Self-Assembled Monolayers of Push-Pull Chromophores as Active Layers and Their Applications.

In recent decades, considerable attention has been focused on the design and development of surfaces with defined or tunable properties for a wide range of applications and fields. To this end, self-assembled monolayers (SAMs) of organic compounds offer a unique and straightforward route of modifying and engineering the surface properties of any substrate. Thus, alkane-based self-assembled monolayers constitute one of the most extensively studied organic thin-film nanomaterials, which have found wide applications in antifouling surfaces, the control of wettability or cell adhesion, sensors, optical devices, corrosion protection, and organic electronics, among many other applications, some of which have led to their technological transfer to industry. Nevertheless, recently, aromatic-based SAMs have gained importance as functional components, particularly in molecular electronics, bioelectronics, sensors, etc., due to their intrinsic electrical conductivity and optical properties, opening up new perspectives in these fields. However, some key issues affecting device performance still need to be resolved to ensure their full use and access to novel functionalities such as memory, sensors, or active layers in optoelectronic devices. In this context, we will present herein recent advances in π-conjugated systems-based self-assembled monolayers (e.g., push-pull chromophores) as active layers and their applications.

Surface Enhancement Using Black Coatings for Sensor Applications.

The resolution of a quartz crystal microbalance (QCM) is particularly crucial for gas sensor applications where low concentrations are detected. This resolution can be improved by increasing the effective surface of QCM electrodes and, thereby, enhancing their sensitivity. For this purpose, various researchers have investigated the use of micro-structured materials with promising results. Herein, we propose the use of easy-to-manufacture metal blacks that are highly structured even on a nanoscale level and thus provide more bonding sites for gas analytes. Two different black metals with thicknesses of 280 nm, black aluminum (B-Al) and black gold (B-Au), were deposited onto the sensor surface to improve the sensitivity following the Sauerbrey equation. Both layers present a high surface roughness due to their cauliflower morphology structure. A high response (i.e., resonant frequency shift) of these QCM sensors coated with a black metal layer was obtained. Two gaseous analytes, H2O vapor and EtOH vapor, at different concentrations, are tested, and a distinct improvement of sensitivity is observed for the QCM sensors coated with a black metal layer compared to the blank ones, without strong side effects on resonance frequency stability or mechanical quality factor. An approximately 10 times higher sensitivity to EtOH gas is reported for the QCM coated with a black gold layer compared to the blank QCM sensor.

Single and binary self-assembled monolayers of phenyl- and pentafluorophenyl-based silane species, and their phase separation with octadecyltrichlorosilane.

In this paper, we first present the study of the formation of phenyltrichlorosilane film and self-assembled monolayers of phenylalkyltrichlorosilane (PATCl), pentafluoro-phenylalkyltrichlorosilane (PFATCl), and a mixture of the two, on silicon covered by its native oxide. These monolayers are shown to grow in two steps with characteristic time constants. The first step is characterized by a similar time constant of growth for all the studied trichlorosilane molecules and attributed to chemisorption. The second step corresponds to the arrangement between molecules, accelerated by the presence of the short alkyl chain (3-4 carbon atoms), and by mixing phenyl and pentafluoro-phenyl terminal moieties, which is accounted for by hydrogen bonding CH···FC and/or attractive quadrupolar interactions within a face-to-face phenyl/pentafluoro-phenyl alternating stack arrangement. Such results should allow improvement of intermolecular stacking within conjugated molecular domains, which is particularly important for molecular electronic devices. In the second part, we studied how PATCl, PFATCl, and their mixture phase separate with octadecyltrichlorosilane (OTS) molecules in various ratios. The way to improve phase separation was studied modifying aromatic ring to ring as well as aromatic-aliphatic interactions. OTS island size and coverage are shown to be smaller with the aromatic phase that involves stronger ring to ring interactions, i.e., attractive interactions between the phenyl species by mixing phenyl and pentafluoro-phenyl rings. The best phase separation is obtained with PFATCl as the aromatic molecule. If nanoislands of aromatic molecules could not be observed in these experiments, we show that they are attainable by mixing OTS and aromatic small organotriethoxysilanes whose grafting kinetics is slower. These results pave the way to the control improvement of the composition and nanostructuration of SAMs, essential for their further use within molecular devices.

Role of molecular properties of ulvans on their ability to elaborate antiadhesive surfaces.

Antiadhesive properties of polysaccharides (such ulvans) once immobilized on a surface are described in the literature but the parameters governing their antifouling properties are not yet well identified. In the present study, the relationship between molecular parameters of ulvans and the inhibition of bacterial adhesion was investigated. To this aim, various ulvans were grafted on silicon wafers under two different experimental immobilization conditions. Results showed that the experimental immobilization conditions and the polysaccharides molecular weight led to specific layer conformations which exhibited a key role in the surface antiadhesive properties.

Antiadhesive activity of ulvan polysaccharides covalently immobilized onto titanium surface.

Bacterial adhesion leading to biofilm formation on the surface of implants is responsible for pathogenesis infections. One promising strategy to reduce the risk of infection consists of modifying implant surfaces by antibacterial coating. In the present study, the ability of ulvan, a non biocidal algal polysaccharide, to limit bacterial adhesion on titanium was investigated. To this end, titanium surfaces were modified by two different ulvans. Polysaccharides were covalently immobilized on titanium surfaces which had been previously functionalized by self assembled monolayers of aminoundecyltrimethoxysilane (AUTMS). Each step in the modification process was characterized by contact angle, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Bacterial adhesion assays showed that immobilized ulvans on titanium surface strongly decreased by about 90% the adhesion of Pseudomonas aeruginosa. Moreover, AFM observations showed that the polysaccharide also inhibited the bacterial spreading on the surface but not cell-to-cell interaction. The permanence of the anti-adhesive effect of the surfaces was finally tested on a non-motile organism, i.e., Staphylococcus epidermidis. The results showed that the effect was maintained for at least 24h.

Solvent induced aggregation of protoporphyrin and octacarboxylphthalocyanine of zinc deposited on gold surface.

In this paper, we studied the influence of solvent on the morphology of zinc protoporphyrin and zinc octacarboxylphthalocyanine films transferred onto gold surface by dipping. In these films, carboxylic acid groups borne in periphery of macrocycles allow anchoring to gold via ionic interaction. First, we followed by UV-Visible absorption spectroscopy the solvation state of these conjugated macrocycles in pure DMF, in pure ethanol and in various ethanol/DMF mixtures. We show that the increase in ethanol proportion promotes interactions between macrocycles. Second, molecular layers of macrocycles spontaneously adsorbed from these various solutions onto gold surface were analyzed by ellipsometry, water contact angle measurements, UV-Visible absorption spectroscopy and atomic force microscopy. Results evidenced the layers were mainly composed of grains whose size of a few nanometers was directly related to the solvation conditions of molecules. In addition, Q band splitting was observed in the absorption spectrum of zinc octacarboxylphthalocyanine grain films which indicates specific organization of those molecules. Therefore solvent is shown to have a profound influence on the nanostructuration of as-prepared macrocycle layers on gold surface by promoting pre-organization in solution, and its composition enables to better control the morphology of those films by tuning the solubilization of macrocycles.