Coal as an abundant source of graphene quantum dots

Photophysical Dynamics in Semiconducting Graphene Quantum Dots Integrated with 2D MoS2 for Optical Enhancement in the Near UV

The hybrid structure of zero-dimensional (0D) graphene quantum dots (GQDs) and semiconducting two-dimensional (2D) MoS₂ has been investigated, which exhibit outstanding properties for optoelectronic devices surpassing the limitations of MoS₂ photodetectors where the GQDs extend the optical absorption into the near-UV regime. The GQDs and MoS₂ films are characterized by Raman and photoluminescence (PL) spectroscopies, along with atomic force microscopy. After outlining the fabrication of our 0D-2D heterostructure photodetectors comprising GQDs with bulk MoS₂ sheets, their photoresponse to the incoming radiation was measured. The hybrid GQD/MoS₂ heterostructure photodetector exhibits a high photoresponsivity R of more than 1200 A W^-1 at 0.64 mW/cm² at room temperature T. The T-dependent optoelectronic measurements revealed a peak R of ∼544 A W^-1 at 245 K, examined from 5.4 K up to 305 K with an incoming white light power density of 3.2 mW/cm². A tunable laser revealed the photocurrent to be maximal at lower wavelengths in the near ultraviolet (UV) over the 400-1100 nm spectral range, where the R of the hybrid GQDs/MoS₂ was ∼775 A W^-1, while a value of 2.33 × 10¹² Jones was computed for the detectivity D* at 400 nm. The external quantum efficiency was measured to be ∼99.8% at 650 nm, which increased to 241% when the wavelength of the incoming laser was reduced to 400 nm. Time-resolved measurements of the photocurrent for the hybrid devices resulted in a rise time τ_rise and a fall time τ_fall of ∼7 and ∼25 ms, respectively, at room T, which are 10× lower compared to previous reports. From our promising results, we conclude that the GQDs exhibit a sizable band gap upon optical excitation, where photocarriers are injected into the MoS₂ films, endowing the hybrids with long carrier lifetimes to enable efficient light absorption beyond the visible and into the near-UV regime. The GQD-MoS₂ structure is thus an enabling platform for high-performance photodetectors, optoelectronic circuits, and quantum devices.

Carbon Dots Fluorescence-Based Colorimetric Sensor for Sensitive Detection of Aluminum Ions with a Smartphone

In this work, blue emission carbon dots (CDs) are synthesized in the one-pot solvothermal method using naringin as precursor. The CDs are used to develop a ratiometric fluorescence sensor for the sensitive analysis of Al3+ with a detection limit of 113.8 nM. A fluorescence emission peak at 500 nm gradually appears, whereas the original fluorescence peak at 420 nm gradually decreases upon the increase in the Al3+ concentration. More importantly, the obvious color change of the CDs probe from blue to green under a 360 nm UV lamp can be identified by a smartphone and combined with the RGB (red/green/blue) analysis. This results in a visual and sensitive analysis of Al3+ with a detection limit of 5.55 μM. Moreover, the high recovery is in the 92.46–104.10% range, which demonstrates the high accuracy of this method for actual samples’ analysis. The use of a smartphone and the RGB analysis greatly simplifies the operation process, saves equipment cost, shortens the detection time, and provides a novel method for the instant, on-site, visual detection of Al3+ in actual samples.

Stimuli responsive multicolour fluorescence emission in carbon nanodots and application in metal free hydrogen evolution from water

The present study convincingly demonstrates visible light induced continuous tuning of the fluorescence of carbon nanodots from yellow to cyan within two hours. These carbon dots are well characterized using field emission gun transmission electron microscopy (FEG-TEM), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), fluorescence spectroscopy and UV-visible spectroscopy. Moreover, solid state fluorescence of different wavelengths has also been obtained from these colour tunable carbon dots by circumventing the common problem of aggregation induced quenching of fluorescence in the solid state. These carbon dots have also shown a very low band gap and the yellow emissive C-dots have been successfully utilized as a photocathode in metal free photo-electrochemical hydrogen evolution from water.

Multicolor Fluorescent Graphene Oxide Quantum Dots for Sensing Cancer Cell Biomarkers

Onion-like carbon nanoparticles were synthesized from diamond nanoparticles to be used as the precursor for graphene oxide quantum dots. Onion-like carbon nanoparticles were exfoliated to produce two types of nanoparticles, graphene oxide quantum dots that showed size-dependent fluorescence and highly stable inner cores. Multicolor fluorescent quantum dots were obtained and characterized using different techniques. Polyacrylamide gel electrophoresis showed a range of emission wavelengths spanning from red to blue with the highest intensity shown by green fluorescence. Using high-resolution transmission electron microscopy, we calculated a unit cell size of 2.47 Å in a highly oxidized and defected structure of graphene oxide. A diameter of ca. 4 nm and radius of gyration of ca. 11 Å were calculated using small-angle X-ray scattering. Finally, the change in fluorescence of the quantum dots was studied when single-stranded DNA that is recognized by telomerase was attached to the quantum dots. Their interaction with the telomerase present in cancer cells was observed and a change was seen after six days, providing an important application of these modified graphene oxide quantum dots for cancer sensing.

A Review on Recent Advancements of Graphene and Graphene-Related Materials in Biological Applications

Graphene is the most outstanding material among the new nanostructured carbonaceous species discovered and produced. Graphene’s astonishing properties (i.e., electronic conductivity, mechanical robustness, large surface area) have led to a deep change in the material science field. In this review, after a brief overview of the main characteristics of graphene and related materials, we present an extensive overview of the most recent achievements in biological uses of graphene and related materials.

Red-fluorescent graphene quantum dots from guava leaf as a turn-off probe for sensing aqueous Hg(ii)

In this work, we have prepared red-fluorescent graphene quantum dots and utilized as a highly selective and sensitive fluorescence turn-off probe for detection of the toxic metal ion Hg²⁺ from guava leaf extract.

Bifunctional Carbon Dots Derived From an Anaerobic Bacterium of Porphyromonas gingivalis for Selective Detection of Fe3+ and Bioimaging

In this study, for the first time, Porphyromonas gingivalis, an anaerobic bacterium, was selected to synthesize carbon dots. The achieved P. gingivalis-carbon dots (Pg-CDs) exhibited strong fluorescence and high stability with capability for dual function as Fe³⁺ sensor and intracellular imaging agent. The detection limit for Fe³⁺ was as low as 1.85 µm. On the other hand, the prepared Pg-CDs were an excellent candidate for biosensor with high biocompatibility.

Graphene Quantum Dot-TiO2 Photonic Crystal Films for Photocatalytic Applications

Photonic crystal structuring has emerged as an advanced method to enhance solar light harvesting by metal oxide photocatalysts along with rational compositional modifications of the materials’ properties. In this work, surface functionalization of TiO₂ photonic crystals by blue luminescent graphene quantum dots (GQDs), n–π* band at ca. 350 nm, is demonstrated as a facile, environmental benign method to promote photocatalytic activity by the combination of slow photon-assisted light trapping with GQD-TiO₂ interfacial electron transfer. TiO₂ inverse opal films fabricated by the co-assembly of polymer colloidal spheres with a hydrolyzed titania precursor were post-modified by impregnation in aqueous GQDs suspension without any structural distortion. Photonic band gap engineering by varying the inverse opal macropore size resulted in selective performance enhancement for both salicylic acid photocatalytic degradation and photocurrent generation under UV–VIS and visible light, when red-edge slow photons overlapped with the composite’s absorption edge, whereas stop band reflection was attenuated by the strong UVA absorbance of the GQD-TiO₂ photonic films. Photoelectrochemical and photoluminescence measurements indicated that the observed improvement, which surpassed similarly modified benchmark mesoporous P25 TiO₂ films, was further assisted by GQDs electron acceptor action and visible light activation to a lesser extent, leading to highly efficient photocatalytic films.

High yield synthesis of graphene quantum dots from biomass waste as a highly selective probe for Fe3+ sensing

Graphene quantum dots (GQDs), a novel type of zero-dimensional fluorescent materials, have gained considerable attention owing to their unique optical properties, size and quantum confinement. However, their high cost and low yield remain open challenges for practical applications. In this work, a low cost, green and renewable biomass resource is utilised for the high yield synthesis of GQDs via microwave treatment. The synthesis approach involves oxidative cutting of short range ordered carbon derived from pyrolysis of biomass waste. The GQDs are successfully synthesised with a high yield of over 84%, the highest value reported to date for biomass derived GQDs. As prepared GQDs are highly hydrophilic and exhibit unique excitation independent photoluminescence emission, attributed to their single-emission fluorescence centre. As prepared GQDs are further modified by simple hydrothermal treatment and exhibit pronounced optical properties with a high quantum yield of 0.23. These modified GQDs are used for the highly selective and sensitive sensing of ferric ions (Fe3+). A sensitive sensor is prepared for the selective detection of Fe3+ ions with a detection limit of as low as 2.5 × 10-6 M. The utilisation of renewable resource along with facile microwave treatment paves the way to sustainable, high yield and cost-effective synthesis of GQDs for practical applications.

Carbon-Based Quantum Dots with Solid-State Photoluminescent: Mechanism, Implementation, and Application

Carbon-based quantum dots (CQDs), including spherical carbon dots and graphene quantum dots, are an emerging class of photoluminescent (PL) materials with unique properties. Great progress has been made in the design and fabrication of high-performance CQDs, however, the challenge of developing solid-state PL CQDs have aroused great interest among researchers. A clear PL mechanism is the basis for the development of high-performance solid-state CQDs for light emission and is also a prerequisite for the realization of multiple practical applications. However, the extremely complex structure of a CQD greatly limits the understanding of the solid-state PL mechanism of CQDs. So far, a variety of models have been proposed to explain the PL of solid-state CQDs, but they have not been unified. This review summarizes the current understanding of the solid-state PL of solid-state CQDs from the perspective of energy band theory and electronic transitions. In addition, the common strategies for realizing solid-state PL in CQDs are also summarized. Furthermore, the applications of CQDs in the fields of light-emitting devices, anti-counterfeiting, fingerprint detection, etc., are proposed. Finally, a brief outlook is given, highlighting current problems, and directions for development of solid-state PL of CQDs.

Application of micro/nanomaterials in adsorption and sensing of active ingredients in traditional Chinese medicine

Traditional Chinese medicine (TCM) has been widely applied for the prevention and cure of various diseases for centuries. Ingredient with pharmacological activity is the key to the application of TCM. Hence, it is of significance to separate and detect active ingredients in TCM effectively. Micro/nanomaterial is the promising candidate for adsorption and sensing due to its unique physical and chemical properties. For years, many efforts have been made to develop functional micro/nanomaterials to realize the effective adsorption or sensing of bioactive compounds in TCM. In this review, we discussed recent progresses in the application of various functional micro/nanomaterials for adsorption or detection (electrochemical detection, fluorescent detection, and colorimetric detection) of active ingredients. Based on the kind of matrix materials, micro/nano-adsorbents or sensors can be classified into following categories: metal-based micro/nanomaterials, porous materials, carbon-based materials, graphene/graphite-liked micro/nanomaterials and hybrid micro/nanomaterials.

Tailoring Multi-Walled Carbon Nanotubes into Graphene Quantum Sheets

Transformation of carbon nanotubes (CNTs) into sub-10 nm pieces is highly required but remains a great challenge. Herein, we report a robust strategy capable of mechanically tailoring pristine multi-walled carbon nanotubes (MWCNTs) into graphene quantum sheets (M-GQSs) with an extremely high yield of up to 44.6 wt %. The method combines silica-assisted ball-milling and sonication-assisted solvent exfoliation and therefore enables reproducible high-yield production of M-GQSs directly from MWCNTs. Remarkable solvent diversity and extraordinary solvability (up to 7 mg/mL) are demonstrated facilitating the solution processing of the M-GQSs. The M-GQSs are essentially monolayers with intrinsic curvature, which could be determinative to their outstanding performances in both dispersions and thin films. Besides the excitation wavelength-, concentration-, and solvent-dependent photoluminescence in dispersions, the solid-state fluorescence and exceptional nonlinear saturation absorption (NSA) in thin films are demonstrated. Particularly, NSA with relative modulation depth up to 46% and saturation intensity down to 1.53 MW/cm² are achieved in M-GQS/poly(methyl methacrylate) hybrid thin films with a loading content of merely 0.2 wt %. Our method opens up a new avenue toward conversion and utilization of CNTs.

Photoluminescence Response in Carbon Nanomaterials to Enzymatic Degradation

Myeloperoxidase (MPO), a key enzyme released by neutrophils during inflammation, has been shown to catalyze the biodegradation of carbon nanomaterials. In this work, we perform photoluminescence studies on the MPO-catalyzed oxidation of graphene oxide (GO) and surfactant-coated pristine (6,5) single-walled carbon nanotubes (SWCNTs). The enzymatic degradation mechanism involves the introduction of defects, which promotes further degradation. Interestingly, the photoluminescence responses of GO and SWCNTs to enzymatic degradation are counterposed. Although the near-infrared (NIR) fluorescence intensity of SWCNTs at 998 nm is either unchanged or decreases depending on the surfactant identity, the blue fluorescence intensity of GO at 440 nm increases with the progression of oxidation by MPO/H₂O₂/Cl^- due to the formation of graphene quantum dots (GQDs). Turn-on GO fluorescence is also observed with neutrophil-like HL-60 cells, indicative of potential applications of GO for imaging MPO activity in live cells. Based on these results, we further construct two ratiometric sensors using SWCNT/GO nanoscrolls by incorporating surfactant-wrapped pristine SWCNTs as the internal either turn-off (with sodium cholate (SC)) or reference (with carboxymethylcellulose (CMC)) sensor. The ratiometric approach enables the sensors to be more stable to external noise by providing response invariant to the absolute intensity emitted from the sensors. Our sensors show linear response to MPO oxidative machinery and hold the promise to be used as self-calibrating carbon nanomaterial-based MPO activity indicators.

Graphene Quantum Dots as Flourishing Nanomaterials for Bio-Imaging, Therapy Development, and Micro-Supercapacitors

Graphene quantum dots (GQDs) are considerably a new member of the carbon family and shine amongst other members, thanks to their superior electrochemical, optical, and structural properties as well as biocompatibility features that enable us to engage them in various bioengineering purposes. Especially, the quantum confinement and edge effects are giving GQDs their tremendous character, while their heteroatom doping attributes enable us to specifically and meritoriously tune their prospective characteristics for innumerable operations. Considering the substantial role offered by GQDs in the area of biomedicine and nanoscience, through this review paper, we primarily focus on their applications in bio-imaging, micro-supercapacitors, as well as in therapy development. The size-dependent aspects, functionalization, and particular utilization of the GQDs are discussed in detail with respect to their distinct nano-bio-technological applications.

Microfluidics for Two-Dimensional Nanosheets: A Mini Review

Since the discovery of graphene, there has been increasing interest in two-dimensional (2D) materials. To realize practical applications of 2D materials, it is essential to isolate mono- or few-layered 2D nanosheets from unexfoliated counterparts. Liquid phase exfoliation (LPE) is the most common technique to produce atomically thin-layered 2D nanosheets. However, low production yield and prolonged process time remain key challenges. Recently, novel exfoliation processes based on microfluidics have been developed to achieve rapid and high yield production of few-layer 2D nanosheets. We review the primary types of microfluidic-based exfoliation techniques in terms of the underlying process mechanisms and the applications of the 2D nanosheets thus produced. The key challenges and future directions are discussed in the above context to delineate future research directions in this exciting area of materials processing.

Solvent Effect on Structural Elucidation of Photoluminescent Graphitic Carbon Nanodots

Photoluminescence (PL) of carbon nanodots (CNDs) is proposed to originate from the polycyclic aromatic carbon-core and in situ synthesized molecular fluorophores. This work reports the CNDs prepared by direct pyrolysis of citric acid only at a prolonged time, 40 h, and their fluorescence emission parameters in a variety of solvents by steady-state and time-resolved emission spectroscopies. The response of fluorescence emission lifetime and emission quenching rate constants to changes in solvent parameters such as polarity and tumbling lifetime were essentially independent, unlike molecular fluorophores that display solvent-dependent emission parameters. Fluorescence emission was quenched in nitromethane additionally indicating to the polycyclic aromatic carbon-core as a predominant structural feature of the CNDs. The quenching of CND emission in the presence of benzophenone that has a strong triplet component in the excited state was observed. Quenching demonstrates the Stern-Volmer behavior and reveals the additional nonradiative decay pathways of CNDs. The main photophysical features of CNDs are discussed in terms of fluorescence emission originating from the excited state of the polycyclic aromatic carbon-core where contribution from the potential molecular fluorophores is considered minimal.

Acid-Free Hydrothermal-Extraction and Molecular Structure of Carbon Quantum Dots Derived from Empty Fruit Bunch Biochar

Carbon quantum dots (CQD) have great potential to be used in various applications due to their unique electrical and optical properties. Herein, a facile, green and eco-friendly hydrothermal method for the preparation of carbon quantum dots was achieved using empty fruit bunch (EFB) biochar as a renewable and abundant carbon source. In the current study, the role of the hydrothermal process was observed and studied by comparing the morphology and optical characteristics of CQD obtained from EFB biochar. Interestingly, based on the high-resolution transmission electron microscopy (HRTEM) result, a considerably similar carbon quantum dots structure can be observed for the EFB biochar sample, showing the similar size and distribution of CQD. To further discuss the extraction of CQD from EFB biochar, a mechanism based on hydrothermal-induced extraction of CQD is proposed. The optimal structure of CQD deduced by density functional theory (DFT) in energy and dipole momentum was about 2057.4905 Hatree and 18.1699 Debye, respectively. This study presents a practical experimental approach in elucidating the molecular structure of photoluminescence CQD based on the Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) results.

Rapid conversion from common precursors to carbon dots in large scale: Spectral controls, optical sensing, cellular imaging and LEDs application

The commercial production of carbon dots will be concerned with the simplicity and energy consumption. Herein, maleic acid and m-phenylenediamine form elegantly simple sources for carbon dots. The two precursors are dissolved in formamid (abbreviated as FA) or N,N-dimethylformamide (abbreviated as DMF) and the dehydration-condensation processes have been performed at 30 min or 120 min under room temperature. No external energy/irradiations, reactants or high temperature will be required and the afforded carbon dots (abbreviated as CDs) are collected by extraction, centrifugation, dialysis and column chromatography. It has been found for the first time the choice of organic solvents has been correlated with emission color. The blue-emitting CDs (abbreviated as B-CDs) and green-emitting CDs (abbreviated as G-CDs) are yielded in FA and DMF respectively. Facts support that the increase of -CONH- units causes red-shift in emissions. The optical sensing of tetracycline has been explored and the detection limit of blue-emitting CDs is as low as 25 nM. Live cells exposed to B-CDs and G-CDs (0.5 mg/ml) show no apparent changes via both Cell Counting Kit-8 and Annexin V/7-AAD analysis. The blue and green fluorescent signals can be easily tracked in cells. It has been demonstrated that the two carbon dots can be fabricated as multiple-color light-emitting diodes (abbreviated as LEDs).

Nonlinear Interactions between Free Electrons and Nanographenes

Free electrons act as a source of highly confined, spectrally broad optical fields that are widely used to map photonic modes with nanometer/millielectronvolt space/energy resolution through currently available electron energy-loss and cathodoluminescence spectroscopies. These techniques are understood as probes of the linear optical response, while nonlinear dynamics has escaped observation with similar degree of spatial detail, despite the strong enhancement of the electron evanescent field with decreasing electron energy. Here, we show that the field accompanying low-energy electrons can trigger anharmonic response in strongly nonlinear materials. Specifically, through realistic quantum-mechanical simulations, we find that the interaction between ≲100 eV electrons and plasmons in graphene nanostructures gives rise to substantial optical nonlinearities that are discernible as saturation and spectral shifts in the plasmonic features revealed in the cathodoluminescence emission and electron energy-loss spectra. Our results support the use of low-energy electron-beam spectroscopies for the exploration of nonlinear optical processes in nanostructures.

Photonic Carbon Dots as an Emerging Nanoagent for Biomedical and Healthcare Applications

As a class of carbon-based nanomaterials, carbon dots (CDs) have attracted enormous attention because of their tunable optical and physicochemical properties, such as absorptivity and photoluminescence from ultraviolet to near-infrared, high photostability, biocompatibility, and aqueous dispersity. These characteristics make CDs a promising alternative photonic nanoagent to conventional fluorophores in disease diagnosis, treatment, and healthcare managements. This review describes the fundamental photophysical properties of CDs and highlights their recent applications to bioimaging, photomedicine (e.g., photodynamic/photothermal therapies), biosensors, and healthcare devices. We discuss current challenges and future prospects of photonic CDs to give an insight into developing vibrant fields of CD-based biomedicine and healthcare.

Carbonized Bark by Laser Treatment for Efficient Solar-Driven Interface Evaporation

Interfacial localization of solar thermal energy conversion to drive evaporation is a promising water treatment technology, especially for gaining pure water in freshwater-deficient areas. Phoenix tree bark is chosen as the raw material mainly because of its low cost and renewability. The carbonized bark with broadened pore sizes possess efficient steam escape channels and light absorption structure. The film with a double-layer structure is constructed through converting the surface of the bark into the carbonized structure under controllable laser treatment. The evaporation efficiency is calculated to be 74% under 1 sun by enhancing the photothermal conversion ability and efficiently opening the surface water transport channels simultaneously. The distillation water exhibits large resistance values (9.65 MΩ) and low concentrations of four primary ions (Na⁺, K⁺, Mg²⁺, and Ca²⁺), which achieves international standard for drinking water. In addition, the carbonized bark also exhibits all-right purified performance toward water evaporation from dye wastewater. The low cost and clean technology provides new inspiration for the future development of applicable solar thermal energy-driven water treatment systems.

Preparation of shape-specific (trilateral and quadrilateral) carbon quantum dots towards multiple color emission

Little progress has been achieved relating to the preparation of shape-specific carbon quantum dots (CQDs) with a well-ordered edge structure and multi-color fluorescence from a single precursor by monitoring and controlling the reaction time. Selecting phloroglucinol (having suitable three-fold symmetry, C3h; symmetry elements: E, C3, C32, σh, S3, S3-1) as a precursor of CQDs is useful for monitoring the shape and structure of CQDs during dehydration mediated controlled growth, which assists to better focus on their formation and PL emission mechanism. We report the rapid synthesis of novel shape-specific (trilateral and quadrilateral) CQDs with multi-color fluorescence emission [blue (B-CQDs), green (G-CQDs), and yellow (Y-CQDs)] by controlling the reaction time. The mechanism of controlled bottom-up growth involves six-membered ring cyclization of the single precursor (phloroglucinol) through the elimination of neighboring active -OH and -H groups in a sulfuric acid medium. Interestingly, wide-range multi-color fluorescence emission of non-nitrogenous CQDs is achieved based on solvatochromism. We consider that the evolution of the tunable photoluminescence (PL) emission can be attributed to both the size of the conjugated domain and oxygen-/sulfur-containing edge electronic states. Furthermore, the multi-color fluorescence CQDs are successfully used as propitious fluorescent probes for multi-color cell (HeLa) and zebra fish larvae imaging owing to an effective intracellular distribution and good biocompatibility.

A polyimide-pyrolyzed carbon waste approach for the scalable and controlled electrochemical preparation of size-tunable graphene

Carbon materials are widely used in numerous fields, thus changing our lives. With the increasing consumption of carbon-based products, the disposal of consequent wastes has become a challenge due to their inert nature, which is hard to degrade, burn, or melt. Here, a recyclable strategy is proposed to deal with the explosive growth of carbon wastes. Through a fast and clean electrochemical method, carbon wastes are converted into functional building blocks of high value, such as graphene and graphene quantum dots (GQDs). For typical polyimide-pyrolyzed carbon (PPC), we establish the relationship between the chemical structure of raw materials and the characteristics of graphene products, including size and yield. The size-tunable graphene ranging from 3 nm to tens of micrometers is prepared by tuning the sp3/sp2 carbon ratio of PPC from 0.5 to 0 at adjustable temperatures (800 °C-2800 °C). Significantly, PPC with a bicontinuous structure (comprising sp2 and sp3) was efficiently cut into GQDs in 2 h with a high yield of 98%. Our protocol offers great potential for the scale-up preparations and applications of GQDs. Besides, we demonstrate that the GQDs performed well as dispersants to disperse hydrophobic carbon nanotubes (0.6 mg mL-1) in water and improved the gravimetric capacitance of graphene-based supercapacitors by 79.4% with 3% GQDs added as nano-fillers.

Recent Advances on Graphene Quantum Dots for Bioimaging Applications

Being a zero-dimensional (0D) nanomaterial of the carbon family, graphene quantum dots (GQDs) showed promising biomedical applications owing to their ultra-small size, non-toxicity, biocompatibility, excellent photo stability, tunable fluorescence, and water solubility, etc., thus capturing a considerable attention in biomedical field. This review summarizes the recent advances made in the research field of GQDs and place special emphasis on their bioimaging applications. We briefly introduce the synthesis strategies of GQDs, including top-down and bottom-up strategies. The bioimaging applications of GQDs are also discussed in detail, including optical [fluorescence (FL)], two-photon FL, magnetic resonance imaging (MRI), and dual-modal imaging. In the end, the challenges and future prospects to advance the clinical bioimaging applications of GQDs have also been addressed.

Direct carbonization of organic solvents toward graphene quantum dots

The bottom-up synthesis of graphene quantum dots (GQDs) using solvothermal methods has attracted considerable attention because of their fewer defects and controllable size/morphology. However, the influence of organic solvents on the preparation of GQDs is still unknown. Herein, a systematic study on the carbonization of organic solvents toward GQDs is reported. The results show that organic solvents with the double bond or benzene ring or double hydrophilic groups could be directly decomposed into GQDs without the addition of catalysts or molecular precursors. The as-synthesized GQDs demonstrate ultra-small size distribution, high stability, tunable excitation wavelength and upconverted fluorescence. Both hematological and histopathological analyses show that the as-synthesized GQDs demonstrate a very good safety profile and excellent biocompatibility. The versatility of this synthesis strategy offers easy control of the surface group, composition, and optical properties of GQDs at the molecular level.

Unravelling the Potential of Graphene Quantum Dots in Biomedicine and Neuroscience

Quantum dots (QDs) are semiconducting nanoparticles that have been gaining ground in various applications, including the biomedical field, thanks to their unique optical properties. Recently, graphene quantum dots (GQDs) have earned attention in biomedicine and nanomedicine, thanks to their higher biocompatibility and low cytotoxicity compared to other QDs. GQDs share the optical properties of QD and have proven ability to cross the blood-brain barrier (BBB). For this reason, GQDs are now being employed to deepen our knowledge in neuroscience diagnostics and therapeutics. Their size and surface chemistry that ease the loading of chemotherapeutic drugs, makes them ideal drug delivery systems through the bloodstream, across the BBB, up to the brain. GQDs-based neuroimaging techniques and theranostic applications, such as photothermal and photodynamic therapy alone or in combination with chemotherapy, have been designed. In this review, optical properties and biocompatibility of GQDs will be described. Then, the ability of GQDs to overtake the BBB and reach the brain will be discussed. At last, applications of GQDs in bioimaging, photophysical therapies and drug delivery to the central nervous system will be considered, unraveling their potential in the neuroscientific field.

Recent Advancement in Bio-precursor derived graphene quantum dots: Synthesis, Characterization and Toxicological Perspective

Graphene quantum dots (GQDs), impressive materials with enormous future potential, are reviewed from their inception, including different precursors. Considering the increasing burden of industrial and ecological bio-waste, there is an urgency to develop techniques which will convert biowaste into active moieties of interest. Amongst the various materials explored, we selectively highlight the use of potential carbon containing bioprecursors (e.g. plant-based, amino acids, carbohydrates), and industrial waste and its conversion into GQDs with negligible use of chemicals. This review focuses on the effects of different processing parameters that affect the properties of GQDs, including the surface functionalization, paradigmatic characterization, toxicity and biocompatibility issues of bioprecursor derived GQDs. This review also examines current challenges and s the ongoing exploration of potential bioprecursors for ecofriendly GQD synthesis for future applications. This review sheds further light on the electronic and optical properties of GQDs along with the effects of doping on the same. This review may aid in future design approaches and applications of GQDs in the biomedical and materials design fields.

A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems

The presence of arsenic in the water system has been a universal problem over the past several decades. Inorganic arsenic ions mainly occur in two oxidation states, As(V) and As(III), in the natural environment. These two oxidation states of arsenic ions are ubiquitous in natural waters and pose significant health hazards to humans when present at or above the allowable limits. Therefore, treatment of arsenic ions has become more stringent based on various techniques (e.g., membrane filtration, adsorption, and ion exchange). This paper aims to review the current knowledge on various functional adsorbents through comparison of removal potential for As on the basis of key performance metrics, especially the partition coefficient (PC). As a whole, novel materials exhibited far better removal performance for As(V) and As(III) than conventional materials. Of the materials reviewed, the advanced sorbent like ZrO(OH)₂/CNTs showcased superior performances such as partition coefficient values of 584.6 (As(V) and 143.8 mol kg^-1 M^-1 (As(III) with excellent regenerability (>90 % of desorption efficiency after three sorption cycles). The results of this review are expected to help researchers to establish a powerful strategy for abatement of arsenic ions in wastewater.

Carbon Dots Conjugated with Vascular Endothelial Growth Factor for Protein Tracking in Angiogenic Therapy

One of the challenges of using growth factors for tissue regeneration is to monitor their biodistributions and delivery to injured tissues for minimally invasive detection. In the present study, tracking of human vascular endothelial growth factor (VEGF) was achieved by chemically linking it to photoluminescent carbon dots (CDs). Carbon dots were synthesized by the hydrothermal method and, subsequently, conjugated with VEGF using carbodiimide coupling. ELISA and western blot analysis revealed that VEGF-conjugated CDs preserve the binding affinity of VEGF to its antibodies. We also show that VEGF-conjugated CDs maintain the functionality of VEGF for tube formation and cell migration. The VEGF-conjugated CDs were also used for in vitro imaging of human umbilical vein endothelial cells. The results of this work suggest that cell-penetrating VEGF-conjugated CDs can be used for growth factor protein tracking in therapeutic and tissue engineering applications.

Facile and Efficient Fabrication of Bandgap Tunable Carbon Quantum Dots Derived From Anthracite and Their Photoluminescence Properties

Low-cost and earth-abundant coal has been considered to have a unique structural superiority as carbon sources of carbon quantum dots (CQDs). However, it is still difficult to obtain CQDs from raw coal due to its compactibility and lower reactivity, and the majority of the current coal-based CQDs usually emit green or blue fluorescence. Herein, a facile two-step oxidation approach (K₂FeO₄ pre-oxidation and H₂O₂ oxidation) was proposed to fabricate bandgap tunable CQDs from anthracite. The K₂FeO₄ pre-oxidation can not only weaken the non-bonding forces among coal molecules which cause the expansion of coal particles, but also form a large number of active sites on the surface of coal particles. The above effects make the bandgap tunable CQDs (blue, green, or yellow fluorescence) can be quickly obtained from anthracite within 1 h in the following H₂O₂ oxidation by simply adjusting the concentration of H₂O₂. All the as-prepared CQDs contain more than 30 at% oxygen, and the average diameters of which are <10 nm. The results also indicate that the high oxygen content only can create new energy states inside the band gap of CQDs with average diameter more than 3.2 ± 0.9 nm, which make the as-prepared CQDs emit green or yellow fluorescence.

Graphene Quantum Dots with High Yield and High Quality Synthesized from Low Cost Precursor of Aphanitic Graphite

It is difficult to keep the balance of high quality and high yield for graphene quantum dots (GQDs). Because the quality is uncontrollable during cutting large 2D nanosheets to small 0D nanodots by top-down methods and the yield is low for GQDs with high quality obtained from bottom-up strategy. Here, aphanitic graphite (AG), a low-cost graphite contains a large amount of small graphite nanocrystals with size of about 10 nm is used as the precursor of graphene oxide quantum dots (GO-QDs) for the first time. GO-QDs with high yield and high quality were successfully obtained directly by liquid phase exfoliating AG without high strength cutting. The yield of these GO-QDs can reach up to 40 wt. %, much higher than that obtained from flake graphite (FG) precursor (less than 10 wt. %). The size of GO-QDs can be controlled in 2-10 nm. The average thickness of GO-QDs is about 3 nm, less than 3 layer of graphene sheet. Graphene quantum dots (GQDs) with different surface properties can be easily obtained by simple hydrothermal treatment of GO-QDs, which can be used as highly efficient fluorescent probe. Developing AG as precursor for GQDs offers a way to produce GQDs in a low-cost, highly effective and scalable manner.

Graphene quantum dots: efficient mechanosynthesis, white-light and broad linear excitation-dependent photoluminescence and growth inhibition of bladder cancer cells

Heteroatom-doped graphene quantum dots (GQDs) have attracted considerable attention due to their potential applications as luminescent materials and in biology. In this work, we developed a solvent-free gram-scale mechanochemical method for the preparation of nitrogen-doped graphene quantum dots (N-GQDs) with the highest solubility (31 mg mL^-1) in water reported to date. Commercial graphite was sheared and cut through grinding with solid melamine and then ground with solid KOH to get sub-5 nm-sized, 1-3-layered N-GQDs. Notably, these N-GQDs exhibit white-light emission and broad excitation-dependent full-color photoluminescence from 463 nm to 672 nm. When the excitation light ranged from 325 nm to 485 nm, these mechanochemically obtained N-GQDs exhibited bright white-light emission. Intriguingly, the change in the emission wavelength has two-stage linear relationships with the change in the excitation wavelength, and the inflection point is at 580 nm (excited at 550 nm). The difference between the emission and excitation wavelengths decreases from 138 to 12 nm, which also shows two-stage linear relationships with the change in the excitation wavelength. It is notable that their PL quantum yields are high, up to 26.6%. Furthermore, we studied the inhibitory effect of as-obtained N-GQDs on bladder cancer cells (UMUC-3); as a result, with the increase of the concentration of N-GQDs, the proliferation of cancer cells was obviously prohibited.

A novel nanobiosorbent of functionalized graphene quantum dots from rice husk with barium hydroxide for microwave enhanced removal of lead (II) and lanthanum (III)

In this study, rice husk was used as a sustainable source to synthesize graphene quantum dots (GQDOs) with 2D morphology. Chemical modification of GQDOs with Ba(OH)₂ was followed to form a novel GQDOs-Ba nanobiosorbent with an increased number of surface hydroxyl groups. The physicochemical properties of GQDOs and GQDOs-Ba were investigated by FT-IR, SEM, TEM, TGA, and XRD. The adsorption parameters of Pb(II) and La(III) onto GQDOs-Ba were optimized using microwave sorption approach. The maximum capacity reached 3400 µmol g^-1 (pH 7), and 1500 µmol g^-1 (pH 5) at 15 s for Pb(II) and La(III), respectively. The adsorption isotherm models by GQDOs-Ba fitted well with Langmuir. The pseudo-second order was agreed by Pb(II) and La(III) ions. The thermodynamic studies elucidated that Pb(II) and La(III) adsorption onto GQDOs-Ba followed a spontaneous model. The GQDOs-Ba nanobiosorbent accomplished excellent removal percentages from different water samples containing lead (98.5%-99.8%) and lanthanum (94.6%-96.2%).

van der Waals Heterojunction between a Bottom-Up Grown Doped Graphene Quantum Dot and Graphene for Photoelectrochemical Water Splitting

van der Waals heterojunctions (vdWHs) formed between 2D materials have attracted tremendous attention recently due to their extraordinary properties, which cannot be offered by their individual components or other heterojunctions. Intriguing electronic coupling, lowered energy barrier, intimate charge transfer, and efficient exciton separation occurring at the atomically sharp interface promise their applications in catalysis, which, however, are largely unexplored. Herein, we demonstrate a 0D/2D vdWH between 0D graphene quantum dots (GQDs) and 2D pristine graphene sheets, simply prepared by ultrasonication of graphite powder using GQDs as intercalation surfactant. Such an all-carbon Schottky-diode-like 0D/2D vdWH is employed for the emerging photoelectrochemical catalysis (water splitting) with high performance. The demonstrated low-cost and scalable bottom-up growth of heteroatom-doped GQDs will greatly promote their widespread applications. Moreover, the mechanisms underlying GQD growth and heterojunction-mediated catalysis are revealed both experimentally and theoretically.

Influence of the solvent environment on luminescent centers within carbon dots

Carbon dots (CDs) are luminescent nanomaterials, with potential use in bioimaging and sensorics. Here, the influence of the surrounding solvent media on the optical properties of CDs synthesized from the most commonly employed precursors, namely citric acid and ethylenediamine, is investigated. The position of optical transitions of CDs can be tuned by the change of pH and solvent polarity. The most striking observation is related to the interaction of CDs with chlorine containing solvents, which results in resolving a set of narrow peaks within both the absorption and PL bands, similar to those observed for polycyclic aromatic hydrocarbons or organic dyes. We assume that the chlorine containing molecules penetrate the surface layers of CDs, which results in an increase of the distance between the luminescent centers; this correlates well with an enhanced D-band in their Raman spectra. A model of CDs composed of a matrix of hydrogenated amorphous carbon with the inclusions of sp2-domains formed by polycyclic aromatic hydrocarbons and their derivatives is suggested; the latter are stacked ensembles of the luminophores and are considered as the origin of the emission of CDs.

Charcoal derived graphene quantum dots for flexible supercapacitor oriented applications

In this article, charcoal is used as a new raw material for the synthesis of high yield graphene quantum dots (GQDs) for supercapacitor application.

Exploration of the potential efficacy of natural resource-derived blue-emitting graphene quantum dots in cancer therapeutic applications

We present a method involving oxidative functionalization followed by a solvothermal cutting technique for the synthesis of strong-blue-emitting GQD nanomaterial in cancer therapy.

Ultralight and highly compressible coal oxide-modified graphene aerogels for organic solvent absorption and light-to-heat conversion

Ultralight, hydrophobic, highly compressible and low-cost coal oxide-modified graphene aerogels exhibit high absorption capacity and high solar thermal conversion efficiency.