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PubMed2005

Surfactant removal and silica condensation during the photochemical calcination of thin film silica mesophases.

The evolution of photochemical surfactant removal and silica condensation from organically templated thin film silica nanocomposites with mesoscopic ordering has been probed using a combined application of Fourier transform infrared (FT-IR) spectroscopy and single wavelength ellipsometry. Thin films of silica nanocomposites were prepared by a previously reported evaporation-induced self-assembly process. Specifically, oxidized silicon and gold substrates were withdrawn at 25 mm/min from a subcritical micelle concentration solution containing an ethylene oxide surfactant as a structure-directing agent and tetraethyl orthosilicate as a silica precursor. Real-time grazing incidence difference FT-IR spectra of the nanocomposite films on gold taken during exposure to short-wavelength ultraviolet light (184-257 nm) show that surfactant removal and silica condensation occur gradually and concomitantly. Surfactant removal and silica reconstructions were found to be nearly complete after 90 min of exposure. Further, a transient feature was observed in the FT-IR spectra around 1713 cm(-1) during the UV exposure process and was assigned to a carbonyl (C=O) stretching mode absorption, reflecting the transient formation of a partially oxidized surfactant intermediate. From these data we propose a stepwise model for surfactant removal from the nanocomposite films. Ellipsometrically determined index of refraction values collected as a function of UV exposure are also shown to support such a stepwise mechanism of surfactant removal from the ordered nanocomposite silica thin film mesophases studied here.

Carbon↗
PubMed2005

Fabrication, characterization, and application in SERS of self-assembled polyelectrolyte-gold nanorod multilayered films.

An effective and facile approach for the preparation of multilayered nanostructure of gold nanorods (Au NRs) has been demonstrated. Linear polyethylenimine (LPEI) was selected as a polymeric adhesive layer, and an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSS) was used as the linker of the positively charged Au NRs in multilayered nanostructure. They were deposited onto the LPEI-modified indium-doped tin oxide (ITO) substrate alternately using the layer-by-layer assembly technique via electrostatic interactions. The plasmonic property of the multilayered nanostructure of Au NRs is tunable by the controlled self-assembly process. FE-SEM was used to study the morphologies of the resulted substrates with Au NRs monolayer membrane and with Au NRs multilayered membrane. More importantly, it was found that the multilayered NRs films could be used as a surface-enhanced Raman spectroscopy (SERS) substrate for probing 4-aminothiophenol (4-ATP).

Journal Article↗
PubMed2006

Modelling layer-by-layer assembly of flexible polyelectrolytes.

We have modelled the layer-by-layer assembly process of flexible polyelectrolytes on flat surfaces. The multilayer has a three-zone structure. An exponential growth is found for the first several layers, followed by a linear growth for subsequent layers evolving toward a steady state. While adjacent layers are highly interpenetrating, stratification can be seen for every four or more layers. The effects of surface charge density, bulk salt concentration, and solvent quality on the thickness and internal structure of the multilayer are also studied. Our results agree with experimental findings.

Letter↗
PubMed2006

Complexation between a macromolecule and an amphiphile by Monte Carlo technique.

Using a simple modified version of Larson's model, we studied the complexation between a macromolecule and an amphiphile in a dilute range of concentrations. The main characteristic of amphiphile molecules, that is, the hydrophobicity of the tails and hydrophilicity of the heads, is used to model the self-assembling process. Contrary to the molecular thermodynamics approaches, no prior shape was considered for the aggregates and the system was allowed to choose the most stable structure. For true ensemble averaging, without any synthetic results, configurational bias Monte Carlo and reptation moves are used to produce a Markov chain of configurations. From the results, it is found that the macromolecule causes the clusters of surfactants to be formed at a concentration much lower than the critical micelle concentration. Furthermore, the shape of the clusters tends to be more spherical, which is in line with theory and experiments. From the results, it is learned how a polymer can change the behavior of an amphiphilic molecule. All of the results are in good qualitative agreement with experimental and molecular thermodynamics results. Furthermore, the model predicts network formation between bound clusters at high concentrations of the surfactant.

Journal Article↗
PubMed2006

Monte Carlo simulations of cylinder-forming ABC triblock terpolymer thin films.

We systematically study the cylinder-forming ABC triblock terpolymer thin films using canonical ensemble Monte Carlo simulations. The simulated annealing procedure is applied to the self-assembling process. By judicious choice of the system dimensions, we elaborately investigate the effect of film thickness on the orientation of the cylinders. This confined triblock terpolymer system exhibits different phase behavior under the weak and strong surface fields. In addition, we also investigate the ensemble-averaged chain orientations and relative density profiles.

Journal Article↗
PubMed2006

Spectroscopic and semiempirical studies of a proton channel formed by the methyl ester of monensin A.

Monensin A is an ionophore able to carry protons and cations through the cell membrane. Its methyl ester (MON1) and its hydrates have been studied in acetonitrile, and its deuterated analogue by Fourier transform infrared (FTIR) and (1)H and (13)C NMR spectroscopies as well as by vapor pressure osmotic and PM5 semiempirical methods. Interestingly, these hydrates show new and unexpected biophysical and biochemical properties. The formation of the hydrates starts with a transfer of a proton from the O(IV)-H hydroxyl group of MON1 to an oxygen atom of a water molecule, which is subsequently hydrated by other water molecules forming the (MON1 + 3H(2)O) species. This hydrate exhibits a ringlike structure in which the water molecules form an almost linear hydrogen-bonded chain. Within this chain, the excess proton fluctuates very fast inside the water cluster as indicated by a continuous absorption in the FTIR spectra. The formation of the (MON1 + 3H(2)O) species is accompanied by a self-assembly process, leading to the formation of a proton channel made up of eight (MON1 + 3H(2)O) units with a length of 60 A, in which the proton can fluctuate over the whole distance. Semiempirical calculations suggest that due to the hydrophobic surface the channel can be incorporated readily in a lipid bilayer. This hypothetical new channel is thought to be able to transport protons through the cell membrane. Thus it is a suitable model for studying proton-transfer processes, and in addition, it may open interesting new fields of application.

Carbon Isotopes↗
PubMed2006

A small-angle scattering study on equilibrium clusters in lysozyme solutions.

We use small-angle scattering experiments to investigate the structural properties of aqueous lysozyme solutions under conditions where the existence of equilibrium clusters has recently been demonstrated (Nature 2004, 432, 492). We also discuss the possible emergence of a low angle scattering contribution, which recently attracted interest due to its appearance in solutions of various proteins. We demonstrate that in lysozyme solutions under our experimental conditions such rising low q intensities can only be observed under special circumstances and can thus not be attributed to the existence of a universal long-range attraction. We then focus on the structural properties of the equilibrium clusters as a function of protein concentration, temperature, and ionic strength. We show that the experimental structure factors obtained from the scattering measurements exhibit the typical cluster-cluster peak q(c) reflecting the mean distance between charged clusters as well as a monomer-monomer peak q(m), which represents the nearest neighbor shell of monomers within a single cluster. The underlying principle for the formation of these structures is the coexistence of two opposing forces, a short-range attraction and a long-range repulsion due to residual charges. We can quantitatively analyze our scattering data by applying a simple equilibrium cluster model and calculate an average cluster aggregation number, N(c). The thus obtained cluster aggregation number increases linearly with volume fraction. We also observe an increasing N(c) as temperature decreases and as the screening of residual charges increases. We point out the importance of the existence of equilibrium clusters and the universality of this phenomenon for self-assembling processes observed in nature. Finally, we discuss the limitations of our simple globular cluster model in view of recent findings from computer simulations.

Diffusion↗
PubMed2004

Mineralization mechanism of calcium phosphates under three kinds of Langmuir monolayers.

Three kinds of Langmuir monolayers formed by dipalmitoylphosphatidylcholine (DPPC), arachidic acid (AA), and octadecylamine (ODA) were used as templates to study the initial stage of nucleation and crystallization of calcium phosphates. It was demonstrated that the combination of calcium ions (or phosphates) to the monolayer/subphase interface is a prerequisite for subsequent nucleation. It was found that calcium phosphate dihydrate (DPCD) formed at 25.0 degrees C for 12 h has a biphasic structure containing both amorphous and crystalline phases. These results showed that calcium phosphates were formed through a multistage assembly process, during which an initial amorphous phase DPCD was followed by a phase transformation into a crystalline phase and then the most stable hydroxyapatite (HAp). This provided new insights into the template-biomineral interaction and a mechanism for biomineralization.

Journal Article↗
PubMed2004

Formation of Langmuir layers and surface modification using new upper-rim fully tethered bipyridinyl or bithiazolyl cyclodextrins and their fluorescent metal complexes.

Seven new amphiphilic cyclodextrins bearing bipyridyl or bithiazolyl moieties at the narrow rim and free hydroxyl or methoxyl groups at the wide rim of the cyclooctaamylose crown were synthesized using a one step "phosphine imide" approach. These ligands form metal complexes that have fluorescence properties with potentials for optical applications. Here, the cyclodextrin derivatives were used as probes for evaluating the role of different moieties in the self-assembly process, providing crucial information in creating functional devices. The behavior of these molecules and of complexes with EuIII in some cases was studied in Langmuir films using surface pressure (pi) and surface potential (deltaV) measurements performed as a function of film compression (compression isotherms). For chosen cyclodextrins, Brewster angle microscopy (BAM) in monolayers was performed. Films formed with derivatives 1, 3, 7, and 2compl were transferred on mica using the Langmuir-Blodgett technique. The properties of the films deposited on mica were analyzed with fluorimetry and, in the case of derivative 7, using fringe of equal chromatic order technique (FECO). The monolayer structure and the fluorescence properties of the Langmuir-Blodgett films indicate that the derivatives studied can be used for preparing cyclodextrin-based optical devices.

Cyclodextrins↗
PubMed2004

Preparation and characterization of n-alkanoic acid self-assembled monolayers adsorbed on 316L stainless steel.

The electrochemical formation and characterization of decanoic, myristic, palmitic, and stearic acid self-assembled monolayers on a native oxide surface of 316L stainless steel have been studied. This work describes a new approach to surface modification of stainless steel in which the self-assembly of n-alkanoic acids is facilitated by applying a potential to the stainless steel in an organic electrolyte solution. While decanoic acid forms a disorganized monolayer as a result of sweeping the potential in an acetonitrile solution containing 0.1 mM of the respective acid, longer acids, that is, myristic and palmitic acids, form highly ordered closed-packed monolayers. This electrochemical approach results in highly reproducible monolayers that are deposited within a shorter time than the traditional assembly process. The monolayers were characterized by cyclic voltammetry, double-layer capacity (ac voltammetry), contact angle measurements, X-ray photoelectron spectroscopy, and external reflection-absorption Fourier transform infrared spectroscopy. The utilization and implications of this modification technique are discussed.

Journal Article↗
PubMed2005

Liquid crystalline mesophases of pluronics (L64, P65, and P123) and transition metal nitrate salts ([M(H2O)6](NO3)2).

The triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers, Pluronics (L64, P65, and P123), form liquid crystalline (LC) mesophases with transition metal nitrate salts (TMS), [M(H(2)O)(n)](NO(3))(2), in the presence and absence of free water in the media. In this assembly process, M-OH(2) plays an important role as observed in a TMS:C(n)EO(m) (C(n)EO(m) is oligo(ethylene oxide) nonionic surfactants) system. The structure of the LC mesophases and interactions of the metal ion-nitrate ion and metal ion-Pluronic were investigated using microscopy (POM), diffraction (XRD), and spectroscopy (FTIR and micro-Raman) techniques. The TMS:L64 system requires a shear force for mesophase ordering to be observed using X-ray diffraction. However, TMS:P65 and TMS:P123 form well structured LC mesophases. Depending on the salt/Pluronic mole ratio, hexagonal LC mesophases are observed in the TMS:P65 systems and cubic and tetragonal LC mesophases in the TMS:P123 systems. The LC mesophase in the water/salt/Pluronic system is sensitive to the concentration of free (H(2)O) and coordinated water (M-OH(2)) molecules and demonstrates structural changes. As the free water is evaporated from the H(2)O:TMS:Pluronic LC mesophase (ternary mixture), the nitrate ion remains free in the media. However, complete evaporation of the free water molecules enforces the coordination of the nitrate ion to the metal ion in all TMS:Pluronic systems.

Journal Article↗
PubMed2005

Evidence of impurities in thiolated single-stranded DNA oligomers and their effect on DNA self-assembly on gold.

The diversity of techniques used in the synthesis, treatment, and purification of the single-stranded DNA oligomers containing a thiol anchor group (SH-ssDNA) has led to a significant variation in the purity of commercially available SH-ssDNA. In this work, we use X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to study how the impurities present in commercially synthesized SH-ssDNA oligomers affected the structure of the resulting DNA films on Au. XPS results indicate that two of the purchased SH-ssDNA oligomers contain excess carbon and sulfur. The molecular fragmentation patterns obtained with ToF-SIMS were used to determine the identity of several contaminants in the DNA films, including poly(dimethylsiloxane) (PDMS), lipid molecules, and sulfur-containing molecules. In particular, the ToF-SIMS results determined that the excess sulfur detected by XPS was due to the presence of dithiothreitol, a reductant often used to cleave disulfide precursors. Furthermore, we found that the SH-ssDNA self-assembly process is affected by the presence of these contaminants. When relatively pure SH-ssDNA is used to prepare the DNA films, the P, N, O, and C atomic percentages were observed by XPS to increase over a 24-h time period. In contrast, surfaces prepared using SH-ssDNA containing higher levels of contaminants did not follow this trend. XPS result indicates that, after the initial SH-ssDNA adsorption, the remaining material incorporated into these films was due to contamination.

DNA↗
PubMed2004

Two-component dendritic gel: effect of spacer chain length on the supramolecular chiral assembly.

The present study investigates in detail the physical gelation of toluene induced by the addition of simple aliphatic diamines to a dendritic L-lysine-based peptide. The gel-phase material obtained was characterized using differential scanning calorimetry, scanning electron microscopy, small-angle X-ray scattering, circular dichroism, 1H NMR, and X-ray diffraction. When the length of the aliphatic diamine is incrementally increased (C6-C12), the thermally reversible gel-sol transition temperature is dramatically increased (4-105 degrees C). This paper shows that the molecular information preset in the diamine is transcribed into the supramolecular assembly on the microscale and that this, in turn, controls the highly tunable macroscopic materials properties. The results also demonstrate the importance of chirality in the assembly process and highlight the role played by the aliphatic diamine in modulating the transcription of chirality from the molecular to the microscopic level.

Journal Article↗
PubMed2004

Fabrication of DNA microarrays on nanoengineered polymeric ultrathin film prepared by self-assembly of polyelectrolyte multilayers.

Microarray-based technology is in need of flexible and cost-effective chemistry for fabrication of oligonucleotide microarrays. We have developed a novel method for the fabrication of oligonucleotide microarrays with unmodified oligonucleotide probes on nanoengineered three-dimensional thin films that are deposited on glass slides by consecutive layer-to-layer adsorption of polyelectrolytes. Unmodified oligonucleotide probes were spotted and immobilized on these multilayered polyelectrolyte thin films (PET) by electrostatic adsorption and entrapment on the porous structure of the PET film. The PET provides higher probe binding capacity and thus higher hybridization signal than that of the traditional two-dimensional aminosilane and poly-L-lysine coated slides. Immobilized probe densities of 3.4 x 10(12)/cm2 were observed for microarray spots on PET with unmodified 50-mer oligonucleotide probes, which is comparable to the immobilized probe densities of alkyamine-modified 50-mer probes end-tethered on an aldehyde-functionalized slide. The study of hybridization efficiency showed that 90% of immobilized probes on PET film are accessible to target DNA to form duplex format in hybridization. The DNA microarray fabricated on PET film has wider dynamic range (about 3 orders of magnitude) and lower detection limit (0.5 nM) than the conventional amino- and aldehyde-functionalized slides. Oligonucleotide microarrays fabricated on these PET-coated slides also had consistent spot morphology. In addition, discrimination of single nucleotide polymorphism of 16S rRNA genes was achieved with the PET-based oligonucleotide microarrays. The PET microarrays constructed by our self-assembly process is cost-effective, versatile, and well suited for immobilizing many types of biological active molecules so that a wide variety of microarray formats can be developed.

Adsorption↗
PubMed2004

Characterization of self-assembled lamellar thermoresponsive silica-hydrogel nanocomposite films.

Mesostructured lamellar nanocomposite films with alternating silica and organic layers containing poly(N-isopropropyl acrylamide) (PNIPAM) were prepared using evaporation-induced self-assembly. A suitable theoretical approach to analyze the small-angle X-ray scattering (SAXS) patterns of oriented lamellar two-phase systems was applied to the SAXS data of films of varying composition, providing details on the self-assembly process, the composition, and the polymerization. In particular, this approach allowed an accurate determination of the thickness of the silica and the organic layer. The applicability of the SAXS approach was carefully tested with simulated data and verified by thermogravimetric analysis (TGA). TGA and (13)C NMR were used to study the polymerization and linkage to the silica matrix. SAXS and time-resolved grazing incidence SAXS revealed that the phase transition of PNIPAM at ca. 32 degrees C leads to a reversible expansion/contraction perpendicular to the layers on a time scale of ca. 30 min.

Journal Article↗
PubMed2005

Dynamic self-assembly of polymer colloids to form linear patterns.

A dynamic self-assembling process is reported which involves drying a droplet of positively charged colloidal suspension on a flat negatively charged hydrophilic surface. This extremely simple method affords lines of colloidal particles with regular 1.5-4.5 microm line spacing and smaller than 2 microm line width over a broad surface area. The ordered region diffracts light to display an iridescent appearance and generates first-order diffraction spots when illuminated by a He-Ne laser. A periodic stick-slip motion of the drying liquid front is observed during the drying process using optical microscopy. The periodic motion must be related to the periodic particle deposition. We propose that the simultaneous deposition of the particles at a fixed distance (i.e., the line spacing) behind the previous line of particles where the contact line is pinned is in turn responsible for the periodic stick-slip motion. The key distinguishing feature of the present system is the attractive interaction between the particles and the surface, which instigates the periodicity of the particle deposition.

Journal Article↗
PubMed2005

Colored thin films prepared from hydrogel microspheres.

Ordered 2-D structures composed of poly(N-isopropylacrylamide) (PNIPAM) microgel particles that had regularity on a sub-micrometer length scale were prepared. By using sterically stabilized PNIPAM microgel particles as components, the ordered array was formed by a self-assembly process. The particle array was prepared by depositing a droplet of the microgel dispersion on a substrate. Atomic force microscopy observation of the resulting thin films revealed that they comprised a monolayer particle array. The periodic structure of the array produced iridescent colors due to optical diffraction. Since a homogeneous particle array can be prepared simply by drying the dispersion, this particle dispersion may be considered as a new ink whose color is generated from the microstructure in the films produced.

Journal Article↗
PubMed2005

Highly photoluminescent multilayer QD-glass films prepared by LbL self-assembly.

A novel and facile preparation method for layer-by-layer (LbL) self-assembled films incorporating quantum dots (QDs) and having intense photoluminescence (PL) from blue to red is presented. Functional sol-gel-derived glass layers prepared by the hydrolysis of 3-aminopropyltrimethoxysilane (APS) or 3-mercaptopropyltrimethoxysilane (MPS) have been used as a linkage between QD layers. Absorption, PL spectroscopy, transmission electron microscopy, and atomic force microscopy were employed for characterization, which revealed that the QDs in the prepared films had a nearly close-packed coverage and were not aggregated. The PL efficiencies of the QDs (CdTe or ZnSe, both are thioglycolic acid-stabilized) dispersed in the films were roughly half that of the initial colloidal solutions but reached 24% before a refractive index correction. The thickness of the red-emitting film with 10 CdTe QD layers was approximately 50 nm. The concentration of QDs in the film derived from the first absorption peak was approximately 0.01 M. Because the PL starts to show a red shift, the obtained concentration is practically the ultimate one in the glass matrix. The mercapto, amino, and carboxyl groups play important roles in LbL self-assembling processes.

Cadmium Compounds↗