Fernando Cacho-Nerin

Free jet micromixer to study fast chemical reactions by small angle X-ray scattering

We present the design, fabrication process, and the first test results of a high aspect ratio micromixer combined with a free jet for under 100 micros time resolved studies of chemical reactions. The whole system has been optimized for synchrotron small angle X-ray scattering (SAXS) experiments. These studies are of particular interest to understand the early stages of chemical reactions, such as the kinetics of nanoparticle formation. The mixer is based on hydrodynamic focusing and works in the laminar regime. The use of a free jet overcomes the fouling of the channels and simultaneously circumvents background scattering from the walls. The geometrical parameters of the device have been optimized using finite element simulations, resulting in smallest features with radius <1 microm, and a channel depth of 60 microm, thus leading to an aspect ratio >60. To achieve the desired dimensions deep X-ray lithography (DXRL) has been employed. The device has been tested. First the focusing effect has been visualized using fluorescein. Then the evolution and stability of the jet, which exits the mixer nozzle at 13 m s(-1), have been characterized. Finally SAXS measurements have been conducted of the formation of calcium carbonate from calcium chloride and sodium carbonate. The fastest measurement is 75 micros after the beginning of the mixing of the reagents. The nanostructural evolution of chemical reactions is clearly discernible.

Shaping Mesoporous Films Using Dewetting on X-ray Pre-patterned Hydrophilic/Hydrophobic Layers and Pinning Effects at the Pattern Edge

Ordered mesoporous silica micrometer-sized structures have been fabricated via selective dewetting of the coating sol on a hydrophilic/hydrophobic fluorinated silica substrate, which had been pre-patterned using deep X-ray lithography with a synchrotron radiation source. We have observed that deposition of mesoporous films on the pre-patterned areas can be used as a design tool for obtaining regions of specific geometry and dimensions. The evaporation of the solution in constrained conditions because of pinning at the pattern edges gives layers with thicker edges. This edge effect appears dependent upon the dimension of the pre-patterned hydrophilic/hydrophobic layer; in smaller patterns, the evaporation is too fast and thickening of the edges is not observed. We have used infrared imaging, optical profilometry, and atomic force microscopy to characterize the patterned layers and the edge effect, produced by pinning at the border of the microstructures.

Densification of sol–gel silica thin films induced by hard X-rays generated by synchrotron radiation

In this article the effects induced by exposure of sol-gel thin films to hard X-rays have been studied. Thin films of silica and hybrid organic-inorganic silica have been prepared via dip-coating and the materials were exposed immediately after preparation to an intense source of light of several keV generated by a synchrotron source. The samples were exposed to increasing doses and the effects of the radiation have been evaluated by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and atomic force microscopy. The X-ray beam induces a significant densification on the silica films without producing any degradation such as cracks, flaws or delamination at the interface. The densification is accompanied by a decrease in thickness and an increase in refractive index both in the pure silica and in the hybrid films. The effect on the hybrid material is to induce densification through reaction of silanol groups but also removal of the organic groups, which are covalently bonded to silicon via Si-C bonds. At the highest exposure dose the removal of the organic groups is complete and the film becomes pure silica. Hard X-rays can be used as an efficient and direct writing tool to pattern coating layers of different types of compositions.

How the chain configuration governs the packing of inverted micelles in the cubic Fd3m-phase

The inverted micellar cubic Fd3m phase has attracted significant interest in drug delivery and is also of special biological relevance in the early steps of fat digestion. Applying small angle X-ray diffraction (SAXD) the stability of the Fd3m phase with respect to the chain configuration has been examined. In particular, the fully hydrated monoelaidin system containing the trans elaidic or its counterpart cis oleic fatty acid was investigated. Re-analyzed data of a fully hydrated monoolein–oleic acid (MO–OA) mixture complete this study (data taken from Luzzati et al.). On the basis of determined electron density maps, minute structural details are presented. The decomposition of the cubic Fd3m nanostructure into its apolar and polar moieties allowed the estimation of chain length and water radius depending on the fatty acid content. An increase in the elaidic chain concentration increases the membrane monolayer thickness, and in particular reduces the sphericity of the polar envelope of the small micelles. Furthermore, the electron density maps revealed the geometry of the space filling 512 and 51264 cages of the Fd3m phase as well as the loci of the greatest packing stress, which are situated at vertices of the 51264 cages that are placed opposite the hexagonal faces.

Tracking morphologies at the nanoscale: Self-assembly of an amphiphilic designer peptide into a double helix superstructure

Hierarchical self-assembly is a fundamental principle in nature, which gives rise to astonishing supramolecular architectures that are an inspiration for the development of innovative materials in nanotechnology. Here, we present the unique structure of a cone-shaped amphiphilic designer peptide. While tracking its concentration-dependent morphologies, we observed elongated bilayered single tapes at the beginning of the assembly process, which further developed into novel double-helix-like superstructures at high concentrations. This architecture is characterized by a tight intertwisting of two individual helices, resulting in a periodic pitch size over their total lengths of several hundred nanometers. Solution X-ray scattering data revealed a marked 2-layered internal organization. All these characteristics remained unaltered for the investigated period of almost three months. In their collective morphology, the assemblies are integrated into a network with hydrogel characteristics. Such a peptide-based structure holds promise as a building block for next-generation nanostructured biomaterials.

Surface passivation improves the synthesis of highly stable and specific DNA-functionalized gold nanoparticles with variable DNA density.

We report a novel and multifaceted approach for the quick synthesis of highly stable single-stranded DNA (ssDNA) functionalized gold nanoparticles (AuNPs). The method is based on the combined effect of surface passivation by (1-mercaptoundec-11-yl)hexa(ethylene glycol) and low pH conditions, does not require any salt pretreatment or high excess of ssDNA, and can be generalized for oligonucleotides of any length or base sequence. The synthesized ssDNA-coated AuNPs conjugates are stable at salt concentrations as high as 3.0 M, and also functional and specific toward DNA-DNA hybridization, as shown from UV-vis spectrophotometry, scanning electron microscopy, gel electrophoresis, fluorescence, and small angle X-ray scattering based analyses. The method is highly flexible and shows an additional advantage of creating ssDNA-AuNP conjugates with a predefined number of ssDNA strands per particle. Its simplicity and tenability make it widely applicable to diverse biosensing applications involving ssDNA functionalized AuNPs.

Patterning block copolymer thin films by deep X-ray lithography

Developing fast, cheap and reliable micro and nanofabrication technologies for block copolymer thin films is a key issue for exploiting the wide potential applications of this class of materials. We have used a synchrotron source of high energy photons (hard X-rays) for developing a lithographic tool that allows direct writing of block-copolymer thin films. We have exposed films prepared by a tri-block copolymer, Pluronic F127, to increasing doses of radiation to evaluate the effect of high energy X-rays on the samples. The as-deposited films show a crystalline structure due to the crystallization of polyethylene oxide chains in Pluronic F127; the exposure to low doses causes a phase change from crystalline to amorphous, as is shown by infrared spectroscopy. Another effect of the exposure to X-rays of the block copolymer films is the surface roughness reduction which has been observed by atomic force microscopy, At higher doses the X-rays erase the film from the substrate allowing the formation of patterned polymeric structures. Deep X-ray lithography has proved to be a very effective tool to pattern block copolymer films by a direct, top-down method.

Thorough small-angle X-ray scattering analysis of the instability of liquid micro-jets in air.

Liquid jets are of interest, both for their industrial relevance and for scientific applications (more important, in particular for X-rays, after the advent of free-electron lasers that require liquid jets as sample carrier). Instability mechanisms have been described theoretically and by numerical simulation, but confirmed by few experimental techniques. In fact, these are mainly based on cameras, which is limited by the imaging resolution, and on light scattering, which is hindered by absorption, reflection, Mie scattering and multiple scattering due to complex air/liquid interfaces during jet break-up. In this communication it is demonstrated that synchrotron small-angle X-ray scattering (SAXS) can give quantitative information on liquid jet dynamics at the nanoscale, by detecting time-dependent morphology and break-up length. Jets ejected from circular tubes of different diameters (100-450 µm) and speeds (0.7-21 m s(-1)) have been explored to cover the Rayleigh and first wind-induced regimes. Various solvents (water, ethanol, 2-propanol) and their mixtures have been examined. The determination of the liquid jet behaviour becomes essential, as it provides background data in subsequent studies of chemical and biological reactions using SAXS or X-ray diffraction based on synchrotron radiation and free-electron lasers.

Mesostructured silica aerosol particles: comparison of gas-phase and powder deposit X-ray diffraction data.

We report on the characterization of mesostructured aerosol silica particles in the gas phase using in situ synchrotron small-angle X-ray scattering (SAXS) in order to unveil the influence of the basic production parameters. The investigated system was based on tetraethylorthosilicate (TEOS) as the inorganic precursor and on cetyltrimethyl-ammonium bromide (CTAB) as the surfactant. The heating temperature, surfactant to silicate ratio, and particle flow rate were thoroughly investigated, and for this purpose, an in-house-built aerosol reactor equipped with a special X-ray observation chamber was used. Complementary fine structural analysis was applied on dried deposits of the silica aerosols comprising direct Fourier transforms as well as simple two-phase model fits. This resulted in robust estimates for the silica wall thickness and surfactant core radius of the hexagonally ordered mesostructure. The particle shape and size distribution were examined by scanning electron microscopy (SEM). The quality of the inner nanostructure was revealed from an analysis of the peak width. The comparison of data from the gas phase and powder deposit shows that, in general, slower drying conditions (heating temperature about 80 °C) and a medium surfactant to Si ratio (about 0.14) lead to nanostructures of the best quality in terms of well-defined long-range organization.

Dynamic Organization of Ligand-Grafted Nanoparticles during Adsorption and Surface Compression at Fluid–Fluid Interfaces

Monolayers of ligand-grafted nanoparticles at fluid interfaces exhibit a complex response to deformation due to an interplay of particle rearrangements within the monolayer, and molecular rearrangements of the ligand brush on the surface of the particles. We use grazing-incidence small-angle X-ray scattering (GISAXS) combined with pendant drop tensiometry to probe in situ the dynamic organization of ligand-grafted nanoparticles upon adsorption at a fluid–fluid interface, and during monolayer compression. Through the simultaneous measurements of interparticle distance, obtained from GISAXS, and of surface pressure, obtained from pendant drop tensiometry, we link the interfacial stress to the monolayer microstructure. The results indicate that, during adsorption, the nanoparticles form rafts that grow while the interparticle distance remains constant. For small-amplitude, slow compression of the monolayer, the evolution of the interparticle distance bears a signature of ligand rearrangements leading to a local decrease in thickness of the ligand brush. For large-amplitude compression, the surface pressure is found to be strongly dependent on the rate of compression. Two-dimensional Brownian dynamics simulations show that the rate-dependent features are not due to jamming of the monolayer, and suggest that they may be due to out-of-plane reorganization of the particles (for instance expulsion or buckling). The corresponding GISAXS patterns are also consistent with out-of-plane reorganization of the nanoparticles.