Water-Induced Structural Changes in the Membrane-Anchoring Monolayers Revealed by Isotope-Edited SERS

Water is required for biological function of tethered bilayer lipid membranes (tBLMs) consisting of a lipid bilayer anchored by a mixed self-assembled monolayer (SAM) to a solid support. In this work, water-induced structural changes in mixed SAMs and bonding with gold substrate were probed in situ by the isotope-edited SERS coupled with the first-principles calculations. The assignment of the bands was based on experimental analysis of deuterated 2-mercaptoethanol (ME-D₄) and quantum chemical modeling of adsorption complexes consisting of a Au cluster of 10 atoms. Evidence for interaction of the gauche conformer with the Au surface through both S and O atoms was obtained from the analysis of the Au–S/Au–O vibrational mode near 303 cm^–1 and adsorption-induced downshift of the C–O stretching band near 1100 cm^–1. First-principles calculations of adsorption complexes revealed shortening of the Au–O bond upon hydrogen bonding of explicit water molecule to the OH group of ME-D₄. Intense bands in the SERS spectrum near 679 and 597 cm^–1 were assigned to C–S vibrational modes of adsorbed ME-D₄ in trans and gauche conformation, respectively. Analysis of relative intensities revealed a decrease in relative amount of trans conformers after 72 h incubation of the SAM in water. At the same time an increase in the population of a long carbon chain molecular anchor (WC14) in the all-trans hydrocarbon chain conformation was detected by SERS. Observed structural changes suggest water-induced clustering of long-chain anchors and conformational transition for short-chain thiols

Synthesis of Silver Nanocubes@Cobalt Ferrite/Graphitic Carbon Nitride for Electrochemical Water Splitting

This study presents the synthesis of graphitic carbon nitride (g-C3N4) and its nanostructures with cobalt ferrite (CoFe2O4) and silver nanocubes (Ag) when using the combined pyrolysis of melamine and the polyol method. The resulted nanostructures were tested as electrocatalysts for hydrogen and oxygen evolution reactions in alkaline media. It was found that Ag@CoFe2O4/g-C3N4 showed the highest current density and gave the lowest overpotential of −259 mV for HER to reach a current density of 10 mA cm−2 in a 1 M KOH. The overpotentials for reaching the current density of 10 mA·cm−2 for OER were 370.2 mV and 382.7 mV for Ag@CoFe2O4/g-C3N4 and CoFe2O4/g-C3N4, respectively. The above results demonstrated that CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 materials could act as bifunctional catalysts due to their notable performances and high stabilities toward hydrogen and oxygen evolution reactions (HER and OER). Total water splitting in practical applications is a promising alternative to noble-metal-based electrocatalysts

Electrical activity of cellobiose dehydrogenase adsorbed on thiols : Influence of charge and hydrophobicity

The interface between protein and material surface is of great research interest in applications varying from implants, tissue engineering to bioelectronics. Maintaining functionality of bioelements depends greatly on the immobilization process. In the present study direct electron transfer of cellobiose dehydrogenase from Humicola insolens (HiCDH), adsorbed on four different self-assembled monolayers (SAMs) formed by 5-6 chain length carbon thiols varying in terminal group structure was investigated. By using a combination of quartz crystal micro balance with dissipation, ellipsometry and electrochemistry the formation and function of the HiCDH film was studied. It was found that the presence of charged pyridinium groups was needed to successfully establish direct electron contact between the enzyme and electrode. SAMs formed from hydrophilic charged thiols achieved nearly two times higher current densities compared to hydrophobic charged thiols. Additionally, the results also indicated proportionality between HiCDH catalytic constant and water content of the enzyme film. Enzyme films on charged pyridine thiols had smaller variations in water content and viscoelastic properties than films adsorbed on the more hydrophobic thiols. This work highlights several perspectives on the underlying factors affecting performance of immobilized HiCDH. (C) 2017 Elsevier B.V. All rights reserved

Reflection Absorption Infrared Spectroscopy Characterization of SAM Formation from 8-Mercapto-N-(phenethyl)octanamide Thiols with Phe Ring and Amide Groups

Multifunctional amide-containing self-assembled monolayers (SAMs) provide prospects for the construction of interfaces with required physicochemical properties and distinctive stability. In this study, we report the synthesis of amide-containing thiols with terminal phenylalanine (Phe) ring functionality (HS(CH2)7CONH(CH2)2C6H5) and the characterization of the formation of SAMs from these thiols on gold by reflection absorption infrared spectroscopy (RAIRS). For reliable assignments of vibrational bands, ring deuterated analogs were synthesized and studied as well. Adsorption time induced changes in Amide-II band frequency and relative intensity of Amide-II/Amide-I bands revealed two-state sigmoidal form dependence with a transition inflection points at 2.2 ± 0.5 and 4.7 ± 0.5 min, respectively. The transition from initial (disordered) to final (hydrogen-bonded, ordered) structure resulted in increased Amide-II frequency from 1548 to 1557 cm−1, which is diagnostic for a strongly hydrogen-bonded amide network in trans conformation. However, the lateral interactions between the alkyl chains were found to be somewhat reduced when compared with well-ordered alkane thiol monolayers

SERS of the Positive Charge Bearing Pyridinium Ring Terminated Self-Assembled Monolayers: Structure and Bonding Spectral Markers

Bifunctional self-assembled monolayers (SAMs) are widely used for construction of surfaces with desirable properties. To predict and control the function of SAMs, molecular level understanding of monolayer architecture is required. In this work, structure and bonding of positive charge bearing SAM of <i>N</i>-(6-mercapto)­hexylpyridinium (MHP) on Au and Ag electrodes was probed by SERS, first-principles calculations, isotopic substitution, and reductive desorption voltammetry. Based on analysis of immersion time- and temperature-dependent SERS spectra as well as DFT calculations, the marker bands for MHP structure, bonding, and orientation have been established. The presence of a band around 1083 cm^–1 was found to be a reliable SERS marker for the all-trans conformation of hydrocarbon chain. Soft C–H stretching mode near 2832 cm^–1 was recognized as a marker for the probing of direct interaction of alkyl chain with the metal. Based on solvent-dependent Raman spectra the pyridinium ring C–H band (ν₂) was proposed to be a marker for environment dielectric constant. The metal–adsorbate bonding marker bands, ν­(Au–S) and ν­(Ag–S), were observed at 260 and 236 cm^–1, respectively. Identified in this work SERS marker bands provide novel spectra–structure correlations applicable to molecular level control of structure, bonding, and stability of bifunctional SAMs

Mediatorless Carbohydrate​/Oxygen Biofuel Cells with Improved Cellobiose Dehydrogenase Based Bioanode

Direct electron transfer (DET) between cellobiose dehydrogenase from Humicola insolens ascomycete (HiCDH) and gold nanoparticles (AuNPs) was achieved by modifying AuNPs with a novel, pos. charged thiol N-​(6-​mercapto)​hexylpyridinium (MHP)​. The DET enabled the use of the HiCDH enzyme as an anodic biocatalyst in the design of a mediatorless carbohydrate​/oxygen enzymic fuel cell (EFC)​. A biocathode of the EFC was based on bilirubin oxidase from Myrothecium verrucaria (MvBOx) directly immobilized on the surface of AuNPs. The following parameters of the EFC based on Au​/AuNP​/MHP​/HiCDH bioanode and Au​/AuNP​/MvBOx biocathode were obtained in quiescent air satd. PBS, pH 7.4, contg.: (i) 5 mM glucose-​open-​circuit voltage (OCV) of 0.65 ± 0.011 V and the maximal power d. of 4.77 ± 1.34 μW cm-​2 at operating voltage of 0.50 V; or (ii) 10 mM lactose-​OCV of 0.67 ± 0.006 V and the maximal power d. of 8.64 ± 1.91 μW cm-​2 at operating voltage of 0.50 V. The half-​life operation times of the EFC were estd. to be at least 13 and 44 h in air satd. PBS contg. 5 mM glucose and 10 mM lactose, resp. Among advantages of HiCDH​/MvBOx FCs are (i) simplified construction, (ii) relatively high power output with glucose as biofuel, and (iii) the absence of the inhibition of the HiCDH based bioanode by lactose, when compared with the best previously reported CDH based bioanode

Electrochemical Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Bonding, Structure, and Ion-Pairing of the Positive Charge Bearing Pyridinium Ring Terminated Monolayer at Smooth Gold Electrode

Electrode potential effect on the molecular structure of adsorbed species at an interface is one of the most important issues in physical electrochemistry. In this study, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to probe the electrochemical potential effect on bonding with surface, ion-pairing, and molecular structure of a positive charge bearing self-assembled monolayer formed from <i>N</i>-(6-mercapto)­hexylpyridinium (MHP) at a smooth gold electrode in aqueous perchlorate solutions. In situ electrochemical SHINERS results show that the Au–S stretching frequency exhibits near linear blue-shift as the electrode potential was tuned to more positive values. The frequency tuning rate was found to be as high as 18.6 ± 0.9 cm^–1/V. Analysis of ion-pairing at the interface revealed a decrease in intensity of the vibrational spectrum of electrostatically attracted perchlorate anions as the electrode potential shifts progressively to more negative values. Spectroscopic evidence for potential-driven conformational changes in the structure of MHP monolayer was revealed. It was found that the negatively charged electrode surface attracts terminal positively charged pyridinium groups, resulting in loss of all-trans conformation in hydrocarbon chains of MHP and forces some methylene groups into direct contact with the metal surface. Surface attracted pyridinium ring groups are not able further to attract perchlorate anions from the solution phase, and the intensity of perchlorate bands in SHINERS spectra decreases