Integrated dual-grating inteferometric detection with polymer microbeams for bio-chemical sensing in liquid environment

An interferometric displacement detection method of polymer microbeams based on integrated dual-grating was developed for bio-chemical sensing in a liquid environment. Sensing microstructures made of polyimide (PI) with four symmetrically arranged L-shaped beams suspending a central plate were designed and fabricated on a glass substrate using a surface micromachining process. With these structures, deflection changes of the microbeams were transduced to out-of-plane movements of the central plate, which were then measured from the interference of light spots diffracted from the integrated gratings using a charge coupled device image measurement setup. To extend the displacement detection range, two adjacent metal gratings corresponding to a microbeam were formed inside and outside a groove with a depth of 55 nm, respectively, to achieve a phase shift of about π/2 in liquid. The detected noise of aluminium (Al) microcantilevers in ethanol slightly increased compared with that in air, and the noise of PI microbeams in phosphate-buffered saline was about three times larger than that of Al cantilevers in ethanol. The preliminary experimental results in ethanol show that the detection method based on polymer microbeams integrated with dual-gratings is feasible in a liquid environment. Antibody-antigen binding detection was also conducted with a rabbit immunoglobulin G (IgG) immobilised chip in 100 μg/ml goat anti-rabbit IgG solution.