Su Ha

Immobilization of glucose oxidase into polyaniline nanofiber matrix for biofuel cell applications

Glucose oxidase (GOx) was immobilized into the porous matrix of polyaniline nanofibers in a three-step process, consisting of enzyme adsorption, precipitation, and crosslinking (EAPC). EAPC was highly active and stable when compared to the control samples of enzyme adsorption (EA) and enzyme adsorption and crosslinking (EAC) with no step of enzyme precipitation. The GOx activity of EAPC was 9.6 and 4.2 times higher than those of EA and EAC, respectively. Under rigorous shaking at room temperature for 56 days, the relative activities of EA, EAC and EAPC, defined as the percentage of residual activity to the initial activity, were 22%, 19% and 91%, respectively. When incubated at 50°C under shaking for 4h, EAPC showed a negligible decrease of GOx activity while the relative activities of EA and EAC were 45% and 48%, respectively. To demonstrate the feasible application of EAPC in biofuel cells, the enzyme anodes were prepared and used for home-built air-breathing biofuel cells. The maximum power densities of biofuel cells with EA and EAPC anodes were 57 and 292 μW/cm(2), respectively. After thermal treatment at 60°C for 4h, the maximum power density of EA and EAPC anodes were 32 and 315 μW/cm(2), representing 56% and 108% of initially obtained maximum power densities, respectively. Because the lower power densities and short lifetime of biofuel cells are serious problems against their practical applications, the present results with EAPC anode has opened up a new potential for the realization of practical biofuel cell applications.

Synthesis and applications of molybdenum (IV) oxide

Molybdenum dioxide (MoO2) is a transition metal oxide with unusual metal-like electrical conductivity and high catalytic activity toward reforming hydrocarbons. This review covers the synthesis techniques used to fabricate MoO2 in a variety of morphologies and particle sizes. Processing from molybdenum ore and reduction from MoO3 are also covered, with emphasis on reduction mechanisms and kinetic considerations. Discussions of various solution-based and gas phase synthesis techniques shed light on strategies to achieve various unique morphologies, which leads into a brief discussion of nanoscale MoO2. Nanoscale MoO2 is of interest for important technological applications including catalysts for partial oxidation of hydrocarbons, solid oxide fuel cell anodes, and high-capacity reversible lithium ion battery anodes.

Nanoscale enzyme reactors in mesoporous carbon for improved performance and lifetime of biosensors and biofuel cells.

Nanoscale enzyme reactors (NERs) of glucose oxidase in conductive mesoporous carbons were prepared in a two-step process of enzyme adsorption and follow-up enzyme crosslinking. MSU-F-C, a mesoprous carbon, has a bottleneck pore structure with mesocellular pores of 26 nm connected with window mesopores of 17 nm. This structure enables the ship-in-a-bottle mechanism of NERs, which effectively prevents the crosslinked enzymes in mesocellular pores from leaching through the smaller window mesopores. This NER approach not only stabilized the enzyme but also expedited electron transfer between the enzyme and the conductive MSU-F-C by maintaining a short distance between them. In a comparative study with GOx that was simply adsorbed without crosslinking, the NER approach was proven to be effective in improving the sensitivity of glucose biosensors and the power density of biofuel cells. The power density of biofuel cells could be further improved by manipulating several factors, such as by adding a mediator, changing the order of adsorption and crosslinking, and inserting a gold mesh as an electron collector.

Highly stable enzyme precipitate coatings and their electrochemical applications.

This paper describes highly stable enzyme precipitate coatings (EPCs) on electrospun polymer nanofibers and carbon nanotubes (CNTs), and their potential applications in the development of highly sensitive biosensors and high-powered biofuel cells. EPCs of glucose oxidase (GOx) were prepared by precipitating GOx molecules in the presence of ammonium sulfate, then cross-linking the precipitated GOx aggregates on covalently attached enzyme molecules on the surface of nanomaterials. EPCs-GOx not only improved enzyme loading, but also retained high enzyme stability. For example, EPC-GOx on CNTs showed a 50 times higher activity per unit weight of CNTs than the conventional approach of covalent attachment, and its initial activity was maintained with negligible loss for 200 days. EPC-GOx on CNTs was entrapped by Nafion to prepare enzyme electrodes for glucose sensors and biofuel cells. The EPC-GOx electrode showed a higher sensitivity and a lower detection limit than an electrode prepared with covalently attached GOx (CA-GOx). The CA-GOx electrode showed an 80% drop in sensitivity after thermal treatment at 50°C for 4 h, while the EPC-GOx electrode maintained its high sensitivity with negligible decrease under the same conditions. The use of EPC-GOx as the anode of a biofuel cell improved the power density, which was also stable even after thermal treatment of the enzyme anode at 50°C. The excellent stability of the EPC-GOx electrode together with its high current output create new potential for the practical applications of enzyme-based glucose sensors and biofuel cells.

Acceptors in ZnO

Zinc oxide (ZnO) has potential for a range of applications in the area of optoelectronics. The quest for p-type ZnO has focused much attention on acceptors. In this paper, Cu, N, and Li acceptor impurities are discussed. Experimental evidence indicates these point defects have acceptor levels 3.2, 1.4, and 0.8 eV above the valence-band maximum, respectively. The levels are deep because the ZnO valence band is quite low compared to conventional, non-oxide semiconductors. Using MoO2 contacts, the electrical resistivity of ZnO:Li was measured and showed behavior consistent with bulk hole conduction for temperatures above 400 K. A photoluminescence peak in ZnO nanocrystals is attributed to an acceptor, which may involve a Zn vacancy. High field (W-band) electron paramagnetic resonance measurements on the nanocrystals revealed an axial center with g⊥ = 2.0015 and g// = 2.0056, along with an isotropic center at g = 2.0035.

Decomposition of methyl species on a Ni(211) surface: investigations of the electric field influence

Density functional theory calculations are performed to examine how an external electric field can alter the reaction pathways on a stepped Ni(211) surface with regard to the decomposition of methyl species. We compare our results to those previously obtained on a close-packed Ni(111) surface and a bimetallic Au/Ni surface. The structures, adsorption energies, and reaction energy barriers of all methyl species on the Ni(211) surface are identified. The calculated results indicate that the presence of an external electric field not only alters the site preferences for the adsorbates on Ni(211), but also significantly changes the adsorption energies of the CHx species. By comparison with our previous results, this electric field effect is smaller than that on Ni(111). The local electric field value is also found to differ at the various adsorption sites for the CH3 group on Ni(211). From the results, a correlation between the calculated local electric fields, the adsorption energies and effective dipole moments values is investigated. The calculations also show that the stepped surfaces are more reactive for the elementary dissociation reactions of the CHx species as compared to the Ni(111) surface. The final conclusion is that a positive electric field strengthens the adsorption energy of reactant CH3, increases the energy barriers of the decomposition of CHx species and weakens the adsorption energies of C and H. This suggests that the formation of pure C atoms deposits will be impeded by an external positive electric field.

Nanobiocatalysis for Enzymatic Biofuel Cells

Enzymatic biofuel cells promise a great potential as a small power source, but their practical applications are being hampered by two serious problems: low power density and short lifetime. This review will describe recent advances of nanobiocatalysis that can potentially solve these two problems, together with some of novel in vivo applications of biofuel cells for harvesting electrical energy from the body fluids of living insects and animals.

Enzyme precipitate coatings of glucose oxidase onto carbon paper for biofuel cell applications.

Enzymatic biofuel cells (BFC) have a great potential as a small power source, but their practical applications are being hampered by short lifetime and low power density. This study describes the direct immobilization of glucose oxidase (GOx) onto the carbon paper in the form of highly stable and active enzyme precipitation coatings (EPCs), which can improve the lifetime and power density of BFCs. EPCs were fabricated directly onto the carbon paper via a three‐step process: covalent attachment (CA), enzyme precipitation, and chemical crosslinking. GOx‐immobilized carbon papers via the CA and EPC approaches were used as an enzyme anode and their electrochemical activities were tested under the BFC‐operating mode. The BFCs with CA and EPC enzyme anodes produced the maximum power densities of 50 and 250 µW/cm2, respectively. The BFC with the EPC enzyme anode showed a stable current density output of >700 µA/cm2 at 0.18 V under continuous operation for over 45 h. When a maple syrup was used as a fuel under ambient conditions, it also produced a stable current density of >10 µA/cm2 at 0.18 V for over 25 h. It is anticipated that the direct immobilization of EPC on hierarchical‐structured electrodes with a large surface area would further improve the power density of BFCs that can make their applications more feasible. Biotechnol. Bioeng. 2012; 109:318–324.

Characterization of a membraneless direct-methanol micro fuel cell

Abstract The performance of a membraneless laminar flow micro fuel cell was evaluated under different operating conditions. The fuel cell was microfabricated in polydimethylsiloxane using standard soft-lithography techniques. It used methanol solution as the fuel for the anode side, and oxygen saturated sulphuric acid for the cathode. The parameters studied were the methanol concentration, flowrate, device width, and the concentration of sulphuric acid in the anode stream. Performance was characterized by V—I plots, stability of open circuit potential (OCP), polarization resistances, and anode polarization curves. We observed behaviour different from that shown thus far by existing laminar flow fuel cells. Our results show that the power output of the device decreases with an increase in the methanol concentration. An increase in the flowrate also decreases the power output of the device. It is shown that these trends are likely caused by the cells internal resistance to proton transport. The addition of sulphuric acid to the fuel significantly decreases this resistance. It was found that the device OCP was not stable over extended operation, and could drop by more than 150 mV in 72 h.

Fabrication of enzyme-based coatings on intact multi-walled carbon nanotubes as highly effective electrodes in biofuel cells

CNTs need to be dispersed in aqueous solution for their successful use, and most methods to disperse CNTs rely on tedious and time-consuming acid-based oxidation. Here, we report the simple dispersion of intact multi-walled carbon nanotubes (CNTs) by adding them directly into an aqueous solution of glucose oxidase (GOx), resulting in simultaneous CNT dispersion and facile enzyme immobilization through sequential enzyme adsorption, precipitation, and crosslinking (EAPC). The EAPC achieved high enzyme loading and stability because of crosslinked enzyme coatings on intact CNTs, while obviating the chemical pretreatment that can seriously damage the electron conductivity of CNTs. EAPC-driven GOx activity was 4.5- and 11-times higher than those of covalently-attached GOx (CA) on acid-treated CNTs and simply-adsorbed GOx (ADS) on intact CNTs, respectively. EAPC showed no decrease of GOx activity for 270 days. EAPC was employed to prepare the enzyme anodes for biofuel cells, and the EAPC anode produced 7.5-times higher power output than the CA anode. Even with a higher amount of bound non-conductive enzymes, the EAPC anode showed 1.7-fold higher electron transfer rate than the CA anode. The EAPC on intact CNTs can improve enzyme loading and stability with key routes of improved electron transfer in various biosensing and bioelectronics devices.