Development of Second and Third Generation Bioelectronics
Abstract: The field of research dealing with the integration of biomolecules with electronic elements to form functional devices is called ‘‘bioelectronics’’. Bioelectronics based on mediated electron transfer (MET) between the biological element and the electrodes are designated as “second generation” while the one based on direct electron transfer (DET) is specified as “third generation”. Implanted bioelectronics for the continuous monitoring of glucose content in human beings is a commercial product in the USA. The reaserch in the field of biofuel cells (BFCs) (bioelectronics which can power other devices) has regained interest for possible use in implantable devices. The following redox enzymes; pyranose oxidase (POx), pyranose dehydrogenase (PDH), diaphorase (DI), tryptophan repressor-binding protein (WrbA) have been wired to electrodes through a low potential Os-redox polymer to construct biosensors and biofuel cells (BFC). Spectroelectrochemical experiments of the enzymes were performed in the presence of a mediator mixture to estimate their formal redox potentials. With POx and PDH, biosensors for glucose and other important mono- and disaccharides were developed. DI and WrbA were used to construct NADH biosensors and the prototype of a two enzymatic system BFC. Single walled carbon nanotubes (SWCNTs) of different lengths were cross-linked with the enzymes and the Os-polymer to increase the catalytic current density. Unlike POx, PDH, DI, WrbA, cellobiose dehydrogenase (CDH) is an extrinsic enzyme, which owns both a catalytic FAD containing domain as well as a domain with a surface exposed haem interconnected through a built-in electron transfer pathway from the active site to the haem, which in turn can also connect with an electrode. CDH is therefore able to communicate directly with electrodes. Length fractionated SWCNTs have been embedded together with CDH to enhance its DET properties. A lactose BFC based on DET and using CDH from Phanerochaete sordida has been shown. When the enzyme was wired together with the Os-polymer the catalytic redox center could be reached and lactose could be oxidized at a much lower potential and higher current densities were recorded. The enzyme was also adsorbed on p-phenylenediamine electrochemically modified SWCNTs glassy carbon electrodes. Due to the positive charges presented on the SWCNTs the haem domain of CDH could better interact with the nanotubes and higher catalytic currents for lactose oxidation were registered. Also the overpotential necessary to oxidise the substrate could be decreased. Myriococcum thermophilum CDH can oxidise glucose at C-1. PDH can oxidise glucose at C-2 and C-3. The two enzymes were coembedded with SWCNTs and an Os-polymer to develop a BFC anode, which could yield up to 3 e from a single glucose molecule. Corynascus thermophilus CDH is very efficient at oxidising glucose at pH 7.4. To increase the anodic catalytic currents for substrate oxidation shortened SWCNTs were cross-linked together with CtCDH on spectrographic graphite electrodes. A 3rd generation glucose biosensor working at physiological conditions and with a linear range for glucose from 0.5 to 30 mM could be developed. Such a 3rd generation biosensor is envisaged in the future to replace the biosensors that are today commercialised and that are based on mediators and have therefore higher fabrication prices and more complicated design
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