Electrically nanowired-enzymes for probe modification and sensor fabrication

Enzymes are highly specific and selective due to their precise, intricate three-dimensional catalytic- structure. Electron transfer in enzymes normally occurs through an active-metal centers or tunneling events that are highly insulated by the surrounding globular protein structure. In case of electrochemically active enzymes/proteins, the distance between the redox-active cofactor and the electrode surface plays key role during direct communication. Therefore, the long electron-tunneling distance can be overcome by introducing mobile redox mediators such as nanostructures specially nanowires which can diffuse into and out of the enzyme active site, ferrying reducing or oxidizing equivalents with them. Therefore, nanowire-conjugated enzymes have gained great interest in the development of biosensor devices and other electrocatalytic-biological applications. Herein we present a comprehensive review about the electrochemical enzyme-based sensor using nanowires. Over the past decade, nanowires were investigated as a versatile platform for various applications including sensors and biosensors because of their high aspect ratio and a high surface-to-volume ratio. This review aimed to summarize some of the recent developments in the enzyme based sensor research that have been achieved with various metallic and non-metallic one-dimensional nanostructure i.e. nanowires. Due to low or no toxicity and biocompatibility, enzymes conjugated with nanowires are still highly specific, sensitive and biologically active. This review demonstrates the potential usability of nanowired-enzymes for the bioanalytical applications. The review includes various types of nanowires, mode of the enzyme integration or immobilization methodologies, probe modification, biosensor fabrication and real or spiked sample testing. Biosensor parameters such as linear range and sensitivity, selectivity and detection limit of reported sensors were also considered herein. We also introduce some of the new nanowire materials which have not yet been used for biosensing or biosensor application. The limitations, challenges and prospects for the use of nanowired-enzymes in electrochemical and other real-time sensing systems as well as fabrication technologies are also discussed in this review.