Sensors for High-Throughput Materials Characterization: 24-channel Array of Quartz Crystal Microbalances
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Sensors for High-Throughput Materials Characterization: 24-channel Array of Quartz Crystal Microbalances William G. Morris and Radislav A. Potyrailo General Electric Global Research, Niskayuna, NY 12309 ABSTRACT We have developed a multichannel materials characterization system based on thickness-shear mode (TSM) sensors, also known as quartz crystal microbalances (QCMs). The sensors are arranged as a 6 x 4 array that is compatible with available 24well plates that can be manipulated with standard robotic equipment. Our sensor system can measure frequencies from 5 MHz to 20 MHz with a noise level of less than 0.1 Hz for a 1 sec acquisition interval. The sensors can be placed in a gas-flow-though cell for studies of vapor-sorption properties, or they can be immersed into a 24-well plate array for studies of materials solubility. The sensor array is connected to the measurement electronics through a multi-conductor cable, enabling the sensors to be operated in a temperature, pressure, or chemical environment, which would otherwise adversely affect the electronic stability. The 24-channel array has been applied to the screening of sensor materials for determination of chlorinated organic solvents at part per billion levels in groundwater wells. The primary screen is the discovery screen where materials are exposed to a single analyte concentration. The secondary screen is the focused evaluation where the best subset of these materials is exposed to analytes and interferences. The tertiary screen involves evaluation of material performance under conditions mimicking the long-term application. Six families of potential sensor materials were examined with rapid downselection by using this approach. INTRODUCTION Several different types of sensors have been used for the measurement of chemical and physical parameters [1-6]. Among these are thickness-shear mode (TSM) devices, which rely on the piezoelectric properties of a material. Typically in the form of a thin disk of single-crystal quartz resonator with electrodes applied to opposite surfaces, the device is connected to an electronic circuit which causes it to oscillate at a frequency related to the transit time of a transverse shear wave through the thickness of the crystal. The oscillation properties of the device are analogous to a mechanical spring with a mass attached to one end. If material is added to the electrode regions, the effect is equivalent to adding mass to an oscillating spring, i.e., the frequency will decrease. Quartz crystal microbalances (QCM) use this principle to measure very small mass increments. Chemical sensors based on the QCM have a material applied to the crystal surfaces that will adsorb and desorb vapor species. As the concentration of a vapor species increases, the mass on the surface of the crystal increases, and the frequency of oscillation decreases. A complete sensor system consists of the quartz crystal resonator, the speciesselective layer, an oscillator circuit, a frequency-measuring instrument, and a calibration curve of
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