Yannick Coffinier

Reduction and Functionalization of Graphene Oxide Sheets Using Biomimetic Dopamine Derivatives in One Step

An easy and environmentally friendly chemical method for the simultaneous reduction and noncovalent functionalization of graphene oxide (GO) using dopamine derivatives is described. The reaction takes place at room temperature under ultrasonication of an aqueous suspension of GO and a dopamine derivative. X-ray photoelectron spectroscopy, FT-IR spectroscopy, and cyclic voltammetry characterizations revealed that the resulting material consists of graphene functionalized with the dopamine derivative. This one-step protocol is applied for simultaneous reduction and functionalization of graphene oxide with a dopamine derivative bearing an azide function. The chemical reactivity of the azide function was demonstrated by a postfunctionalization with ethynylferrocene using the Cu(I) catalyzed 1,3-dipolar cyloaddition.

Quantitative testing of robustness on superomniphobic surfaces by drop impact

The quality of a liquid-repellent surface is quantified by both the apparent contact angle θ(0) that a sessile drop adopts on it and the value of the liquid pressure threshold the surface can withstand without being impaled by the liquid, hence maintaining a low-friction condition. We designed surfaces covered with nanowires obtained by the vapor-liquid-solid (VLS) growth technique that are able to repel most of the existing nonpolar liquids including those with very low surface tension as well as many polar liquids with moderate to high surface tension. These superomniphobic surfaces exhibit apparent contact angles ranging from 125 to 160° depending on the liquid. We tested the robustness of the surfaces against impalement by carrying out drop impact experiments. Our results show how this robustness depends on Youngs contact angle θ(0) related to the surface tension of the liquid and that the orientational growth of nanowires is a favorable factor for robustness.

Engineering Sticky Superomniphobic Surfaces on Transparent and Flexible PDMS Substrate

Following the achievement of superhydrophobicity which prevents water adhesion on a surface, superomniphobicity extends this high repellency property to a wide range of liquids, including oils, solvents, and other low surface energy liquids. Recent theoretical approaches have yield to specific microstructures design criterion to achieve such surfaces, leading to superomniphobic structured silicon substrate. To transfer this technology on a flexible substrate, we use a polydimethylsiloxane (PDMS) molding process followed by surface chemical modification. It results in so-called sticky superomniphobic surfaces, exhibiting large apparent contact angles (>150°) along with large contact angle hysteresis (>10°). We then focus on the modified Cassie equation, considering the 1D aspect of wetting, to explain the behavior of droplets on these surfaces and compare experimental data to previous works to confirm the validity of this model.

Culture of mammalian cells on patterned superhydrophilic/superhydrophobic silicon nanowire arrays

Interfacing nanowires and living cells is highly interesting in various fields including biomedical implants, biosensors or drug delivery. Vertically aligned silicon nanowire (SiNW) arrays prepared by the stain etching technique were investigated in this study. Chemical modification with octadecyltrichlorosilane (OTS) led to the formation of superhydrophobic SiNW surface with a contact angle around 160°. A micropatterned superhydrophilic/superhydrophobic SiNW surface was fabricated using standard optical lithography techniques. Here we report on Chinese Hamster Ovary K1 (CHO) cell culture on patterned superhydrophilic/superhydrophobic silicon nanowire surfaces. It was found that the cells adhered selectively to the superhydrophilic regions while cell adhesion was almost completely suppressed on the superhydrophobic surface. Transmission electron microscopy analysis showed that the cell cytoplasmic projections penetrate the hydrophilic silicon nanowires layer and coat the nanowires, leading to an intimate surface contact and thus a strong adhesion. On the superhydrophobic surface, the cell cytoplasmic projections remained on the top of wires. The nonfouling of the superhydrophobic SiNW substrate was attributed to a stable Cassie–Baxter state, limiting the contact with the culture medium. Another interesting finding from this study is the corrosion of the superhydrophilic SiNW surface in phosphate-buffered saline (PBS) solution.

Biomolecule and Nanoparticle Transfer on Patterned and Heterogeneously Wetted Superhydrophobic Silicon Nanowire Surfaces

We report on the use of patterned superhydrophobic silicon nanowire surfaces for the efficient, selective transfer of biological molecules and nanoparticles. Superhydrophilic patterns are prepared on superhydrophobic silicon nanowire surfaces using standard optical lithography. The resulting water-repellent surface allows material transfer and physisorption to the superhydrophilic islands upon exposure to an aqueous solution containing peptides, proteins, or nanoparticles.

Extreme resistance of superhydrophobic surfaces to impalement: reversible electrowetting related to the impacting/bouncing drop test.

The paper reports on the comparison of the wetting properties of superhydrophobic silicon nanowires (NWs), using drop impact impalement and electrowetting (EW) experiments. A correlation between the resistance to impalement on both EW and drop impact is shown. From the results, it is evident that when increasing the length and density of NWs (i) the thresholds for drop impact and EW irreversibility increase and (ii) the contact-angle hysteresis after impalement decreases. This suggests that the structure of the NW network could allow for partial impalement, hence preserving the reversibility, and that EW acts the same way as an external pressure. The most robust of our surfaces shows a threshold to impalement higher than 35 kPa, while most of the superhydrophobic surfaces tested so far have impalement thresholds smaller than 10 kPa.

Sliding Droplets on Superomniphobic Zinc Oxide Nanostructures

This study reports on liquid-repellency of zinc oxide nanostructures (ZnO NS). The ZnO NS are synthesized by an easy and fast chemical bath deposition technique. Three different nanostructured surfaces consisting of nanorods, flowers, and particles are prepared, depending on the deposition time and the presence of ethanolamine in the reaction mixture. Chemical functionalization of the ZnO NS with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS) in liquid (PFTS L) and vapor phase (PFTS V) or through octafluorobutane (C(4)F(8)) plasma deposition led to the formation of superomniphobic surfaces. A comprehensive characterization of the wetting properties (static contact angle and contact angle hysteresis) has been performed using liquids composed of deionized water and various concentrations of ethanol (surface tension between 35 and 72.6 mN/m). Depending on the nanostructures morphology, coating nature and liquid employed, high static apparent contact angles θ ≈ 150-160°, and low contact angle hysteresis Δθ ≈ 0° are obtained. The different ZnO NS are characterized using scanning electron microscopy (SEM) and contact angle measurements. The results reported in this work permit preparation of sliding omniphobic surfaces using a simple and low cost technique.

Reversible electrowetting on superhydrophobic double-nanotextured surfaces.

The paper reports on wetting, electrowetting (EW), and systematic contact angle hysteresis measurements after electrowetting of superhydrophobic silicon nanowire surfaces (NWs). The surfaces consist of C4F8-coated silicon nanowires grown on Si/SiO2 substrate. Different surfaces modulating (i) the dielectric layer thickness and (ii) the nanotexturation were investigated in this study. It was found that the superhydrophobic NWs display different EW behaviors according to their double nanotexturation with varying droplet impalement levels. Some surfaces exhibited a total reversibility to EW with no impalement (contact angle variation of 35+/-2 degrees at 190 VTRMS with deionized water), whereas other surfaces showed nonreversible behavior to EW with partial droplet impalement. A scenario is proposed to explain the unique properties of these surfaces.

The antimicrobial effect of silicon nanowires decorated with silver and copper nanoparticles.

The paper reports on the preparation and antibacterial activity of silicon nanowire (SiNW) substrates coated with Ag or Cu nanoparticles (NPs) against Escherichia coli (E. coli) bacteria. The substrates are easily prepared using the metal-assisted chemical etching of crystalline silicon in hydrofluoric acid/silver nitrate (HF/AgNO3) aqueous solution. Decoration of the SiNWs with metal NPs is achieved by simple immersion in HF aqueous solutions containing silver or copper salts. The SiNWs coated with Ag NPs are biocompatible with human lung adenocarcinoma epithelial cell line A549 while possessing strong antibacterial properties to E. coli. In contrast, the SiNWs decorated with Cu NPs showed higher cytotoxicity and slightly lower antibacterial activity. Moreover, it was also observed that leakage of sugars and proteins from the cell wall of E. coli in interaction with SiNWs decorated with Ag NPs is higher compared to SiNWs modified with Cu NPs.

Preparation of superhydrophobic and oleophobic diamond nanograss array

The paper reports on the fabrication of superhydrophobic boron-doped diamond nanograss surfaces. The boron-doped diamond nanograss array (BDD NG) was prepared simply by reactive ion etching (RIE) with oxygen plasma of highly boron-doped (the boron doping level is 8 × 1019 B cm−3) polycrystalline diamond electrodes. Depending on the RIE conditions, substrates with different nanograss densities can be prepared. Chemical functionalization of the NG array surface with octadecyltrichlorosilane (OTS) or 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS) led to the formation of superhydrophobic surfaces. Under optimized conditions, BDD NG surfaces with contact angles as high as 160° and quasi null hysteresis (< 2°) can be reached. A water droplet deposited on the surface tends to roll off when the NG surface was tilted by a few degrees. Furthermore, the BDD NG array chemically modified with PFTS displays oleophobic properties. A contact angle of 94° was measured for hexadecane. The resulting surfaces were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and contact angle (CA) measurements.