George C. Anyfantis

Magnetically Driven Floating Foams for the Removal of Oil Contaminants from Water

In this study, we present a novel composite material based on commercially available polyurethane foams functionalized with colloidal superparamagnetic iron oxide nanoparticles and submicrometer polytetrafluoroethylene particles, which can efficiently separate oil from water. Untreated foam surfaces are inherently hydrophobic and oleophobic, but they can be rendered water-repellent and oil-absorbing by a solvent-free, electrostatic polytetrafluoroethylene particle deposition technique. It was found that combined functionalization of the polytetrafluoroethylene-treated foam surfaces with colloidal iron oxide nanoparticles significantly increases the speed of oil absorption. Detailed microscopic and wettability studies reveal that the combined effects of the surface morphology and of the chemistry of the functionalized foams greatly affect the oil-absorption dynamics. In particular, nanoparticle capping molecules are found to play a major role in this mechanism. In addition to the water-repellent and oil-absorbing capabilities, the functionalized foams exhibit also magnetic responsivity. Finally, due to their light weight, they float easily on water. Hence, by simply moving them around oil-polluted waters using a magnet, they can absorb the floating oil from the polluted regions, thereby purifying the water underneath. This low-cost process can easily be scaled up to clean large-area oil spills in water.

Air-stable ambipolar organic transistors

The authors report on ambipolar organic transistors based on the soluble dithiolene derivative (diphenylethylenedithiolato)(1,3-dithiol-2-thione-4,5-dithiolato)nickel [Ni(dpedt)(dmit)]. Due to its small band gap, efficient injection of holes and electrons from gold source/drain electrodes is possible. Both carrier mobilities are estimated to be approximately equal with maximum value on the order of 10−4cm2∕Vs. The transistors exhibit excellent ambient stability with a shelve lifetime exceeding 3months. The pronounced stability of Ni(dpedt)(dmit) as well as of several other molecules studied here is correlated to their redox potential. The present findings can be used as a general guide towards design and synthesis of air-stable ambipolar/n-channel molecules.

Toxicity assessment of silica coated iron oxide nanoparticles and biocompatibility improvement by surface engineering.

We have studied in vitro toxicity of iron oxide nanoparticles (NPs) coated with a thin silica shell (Fe3O4/SiO2 NPs) on A549 and HeLa cells. We compared bare and surface passivated Fe3O4/SiO2 NPs to evaluate the effects of the coating on the particle stability and toxicity. NPs cytotoxicity was investigated by cell viability, membrane integrity, mitochondrial membrane potential (MMP), reactive oxygen species (ROS) assays, and their genotoxicity by comet assay. Our results show that NPs surface passivation reduces the oxidative stress and alteration of iron homeostasis and, consequently, the overall toxicity, despite bare and passivated NPs show similar cell internalization efficiency. We found that the higher toxicity of bare NPs is due to their stronger in-situ degradation, with larger intracellular release of iron ions, as compared to surface passivated NPs. Our results indicate that surface engineering of Fe3O4/SiO2 NPs plays a key role in improving particles stability in biological environments reducing both cytotoxic and genotoxic effects.

Spatially controlled surface energy traps on superhydrophobic surfaces.

Water wetting and adhesion control on polymeric patterns are achieved by tuning the configuration of their surfaces structural characteristics from single to dual and triple length-scale. In particular, surfaces with combined micro-, submicrometer-,and nanoroughness are developed, using photolithographically structured SU-8 micro-pillars as substrates for the consecutive spray deposition of polytetrafluoroethylene (PTFE) submicrometer particles and hydrophobically capped iron oxide colloidal nanoparticles. The PTFE particles alone or in combination with the nanoparticles render the SU-8 micropillars superhydrophobic. The water adhesion behaviour of the sprayed pillars is more complex since they can be tuned gradually from totally adhesive to completely non adhesive. The influence of the hierarchical geometrical features of the functionalized surfaces on this behaviour is discussed within the frame of the theory. Specially designed surfaces using the described technique are presented for selective drop deposition and evaporation. This simple method for liquid adhesion control on superhydrophobic surfaces can find various applications in the field of microfluidics, sensors, biotechnology, antifouling materials, etc.

Control of the water adhesion on hydrophobic micropillars by spray coating technique

We present an alternative approach for controlling the water adhesion on solid superhydrophobic surfaces by varying their coverage with a spray coating technique. In particular, micro-, submicro-, and nanorough surfaces were developed starting from photolithographically tailored SU-8 micropillars that were used as substrates for spraying first poly(tetrafluoroethylene) submicrometer particles and subsequently iron oxide nanoparticles. The sprayed particles serve to induce surface submicrometer and nanoscale roughness, rendering the SU-8 patterns superhydrophobic (apparent contact angle values of more than 150°), and also to tune the water adhesion between extreme states, turning the surfaces from “non-sticky” to “sticky” while preserving their superhydrophobicity. The influence of the chemical properties and of the geometrical characteristics of the functionalized surfaces on the wetting properties is discussed within the frame of the theory. This simple method can find various applications in the fabrication of microfluidic devices, smart surfaces, and biotechnological and antifouling materials.

Formation and magnetic manipulation of periodically aligned microchains in thin plastic membranes

We demonstrate the fabrication of polymeric membranes that incorporate a few layers of periodically aligned magnetic microchains formed upon the application of variable magnetic fields. A homogeneous solution containing an elastomeric polymer and a small amount of colloidal magnetic nanoparticles is spin coated on glass slides, thereby forming thin magnetic membranes of ca. 10 μm thickness. Subsequent application of a homogeneous magnetic field results in the orientation of the magnetic clusters and their further motion into the matrix along the field lines forming layers of aligned chains. The study of the kinetics of alignment demonstrates that the chains are formed in the first hour of exposure to the magnetic field. Above all, a detailed microscopy study reveals that the dimensions and the periodicity of the microchains are effectively controlled by the intensity of the magnetic field, in good agreement with the theoretical simulations. This ability to form and manipulate the size and the distribution...

Localized synthesis of gold nanoparticles in anisotropic alginate structures

We present a method to create alginate nanocomposite objects having two regions with distinct chemical composition by spatially confining the synthesis of gold nanoparticles (Au NPs) in the polysaccharide matrix. Controlling the floating of sodium alginate drops on calcium chloride solution containing specific concentrations of gold ions, we demonstrate the formation of bicolour structures with a mushroom-like shape. Their immersed portion (the cap) was exclusively involved in the uptake and reduction of gold precursor, acquiring the typical purple colour of Au NPs; whereas, the emerged portion (the stem) did not participate in the in situ synthesis and retained the white colour of calcium alginate. Moreover, we noted that the localized growth of Au NPs was strongly related to the initial amount of gold precursor. In fact for high concentrations, the gold ions were just encapsulated inside the gel matrix and no reduction was observed in the wet structures neither in the cap nor in the stem. The reported procedure is simple and capable of directing the synthesis of Au NPs into selected areas of the alginate matrix, making possible the fabrication of a novel class of alginate structures with an anisotropic character and potential applications in the area of drug delivery and biosensors.

Organic-Inorganic Hybrid Compounds Based on Lead Halide Units: Preparation from Melts and through Grinding Effects

Organic-inorganic hybrid compounds based on lead halide units of the formula (CH3NH3)2- (CH3C6H4CH2NH3)2Pb3(BrxY1−x)10 (Y = Cl, I) were prepared from melts, and their optical absorption as well as photoluminescence spectra, before and after grinding the compounds, were investigated. The luminescence of the materials after grinding is strong and is observed with naked eye. By changing the mole fraction (x), the strong photoluminescence in the range from ca. 400 nm to ca. 600 nm could be tuned. These effects indicate that the hybrid compounds are promising materials for potential applications. Graphical Abstract Organic-Inorganic Hybrid Compounds Based on Lead Halide Units: Preparation from Melts and through Grinding Effects

Laser-induced localized formation of silver nanoparticles on chitosan films: study on particles size and density variation

We present the in situ localized formation of silver nanoparticles in chitosan films by pulsed UV laser irradiation, as a result of the photoreduction of the silver nitrate precursor loaded throughout the volume of the polymer. The UV pulsed irradiation is also found to be responsible for the photofragmentation of the previously formed nanoparticles, leading to their average size reduction as the number of pulses increases. In fact, their diameter changes from ~150 to ~30 nm for irradiation with 5 to 200 pulses, respectively. After irradiation the formation of nanoparticles continues for several days, since the already formed nanoparticles act as seeds for the reduction of the unreacted precursor. Indeed, few weeks after irradiation, the chitosan films present a metallic mirror-like appearance on the previously irradiated areas, as they are fully covered by silver. Taking advantage of all these simultaneous mechanisms, and controlling the number of pulses and elapsed time after irradiation, Ag nanoparticles of specific size can be formed in situ on desired areas of the film. Through this process is envisioned the fabrication of nanocomposites with functional properties.

Tailoring the morphology of poly(ethylene oxide)/silver triflate blends: from crystalline to self-assembled nanofibrillar structures

Interaction of polyethylene oxide (PEO) with transition metal triflates is a newly emerging research area due to its numerous application fields, such as thin-film power conversion devices and sensors. In the present study, we demonstrate, for the first time, that PEO can solvate silver triflate organic salts in large quantities when formic acid is used as a common solvent for both. Nanocomposites with unique structural and electrical properties are fabricated by simply drop casting formic acid solutions of PEO and silver triflate salts. We present a detailed experimental study on the characterization of morphological and electrical properties of PEO-silver triflate nanocomposite films as a function of silver triflate concentration and discuss their potential applications as humidity sensors. In particular, by increasing the concentration of the salt in the initial solution the morphological features of the formed nanocomposites can be varied from well defined microcrystals to amorphous nanofibers. Of special interest are the nanocomposite films fabricated from a 1:1 (PEO-unit:Ag(+)) molar ratio, since they consist of self-assembled nanofibrillar structures, which exhibit good electrical conductivity as well as highly repeatable sensitivity towards humidity.