S. Truax

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 resonators 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.

Temperature compensation method for resonant microsensors based on a controlled stiffness modulation

A strategy to compensate for frequency drifts caused by temperature changes in resonant microstructures is presented. The proposed compensation method is based on a controlled stiffness modulation of the resonator by an additional feedback loop to extract the frequency changes caused by temperature changes. The feasibility of the suggested method is verified experimentally by compensating for temperature-induced frequency fluctuations of a micromachined resonator. The developed compensation scheme requires only one additional feedback loop and is applicable to any resonant microstructure featuring excitation and detection elements.

Gas and liquid phase sensing of volatile organics with disk microresonator

The sensing of volatile organic compounds (VOCs) using a MEMS resonator with an in-plane vibrational mode is reported. VOCs are detected in both the gas and liquid phases by a polymer-coated disk microresonator, which is operated as the frequency determining element in an amplifying feedback loop. The functionalized disk microresonators exhibit a short term frequency stability of 1.1 times 10-7 in air and 3.4 times 10-6 in water. Using polymer membranes as chemically sensitive layers, different concentrations of o-xylene, benzene, octane, trichloroethane, and toluene have been detected in the gas phase, with the limit of detection for o-xylene being 2.2 ppm. M-xylene has been detected in the liquid phase with a limit of detection of 1.9 ppm.

Exploring the resolution of different disk-type chemical sensors

Chemical sensitivities and limits of detection to volatile organic compounds (VOCs) for polymer-coated disk resonators in the gas phase are reported. The dependence of the sensor performance on the device geometry is investigated. The use of rotational inplane modes results in high frequency stabilities, with Allan variances as low as 7.8×10−9 being achieved for 25µm thick resonators. A limit of detection of 18.3 ppm for toluene was obtained for a 5 µm thick device coated with an only 20 nm thick polymer layer.

On the relative sensitivity of mass-sensitive chemical microsensors

In this work, the chemical sensitivity of mass-sensitive chemical microsensors with a uniform layer sandwich structure vibrating in their lateral or in-plane flexural modes is investigated. It is experimentally verified that the relative chemical sensitivity of such resonant microsensors is -to a first order- independent of the microstructures in-plane dimensions and the flexural eigenmode used, and only depends on the layer thicknesses and densities as well as the sorption properties of the sensing film. Important implications for the design of mass-sensitive chemical microsensors are discussed, whereby the designer can focus on the layer stack to optimize the chemical sensitivity and on the in-plane dimensions and mode shape to optimize the resonators frequency stability.

Selectivity enhancement strategy for cantilever-based gas-phase VOC sensors through use of peptide-functionalized carbon nanotubes

A major challenge in the development of chemical sensors for volatile organic compounds (VOC) has been finding sensitive films, which selectively partition different volatile organics. This work presents a selectivity enhancement strategy using carefully chosen peptides to preferentially interact with different VOCs based on the polarity of the analytes. Carbon nanotubes (CNT) grown at low-temperature are used as a scaffold for the peptides. The CNTs are grown on top of mass-sensitive cantilever-based sensors and provide a large surface area for peptide binding, thus helping to increase the sensitivity of the sensors. Tests show that the peptides do in fact interact differently with VOCs based on the polarity of the compound. Achieved detection limits are in the low parts-per-million range.

Frequency Drift Compensation in Mass-Sensitive Chemical Sensors based on Periodic Stiffness Modulation

The successful compensation of frequency drift in a mass-sensitive chemical microsensor is demonstrated. The proposed compensation method uses a periodic stiffness modulation, generated by a second feedback loop, to monitor the microresonators quality factor (Q-factor). The Q-factor is solely obtained from frequency measurements and monitored along with the measurand-induced frequency shift during normal closed-loop sensor operation. This simultaneous measurement of Q-factor and frequency shift enables the compensation of frequency drift induced by environmental disturbances using the extracted Q-factor. The feasibility of drift compensation has been demonstrated by implementing the compensation scheme into a closed-loop chemical sensing system and performing gas-phase chemical measurements.

Assessing polymer sorption kinetics using micromachined resonators

This paper introduces a new approach to investigate polymer sorption kinetics by weighing the polymer films using micromachined in-plane resonators. A custom gas-testing setup enables fast analyte concentration changes, which are necessary to study analyte diffusion into thin polymer coatings deposited on top of the microresonators. Short-term frequency stabilities of the microresonators in the 10−8 range in air yield sub-picogram mass resolution and enable real-time measurement of analyte uptake into µm-thick films with time constants ranging from seconds to minutes. As an example, the diffusion of alcohols and aromatic hydro-carbons into poly(epichlorohydrin) and poly(isobuty-lene) films, which are of interest for chemical sensing applications, has been investigated.

Effect of polymer thickness on the chemical sensing behavior of polymer-coated mass-sensitive disk resonators

The chemical sensing behavior of disk microresonators, as it is affected by the thickness of polymer coatings applied to them, is reported. The disk microresonators were fabricated using a CMOS-compatible silicon bulk micro machining process. The disks were subsequently coated with layers of (poly) isobutylene (PIB) as a sensing film and exposed to gaseous benzene, toluene, and m-xylene. The experimental results agree well with analytical models derived for both the chemical sensitivity and limit of detection (LOD). Minimal LODs measured for benzene, toluene, and m-xylene were 5.3 ppm, 1.2 ppm, and 0.6 ppm, respectively.

Chemical microsystem based on integration of microresonant sensor and CMOS ASIC

A silicon-based microsystem consisting of a mass-sensitive resonant sensor and a CMOS ASIC containing feedback circuitry is demonstrated for portable sensing applications. The feedback circuitry sustains oscillation of the resonant sensor at its mechanical resonance frequency ranging between 200 and 800 kHz. The microsystem has been used for detection of volatile organic compounds in the gas-phase, and a limit of detection of 13 ppm for toluene is obtained with a frequency stability of 16 mHz.