Chen-Zhong Li

Membraneless enzymatic biofuel cells based on graphene nanosheets.

The possibility of employing graphene sheets as a potential candidate for the construction of biofuel cells is reported in this paper. Initially, graphene sheets were chemically synthesized and characterized by surface characterization techniques. Following this, graphene was employed to fabricate the anode and cathode in the biofuel cell. The anode of the biofuel cell consists of a gold electrode on which we co-immobilized graphene - glucose oxidase using silica sol-gel matrix. Voltammetric measurements were conducted to quantitatively evaluate the suitability of employing graphene sheets as an electrode dopant and its performance was compared with single walled carbon nanotubes (SWCNTs). The cathode of the biofuel cell was constructed in a similar method except that graphene was co-immobilized with bilirubin oxidase. Finally, two membraneless enzymatic biofuel cells, one using graphene sheets and the other using SWCNTs, were constructed and their performances were compared. Upon comparison, graphene based biofuel cell exhibited a maximum power density of about 24.3+/-4 microW (N=3), which is nearly two times greater than that of the SWCNTs biofuel cell, and the performance of the graphene biofuel cell lasted for 7 days.

Hydrogenation: a simple approach to realize semiconductor-half-metal-metal transition in boron nitride nanoribbons.

The intriguing electronic and magnetic properties of fully and partially hydrogenated boron nitride nanoribbons (BNNRs) were investigated by means of first-principles computations. Independent of ribbon width, fully hydrogenated armchair BNNRs are nonmagnetic semiconductors, while the zigzag counterparts are magnetic and metallic. The partially hydrogenated zigzag BNNRs (using hydrogenated BNNRs and pristine BNNRs as building units) exhibit diverse electronic and magnetic properties: they are nonmagnetic semiconductors when the percentage of hydrogenated BNNR blocks is minor, while a semiconductor-->half-metal-->metal transition occurs, accompanied by a nonmagnetic-->magnetic transfer, when the hydrogenated part is dominant. Although the half-metallic property is not robust when the hydrogenation ratio is large, this behavior is sustained for partially hydrogenated zigzag BNNRs with a smaller degree of hydrogenation. Thus, controlling the hydrogenation ratio can precisely modulate the electronic and magnetic properties of zigzag BNNRs, which endows BN nanomaterials many potential applications in the novel integrated functional nanodevices.

Molecular detection based on conductance quantization of nanowires

We have studied molecular adsorption onto stable metallic nanowires fabricated with an electrochemical method. Upon the adsorption, the quantized conductance decreases, typically, to a fractional value, which may be attributed to the scattering of the conduction electrons by the adsorbates. The further conductance change occurs when the nanowire is exposed to another molecule that has stronger adsorption strength. Because the quantized conductance is determined by a few atoms at the narrowest portion of each nanowire, adsorption of a molecule onto the portion is enough to change the conductance, which may be used for chemical sensors.

Conductance of polymer nanowires fabricated by a combined electrodeposition and mechanical break junction method

We electrochemically deposit conducting polymer to bridge two closely placed electrodes, and then form a polymer nanowire by stretching the polymer bridge with the electrodes. During stretching, the conductance increases initially as the polymer chains are aligned in parallel, and then decreases in a stepwise fashion, due to abrupt changes in the nanowire thickness. We study the current–voltage (I–V) characteristics of the nanowire as a function of its electrochemical potential in an analogous fashion to the control of the gate voltage in semiconductor devices. Depending on the potential, the I–V curves vary from ohmic to rectifying characteristics.

Quantum transport in metallic nanowires fabricated by electrochemical deposition/dissolution

A nonmechanical method for fabricating a metallic narrow constriction between two electrodes using electrochemical deposition is described. The width of the constriction can be adjusted by slowly dissolving metal atoms away or redepositing atoms onto the constriction which can be controlled flexibly by the electrodes’ potentials. Well-defined plateaus near the integer numbers of the conductance quantum have been observed in these constrictions at room temperature. Since no mechanical movements are involved, this method has the potential of fabricating nanoconstrictions with long term stability.

Nanomedicine: Magnetic Nanoparticles and their Biomedical Applications

During this past decade, science and engineering have seen a rapid increase in interest for nanoscale materials with dimensions less than 100 nm, which lie in the intermediate state between atoms and bulk (solid) materials. Their attributes are significantly altered relative to the corresponding bulk materials as they exhibit size dependent behavior such as quantum size effects (depending on bulk Bohr radius), optical absorption and emission, coulomb staircase behavior (electrical transport), superparamagnetism and various unique properties. They are active components of ferrofluids, recording tape, flexible disk recording media along with potential future applications in spintronics: a new paradigm of electronics utilizing intrinsic charge and spin of electrons for ultra-high-density data storage and quantum computing. They are used in a gamut of biomedical applications: bioseparation of biological entities, therapeutic drugs and gene delivery, radiofrequency-induced destruction of cells and tumors (hyperthermia), and contrast-enhancement agents for magnetic resonance imaging (MRI). The magnetic nanoparticles have optimizable, controllable sizes enabling their comparison to cells (10-100 µm), viruses (20-250 nm), proteins (3-50 nm), and genes (10-100 nm). Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) provide necessary characterization methods that enable accurate structural and functional analysis of interaction of the biofunctional particles with the target bioentities. The goal of the present discussion is to provide a broad review of magnetic nanoparticle research with a special focus on the synthesis, functionalization and medical applications of these particles, which have been carried out during the past decade, and to examine several prospective directions.

Paper based point-of-care testing disc for multiplex whole cell bacteria analysis

Point-of-care testing (POCT) of infectious bacterial agents offers substantial benefits for disease diagnosis, mainly by shortening the time required to obtain results and by making the test available bedside or at remote care centers. Immunochromatographic lateral flow biosensors offer a low cost, highly sensitive platform for POCT. In this article, we describe the fabrication and testing of a multiplex immuno-disc sensor for the specific detection of Pseudomonas aeruginosa and Staphylococcus aureus. Antibody conjugated gold nanoparticles were used as the signaling agents. The detection range of the bacteria lies within 500-5000 CFU/ml. The advantage of the immuno-disc sensor is that it does not require any preprocessing of biological sample and is capable of whole cell bacterial detection. We also describe the design and fabrication of a compact portable device which converts the color intensity of the gold nanoparticles that accumulate at the test region into a quantitative voltage reading proportional to the bacterial concentration in the sample. The combination of the immuno-disc and the portable color reader provides a rapid, sensitive, low cost, and quantitative tool for the detection of a panel of infectious agents present in the patient sample.

Quantized tunneling current in the metallic nanogaps formed by electrodeposition and etching

We have studied electron tunneling across the gap between two electrodes as the gap is varied by electrodeposition and etching. The tunneling current tends to change in a stepwise fashion, corresponding to a discrete change of the gap width. The stepwise change is due to the discrete nature of atoms and a series of structural relaxations of the atoms at the electrodes between stable configurations upon deposition and etching. By stabilizing the tunneling current on various steps using a feedback loop, we have demonstrated that stable molecular-scale gaps can be fabricated with subangstrom precision.

Self-assembly of aromatic thiols on Au(111)

We have studied the self-assembly of two aromatic thiols, 4-mercaptopyridine (4MPY ) and 4-hydroxythiophenol (4HTP), on Au(111) from aqueous solutions with in situ scanning tunneling microscopy and ex situ X-ray photoelectron and Auger electron spectroscopies. Upon adsorption of the molecules, islands of nanometer scale appear immediately on flat Au(111) terraces, which is in sharp contrast to the formation of depressions in the selfassembly of the n-alkanethiol monolayers. The islands increase both in size and number initially, and decrease slowly after reaching maximum values in about 15 min. The formation of the islands can be attributed to the Au atoms on the surface which become highly mobile due to the strong binding to the thiols. The decrease of the islands at the late stage is due to the fact that atoms in small islands diVuse into nearby big islands and surface step edges. Both 4MPY and 4HTP molecules are randomly distributed on the surface immediately after adsorption, but slowly arrange into domains of ordered structures. For 4MPY the ordered structure has lattice constants, a=5.1±0.3 A ˚ , b=

Fabrication of stable metallic nanowires with quantized conductance

A metallic nanowire with quantized conductance was fabricated by electrochemically etching a narrow portion of a metallic wire supported on a solid substrate down to the atomic scale. The width of the nanowire was controlled flexibly by etching atoms away or depositing atoms back onto the wire with the electrochemical potential. Using a feedback loop this method can, at will, fabricate a single or an array of long-term stable nanowires with a pre-selected quantized conductance. These stable nanowires may be used in devices as digitized conductors and as sensors that detect chemicals in the air or in solutions. Using the conductance quantization as a feedback, this method may be used to fabricate nanoelectrodes by etching off the last few atoms in the thinnest portion of each nanowire. These nanoelectrodes may be connected to single molecules in molecular devices.