Direct, Electrocatalytic Oxygen Reduction by Laccase on Anthracene-2-methanethiol-Modified Gold

Abstract

Laccase, a multicopper oxidase, catalyses the four electron reduction of oxygen to water. Upon adsorption to an electrode surface, laccase is known to reduce oxygen at overpotentials lower than the best noble metal electrocatalysts usually employed. While the electrocatalytic activity of laccase is well established on carbon electrodes, laccase does not typically adsorb to better defined noble metal surfaces in an orientation that allows for efficient electrocatalysis. In this work, we utilized anthracene-2-methanethiol (AMT) to modify the surface of Au electrodes and examined the electrocatalytic activity of adsorbed laccase. AMT facilitated the adsorption of laccase, and the onset of electrocatalytic oxygen reduction was observed as high as 1.13 VRHE. We observed linear Tafel behavior with a 144 mV/dec slope, consistent with an outer sphere single electron transfer from the electrode to a Cu site in the enzyme as the rate determining step of the oxygen reduction mechanism.

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“…It is worth noting that this LAC is structurally different to the others, being organized as a trimer. [187] SAM-functionalized gold electrodes and gold nanoparticles (NPs) with anthracenethiol derivatives, [188] or phenyl groups ending either with amino or carboxyl functions were also evaluated as platforms for efficient orientation of Trametes versicolor (Tv), Tt and Trametes hirsute (Th) LACs. [189][190][191] PMIRRAS, QCM, and electrochemistry were coupled and allowed a specific orientation linked to a different activity of the enzyme depending on the functionality borne by the SAM to be demonstrated.…”

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“…where PI and NI are the peroxy and native intermediates, respectively [49]. Considering the expressions in equations 2a and 2b, the rate determining step for each process can be proposed [45]. In the case of Lac-erGO-GC electrode, an α transfer coefficient close to 1/2 is obtained from the Tafel slope value of 129 mV dec -1 , which is the expected value for an ideal one electron transfer process at ambient temperature [50].…”

“…In order to study the purely kinetic contribution of the electrodes towards ORR [45], polarization curves of the Lac-erGO-GC and BOx-erGO-GC electrodes were obtained by subtracting the blank cathodic scans measured under N 2 to the ones measured under O 2 , thus eliminating the capacitive currents of the electrodes ( Figure 9A). Tafel plots were obtained from the polarization curves after correction of the ohmic drop caused by the resistances of the system (mainly by ionic resistance of the electrolyte) and of the mass transport limitations of oxygen from the solution to the electrode ( Figure 9B) [45].…”

“…Tafel plots were obtained from the polarization curves after correction of the ohmic drop caused by the resistances of the system (mainly by ionic resistance of the electrolyte) and of the mass transport limitations of oxygen from the solution to the electrode ( Figure 9B) [45]. The iR drop was corrected using the R s value obtained from the intersection of the semicircle at the high frequency region of Nyquist plot, whereas the mass transport corrected current density was calculated using equation (1) [45], derived from the Koutecky-Levich equation for a first order reaction:…”

“…150 mV difference in onset potential is attributed to the difference in the redox potential of the copper T1 site of each enzyme [20], as the different proton activity of the electrolytes (2.8 pH units) is not considered to have an effect due to the thermodynamic potential given by the Nernst relationship [47]. In order to propose an oxygen reduction reaction mechanism for the enzymatic electrodes, multistep and multi-electron reactions are considered with both an α transfer coefficient (a) and Tafel (b) expressions as follows [45]:…”