Synthesis and Characterization of Graphene Quantum Dots and Nanocomposites for Biomedical Applications
Morell, Gerardo (Consejero)
Graphene quantum dots (GQDs) are nanoparticles derived from graphene; their size is in the 2-20 nm range, and they have shown considerable potential in biological applications due to their unique optical properties, high biocompatibility, and high surface to volume ratio. Furthermore, the synthesis of nanocomposites of GQDs may enhance their intrinsic properties and make them promising for a wide range of applications. Currently, hydrothermal, electrochemical, and reduction methods are commonly used to synthesize GQDs and their nanocomposites via multiple steps; such methods require large amounts of reagents to produce low yields of nanomaterials. This thesis focuses on an alternative novel method developed to synthesize GQDs and their nanocomposites using a single step pulsed laser synthesis (PLS) and producing them at high yield. The synthesized nanoparticles were fully characterized and analyzed using spectroscopic techniques and microscopies to explore their potential in biomedical applications. They have shown considerable potential for various biomedical applications such as their use as fluorescent nano-probes in confocal microscopy, as antibacterial agents against different types of bacteria, and as a component of cancer therapy. The first chapter of the thesis briefly gives a comprehensive introduction on previous studies’ advances and explains the motivation for the work. The introduction explains the development and the synthesis of carbon-based nanomaterials in general, as well as the recent advances in using nanomaterials in the biomedical field. Chapter II describes the fabrication of different carbon-based nanostructures using pulsed laser synthesis (PLS) and provides plausible mechanisms for their formation. We explain our novel approach to synthesize GQDs using PLS and we propose a mechanism of synthesis for that approach. The proposed mechanism is consistent with the mechanisms of other reported approaches to the synthesis of carbon nanomaterials by PLS, and the data obtained by electron microscopy and Raman spectroscopy support our proposed mechanism. In chapter III, the GQDs are fully characterized by microscopic and spectroscopic techniques such as: transmission electron microscopy, atomic force microscopy, Electron Energy Loss Spectroscopy (EELS), Raman spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV-Visible spectroscopy and PL spectroscopy. The microscopic data obtained show that they measure 2-6 nm across and are about 1–3 layers thick. They are soluble in water and exhibit strong intrinsic fluorescence in the visible region. Moreover, we used them to label bacterial cells for confocal microscopy imaging. Confocal microscopy images of bacteria exposed to GQDs show their suitability as nano-probes in high-contrast bioimaging. Chapter IV discusses our development of a PEGylated nanocomposites of silver nanoparticles decorated with graphene quantum dots (Ag-GQDs) using PLS. The PEGylation of the nanocomposites increases their cell uptake and solubility in aqueous solutions. The antibacterial activity of Ag-GQDs was evaluated and compared to that of bare GQDs and commercial silver nanoparticles (Ag-NPs) against both Gram-negative and Gram-positive bacteria, using Pseudomonas aeruginosa and Staphylococcus aureus as model bacteria, respectively. Concentration values of 25 and 50 μg/mL are required for Ag-GQDs to inhibit (MIC) the growth of S. aureus and P. aeruginosa bacteria, respectively. The Fractional Inhibitory Concentration (FIC) index is below 0.5 indicating that there is a synergistic effect between Ag-NPs and GQDs. Kirby-Bauer tests showed that Ag-GQDs inhibit P. aeruginosa and S. aureus, in contrast to bare GQDs and Ag-NPs alone. Cell viability of normal mammalian cells treated with Ag-GQDs show that cell viability is maintained at 100 % of cells treated with Ag-GQDs. The decoration of Ag-NPs with GQDs minimizes their cytotoxicity in mammalian cells and increases their biocompatibility. Ag-GQDs have potential applications in the fabrication of antibacterial coatings, self-sterile textiles, and personal care products. Chapter V presents the testing of the potential application of PEGylated Ag-GQDs in two different cancer therapeutic modalities (i.e., chemotherapy and photodynamic therapy). The PEGylated Ag-GQDs demonstrated great ability to deliver the chemotherapy drug Doxorubicin (DOX) to HeLa and DU-145 cancer cells. They have also demonstrated an anticancer activity by inducing apoptosis in cancer cells, which enhance the treatment efficacy in chemotherapeutic modality. Moreover, we tested their applicability in photodynamic therapy (PDT) and our results indicate that there is an increase in their cytotoxicity under irradiation with visible light. Employing Ag-GQDs in the combination of PDT and chemotherapy enhances treatment efficiency against both cancer types. Finally, the research findings are summarized in chapter VI. The chapter also includes an overview of further future work based on the outcomes discussed in the thesis.