Assembly of electroactive layer-by-layer films of myoglobin and ionomer poly(ester sulfonic acid)

Layer-by-layer films were assembled on solid substrates by alternate adsorption of negatively charged ionomer poly(ester sulfonic acid) or Eastman AQ55 from its aqueous dispersion and positively charged myoglobin (Mb) from its solution at pH 4.5. The film assembly process was monitored by cyclic voltammetry (CV), UV-vis spectroscopy, and quartz crystal microbalance (QCM). [AQ/Mb](n) films grown on pyrolytic graphite (PG) electrodes showed a pair of well-defined and nearly reversible CV peaks at about -0.20 V vs Ag/AgCl in pH 5.5 buffers, characteristic of the Mb heme Fe(III)/Fe(II) redox couple. Although the amount of Mb adsorbed in each bilayer was essentially the same, the fraction of electroactive Mb decreased dramatically with an increase of bilayer number (n). Soret absorption bands of [AQ/Mb](n) films on glass slides suggest that Mb in the films retains its native state in the medium pH range. Trichloroacetic acid, oxygen, and hydrogen peroxide were electrochemically catalyzed by [AQ/Mb](6) films with significant lowering of reduction overpotential.     

Electrochemical Studies of Thiol Self‐Assembled Monolayers at a Heated Gold‐Wire Microelectrode

Mercaptoundecanoic acid (MUA) and glutathione (GSH) self‐assembled monolayers were prepared on gold‐ wire microelectrode. Cyclic voltammetry was used to investigate the influence of temperature on electrochemical behaviors of Fe(CN)₆^3?/4? and Ru(NH₃)₆^3+/2+ at these SAMs modified electrodes in aqueous solution. It is found that temperature shows great influence on electron transfer (ET) and mass transport (MT) for the two SAMs modified electrodes and the influence of temperature depends on the charge properties of the redox couples and terminal groups of SAMs and the structure of the monolayer on gold surface. The temperature can greatly increase MT rate of Fe(CN)₆^3?/4? at both MUA and GSH modified electrodes. However, the increased MT rate doesn't have any effect on the CV's for Fe(CN)₆^3?/4? /MUA system. For Ru(NH₃)₆^3+/2+ , temperature can greatly improve the electrochemical reaction in both MUA and GSH modified electrodes, which is ascribed to temperature‐induced diffusion and convection and the electrostatic interaction between Ru(NH₃)₆^3+/2+ and negatively charged carboxyl groups on the electrode surface.     

Electrochemical assembly and potential-dependent plasmon absorption of au nanoclusters covered with a 4-aminothiophenol self-assembled monolayer

Gold nanoclusters covered with 4-aminothiophenol (4-ATP) self-assembled monolayers (SAMs) were electrochemically assembled on an Au or ITO electrode. The assembly mechanism is discussed on the basis of results of electrochemical, FT-IR, and XPS measurements. The intensity of plasmon absorption of the gold nanocluster assembly was shown to be dependent on applied potential as a result of electrochemical doping/undoping of a counteranion in the polyaniline film.     

Surface‐Tuned Electron Transfer and Electrocatalysis of Hexameric Tyrosine‐Coordinated Heme Protein

Molecular modeling, electrochemical methods, and quartz crystal microbalance were used to characterize immobilized hexameric tyrosine‐coordinated heme protein (HTHP) on bare carbon or on gold electrodes modified with positively and negatively charged self‐assembled monolayers (SAMs), respectively. HTHP binds to the positively charged surface but no direct electron transfer (DET) is found due to the long distance of the active sites from the electrode surfaces. At carboxyl‐terminated surfaces, the neutrally charged bottom of HTHP can bind to the SAM. For this “disc” orientation all six hemes are close to the electrode and their direct electron transfer should be efficient. HTHP on all negatively charged SAMs showed a quasi‐reversible redox behavior with rate constant k_s values between 0.93 and 2.86 s^?1 and apparent formal potentials ${E{{0{^{\prime }}\hfill \atop {\rm app}\hfill}}}$ between ‐131.1 and ‐249.1 mV. On the MUA/MU‐modified electrode, the maximum surface concentration corresponds to a complete monolayer of the hexameric HTHP in the disc orientation. HTHP electrostatically immobilized on negatively charged SAMs shows electrocatalysis of peroxide reduction and enzymatic oxidation of NADH.     

Electrochemistry at chemically assembled single-wall carbon nanotube arrays

Single-wall carbon nanotubes (SWNTs) chemically assembled on gold substrates were employed as electrodes to investigate the charge transfer process between SWNTs and the underlying substrates. Cyclic voltammetry (CV) indicates that the assembled SWNTs allow electron communication between a gold electrode and the redox couple in solution, though the SWNTs are linked directly onto the insulating monolayer of 11-amino-n-undecanethiol (AUT) on the Au substrate. An electron transfer (ET) mechanism, which contains an electron tunneling process across the AUT monolayer, is proposed to explain the CV behavior of Au/AUT/SWNT electrodes. Electrochemical measurements show that the apparent electron tunneling resistance, which depends on the surface density of assembled SWNTs, has apparent effects similar to those of solution resistance on CV behavior . The theory of solution resistance is used to describe the apparent tunneling resistance. The experimental results of the dependence of ET parameter psi on the potential scan rate upsilon are in good agreement with the theoretical predictions. Kinetic studies of the chemical assembly of SWNTs by atomic force microscopic (AFM), electrochemical, and Raman spectroscopic methods reveal that two distinct assembly kinetics exist: a relatively fast step that is dominated by the surface reaction, and a successive slow step that is governed by bundle formation.     

Electrochemical studies of derivatized thiol self-assembled monolayers on gold electrode in the presence of surfactants.

Electrochemical impendence spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed to investigate the barrier properties and electron transfer of derivatized thiol self-assembled monolayers (SAMs) on gold in the presence of surfactants. The thiol derivatives used included 2-mercaptoethanesulfonic acid (MES), 2-mercaptoacetic acid (MAA), and N-acetyl-L-cysteine (NAC). A simple equivalent circuit was derived to fit the impedance spectra very well. The negative redox probe [Fe(CN)6](3-/4-) was selected to indicate the electron-transfer efficiency on the interface of the studied electrodes. It was found that by changing the surface structure of SAMs, different surfactants could regulate the barrier properties and electron-transfer efficiency in different ways. A positively charged surfactant lowered the electrostatic repulsion between the negative redox probe and negatively charged surface groups of a monolayer, while enhancing the reversibility of electron transfer by virtue of increasing the redox probe concentration within the electric double-layer region. A neutral surfactant showed no significant effect, while a negative surfactant hindered the access and reaction of redox probe by electrostatic repulsion of same-sign charges.     

Detection of Tellurium in Presence of Extra Arsenic on Gold Electrodes Modified By Benzenedithiol Self‐Assembled Monolayers

Electrochemical detection of tellurium is interfered with arsenic due to their similar redox potentials. Gold electrodes modified by benzenedithiols (BDT) self‐assembled monolayers (SAMs) were explored to determine Te(IV) in the presence of extra As(III) in this work. SAMs of benzenedithiols on the gold surface allows for selective extraction of Te (IV) over As(III), while blocking the redox of As(III). It provides potential applications to tellurium toxicity analysis, biological membrane study and identification of geochemistry samples.     

Electrochemical studies of blocking properties of solid supported tethered lipid membranes on gold

The insulating properties of self-assembled thiolipid monolayers and tethered lipid bilayers on polycrystalline gold electrodes were studied by means of cyclic voltammetry (CV). These films were formed by two-step self-assembly processes. Electrochemical measurements of the heterogeneous electron transfer rate constant of different redox couples such as potassium ferrocyanide (K(4)[Fe(CN)(6)]) and dopamine (DP) were used to examine the molecular integrity and structural defects and pinholes within the monolayers. We demonstrate by means of cyclic voltammetry that the bilayer lipid membranes tethered to the gold surface are blocking, stable, yet retaining their dynamic properties and can be used as a model of the cell membrane.     

Electron transfer in SAM/cytochrome/polyelectrolyte hybrid systems on electrodes: a time-resolved surface-enhanced resonance Raman study

Silver electrodes were covered with mixed self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) and 11-mercaptoundecanol (MU) and subsequently coated with alternating layers of cytochrome c (Cyt) and poly(anilinesulfonic acid) (PASA). The immobilized protein is electroactive and retains its native structure. Compared to the case of systems on gold electrodes, the stability of the assembly was found to be decreased. The redox process of Cyt is accompanied by reversible oxidation-reduction of PASA as revealed by the comparative surface-enhanced resonance Raman (SERR) analysis of assemblies including Cyt and the redox-inactive apo-cytochrome c. Time-resolved SERR experiments show a fast electron exchange between the protein and the polyelectrolyte that may play a supporting role in the electric communication of thicker multilayer assemblies employed as sensors.     

Surface‐Confined Electropolymerization of Methylene Blue on Gold Electrodes

We report on the electrochemical behaviour and electropolymerization of self‐assembled monolayers (SAMs) of methylene blue (MB) on gold electrodes. The SAMs of MB on gold electrodes were prepared by immersing the substrates into a solution of 1.0 mM MB in absolute ethanol for different times at room temperature. Cyclic voltammetry experiments exhibited that reductive desorption of MB monolayer takes place at three different potentials on polycrystalline gold electrodes, while reductive desorption of MB monolayer consists of only one peak on single crystal Au(111) substrates. Calculated charge densities for different immersion times indicated that optimal immersion time for self‐assembly of MB is 96 h. Electropolymerization of SAMs of MB on gold electrode was achieved by applying 0.95 V for 1 s in 0.1 M borate buffer solution (pH: 9.0). It was observed that poly(MB) monolayers are highly stable in acidic media. ATR‐FTIR and UV‐vis spectra exhibited differences between monomer and polymer monolayers, which are attributed to surface‐confined electropolymerization. STM image of poly(MB) monolayer on Au(111) substrate revealed a surface that is covered by well‐ordered, collateral nanowires with an average size of 3 nm.     

Driving forces for layer-by-layer self-assembly of films of SiO2 nanoparticles and heme proteins

Heme protein hemoglobin (Hb) or myoglobin (Mb) and silica nanoparticles in a variety of charge states were assembled layer-by-layer into films on solid surfaces to investigate the driving forces for film assembly. Cyclic voltammetry (CV), quartz crystal microbalance (QCM), X-ray photoelectron spectroscopy (XPS), and UV-vis and reflectance absorption infrared (RAIR) spectroscopy were used to characterize the different [SiO2/protein]n films. Even when the proteins and silica were both negatively charged, stable layer-by-layer [SiO2/protein]n films were successfully fabricated, although amounts of protein were smaller than when nanoparticles and proteins had opposite charges. Results suggest the importance of localized Coulombic attractions between the negative nanoparticle surface and positively charged amino acid residues on the Mb or Hb surfaces in the assembly and for the stability of [SiO2/protein]n films.     

Deuterium Isotope Effects Upon the Redox‐switching of the Viscosity of DNA Layers Observed by Electrochemical Quartz Crystal Micro‐balance

We report on H/D isotope effects observed upon quick redox‐switching of the viscoelasticity of self‐assembled monolayers of single‐stranded DNA (ssDNA‐SAM) observed by electrochemical quartz‐crystal micro‐balance (EQCM) of three redox‐active small molecules that travel through the DNA layer on gold electrodes. We have recently reported hexammine cobalt(III) (CoHex) to have the largest voltammetric isotope effect while hexammine ruthenium(III) (RuHex) does not show this effect. Daunomycin, on the other hand showed a significant redox potential shift up to ?80 mV. A thin‐layer model may explain this voltammetric behavior. RuHex covers the negatively charged DNA strand and provides considerable conductivity, while CoHex and daunomycin do not. Latest results regarding the reproducible frequency responses indicate considerable isotope effects also in EQCM measurements depending on the redox molecule interacting with the ssDNA‐SAM. These effects will provide new opportunities in drug screening and studies of DNA damage by toxic chemicals.     

Graphene as a spacer to layer-by-layer assemble electrochemically functionalized nanostructures for molecular bioelectronic devices

This study demonstrates the capability of graphene as a spacer to form electrochemically functionalized multilayered nanostructures onto electrodes in a controllable manner through layer-by-layer (LBL) chemistry. Methylene green (MG) and positively charged methylimidazolium-functionalized multiwalled carbon nanotubes (MWNTs) were used as examples of electroactive species and electrochemically useful components for the assembly, respectively. By using graphene as the spacer, the multilayered nanostructures of graphene/MG and graphene/MWNT could be readily formed onto electrodes with the LBL method on the basis of the electrostatic and/or π-π interaction(s) between graphene and the electrochemically useful components. Scanning electron microscopy (SEM), ultraviolet-visible spectroscopy (UV-vis), and cyclic voltammetry (CV) were used to characterize the assembly processes, and the results revealed that nanostructure assembly was uniform and effective with graphene as the spacer. Electrochemical studies demonstrate that the assembled nanostructures possess excellent electrochemical properties and electrocatalytic activity toward the oxidation of NADH and could thus be used as electronic transducers for bioelectronic devices. This potential was further demonstrated by using an alcohol dehydrogenase-based electrochemical biosensor and glucose dehydrogenase-based glucose/O(2) biofuel cell as typical examples. This study offers a simple route to the controllable formation of graphene-based electrochemically functionalized nanostructures that can be used for the development of molecular bioelectronic devices such as biosensors and biofuel cells.     

A Comparative Study of Modifying Gold and Carbon Electrode with 4‐Sulfophenyl Diazonium Salt

A comparison of the reductive adsorption behavior of 4‐sulfophenyl diazonium salt and subsequent electrochemical reactivity on gold relative to carbon was studied with some significant differences observed. The ability of the 4‐sulfophenyl layer adsorbed onto gold to block access of the redox probe ferricyanide to the underlying electrodes, as determined via cyclic voltammetry was inferior to the same layers formed on glassy carbon electrodes thus indicating a more open, porous layer formed on gold. More significantly, the 4‐sulfophenyl layers are shown to be far less electrochemically stable on gold than on glassy carbon. Electrochemical and X‐ray photoelectron spectroscopy (XPS) evidence suggests the instability is due to cleavage of the bond between sulfonate functional group and phenyl ring. These results provide further evidence that although aryl diazonium salt layers are relatively stable on gold surfaces compared with alkanethiol based self‐assembled monolayer (SAMs), the stability is not as high as is observed on carbon.     

Layer-by-layer films of hemoglobin or myoglobin assembled with zeolite particles: electrochemistry and electrocatalysis

Positively charged hemoglobin (Hb) or myoglobin (Mb) at pH 5.0 in solutions and negatively charged zeolite particles in dispersions were alternately adsorbed onto solid surfaces forming [zeolite/protein](n) layer-by-layer films, which was confirmed by quartz crystal microbalance (QCM) and cyclic voltammetry (CV). The protein films assembled on pyrolytic graphite (PG) electrodes exhibited a pair of well-defined, nearly reversible CV peaks at about -0.35 V vs. SCE at pH 7.0, characteristic of the heme Fe(III)/Fe(II) redox couples. Hydrogen peroxide (H(2)O(2)) and nitrite (NO(2)(-)) in solution were catalytically reduced at [zeolite/protein](7) film modified electrodes, and could be quantitatively determined by CV and amperometry. The shape and position of infrared amide I and II bands of Hb or Mb in [zeolite/protein](7) films suggest that the proteins retain their near-native structure in the films. The penetration experiments of Fe(CN)(6)(3-) as the electroactive probe into these films and scanning electron microscopy (SEM) results indicate that the films possess a great amount of pores or channels. The porous structure of ]zeolite/protein](n) films is beneficial to counterion transport, which is crucial for protein electrochemistry in films controlled by the charge-hopping mechanism, and is also helpful for the diffusion of catalysis substrates into the films. The proteins with negatively charged net surface charges at pH 9.0 were also successfully assembled with like-charged zeolite particles into layer-by-layer films, although the adsorption amount was less than that assembled at pH 5.0. The possible reasons for this were discussed, and the driving forces were explored.     

Investigating Electrochemical Stability and Reliability of Gold Electrode‐electrolyte Systems to Develop Bioelectronic Nose Using Insect Olfactory Receptor

This article aims to demonstrate an electrochemically stable and reliable gold electrode‐electrolyte system to develop an insect odorant receptor (Drosophila melanogaster Or35a) based bioelectronic nose. Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) of bare gold electrodes, after modification with the self‐assembled monolayer (SAM) of 6‐mercaptohexanoic acid (MHA) and after immobilization with Or35a integrated into the lipid bilayers of liposomes were conducted in the presence of four different redox probes. Potassium ferri/ferrocyanide [Fe(CN)₆]^3?/[Fe (CN)₆]^4? and hydroquinone (H₂Q) redox probes revealed variable and irreversible signals at the time scale of our measurements, with atomic force microscopy (AFM) images and x‐ray photoelectron spectroscopy (XPS) results suggesting gold surface etching due to the presence of CN^? ions in case of [Fe(CN)₆]^3?/[Fe (CN)₆]^4?. Although the hexaammineruthenium complex showed stable electrochemical behaviour at all stages of biosensor development, changes in CV and EIS readings after each surface modifications were insignificant. PBS buffer as a non‐Faradaic medium, was found to provide reliable systems for electrochemical probing of modified gold electrodes with Or35a/liposomes in aqueous media. Using this system, we have shown that this novel biosensor can detect its known odorant E2‐hexenal selectively compared to methyl salicylate down to femtomolar concentration.     

Supported lipid bilayers lifted from the substrate by layer-by-layer polyion cushions on self-assembled monolayers

The formation of lipid bilayers, lifted from the solid substrate by layer-by-layer polyion cushions, on self-assembled monolayers (SAMs) on gold was investigated by surface plasmon resonance (SPR) and fluorescence recovery after photobleaching (FRAP). The polyions poly(diallyldimethylammonium chloride) (PDDA) and polystyrene sulfonate (PSS) sodium salt were used for the layer-by-layer polyion macromolecular assembly. The cushion was formed by electrostatic interaction of PDDA/PSS/PDDA layers with a negatively charged surface of an SAM of 11-mercaptoundecanoic acid (MUA) on gold. The lipid bilayer membranes were deposited by vesicle fusion with different compositions of SOPS (an anionic lipid, 1-stearoyl-2-oleoyl-phosphatidylserine) and POPC (a zwitterionic lipid, 1-palmitoyl-2-oleoylphosphatidylcholine). In the case of pure SOPS and for lipid mixtures with a POPC composition up to 25%, single bilayers were deposited. FRAP experiments showed that single bilayers supported on PDDA/PSS/PDDA/MUA were mobile at room temperature, with lateral coefficients of approximately (1.2–2.1)×10⁻⁹ cm²/s. The kinetics of the addition of the ion-channel-forming peptide protegrin-1 to the supported bilayers was detected by SPR. A two-step interaction was observed, similar to the association behavior of protegrin-1 with bilayers supported on PDDA/MUA. The results are similar to that of supported lipid bilayers without a layer-by-layer cushion. The model membrane system in this work is a potential biosensor for mimicking the natural activities of biomolecules and is a possible tool to investigate the fundamental properties of biomembranes.     

Selective anion sensing based on tetra-amide calix[6]arene derivatives in solution and immobilized on gold surfaces via self-assembled monolayers

Two anion receptors, 1 and 2, based on the calix[6]crown-4 architecture were synthesized and characterized by NMR (1H, 13C, COSY), UV-vis, and MALDI-MS. 1H NMR measurements demonstrate that receptors 1 and 2 exhibit the highest binding affinity for fluoride ions compared to other anions including Cl-, Br-, NO3-, HSO4-, H2PO4-, and AcO-. The binding constants of 1 with F- and AcO- are 326 (+/-32) and 238 (+/-23) M-1, whereas those of 2 with F- and AcO- are 222 (+/-25) and 176 (+/-21) M-1. The fluorescent titration of 2 with various anions such as Cl-, Br-, NO3-, HSO4-, and H2PO4- led to essentially no change in excimer emission and a slight enhancement of monomer emission. In contrast, a dramatic change was observed in the fluorescence spectra upon the addition of F- and AcO- to 2. Self-assembled monolayers (SAMs) of 1 were formed on gold surfaces and characterized by reductive desorption and other techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy were used to monitor anion recognition by the SAM-modified gold electrodes. The gold electrodes modified by SAMs of 1, upon binding with the F- anion, exhibit a dramatic increase in charge-transfer resistance (Rct) values. This is due to the repulsion between the negatively charged electrode surfaces and the negatively charged Fe(CN)6(3-/4-) redox probe in the electrolyte solution. In contrast, smaller increases in Rct values were observed in the cases of other monovalent anions investigated.     

Effects of ion-pairing on rate of electron transfer between immobilized gold nanoclusters and soluble redox probes

Hydrophilic gold nanoclusters were tethered onto gold electrodes modified with mixed 1-octane thiol/1,9-nonane dithiol monolayers. The heterogeneous electron transfer (ET) kinetics of soluble redox species in the supporting electrolyte were investigated at these electrodes by cyclic voltammetry (CV) in the presence and absence of the ion-pairing anions and . The redox species investigated, [Fe(CN)₆]^3−/4− and [Co(C₁₂H₈N₂)₃]^3+/2+ where oppositely charged. The results presented here reveal that the rate of ET for the negatively charged redox species decreases with decreasing ionic charging time constant of the electrolyte. The opposite trend is observed for the positively charged redox species.     

Biosensors based on gold nanoelectrode ensembles and screen printed electrodes

Gold nanowires were synthesized within polycarbonate membranes according to an electroless deposition method, obtaining nanoelectrode ensembles (NEEs) with special electrochemical features. NEEs were coupled with home-produced carbon graphite screen printed electrodes and the electrochemical properties of the original nanoelectrode ensemble on screen printed substrate (NEE/SPS) assembly has been tested for sensors application. Glucose oxidase has been used as model enzyme in order to verify the feasibility of disposable gold NEE/SPS biosensors. Finally, different immobilisation and electrochemical deposition techniques based on either self assembled monolayers of cysteamine (CYS) or amino-propyl-triethoxysilane (APTES) and conductive polyaniline (PANI) molecular wires were used. Spatial patterning of the enzyme on the polycarbonate surface and of PANI wires on gold nanoelectrodes was obtained. Possible direct electron transfer between the enzyme and the PANI modified gold nanoelectrodes has been evaluated.