Microcantilevers that are properly functionalized with chemo- or bioselective coatings have been shown to be extremely sensitive to chemical and biological analytes in both vapor and liquid media. Microcantilevers therefore exhibit great promise as molecular and atomic recognition sensors for an extremely diverse set of applications including environmental monitoring, contamination detection, and recognition of explosives and chemical and biological agents. Microcantilever operation is characterized by chemical reaction or adsorption of molecular species at the microcantilever surface which results in a change in the microcantelever's deflection and in properties such as its resonance frequency. While these induced changes can be very small (sub-nanometer cantilever deflection, for example), they are readily measurable with a laser beam reflection technique developed for atomic force microscope (AFM) cantilever measurements. To realize the full promise of microcantilever sensor technology, arrays of individually functionalized microcantilevers are required to enable the simultaneous detection of multiple target molecules while rejecting interference from other chemical species in the environment as well as stochastic noise such as thermal fluctuations. A critical problem is that current cantilever measurement methods do not lend themselves to practical implementation for large numbers of cantilevers that are suitable for high sensitivity operation in both vapor and liquid ambients. Asolution to this problem based on waveguide cantilevers combined with a receiver waveguide splitter structure that enables differential detection of cantilever deflection has been proposed.
Combining these elements with compact waveguide components and with grating couplers to enable fiber coupling to off-chip detectors and an off-chip optical source permits the realization of small microcantilever-based sensors with interchangeable or disposable microcantilever array chips. Compatibility of the waveguide structures with batch microfabrication techniques suggests that such arrays can be produced very inexpensively. Initial analysis indicates that the proposed waveguide cantilever array sensor offers an attractive path for the realization of practical, highly functional sensors.
The proposed work concentrates on an important aspect of the development of a general photonic microcantilever array-based sensor platform that relies on a compact integrated optical readout mechanism to realize scalable, small area microcantilever arrays. A successful research and development effort will lay the foundation for microcantilever arrays to become a common and easy to use high sensitivity sensor for many application areas relevant to MASINT missions. The elements of waveguide microcantilever sensors to be explored in the proposed work build directly on the experience and expertise of the PI and his group.
Microcantilever array sensors have applications in a broad variety of DoD and non-DoD arenas. These include homeland defense, pathology diagnositics, industrial process control, biomedical instrumentation and research, gene assays, proteomic research, and microfluidics.
The purpose of this grant was to (1) develop a method of splitting light into many waveguides on a silicon chip such that very little chip area is required, (2) scale up the number of microcantilever sensor structures on a chip, and (3) demonstrate biochemical sensing with the microcantilevers. The first purpose was accomplished successfully using new nanotrench structures. This solved a problem that has been recognized since the beginning of attempts to integrate waveguide devices on a single substrate in the early 1980’s, namely, splitting light from a single waveguide into many waveguides in a very compact area. We were also successful in accomplishing the second purpose, and demonstrated up to 64 microcantilevers on an individual chip, each with its own photonic deflection readout structure. We demonstrated that these structures enable the deflection state of each microcantilever to be determined with exquisite sensitivity. The third purpose focused on applying the photonic and microcantilever structures to sensing biomolecules. Because of the sensitivity of our system we were able to detect target biomolecules for a biotin/streptavidin model molecular system. However, we found that the effect was unexpectedly small compared to what we were led to believe based on the literature. Future work should focus on how to obtain larger sensor responses.
