Guest-controlled self-sorting in assemblies driven by the hydrophobic effect

The extent of self-sorting in systems comprised of two different deep-cavity cavitands is investigated. The nature of the guest(s) encapsulated in the resulting assemblies is shown to profoundly influence the extent of self-sorting.     

Buildup of gold nanoparticle multilayer thin films based on the covalent-bonding interaction between boronic acids and polyols

A novel method for the fabrication of gold nanoparticle multilayer films based on the covalent-bonding interaction between boronic acid and polyols, poly(vinyl alcohol) (PVA), was developed. The multilayer buildup was monitored by UV-vis absorbance spectroscopy, which showed a linear increase of the film absorbance with the number of adsorbed Au layers and indicated the stepwise and uniform assembling process. The atomic force microscopy (AFM) image showed that a compact gold multilayer thin film was successfully assembled. The residual boronic acid group on the surface of thin film could incorporate glycosylated-protein horseradish peroxidase (HRP), and good catalytic activity for H2O2 could be observed.     

Internal structure of polyelectrolyte multilayer assemblies

The paper deals with the correlation between the internal structure and dynamics of polyelectrolyte multilayers on one hand and their functional properties on the other hand. It considers different concepts of multilayer formation like driving forces, adsorption kinetics, mode of growth and stability aspects. A further focus is the control of internal structure and dynamics which is of high impact with respect to the design of stimuli-responsive material.     

Spontaneous assembly of organic thiocyanates on gold sufaces. Alternative precursors for gold thiolate assemblies

Thiolate self-assembly on gold has proven to be a valuable technique for assembling monolayers on a wide variety of substrates. However, the oxidative instability of the thiols, especially aromatic thiols and alpha,omega-dithiols, presents several difficulties. Shown here is that thiocyanates, easily synthesized stable thiol derivatives, can be directly assembled on gold surfaces with no auxiliary reagents required. Assembly is complete in 24 h and leaves a similar gold thiolate structure as seen in typical thiol self-assembled monolayers.     

Hydrogen-bonding based multilayer assemblies by self-deposition of dendrimer

We reported on hydrogen-bonding directed Layer-by-Layer assemblies by self-deposition of a kind of dendrimer bearing carboxyl groups on its periphery that act as hydrogen bonding donor as well as hydrogen bonding acceptor.     

Design of hydrophobic polyoxometalate hybrid assemblies beyond surfactant encapsulation

Grafting of C-6, C-16 and C-18 alkyl chains onto the hydrophilic Mn-Anderson clusters (compounds 2-4) has been achieved. Exchange of the tetrabutyl ammonium (TBA) with dimethyldioctadecyl ammonium (DMDOA) results in the formation of new polyoxometalate (POM) assemblies (compounds 5-6), in which the POM cores are covalently functionalized by hydrophilic alkyl-chains and enclosed by surfactant of DMDOABr. As a result, we have been able to design and synthesize POM-containing hydrophobic materials beyond surfactant encapsulation. In solid state, scanning electron and transmission electron microscopy (SEM and TEM) studies of the TBA salts of compounds 3 and 4 show highly ordered, uniform, reproducible assemblies with unique segmented rodlike morphology. SEM and TEM studies of the DMDOA salts of compounds 5 and 6 show that they form spherical and sea urchin 3D objects in different solvent systems. In solution, the physical properties of compound 5 and 6 (combination of surfactant-encapsulated cluster (SEC) and surface-grafted cluster (SGC)) show a liquid-to-gel phase transition in pure chloroform below 0 degrees C, which are much lower than other reported SECs. By utilizing light scattering measurements, the nanoparticle size for compounds 5 and 6 were measured at 5 degrees C and 30 degrees C, respectively. Other physical properties including differential scanning calorimetry have been reported.     

An extraordinary electrocatalytic reduction of oxygen on gold nanoparticles-electrodeposited gold electrodes

The cathodic reduction of oxygen has been investigated at a gold nanoparticles-electrodeposited gold electrode in 0.5 M H₂SO₄ solution. Two well-defined reduction peaks were observed at +50 and −250 mV vs. Ag/AgCl/KCl (sat.). Those two peaks indicated a 2-step 4-electron reduction pathway of O₂ in this strong acidic medium. The former peak was ascribable to the 2-electron reduction of O₂ to H₂O₂, while the latter was assigned to the reduction of H₂O₂ to H₂O. The observed electrocatalysis for the reduction of O₂ is attributable to the extraordinary catalytic activity of the gold nanoparticles over the bulk gold electrode, at which the 2-electron reduction peak of O₂ to H₂O₂ was observed at −200 mV.     

Sub-monolayer assemblies of octanethiol and octadecylthiol at gold electrodes for the direct analysis of 4,4'-oxydianiline in wastewaters and shoe-dyeing samples

Self-assembled monolayers (SAMs) of di-n-octadecyldisulphide (C(18)) and n-octanethiol (C(8)) were prepared on gold electrodes. From the studies carried out by cyclic voltammetry and square wave voltammetry, it was observed that the electrochemical behaviour of 4,4'-oxydianiline on these electrodes is affected by the length chain of the alkanethiol. After the optimization of all the variables involved in the electrochemical response of 4,4'-oxydianiline by square wave voltammetry employing the modified electrodes, it is possible the determination of 4,4'-oxydianiline with a detection limit of 0.04microg/mL (C(18)) and 0.06microg/mL (C(8)) and determination limits of 0.12 and 0.22microg/mL, respectively. The calculated Er (%)(n=10) and R.S.D.(%)(n=10) values were minor than 2.2% and 3.7%. The proposed methods were successfully applied to the analysis of oxydianiline in wastewater and shoe-dyeing samples.     

Protein immobilisation on perpendicularly aligned gold tipped nanorod assemblies

A multi component assembly consisting of the redox protein cytochrome c (cyt c) immobilised onto vertically aligned gold tipped semiconductor nanorods is described. Cyt c was successfully immobilised using a thiol linker. A faradaic response demonstrated that the protein is electroactive in this ultra high density array.     

Recognition of bile acids at cyclodextrin-modified gold electrodes.

Lipoylamino-beta- and gamma-cyclodextrin (LP-beta-CD and LP-gamma-CD, respectively) were adsorbed at the surface of gold electrodes by sulfur-gold bonding. The resultant electrodes exhibited quasi-reversible voltammograms for the redox reaction of Fe(CN)6(3-/4-) in aqueous solutions, with peak-to-peak separation (deltaEp) being 85 mV at 20 mV s(-1) as a potential sweep rate. When bile acids are added to the solution, deltaEp values increased to 200-300 mV with increasing the concentration of bile acids. A Langmuir-type adsorption analyses satisfactorily afforded the binding constants (Ksurf) of the surface-confined LP-beta-CD and LP-gamma-CD with the bile acids. The obtained Ksurf values of LP-gamma-CD are 5.0-50 times larger than the corresponding binding constants of gamma-CD in homogeneous aqueous solutions. Cyclic voltammetric experiments with positively, negatively, and non-charged adamantane derivatives as well as pH titration experiments revealed that the retardation of the electrode reaction of negatively charged Fe(CN)6(3-/4-) caused by bile acids was attributable (1) to electric potential changes due to the accumulation of the negative charges at the electrode surface, and (2) to an increase in the hydrophobicity of the electrode surface due to the binding of hydrophobic bile acids to the LP-beta-CD and LP-gamma-CD membranes.     

Oxygen-reducing electrodes based on layer-by-layer assemblies of cytochrome c and laccasse

Electroactive multilayer assemblies combining the redox protein cytochrome c and the enzyme laccase were fabricated by the layer-by-layer adsorption technique on gold electrodes and were shown to be capable of direct oxygen reduction. Laccase from trametes versicolor was electrostatically immobilized on multilayer films consisting of cytochrome c and the polyelectrolyte polyanilinesulfonic acid. The layer formation was monitored by quartz crystal microbalance. The electrochemical behavior of the electrodes was investigated by cyclic voltammetry. The resulting assembly exhibited a catalytic oxygen reduction current. This indicates a multi-step electron transport chain from the electrode via the cytochrome c layers towards laccase, and finally, to molecular oxygen. The catalytic efficiency of the electrodes was examined in the pH range from 4.5 to 7.0, showing highest enzymatic oxygen reduction at pH 4.5. Furthermore, the catalytic current was found to correlate linearly with the oxygen content of the solution. This suggests that the overall current is limited by the catalytic reduction of oxygen by the laccase rather than by the preceding electron transfer steps.     

Infrared reflexion-absorption measurements on emersed gold electrodes

Infrared reflexion-absorption spectroscopy was performed on gold electrodes which had been emersed from aqueous potassium cyanate electrolytes at different potentials. New information was, thereby, obtained on the process of emersion and the electrosorption of cyanate on gold.The polycrystalline gold electrodes could be emersed without any adhering solution (“hydrophobic”) only when traces of organic substances were present in the electrolyte. On such emersed electrodes the potential-dependent adsorption of cyanate was observed by the asymmetric stretching vibration at 2185 cm^?1, and by hot bands. At the potential of anodic gold oxide formation the surface concentration of cyanate was found to drop to near zero.     

Setting the environmental conditions for controlling gold nanoparticle assemblies

Fundamental insights into the factors that control the properties and structure of gold nanoparticle (AuNP) based assemblies enable the design and construction of new materials. The dimensions (shape and size) and the optical properties of AuNP assemblies are affected by the electronic properties of the organic cross-linker and the nature of the AuNPs.     

Electrochemical properties of sulfur adsorbed on gold electrodes

The electrochemical properties of sulfur adsorbed on gold electrodes were studied in 10^?5M solutions of S^2? in 1 M NaOH. In general, ∵_S is less than a monolayer. At E=0.05 V only, a monolayer will be formed after long times. The sulfur layer is stable in the potential range between ?0.6 and +0.4 V. At lower potentials, sulfur can be desorbed cathodically (charge Q_red), but at higher potentials, where layers of gold oxide are formed, the sulfur is oxidized anodically (charge Q_ox). From the ratio Q_red·6/Q_ox=γ, the electrosorption valency γ=?2 is obtained. This means, that the sulfide ions are almost completely discharged during adsorption. The same layer can be formed by adsorption from polysulfide solutions, which can be explained by a break of the sulfur bond and adsorption of single sulfur atoms. The double layer capacity decreases during adsorption of sulfur indicating the formation of an insulating sulfur layer with a dielectric constant of about 2. The anodic adsorption of sulfide ions is limited by diffusion only. For longer polarisation times, the coverage is independent of time, i.e. place exchange reactions between Au and S can be excluded. The cathodic desorption as well as the anodic oxidation of the adsorbed sulfur are potential dependent charge transfer processes, as can be concluded from potentiodynamic measurements with various sweep rates.     

Blocking behavior of self-assembled monolayers on gold electrodes

Self-assembled monolayers (SAMs) with metal electrodes, especially thiols on gold, are the subject of this investigation because of the unique properties of SAM-modified surfaces. Normal alkanethiols are used to modify the surface of a conventional gold electrode to block certain ions such as Pb(II) and Cu(II) from the surface of the electrode. Normal alkanethiols are also used to study the SAM-gold interfacial adsorption-desorption behavior of the self-assembled monolayer. The effects of varying chain length of SAMs, varying concentration of the alkanethiol solutions, immersion time of the pure gold electrode in the SAM solution, and the stability of a SAM-modified gold electrode in fresh chloroform are investigated using the oxidation-reduction peaks of gold. Conditions that optimize the surface coverage and the uniformity of the SAMs have been determined. Normal alkanethiols proved to be a good insulator on the electrode surface. Received: 16 January 1997 / Accepted: 4 March 1997     

Electrochemical characterization of self-assembled thiol-porphyrin monolayers on gold electrodes by SECM.

Herein, the scanning electrochemical microscopy (SECM) approach is applied to study the formation of thiol-porphyrin self-assembled monolayer (SAMs). Using cyclic voltammetry (CV), the formation process is characterized adopting different probe molecules. The observed phenomena indicate that the formation process is affected by solution properties and the molecular structure of the probe molecules. In K(3)Fe(CN)(6) , the SAMs show a strong electron-transfer (ET) blocking effect on a pure porphyrin-modified electrode. However, addition of metal ions to the porphyrin molecules leads to ET. A consistent tendency is observed throughout the modification process using CV and SECM methods. Furthermore, k(eff) values, the apparent heterogeneous rate constants, obtained for different modification periods affirm the validity of these results. SECM images are used to collect surface information in the course of the modification process when the substrate potential is 0.5 V versus Ag/AgCl. The effect of the substrate potential indicates that the oxidation of the porphyrin molecules is supported by more positive potentials because of the similar bimolecular reaction of the porphyrin ring with positive charge and the probe molecules with negative charge.     

Electrochemical characterization of polyelectrolyte/gold nanoparticle multilayers self-assembled on gold electrodes

Polyelectrolyte/gold nanoparticle multilayers composed of poly(l-lysine) (pLys) and mercaptosuccinic acid (MSA) stabilized gold nanoparticles (Au NPs) were built up using the electrostatic layer-by-layer self-assembly technique upon a gold electrode modified with a first layer of MSA. The assemblies were characterized using UV-vis absorption spectroscopy, cyclic and square-wave voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy. Charge transport through the multilayer was studied experimentally as well as theoretically by using two different redox pairs [Fe(CN)(6)](3-/4-) and [Ru(NH(3))(6)](3+/2+). This paper reports a large sensitivity to the charge of the outermost layer for the permeability of these assemblies to the probe ions. With the former redox pair, dramatic changes in the impedance response were obtained for thin multilayers each time a new layer was deposited. In the latter case, the multilayer behaves as a conductor exhibiting a strikingly lower impedance response, the electric current being enhanced as more layers are added for Au NP terminated multilayers. These results are interpreted quite satisfactorily by means of a capillary membrane model that encompasses the wide variety of behaviors observed. It is concluded that nonlinear slow diffusion through defects (pinholes) in the multilayer is the governing mechanism for the [Fe(CN)(6)](3-/4-) species, whereas electron transfer through the Au NPs is the dominant mechanism in the case of the [Ru(NH(3))(6)](3+/2+) pair.