Porous structural colored hydrogel for visual detection and removal of toxic ionic species

Abstract

Smart polymeric hydrogel composed of a three-dimensionally cross-linked polymer network with a large amount of water has great potential for the development of sensor because it undergoes volume changes in response to the changes in a wide variety of stimuli including temperature, pH, ionic strength, pressure, and electric or magnetic fields. Although hydrogels have high potential for sensors, optical devices, and displays but the diffusion limited volume change of the bulk hydrogel restricts their widespread use. On the other hand, porous hydrogel exhibits a very fast response kinetic attributable to the interconnecting porous structure which makes solvent discharge easier. The periodically ordered porous hydrogels can be fabricated by a templating method using a colloidal crystal with a face-centered cubic (fcc) lattice composed of closest-packing monodispersed amorphous SiO2 particles as a template. If the porous hydrogel retains the fine structure of the precursor colloidal crystal and the volume change is isotropic, the value of wavelength maximum,λmax diffracted from the (111) planes in the porous gel can be expressed by Bragg’s law combined with Snell’s law. When the lattice spacing of the resulting porous hydrogel is smaller than the wavelength of incident light, the reflections can be observed visually as structural color. Moreover, the structural color of the porous hydrogel is changed rapidly with the swelling ratio of the hydrogels because of the interconnecting porous structure allows the solvent to diffuse more rapidly into/out of the networks. These structural colored porous hydrogels reach to swelling equilibrium very fast and upsurge their use for the development of visible color sensor. Some ionic species, such as arsenic, lead, mercury, cadmium, and chromium come into the living body from food circle owing to their ubiquitous and recalcitrant which poses an imminent health risk. A number of methods have been developed for early detection and removal of toxic ionic species but due to their less efficacy limited success has been achieved to date. Here, a structural color porous hydrogel will be prepared using templating techniques by free radical co-polymerization of thermosensitive N-isopropylacrylamide (NIPAm) as major monomer with ionic co-monomer/s in presence of a suitable crosslinker and initiator. The synthesized porous hydrogel will exhibit different colors via up taking varying amount of toxic ionic species from the food samples for their prior detection and removal.