| A MEMS-based piezoelectric biosensor has been successfully fabricated at the University of Minnesota Microtechnology Laboratory. The biosensor operates at resonance and, in monitoring the resonance frequency, mass loading events can be detected. The biosensor consists of a silicon nitride cantilever beam of dimensions 1000 mum long by 300 mum wide. Atop the silicon nitride supporting structure is deposited a lead zirconium titanate (PZT) capacitor structure. The PZT structure provides a means of piezoelectric actuation and sensing. The thin film PZT for this work was spin-coated onto the devices using a metal-organic deposition method, resulting in a PZT thickness of 0.5 mum. The average remanent polarization (+Pr) of the PZT film was measured at 12.6 muC/cm2 with an average relative dielectric constant (K) of 1927. Upon actuation of the PZT, which in turn oscillates the cantilever beam, resonant modes at 3 kHz, 230 kHz, and 1.4 MHz were observed. Monitoring these frequencies, mass loading phenomena were evaluated. First, salt (NaCl) solutions were loaded onto the sensors with varying concentrations. An increasing percent shift in resonance frequency (0% to 0.12%) was observed as the NaCl concentration was increased from 0 g/L to 4 g/L. Finally, the cantilever beam devices were coated with a biotinylated molecule to form a biosensor. The biotin biomolecule readily binds with the glycoprotein, avidin. The biosensors were then exposed to various concentrations of avidin. Again, an increasing percent shift in the resonance frequency (0% to 3.76%) was observed as the avidin concentration was increased from 0 mg/mL to 0.01 mg/mL. The results from the salt mass loading experiments and the avidin binding experiments show promise for the feasibility of a microfabricated piezoelectric cantilever beam biosensor. |
