Electron Transfer Between Enzymes and Electrodes

Abstract

Efficient electron transfer between redox enzymes and electrocatalytic surfaces plays a significant role in development of novel energy conversion devices as well as novel reactors for production of commodities and fine chemicals. Major application examples are related to enzymatic fuel cells and electroenzymatic reactors, as well as enzymatic biosensors. The two former applications are still at the level of proof-of-concept, partly due to the low efficiency and obstacles to electron transfer between enzymes and electrodes. This chapter discusses the theoretical backgrounds of enzyme/electrode interactions, including the main mechanisms of electron transfer, as well as thermodynamic and kinetic aspects. Additionally, the main electrochemical methods of study are described for selected examples. Finally, some recent advancements in the preparation of enzyme-modified electrodes as well as electrodes for soluble co-factor regeneration are reviewed.

Appendix: List of Symbols

A :

Amplitude (V)

a :

Internal active surface area (${\mathrm{m}}_{\mathrm{act}}^2\ {\mathrm{m}}_{\mathrm{geo}}^{-3}\Big)$

A_geo :

Geometrical surface area of electrode (${\mathrm{m}}_{\mathrm{geo}}^2\Big)$

c :

Volumetric concentration (mol m⁻³)

c _DL :

Double layer capacitance (F m⁻²)

D :

Diffusion coefficient of species (${\mathrm{m}}_{\mathrm{geo}}^2\ {\mathrm{s}}^{-1}\Big)$

E :

Electrode potential (V)

F:

Faraday’s constant = 96,485 (C mol⁻¹)

f :

Frequency (Hz)

G :

Flow rate (m³ s⁻¹)

g _k :

Diffusion flux (k = 1,2,3) ($\mathrm{mol}\ {\mathrm{m}}_{\mathrm{geo}}^{-2}$s⁻¹)

I :

Current (A)

i :

Imaginary number

Im(Z):

Imaginary part of electrochemical impedance Z (Ω m²)

j :

Current density ($\mathrm{A}\ {\mathrm{m}}_{\mathrm{geo}}^{-2}\Big)$

k ₁, k _m :

Reaction constants of enzyme substrate (m³ mol⁻¹ s⁻¹/m² mol⁻¹ s⁻¹)

and enzyme mediator reactions

k _ei :

Kinetic constant of the (s⁻¹)

Electrochemical reaction (i = 1,2)

K _M :

Michaelis-Menten constant (mol m⁻³)

L :

Catalyst layer thickness (m _geo)

n :

Number of electrons

P :

Power density (W m⁻²)

r :

Reaction rate (mol s⁻¹ m⁻²)

R:

Universal gas constant = 8.314 (J mol⁻¹ K⁻¹)

R _Ω :

Electrolyte resistance (Ω m²)

Re(Z):

Real part of electrochemical impedance Z (Ω m²)

T :

Temperature (K)

U :

Cell potential (V)

v _k :

Average molar velocity (k = 1,2,3) (m s⁻¹)

w:

Rotation rate of rotating disc electrode (rad s⁻¹)

Y:

Linear frequency response function (Ω⁻¹ m⁻²)

|Z|:

Magnitude of electrochemical impedance Z (Ω m²)

Z :

Electrochemical impedance (Ω m²)

z _k :

Space coordinate (k = 1,2,3)

v :

Sweep rate (V s⁻¹)

1.1 Greek

ν:

Stoichiometric coefficient

η :

Overpotential (V)

φ:

Phase shift (^o)

Γ :

Surface concentration (mol m⁻²)

η _i :

Efficiency (i = th, ec,fuel)

${\Delta}_f{G}_i^o$ :

Standard Gibbs free energies of formation of component “i” (kJ mol⁻¹)

Δ_r G ^o :

Standard Gibbs free energy change of reaction (kJ mol⁻¹)

Δ_r H ^o :

Standard enthalpy change of reaction (kJ mol⁻¹)

ϕ_E, ϕ_I :

Potentials of electron-and ion- conducting phases, respectively (V)

γ_E, γ_I :

Electron-and ion conductivities ($\mathrm{S}\ {\mathrm{m}}_{\mathrm{geo}}^{-1}$)

α,β:

Transfer coefficients of electrochemical steps

δ:

Diffusion layer thickness (m_geo)

ε:

Void fraction (${\mathrm{m}}^3{\mathrm{m}}_{\mathrm{geo}}^{-3}\Big)$

ι:

Local current density ($\mathrm{A}\ {\mathrm{m}}_{\mathrm{act}}^{-2}\Big)$

ω:

Angular frequency (rad s⁻¹)

1.2 Super- and Sub-scripts

A,C,cell:

Anode, cathode and cell respectively

act, geo:

Active and geometrical respectively

CL, DL:

Catalyst layer and diffusion layer respectively

e0:

Electrochemical reaction step

ec:

Electrochemical

I, E:

Ion and electron conducting phase respectively

o:

Standard conditions

o,#:

At pH 7

Ohm:

Ohmic

ox,red:

Oxidized and reduced states respectively

S:

Substrate

sim, exp.:

Simulation and experiment respectively

SS:

Steady state

th:

Thermodynamic

1.3 List of Abbreviations

A:

Anode

Ag/AgCl:

Silver/silver chloride reference electrode

C:

Cathode

CC:

Current collector

CE:

Counter electrode

CL:

Catalyst layer

DET:

Direct electron transfer

DET_SS:

Direct electron transfer steady state

E:

Enzyme

EM:

Enzyme mediator complex

ES:

Enzyme substrate complex

FAD:

Flavin adenine dinucleotide

FMN:

Flavin mononucleotide

GOx:

Glucose oxidase

HRP:

Horseradish peroxidase

Int:

Intermediate

Medi (i = ox,red):

Oxidized and reduced forms of a mediator

MET:

Mediated electron transfer

NAD:

Nicotinamide adenine dinucleotide

NADP:

Nicotinamide adenine dinucleotide phosphate

P:

Product

RE:

Reference electrode

RH:

Organic substrate

S:

Substrate

SAMs:

Self-assembled monolayers

SCE:

Saturated calomel electrode

SHE:

Standard hydrogen electrode

WE:

Working electrode