Sulfur quantum dots as sustainable materials for biomedical applications: Current trends and future perspectives

Discovered over a decade ago, sulfur quantum dots (SQDs) have rapidly emerged as a sustainable, safe, and inexpensive quantum material. Sustainably synthesizing SQDs using sublimed sulfur powders, typically produced as waste in industrial petrochemical refining processes, has attracted researchers to use these functional quantum materials in various research fields. SQDs quickly found applications in various research fields, such as electronics, environmental sensing, food packaging, and biomedical engineering. Although low production yields, time-consuming and energy-intensive synthetic methods, and low photoluminescence quantum yield (PLQY) have been some problems, researchers have found ways to improve synthetic methods, develop passivating agents, and systematically modify reaction schemes and energy sources to achieve large-scale synthesis of stable SQDs with high PLQY. Nonetheless, SQDs have succeeded tremendously in biomedical and related applications due to their low toxicity, antibacterial and antioxidant properties, biocompatibility, appropriate cellular uptake, and photoluminescent properties. Although the bioimaging applications of SQDs have been extensively studied, their other reported properties indicate their suitability for use as antimicrobial agents, free radical scavengers, and drug carriers in other biomedical applications, such as tissue regeneration, wound healing, and targeted drug delivery.

Coal waste-derived synthesis of yellow oxidized graphene quantum dots with highly specific superoxide dismutase activity: characterization, kinetics, and biological studies

The disintegration of coal-based precursors for the scalable production of nanozymes relies on the fate of solvothermal pyrolysis. Herein, we report a novel economic and scalable strategy to fabricate yellow luminescent graphene quantum dots (YGQDs) by remediating unburnt coal waste (CW). The YGQDs (size: 7-8 nm; M.W: 3157.9 Da) were produced using in situ "anion-radical" assisted bond cleavage in water (within 8 h; at 121 °C) with yields of ∼87%. The presence of exposed surface and edge groups, such as COOH, C-O-C, and O-H, as structural defects accounted for its high fluorescence with εmax ∼530 nm at pH 7. Besides, these defects also acted as radical stabilizers, demonstrating prominent anti-oxidative activity of ∼4.5-fold higher than standard ascorbic acid (AA). In addition, the YGQDs showed high biocompatibility towards mammalian cells, with 500 μM of treatment dose showing <15% cell death. The YGQDs demonstrated specific superoxide dismutase (SOD) activity wherein 15 μM YGQDs equalled the activity of 1-unit biological SOD (bSOD), measured using the pyrogallol assay. The Km for YGQDs was ∼10-fold higher than that for bSOD. However, the YGQDs retained their SOD activity in harsh conditions like high temperatures or denaturing reactions, where the activity of bSOD is completely lost. The binding affinity of YGQDs for superoxide ions, measured from isothermal calorimetry (ITC) studies, was only 10-fold lower than that of bSOD (Kd of 586 nM vs. 57.3 nM). Further, the pre-treatment of YGQDs (∼10-25 μM) increased the cell survivability to >75-90% in three cell lines during ROS-mediated cell death, with the highest survivability being shown for C6-cells. Next, the ROS-induced apoptosis in C6-cells (model for neurodegenerative diseases study), wherein YGQDs uptake was confirmed by confocal microscopy, showed ∼5-fold apoptosis alleviation with only 5 μM pretreatment. The YGQDs also restored the expression of pro-inflammatory Th1 cytokines (TNF-α, IFN-γ, IL-6) and anti-inflammatory Th2 cytokines (IL-10) to their basal levels, with a net >3-fold change observed. This further explains the molecular mechanism for the antioxidant property of YGQDs. The high specific SOD activity associated with YGQDs may provide the cheapest alternative source for producing large-scale SOD-based nanozymes that can treat various oxidative stress-linked disorders/diseases.

Upcycling Waste: Citrus limon Peel-Derived Carbon Quantum Dots for Sensitive Detection of Tetracycline in the Nanomolar Range

In this work, a sustainable method was developed for the production of water-soluble carbon quantum dots employing a green approach. The synthetic protocol was employed using the microwave pyrolysis technique, while lemon peel 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, FTIR, X-ray diffraction, Raman spectroscopic analysis, and TEM techniques. The 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, conditions, and pH stability. LP-CQDs displayed encouraging results for tetracycline (TC) detection using a PL turn-off approach. The sensitivity of LP-CQDs toward TC was seen in a nanomolar range having a 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 a deeper understanding of the sensing mechanism. Fabricated LP-CQDs showed fluorescence quenching toward TC, elucidated by the inner filter effect (IFE) mechanism. The synthesized nanoprobe demonstrated a lesser detection limit with a broad linear range, enabling facile, cheap, environmentally friendly, and fast detection of TC. Practicality of the detection method was assessed through 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 fluorescent inks and to fabricate paper-based fluorescent strips. This study lays the door for the sensing platform of LP-CQDs toward detection of TC, which may impact the potential role of environmental sustainability.

Sustainable Preparation of Graphene Quantum Dots from Leaves of Date Palm Tree

The date palm (Phoenix dactylifera), a subtropical and tropical tree, included in the family Palmae (Arecaceae) is one of the oldest cultivated plants of mankind. Date palm is a major agricultural product in the semi-arid and arid areas of the world, particularly in Arab countries. These trees generate high quantities of agricultural waste in the form of dry leaves, seeds, etc. In this study, dried date palm leaves were used as green precursors for synthesizing graphene quantum dots (GQDs). This work reported the preparation of GQDs using two different sustainable methods. GQD-1 was developed using a simple, hydrothermal technique at 200 °C for 12 h in water, with no requirement of reducing or passivizing agents or organic solvents. GQD-2 was prepared using a hydrothermal technique at 200 °C for 12 h in water, with the usage of just distilled water and absolute ethanol. The compositional analysis of the leaf extract was performed, along with the morphological, compositional, and optical examination of the sustainably developed GQDs. The characterization results confirmed the successful formation of GQDs, with average sizes ranging from 3.5 to 8 nm. This study helps to obtain GQDs in an economical, eco-friendly, and biocompatible manner and can assist in large-scale production and in recycling date palm tree waste products from Middle East countries into value-added products.

Orange Pomace-Derived Fluorescent Carbon Quantum Dots: Detection of Dual Analytes in the Nanomolar Range

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, Fourier transform infrared, Raman, and transmission electron microscopic techniques. The average size of the 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 toward Cr⁶⁺ and 4-NP was found up to the nanomolar range with the limit of detection values of 59.6 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 insights into the sensing mechanism. The synthesized CQDs exhibited fluorescence quenching toward incrementing the quencher concentration, which was rationalized by the inner filter effect through time-correlated single-photon counting measurements. The CQDs fabricated in the current work exhibited a lower detection limit and a 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 toward the developed probes. This research paves the way for developing CQDs with superior characteristics utilizing orange pomace (biowaste precursor).

Glutathione-modified graphene quantum dots as fluorescent probes for detecting organophosphorus pesticide residues in Radix Angelica Sinensis

A novel fluorescent sensor was developed in this study based on glutathione-functionalized graphene quantum dots (GQDs@GSH) to detect organophosphorus pesticide residues in Radix Angelica Sinensis. GQDs@GSH was synthesized by a one-step pyrolysis method with a fluorescence quantum yield as high as 33.9% and its structure was characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. GQDs@GSH exhibited excellent fluorescence property showing strong blue fluorescence under UV irradiation. The fluorescence of GQDs@GSH could be quenched by Fe³⁺ by electron transfer and the quenched fluorescence could be recovered due to the strong chelating and reducing ability of phytic acid (PA). Under the catalyzation of acetylcholinesterase (AChE) and choline oxidase (ChOx), acetylcholine (ACh) could be decomposed to H₂O₂, which could further oxidize Fe²⁺ to Fe³⁺ thus quenching the fluorescence of GQDs@GSH once again. Coumaphos, a kind of organophosphorus pesticide, could inhibit AChE activity, thus making the quenched fluorescence turn on again. Several parameters influencing the fluorescence response such as Fe³⁺, PA, ACh and coumaphos concentration, pH value and reaction time were optimized. Based on such a fluorescence "off-on-off-on" ngkmechanism, GQDs@GSH was successfully applied to the detection of coumaphos in Radix Angelica Sinensis. A good linear relationship between the fluorescence intensity and coumaphos concentration was obtained in the range of 0.1-10.0 μmol·L^-1. By a standard addition method, the recoveries were measured to be 101.44-117.90% with RSDs lower than 1.98%. The biosensor system is simple, sensitive and accurate. It has a good application prospect in the detection of organophosphorus pesticide residues in traditional Chinese medicine and agricultural products, and also expanded the application scope for glutathione as a highly selective biological molecule.

Versatile Carbon Nanofiber-Based Sensors

Carbon nanofibers (CNFs) display colossal potential in different fields like energy, catalysis, biomedicine, sensing, and environmental science. CNFs have revealed extensive uses in various sensing platforms due to their distinctive structure, properties, function, and accessible surface functionalization capabilities. This review presents insight into various fabrication methods for CNFs like electrospinning, chemical vapor deposition, and template methods with merits and demerits of each technique. Also, we give a brief overview of CNF functionalization. Their unique physical and chemical properties make them promising candidates for the sensor applications. This review offers detailed discussion of sensing applications (strain sensor, biosensor, small molecule detection, food preservative detection, toxicity biomarker detection, and gas sensor). Various sensing applications of CNF like human motion monitoring and energy storage and conversion are discussed in brief. The challenges and obstacles associated with CNFs for futuristic applications are discussed. This review will be helpful for readers to understand the different fabrication methods and explore various applications of the versatile CNFs.

Study on the microstructure of the symbiosis of coal-based graphene and coal-based graphene quantum dots: preparation and characterization

Coal-based graphene sheets (GS) and coal-based graphene quantum dots (GQDs) are usually prepared separately. In this paper, symbiosis of coal-based GS and coal-based GQDs was successfully prepared with our proposed preparation method by using three raw coals with different reflectance (collected from Qinshui coalfield, Shanxi Province) as carbon sources. The results showed that coal-based GS and coal-based GQDs can exist stably in the symbiosis and are distributed in different layers, and the GQDs are freely distributed between layers of GS. The average number of GS (N_ave) in the three symbiosis is about 7 and the average interlayer spacing (d₀₀₂) is about 0.3887 nm. The average diameter of GQDs in the three symbiosis is about 4.255 nm and the averaged₀₀₂is about 0.230 nm. The averageN_aveof the three symbiosis was about 3 and the averaged₀₀₂is about 0.361 nm. The morphology and crystal parameters of symbiosis is more similar to that of graphene, the elements are only carbon and oxygen. In the prepared symbiosis, the higher the reflectance of raw coal, the smoother the lattice skeleton and the less vortex-layer structure of GS, and the larger the diameter and the denser the six membered ring of GQDs. The C and O functional groups of the prepared symbionts are similar. The higher the reflectance of coal, the higher the content of C-C/C=C. Under ultraviolet light, the prepared products all emit blue, and the higher the reflectance of coal, the higher the ultraviolet absorption, and the stronger the fluorescence intensity.