Photorefractive device using self-assembled monolayer coated indium-tin-oxide electrodes

Photorefractive (PR) performances of methyl-substituted poly(triarylamine) (PTAA)-based PR device were demonstrated using chemically modified electrodes (CMEs) of self-assembled monolayer (SAM) coated indium-tin-oxide (ITO) electrodes. The SAM-ITO electrodes successfully suppressed dark current which blocks the formation of space-charge field and also causes the dielectric breakdown. The PR device consisted of composite of PTAA, 4-azacycloheptylbenzylidenemalononitrile (7-DCST), N-ethylcarbazole (ECz), and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between the SAM-ITO electrodes. The PR devices showed the PR performances: optical gain Γ, refractive index Δn, diffraction efficiency η, response time τ, sensitivity S, phase shift Φ, trap-limited field E_q, number density of traps N_T, and space-charge field E_SC. The remarkable response time of 11.3 ms was achieved at the low electric field of 20 V μm⁻¹, which was comparable to the response time of high-definition television (HDTV) quality of 16 ms. Our approach will widen the usage of higher mobility materials to photorefractive field and give us more favorable materials to achieve the best performance of photorefractivity in the future.     

A Simple Bioconjugate Attachment Protocol for Use in Single Molecule Force Spectroscopy Experiments Based on Mixed Self-Assembled Monolayers

Single molecule force spectroscopy is a technique that can be used to probe the interaction force between individual biomolecular species. We focus our attention on the tip and sample coupling chemistry, which is crucial to these experiments. We utilised a novel approach of mixed self-assembled monolayers of alkanethiols in conjunction with a heterobifunctional crosslinker. The effectiveness of the protocol is demonstrated by probing the biotin-avidin interaction. We measured unbinding forces comparable to previously reported values measured at similar loading rates. Specificity tests also demonstrated a significant decrease in recognition after blocking with free avidin.     

High-sensitivity and high-resolution low-energy ion scattering

Low-Energy Ion Scattering (LEIS or ISS) is used to selectively analyze the atomic composition of the outer atomic layer of surfaces. In addition, the spectrum gives (non-destructively) the in-depth distribution. Using a double toroidal energy analyzer with parallel energy detection and time-of-flight filtering a high sensitivity and mass resolution of LEIS is achieved. This is demonstrated for a highly dispersed catalyst of Pt/Au on γ-alumina. The improved depth resolution is illustrated for self-assembled monolayers of alkanethiols (12-20 carbon atoms) on gold. Even for these low Z carbon atoms a clear shift of 8 eV/carbon atom is observed (using 1.5 keV ⁴He⁺ ion scattering). This opens many new possibilities for studies of ultra-thin diffusion barriers, high-k dielectrics and biosensors.     

Computer simulation and SERR detection of cytochrome c dynamics at SAM-coated electrodes

In this paper we present a combined experimental and theoretical study of the heterogeneous electron transfer reaction of cytochrome c electrostatically adsorbed on metal electrodes coated with monolayers of 6-mercaptohexanoic acid. Molecular dynamics simulations and pathways calculations show that adsorption of the protein leads to a broad distribution of orientations and, thus, to a correspondingly broad distribution of electron transfer rate constants due to the orientation-dependence of the electronic coupling parameter. The adsorbed protein exhibits significant mobility and, therefore, the measured reaction rate is predicted to be a convolution of protein dynamics and tunnelling probabilities for each orientation. This prediction is confirmed by time-resolved surface enhanced resonance Raman which allows for the direct monitoring of protein (re-)orientation and electron transfer of the immobilised cytochrome c. The results provide a consistent explanation for the non-exponential distance-independence of electron transfer rates usually observed for proteins immobilized on electrodes.     

Fabrication of a stable, switchable An/SAM-Au electrode with tunable electron transfer and excellent electrochemical properties

This paper describes the electrochemical properties, such as electrode reactivity, and interfacial capacitance of an anthracene/self-assembled monolayer (An/SAM) Au electrode prepared through controllable immersion of the dodecanethiol (C₁₂SH) SAM-Au electrode in a THF solution of anthracene (An). The C₁₂SH SAM almost totally blocks heterogeneous electron transfer between the bare Au electrode and Fe(CN)₆^3−/4− in solution, but adsorption of the An molecules onto the SAM-Au electrode restores electron transfer. The prepared An/SAM-Au electrode possesses good reactivity without a remarkable barrier to heterogeneous electron transfer. On the other hand, the electroactive form of the An/SAM-Au electrode can be converted back to the non-electroactive form by immersing the electrode into pure THF.     

Electrochemical properties of self-assembled monolayer based on mono-(6-deoxy-6-mercapto)-β-cyclodextrin toward controlled molecular recognition

The present work describes the construction of nanoporous assemblies based on single self-assembled monolayer (SAM) of mono-(6-deoxy-6-mercapto)-β-cyclodextrin (βCDSH) and two-component monolayers of βCDSH and 11-mercaptoundecanoic acid. An investigation of its supramolecular properties toward the tricyclic antidepressants drugs chlorprothixene and imipramine is also presented. The adsorption process was investigated by using surface plasmon resonance and the selectivity of the SAM was verified by using electroactive species permeable and not permeable into the cyclodextrin SAM, such as ferrocenemonocarboxylic acid, hexacyanoferrate, methylene blue and hexamineruthenium (II). The redox probe was chosen based on its capability to permeate the grafted cyclodextrin molecules and its electrochemical characteristics. It was applied to imipramine and chlorprothixene competitive assay by the mono-(6-deoxy-6-mercapto)-β-cyclodextrin self-assembled cavity. Finally, the pK_a of the surface and its effects on thee host-guest interaction properties were also investigated.     

Adhesion of Polymer Monolayers to the Water Subphase. Effect of Solvents and Polymer Functionality

The properties and the organization of poly (D, L-lactic acid) monolayers spread at the air/water interface were shown to be dramatically dependent on whether these monolayers have been spread from a good or a bad solvent. Whereas a good solvent, such as chloroform, favored the deployment of polymer chains from their coiled structure in solution to the unfolded structure, a bad solvent, as exemplified by acetone, enhanced strong intersegment interactions resulting in the formation of microdomains capable of respreading and favored adhesion of the polymer monolayer to water.

Independently carried out experiments with monolayers of ester derivatives of hyaluronic acid demonstrated that the nature of a chemical group substituted on the glucuronic acid moieties of the polymer can considerably influence surface properties of these monolayers. Thus, if monolayers of the ethyl ester derivative were shown to be rather compressible, those of the benzyl ester derivative were more rigid and, relative to the ethyl ester derivative, they exhibited increased adhesion to the water subphase within a wide range of areas.

It has been shown that the van Oss-Chaudhury-Good theoretical approach applied to the calculation of interfacial free energies of these monolayers with water, obtained from the contact angle data on their Langmuir-Blodgett films, was perfectly adapted to explain the observed differences in their adhesion properties.