Phosphonic Acid Monolayers Research Articles

Modification of silica by an organic monolayer in aqueous medium using octylphosphonic acid and aluminium species

In the present work we report a “green” method for the grafting of organic monolayers at the surface of silica nanoparticles using water as a solvent. This method is based on the use of water-stable phosphonic acid coupling molecules grafted on an intermediate layer of aluminium species, as the sensitivity of Si–O–P bonds toward hydrolysis precludes a direct anchoring onto silica. Two approaches were explored: anchoring of octylphosphonic acid on aluminated silica and one-pot modification of silica by AlCl3 then by octylphosphonic acid. The modified powders were characterized using elemental analysis, 31P and 27Al MAS NMR spectroscopy, FTIR spectroscopy and N2 physisorption. The one-pot approach appears particularly promising, as it allows the anchoring on silica of phosphonic acid monolayers with controlled densities in a significantly shorter time.

Reduction of 3T3 fibroblast adhesion on SS316L by methyl-terminated SAMs

Inhibiting the non-specific adhesion of cells and proteins to biomaterials such as stents, catheters and guide wires is an important interfacial issue that needs to be addressed in order to reduce surface-related implant complications. Medical grade stainless steel 316L was used as a model system to address this issue. To alter the interfacial property of the implant, self-assembled monolayers of long chain phosphonic acids with –CH 3, –COOH, and –OH tail groups were formed on the native oxide surface of medical grade stainless steel 316 L. The effect of varying the tail groups on 3T3 fibroblast adhesion was investigated. The methyl-terminated phosphonic acid significantly prevented cell adhesion however presentation of hydrophilic tail groups at the interface did not significantly reduce cell adhesion when compared to the control stainless steel 316L.

Theoretical study of the surface modification of indium tin oxide with trifluorophenyl phosphonic acid molecules: impact of coverage density and binding geometry

The interface composed of 3,4,5-trifluorophenyl phosphonic acid (PA) molecules chemisorbed on the indium-tin oxide (ITO) surface was studied at the density functional theory level. The impact of coverage density and binding geometry of the PA molecules on the ITO surface characteristics has been investigated systematically by increasing the PA surface coverage density from one to four PA molecules per surface unit cell. The calculations point to several bonding schemes where the phosphonate (PO3) moieties are directly bonded to the surface In/Sn atoms and/or form hydrogen bonds with the hydroxyl groups present on the ITO surface. Both PA coverage density and binding geometry strongly influence the work-function modifications of the ITO surface upon chemisorption. The oxygen core-level binding energy shifts have been calculated and compare favorably with the experimental X-ray photoelectron spectroscopy data. Substantial depolarization effects are observed within the PA monolayer at high coverage densities.

The impact of self-assembled monolayer thickness in hybrid gate dielectrics for organic thin-film transistors

We have investigated the electrical characteristics of hybrid dielectrics with a thickness of 6 nm or less that are composed of a plasma-grown aluminum oxide (AlO x ) layer and a self-assembled monolayer (SAM) of an aliphatic phosphonic acid. The impact of the quality of the AlO x layer on the insulating properties of the double-layer dielectrics was assessed by comparing two different oxidation procedures, and the influence of the thickness of the organic SAM was evaluated by employing molecules with five different chain lengths. In order to decouple the relative contributions of the oxide and the SAM to the performance of the double-layer dielectrics we have also performed cyclic voltammetry measurements on indium tin oxide (ITO)/SAM devices without AlO x layer. Finally, we have evaluated how the quality of the AlO x layer and the thickness of the SAM affect the performance of low-voltage organic thin-film transistors (TFTs) that employ the thin AlO x /SAM dielectrics as the gate dielectric. The results confirm the important role of the SAM in determining the breakdown voltage, in limiting the current density, and in compensating the somewhat lower quality of AlO x layers produced under mild plasma conditions.

Photopatterning, Etching, and Derivatization of Self-Assembled Monolayers of Phosphonic Acids on the Native Oxide of Titanium

Electron-hole pair formation at titania surfaces leads to the formation of reactive species that degrade organic materials. Here, we describe the degradation of self-assembled monolayers of alkylphosphonic acids on the native oxide of titanium following exposure to UV light. The rate of degradation was found to decrease as the length of the adsorbate molecule increased. Increasing order in the monolayer, resulting from the enhanced dispersion forces between longer adsorbates, impedes the progress of oxygen-containing molecules to the oxide surface and slows the rate of oxidation. Rates of degradation on titanium oxide are substantially greater than rates of degradation on aluminum oxide because of the photocatalytic effect of the titanium oxide substrate. Micrometer-scale patterns may be fabricated readily using a UV laser in conjunction with a mask, and nanometer-scale patterns may be fabricated using a scanning near-field optical microscope coupled to a UV laser. Photodegraded adsorbates may be replaced by contrasting molecules to yield chemical contrast. Such patterned materials have been utilized to fabricate patterns from polymer nanoparticles. The resist character is switchable--at lower exposures, the monolayer behaves as a positive tone resist, but at higher exposures, it exhibits negative tone behavior. Patterned samples may also be utilized as resists for solution-phase etching of the underlying substrate.

Polystyrene Formation on Monolayer-Modified Nitinol Effectively Controls Corrosion

A surface-initiated polymerization of styrene on carboxylic acid terminated phosphonic monolayers was utilized to increase the corrosion resistance of nitinol and nickel oxide surfaces. Alkyl chain ordering, organic reactions, wettability, and film quality of the monolayers and polymers were determined by infrared spectroscopy, atomic force microscopy, matrix-assisted laser desorption ionization spectrometry, and water contact angles. The polystyrene film proved to be a better corrosion barrier than phosphonic acid monolayers by analysis with cyclic voltammetry and electrochemical impedance spectroscopy. The protection efficiency of the polystyrene film on nitinol was 99.4% and the monolayer was 42%.

Theoretical Characterization of the Indium Tin Oxide Surface and of Its Binding Sites for Adsorption of Phosphonic Acid Monolayers

We present a theoretical characterization, based on density functional theory, of the indium tin oxide surface and of the nature of the binding sites that determine the adsorption of phosphonic acid monolayers. The validity of our theoretical description is assessed by comparing our first-principles results to X-ray photoelectron spectroscopy data.

High-Field 17O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania

High-field 17O MAS NMR was used to investigate the binding of self-assembled monolayers of 17O-enriched phosphonic acids deposited on a titania anatase support. The spectra were recorded at two different magnetic fields (9.4 and 17.6 T), to improve the reliability of the simulations of the different resonances. The spectra recorded at 17.6 T offer an excellent resolution between the different oxygen sites, P═O, P−O−H, and P−O−Ti, thus greatly facilitating their quantification. The data reported here give direct evidence of the extensive formation of Ti−O−P bonds in the surface modification of titania by phosphonic acids. The presence of residual P═O and P−O−H sites indicates the presence of several different binding modes in phosphonic acid monolayers. The chemical shift of P−O−Ti sites is consistent with bridging (as opposed to chelating) modes.

Micrometer and Nanometer Scale Photopatterning of Self-Assembled Monolayers of Phosphonic Acids on Aluminum Oxide

Self-assembled monolayers of alkylphosphonic acids (APA) have been patterned at the micrometer and nanometer scale by photochemical methods. Exposure of APA monolayers to light with a wavelength of 244 nm leads to scission of the C−P bond and desorption of the alkyl chain from the surface. Immersion of the specimen in a solution of a second APA leads to adsorption at the surface in the exposed regions. Immersion in a solution of hydroxide ions leads to etching of the exposed regions of the aluminum film. UV exposure appears to anneal the aluminum oxide film, conferring increased resistance to etching at high exposures and a switching the function of the monolayer from a positive to a negative tone resist. At intermediate exposures, nanoscale trenches form at the perimeters of exposed regions. Near-field exposure of the phosphonic acid structures, using a UV laser coupled to a scanning near-field optical microscope, yields well-defined nanostructured features following development of the pattern in aqueous base, exemplified by the fabrication of 100 nm wide trenches.

Towards a Nano-Scale Electrochemical and Spectroscopic Characterization of Organic Layers on Oxide Surfaces

The presented works prepares the ground for galvanic deposition of metallic overlayers on metal oxide surfaces functionalized by phosphonic acid monolayers. In view of this goal, it is necessary to control the most important factors of the electrodeposition process (e.g. flatness and uniformity of distribution). Because of these requirements, in this work, we study the morphology and the interaction between PO(OH)2-functionalities and an oxide surface. The analysis is carried out using complementary surface analysis techniques, like Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). We have succeeded in creating a blank surface with a RMS roughness of less than 1 nm and a hydroxyl fraction of 17%. The organic deposition was followed in time and it was observed that Volmer Weber multilayer growth was present. Also the influence of carbon contamination was characterized and a methodology is presented to correct the experimental data for this ambient contamination.

Phosphonic Acid Monolayers for Binding of Bioactive Molecules to Titanium Surfaces

Two different phosphonic acid monolayer films for immobilization of bioactive molecules such as the protein BMP-2 on titanium surfaces have been prepared. Monolayers of (11-hydroxyundecyl)phosphonic acid and (12-carboxydodecyl)phosphonic acid molecules were produced by a simple dipping process (the T-BAG method). The terminal functional groups on these monolayers were activated (carbonyldiimidazole for hydroxyl groups and N-hydroxysuccinimide for carboxyl groups) to bind amine-containing molecules. The reactivity of the surfaces was investigated using trifluoroethylamine hydrochloride and BMP-2. Each step of the surface modification procedure was characterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry.

Formation of Self-Assembled Monolayers of Alkylphosphonic Acid on the Native Oxide Surface of SS316L

Phosphonate-steel interactions have been industrially significant for decades, but details of the phosphonate-steel interface have not yet been characterized. Self-assembled monolayers of phosphonic acids were formed on stainless steel 316L by room-temperature solution deposition. The acids are covalently bound to the surface as phosphonates in a bidentate manner, as determined by diffuse reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Complete coverage of the surface is confirmed by contact angle measurement and atomic force microscopic imaging. This method of monolayer formation contrasts the requirement for heating and long reaction times found to be necessary to form phosphonate monolayers on other metal oxide substrates, such as titanium and silicon.

Thermal stability of organic monolayers chemically grafted to minerals

The thermal degradation of monolayers of alkylsilicon hydrides (RSiH 3), alkylphosphonic acids (RP(O)(OH) 2), and alkyldimethylchlorosilanes (R(CH 3) 2SiCl) supported on silica, alumina, titania, zirconia, and calcium hydroxyapatite particles was investigated using thermal gravimetric analysis (TGA). The common feature in the TGA was a dramatic weight loss between ∼200 and ∼600 °C (N 2 atmosphere), which was not present for bare minerals, and, thus, was assigned to the degradation of the surface grafted organic species. The onset of weight loss and the temperature of maximal weight loss rate ( T MAX ) showed no dependence on the mineral and were determined by the nature of the chemical group directly attached to the surface. The T MAX values were ∼ 400 – 450 °C (monolayers of RSiH 3 and RP(O)(OH) 2) and ∼250 °C (monolayers of RSi(CH 3) 2Cl). The activation energies ( E A ) of the degradation processes were determined using TGA at multiple heating rates. Surprisingly, the substrate material did not show a significant effect on E A . For example, all the monolayers of octadecyl groups (R = C 18H 37) supported on silica, alumina, titania, and zirconia showed closed E A values ∼ 265 ± 15 kJ / mol . The weight loss, IR, MS, and chemical analysis data suggested that monolayer degradation occurred through pyrolysis of the hydrocarbon moieties (R groups) via cleavage of the C C and Si C (monolayers of silanes) or C C and P C bonds (monolayers of phosphonic acids). The inorganic portion of the molecule remained attached to the surface of the mineral (presumably as silicates or phosphates).

Attachment of Polymer Films to Aluminium Surfaces by Photochemically Active Monolayers of Phosphonic Acids

AbstractSummary: We describe a simple photochemical process that allows the covalent attachment of a variety of different polymers at room temperature onto aluminium surfaces. The system is based on a photoreactive benzophenone derivative that is bound to aluminium surfaces by a phosphonic acid anchor. The synthesis of the phosphonic acid is described and the immobilization of this compound is studied by X‐ray photoelectron and FT‐IR spectroscopy. After deposition of the polymeric coating, UV light illumination at 365 nm, and solvent extraction of the substrate, polymer monolayers are obtained that are chemically bound to the surface.Covalent attachment of polymers to the aluminium‐bound benzophenone phosphonic acid.magnified imageCovalent attachment of polymers to the aluminium‐bound benzophenone phosphonic acid.

Comparative study of self‐assembly of a range of monofunctional aliphatic molecules on magnetron‐sputtered aluminium

AbstractThe adsorption of a range of organic molecules from toluene onto the oxidized surface of magnetron‐sputtered aluminium metal is studied using sessile drop water contact angle measurements. Molecules with different head group functionalities and various chain lengths are considered, including alkyl carboxylic acids, alkyl phosphonic acids, alkyl amines, alkyl trimethoxysilanes, alkyl trichlorosilanes and epoxy alkanes. Alkyl phosphonic and carboxylic acids are identified as readily forming the most well‐packed monolayers on the aluminium surface, whereas the others adsorb less well and the chlorosilanes polymerize as a result of combination with moisture to form a thick deposit. The high‐adsorption‐density monolayers of alkyl phosphonic and carboxylic acids were studied using polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) and x‐ray photoelectron spectroscopy (XPS): PM‐IRRAS reveals relatively poorer ordering of the C10 alkyl carboxylic acid monolayer compared with that formed from the phosphonic acid, and XPS data suggest that this is likely to relate to a lower ability to displace preadsorbed volatile organic compounds. Copyright © 2004 John Wiley & Sons, Ltd.

Langmuir films of ( α-amino) phosphorus amphiphiles on various ion-containing subphases

Monolayers of amphiphilic ( α-amino)phosphonocarboxylic and ( α-amino)phosphonic acids have been formed by adsorption at the air/water interface. The influence of both the ionic strength and the pH of the subphase on the stability and compactness of the monolayers have been studied. The stability and the compactness of the Langmuir films are enhanced by introduction of metallic ions such as Ca 2+ or Mg 2+ in the subphases. These effects are more pronounced with Ca 2+. These metal ions can form dimeric complexes with the phosphorus moieties of the surfactant polar heads and therefore bring the amphiphiles closer. For the less hydrophobic derivative, complexation with Ca 2+ or Mg 2+ is required to ensure the formation of a stable monomolecular film. For both phosphonocarboxylic and phosphonic compounds, models have been proposed to represent the complexation phenomenon at the air/water interface.

Bodycote International Prize Paper Competition: Shortlisted Characterisation of Alkyl Phosphonic Acid Monolayers Self Assembled on Hydrated Surface of Aluminium

Self assembly of alkyl phosphonic acids (APAs) on magnetron sputtered aluminium (MS-Al) has been investigated with particular attention on the role of the aluminium surface chemistry produced by conditioning through ambient atmospheric and ethanoic methyl phosphonic acid solution immersion. For the longest molecule investigated, decyl phosphonic acid, contact angle measurement revealed rapid formation of relatively disordered self assembled monolayers (SAMs) from a 5 mM ethanoic solution. Evidence of a reaction of the phosphonic acid head group in a condensation reaction with the hydrox yls at the alumina surface to form a phosphonate was obtained using polarisation modulation infrared reflection absorption spectroscopy (PM-IRRAS). The concentration of hydrox yl groups in the ox ide at the surface of the M S-Al, determined by X-ray photoelectron spectroscopy (XPS), increased on immersion in ethanol owing to the presence of dissolved impurity water. Adsorption of hydrocarbon impurities is also evident. Preadsorption of methyl phosphonic acid inhibited ox ide growth and stabilised the wettability of the MS-Al stored in ethanol, although it did not increase the order of self assembled decyl phosphonic acid (DPA) compared with conditioning of the surface by atmospheric exposure.

Elimination of cross-talk and modulation of function in an organized heterosupramolecular assembly.

A close-packed monolayer of TiO2 nanocrystals was deposited on a conducting glass support using Langmuir-Blodgett (LB) techniques and fired. A close-packed mixed monolayer of eicosyl phosphonic acid (I) and the viologen. 1,1'-dieicosyl-4,4'-bipyridinium dichloride (II) was then deposited on the TiO2 substrate, also using LB techniques. At sufficiently high dilutions of II in II a single viologen molecule is adsorbed with a known orientation at the surface of each nanocrystal. The resulting assembly was incorporated as the working electrode in an electrochemical cell. Under open circuit conditions, bandgap excitation of a TiO2 nanocrystal results in electron transfer to a viologen molecule. No electron transfer between the viologen molecules adsorbed at different nanocrystals is observed. At a positive applied potential, electron transfer following bandgap excitation is largely suppressed. Considered are the implications of these findings for the development of practical devices based on modulatable function addressable on the nanometer scale.

Self-Assembled Monolayers on Engineering Metals: Structure, Derivatization, and Utility

A methodology for the formation and derivatization of self-assembled monolayers (SAMs) of alkyl phosphonic acids on common engineering metals such as steel, stainless steel, aluminum, copper, and brass is described. This methodology is shown to be a versatile route for surface modification of such substrates. α,ω-Metal bisphosphonate SAMs are shown to be receptive to complexation by organic acids and acid-containing polymers such as fluoropolymers and ethylene-co-methacrylic acid. This latter attribute is exploited in the construction of polymer/SAM/metal interfaces of surprising durability. The durability of the interface is a strong function of the SAM chain length.

Structures of Monolayers of Long-Chain Aliphatic Acids Deposited on Metal, Conducting Glass, and Nanocrystalline Semiconductor Substrates Using Langmuir−Blodgett Techniques

The structures of monolayers of long-chain aliphatic carboxylic and phosphonic acids deposited, using Langmuir−Blodgett techniques, on metal (silver and gold), conducting glass (fluorine-doped tin oxide), and nanostructured semiconductor (titanium dioxide) substrates have been studied. In the course of these studies, the effects of pH, dissolved ions, and conditioning regime on the structure of the monolayer present at the air/water interface have been examined. Also examined have been the effects of different deposition conditions and different substrate materials on the structure of the deposited monolayer. These findings represent an extension of such studies to molecules, namely, phosphonic acids, and substrates, namely, nanostructured TiO2 films, of increasing scientific and technological importance.