Photonic crystal hydrogel material for the sensing of toxic mercury ions (Hg2+) in water

Abstract

We report the development of a new photonic crystal hydrogel for the sensing of highly toxic mercury ion (Hg²⁺) in water. This new sensing material optically reports the Hg²⁺ concentration in watervia diffraction of visible light from polymerized crystalline colloidal array (PCCA). The PCCA consists of light diffracting crystalline colloidal array (CCA) of monodisperse, highly charged polystyrene particles, which are polymerized within the polyacrylamide hydrogel. The changes in hydrogel surroundings trigger the volume change of hydrogel, which alters the lattice spacing of CCA and hence shifts the diffraction wavelength of light. The ions responsive urease coupled PCCA (UPCCA) hydrolyzes the urea and produces the HCO₃⁻ and NH₄⁺ ions inside the hydrogel. These ions induce the charge-screening of the polyacrylamide carboxylates by decreasing the electrostatic repulsion between carboxylates and the polyacrylamide backbone relaxes, causing the shrinkage of hydrogel. Hence the UPCCA exhibits the blue shift of the diffracted wavelength. Hg²⁺ being the principal inhibitor of the urease-urea hydrolysis perturbs urea hydrolysis by the UPCCA when UPCCA is exposed to Hg²⁺ along with urea and hence suppresses the production of ions. This intervenes the shrinkage of hydrogel and does not allow the hydrogel to shrink as it shrinks in absence of Hg²⁺. Therefore the PCCA net blue shift decreases in the presence of Hg²⁺ along with urea compared to only urea. The extent of this hydrogel volume change is a function of Hg²⁺ concentrations. This UPCCA photonic crystal sensor detects ultra low (1 ppb) concentration of Hg²⁺ in water, exhibits reversibility and displays very high selectivity towards Hg²⁺. The uncompetitive inhibition nature of the urease enzyme, when it is covalently attached in the polymer hydrogel backbone, is the driving force for the very high selectivity and reversibility of the UPCCA sensor.