The objective of this research is to develop efficient and highly sensitive resonant microcantilever sensors for liquid-phase (bio)chemical sensing applications by reducing fluid damping and effective fluid mass through proper design and, thus, improving the sensitivity and resolution of the sensor. The approach is to utilize optimal cantilever geometries and more general excitation characteristics. Specifically, the lateral bending mode will be investigated. The use of in-plane rather than out-of-plane forces demonstrates how the intelligent control of electromechanical load characteristics may be used to drastically enhance performance, even in liquids.
The intellectual merit lies in a more fundamental understanding of the coupling between a vibrating microstructure (cantilever beam) and the surrounding fluid. The developed analytical models will ultimately guide the design of more efficient microsensors, not only for the bio-chemical sensing applications targeted here, but for resonant sensors operating in a non-vacuum environment in general.
The broader impact stems from the availability of the improved cantilever-based sensing platforms as a sensing platform for liquid-phase (bio)chemical sensing and as an educational tool coupling device physics, state-of-the-art microfabrication, sensor technology and systems engineering. On the sensing system side, a performance improvement of up to two orders of magnitude in the liquid phase could be truly transformative, enabling the development of novel handheld sensing systems for applications spanning environmental monitoring, detection of hazardous compounds, and medical diagnosis. On the educational side, cantilevers represent one of the simplest microstructures and are thus ideally suited for graduate and undergraduate student training and K-12 outreach programs.
The overall objective of this project is to develop efficient and highly sensitive resonant microcantilever (microbeams fabricated from silicon) sensors for liquid-phase (bio)chemical sensing applications by reducing fluid damping and effective fluid mass through proper design and, thus, improving the sensitivity and resolution of the sensor. To this end, in particular the micromachined cantilever beams vibrating in their own plane rather than the more conventional out-of-plane (like a diving board) vibration are investigated. The project combines experimental and theoretical efforts to (1) design and fabricate the cantilever-based resonant sensors, (2) develop adequate analytical and numerical models to understand fluid mass loading and damping effects in the resonant microbeams and ultimately optimize the ability of these devices to detect compounds in liquids, and (3) apply the gained knowledge to demonstrate the implementation of improved gas- and liquid-phase biochemical sensors. A key demand in environmental monitoring is creating analytical instruments that are portable or hand-held and allow for on-site measurements. State-of-the-art laboratory techniques are of limited applicability for in-field deployment or use by first responders. Thus, for targeted analyte detection, chemically sensitized microsensors are needed. Recently there has been substantial interest in cantilever-based devices, which involve small structural beams being fabricated as part of a silicon microchip. These devices operate as chemical sensors by virtue of a sensitive coating that is tuned to absorb the target substance, if present in the environment. Several studies have been dedicated to cantilever-based chemical sensors in gas or air, while fewer attempts have been made to utilize cantilevers in liquids. This is because, when a cantilever vibrates in a viscous fluid, the fluid offers a considerable resistance to beam motion, especially for conventional cantilevers, which vibrate out-of-plane like a diving board. The use of other vibration modes could be promising for reducing the fluid resistance and thereby significantly improving the sensor’s limit of detection in liquid environments. In this project, we have successfully investigated and developed efficient and highly sensitive microsensors for the detection of targeted chemicals or biological substances in liquids. The proposed device is based on exciting resonant vibrations of thin microcantilevers (microbeams fabricated from silicon) and recording the change of a particular resonance frequency upon sorption of analytes into a chemically sensitive coating, thus weighing the sorbed analyte molecules. The microcantilevers vibrate in their own plane, resulting in a more efficient motion in the surrounding liquid because of the more "streamlined" orientation of the microbeams. Utilizing the cantilever’s in-plane flexural mode limits the fluid resistance primarily to its frictional resistance along the two larger cantilever surfaces. In particular, the device minimizes the energy losses in the viscous fluid and the liquid mass that is disturbed during the vibration. To explore the feasibility of chemical sensing with the resonant microsensors, the microbeams were coated with chemically sensitive polymer films and exposed to different chemical compounds both in air and in liquid. When the chemical substance is absorbed into the film, it causes a corresponding decrease in the beam’s resonant frequency, which may be correlated to its concentration in the surrounding medium. Actual traces of organic compounds in water, that are known carcinogens, have been detected in laboratory prepared solutions. Preliminary biosensing experiments were also performed by chemically modifying the sensor surfaces to detect biomolecules in solutions. Antibodies were detected with estimated detection limits close to those needed for detecting biomarkers in bodily fluids for diagnostics. The new design is shown to significantly improve the chemical/biochemical sensitivity and resolution of the sensor. As a sensing platform for liquid-phase (bio)chemical sensing, a performance improvement of up to one order of magnitude in liquid is seen to be truly transformative, enabling the development of novel handheld sensing systems for applications spanning environmental monitoring, detection of hazardous compounds, and medical diagnosis. The gained knowledge will be applied to initiate the implementation of improved gas- and liquid-phase biochemical sensors. Applications are numerous, including inexpensive, portable, and reliable devices to detect pollutants in air or water, chemical warfare agents in military scenarios, and particular substances in blood.
