Carbon Dots Rooted Agarose Hydrogel Hybrid Platform for Optical Detection and Separation of Heavy Metal Ions

A robust solid sensing platform for an on-site operational and accurate detection of heavy metal is still a challenge. We introduce chitosan based carbon dots rooted agarose hydrogel film as a hybrid solid sensing platform for detection of heavy metal ions. The fabrication of the solid sensing platform is centered on simple electrostatic interaction between the NH₃⁺ group present in the carbon dots and the OH^– groups present in agarose. Simply on dipping the hydrogel film strip into the heavy metal ion solution, in particular Cr⁶⁺, Cu²⁺, Fe³⁺, Pb²⁺, Mn²⁺, the strip displays a color change, viz., Cr⁶⁺ → yellow, Cu²⁺ → blue, Fe³⁺ → brown, Pb²⁺ → white, Mn²⁺ → tan brown. The optical detection limit of the respective metal ion is found to be 1 pM for Cr⁶⁺, 0.5 μM for Cu²⁺, and 0.5 nM for Fe³⁺, Pb²⁺, and Mn²⁺ by studying the changes in UV–visible reflectance spectrum of the hydrogel film. Moreover, the hydrogel film finds applicability as an efficient filtration membrane for separation of these quintet heavy metal ions. The strategic fundamental feature of this sensing platform is the successful capability of chitosan to form colored chelates with transition metals. This proficient hybrid hydrogel solid sensing platform is thus the most suitable to employ as an on-site operational, portable, cheap colorimetric-optical detector of heavy metal ion with potential skill in their separation. Details of the possible mechanistic insight into the colorimetric detection and ion separation are also discussed