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Contact electrochemical replication (CER) is a novel pattern replication methodology advanced in this laboratory that offers the unprecedented capability of direct one-step reproduction of monolayer surface patterns consisting of hydrophilic domains surrounded by a hydrophobic monolayer background (hydrophilic @ hydrophobic monolayer patterns), regardless of how the initial "master" pattern was created. CER is based on the direct electrochemical transfer of information, through aqueous electrolyte bridges acting as an information transfer medium, between two organosilane monolayers self-assembled on smooth silicon wafer surfaces. Upon the application of an appropriate voltage bias between a patterned monolayer/silicon specimen playing the role of "stamp" and a monolayer/silicon specimen playing the role of "target", the hydrophilic features of the stamp are copied onto the hydrophobic surface of the target. It is shown that this electrochemical printing process may be implemented under a variety of experimental configurations conducive to the formation of nanometric electrolyte bridges between stamp and target; however, using plain liquid water for this purpose is, in general, not satisfactory because of the high surface tension, volatility, and incompressibility of water. High-fidelity replication of monolayer patterns with variable size of hydrophilic features was achieved by replacing water with a sponge-like hydrogel that is nonvolatile, compressible, and binds specifically to the hydrophilic features of such patterns. Since any copy resulting from the CER process can equally perform as stamp in a subsequent CER step, this methodology offers the rather unique option of multiple parallel reproduction of an initially fabricated master pattern.
ACS Nano 2008 Dec 23
PMID:Contact electrochemical replication of hydrophilic-hydrophobic monolayer patterns. 1920 92

We report on an scanning tunneling microscopy study of the nanocrystallite phases of TiO(2) formed via reactive-layer-assisted deposition in ultrahigh vacuum. The synthesis used reaction of a thin layer of water, on a Au(111) substrate at 130 K, with low-coverage vapor-deposited Ti. The effects of annealing temperature and reactant coverage were investigated. Large-scale (>20 nm) patterns in the surface distribution of nanoparticles were observed with the characteristic length-scale of the pattern correlating with the thickness of the initial layer of H(2)O. The phenomenon is explained as being due to the formation of droplets of liquid water at temperatures between 130 and 300 K. After the surface was annealed to 400 K, the individual titania nanoparticles formed by this process had diameters of 0.5-1 nm. When the surface was annealed to higher temperatures, nanoparticles coalesced and for annealing temperatures of 900 K compact nanocrystals formed with typical dimensions of 5-20 nm. Three distinct classes of nanocrystallites were observed and their atomic structure and composition investigated and discussed.
ACS Nano 2008 Jul
PMID:Scanning tunneling microscopy study of titanium oxide nanocrystals prepared on Au(111) by reactive-layer-assisted deposition. 1920 2

Molecular level alignment of components and optimum morphology of hybrid materials are of great interest in many applications. Morphology control has been extensively used as a direct tool in the evaluation of interactions and assemblies of components in thin films. It is believed that preparation method and composition are powerful tools to direct the morphology, particularly in self-assembled systems such as fullerene-based hybrid materials. The present report outlines a synergistic self-assembly of fullerenes (C(60)) and functionalized poly (p-phenylene) (PPP) to develop nanofibers with high aspect ratios. Nanostructured PPP-C(60) hybrids were prepared by direct casting of the dilute solution on solid substrates and on water under ambient conditions. The formation of whiskers with high aspect ratio and investigation of interesting photophysical properties are discussed. An amphiphilic PPP was used as a template for preparing nanohybrids of C(60) at ambient temperature and conditions.
ACS Nano 2008 Jul
PMID:Amphiphilic poly(p-phenylene)-driven multiscale assembly of fullerenes to nanowhiskers. 1920 11

Core-shell ZnO:MgO nanocrystals have been synthesized by a sequential preparative procedure and capped with carboxymethyl beta-cyclodextrin (CMCD) cavities, thereby rendering the surface of the nanocrystals hydrophilic and the particles water-soluble. The water-soluble CMCD-capped ZnO:MgO nanocrystals emit strongly in the visible region (450-680 nm) on excitation by UV radiation and are stable over extended periods and over a range of pH values. The integrity of the cyclodextrin cavities is preserved on capping and retains their capability for complexation of hydrophobic species in aqueous solutions. Here we report the use of the water-soluble cyclodextrin-capped ZnO:MgO nanocrystals as energy donors for fluorescence resonance energy transfer studies. The organic dye Nile Red has been included within the anchored cyclodextrin cavities to form a noncovalent CMCD ZnO:MgO-Nile Red assembly in aqueous solution. Significant Nile Red fluorescence at 640 nm is observed on band gap excitation of the ZnO:MgO in the UV, indicating efficient resonance energy transfer (RET) from the nanocrystals to the included dye. The number of acceptor molecules interacting with a single donor in the CMCD ZnO:MgO-Nile Red assembly may be altered by controlling the filling up of the anchored cavities by Nile Red, leading to a variation in the efficiency of resonance energy transfer. The donor-acceptor distance was estimated from the efficiency measurements. The Nile Red emission following RET shows a pronounced thermochromic shift, suggesting the possible use of the CMCD ZnO:MgO-Nile Red assembly as thermometers in aqueous solutions.
ACS Nano 2008 Jul
PMID:Resonance energy transfer from beta-cyclodextrin-capped ZnO:MgO nanocrystals to included Nile Red guest molecules in aqueous media. 1920 17

Vanadium dioxide (VO(2)) is a well-known semiconductor material with a band gap of 0.7 eV, and is seldom used as a photocatalyst. We report here a new crystal structure for nanostructured VO(2), with body-centered-cubic (bcc) structure and a large optical band gap of approximately 2.7 eV, which surprisingly shows excellent photocatalytic activity in hydrogen production. The bcc VO(2) phase exhibited a high quantum efficiency of approximately 38.7% when synthesized as nanorods. Using films of the aligned VO(2) nanorods, the hydrogen production rate can be tuned by varying the incident angle of UV light on the films and reaches a high rate of 800 mmol/m(2)/h from a mixture of water and ethanol under UV light, at a power density of approximately 27 mW/cm(2), allowing possible commercial application of this material as photoassisted hydrogen generators.
ACS Nano 2008 Jul
PMID:Nanostructured VO2 photocatalysts for hydrogen production. 1920 20

Molecular dynamics simulations of the orientational dynamics of water molecules confined in narrow carbon nanotubes and nanorings reveal that confinement leads to strong anisotropy in the orientational relaxation. The relaxation of the aligned dipole moments, occurring on a time scale of nanoseconds, is 3 orders of magnitude slower than that of bulk water. In contrast, the relaxation of the vector joining the two hydrogens is ten times faster compared to bulk, with a time scale of about 150 fs. The slow dipolar relaxation is mediated by the hopping of orientational defects, which are nucleated by the water molecules outside the tube, across the linear water chain.
ACS Nano 2008 Jun
PMID:Strongly anisotropic orientational relaxation of water molecules in narrow carbon nanotubes and nanorings. 1920 36

Water nanoclusters confined to zeolitic cavities have been extensively investigated by various experimental techniques. We report a series of molecular dynamics simulations at different temperatures and for water nanoclusters of different sizes in order to attempt an atomistic interpretation of the properties of these systems. The cavities of zeolite NaA are spherical in shape and about 1 nm in diameter and can host nanoclusters of water containing nearly up to 24 water molecules. A modified interaction potential, yielding a better reproduction of experimental hydration energy and water diffusivity across a number of different zeolites, is proposed. Molecular dynamics simulations reproduce the known experimental structural features obtained by X-ray diffraction. Variations of simulated vibrational IR and IINS spectra with temperature and size of nanoclusters are in good agreement with experiment. The simulated water nanoclusters in zeolite NaA are found to be too small to crystallize and, at low temperature, behave as amorphous ice, in agreement with recent experimental results for similar water nanoclusters in reverse micelles.
ACS Nano 2008 Aug
PMID:Dynamical properties of confined water nanoclusters: Simulation study of hydrated zeolite NaA: structural and vibrational properties. 1920 62

The size below which anatase nanoparticles become more stable than rutile nanoparticles (crossover diameter) is dependent on the environment of the nanoparticles. It is smaller for nanoparticles in vacuum than those in water and continues to decrease with increase in temperature. Phase transformation between anatase and rutile phases is facilitated by enhanced ionic mobility at temperatures near the melting point of the nanoparticles. Multiparticle multiphase molecular dynamics simulations of TiO(2) nanoparticles undergoing sintering-induced phase transformations are reported here. Over the time scales accessible to molecular dynamics simulations, we found that the final sintering agglomerate transformed to the rutile phase, provided one of the sintering nanoparticles was rutile, while sintering of anatase and amorphous nanoparticles resulted in a brookite agglomerate. No such phase transformations were observed at temperatures away from nanoparticle's melting temperatures.
ACS Nano 2008 Aug
PMID:Phase transformations during sintering of titania nanoparticles. 1920 64

We report the engineering of a novel lipid-polymer hybrid nanoparticle (NP) as a robust drug delivery platform, with high drug encapsulation yield, tunable and sustained drug release profile, excellent serum stability, and potential for differential targeting of cells or tissues. The NP comprises three distinct functional components: (i) a hydrophobic polymeric core where poorly water-soluble drugs can be encapsulated; (ii) a hydrophilic polymeric shell with antibiofouling properties to enhance NP stability and systemic circulation half-life; and (iii) a lipid monolayer at the interface of the core and the shell that acts as a molecular fence to promote drug retention inside the polymeric core, thereby enhancing drug encapsulation efficiency, increasing drug loading yield, and controlling drug release. The NP is prepared by self-assembly through a single-step nanoprecipitation method in a reproducible and predictable manner, making it potentially suitable for scale-up.
ACS Nano 2008 Aug
PMID:Self-assembled lipid--polymer hybrid nanoparticles: a robust drug delivery platform. 1920 74

Because of their unique structure and composition, single-wall carbon nanotubes (SWNTs) are at the interface between molecules and crystalline solids. They also present properties that are ideal for making lightweight, inexpensive, and flexible electronics. The raw material is composed of a heterogeneous mixture of SWNTs that differ in helicity and diameter and, therefore, requires purification and separation. In a series of groundbreaking experiments, a robust process serving this purpose was developed based on SWNTs encapsulated in surfactants and water. Ultracentrifugation in a density gradient combined with surfactant mixtures provided buoyant density differences, enabling enrichment for both diameter and electronic properties. A new paper in this issue explores further the process through the hydrodynamic properties of SWNT-surfactant complexes. The study reveals that we have just begun to uncover the dynamics and properties of nanotube-surfactant interactions and highlights the potential that could be gained from a better understanding of their chemistry. The time scale of integration of carbon nanotubes into electronics applications remains unclear, but the recent developments in sorting out SWNTs paves the way for improving on the properties of network-based SWNTs.
ACS Nano 2008 Nov 25
PMID:Sorting carbon nanotubes for electronics. 1920 95


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