Volume 66, Issue 5 - Topical Issue: Ultrathin organic, inorganic, hybrid, and living cell coatings

Polymer thin films offer a versatile and ubiquitous platform for a wide variety of real-world applications in biomedicine, nanotechnology, catalysis, photovoltaic devices, and energy conversion and storage. Depending on the chemical composition of the polymers and the associated microenvironment, the physicochemical properties (biocompatibility, stability, wettability, adhesion, morphology, surface free energy, and others) of polymer films can be tuned for a specific application through precisely controlled surface synthesis and the incorporation of desirable and responsive functional groups. In this short review, we first summarise the methods most commonly used for the fabrication of polymer thin films. Then we discuss how these polymer thin films can be used in a selection of biomedical applications in antifouling materials and biosensors. Some directions for the rational design of polymer thin films to achieve a specific function or application are also provided.

This review provides a short overview of polymeric thin films incorporating molecular imprints within their 3D macromolecular structure as synthetic recognition elements and prepared by in situ polymerization for surface plasmon resonance application. This review starts with a brief reminder of the principle of surface plasmon resonance detection. The second section is focused on molecularly imprinted materials. Bulk and thin film polymer formats can be obtained by free radical polymerization, where the functional monomer interacts specifically with the template and the cross-linker controls the rigidity of the imprinted cavities. Grafting polymerization is presented as a method of choice for covalent attachment of ultra-thin molecularly imprinted films on a surface plasmon resonance metallic substrate. Examples of electropolymerized thin films are also provided. In the rest of this contribution, surface plasmon resonance applications of molecularly imprinted polymers reported mainly over the last two years are presented with respect to the preparation mode. Also, applications of gold nanoparticle/molecularly imprinted polymer composites for the design of surface plasmon resonance-based sensors with enhanced sensitivity due to the phenomenon of localized surface plasmon resonance induced by the presence of gold nanoparticles are summarized.

The paper presents a short review on the synthesis, characterisation and selected medical applications of poly(styrene/α-tert-butoxy-ω-vinylbenzyl-polyglycidol) (P(S/PGL)) microspheres. The soap-free emulsion-polymerisation of styrene and α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer (PGL) in water yielded core-shell microspheres with a low particle-diameter dispersity (ratio of the weight average particle diameter and the number average particle diameter). The interfacial fraction of PGL units, estimated by XPS, was in the range of 0–42 mole % depending on the concentration of the macromonomer in the polymerisation feed. The studies of adsorption of model proteins showed that the surface fraction of adsorbed protein was significantly reduced when the PGL interfacial fraction was higher than 40 mole %. The P(S/PGL) particles with covalently immobilised proteins were used for the preparation of photonic crystal assemblies suitable for applications in optical biosensors and the medical diagnostic test for the detection of Helicobacter pylori antibodies in the blood serum.

This review emphasises the role of aryl diazonium compounds as a new class of coupling agents for grafting polymer thin layers onto carbon, diamond, metals, metal oxides, alloys, semi-conductors, ceramics, and polymers. Physical and chemical methods are first reported for anchoring aryl layers to the surfaces, then the review concentrates on the modification of the above substrates by thin polymer films via a range of the “grafting from” and “grafting onto” strategies. Some applications are described which highlight the important role that diazonium salts will continue to play in the near future in the polymer and surface sciences.

Polypyrrole is one of the most frequently studied conducting polymers, having high electrical conductivity and stability, suitable for multi-functionalised applications. Coatings of chemically synthesised polypyrrole applied onto various organic and inorganic materials, such as polymer particles and films, nanoparticles of metal oxides, clay minerals, and carbon nanotubes are reviewed in this paper. Its primary subject is the formation of new materials and their application in which chemical oxidative polymerisation of pyrrole was used. These combined materials are used in antistatic applications, such as anti-corrosion coating, radiation-shielding, but also as new categories of sensors, batteries, and components for organic electronics are created by coating substrates with conducting polymer layers or imprinting technologies.

Any surface immersed in the aqueous reaction mixture used for the preparation of polyaniline becomes coated with a polyaniline film of submicrometre thickness. In this way, various materials can be modified by an overlayer of conducting polymer. The present review illustrates the role of infrared, Raman, and UV-VIS spectroscopies in the studies of polyaniline film growth. Spectroscopic methods are crucial in the evaluation of the performance of polyaniline films alone or in combination with nanoparticles of noble metals. The assessment of film ageing and stability can be followed conveniently by these methods. Carbonization of polyaniline films to nitrogen-containing carbon analogues is also discussed.

Chemistry and physics of thin semiconducting layers of various types are subjects of intense research. Especially when nanotechnology methods such as self-assembly are involved, amazing structural and/or functional properties may appear. Also modern physical methods using variously organized plasma arrangements are able to produce uniform structures with distinctive functionality. In this review, based virtually on our own work, discussions on the preparation, structure, morphology, and function of titanium(IV) oxide nanoscopic thin films are presented. It was shown that structurally and functionally similar titanium(IV) oxide films can be prepared via completely different preparation techniques. Function tests were arranged as “primary”, covering the assessment of the light induced charge separation efficiency, and “secondary”, based on photocatalytic surface oxidations.

Self-cleaning photocatalytic TiO2 films are beneficial since they reduce the maintenance cost and enhance the efficiency of various optical systems, especially thermal and photovoltaic solar systems. However, the presence of a TiO2 layer on glass reduces the transmission of incident light, which leads to a decrease in efficiency. This drawback can be overcome by applying a layer of anti-reflective coating beneath the TiO2 layer. Generally, the anti-reflective layer is porous silica. The presence of the anti-reflective layer compensates for the loss of light transmittance caused by the photocatalytic TiO2 top layer. This paper reviews some of the previous and the latest fundamental studies in the literature on anti-reflective, self-cleaning and multi-functional films.

The development of new electrocatalysts with the aim of enhancing the rate of electrochemical reactions has been a long-term goal of electrochemists. In part, this is due to the great importance of electrocatalysts in energy generation and environmental concerns. In this review, various methods of the preparation of nanostructured electrocatalysts and their applications after attachment to the electrode surface are described. Diazonium chemistry has been extensively used for the preparation and attachment of nanostructured electrocatalysts and this review thus describes the recent developments and applications of this chemistry in electrocatalysis. The preparation of nanostructured electrocatalysts including grafted molecular films and metal nanoparticles physically adsorbed on electrode surfaces and those attached to the surface by molecular links using diazonium chemistry is reviewed. Two methods for the attachment of nanoparticles by simple physical adsorption and by electrochemical deposition on molecular films are described and the electrochemical response of nanostructured electrocatalysts for some of the most common electrochemical reactions is discussed.

We have investigated the morphology, charge transport, and gas-sensing characteristics of thin films of cobalt phthalocyanine (CoPc) deposited on glass and sapphire substrates, using molecular beam epitaxy (MBE). CoPc films deposited on glass were found to be highly disordered. The ambient oxygen was found to be chemisorbed and created deep trap states, which led not only to hysteretic current-voltage (J-V) characteristics but also reduce the charge mobility. These properties render them unsuitable for gas-sensing. On the other hand, films deposited on sapphire were polycrystalline, which was attributed to an improved molecule-substrate interaction. The physically sorbed oxygen only created shallow traps, and the J-V characteristics were non-hysteretic, rendering them suitable for gas-sensing applications. It was demonstrated that the ultrathin (20 nm) CoPc films deposited on sapphire acted as highly sensitive and selective sensors for chlorine present in the w Cl concentration range of 5 × 10−9−2 × 10−6 (5–2000 ppb).

Ni-W alloy coatings were deposited by applying current pulses with different pulse parameters at 60°C onto mild steel substrates from aqueous electrolytes with different tungstate concentration. Morphology and composition of the alloys were analyzed by SEM and EDX, respectively. XRD was used to determine metallic phases. Scanning electron micrographs revealed that deposition parameters had a strong effect on the morphology of the coatings. Increasing the duty cycle or decreasing the off time led to a compact morphology. Corrosion properties of the coatings were investigated by potentiodynamic polarization in a chloride medium. It was found that compact morphology of the deposits and high content of tungsten in the coating contribute to satisfactory corrosion results of Ni-W alloy coatings under the conditions studied.

Processing the aerosol of metal salts in non-equilibrium plasma represents a promising technique that combines the advantages of spray pyrolysis with the high reactivity of plasmas at nearlaboratory temperature in order to produce mixed-oxides and perovskite materials. The aim of this paper is to describe the principles of this new technique and to present the various applications and latest developments. This technique’s capacity to deposit various mixed metal oxides with precise stoichiometry is demonstrated. It is shown that oxidant plasma species play a key role in the chemical transformation of starting materials into oxides at laboratory temperature, while the configuration of the reactor determines the morphology and texture of the deposited layers. Two different reactor configurations are presented. The porous layers of LaxSr1−x MnO3 as the cathode for fuel cells were synthesised in a wave shock reactor configuration, while nanostructured ZnO-Al layers to form a transparent conductive cathode for photovoltaic cells were deposited in the spray plasma reactor of the latest generation for this technique. The experimental results emphasise the role of plasma species in the rate of chemical reactions and in the chemical composition of the deposited layers.

At present, coatings are mostly produced by CVD, PVD methods, plasma, and detonation spraying. Molten salts for coatings deposition are also becoming highly attractive. They provide wide possibilities for coating production of hafnium by electrodeposition, electrochemical synthesis, and precise surface alloying. Consideration is given to all these methods. The production of heat-resistant coatings from hafnium and niobium-hafnium alloys, and the electrochemical syntheses of copperhafnium solder and HfB2 coatings are also discussed.

This study is directed towards investigating the role of the surface treatment of clay particles on the rheological and thermomechanical behaviour of clay-epoxy blends. Nanocomposites were prepared by mixing small amounts (5–10 mass %) of commercial organoclays or raw clays with an epoxy system commonly used in civil engineering. Rheological characterisations in the liquid state revealed a pronounced thixotropic character of the organoclay-based systems, which all exhibited a shear-thinning behaviour above a critical stress threshold (yield stress), depending on both the intensity of interfacial interactions and the degree of filler dispersion. On the other hand, systems based on raw clay particles behaved like Newtonian fluids, in the same way as the unreinforced polymer matrix. Complementary dynamic mechanical analyses (DMA) performed on the cured cross-linked nanocomposites also showed significant changes in the viscoelastic behaviour of the epoxy matrix due to the introduction of organoclays, whereas only minor variations were observed following the introduction of raw fillers. These results were consistent with nanoscale morphological characterisations performed by conventional X-ray diffraction (XRD) on the various hybrid systems. In this context, rheology and DMA appear as attractive alternative methods for assessing the filler dispersion at a macroscopic (and possibly more relevant) scale. This research is of practical interest for civil engineers, since clay reinforced-epoxies could in the future be used as coating materials with enhanced barrier performances, in order to protect infrastructures against environmental ageing or corrosion.

The seeding of endothelial cells on biomaterial surfaces has become a major challenge to achieve better haemocompatibility of these surfaces. Multilayers of polyelectrolytes formed by the layerby-layer method are promising in this respect. In this study, the interactions of endothelial cells with multilayered polyelectrolytes films were investigated. The build-ups were prepared by selfassembled alternatively adsorbed polyanions and polycations functionalised with fibronectin and collagen. Anionic poly(sodium 4-styrenesulfonate) and cationic poly(allylamine hydrochloride) polyelectrolytes were chosen as a model system. Elaborated surfaces were characterised by electrochemical impedance spectroscopy and cyclic voltammetry. The modified electrode showed good reversible electrochemical properties and high stability in an electrolyte solution. The film ohmic resistance was highest when the film was coated with fibronectin; the parameters so determined were correlated with atomic force microscopy images. Cell colorimetric assay (WST-1) and immunofluorescence were used to quantify the cell viability and evaluate the adhesion properties. When cultured on a surface where proteins were deposited, cells adhered and proliferated better with fibronectin than with collagen. In addition, a high surface free energy was favourable to adhesion and proliferation (48.8 mJ m−2 for fibronectin and 39.7 mJ m−2 for collagen, respectively). Endothelial cells seeded on functionalised-polyelectrolyte multilayer films showed a good morphology and adhesion necessary for the development of a new endothelium.