Thermally induced electrode protection against biofouling

This paper demonstrates that a combined thermal and electrochemical conditioning step can greatly minimize electrode blocking. We detected 50 ppm dopamine after a blocking step in 1000 ppm gelatine solution. Only a treatment of the electrode at −1.5 V and 61.5 °C can reveal the voltammetric dopamine signals to 82%. The increase of the peak separation of the cyclic voltammograms obtained in 50 ppm dopamine is limited to 14%, whereas negative polarization (−1.5 V) alone leads to a 31% increase compared to 109% upon thermal and 105% without any conditioning. The positive effects can be addressed to an enforced reductive degradation and accelerated removal of the blocking agents. Also the formation of hydrogen bubbles might play a significant role. Thermo-electrochemical treatment holds great promise for electrochemical sensors and detectors which are applied for long-term monitoring of samples that contain blocking matrices.     

Mesoporous silica thin film mechanized with a DNAzyme-based molecular switch for electrochemical biosensing

A novel electroanalytical strategy for copper and ascorbic acid detection was developed by using a nanostructured electrode surface mechanized with a DNAzyme-based molecular gate. This sensing interface was constructed by first electrodeposition of a mesoporous silica thin film on Au electrodes and further assembly of a Cu(II)-specific DNAzyme. The biosensing assay was based on the Cu(II) and ascorbic acid responsible activation of the DNAzyme, which acted as a molecular switch able to control the diffusion of the Fe(CN)₆^{3 −/4 −} electrochemical probe through the nanochannels of the mesoporous film.     

Mass transport and electrode accessibility through periodic self-assembled nanoporous silica thin films

Ordered nanoporous silica films have attracted great interest for their potential use to template nanowires for photovoltaics and thermoelectrics. However, it is crucial to develop films such that an electrode under the nanoporous film is accessible to solution species via facile mass transport through well-defined pores. Here, we quantitatively measure the electrode accessibility and the effective species diffusivity for nearly all the known nanoporous silica film structures formed by evaporation-induced self-assembly upon dip-coating or spin-coating. Grazing-angle of incidence small-angle X-ray scattering was used to verify the nanoscale structure of the films and to ensure that all films were highly ordered and oriented. Electrochemical impedance spectroscopy (EIS) was then used to assess the transport properties. A model has been developed that separates the electrode/film kinetics and the film transport properties from the film/solution interface and bulk solution effects. Accounting for this, the accessible area of the nanoporous film coated FTO electrode (1-theta) is obtained from the high-frequency data, while the effective diffusivity of the ferrocene dimethanol (D(FDM)) redox couple is obtained from intermediate frequencies. It was found that the degree of order and orientation in the film, in addition to the symmetry/topology, is a dominant factor that determines these two key parameters. The EIS data show that the (211) oriented double gyroid, (110) oriented distorted body center cubic, and (211) distorted primitive cubic silica films have significant accessibility (larger than 26% of geometric area). However, the double-gyroid films showed the highest diffusivity by over an order of magnitude. Both the (10) oriented 2D hexagonal and (111) oriented rhombohedral films were found to be highly blocking with only small accessibility due to microporosity. The impedance data were also collected to study the stability of the nanoporous silica films in aqueous solutions as a function of pH. The distorted primitive silica film showed much faster degradation in pH 7 solution when compared to a blocking film such as the 2D hexagonal. However, silica films maintained their structure at pH 2 for at least 12 h.     

Mesoporous silicon oxynitride thin films

Highly-ordered, pore-modified with amine groups, and glass-like mesoporous silicon oxynitride thin films were prepared by heat treatment of as-synthesized mesoporous silica thin films in a flowing ammonia environment at high temperatures.     

Mesoporous silica thin films as a spatially extended probe of interfacial electric fields for amplified signal transduction in surface plasmon resonance spectroscopy

A new simpler concept about the signal amplification of surface plasmon resonance (SPR) that is based on the utilization of mesoporous silica thin films is demonstrated. As compared to monolayer based coatings, mesoporous silica thin films of approximately 200 nm extend the interaction arena away from the metal, thus permitting the integration of the change in optical contrast at different distances from the sensor surface.     

Evaporation induced self-assembly of templated silica and organosilica thin films on various electrode surfaces

A new one-step method is reported for the deposition of hybrid mesoporous thin films on various electrode surfaces (gold, platinum, glassy carbon). Deposition was achieved by spin-coating sol–gel mixtures in the presence of a surfactant template to get mesostructured thin layers on the various conducting substrates. Film formation occurred by evaporation induced self-assembly (EISA) involving the hydrolysis and (co)condensation of silane and/or organosilane precursors on the electrode surface. Extraction of the surfactant from the ordered mesoporous films led to a large increase of mass transport rates into the materials and imparted high accessibility to the organic moieties in case of functionalized mesoporous overlayers. The electrochemical properties of the film-modified electrodes have been studied by cyclic voltammetry (CV), and also via the chemical accumulation of mercury ions prior to their stripping analysis by differential pulse voltammetry (i.e. for thiol-functionalized thin films). Some evidences to support the formation of self-assembled monolayers (SAMs) on electrodes, have been also discussed. The formation of well-adhering mesoporous thin films on solid electrode surfaces is expected to have a high impact on the development of new electrochemical sensors.     

Mesoporous aluminophosphate thin films with cubic pore arrangement

Mesoporous aluminophosphate thin films with 3D cubic (Im3m) pore arrangement were synthesized for the first time. Thin films were templated with block copolymer nonionic templates Pluronic F127 and F108 and deposited on a glass substrate by dip-coating. In situ SAXS investigations show the formation of a highly ordered mesostructure upon the dip-coating process, which remains stable up to at least 670 K. A cubic mesostructure was observed also by TEM. Template removal process was monitored by TG and FT-IR. A transition from an amorphous aluminophosphate gel to a well-defined aluminophosphate framework was observed by MAS NMR.     

Manipulation of molecular transport into mesoporous silica thin films by the infiltration of polyelectrolytes

The design of hybrid mesoporous materials incorporating polymeric assemblies as versatile functional units has become a very fertile research area offering major opportunities for controlling molecular transport through interfaces. However, the creation of such functional materials depends critically on our ability to assemble polymeric units in a predictable manner within mesopores with dimensions comparable to the size of the macromolecular blocks themselves. In this work, we describe for the first time the manipulation of the molecular transport properties of mesoporous silica thin films by the direct infiltration of polyelectrolytes into the inner environment of the 3D porous framework. The hybrid architectures were built up through the infiltration-electrostatic assembly of polyallylamine (PAH) on the mesopore silica walls, and the resulting systems were studied by a combination of experimental techniques including ellipso-porosimetry, cyclic voltammetry and X-ray photoelectron spectroscopy, among others. Our results show that the infiltration-assembly of PAH alters the intrinsic cation-permselective properties of mesoporous silica films, rendering them ion-permeable mesochannels and enabling the unrestricted diffusion of cationic and anionic species through the hybrid interfacial architecture. Contrary to what happens during the electrostatic assembly of PAH on planar silica films (quantitative charge reversal), the surface charge of the mesoporous walls is completely neutralized upon assembling the cationic PAH layer (i.e., no charge reversal occurs). We consider this work to have profound implications not only on the molecular design of multifunctional mesoporous thin films but also on understanding the predominant role of nanoconfinement effects in dictating the functional properties of polymer-inorganic hybrid nanomaterials.     

Mesoporous hybrid thin films: the physics and chemistry beneath

Mesoporous films containing organic or biological functions within an organised array of cavities are produced by combining sol-gel, self-assembly of supramolecular templates and surface chemistry. This paper reviews the essential physics and chemical concepts behind the synthesis of these complex multifunctional materials.     

Gelatine thin films as biomimetic surfaces for silica particles formation

The formation of silica nanostructures by several living organisms, such as diatoms or sponges, involves specific macromolecules that control the growth and the organization of silica nanoparticles. In order to investigate if a single molecular system could perform both particle size control and morphological template, gelatine thin films of various concentration and strength were prepared as biomimetic models and their reactivity towards sodium silicate aqueous solutions was studied. Simultaneous formation of silica particles in the nanometric and micrometric size range was observed. The former corresponds to colloids grown at the surface of the gelatine films and the latter to particles induced by gelatine chain brushes formed at the film/water interface. These results are in good agreement with well-known principles of biomineralization and suggest that multi-molecular systems, rather than single components, are responsible for biogenic silica nanostructure formation.     

Imaging of crystal morphology and molecular simulations of surface energies in pentacene thin films

We have investigated the crystal growth of the organic semiconductor pentacene by complementing molecular simulations of surface energies with experimental images of pentacene films. Pentacene thin films having variations in thickness and grain size were produced by vacuum sublimation. Large (approximately 20 microm) faceted crystals grew on top of the underlying polycrystalline thin film. The films were characterized using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Single crystals most commonly grew in a truncated diamond shape with the largest crystal face, (001), growing parallel to the substrate. Crystal morphologies and surface energies were calculated using force field-based molecular simulations. The (001) surface was found to have the lowest energy, at 76 mJ/m(2), which was consistent with experimental observations of crystal face size. It was demonstrated that the morphology of the large faceted crystals approached the equilibrium growth shape of pentacene. From contact angle measurements, the critical surface tension of textured pentacene thin films in air was determined to be 34 mJ/m(2).     

Spectroscopic consideration of the surface potential built across phthalocyanine thin films on a metal electrode

The nonlinear optical properties of tert-butyl phthalocyanine copper Langmuir-Blodgett (CuttbPc LB) films and vacuum-evaporated phthalocyanine copper (CuPc) films deposited on a metal surface were investigated by second-harmonic generation (SHG) spectroscopy. At the organic/metal interface, a space charge field is formed due to the presence of excess charge injected from a metal electrode to the organic layer. Since the Pc molecule has D4h symmetry, an inversion center is present and the optical SH process is not allowed under the electric-dipole approximation. However, the space charge field at the interface directly influences the symmetric structure of the electrons in the Pc molecule. We investigated the contributions of the surface potential to the SHG using Pc LB and vacuum-evaporated films deposited on aluminum (Al) and gold (Au) metal electrodes, where a distinctive difference in the spectrum for the Pc films on the Al and Au surfaces was observed. The contribution of the surface potential was revealed based on the resonant conditions of the SH process, taking into account the electric-quadrupole transition and dc-field-induced electric-dipole transition.     

Preparation and characterization of mesoporous silica thin films from polyethoxysiloxanes

Tetraethoxysilane (TEOS) and polyethoxysiloxanes (PEOSs; prepared by the acid‐catalyzed hydrolytic polycondensation of TEOS) were subjected to the sol–gel process in the presence of cetyltrimethylammonium bromide (CTAB), respectively. The PEOSs with M_w 700–26,000, as prepared by sol–gel coating of TEOS and PEOS under various conditions, were used. Uniform and crack‐free thin films of thickness 276–613 nm were prepared by spin‐coating of a PEOS solution containing CTAB. When the coating films were sintered at 400 °C, the combustion of ethoxy groups and CTAB took place to provide porous silica thin films. The structure of the thin films was found to be dependent on the molecular weight of PEOS and the molar ratio of CTAB/Si: lamellar or hexagonal phase was observed for M_w less than 15,000 and for CTAB/Si molar ratios greater than 0.10. Honeycomb structures were observed for M_w less than 5000 and for CTAB/Si molar ratios of 0.15. The honeycomb structure was also observed by atomic force microscopy and transmission electron microscope. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2542–2550, 2006     

Protein UTLC-MALDI-MS using thin films of submicrometer silica particles

Slides for ultra thin-layer chromatography (UTLC) were made by coating nonporous silica particles, chemically modified with polyacrylamide, as 15 μm films on glass or silicon. Three proteins, myoglobin, cytochrome c and lysozyme, are nearly baseline resolved by the mechanism of hydrophilic interaction chromatography. A plate height as low as 3 μm, with 3900 plates, is observed in 14 mm. Varying silica particle diameter among 900, 700 and 350 nm showed that decreasing particle diameter slightly improves resolution but slows the separation. Matrix-assisted laser desorption/ionization (MALDI)-MS of the proteins after separation is demonstrated by wicking sufficient sinapinic acid into the separation medium.     

Pure silica BETA colloidal zeolite assembled in thin films

Pure silica nanoscale zeolite BETA with monomodal particle size distribution was synthesized from a colloidal precursor solution and successfully applied for the preparation of hydrophobic ultrathin films on silicon wafers via spin coating.     

Elaboration of thin colloidal silica films with controlled thickness and wettability

Silica films with controlled thickness and wettability have been formed by sequential adsorption of colloidal silica nanoparticles and a cationic polyelectrolyte (poly(allylamine hydrochloride) or poly(diallyldimethylammonium chloride)) was used as the binding agent. Whatever be the conditions used, the structure of films appeared dense and non-porous. Thicknesses varying from 12 to 430 nm and wettability varying from 5 to 60° were obtained when the pH or concentration of the silica solution was varied. Quartz crystal microbalance measurements evidenced the formation of regular and reproducible thin films mainly composed of silica nanoparticles. These films contained few polycations due to the formation of long-distance charge pairs between silica nanoparticles and polycations.     

Three-dimensional molecular packing of thin organic films of PTCDI-C8 determined by surface X-ray diffraction

We have determined the full molecular 3D packing of thin organic films of the archetypical organic n-type semiconductor N, N'-dioctyl-3,4:9,10-perylene tetracarboxylic diimide (PTCDI-C 8) by surface X-ray crystallography. We show that PTCDI-C 8 forms smooth layered films on Al 2O 3 (11-20) with an outstanding degree of molecular order. The thin-film structure is found to consist of a triclinic unit cell with the plane of the aromatic core tilted by 67 +/- 2 degrees with respect to the surface plane, which differs significantly from the bulk structure. The 3D crystallites extend with vertical coherent order across the entire film thickness.     

Mesoporous and biocompatible surface active silica aerogel synthesis using choline formate ionic liquid

In this paper, we report the preparation and characterization of mesoporous and biocompatible transparent silica aerogel by the sol-gel polymerization of tetraethyl orthosilicate using ionic liquid. Choline cation based ionic liquid allows the silica framework to form in a non collapsing environment and controls the pore size of the gel. FT-IR spectra reveal the interaction of ionic liquid with surface -OH of the gel. DSC thermogram giving the evidence of confinement of ionic liquid within the silica matrix, which helps to avoid the shrinkage of the gel during the aging process. Nitrogen sorption measurements of gel prepared with ionic liquid exhibit a low surface area of 100.53 m2/g and high average pore size of 3.74 nm. MTT assay proves the biocompatibility and cell viability of the prepared gels. This new nanoporous silica material can be applied to immobilize biological molecules, which may retain their stability over a longer period.     

Mesoporous silica as nanoreactor for olefin polymerization

Various metal-containing MCM-41(Metal-MCM-41) were prepared by the post-synthesis method with organometallic compound or alkoxide and used as a nanoreactor for olefin polymerization. Strong Lewis acid sites generated on Metal-MCM-41 could activate the metallocene catalyst rac-ethylene(bisindenyl)zirconium dichloride (rac-Et(Ind)₂ZrCl₂) effectively, resulting in the formation of active sites for polymerizations of ethylene and propylene. This suggests that Metal-MCM-41 is useful as a heterogenized cocatalyst. Ti-, Zr-, Hf-, Mn- and Zn-MCM-41 combined with alkylaluminum (without metallocene catalyst) were also found to provide isotactic polypropylene with a broad molecular weight distribution. By analyzing the characteristics of polypropylenes both inside and outside the mesopores, the polymerization behavior under extreme confined geometry was discussed.