A bioinspired approach to fabricate fluorescent nanotubes with strong water adhesion by soft template electropolymerization and post-grafting.

HYPOTHESIS: In this original work, we aim to control both the surface wetting and fluorescence properties of extremely ordered and porous conducting polymer nanotubes prepared by soft template electropolymerization and post-grafting. For reaching this aim, various substituents of different hydrophobicity and fluorescence were post-grafted and the post-grafting yields were evaluated by surface analyses. We show that the used polymer is already fluorescent before post-grafting while the post-grafting yield and as a consequence the surface hydrophobicity highly depend on the substituent. EXPERIMENTS: Here, we have chosen to chemically grafting various fluorinated and aromatic substituents using a post-grafting in order to keep the same surface topography. Flat conducting polymer surfaces with similar properties have been also prepared for determining the surface energy with the Owens-Wendt equation and estimating the post-grafting yield by X-ray Photoemission Spectroscopy (XPS) and Time of Flight Secondary Emission Spectrometry (ToF-SIMS). For example, using fluorinated chains of various length (C4F9, C6F13 and C8F17), it is demonstrated that the surface hydrophobicity and oleophobicity do not increase with the fluorinated chain length due to the different post-grafting yields and because of the presence of nanoroughness after post-grafting. FINDINGS: These surfaces have high apparent water contact angle up to 130.5° but also strong water adhesion, comparable to rose petal effect even if there are no nanotubes on petal surface. XPS and ToF-SIMS analyses provided a detailed characterisation of the surface chemistry with a qualitative classification of the grafted surfaces (F6 > F4 > F8). SEM analysis shows that grafting does not alter the surface morphology. Finally, fluorescence analyses show that the polymer surfaces before post-treatment are already nicely fluorescent. Although the main goal of this paper was and is to understand the role of surface chemistry in tailoring the wetting properties of these surfaces rather than provide specific application examples, we believe that the obtained results can help the development of specific nanostructured materials for potential applications in liquid transport, or in stimuli responsive antimicrobial surfaces.

Micellar formation by soft template electropolymerization in organic solvents.

HYPOTHESIS: The formation of porous nanostructures on surfaces and the control of their size and shape is fundamental for various applications. The creation of nanotubes is particularly difficult to implement without the aid of hard and rigid templates. Recently, methods that form nanotubular structures in a straightforward manner and without direct templating, e.g. soft templating, have been highly sought after. Here we propose the use of "soft templating" via self-assembly of conducting monomers during electropolymerization in organic solvents as a mean to form porous, nanotubular features. EXPERIMENTS: Naphtho[2,3-b]thieno[3,4-e][1,4]dioxine (NaphDOT) is employed as monomer for electropolymerizations conducted in dichloromethane and chloroform containing varying amounts of water. SEM analyses of the resulting surfaces confirms the strong capacity of NaphDOT to form vertically aligned nanotubes. Polymerization solutions analyzed by DLS and TEM reveal the presence of micelles prior to electropolymerization, and the size of the micelles correlates with the inner diameter of the nanotubes formed. FINDINGS: We show that micelles in polymerization solutions are stabilized by both monomers and electrolytes. We propose a mechanism where reverse micelles are forming a soft-template responsible for the formation of porous nanostructures during electropolymerization in organic, non-polar solvents. In this mechanism, the monomer and electrolyte assume the role of surfactant in the reverse micelle system.

A soft template approach to various porous nanostructures from conjugated carbazole-based monomers.

HYPOTHESIS: Controlling the size and the shape of nanostructures on surfaces is fundamental for various applications while the formation of porous structures such as nanotubes is particularly difficult. The templateless electropolymerization is a choice process that not only forms nanostructured surfaces, but also can tune their morphologies using different monomers. EXPERIMENTS: In this work, we used this soft-template and surfactant free electropolymerization in organic solvent to deposit for the first time carbazole-based monomers. Five different conjugated carbazole-based monomers are tested here. FINDINGS: We show that the shape of surfaces nanostructures is highly dependent on the amount of water present in the organic solvent and on the molecular structure of the carbazole monomers. Different morphologies are obtained from fibers to vertically aligned nanotubes and even porous membranes, depending on the monomer and on the electropolymerization method. The nanostructured surfaces reach superhydrophobic properties and their dynamical non-wetting behavior varies with the monomer and the electrochemical parameters.

A bioinspired strategy for designing well-ordered nanotubular structures by templateless electropolymerization of thieno[3,4-b]thiophene-based monomers.

Here, a bioinspired strategy is used to prepare well-ordered nanotubular structures, as observed in animals and plants, such as gecko toe pads or corals. The nanotubes are obtained by templateless electropolymerization of thieno[3,4-b]thiophene-based monomers with various aromatic groups in an organic solvent (dichloromethane). The most interesting and robust structures were obtained with carbazole and pyrene substituents to the base monomer structure, since these groups participate significantly in the polymerization and also have strong π-stacking interactions. The addition of water to electropolymerization solvent significantly impacted the formation of nanotubes, as it caused the release of a significant amount of H2 and O2 bubbles, depending on the electropolymerization method. Identifying templateless approaches to vary nanotubular structures is very interesting, as these materials are sought-after for applications in water harvesting systems. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 3)'.

Tuning nanotubular structures by templateless electropolymerization with thieno[3,4-b]thiophene-based monomers with different substituents and water content.

Here, templateless electropolymerization is employed to produce nanotubular structures from various thieno[3,4-b]thiophene-based monomers that differ in substituent structure and size, as well as the linker connecting the thieno[3,4-b]thiophene core and substituent. The formation of densely packed vertically aligned are obtained from monomers with a pyrene substituent and when a significant amount of water (CH2Cl2 + H2O) is included in the solvent. The geometrical parameters of the nanotubes are highly dependent on the electopolymerization method. A significant amount of air is trapped within the structure of the densely packed open nanotubes obtained with Qs = 100 mC cm-2 causing an increase in water contact angle (θw) up to 82.6° (intermediate state between the Wenzel and the Cassie-Baxter state), and θw can become even more hydrophobic by further modifying the deposition method or the electrolyte.

Hybrid surfaces combining electropolymerization and lithography: fabrication and wetting properties.

In the current work, we developed a novel method to fabricate hybrid surfaces consisting of mixed hydrophilic/superhydrophobic properties. These surfaces specifically consist of a regular array of hydrophilic pillars (displaying a receding contact angle lower than 90°) surrounded by a superhydrophobic thinner layer made via the electropolymerization of a fluorinated monomer. Then, we determined the wetting properties of various forms of this complex surface, i.e., displaying different surface properties, by specifically determining their advancing (θa) and receding (θr) contact angles. Two main parameters were varied: the pillar density (from 21.2% to 6.5% based on using a spacing d between pillars varying from 25 to 45 micrometers) and the polymer charge density (from 0 to 100 mC cm-2). We observed that, for low charge density values, only the ground surface was covered by the hydrophobic polymers; while for higher charge density values, polymerization reached higher levels on the lateral surfaces of the nonconductive cylindrical pillars, eventually up to their top surfaces and covering them for the highest charge densities. This feature gave us an additional parameter that we could use to control the surface wettability. We also found that contact angles (advancing and receding) increased markedly with increasing polymer charge density above a critical value (which was higher for receding angles). And we measured advancing and receding contact angles to, respectively, increase and decrease with increasing pillar density. We interpreted qualitatively these behaviors, the main point being the importance of the impalement (null, partial or total).

Designing Nanoporous Membranes through Templateless Electropolymerization of Thieno[3,4-b]thiophene Derivatives with High Water Content.

In this work, we present the synthesis of original thieno[3,4-b]thiophene monomers with rigid substituents (e.g., perfluorinated chains, and aromatic groups) and demonstrate the ability to prepare nanotubular and nanoporous structures via templateless, surfactant-free electropolymerization in organic solvents (dichloromethane). For the majority of synthesized monomers, including a significant amount of water in the electropolymerization solvent leads to the formation of nanoporous membranes with tunable size and surface hydrophobicity. If water is not included in the electropolymerization solvent, most of the surfaces prepared are relatively smooth. Tests with different water contents show that the formation of nanoporous membranes pass through the formation of vertically aligned nanotubes and that the increase in water content induces an increase in the number of nanotubes while their diameter and height remain unchanged. An increase in surface hydrophobicity is observed with the formation of nanopores up to ≈300 nm in diameter, but as the nanopores further increase in diameter, the surfaces become more hydrophilic with an observed decrease in the water contact angle. These materials and the ease with which they can be fabricated are extremely interesting for applications in separation membranes, opto-electronic devices, as well as for sensors.

Dynamic Wetting Properties of Mesh Substrates with Tunable Water Adhesion.

In nature, wetting phenomena are present nearly everywhere and are a source of inspiration for liquid transportation. A good understanding of the underlying dynamic phenomena that governs wettability is therefore extremely important for researchers involved in bio-inspired surfaces. Herein, we study the adhesive behavior with water of mesh substrates modified with structured copolymers in order to tune the surfaces from parahydrophobic states (high water adhesion) to superhydrophobic states (low water adhesion). Using the ejection test method (ETM), a new technique that consists of the ejection of water droplets deposited onto a substrate with the aid of a catapult system, we experimentally demonstrate that the elasticity of the mesh substrate can be exploited for efficient vertical actuation of droplets.

Designing bioinspired coral-like structures using a templateless electropolymerization approach with a high water content.

Here, with the aim of obtaining densely packed porous nanostructures of various shape using templateless electropolymerization in organic solvent (dichloromethane), original thieno[3,4- b]thiophene-based monomers with different substituents are studied. First of all, the adding of water in solution has a huge influence on the formation of porous structures because a much higher amount of gas (O2 and/or H2) is released. Rigid substituents such as aromatic groups have a beneficial effect on the formation of nanotubular structures contrary to flexible ones such as alkyl chains. Special results are obtained with the pyrene substituent (Thieno-Pyr). With this monomer, coral-like structures are obtained. These structures are obtained by the formation first on long nanotubular structures and their sagging due to their weight. Then, the released gas is trapped inside these structures leading to huge craters. These exceptional surfaces could be used in the future in various potential applications such as in drug delivery, cell growth, sensors, optical devices or surface adhesion. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 2)'.

Micro- and nanoscopic observations of sexual dimorphisms in Mecynorhina polyphemus confluens (Kraatz, 1890) (Coleoptera, Cetoniidae, Goliathini) and consequences for surface wettability.

In the animal kingdom, macroscopic variations in size, color, and even hairiness are frequently observed between male and female, making the sex of various species easy to discern. In the case of insects, similar variances also exist. While direct observation is a quick and efficient way to differentiate between sexes, there are also variations which are unseen to the naked eye and occur on a micro- or nanoscopic scale. Sometimes, these micro/nanoscopic variations can lead to significant variations in surface properties as a function of sex. Such is the case for the Mecynorhina polyphemus confluens (Kraatz, 1890). In this work, we characterize these micro- and nanoscale differences, and describe their impact on the surface properties (e.g. wettability). It is found that water interacts quite differently with the surface of the cuticle of Mecynorhina polyphenus confluens, depending on the specimen sex. On a female, water spreads readily across the elytra indicating hydrophilic behavior. However, on the surface of the male elytra, strong hydrophobicity is observed. Microscopic observations reveal differences in microscale surface morphology across the male and female cuticle. These observations contribute to a better, global understanding of the wettability behavior observed on M. polyphemus confluens.

Fabrication of Superhydrophobic Hierarchical Surfaces by Square Pulse Electrodeposition: Copper-Based Layers on Gold/Silicon (100) Substrates.

Copper based layers were fabricated on gold/silicon (100) substrates by using square pulse electrodeposition at different deposition temperatures. The predominant crystalline plane on Cu2 O samples at temperatures higher than 30 °C is (111), which is the most hydrophobic facet of Cu2 O cubic structure. Different crystallite structures such as semivertical leaves, fractal trees, and octahedral pyramids were formed on the surface. These water-repellent samples have hierarchical structures, including octahedral pyramid microstructures with small spherical balls on them and well-branched micrometric vertical leaves on the surface. They provide a suitable surface for trapping air pockets inside the structure and increasing the water contact angle up to 154°. This approach may be applicable to the large-scale preparation of water-repellent surfaces as superhydrophobicity can be achieved in a one-step deposition process without any secondary modifications.

Experimental Characterization of Droplet Adhesion: The Ejection Test Method (ETM) Applied to Surfaces with Various Hydrophobicity.

We study the wetting and the adhesive behavior of substrates made by electropolymerization of copolymers of pyrene substituted with fluoroalkyl and adamantyl groups. The hydrophobicity and water adhesion properties can be tuned by the molar percentage (mol %) of each pyrene monomer so that the substrate properties can vary from superhydrophobic to parahydrophobic, with respectively low and high water adhesion. The ejection test method (ETM) is proposed as an original tool to discriminate and characterize such substrates. Using a catapult-like apparatus, a droplet initially at rest on the surface is subject to a large acceleration and is subsequently ejected. Depending on the surface properties and initial catapult acceleration, the ejection is more or less efficient and occurs with or without fragmentation of the droplet. The ETM is shown to be a complementary test to the lateral adhesion and hysteresis classical measurements. This work is of importance for the understanding of adhesion phenomena on various surfaces and for a better quantitative characterization of their adhesive properties.

Parahydrophobic and Nanostructured Poly(3,4-ethylenedioxypyrrole) and Poly(3,4-propylenedioxypyrrole) Films with Hyperbranched Alkyl Chains.

Here, we control the surface hydrophobicity and the adhesion of water droplets by electrodeposition of poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,4-propylenedioxypyrrole) (PProDOP) with branched alkyl chains placed preferentially on the bridge to favor the formation of nanofibers. Branched alkyl chains of various sizes from very short (C3) to hyperbranched (C18) are studied because they have lower surface hydrophobicity than long alkyl or fluoroalkyl chains (preferable for parahydrophobic properties). The electrodeposition is much more favored with the PEDOP derivatives because the ProDOP films are more soluble. However, the formation of nanoparticles is favored with the PEDOP polymers in contrast to the formation of fibers, resembling the wax nanoclusters observed on lotus leaves, with the ProDOP polymers. With both these PEDOP and PProDOP derivatives, it is possible to reach parahydrophobic properties characterized by a sticking behavior toward water droplets. This kind of surfaces could be used in the future in water harvesting systems, for example.

Surface Nanostructuration and Wettability of Electrodeposited Poly(3,4-ethylenedioxypyrrole) and Poly(3,4-propylenedioxypyrrole) Films Substituted by Aromatic Groups.

In the aim to obtain parahydrophobic materials (both high contact angles and high hysteresis) for possible applications in water harvesting systems, we report the synthesis of novel 3,4-ethylenedioxypyrrole (EDOP) and 3,4-propylenedioxypyrrole (ProDOP) monomers with aromatic rings on the 3,4-alkylenedioxy bridge and the resulting conducting polymer films were prepared by electropolymerization. We show that the surface properties can be tuned by the nature of the aromatic ring (phenyl, biphenyl, diphenyl, naphthalene, fluorene, and pyrene) and the polymerizable core (EDOP or ProDOP). The best results are obtained with both EDOP and diphenyl, with which extremely high hydrophobic properties (up to 116°) are obtained, even if the polymers are intrinsically hydrophilic. These surfaces could be applied in the future, for example, in water harvesting systems or in water/oil separation membranes. The synthesis strategy is extremely interesting, and many other molecules will be envisaged in the future.

Formation of Nanofibers with High Water Adhesion by Electrodeposition of Films of Poly(3,4-ethylenedioxypyrrole) and Poly(3,4-propylenedioxypyrrole) Substituted by Alkyl Chains.

Invited for this month's cover are the collaborating groups of Dr. Thierry Darmanin at Université Côte d'Azur, France, and Dr. Alioune Diouf at Université Cheikh Anta Diop de Dakar, Senegal. The front cover shows nanofibrous surfaces prepared by the electropolymerization of 3,4-ethylenedioxypyrrole (EDOP) and 3,4-propylenedioxypyrrole (ProDOP) derivatives in which an alkyl chain is grafted to the 3,4-alkylenedioxy bridge. Depending on whether an EDOP or a ProDOP monomer is used, and the length of the alkyl chain, the surface properties of the resulting polymer are very different. Read the full text of the article at 10.1002/cplu.201800279.

Formation of Nanofibers with High Water Adhesion by Electrodeposition of Films of Poly(3,4-ethylenedioxypyrrole) and Poly(3,4-propylenedioxypyrrole) Substituted by Alkyl Chains.

With the aim of controlling the surface hydrophobicity and water adhesion, nanofibrous surfaces were prepared by electropolymerization of 3,4-ethylenedioxypyrrole (EDOP) and 3,4-propylenedioxypyrrole (ProDOP) derivatives having various alkyl chains (C3 to C17 ) grafted to a 3,4-alkylenedioxy bridge. Depending on whether an EDOP or a ProDOP was used, and the length of the alkyl chain, the surface properties of the resulting polymer were very different. The formation of nanofibrous surfaces was much more favored for ProDOP derivatives. The alkyl chain length has also a huge influence on the formation of nanofibers, and alkyl chains of intermediate length (C9 and C11 ) gave the best results. Apparent contact angles (θw ) of up to 150° were obtained, and the water adhesion properties were highly variable. This work is important for many applications in which the control of interaction forces with a medium is required, such as in oil-water separation membranes or water-harvesting systems.

Variation of Goliathus orientalis (Moser, 1909) Elytra Nanostructurations and Their Impact on Wettability.

Among the different species of flower beetles, there is one of particular notoriety: the Goliath beetle. This large insect can grow up to 11 cm long and is well-known for its distinctive black and white shield. In this paper, we focus on a particular Goliathus species: G. orientalis (Moser, 1909). We investigated the variations in properties of both the black and white parts of the upper face of G. orientalis; more precisely, the variation in surface properties with respect to the wettability of these two parts. This work reveals that the white parts of the shield have a higher hydrophobic character when compared to the black regions. While the black parts are slightly hydrophobic (θ = 91 ± 5°) and relatively smooth, the white parts are highly hydrophobic (θ = 130 ± 3°) with strong water adhesion (parahydrophobic); similar to the behavior observed for rose petals. Roughness and morphology analyses revealed significant differences between the two parts, and, hence, may explain the change in wettability. The white surfaces are covered with horizontally aligned nanohairs. Interestingly, vertically aligned microhairs are also present on the white surface. Furthermore, the surfaces of the microhairs are not smooth, they contain nanogrooves that are qualitatively similar to those observed in cactus spines. The nanogrooves may have an extremely important function regarding water harvesting, as they preferentially direct the migration of water droplets; this process could be mimicked in the future to capture and guide a large volume of water.

pH-Driven Wetting Switchability of Electrodeposited Superhydrophobic Copolymers of Pyrene Bearing Acid Functions and Fluorinated Chains.

A smart stimuli-responsive surface was fabricated by the electro-copolymerization of pyrene monomers followed by base and acid treatment. Copolymers of pyrenes bearing fluorinated chains (Py-nF6 ) and acid functions (Py-COOH) were produced with different molar concentrations of each monomer (0, 25, 50, 75, and 100 % of Py-nF6 vs. Py-COOH) by an electrochemical process. Two different perfluorinated pyrenes containing ester and amide groups were used to reach superhydrophobic properties. The relation of those bonds with the final properties of the surface was explored. The pH-sensitive group of Py-COOH allowed the surfaces to be reversibly switched from superhydrophobic (water contact angle>θw >150° and very low hysteresis) to hydrophilic (θw <90°). The amide and ester bonds influenced the recovery of the original wettability after both base and acid treatment. Although the fluorinated homopolymer with ester bonds was insensitive to base and acid treatment due to its superhydrophobic properties with ultralow water adhesion, the recovery of the original wettability for the copolymers was much more important with amide bonds due to the amide functional groups be more resistant to the hydrolysis reaction. This strategy offered the opportunity to access superhydrophobic films with switchable wettability by simple pH treatment. The films proved to be a good tool for use in biological applications, for example, as a bacterial-resistant film if superhydrophobic and as a bacterial-adherent film if hydrophilic.

Superpropulsion of Droplets and Soft Elastic Solids.

We investigate the behavior of droplets and soft elastic objects propelled with a catapult. Experiments show that the ejection velocity depends on both the projectile deformation and the catapult acceleration dynamics. With a subtle matching given by a peculiar value of the projectile/catapult frequency ratio, a 250% kinetic energy gain is obtained as compared to the propulsion of a rigid projectile with the same engine. This superpropulsion has strong potentialities: actuation of droplets, sorting of objects according to their elastic properties, and energy saving for propulsion engines.

The design of superhydrophobic stainless steel surfaces by controlling nanostructures: A key parameter to reduce the implantation of pathogenic bacteria.

Reducing bacterial adhesion on substrates is fundamental for various industries. In this work, new superhydrophobic surfaces are created by electrodeposition of hydrophobic polymers (PEDOT-F4 or PEDOT-H8) on stainless steel with controlled topographical features, especially at a nano-scale. Results show that anti-bioadhesive and anti-biofilm properties require the control of the surface topographical features, and should be associated with a low adhesion of water onto the surface (Cassie-Baxter state) with limited crevice features at the scale of bacterial cells (nano-scale structures).