Sunlight-Activated Phase Transformation in Carbon Dot-Hydrogel Facilitates Water Purification and Optical Switching

A multifunctional hydrogel comprising carbon dots (C-dots), carboxymethylcellulose (CMC), and poly-(N-isopropylacrylamide) (PNIPAM) has been synthesized. The microporous C-dot–CMC–PNIPAM hydrogel constitutes an effective sunlight-mediated water purification platform and an optical switch, facilitated through the hydrophilic-to-hydrophobic transformation of PNIPAM. The C-dots, in particular, play crucial roles in the system—functioning as photothermal agents for inducing the phase transition of the hydrogel, enhancing its mechanical stability, enabling water purification through effective adsorption of salts and pollutants, and furnishing the optical switching functionality through their colorimetric properties. The composite hydrogel, prepared from recyclable, green, and inexpensive reagents, may serve as a practical solar-mediated vehicle for water desalination and purification, an anticounterfeit smart ink, and an "on/off" optical switch.