Resonant Chemical Sensors Research Articles

A unified model for electrostatic sensors in fluid media

We present a unified model of electrostatic sensors comprising cantilever microbeam resonators in fluid media. The model couples Euler–Bernoulli beam equation to the nonlinear Reynolds equation. Static, damped eigenvalue, and dynamic reduced-order models were developed and validated by comparing a nonlinear frequency response of a gas sensor to its experimentally measured counterpart. Experiments were conducted to verify the capability of the developed model to predict the out-of-plane and in-plane natural frequencies of the sensor. The models were also used to investigate the potential operation of electrostatic chemical sensors based on different sensing mechanisms. While in-plane and out-of-plane vibration modes were found to be viable alternatives for resonant gas sensors, only in-plane modes were suitable to implement resonant chemical sensors due to the added mass and damping of liquid media. Similarly, higher-order modes were found more sensitive than lower order modes. Further, evidence was found for elastic interaction between out-of-plane modes and liquids in the channel underneath them but none for in-plane modes. Finally, the model predicts that in-plane modes provide the multi-valuedness necessary to implement bifurcation chemical sensors in liquid media.

CMUT-based resonant gas sensor array for VOC detection with low operating voltage

With the anticipation for a more connected world through the Internet of Things, there is still a strong demand for miniaturized chemical sensors. Here, we report on a miniaturized resonant chemical sensor based on a Capacitive Micromachined Ultrasonic Transducer (CMUT) with a low operating voltage suitable for portable gas sensor applications. Previously reported CMUT chemical sensors required a DC operating voltage (16∼50 V) higher than the supply voltages of common circuits (e.g., 1.8–5 V). Thus, additional circuitry such as a charge pump circuit often was required as a part of the sensor interface circuits to supply the DC voltage to CMUT. This resulted in additional power consumption and a larger footprint. In this work, the vacuum gap of the CMUT which determines the operating voltage was reduced to 50 nm through the development of a double oxidation process with a high wafer-level yield. We achieved a significantly smaller pull-in voltage (<10 V) for a 500-nm-thick CMUT resonant sensor. The CMUT was operated at approximately 80% of the pull-in voltage (∼8 V) which does not require additional voltage supply to that of the real-time read-out circuitry. Based on the resonant frequency of 6.7 MHz, the theoretical sensitivity of the fabricated CMUT resonant sensor was 0.8 Hz/fg. By coating four different chemically-sensitive polymer layers, we confirmed the operation of the CMUT with low operating voltage as a chemical sensor. Frequency shifts due to the chemical reaction of volatile organic compounds were observed and analyzed through principal component analysis. This work demonstrates the potential of the developed CMUT with low bias voltage as a key component of a portable chemical sensor system.

Length-extensional resonating gas sensors with IC-foundry compatible low-cost fabrication in non-SOI single-wafer

This study presents ‘dog-bone’ structured bulk-mode resonant gas sensors that can be low-cost volume fabricated in standard IC foundries. Without the need for MEMS-specific processes like double-side aligned photolithography and wafer-to-wafer bonding, the sensor fabrication can be completed in standard IC-foundries, thereby showing advantage in low-cost mass production and application. Based on a diaphragm-on-cavity structure made by a MIS (micro-openings inter-etch and sealing) micromachining technology in ordinary (non-SOI) single-wafer, the resonator fabrication is further completed with simple dry etching and metal interconnection. With similar fabrication scheme to semiconductor IC chips, herein the 3-D micromachining processes are only implemented from the front-side of the wafer (with unpolished wafer backside) and all the fabrication steps utilize IC-foundry available processes. After loading with gas sensitive material, the resonant sensors are ready for application. For comparison, traditional sensors with identical resonator structure are also fabricated in SOI wafer with double-sided process. Besides the measured similar resonance performance for the two sensors, the new sensor is advantageous in smaller dimensions (0.5mm×0.5mm) and much lower volume-fabrication cost (in ASMC, a 6-inch IC foundry company in Shanghai) of one US-cent per sensor. By loading an organo-phosphor sensitive chemical-group modified hyper-branched polymer onto the new resonator, the disposable biochemical micro-sensor is used to rapidly detect organophosphorous pesticide residue for food-safety and agriculture applications. The resonant chemical sensor experimentally performs linear and repeatable detection of ppm-level organo-phosphorous vapor.

Time frequency property for a micro resonant gas sensor

In this paper, combining dynamics equation of the resonant cantilever with kinetic equation of the adsorption reaction between coatings and the measured gas, a mechanical chemical coupled dynamics equation is proposed. Using the method of multiple scales, the mechanical chemical coupled dynamics equation is resolved. From the solution, using Hilbert transform, changes of the resonant frequency along with time are obtained. The time frequency property of the resonant cantilever during gas adsorption reaction is investigated. Results show that different gases can be distinguished from each other by means of the different time frequency property. Using the method, the measured time is short and there is not special requirement for adsorption reaction rate between coatings and the measured gas. It explores a novel idea for micro resonant chemical sensors.

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.

Mesoporous Thin-Film on Highly-Sensitive Resonant Chemical Sensor for Relative Humidity and CO2 Detection

Distributed sensing of gas-phase chemicals is a promising application for mesoporous materials when combined with highly sensitive miniaturized gas sensors. We present a direct application of a mesoporous silica thin film on a highly sensitive miniaturized resonant chemical sensor with a mass sensitivity at the zeptogram scale for relative humidity and CO(2) detection. Using mesoporous silica thin-film, we report one of the lowest volume resolutions and a sensitive detection of 5.1 × 10(-4)% RH/Hz to water vapor in N(2), which is 70 times higher than a device with a nontemplated silica layer. In addition, a mesoporous thin-film that is functionalized with an amino-group is directly applied on the resonant sensor, which exhibits a volume sensitivity of 1.6 × 10(-4)%/Hz and a volume resolution of 1.82 × 10(-4)% to CO(2) in N(2).

Chemical Vapor Detection Using a Capacitive Micromachined Ultrasonic Transducer

Distributed sensing of gas-phase chemicals using highly sensitive and inexpensive sensors is of great interest for many defense and consumer applications. In this paper we present ppb-level detection of dimethyl methylphosphonate (DMMP), a common simulant for sarin gas, with a ppt-level resolution using an improved capacitive micromachined ultrasonic transducer (CMUT) as a resonant chemical sensor. The improved CMUT operates at a higher resonant frequency of 47.7 MHz and offers an improved mass sensitivity of 48.8 zg/Hz/μm(2) by a factor of 2.7 compared to the previous CMUT sensors developed. A low-noise oscillator using the CMUT resonant sensor as the frequency-selective device was developed for real-time sensing, which exhibits an Allan deviation of 1.65 Hz (3σ) in the presence of a gas flow; this translates into a mass resolution of 80.5 zg/μm(2). The CMUT resonant sensor is functionalized with a 50-nm thick DKAP polymer developed at Sandia National Laboratory for dimethyl methylphosphonate (DMMP) detection. To demonstrate ppb-level detection of the improved chemical sensor system, the sensor performance was tested at a certified lab (MIT Lincoln Laboratory), which is equipped with an experimental chemical setup that reliably and accurately delivers a wide range of low concentrations down to 10 ppb. We report a high volume sensitivity of 34.5 ± 0.79 pptv/Hz to DMMP and a good selectivity of the polymer to DMMP with respect to dodecane and 1-octanol.

Bifurcation-based mass sensing using piezoelectrically-actuated microcantilevers

In conventional implementations, resonant chemical and biological sensors exploit chemomechanically-induced frequency shifts, which occur in linear systems, for analyte detection. In this letter, an alternative sensing approach, based upon dynamic transitions across saddle-node bifurcations is investigated. This technique not only has the potential to render improved sensor metrics but also to eliminate frequency tracking components from final device implementations. The present work details proof-of-concept experiments on bifurcation-based sensing, which were conducted using selectively functionalized, piezoelectrically-actuated microcantilevers. Preliminary results reveal the proposed sensing technique to be a viable alternative to existing resonant sensing methods.

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.

Fabrication of free-standing porous silicon microstructures

The high specific surface of porous silicon and its high reactivity makes this material a good candidate for chemical sensors based on electrical or electromechanical devices. In this paper, several processes are presented to realize free-standing porous silicon microstructures (membranes and cantilever beams). Good resonance quality factors were measured (Q = 110 and 760 respectively) demonstrating that porous silicon is a suitable material for resonant chemical sensors.

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 CMOS-compatible bulk technology for the fabrication of magnetically actuated microbalances for chemical sensing

A resonant chemical sensor, fabricated in a standard, CMOS-compatible technology followed by a two-mask post-processing, is presented. A silicon dioxide plate, suspended to two torsional springs, is released from the silicon substrate by means of a TMAH anisotropic wet etch. Two metal loops are integrated on the plate. The first, in conjunction with an external micromagnet, is used to drive the plate into mechanical resonance, while the second loop is used for detection. In accordance with the microbalance principle, a shift of the resonance frequency can be related to the adsorption on the plate of the chemical species under test. The sensor design, and first results on the implementation of the post-processing technology, are presented.

Coupled determination of gravimetric and elastic effects on two resonant chemical sensors: Love wave and microcantilever platforms

The objective of this paper is to couple theoretical and experimental results from microcantilevers and Love-wave acoustic devices in order to identify and separate mass loading effects from elastic effects. This is important in the perspective of sensing applications. For that, a thin-film polymer is deposited on both resonant platforms. It is demonstrated that microcantilevers are essentially mass sensitive. They allow one to determine the polymer layer thickness, which is validated by optical profilometry measurements. Then, taking into account this thickness, theoretical modeling and experimental measurements with Love-wave devices permit one to estimate an equivalent elastic shear modulus of the thin-film polymer at high frequency. Results are interesting if one is to fully understand and optimize (bio)chemical sensor responses.

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.

Dual-response resonant chemical sensors for multianalyte analysis

We developed an approach to increase the information content from resonant, thickness-shear mode (TSM) sensors. These sensors are intended for integration into a microanalytical system that will periodically collect and precondition a predetermined volume of sample and will further present it to the sensor for analysis. Each side of the TSM resonator is coated with a different sensor film. During measurements, only one side at a time is exposed to an analyzed vapor while another side is exposed to a blank gas from a scrubber. Thus, analyte-related signals are generated independently from each side of the TSM sensor. Such dual-response sensing approach reduces the number of sensing elements by a factor of two, leading to further system miniaturization and improved power budget in sensor operation. We demonstrated our dual-response sensor concept with a high-performance, low noise TSM resonator ( f 0 = 10 7 Hz, RMS noise <0.05 Hz) arranged in a low volume (<200 μL) dual flow cell. With a single resonator, we successfully discriminated and quantified toxic industrial and environmental concern vapors such as toluene and trichloroethylene and two isomers of dichloroethylene (DCE) such as cis-1,2-DCE and trans-1,2-DCE.

Sensor Technologies for Monitoring Metabolic Activity in Single Cells—Part II: Nonoptical Methods and Applications

A review of solid-state chemical and electrochemical sensors to detect metabolic activity at the extracellular, single-cell level is presented in the context of the development of lab-on-a-chip research instrumentation. Metabolic processes in cells are briefly reviewed with the goal of quantifying the role of metabolites within the cell. Sensors reviewed include both research and commercial devices that can noninvasively detect extracellular metabolites, including oxygen, carbon dioxide, and glucose. Metabolic activity can also be sensed nonselectively by measuring pH gradients. Performance metrics, such as sensitivity, sensor size, drift, time response, and sensing range, are included when available. Highly suitable sensor technologies for monitoring cellular metabolic activity include electrochemical sensors, scanning electrochemical microscopy, ion-sensitive field effect transistor sensors, and solid-state light-addressable potentiometric sensors. Other less-suitable, but still potentially viable, solid-state sensing technologies are also reviewed briefly, including resonant chemical sensors (surface acoustic wave and quartz crystal microbalance), conductivity or impedance sensors, and sensors with multiple transduction stages. Specific biological applications which benefit from detection of extracellular metabolic events at the single-cell level are discussed to provide context to the practical use of these sensor technologies; these applications include case studies of various diseases (cancer, diabetes, mitochondrial disorders. etc.), cell and tissue differentiation; cell and tissue storage; cell life cycle and basic cellular processes; and developmental biology.

Investigation of acoustic load sensitivity of quartz crystal resonator and related sensors

Linked with the characteristic of quartz crystal resonant chemical sensors, surface acoustic load sensitivity of a quartz crystal resonator (QCR), coated or not coated, is analyzed. Taking surface coating material and surface sorption into account, a multi-layer mechanical model is set up for the QCR. Starting from this model, formulae, which can be generally used for calculating relationship between resonant parameters and surface state, are deduced.