Electronic Readout of Microchannel Resonators for Precision Mass Sensing in Solution

Microfabricated transducers have enabled new approaches for detection of biomolecules and cells. Integration of electronics with these tools simplify systems and provide platforms for robust use outside of the laboratory setting. Suspended microchannel resonators (SMRs) are sensitive microfluidic platforms used to precisely measure the buoyant mass of single cells and monolayers of protein in fluid environments. Conventionally, microcantilever deflection is measured by the optical-lever technique, wherein a laser beam is reflected off the cantilever onto a position sensitive photodiode. This thesis introduces microchannel resonators with electronic readout, eliminating the use of external optical components for resolving the sensor's resonant frequency. Piezoresistors have been fabricated on SMRs through ion implantation integrated with the existing SMR fabrication process. We fabricated two designs: one with a cantilever length of 210 pm and resonant frequency of -347 kHz, and the other with a cantilever length of 406 pm and resonant frequency of ~92 kHz. The work here builds upon knowledge of signal transduction from static and dynamic cantileverbased sensors because the piezoresistors are implemented on vacuum encapsulated devices containing fluid. Electronic readout is shown to resolve the microchannel resonance frequency with an Allan variance of 5 x 10-18 (210 pm) and 2 x 1017 (406 pm) using a 100ms gate time, corresponding to a mass resolution of 0.1 and 0.4 fg respectively. This mass resolution calculated from piezoresistive readout frequency stability, is approximately 3X better than optical readout for the 210 pm device and 1.3X for the 406 pm device using the same gate time. Resolution is expected to improve with further optimization of the system. To demonstrate the readout, histograms of the buoyant masses of a mixture of size standard polystyrene beads (with nominal diameters 1.6, 1.8, and 2.0 pm) and budding yeast cells were made. Thesis Supervisor: Scott R. Manalis Title: Associate Professor of Biological and Mechanical Engineering