Refractive Index Change in Nanoscale Thermosensitive Hydrogel for Optoelectronic and Biophotonic Applications
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1060-LL06-08
Refractive Index Change in Nanoscale Thermosensitive Hydrogel for Optoelectronic and Biophotonic Applications Brett W Garner, Zhibing Hu, Floyd D. McDaniel, and Arup Neogi Department of Physics, University of North Texas, 211 Avenue A, Denton, TX, 76203 ABSTRACT The familiar property responses of isopropylacrylamide hydrogel, cause the hydrogel to undergo a discontinuous volumetric phase across a critical level of stimuli. Poly(Nisopropylacrylamide) hydrogel is a recognized response to global temperature stimuli across the low critical solution temperature (LCST). The basic PNIPAM hydrogel undergoes its phase change at a LCST of 34°C, where water is expelled from the interior of the gel microsphere, as the temperature increases past LCST. The responsiveness of PNIPAM hydrogel offers the potential for controlling optical properties of a medium such as refractive index and scattering index. In the present work we present the refractive index of microsphere/microbeaded hydrogel structures necessary for optical application. The particle size of the hydrogels nanostructures at room temperature is observed to be 300-400 nm as estimated by dynamic light scattering and agree well with scanning electron microscopy measurements. The temperature dependent refractive index change of hydrogel microspheres have been measured using variable angle spectroscopic ellipsometry and shows a 10% change as the temperature changes from 33°C to 34°C. INTRODUCTION Self-assembling of colloidal systems has been extensively studied for the past two decades. Smart polymers or polymers, which respond to external physical stimuli such as temperature, pH, salinity and electric and magnetic fields exhibit unique attributes.1 These polymers have created an intense interest in the medical field for applications relating to controlled drug release,2 cell adhesion mediators3, tunable optics and bio-sensors,4 and molecular imaging5. Poly-N-isopropylacrylamide (PNIPA) in particular is a gel system that exhibits strong temperature dependent characteristics. PNIPA exhibits a sharp volume phase transition at a critical temperature.6,7,8,9,10 The PNIPA nanoparticles have new advantages in comparison with previous systems: (i) since the gel nanoparticles contain up to 97 % water, their density and refractive index are nearly matched to their surrounding water. Due to proximity of refractive index matching or index matching liquid is required for bio-imaging. (ii) The size of NIPA nanoparicles is tunable by external stimuli. (iii) The nanoparticles have functionality such as an ability to entrap and deliver drugs. (iv) The phase transition of the NIPA nanoparticle assembly can be probed by varying either temperature or nanoparticle concentration under essentially microgravity (due to density matching. Poly-N-isopropylacrylamide (PNIPA) gel systems exhibit strong temperature dependent characteristics; in particular, they exhibit a sharp volume phase transition at a critical temperature. Below the lower critical solution temperature (LCST), polymer
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