Synthesis, characterization and metal ions sensing applications of meta-benziporphodimethene-embedded polyacrylamide/carboxymethyl guargum polymeric hydrogels in water

We have developed a robust procedure for the visual on-site detection of zinc, cadmium and mercury metal ions in an aqueous medium using a polymeric hydrogel matrix-based colorimetric sensor. Zn2+, Cd2+and Hg2+, owing to their biological significant value and environmental harm, have attracted more attention. The selective detection of Zn2+, Cd2+and Hg2+ metal ions has always been challenging due to their closed-shell d10 electronic configuration which makes them spectroscopically silent. A polyacrylamide/carboxymethyl guargum polymeric hydrogel-based metal ion sensor was synthesized by in situ embedding 11, 16-bis (phenyl)-6, 6, 21, 21-tetramethyl-m-benzi-6,21-porphodimethene (meta-BPDM) in a host hydrogel. The meta-BPDM-embedded hydrogel shows high stability, sensitivity and selectivity when it is dipped into the aqueous solutions of Zn2+, Cd2+and Hg2+ metal ions. During detection, the binding of these metal ions in hydrogel causes hydrogel to change from red to bluish-green which was visually detected and confirmed by UV–visible spectroscopy. The meta-BPDM-embedded polymeric hydrogel was characterized by using solid-state UV–visible spectroscopy, scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, Fourier-transform infrared spectroscopy, energy-dispersive X-ray analysis and its sensing properties were studied. The meta-BPDM-embedded polymeric hydrogel was found to be an excellent colorimetric sensor for Zn2+, Cd2+ and Hg2+ in aqueous medium without leaching of meta-BPDM from the hydrogel. The selectivity to sense these ions is mainly dependent on the binding constant of these metal ions with the meta-BPDM embedded in the hydrogel. The sensitivity of the hydrogel was 0.5, 1, and 2 mg/L with Hg2+, Zn2+ and Cd2+, respectively.