Planar Silica Substrates Research Articles

Toward a method of understanding the complexation of rare earth element by functionalized organosilanes in aqueous media

In the context of rare earth element extraction using solid-liquid process, hybrid materials having specific functional group were largely studied. However, there is a lack of in situ characterization method to understand how the functional group behaves when interacts with the rare earth element. Here, we propose a new experimental method to understand the interaction between immobilized functional group and REE in aqueous media using a well-characterized model system coupled to surface analyses. Model system was prepared by grafting diglycolamide functionalized silane on planar silica substrate. In order to have a clear description of the morphology and the structure of the model system, the silane grafting process was characterized using contact angle measurement, X-Ray Reflectivity, peak force measurements by Atomic Force Microscopy and in situ analysis by Fourier Transformed Infrared Spectroscopy in Attenuated Total Reflection. Afterwards, the behavior of the grafted molecules was investigated in neodymium aqueous solution at various pH by in situ infrared spectroscopy and hard X-ray reflectivity characterizations. Here, we highlight that the complexation of Nd3+ by the diglycolamide function is correlated with its dehydration and a modification of the molecule configuration.

Depletion of Polyelectrolytes near Like-Charged Substrates Probed by Optical Reflectivity

Salt-free polyelectrolyte solutions form a polyelectrolyte-free depletion layer near like-charged solid interfaces. This evidence is provided by optical reflectivity measurements from planar silica substrates in contact with aqueous solutions of sodium poly(styrene sulfonate). Analysis of the observed reflectivity signal, which features unusual negative signals and optical matching points, with a two-slab model demonstrates the existence of such a polyelectrolyte-free depletion layer. Moreover, the applied model permits the extraction of the corresponding layer thickness, which typically is several tenths of nanometers. This thickness decreases as the inverse square root of the polyelectrolyte concentration, but the actual values exceed the expected Debye length substantially. These results clearly provide independent evidence for the existence of a polyelectrolyte-free depletion layer, as previously suggested on the basis of direct force measurements with the atomic force measurement.

Particle Deposition to Silica Surfaces Functionalized with Cationic Polyelectrolytes

Positively charged water-solid interfaces are prepared by adsorption of a cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) from aqueous solutions to planar silica substrates. These substrates are characterized by atomic force microscopy (AFM), optical reflectivity, and streaming current measurements. By tuning the amount of adsorbed polyelectrolyte, the surface charge of the substrate can be systematically varied. These substrates are further used to study deposition of sulfate latex nanoparticles, which is also accomplished by optical reflectivity. This deposition process is found to be consistent with an extension of the random sequential adsorption (RSA) model in a semi-quantitative fashion. Such deposition studies were further used to ascertain that the substrates obtained by in situ and ex situ functionalization behave in an identical fashion.

Enrichment of Charged Monomers Explains Non-monotonic Polymer Volume Fraction Profiles of Multi-stimulus Responsive Copolymer Brushes.

Multi-stimulus responsive poly(2-(2-methoxyethoxy)ethyl methacrylate-co-2-(diethylamino)ethyl methacrylate) [P(MEO2MA-co-DEA)] 80:20 mol % copolymer brushes were synthesized on planar silica substrates via surface-initiated activators continuously regenerated via electron transfer atom transfer radical polymerization. Brush thickness was sensitive to changes in pH and temperature as monitored with ellipsometry. At low pH, the brush is charged and swollen, while at high pH, the brush is uncharged and more collapsed. Clear thermoresponsive behavior is also observed with the brush more swollen at low temperatures compared to high temperatures at both high and low pH. Neutron reflectometry was used to determine the polymer volume fraction profiles (VFPs) at various pH values and temperatures. A region of lower polymer content, or a depletion region, near the substrate is present in all of the experimental polymer VFPs, and it is more pronounced at low pH (high charge) and less so at high pH (low charge). Polymer VFPs calculated through numerical self-consistent field theory suggest that enrichment of DEA monomers near the substrate results in the experimentally observed non-monotonic VFPs. Adsorption of DEA monomers to the substrate prior to initiation of polymerization could give rise to DEA segment-enriched region proximal to the substrate.

Theory of plasmonic properties of hyper-doped silicon nanostructures

The presence of a Localized Surface Plasmon Resonance in doped semiconductor nanostructures opens a new field for plasmonics and metasurfaces. Semiconductor nanostructures can be easily processed, have weak dissipation losses, and the plasmon resonance can be tuned from the mid- to the near-infrared spectral range by changing the dopant concentration (in complement to the constituent material, the size and shape of the nanostructure). We present in this paper an extension of the Green Dyadic Method applied to the case of doped silicon nanostructures of arbitrary shape on a planar silica substrate. The method is used to compute both far- and near-field optical properties, such as the extinction efficiency and the electromagnetic near-field intensity inside and around any doped silicon nanostructure, respectively. This theoretical approach provides an important tool for active dopant characterization in doped semiconductor nanostructures, for near-field imaging of complex nanoantennas produced by electron beam lithography, and for the definition of doped semiconductor-based metasurfaces.

Adsorption and fusion of hybrid lipid/polymer vesicles onto 2D and 3D surfaces.

We investigated the formation of hybrid lipid/polymer (1,2-dioleoyl-sn-glycero-3-phosphocholine and poly(ethylene oxide-b-butadiene); DOPC/EO22Bd37) films onto planar silica surfaces. Using laser scanning confocal microscopy, atomic force microscopy, and quartz crystal microbalance analysis, we monitored the adsorption and fusion of hybrid lipid/polymer vesicles onto planar borosilicate glass cleaned via chemical etching or RF/air plasma treatment. In addition we used cryo-electron microscopy to characterize film formation on mesoporous silica nanoparticles. As the polymer content in the vesicles increased, the resulting hybrid lipid/polymer films on borosilicate glass - cleaned by chemical etching or plasma treatment - were more heterogeneous, indicating a large number of adsorbed vesicles rather than continuous bilayer films at higher polymer loadings. The observed lateral fluidity of both DOPC and hybrid lipid/polymer films also decreased substantially with increasing polymer fraction and was found to be relatively insensitive to changes in pH. Films prepared from vesicles with higher polymer loadings were completely immobile. We also found that polymer vesicles did not interact with clean plasma-treated glass surfaces, which may be due to elevated OH and Si-OH on plasma-treated surfaces. Conformal hybrid lipid/polymer coatings consistent with bilayers could be formed on mesoporous silica nanoparticles and imaged via cryo-electron microscopy. These results expand the library of biocompatible materials that can be used for coating silica-based materials and nanoparticles.

Adsorption of monovalent and divalent cations on planar water-silica interfaces studied by optical reflectivity and Monte Carlo simulations

Adsorption on planar silica substrates of various monovalent and divalent cations from aqueous solution is studied by optical reflectivity. The adsorbed amount is extracted by means of a thin slab model. The experimental data are compared with grand canonical Monte Carlo titration simulations at the primitive model level. The surface excess of charge due to adsorbed cations is found to increase with pH and salt concentration as a result of the progressive dissociation of silanol groups. The simulations predict, in agreement with experiments, that the surface excess of charge from divalent ions is much larger than from monovalent ions. Ion-ion correlations explain quantitatively the enhancement of surface ionization by multivalent cations. On the other hand, the combination of experimental and simulation results strongly suggests the existence of a second ionizable site in the acidic region. Variation of the distance of closest approach between the ions and surface sites captures ion specificity of water-silica interfaces in an approximate fashion.

Converging layer-by-layer polyelectrolyte microcapsule and cubic lyotropic liquid crystalline nanoparticle approaches for molecular encapsulation

Microcapsules created by the layer-by-layer (LbL) polyelectrolyte adsorption technique have been sub-compartmentalised by embedding cubic mesophase lipid nanoparticles (cubosomes™) into the capsule shell wall. Monoolein and phytantriol cubosomes™ containing fluorescent lipid and/or positively charged surfactant were first analysed for stability viadynamic light scattering, microelectrophoresis, and small angle X-ray scattering techniques. Once nanoparticle stability was confirmed, cubosomes™ were embedded within a multilayer assembly of oppositely charged polyelectrolytes [poly(allylamine hydrochloride) and poly(styrene sulfonate)] on planar silica substrates. Deposition of each layer was monitored using a quartz crystal microbalance with dissipation monitoring. These findings were then correlated to the growth of polyelectrolyte films incorporating cubosomes™ onto silica microparticles, where ζ-potential measurements were used to monitor the deposition of each subsequent layer. Small angle X-ray scattering experiments provided verification that cubosomes™ remained structurally intact when embedded within the polyelectrolyte matrix. Upon removal of the silica core, stable microcapsules containing one layer of embedded cubic nanoparticles were obtained. A diversity of molecular encapsulation matrices is offered through the capsule core, polyelectrolyte layers, and the embedded cubosomes™ of these sub-compartmentalised, nanostructured microcapsules.

Layer-by-Layer Deposition of Polyelectrolyte Macroinitiators for Enhanced Initiator Density in Surface-Initiated ATRP

The layer-by-layer (L-b-L) deposition of oppositely charged polyelectrolytic macroinitiators has been demonstrated on planar silica substrates. The build-up of the macroinitiator multilayers was monitored by ellipsometry (up to 21 layers) and dual polarization interferometry (up to 17 layers) and good agreement was found between these techniques. The increase in L-b-L thickness was approximately linear, with an average thickness of 2.3 A per layer of deposited macroinitiator. Surface-initiated ATRP of a model nonionic methacrylic monomer, 2-hydroxyethyl methacrylate (HEMA) in a 1:1 methanol/water mixture was conducted at ambient temperature. Increasing the number of macroinitiator layers led to a significant increase in PHEMA brush thickness up to 110 nm, which is attributed to the greater surface grafting density. PHEMA brush thicknesses obtained after 22 h showed a linear dependence on the number of layers of deposited macro-initiator, with all layers exhibiting near-identical growth kinetics. X-ray photoelectron spectroscopy was used to monitor L-b-L assembly and also to confirm PHEMA growth. This technique indicated the loss of small counterions from the multilayers during L-b-L deposition and confirmed an increase in the surface density of bromoester initiator groups as the number of deposited macroinitiator layers was increased. For 17 macroinitiator layers, the bromoester initiator density is estimated to be approximately 4.9 +/- 0.2 nm (-2) from the DPI data. This is comparable to that calculated for ATRP initiator monolayers obtained by either thiol or silane chemistry. Ellipsometry suggested that the macroinitiator multilayers were weakly hydrated prior to the in situ HEMA polymerization. AFM studies indicated that the PHEMA brushes had appreciable surface roughness, but this roughness became negligible compared to the brush thickness with increasing macroinitiator layers.

UV-Patterned Poly(ethylene glycol) Matrix for Microarray Applications

A versatile method to fabricate polymeric matrixes for microarray applications is demonstrated. Several different design strategies are presented where a variety of organic films, such as plastic polymers and self-assembled monolayers (SAMs) on planar silica and gold substrates, act as supports for the graft polymerization procedure. An ensemble of poly(ethylene glycol) methacrylate monomers are combined to obtain a matrix with desired properties: low nonspecific binding and easily accessible groups for postimmobilization of ligands. The free radical graft polymerization process occurs under irradiation with UV light in the 254-266 nm range, which offers the possibility to introduce patterns by means of a photomask. The arrays are created on inert and homogeneous coatings prepared either by graft polymerization of a methoxy-terminated PEG-methacrylate or self-assembly of a methoxy-terminated oligo(ethylene glycol) thiol. Carboxylic acid groups, introduced in the array spots either during graft polymerization or upon wet chemical conversion of hydroxyls, grant the capability to immobilize proteins and other molecules via free amine groups. Immobilization of fluorescent species as well as biotin followed by exposure to a fluorescently labeled antibody directed toward biotin display both excellent integrity of the spots and low nonspecific binding to the surrounding framework. Beside patterns of uniform height and size, an array of spots with varying thickness (a sort of gradient) is demonstrated. Such gradient samples enable us to address critical issues regarding the mechanism(s) behind spatially resolved free radical polymerization of methacrylates. It also offers a convenient route to optimize the matrix properties with respect to thickness, loading capacity, protein diffusion/penetration, and nonspecific binding.

Thin adsorbed films of a strong cationic polyelectrolyte on silica substrates

The adsorption of poly(diallyldimethyl ammonium chloride) (DADMAC) on planar silica substrates was examined as a function of ionic strength and pH. The study was carried out with reflectometry in an impinging-jet cell and complemented by atomic force microscopy (AFM) and ellipsometry investigations. The adsorption process is initially transport limited, whereby the adsorption rate increases somewhat with increasing ionic strength. This effect is caused by a simultaneous decrease of the hydrodynamic radius of the polymer. After a transient period, the adsorption process saturates and leads to an adsorption plateau. The plateau value increases strongly with increasing ionic strength. This increase can be explained by progressive screening of the electrostatic repulsion between the adsorbing polyelectrolyte chains, as can be rationalized by a random sequential adsorption (RSA) model. The adsorbed amount further increases with increasing pH, and this effect is probably caused by the corresponding increase of the surface charge of the silica substrate.

MCVD planar substrates for UV-written waveguide devices

A method for fabricating planar silica substrates via modified chemical vapour deposition (MCVD) and a modified fibre drawing technique is presented. Long lengths of planar material are generated from a single substrate offering a potentially low-cost alternative to existing planar substrate deposition processes. Buried straight and splitting waveguide channels, as well as Bragg gratings, are inscribed in the planar material using direct UV-writing technology, and the results are reported.

Deposition of nanosized latex particles onto silica and cellulose surfaces studied by optical reflectometry

Deposition of positively charged nanosized latex particles onto planar silica and cellulose substrates was studied in monovalent electrolyte solutions at pH 9.5. The deposition was probed in situ with optical reflectometry in a stagnation point flow cell. The surface coverage can be estimated reliably with island film theory as well as with a homogeneous film model, as confirmed with atomic force microscopy (AFM). The deposition kinetics on the bare surface was of first order with respect to the particle concentration, whereby the deposition rate was close to the value expected for a perfect collector. The efficiency coefficient, which was defined as the ratio of the experimental and theoretical deposition rate constants, was in the range from 0.3 to 0.7. Subsequently, the surface saturated and a limiting maximum coverage was attained (i.e., blocking). These trends were in qualitative agreement with predictions of the random sequential absorption (RSA) model, where electrostatic interactions between the particles were included. It was observed, however, that the substrate strongly influenced the maximum coverage, which was substantially higher for silica than for cellulose. The major conclusion of this work was that the nature of the substrate played an important role in a saturated layer of deposited colloidal particles.

Direct Writing of Metal Nanoparticle Films Inside Sealed Microfluidic Channels

Herein we demonstrate the ability to pattern Ag nanoparticle films of arbitrary geometry inside sealed PDMS/TiO2/glass microfluidic devices. The technique can be employed with aqueous solutions at room temperature under mild conditions. A 6 nm TiO2 film is first deposited onto a planar Pyrex or silica substrate, which is subsequently bonded to a PDMS mold. UV light is then exposed through the device to reduce Ag+ from an aqueous solution to create a monolayer-thick film of Ag nanoparticles. We demonstrate that this on-chip deposition method can be exploited in a parallel fashion to synthesize nanoparticles of varying size by independently controlling the solution conditions in each microchannel in which the film is formed. The film morphology was checked by atomic force microscopy, and the results showed that the size of the nanoparticles was sensitive to solution pH. Additionally, we illustrate the ability to biofunctionalize these films with ligands for protein capture. The results indicated that this could be done with good discrimination between addressed locations and background. The technique appears to be quite general, and films of Pd, Cu, and Au could also be patterned.

A Combinatorial Approach to Study Solvent-Induced Self-Assembly of Mixed Poly(methyl methacrylate)/Polystyrene Brushes on Planar Silica Substrates: Effect of Relative Grafting Density

This article reports the study of the effect of relative grafting densities of two polymer chains on solvent-induced self-assembly of mixed poly(methyl methacrylate) (PMMA)/polystyrene (PS) brushes through a combinatorial approach. Gradient-mixed PMMA/PS brushes were synthesized from a gradient-mixed initiator-terminated monolayer by combining atom transfer radical polymerization (ATRP) and nitroxide mediated radical polymerization (NMRP) in a two-step process. The gradient-mixed initiator-terminated monolayer was fabricated by first formation of a gradient in density of an ATRP initiator through vapor diffusion followed by backfilling of an NMRP-initiator-terminated trichlorosilane. After treatment of a gradient-mixed brush whose PS Mn was slightly lower than that of PMMA with glacial acetic acid, a selective solvent for PMMA, relatively ordered nanodomains were observed in the region where the ratio of PS to PMMA grafting density (number of polymer chains/nm2) was in the range from 0.67 to 2.17 and the overall grafting density was approximately 0.85 polymer chains/nm2. Contact angle hysteresis were high (> or =40 degrees ) in this region and XPS studies confirmed that the PMMA chains were enriched at the outermost layer. The nanodomains are speculated to be of a micellar structure with PS chains forming the core shielded by PMMA chains.

Self-reorganization of mixed poly(methyl methacrylate)/polystyrene brushes on planar silica substrates in response to combined selective solvent treatments and thermal annealing

This article reports reversible changes in surface morphology and surface wettability of mixed poly(methyl methacrylate) (PMMA)/polystyrene (PS) brushes with PS Mn slightly lower than that of PMMA in response to combined selective solvent treatments and thermal annealing. The mixed brushes were synthesized from asymmetric difunctional initiator-terminated monolayers by controlled radical polymerizations. Two sets of experiments were performed. The first set involved treating the samples with glacial acetic acid (AA), a selective solvent for PMMA, followed by treatment with cyclohexane, a selective solvent for PS, or thermal annealing. The surfaces were rough and composed of relatively ordered nanodomains after AA treatment. Subsequent cyclohexane treatment or thermal annealing made the surfaces flattened and surface analysis indicated that the outermost layers were occupied by PS. The second set of experiments involved treating the samples with cyclohexane or thermal annealing, followed by AA treatment. The surfaces were smooth after cyclohexane treatment or thermal annealing and became very rough in the subsequent AA treatment. The nanodomains formed in AA treatment were speculated to be micelles with PS forming the core shielded by PMMA chains.

Solvent-induced self-assembly of mixed poly(methyl methacrylate)/polystyrene brushes on planar silica substrates: molecular weight effect.

The effect of molecular weight on the solvent-induced self-assembly of mixed poly(methyl methacrylate) (PMMA)/polystyrene (PS) brushes on silicon wafers was studied. For a series of mixed brushes with a fixed PMMA M(n) and systematically changed PS M(n), a transition in water advancing contact angle (theta(a)) from 74 degrees, the value for a flat PMMA surface, to 91 degrees, the value for a flat PS film, was observed with increasing PS M(n) after treatment with CHCl(3). Atomic force microscopy studies showed smooth surfaces for all samples. While no significant changes in surface morphologies were observed after treatment with cyclohexane, a selective solvent for PS, contact angle and XPS studies indicated that the mixed brushes with a PS M(n) slightly smaller than that of PMMA underwent self-reorganization, exhibiting a different theta(a). Intriguing surface morphologies composed of relatively ordered nanoscale domains were found from mixed brushes with PS M(n) slightly smaller than or similar to that of PMMA after treatment with acetic acid, a selective solvent for PMMA. The nanodomains are speculated to be of a micellar structure, with PS chains forming a core shielded by PMMA chains.

Self-assembled patterns of iron oxide nanoparticles by hydrothermal chemical-vapor deposition

Here, we report a hydrothermal chemical-vapor deposition process, which produces self-assembled patterns of iron oxide nanoparticles. By exposing a planar silica substrate to a prevaporized mixture of water, ferrocene [Fe(C5H5)2] and xylene (C8H10), at temperatures of ∼1000 °C, Fe2O3 nanoparticles are deposited on the substrate surface, in regular circular patterns. The particle sizes are less than 100 nm, and are organized into submicron-size patterns. The same process without water produces arrays of carbon nanotubes catalyzed by iron nanoparticles that are formed by the decomposition of ferrocene molecules.

Creation of Radial Patterns of Carbonated Silica Fibers on Planar Silica Substrates

Creation of Radial Patterns of Carbonated Silica Fibers on Planar Silica Substrates

Lateral Diffusion of Molecules Partitioned into Silica-Bound Alkyl Chains: Influence of Chain Length and Bonding Density

Lateral diffusion of a hydrophobic fluorescent molecule partitioned into monomeric alkyl chains bound to a planar silica substrate was measured as a function of chain density and chain length. Measurement of fluorescence recovery after patterned photobleaching was used to observe the diffusional relaxation of a concentration profile of probe molecules over distances of micrometers. The diffusion rate of the probe molecule partitioned into C-18 chains decreased with decreasing chain coverage. As the chain length was reduced from C-18 to C-8 and C-4, the rate of diffusion also decreased. These results, when combined with results from a previous study of the effect of overlaying solvent on diffusion rate (Hansen, R. L.; Harris, J. M. Anal. Chem. 1995, 67, 492-498), are consistent with a domain model for long-range transport of partitioned molecules through the bound ligands. Fluorescence recovery experiments in which diffusion is monitored over a distance of micrometers offer a unique means to probe long-range structure of surface-immobilized alkyl chains.