10.1002/cctc.201800032
ChemCatChem
COMMUNICATION
Size Does Matter – Mediation of Electron Transfer by Gold
Clusters in Bioelectrocatalysis
Michal Kizling,[b] Maciej Dzwonek,[a] Agnieszka Więckowska,[a] Renata Bilewicz*[a]
Abstract: Metal nanostructures are often used in bioelectrocatalytic
systems to increase the electrode surface area or to improve the
conductivity of biofilms. We demonstrate, that decreasing the size of
gold nanoparticles below 2 nm may result in a change of the
mechanism of electron transfer (ET) between the enzyme active site
and the electrode from direct to mediated ET. Clusters with
diameters smaller than 2 nm exhibited molecule-like behavior
reflected in the appearance of oxidation and reduction peaks
separated by a clearly developed HOMO - LUMO gap. The redox
activity of the nanoparticles was found to contribute to the ET
mechanism of fructose dehydrogenase switching it to gold cluster
mediated electron transfer instead of direct ET. In the presence of
gold clusters at the electrode, the overpotential of the catalyzed
fructose oxidation reaction was 100 mV lower and the catalytic
reaction rate constant was 2.5 times larger confirming the unique
mediating role of the Au clusters..
often related to an expansion of the real electrode surface, and
is not a result of thermodynamics; such as a decrease in the
overpotential or a change in the reaction kinetics. Pankratov et.
al. have shown that AuNPs (20 – 100 nm) can be utilized as a
support for laccase immobilization. Nanoparticles that are
considerably larger than the enzyme molecules, only enlarge the
surface area of the electrode and an observed increase in the
obtained current output can be directly attributed to the increase
in the catalysts concentration[5]. Similar conclusions were
presented for smaller nanoparticles with diameters ranging from
5[6] and 7[7] nm. Gold nanoparticle-enzyme hybrid systems have
been shown to provide an electron relay pathway to the redox
center region[8]. Nonspecific molecular junctions have been
investigated by employing molecular wires, in conjunction with
nanoparticles, to connect electrodes to specific protein sites.
Such constructs differ considerably from a traditional molecular
wire, and were shown to enhance the electron transfer rates by
the electrostatic binding of the carboxylate terminus on the
nanoparticles to lysine residues in the heme regions of
cytochrome C[9]. Abad et. al. have shown that wiring galactose
oxidase to the metal redox center can give a considerable
improvement in the electrical contact and allow direct electron
transfer[10]. The use of genetically modified glucose oxidase with
a free thiol group for site-specific covalent binding to the AuNPs
has also been investigated, and the results obtained show a
significant decrease in the glucose oxidation overpotential. The
enhanced rate of electron transfer was ascribed to the
Gold nanoparticles (AuNPs) and their applications in
bioelectrocatalysis, have raised a significant interest in the last
few years[1]. Due to their high affinity to thiols they are an
excellent platform for introducing functional moieties and for the
surface attachment of biological species in a controlled manner.
AuNPs provide variability of chemical and physical properties
which are strongly dependent on the nanoparticle’s size. Optical
effects, such as surface plasmon bands due to collective
excitation of conduction electrons, are observed for
nanoparticles with diameters above ~2 nm. For diameters below
this value, the plasmon bands diminish and new bands are
developed corresponding to electronic transitions[2]. The
electrochemical properties of AuNPs also vary with size and
three types of behavior are observed: bulk gold exhibits
continuum behavior, small nanoparticles show quantum double-
layer charging, while the smallest – clusters, exhibit “molecule-
like” behavior. Voltammograms of AuNPs, with diameters lower
than 2 nm, give voltammetry current peaks corresponding to a
single electron transfer processes to or from the nanoparticle’s
metallic core[3], whereas molecule-like processes are reflected in
the oxidation and reduction signals, separated by a clearly
developed HOMO-LUMO gap[4].
nanostructures character[11]
.
In our previous work we constructed an enzymatic fuel cell
based on porous vitreous carbon and AuNPs with two different
sizes: 2.8 nm and 14 nm. We suggested that application of small
gold nanoparticles in an enzymatic fuel cell, modifies the
thermodynamics of the electrochemical reaction due to the
redox activity of the metallic core[12]. In this study, we explore a
bioelectrocatalytic system composed of fructose dehydrogenase
(FDH) adsorbed onto
a two–dimensional self–assembled
monolayer of AuNPs and how the role nanoparticles size
(ranging from 3.5 to 1.0 nm) plays on the activity of the system.
D-Fructose dehydrogenase is a heterotrimeric membrane-bound
enzyme complex, insensitive to oxygen[13] consisting of three
domains. Subunit I is the catalytic dehydrogenase domain with a
covalently bound flavin adenine dinucleotide (FAD) cofactor,
where D-(-)-fructose is involved in a 2H+/2e- oxidation reaction to
form 5-dehydro-D-(-)-fructose. Subunit II is equivalent to the
cytochrome domain and acts as electron acceptor to subunit I, it
also contains three heme C moieties covalently bound to the
enzyme scaffold and two of these domains are involved in the
one-by-one ET pathway. Finally, subunit III, which is not
involved in the ET pathway, but rather plays a key role in the
stability of the enzyme complex. FDH has been used as catalyst
in bioelectrocatalytic devices development for both direct and
Gold nanoparticles (AuNPs) are successfully utilized for
nanostructuring electrode surfaces in protein-based electronics.
In most cases, “planar” electrode surfaces without porous
nanostructure, show insufficient or no electron transfer (ET)
between the immobilized enzyme active sites and the electrode.
The commonly offered explanation for “enzyme nanowiring” is a
desirable orientation of the proteins on the surface to shorten the
ET distance. An increase in the catalytic current, originating from
nanostructuration of the electrode surface, is typically observed.
However, it should be emphasized that the observed effect is
mediated ET (DET and MET, respectively) modes[14]
.
[a]
[b]
M. Dzwonek, A. Więckowska, R. Bilewicz
Faculty of Chemistry
University of Warsaw
Pasteura 1, 02-093, Warsaw, Poland
E-mail: bilewicz@chem.uw.edu.pl
M. Kizling
College of Inter Faculty Individual Studies in Mathematics and
Natural Sciences
University of Warsaw
Stefana Banacha 2C, 02-097, Warsaw, Poland
Supporting information for this article is given via a link at the end of
the document.
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