Fabrication of intelligent photonic crystal hydrogel sensors for selective detection of trace mercury ions in seawater

A hydrogel sensing material has been fabricated using three dimensional (3-D), polymerized colloidal photonic crystals (PCPCs) for highly sensitive and selective detection of mercury ions (Hg2+) in seawater. Due a to periodically ordered lattice of the embedded photonic crystals, the hydrogel diffracts light in visible spectral range according to the Bragg's law. The hydrogel has been functionalized by spatially distributed -SH groups through cleaving -S-S-bonds in grafted N, N'-cystaminebisacrylamide molecules and can selectively bind with Hg2+ ions to form -S-Hg-S-bridge bonds in seawater. The Hg2+ binding causes the volume of the hydrogel to shrink, corresponding to a wavelength shift of the light diffracted by the hydrogel. The shifted wavelength is proportional to the amount of bound Hg2+ ions, which enables the quantitative evaluation of Hg2+ ions with a limit of detection at 10 +/- 9 M level in seawater, as measured by a portable spectrometer. With this intelligent hydrogel sensors, we demonstrate a simple method for the rapid in situ monitoring and detection of Hg2+ ions at low concentrations in seawater.