Synthesis and physical characterization of carbon quantum dots from watermelon seed towards a biological application

Abstract

Carbon quantum dots (CQDs) incorporated into hydrogels are promising materials for drug delivery applications, especially wound dressings. Its green synthesis, using biomass such as watermelon seeds, offers important economic, environmental, and technological advantages. This aligns with the principles of the circular economy and supports their potential for biomedical use. In this work, the CQDs were synthesized via the hydrothermal method from watermelon seeds. Their physicochemical properties were thoroughly characterized using multiple techniques, including high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), fluorescence microscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Hydrogels were prepared using polyvinyl alcohol (PVA) and hydroxypropyl methylcellulose (HPMC) to assess CQDs’ antimicrobial activity and cytotoxicity, both in their pure form and integrated into the hydrogels. HRTEM analysis revealed that CQDs exhibited a quasi-spherical morphology with an average diameter of approximately 12–13 nm, as well as, AFM measurements confirmed a similar size of diameter distribution with an average height of 0.385 nm. Raman spectroscopy identified two dominant peaks at $1340{\mathrm{cm}}^{-1}$ and $1590{\mathrm{cm}}^{-1}$, corresponding to the disordered D-band and the crystalline G-band, respectively. FTIR spectroscopy indicated the presence of functional groups, including hydroxyl, amine, $s{p}^2$ and $s{p}^3$ hybridized CH bonds, carbonyl, alkene/alkyne, amide, ether, and CO bonds. XPS analysis confirmed the presence of carbon, nitrogen, and oxygen elements, while fluorescence microscopy revealed strong, sustained photoluminescence in the blue range. Lastly, biological tests showed that CQDs, in isolation, did not exhibit significant antimicrobial activity. However, cytotoxicity assessments demonstrated that CQDs in pure form were non-toxic at a concentration of 0.03 mg/ml. Conversely, when integrated into PVA and HPMC hydrogels, a toxic effect was observed at the same concentration. When combined with HPMC alone, a slight toxicity was observed. These findings suggest that CQDs’ high photoluminescence and minimal cytotoxicity make them excellent candidates for hydrogel-based drug delivery systems in wound care applications.