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Microdroplets have great potential for high-throughput biochemical screening. We report the design of an integrated microfluidic device for droplet formation, incubation and screening. Picolitre water-in-oil droplets can be stored in a reservoir that contains approximately 10(6) droplets. In this reservoir droplets are stable for at least 6 h, which gives an extended timescale for biochemical experiments. We demonstrate the utility of the system by following the in vitro expression of green fluorescent protein. The high efficiency allows protein expression from a single molecule of DNA template, creating "monoclonal droplets" in which genotype and phenotype are combined in one emulsion compartment.

This paper describes the electroactuation of microcantilevers coated on one side with cationic polyelectrolyte brushes. We observed very strong cantilever deflection by alternating the potential on the cantilever between +0.5 and -0.5 V at frequencies up to 0.25 Hz. The actuation resulted from significant increases in the expansive stresses in the polymer brush layer at both negative and positive potentials. However, the deflection at negative bias was significantly larger. We have developed a theoretical framework that correlates conformational changes of the polymer chains in the brush layer with the reorganization of ions due to the potential bias. The model predicts a strong increase in the polymer volume fraction, close to the interface, which results in large expansive stresses that bend the cantilever at negative potentials. The model also predicts that the actuation responds much stronger to negative potentials than positive potentials, as observed in the experiments.

Cyclic voltammetry (CV) was employed to characterize the electrochemical behavior of polyelectrolyte brushes with immobilized electroactive counterions in response to external changes in concentration and composition of the supporting electrolyte and as a function of brush thickness. Poly(methacryloyloxy)ethyl-trimethyl-ammonium chloride (PMETAC) brushes were synthesized on Au substrates via atom transfer radical polymerization followed by ion-exchange with ferricyanide ions ([Fe(CN)6]3-) as redox probes. CV measurements of the modified PMETAC brushes showed the typical electrochemical response corresponding to a surface-confined electroactive species and the redox counterions, as [Fe(CN)6]3- species form stable ion pairs with the quaternary ammonium groups of the brush. The electron-transfer features of PMETAC brushes with different thicknesses, as characterized by CV and UV-vis spectroscopy, revealed that the charge density probed by CV was lower than the charge density measured by UV-vis spectroscopy. The electrode current decreased significantly with increasing concentration of supporting electrolyte due to the effect of the Donnan potential. Hydrophobic counterions, ClO4-, which induced brush collapse, lead to significantly reduced electrode currents.

Here we demonstrate a new method for droplet fusion based on a surface energy pattern on the walls of a microfluidic device, that does not require active elements nor accurate synchronization of the droplets.

The rational design, synthesis, and characterization of a series of novel perfluorinated dendrons 14a,b, 25a,b, 26a,b, and 18 are described. The dendrimers were designed to have a thiol at the focal point for attachment to a gold surface to enable the fabrication of self-assembled monolayers (SAMs). Perfluorinated tails were attached to the periphery to provide solubility in supercritical carbon dioxide, and to increase the hydrophobicity and the stability of self-assembled monolayers formed. Mitsunobu reactions were utilized to provide high-yielding steps allowing large-scale production of the novel dendrimers.

The conformational behavior of poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes with different chain density in the presence of large benzyltributylammonium chloride (BTBAC) ions has been studied by a Quartz Crystal Microbalance with Dissipation (QCM-D) and Scanning Force Microscopy. Dense brushes do not collapse in the presence of BTBAC solutions of increasing ionic strength, contrary to what is observed in the presence of NaCl. Brush collapse can be observed for low-ionic-strength solutions of BTBAC when the brush density has been reduced. These phenomena can be explained by considering the Hofmeister series as well as ion size and free space in the brush.

We demonstrate the uniaxial alignment of a liquid-crystalline conjugated polymer, poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) by means of nanoconfinement during nanoimprinting. The orientation of the conjugated backbones was parallel to the nanolines imprinted into the polymer film. Polarized UV-vis absorption and photoluminescence spectra were measured to quantify the degree of alignment, showing that the polarization ratio and uniaxial molecular order parameter were as high as 66 and 0.97, respectively. The aligned F8BT film was used as the active layer in a PLED, which resulted in polarized electroluminescence with a polarization ratio of 11. Ambipolar PFET in a top-gate configuration with aligned F8BT as the active semiconducting layer showed mobility enhancement when the chains were aligned parallel to the transport direction. Mobility anisotropies for hole and electron transport were 10-15 and 5-7, respectively, for current flow parallel and perpendicular to the alignment direction.

This article describes the aqueous atom transfer radical polymerization synthesis of poly(3-sulfopropylmethacrylate) brushes onto gold and Si/SiO2 surfaces in a controlled manner. The effect of Cu(I)/Cu(II) ratio was examined, and a quartz crystal microbalance was used to study the kinetics of the brush synthesis. The synthesized brushes displayed a thickness from a few nanometers to several hundred nanometers and were characterized using atomic force microscopy, ellipsometry, Fourier transform infrared spectroscopy (FTIR), contact angle measurements, and X-ray photoelectron spectroscopy (XPS). The as-synthesized sulfonate brushes had very good ion-exchange properties for the ions tested in this study, i.e., Na+, K+, Cu2+, and Ag+. FTIR and XPS show that the metal ions are coordinating to sulfonate moieties inside the brushes. The brushes were easily loaded with silver ions, and the effect of silver ion concentration on silver loading of the brush was examined. The silver-loaded brushes were shown to be antibacterial toward both gram negative and gram positive bacteria. The silver leaching was studied through leaching experiments into water, NaNO3, and NaCl (physiological medium). The results from these leaching experiments are compared and discussed in the article.

This paper describes a fabrication strategy based on polymer brushes (20-150 nm thick) and soft lithographic techniques, for creating hydrophobic, cross-linked, laterally patterned polymer films. The hydrophobicity of the resulting micrometer-scale "quasi-2D" objects is shown to allow the polymer to act as an etch resist. By adjusting the etching time, we demonstrate that underetching of the gold from underneath the edges of the laterally patterned films can be used to create free-standing polymer-gold hybrid structures. These structures retain their structural integrity when lifted wholly or partially from the substrate and can hence be imaged in suspension. Characterization of the quasi-2D objects was carried out using atomic force microscopy (AFM), ellipsometry, optical microscopy, and Fourier transform infrared spectroscopy (FTIR). A continuous film, containing embedded polymer-gold objects, can be lifted, folded, and re-deposited onto a substrate without damaging the conductivity of the embedded metallic objects.

Cyclic voltammetry and impedance spectroscopy were employed to probe the responsive properties of polyelectrolyte brushes. Poly[(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes over 100 nm thick on gold substrates were synthesized via surface-initiated atom-transfer radical polymerization and quaternized with methane iodide to obtain cationic brushes (Q-PDMAEMA). Q-PDMAEMA brushes respond to electrolytes by exhibiting swollen and collapsed states. Swollen brushes allow good permeability of electroactive probes, while collapsed states block electron transport. Electrolytes have different impacts on the electrochemical properties of Q-PDMAEMA. Some salts (NaNO3) cause brush collapse due to charge screening, while others such as those with more hydrophobic anions (ClO4-, PF6-, and Tf2N-) induce brush collapse because of solubility changes. The collapsed brushes exhibit intrinsically different resistance as probed with impedance. Charged screened brushes retain good permeability to electroactive probes. Strongly coordinating hydrophobic anions lead to insoluble brushes, resulting in a high resistance. These results show that electrochemical impedance spectroscopy is a powerful technique to probe the properties and structure of polyelectrolyte brushes.

This article describes a general synthetic route to laterally distinctive multicomponent polymer brushes on gold. The procedure involves repeated surface patterning using microcontact printing (muCP) of initiator-terminated thiols without backfilling with inert thiols and surface-initiated atomic transfer radical polymerization steps. In between brush growth, the remaining initiator moieties are deactivated to avoid reinitiation on existing brushes. Optical and fluorescence microscopy, atomic force microscopy, attenuated total reflectance Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy have been used to characterize every step of this procedure. We found that brushes can be grown from initiator-modified surfaces that contain bare gold areas and that these areas remain available for further patterning using muCP. To demonstrate the flexibility of this approach, surfaces containing four different polymer brushes in patterns ranging from 2 x 4 microm lines to 20 x 20 microm squares were fabricated. The range of chemical functionalities incorporated includes cationic and anionic polyelectrolytes, as well as thermally responsive polymers.

Polymer brushes are assemblies of macromolecules chemically tethered at one end to a substrate. They provide an alternative to self-assembled monolayers because of the intrinsically large size of the building blocks and the ensuing entropic contribution to the film morphology. In this article, an overview of a number of representative polymer brush systems will be presented and their potential application for surfaces with controlled wettability, smart surfaces and nanoactuators will be explored in some detail.

Ion-exchange in surface-initiated polyelectrolyte brushes provides a versatile route to the formation of catalytically active surfaces for electroless deposition of Cu. The advantage of this procedure is the covalent anchoring of the catalyst support layer, eliminating delamination of the metal film, even when deposited onto PDMS substrates. Furthermore, by tuning the concentration of PdCl4(2-) ions in the brushes, the rate of deposition and hence the thickness of the overall film can be controlled easily.

This paper reports a new patterning method, which utilizes NaOH to facilitate the irreversible binding between the PDMS stamp and substrates and subsequent cohesive mechanical failure to transfer the PDMS patterns. Our method shows high substrate tolerance and can be used to "print" various PDMS geometries on a wide range of surfaces, including Si100, glass, gold, polymers, and patterned SU8 photoresist. Using this technique, we are able to locally change the wettability of substrate surfaces by printing well-defined PDMS architectures on the patterned SU8 photoresist. It is possible to generate differential wetting and dewetting properties in microchannels and in the PDMS printed area, respectively.

In this communication we report the first preparation of ionic liquid-modified carbon nanotubes with reversibly switchable solubility between aqueous and organic solvents, induced by anion exchange.

Microcantilever bending can be reversibly driven by conformational changes of phosphate containing polyelectrolyte brushes when exposed to different pH or salt solutions. The deflection of the cantilevers allows a detailed analysis of the properties of polymer brushes, while these systems are also a first step toward polymer-based nanoactuators.

We show that charge-transporting polymer chains in the brush conformation can be synthesized from a variety of substrates of interest, displaying a high degree of stretching and showing up to a 3 orders of magnitude increase in current density normal to the substrate as compared with a spin-coated film. These nanostructured polymeric films may prove to be suitable for electronic devices based on molecular semiconductors as current fabrication techniques often provide little control over film structure.

The confinement of LCE materials into surface monodomains via micropatterning leads to the formation of reversible, shape-shifting surface patterns. The individual features are liquid crystalline monodomains, and by switching to an isotropic state using light or heat, the features switch between imprinted circular features and anisotropic liquid crystalline features.

Surface initiated polymer brushes containing phosphate side groups have been used to reversibly switch wettability and hydration between three different states.

When organic materials are placed in environments which physically confine the materials at the nanometre scale, interfacial effects and confinement-induced loss of entropy can significantly alter materials' properties such as the glass transition temperature as well as the nanoscale morphology as compared to a "free" system. In block copolymers, nanoconfinement leads to a range of unusual self-organized nanoscale morphologies. In this article, attempts to induce nanoconfinement effects in new polymer systems as well as at interfaces will be highlighted and some possible future implications for organic synthesis and biology will be discussed.

We present a new approach to achieving order in molecular semiconductors via alignment of polymer chains using surface-initiated polymerization. Polyacrylate brushes grown from transparent conducting electrodes, with triarylamine side groups as hole-transporting components, show characteristics of high mobilities for hole transport. Solution processing a second component with favorable enthalpic interactions can form a composite with mesoscale order and be exploited for heterojunction diodes. We find substantial uptake of CdSe nanocrystals (with diameter in the range 2.5-2.8 nm), and such composites show photovoltaic quantum efficiencies of up to 50%.

Samples of polymeric materials generally have no intrinsic shape; rather their macroscopic form is determined by external forces such as surface tension and memory of shear (for example, during extrusion, moulding or embossing). Hence, in the molten state, the thermodynamically most stable form for polymer (nano)particles is spherical. Here, we present the first example of polymer nanoparticles that have an intrinsic non-spherical shape. We observe the formation of high-aspect-ratio ellipsoidal polymer nanoparticles, of controlled diameter, made from main-chain liquid crystalline polymers using a mini-emulsion technique. The ellipsoidal shape is shown to be an equilibrium (reversible) characteristic and a direct result of the material shape memory when a liquid crystal nanoparticle is in its monodomain form.

We report the synthesis, characterisation and self-assembly of a nanoscopic Fe24 cluster using an organic dendrimer as a molecular substrate for building up well-defined molecular nanostructures., (Copyright 2004 The Royal Society of Chemistry)

The manufacture of high-performance electronic devices with micrometre or even submicrometre dimensions by solution processing and direct printing, requires the ability to control accurately the flow and spread of functional liquid inks on surfaces. This can be achieved with the help of surface-energy patterns causing inks to be repelled and dewetted from pre-defined regions of the substrate. To exploit this principle for the fabrication of submicrometre device structures, a detailed understanding of the factors causing ink droplets to dewet on patterned surfaces is required. Here, we use hydrophobic surface-energy barriers of different geometries to study the influence of solution viscosity, ink volume, and contact angle on the process of dewetting of inkjet-printed droplets of a water-based conducting polymer. We demonstrate polymer field-effect transistor devices with channel length of 500 nm fabricated by surface-energy-assisted inkjet printing.

Polymer brushes produced by controlled surface-initiated polymerization provide a route to surfaces coated with well-defined thin polymer films that are covalently bound to the substrate. All of the major controlled polymerization techniques have been applied to the synthesis of polymer brushes and examples of each are presented here. Many examples of brush synthesis in the literature have used the living atom transfer radical polymerization (ATRP) system, and in this tutorial review a particular focus is given to examples of this technique.

We have achieved a significant breakthrough in the synthesis of polyelectrolyte brushes of controlled thickness and density, which has been demonstrated by the synthesis of triblock copolymer brushes composed of cationic, neutral, and anionic segments.

This Letter reports the measurement of electro-osmotic flows (EOF) in microchannels with surface charge patterned on the 200 microm scale. We have investigated two classes of patterns: (1) Those in which the surface charge varies along a direction perpendicular to the electric field used to drive the EOF; this type of pattern generates multidirectional flow along the direction of the field. (2) Those in which the surface charge pattern varies parallel to the field; this pattern generates recirculating cellular flow, and thus causes motion both parallel and perpendicular to the external field. Measurements of both of these flows agree well with theory in the limit of thin double layers and low surface potential.

The incorporation of dialkyl sulfide side chains in metallodendrimers is a simple method for their insertion into a monolayer of decanethiol formed by self-assembly on a gold surface. The dendrimer binds through the sulfide group to a defect in the monolayer on the gold surface (see picture). The surface concentration of the isolated dendrimer adsorbate can be regulated by the adsorption time (for example, 55 adsorbates on a surface of 200x200 nm(2) after 20 h).