Kefeng Zeng

A wireless, passive carbon nanotube-based gas sensor

A gas sensor, comprised of a gas-responsive multiwall carbon nanotube (MWNT)-silicon dioxide (SiO/sub 2/) composite layer deposited on a planar inductor-capacitor resonant circuit is presented here for the monitoring of carbon dioxide (CO/sub 2/), oxygen (O/sub 2/), and ammonia (NH/sub 3/). The absorption of different gases in the MWNT-SiO/sub 2/ layer changes the permittivity and conductivity of the material and consequently alters the resonant frequency of the sensor. By tracking the frequency spectrum of the sensor with a loop antenna, humidity, temperature, as well as CO/sub 2/, O/sub 2/ and NH/sub 3/ concentrations can be determined, enabling applications such as remotely monitoring conditions inside opaque, sealed containers. Experimental results show the sensor response to CO/sub 2/ and O/sub 2/ is both linear and reversible. Both irreversible and reversible responses are observed in response to NH/sub 3/, indicating both physisorption and chemisorption of NH/sub 3/ by the carbon nanotubes. A sensor array, comprised of an uncoated, SiO/sub 2/ coated, and MWNT-SiO/sub 2/ coated sensor, enables CO/sub 2/ measurement to be automatically calibrated for operation in a variable humidity and temperature environment.

Time domain characterization of oscillating sensors: Application of frequency counting to resonance frequency determination

A frequency counting technique is described for determining the resonance frequency of a transiently excited sensor; the technique is applicable to any sensor platform where the characteristic resonance frequency is the parameter of interest. The sensor is interrogated by a pulse-like excitation signal, and the resonance frequency of the sensor subsequently determined by counting the number of oscillations per time during sensor ring-down. A repetitive time domain interrogation technique is implemented to overcome the effects of sensor damping, such as that associated with mass loading, which reduces the duration of the sensor ring-down and hence the measurement resolution. The microcontroller based, transient frequency counting technique is detailed with application to the monitoring of magnetoelastic sensors [C. A. Grimes, D. Kouzoudis, and C. Mungle, Rev. Sci. Instrum. 71, 3822 (2000)], with a measurement resolution of 0.001% achieved in approximately 40 ms.

A wireless magnetoelastic biosensor for the direct detection of organophosphorus pesticides

An organophosphorus (OP) pesticide sensor was fabricated by applying a pH-sensitive polymer coating and organophosphorus hydrolase (OPH) enzyme onto the surface of a magnetoelastic sensor, the magnetic analogue of the better-known surface acoustic wave sensor. Organophosphorus hydrolase catalyses the hydrolysis of a wide range of organophosphorus compounds, which changes the pH in the hydrogel. This article describes the application of the magnetoelastic sensor for the detection of OP pesticides by measuring the changes in viscoelasticity caused by the swelling/shrinking of the pH-responsive polymer when exposed to the pesticides. The sensor was successfully used to detect paraoxon and parathion down to a concentration of 1 x 10(-7) and 8.5 x 10(-7) M respectively.

Quantification of multiple bioagents with wireless, remote-query magnetoelastic microsensors

This paper presents a micromagnetoelastic sensor array for simultaneously monitoring multiple biological agents. Magnetoelastic sensors, made of low-cost amorphous ferromagnetic ribbons, are analogous and complementary to piezoelectric acoustic wave sensors, which track parameters of interest via changes in resonance behavior. Magnetoelastic sensors are excited with magnetic ac fields, and, in turn, they generate magnetic fluxes that can be detected with a sensing coil from a distance. As a result, these sensors are highly attractive, not only due to their small size and low cost, but also because of their passive and wireless nature. Magnetoelastic sensors have been applied for monitoring pressure, temperature, liquid density, and viscosity, fluid How velocity and direction, and with chemical/biological responsive coatings that change mass or elasticity, various biological and chemical agents. In this paper, we report the fabrication and application of a six-sensor array for simultaneous measurement of Escherichia coli O157:H7, staphylococcal enterotoxin B, and ricin. In addition, the sensor array also monitors temperature and pH so the measurements are independent from these two parameters

Threshold-crossing counting technique for damping factor determination of resonator sensors

The behavior of resonator-type sensors at resonance is characterized by two fundamental parameters: resonance frequency and damping factor (or Q-factor). Practical applications require accurate and efficient measurements of these two parameters. Using magnetoelastic resonant sensors as a test case earlier work [K. Zeng, K. G. Ong, C. Mungle, and C. A. Grimes, Rev. Sci. Instrum. 73, 4375 (2002)] demonstrated the ability to determine resonance frequency by counting the number of cycles in the transient response of a pulsewise excited sensor. Presented in this paper is a novel technique for measuring the damping factor of a resonant magnetoelastic sensor, or any resonator type sensor, using threshold-crossing counting of the transient response. The damping factor determination technique eliminates the need for a lock-in amplifier or FFT analysis as in the conventional method of quality factor estimation from spectrum analysis, significantly simplifying the electronic implementation as well as improving measu...

Wireless Magnetoelastic Physical, Chemical, and Biological Sensors

Magnetoelastic sensors belong to the class of resonator sensors and can be considered the magnetic analog of piezoelectric sensors; tracking the resonance behavior, typically characterized by the resonance frequency and/or amplitude, enables a wide variety of applications, including chem/bio-sensing and environmental monitoring. The practical use of the sensors is facilitated with the smart microprocessor-based monitoring electronics that accurately and efficiently characterizes the pulsewise excited sensor response. The low cost and remote query nature of the sensors make the sensor platform ideally suited for applications where the sensors have to be placed inside sealed, optically opaque containers and disposable use is desired. The applications of the sensors described herein are focused on, but not limited to, the quantification of endotoxin concentrations, blood coagulation monitoring, the measurement of gastroesophageal pH, and glucose sensing