Graphene quantum dots: emergent nanolights for bioimaging, sensors, catalysis and photovoltaic devices

Similar to the popular older cousins, luminescent carbon dots (C-dots), graphene quantum dots or graphene quantum discs (GQDs) have generated enormous excitement because of their superiority in chemical inertness, biocompatibility and low toxicity. Besides, GQDs, consisting of a single atomic layer of nano-sized graphite, have the excellent performances of graphene, such as high surface area, large diameter and better surface grafting using π-π conjugation and surface groups. Because of the structure of graphene, GQDs have some other special physical properties. Therefore, studies on GQDs in aspects of chemistry, physical, materials, biology and interdisciplinary science have been in full flow in the past decade. In this Feature Article, recent developments in preparation of GQDs are discussed, focusing on the main two approaches (top-down and bottom-down). Emphasis is given to their future and potential development in bioimaging, electrochemical biosensors and catalysis, and specifically in photovoltaic devices that can solve increasingly serious energy problems.     

Strongly green-photoluminescent graphene quantum dots for bioimaging applications

Strongly fluorescent graphene quantum dots (GQDs) have been prepared by one-step solvothermal method with PL quantum yield as high as 11.4%. The GQDs have high stability and can be dissolved in most polar solvents. Because of fine biocompatibility and low toxicity, GQDs are demonstrated to be excellent bioimaging agents.     

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).

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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).

     

Computational characterization of organic photovoltaic devices

We present recent progresses on applying the theoretical models and computational tools in assessing the performance of organic solar cells, especially the bulk heterojunction solar cells. Both the continuum device model and the dynamic Monte Carlo model are developed to investigate the photocurrent-voltage characteristics based on the exciton and charge carrier dynamics. Insights into key factors influencing the organic photovoltaic performances have been gained from these studies.     

S-doped graphene quantum dots as nanophotocatalyst for visible light degradation

S-doped graphene quantum dots (S-GQDs) with well crystallization and monodispersity were prepared and applied as novel nanophotocatalyst for visible light degradation of basic fuchsin.     

Random benzotrithiophene-based donor-acceptor copolymers for efficient organic photovoltaic devices

A series of benzotrithiophene-containing random terpolymers for polymer solar cells is reported. Through variations of the two other components in the terpolymers, the absorption profile and the frontier energy levels are optimized and maximum power conversion efficiencies are nearly doubled (5.14%) relative to the parent alternating copolymer.     

Nitrogen-doped graphene quantum dots with oxygen-rich functional groups

Graphene quantum dots (GQDs) represent a new class of quantum dots with unique properties. Doping GQDs with heteroatoms provides an attractive means of effectively tuning their intrinsic properties and exploiting new phenomena for advanced device applications. Herein we report a simple electrochemical approach to luminescent and electrocatalytically active nitrogen-doped GQDs (N-GQDs) with oxygen-rich functional groups. Unlike their N-free counterparts, the newly produced N-GQDs with a N/C atomic ratio of ca. 4.3% emit blue luminescence and possess an electrocatalytic activity comparable to that of a commercially available Pt/C catalyst for the oxygen reduction reaction (ORR) in an alkaline medium. In addition to their use as metal-free ORR catalysts in fuel cells, the superior luminescence characteristic of N-GQDs allows them to be used for biomedical imaging and other optoelectronic applications.     

FRET-based modified graphene quantum dots for direct trypsin quantification in urine

A versatile nanoprobe was developed for trypsin quantification with fluorescence resonance energy transfer (FRET). Here, fluorescence graphene quantum dot is utilized as a donor while a well-designed coumarin derivative, CMR2, as an acceptor. Moreover, bovine serum albumin (BSA), as a protein model, is not only served as a linker for the FRET pair, but also a fluorescence enhancer of the quantum dots and CMR2. In the presence of trypsin, the FRET system would be destroyed when the BSA is digested by trypsin. Thus, the emission peak of the donor is regenerated and the ratio of emission peak of donor/emission peak of acceptor increased. By the ratiometric measurement of these two emission peaks, trypsin content could be determined. The detection limit of trypsin was found to be 0.7 μg/mL, which is 0.008-fold of the average trypsin level in acute pancreatitis patient's urine suggesting a high potential for fast and low cost clinical screening.     

Preparation and biodistribution of 131I-labeled graphene quantum dots

Uranium sorption from commercial phosphoric acid was carried out onto kaolinite and metakaolinite. The different factors affecting uranium adsorption have been investigated. The obtained results show that, the sorption process is applicable for the high strength phosphoric acid, 40% P₂O₅, and diluted phosphoric acid, 20% P₂O₅, but it preferred to the diluted phosphoric acid. The kinetic models applied to the sorption rate data were evaluated for Lagergren first order and the pseudo second order models. From the results, the uranium sorption from commercial phosphoric acid, 40% P₂O₅, onto kaolinite and metakaolinite was found to occur through physical sorption process.     

氮掺杂石墨烯量子点用于定量分析牛血清白蛋白的含量

合成了氮掺杂石墨烯量子点,并基于茜素红-氮掺杂石墨烯量子点之间的相互作用形成氢键复合物,茜素红可以对所合成的氮掺杂石墨烯量子点产生明显的荧光猝灭作用(荧光关),氮掺杂石墨烯量子点荧光强度的变化(F0/F)与茜素红浓度(2.78~23.59 nmol/L)具有良好的线性关系,检出限为1.24 nmol/L;继续向该溶液中加入牛血清白蛋白,会使原已发生荧光猝灭的氮掺杂石墨烯量子点溶液的荧光发射强度得以恢复(荧光开),且荧光发射强度的恢复与牛血清白蛋白浓度(0.1~0.375 g/L)之间具有良好的线性关系,检出限为0.011 g/L。此外,该"关-开"荧光检测体系被用来定量分析人尿液中的牛血清白蛋白含量,方法已用于实际尿液样品的定量分析。     

Facile preparation and upconversion luminescence of graphene quantum dots

A facile hydrazine hydrate reduction of graphene oxide (GO) with surface-passivated by a polyethylene glycol (PEG) method for the fabrication of graphene quantum dots (GQDs) with frequency upconverted emission is presented. And we speculate on the upconversion luminescence due to the anti-Stokes photoluminescence (ASPL), where the δE between the π and σ orbitals is near 1.1 eV.     

Fluorescent determination of graphene quantum dots in water samples

This work presents a simple, fast and sensitive method for the preconcentration and quantification of graphene quantum dots (GQDs) in aqueous samples. GQDs are considered an object of analysis (analyte) not an analytical tool which is the most frequent situation in Analytical Nanoscience and Nanotechnology. This approach is based on the preconcentration of graphene quantum dots on an anion exchange sorbent by solid phase extraction and their subsequent elution prior fluorimetric analysis of the solution containing graphene quantum dots. Parameters of the extraction procedure such as sample volume, type of solvent, sample pH, sample flow rate and elution conditions were investigated in order to achieve extraction efficiency. The limits of detection and quantification were 7.5 μg L⁻¹ and 25 μg L⁻¹, respectively. The precision for 200 μg L⁻¹, expressed as %RSD, was 2.8%. Recoveries percentages between 86.9 and 103.9% were obtained for two different concentration levels. Interferences from other nanoparticles were studied and no significant changes were observed at the concentration levels tested. Consequently, the optimized procedure has great potential to be applied to the determination of graphene quantum dots at trace levels in drinking and environmental waters.     

Flexible organic photovoltaic devices based on oligothiophene derivatives

For the purpose of developing flexible organic photovoltaic devices, we have fabricated two flexible devices using 5-formyl- 2,2′：5′,2″：5″,2′″-quaterthiophene （4T-CHO）, 5-formyl-2,2′：5′, 2″：5″,2′″：5′″,2″″-quinquethiophene （5T-CHO） and 3,4,9,10-perylenetertracarboxylic dianhydride （PTCDA）. The PET-ITO/4T-CHO/PTCDA/A1 device has an open circuit voltage （Voc） of 1.56 V, photoelectric conversion efficiency of 0.77%. The PET-ITO/5T-CHO/PTCDA/A1 device has a Voc of 1.70 V, photoelectric conversion efficiency of 0.84%. The two flexible devices have high Voc （1.56 and 1.70 V）. It is possible that intermolecular hydrogen bonding between -CHO group of nT-CHO and carboxylic dianhydride of PTCDA contributes to enhancing the efficiency by promoting interfacial electron transfer and eliminating the subconducting band trap sites.     

Conjugation of quantum dots with graphene for fluorescence imaging of live cells

It is difficult to achieve fluorescent graphene-quantum dots (QDs) conjugation because graphene quenches the fluorescence of the QDs. In the present study, the conjugation of graphene (reduced graphene oxide, RGO) with QDs via a bridge of bovine serum albumin (BSA) provides a novel highly fluorescent nano probe for the first time. BSA capped QDs are firmly grafted onto polyethylenimine (PEI)/poly(sodium 4-styrenesulfonate) (PSS) coated RGO (graphene-QDs) via electrostatic layer by layer assembly. The strong luminescence of the graphene-QDs provides a potential for non-invasive optical in vitro imaging. The graphene-QDs are used for in vitro imaging of live human carcinoma (Hela) cells. Graphene-QDs could be readily up-taken by Hela cells in the absence of specific targeting molecules, e.g., antibodies or folic acid, and no in vitro cytotoxicity is observed at 360 μg mL(-1) of the graphene-QDs. The results for the imaging of live cells indicated that the cell-penetrating graphene-QDs could be a promising nano probe for intracellular imaging and therapeutic applications.     

Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology

Multicolor photoluminescent graphene quantum dots (GQDs) with a uniform size of ～60 nm diameter and 2-3 nm thickness were prepared by using unsubstituted hexa-peri-hexabenzocoronene as the carbon source. This result offers a new strategy to fabricate monodispersed GQDs with well-defined morphology.     

Lateral flow devices for nucleic acid analysis exploiting quantum dots as reporters

There is a growing interest in the development of biosensors in the form of simple lateral flow devices that enable visual detection of nucleic acid sequences while eliminating several steps required for pipetting, incubation and washing out the excess of reactants. In this work, we present the first dipstick-type nucleic acid biosensors based on quantum dots (QDs) as reporters. The biosensors enable sequence confirmation of the target DNA by hybridization and simple visual detection of the emitted fluorescence under a UV lamp. The ‘diagnostic’ membrane of the biosensor contains a test zone (TZ) and a control zone (CZ). The CZ always fluoresces in order to confirm the proper function of the biosensor. Fluorescence is emitted from the TZ, only when the specific nucleic acid sequence is present. We have developed two general types of QD-based nucleic acid biosensors, namely, Type I and Type II, in which the TZ consists of either immobilized streptavidin (Type I) or immobilized oligodeoxynucleotides (Type II). The control zone consists of immobilized biotinylated albumin. No purification steps are required prior to the application of the DNA sample on the strip. The QD-based nucleic acid biosensors performed accurately and reproducibly when applied to (a) the visual detection of PCR amplification products and (b) visual genotyping of single nucleotide polymorphisms (SNPs) in human genomic DNA from clinical samples. As low as 1.5 fmol of double-stranded DNA were clearly detected by naked eye and the dynamic range extended to 200 fmol. The %CV were estimated to be 4.3–8.2.     

Interfacial modification of organic photovoltaic devices by molecular self-organization

This feature article focuses on the relationship between the interfacial structures constructed by molecular self-organization and the properties of organic photovoltaic devices. The use of self-assembled monolayers (SAMs) is reviewed for metal and metal oxide/organic interfaces, while surface-segregated monolayers (SSMs) are introduced as a new method for the modification of organic/organic interfaces. Research up to now has clearly demonstrated the effectiveness of the control of energy levels and other properties at the interfaces to enhance photovoltaic performance. The possibility of more precise control of the interfacial structures is also discussed.     

Stamping transfer of a quantum dot interlayer for organic photovoltaic cells

An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the QD solution. The coverage of QD particles on the active layer was verified by TEM analysis and fluorescence images. After insertion of the QD interlayer between the active layer and metal cathode, the photovoltaic performances of the organic solar cell were clearly enhanced. By ultraviolet photoelectron spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer reduces charge recombination by blocking the holes moving to the cathode from the active layer and facilitating efficient collection of the electrons from the active layer to the cathode.     

Functionalized N-doped graphene quantum dots for electrochemical determination of cholesterol through host-guest inclusion

Functionalized nitrogen-doped graphene quantum dots (N-GQD) with mean particle size of 8.5?±?0.5 nm were covalently linked to β-cyclodextrin (β-CD) to form a β-CD@N-GQD nanoprobe. The probe is shown to enable voltammetric determination of cholesterol via selective host-guest recognition and by using ferrocene (FC) as the redox indicator. FC is first included in β-cyclodextrin. Cholesterol has a higher affinity for β-CD (in comparison to FC). It forms a strong inclusion complex with β-CD and can replace FC from its cavities. The quantity of released FC is proportional to the concentration of cholesterol. The differential pulse voltammetric signal for FC (with a peak at typically 0.22 V vs Ag/AgCl) increases linearly in the 0.5–100 μM cholesterol concentration range, with a limit of detection as low as 80 nM. The assay is found to be highly selective over 15 potentially interfering species. The method was successfully applied to the detection of cholesterol in spiked serum samples which gave recoveries between 96 and 101%. The probe can be stored for at least 28 days after which the activity still is 87%.

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Graphical abstract This scheme illustrates the detection of cholesterol by differential pulse voltammetry (DPV) technique. The β-cyclodextrin functionalized nitrogen-doped graphene quantum dot (β-CD@N-GQD) probe was developed to enable voltammetric determination of cholesterol using selective host-guest recognition.

     

131I-labeled PEG and folic acid co-functionalized graphene quantum dots for tumor-targeted imaging

Journal of Radioanalytical and Nuclear Chemistry - In this study, we successfully synthesized polyethylene glycol (PEG) and folic acid (FA) co-functionalized graphene quantum dots (GQDs) to improve...