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)'.

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.

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 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)'.

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.

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.

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.

Switchable Surface Wettability by Using Boronic Ester Chemistry.

Here, we report for the first time the use of a boronic ester as an efficient tool for reversible surface post-functionalization. The boronic ester bond allows surfaces to be reversibly switched from hydrophilic to hydrophobic. Based on the well-known boronic acid/glycol affinity, this strategy offers the opportunity to play with surface hydrophobic properties by adding various boronic acids onto substrates bearing glycol groups. The post-functionalization can then be reversed to regenerate the starting glycol surface. This pathway allows for the preparation of various switchable surfaces for a large range of applications in biosensors, liquid transportation, and separation membranes.

3,4-Dialkoxypyrrole for the Formation of Bioinspired Rose Petal-like Substrates with High Water Adhesion.

Self-organization is commonly present in nature and can lead to the formation of surface structures with different wettabilities. Indeed, in nature superhydrophobic (low water adhesion) and parahydrophobic (high water adhesion) properties exist, such as in lotus leaves and red roses, respectively. The aim of this work is to prepare parahydrophobic properties by electrodeposition. For this, pyrrole derivatives with two alkoxy groups of various lengths (from 1 to 12) were synthesized in 8 steps by adapting a method developed by Merz et al. We show that the alkyl chain length has a huge influence on the polymer solubility and as a consequence on the surface morphology and hydrophobicity. Moreover, the alkyl chain length should be at least greater than eight carbons in order to obtain parahydrophobic properties. The properties are also controlled by the electrolyte nature. These materials can be used for many potential applications in water harvesting and transportation and separation membranes.

pH- and voltage-switchable superhydrophobic surfaces by electro-copolymerization of EDOT derivatives containing carboxylic acids and long alkyl chains.

In order to produce pH- and voltage-switchable superhydrophobic surfaces, PEDOT derivatives containing various proportions of a EDOT monomer containing carboxylic groups (EDOT-COOH) and EDOT monomer-containing dodecyl chains (EDOT-O-H12) are elaborated. The surface morphology and roughness depend highly on the proportion of the monomers. Superhydrophobic properties are reached for a mol% of EDOT-COOH between 0 and 25%. It is possible to switch from superhydrophobic to hydrophilic (θ(water) until about 45°) by electrochemical reduction at low voltage (-1 V vs SCE) to remove the doping anions, following by treatment with NaOH to change the carboxylic groups into carboxylate. By elaborating smooth surfaces of each polymer, the effect of each treatment is reported. The reversibility of the reactions is also reported.

Superhydrophobic conducting polymers based on hydrocarbon poly(3,4-ethylenedioxyselenophene).

We report the first studied example of wettability of conducting polymers based on poly(3,4-ethylenedioxyselenophene). Hydrocarbon 3,4-ethylenedioxyselenophene (C2H5 to C12H25) derivatives are synthesized and electropolymerized in propylene carbonate using lithium tetrafluoroborate (LiBF4) and lithium bistrifluoromethanesulfonimidate (LiTf2N) as electrolyte. Highly structured films are obtained from a C8H17 chain independent of the electrolyte used. Superhydrophobic properties with high adhesion are specifically reached if using a C12H25 chain and LiBF4. The surface morphology is porous and highly structured as the films consist of assemblies of nanofibrils.

One F-octyl versus two F-butyl chains in surfactant aggregation behavior.

An easy synthetic procedure in two or three steps from perfluoroalkylethyl iodide derivatives led to six novel fluorinated carboxylates monomeric and gemini surfactants with one or two hydrophobic tails, respectively: RF(C2H4)CH(CO2(-))2,2Na(+) and [RF(C2H4)]2C(CO2(-)),Na(+), where RF = C4F9, C6F13, and C8F17. These anionic surfactants exhibited very low surface tension from 15 to 33 mN/m as well as low critical micelle concentration until 1.3 × 10(-4) mol/L. Furthermore, the surface properties of the gemini compound with two short fluoroalkyl chains (RF = C4F9) were found to be almost equal to those of the monomeric surfactant with one long fluoroalkyl chain (RF = C8F17), which could provide an interesting alternative to the bioaccumulative long-chain perfluorinated surfactant.

Surface structuration (micro and/or nano) governed by the fluorinated tail lengths toward superoleophobic surfaces.

As compared to superhydrophobic surfaces, the challenge to obtain superoleophobic properties, surfaces against low-surface-tension probe liquids such as hexadecane, is very important because of their high tendency to wet. From the molecular design of the monomer, it is possible to obtain in one step superoleophobic surfaces by electrodeposition. Hence, we report the synthesis and the characterization of an original series of fluorinated 3,4-ethylenedioxypyrrole (EDOP) derivatives. The electrodeposited polymer films are characterized by contact angle measurements (static and dynamic with various probe liquids), optical profilometry, and scanning electron microscopy. In the view toward reaching superoleophobic properties, a common approach is to increase the number of fluoromethylene units of the surface post-treatment agent. Here, surprisingly, it is possible, in one step, to reach more efficient antioil surface properties by decreasing the length of the fluorinated tail (F-octyl to F-hexyl). This fact can be explained by a double scale of structuration (micro and nano) induced using only F-hexyl tails.

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.

Superhydrophobic fiber mats by electrodeposition of fluorinated poly(3,4-ethyleneoxythiathiophene).

The control of surface morphology and wettability is crucial in the development of superhydrophobic surfaces, which implies new strategy and molecular design. In this Article, we report the synthesis, characterization, and electrochemical properties of original 3,4-ethyleneoxythiathiophenes (EOTT) as platform molecules and its derivatives bearing a semifluorinated chain of various length (F-octyl, F-hexyl, F-butyl, and F-ethyl). We report the influence of the fluorinated chain length as well as the presence of sulfur atoms in the monomer on the surface construction and nonwetting properties of the corresponding electrodeposited polymer films. Surprisingly, these films exhibit the possibility to obtain extremely long polymer fibers with a possible control of their length by a careful choice in the monomer structure. We show that the presence of sulfur atoms in the monomer structure seems to be necessary to modulate the formation of extremely long polymer fibers by aggregation of smaller polymer fibrils. In this Article, the formation of superhydrophobic material (contact angle above 150°) for four, six, and eight fluoromethylene units but also highly hydrophobic surfaces (contact angle above 125°) from extremely short chains (two fluoromethylene units) is also demonstrated.

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.

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.

Hydrocarbon versus fluorocarbon in the electrodeposition of superhydrophobic polymer films.

To elaborate on superhydrophobic surfaces, we report the electrochemical synthesis, surface morphology, and wettability of hydrocarbon conductive polymer films obtained by the electrodeposition of polythiophene, poly(3,4-ethylenedioxythiophene) (i.e., PEDOT), and poly(3,4-ethylenedioxypyrrole) (i.e., PEDOP) derivatives. Highly hydrophobic films were obtained from n-C(14)H(29) and n-C(8)H(17) chains in the cases of polythiophenes and PEDOP, respectively. By contrast, superhydrophobic films were formed by the deposition of PEDOT substituted with n-C(10)H(21) chains (PEDOT-methyl undecanoate): static contact angle ≈ 160.6°, hysteresis ≈ 2°, and sliding angle ≈ 3°. Their surface properties were compared to those of previously reported fluorinated analogues. The water-repellent properties of PEDOT-methyl undecanoate were similar to the best surface properties obtained with fluorinated monomers. Even if the main approach for the chemical factor to build up superhydrophobic surfaces is via a coating of a fluorinated compound, this work confirms that the formation of fractal surfaces is able to achieve super-anti-wetting properties within a hydrocarbon series (less expensive with a favorable ecotoxic approach), and it opens a new path to bioinspired surfaces.

Connector ability to design superhydrophobic and oleophobic surfaces from conducting polymers.

In the aim of creating superoleophobic surfaces using monomers with short perfluorinated chains, to avoid drawbacks associated with PFOA, original semifluorinated (C(4)F(9), C(6)F(13)) 3,4-ethylenedioxypyrrole derivatives were synthesized. These monomers were obtained using the faster synthetic method than previously described with some analogues, characterized and electrochemically polymerized on gold plates. The obtained surfaces exhibited superhydrophobic (contact angle with water of 157 degrees and 158 degrees, respectively) and oleophobic properties (contact angle with hexadecane: 88 degrees and 108 degrees, respectively). The comparison between these new monomers and already published analogue EDOP6 confirms the importance of the bipolaronic form of conductive polymer for obtaining surface nanoporosity and as a consequence improving surface oleophobicity. Thus, little change in the molecule design of the connector and the spacer of the monomer can have a significant influence on the surface oleophobicity.