The Application of Engineered Glucose Dehydrogenase to a Direct Electron–Transfer‐Type Continuous Glucose Monitoring System and a Compartmentless Biofuel Cell

Third generation impedimetric sensor employing direct electron transfer type glucose dehydrogenase

Faradaic electrochemical impedance spectroscopy (faradaic EIS) is an attractive measurement principle for biosensors. However, there have been no reports on sensors employing direct electron transfer (DET)-type redox enzymes based on faradaic EIS principle. In this study, we have attempted to construct the 3rd-generation faradaic enzyme EIS sensor, which used DET-type flavin adenine dinucleotide (FAD) dependent glucose dehydrogenase (GDH) complex, to elucidate its characteristic properties as well as to investigate its potential application as the future immunosensor platform. The gold disk electrodes (GDEs) with DET-type FADGDH prepared using self-assembled monolayer (SAM) showed the glucose concentration dependent impedance change, which was confirmed by the change in the charge transfer resistance (R_ct). The Δ(1/R_ct) values were also affected by DC bias potential and the length of SAM. Based on the Nyquist plot and Bode plot simulations, glucose sensing by imaginary impedance monitoring under fixed frequency (5 mHz) was carried out, revealing the higher sensitivity at low glucose concentration with wider linear range (0.02-0.2 mM). Considering this high sensitivity toward glucose, the 3rd-generation faradaic enzyme EIS sensor would provide alternative platform for future impedimetric immunosensing system, which does not use redox probe.

Direct enzymatic bioelectrocatalysis: differentiating between myth and reality

Enzymatic bioelectrocatalysis is being increasingly exploited to better understand oxidoreductase enzymes, to develop minimalistic yet specific biosensor platforms, and to develop alternative energy conversion devices and bioelectrosynthetic devices for the production of energy and/or important chemical commodities. In some cases, these enzymes are able to electronically communicate with an appropriately designed electrode surface without the requirement of an electron mediator to shuttle electrons between the enzyme and electrode. This phenomenon has been termed direct electron transfer or direct bioelectrocatalysis. While many thorough studies have extensively investigated this fascinating feat, it is sometimes difficult to differentiate desirable enzymatic bioelectrocatalysis from electrocatalysis deriving from inactivated enzyme that may have also released its catalytic cofactor. This article will review direct bioelectrocatalysis of several oxidoreductases, with an emphasis on experiments that provide support for direct bioelectrocatalysis versus denatured enzyme or dissociated cofactor. Finally, this review will conclude with a series of proposed control experiments that could be adopted to discern successful direct electronic communication of an enzyme from its denatured counterpart.

BioCapacitor: A novel principle for biosensors

Studies regarding biofuel cells utilizing biocatalysts such as enzymes and microorganisms as electrocatalysts have been vigorously conducted over the last two decades. Because of their environmental safety and sustainability, biofuel cells are expected to be used as clean power generators. Among several principles of biofuel cells, enzyme fuel cells have attracted significant attention for their use as alternative energy sources for future implantable devices, such as implantable insulin pumps and glucose sensors in artificial pancreas and pacemakers. However, the inherent issue of the biofuel cell principle is the low power of a single biofuel cell. The theoretical voltage of biofuel cells is limited by the redox potential of cofactors and/or mediators employed in the anode and cathode, which are inadequate for operating any devices used for biomedical application. These limitations inspired us to develop a novel biodevice based on an enzyme fuel cell that generates sufficient stable power to operate electric devices, designated "BioCapacitor." To increase voltage, the enzyme fuel cell is connected to a charge pump. To obtain a sufficient power and voltage to operate an electric device, a capacitor is used to store the potential generated by the charge pump. Using the combination of a charge pump and capacitor with an enzyme fuel cell, high voltages with sufficient temporary currents to operate an electric device were generated without changing the design and construction of the enzyme fuel cell. In this review, the BioCapacitor principle is described. The three different representative categories of biodevices employing the BioCapacitor principle are introduced. Further, the recent challenges in the developments of self-powered stand-alone biodevices employing enzyme fuel cells combined with charge pumps and capacitors are introduced. Finally, the future prospects of biodevices employing the BioCapacitor principle are addressed.