No abstract

Enzymes are powerful catalysts for biosensor and biofuel cell electrodes due to their unique substrate specificity. This specificity is defined by the amino acid chain's complex three-dimensional structure based on non-covalent forces, being also responsible for the very limited enzyme lifetime of days to weeks. Many electrochemical applications, however, would benefit from lifetimes over months to years. This mini-review provides a critical overview of strategies and ideas dealing with the problem of short enzyme lifetime, which limits the overall lifetime of bioelectrochemical electrodes. The most common approaches aim to stabilize the enzyme itself. Various immobilization techniques have been used to reduce flexibility of the amino acid chain by introducing covalent or non-covalent binding forces to external molecules. The enzyme can also be stabilized using genetic engineering methods to increase the binding forces within the protein or by optimizing the environment in order to reduce destabilizing interactions. In contrast, renewing the inactivated catalyst decouples overall system lifetime from the limited enzyme lifetime and thereby promises theoretically unlimited electrode lifetimes. Active catalyst can be supplied by exchanging the electrolyte repeatedly. Alternatively, integrated microorganisms can display the enzymes on their surface or secrete them to the electrolyte, allowing unattended power supply for long-term applications.

No abstract

Enzymatic biofuel cells (BFCs) show great potential for the direct conversion of biochemically stored energy from renewable biomass resources into electricity. However, enzyme purification is time-consuming and expensive. Furthermore, the long-term use of enzymatic BFCs is hindered by enzyme degradation, which limits their lifetime to only a few weeks. We show, for the first time, that crude culture supernatant from enzyme-secreting microorganisms (Trametes versicolor) can be used without further treatment to supply the enzyme laccase to the cathode of a mediatorless BFC. Polarization curves show that there is no significant difference in the cathode performance when using crude supernatant that contains laccase compared to purified laccase in culture medium or buffer solution. Furthermore, we demonstrate that the oxygen reduction activity of this enzymatic cathode can be sustained over a period of at least 120 days by periodic resupply of crude culture supernatant. This is more than five times longer than control cathodes without the resupply of culture supernatant. During the operation period of 120 days, no progressive loss of potential is observed, which suggests that significantly longer lifetimes than shown in this work may be possible. Our results demonstrate the possibility to establish simple, cost efficient, and mediatorless enzymatic BFC cathodes that do not require expensive enzyme purification procedures. Furthermore, they show the feasibility of an enzymatic BFC with an extended lifetime, in which self-replicating microorganisms provide the electrode with catalytically active enzymes in a continuous or periodic manner.

Graphical abstract