Visible-Light-Driven Photocatalytic H2 Production Using Composites of Co-Al Layered Double Hydroxides and Graphene Derivatives

The direct conversion of solar energy into chemical energy represents an enormous challenge for current science. One of the commonly proposed photocatalytic systems is composed of a photosensitizer (PS) and a catalyst, together with a sacrificial electron donor (ED) when only the reduction of protons to H2 is addressed. Layered double hydroxides (LDH) have emerged as effective catalysts. Herein, two Co-Al LDH and their composites with graphene oxide (GO) or graphene quantum dots (GQD) have been prepared by coprecipitation and urea hydrolysis, which determined their structure and so their catalytic performance, giving H2 productions between 1409 and 8643 μmol g-1 using a ruthenium complex as PS and triethanolamine as ED at 450 nm. The influence of different factors, including the integration of both components, on their catalytic behavior, has been studied. The proper arrangement between the particles of both components seems to be the determining factor for achieving a synergistic interaction between LDH and GO or GQD. The novel Co-Al LDH composite with intercalated GQD achieved an outstanding catalytic efficiency (8643 μmol H2 g-1) and exhibited excellent reusability after 3 reaction cycles, thus representing an optimal integration between graphene materials and Co-Al LDH for visible light driven H2 photocatalytic production.

Application of gold nanoclusters in fluorescence sensing and biological detection

Gold nanoclusters (Au NCs) exhibit broad fluorescent spectra from visible to near-infrared regions and good enzyme-mimicking catalytic activities. Combined with excellent stability and exceptional biocompatibility, the Au NCs have been widely exploited in biomedicine such as biocatalysis and bioimaging. Especially, the long fluorescence lifetime and large Stokes shift attribute Au NCs to good probes for fluorescence sensing and biological detection. In this review, we systematically summarized the molecular structure and fluorescence properties of Au NCs and highlighted the advances in fluorescence sensing and biological detection. The Au NCs display high sensitivity and specificity in detecting iodine ions, metal ions, and reactive oxygen species, as well as certain diseases based on the fluorescence activities of Au NCs. We also proposed several points to improve the practicability and accelerate the clinical translation of the Au NCs.

Quantum Material-Based Self-Propelled Microrobots for the Optical "On-the-Fly" Monitoring of DNA

Quantum dot-based materials have been found to be excellent platforms for biosensing and bioimaging applications. Herein, self-propelled microrobots made of graphene quantum dots (GQD-MRs) have been synthesized and explored as unconventional dynamic biocarriers toward the optical "on-the-fly" monitoring of DNA. As a first demonstration of applicability, GQD-MRs have been first biofunctionalized with a DNA biomarker (i.e., fluorescein amidite-labeled, FAM-L) via hydrophobic π-stacking interactions and subsequently exposed toward different concentrations of a DNA target. The biomarker-target hybridization process leads to a biomarker release from the GQD-MR surface, resulting in a linear alteration in the fluorescence intensity of the dynamic biocarrier at the nM range (1-100 nM, R2 = 0.99), also demonstrating excellent selectivity and sensitivity, with a detection limit as low as 0.05 nM. Consequently, the developed dynamic biocarriers, which combine the appealing features of GQDs (e.g., water solubility, fluorescent activity, and supramolecular π-stacking interactions) with the autonomous mobility of MRs, present themselves as potential autonomous micromachines to be exploited as highly efficient and sensitive "on-the-fly" biosensing systems. This method is general and can be simply customized by tailoring the biomarker anchored to the GQD-MR's surface.

Fluorescence Sensing Mechanisms of Versatile Graphene Quantum Dots toward Commonly Encountered Heavy Metal Ions

Graphene quantum dots (GQDs) have received tremendous attention as fluorescent probes for detection of diverse heavy metal ions (HMIs). Nevertheless, the fluorescence sensing mechanisms of versatile GQDs with respect to different HMIs remain elusive. Herein, the fluorescence sensing behaviors and mechanisms of GQDs with amino and carboxyl groups toward commonly encountered Cr6+, Fe3+, Cu2+, Cr3+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, and Hg2+ under different pH conditions are systemically explored. The results show that the fluorescence of GQDs can be enhanced by Zn2+/Cd2+ and quenched by other HMIs at pH 5.8, while it can be enhanced by HMIs except Cr6+/Fe3+/Cu2+ at pH 2.0. Systematic studies verify that the fluorescence quenching/enhancing is mediated by the synergistic effect of the inner filter effect (IFE) and the photoinduced electron transfer (PET) or metal orbital-controlled chelation-quenched/enhanced fluorescence (CHQF/CHEF) effect. The strong and weak IFEs of Cr6+/Fe3+ and Cr3+/Cu2+, respectively, are one of the reasons for the fluorescence quenching, while other HMIs have no IFE. Moreover, the PET effect caused by the interaction of GQDs with Hg2+ at pH 5.8 and the CHQF/CHEF effect caused by the interaction of GQDs with other HMIs are also crucial for fluorescence quenching/enhancing. The findings suggest that the pH condition, the existing forms of functional groups on GQDs, and the complexation states of HMIs in aqueous systems dominate the PET and CHQF/CHEF effects. The elucidating of the fluorescence sensing mechanisms of GQDs toward different HMIs paves the way for developing versatile sensing platforms for monitoring of HMI contamination.

Surface Area of Graphene Governs Its Neurotoxicity

Due to their unique physicochemical properties, graphene and its derivatives are widely exploited for biomedical applications. It has been shown that graphene may exert different degrees of toxicity in in vivo or in vitro models when administered via different routes and penetrated through physiological barriers, subsequently being distributed within tissues or located within cells. In this study, in vitro neurotoxicity of graphene with different surface areas (150 and 750 m²/g) was examined on dopaminergic neuron model cells. SH-SY5Y cells were treated with graphene possessing two different surface areas (150 and 750 m²/g) in different concentrations between 400 and 3.125 μg/mL, and the cytotoxic and genotoxic effects were investigated. Both sizes of graphene have shown increased cell viability in decreasing concentrations. Cell damage increased with higher surface area. Lactate dehydrogenase (LDH) results have concluded that the viability loss of the cells is not through membrane damage. Neither of the two graphene types showed damage through lipid peroxidation (MDA) oxidative stress pathway. Glutathione (GSH) values increased within the first 24 and 48 h for both types of graphene. This increase suggests that graphene has an antioxidant effect on the SH-SY5Y model neurons. Comet analysis shows that graphene does not show genotoxicity on either surface area. Although there are many studies on graphene and its derivatives on their use with different cells in the literature, there are conflicting results in these studies, and most of the literature is focused on graphene oxide. Among these studies, no study examining the effect of graphene surface areas on the cell was found. Our study contributes to the literature in terms of examining the cytotoxic and genotoxic behavior of graphene with different surface areas.

Antibacterial Carbon Dots-Based Composites

The emergence and global spread of bacterial resistance to conventionally used antibiotics have highlighted the urgent need for new antimicrobial agents that might replace antibiotics. Currently, nanomaterials hold considerable promise as antimicrobial agents in anti-inflammatory therapy. Due to their distinctive functional physicochemical characteristics and exceptional biocompatibility, carbon dots (CDs)-based composites have attracted a lot of attention in the context of these antimicrobial nanomaterials. Here, a thorough assessment of current developments in the field of antimicrobial CDs-based composites is provided, starting with a brief explanation of the general synthesis procedures, categorization, and physicochemical characteristics of CDs-based composites. The many processes driving the antibacterial action of these composites are then thoroughly described, including physical destruction, oxidative stress, and the incorporation of antimicrobial agents. Finally, the obstacles that CDs-based composites now suffer in combating infectious diseases are outlined and investigated, along with the potential applications of antimicrobial CDs-based composites.

Recent Progress of Carbon Dots for Air Pollutants Detection and Photocatalytic Removal: Synthesis, Modifications, and Applications

Rapid industrialization has inevitably led to serious air pollution problems, thus it is urgent to develop detection and treatment technologies for qualitative and quantitative analysis and efficient removal of harmful pollutants. Notably, the employment of functional nanomaterials, in sensing and photocatalytic technologies, is promising to achieve efficient in situ detection and removal of gaseous pollutants. Among them, carbon dots (CDs) have shown significant potential due to their superior properties, such as controllable structures, easy surface modification, adjustable energy band, and excellent electron-transfer capacities. Moreover, their environmentally friendly preparation and efficient capture of solar energy provide a green option for sustainably addressing environmental problems. Here, recent advances in the rational design of CDs-based sensors and photocatalysts are highlighted. An overview of their applications in air pollutants detection and photocatalytic removal is presented, especially the diverse sensing and photocatalytic mechanisms of CDs are discussed. Finally, the challenges and perspectives are also provided, emphasizing the importance of synthetic mechanism investigation and rational design of structures.

A combination of a graphene quantum dots-cationic red dye donor-acceptor pair and cucurbit[7]uril as a supramolecular sensor for ultrasensitive detection of cancer biomarkers spermine and spermidine

In a unique approach, the combination of a donor-acceptor pair of hydroxy graphene quantum dots (GQDs-OH) and a red-emissive donor-two-acceptor (D-2-A) type dye with pyridinium units (BPBP) and the well-known host cucurbit[7]uril (CB[7]) has been exploited as a supramolecular sensing assembly for the detection of cancer biomarkers spermine and spermidine in aqueous media at the sub-ppb level based on the affinity-driven exchange of guests from the CB[7] portal. In the binary conjugate, green fluorescent GQDs-OH transfers energy to trigger the emission of the dye BPBP and itself remains in the turn-off state. CB[7] withdraws the dye from the surface of GQDs-OH by strong host-guest interactions with its portal, making GQDs-OH fluoresce again to produce a ratiometric response. In the presence of spermine (SP) or spermidine (SPD), their strong affinity with CB[7] forces the ejection of the fluorophore to settle on the GQDs-OH surface, and the strong green emission of GQDs-OH turns off to device a supramolecular sensor for the detection of SP/SPD. The DFT studies revealed interesting excited-state charge-transfer conjugate formation between BPBP and GQDs leading to turn-on emission of the dye, and further supported the stronger binding modes of BPBP-CB[7], indicating the retrieval of the emission of GQDs. The assembly-disassembly based sensing mechanism was also established by Job's plot analysis, particle size analysis, zeta potential, time-resolved spectroscopy, ITC studies, microscopic studies, etc. The supramolecular sensing assembly is highly selective to SP and SPD, and showed nominal interference from other biogenic amines, amino acids, various metal ions, and anions. The limits of detection (LODs) were 0.1 ppb and 0.9 ppb for spermine and spermidine, respectively. The potential for the real-world application of this sensing assembly was demonstrated by spiking SP and SPD in human urine and blood serum with a high %recovery.

Synthesis of Metal–Organic Frameworks Quantum Dots Composites as Sensors for Endocrine-Disrupting Chemicals

Hazardous chemical compounds such as endocrine-disrupting chemicals (EDCs) are widespread and part of the materials we use daily. Among these compounds, bisphenol A (BPA) is the most common endocrine-disrupting chemical and is prevalent due to the chemical raw materials used to manufacture thermoplastic polymers, rigid foams, and industrial coatings. General exposure to endocrine-disrupting chemicals constitutes a serious health hazard, especially to reproductive systems, and can lead to transgenerational diseases in adults due to exposure to these chemicals over several years. Thus, it is necessary to develop sensors for early detection of endocrine-disrupting chemicals. In recent years, the use of metal–organic frameworks (MOFs) as sensors for EDCs has been explored due to their distinctive characteristics, such as wide surface area, outstanding chemical fastness, structural tuneability, gas storage, molecular separation, proton conductivity, and catalyst activity, among others which can be modified to sense hazardous environmental pollutants such as EDCs. In order to improve the versatility of MOFs as sensors, semiconductor quantum dots have been introduced into the MOF pores to form metal–organic frameworks/quantum dots composites. These composites possess a large optical absorption coefficient, low toxicity, direct bandgap, formidable sensing capacity, high resistance to change under light and tunable visual qualities by varying the size and compositions, which make them useful for applications as sensors for probing of dangerous and risky environmental contaminants such as EDCs and more. In this review, we explore various synthetic strategies of (MOFs), quantum dots (QDs), and metal–organic framework quantum dots composites (MOFs@QDs) as efficient compounds for the sensing of ecological pollutants, contaminants, and toxicants such as EDCs. We also summarize various compounds or materials used in the detection of BPA as well as the sensing ability and capability of MOFs, QDs, and MOFs@QDs composites that can be used as sensors for EDCs and BPA.

Graphene quantum dots: synthesis, properties, and applications to the development of optical and electrochemical sensors for chemical sensing

GQDs exhibits exceptional electrochemical activity owing to their active edge sites that make them very attractive for biosensing applications. However, their use in the design of new biosensing devices for application to the detection and quantification of toxins, pathogens, and clinical biomarkers has so far not investigated in detail. In this regard, herein we provide a detailed review on various methodologies employed for the synthesis of GQDs, including bottom-up and top-down approaches, with a special focus on their applications in biosensing via fluorescence, photoluminescence, chemiluminescence, electrochemiluminescence, fluorescence resonance energy transfer, and electrochemical techniques. We believe that this review will shed light on the critical issues and widen the applications of GQDs for the design of biosensors with improved analytical response for future applications. HIGHLIGHTS: • Properties of GQDs play a critical role in biosensing applications. • Synthesis of GQDs using top-down and bottom-up approaches is discussed comprehensively. • Overview of advancements in GQD-based sensors over the last decade. • Methods for the design of selective and sensitive GQD-based sensors. • Challenges and opportunities for future GQD-based sensors.

Optical Detection of Acetone Using "Turn-Off" Fluorescent Rice Straw Based Cellulose Carbon Dots Imprinted onto Paper Dipstick for Diabetes Monitoring

Persistent bad breath has been reported as a sign of serious diabetes health conditions. If an individual's breath has a strong odor of acetone, it may indicate high levels of ketones in the blood owing to diabetic ketoacidosis. Thus, acetone gas in the breath of patients with diabetes can be detected using the current easy-to-use fluorescent test dipstick. In another vein, rice straw waste is the most well-known solid pollutant worldwide. Thus, finding a simple technique to change rice straw into a valuable material is highly important. A straightforward and environmentally friendly approach for reprocessing rice straw as a starting material for the creation of fluorescent nitrogen-doped carbon dots (NCDs) has been established. The preparation process of NCDs was carried out via one-pot hydrothermal carbonization using NH₄OH as a passivation substance. A testing strip was developed on the basis of cellulose CD nanoparticles (NPs) immobilized onto cellulose paper assay. The NCDs demonstrated a quantum yield of 23.76%. A fluorescence wavelength was detected at 443 nm upon applying an excitation wavelength of 354 nm. NCDs demonstrated remarkable selectivity for acetone gas as their fluorescence was definitely exposed to quenching by acetone as a consequence of the inner filter effect. A linear correlation was observed across the concentration range of 0.5-150 mM. To detect and measure acetone gas, the present cellulose paper strip has a "switch off" fluorescent signal. A readout limit was accomplished for an aqueous solution of acetone as low as 0.5 mM under ambient conditions. The chromogenic fluorescence of the cellulose assay responsiveness depends on the fluorescence quenching characteristic of the cellulose carbon dots in acetone. A thin fluorescent cellulose carbon dot layer was deposited onto the surface of cellulose strips by a simple impregnation process. CDs were made using NP morphology and analyzed using infrared spectroscopy and transmission electron microscopy. The carbon dot distribution on the paper strip was evaluated by scanning electron microscope and energy-dispersive X-ray analysis. The absorption and fluorescence spectral analyses were investigated. The paper sheets' mechanical qualities were also examined.

Applications of graphene-based electrochemical and optical biosensors in early detection of cancer biomarkers

Graphene is a one-atom-thick carbon compound, which holds promises for detecting cancer biomarkers along with its derivatives. The atom-wide graphene layer is ideal for cancer biomarker detection due to its unique physicochemical properties like increased electrical and thermal conductivity, optical transparency, and enhanced chemical and mechanical strength. The scientific aim of any biosensor is to create a smaller and portable point of care device for easy and early cancer detection; graphene is able to live up to that. Apart from tumour detection, graphene-based biosensors can diagnose many diseases, their biomarkers, and pathogens. Many existing remarkable pieces of research have proven the candidacy of nanoparticles in most cancer biomarkers detection. This article discusses the effectiveness of graphene-based biosensors in different cancer biomarker detection. This article provides a detailed review of graphene and its derivatives that can be used to detect cancer biomarkers with high specificity, sensitivity, and selectivity. We have highlighted the synthesis procedures of graphene and its products and also discussed their significant properties. Furthermore, we provided a detailed overview of the recent studies on cancer biomarker detection using graphene-based biosensors. The different paths to create and modify graphene surfaces for sensory applications have also been highlighted in each section. Finally, we concluded the review by discussing the existing challenges of these biosensors and also highlighted the steps that can be taken to overcome them.

Carbon Dots: Synthesis, Properties and Applications

Carbon dots (CDs) are known as the rising star of carbon-based nanomaterials and, by virtue of their unique structure and fascinating properties, they have attracted considerable interest in different fields such as biological sensing, drug delivery, photodynamic therapy, photocatalysis, and solar cells in recent years. Particularly, the outstanding electronic and optical properties of the CDs have attracted increasing attention in biomedical and photocatalytic applications owing to their low toxicity, biocompatibility, excellent photostability, tunable fluorescence, outstanding efficient up-converted photoluminescence behavior, and photo-induced electron transfer ability. This article reviews recent progress on the synthesis routes and optical properties of CDs as well as biomedical and photocatalytic applications. Furthermore, we discuss an outlook on future and potential development of the CDs based biosensor, biological dye, biological vehicle, and photocatalysts in this booming research field.

Facile Synthesis of L-Cysteine Functionalized Graphene Quantum Dots as a Bioimaging and Photosensitive Agent

Nowadays, a larger number of aggressive and corrosive chemical reagents as well as toxic solvents are used to achieve structural modification and cleaning of the final products. These lead to the production of residual, waste chemicals, which are often reactive, cancerogenic, and toxic to the environment. This study shows a new approach to the modification of graphene quantum dots (GQDs) using gamma irradiation where the usage of reagents was avoided. We achieved the incorporation of S and N atoms in the GQD structure by selecting an aqueous solution of L-cysteine as an irradiation medium. GQDs were exposed to gamma-irradiation at doses of 25, 50 and 200 kGy. After irradiation, the optical, structural, and morphological properties, as well as the possibility of their use as an agent in bioimaging and photodynamic therapy, were studied. We measured an enhanced quantum yield of photoluminescence with the highest dose of 25 kGy (21.60%). Both S- and N-functional groups were detected in all gamma-irradiated GQDs: amino, amide, thiol, and thione. Spin trap electron paramagnetic resonance showed that GQDs irradiated with 25 kGy can generate singlet oxygen upon illumination. Bioimaging on HeLa cells showed the best visibility for cells treated with GQDs irradiated with 25 kGy, while cytotoxicity was not detected after treatment of HeLa cells with gamma-irradiated GQDs.

Effect of Solvent on Fluorescence Emission from Polyethylene Glycol-Coated Graphene Quantum Dots under Blue Light Illumination

To explore aggregate-induced emission (AIE) properties, this study adopts a one-pot hydrothermal route for synthesizing polyethylene glycol (PEG)-coated graphene quantum dot (GQD) clusters, enabling the emission of highly intense photoluminescence under blue light illumination. The hydrothermal synthesis was performed at 300 °C using o-phenylenediamine as the nitrogen and carbon sources in the presence of PEG. Three different solvents, propylene glycol methyl ether acetate (PGMEA), ethanol, and water, were used for dispersing the PEG-coated GQDs, where extremely high fluorescent emission was achieved at 530-550 nm. It was shown that the quantum yield (QY) of PEG-coated GQD suspensions is strongly dependent on the solvent type. The pristine GQD suspension tends to be quenched (i.e., QY: ~1%) when dispersed in PGMEA (aggregation-caused quenching). However, coating GQD nanoparticles with polyethylene glycol results in substantial enhancement of the quantum yield. When investigating the photoluminescence emission from PEG-coated GQD clusters, the surface tension of the solvents was within the range of from 26.9 to 46.0 mN/m. This critical index can be tuned for assessing the transition point needed to activate the AIE mechanism which ultimately boosts the fluorescence intensity. The one-pot hydrothermal route established in this study can be adopted to engineer PEG-coated GQD clusters with solid-state PL emission capabilities, which are needed for next-generation optical, bio-sensing, and energy storage/conversion devices.

Clitoria ternatea Mediated Synthesis of Graphene Quantum Dots for the Treatment of Alzheimer's Disease

The main purpose of the present study was to synthesize graphene quantum dots (GQDs)from the flowers of Clitoria ternatea with the help of one-pot microwave-assisted green synthesis for the treatment of Alzheimer's disease. Further, the synthesized graphene quantum dots show a particle size of 10 nm ±1.3, a PDI of 0.354 ± 1.8, and a ζ potential of -46 ± 0.4, indicating the good stability of the quantum dots. With the help of scanning electron microscopy (SEM) and transfer electron microscopy (TEM) examination, the surface microscopic behavior of the synthesized quantum dots was determined. The presence of functional groups in the quantum dots was determined by Fourier-transform infrared spectroscopy (FTIR) study, the chemical state information on the sample was determined with the help of X-ray photoelectron spectroscopy (XPS), and the surface area of the dots was determined with the help of a surface area analyzer. With the help of a radial arm maze and water morris maze assay, the learning and memory capacity of the quantum dots was assessed, and the results show that the ctGQDs significantly decreased the transfer latency to reach the baited arm in 10.37 ± 1.65 s or to the hidden platform in 18.42 ± 0.99 s in 7 days. The synthesized quantum dots show more inhibition of the acetyl cholinesterase enzyme, i.e., 86.32 ± 1.52%, as compared to that of pure donepezil, i.e., 72.46 ± 2.21%. ctGQDs considerably increased the level of glutathione and protein and decreased the level of lipid peroxide and nitric oxide. The histopathological image of ctGQDs shows more preservation of small pyramidal cell and treats the disorganization of the cells. These results suggest that the quantum dots significantly crossed the blood-brain barrier since they were small in size and were effective in reducing Alzheimer-like symptoms in rodents, and thus, it can be concluded that Clitoria ternatea flowers can be used as an adjuvant in the treatment of Alzheimer's.

Recent advances in nanoscale materials for antibody-based cancer theranostics

The quest for advanced management tools or options of various cancers has been on the rise to efficiently reduce their risks of mortality without the demerits of conventional treatments (e.g., undesirable side effects of the medications on non-target tissues, non-targeted distribution, slow clearance of the administered drugs, and the development of drug resistance over the duration of therapy). In this context, nanomaterials-antibody conjugates can offer numerous advantages in the development of cancer theranostics over conventional delivery systems (e.g., highly specific and enhanced biodistribution of the drug in targeted tissues, prolonged systemic circulation, low toxicity, and minimally invasive molecular imaging). This review comprehensively discusses and evaluates recent advances in the application of nanomaterial-antibody bioconjugates for cancer theranostics for the further advancement in the control of diverse cancerous diseases. Further, discussion is expanded to cover the various challenges and limitations associated with the design and development of nanomaterial-antibody conjugates applicable towards better management of cancer.

Determination of aminophylline based on fluorescence quenching of amino-functionalized graphene quantum dots induced by photoilluminated riboflavin-aminophylline system

A new method based on fluorescence spectroscopy for the sensitive determination of aminophylline (AP), an antiasthmatic drug, was developed in this work. Amino-functionalized graphene quantum dots (afGQDs) were synthesized based on a two-step method and they were characterized by transmission electron microscope, UV-vis absorption spectrum and infrared spectrum. The fluorescence of afGQDs was quenched by riboflavin (Rf) via both dynamic quenching and inner filter effect. Photoilluminated Rf-AP system in the presence of oxygen produced hydroxyl radicals (OH). The latter accepted electrons from afGQDs owing to a photo-induced electron transfer process and led to the further fluorescence decline. The changing extent of the fluorescence intensity was found to be proportional to the concentration of AP in the range of 0.10-10 μg mL^-1 and the limit of detection arrived at 40 ng mL^-1. The proposed method was successfully employed for the determination of AP in a pharmaceutical sample and the recovery rate varied in the range of 99%-106%.

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.

Polyethylene Glycol6000/carbon Nanodots as Fluorescent Bioimaging Agents

Photoluminescent nanomaterials have immense potential for use in biological systems due to their excellent fluorescent properties and small size. Traditional semiconductor quantum dots are heavy-metal-based and can be highly toxic to living organisms, besides their poor photostability and low biocompatibility. Nano-sized carbon quantum dots and their surface-modified counterparts have shown improved characteristics for imaging purposes. We used 1,3, 6-trinitropyrene (TNP) and polyethylene glycol6000 (PEG6000) in a hydrothermal method to prepare functional polyethylene glycol6000/carbon nanodots (PEG6000/CDs) and analyzed their potential in fluorescent staining of different types of bacteria. Our results demonstrated that PEG6000/CDs stained the cell pole and septa of gram-positive bacteria B. Subtilis and B. thuringiensis but not those of gram-negative bacteria. The optimal concentration of these composite nanodots was approximately 100 ppm and exposure times varied across different bacteria. The PEG6000/CD composite had better photostability and higher resistance to photobleaching than the commercially available FM4-64. They could emit two wavelengths (red and green) when exposed to two different wavelengths. Therefore, they may be applicable as bioimaging molecules. They can also be used for differentiating different types of bacteria owing to their ability to differentially stain gram-positive and gram-negative bacteria.

Molecularly imprinted polymer containing fluorescent graphene quantum dots as a new fluorescent nanosensor for detection of methamphetamine

A novel fluorescent nanosensor based on graphene quantum dots embedded within molecularly imprinted polymer (GQDs@MIP) was developed for detection and determination of methamphetamine (METH). The resulting GQDs@MIP nanocomposite exhibited higher methamphetamine selectivity in comparison with corresponding non-imprinted polymer (GQDs@NIP). Characterization of the GQDs@MIP nanocomposite was done by nitrogen adsorption and desorption analysis (BET method), transmission electron microscopy (TEM), photoluminescence (PL), ultraviolet-visible (UV-Vis), and Fourier transform infrared (FT-IR) spectroscopies. The fluorescence intensity of GQDs@MIP was efficiently quenched in the presence of methamphetamine template molecules while no quenching was observed in the presence of other analytes such as amphetamine, ibuprofen, codeine, and morphine. Using this method, the detection limit of 1.7 μg/L was obtained for methamphetamine determination.

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.

One-step preparation of single-layered graphene quantum dots for the detection of Fe3

Single-layer graphene quantum dots are highly desirable while their facile and controllable preparations remain challenging. Herein, single-layered graphene quantum dots (sl-GQDs) were developed via a facile one-step hydrothermal synthesis, with citric acid and β-cyclodextrin (CD) as starting materials. The sl-GQDs decorated with CD molecules emit green fluorescence with a quantum yield of 5.34%, and exhibit a good response exclusively to ferric ions for their structural oxygenous groups. The linear range of the proposed sensor for ferric ions was found in a wide concentration range of 0-85 μM. The detection limit is about 0.26 μM. The sl-GQDs based sensing platform also demonstrates its feasibility in real water sample analysis with recoveries of 93.8%-101.5%.

Quantum Yield Enhancement in Graphene Quantum Dots via Esterification with Benzyl Alcohol

The quantum yield of graphene quantum dots was enhanced by restriction of the rotation and vibration of surface functional groups on the edges of the graphene quantum dots via esterification with benzyl alcohol; this enhancement is crucial for the widespread application of graphene quantum dots in light-harvesting devices and optoelectronics. The obtained graphene quantum dots with highly graphene-stacked structures are understood to participate in π-π interactions with adjacent aromatic rings of the benzylic ester on the edges of the graphene quantum dots, thus impeding the nonradiative recombination process in graphene quantum dots. Furthermore, the crude graphene quantum dots were in a gel-like solid form and showed white luminescence under blue light illumination. Our results show the potential for improving the photophysical properties of nanomaterials, such as the quantum yield and band-gap energy for emission, by controlling the functional groups on the surface of graphene quantum dots through an organic modification approach.

Excited States and Optical Properties of Hydrogen-Passivated Rectangular Graphenes: A Computational Study

In this paper, we perform large-scale electron-correlated calculations of optoelectronic properties of rectangular graphene-like polycyclic aromatic hydrocarbon molecules. Theoretical methodology employed in this work is based upon Pariser-Parr-Pople (PPP) π-electron model Hamiltonian, which includes long-range electron-electron interactions. Electron-correlation effects were incorporated using multi-reference singles-doubles configurationinteraction (MRSDCI) method, and the ground and excited state wave functions thus obtained were employed to calculate the linear optical absorption spectra of these molecules, within the electric-dipole approximation. As far as the ground state wave functions of these molecules are concerned, we find that with the increasing size, they develop a strong diradical open-shell character. Our results on optical absorption spectra are in very good agreement with the available experimental results, outlining the importance of electron-correlation effects in accurate description of the excited states. In addition to the optical gap, spin gap of each molecule was also computed using the same methodology. Calculated spin gaps exhibit a decreasing trend with the increasing sizes of the molecules, suggesting that the infinite graphene has a vanishing spin gap.

Graphene quantum dots: Synthesis, characterization, cell viability, genotoxicity for biomedical applications

We report the synthesis and applications of a novel N-doped graphene quantum dots (GQDs) using hydrothermal reaction between citric acid and p-aminophenol. The synthesized N-doped GQDs have been characterized physico-chemically and evaluated its antioxidant, antimicrobial, DNA binding and cleavage activities. siRNA loading studies were performed and their effects on cells were evaluated. Obtained results indicate that monodisperse solution of N-doped GQDs has been obtained with particles size ca. ∼10.9 ± 1.3 nm. UV–Vis spectroscopy studies of the interactions between the N-doped GQDs and calf thymus DNA (CT-DNA) showed that the compound interact with CT-DNA via both intercalative and electrostatic binding. The DNA cleavage study showed that the N-doped GQDs cleaved DNA without any external agents. The antioxidant activity of N-doped GQDS was very active when compared to BHT. As the concentration of the compound increased, the antioxidant activity also increased. Cell viability assay demonstrated that the Ndoped GQDs showed cell viability (70%) when the concentration reached 200 μg/mL for A549 and also MDA-MB-231, 150 μg/mL for NIH-3T3 cell lines at 24 h incubation. N-doped GQDs were coated with Eudragit RS 100 and EphA2-siRNA was loaded. As a result of the studies on these formulations, it was concluded that there may be significant effects on A549 cells. The microscopy results revealed that N-doped GQDs was quickly internalized into the cell. Our novel N-doped-GQDs with siRNA are candidate for in situ tumor suppression via DNA and mRNA breakage.

Graphene quantum dots unraveling: Green synthesis, characterization, radiolabeling with 99mTc, in vivo behavior and mutagenicity

Graphene is one of the crystalline forms of carbon, along with diamond, graphite, carbon nanotubes, and fullerenes, and is considered as a revolutionary and innovating product. The use of a graphene-based nanolabels is one of the latest and most prominent application of graphene, especially in the field of diagnosis and, recently, in loco radiotherapy when coupled with radioisotopes. However, its biological behavior and mutagenicity in different cell or animal models, as well as the in vivo functional activities, are still unrevealed. In this study we have developed by a green route of synthesizing graphene quantum dots (GQDs) and characterized them. We have also developed a methodology for direct radiolabeling of GQDs with radioisotopes.Finally; we have evaluated in vivo biological behavior of GQDs using two different mice models and tested in vitro mutagenicity of GQDs. The results have shown that GQDs were formed with a size range of 160-280 nm, which was confirmed by DRX and Raman spectroscopy analysis, corroborating that the green synthesis is an alternative, environmentally friendly way to produce graphene. The radiolabeling test has shown that stable radiolabeled GQDs can be produced with a high yield (>90%). The in vivo test has demonstrated a ubiquitous behavior when administered to healthy animals, with a high uptake by liver (>26%) and small intestine (>25%). Otherwise, in an inflammation/VEGF hyperexpression animal model (endometriosis), a very peculiar behavior of GQDs was observed, with a high uptake by kidneys (over 85%). The mutagenicity test has demonstrated A:T to G:C substitutions suggesting that GQDs exhibits mutagenic activity.

Biomolecule-derived quantum dots for sustainable optoelectronics

The diverse chemical functionalities and wide availability of biomolecules make them essential and cost-effective resources for the fabrication of zero-dimensional quantum dots (QDs, also known as bio-dots) with extraordinary properties, such as high photoluminescence quantum yield, tunable emission, photo and chemical stability, excellent aqueous solubility, scalability, and biocompatibility. The additional advantages of scalability, tunable optical features and presence of heteroatoms make them suitable alternatives to conventional metal-based semiconductor QDs in the field of bioimaging, biosensing, drug delivery, solar cells, photocatalysis, and light-emitting devices. Furthermore, a recent focus of the scientific community has been on QD-based sustainable optoelectronics due to the primary concern of partially mitigating the current energy demand without affecting the environment. Hence, it is noteworthy to focus on the sustainable optoelectronic applications of biomolecule-derived QDs, which have tunable optical features, biocompatibility and the scope of scalability. This review addresses the recent advances in the synthesis, properties, and optoelectronic applications of biomolecule-derived QDs (especially, carbon- and graphene-based QDs (C-QDs and G-QDs, respectively)) and discloses their merits and disadvantages, challenges and future prospects in the field of sustainable optoelectronics. In brief, the current review focuses on two major issues: (i) the advantages of two families of carbon nanomaterials (i.e. C-QDs and G-QDs) derived from biomolecules of various categories, for instance (a) plant extracts including fruits, flowers, leaves, seeds, peels, and vegetables; (b) simple sugars and polysaccharides; (c) different amino acids and proteins; (d) nucleic acids, bacteria and fungi; and (e) biomasses and their waste and (ii) their applications as light-emitting diodes (LEDs), display systems, solar cells, photocatalysts and photo detectors. This review will not only bring a new paradigm towards the construction of advanced, sustainable and environment-friendly optoelectronic devices using natural resources and waste, but also provides critical insights to inspire researchers ranging from material chemists and chemical engineers to biotechnologists to search for exciting developments of this field and consequently make an advance step towards future bio-optoelectronics.

In Vivo and In Vitro Monitoring of Amyloid Aggregation via BSA@FGQDs Multimodal Probe

Early detection of peptide aggregate intermediates is quite challenging because of their variable and complex nature as well as due to lack of reliable sensors for diagnosis. Herein, we report the detection of monomers and oligomers using specified fluorescence and a magnetic resonance imaging (MRI) multimodal probe based on bovine-serum-albumin-capped fluorine functionalized graphene quantum dots (BSA@FGQDs). This probe enables in vitro fluorescence-based monitoring of human islet amyloid polypeptide (hIAPP), insulin, and amyloid β_(1-42) (Aβ₄₂) monomers and oligomers during the fibrillogenesis dynamic. Up to 90% fluorescence quenching of BSA@FGQDs probe upon addition of amyloid monomers/oligomers was observed due to static quenching and nonradiative energy transfer. Moreover, the BSA@FGQDs probe shows 10 times higher signals in detecting amyloid intermediates and fibrils than that of conventional thioflavin dye. A negative Δ G° value (-36.21 kJ/mol) indicates spontaneous interaction of probe with the peptide. These interactions are hydrogen bonding and hydrophobic as proved by thermodynamic parameters. Visual binding clues of BSA@FGQDs with different morphological states of amyloid protein was achieved through electron microscopy. Furthermore, intravenous and intracranial injection of BSA@FGQDs probe in Alzheimer model mice brain enabled in vivo detection of amyloid plaques in live mice brain by ¹⁹F MRI through contrast enhancement. Our proposed probe not only effectively monitors in vitro fibrillation kinetics of number of amyloid proteins with higher sensitivity and specificity than thioflavin dye, but also, the presence of a ¹⁹F center makes BSA@FGQDs an effective probe as a noninvasive and nonradiative in vivo detection probe for amyloid plaques.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

Highly selective naked eye detection of vitamin B1 in the presence of other vitamins using graphene quantum dots capped gold nanoparticles

The application of Au-GQDs towards the sensitive determination of thiamine was demonstrated.

Boron-doped graphene quantum dots: an efficient photoanode for a dye sensitized solar cell

A cost effective boron doped graphene quantum dot from boron carbide graphene by microwave reactor assisted process that can tune the properties of metal oxides for DSSC application is proposed by D. K. Pattanayak, S. Kundu, T. N. Narayanan and co-workers.

Mint leaf derived carbon dots for dual analyte detection of Fe(iii) and ascorbic acid

Highly luminescent carbon dots (CDs) are obtained from mint leaves adopting a simple and cost effective route devoid of additional chemical reagents and functionalization.