Synthesis of Graphene Quantum Dots (GQDs) and Carbon Quantum Dots (CQDs) for their Sensing Applications

Abstract

The thesis entitled “Synthesis of Graphene Quantum Dots (GQDs) and Carbon Quantum Dots (CQDs) for their Sensing Applications” is divided into five Chapters. Chapter-1. It describes the background of nanomaterials, carbon-based nanomaterials including graphene quantum dots (GQDs) and carbon quantum dots (CQDs) along with literature survey and scope of the work. This Chapter discusses the detailed information about GQDs and CQDs with their optical properties, synthesis methods, fluorescence quenching and restoration mechanisms. Importance of coal-derived GQDs and natural-resource derived CQDs were also described in this chapter. Various sensing applications for the detection of different analytes was also explained. Chapter-2. Coal is an abundant, low-cost, and high carbon content energy source on the planet. Herein, water-soluble graphene quantum dots (GQDs) were prepared from Indian anthracite and bituminous coals labelled as A-GQDs and B-GQDs, respectively by the facile one-step wet chemical route. The size of B-GQDs (7.38 nm ± 0.13) was smaller than A-GQDs (10.25 nm ± 0.19) which was analysed from TEM. The effects of pH, temperature and irradiation time on the fluorescence of the GQDs have been studied which affirmed the stability of GQDs. Moreover, both the synthesized GQDs exhibited tunable photoluminescence (PL) properties and excitation wavelength dependency. The developed GQDs was high selective and sensitivity for Mnn+ (2+, 7+) ions. However, the as-prepared GQDs showed significant change in both the steady state absorption and fluorescence intensity but remain unaltered in the excited state lifetime values in the presence of different Mnn+ quenchers, which validated the static quenching mechanism between GQDs@Mnn+ nanoprobes. The prepared nanosensor showed high sensitivity for GSH sensing with a good detection limit of 27 M for BGQD@Mn7+. Accrediting low-cost productivity and intriguing photoluminescence properties, the current study reveals new potential for sustainable synthesis of GQDs with novel ascribed attributes. Chapter-3. Eco-friendly, water-soluble, and fluorescent carbon quantum dots (CQDs) with an average size of 8.3 nm were synthesized from rice husk (RH) using the hydrothermal method, and the CQDs were labelled as rice-husk carbon quantum dots (RH-CQDs). The composition and surface functionalities were studied using X-ray photoelectron spectroscopy, FT-IR, and Raman spectroscopy. A study on the impact of pH and irradiation time on fluorescence xv affirmed the stability of RH-CQDs. The as-synthesized nanosensor has high selectivity and sensitivity for Fe3+ ions. Several photophysical studies were performed to investigate the interaction between RH-CQDs and Fe3+. From the time-correlated single-photon technique (TCSPC), the average lifetime value of RH-CQDs significantly decreases in the presence of Fe3+ which supports a dynamic quenching mechanism. The developed sensor exhibited excellent sensitivity with a detection limit in the nanomolar range (149 nM) with a wide linear range of 0-1300 nM for Fe3+ ions. The prepared nanosensor was also used to detect Fe3+ in a tablet supplement with high recoveries. Moreover, the RH-CQDs nanoprobe was used to detect other analytes (fluoroquinolones) using the fluorescent enhancement technique. It showed high selectivity and sensitivity toward Ofloxacin (OFX) and Ciprofloxacin (CPX). The detection limit calculated was 150 nM and 127 nM with a linearity range of 50-1150 nM for OFX and CPX, respectively. The enhancement of average lifetime value and quantum yield in the presence of OFX and CPX favors the increased fluorescence property of RH-CQDs through hydrogen bonding and charge-transfer complex. In this work, integration of two different mechanisms (fluorescence quenching and fluorescence enhancement) was followed to construct a single sensing platform for accurate quantification of dual-mode nanosensors for the detection of metal ions and fluoroquinolones by excited state electron transfer and hydrogen bonding mechanism respectively. This strategy also stimulates the detection of more than one analyte. Chapter-4. Green-emissive carbon quantum dots (CQDs) with exclusive chemosensing aspects were synthesized from orange pomace as a biomass-based precursor via a facile microwave method without using any chemicals. The synthesis of highly fluorescent CQDs with inherent Nitrogen was confirmed through X-ray diffraction, X-ray photoelectron, FT-IR, Raman, and Transmission electron microscopic techniques. The average size of synthesized CQDs was found to be 7.5 nm. These fabricated CQDs displayed excellent photostability, water solubility, and outstanding fluorescent quantum yield, i.e., 54.26%. The synthesized CQDs showed promising results for the detection of Cr6+ ions and 4-nitrophenol (4-NP). The sensitivity of CQDs towards Cr6+ and 4-NP was found up to the nanomolar range with the limit of detection (LOD) values 59.6 nM and 14 nM, respectively. Several analytical performances were thoroughly studied for high precision of dual analytes of the proposed nanosensor. Various photophysical parameters of CQDs (quenching efficiency, binding constant, etc.) were analyzed in the presence of dual analytes to gain more insight into the sensing mechanism. The synthesized CQDs exhibited fluorescence quenching towards incrementing the quencher xvi concentration, which was rationalized by the inner filter effect (IFE) through time-correlated single photon counting (TCSPC) measurements. The CQDs fabricated in the current work exhibited a lower detection limit and wide linear range through the simple, eco-friendly, and rapid detection of Cr6+ and 4-NP ions. To evaluate the feasibility of the detection approach, real sample analysis was conducted, demonstrating satisfactory recovery rates and relative standard deviations towards developed probes. This research paves the way for developing CQDs with superior characteristics utilizing orange pomace (biowaste precursor). Chapter-5. A sustainable way was developed for the production of water-soluble carbon quantum dots employing green approach. The synthetic protocol was employed using microwave pyrolysis technique, while lemon peel was served as a carbon precursor. Fabrication of highly fluorescent lemon-peel derived CQDs (LP-CQDs) having inherent nitrogen functionality was validated by X-ray photoelectron spectroscopy, FT-IR, X-ray diffraction, Raman spectroscopic analysis, and TEM techniques. Average particle size of fabricated LP-CQDs was 4.46 nm. LP-CQDs yielded a remarkable quantum yield of 49.5%, which displayed excellent salinity, photostability, storage time, and pH stability. LP-CQDs displayed encouraging results for tetracycline (TC) detection using a PL turn-off approach. The sensitivity of LP-CQDs towards TC was seen in nanomolar range having detection limit of 50.4 nM. Method validation was comprehensively studied to ensure the precision of the nanosensor. A complete analysis of different photophysical parameters of LP-CQDs was performed with TC to gain deeper understanding of the sensing mechanism. Fabricated LPCQDs showed fluorescence quenching towards TC, elucidated by inner filter effect (IFE) mechanism. The synthesized nanoprobe demonstrated a lesser detection limit with broad linear range enabling facile, cheap, environmentally friendly, and fast detection of TC. Practicality of the detection method was assessed through the analysis of real samples, resulting in satisfactory recovery percentage and relative standard deviation with respect to the developed probes. Furthermore, LP-CQDs were used as a fluorescent ink and to fabricate the paper-based fluorescent strips. This study lays the door for the sensing platform of LP-CQDs towards detection of TC, which may impact potential role for environmental sustainability.