Environmental sensors based on micromachined cantilevers with integrated read-out

Polymer Ring-Flexure-Membrane Suspended Gate FET Gas Sensor: Design, Modelling and Simulation

This work reports the design, modelling, and simulation of a novel polymer MEMS gas sensor platform called a ring-flexure-membrane (RFM) suspended gate field effect transistor (SGFET). The sensor consists of a suspended polymer (SU-8) MEMS based RFM structure holding the gate of the SGFET with the gas sensing layer on top of the outer ring. During gas adsorption, the polymer ring-flexure-membrane architecture ensures a constant gate capacitance change throughout the gate area of the SGFET. This leads to efficient transduction of the gas adsorption-induced nanomechanical motion input to the change in the output current of the SGFET, thus improving the sensitivity. The sensor performance has been evaluated for sensing hydrogen gas using the finite element method (FEM) and TCAD simulation tools. The MEMS design and simulation of the RFM structure is carried out using CoventorWare 10.3, and the design, modelling, and simulation of the SGFET array is carried out using the Synopsis Sentaurus TCAD. A differential amplifier circuit using RFM-SGFET is designed and simulated in Cadence Virtuoso using the lookup table (LUT) of the RFM-SGFET. The differential amplifier exhibits a sensitivity of 2.8 mV/MPa for a gate bias of 3 V and a maximum detection range of up to 1% hydrogen gas concentration. This work also presents a detailed fabrication process integration plan to realize the RFM-SGFET sensor using a tailored self-aligned CMOS process adopting the surface micromachining process.

Micro-Kelvin Resolution at Room Temperature Using Nanomechanical Thermometry

Ultrahigh sensitivity temperature measurement is becoming increasingly relevant for different scientific and technological fields from fundamental physics to high-precision engineering applications. Here, we demonstrate the use of a nanomechanical resonator-free standing silicon nitride membranes with thicknesses in the nanoscale-for room temperature thermometry reaching an unprecedented resolution of 15 μK. These devices were characterized by using an interferometric system at high vacuum, where there are only two possible mechanisms for heat transfer: thermal conductivity and radiation. While the expected behavior should be to decrease the frequency of the mechanical resonance due to the thermoelastic effect, we observe that the nanomechanical response can be both positive and negative depending on the thermal flux: a heat point source always shifts the mechanical resonance to lower frequencies, while a distributed heat source shifts the resonance to higher frequencies.

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.

Graphene nanoribbons as flexible docks for chemiresistive sensing of gas phase explosives

Interpretation of chemiresistive sensor measurements is made difficult by the fact that similar conductance changes may be produced by different adsorbed species. This fundamental ambiguity may be addressed by formulating a new docking paradigm. Instead of decorating graphene with ligands whose structure is well suited to bind with a particular target molecule, a generic dock in the form of a flexible, semiconducting graphene nanoribbon (GNR) may be employed. If the deformed shape of the GNR is then varied, via mechanical actuation, a two dimensional signature (sensor current versus bias voltage and GNR deformation) of the target molecule may be obtained. Ab initio modeling results indicate that this signature may be used to distinguish explosives from background gases and to discriminate between chemically similar explosives.

A Real-Time Thermal Self-Elimination Method for Static Mode Operated Freestanding Piezoresistive Microcantilever-Based Biosensors

Here, we provide a method and apparatus for real-time compensation of the thermal effect of single free-standing piezoresistive microcantilever-based biosensors. The sensor chip contained an on-chip fixed piezoresistor that served as a temperature sensor, and a multilayer microcantilever with an embedded piezoresistor served as a biomolecular sensor. This method employed the calibrated relationship between the resistance and the temperature of piezoresistors to eliminate the thermal effect on the sensor, including the temperature coefficient of resistance (TCR) and bimorph effect. From experimental results, the method was verified to reduce the signal of thermal effect from 25.6 μV/°C to 0.3 μV/°C, which was approximately two orders of magnitude less than that before the processing of the thermal elimination method. Furthermore, the proposed approach and system successfully demonstrated its effective real-time thermal self-elimination on biomolecular detection without any thermostat device to control the environmental temperature. This method realizes the miniaturization of an overall measurement system of the sensor, which can be used to develop portable medical devices and microarray analysis platforms.

Polymeric Flexible Immunosensor Based on Piezoresistive Micro-Cantilever with PEDOT/PSS Conductive Layer

In this paper, a fully polymeric micro-cantilever with the surface passivation layer of parylene-C and the strain resistor of poly(3,4-ethylenedioxythiophene)/poly (styrene sulfonate) (PEDOT/PSS) was proposed and demonstrated for immunoassays. By optimizing the design and fabrication of the polymeric micro-cantilever, a square resistance of 220 Ω/□ for PEDOT/PSS conductive layer have been obtained. The experimental spring constant and the deflection sensitivity were measured to be 0.017 N/m and 8.59 × 10-7 nm-1, respectively. The biological sensing performances of polymeric micro-cantilever were investigated by the immunoassay for human immunoglobulin G (IgG). The immunosensor was experimentally demonstrated to have a linear behavior for the detection of IgG within the concentrations of 10~100 ng/mL with a limit of detection (LOD) of 10 ng/mL. The experimental results indicate that the proposed polymeric flexible conductive layer-based sensors are capable of detecting trace biological substances.

A Review on Surface Stress-Based Miniaturized Piezoresistive SU-8 Polymeric Cantilever Sensors

In the last decade, microelectromechanical systems (MEMS) SU-8 polymeric cantilevers with piezoresistive readout combined with the advances in molecular recognition techniques have found versatile applications, especially in the field of chemical and biological sensing. Compared to conventional solid-state semiconductor-based piezoresistive cantilever sensors, SU-8 polymeric cantilevers have advantages in terms of better sensitivity along with reduced material and fabrication cost. In recent times, numerous researchers have investigated their potential as a sensing platform due to high performance-to-cost ratio of SU-8 polymer-based cantilever sensors. In this article, we critically review the design, fabrication, and performance aspects of surface stress-based piezoresistive SU-8 polymeric cantilever sensors. The evolution of surface stress-based piezoresistive cantilever sensors from solid-state semiconductor materials to polymers, especially SU-8 polymer, is discussed in detail. Theoretical principles of surface stress generation and their application in cantilever sensing technology are also devised. Variants of SU-8 polymeric cantilevers with different composition of materials in cantilever stacks are explained. Furthermore, the interdependence of the material selection, geometrical design parameters, and fabrication process of piezoresistive SU-8 polymeric cantilever sensors and their cumulative impact on the sensor response are also explained in detail. In addition to the design-, fabrication-, and performance-related factors, this article also describes various challenges in engineering SU-8 polymeric cantilevers as a universal sensing platform such as temperature and moisture vulnerability. This review article would serve as a guideline for researchers to understand specifics and functionality of surface stress-based piezoresistive SU-8 cantilever sensors.

Microcantilever array instrument based on optical fiber and performance analysis

We developed a microcantilever array biosensor instrument based on optical readout from a microcantilever array in fluid environment. The microcantilever signals were read out sequentially by laser beams emitted from eight optical fibers. The optical fibers were coupled to lasers, while the other ends of the fibers were embedded in eight V-grooves with 250 μm pitch microfabricated from a Si wafer. Aspherical lens was used to keep the distance between lasers. A programmable logic controller was used to make the system work stably. To make sure that the output of lasers was stable, a temperature controller was set up for each laser. When the deflection signal was collected, lasers used here were set to be on for at least 400 ms in each scanning cycle to get high signal-to-noise ratio deflection curves. A test was performed by changing the temperature of the liquid cell holding a microcantilever array to verify the consistent response of the instrument to the cantilever deflections. The stability and conformance of the instrument were demonstrated by quantitative detection of mercury ions in aqueous solution and comparison detection of clenbuterol by setting test and reference cantilevers. This microcantilever array detection instrument can be applied to highly sensitive detection of chemical and biological molecules in fluid environment.

Nonlinear-Based MEMS Sensors and Active Switches for Gas Detection

The objective of this paper is to demonstrate the integration of a MOF thin film on electrostatically actuated microstructures to realize a switch triggered by gas and a sensing algorithm based on amplitude tracking. The devices are based on the nonlinear response of micromachined clamped-clamped beams. The microbeams are coated with a metal-organic framework (MOF), namely HKUST-1, to achieve high sensitivity. The softening and hardening nonlinear behaviors of the microbeams are exploited to demonstrate the ideas. For gas sensing, an amplitude-based tracking algorithm is developed to quantify the captured quantity of gas. Then, a MEMS switch triggered by gas using the nonlinear response of the microbeam is demonstrated. Noise analysis is conducted, which shows that the switch has high stability against thermal noise. The proposed switch is promising for delivering binary sensing information, and also can be used directly to activate useful functionalities, such as alarming.

Model development and boundary interaction force control of a piezoresistive-based microcantilever

Robots with micro- and nanoresolution of motion are becoming more practical and useful in many precision manufacturing processes and industries such as medical instruments and imaging tools. Apparently, the most important features of these devices are their precision and durability. As accuracy increases, more delicate tasks may be performed. Along this line, a spatial micromanipulator with three revolute–revolute–prismatic joints while equipped with nanometer motion resolution is considered here. At the end of the micromanipulator, a piezoresistive-based microcantilever operates as a force sensor to quantify the amount of the strain generated in the microcantilever and transduces it into a proper voltage for force sensing applications. In terms of the controller design, the value of the produced voltage can further be implemented as the feedback entering into the control loop and making the control unit to produce appropriate signals for manipulating the robot arm. A challenging and important problem is the need to control the applied boundary forces at the contact zone with external objects (specifically the biological samples). The ability to control the interaction force is of most interest today which has numerous applications in precision manufacturing and biomedical engineering. For this purpose, two types of controllers are presented here: a Lyapunov-based proportional-derivative (PD) controller and a robust adaptive (RA) controller. The performance and the stability of these two controllers are examined and discussed thoroughly in this paper so that it can be interpreted that the robust adaptive controller is robust under presence of uncertainties in force tracking control purposes.

Analyzing the Effect of Capillary Force on Vibrational Performance of the Cantilever of an Atomic Force Microscope in Tapping Mode with Double Piezoelectric Layers in an Air Environment

The aim of this paper is to determine the effects of forces exerted on the cantilever probe tip of an atomic force microscope (AFM). These forces vary according to the separation distance between the probe tip and the surface of the sample being examined. Hence, at a distance away from the surface (farther than d(on)), these forces have an attractive nature and are of Van der Waals type, and when the probe tip is situated in the range of a₀≤ d(ts) ≤ d(on), the capillary force is added to the Van der Waals force. At a distance of d(ts) ≤ a₀, the Van der Waals and capillary forces remain constant at intermolecular distances, and the contact repulsive force repels the probe tip from the surface of sample. The capillary force emerges due to the contact of thin water films with a thickness of h(c) which have accumulated on the sample and probe. Under environmental conditions a layer of water or hydrocarbon often forms between the probe tip and sample. The capillary meniscus can grow until the rate of evaporation equals the rate of condensation. For each of the above forces, different models are presented. The smoothness or roughness of the surfaces and the geometry of the cantilever tip have a significant effect on the modeling of forces applied on the probe tip. Van der Waals and the repulsive forces are considered to be the same in all the simulations, and only the capillary force is altered in order to evaluate the role of this force in the AFM-based modeling. Therefore, in view of the remarkable advantages of the piezoelectric microcantilever and also the extensive applications of the tapping mode, we investigate vibrational motion of the piezoelectric microcantilever in the tapping mode. The cantilever mentioned is entirely covered by two piezoelectric layers that carry out both the actuation of the probe tip and the measuringof its position.

Nanomechanical analysis of the adsorption and desorption of water vapor on porous surfaces

We fabricated nanoporous microcantilevers using anodic aluminum oxide (AAO) and measured the variations in the resonance frequency and deflection of the cantilever during the adsorption and desorption of water vapor.

Device for filamentous fungi growth monitoring using the multimodal frequency response of cantilevers

Filamentous fungi cause opportunistic infections in hospital patients. A fast assay to detect viable spores is of great interest. We present a device that is capable of monitoring fungi growth in real time via the dynamic operation of cantilevers in an array. The ability to detect minute frequency shifts for higher order flexural resonance modes is demonstrated using hydrogel functionalised cantilevers. The use of higher order resonance modes sees the sensor dependent mass responsivity enhanced by a factor of 13 in comparison to measurements utilizing the fundamental resonance mode only. As a proof of principle measurement, Aspergillus niger growth is monitored using the first two flexural resonance modes. The detection of single spore growth within 10 h is reported for the first time. The ability to detect and monitor the growth of single spores, within a small time frame, is advantageous in both clinical and industrial settings.

Development of multi-environment dual-probe atomic force microscopy system using optical beam deflection sensors with vertically incident laser beams

We developed a dual-probe atomic force microscopy (DP-AFM) system with two cantilever probes that can be operated in various environments such as in air, vacuum, and liquid. The system employs the optical beam deflection method for measuring the deflection of each cantilever mounted on a probe scanner. The cantilever probes mounted on the probe scanners are attached to inertia sliders, which allow independent control of the probe positions. We constructed three types of probe scanners (tube, shear-piezo, and tripod types) and characterized their performance. We demonstrated AFM imaging in ambient air, vacuum, and ultrapure water, and also performed electrical measurement and pick-up manipulation of a Au nanorod using the DP-AFM system.

Electrical detection of C-reactive protein using a single free-standing, thermally controlled piezoresistive microcantilever for highly reproducible and accurate measurements

This study demonstrates a novel method for electrical detection of C-reactive protein (CRP) as a means of identifying an infection in the body, or as a cardiovascular disease risk assay. The method uses a single free-standing, thermally controlled piezoresistive microcantilever biosensor. In a commonly used sensing arrangement of conventional dual cantilevers in the Wheatstone bridge circuit, reference and gold-coated sensing cantilevers that inherently have heterogeneous surface materials and different multilayer structures may yield independent responses to the liquid environmental changes of chemical substances, flow field and temperature, leading to unwanted signal disturbance for biosensing targets. In this study, the single free-standing microcantilever for biosensing applications is employed to resolve the dual-beam problem of individual responses in chemical solutions and, in a thermally controlled system, to maintain its sensor performance due to the sensitive temperature effect. With this type of single temperature-controlled microcantilever sensor, the electrical detection of various CRP concentrations from 1 µg/mL to 200 µg/mL was performed, which covers the clinically relevant range. Induced surface stresses were measured at between 0.25 N/m and 3.4 N/m with high reproducibility. Moreover, the binding affinity (KD) of CRP and anti-CRP interaction was found to be 18.83 ± 2.99 µg/mL, which agreed with results in previous reported studies. This biosensing technique thus proves valuable in detecting inflammation, and in cardiovascular disease risk assays.

A self-sensing piezoelectric microcantilever biosensor for detection of ultrasmall adsorbed masses: theory and experiments

Detection of ultrasmall masses such as proteins and pathogens has been made possible as a result of advancements in nanotechnology. Development of label-free and highly sensitive biosensors has enabled the transduction of molecular recognition into detectable physical quantities. Microcantilever (MC)-based systems have played a widespread role in developing such biosensors. One of the most important drawbacks of all of the available biosensors is that they all come at a very high cost. Moreover, there are certain limitations in the measurement equipments attached to the biosensors which are mostly optical measurement systems. A unique self-sensing detection technique is proposed in this paper in order to address most of the limitations of the current measurement systems. A self-sensing bridge is used to excite piezoelectric MC-based sensor functioning in dynamic mode, which simultaneously measures the system's response through the self-induced voltage generated in the piezoelectric material. As a result, the need for bulky, expensive read-out equipment is eliminated. A comprehensive mathematical model is presented for the proposed self-sensing detection platform using distributed-parameters system modeling. An adaptation strategy is then implemented in the second part in order to compensate for the time-variation of piezoelectric properties which dynamically improves the behavior of the system. Finally, results are reported from an extensive experimental investigation carried out to prove the capability of the proposed platform. Experimental results verified the proposed mathematical modeling presented in the first part of the study with accuracy of 97.48%. Implementing the adaptation strategy increased the accuracy to 99.82%. These results proved the measurement capability of the proposed self-sensing strategy. It enables development of a cost-effective, sensitive and miniaturized mass sensing platform.

Sensorless enhancement of an atomic force microscope micro-cantilever quality factor using piezoelectric shunt control

The image quality and resolution of the Atomic Force Microscope (AFM) operating in tapping mode is dependent on the quality (Q) factor of the sensing micro-cantilever. Increasing the cantilever Q factor improves image resolution and reduces the risk of sample and cantilever damage. Active piezoelectric shunt control is introduced in this work as a new technique for modifying the Q factor of a piezoelectric self-actuating AFM micro-cantilever. An active impedance is placed in series with the tip oscillation voltage source to modify the mechanical dynamics of the cantilever. The benefit of using this control technique is that it removes the optical displacement sensor from the Q control feedback loop to reduce measurement noise in the loop and allows for a reduction in instrument size.

A micro-cantilever sensor chip based on contact angle analysis for a label-free troponin I immunoassay

Cantilever sensors have been extensively explored as a promising technique for real-time and label-free analyses in biological systems. A major sensing principle utilized by state-of-the-art cantilever sensors is based on analyte-induced surface stress changes, which result in static bending of a cantilever. The sensor performance, however, suffers from the intrinsically small change in surface stress induced by analytes, especially for molecular recognition such as antigen-antibody binding. Through the contact angle change on a tailored solid surface, it is possible to convert a tiny surface stress into a capillary force-a much larger physical quantity needed for a practical sensor application. In this work, a micro-cantilever sensor based on contact angle analysis (CAMCS) was proposed to effectively enhance the sensitivity of a sensor in proportion to the square of the length to thickness ratio of the cantilever structure. CAMCS chips were fabricated using a standard complementary-metal-oxide-semiconductor (CMOS) process to demonstrate a 1250-fold enhancement in the sensitivity of surface stress to bioanalyte adsorption using a piezoresistive sensing method. A real-time and label-free troponin I (cTnI) immunoassay, which is now widely used in clinics and considered a gold standard for the early diagnosis and prognosis of cardiovascular disease, was performed to demonstrate cTnI detection levels as low as 1 pg mL(-1). The short detection time of this assay was within several minutes, which matches the detection time of commercially available instruments that are based on fluorescence-labeling techniques.

Monolithic composite “pressure + acceleration + temperature + infrared” sensor using a versatile single-sided “SiN/Poly-Si/Al” process-module

We report a newly developed design/fabrication module with low-cost single-sided "low-stress-silicon-nitride (LS-SiN)/polysilicon (poly-Si)/Al" process for monolithic integration of composite sensors for sensing-network-node applications. A front-side surface-/bulk-micromachining process on a conventional Si-substrate is developed, featuring a multifunctional SiN/poly-Si/Al layer design for diverse sensing functions. The first "pressure + acceleration + temperature + infrared" (PATIR) composite sensor with the chip size of 2.5 mm × 2.5 mm is demonstrated. Systematic theoretical design and analysis methods are developed. The diverse sensing components include a piezoresistive absolute-pressure sensor (up to 700 kPa, with a sensitivity of 49 mV/MPa under 3.3 V supplied voltage), a piezoresistive accelerometer (±10 g, with a sensitivity of 66 μV/g under 3.3 V and a -3 dB bandwidth of 780 Hz), a thermoelectric infrared detector (with a responsivity of 45 V/W and detectivity of 3.6 × 107 cm·Hz1/2/W) and a thermistor (-25-120 °C). This design/fabrication module concept enables a low-cost monolithically-integrated "multifunctional-library" technique. It can be utilized as a customizable tool for versatile application-specific requirements, which is very useful for small-size, low-cost, large-scale sensing-network node developments.

Analysis of detection enhancement using microcantilevers with long-slit-based sensors

The present work analyzes theoretically and verifies the advantage of utilizing rectangular microcantilevers with long-slits in microsensing applications. The deflection profile of these microcantilevers is compared with that of typical rectangular microcantilevers under the action of dynamic disturbances. Various force-loading conditions are considered. The theory of linear elasticity for thin beams is used to obtain the deflection-related quantities. The disturbance in these quantities is obtained based on wave propagation and beam vibration theories. It is found that detections of rectangular microcantilevers with long-slits based on maximum slit opening length can be more than 100 times the deflections of typical rectangular microcantilevers. Moreover, the disturbance (noise effect) in the detection quantities of the microcantilever with long-slits is found to be always smaller than that of typical microcantilevers, regardless of the wavelength, force amplitude, and the frequency of the dynamic disturbance. Eventually, the detection quantities of the microcantilever with long-slits are found to be almost unaffected by dynamic disturbances, as long as the wavelengths of these disturbances are larger than 3.5 times the microcantilever width. Finally, the present work recommends implementation of microcantilevers with long-slits as microsensors in robust applications, including real analyte environments and out of laboratory testing.

Atomic force microscopy as a tool applied to nano/biosensors

This review article discusses and documents the basic concepts and principles of nano/biosensors. More specifically, we comment on the use of Chemical Force Microscopy (CFM) to study various aspects of architectural and chemical design details of specific molecules and polymers and its influence on the control of chemical interactions between the Atomic Force Microscopy (AFM) tip and the sample. This technique is based on the fabrication of nanomechanical cantilever sensors (NCS) and microcantilever-based biosensors (MC-B), which can provide, depending on the application, rapid, sensitive, simple and low-cost in situ detection. Besides, it can provide high repeatability and reproducibility. Here, we review the applications of CFM through some application examples which should function as methodological questions to understand and transform this tool into a reliable source of data. This section is followed by a description of the theoretical principle and usage of the functionalized NCS and MC-B technique in several fields, such as agriculture, biotechnology and immunoassay. Finally, we hope this review will help the reader to appreciate how important the tools CFM, NCS and MC-B are for characterization and understanding of systems on the atomic scale.

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.

Analysis of deflection enhancement using epsilon assembly microcantilevers based sensors

The present work analyzes theoretically and verifies the advantage of utilizing ɛ-microcantilever assemblies in microsensing applications. The deflection profile of these innovative ɛ-assembly microcantilevers is compared with that of the rectangular microcantilever and modified triangular microcantlever. Various force-loading conditions are considered. The theorem of linear elasticity for thin beams is used to obtain the deflections. The obtained defections are validated against an accurate numerical solution utilizing finite element method with maximum deviation less than 10 percent. It is found that the ɛ-assembly produces larger deflections than the rectangular microcantilever under the same base surface stress and same extension length. In addition, the ɛ-microcantilever assembly is found to produce larger deflection than the modified triangular microcantilever. This deflection enhancement is found to increase as the ɛ-assembly's free length decreases for various types of force loading conditions. Consequently, the ɛ-microcantilever is shown to be superior in microsensing applications as it provides favorable high detection capability with a reduced susceptibility to external noises. Finally, this work paves a way for experimentally testing the ɛ-assembly to show whether detective potential of microsensors can be increased.

High throughput label-free platform for statistical bio-molecular sensing

Sensors are crucial in many daily operations including security, environmental control, human diagnostics and patient monitoring. Screening and online monitoring require reliable and high-throughput sensing. We report on the demonstration of a high-throughput label-free sensor platform utilizing cantilever based sensors. These sensors have often been acclaimed to facilitate highly parallelized operation. Unfortunately, so far no concept has been presented which offers large datasets as well as easy liquid sample handling. We use optics and mechanics from a DVD player to handle liquid samples and to read-out cantilever deflection and resonant frequency. Also, surface roughness is measured. When combined with cantilever deflection the roughness is discovered to hold valuable additional information on specific and unspecific binding events. In a few minutes, 30 liquid samples can be analyzed in parallel, each by 24 cantilever-based sensors. The approach was used to detect the binding of streptavidin and antibodies.

Transient deflection response in microcantilever array integrated with polydimethylsiloxane (PDMS) microfluidics

We report the integration of a nanomechanical sensor consisting of 16 silicon microcantilevers with polydimethylsiloxane (PDMS) microfluidics. For microcantilevers positioned near the bottom of a microfluidic flow channel, a transient differential analyte concentration for the top versus bottom surface of each microcantilever is created when an analyte-bearing fluid is introduced into the flow channel (which is initially filled with a non-analyte containing solution). We use this effect to characterize a bare (nonfunctionalized) microcantilever array in which the microcantilevers are simultaneously read out with our recently developed high sensitivity in-plane photonic transduction method. We first examine the case of non-specific binding of bovine serum albumin (BSA) to silicon. The average maximum transient microcantilever deflection in the array is -1.6 nm, which corresponds to a differential surface stress of only -0.23 mN m(-1). This is in excellent agreement with the maximum differential surface stress calculated based on a modified rate equation in conjunction with finite element simulation. Following BSA adsorption, buffer solutions with different pH are introduced to further study microcantilever array transient response. Deflections of 20-100 nm are observed (2-14 mN m(-1) differential surface stress). At a flow rate of 5 μL min(-1), the average measured temporal width (FWHM) of the transient response is 5.3 s for BSA non-specific binding and 0.74 s for pH changes.

Mass-sensitive detection of gas-phase volatile organics using disk microresonators

The detection of volatile organic compounds (VOCs) in the gas phase by mass-sensitive disk microresonators is reported. The disk resonators were fabricated using a CMOS-compatible silicon micromachining process and subsequently placed in an amplifying feedback loop to sustain oscillation. Sensing of benzene, toluene, and xylene was conducted after applying controlled coatings of an analyte-absorbing polymer. An analytical model of the resonator's chemical sensing performance was developed and verified by the experimental data. Limits of detection for the analytes tested were obtained, modeled, and compared to values obtained from other mass-sensitive resonant gas sensors.

High throughput optical readout of dense arrays of nanomechanical systems for sensing applications

We present an instrument based on the scanning of a laser beam and the measurement of the reflected beam deflection that enables the readout of arrays of nanomechanical systems without limitation in the geometry of the sample, with high sensitivity and a spatial resolution of few micrometers. The measurement of nanoscale deformations on surfaces of cm(2) is performed automatically, with minimal need of user intervention for optical alignment. To exploit the capability of the instrument for high throughput biological and chemical sensing, we have designed and fabricated a two-dimensional array of 128 cantilevers. As a proof of concept, we measure the nanometer-scale bending of the 128 cantilevers, previously coated with a thin gold layer, induced by the adsorption and self-assembly on the gold surface of several self-assembled monolayers. The instrument is able to provide the static and dynamic responses of cantilevers with subnanometer resolution and at a rate of up to ten cantilevers per second. The instrumentation and the fabricated chip enable applications for the analysis of complex biological systems and for artificial olfaction.

An analytical model of joule heating in piezoresistive microcantilevers

The present study investigates Joule heating in piezoresistive microcantilever sensors. Joule heating and thermal deflections are a major source of noise in such sensors. This work uses analytical and numerical techniques to characterise the Joule heating in 4-layer piezoresistive microcantilevers made of silicon and silicon dioxide substrates but with the same U-shaped silicon piezoresistor. A theoretical model for predicting the temperature generated due to Joule heating is developed. The commercial finite element software ANSYS Multiphysics was used to study the effect of electrical potential on temperature and deflection produced in the cantilevers. The effect of piezoresistor width on Joule heating is also studied. Results show that Joule heating strongly depends on the applied potential and width of piezoresistor and that a silicon substrate cantilever has better thermal characteristics than a silicon dioxide cantilever.

Self-assembling siloxane bilayer directly on SiO2 surface of micro-cantilevers for long-term highly repeatable sensing to trace explosives

This paper presents a novel sensing layer modification technique for static micro-cantilever sensors that detect trace explosives by measuring specific adsorption-induced surface stress. For the first time, a method of directly modifying a siloxane sensing bilayer on an SiO(2) surface is proposed to replace the conventional self-assembled monolayers (SAMs) of thiols on Au to avoid the trouble from long-term unstable Au-S bonds. For modifying the long-term reliable sensing bilayer on the piezoresistor-integrated micro-cantilevers, a siloxane-head bottom layer is self-assembled directly on the SiO(2) cantilever surface, which is followed by grafting another explosive-sensing-group functionalized molecule layer on top of the siloxane layer. The siloxane-modified sensor has experimentally exhibited a highly resoluble response to 0.1 ppb TNT vapor. More importantly, the repeated detection results after 140 days show no obvious attenuation in sensing signal. Also observed experimentally, the specific adsorption of the siloxane sensing bilayer to TNT molecules causes a tensile surface stress on the cantilever. Herein the measured tensile surface stress is in contrast to the compressive surface stress normally measured from conventional cantilever sensors where the sensitive thiol-SAMs are modified on an Au surface. The reason for this newly observed phenomenon is discussed and preliminarily analyzed.

Piezoresistive cantilever array sensor for consolidated bioprocess monitoring

Cellulolytic microbes occur in diverse natural niches and are being screened for industrial modification and utility. A microbe for consolidated bioprocessing (CBP) development can rapidly degrade pure cellulose and then ferment the resulting sugars into fuels. To identify and screen for novel microbes for CBP, we have developed a piezoresistive cantilever array sensor which is capable of simultaneous monitoring of glucose and ethanol concentration changes in a phosphate buffer solution. 4-mercaptophenylboronic acid and polyethyleneglycol-thiol are employed to functionalize each piezoresistive cantilever for glucose and ethanol sensing, respectively. Successful concentration measurements of glucose and ethanol with minimal interferences are obtained with our cantilever array sensor.

Demonstration of microcantilever array with simultaneous readout using an in-plane photonic transduction method

We demonstrate a microcantilever array with an in-plane photonic transduction method for simultaneous readout of each microcantilever. The array is fabricated on a silicon-on-insulator substrate. Rib waveguides in conjunction with a compact waveguide splitter network comprised of trench-based splitters and trench-based bends route light from a single optical input to each microcantilever on the chip. Light propagates down a rib waveguide integrated into the microcantilever and, at the free end of the microcantilever, crosses a small gap. Light is captured in static asymmetric multimode waveguides that terminate in Y-branches, the outputs of which are imaged onto an InGaAs line scan camera. A differential signal for each microcantilever is simultaneously formed from the two outputs of the corresponding Y-branch. We demonstrate that reasonable signal uniformity is obtained with a scaled differential signal for seven out of nine surviving microcantilevers in an array.

In vivo wireless ethanol vapor detection in the Wistar rat

Traditional alcohol studies measure blood alcohol concentration to elucidate the biomedical factors that contribute to alcohol abuse and alcoholism. These measurements require large and expensive equipment, are labor intensive, and are disruptive to the subject. To alleviate these problems, we have developed an implantable, wireless biosensor that is capable of measuring alcohol levels for up to six weeks. Ethanol levels were measured in vivo in the interstitial fluid of a Wistar rat after administering 1 g/kg and 2 g/kg ethanol by intraperitoneal (IP) injection. The data were transmitted wirelessly using a biosensor selective for alcohol detection. A low-power piezoresistive microcantilever sensor array was used with a polymer coating suitable for measuring ethanol concentrations at 100% humidity over several hours. A hydrophobic, vapor permeable nanopore membrane was used to screen liquid and ions while allowing vapor to pass to the sensor from the subcutaneous interstitial fluid.

Towards Microcantilever-based Force Sensing and Manipulation: Modeling, Control Development and Implementation

This paper presents a distributed-parameters-based modeling framework for piezoresistive microcantilever (MC)-based force sensors used in a variety of cantilever-based nanomanipulation processes. Current modeling practices call for a simple lumped-parameters approach rather than modeling the piezoresistive MC itself. Owing to the widespread applications of such MCs in nanoscale force sensing or non-contact atomic force microscopy with nano-Newton to pico-Newton force measurement requirements, precise modeling of the piezoresistive MCs is essential. Instead of the previously used lumped-parameters modeling, a distributed-parameters modeling framework is proposed and developed here to arrive at the most complete model of the piezoresistive MC including tip-mass, tip-force and base movement considerations. In order to have online control and real-time sensor feedback, a closed-form model of the piezoresistive MC, which expresses the MC's piezoresistive output voltage as a function of tip force and base motion, is highly desirable. Along this line of reasoning, a closed-form model for the piezoresistive MC is presented. Following mathematical modeling, both numerical simulations and experimental results are presented to demonstrate the accuracy of the proposed distributed-parameters model when compared with the previously reported lumped-parameters modeling approach. Utilizing the developed model, a modified robust controller with perturbation estimation is adopted to target the problem of slow imaging acquisition and manipulation at the nanoscale. It is shown that the proposed controller can stabilize such nanomanipulation process in less than a second. Experimental results are presented to demonstrate the stability and performance characteristics of the designed controller. Such modeling and control development could pave the way for MC-based nanomanipulation and nanopositioning.

Review: Semiconductor Piezoresistance for Microsystems

Piezoresistive sensors are among the earliest micromachined silicon devices. The need for smaller, less expensive, higher performance sensors helped drive early micromachining technology, a precursor to microsystems or microelectromechanical systems (MEMS). The effect of stress on doped silicon and germanium has been known since the work of Smith at Bell Laboratories in 1954. Since then, researchers have extensively reported on microscale, piezoresistive strain gauges, pressure sensors, accelerometers, and cantilever force/displacement sensors, including many commercially successful devices. In this paper, we review the history of piezoresistance, its physics and related fabrication techniques. We also discuss electrical noise in piezoresistors, device examples and design considerations, and alternative materials. This paper provides a comprehensive overview of integrated piezoresistor technology with an introduction to the physics of piezoresistivity, process and material selection and design guidance useful to researchers and device engineers.

In-plane photonic transduction of silicon-on-insulator microcantilevers

We demonstrate an in-plane photonic transduction method for microcantilevers, which have been widely investigated for sensor applications. In our approach the microcantilever is etched to form a single mode rib waveguide. Light propagates down the microcantilever and crosses a small gap at the free end of the microcantilever, some of which is captured by an asymmetrical multimode waveguide that terminates in a Y-branch. The Y-branch outputs are used to form a differential signal that is monotonically dependent on microcantilever deflection. The measured differential signal matches simulation when microcantilever rotation is properly accounted for. The measured differential signal sensitivity is 1.4 x 10(-4) nm(-1) and the minimum detectable deflection is 0.35 nm.

SU-8 Cantilevers for Bio/chemical Sensing; Fabrication, Characterisation and Development of Novel Read-out Methods

Here, we present the activities within our research group over the last five yearswith cantilevers fabricated in the polymer SU-8. We believe that SU-8 is an interestingpolymer for fabrication of cantilevers for bio/chemical sensing due to its simple processingand low Young's modulus. We show examples of different integrated read-out methodsand their characterisation. We also show that SU-8 cantilevers have a reduced sensitivity tochanges in the environmental temperature and pH of the buffer solution. Moreover, weshow that the SU-8 cantilever surface can be functionalised directly with receptormolecules for analyte detection, thereby avoiding gold-thiol chemistry.

Uncooled infrared imaging device based on optimized optomechanical micro-cantilever array

It is a major issue to improve the thermo-mechanical sensitivity of uncooled optomechanical focal plane arrays (FPAs) for infrared imaging. This work presents an optimized multi-fold interval metallized leg (IML) configuration to increase the thermo-mechanical sensitivity of an uncooled optomechanical bi-material micro-cantilever array. The inclination angle changes of the cantilever elements are measured in the IR imaging system using an optical readout with a knife-edge filtering operation in the spectrum plane. The multi-fold IML configuration consists of alternately connected unmetallized and metallized legs. With the optimized fold number, the thermo-mechanical sensitivity of a micro-cantilever array can be amplified to two times of one-fold IML for a 120 microm x 120 microm element with 1 microm thick SiNx/0.2 microm thick Au films. Room temperature objects are imaged with the fabricated FPA containing 160 x 160 elements and a 12-bit CCD. Further modeling analysis shows that the experimental results are well accordant with the theoretical calculation. An important practical feature of the implemented approach is its straightforward fabrication for a large FPA, without growing complexity and cost.

Readout of micromechanical cantilever sensor arrays by Fabry-Perot interferometry

The increasing use of micromechanical cantilevers in sensing applications causes a need for reliable readout techniques of micromechanical cantilever sensor (MCS) bending. Current optical beam deflection techniques suffer from drawbacks such as artifacts due to changes in the refraction index upon exchange of media. Here, an adaptation of the Fabry-Perot interferometer is presented that allows simultaneous determination of MCS bending and changes in the refraction index of media. Calibration of the instrument with liquids of known refraction index provides an avenue to direct measurement of bending with nanometer precision. Versatile construction of flow cells in combination with alignment features for substrate chips allows simultaneous measurement of two MCS situated either on the same, or on two different support chips. The performance of the instrument is demonstrate in several sensing applications, including adsorption experiments of alkanethioles on MCS gold surfaces, and measurement of humidity changes in air.

An experimental investigation of analog delay generation for dynamic control of microsensors and atomic force microscopy

We present an implementation of pure-time-delay generation in analog signals located in the kilo-Hertz frequency band. The controlled constant delays that are produced engage in a feedback system to investigate the dynamic response of microcantilevers. Delayed systems offer a vast richness of eigenvalues resulting in the possibility of excitations at frequencies other than that of the fundamental mode. Different cantilever actuation and delay generation approaches are investigated and compared, and detailed experimental observation of the dynamic response of the system is presented. Based on our results, an acoustic excitation is devised that may be used as an efficient sensor.

Uncooled IR imaging using optomechanical detectors

In this study, we present an uncooled infrared imaging detector using knife-edge filter optical readout method. The tilt angle change of each cantilever in a focal plane array (FPA) can be simultaneously detected with a resolution of 10(-5) degrees. A deformation magnifying substrate-free microcantilever unit is specially designed. The multi-fold legs of microcantilever are interval metal coated to form a thermal deformation magnifying structure. Thermal and thermomechanical performance of this microcantilever unit are modeled and analyzed. An FPA with 100 x 100 pixels is fabricated and thermal images of human body are obtained by this detector.

Multiwell micromechanical cantilever array reader for biotechnology

We use a multiwell micromechanical cantilever sensor (MCS) device to measure surface stress changes induced by specific adsorption of molecules. A multiplexed assay format facilitates the monitoring of the bending of 16 MCSs in parallel. The 16 MCSs are grouped within four separate wells. Each well can be addressed independently by different analyte liquids. This enables functionalization of MCS separately by flowing different solutions through each well. In addition, each well contains a fixed reference mirror which allows measuring the absolute bending of MCS. In addition, the mirror can be used to follow refractive index changes upon mixing of different solutions. The effect of the flow rate on the MCS bending change was found to be dependent on the absolute bending value of MCS. Experiments and finite element simulations of solution exchange in wells were performed. Both revealed that one solution can be exchanged by another one after 200 microl volume has flown through. Using this device, the adsorption of thiolated DNA molecules and 6-mercapto-1-hexanol on gold surfaces was performed to test the nanomechanical response of MCS.

Liquid-Phase Chemical and Biochemical Detection Using Fully Integrated Magnetically Actuated Complementary Metal Oxide Semiconductor Resonant Cantilever Sensor Systems

A novel resonant cantilever sensor system for liquid-phase applications is presented. The monolithic system consists of an array of four electromagnetically actuated cantilevers with transistor-based readout, an analog feedback circuit, and a digital interface. The biochemical sensor chip with a size of 3 mm x 4.5 mm is fabricated in an industrial complementary metal oxide semiconductor (CMOS) process with subsequent CMOS-compatible micromachining. A package, which protects the electrical components and the associated circuitry against liquid exposure, allows for a stable operation of the resonant cantilevers in liquid environments. The device is operated at the fundamental cantilever resonance frequency of approximately 200 kHz in water with a frequency stability better than 3 Hz. The use of the integrated CMOS resonant cantilever system as a chemical sensor for the detection of volatile organic compounds in liquid environments is demonstrated. Low concentrations of toluene, xylenes, and ethylbenzene in deionized water have been detected by coating the cantilevers with chemically sensitive polymers. The liquid-phase detection of analyte concentrations in the single-ppm range has been achieved. Furthermore, the application of this sensor system to the label-free detection of biomarkers, such as tumor markers, is shown. By functionalizing the cantilevers with anti-prostate-specific antigen antibody (anti-PSA), the corresponding antigen (PSA) has been detected at concentration levels as low as 10 ng/mL in a sample fluid.

Detection of trace organophosphorus vapor with a self-assembled bilayer functionalized SiO2 microcantilever piezoresistive sensor

Using piezoresistive SiO2 microcantilever technology, we present an ultra-sensitive chemical sensor for trace organophosphorus vapor detection. A self-assembled composite layer of Cu2+/11-mercaptoundecanoic acid is modified on the surface of the sensing cantilever as a specific coating to capture P=O containing compounds. Experimental results indicate that the sensor can be quite sensitive to DMMP vapor (well known as a simulant of nerve agent). The signal-noise-limited detection resolution of the sensor is experimentally obtained as low as several parts per billion. Besides that the sensor can yield reversible and repeatable response to DMMP vapor, adsorption of DMMP on the self-assembled composite layer is well fit to the Langmuir isotherm model.