Optofluidic Waveguiding for Biomedical Sensing
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Optofluidic Waveguiding for Biomedical Sensing Thomas A. Wall1 and Joshua Parks2 and Kaelyn D. Leake2 and Holger Schmidt2 and Aaron R. Hawkins1 1 Brigham Young University, Provo, Utah 84602, U.S.A. 2 University of California, Santa Cruz, 1156 High St, Santa Cruz, CA 95064, U.S.A. ABSTRACT We review an optofluidic waveguiding lab-on-a-chip used to sense bioparticles. The sensor uses a liquid filled Anti-Resonant Reflecting Optical Waveguide (ARROW) that is interfaced with standard ridge waveguides. The ridge waveguides are coupled to off-chip lasers and detectors via optical fiber. A perpendicular intersection between the ARROW and a ridge waveguide is especially useful for detecting fluorescently tagged particles. Light coupled into the ridge waveguide can fluorescently excite these particles within a very small volume. Fluorescent signal can then be guided within the ARROW and subsequently off chip to a detector. We also discuss how such devices are fabricated. Both the ARROW and ridge waveguides are made using alternating thin films of tantalum oxide and silicon dioxide on silicon substrates. These thin films are deposited by either sputtering or plasma enhanced chemical vapor deposition (PECVD). The waveguides are patterned using a combination of standard photolithographic processes, reactive ion etching, and sacrificial etching. Low-loss optical guiding is very dependent on both the waveguide structure and the materials used. The latest processes for maximizing detection sensitivity are reviewed. We also present results using the optofluidic waveguiding sensor for detecting a variety of different types of particles such as fluorescently labeled nanobeads, viruses, ribosomes, and RNA. INTRODUCTION Optofluidics is the combination of optics and fluidics on the microscale. Recently optofluidic platforms have become popular for use as biosensors [1, 2]. Biosensing devices have been fabricated using several different techniques, which include interferometry, evanescent fields, fluorescence, and spectral absorption [1, 3-5]. We present an optofluidic waveguiding lab-on-a-chip used to sense bioparticles called the Liquid-Core ARROW Biosensor [6]. The Liquid-Core ARROW Biosensor is an optofluidic device that uses intersecting planar waveguides to force light to interact with a small volume of a liquid sample. Our biosensor makes use of the fluorescence of particles flowing past a detection point within the device in order to perform direct detection of single bioparticles [6]. Single nanoparticle detection requires the biosensor to have a high sensitivity. The materials and techniques used to fabricate the device greatly affect its sensitivity. This paper will outline some of the crucial decisions involving optimal materials, how to deposit the material onto a silicon substrate, the thickness of the materials, and device geometries. THEORY
We have developed an optofluidic ARROW biosensor that is capable of single bioparticle detection. Figure 1 depicts the general setup for our biosensor. The biosensor is fabricated on
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