Silicon Piezoresistive Microcantilever For Potentiometric Sensing

There has been extensive research focus on miniaturization of chemical sensors due to their applications in various fields, e.g., environmental monitoring, medical diagnosis, and national security. In general, a miniaturized chemical sensor inevitably depends on functionalization layers, which alter their electrical properties and change electrical conductance of the functionalization layers due to adsorption/desorption of target molecules. The amperometric chemical sensing devices have been shrunk down from micro to nano scale during last decades. In addition to conductance, it is very attractive to apply surface potential as a signature for chemical detection. In this work, miniaturization of both amperometric and potentiometric chemical sensors is investigated to achieve merits such as high sensitivity, low power consumption, and fast response. In the first part of this work - amperometric sensors, InN nanowires with thick In2O3 shell layer are demonstrated for NO2 detection down to 45 ppb in a field effect transistor (FET) configuration. The application of polydimethylsiloxane (PDMS) thin film in highly sensitive oxygen sensor is demonstrated, which is designed for oxygen content measurements within heart and blood vessels. For the oxygen sensor, the best sensitivity was~2.75 μA for 1% change in oxygen content of the surrounding media, with a noise limited resolution of ~6.18 ppm oxygen. In addition, the electric properties of bio-nanofiber based on tobacco mosaic virus are characterized using conductive Atomic Force Microscopy and its electrical conductivity (~3×10-5 Ωcm-1) is reported firstly, offering possibility in application in volatile organic compounds detection. In the second part of my work - potentiometric sensors, a chemical sensor based on potentiometric measurement is demonstrated using Silicon microcantilever and sensing substrate. This technique depends on monitoring surface work function (SWF) change due to molecular adsorption/desorption. Compared to conventional microcantilever sensors, based on tracking static deflection or resonant frequency, the potentiometric sensor does not require a functional layer on the microcantilever but on the grounding substrate. Implementing this technique with an Atomic Force Microscope, NO2 detection is demonstrated using SiC as active sensing substrate. The potentiometric microcantilever sensor requires a new paradigm for signal transduction and detection beyond the optical approach that can support simple and accurate signal readout. Thus, an electrical readout system comprising Silicon microcantilevers and piezoresistors embedded at the high stress region in cantilever is developed, which facilitates miniaturization of the sensor. The dimension-related quality factor and spring constant of microcantilevers are investigated to improve their sensitivity. Using fabricated Si piezoresistive microcantilever and Pt coated substrate, an miniaturized potentiometric sensor is demonstrated with detection capability of 200 ppm H2