11 research outputs found
Polymer coating for improved redox-polymer-mediated enzyme electrodes: A mini-review
Redox polymers have been widely used to facilitate and stabilize bioelectrochemical communication between the active sites of enzymes and electrodes, enabling development of high-performance enzyme electrodes for sensing, provision of power, and in synthesis. This review offers a brief overview of recent efforts in the use of additional
polymer layers on top of a redox polymer/enzyme layer, which can both improve the performance and expand the functionality of the resulting bioelectrodes
Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces
Metalloporphyrins
are active sites in metalloproteins and synthetic
catalysts. They have also been studied extensively by electrochemistry
as well as being prominent targets in electrochemical scanning tunneling
microscopy (STM). Previous studies of FePPIX adsorbed on graphite
and alkylthiol modified Au electrodes showed a pair of reversible
FeÂ(III/II)ÂPPIX peaks at about â0.41 V (vs NHE) at high solution
pH. We recently used iron protoporphyrin IX (FePPIX) as an intercalating
probe for long-range electrochemical electron transfer through a G-quadruplex
oligonucleotide (DNAzyme); this study disclosed two, rather than a
single pair of voltammetric peaks with a new and dominating peak,
shifted 200 mV positive relative to the ââ0.4 V peak.
Prompted by this unexpected observation, we report here a study of
the voltammetry of FePPIX itself on single-crystal Au(111), (100),
and (110) and polycrystalline Au electrode surfaces. In all cases
the dominating pair of new FeÂ(III/II)ÂPPIX redox peaks, shifted positively
by more than 200 mV compared to those of previous studies appeared.
This observation is supported by density functional theory (DFT) which
shows that strong dispersion forces in the FePPIX/Au electronic interaction
drive the midpoint potential toward positive values. The FePPIX spin
states depend on interaction with the Au(111) interface, converting
all the FeÂ(II)/(III)ÂPPIX species into low-spin states. These results
support electrochemical evidence for the nature of the electronic
coupling between FePPIX and Au-surfaces, and the electronic states
of adsorbate molecules, with a bearing also on recent reports of magnetic
FePPIX/AuÂ(111) interactions in ultrahigh vacuum (UHV)
Vpliv ĆĄportnorekreativne vadbe na gibljivost in moÄ gibalno oviranega otroka
Among
the low-index single-crystal gold surfaces, the Au(110) surface
is the most active toward molecular adsorption and the one with fewest
electrochemical adsorption data reported. Cyclic voltammetry (CV),
electrochemically controlled scanning tunneling microscopy (EC-STM),
and density functional theory (DFT) calculations have been employed
in the present study to address the adsorption of the four nucleobases
adenine (A), cytosine (C), guanine (G), and thymine (T), on the Au(110)-electrode
surface. Au(110) undergoes reconstruction to the (1 Ă 3) surface
in electrochemical environment, accompanied by a pair of strong voltammetry
peaks in the double-layer region in acid solutions. Adsorption of
the DNA bases gives featureless voltammograms with lower double-layer
capacitance, suggesting that all the bases are chemisorbed on the
Au(110) surface. Further investigation of the surface structures of
the adlayers of the four DNA bases by EC-STM disclosed lifting of
the Au(110) reconstruction, specific molecular packing in dense monolayers,
and pH dependence of the A and G adsorption. DFT computations based
on a cluster model for the Au(110) surface were performed to investigate
the adsorption energy and geometry of the DNA bases in different adsorbate
orientations. The optimized geometry is further used to compute models
for STM images which are compared with the recorded STM images. This
has provided insight into the physical nature of the adsorption. The
specific orientations of A, C, G, and T on Au(110) and the nature
of the physical adsorbate/surface interaction based on the combination
of the experimental and theoretical studies are proposed, and differences
from nucleobase adsorption on Au(111)- and Au(100)-electrode surfaces
are discussed
Nanoscale Confinement and Fluorescence Effects of Bacterial Light Harvesting Complex LH2 in Mesoporous Silicas
Many key chemical and biochemical reactions, particularly
in living
cells, take place in confined space at the mesoscopic scale. Toward
understanding of physicochemical nature of biomacromolecules confined
in nanoscale space, in this work we have elucidated fluorescence
effects of a light harvesting complex LH2 in nanoscale chemical environments.
Mesoporous silicas (SBA-15 family) with different shapes and pore
sizes were synthesized and used to create nanoscale biomimetic environments
for molecular confinement of LH2. A combination of UVâvis absorption,
wide-field fluorescence microscopy, and in situ ellipsometry supports
that the LH2 complexes are located inside the silica nanopores. Systematic
fluorescence effects were observed and depend on degree of space confinement.
In particular, the temperature dependence of the steady-state fluorescence
spectra was analyzed in detail using condensed matter band shape theories.
Systematic electronic-vibrational coupling differences in the LH2
transitions between the free and confined states are found, most likely
responsible for the fluorescence effects experimentally observed
Complexity of Gold Nanoparticle Formation Disclosed by Dynamics Study
Although chemically
synthesized gold nanoparticles (AuNPs) from
gold salt (HAuCl<sub>4</sub>) are among the most studied nanomaterials,
understanding the formation mechanisms is a challenge mainly due to
limited dynamics information. A range of in situ methods with down
to millisecond (ms) time resolution have been employed in the present
report to monitor time-dependent physical and chemical properties
in aqueous solution during the chemical synthesis. Chemical synthesis
of AuNPs is a reduction process accompanied by release of ions and
protons, and formation of solid particles. Dynamic information from
redox potential, pH, conductivity, and turbidity of the solution enables
distinct observation of reduction and nucleation/growth of AuNPs phases.
The dynamics of the electrochemical potential shows that reduction
of gold salt (HAuCl<sub>4</sub> and its hydrolyzed forms) occurs via
intermediate [AuCl<sub>2</sub>]<sup>â</sup> to form Au atoms
during the early stage of the synthesis process. pH- and conductivity-dynamics
point further clearly to formation of coating layers on AuNPs and
adsorbate exchange between MES and starch
Voltammetry and Single-Molecule in Situ Scanning Tunneling Microscopy of Laccases and Bilirubin Oxidase in Electrocatalytic Dioxygen Reduction on Au(111) Single-Crystal Electrodes
Laccases (E.C. 1.10.3.2) are multicopper oxidases catalytically active in the oxidation of diphenolics and related compounds by molecular dioxygen. The laccases contain a single-copper type I center and a trinuclear cluster of a single-copper type II and a dinuclear type III center. The oxidation of four equivalents of substrate near the type I copper and the sequential transfer of electrons to the trinuclear cluster are coupled with four-electron reduction of O<sub>2</sub> to H<sub>2</sub>O at the latter site. Extensive efforts have been given to kinetic and structural characterization of numerous laccases to elucidate the catalytic mechanism, where laccase (sub)monolayer voltammetry has been a core approach. In this report, we address voltammetry and electrocatalysis of O<sub>2</sub> reduction of (sub)monolayers of several laccases in new ways. These are based on the use of single-crystal, atomically planar bare Au(111)-electrode surfaces or surfaces modified by thiol-based self-assembled molecular monolayers. These well-defined surfaces enable introducing electrochemical scanning tunneling microscopy directly in aqueous biological media in which the enzymes are operative (in situ STM), to the level of resolution of the single enzyme molecule in electrocatalytic action. Enzyme-electrode electronic contact and intramolecular electron transfer triggered by the electrode potential or by O<sub>2</sub>âsubstrate binding to the enzyme, followed at the single-molecule level, are the most important observations of this study
Chiral Selectivity in Inter-reactant Recognition and Electron Transfer of the Oxidation of Horse Heart Cytochrome <i>c</i> by Trioxalatocobaltate(III)
Outer-sphere
electron transfer (ET) between optically active transition-metal complexes
and either other transition-metal complexes or metalloproteins is
a prototype reaction for kinetic chirality. Chirality as the ratio
between bimolecular rate constants of two enantiomers mostly amounts
to 1.05â1.2 with either the Î or Î form the more
reactive, but the origin of chirality in ET parameters such as work
terms, electronic transmission coefficient, and nuclear reorganization
free energy has not been addressed. We report a study of ET between
the Î-/Î-[CoÂ(Ox)<sub>3</sub>]<sup>3â</sup> pair
(Ox = oxalate) and horse heart cytochrome <i>c</i> (cyt <i>c</i>). This choice is prompted by strong ion-pair formation
that enables separation into inter-reactant interaction (chiral ârecognitionâ)
and ET within the ion pair (âstereoselectivityâ). Chiral
selectivity was first addressed experimentally. Î-[CoÂ(Ox)<sub>3</sub>]<sup>3â</sup> was found to be both the more strongly
bound and faster reacting enantiomer expressed respectively by the
ion-pair formation constant <i>K</i><sub>X</sub> and ET
rate constant <i>k</i><sub>ET</sub><sup>X</sup> (X = Î and Î), with <i>K</i><sub>Î</sub>/<i>K</i><sub>Î</sub> and <i>k</i><sub>ET</sub><sup>Î</sup>/<i>k</i><sub>ET</sub><sup>Î</sup> both â1.1â1.2. <i>rac</i>-[CoÂ(Ox)<sub>3</sub>]<sup>3â</sup> behavior is intermediate between those
of Î- and Î-[CoÂ(Ox)<sub>3</sub>]<sup>3â</sup>.
Chirality was next analyzed by quantum-mechanical ET theory combined
with density functional theory and statistical mechanical computations.
We also modeled the ion pair K<sup>+</sup>·[CoÂ(Ox)<sub>3</sub>]<sup>3â</sup> in order to address the influence of the solution
ionic strength. The complex structure of cyt <i>c</i> meant
that this reactant was represented solely by the heme group including
the chiral axial ligands L-His and L-Met. Both singlet and triplet
hemes as well as hemes with partially deprotonated propionic acid
side groups were addressed. The computations showed that the most
favorable inter-reactant configuration involved a narrow distance
and orientation space very close to the contact distance, substantiating
the notion of a reaction complex and the equivalence of the binding
constant to a bimolecular reaction volume. The reaction is significantly
diabatic even at these short inter-reactant distances, with electronic
transmission coefficients Îș<sub>el</sub><sup>X</sup> = 10<sup>â3</sup>â10<sup>â2</sup>. The computations demonstrated chirality in both <i>K</i><sub>X</sub> and Îș<sub>el</sub><sup>X</sup> but no chirality in the reorganization and
reaction free energy (driving force). As a result of subtle features
in both <i>K</i><sub>X</sub> and Îș<sub>el</sub><sup>X</sup> chirality, the âoperationalâ
chirality Îș<sub>ET</sub><sup>Î</sup><i>K</i><sub>Î</sub>/Îș<sub>ET</sub><sup>Î</sup><i>K</i><sub>Î</sub> emerges larger than unity (1.1â1.2)
from the molecular modeling as in the experimental data
Polycation Induced Potential Dependent Structural Transitions of Oligonucleotide Monolayers on Au(111)-Surfaces
We have studied self-assembled molecular monolayers (SAMs)
of several
3âČ-C3-SH conjugated single-strand (ss) and double-strand (ds)
20-base oligonucleotides (ONs) immobilized on single-crystal, atomically
planar Au(111)-electrode surfaces in the presence of the triply positively
charged base spermidine (Spd). This cation binds strongly to the polyanionic
ON backbone and stabilizes the ds-form relative to the ss-form. A
combination of chemical ON synthesis, melting temperature measurements,
cyclic voltammetry (CV), and <i>in situ</i> scanning tunneling
microscopy (STM) in aqueous biological buffer under electrochemical
potential control was used. Spd binding was found to increase notably
the ds-ON melting temperature. CV displays capacitive features associated
with ss- and ds-ON. A robust capacitive peak around â0.35 V
versus saturated calomel electrode (SCE), specific to ds-ON and highly
sensitive to base pair mismatches, was consistently observed. The
peak is likely to be caused by surface structural reorganization around
the peak potential and located close to reported peak potentials of
several DNA intercalating or covalently tethered redox molecules reported
as probes for long-range electron transfer
Single-Molecule Electrochemical Gating in Ionic Liquids
The single-molecular conductance of a redox active molecular
bridge
has been studied in an electrochemical single-molecule transistor
configuration in a room-temperature ionic liquid (RTIL). The redox
active pyrrolo-tetrathiafulvalene (pTTF) moiety was attached to gold
contacts at both ends through â(CH<sub>2</sub>)<sub>6</sub>Sâ groups, and gating of the redox state was achieved with
the electrochemical potential. The water-free, room-temperature, ionic
liquid environment enabled both the monocationic and the previously
inaccessible dicationic redox states of the pTTF moiety to be studied
in the in situ scanning tunneling microscopy (STM) molecular break
junction configuration. As the electrode potential is swept to positive
potentials through both redox transitions, an ideal switching behavior
is observed in which the conductance increases and then decreases
as the first redox wave is passed, and then increases and decreases
again as the second redox process is passed. This is described as
an âoffâonâoffâonâoffâ conductance
switching behavior. This molecular conductance vs electrochemical
potential relation could be modeled well as a sequential two-step
charge transfer process with full or partial vibrational relaxation.
Using this view, reorganization energies of âŒ1.2 eV have been
estimated for both the first and second redox transitions for the
pTTF bridge in the 1-butyl-3-methylimidazolium trifluoromethanesulfonate
(BMIOTf) ionic liquid environment. By contrast, in aqueous environments,
a much smaller reorganization energy of âŒ0.4 eV has been obtained
for the same molecular bridge. These differences are attributed to
the large, outer-sphere reorganization energy for charge transfer
across the molecular junction in the RTIL
Controlling the Stereochemistry and Regularity of Butanethiol Self-Assembled Monolayers on Au(111)
The
rich stereochemistry of the self-assembled monolayers (SAMs)
of four butanethiols on Au(111) is described, the SAMs containing
up to 12 individual C, S, or Au chiral centers per surface unit cell.
This is facilitated by synthesis of enantiomerically pure 2-butanethiol
(the smallest unsubstituted chiral alkanethiol), followed by in situ
scanning tunneling microscopy (STM) imaging combined with density
functional theory molecular dynamics STM image simulations. Even though
butanethiol SAMs manifest strong headgroup interactions, steric interactions
are shown to dominate SAM structure and chirality. Indeed, steric
interactions are shown to dictate the nature of the headgroup itself,
whether it takes on the adatom-bound motif RS<sup>âą</sup>AuÂ(0)ÂS<sup>âą</sup>R or involves direct binding of RS<sup>âą</sup> to face-centered-cubic or hexagonal-close-packed sites. Binding
as RS<sup>âą</sup> produces large, organizationally chiral domains
even when R is achiral, while adatom binding leads to rectangular
plane groups that suppress long-range expression of chirality. Binding
as RS<sup>âą</sup> also inhibits the pitting intrinsically associated
with adatom binding, desirably producing more regularly structured
SAMs