Detection Limits of Captured Protein on the BioCD
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Detection Limits of Captured Protein on the BioCD Chad Barden1, Ming Zhao2, Xuefeng Wang2 and David D. Nolte2 1 Quadraspec, Inc., West Lafayette, IN 47906 2 Dept. of Physics, Purdue University, West Lafayette, IN 47907, U.S.A. ABSTRACT The BioCD is a spinning biochip that uses quadrature laser interferometry to detect captured protein on the disc surface. We describe the detection limits of protein binding on this optical biosensor. The fundamental metrology limit is 1 picometer for a single 100-micron diameter spot. Under assay conditions for prostate specific antigen, we can detect 25 pg/ml at 10 assays per disc. INTRODUCTION The BioCD is an interferometric biosensor that detects protein captured by antibody arrays. It has two key attributes that separate this technology from other optical or interferometric biosensor technologies. The first is the intrinsic scalability of surface-normal interferometric detection with capacity for hundreds or thousands of assays per disc (Fig. 1) because the footprint per measurement can be as small as a square micron. The second is the high-speed laser scanning that moves the detection frequency far off 1/f noise, allowing repeatable surface height sensitivity to below 10 picometers. These two simple attributes provide the potential for high-speed label-free multi-analyte assays with future applications in diagnostics, prognostics and drug discovery. The sensor readout is performed on a spinning disc using a common-path interferometric configuration that is stable and sensitive to sub-monolayer coverage of captured protein [1, 2]. Protein is detected using phase quadrature that converts phase to intensity modulation using local generation of signal and reference to lock the relative phase of the waves [3, 4]. Several different classes of the BioCD have been developed, differentiated by the means of generating the phase-locked reference. These include the microdiffraction (MD) class [5], the phase contrast (PC) class [3], the adaptive optical (AO) class [6], and the in-line (IL) class [7]. Of these different quadrature classes, the in-line BioCD has the highest sensitivity with a scaling detection sensitivity of 0.25 pg/mm [8]. The minimum detectable mass is set by simple scaling relations. The metrology limit is set by surface roughness combined with repositioning offset between pre- and post-incubation scans. Optimal sensitivity is achieved by critical sampling of protein spots in arrays. We imaged a single 100 micron wide protein spot with focal spot sizes of 1, 5 and 10 microns and observe a square-root scaling as a function of the number of pixels per protein spot. The current generation of the BioCD is based on silicon. We discuss recent progress in the silicon BioCD that consists of patterned protein on thermal oxide on silicon. The thermal oxide provides the condition for in-line interferometric quadrature for stable common-path interferometry of bound protein on the disc surface.
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