Microcantilever-based Sensors Research Articles

Highly vertical 3D bio-inspired hierarchical and multiscale superstructures on microcantilever for gas sensing of organophosphorous agents

Microgravimetric sensors offer promising solution to detect volatile compounds. However, they suffer from a low surface-to-volume ratio limiting the maximum amount of adsorbate that can be fixed on their 2D surface. In this study, we report a bio-inspired method to elaborate highly vertical 3D hierarchical and multiscale superstructures in order to greatly increase the surface area of the microcantilever and to capture more molecules in vapor phase. However, building such superstructures on the microcantilever surface remains challenging in terms of structural design and synthesis while preserving a sufficient response of the sensor at low gas concentration. These particular superstructures are inspired from the antenna of the silkmoth Bombyx mori which is composed of a stem with branches on either side, themselves covered with sensilla. A patterned-microcantilever with high-aspect-ratio silicon micropillars is therefore used as support to generate a dense array of titanium dioxide nanorods decorated by goethite nanobranches. 3D hierarchical superstructures show a high enhancement in the response of the sensor to vapor of dimethyl methylphosphonate, a common simulant of sarin considered as one of the most toxic molecules. This design generates mass and surface stress effects under exposure to low vapor concentrations allowing to reach a limit of detection of 3 ppb. The sensor shows also a fast response to a wide range of concentrations. These results provide promising prospects to improve the detection potential of microcantilever-based sensors by using complex superstructures and dedicated materials to generate stress effect on the microcantilever surface and overcome the conventional microcantilever sensors based only on mass variation.

Screen-Printed Microcantilevers Coated With Functionalized Mesocellular Foam Silica for Detection of Solvent Vapors

This work presents an application of mass-sensitive microcantilevers coated with a new alternative of sensitive materials that is hexamethyldisilazane functionalized mesocellular foam silica (HMDS-MCF-Si) for detection of solvent vapors. The microcantilevers (3 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times2$ </tex-math></inline-formula> mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times0.1$ </tex-math></inline-formula> mm) based on a lead zirconate titanate (PZT) beam sandwiched by thin gold (Au) electrodes were fabricated by screen-printing. Mesocellular foam silica (MCF-Si) was synthesized and used as the sensitive material due to its high specific surface area and pore volume. This material was functionalized by hexamethyldisilazane (HMDS) to enhance hydrophobicity. By a small deposition of HMDS-MCF-Si, the obtained devices demonstrated longitudinal vibration mode with the quality factor of approximately 200 at ambient temperatures. Responsiveness of the microcantilever coated with HMDS-MCF-Si to each solvent vapor at room temperature was compared. The largest response was obtained from exposure to toluene vapor with the sensitivity of 0.85 ppm/ppmV (218 mHz/ppmV) and a limit of detection (LOD) of 28 ppmV. The sensitivity dropped by almost six times when exposed to the higher polar vapor of ethanol. Very low response to water vapor (0.05 ppm/ppmV) confirmed the hydrophobic behavior of HMDS-MCF-Si. Detectable performances under different vapors of our device can be utilized as a guide for developing sensitive materials and resonant microcantilever-based vapor sensors.

Additive Manufacturing of Microcantilevers of Varying Stiffness for Sensing Applications

Introduction Microcantilevers have numerous applications in physical, biological and chemical sensing. They have been employed for determination of the fluid viscosity and density [1], detection of biomolecular interaction [2], and part per billion (ppb) level of ammonia in ambient atmosphere [3]. The silicon-based microcantilevers are the most common type of these microdevices, however the biological and physical applications of the polymeric and polymer-composite microcantilevers have been reported in the literature [4], [5]. The polymeric cantilevers have lower Young’s modulus compared to the ones made of silicon and are more sensitive for static deflection measurement [6]. The polydimethylsiloxane (PDMS) and SU-8 are examples of polymers used for fabricating cantilevers through the soft-lithography process. Although the soft-lithography is a matured process for fabrication of the micro-electromechanical systems (MEMS), it is time consuming and costly in comparison with the additive manufacturing process. This may justify further investigations on employing Additive Manufacturing (AM) technologies in fabrication of the MEMS devices. In this study, we report additive manufacturing of the polymeric microcantilevers with different dimensions using SLA 3D printing technology for employing in micro-sensing applications. Method Four 300 μm width microcantilevers with different lengths and thicknesses, were 3D printed using the Form 2 SLA 3D printer. Table 1 presents the dimensions of the cantilevers. Figure 1 shows the images of the green parts on the building platform and the final parts after post processing. 50 μm was chosen as a thickness of each layer for printing. The building material was Flexible resin (Formlabs, USA). Table 2 presents the characteristics of the printing material. The printing process of all cantilevers took 2.5 hours. To investigate the mechanical property of printed cantilevers, the linear stiffness of each part was determined by the static load-deflection tests. The tests were performed by imposing the deflection to the tip of cantilever and recording the corresponding load. The maximum deflection applied to cantilevers was 300 μm. The FemtoTools FT-RS1002 Microrobotic Measurement System was employed to perform the measurements using the mounted sensing probe with the needle tip section size of (50 × 50 μm), force range of ±100000 μN, and resolution of 5 μN. Figure 2 illustrates the front and side views of the sensing probe in contact with the one of cantilevers at the beginning of the measurement test. Results and Conclusions The length and thickness of the microcantilever affect its sensitivity. Figure 3 displays the force-deflection graphs of microcantilevers with different length-thickness ratios obtained from the tests explained in methodology. Due to the small deflection imposed to all plastic microcantilevers with respect to their dimensions, the results show the bilinear response of the cantilevers. The linear relation between the force and the deflection can be observed when the samples deflects beyond the 80 μm. Despite various dimensions of the fabricated microcantilevers, the graph confirms that the length-thickness ratio affects the stiffness of the beams which is determined by the slope of each graphs. Table 3 presents the stiffness of fabricated samples. The graphs show increasing the length-ratio of the microbeams results in increasing the stiffness of the beam. The mechanical stiffnesses of the low length-thickness ratio microcantilevers, samples 1 and 2, are comparable with the PDMS cantilever’s stiffness reported in reference [7] . This reveals the possibility of employing the SLA 3D printing method in fabrication of the operational microcantilevers. This manufacturing method offers the simplicity, time and cost reduction in fabrication of the microcantilevers. References Kim, Deokman, et. al. "Determination of fluid density and viscosity by analyzing flexural wave propagations on the vibrating micro-cantilever." Sensors 17, no. 11 (2017): 2466.Amritsar, Jeetender, et. al. "Conformational detection of heat shock protein through bio-interactions with microstructures." Research on Biomedical Engineering 36, no. 1 (2020): 89-98.Liu, Manyi, et. al. "Revealing humidity-enhanced NH3 sensing effect by using resonant microcantilever." Sensors and Actuators B: Chemical 257 (2018): 488-495.Seena, V., et. al. "Polymer microcantilever biochemical sensors with integrated polymer composites for electrical detection." Solid State Sciences 11, no. 9 (2009): 1606-1611.Sadabadi, Hamid, and Muthukumaran Packirisamy. "Nano-integrated suspended polymeric microfluidics (SPMF) platform for ultra-sensitive bio-molecular recognition of bovine growth hormones." Scientific reports 7, no. 1 (2017): 1-10.Chaudhary, Monika, and Amita Gupta. "Microcantilever-based sensors." Defence Science Journal 59, no. 6 (2009): 634-641.Nezhad, Amir Sanati, et. al. "PDMS microcantilever-based flow sensor integration for lab-on-a-chip." IEEE Sensors journal 13, no. 2 (2012): 601-609. Figure 1

Review: Cantilever-Based Sensors for High Speed Atomic Force Microscopy.

This review critically summarizes the recent advances of the microcantilever-based force sensors for atomic force microscope (AFM) applications. They are one the most common mechanical spring–mass systems and are extremely sensitive to changes in the resonant frequency, thus finding numerous applications especially for molecular sensing. Specifically, we comment on the latest progress in research on the deflection detection systems, fabrication, coating and functionalization of the microcantilevers and their application as bio- and chemical sensors. A trend on the recent breakthroughs on the study of biological samples using high-speed atomic force microscope is also reported in this review.

A Versatile Mass-Sensing Platform With Tunable Nonlinear Self-Excited Microcantilevers

A versatile mass-sensing platform based on the nonlinear dynamical response of a microcantilever embedded in a self-excitation feedback loop is proposed. It is experimentally shown that the delay imposed in the feedback loop by an adjustable phase-shifter can be used to finely tune this system to work in three different modalities, according to the desired mass sensing application: 1) as a continuous mass sensor, where the oscillation frequency smoothly responds to changes in the mass added to the resonator; 2) as a threshold sensor, where a sudden change in the oscillation frequency is triggered by an arbitrarily small change of mass added to the cantilever; and 3) as a stable microresonator, whose oscillation frequency is almost not affected by environmental conditions, such as changes in added mass, or in density/viscosity of the surrounding fluid. This variety of dynamical responses was registered for a wide range of added masses, in the form of beads individually attached to the cantilever. A complete analytical model to explain the observed experimental results is derived and shows a strong agreement with the measured data. The high resolution and signal-to-noise ratio, as well as the threshold and stable sensing modalities obtained with this closed-loop technique, are not available in the current open-loop microcantilever-based mass sensors.

Gas Sensing System Composed of IC-Foundry-Compatible Microcantilever, Advanced Sensing Material, and Intelligent Signal Processing

Gas sensors based on resonant microcantilevers have drawn significant attention in the application of trace molecule detection. In order to realize the practical application of microcantilever-based sensors, three critical points still must be addressed: inexpensive and batch-fabricated microcantilevers, high-performance sensing material and easily operated signal-processing circuit with intelligent software. Herein, a gas sensing system with a multiand- interdisciplinary collaboration mode is proposed. A low-cost and IC-foundry-compatible fabrication technology of the resonant microcantilever is developed. For the example of trace aniline detection, a scalable metal-organic-framework (MOF) material of MOF-5 with ultrahigh surface area and abundant specific aniline-adsorption sites is prepared. For sensing signal processing, an interface circuit with intelligent data processing capability is designed. Composed of the low-cost microcantilever, the advanced MOF-5 sensing material, and the intelligent signal process, the new gas sensing system shows satisfactory performance for trace aniline vapor detection.

Size-dependent effects of surface stress on resonance behavior of microcantilever-based sensors

The mass and surface stress loadings in microcantilever-based sensors, caused by surface adsorption of target analytes/biomolecules, change their resonance behavior. Despite numerous efforts, the mechanisms of surface stress impact on the resonance behavior of microcantilevers are not fully understood. This results in substantial discrepancies between theoretically predicted and experimentally measured resonance frequency shifts in wide microcantilevers with aspect ratios (width-to-thickness) larger than 10, commonly found in chemical and biological sensing applications. This also impedes the proper decoupling of the effects of mass and surface stress loading and hinders the development of accurate microcantilever-based sensors. In this work, we attempt to decrease the discrepancy found between theoretical models and experiments by introducing a new mechanism that correlates the nonlinear geometric effects associated with surface stress-induced variation of microcantilever biaxial curvature to its frequency and modal responses, with a correlation intensity that strongly depends on the microcantilever aspect ratio. A new comprehensive analytical model combining the new mechanism with currently available theoretical models is proposed and used to study the resonance behavior of microcantilevers with different deformations and sizes. Using the new model, it is shown that in wide microcantilevers, the theoretically predicted resonance frequency shifts agree with experimental and finite element simulations results within 30% whereas the predictions by other currently available theoretical models are off by more than one order of magnitude. The new model is also used to investigate the modal response of microcantilevers where it anticipates the bending-extensional mode coupling associated with the flexural vibration modes of microcantilevers with a coupling strength that depends on the microcantilever curvature. The presented model is expected to provide a new tool to decouple the effects of mass and surface stress loadings on the resonance behavior of microcantilevers used in chemical and biological sensors.

Design and Modeling of MEMS-Based Trace-Level Moisture Measurement System for GIS Applications in Smart Grid Environment

This paper presents a system to monitor the presence of trace moisture in SF6 gas insulated switchgear (GIS) employing a micro-cantilever sensor. The main objective of the proposed system design is to continuously monitor the moisture concentration inside the GIS for enhanced safety. Microcantilever-based sensors reduce the overall size of the measurement system as well as enhance the measurement accuracy and sensitivity. A new system approach is proposed to detect trace moisture by monitoring the change in Q-factor of the microcantilever. It makes the measurement of resonant frequency of the microcantilever easier to implement and analyze. The system is made Internet of Things (IoT) enabled using Raspberry Pi. It allows easy local and remote accessibility of sensor data. The usefulness of the proposed system lies in its continuous monitoring ability, compact nature, and easy integration and accessibility, which are some of the essential and current requirements of the smart grid environment.

Real-time stochastic response analysis as a tool for monitoring cantilever mechanical properties

Microcantilever-based sensors are very promising devices for biochemical applications. They are usually operated in two modes. In the first one, a microcantilever static bending induced by the surface stress is observed. In the second mode, resonant frequency shift caused by mass loading is measured. The second mode requires an external force to excite cantilever vibrations. There is a possibility to use a stochastic excitation signal to estimate frequency shift as well as other interesting mechanical properties, such as effective spring constant or damping. Cantilever thermal self-vibrations or artificially generated white noise are very convenient examples of such a stochastic excitation signal.In the paper a real-time stochastic response analysis (RTSA) technique is presented. It is based on autoregressive moving average (ARMA) process modeling. Estimated model parameters are used for calculation of the eigenfrequency, quality factor and effective spring constant of a given vibration mode. The description of the entire procedure is presented, along with the results of simulations. The results confirm validity of the proposed ARMA model and show expected estimation errors for an illustrative set of cantilevers.The proposed algorithm is also applied to monitor the quality factor and resonant frequency of an electromagnetically-actuated microcantilever. The stochastic signal used to excite the cantilever is generated by a very simple white noise generator. The RTSA enables simultaneous monitoring of the cantilever resonant frequency and quality factor.The proposed solution is an interesting option in applications, in which simplicity and cost of the measurement system are key issues.

Geometry optimization of uncoated silicon microcantilever-based gas density sensors

In the absence of coating, the only way to improve the sensitivity of silicon microcantilever-based density sensors is to optimize the device geometry. Based on this idea, several microcantilevers with different shapes (rectangular-, U- and T-shaped microstructures) and dimensions have been fabricated and tested in the presence of hydrogen/nitrogen mixtures (H2/N2) of various concentrations ranging from 0.2% to 2%. In fact, it is demonstrated that wide and short rectangular cantilevers are more sensitive to gas density changes than U- and T-shaped devices of the same overall dimensions, and that the thickness does not affect the sensitivity despite the fact that it affects the resonant frequency. Moreover, because of the phase linearization method used for the natural frequency estimation, detection of a gas mass density change of 2mg/l has been achieved with all three microstructures. In addition, noise measurements have been used to estimate a limit of detection of 0.11mg/l for the gas mass density variation (corresponding to a concentration of 100ppm of H2 in N2), which is much smaller than the current state of the art for uncoated mechanical resonators.

Effect of Conformational Entropy on the Nanomechanics of Microcantilever-Based Single-Stranded DNA Sensors

An entropy-controlled bending mechanism is presented to study the nanomechanics of microcantilever-based single-stranded DNA (ssDNA) sensors. First; the conformational free energy of the ssDNA layer is given with an improved scaling theory of thermal blobs considering the curvature effect; and the mechanical energy of the non-biological layer is described by Zhang’s two-variable method for laminated beams. Then; an analytical model for static deflections of ssDNA microcantilevers is formulated by the principle of minimum energy. The comparisons of deflections predicted by the proposed model; Utz–Begley’s model and Hagan’s model are also examined. Numerical results show that the conformational entropy effect on microcantilever deflections cannot be ignored; especially at the conditions of high packing density or long chain systems; and the variation of deflection predicted by the proposed analytical model not only accords with that observed in the related experiments qualitatively; but also appears quantitatively closer to the experimental values than that by the preexisting models. In order to improve the sensitivity of static-mode biosensors; it should be as small as possible to reduce the substrate stiffness.

Preliminary results of the feasibility of hydrogen detection by the use of uncoated silicon microcantilever-based sensors

Hydrogen is a key parameter to monitor radioactive disposal facility such as the envisioned French geological repository for nuclear wastes. The use of microcantilevers as chemical sensors usually involves a sensitive layer whose purpose is to selectively sorb the analyte of interest. The sorbed substance can then be detected by monitoring either the resonant frequency shift (dynamic mode) or the quasi-static deflection (static mode). The objective of this paper is to demonstrate the feasibility of eliminating the need for the sensitive layer in the dynamic mode, thereby increasing the long-term reliability. The microcantilever resonant frequency allows probing the mechanical properties (mass density and viscosity) of the surrounding fluid and, thus, to determine the concentration of a species in a binary gaseous. Promising preliminary work has allowed detecting concentration of 200 ppm of hydrogen in air with non-optimized geometry of silicon microcantilever with integrated actuation and read-out.

New characterization methods for monitoring small resonant frequency variation: Experimental tests in the case of hydrogen detection with uncoated silicon microcantilever-based sensors

The uncoated silicon microcantilever (USMC) operated in the dynamic mode is a new concept in the field of microcantilever-based chemical sensors. Due to the absence of a sensitive layer, this kind of microsensor can only be used for specific applications where it is known that only one chemical species may be varying in concentration, such as monitoring hydrogen release in radioactive waste disposal facilities. Usually, the relative variation of the USMC resonant frequency expected for low concentrations (≤2%) of hydrogen in nitrogen is below 50ppm. As a result, the measurement of both the resonant frequencies, fr, and the quality factor, Q, by classical methods, based on the gain spectrum (resonant peak and −3dB bandwidth), is not sufficiently accurate. In this paper, new measurement methods for monitoring fr and Q variations are proposed: (1) variation of gain and phase at fixed frequencies and (2) polynomial approximations of gain and phase spectra. The performance study of these characterization methods shows that monitoring fr by using phase linearization yields the best signal-to-noise ratio (e.g., 100 at 0.6% of H2 in N2), with 0.02% as a limit of detection for hydrogen.

Activation process of reversible Pd thin film hydrogen sensors

Microcantilever-based thin film palladium hydrogen sensors have high selectivity and sensitivity. Reproducibility and accuracy of the sensor performance depend on the activation process of the polycrystalline palladium film deposited on the cantilever. When the hydrogen is in solid solution (α-phase), the cantilever bending is mostly governed by the residual film stress induced by the swelling at the grain boundaries in the film. When the palladium hydride (β-phase) starts to be formed, the cantilever undergoes a large deflection due to hydrogen absorption-induced film swelling (10% change in volume). Differences in the phase diagrams of the palladium hydride for two film thicknesses show that the cantilever bending is governed by hydrogen uptake as well as the release of the residual stress of the film through cyclic exposure and cycling number.

Resonance Frequency Change in Microcantilever-Based Sensor due to Humidity Variation

Microcantilever-based sensors have attracted interest in the last decade because of their small size, rapid detection and high sensitivity. This sensor can be applied in the many fields, i.e. physics, chemistry, biology, biochemistry, medical, and environment. In this paper, we describe microcantilever-based sensor for environmental monitoring, especially for a humidity detection. This sensor was operated in dynamic mode where a change in mass or spring constant of the microcantilever provides the resonance frequency change. Here, a change of humidity is detected by the resonance frequency and the amplitude changes. It is found that the increase in the humidity causes the decreasing the resonance frequency but increasing the amplitude. This result opens up the possibility of the humidity detection using microcantilever-based sensor.

Comprehensive distributed-parameters modeling and experimental validation of microcantilever-based biosensors with an application to ultrasmall biological species detection

Nanotechnological advancements have made a great contribution in developing label-free and highly sensitive biosensors. The detection of ultrasmall adsorbed masses has been enabled by such sensors which transduce molecular interaction into detectable physical quantities. More specifically, microcantilever-based biosensors have caught widespread attention for offering a label-free, highly sensitive and inexpensive platform for biodetection. Although there are a lot of studies investigating microcantilever-based sensors and their biological applications, a comprehensive mathematical modeling and experimental validation of such devices providing a closed form mathematical framework is still lacking. In almost all of the studies, a simple lumped-parameters model has been proposed. However, in order to have a precise biomechanical sensor, a comprehensive model is required being capable of describing all phenomena and dynamics of the biosensor. Therefore, in this study, an extensive distributed-parameters modeling framework is proposed for the piezoelectric microcantilever-based biosensor using different methodologies for the purpose of detecting an ultrasmall adsorbed mass over the microcantilever surface. An optimum modeling methodology is concluded and verified with the experiment. This study includes three main parts. In the first part, the Euler–Bernoulli beam theory is used to model the nonuniform piezoelectric microcantilever. Simulation results are obtained and presented. The same system is then modeled as a nonuniform rectangular plate. The simulation results are presented describing model's capability in the detection of an ultrasmall mass. Finally the last part presents the experimental validation verifying the modeling results. It was shown that plate modeling predicts the real situation with a degree of precision of 99.57% whereas modeling the system as an Euler–Bernoulli beam provides a 94.45% degree of precision. The detection of ultrasmall immobilized enzyme molecules over the cantilever surface was achieved using a self-exciting piezoelectric-based microcantilever in the dynamic mode.

Chemisorption-Induced Resonance Frequency Shift of a Microcantilever

The autonomy and property of atoms/molecules adsorbed on the surface of a microcantilever can be probed by measuring its resonance frequency shift due to adsorption. The resonance frequency change of a cantilever induced by chemisorption is theoretically studied. Oxygen chemisorbed on the Si(100) surface is taken as a representative example. We demonstrate that the resonant response of the cantilever is mainly determined by the chemisorption-induced bending stiffness variation, which depends on the bond configurations formed by the adsorbed atoms and substrate atoms. This study is helpful for optimal design of microcantilever-based sensors for various applications.

Deflection and Resonance Frequency Shift of a Microcantilever Induced by Chemisorption: Oxygen on Si(100)

An energy-based model is presented to predict the deflection and resonance frequency shift of a microcantilever induced by chemisorption. The chemisorption of oxygen on Si(100) surface is taken as a representative example. The connection between the continuum-level cantilever responses (e.g. static deflection, resonance frequency) and the molecular-level interactions (covalent bond interactions) are established. The mechanical behavior of the cantilever on the density of adsorbed atoms/molecules and the property of the substrate is investigated. This study is helpful for designing microcantilever-based sensors which have many technologically important applications

A novel method for simultaneous readout of static bending and multimode resonance-frequency of microcantilever-based biochemical sensors

We propose a novel method for simultaneous readout of static bending and multimode resonance-frequency of a microcantilever. The method, as opposed to the currently most common approach, requires no external excitation of microcantilever's vibrations. It utilizes intrinsic thermal Brownian noise of the microcantilever. It allows monitoring the frequencies of several flexural and torsional vibration modes simultaneously along with static bending and twisting in both gaseous and liquid environments. We have developed a complete measurement setup for investigating microcantilever-based sensors. The setup consists of a measurement head with optical detection system, a liquid cell with working volume of 100 μl, all homemade, and PC-based processing software. We report a successful application of proposed method and developed system to investigation of microcantilever-based sensors in gaseous and liquid environments, with achieved time and frequency (mass) resolution competitive to previously reported methods.

A novel method of temperature compensation for piezoresistive microcantilever-based sensors

Microcantilever with integrated piezoresistor has been applied to in situ surface stress measurement in the field of biochemical sensors. It is well known that piezoresistive cantilever-based sensors are sensitive to ambient temperature changing due to highly temperature-dependent piezoresistive effect and mismatch in thermal expansion of composite materials. This paper proposes a novel method of temperature drift compensation for microcantilever-based sensors with a piezoresistive full Wheatstone bridge integrated at the clamped ends by subtracting the amplified output voltage of the reference cantilever from the output voltage of the sensing cantilever through a simple temperature compensating circuit. Experiments show that the temperature drift of microcantilever sensors can be significantly reduced by the method.