SULFUR DOPED GRAPHENE QUANTUM DOTS FOR BIOMEDICAL AND OPTOELECTRONICS APPLICATIONS

Abstract

Since the discovery of graphene by Geim and Novoselov in 2004, its exceptional characteristics such as high electrical conductivity, good thermal stability and high mechanical stiffness have made it an indispensable material for many interesting applications in several areas. However, the zero-bandgap, low aqueous solubility and low absorptivity impede the effective employment of graphene in many applications. To overcome these limitations, the structural regulations of graphene have gained significant attention from the scientists. They have concluded that if the large graphene sheet is censored into small fragments, it exhibits interesting characteristics including (zigzag and armchair) edge sites, quantum confinement effects and oxygen enriched functional groups. In pursuit of this, recently, graphene quantum dots (GQDs) have emerged as a promising quasi zero-dimensional material of the graphene family. Owing to their unique set of characteristics including bright luminescence, ease of synthesis, favorable biocompatibility, quantum confinement effect, excellent aqueous solubility and tunable bandgap, GQDs have been applied in several biomedical and optoelectronics applications. Additionally, since the synthesis methods of GQDs avert the use of toxic heavy metals and sophisticated instruments, they have been considered to be more promising alternate over the traditional semiconductor quantum dots. Further, the ability to tailor the desired physical, chemical and optical properties of GQDs with ease during the synthesis process makes them more attractive and versatile as compared to other allotropes of carbon such as fullerene, graphene, graphene nanoribbons, carbon nanotubes, etc. Among the several attractive properties that GQDs possess, photoluminescence (PL) has fascinated the scientists and researchers worldwide in the last few years. Although GQDs hold various attractive properties, their comparatively low fluorescence and quantum yield limits their commercial application in several fields. The aforementioned facts captured our interest immensely motivating us to focus the present thesis on enhancing the optical, electronic and biological properties of GQDs through the sulfur atom doping for various biomedical and optoelectronics applications. The present thesis starts with the systematic investigation of the effect of sulfur doping on the fluorescence and quantum yield of graphene quantum dots. In this, sulfur doped GQDs (S-GQDs) were synthesized by simply pyrolyzing citric acid (CA) as a source of carbon and 3-Mercaptopropionic acid as a source of sulfur dopant. The quantum yield and fluorescence I behavior of S-GQDs may be affected by various parameters, such as the ratio of carbon to sulfur source, the temperature and reaction time of pyrolysis process used for synthesis. Therefore, as a part of initial investigation, these synthesis reaction parameters were optimized to obtain highly fluorescent S-GQDs with enhanced optical and electronic properties. Next, the differently synthesized S-GQDs were extensively characterized by various analytical techniques to confirm the successful doping of S atom in GQDs framework. Further, for an in-depth analysis of experimental results and the underlying phenomena, the theoretical simulations via density functional theory (DFT) were employed to confirm the doping of S atom through evaluation of chemical structure, sites of doping and change in electron density caused by S-dopant. Conclusively, the as synthesized S-GQDs exhibited excellent solubility in water, strong fluorescence and desirably high quantum yield (57.44%) as compared to undoped GQDs (11.56%) and opened a new window for exploring their potential applications in sensing, therapeutics, bioimaging and optoelectronics. Further, the optical sensing potential of as-prepared S-GQDs was assessed through the detection of a bioflavonoid (quercetin) and highly mutagenic nitroaromatic (picric acid, PA) compound. The fluorescence intensity (FI) of S-GQDs was efficiently quenched in the presence of both analytes due to the existing inner filter effect and the dominant static quenching mechanism. Under the optimal analytical conditions, the developed sensing probe showed a linear relationship between the quenched FI of the S-GQDs and analytes concentration in the range (0.1-100 μΜ for PA and 0-50 μΜ for quercetin) with a lowest detection limit (0.093 μΜ for PA and 0.006 μg/mL for quercetin). To authenticate the real-time application of S-GQDs as a potential fluorescent probe, red wine and rain water samples having different quercetin and PA concentrations, respectively, were used for quantitative analysis, after the optimization of several analytical parameters. As we know, the excessive use of traditional antibiotic and antibacterial agents has globally increased the growth of antibiotic-resistant bacteria that poses serious health risks. Therefore, the development of new generation antibacterial or antimicrobial agents for effective inhibition of bacterial growth is highly desired. In this direction, a one-step synthesis approach for the preparation of a nanocomposite composed of silver nanoparticles (AgNPs) decorated with sulfur doped graphene quantum dots (S-GQDs) was adopted to investigate the synergistic antibacterial activity and cell viability. The characterization results demonstrated that the uniform II surface decoration of AgNPs with S-GQDs ensured the stability and dispersibility of the Ag@S-GQDs nanocomposite. The as-prepared Ag@S-GQDs nanocomposite exhibited enhanced antibacterial activity as compared to AgNPs and S-GQDs with minimum inhibitory concertation (MIC) values as low as 70 and 35 μg mL-1 to inhibit the growth of P. aeruginosa and S. aureus model bacterial strains, respectively. The fractional inhibition concentration (FIC) index result of less than 0.5 demonstrated that the surface passivation of AgNPs with S-GQDs encourages a synergistic effect between AgNPs and S-GQDs in inhibiting the bacterial growth. In addition, the Ag@S-GQDs nanocomposite revealed superior cell viability for HEK 293 cell lines in comparison to bare AgNPs and S-GQDs. Next, bioimaging is a considerably advanced technique in the field of biomedical research, therefore, in order to investigate the potential of S-GQDs for bioimaging applications, a facile in-situ synthesis was used to prepare folic acid-conjugated sulfur-doped graphene quantum dots (FA-SGQDs) and differentiate folate receptor (FR)-positive and FR-negative cancer cells through the targeted bioimaging technique. The as synthesized FA-SGQDs showed excellent antioxidant activity and favorable biocompatibility. The existence of FA residue on their surface facilitated them to recognize and target FR positive cancer cells with ease, which was confirmed by fluorescence microscopy imaging experiments on MCF-7 (FR-positive) and CHO (FR-negative) cells. Overall, these FA-SGQDs have shown great potential to be used as a targeted fluorescent nanoprobe for cancer studies in prognosis, early diagnosis, and subsequently in treatment. Further, the role of as-prepared S-GQDs for the development of next generation optoelectronic devices was also explored by incorporating the freshly prepared S-GQDs into methylammonium lead bromide (MAPbBr3) perovskite precursors solution and systematically investigated the effect of different amounts of S-GQDs on the morphological, optical and electron transfer properties of fabricated thin films. The experimental findings revealed that multiple surface functional groups, quantum confinement and desirable electronic conductivity in S-GQDs help to passivate perovskite surface by reducing the surface and grain boundary traps. Interestingly, the incorporation of S-GQDs increased the light absorption of MAPbBr3 along with faster electron transfer across their interfaces. Hence, this strategy of S-GQDs incorporation presents a versatile and novel way to prepare highly efficient perovskite thin films for developing next generation solar cells, light emitting diodes and other optoelectronic devices.