Recent developments in faradaic bioelectrochemistry

Cyclodextrins (CDs) have captivated many scientists since their discovery back in 1891. The intriguing structure of these materials has attracted chemists, physicists, biologists, engineers, and many others in attempts to take advantage of the unique properties. Cyclodextrins are among the most important molecular receptors studied in supramolecular chemistry. They have raised many questions and provided answers to academic and industrial problems, and as a result of these efforts, there is now what we may call a ‘cyclodextrin science’ that involves the search for improved properties and widespread applications of these molecules. The properties and applications of CDs have been the subject of many reviews1-6 and books.7-11 The last compilation appeared in 1998 in a special issue of Chemical ReViews edited by D’Souza and Lipkowitz.12 Readers interested in this field can find information on the most relevant aspects of CDs, including application of NMR spectroscopy, computational chemistry, and X-ray crystallographysamong other techniquessfor the elucidation of the structures of CDs and their complexes. For this reason, it is not our intention to cover all of these topics but to focus on the most recent applications of CDs in the biotechnological field.

Supramolecular chemistry of cyclodextrins in enzyme technology.

Electrocatalytic oxidation of NAD(P) H at mediator-modified electrodes.

Redox enzymes have tremendous potential as catalysts for preparative organic chemistry. Their usually high selectivity, paired with their catalytic efficiency under mild reaction conditions, makes them potentially very valuable tools for synthesis. The number of interesting monooxygenases, dehydrogenases, reductases, oxidases, and peroxidases is steadily increasing and the tailoring of a given biocatalyst is more and more becoming standard technology. However, their cofactor dependency still represents a major impediment en route to true preparative applicability. Currently, three different approaches to deal witch this 'cofactor challenge' are being pursued: using whole cells, biomimetic approaches comprising enzymatic cofactor regenerations systems, and 'unconventional' nonenzymatic regeneration. The latter technique offers the promise of enabling simple, easily applicable, and robust reaction schemes, for example, by circumventing the 'cofactor challenge' and introducing redox power directly to the enzyme's active sites.

Biocatalytic Redox Reactions for Organic Synthesis: Nonconventional Regeneration Methods

Laccase was earlier wired to yield an O2 electroreduction catalyst greatly outperforming platinum and its alloys. Here we describe the design, synthesis optimization of the composition, and characterization of the +0.55 V (AgAgCl) laccase-wiring redox hydrogel, with an apparent electron diffusion coefficient (Dapp) of 7.6 × 10-7 cm2 s-1. The high Dapp results in the tethering of redox centers to the polymer backbone through eight-atom-long spacer arms, which facilitate collisional electron transfer between proximal redox centers. The O2 flux-limited, true-area-based current density was increased from the earlier reported 560 to 860 μA cm-2. When the O2 diffusion to the 7-μm-diameter carbon fiber cathode was cylindrical, half of the O2 flux-limited current was reached already at 0.62 V and 90% at 0.56 V vs Ag/AgCl, merely −0.08 and −0.14 V versus the 0.7 V (Ag/AgCl) reversible O2/H2O half-cell potential at pH 5.

A laccase-wiring redox hydrogel for efficient catalysis of O2 electroreduction.

A novel neutral ferrocene-containing polyacrylamide-based redox gel was prepared from the copolymerization of vinylferrocene (VF) with acrylamide and N,N'-methylenebisacrylamide in a one-step procedure. Hydroxypropyl-,8-cyclodextrin (HPCD) was used to convert water-insoluble VF into the water-soluble VF-HPCD inclusion complex, allowing the copolymerization to be performed in aqueous solution. A new ternary catalyst, consisting of flavin mononucleotide, H 2 O 2 , and N,N,N',N'-tetramethylethylenediamine, was developed for initiating the polymerization. A redox gel electrode, fabricated by covering the redox gel on the surface of a carbon paste electrode with a dialysis membrane, exhibits quasi-reversible electrochemical characteristics. The electrocatalytic properties of a glucose biosensor prepared by entrapping glucose oxidase in the redox gel were investigated. The simplicity and reproducibility of its preparation make this new redox gel a good candidate for biosensor applications.

Characterization of a Ferrocene-Containing Polyacrylamide-Based Redox Gel for Biosensor Use

NADH and NADPH sensors were developed by entrapping lipoamide dehydrogenase (LD) and glutathione reductase (GR), respectively, in a redox gel formed by the copolymerization of vinylferrocene with a...

NAD(P)H Sensors Based on Enzyme Entrapment in Ferrocene-Containing Polyacrylamide-Based Redox Gels

The charge transport properties of the redox gels formed from the copolymerization of vinylferrocene (VF) with acrylamide and N,N‘-methylenebisacrylamide (PVAB gels) are described. The apparent electron diffusion coefficients (Dap) measured by cyclic voltammetry and chronocoulometry exhibit an upward bowlike curvature vs immobilized VF concentration ([VF]imm) within the range 0.1−1.60 mM in 0.10 M supporting electrolyte (NaH2PO4/Na2HPO4, NaClO4, or NaNO3). The range of measured Dap values [(0.6−6.0) × 10-7 cm2 s-1] can be interpreted in terms of the mean-field model developed by Blauch and Saveant (Blauch, D. N.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 3323). The large Dap values are attributed to the large λ (range of molecular motion permitted to the ferrocene residues) and large kex (bimolecular electron self-exchange rate constant of the ferrocenes in the PVAB gels). It was further observed that Dap decreases on increasing the electrolyte (NaClO4 or NaNO3) concentration from 0.10 to 0.50 M, especi...

Charge Transport in Ferrocene-Containing Polyacrylamide-Based Redox Gels

Enzyme electrodes were fabricated by entrapping glucose oxidase (GOx) in charged ferrocene-containing redox gels formed by the copolymerization of vinylferrocene−hydroxypropyl-β-cyclodextrin inclusion complex, acrylamide, and N,N ‘-methylenebis(acrylamide), together with a positively or negatively charged monomer at pH 7, in the presence of GOx in aqueous solution. The one-step polymerization procedure was initiated by a ternary catalyst system consisting of flavin mononucleotide, H2O2, and N,N,N ‘,N ‘-tetramethylethylenediamine. The effects of varying the concentration and structure of the charged monomers on the electrochemical properties of the charged redox gels with and without GOx were investigated, and the pH−activity profiles of entrapped GOx were examined. The storage and operational stabilities of the sensors were established, and the influence of the interferents ascorbic acid, acetaminophen, and uric acid, which are present in body fluids, on the amperometric signals of the sensors was determined.

Hai-zhi Bu

Papers

Characterization of a Ferrocene-Containing Polyacrylamide-Based Redox Gel for Biosensor Use

NAD(P)H Sensors Based on Enzyme Entrapment in Ferrocene-Containing Polyacrylamide-Based Redox Gels

Charge Transport in Ferrocene-Containing Polyacrylamide-Based Redox Gels

Modification of Ferrocene-Containing Redox Gel Sensor Performance by Copolymerization of Charged Monomers