Host-guest interactions at self-assembled monolayers of cyclodextrins on gold

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electro-chemistry and wettability studies. On average about 70% of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.     

Fluorescence lifetime fluctuations of single molecules probe local density fluctuations in disordered media: a bulk approach

We investigated the nanometer scale mobility of polymers in the glassy state by monitoring the dynamics of embedded single fluorophores. Recently we reported on fluorescence lifetime fluctuations which reflect the segmental rearrangement dynamics of the polymer in the surroundings of the single molecule probe. Here we focus on the nature of these fluorescence lifetime fluctuations. First the potential role of quenching and molecular conformational changes is discussed. Next we concentrate on the influence of the radiative density of states on the spontaneous emission of individual dye molecules embedded in a polymer. To this end we present a theory connecting the effective-medium theory to a cell-hole model, originating from the Simha-Somcynsky free-volume theory. The relation between the derived distributions of free volume and fluorescence lifetime allows one to determine the number of segments involved in the local rearrangement directly from experimental data. Results for two different polymers as a function of temperature are presented.     

Electrochemistry of surface-grafted stimulus-responsive monolayers of poly(ferrocenyldimethylsilane) on gold

Poly(ferrocenyldimethylsilane)s with various degrees of polymerization and featuring a thiol end group were chemically end-grafted onto gold substrates by self-assembly, forming redox-active monolayers. The monolayers were characterized by contact angle measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Layer thickness values were determined by surface plasmon resonance spectroscopy and ellipsometry. The electrochemical properties of these films in aqueous NaClO(4) were studied using cyclic voltammetry (CV), differential pulse voltammetry, chronoamperometry, and chronocoulometry. Cyclic voltammograms showed two reversible redox peaks, indicating a stepwise oxidation of the electroactive sites. The first oxidation step showed reversible behavior at low scan rates and quasi-reversible behavior at higher scan rates. Peak currents (i(p)) plotted against the square root of scan rates (v(1/2)) for the first oxidation peak and for the corresponding reduction peak exhibited a linear dependence, indicating that the oxidation process in the first step is controlled by the diffusion of counterions into the polymer film. For the second oxidation peak and the corresponding reduction peak, i(p) varied linearly with v. This redox behavior is characteristic of surface-immobilized electroactive layers. The higher reversibility of the second oxidation and reduction waves in the CV experiments was explained from the solvation of the surface-grafted poly(ferrocenylsilane) (PFS) chains, which depends on the degree of oxidation. Oxidized PFS films are swollen in the aqueous electrolyte solutions, leading to a higher segmental mobility of the polymer chains and a much increased counterion mobility within the film. Kinetic parameters for the redox processes were obtained from chronocoulometry experiments.     

A Brush‐Gel/Metal‐Nanoparticle Hybrid Film as an Efficient Supported Catalyst in Glass Microreactors

A polymer‐brush‐based material was applied for the formation and in situ immobilization of silver and palladium nanoparticles, as a catalytic coating on the inner wall of glass microreactors. The brush film was grown directly on the microchannel interior by means of atom‐transfer radical polymerization (ATRP), which allows control over the polymer film thickness and therefore permits the tuning of the number of nanoparticles formed on the channel walls. The wide applicability of the catalytic devices is demonstrated for the reduction of 4‐nitrophenol and for the Heck reaction.     

α‐Chymotrypsin‐Catalyzed Reaction Confined in Block‐Copolymer Vesicles

Herein the reactivity of the enzyme α‐chymotrypsin in the confinement of polystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA) vesicles was investigated. Enzyme and substrate molecules were encapsulated in PS‐b‐PAA vesicles with internal diameters ranging from 26 nm to 165 nm during the formation of the vesicles. While the loading efficiencies of enzyme and substrate molecules were practically identical for vesicles of identical size, they were found to increase with decreasing vesicle size. The kinetics of the α‐chymotrypsin catalyzed hydrolysis of N‐succinyl‐Ala‐Ala‐Phe‐7‐amido‐4‐methylcoumarin (AMC) was evaluated following the increase of the absorption of the product 7‐amino‐4‐methylcoumarin by UV/Vis spectroscopy. The values of the catalytic turnover number obtained for reactions inside vesicles with different sizes showed an increase of up to fourteen times compared to the bulk value with decreasing vesicle volume, while the values of the Michaelis–Menten constant decreased, respectively. This increase in reactivity of α‐chymotrypsin is attributed to the effect of vesicle–wall interactions in the finite encapsulated space, where the reagents could diffuse, leading to enhanced collision frequencies.     

pH Dependent Elasticity of Polystyrene‐block‐poly(acrylic acid) Vesicle Shell Membranes by Atomic Force Microscopy

We assess the elastic properties of PS‐b‐PAA vesicle membranes under different pH values by AFM force measurements. We find that based on the shell deformation theory, the values of the estimated apparent Young's modulus of the vesicle membranes decrease as the pH of the solution increases. The onset of the decrease of E coincides with the surface pK_a determined from ζ‐potential measurements. This decrease of E at higher pH is attributed to electrostatic repulsion between the deprotonated PAA chains resulting in the thinning of the vesicle membrane.

     

Breathing Pores on Command: Redox‐Responsive Spongy Membranes from Poly(ferrocenylsilane)s

Redox‐responsive porous membranes can be readily formed by electrostatic complexation between redox active poly(ferrocenylsilane) PFS‐based poly(ionic liquid)s and organic acids. Redox‐induced changes on this membrane demonstrated reversible switching between more open and more closed porous structures. By taking advantage of the structure changes in the oxidized and reduced states, the porous membrane exhibits reversible permeability control and shows great potential in gated filtration, catalysis, and controlled release.     

Quantitative nanotribology by AFM: a novel universal calibration platform

The quantitative determination of friction forces by atomic force microscopy (AFM) in nanotribology requires the conversion of the output voltage signal of the sector area-sensitive photodiode to force using (a) the torsional spring constant of the cantilever and (b) the lateral sensitivity of the photodiode. Many existing methods provide calibration factors with large errors and suffer from poor reproducibility. We report on the fabrication, validation, and application of a new, universally applicable standard specimen that enables one to accurately calibrate all types of AFM cantilevers and tips for quantitative friction force measurements. The Si(100) calibration standard, which exhibits 30 and 50 mum wide notches with tilt angles theta between 20 degrees and 35 degrees with respect to the wafer surface, was fabricated by focused ion beam (FIB) milling. The quantification of friction forces obtained on this universal standard specimen using a direct method (the improved wedge calibration method, as introduced by Ogletree, Carpick, and Salmeron Rev. Sci. Instrum. 1996, 67, 3298-3306), which yields (a) and (b) simultaneously, was critically tested for various types of Si3N4 integrated cantilever-tip assemblies. The error in the calibration factors obtained was found to be ca. 5%, which is a significant improvement compared to errors of 30-50% observed for the often applied two-step calibration procedures of cantilever lateral force constant and photodiode sensitivity. As demonstrated for oxidized Si(100), thin films of poly(methyl methacrylate) (PMMA), and micropatterned self-assembled monolayers (SAMs) on gold, the calibration of various V-shaped and single beam cantilevers based on the application of the new universal standard in conjunction with the direct wedge method proposed allows one to conveniently perform quantitative nanotribological measurements for a wide range of materials and applications.