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 Fe­(III/II)­PPIX peaks at about −0.41 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 ≈−0.4 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 Fe­(III/II)­PPIX 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 Fe­(II)/(III)­PPIX 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/Au­(111) interactions in ultrahigh vacuum (UHV)

Vpliv športnorekreativne vadbe na gibljivost in moč 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 × 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 UV–vis 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 (HAuCl₄) 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 (HAuCl₄ and its hydrolyzed forms) occurs via intermediate [AuCl₂]⁻ 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 O₂ to H₂O 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 O₂ 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 O₂–substrate 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 1.05–1.2 with either the Λ or Δ 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 Λ-/Δ-[Co­(Ox)₃]^3– 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 “recognition”) and ET within the ion pair (“stereoselectivity”). Chiral selectivity was first addressed experimentally. Λ-[Co­(Ox)₃]^3– was found to be both the more strongly bound and faster reacting enantiomer expressed respectively by the ion-pair formation constant <i>K</i>_X and ET rate constant <i>k</i>_ET^X (X = Λ and Δ), with <i>K</i>_Λ/<i>K</i>_Δ and <i>k</i>_ET^Λ/<i>k</i>_ET^Δ both ≈1.1–1.2. <i>rac</i>-[Co­(Ox)₃]^3– behavior is intermediate between those of Λ- and Δ-[Co­(Ox)₃]^3–. 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⁺·[Co­(Ox)₃]^3– 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 κ_el^X = 10^–3–10^–2. The computations demonstrated chirality in both <i>K</i>_X and κ_el^X but no chirality in the reorganization and reaction free energy (driving force). As a result of subtle features in both <i>K</i>_X and κ_el^X chirality, the “operational” chirality κ_ET^Λ<i>K</i>_Λ/κ_ET^Δ<i>K</i>_Δ emerges larger than unity (1.1–1.2) 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 3′-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 −0.35 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 −(CH₂)₆S– 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 “off–on–off–on–off” 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 ∼1.2 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 ∼0.4 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^•Au­(0)­S^•R or involves direct binding of RS^• to face-centered-cubic or hexagonal-close-packed sites. Binding as RS^• 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^• also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs