Speaker: Gaurav Kathel (EE12S029)
Current bio-sensing applications require sensors which are compact and capable of label free detection rapidly, with high accuracy and precision. Micro-fabricated mechanical structures like cantilevers have these characteristics. When operated at its resonance frequency in vacuum microcantilever can be used as an extremely sensitive mass sensor detecting upto femtogram or lower. However detection of bio molecules often has to be done in liquid where its performance is severely affected by hydrodynamic loading and damping, resulting in low quality factors and large error. Therefore, there is a need for better measurement and analysis procedure to improve the lower limit of detection (mmin) and mass estimation.
Improvement in the detection limit was obtained by operating microcantilever at higher modes. Two measurement techniques were employed (i) frequency sweeping (FS) and (ii) continuous tracking using Phase Locked Loop (PLL). In FS, mass calculation using multiple resonant peaks improved detection accuracy. However, spurious modes also occurred in frequency spectra and regular modes were then identified by plotting fn-fn-1 vs. the mode number (n). Standard deviations of all resonant peak values determined the minimum frequency shift achievable and hence mmin found to be >1 ng for all modes. Alternate to FS, PLL tracking can provide much more accurate result, if the baseline is stable. This was achieved by varying Proportional (P) & Integral (I) gain values between 20 and 10000. Optimized values of P=350 and I=800 gave the least error of 5 Hz in DI water at 14th mode which led to a mmin of 14 pg.
To improve the estimation accuracy of the attached mass, two analysis methods were introduced. In first method, parameters in the expression for the resonance frequency in fluids were fixed using Monte-Carlo optimization. In second method, cantilever dimensions were measured and other parameters were extracted from spectra in air and liquid. The attached mass was calculated by taking the weighted average of frequency shifts from both these methods. These analysis methods along with PLL were then used to study the Antibody-Antigen interactions on Gold coated cantilevers.
To further study the effect of fluid medium and different boundary conditions on cantilever resonance, simulations were done using COMSOL Multiphysics. Dimension optimization was also performed to achieve mmin of 10 femtogram in air and fabrication process flow was proposed for the same.