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

Investigation into the fluorescence quenching behaviors and applications of carbon dots

Carbon dots (CDs) are novel fluorescent materials with low toxicity and good biocompatibility. Herein, the collisional/dynamic and photoluminescence (PL) center destruction quenching behaviors of a novel type of CDs were investigated. Moreover, the quenching behaviors of the CDs were exploited in applications. Firstly, dynamic PL quenching was achieved by Fe(3+) ions, which was proved by the Stern-Volmer equation, temperature dependent quenching and fluorescence lifetime measurements. Furthermore, a hemin sensor based on the Fe(3+)/CDs system was achieved. Secondly, quenching induced by PL center destruction was caused by hydroxyl radicals (˙OH), which were produced by high power UV light or the H2O2/Fe(2+) system; thus an H2O2 sensor with a low detection limit (0.9 ppb) was realized. Finally, we assumed that the CDs are really composed of cross-linked molecular clusters, and that the PL centers of the as prepared CDs are certain molecular/chemical groups.

Synthesis of highly fluorescent nitrogen-doped graphene quantum dots for sensitive, label-free detection of Fe (III) in aqueous media

Heteroatom doping can drastically alter the electronic characteristics of graphene quantum dots (GQDs), thus resulting in unusual properties and related applications. Herein, we develop a simple and low-cost synthetic strategy to prepare nitrogen-doped GQDs (N-GQDs) through hydrothermal treatment of GQDs with hydrazine. The obtained N-GQDs with oxygen-rich functional groups exhibit a strong blue emission with 23.3% quantum yield (QY). Compared to GQDs, the N-GQDs exhibit enhanced fluorescence with blue-shifted energy. Due to the selective coordination to Fe(3+), the N-GQDs can be used as a green and facile sensing platform for label-free sensitive and selective detection of Fe (III) ions in aqueous solution and real water samples. The N-GQDs fluorescence probe shows a sensitive response to Fe(3+) in a wide concentration range of 1-1945μM with a detection limit of 90nM (s/N=3). Interestingly, it is also found that both dynamic and static quenching processes occur for the detection of Fe(3+) by N-GQDs, while the quenching effect of Fe(3+) on the fluorescence of GQDs is achieved by affecting the surface states of GQDs.

Graphene quantum dots as fluorescence probes for turn-off sensing of melamine in the presence of Hg(2+)

A rapid and sensitive fluorescence sensing system for melamine based on charge transfer quenching of the fluorescence of graphene quantum dots (GQDs) in the presence of Hg(2+) is proposed. The synthesized GQDs were strongly luminescent with predominantly aromatic sp(2) domains. Melamine could coordinate with mercury through nitrogen atoms in both its amine and triazine groups and bring more Hg(2+) to the surface of GQDs through π-π stacking, thus leading to quenching of the GQDs' fluorescence. The quenching mechanism was investigated in detail and ascribed to charge transfer from the GQDs to Hg(2+) with melamine acting as the linkage agent. The melamine demonstrated a linear range 0.15-20 μM and a detection limit of 0.12 μM, which was far below the regulatory level, suggesting the promising practical usage of this sensing system. This sensing system also possessed high selectivity for melamine in the presence of possible interferences. Finally, this novel sensor was successfully applied for melamine detection in raw milk and satisfactory recovery was achieved.

Nitrogen-doped carbon-based dots prepared by dehydrating EDTA with hot sulfuric acid and their electrocatalysis for oxygen reduction reaction

Nitrogen doped carbon-based dots were synthesized by dehydrating ethylenediaminetetraacetic acid, and were used for electro-catalyzing oxygen reduction.

Green and size-controllable synthesis of photoluminescent carbon nanoparticles from waste plastic bags

Hydrothermal treatment of various waste plastic bags in low-concentration H₂O₂ solutions for green and size-controllable synthesis of photoluminescent carbon nanoparticles.

Direct growth of vertically-oriented graphene for field-effect transistor biosensor

A sensitive and selective field-effect transistor (FET) biosensor is demonstrated using vertically-oriented graphene (VG) sheets labeled with gold nanoparticle (NP)-antibody conjugates. VG sheets are directly grown on the sensor electrode using a plasma-enhanced chemical vapor deposition (PECVD) method and function as the sensing channel. The protein detection is accomplished through measuring changes in the electrical signal from the FET sensor upon the antibody-antigen binding. The novel biosensor with unique graphene morphology shows high sensitivity (down to ~2 ng/ml or 13 pM) and selectivity towards specific proteins. The PECVD growth of VG presents a one-step and reliable approach to prepare graphene-based electronic biosensors.

In vivo biodistribution and toxicology of carboxylated graphene quantum dots

Photoluminescent graphene quantum dots (GQDs) have fascinating optical and electronic properties with numerous promising applications in biomedical engineering. In this work, we first studied the in vivo biodistribution and the potential toxicity of carboxylated photoluminescent GQDs. KB, MDA-MB231, A549 cancer cells, and MDCK normal cell line were chosen as in vitro cell culture models to examine the possible adverse effects of the carboxylated photoluminescent GQDs. The carboxylated GQDs are desirable for increased aqueous solubility. All cancer cells efficiently took up the carboxylated GQDs. No acute toxicity or morphological changes were noted in either system at the tested exposure levels. A long-term in vivo study revealed that the GQDs mainly accumulated in liver, spleen, lung, kidney, and tumor sites after intravenous injection. To reveal any potential toxic effect of the GQDs on treated mice, serum biochemical analysis and histological evaluation were performed. The toxicity results from serum biochemistry and complete blood count study revealed that the GQDs do not cause appreciable toxicity to the treated animals. Finally, we observed no obvious organ damage or lesions for the GQDs treated mice after 21 days of administration at 5 mg/kg or 10 mg/kg dosages. With adequate studies of toxicity, both in vitro and in vivo, photoluminescent GQDs may be considered for biological application.

Fabrication of highly fluorescent graphene quantum dots using L-glutamic acid for in vitro/in vivo imaging and sensing

A facile bottom-up method for the synthesis of highly fluorescent graphene quantum dots (GQDs) has been developed using a one-step pyrolysis of a natural amino acid, L-glutamic acid, with the assistance of a simple heating mantle device. The developed GQDs showed strong blue, green and red luminescence under the irradiation of ultra-violet, blue and green light, respectively. Moreover, the GQDs emitted near-infrared (NIR) fluorescence in the range of 800-850 nm with the excitation-dependent manner. This NIR fluorescence has a large Stokes shift of 455 nm, providing significant advantage for sensitive determination and imaging of biological targets. The fluorescence properties of the GQDs, such as quantum yields, fluorescence life time, and photostability, were measured and the fluorescence quantum yield was as high as 54.5 %. The morphology and composites of the GQDs were characterized using TEM, SEM, EDS, and FT-IR. The feasibility of using the GQDs as a fluorescent biomarker was investigated through in vitro and in vivo fluorescence imaging. The results showed that the GQDs could be a promising candidate for bioimaging. Most importantly, compared to the traditional quantum dots (QDs), the GQDs is chemically inert. Thus, the potential toxicity of the intrinsic heavy metal in the traditional QDs would not be a concern for GQDs. In addition, the GQDs possessed an intrinsic peroxidase-like catalytic activity that was similar to the graphene sheets and carbon nanotubes. Coupled with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), the GQDs can be used for the sensitive detection of hydrogen peroxide with a limit of detection of 20 μM.

Dimension-tailored functional graphene structures for energy conversion and storage

Functional graphene nanostructures of interesting physical and chemical properties have been attracting lots of research effort. In this feature article we focus on some of the recent work of dimension-tailed graphene contributed by us and others, and review the current trends in the tunable and controllable preparation of functionalized graphene architectures ranging from zero-dimensional quantum dots to three-dimensional networks. Additionally, recent progresses in applying these dimension-tailored graphene structures in energy conversion and storage are explicitly discussed, particularly in devices such as solar cells, actuators, fuel cells, supercapacitors, etc., presenting the great prospect of functional graphene structures in this dynamic research field.