Modeling of Stimulated Hydrogel Volume Changes in\nPhotonic Crystal Pb²⁺ Sensing Materials

Abstract

We modeled the stimulated hydrogel volume transitions of a material which binds Pb²⁺ and is\nused as a photonic crystal chemical sensing material. This material consists of a polymerized crystalline\ncolloidal array (PCCA) hydrogel which contains a crown ether molecular recognition group. The PCCA is\na polyacrylamide hydrogel which embeds a crystalline colloidal array (CCA) of monodisperse polystyrene\nspheres of ∼100 nm. The array spacing is set to diffract light in the visible spectral region. Changes in the\nhydrogel volume induced by Pb²⁺ binding alter the array spacing and shift the diffracted wavelength. This\nsystem allows us to sensitively follow the hydrogel swelling behavior which results from the immobilization\nof the Pb²⁺ by the crown ether chelating groups. Binding of the Pb²⁺ immobilizes its counterions. This\nresults in a Donnan potential, which results in an osmotic pressure which swells the hydrogel. We continue\nhere our development of a predictive model for hydrogel swelling based on Flory's theory of gel swelling.\nWe are qualitatively able to model the PCCA swelling but cannot correctly model the large responsivity\nobserved at the lowest Pb²⁺ concentrations which give rise to the experimentally observed low detection\nlimits for Pb²⁺. These PCCA materials enable stimulated hydrogel volume transitions to be studied.