Biosensing using Piezoelectric-based Cantilever-like Micro-electromechanical Systems

Field: Engineering (Physical Sciences)

The research objective presented in this dissertation is to develop novel micro-electromechanical systems (MEMS) for sensitive and rapid detection of biomarkers and toxin-producing bacteria via ribonucleic acid (RNA)-based targets in liquid environments with high levels of interfering background biological species. A secondary objective is to improve fundamental understanding of electrical and mechanical dynamical coupling in piezoelectric-based cantilever-like MEMS. Detection of biologics, such as cells, proteins, and nucleic acids, directly in their native environments is important due to need for medical diagnostics, food safety and environmental monitoring. However, standard assays which typically use a combination of molecular-level techniques are especially challenged by interfering species present in the native sample environments which cause challenges of long time-to-results (TTR) and high variance in measurement. Thus, the ability to control the outcome of a biological process through assay-informed corrective measures is comprimised by such lag time which is highly important if the process is one's health as in diagnostics. Alternatively, novel biosensors consisting of a dynamic-mode piezoelectric cantilever (PEMC) design provide detection with competitive sensitivity, but with reduced associated TTR, making the technology attractive for application in point-of-care settings. Selective biosensing occurs by covalently linking a complementary DNA strand for bio-recognition directly to the gold-coated piezoelectric transducer element. Thus, target binding in PEMCs is transduced in terms of cantilever resonant frequency shift caused by associated effective mass increase. A biosensor-based assay for detection of microRNA let-7a, a potential lung cancer biomarker, was developed with limit of detection (LOD) at ~ 5 femtomolar, corresponding to an absolute detection limit of ~ 5 attomoles, directly in human serum environments. Such sensitivity was comparable to the other available assays and the measurement showed improved TTR and ability to detect in complex background. A biosensor-based assay for detection of M. aeruginosa, a toxin-producing cyanobacteria important in source water monitoring, was developed with limit of detection of 5 cells/mL with associated TTR less than one hour. Such studies demonstrate the application of PEMC-based biosensors to two important problems of early disease diagnosis in the healthcare sector and early detection of toxins in clean drinking water in the environmental monitoring sector, respectively. The condition for high coupling between mechanical resonant frequencies used in mass sensing and change in bulk electrical impedance at resonance of the piezoelectric material was found to be directly related to magnitude of net charge accumulation on crystal faces. Net charge accumulation was enhanced by numerous techniques including asymmetry in electrode, anchor, or composite layer geometry, which enabled development of a single-layer, integrated self-sensing and -exciting MEMS platform. In situ characterization of measurement was provided by integration of simultaneous electrochemical sensing capabilities with the already present mass-sensing capabilities. The resulting simultaneous dual-mode biosensing platform not only provided enhanced reliability in biosensing assays, but also revealed the inherent anti-fouling properties of PEMC sensors found to be due to presence of continuous surface vibration. Flow visualization studies also revealed acoustic streaming exists under biosensing conditions in PEMC.