Xueqiu You

Electrochemistry at the Edge of a Single Graphene Layer in a Nanopore

We study the electrochemistry of single layer graphene edges using a nanopore-based structure consisting of stacked graphene and Al(2)O(3) dielectric layers. Nanopores, with diameters ranging from 5 to 20 nm, are formed by an electron beam sculpting process on the stacked layers. This leads to a unique edge structure which, along with the atomically thin nature of the embedded graphene electrode, demonstrates electrochemical current densities as high as 1.2 × 10(4) A/cm(2). The graphene edge embedded structure offers a unique capability to study the electrochemical exchange at an individual graphene edge, isolated from the basal plane electrochemical activity. We also report ionic current modulation in the nanopore by biasing the embedded graphene terminal with respect to the electrodes in the fluid. The high electrochemical specific current density for a graphene nanopore-based device can have many applications in sensitive chemical and biological sensing, and energy storage devices.

Zinc oxide inverse opal enzymatic biosensor

We report ZnO inverse opal- and nanowire (NW)-based enzymatic glucose biosensors with extended linear detection ranges. The ZnO inverse opal sensors have 0.01–18 mM linear detection range, which is 2.5 times greater than that of ZnO NW sensors and 1.5 times greater than that of other reported ZnO sensors. This larger range is because of reduced glucose diffusivity through the inverse opal geometry. The ZnO inverse opal sensors have an average sensitivity of 22.5 μA/(mM cm2), which diminished by 10% after 35 days, are more stable than ZnO NW sensors whose sensitivity decreased by 10% after 7 days.

Low voltage electrowetting on atomic-layer-deposited aluminum oxide

Electrowetting on dielectric (EWOD) is useful in manipulating droplets for digital (droplet-based) microfluidics, but its high driving voltage over several tens of volts has been a barrier to overcome. This paper presents the characteristics of EWOD device with aluminum oxide (Al2O3, εr≈10), deposited by atomic layer deposition (ALD), as the high-k dielectric for lowering the EWOD driving voltage substantially. The EWOD device of the single-plate configuration was fabricated by several steps for the control electrode array of 1 mm × 1 mm squares with 50μm space, the dielectric layer of 127nm thick ALD Al2O3, the reference electrode of 20μm wide line electrode, and the hydrophobic surface treatment by Teflon-AF coating, respectively. We observed the movement of a 2μl water droplet in an air environment, applying a voltage between one of the control electrodes and the reference electrode in contact with the droplet. Exponentially increasing droplet velocity with the applied voltage was obtained below 15V. The measured threshold voltage to move the droplet was as low as 3V which is the lowest voltage reported so far in the EWOD researches. This result opens a possibility of manipulating droplets, without any surfactant or oil treatment, at only a few volts by EWOD using ALD Al2O3 as the dielectric.

Power Generating Characteristics of Zinc Oxide Nanorods Grown on a Flexible Substrate by a Hydrothermal Method

This paper describes the power generating property of hydrothermally grown ZnO nanorods on a flexible polyethersulfone (PES) substrate. The piezoelectric currents generated by the ZnO nanorods were measured when bending the ZnO nanorod by using I-AFM, and the measured piezoelectric currents ranged from 60 to 100 pA. When the PtIr coated tip bends a ZnO nanorod, piezoelectrical asymmetric potential is created on the nanorod surface. The Schottky barrier at the ZnO-metal interface accumulates elecntrons and then release very quickly generating the currents when the tip moves from tensile to compressed part of ZnO nanorod. These ZnO nanorods were grown almost vertically with the length of 300-500 ㎚ and the diameter of 30-60 ㎚ on the Ag/Ti/PES substrate at 90℃ for 6 hours by hydrothermal method. The metal-semiconductor interface property was evaluated by using a HP 4145B Semiconductor Parameter Analyzer and the piezoelectric effect of the ZnO nanorods were evaluated by using an I-AFM. From the measured I-V characteristics, it was observed that ZnO-Ag and ZnO-Au metal-semiconductor interfaces showed an ohmic and a Schottky contact characteristics, respectively. ANSYS finite element simulation was performed in order to understand the power generation mechanism of the ZnO nanorods under applied external stress theoretically.

Lithography-free fabrication of bistability graphene FET biosensor

A novel non-lithographic, contamination-free technique for the fabrication of graphene field effect transistors (GFET) is presented. The GFET was modified with C reactive protein aptamer. Iron nanoparticles were immobilized on the GFET channel surface by using an aptamer-target protein (CRP)-aptamer sandwich method. Bistability signal was obtained by backgate voltage up/down sweeping. The bistability phenomenon caused by the target protein induced NPs adsorption demonstrates a novel high sensitive biosensing method.

Enzymatic glucose biosensor based on porous ZnO/Au electrodes

This paper describes a glucose biosensor based on glucose oxidase (GOx) immobilized on porous ZnO/Au electrodes. The ZnO porous electrodes were fabricated by electrochemical deposition of zinc oxide on patterned Au electrodes using polystyrene (PS) spheres as templates. Uniform pore size and highly ordered ZnO pore arrangement were observed from SEM images. X-ray diffraction (XRD) patterns revealed a single crystalline nature of the porous ZnO. This porous structure provides high enzyme loading capacity and long-term stability. In a pH 7.4 phosphate buffer solution, the positively charged high isoelectric point (IEP) ZnO pores enhance the adsorption of negatively charged low IEP GOx through electrostatic attractive force. Once GOx molecules are immobilized within the ZnO pores, the bottleneck structure resulting from the connected pores hinders leaching of GOx from the pores. The resulting enzymatic biosensor showed a linear detection range from 1mM to 18mM, and sensitivity of 10.89μA/ (mM·cm2) with good selectivity and long-term stability.

Rapidly dissolving silk protein microneedles for transdermal drug delivery

This paper presents rapidly dissolving fibroin microneedles for the first time. A reverse PDMS microneedles mold was first created and drug-contained fibroin solution was poured into this reverse PDMS microneedles mold. Fibroin microneedles were successfully fabricated after drying and detaching the solidified fibroin structure from the PDMS mold. These fibroin microneedles serve as a matrix to incorporate drug molecules while maintaining the drug activity. The dimensions of the fabricated fibroin microneedles are 500 μm in length, 200 μm in diameter at the base, and 5 μm in radius at the tip. These fibroin matrix microneedles can dissolve within one minute under the skin to release the drug molecules and the dissolved fibroin in the skin generates noninflammatory amino acid degradation products usable in cell metabolic functions. The fibroin microneedles containing methylene blue as a drug were fabricated and their surface morphology, internal layered structure, mechanical property, and the dissolving characteristics were examined. These rapidly dissolving fibroin microneedles provide more benefit than conventional syringes for painless transdermal drug delivery.