Fig. 1: Preparation of CDs and PVA–CDs–SP hydrogel.

a Schematic diagram of the preparation of CDs. b Transmission electron microscopy (TEM) images of CDs and its particle size distribution (inset). c Raman spectrum of CDs. d X-ray diffraction (XRD) spectrum of CDs. e Schematic diagram of the preparation of PVA–CDs–SP hydrogel. f Scanning electron microscopy (SEM) images of the surfaces (up) and cross-sections (down) of PVA, PVA–CDs, and PVA–CDs–SP hydrogel.