Multi-functional rhodamine-based chitosan hydrogels as colorimetric Hg²⁺ adsorbents and pH-triggered biosensors

Water contamination from heavy metal ions is a major global environmental concern. Adsorbents based on biomaterials have been demonstrated to possess remarkable removal efficiency for metal ions, but the adsorption model of biosorbents is not clear and much efforts should be devoted to study the adsorption behaviors and understand the adsorption mechanism.

The multifunctional rhodamine-modified chitosan (RMC) hydrogel for Hg²⁺ adsorption with fluorescent turn-ON properties was fabricated through grafting the rhodamine-modified poly (ethylene glycol) benzaldehyde (RM-PEG) onto the hydrogel network serving as the fluorescence/colorimetric sensing receptor. The adsorption behaviors and colorimetric sensing mechanism of RMC hydrogel towards Hg²⁺ were investigated in detail.

RMC hydrogel can remove more than 96.5% of Hg²⁺ from aqueous solution with significant fluorescence response and colorimetric change. The high adsorption selectivity and colorimetric sensing mechanism of RMC hydrogel towards Hg²⁺ can be explained by the hard and soft acid/base (HSAB) theory. The O atom in hydroxyl and carbonyl groups together with the N atom in amine/imine groups of RMC hydrogel play a vital role in the adsorption of Hg²⁺, while the colorimetric response and fluorescence enhancement of the hydrogel after adsorption are attributed to the specific spiro-lactam structure of rhodamine moieties. The adsorption isotherms and kinetics were investigated and well described by Freundlich isotherm and pseudo-second-order kinetic model. Furthermore, RM-PEG showed low cytotoxicity towards mouse embryonic fibroblast cells and RMC hydrogel can be used as a fluorescent pH indicator from 4.2 to 7.4, demonstrating the potential applications of RMC hydrogel in biological diagnosis.