Portable biosensor with coincident imaging and diffractometry
Abstract
Portable and point-of-care biosensors are highly desirable in multiple fields including medical diagnosis, environmental monitoring and food quality control. Such systems need to be sensitive, robust, compact and user-friendly. Diffractometry has been demonstrated to possess these characteristics and has been used extensively in many different forms/versions that range from waveguides to particle-based self-assembled diffraction gratings. Diffraction measurements are usually performed in specifically-built optical setups that are relatively large, intricate and therefore immobile due to the requirement of multiple pieces of equipment (optical components, microscopes and computers). This not only introduces difficulties due to multiple alignment steps but also makes the system virtually impossible to use by an unskilled person. This is one reason why most biosensing schemes developed in academia do not leave the developers' laboratory. A portable package that enables multiple modalities of optical analysis (including diffraction measurements and regular brightfield microscopy) with "all" of the computational and electronic peripherals built-in, would be extremely useful. This dissertation reports on the development of an all-in-one portable system that serves as an advanced multi-modal imaging and analysis tool. This system performs imaging and diffraction measurements coincidently (on the same interrogation spot) by two illumination sources - white light and coherent light (diode laser) - arranged in an in-line format. The package accommodates all of the necessary optical components, such as lenses and beam-splitters, printed circuit boards and electronic components, a touch-screen display and an on-board computer that can be programmed to execute specific imaging tasks. The package is highly portable, versatile and measures only 21.5 x 13.5 x 14 cm 3. We demonstrate the design and application of the portable system in the context of bead-based diffraction gratings where the grating is formed by self-assembly of microspheres or "beads" mediated by biomolecular recognition. In this scheme, molecule-coupled beads selectively bind to a microcontact "printed" template pattern that is composed of alternating lines. The binding of the beads forms a diffraction grating which produces a specific diffraction pattern when illuminated with a coherent light source. At the same time the chip surface needs to be interrogated using brightfield microscopy. For this purpose bead counting and diffraction measurement programs were developed and used with the images acquired by the system. Numerical simulations of the bead-based gratings are also performed to understand the underlying mechanism of diffraction. The measurements agree with the simulation predictions as well as demonstrate the unique capability of the device to perform imaging and diffractometry at the same spot on the chip. The unique design of the portable package and the analysis software not only significantly reduce the entire process of optical chip analysis to a few simple steps but also completely isolate the user from the underlying complexity of an engineering system.
Degree
Ph.D.
Advisors
Savran, Purdue University.
Subject Area
Mechanical engineering
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