Xuejiao Zhou

Photo-Fenton Reaction of Graphene Oxide: A New Strategy to Prepare Graphene Quantum Dots for DNA Cleavage

Graphene quantum dots (GQDs) are great promising in various applications owing to the quantum confinement and edge effects in addition to their intrinsic properties of graphene, but the preparation of the GQDs in bulk scale is challenging. We demonstrated in this work that the micrometer sized graphene oxide (GO) sheets could react with Fenton reagent (Fe(2+)/Fe(3+)/H(2)O(2)) efficiently under an UV irradiation, and, as a result, the GQDs with periphery carboxylic groups could be generated with mass scale production. Through a variety of techniques including atomic force microscopy, X-ray photoelectron spectroscopy, gas chromatography, ultraperformance liquid chromatography-mass spectrometry, and total organic carbon measurement, the mechanism of the photo-Fenton reaction of GO was elucidated. The photo-Fenton reaction of GO was initiated at the carbon atoms connected with the oxygen containing groups, and C-C bonds were broken subsequently, therefore, the reaction rate depends strongly on the oxidization extent of the GO. Given the simple and efficient nature of the photo-Fenton reaction of GO, this method should provide a new strategy to prepare GQDs in mass scale. As a proof-of-concept experiment, the novel DNA cleavage system using as-generated GQDs was constructed.

Assembly of Graphene Oxide–Enzyme Conjugates through Hydrophobic Interaction

Biochemical and biomedical applications of graphene oxide (GO) critically rely on the interaction of biomolecules with it. It has been previously reported that the biological activity of the GO-enzyme conjugate decreases due to electrostatic interaction between the enzymes and GO. Herein, the immobilization of horseradish peroxidase (HRP) and oxalate oxidase (OxOx) on chemically reduced graphene oxide (CRGO) are reported. The enzymes can be adsorbed onto CRGO directly with a tenfold higher enzyme loading than that on GO, and maximum enzyme loadings reach 1.3 and 12 mg mg(-1) for HRP and OxOx, respectively. Significantly, the more CRGO is reduced, the higher the enzyme loading. The CRGO-HRP conjugates also exhibit higher enzyme activity and stability than GO-HRP. Excellent properties of the CRGO-enzyme conjugates are attributed to hydrophobic interaction between the enzymes and the CRGO. The hydrophobic interaction mode of the CRGO-enzyme conjugates can be applied to other hydrophobic proteins, and thus could dramatically improve the performance of immobilized proteins. The results indicate that CRGO is a potential substrate for efficient enzyme immobilization, and is an ideal candidate as a macromolecule carrier and biosensor.

Fluorescent aptamer-functionalized graphene oxide biosensor for label-free detection of mercury(II)

Label-free fluorescent detection of Hg(2+) has been realized via quenching of fluorescence of graphene oxide (GO). The water-soluble GO sheets, which are functionalized with single-stranded DNA aptamer, exhibit strong fluorescence emission at 600 nm under the excitation of 488 nm in the absence of Hg(2+) ions. When Hg(2+) ions appear in the aqueous solution, Hg(2+) ions are sandwiched between the hairpin-shaped double-stranded DNA due to the formation of the thymine-Hg(2+)-thymine complex, which holds the Hg(2+) ions in proximity to the surface of GO sheets. As a result, the fluorescence emission of GO is quenched. The present GO-based sensor shows a limit of detection as low as 0.92 nM and excellent selectivity toward Hg(2+) over a wide range of metal ions. The present work indicates that GO is a promising fluorescent probe for detection of metal ions and biomolecules.

Fingerprinting photoluminescence of functional groups in graphene oxide

Chemically modified graphene oxide (GO) sheets exhibit three “fingerprinting” photoluminescent (PL) peaks, which originate from the σ* → n, π* → π and π* → n electronic transitions between the antibonding and the bonding molecular orbitals. The three PL peaks are associated with the C–OH, the aromatic CC and the CO functional groups in the GO sheets, respectively. The relative intensities of the three PL peaks are modulated by varying the oxygen-containing functional groups. The three PL emission peaks exhibit a red-shift with an increase in the excitation wavelength. The difference between the emission peak and the excitation wavelength shows a constant Stokes shift of 53.3 nm, 112.1 nm and 217.9 nm for the σ* → n, π* → π and π* → n transitions, respectively.

Insight into the Cellular Internalization and Cytotoxicity of Graphene Quantum Dots

Graphene quantum dots (GQDs), owing to their unique morphology, ultra-small lateral sizes, and exceptional properties, hold great promise for many applications, especially in the biomedical field. In this work, the cellular internalization, distribution, and cytotoxicity of the GQDs are explored complementarily using transmission electron microscopy, confocal laser scanning microscopy, UV-vis, and fluorescence spectroscopies, and flow cytometry with human gastric cancer MGC-803 and breast cancer MCF-7 cells. It is demonstrated that the GQDs are internalized primarily through caveolae-mediated endocytosis. The effects of GQDs on the cell viability, internal cellular reactive oxygen species (ROS) level, mitochondrial membranes potential, and cell cycles show that the cytotoxicity of GQDs is lower than that of the micrometer-sized graphene oxide (GO). The low cytotoxicity and size consistence render GQDs appropriate for biomedical application.

DNA cleavage system of nanosized graphene oxide sheets and copper ions.

The exploration of efficient DNA intercalative agents (intercalators) is essential for understanding DNA scission, repair, and signal transduction. In this work, we explored systematically the graphene oxide (GO) interaction with DNA molecules using fluorescence spectroscopic (FL) and circular dichroism (CD) studies, gel electrophoresis, and DNA thermal denaturation. We demonstrated that the GO nanosheets could intercalate efficiently into DNA molecules. Significantly, we illustrated that the scission of DNA by GO sheets combining with copper ions could take place pronouncedly. The scission of DNA by the GO/Cu(2+) system is critically dependent on the concentrations of GO and Cu(2+) and their ratio. DNA cleavage ability exhibited by the GO with several other metal ions and the fact that GO/Cu(2+)-cleaved DNA fragments can be partially relegated suggest that the mechanism of DNA cleavage by the GO/metal ion system is oxidative and hydrolytic. The result reveals that the GO/Cu(2+) could be used as a DNA cleaving system that should find many practical applications in biotechnology and as therapeutic agents.

Enhancing Cell Nucleus Accumulation and DNA Cleavage Activity of Anti-Cancer Drug via Graphene Quantum Dots

Graphene quantum dots (GQDs) maintain the intrinsic layered structural motif of graphene but with smaller lateral size and abundant periphery carboxylic groups, and are more compatible with biological system, thus are promising nanomaterials for therapeutic applications. Here we show that GQDs have a superb ability in drug delivery and anti-cancer activity boost without any pre-modification due to their unique structural properties. They could efficiently deliver doxorubicin (DOX) to the nucleus through DOX/GQD conjugates, because the conjugates assume different cellular and nuclear internalization pathways comparing to free DOX. Also, the conjugates could enhance DNA cleavage activity of DOX markedly. This enhancement combining with efficient nuclear delivery improved cytotoxicity of DOX dramatically. Furthermore, the DOX/GQD conjugates could also increase the nuclear uptake and cytotoxicity of DOX to drug-resistant cancer cells indicating that the conjugates may be capable to increase chemotherapy efficacy of anti-cancer drugs that are suboptimal due to the drug resistance.

Individual nanocomposite sheets of chemically reduced graphene oxide and poly(N-vinyl pyrrolidone): preparation and humidity sensing characteristics

Individual nanocomposite sheets of chemically reduced graphene oxide (CRG) and poly(N-vinyl pyrrolidone) (PVP), namely CRG/PVP, have been fabricated through a simple one-pot procedure. The structure and composition of the as-prepared CRG/PVP sheets were complementarily characterized using solid-state 13C NMR, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and other spectroscopic measurements, demonstrating that the PVP molecules were chemically grafted on the CRG surfaces. The electrical conductivity of the individual CRG/PVP sheets was measured at different levels of relative humidity (RH) using a conductive atomic force microscopy (CAFM) system, revealing that the electrical conductivity of a CRG/PVP sheet is sensitive to RH variation with a response time of a few seconds. Given the easy mass scale production and improved electrical conductivity, we envisage that the CRG/PVP nanocomposite sheets should have a broad spectrum of applications in electrical conductivity based sensors.

Control on the formation of Fe3O4 nanoparticles on chemically reduced graphene oxide surfaces

Fe3O4 nanoparticles were generated on chemically reduced graphene oxide (CRG) surfaces through a controlled hydrothermal procedure. The as-synthesised nanoscaled Fe3O4/CRG composites were characterized complementarily using scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, and Raman spectroscopy. It was illustrated that the initial ratio of Fe2+/GO, the pH of reaction solution, and the hydrothermal reaction time all affect the morphology of the nanoscaled Fe3O4 in the composites. The electrical and magnetic characteristics of Fe3O4/CRG composite were studied systematically. The results indicate that the GO sheets were reduced by Fe(OH)2 and their electronic conjugation states were restored readily during the hydrothermal process. This might be developed as a scalable route to prepare magnetic Fe3O4/CRG nanocomposites.

Stabilization and Induction of Oligonucleotide i-Motif Structure via Graphene Quantum Dots

DNA i-motif structures have been found in telomeric, centromeric DNA and many in the promoter region of oncogenes; thus they might be attractive targets for gene-regulation processes and anticancer therapeutics. We demonstrate in this work that i-motif structures can be stabilized by graphene quantum dots (GQDs) under acidic conditions, and more importantly GQDs can promote the formation of the i-motif structure under alkaline or physiological conditions. We illustrate that the GQDs stabilize the i-motif structure through end-stacking of the bases at its loop regions, thus reducing its solvent-accessible area. Under physiological or alkaline conditions, the end-stacking of GQDs on the unfolded structure shifts the equilibrium between the i-motif and unfolded structure toward the i-motif structure, thus promoting its formation. The possibility of fine-tuning the stability of the i-motif and inducing its formation would make GQDs useful in gene regulation and oligonucleotide-based therapeutics.