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Synthesis, advanced characterization, and optoelectronic/biological applications of novel graphene quantum dots
Hasan, Md Tanvir
Hasan, Md Tanvir
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2020
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In this work, we focus on developing novel graphene quantum dots (GQDs) utilizing two separate synthetic approaches (bottom-up/top-down) and exploring their optoelectronic and bioimaging applications. Nitrogen-doped (N-GQDs) and nitrogen/sulfur co-doped (NS-GQDs) GQDs are synthesized with glucosamine/thiourea precursor via a simple/cost-effective microwave-facilitated hydrothermal method yielding water-soluble GQDs showing excitation-dependent fluorescence in the visible and near-infrared with high quantum yield. These GQDs are applied further to fabricate electroluminescence (EL) devices, solar cells, and UV photodetectors. The fabricated EL devices with N-GQDs as an emissive layer show bright EL emission with moderate turn-on voltage (~7V) indicating that the synthesized quantum dots are a promising material for low cost eco-friendly organic LEDs. The optical properties of as-prepared N-GQDs are optimized further via the controlled ozone treatment providing significant enhancement of photovoltaic performances of GQDs-based solar cells such as fill factor (28 to 86.40%), PCE (0.41 to 2.64%), Isc (1.13 to 4.8mAcm -2 ), and Voc (0.57 to 0.83V) over those with untreated N-GQDs. Further significant modification of optical/structural properties of N-GQDs takes place under short (254 nm)/mid (302 nm)/long-wave (365 nm) UV irradiation leading us to fabricate UV photodetectors using untreated/UV-treated NGQDs as a sensitizer. These devices provide highly reproducible and fast photo-switching characteristics with high photosensitivity up to 0.59 A/W and excellent photodetectivity up to 1.03X10 11 Jones. In the separate work on biological applications, these GQDs are utilized for multi-color in-vitro imaging and pH-dependent fluorescence-based ratiometric cancer detection. On the other hand, a separate UV-driven aqueous top-down approach was developed to synthesize novel fluorescing GQDs (RGQDs) from optically inert reduced graphene oxide. Those exhibit partially excitation-independent emission in the visible and NIR-I (~950 nm) with quantum yields from ~1.4 to ~8%. As-synthesized RGQDs with an average size of 3.54¿0.05 nm are biocompatible with ~80% cell viability up to a high 1mg/mL concentrations as verified via MTT and Luminescence cytotoxicity assays. Due to advantageous high tissue penetration depth NIR emission, small size, high water solubility, and biocompatibility, RGQDs are utilized as both in-vitro/in-vivo bioimaging agents. In vitro study indicates effective cellular internalization with observable intracellular NIR/VIS fluorescence from RGQDs compared to the control, whereas in-vivo/ex-vivo mice imaging shows RGQD accumulation mainly in the spleen, kidney, liver, and intestine suggesting a promising material for image-guided drug delivery.
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Physics and Astronomy