Identification of Molecular Fluorophore as a Component of Carbon Dots able to Induce Gelation in a Fluorescent Multivalent-Metal-Ion-Free Alginate Hydrogel.

We introduce a simple approach to fabricate fluorescent multivalent metal ion-free alginate hydrogels, which can be produced using carbon dots accessible from natural sources (citric acid and L-cysteine). Molecular fluorophore 5-oxo-2,3-dihydro-5H-[1,3]-thiazolo[3,2-a] pyridine-3,7-dicarboxylic acid (TPDCA), which is formed during the synthesis of carbon dots, is identified as a key segment for the crosslinking of hydrogels. The crosslinking happens through dynamic complexation of carboxylic acid groups of TPDCA and alginate cages along with sodium ions. The TPDCA derived hydrogels are investigated regarding to their thermal, rheological and optical properties, and found to exhibit characteristic fluorescence of this aggregated molecular fluorophore. Moreover, gradient hydrogels with tunable mechanical and optical properties and controlled release are obtained upon immersion of the hydrogel reactors in solutions of divalent metal ions (Ca, Cu, and Ni) with a higher affinity to alginate.