveal that both ACET flow and DEP motion act in the same plane but in different directions to achieve high-precision separation between particles. This work provides new design ideas for solving the very tricky Joule heating interference in the DEP separation process, which paves the way for further improving the throughput of the DEP-based (bio)microparticle separation system.

AC Electrothermal Effect Promotes Enhanced Solute Mixing in a Wavy Microchannel.

For liquids used in biological applications, a smaller diffusion coefficient results in a longer mixing time. We discuss, in this endeavor, the promising potential of the AC electrothermal (ACET) effect toward modulating enhanced mixing of electrolytic liquids with higher convective strength in a novel wavy micromixer. To this end, we develop a modeling framework and numerically solve the pertinent transport equations in a three-dimensional (3D) configuration numerically. By benchmarking the developed modeling framework with the experimental results available in this paradigm, we aptly demonstrate the maximum temperature rise, flow topology, species concentration field, and mixing efficiency in the proposed configuration for a set of parameters pertinent to this analysis. We find that the maximum temperature increase in the wavy micromixer, owing to the electrothermal effect, is less than 10 K even for the higher strength of the applied voltage, implying nondegradation of biological substances within the liquid sample. We report that the formation of significant lateral flow closer to the electrodes leads to a highly three-dimensional ACET flow field, which has a significant impact on the mixing efficiency for the range of diffusive Peclet numbers considered. We also report that the wave amplitude of the mixer, when intervening with the diffusive Peclet number, strongly impacts the mixing efficiency. As witnessed in this endeavor, for the smaller diffusive Peclet number, the mixing efficiency increases with amplitude, while the effect becomes the opposite for the higher Peclet number. The results of this study seem to provide an adequate basis for the design of a novel micromixer intended for enhanced solute mixing in realistic microfluidic applications.

Optimization of Planar Interdigitated Microelectrode Array for Enhanced Sensor Responses

Immunoassays play a pivotal role in detecting and quantifying specific proteins within biological samples. However, its sensitivity and turnaround time are constrained by the passive diffusion of target molecules towards the sensors. ACET (Alternating Current Electrothermal) enhanced reaction emerges as a solution to overcome this limitation. The ACET-enhanced biosensor works by inducing vortices through electrothermal force, which stirs the analyte within the microchannel and promotes a reaction process. In this study, a comprehensive two-dimensional finite element study is conducted to optimize the binding efficiency and detection time of an ACET-enhanced biosensor without external pumping. Optimal geometries for interdigitated electrodes are estimated to achieve significant improvements in terms of probe utilization and enhancement factor. The study’s findings demonstrate enhancement factors of 3.21, 2.15, and 3.09 along with 71.22%, 75.80%, and 57.52% normalized binding for C-reactive protein (CRP), immunoglobulin (IgG), and SARS-CoV-2, respectively.

One-step and real-time detection of Hg2+ in brown rice flour using a biosensor integrated with AC electrothermal enrichment.

Mercury (Hg2+) is one of the most toxic heavy metals in farm products, so rapid detection of trace Hg2+ has always been sought after with high interest. Herein, we report a biosensor to specifically recognize Hg2+ in leaching solutions of brown rice flour. This sensor is simple and of low cost, with a very short assay time of 30s. Another merit is the ultra-low limit of detection (LOD) at fM level. In addition, the specific aptamer probe realizes a good selectivity above 105: 1 against the interferences. This sensor is developed based on an aptamer-modified gold electrode array (GEA) for capacitive sensing. Alternating current electrothermal (ACET) enrichment is induced during the AC capacitance acquirement. Thus, the enrichment and detection are coupled as a single step, and pre-concentration is needless. Owing to the sensing mechanism of solid-liquid interfacial capacitance and ACET enrichment, Hg2+ level can be sensitively and rapidly reflected. Also, the sensor has a wide linear range from 1 fM to 0.1nM and a shelf life of 15days. This biosensor shows advantages on overall performance, enabling easy-to-operate, real-time, and large-scale Hg2+ detection in farm products.

Competing effects of buoyancy-driven and electrothermal flows for Joule heating-induced transport in microchannels

Ionic fluids subjected to externally applied electric fields experience Joule heating, which increases with the increased electric field and ionic conductivity of the medium. Temperature gradients induced by Joule heating can create buoyancy-driven flows produced by local density changes, as well as electrothermal transport due to the temperature-dependent variations in fluid permittivity and conductivity. This manuscript considers Joule heating-induced transport in microchannels by a pair of electrodes under alternating current electric fields. Resulting buoyancy-driven and alternating current electrothermal (ACET) flows are investigated theoretically, numerically and experimentally. Proper normalizations of the governing equations led to the ratio of the electrothermal and buoyancy velocities, as a new non-dimensional parameter, which enabled the construction of a phase diagram that can predict the dominance of ACET and buoyancy-driven flows as a function of the channel size and electric field. Numerical results were used to verify the phase diagram in various height microchannels for different ionic conductivity fluids and electric fields, while the numerical results were validated using the micro-particle-image velocimetry technique. The results show that ACET flow prevails when the channel dimensions are small, and the electric potentials are high, whereas buoyancy-driven flow becomes dominant for larger channel heights. The present study brings insights into Joule heating-induced transport phenomena in microfluidic devices and provides a pathway for the design and utilization of ACET-based devices by properly considering the co-occurring buoyancy-driven flow.

Numerical investigation of microchannel geometry for effective on-chip biofluid delivery by AC electrothermal effect.

Although there exist tremendous needs for on-chip biofluid delivery, research in this field has yielded limited numbers of devices for real-world applications. One challenge is the difficulty for micropumps to meet the requirements of being low cost to fabricate, easy to integrate and effective for intended applications at the same time. This research focuses on AC electrothermal (ACET) micropumps based on planar interdigitated electrodes, due to their practicality in fabrication and operation, and compatibility with biochemical fluids. Our prior work has optimized the design of electrode dimensions for a fixed microchannel design. This work finds that microchannel dimensions can also affect ACET micropumps significantly, with respect to flow rate and electric impedance loading. This work first considers the constraints arising from impedance loading by ACET micropumps on power supplies, then the investigation describes several key parameters (threshold height, saturation thickness), to arrive at an appropriate microchannel geometry for the effective delivery of biofluids. The optimized microchannel is expected to incorporate well into a multifunctional lab-chip system to transport biofluids efficiently.

Design parameters optimization of an electrothermal flow biosensor for the SARS-CoV-2 S protein immunoassay.

To combat the coronavirus disease 2019 (COVID-19), great efforts have been made by scientists around the world to improve the performance of detection devices so that they can efficiently and quickly detect the virus responsible for this disease. In this context we performed 2D finite element simulation on the kinetics of SARS-CoV-2 S protein binding reaction of a biosensor using the alternating current electrothermal (ACET) effect. The ACET flow can produce vortex patterns, thereby improving the transportation of the target analyte to the binding surface and thus enhancing the performance of the biosensor. Optimization of some design parameters concerning the microchannel height and the reaction surface, such as its length as well as its position on the top wall of the microchannel, in order to improve the biosensor efficiency, was studied. The results revealed that the detection time can be improved by 55% with an applied voltage of 10 Vrms and an operating frequency of 150 kHz and that the decrease in the height of the microchannel and in the length of the binding surface can lead to an increase in the rate of the binding reaction and therefore decrease the biosensor response time. Also, moving the sensitive surface from an optimal position, located in front of the electrodes, decreases the performance of the device.

3D simulation of microfluidic biosensor for SARS-CoV-2 S protein binding kinetics using new reaction surface design.

In this study, we performed 3D finite element simulations on the binding reaction kinetics of SARS-CoV-2 S protein (target analyte) and its corresponding immobilized antibody (ligand) in a heterogeneous microfluidic immunoassay. Two types of biosensors with two different shapes and geometries of the reaction surface and electrodes were studied. Alternating current electrothermal (ACET) force was applied to improve the binding efficiency of the biomolecular pairs by accelerating the transport of analytes to the binding surface. The ACET force stirs the flow field, thereby reducing the thickness of the diffusion boundary layer, often developed on the reaction surface due to the slow flow velocity, low analyte diffusion coefficient, and surface reaction high rate. The results showed that the detection time of one of the biosensors can be improved by 69% under an applied voltage of 10 Vrms and an operating frequency of 100 kHz. Certain control factors such as the thermal boundary conditions as well as the electrical conductivity of the buffer solution were analyzed in order to find the appropriate values to improve the efficiency of the biosensor.

Front Cover: Enhancing affinity‐based electroanalytical biosensors by integrated AC electrokinetic enrichment—A mini review

DOI: 10.1002/elps.202100168 The cover picture shows how affinity-based electroanalytical biosensors can be enhanced by integrated AC electrokinetic (ACEK) effects. ACEK effects, including AC electrothermal (ACET) effect, AC electroosmosis (ACEO) effect and dielectrophoretic (DEP) effect, are produced by applying AC signals to the interdigitated electrode arrays. ACEK effects induce directed movement of microflows and particles (e.g., proteins, nucleic acids, bacteria, small molecules, etc.) in the solution, so that the analytes in the solution can be quickly routed toward the surface of biosensors, thus improving the binding efficiency between the analytes and the probes on the surface of the sensor. Studies have shown that the method holds great promise for achieving high sensitivity, low detection limit and rapid turnaround.

AC electrothermal assisted plasmonic biosensor for detection of low-concentration biological analytes

The main challenges of the surface plasmon resonances (SPR) based biosensors are mass transport limitations, which makes them more complicated in the detection of low-concentration biological analytes. In this research work, they were coupled to a microfluidic system in which the mass transport limitation in the reaction region improved by employing an AC electrothermal (ACET) concentrator. Multi-physical modeling based on finite-element-method was done to study an ACET-enhanced plasmonic biosensor inside a microfluid channel. The effect of vital parameters (like electrical excitation and analyte concentration) on ACET flow and equilibrium reaction time are investigated. The obtained numerical results reveal that the proposed method enhances the analyte-receptor binding reaction and effectively reduces the response time. To identify refractive index changes, the surface plasmon resonances of a 50 nm silver film on a glass substrate is used. The optical characteristics like sensitivity (S) and figure of merit (FOM) were obtained. The sensitivity of higher than 300 nm/RIU and FOM of higher than 40 are calculated. Our work can be applied to future sensors to detect low concentration biological analytes.

Real-time Cd2+ detection at sub-femtomolar level in various liquid media by an aptasensor integrated with microfluidic enrichment

Cadmium ion (Cd2+) is a highly toxic heavy metal pollutant with long biological half-life and strong bioaccumulation, which may cause irreversible damage to human health even at low-dose intake. Sensitive and specific detection of ultra-trace Cd2+ is of great significance in food safety and environmental monitoring. In this work, a simple-to-use aptasensor to rapidly detect Cd2+ in various liquid media is developed based on a DNA aptamer modified gold interdigitated electrode (IDE). Capacitance at the solid-liquid interface is utilized to reflect the change caused by adsorbed Cd2+. With the microfluidic enrichment by AC electrothermal (ACET) effect, Cd2+ ions are sped up toward the IDE surface and captured by the probes instantaneously. Owing to the integration of target enrichment and capacitance detection, the sample-to-result time of this sensor is as short as 30 s. This aptasensor as a quantitative detector has a linear response range from 0.45 fM to 4.5 pM, with a limit of detection of 253.16 aM. The selectivity ratio reaches 2320: 1 against non-target metal ions. By detection of Cd2+ spiked in tap water and olive oil as well as natural residual Cd2+ in rice leaching solution, the sensor’s applicability and reliability in different practical liquid media are demonstrated.

Simulation of the Slip Velocity Effect in an AC Electrothermal Micropump.

The principal aim of this study was to analyze the effect of slip velocity at the microchannel wall on an alternating current electrothermal (ACET) flow micropump fitted with several pairs of electrodes. Using the finite element method (FEM), the coupled momentum, energy, and Poisson equations with and without slip boundary conditions have been solved to compute the velocity, temperature, and electrical field in the microchannel. The effects of the frequency and the voltage, and the electrical and thermal conductivities, respectively, of the electrolyte solution and the substrate material, have been minutely analyzed in the presence and absence of slip velocity. The slip velocity was simulated along the microchannel walls at different values of slip length. The results revealed that the slip velocity at the wall channel has a significant impact on the flow field. The existence of slip velocity at the wall increases the shear stress and therefore enhances the pumping efficiency. It was observed that higher average pumping velocity was achieved for larger slip length. When a glass substrate was used, the effect of the presence of the slip velocity was more manifest. This study shows also that the effect of slip velocity on the flow field is very important and must be taken into consideration in an ACET micropump.

Alternating Current Electrothermal Flow for Cooling of Localized Hot Spots in Microelectronic Devices

In this article, we present our results on an energy-efficient cooling technology for localized microprocessor hot spots using alternating current electrothermal (ACET) flows. The electrothermal process deploys an electrokinetic transport mechanism without requiring any external prime mover and can be shown to be highly effective in reducing hot spot temperatures below their allowable limits. The proposed technique leverages the heat source(s) itself to drive the fluid in the electrothermal cooling mechanism, thereby making it efficient. Our parametric analyses present the optimal range of fluid properties, namely, electrical conductivity, where the cooling effectiveness is maximum. Furthermore, the impact of geometrical parameters as well as input voltages has been characterized. These results establish the potential of the electrothermal cooling and can open up a new avenue to be explored for thermal management of miniaturized devices and systems.

Insulator-based dielectrophoretic antifouling of nanoporous membrane for high conductive water desalination

Nanoporous membrane (NPM) based water desalination plays an essential role in remedying the global water scarcity crisis. However, the NPM always suffers from fouling issue because of various nanoparticles existed in sea/waste water. Based on the electrical insulation property of NPM and its ultra-small pore size, insulator-based dielectrophoresis (iDEP) is promising for manipulating the nanoparticle behavior in NPM and serves as an effective antifouling technique. Besides, the alternating current electrothermal (ACET) flow occurs due to the temperature dependent variation of water dielectric property, and it also affects the nanoparticle behavior via electrohydrodynamic (EHD) force. In this paper, the AC electrokinetic antifouling mechanism of NPM for water desalination is numerically investigated under several parametric conditions. The results indicate that iDEP force is effective for nanoparticle antifouling in NPM while ACET EHD force has a suppressing influence. Fortunately, due to its dominant magnitude, iDEP force makes the AC electrokinetic based NPM antifouling technique successful. Furthermore, the NPM with circular cone nanopore is highly recommended instead of that with circular cylinder nanopore in order to eliminate the nanoparticle “trapping zone” during antifouling process. The current investigation demonstrates the potential of handling NPM fouling issue using AC electrokinetics for membrane based water desalination technologies.

Combined alternating current electrothermal and dielectrophoresis-induced tunable patterning to actuate on-chip microreactions and switching at a floating electrode

A unique platform to fabricate tunable micropatterns at an electrically floating electrode has been developed via an enhanced combination of dielectrophoresis (DEP) and alternating current electrothermal (ACET) processes. The design of the platform uses two pairs of driving electrodes to generate a tunable electric field at a floating electrode, which in turn allows for the formation of patterned cells and fluids with desired features. By regulating the input configuration of the ac signals, yeast cells can be either patterned at the field stagnant region by a synergistic combination of negative DEP and ACET flow, or partially captured at the electrode edges subjected to a strong positive DEP, to form varying cell patterns. Additionally, nanoparticles were used to characterize the ACET-based flow pattern formation. Furthermore, the patterns were successfully exploited as an electrokinetic actuator to actuate continuous droplet switching, fluid mixing and microreactions. Implementing the simultaneous nanoparticle synthesis and guiding demonstrated that the combined field pattern is capable of manipulating both particulate samples and fluids. The proposed micropatterning technique may provide new insights for sample manipulation, on-chip function development and integration with other analytical components (i.e., biochemical sensors, chemical detectors, cell culture bioreactors) owing to a facile operation, easy integration and multifunctionality.