NSF STTR grant IIP-07122360 Stefan Cular, Subramanian Sankaranarayanan, and Reetu Singh from SRL

1. Patterned Electrodes For Thickness Shear Mode Quartz Resonators To Achieve Uniform Mass Sensitivity DistributionAnthony Richardson and Venkat R. Bhethanabotla Sensors Research Laboratory, Department of Chemical and Biomedical Engineering Abstract Analytical Model Mass Sensitivity Measurements • Fabricated devices were tested using the following apparatus: Development of an electrode-modified thickness shear mode (TSM) quartz resonator that is responsive to nanogram mass loadings, while exhibiting a mass sensitivity profile that is independent of material placement on the sensor platform, is the aim of this study A ring electrode design predicted by an analytical theory for sensitivity distribution to achieve the desired uniform mass sensitivity distribution is presented in this work. Using a microvalve capable of depositing nanogram droplets of a polymer solution, and a linear stepping stage for radial positioning of these droplets across the sensor platform, measurements of the mass sensitivity distributions were conducted and presented. The measurements agree well with theory. Further improvements are possible to achieve better uniformity and to reduce the instability of resonant frequency of these devices. • The mass sensitivity profile of any TSM device depends on: • Electrode geometry • Electrode mass loading factor, R • For the simple “n-m” solid and single ring electroded cases: • General solution to u1: (a) (b) Experimental Apparatus Microvalve capable of dispensing reproducible nanoliter droplets of polymer/solvent solutions SS Test cell with drilled channels for water circulation to maintain system T and facilitate droplet evaporation Linear and rotary stages for accurate micro-positioning of droplets across device surface particle displacement amplitude (c) (d) mass sensitivity Motivation • A resulting nanogram balance would: • Eliminate issues of non-uniformity in mass sensitivity across sensor platform • Reduce reliance on analytical and mechanical balances for characterizing absolute mass • Microgram mass sensitivity and high vibration instability • Provide an inexpensive gravimetric technique for measuring NVR in HP solvents “n-m” electrode ring electrode Results • The figure to the right shows the the measured mass sensitivity profile for a 4-10 mm ring electrode with R = 0.0025 • Results agree well with theory, however: • Remnant bimodal response • Uncertainty in electrode thickness • Scatter in data • Inherent frequency instability of ring device • The Gaussian profile in the “n-m” case depicts the non-uniformity in mass sensitivity • The bimodal response in the ring case makes it a viable candidate to produce a flat sensitivity distribution “n-m” electrode TSM Resonator ring electrode • Operation: • A constant voltage put across the electrodes prompts oscillation of the quartz crystal near resonance (MHz frequencies) • Piezoelectric effect • Advantages of Application: • Ease of design and implementation • Utilization of shear-horizontal wave • Provides rapid response and high sensitivity to mass loadings • Robust, reusable, and inexpensive • Disadvantages • Non-uniformity in mass sensitivity across surface Analytical Model Results Conclusions R = 0.0033 R = 0.0025 • The figure to the right shows the effect of varying R from 0.0025 to 0.0088 for a 4-10 mm ring electrode • For many ring electrode configurations, there exists an optimal R value that produces a uniform mass sensitivity distribution • A novel technique for measuring mass sensitivity profiles of TSM devices is presented • Ring electrode devices are shown to theoretically yield flat sensitivity distributions • Results from tested devices agree well with theory, however, further design optimizations are necessary to eliminate the bimodal response and increase frequency stability R = 0.0042 R = 0.0088 Acknowledgements • NSF STTR grant IIP-07122360 • Stefan Cular, Subramanian Sankaranarayanan, and Reetu Singh from SRL • Allan Smith and John Furry from Masscal Scientific Instruments, Inc. • Fabien Josse from Marquette University