| dc.description | 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. | |
