pH responsive polymer cushions for probing membrane environment interactions.

A robust and straightforward method for the preparation of lipid membranes upon dynamically responsive polymer cushions is reported. Structural characterization demonstrates that complete, well-packed membranes with tunable mobility can be constructed on the polymeric cushion. With this system, membrane conformational changes induced by cellular cytoskeleton interactions can be modeled. The membrane can be tailored to screen the cushion from changes in pH or allow rapid response to the pH environment by incorporation of protein ion channels. This elementary system offers a means to replicate the conformational changes that occur with the cellular cytoskeleton and has great potential for fundamental biophysical studies of membrane properties and membrane-protein interactions decoupled from the underlying solid support.

A stripe-to-droplet transition driven by conformational transitions in a binary lipid-lipopolymer mixture at the air-water interface.

We report the observation of an unusual stripe-droplet transition in precompressed Langmuir monolayers consisting of mixtures of poly(ethylene) glycol (PEG) amphiphiles and phospholipids. This highly reproducible and fully reversible transition occurs at approximately zero surface pressure during expansion (or compression) of the monolayer following initial compression into a two-dimensional solid phase. It is characterized by spontaneous emergence of an extended, disordered stripe-like morphology from an optically homogeneous phase during gradual expansion. These stripe patterns appear as a transient feature and continuously progress, involving gradual coarsening and ultimate transformation into a droplet morphology upon further expansion. Furthermore, varying relative concentrations of the two amphiphiles and utilizing amphiphiles with considerably longer ethylene glycol headgroups reveal that this pattern evolution occurs in narrow concentration regimes, values of which depend on ethylene oxide headgroup size. These morphological transitions are reminiscent of those seen during a passage through a critical point by variations in thermodynamic parameters (e.g., temperature or pressure) as well as those involving spinodal decomposition. While the precise mechanism cannot be ascertained using present experiments alone, our observations can be reconciled in terms of modulations in competing interactions prompted by the pancake-mushroom-brush conformational transitions of the ethylene glycol headgroup. This in turn suggests that the conformational degree of freedom represents an independent order parameter, or a switch, which can induce large-scale structural reorganization in amphiphilic monolayers. Because molecular conformational changes are pervasive in biological membranes, we speculate that such conformational transition-induced pattern evolution might provide a physical mechanism by which membrane processes are amplified.

Identification of acidic phosphorus-containing ligands involved in the surface chemistry of CdSe nanoparticles prepared in tri-N-octylphosphine oxide solvents.

The surface ligand composition of CdSe nanoparticles prepared using technical grade tri-n-octylphosphine oxide (TOPO) was investigated using a nucleophilic ligand displacement methodology and (31)P {(1)H} NMR spectroscopy. 4-(N,N-Dimethylamino)pyridine (DMAP) and benzyltrimethylammonium propionate were added to tetrahydrofuran solutions of CdSe nanoparticles prepared in technical grade TOPO. DMAP was shown to be a sufficiently strong nucleophile to displace the more weakly coordinating ligands, TOPO, TOPSe, di-n-octylphosphinate, and n-octylphosphonate (OPA). Benzyltrimethylammonium propionate was shown to be a stronger nucleophile than DMAP in that it could displace all the aforementioned surface-bound ligands as well as a previously unidentified surface-bound phosphorus species. Independent synthesis and (31)P {(1)H} NMR spectral matching confirmed that the new species was P,P'-(di-n-octyl) dihydrogen pyrophosphonic acid (PPA). The PPA was shown to form during the nanoparticle synthesis via the dehydrative condensation of OPA. CdSe nanoparticle syntheses were performed using pure TOPO and added OPA, and subsequent displacement experiments showed that OPA and PPA were the predominant surface-bound ligands. CdSe nanoparticle syntheses were performed using pure TOPO and added PPA, and subsequent displacement experiments showed that PPA was the predominant surface-bound ligand. PPA was also shown to have the greatest affinity for the nanoparticle surface of all the ligands investigated. Thus, a model for the surface ligand composition could be developed for nanoparticles prepared using technical grade TOPO or other high-boiling solvents with added acidic phosphorus compounds.

High-efficiency stepwise contraction and adsorption nanolithography.

A new miniaturization protocol is demonstrated using stretching and relaxation of an elastomer substrate. A designed microstructure is formed on the stretched substrate and subsequently becomes miniaturized when the substrate relaxes. More importantly, the miniaturized structures can be transferred onto a new substrate for further miniaturization or can be utilized as stamps for nanolithography of designated materials. As an example of this approach, an elastic mold was first cast from a Si mold containing periodic line arrays of 1.5-microm line width. Upon relaxation, line width is reduced to 240 nm. The new elastomer may be used as stamps for micro- and nanofabrication of materials such as proteins. The polymer surface roughness or wrinkling behavior at nanoscale is found to follow classic stability model in solid mechanics. This observation provides means to design and control the surface roughness to meet specific requirements.

Biomolecule detection via target mediated nanoparticle aggregation and dielectrophoretic impedance measurement.

A new biosensing system is described that is based on the aggregation of nanoparticles by a target biological molecule and dielectrophoretic impedance measurement of these aggregates. The aggregation process was verified within a microchannel via fluorescence microscopy, demonstrating that this process can be used in a real time sensor application. Positive dielectrophoresis is employed to capture the nanoparticle aggregates at the edge of thin film electrodes, where their presence is detected either by optical imaging via fluorescence microscopy or by measuring the change in electrical impedance between adjacent electrodes. The electrical detection mechanism demonstrates the potential for this method as a micro total analysis system (microTAS).

Improving Hematite's Solar Water Splitting Efficiency by Incorporating Rare-Earth Upconversion Nanomaterials.

Confounded by global energy needs, much research has been devoted to convert solar energy to various usable forms, such as chemical energy in the form of hydrogen via water splitting. Most photoelectrodes, such as hematite, utilize UV and visible radiation, whereas ∼40% infrared (IR) energy remains unconverted. This work represents our initial attempt to utilize IR radiation, that is, adding rare-earth materials to existing photoelectrodes. A simple substrate composed of hematite film and rare-earth nanocrystals (RENs) was prepared and characterized. Spectroscopy evidence indicates that the RENs in the composite absorb IR radiation (980 nm) and emit at 550 and 670 nm. The emitted photons are absorbed by surrounding hematite films, leading to improvement of water splitting efficiency as measured by photocurrent enhancement. This initial work demonstrates the feasibility and concept of using RENs for utilizing more solar radiation, thus improving the efficiency of existing solar materials and devices.

Fabrication and characterization of rare-earth-doped nanostructures on surfaces.

This article presents a simple and practical means to produce rare-earth-based nanostructures, as well as a combined characterization of structure and optical properties in situ. A nanosphere lithography strategy combined with surface chemistry enables the production of arrays of ß-NaYF(4):Yb,Er nanorings inlaid in an octadecyltrichlorosilane matrix. These arrays of nanorings are produced over the entire support, such as a 1 cm(2) glass coverslip. The dimension of nanorings can be varied by changing the deposition conditions. A home-constructed, multifunctional microscope integrating atomic force microscopy, near-field scanning optical microscopy, and far-field optical microscopy and spectroscopy is utilized to characterize the nanostructures. This in situ and combined characterization is important for rare-earth-containing nanomaterials in order to correlate local structure with upconversion photoluminescence. Knowledge gained from the investigation should facilitate materials design and optimization, for instance, in the context of photovoltaic devices and biofluorescent probes.

Asymmetric dinuclear copper(I) complexes of bis-(2-(2-pyridyl)ethyl)-2-(N-toluenesulfonylamino)ethylamine with short copper-copper distances.

Addition of two equivalents of CuCl to deprotonated bis-(2-(2-pyridyl)ethyl)-2-(N-toluenesulfonylamino)ethylamine (PETAEA) and its derivatives yielded new types of dinuclear Cu(I) complexes, Cu(mu-PETAEA)CuCl, Cu(mu-PEMAEA)CuCl, and Cu(mu-PENAEA)CuCl (PEMAEA is the 4-methoxyphenyl derivative of PETAEA and PENAEA is the 4-nitrophenyl derivative), exhibiting a four coordinate N(4)Cu center, a two coordinate NCuCl center, and a metal-metal distance within the range of 2.6572(8) to 2.6903(3) A. Analysis of the covalent radii for four coordinate and two coordinate copper(I), the acute copper-nitrogen-copper angles, and density functional theory (DFT) calculations suggest a weak attraction between the two copper atoms. The complexes apparently formed in a two-step process with the formation of the tetracoordinate mononuclear complex preceding the coordination of a second equivalent of CuCl to the lone pair of the sulfonamidate ligand.

Nanoparticle agglutination: acceleration of aggregation rates and broadening of the analyte concentration range using mixtures of various-sized nanoparticles.

SiO(2) particles of various sizes were prepared and surface modified with biotin-chain-end-functionalized poly(ethylene glycol). Dispersions of these particles were prepared, and their aggregation was induced upon the addition of avidin. The aggregate size and growth rate were monitored by DLS analysis, and SEM and TEM images of freeze-dried samples of the aggregate solutions were used to confirm the DLS data and to image the aggregate size and dimension. A linear correspondence between apparent diameter and time was observed, and both the 20 and 300 nm particles aggregated at slower rates than for the 40 nm particles. These observations were attributed to differences in the average functionality of the systems and the different initial concentrations of particles and avidin. The observed aggregation rates of binary combinations of the three particle sizes (i.e., 20 + 40 nm or 40 + 300 nm) were faster than those of the single-sized mixtures. These results were attributed to the faster flux of smaller particles toward larger particles in the mixture solutions as well as to the potentially larger number of productive collisions possible between mixtures of small particles and large particles versus only similarly sized particles. Combinations of the three sizes of particles were studied to find an empirical optimum formulation for generating large aggregates on a short time scale and over a wide range of analyte concentrations.

Measuring the size dependence of Young's modulus using force modulation atomic force microscopy.

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.

Copper(I) and -(II) complexes of neutral and deprotonated N-(2,6-diisopropylphenyl)-3-[bis(2-pyridylmethyl)amino]propanamide.

As part of a study of atom-transfer radical polymerization (ATRP) catalysts, four new copper(I) and -(II) compounds of a new monoanionic, tripodal tetradentate ligand, N-(2,6-diisopropylphenyl)-3-[bis(2-pyridylmethyl)amino]propanamide (DIPMAP), were prepared. Ligand synthesis followed from the addition-elimination reaction of 2,6-diisopropylaniline with acryloyl chloride and then a Lewis acid catalyzed Michael addition of bis(2-pyridylmethyl)amine to this product. The ligand was complexed to CuCl to yield monomeric Cu(DIPMAP)Cl featuring an intramolecular hydrogen bond between the free amide hydrogen and the coordinated chloride ligand. Deprotonation of the amide hydrogen in Cu(DIPMAP)Cl using n-BuLi led to the incorporation of LiCl in the resulting product, Li2Cu2(DIPMAP)2Cl2. This complex exhibited an unusual dimeric structure, with the amine nitrogens of one ligand coordinated to a lithium ion, the amide oxygen of the same ligand bridging between the lithium ions, and the amidate nitrogen of that ligand coordinated to a CuCl unit that has a structure analogous to dihalocuprate ions. Deprotonation of Cu(DIPMAP)Cl using KOtBu yielded an alkali-metal chloride free product, Cu2(DIPMAP)2, that also exhibited a dimeric structure in which the three amine nitrogens of one ligand were coordinated to one CuI ion and the amidate nitrogen of the same ligand was coordinated to the other CuI ion. Cu2(DIPMAP)2 was effective in abstracting halogen atoms from organic halides, but in the attempted ATRP of tert-butyl acrylate, molecular weight versus conversion behavior reminiscent of a redox-initiated polymerization was observed. DIPMAP was coordinated to CuBr2 to yield [Cu(DIPMAP)Br]Br with a square-pyramidal structure. The amide hydrogen in this complex could be deprotonated using KOtBu to form complex [DIPMAP]CuBr. Spectral characterization of complex confirmed deprotonation of the ligand and that it most likely had an axially distorted trigonal-bipyramidal structure, although crystals suitable for X-ray analysis could not be obtained. Solution oxidation of Cu2(DIPMAP)2 using CBr4 yielded a product, complex, whose spectral signatures did not match those of complex. The dimeric structure of Cu2(DIPMAP)2 might be a significant contributing factor to the slow rate of deactivation observed in atom-transfer reactions using Cu2(DIPMAP)2 as the catalyst.

Identification and characterization of monoanionic tripodal tetradentate ligand complexes of copper(I) and copper(II) involved in halogen atom transfer reactions.

The copper(I) complex of bis-(2-(2-pyridyl)-ethyl)-(2-(N-p-toluenesulfonamido)-ethyl)amine (PETAEA), a monoanionic, tripodal tetradentate ligand, was prepared, characterized, and shown to be an effective catalyst for atom transfer radical polymerization (ATRP). A model atom transfer reaction of Cu(PETAEA) with 1-phenylethyl bromide and TEMPO radical trapping agent was studied. The copper(II) complex formed in this reaction was identified by comparison of its spectroscopic data with that of Cu(PETAEA)Br prepared by an independent synthesis. Kinetic and spectroscopic data indicated that the reaction mechanism involved simple atom transfer from the alkyl halide to the Cu(PETAEA) to form the Cu(PETAEA)Br, and no other intermediates were involved. The solid-state structures of the copper(I) and (II) complexes appeared to be maintained in solution, so this system is an atom transfer reaction in which all of the reactive species are identified and characterized.