Optical Detection of Polyacrylamide Swelling Behavior in a Porous Silicon Sensor
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Optical Detection of Polyacrylamide Swelling Behavior in a Porous Silicon Sensor Lisa M. Bonanno1 and Lisa A. DeLouise1,2 1
Department of Biomedical Engineering, University of Rochester, 601 Elmwood Ave., Rochester, NY 14642, USA 2 Department of Dermatology, University of Rochester School of Medicine and Dentistry, 601 Elmwood Ave., Rochester, NY 14642, USA ABSTRACT This work proves that a 1-D porous silicon (PSi) sensor is capable of monitoring the optical changes in a polyacrylamide (PAAm) hydrogel that correlate with swelling capacity. The PSi device was impregnated with PAAm hydrogel with varying crosslinking density and total solids. The optical response of the PSi sensor was utilized to distinguish the changes in refractive index of hydrogels with varying cross-linking densities. Refractive index values calculated by the composite hydrogel-PSi sensor response agree well (≤1% difference) with values measured using a bench-top refractometer. This work serves to build a foundation for creating a composite biochemical-responsive hydrogel-PSi sensor in which competitive binding of a target analyte would cause a reduction in hydrogel cross-linking density. Long-term goals of this work are to exploit the volume sensitivity of a PSi sensor and leverage the added optical response of the responsive hydrogel to increase target detection sensitivity in an affinity based biosensor. INTRODUCTION Current sensitivity limitations have restricted commercial development of PSi sensors in laboratory diagnostic screening or in point-of-care biosensing applications. Today’s standard lab bioanalytical techniques Enzyme Linked Immuno-Sorbant Assay and Surface Plasmon Resonance have detection limits on the order of ~0.028 ng ml-1 and ~1 ng ml-1 respectively [1,2]. Recent developments in intricate PSi etching techniques and signal processing have improved the limit of detection in PSi sensors to be ~100 ng ml-1 for immunoassays [3] and ~10,000 ng ml-1 [4] for small molecule detection. However, in order for these devices to be useful in the majority of applications for disease diagnosis and drug screening the limit of detection must be improved further. Our approach for enhancing sensitivity without the need for secondary label amplification will be to incorporate a target analyte-responsive hydrogel (TRAP-gel) into the PSi sensor. Chemical-responsive hydrogels in which the binding of a target analyte breaks cross-links in the hydrogel structure to cause swelling have been successfully used to detect protein, oligonucleotide, or ligand-binding [5,6,7]. Quantification of the responsive swelling due to the resulting increase in osmotic pressure has been monitored by different optical techniques. These include bright field microscopy to monitor changes in the focal length of a hydrogel microlense [7] and a visual color change due to changes in diffraction of light from a crystalline colloidal array of polymer spheres [6]. Here, we
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