Chunhai Fan

Graphene-Based Antibacterial Paper

Graphene is a monolayer of tightly packed carbon atoms that possesses many interesting properties and has numerous exciting applications. In this work, we report the antibacterial activity of two water-dispersible graphene derivatives, graphene oxide (GO) and reduced graphene oxide (rGO) nanosheets. Such graphene-based nanomaterials can effectively inhibit the growth of E. coli bacteria while showing minimal cytotoxicity. We have also demonstrated that macroscopic freestanding GO and rGO paper can be conveniently fabricated from their suspension via simple vacuum filtration. Given the superior antibacterial effect of GO and the fact that GO can be mass-produced and easily processed to make freestanding and flexible paper with low cost, we expect this new carbon nanomaterial may find important environmental and clinical applications.

Electrochemical interrogation of conformational changes as a reagentless method for the sequence-specific detection of DNA

We report a strategy for the reagentless transduction of DNA hybridization into a readily detectable electrochemical signal by means of a conformational change analogous to the optical molecular beacon approach. The strategy involves an electroactive, ferrocene-tagged DNA stem–loop structure that self-assembles onto a gold electrode by means of facile gold-thiol chemistry. Hybridization induces a large conformational change in this surface-confined DNA structure, which in turn significantly alters the electron-transfer tunneling distance between the electrode and the redoxable label. The resulting change in electron transfer efficiency is readily measured by cyclic voltammetry at target DNA concentrations as low as 10 pM. In contrast to existing optical approaches, an electrochemical DNA (E-DNA) sensor built on this strategy can detect femtomoles of target DNA without employing cumbersome and expensive optics, light sources, or photodetectors. In contrast to previously reported electrochemical approaches, the E-DNA sensor achieves this impressive sensitivity without the use of exogenous reagents and without sacrificing selectivity or reusability. The E-DNA sensor thus offers the promise of convenient, reusable detection of picomolar DNA.

Target-Responsive Structural Switching for Nucleic Acid-Based Sensors

Interest in the development of sensitive, selective, rapid, and cost-effective biosensors for biomedical analysis, environmental monitoring, and the detection of bioterrorism agents is rapidly increasing. A classic biosensor directly transduces ligand-target binding events into a measurable physical readout. More recently, researchers have proposed novel biosensing strategies that couple ligand-induced structural switching of biomolecules with advanced optical and electronic transducers. This approach has proven to be a highly general platform for the development of new biosensors. In this Account, we describe a series of electrochemical and optical nucleic acid sensors that use target-responsive DNA structures. By employing surface-confined DNA structures with appropriate redox labels, we can monitor target-induced structural switching of DNA or aptamer-specific small molecule probes by measuring electrochemical currents that are directly associated with the distance between the redox label and the electrode surface. We have also demonstrated significant improvements in sensing performance through optimization of the DNA self-assembly process at electrode surfaces or the introduction of nanomaterial-based signal amplification. Alternatively, gold nanoparticles interact differently with folded and unfolded DNA structures, which provides a visual method for detecting target-induced structural switching based on the plasmonic change of gold nanoparticles. This novel method using gold nanoparticles has proven particularly suitable for the detection of a range of small-molecule targets (e.g., cocaine) and environmentally toxic metal ions (e.g., Hg(2+)). Rational sequence design of DNA aptamers improves the sensitivity and increases the reaction kinetics. Recently, we have also designed microfluidic devices that allow rapid and portable mercury detection with the naked eye. This Account focuses on the use of bulk and nanoscale gold and DNA/aptamer molecules. We expect that researchers will further expand the analyte spectrum and improve the sensitivity and selectivity of nucleic acid sensors using functional biomolecules, such as DNAzymes, peptide aptamers and engineered proteins, and nanomaterials of different sizes, dimensions and compositions, such as carbon nanotubes, graphene, silicon nanowires, and metal nanoparticles or nanorods.

Single-Layer MoS2-Based Nanoprobes for Homogeneous Detection of Biomolecules

A single-layer MoS2 nanosheet exhibits high fluorescence quenching ability and different affinity toward ssDNA versus dsDNA. As a proof of concept, the MoS2 nanosheet has been successfully used as a sensing platform for the detection of DNA and small molecules.

Protein Corona-Mediated Mitigation of Cytotoxicity of Graphene Oxide

Graphene is a single layer of sp(2)-bonded carbons that has unique and highly attractive electronic, mechanical, and thermal properties. Consequently, the potential impact of graphene and its derivatives (e.g., graphene oxide, GO) on human and environmental health has raised considerable concerns. In this study, we have carried out a systematic investigation on cellular effects of GO nanosheets and identified the effect of fetal bovine serum (FBS), an often-employed component in cell culture medium, on the cytotoxicity of GO. At low concentrations of FBS (1%), human cells were sensitive to the presence of GO and showed concentration-dependent cytotoxicity. Interestingly, the cytotoxicity of GO was greatly mitigated at 10% FBS, the concentration usually employed in cell medium. Our studies have demonstrated that the cytotoxicity of GO nanosheets arises from direct interactions between the cell membrane and GO nanosheets that result in physical damage to the cell membrane. This effect is largely attenuated when GO is incubated with FBS due to the extremely high protein adsorption ability of GO. The observation of this FBS-mitigated GO cytotoxicity effect may provide an alternative and convenient route to engineer nanomaterials for safe biomedical and environmental applications.

Functional nanoprobes for ultrasensitive detection of biomolecules

There has been great interest in developing new nucleic acid and protein detection methods for both clinical and numerous non-clinical applications. In a long-lasting effort to improve the detection ability of bioassays, functional nanomaterials have been actively explored to greatly enhance the sensitivity during the last two decades. This tutorial review focuses on recent progress in biosensor development by exploiting several unique optical, electronic and catalytic properties of a range of nanomaterials, such as gold nanoparticles, quantum dots, silicon nanowires, carbon nanotubes and graphene. In addition, a perspective on new opportunities offered by emerging technologies (e.g. DNA nanotechnology) is provided.

The cytotoxicity of cadmium-based quantum dots ☆

Semiconductor Quantum dots (QDs) have raised great attention because of their superior optical properties and wide utilization in biological and biomedical studies. More recently, there have been intense concerns on cytotoxicity assessment of QDs. Most QDs are made of heavy metal ions (e.g., Cd(2+)), which may result in potential in vitro toxicity that hampers their practical applications. In this article, we aim to summarize recent progress on mechanistic studies of cytotoxicity of II-IV QDs. We have studied the cytotoxicity of a series of aqueous synthesized QDs (aqQDs), i.e. CdTe, CdTe/CdS core-shell structured and CdTe/CdS/ZnS core-shell-shell structured aqQDs. Our results suggested that released cadmium ions are responsible for the observed cytotoxicity of cadmium-based QDs. The fact that CdTe/CdS/ZnS core-shell-shell structured QDs are nearly nontoxic to cells further confirmed the role of released cadmium ions on cytotoxicity, and the effective protection of the ZnS shell. However, intracellular level of Cd(2+) ions cannot be the only reason since the comparison with CdCl(2)-treated cells suggests there are other factors contributed to the cytotoxicity of aqQDs. Our studies on genome-wide gene expression profiling and subcellular localization of aqQDs with synchrotron-based scanning transmission X-ray microscopy (STXM) further suggest that the cytotoxicity of CdTe QDs not only comes from the release of Cd(2+) ions but also intracellular distribution of QD nanoparticles in cells and the associated nanoscale effects.

Beyond superquenching: Hyper-efficient energy transfer from conjugated polymers to gold nanoparticles

Gold nanoparticles quench the fluorescence of cationic polyfluorene with Stern–Volmer constants (KSV) approaching 1011 M—1, several orders of magnitude larger than any previously reported conjugated polymer–quencher pair and 9–10 orders of magnitude larger than small molecule dye–quencher pairs. The dependence of KSV on ionic strength, charge and conjugation length of the polymer, and the dimensions (and thus optical properties) of the nanoparticles suggests that three factors account for this extraordinary efficiency: (i) amplification of the quenching via rapid internal energy or electron transfer, (ii) electrostatic interactions between the cationic polymer and anionic nanoparticles, and (iii) the ability of gold nanoparticles to quench via efficient energy transfer. As a result of this extraordinarily high KSV, quenching can be observed even at subpicomolar concentrations of nanoparticles, suggesting that the combination of conjugated polymers with these nanomaterials can potentially lead to improved sensitivity in optical biosensors.

Visual Cocaine Detection with Gold Nanoparticles and Rationally Engineered Aptamer Structures

A novel bioassay strategy is designed to detect small-molecule targets such as cocaine, potassium, and adenosine, based on gold nanoparticles (AuNPs) and engineered DNA aptamers. In this design, an aptamer is engineered to be two pieces of random, coil-like single-stranded DNA, which reassembles into the intact aptamer tertiary structure in the presence of the specific target. AuNPs can effectively differentiate between these two states via their characteristic surface-plasmon resonance-based color change. Using this method, cocaine in the low-micromolar range is selectively detected within minutes. This strategy is also shown to be generic and applicable to the detection of several other small-molecule targets.

Self-Assembled Multivalent DNA Nanostructures for Noninvasive Intracellular Delivery of Immunostimulatory CpG Oligonucleotides

Designed oligonucleotides can self-assemble into DNA nanostructures with well-defined structures and uniform sizes, which provide unprecedented opportunities for biosensing, molecular imaging, and drug delivery. In this work, we have developed functional, multivalent DNA nanostructures by appending unmethylated CpG motifs to three-dimensional DNA tetrahedra. These small-sized functional nanostructures are compact, mechanically stable, and noncytotoxic. We have demonstrated that DNA nanostructures are resistant to nuclease degradation and remain substantially intact in fetal bovine serum and in cells for at least several hours. Significantly, these functional nanostructures can noninvasively and efficiently enter macrophage-like RAW264.7 cells without the aid of transfection agents. After they are uptaken by cells, CpG motifs are recognized by the Toll-like receptor 9 (TLR9) that activates downstream pathways to induce immunostimulatory effects, producing high-level secretion of various pro-inflammatory cytokines including tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-12. We also show that multivalent CpG motifs greatly enhance the immunostimulatory effect of the nanostructures. Given the high efficacy of these functional nanostructures and their noncytotoxic nature, we expect that DNA nanostructures will become a promising tool for targeted drug delivery.