Yuwei Hu

Fluorescence resonance energy transfer quenching at the surface of graphene quantum dots for ultrasensitive detection of TNT

This paper for the first time reports a chemical method to prepare graphene quantum dots (GQDs) from GO. Water soluble and surface unmodified GQDs, serving as a novel, effective and simple fluorescent sensing platform for ultrasensitive detection of 2,4,6-trinitrotoluene (TNT) in solution by fluorescence resonance energy transfer (FRET) quenching. The fluorescent GQDs can specifically bind TNT species by the π-π stacking interaction between GQDs and aromatic rings. The resultant TNT bound at the GQDs surface can strongly suppress the fluorescence emission by the FRET from GQDs donor to the irradiative TNT acceptor through intermolecular polar-polar interactions at spatial proximity. The unmodified GQDs can sensitively detect down to ~0.495 ppm (2.2 μM) TNT with the use of only 1 mL of GQDs solution. The simple FRET-based GQDs reported here exhibit high and stable fluorescence. Eliminating further treatment or modification, this method simplifies and shortens the experimental process. It possesses good assembly flexibility and can thus find many applications in the detection of ultratrace analytes.

Decorated graphene sheets for label-free DNA impedance biosensing

An efficient DNA impedance biosensing platform is constructed, in which positively charged N,N-bis-(1-aminopropyl-3-propylimidazol salt)-3,4,9,10-perylene tetracarboxylic acid diimide (PDI) is anchored to graphene sheets. The π-π stacking and electronic interactions are elucidated by the distinct absorption features in UV-vis spectra and by quenching perylene fluorescence in contact with graphene. The rational design and tailoring of graphene surface invest it with desired properties (dispersive, structural, photoelectrical and conductive, etc.) and boost its application. Electrostatic interaction between PDIs positively charged imidazole rings and negatively charged phosphate backbones of single-stranded DNA (ssDNA) facilitates ssDNA immobilization. This manner is different from these mainly based on the attraction between the rings in DNA bases and the hexagonal cells of graphene, which is disturbed after hybridization and causes the leaving of formed double-stranded DNA from graphene surface. The electrostatic ssDNA grafting occupies phosphate backbones and particularly leaves the bases available for efficient hybridization. DNA immobilization and hybridization lead to PDI/graphene interfacial property changes, which are monitored by electrochemical impedance spectroscopy and adopted as the analytical signal. The conserved sequence of the pol gene of human immunodeficiency virus 1 is satisfactorily detected via this PDI/graphene platform and shows high reproducibility, selectivity.

Hollow flower-like AuPd alloy nanoparticles: One step synthesis, self-assembly on ionic liquid-functionalized graphene, and electrooxidation of formic acid

A novel hollow AuPd (hAuPd) alloy nanostructure with a rough surface was fabricated via a facile one-pot simultaneous reduction of Au(III) and Pd(I) and then assembled on ionic liquid-grafted graphene sheets by electrostatic interaction to form graphene–metal hybrid nanomaterials under mild conditions. The resulting hollow alloy nanostructure and graphene nanocomposites were then characterized using many techniques, such as transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), element analysis mapping, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), confirming that the alloy nanoparticles with hollow cores had been successfully synthesized by one step galvanic replacement and attached firmly onto the graphene sheets. The electrocatalytic ability of the resulting nanocomposites for direct oxidation of formic acid was also explored. The hollow AuPd alloy nanospheres, especially the graphene-supported nanocomposites, exhibited striking electrocatalytic activities which show potential application in fuel cells.

Green-synthesized gold nanoparticles decorated graphene sheets for label-free electrochemical impedance DNA hybridization biosensing

Sensitive electrochemical impedance assay of DNA hybridization by using a novel graphene sheets platform was achieved. The graphene sheets were firstly functionalized with 3,4,9,10-perylene tetracarboxylic acid (PTCA). PTCA molecules separated graphene sheets efficiently and introduced more negatively-charged -COOH sites, both of which were beneficial to the decoration of graphene with gold nanoparticles. Then amine-terminated ionic liquid (NH₂-IL) was applied to the reduction of HAuCl₄ to gold nanoparticles. The green-synthesized gold nanoparticles, with the mean diameter of 3 nm, dispersed uniformly on graphene sheets and its outer layer was positively charged imidazole termini. Due to the presence of large graphene sheets and NH₂-IL protected gold nanoparticles, DNA probes could be immobilized via electrostatic interaction and adsorption effect. Electrochemical impedance value increased after DNA probes immobilization and hybridization, which was adopted as the signal for label-free DNA hybridization detection. Unlike previously anchoring DNA to gold nanoparticles, this label-free method was simple and noninvasive. The conserved sequence of the pol gene of human immunodeficiency virus 1 was satisfactorily detected via this strategy.

Perylene derivative-bridged Au–graphene nanohybrid for label-free HpDNA biosensor

Along with the challenges of wet-chemically preparing graphene-based nanohybrids, for example easy aggregation, low-stability in solution environment and insufficient loading amount, here we report the preparation and application of a type of π-conjugated molecule, perylenetetracarboxylic acid di-imide (PDI)-functionalized graphene material with high density of gold nanoparticles (AuNPs). In this nanohybrid, the PDI molecule comprises five-connected benzene rings and positively charged terminals composed of two symmetrical imidazole rings and amine groups, which offers the intrinsic driving force for π-π interactions with graphene and also serves as the active sites for immobilization of AuNPs. Transmission electron microscopy results demonstrated that AuNPs were uniformly dispersed and densely covered the PDI-functionalized graphene compared to the control experiment without PDI. To prove its biological application, the Au-PDI-graphene nanohybrid was chosen as a sensing material for fabricating a label-free electrochemical impedance hairpin DNA (hpDNA) biosensor for detection of human immunodeficiency virus 1 gene. When hpDNA was hybridized, it exhibited a sensitive electrochemical impedance variation on an Au-PDI-graphene modified electrode. This fabricated hpDNA biosensor reveals a wide linear detection range and a relatively low detection limit. Thanks to its high stability and efficient electrochemical impedance sensitivity, this nanohybrid would offer a broad range of possible DNA sequences for specific applications in biodiagnostics and bionanotechnology.

Direct electron transfer of cytochrome c at mono-dispersed and negatively charged perylene–graphene matrix

Mono-dispersed 3,4,9,10-perylene tetracarboxylic acid (PTCA) functionalized graphene sheets (PTCA-graphene) were fabricated by a chemical route and dispersed well in aqueous solution. PTCA-graphene with plenty of -COOH groups as electrostatic absorbing sites were beneficial to the loading of Cytochrome c (Cyt c). Cyt c, which was tightly immobilized on the PTCA-graphene modified glassy carbon electrode, maintained its natural conformation. Direct electron transfer of Cyt c and the electro-catalytic activity towards the reduction of H2O2 were also achieved. It has been substantiated that PTCA-graphene is a preferable biocompatible matrix for Cyt c.

Perylene ligand wrapping G-quadruplex DNA for label-free fluorescence potassium recognition

A perylene ligand, N,N-bis-(1-aminopropyl-3-propylimidazol salt)-3,4,9,10-perylene tetracarboxylic acid diimide ligand (PDI), which consisted of π-conjugated perylene moiety and hydrophilic side chains with positively charged imidazole rings, was used to wrap G-quadruplex for fluorescence turn-on K(+) recognition. Electrostatic attraction between PDIs positively charged imidazole rings and DNAs negatively charged phosphate backbones enabled PDI to accumulate on DNA. Upon trapping K(+), these G-rich DNA sequences transitioned to G-quadruplex. Subsequently, PDI ligands wrapped G-quadruplex, in which the flat aromatic core of PDI ligand interacted with G-quartet through π-π stacking and the side chains were positioned in grooves through electrostatic interactions. Consequently, the interaction mode change and conformational transition from PDI stacked G-sequence to PDI wrapped G-quadruplex led to PDI fluorescence enhancement, which was readily monitored as the detection signal. This strategy excluded the sequence tagging step and exhibited high selectivity and sensitivity towards K(+) ion with the linear detection range of 10-150 nM. Besides, PDI ligands may hold diagnostic and therapeutic application potentials to human telomere and cancer cells.

Graphene for DNA Biosensing

Graphene (or GO) is an excellent candidate for biomolecules anchoring and detection due to its large surface area (up to 2,630 m2/g) and unique sp2 (sp2/sp3)-bonded network. According to the binding affinity difference between single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) to graphene sheet, GO has been successfully adopted as a platform to discriminate DNA sequences. Fluorescent, electrochemical, electrical, surface-enhanced Raman scattering (SERS) and other methods have been utilized to achieve the sensitive, selective, and accurate DNA recognition. Both theoretical and experimental results illustrate that ssDNA sequences are adsorbed on the surface of graphene sheet with all nucleobases lying nearly flat. DNA or RNA sequencing through graphene nanopore, nanogap, and nanoribbon has also attracted much interest because it is a label-free, amplification-free, and single-molecule approach that can be scaled up for high-throughput DNA or RNA analysis. Meanwhile, miRNA detection is achieved by forming DNA–miRNA duplex helixes, and strong emission is observed due to the poor interaction between the helix and GO.

Graphene for Amino Acid, Peptide, Protein, and Enzyme Detection

The excellent biocompatibility, unique structure, and properties of graphene make it suitable for various biomolecules detection. Theoretical and experimental studies were carried out to illustrate the interaction of graphene with aromatic amino acids, amino acids of peptide, protein, and enzyme. The aromatic rings of these amino acids prefer to orient in parallel with respect to the graphene basal plane, which bears the signature of weak π–π stacking. The polarizability of amino acids may play a critical role in the binding strength with graphene, which has implications toward developing biosensors. For example, immunosensors have gained much attention in many biomedical research and clinical diagnostics, as a promising approach for selective and sensitive analysis. Immunosensors based on specific antigen–antibody recognition show great potentials with above merits and highly designable property in practical applications. In a word, graphene shows its great potentials in simplicity, sensitivity, and selectivity, etc., as the substrate for anchoring other materials or the platform for direct biomolecular detection.

Graphene in Drug Delivery, Cellular Imaging, Bacteria Inhibition, Versatile Targets Bioassays

The evaluation of potential risk of graphene to human body has been carried out due to its exceptional promise in various applications, such as glucose detection, drug delivery, and cellular imaging. Small nanosheets entered cells mainly through clathrin-mediated endocytosis, and the increase in graphene size enhanced phagocytotic uptake of the nanosheets. Cellular imaging and drug delivery have also been achieved by using biocompatible material-grafted graphene platforms. Graphene can also be used in pathogen and bacteria inhibition in the form of graphene nanowalls deposited on stainless steel substrates. Recently, versatile bioassay represents one of the main challenges in bioanalytical applications. The large surface area and universal fluorescence quenching ability of graphene sheets enable it to be an efficient platform for multiplex targets assay in a fluorescent manner. Based on this, multiplex analytes, such as DNA, thrombin, Ag+, Hg2+, cysteine and DNA, thrombin, ATP in logic gate operations, have been successfully detected with high sensitivity and simplicity.