Blue copper proteins as bioelements for bioelectronic devices
Abstract: This thesis is focused on bioelements for biological electric power sources, specifically, on blue copper proteins with and without an intrinsic biocatalytic activity, i.e. ability to reduce oxygen directly to water. These proteins, viz. different laccases, ceruloplasmin, and rusticyanin, were characterised in detail and employed for the construction of both self-charging and conventional biosupercapacitors. First, similarities and particularities of oxygen electroreduction vs. bioelectroreduction were reviewed. Moreover, being a promising candidate for the construction of autotolerant implantable biocathodes, the electrochemistry of human ceruloplasmin was revisited. For the first time, a clear bioelectrocatalytic reduction of oxygen on ceruloplasmin modified electrodes was shown. Second, computational design combined with directed evolution resulted in a high redox potential mutated laccase, GreeDo, with increased redox potential of the T1 site, increased activity towards high redox potential mediators, as well as enhanced stability. Third, GreeDo was electrochemically characterised in detail. The mutant exhibited higher open circuit potential values and onset potentials for oxygen bioelectroreduction compared to the parental laccase, OB-1. Moreover, the operational stability of GreeDo modified graphite electrodes was found to be more than 2 h in a decidedly acidic electrolyte, in agreement with the extended operational and storage stabilities of the enzyme in acidic solutions. Fourth, multi-cell single-electrolyte glucose/ oxygen biodevices with adjustable open-circuit and operating voltages, which are independent on the difference in equilibrium redox potentials of the two redox couples, gluconolactone/glucose and oxygen/water, viz. 1.18 V, but dependent on the number of half-cells in the biodevice construction, were designed and tested. The biodevices were made from tubular graphite electrodes with electropolymerised poly(3,4-ethylenedioxythiophene) modified with Trametes hirsuta laccase and Neurospora crassa cellobiose dehydrogenase as the cathodic and anodic biocatalysts, respectively. Due to the interplay between faradaic and non-faradaic electrochemical processes, as well as between ionic and electronic conductivities, the open-circuit voltage of the self-charged biodevice is extraordinarily high, reaching 3 V, when seven biosupercapacitors operating in a common electrolyte were connected in series. Moreover, glucose/oxygen biodevices could be externally discharged at an operating voltage exceeding the maximal limiting open-circuit value of 1.24 V for the complete glucose oxidation. Last but not least, a conventional biosupercapacitor, i.e. a biodevice lacking self-charging ability, was composed of Acidithiobacillus ferrooxidans rusticyanin modified gold electrodes. The complete biodevices as well as separate electrodes were thoroughly characterised electrochemically. The symmetrical biosupercapacitor based on two identical gold electrodes modified with rusticyanin is able to capacitively store electricity and deliver electric power, accumulated mostly in the form of biopseudocapacitance, when charged and discharged externally.
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