Graphene quantum dots in analytical science

S,N co-doped graphene quantum dots-induced ascorbic acid fluorescent sensor: Design, characterization and performance

In this work, new detection route for ascorbic acid was designed. First, highly luminescent sulfur and nitrogen doped graphene quantum dots (S,N-GQDs) were prepared via simple hydrothermal method using citric acid and thiourea as the C, N and S sources respectively. The prepared S,N-GQDs are characterized by XRD, HRTEM, FTIR, EDS and PL. Investigations showed that prepared S,N-GQDs have a good photostability and excitation-dependent emission fluorescence. Prepared S,N-GQDs showed maximum excitation wavelength and emission wavelength at 400 and 462 nm, respectively. In the following, prepared S,N-GQDs were applied as a photoluminescence probe for detection of ascorbic acid (AA). The designed sensor was based on "off-on" detection mode. The developed sensor had a linear response to AA over a concentration range of 10-500 μM with a detection limit of 1.2 μM. The regression equation is Y = 0.0014 X + 1.2036, where Y and X denote the fluorescence peak intensity and AA concentration, respectively.

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.

A Highly Efficient Chemiluminescence System Based on an Enhancing Effect of Ag Nanoclusters/Graphene Quantum Dots Mixture for Ultrasensitive Detection of Rabeprazole

Herein, an efficient chemiluminescence (CL) reaction with a high emission intensity is reported based on a synergistic improving effect of silver nanoclusters (AgNCs) and graphene quantum dots (GQDs). First, the syntheses of AgNCs and GQDs were simply performed by the chemical reducing of AgNO₃ and a thermal treatment of glucose, respectively. After the characterization steps, the beneficial behavior of the prepared nanomaterial was investigated in CL systems. The oxidation reaction of KMnO₄-rhodamine B produced weak CL emission. However, the presence of AgNCs and GQDs led to a synergetic enhancing effect, and thus higher emission was obtained. A possible mechanism was investigated for this effect using absorption and fluorescence experiments. Furthermore, rabeprazole showed a relatively selective enhancing impact on the CL emission. The CL intensity was linearly increased in the rabeprazole concentration range of 4 - 133 ng mL^-1 with a detection limit (3S_b/m) of 1.1 ng mL^-1. The developed CL method was utilized for the measurement of Rbp in biological samples with acceptable precision and accuracy.

New carbon dots based on glycerol and urea and its application in the determination of tetracycline in urine samples

The current study proposes a fast one-pot microwave assisted synthesis of new carbon dots (CDs) based on glycerol and urea. The novel carbon nanoparticles (GUCDs) have been appropriately characterized and exhibited good luminescent properties with a quantum yield of about 9.8%. Interestingly, the GUCDs are able to selectively interact with tetracycline class antibiotics, which produce a decrease in the native fluorescence of the CDs. On the base of these features, a new analytical method has been developed for the determination of tetracycline. The proposed method has shown satisfactory analytical parameters, such as good linearity range -between 0.5 and 25 μM (R² = 0.999₇)- and an acceptable detection limit (165 nM). Moreover, the new method has been successfully applied for tetracycline determination in urine samples with good recoveries (94.7-103%) and precision (4.6 RSD%).

Ultrasensitive chemiluminescence assay for cimetidine detection based on the synergistic improving effect of Au nanoclusters and graphene quantum dots

A novel and sensitive chemiluminescence (CL) procedure based on the synergetic catalytic effects of gold nanoclusters (Au NCs) and graphene quantum dots (GQDs) was developed for the reliable measurement of cimetidine (CM). The initial experiments showed that the KMnO₄ -based oxidation of alkaline rhodamine B (RhoB) generated a very weak CL emission, which was intensively enhanced in the simultaneous presence of Au NCs and GQDs. CL intermediates can be adsorbed and gathered on the surface of Au NCs, becoming more stable. GQDs participate in the energy transferring processes and facilitate them. These improving effects were simultaneously obtained by adding both Au NCs and GQDs into the RhoB-KMnO₄ reaction. Consequently, the increasing effect of the Au NCs/GQDs mixture was more than that of pure Au NCs or GQDs, and a new nano-assisted powerful CL system was achieved. Furthermore, a marked quenching in the emission of the introduced CL system was observed in the presence of CM, so the system was examined to design a sensitive sensor for CM. After optimization of influencing parameters, the linear lessening in CL emission intensity of KMnO₄ -RhoB-Au NCs/GQDs was verified for CM concentrations in the range 0.8-200 ng ml^-1 . The limit of detection (3S_b /m) was 0.3 ng ml^-1 . Despite being a simple CL method, good sensitivity was obtained for CM detection with reliable results for CM determination in human urine samples.

Advances in Molecularly Imprinting Technology for Bioanalytical Applications

In recent years, along with the rapid development of relevant biological fields, there has been a tremendous motivation to combine molecular imprinting technology (MIT) with biosensing. In this situation, bioprobes and biosensors based on molecularly imprinted polymers (MIPs) have emerged as a reliable candidate for a comprehensive range of applications, from biomolecule detection to drug tracking. Unlike their precursors such as classic immunosensors based on antibody binding and natural receptor elements, MIPs create complementary cavities with stronger binding affinity, while their intrinsic artificial polymers facilitate their use in harsh environments. The major objective of this work is to review recent MIP bioprobes and biosensors, especially those used for biomolecules and drugs. In this review, MIP bioprobes and biosensors are categorized by sensing method, including optical sensing, electrochemical sensing, gravimetric sensing and magnetic sensing, respectively. The working mechanism(s) of each sensing method are thoroughly discussed. Moreover, this work aims to present the cutting-edge structures and modifiers offering higher properties and performances, and clearly point out recent efforts dedicated to introduce multi-sensing and multi-functional MIP bioprobes and biosensors applicable to interdisciplinary fields.

Photophysical properties and photodynamic therapy activity of ameso-tetra(4-carboxyphenyl)porphyrin tetramethyl ester–graphene quantum dot conjugate

Ameso-tetra(4-carboxyphenyl)porphyrin tetramethyl ester and the Zn and GaCl derivatives were π–π stacked with graphene quantum dots to form conjugates and their photophysical and photodynamic therapy properties were investigated.

An rGQD/chitosan nanocomposite-based pH-sensitive probe: application to sensing in urease activity assays

We used the intriguing pH-responsive protonation/deprotonation transitions of chitosan and the fluorescence properties of reduced graphene quantum dots to design a novel pH probe and realize the real-time monitoring of urease activity.

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.

Graphene quantum dots as nanoprobes for fluorescent detection of propofol in emulsions

We report a new fluorescent detection method for propofol based on graphene quantum dots (GQDs). Citric acid (CA) was selected as the carbon precursor, and fluorescent GQDs were prepared by carbonizing CA. The product, which efficiently quenched the fluorescence of GQDs, could be obtained through the oxidation of propofol in the presence of horseradish peroxidase and hydrogen peroxide. The fluorescence intensity ratio of GQDs (F/F ₀) was positively correlated with the concentration of propofol, which ranged within 5.34-89.07 mg l^-1, the limit of detection was 0.5 mg l^-1 and the limit of quantity was 5.34 mg l^-1. The developed fluorescence method reported in the present study is simple, sensitive, reproducible, and can serve in determining propofol contents in emulsions.

Specific quantification of atropine using molecularly imprinted polymer on graphene quantum dots

Herein, development of a reliable and specific fluorometric assay was disclosed for the sensitive detection of atropine. The method was designed using the surface molecularly imprinted polymer on high fluorescent graphene quantum dots (GQDs). Molecularly imprinted polymer capped GQDs (MIP-GQDs) were prepared through the common co-polymerization reaction of 3-(3-aminopropyl) triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS), act as the main functional and cross-linking monomers, respectively. The used template for this reaction was atropine. The created blue luminescent MIP-GQDs composite, which had a great affinity to adsorb atropine from the sample solution, could lead to a notable fluorescence quenching. In fact, GQDs act as the recognizing antenna for adsorbed atropine into the specific MIP sites. The linear association between the observed quenching effect and atropine concentration was exploited to design a selective assay to the detection of atropine. After optimization process, a linear calibration graph was achieved in the atropine concentration range of 0.5-300 ng mL^-1 with a detection limit of 0.22 ng mL^-1. Exploitation of high specific MIP technique along with high fluorescent GQDs provided a highly selective and sensitive assay for atropine as a model analyte. It was adequately utilized for the analysis of atropine in biological samples.

Microwave-assisted synthesis of thymine-functionalized graphitic carbon nitride quantum dots as a fluorescent nanoprobe for mercury(II)

A microwave-assisted hydrothermal method was employed to prepare thymine-modified graphitic carbon nitride quantum dots (T-gCNQDs) which are shown to be a novel fluorescent nanoprobe for Hg(II). They exhibit excellent optical properties (blue emission with a fluorescence quantum yield of 46%) and water solubility. The incorporation of thymine into the gCNQDs results in an enhancement in photoluminescence properties. It is found that fluorescence, best measured at excitation/emission wavelengths of 350/445 nm, is much more strongly quenched by Hg(II) compared to the thymine-free nanoprobe. The quenching is highly selective even in the presence other metal ions. This is ascribed to the formation of T-Hg(II)-T base complexes. Fluorescence drops linearly in the 1.0 to 500 nM Hg(II) concentration range, and the limit of detection is 0.15 nM. The method was applied to the determination of Hg(II) in spiked samples of tap and pond water. Recoveries were found to be >95%, thus demonstrating the practical applicability of the assay. Graphical abstract A microwave-assisted hydrothermal route was employed to prepare thymine-functionalized graphitic carbon nitride QDs (T-gCNQDs). A selective fluorescence quenching mechanism occurred between T-gCNQDs and Hg(II) due to thymine functionalization. T-gCNQDs was utilized to detect Hg(II) in real samples.

Electrochemiluminescent aptasensor for thrombin using nitrogen-doped graphene quantum dots

An electrochemiluminescent (ECL) aptamer based method is described for the determination of thrombin. Three-dimensional nitrogen-doped graphene oxide (3D-NGO) was placed on a glassy carbon electrode (GCE) to provide an electrode surface that displays excellent electrical conductivity and acts as a strong emitter of ECL. The modified electrode was further coated with chitosan via electrodeposition. Finally, the amino-modified aptamer was immobilized on the modified GCE. The interaction between thrombin and aptamer results in a decrease in ECL. The assay has a linear response in the 1 fM to 1 nM thrombin concentration range and a 0.25 fM lower detection limit (at an S/N ratio of 3). The method was applied to the determination of thrombin in spiked human plasma samples, and recoveries ranged between 94 and 105% (with RSDs of <3.6%). The calibration plot was recorded at potential and wavelength of fluorescence emission (wavelength: 445 nm; potential: 0 to -2 V). Graphical abstract A bare glassy carbon electrode (GCE) does not display electrochemiluminescence (ECL). If, however, nitrogen-doped graphene quantum dots, chitosan, and three-dimensional nitrogen-doped graphene oxide (NGQD-chitosan/3D-NGO) are electrodeposited on the GCE, strong ECL can be observed. The ECL intensity decreased after aptamer and bovine serum albumin (BSA) were dropped onto the electrode (curve a). However, the ECL further decreases after addition of thrombin (TB; curve b).

Graphene Quantum Dot-Aerogel: From Nanoscopic to Macroscopic Fluorescent Materials. Sensing Polyaromatic Compounds in Water

Fluorescence based on quantum confinement is a property restricted to the nanoscopic range. The incorporation of nanoparticles in a three-dimensional polymeric network could afford macroscopic scaffolds that show nanoscopic properties. Moreover, if these scaffolds are based on strong bonds, the stability of the resulting materials can be preserved, thus enhancing their final applications. We report for the first time the preparation of a graphene quantum dot (GQD) composite based on a cationic covalent network. This new material has unusual features: (i) the final composite remains stable after several swelling-deswelling cycles, thus demonstrating strong interactions between GQDs and the polymeric material, and therefore it could be used as a portable system. (ii) Fluorescence emission in the composite and in solution is quasi-independent to the excitation wavelength. (iii) However, and in contrast to the behavior observed in GQD solutions, the fluorescence of the composite remains unaltered over a wide pH range and in the presence of different ions commonly found in tap water. (iv) Fluorescence quenching is only observed as a consequence of molecules that bear aromatic systems, and this could be applied to the preparation of in situ water sensors.

Graphene Oxide Quantum Dots Exfoliated From Carbon Fibers by Microwave Irradiation: Two Photoluminescence Centers and Self-Assembly Behavior

Graphene oxide quantum dots (GOQDs) attract great attention for their unique properties and promising application potential. The difficulty in the formation of a confined structure, and the numerous and diverse oxygen-containing functional groups results in a low emission yield to GOQDs. Here, GOQDs with a size of about 5 nm, exfoliated from carbon fibers by microwave irradiation, are detected and analyzed. The exfoliated GOQDs are deeply oxidized and induce large numbers of epoxy groups and ether bonds, but only a small amount of carbonyl groups and hydroxyl groups. The subdomains of sp² clusters, involving epoxy groups and ether bonds, are responsible for the two strong photoluminescence emissions of GOQDs under different excitation wavelengths. Moreover, GOQDs tend to self-assemble at the edges of their planes to form self-assembly films (SAFs) with the evaporation of water. SAFs can further assemble into different 3D patterns with unique microstructures such as sponge bulk, sponge ball, microsheet, sisal, and schistose coral, which are what applications such as supercapacitors, cells, catalysts, and electrochemical sensors need. This method for preparation of GOQDs is easy, quick, and environmentally friendly, and this work may open up new research interests about GOQDs.

Investigation of Sensitivities and Drift Effects of the Arrayed Flexible Chloride Sensor Based on RuO₂/GO at Different Temperatures

We investigate the temperature effect on sensing characteristics and drift effect of an arrayed flexible ruthenium dioxide (RuO₂)/graphene oxide (GO) chloride sensor at different solution temperatures between 10 °C and 50 °C. The average sensor sensitivities according to our experimental results were 28.2 ± 1.4 mV/pCl (10 °C), 42.5 ± 2.0 mV/pCl (20 °C), 47.1 ± 1.8 mV/pCl (30 °C), 54.1 ± 2.01 mV/pCl (40 °C) and 46.6 ± 2.1 mV/pCl (50 °C). We found the drift effects of an arrayed flexible RuO₂/GO chloride sensor in a 1 M NaCl solution to be between 8.2 mV/h and 2.5 mV/h with solution temperatures from 10 °C to 50 °C.

Progress in utilisation of graphene for electrochemical biosensors

This review discusses recent graphene (GR) electrochemical biosensor for accurate detection of biomolecules, including glucose, hydrogen peroxide, dopamine, ascorbic acid, uric acid, nicotinamide adenine dinucleotide, DNA, metals and immunosensor through effective immobilization of enzymes, including glucose oxidase, horseradish peroxidase, and haemoglobin. GR-based biosensors exhibited remarkable performance with high sensitivities, wide linear detection ranges, low detection limits, and long-term stabilities. Future challenges for the field include miniaturising biosensors and simplifying mass production are discussed.

A versatile platform for the highly efficient preparation of graphene quantum dots: photoluminescence emission and hydrophilicity-hydrophobicity regulation and organelle imaging

In this work, a versatile platform for the highly efficient preparation of graphene quantum dots (GQDs) with diverse properties was developed. First of all, an excess amount of oxidants and an additional high temperature step of the Hummers' method for the synthesis of graphene oxide (GO) was applied to obtain nanosized graphene oxide (NGO). Then, high quality GQDs (quantum yields up to 18.2%) with different photoluminescence emission wavelengths, adjustable hydrophilicity-hydrophobicity, and selective cell organelle imaging capacity can be facilely achieved through a one-pot hydrothermal reaction between the NGO and ammonia, fatty primary amines, or amino-substituted organelle targetable compounds, respectively. The superior features of the as-developed method are extremely high conversion ratio (ca. 60 wt% from graphite to the functional GQDs) and great expandability. Such a high conversion ratio is deemed to be due to effectively decreasing aggregation of the NGO (in comparison with GO) during the post-treatment process. This work provides a robust strategy for the highly efficient preparation of GQDs with diverse properties and functions, and is believed to be beneficial for boosting their applications in the future.

One-pot synthesis of graphene quantum dots and simultaneous nanostructured self-assembly via a novel microwave-assisted method: impact on triazine removal and efficiency monitoring

Graphene quantum dot (GQDs) assemblies from a one-step microwave reaction as bifunctional materials in remediation of triazines.

Nanospace-confined preparation of uniform nitrogen-doped graphene quantum dots for highly selective fluorescence dual-function determination of Fe3+ and ascorbic acid

N-Doped graphene quantum dots (N-GQDs) combine the advantages of N-doped carbon and quantum dot materials, displaying enhanced performance in electrocatalysis, drug delivery, sensing and so on. In this work, novel hydrotropic N-GQDs with controlled size are obtained for the first time via a nanospace-confined preparation strategy, in which HNO₃ vapour serves as scissors for quickly cutting the N-doped carbon nanolayer in the confined nanospace of reusable mesoporous molecular sieves. The as-prepared N-GQDs exhibit a uniform lateral size of about 2.4 nm, high photostability and yellow fluorescence, which is strongly quenched upon addition of ferric ions due to the coordination between ferric ions and N/O-rich groups of the N-GQDs surface. Significantly, the fluorescence response to Fe³⁺ is linear in the 0.5 to 40 μM concentration range and the N-GQDs showed good selectivity and satisfying recovery for ferric ion detection in tap water. Noteworthily, the quenched fluorescence by Fe³⁺ can be recovered by adding ascorbic acid (AA), which efficiently destroyed the coordination between Fe³⁺ and N-GQDs. Based on this principle, the N-GQDs were used to successfully construct an AA sensor, exhibiting a wide linearity range (between 0.5 and 90 μM) with a low detection of limit (80 nM at S/N = 3) and better selectivity towards AA compared with other common physiological substances. Finally, the constructed fluorescence sensor was employed successfully for AA determination in fish blood with satisfactory recovery ranging from 95.3 to 106.2%. The results indicate that N-GQDs synthesized by the nanospace-confined strategy are promising in biosensor fabrication.

Nanospace-confined synthesis of N-GQDs was successfully achieved via a mesoporous silica recycling and vapor cutting route for label-free fluorescence dual-function detection of Fe³⁺ and AA.

Dual mode detection of amifostine based on gold nanoparticles and sulfanilic acid functionalized graphene quantum dots

Visual and fluorescence detection of amifostine based on the inner filter effect of gold nanoparticles on sulfanilic acid functionalized graphene quantum dots.

Analytical Nanoscience and Nanotechnology: Where we are and where we are heading

The main aim of this paper is to offer an objective and critical overview of the situation and trends in Analytical Nanoscience and Nanotechnology (AN&N), which is an important break point in the evolution of Analytical Chemistry in the XXI century as they were computers and instruments in the second half of XX century. The first part of this overview is devoted to provide a general approach to AN&N by describing the state of the art of this recent topic, being the importance of it also emphasized. Secondly, particular but very relevant trends in this topic are outlined: the analysis of the nanoworld, the so "third way" in AN&N, the growing importance of bioanalysis, the evaluation of both nanosensors and nanosorbents, the impact of AN&N in bioimaging and in nanotoxicological studies, as well as the crucial importance of reliability of the nanotechnological processes and results for solving real analytical problems in the frame of Social Responsibility (SR) of science and technology. Several reflections are included at the end of this overview written as a bird's eye view, which is not an easy task for experts in AN&N.

Fe3O4/hydroxyapatite/graphene quantum dots as a novel nano-sorbent for preconcentration of copper residue in Thai food ingredients: Optimization of ultrasound-assisted magnetic solid phase extraction

Fe₃O₄/hydroxyapatite/graphene quantum dots (Fe₃O₄/HAP/GQDs) nanocomposite was synthesized and used as a novel magnetic adsorbent. This nanocomposite was characterized using scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and magnetization property. The Fe₃O₄/HAP/GQDs was applied to pre-concentrate copper residues in Thai food ingredients (so-called "Tom Yum Kung") prior to determination by inductively coupled plasma-atomic emission spectrometry. Based on ultrasound-assisted extraction optimization, various parameters affecting the magnetic solid-phase extraction, such as solution pH, amount of magnetic nanoparticles, adsorption and desorption time, and type of elution solvent and its concentration were evaluated. Under optimal conditions, the linear range was 0.05-1500ngmL^-1 (R²>0.999), limit of detection was 0.58ngmL^-1, and limit of quantification was 1.94ngmL^-1. The precision, expressed as the relative standard deviation of the calibration curve slope (n=5), for intra-day and inter-day analyses was 0.87% and 4.47%, respectively. The recovery study of Cu for real samples was ranged between 83.5% and 104.8%. This approach gave the enrichment factor of 39.2, which guarantees trace analysis of Cu residues. Therefore, Fe₃O₄/HAP/GQDs can be a potential and suitable candidate for the pre-concentration and separation of Cu from food samples. It can easily be reused after treatment with deionized water.

Graphene quantum dots coordinated to mercaptopyridine-substituted phthalocyanines: Characterization and application as fluorescence "turn ON" nanoprobes

This study reports on the design of novel nanoconjugates of graphene quantum dots (GQDs) and tetra or octa-mercaptopyridine-substituted zinc and aluminium phthalocyanines (Pcs) deployed as fluorescence "turn ON" nanoprobes. The phthalocyanines were separately adsorbed onto the planar structure of graphene quantum dots (GQDs) via π-π stacking interaction to form GQDs-mercaptopyridine Pcs nanoconjugates. The quaternized Pc complexes could also interact with the GQDs through electrostatic attraction due to the positive charges on the Pcs ring substituents and the negative charges on the surface of GQDs. The fluorescence emission of the GQDs was quenched upon coordination to the respective Pcs. However, the fluorescence emission was "turned ON" in the presence of Hg²⁺ employed as a test analyte. The mechanism of the "turn ON" of the GQDs emission in the nanoconjugates is ascribed to the strong affinity of Hg²⁺ to bind with the bridging sulfur on the Pcs periphery thereby disrupting the π-π stacking interaction between the GQDs and the Pcs with a consequent "turn ON" of the coordinated GQDs' fluorescence.

In-situ Evidence of the Redox-State Dependence of Photoluminescence in Graphene Quantum Dots

Changes in the optical properties of graphene quantum dots (GQD) during electrochemical reduction and oxidation were investigated by photoluminescence (PL) spectroelectrochemistry, which provided direct in situ evidence of the dependence of GQD luminescence on their redox state. We demonstrated that GQD PL intensity was enhanced upon reduction (quantum yield increased from 0.44 to 0.55) and substantially bleached during oxidation (quantum yield ∼0.12). Moreover, PL emission blue/red-shifted upon GQD reduction/oxidation, rendering information about electronic transitions involved in the redox processes, namely, the π → π* and the n → π* transitions between energy levels of the aromatic sp² domains and the functional groups, respectively. PL intensity changes during GQD reduction/oxidation resulted from a variation in structural changes in GQD as a result of charge injection, as corroborated by in situ Raman spectroelectrochemistry.

Application of graphene quantum dots functionalized with thymine and thymine-appended zinc phthalocyanine as novel photoluminescent nanoprobes

Thymine Zn phthalocyanine turned off the fluorescence of graphene quantum dots, which was subsequently turned on by Hg²⁺ for its sensitive detection.

Graphene Quantum Dot-MnO2 Nanosheet Based Optical Sensing Platform: A Sensitive Fluorescence "Turn Off-On" Nanosensor for Glutathione Detection and Intracellular Imaging

Glutathione (GSH) monitoring has attracted extensive attention because it serves a vital role in human pathologies. Herein, a convenient fluorescence "turn off-on" nanosensor based on graphene quantum dots (GQDs)-manganese dioxide (MnO2) nanosheet has been designed for selective detection of GSH in living cells. The fluorescence intensity of GQDs can be quenched by MnO2 nanosheets via a fluorescence resonance energy transfer. However, GSH can reduce MnO2 nanosheets to Mn(2+) cations and release GQDs, causing sufficient recovery of fluorescent signal. The MnO2 nanosheets serve as both fluorescence nanoquencher and GSH recognizer in the sensing platform. The sensing platform displayed a sensitive response to GSH in the range of 0.5-10 μmol L(-1), with a detection limit of 150 nmol L(-1). Furthermore, the chemical response of the GQDs-MnO2 nanoprobe exhibits high selectivity toward GSH over other electrolytes and biomolecules. Most importantly, the promising platform was successfully applied in monitoring the intracellular GSH in living cells, indicating its great potential to be used in disease diagnosis. Meanwhile, this GQDs-MnO2 platform is also generalizable and can be easily expanded to the detection and imaging of other reactive species in living cells.

Free-Radical-Assisted Rapid Synthesis of Graphene Quantum Dots and Their Oxidizability Studies

This work reports a modified electrochemical method for rapid and large-scale preparing graphene quantum dots (GQDs) by introduction of active free radicals, which were produced by hydrogen peroxide or ultraviolet radiation. These free radicals can deepen the oxidized or reduced level of working electrode in electrochemical process and thus lead to GQDs with high concentration and small size, but different surface oxidized degree. The improved oxidation and reduction mechanism were analyzed in this work. Meanwhile, the optical properties and oxidizability of GQDs with different surface oxidized degree were investigated. It is found that these GQDs can be used as an oxidizing agent and their oxidizability is related to the degree being oxidized.

Fluorescent nanoprobes for sensing and imaging of metal ions: recent advances and future perspectives

Recent advances in nanoscale science and technology have generated nanomaterials with unique optical properties. Over the past decade, numerous fluorescent nanoprobes have been developed for highly sensitive and selective sensing and imaging of metal ions, both in vitro and in vivo. In this review, we provide an overview of the recent development of the design and optical properties of the different classes of fluorescent nanoprobes based on noble metal nanomaterials, upconversion nanoparticles, semiconductor quantum dots, and carbon-based nanomaterials. We further detail their application in the detection and quantification of metal ions for environmental monitoring, food safety, medical diagnostics, as well as their use in biomedical imaging in living cells and animals.