Carbon Dot/Poly(methylacrylic acid) Nanocomposite Hydrogels with High Toughness and Strong Fluorescence

Fluorescent hydrogels have promising applications in biomedical and engineering fields such as information display and flexible electronics, which usually suffer from poor mechanical properties. Herein, we report a facile strategy to fabricate nanocomposite hydrogels with high toughness and strong fluorescence by soaking poly(methylacrylic acid) (PMAAc) hydrogels in a carbon dot (CD) suspension and then re-equilibrating in water. The equilibrated hydrogels exhibited strong fluorescence because of CDs trapped inside the gel matrix due to the formation of hydrogen bonds between the amino group of CDs and carboxylic group of PMAAc. When the CD suspensions had an appropriate concentration, the mechanical properties of hydrogels were greatly enhanced; the tensile breaking stress and Young's modulus can reach high values up to 5.8 and 158 MPa, respectively. We revealed that the mechanical enhancement of hydrogels mainly originated from the residual ethylenediamine in CD suspension, which formed strong hydrogen bonds with the gel matrix. Since the fluorescence of CD can be quenched by ferric ions, patterning the fluorescent hydrogel was realized by site-specific diffusion of ferric ions through a silhouetted mask to the underneath CD-composited hydrogel sheet. This kind of tough and fluorescent hydrogel should find applications in diverse fields for chemical sensing, information display, etc.