Microcantilever Chemical Sensors Research Articles

Silicon micro-cantilever chemical sensors fabricated in double-layer silicon-on-insulator (SOI) wafer

Micro-cantilever piezoresistive sensors are optimally designed and fabricated in a double-layer silicon-on-insulator (SOI) wafer. The sensor geometry is optimized by placing the sensing piezoresistor at the cantilever root region to increase effective piezoreisistive sensing area. According to finite-element simulation results, high sensitivity can be obtained by design the cantilever into a wide and short shape. In order to use single-crystalling silicon to fabricate both the cantilever and the piezoresistor for high sensitivity, double-layer SOI wafer, which has two active layers and two insulating layers, is proposed to fabricate the self-sensing micro-cantilever sensor. The piezoresistor is made of the top active-layer single-crystalline silicon. Without p---n junction isolation, such a piezoresistor can be free from leakage-current relative noise that helps to achieve fine sensing resolution. The bottom active-layer is used to form the cantilever, with well controlled cantilever thickness and high fabrication yield. With the top surface of the micro-cantilever is modified with the functionalized self-assembled monolayer, detection of trace-concentration Trinitrotoluene (TNT) vapor is experimentally carried out, with reproducible sensing response to 7.6 ppb TNT.

Micro-/Nanocombined Gas Sensors With Functionalized Mesoporous Thin Film Self-Assembled in Batches Onto Resonant Cantilevers

This paper reports a novel top-down/bottom-up combined resonant microcantilever chemical sensor, where the nanosensing material of a functionalized mesoporous thin film (MTF) is directly self-assembled on the sensing region of the integrated microcantilever. By using the batch-producible nano-on-micro construction technique, a large number of such sensors can be batch fabricated with uniform performance and low cost. More importantly, the sensing molecule terminals can be simultaneously constructed at the pore inner surface when the MTF is directly grown on the cantilever. With -NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -group-functionalized MTF directly grown onto the surface of the cantilever free end, the micro-/nanocombined gravimetric sensor has experimentally exhibited quick response and highly sensitive detection of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas.

Self-assembly and sensing-group graft of pre-modified CNTs on resonant micro-cantilevers for specific detection of volatile organic compound vapors

This paper reports MWCNT (multi-wall carbon nano-tube)-modified resonant micro-cantilever chemical sensors for detection of trinitrotoluene (TNT) vapor. The MWCNTs are pre-modified and then area-selectively self-assembled at the free-end gold pad of a micro-cantilever, in which a resonance-exciting heater and a signal-readout piezoresistive Wheatstone bridge are integrated. Featuring a high specific surface area, the MWCNTs are further functionalized with TNT-sensitive groups by grafting onto the sidewalls of the MWCNTs. To lower the non-specific absorption of water and other small organic molecules, the SiO2 surface of the micro-cantilever was also pre-treated for hydrophobicity and oleophobicity by self-assembling a monolayer of heptadecafluorodecyltrimethoxysilane. The results of our sensing experiments have shown a capability to rapidly detect ppb-level TNT vapor, and a high specificity of the functionalized groups to TNT molecules. The experiment has also confirmed a good long-term stability in detecting sensitivity.

A chemisorption-based microcantilever chemical sensor for the detection of trimethylamine

This paper presents a new method for detecting trimethylamine (TMA) in gas and liquid phases using a piezoresistive microcantilever sensor functionalized with a self-assembled monolayer (SAM). The microcantilever consists of two silicon dioxide layers and a single crystalline silicon piezoresistor in-between. This configuration allows microcantilevers to achieve high sensitivity because of the low stiffness of silicon dioxide and the high piezoresistive coefficients of single crystalline silicon. The surface of the microcantilever is chemically functionalized with a self-assembled monolayer of 11-mercaptoundecanoic acid (11-MUA) via Au–SH covalent bonding on a gold film deposited on the microcantilever. The 11-MUA SAM adsorbs TMA through hydrogen bonding between the SAM and TMA molecules, and the microcantilever is deflected by the changes in the surface stress as a result of the hydrogen bonding based chemisorption. Experimental results show that the deflection of the microcantilever depends on the concentration of TMA, and the minimum detection limits of 10 mg/L for liquid TMA and 1.65 μg/L for gas TMA are achieved. The selectivity of TMA with respect to ethanol, acetone, and butanone is also investigated.

An analytical model for transient deformation of viscoelastically coated beams: Applications to static-mode microcantilever chemical sensors

The problem governing the transient deformation of an elastic cantilever beam with viscoelastic coating, subjected to a time-dependent coating eigenstrain, is mathematically formulated. An analytical solution for an exponential eigenstrain history, exact within the context of beam theory, is obtained in terms of the coating and base layer thicknesses, the elastic modulus of the base material, the initial coating modulus, the coating relaxation percentage (0%–100%), and the time constants of the coating’s relaxation process and its eigenstrain history. Approximate formulas, valid for thin coatings, are derived as special cases to provide insight into system behavior. Main results include (1) the time histories of the beam curvature and the coating stresses, (2) a criterion governing the response type (monotonic or “overshoot” response), and (3) simple expressions for the overshoot ratio, defined as the peak response scaled by the steady-state response, and the time at which the peak response occurs. Applications to polymer-coated microcantilever-based chemical sensors operating in the static mode are discussed.

Theoretical Analysis of Strong-Axis Bending Mode Vibrations for Resonant Microcantilever (Bio)Chemical Sensors in Gas or Liquid Phase

The frequency stability, sensitivity, and limit of detection of a coated-cantilever chemical sensor operating in a dynamic are mainly determined by its mechanical quality factor. While a coated-cantilever operating in the gas phase exhibits a large reduction in quality factor, immersion in liquids results in an even greater reduction in the Q-factor due to displaced fluid mass and losses in the surrounding liquid. In this paper, two different vibration modes are studied in order to minimize both the losses induced by the surrounding medium and the displaced fluid mass, thus increasing the quality factor and sensitivity and improving (decreasing) the detection limit of the biochemical microsensor. The two particular vibration modes both involve first mode flexural vibrations (but in different orthogonal planes), and are referred to herein as bending (WAB) and bending (SAB). Using Sader's model, the expressions for both the quality factor and the resonant frequency are analyzed for the case of immersion in a viscous fluid. The results indicate that the strong-axis has certain advantages over the more conventional weak-axis in enhancing the sensor sensitivity and detection limit, even for the case in which the WAB and SAB devices have identical resonant frequencies

Effect of Coating Viscoelasticity on Quality Factor and Limit of Detection of Microcantilever Chemical Sensors

Microcantilevers with polymer coatings hold great promise as resonant chemical sensors. It is known that the sensitivity of the coated cantilever increases with coating thickness; however, increasing this thickness also results in an increase of the frequency noise due to a decrease of the quality factor. By taking into account only the losses associated with the silicon beam and the surrounding medium, the decrease of the quality factor cannot be explained. In this paper, an analytical expression is obtained for the quality factor, which accounts for viscoelastic losses in the coating. This expression explains the observed decrease of the quality factor with increasing polymer thickness. This result is then used to demonstrate that an optimum coating thickness exists that will maximize the signal-to-noise ratio and, thus, minimize the sensor limit of detection

A microcantilever chemical sensors optimization by taking into account losses

The current trend in increasing microcantilever resonant frequencies leads to relatively low quality factors for microcantilevers resonating in a gas medium. In order to increase the quality factor of such high-frequency microcantilevers, an approach that takes into account the different types of losses is provided to specify appropriate values of microcantilever aspect ratios (length to thickness, width to thickness). Conventional values of aspect ratios result in low quality factors, while those proposed here lead to slightly higher values of quality factor.

An alternative solution to improve sensitivity of resonant microcantilever chemical sensors: comparison between using high-order modes and reducing dimensions

By measuring shifts in the resonant frequency of a silicon microcantilever coated with a sensitive layer, it is possible to obtain a competitive chemical microsensor. In fact, the sensitivity of such a microsensor is improved when used in high resonant frequency device. Usually, to increase the resonant frequency, smaller microcantilevers are used. The limitation on increasing the natural frequency by reducing the microstructure size is often due to the limitation of the deflection measurement principle. Furthermore, it could also be difficult to obtain reproducible sensitive coatings on very small surfaces. In order to achieve high sensitivities corresponding to high frequencies without decreasing the microstructure size, high-order flexural modes can be considered. In this paper, it is demonstrated that the theoretical performance of the resonant microcantilever chemical sensor is essentially due to the resonant frequency value and not due to the mode order. The advantages of increasing the resonant frequency by using high-order modes is also confirmed experimentally.

A pH Sensor Based on a Microcantilever Coated with Intelligent Hydrogel

Hydrogels that contain different amounts of amino groups were used to modify microcantilevers for the pH measurements. These microcantilevers deflected upon exposure to various pH solutions due to the swelling and shrinking of the hydrogels. The microcantilever deflection, as a function of pH, is nearly linear in a wide pH range. A significant 1000 nm/pH bending response was observed from a Gel‐4 coated microcantilever, which could be used for precise pH measurements. Such hydrogel coated microcantilevers could potentially be used to prepare microcantilever chemical and biological sensors when molecular recognition agents are immobilized into the polymer.

Enhancing chemi-mechanical transduction in microcantilever chemical sensing by surface modification

The use of chemically selective thin-film coatings has been shown to enhance both the chemical selectivity and sensitivity of microcantilever (MC) chemical sensors. As an analyte absorbs into the coating, the coating can swell or contract causing an in-plane stress at the associated MC surface. However, much of the stress upon absorption of an analyte may be lost through slippage of the chemical coatings on the MC surface, or through relaxation of the coating in a manner that minimizes stress to the cantilever. Structural modification of MC chemical sensors can improve the stress transduction between the chemical coating and the MC. Surfaces of silicon MC were modified with focused ion beam milling. Sub-micron channels were milled across the width of the MC. Responses of the nanostructured, coated MCs to 2,3-dihydroxynaphthalene and a series of volatile organic compounds (VOCs) were compared to smooth, coated MCs. The analytical figures of merit for the nanostructured, coated MCs in the sensing of VOCs were found to be better than the unstructured MCs. A comparison is made with a previously reported method of creating disordered nanostructured MC surfaces.

An embedded polymer piezoresistive microcantilever sensor

We have developed a new type of chemical microsensor based on piezoresistive microcantilever technology. In this embedded polymer microsensor, a piezoresistive microcantilever is partially “embedded” into a polymeric material. Swelling of the polymer upon analyte exposure is measured as a simple resistance change in the embedded cantilever. Arrays of these sensors, each employing a different polymeric material, provide for the identification of a wide range of chemical vapor analytes. Advantages of this system over previous “surface” piezoresistive microcantilever chemical sensors include enhanced mechanical simplicity (no mechanical approach necessary), greater resistance to shock or movement, and lower cost.