Abstract: A novel method is disclosed wherein polymerized crystalline colloidal array (PCCA) chemical sensing materials are used to detect the presence of certain chemical species in high ionic strength solutions, such as bodily fluids. The PCCA sensors consist of a mesoscopically periodic array of colloidal particles polymerized into a hydrogel. The array of colloidal particles diffracts light in the visible spectral region due to the periodic spacing of the particles. The PCCA materials also contain molecular recognition components that bind to the chemical species being detected. The binding or the chelation of the chemical species of interest results in a Donnan potential that swells the hydrogel and in turn red shifts the diffracted light proportionately to the concentration of the chemical species. However, no significant red shift response may occur for PCCA chemical sensors in high ionic strength solutions containing chemical species of interest.