Alan S. Campbell

Enzyme Catalytic Efficiency: A Function of Bio–Nano Interface Reactions

Biocatalyst immobilization onto carbon-based nanosupports has been implemented in a variety of applications ranging from biosensing to biotransformation and from decontamination to energy storage. However, retaining enzyme functionality at carbon-based nanosupports was challenged by the non-specific attachment of the enzyme as well as by the enzyme-enzyme interactions at this interface shown to lead to loss of enzyme activity. Herein, we present a systematic study of the interplay reactions that take place upon immobilization of three pure enzymes namely soybean peroxidase, chloroperoxidase, and glucose oxidase at carbon-based nanosupport interfaces. The immobilization conditions involved both single and multipoint single-type enzyme attachment onto single and multi-walled carbon nanotubes and graphene oxide nanomaterials with properties determined by Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Our analysis showed that the different surface properties of the enzymes as determined by their molecular mapping and size work synergistically with the carbon-based nanosupports physico-chemical properties (i.e., surface chemistry, charge and aspect ratios) to influence enzyme catalytic behavior and activity at nanointerfaces. Knowledge gained from these studies can be used to optimize enzyme-nanosupport symbiotic reactions to provide robust enzyme-based systems with optimum functionality to be used for fermentation, biosensors, or biofuel applications.

Membrane/mediator-free rechargeable enzymatic biofuel cell utilizing graphene/single-wall carbon nanotube cogel electrodes.

Enzymatic biofuel cells (EBFCs) utilize enzymes to convert chemical energy present in renewable biofuels into electrical energy and have shown much promise in the continuous powering of implantable devices. Currently, however, EBFCs are greatly limited in terms of power and operational stability with a majority of reported improvements requiring the inclusion of potentially toxic and unstable electron transfer mediators or multicompartment systems separated by a semipermeable membrane resulting in complicated setups. We report on the development of a simple, membrane/mediator-free EBFC utilizing novel electrodes of graphene and single-wall carbon nanotube cogel. These cogel electrodes had large surface area (∼ 800 m(2) g(-1)) that enabled high enzyme loading, large porosity for unhindered glucose transport and moderate electrical conductivity (∼ 0.2 S cm(-1)) for efficient charge collection. Glucose oxidase and bilirubin oxidase were physically adsorbed onto these electrodes to form anodes and cathodes, respectively, and the EBFC produced power densities up to 0.19 mW cm(-2) that correlated to 0.65 mW mL(-1) or 140 mW g(-1) of GOX with an open circuit voltage of 0.61 V. Further, the electrodes were rejuvenated by a simple wash and reloading procedure. We postulate these porous and ultrahigh surface area electrodes will be useful for biosensing applications, and will allow reuse of EBFCs.

Improved power density of an enzymatic biofuel cell with fibrous supports of high curvature

Enzyme immobilization onto gold- or carbon nanotube-based nanomaterials has driven recent advances in the development of enzymatic biofuel cells (EBFCs). Enzyme–gold and enzyme–carbon nanotube interfaces are conducive to achieving efficient electron transfer between the enzyme active site and an electrode along with high enzyme loadings. Herein, we investigate the performance dependence of EBFCs on the surface curvature, specific surface area (SSA) and pore size of underlying enzyme supports. One of the supports was gold/multi-wall carbon nanotube (MWCNT) fiber paddles that were formed by depositing gold nanoparticles and MWCNTS onto electrospun polyacrylonitrile fibers with a diameter of 1 μm and a SSA of 3.6 m2 g−1 with micrometer sized pores. The other support was graphene-coated single-wall carbon nanotube (SWCNT) gels, which had 1 nm thick struts, 686 m2 g−1 SSA, and pores of diameter ≤ 15 nm. Glucose oxidase (GOX) and bilirubin oxidase (BOD) were immobilized onto each material to form enzymatically active anodes and cathodes, respectively. EBFCs constructed using gold/MWCNT fiber paddle electrodes yielded power densities of 0.4 μW cm−2 with an open circuit voltage of 0.22 V and GOX loadings of 2.0 × 10−10 mol cm−2. In comparison, EBFCs utilizing graphene-coated SWCNT gel electrodes had 10-fold lower GOX loadings (1.0 × 10−11 mol cm−2), but still produced 10-fold greater power densities (≈3.6 μW cm−2) and an open circuit voltage of 0.22 V. We postulate that a greater fraction of GOX supported on graphene-coated SWCNTs that had high curvature retained their biochemical functionality. Further, this study provides a design principle for improving enzymatic power generation.

A Systematic Study of the Catalytic Behavior at Enzyme–Metal-Oxide Nanointerfaces

Metal-oxide nanoparticles with high surface area, controllable functionality and thermal and mechanical stability provide high affinity for enzymes when the next generation of biosensor applications are being considered. We report on the synthesis of metal-oxide-based nanoparticles (with different physical and chemical properties) using hydrothermal processing, photo-deposition and silane functionalization. Physical and chemical properties of the user-synthesized nanoparticles were investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and Raman scattering, respectively. Thus, characterized metal-oxide-based nanoparticles served as nanosupports for the immobilization of soybean peroxidase enzyme (a model enzyme) through physical binding. The enzyme–nanosupport interface was evaluated to assess the optimum nanosupport characteristics that preserve enzyme functionality and its catalytic behavior. Our results showed that both the nanosupport geometry and its charge influence the functionality and catalytic behavior of the bio-metal-oxide hybrid system.