Charge transport through redox active [H7P8W48O184]33- polyoxometalates self-assembled onto gold surfaces and gold nanodots.

Polyoxometalates (POMs) are redox-active molecular oxides, which attract growing interest for their integration into nano-devices, such as high-density data storage non-volatile memories. In this work, we investigated the electrostatic deposition of the negatively charged [H7P8W48O184]33- POM onto positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs) preformed onto gold substrates or onto an array of gold nanodots. The ring-shaped [H7P8W48O184]33- POM was selected as an example of large POMs with high charge storage capacity. To avoid the formation of POM aggregates onto the substrates, which would introduce variability in the local electrical properties, special attention has to be paid to the preformed SAM seeding layer, which should itself be deprived of aggregates. Where necessary, rinsing steps were found to be crucial to eliminate these aggregates and to provide uniformly covered substrates for subsequent POM deposition and electrical characterizations. This especially holds for commercially available gold/glass substrates while these rinsing steps were not essential in the case of template stripped gold of very low roughness. Charge transport through the related molecular junctions and nanodot molecule junctions (NMJs) has been probed by conducting-AFM. We analyzed the current-voltage curves with different models: electron tunneling though the SAMs (Simmons model), transition voltage spectroscopy (TVS) method or molecular single energy level mediated transport (Landauer equation) and we discussed the energetics of the molecular junctions. We concluded to an energy level alignment of the alkyl spacer and POM lowest occupied molecular orbitals (LUMOs), probably due to dipolar effects.

One-pot synthesis of a new generation of hybrid bisphosphonate polyoxometalate gold nanoparticles as antibiofilm agents.

A reduced polyoxovanadate functionalized with bisphosphonate molecules was synthesized and used to prepare in one step hybrid organic-inorganic polyoxometalate decorated gold nanoparticles. These new composites were shown to strongly inhibit P. aeruginosa and S. epidermidis biofilm growth, with the three components constituting the nanoparticles (Au0 core, vanadium and alendronate) acting synergistically.

Correction: One-pot synthesis of a new generation of hybrid bisphosphonate polyoxometalate gold nanoparticles as antibiofilm agents.

[This corrects the article DOI: 10.1039/C9NA00401G.].

Supramolecular chiral self-assemblies of Gly-Pro dipeptides on metallic fcc(110) surfaces.

Adsorption of the Glycine-Proline (Gly-Pro) dipeptide has been investigated using surface science complementary techniques on Au(110) and Ag(110), showing some interesting differences both in the chemical form and surface organization of the adsorbed peptide. On Au(110), Gly-Pro mainly adsorbs in neutral form (COOH/NH2), at low coverage or for a short interaction time; the surface species become zwitterionic at a higher coverage or longer interaction time. These changes are accompanied by a complete reorganization of the molecules at the surface. On Ag(110), only anionic molecules (COO-/NH2) were detected on the surface and only one type of arrangement was observed. These results will be compared to some previously obtained on Cu(110), thus providing a unique comparison of the adsorption of the same di-peptide on three different metal surfaces; the great influence of the substrate on both the chemical form and the arrangement of adsorbed di-peptides was made clear.

Low-energy electron induced resonant loss of aromaticity: consequences on cross-linking in terphenylthiol SAMs.

Aromatic self-assembled monolayers (SAMs) can be used as negative tone electron resists in functional surface lithographic fabrication. A dense and resistant molecular network is obtained under electron irradiation through the formation of a cross-linked network. The elementary processes and possible mechanisms involved were investigated through the response of a model aromatic SAM, p-terphenylthiol SAM, to low-energy electron (0-10 eV) irradiation. Energy loss spectra as well as vibrational excitation functions were measured using High Resolution Electron Energy Loss Spectroscopy (HREELS). A resonant electron attachment process was identified around 6 eV through associated enhanced excitation probability of the CH stretching modes ν(CH)(ph) at 378 meV. Electron irradiation at 6 eV was observed to induce a peak around 367 meV in the energy loss spectra, attributed to the formation of sp(3)-hybridized CHx groups within the SAM. This partial loss of aromaticity is interpreted to be the result of resonance formation, which relaxes by reorganization and/or CH bond dissociation mechanisms followed by radical chain reactions. These processes may also account for cross-linking induced by electron irradiation of aromatic SAMs in general.

Selective terminal function modification of SAMs driven by low-energy electrons (0-15 eV).

Low-energy electron induced degradation of a model self-assembled monolayer (SAM) of acid terminated alkanethiol was studied under ultra-high vacuum (UHV) conditions at room and low (~40 K) temperatures. Low-energy electron induced chemical modifications of 11-mercaptoundecanoic acid (MUA, HS-(CH2)10-COOH) SAMs deposited on gold were probed in situ as a function of the irradiation energy (<11 eV) by combining two complementary techniques: High Resolution Electron Energy Loss Spectroscopy (HREELS), a surface sensitive vibrational spectroscopy technique, and Electron Stimulated Desorption (ESD) analysis of neutral fragments. The SAM's terminal functions were observed to be selectively damaged at around 1 eV by a resonant electron attachment mechanism, observed to decay by CO, CO2 and H2O formation and desorption. CO2 and H2O were also directly identified at low temperature by vibrational analysis of the irradiated SAMs. At higher irradiation energy, both terminal functions and spacer alkyl chains are damaged upon electron irradiation, by resonant and non-resonant processes.

Antibacterial surfaces developed from bio-inspired approaches.

Prevention of bacterial adhesion and biofilm formation on the surfaces of materials is a topic of major medical and societal importance. Various synthetic approaches based on immobilization or release of bactericidal substances such as metal derivatives, polyammonium salts and antibiotics were extensively explored to produce antibacterial coatings. Although providing encouraging results, these approaches suffer from the use of active agents which may be associated with side-effects such as cytotoxicity, hypersensibility, inflammatory responses or the progressive alarming phenomenon of antibiotic resistance. In addition to these synthetic approaches, living organisms, e.g. animals and plants, have developed fascinating strategies over millions of years to prevent efficiently the colonization of their surfaces by pathogens. These strategies have been recently mimicked to create a new generation of bio-inspired biofilm-resistant surfaces. In this review, we discuss some of these bio-inspired methods devoted to the development of antibiofilm surfaces. We describe the elaboration of antibacterial coatings based on natural bactericidal substances produced by living organisms such as antimicrobial peptides, bacteriolytic enzymes and essential oils. We discuss also the development of layers mimicking algae surfaces and based on anti-quorum-sensing molecules which affect cell-to-cell communication. Finally, we report on very recent strategies directly inspired from marine animal life and based on surface microstructuring.

Salt concentration and pH-dependent adsorption of two polypeptides on planar and divided alumina surfaces. In situ IR investigations.

The adsorption of proteins is the first process to take place when a solid is immersed in a biological fluid; though not yet thoroughly understood at a molecular level, this process is also known to be strongly influenced by the presence of salt in solution or by pH changes. In the present work, poly-L-glutamic acid (PG) and poly-L-lysine (PL) were selected to mimic the behavior of some protein fragments. Their adsorption was investigated by infrared spectroscopy in various modes, both on planar and on divided (powder) surfaces of aluminum oxide. These two peptides were shown to have different behaviors when adsorbed from solutions with or without CaCl2 and at various pH values. Polarization modulation-reflection absorption infrared spectroscopy, applied in a special cell designed to characterize the solid surface in contact with the liquid, enabled the observation of the influence of pH and salts upon polypeptide adsorption. At pH values higher than 5 and in the presence of CaCl2 in solution, a net increase of the PG adsorbed amount is observed, whereas no such effect could be detected for PL. Specific interactions between the COO- groups on the side chains and the surface, or between those of two different molecules, was inferred. Interestingly, similar conclusions could be drawn for the surface of alumina powders contacted with solutions of PG and PL and characterized by attenuated total reflectance IR. This work demonstrates the potential for IR investigations of solid oxide-liquid interfaces combining the study of planar and finely divided surfaces.