Direct electron transfer of other active enzymes

Direct electron transfer of other active enzymes... 

Electronic communication between electrode surfaces and biocatalysts can be achieved by direct electron transfer if the active site of the biocatalyst is not located too remote from the protein surface, as discussed elsewhere in this book (Chapter 17). Direct electron transfer is an attractive process for fuel cells as no other molecules except the substrate and the enzyme are involved in the electrocatalytic reaction, as depicted in the schematic in Fig. 12.2. The enzyme is the relay for the electron transfer between the substrate and the electrode surface. Recent advances in tailoring surface nanostructural features to match the size of co-substrate channels in biocatalysts, and in reconstituting active prosthetic groups tethered to, and communicating electronically with, surfaces, with apo-enzymes, are elegant demonstrations of direct electron transfer to biocatalyst active sites that were previously considered inaccessible to electrode surfaces [8-17]. 

Au nanoparticle (AuNP) electrodes prepared through various other deposition strategies have been extensively used for protein voltammetry and in electrochemical biosensing applica-tions. ° ° Electrodes with layers of HRP-modified AuNPs (prepared through covalent modification, self-assembly, or layer-by-layer techniques) have been shown to exhibit electrocatalytic activity toward the reduction of H2O2 through direct electron transfer between electrode and enzyme redox... 

However, because of the mostly very slow electron transfer rate between the redox active protein and the anode, mediators have to be introduced to shuttle the electrons between the enzyme and the electrode effectively (indirect electrochemical procedure). As published in many papers, the direct electron transfer between the protein and an electrode can be accelerated by the application of promoters which are adsorbed at the electrode surface [27], However, this type of electrode modification, which is quite useful for analytical studies of the enzymes or for sensor applications is in most cases not stable and effective enough for long-term synthetic application. Therefore, soluble redox mediators such as ferrocene derivatives, quinoid compounds or other transition metal complexes are more appropriate for this purpose. 

CNTs and other nano-sized carbon structures are promising materials for bioapplications, which was predicted even previous to their discovery. These nanoparticles have been applied in bioimaging and drag delivery, as implant materials and scaffolds for tissue growth, to modulate neuronal development and for lipid bilayer membranes. Considerable research has been done in the field of biosensors. Novel optical properties of CNTs have made them potential quantum dot sensors, as well as light emitters. Electrical conductance of CNTs has been exploited for field transistor based biosensors. CNTs and other nano-sized carbon structures are considered third generation amperometric biosensors, where direct electron transfer between the enzyme active center and the transducer takes place. Nanoparticle functionalization is required to achieve their full potential in many fields, including bio-applications. 

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