Graphene Quantum Dot as a Green and Facile Sensor for Free Chlorine in Drinking Water

Zinc-Dithizone Complex Engineered Upconverting Nanosensors for the Detection of Hypochlorite in Living Cells

Current chemo/biosensors for hypochlorous acid or hypochlorite detections are usually limited to the submicromolar level because of their insufficient sensitivity, which is a problem because the concentrations in biological matrices is generally on the nanomolar scale or even lower. Developing a probe with a high enough sensitivity remains a challenge. Using the minimal background fluorescence of upconversion nanocrystals to our advantage, we herein report on an energy-transfer mechanism-based upconversion luminescent nanosensor for the sensitive and selective detection of hypochlorite in aqueous solution. In this nanosensor water-dispersible upconversion nanoparticles act as the energy donor and a novel hypochlorite-responsive coordination complex Zn(DZ)3 is employed as the energy acceptor. The quenched upconversion luminescence, induced by the Zn(DZ)3 complex, can be efficiently recovered after addition of hypochlorite through the selective oxidative breakage of the Zn-S-C bonds in the Zn(DZ)3 complex, which was verified by mass spectrometry. The detection limit for hypochlorite of this sensing system is as low as 3 nM. Furthermore, this newly coordination-complex engineered upconversion nanosensor is successfully applied to image different amounts of exogenous hypochlorite in living HeLa cells.

A new mild, clean and highly efficient method for the preparation of graphene quantum dots without by-products

We demonstrated that graphene oxide (GO) can be oxidized and cut into graphene quantum dots (GQDs) by hydroxyl radicals (˙OH), which is obtained by the catalytic decomposition of hydrogen peroxide (H₂O₂) with a tungsten oxide nanowire (W₁₈O₄₉) catalyst. The clean oxidizing agent (H₂O₂) and the solid catalyst lead to a simple GQD preparing method without any by-products. The obtained GQD aqueous solution can be directly applied to fluorescence imaging in vitro without any further purification. The effect of the W₁₈O₄₉ catalyst on the ˙OH formation is discussed, and the size of GQDs can be controlled via changing the concentration of hydroxyl radicals.

Fabrication of graphene oxide decorated with nitrogen-doped graphene quantum dots and its enhanced electrochemiluminescence for ultrasensitive detection of pentachlorophenol

Nitrogen-doped graphene quantum dots (NGQDs), as a new class of quantum dots, have potential applications in fuel cells and optoelectronics fields due to their electrocatalytic activity, tunable luminescence and biocompatibility. Herein, a facile hydrothermal approach for cutting nitrogen-doped graphene into NGQDs has been proposed for the first time. The resulting NGQDs were homogeneously modified onto the surface of graphene oxide (GO) to form NGQDs-GO nanocomposites. Compared with NGQDs, the as-prepared NGQDs-GO nanocomposites exhibited excellent electrochemiluminescence (ECL) performances including 3.8-fold enhancement of ECL intensity and a decrease by 200 mV of the ECL onset potential, which are ascribed to the introduction of GO. Based on the selective inhibitory effect of pentachlorophenol (PCP) on the ECL intensity of the NGQDs-GO system, a novel ECL sensor for PCP concentration determination was constructed, with a wide linear response ranging from 0.1 to 10 pg mL(-1) and a detection limit of 0.03 pg mL(-1). The practicability of the sensing platform in real water samples showed satisfactory results, which could open the possibility of using NGQDs-based nanocomposites in the electroanalytical field.

Graphene Quantum Dot Layers with Energy-Down-Shift Effect on Crystalline-Silicon Solar Cells

Graphene quantum dot (GQD) layers were deposited as an energy-down-shift layer on crystalline-silicon solar cell surfaces by kinetic spraying of GQD suspensions. A supersonic air jet was used to accelerate the GQDs onto the surfaces. Here, we report the coating results on a silicon substrate and the GQDs' application as an energy-down-shift layer in crystalline-silicon solar cells, which enhanced the power conversion efficiency (PCE). GQD layers deposited at nozzle scan speeds of 40, 30, 20, and 10 mm/s were evaluated after they were used to fabricate crystalline-silicon solar cells; the results indicate that GQDs play an important role in increasing the optical absorptivity of the cells. The short-circuit current density was enhanced by about 2.94% (0.9 mA/cm(2)) at 30 mm/s. Compared to a reference device without a GQD energy-down-shift layer, the PCE of p-type silicon solar cells was improved by 2.7% (0.4 percentage points).

Natural carbon-based dots from humic substances

For the first time, abundant natural carbon-based dots were found and studied in humic substances (HS). Four soluble HS including three humic acids (HA) from different sources and one fulvic acids (FA) were synthetically studied. Investigation results indicate that all the four HS contain large quantities of Carbon-based dots. Carbon-based dots are mainly small-sized graphene oxide nano-sheets or oxygen-containing functional group-modified graphene nano-sheets with heights less than 1 nm and lateral sizes less than 100 nm. Carbon-based nanomaterials not only contain abundant sp2-clusters but also a large quantity of surface states, exhibiting unique optical and electric properties, such as excitation-dependent fluorescence, surface states-originated electrochemiluminescence, and strong electron paramagnetic resonance. Optical and electric properties of these natural carbon-based dots have no obvious relationship to their morphologies, but affected greatly by their surface states. Carbon-based dots in the three HS have relative high densities of surface states whereas the FA has the lowest density of surface states, resulting in their different fluorescence properties. The finding of carbon-based dots in HS provides us new insight into HS, and the unique optical properties of these natural carbon-based dots may give HS potential applications in areas such as bio-imaging, bio-medicine, sensing and optoelectronics.

Masking agent-free and channel-switch-mode simultaneous sensing of Fe(3+) and Hg(2+) using dual-excitation graphene quantum dots

A novel channel-switch-mode strategy for simultaneous sensing of Fe(3+) and Hg(2+) is developed with dual-excitation single-emission graphene quantum dots (GQDs). By utilizing the dual-channel fluorescence response performance of GQDs, this strategy achieved a facile, low-cost, masking agent-free, quantitative and selective dual-ion assay even in mixed ion samples and practical water samples.

Scale-Up Synthesis of Fragrant Nitrogen-Doped Carbon Dots from Bee Pollens for Bioimaging and Catalysis

Fragrant nitrogen-doped carbon dots of gram scale can be prepared from commercial bee pollens by a hydrothermal process. These carbon dots of 1-2 nm in size show promising applications in cellular imaging and catalysis/photocatalysis.

A sensor based on blue luminescent graphene quantum dots for analysis of a common explosive substance and an industrial intermediate, 2,4,6-trinitrophenol

A rapid and effective sensor for the analysis of nitrophenol-based explosive substances, represented by trinitrophenol (TNP), has been developed with the use of the blue luminescent graphene quantum dots (GQDs); these GQDs are derived from citric acid by a pyrolysis procedure. They emit strong blue fluorescence at 450 nm after excitation at 365 nm, and TNP can quench this fluorescence because a fluorescence resonance energy transfer occurs. The quenching ratio (F0-F)/F0 was related linearly to the concentration of TNP in the range of 0.1-15 μmol L(-1) with a detection limit of 0.091 μmol L(-1) (S/N=3). The developed method exhibits high sensitivity, good linearity and reliable reproducibility for the quantitative analysis of TNP in water samples. The GQDs were used directly without any further treatment or complicated modification.

Electrospinning graphene quantum dots into a nanofibrous membrane for dual-purpose fluorescent and electrochemical biosensors

Graphene quantum dots (GQDs) have become increasingly important for applications in energy materials, optical devices and biosensors. Here we report a facile technique to fabricate a nanofibrous membrane of GQDs by electrospinning water-soluble GQDs with polyvinyl alcohol (PVA) directly. The structure and fluorescence properties of the fabricated PVA/GQD nanofibrous membrane were investigated using scanning and transmission electron microscopy, and fluorescence microscopy. It was found that the electrospun PVA/GQD nanofibrous membrane has a three-dimensional structure with a high surface area to volume ratio, which is beneficial for the adsorption of electrolytes and the diffusion of reactants. For the first time, the created PVA/GQD nanofibrous membrane was utilized to fabricate dual-purpose fluorescent and electrochemical biosensors for highly sensitive determination of hydrogen peroxide (H₂O₂) and glucose. The experimental results indicated that the fluorescence intensity of the nanofibrous membrane decreased linearly with increasing H₂O₂ concentration, because the addition of H₂O₂ leads to fluorescence quenching of the GQDs, which endows the fabricated nanofibrous membrane with fluorescence activity. Besides, after binding glucose oxidase onto the created nanofibrous membrane, the fabricated nanofibrous membrane showed high sensitivity and selectivity for glucose detection. In addition, the PVA/GQD nanofibrous membrane can also be directly electrospun onto an electrode for electrochemical detection of H₂O₂. This novel nanofibrous membrane exhibits excellent catalytic performance and fluorescence activity, and therefore has potential applications for the highly stable, sensitive, and selective detection of H₂O₂ and glucose.

Electrochemical probing of carbon quantum dots: not suitable for a single electrode material

We demonstrate that the easy aggregation, rapid stacking and high oxygen-functional groups of GQDs have a negative impact on the electrochemical properties. GQDs are no better than graphene as an excellent single electrode material.

A Carbon Nanodots-Based Fluorescent Turn-On Probe for Iodide

The human body requires iodine to develop and maintain proper metabolic balance. Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of intellectual disability. Small amounts of iodine are needed for good health. However, large doses can eventually cause iodide goitre, hypothyroidism or myxedema. Children are especially sensitive to the effects of iodine. Because humans can be exposed to iodide via several different food chains, the development of on-site, real-time and reliable sensors for iodide is of great interest to ensure early diagnosis and improve management. We propose here a simple and low cost, yet sensitive and selective fluorescent ‘turn-off-on’ assay for rapid determination of iodide based on a combined carbon nanodots (CDs) and Hg2+ system. The fluorescence of CDs that was quenched by Hg2+ was restored and ‘turned on’ in the presence of iodide, which triggered a competitive reaction among CDs, Hg2+ and iodide. The recovered fluorescence intensity varied linearly with the concentration of iodide in the range of 0.05–5 μmol L–1, with a limit of detection as low as 46 nmol L–1. This approach shows excellent selectivity for iodine over the other anions.

Reduction degree and property study of graphene nanosheets prepared with different reducing agents and their applicability as a carrier of the Ru(phen)3Cl2 luminescent sensor for DNA detection

Recently, graphene nanosheets (GNS) have been widely investigated and used in capacitors, catalysts, biological/chemical sensors, etc.

Nitrogen and sulfur codoped graphene quantum dots as a new fluorescent probe for Au3+ ions in aqueous media

The N and S codoped GQDs can be used as a promising label free fluorescent probe for the sensitive and selective detection of Au³⁺ in aqueous media, which provides a new application of the functionalized GQDs to the detection of metal ions.

Valence-tautomeric infinite coordination polymer nanoparticles for encapsulation of rhodamine B and its potential application for colorimetric and fluorescence dual mode sensing of hypochlorite

The colorimetric and fluorescence ClO⁻ sensing based on the stimulus response of valence-tautomeric RhB@{Co(3,5-dbsq)(3,5-dbcat)(bix)} ICP nanoparticles.

Synthesis of a carbon-dot-based photoluminescent probe for selective and ultrasensitive detection of Hg2+ in water and living cells

Nitrogen and sulphur co-doped carbon dots with high PL quantum yield and photostability have been synthesized by a simple hydrothermal synthesis and successfully used for bioimaging of Hg²⁺ in living cells.

Low temperature synthesis of phosphorous and nitrogen co-doped yellow fluorescent carbon dots for sensing and bioimaging

A simple and high-output strategy for the fabrication of yellow fluorescent phosphorous and nitrogen co-doped carbon dots (P,N-CDs) is developed. P,N-CDs possess distinctive photoluminescence properties, low toxicity, and biocompatible. P,N-CDs are potentially useful for versatile applications such as pH sensing, in vitro and in vivo imaging.

An ionic liquid promoted microwave-hydrothermal route towards highly photoluminescent carbon dots for sensitive and selective detection of iron(iii)

Carbon dots with a high photoluminescence efficiency of ∼22.58% are obtained by a facile microwave-hydrothermal treatment of rice straw with the presence of ionic liquid.

A novel triangular silver nanoprisms-based surface plasmon resonance assay for free chlorine

In this study, a novel assay for the detection of free chlorine is proposed for the first time.

How do nitrogen-doped carbon dots generate from molecular precursors? An investigation of the formation mechanism and a solution-based large-scale synthesis

A solution-based method is developed for rapid and scalable synthesis of highly fluorescent carbon dots in a monoethanolamine system, and an interesting formation process of carbon dots is directly observed.

Oxygen-driven, high-efficiency production of nitrogen-doped carbon dots from alkanolamines and their application for two-photon cellular imaging

A novel oxygen-driven method has been developed for low-cost, large-scale, and high-efficiency production of nitrogen-doped carbon dots (N-C-dots) using various alkanolamines as both precursors and solvents.

Thermal responsive fluorescent nanocomposites based on carbon dots

PNIPAM & CDs composites have been used as a sensor of temperature. At around 32 °C, the fluorescent intensity decrease sharply because of CDs' self-quenching effect.

Sensing applications of luminescent carbon based dots

Carbon based dots (CDs) including carbon quantum dots and graphene quantum dots exhibit unique luminescence properties, such as photoluminescence (PL), chemiluminescence (CL) and electrochemiluminescence (ECL).

A label-free fluorescent assay for free chlorine in drinking water based on protein-stabilized gold nanoclusters

Bovine serum albumin stabilized Au nanoclusters (BSA-AuNCs) were demonstrated as a novel fluorescence probe for sensitive and selective detection of free chlorine in drinking water. The fluorescence of BSA-AuNCs was found to be quenched effectively by the free chlorine, and the decrease in fluorescence intensity of BSA-AuNCs allowed the sensitive detection of free chlorine in the range of 0.8-800 μM. The detection limit is 0.50 μM at a signal-to-noise ratio of 3. The present fluorescent assay for free chlorine possesses low detection limit, wide linear range and good selectivity. Real tap water samples were analyzed with satisfactory results, which suggested its potential for water quality analysis.

Large scale synthesis of graphene quantum dots (GQDs) from waste biomass and their use as an efficient and selective photoluminescence on-off-on probe for Ag(+) ions

Graphene quantum dots (GQDs) are synthesized from bio-waste and are further modified to produce amine-terminated GQDs (Am-GQDs) which have higher dispersibility and photoluminescence intensity than those of GQDs. A strong fluorescence quenching of Am-GQDs (switch-off) is observed for a number of metal ions, but only for the Ag(+) ions is the original fluorescence regenerated (switch-on) upon addition of L-cysteine.

Photoluminescence, chemiluminescence and anodic electrochemiluminescence of hydrazide-modified graphene quantum dots

Single-layer graphene quantum dots (SGQDs) were refluxed with hydrazine (N2H4) to prepare hydrazide-modified SGQDs (HM-SGQDs). Compared with SGQDs, partial oxygen-containing groups have been removed from HM-SGQDs. At the same time, a lot of hydrazide groups have been introduced into HM-SGQDs. The introduced hydrazide groups provide HM-SGQDs with a new kind of surface state, and give HM-SGQDs unique photoluminescence (PL) properties such as blue-shifted PL emission and a relatively high PL quantum yield. More importantly, the hydrazide-modification made HM-SGQDs have abundant luminol-like units. Accordingly, HM-SGQDs exhibit unique and excellent chemiluminescence (CL) and anodic electrochemiluminescence (ECL). The hydrazide groups of HM-SGQDs can be chemically oxidized by the dissolved oxygen (O2) in alkaline solutions, producing a strong CL signal. The CL intensity is mainly dependent on the pH value and the concentration of O2, implying the potential applications of HM-SGQDs in pH and O2 sensors. The hydrazide groups of HM-SGQDs can also be electrochemically oxidized in alkaline solutions, producing a strong anodic ECL signal. The ECL intensity can be enhanced sensitively by hydrogen peroxide (H2O2). The enhanced ECL intensity is proportional to the concentration of H2O2 in a wide range of 3 μM to 500 μM. The detection limit of H2O2 was calculated to be about 0.7 μM. The results suggest the great potential applications of HM-SGQDs in the sensors of H2O2 and bio-molecules that are able to produce H2O2 in the presence of enzymes.

Dopamine fluorescent sensors based on polypyrrole/graphene quantum dots core/shell hybrids

A facilely prepared fluorescent sensor was developed for dopamine (DA) detection with high sensitivity and selectivity based on polypyrrole/graphene quantum dots (PPy/GQDs) core/shell hybrids. The composites exhibit strong fluorescence emission, which is dramatically enhanced as high as three times than pristine GQDs. The prepared sensor allows a highly sensitive determination of DA by fluorescent intensity decreasing with the addition of DA and presents a good linearity in range of 5-8000 nM with the detection limit of 10 pM (S/N = 3). Furthermore, the application of the proposed approach have been demonstrated in real samples and showed promise in diagnostic purposes.

A visible-light-driven composite photocatalyst of TiO2 nanotube arrays and graphene quantum dots

TiO2 nanotube arrays are well-known efficient UV-driven photocatalysts. The incorporation of graphene quantum dots could extend the photo-response of the nanotubes to the visible-light range. Graphene quantum dot-sensitized TiO2 nanotube arrays were synthesized by covalently coupling these two materials. The product was characterized by Fourier-transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and UV-vis absorption spectroscopy. The product exhibited high photocatalytic performance in the photodegradation of methylene blue and enhanced photocurrent under visible light irradiation.

Synthesis-modification integration: one-step fabrication of boronic acid functionalized carbon dots for fluorescent blood sugar sensing

In this paper, we have presented a novel strategy to fabricate fluorescent boronic acid modified carbon dots (C-dots) for nonenzymatic blood glucose sensing applications. The functionalized C-dots are obtained by one-step hydrothermal carbonization, using phenylboronic acid as the sole precursor. Compared with conventional two-step fabrication of nanoparticle-based sensors, the present "synthesis-modification integration" strategy is simpler and more efficient. The added glucose selectively leads to the assembly and fluorescence quenching of the C-dots. Such fluorescence responses can be used for well quantifying glucose in the range of 9-900 μM, which is 10-250 times more sensitive than that of previous boronic acid based fluorescent nanosensing systems. Due to "inert" surface, the C-dots can well resist the interferences from various biomolecules and exhibit excellent selectivity. The proposed sensing system has been successfully used for the assay of glucose in human serum. Due to simplicity and effectivity, it exhibits great promise as a practical platform for blood glucose sensing.