Carboxylic Self-assembled Monolayers Research Articles

Anchoring groups for ordered and highly stable monomolecular films on naturally oxidized aluminum – phosphonate versus carboxylate

Modification of metal oxide surfaces by self-assembled monolayers (SAMs) is attracting growing attention. Carboxylic (CA) and phosphonic (PA) acids are most popular anchoring groups in this context, applied to create hydrophobic coatings, biosensors, and organic field-effect transistors. The efficiency of these devices is crucially affected by the structural quality and stability of the SAM. In this context, we studied the effect of the preparation procedure (solvent, incubation time, and temperature) on the quality of PA SAMs on naturally oxidized aluminum. We demonstrate formation of PA SAMs matching structural quality of archetypical alkanethiols on gold. Next, we compare their stability with analogous CA SAMs. Our data show that PA SAMs not only exhibit much higher hydrolytic stability but are also capable to protect aluminum substrate from oxidation and degradation. Importantly, even small improvement of the PA SAMs quality, translates into enormous increase of their hydrolytic stability crucial for majority of applications. Finally, we demonstrate that the thermal stability of PA on oxidized aluminum is by ∼250 K higher than CA SAMs, and by ∼200 K higher than for thiols on gold, which makes PA SAMs an excellent candidate for organic electronics, where overheating problems and high-temperature fabrication procedures are common issues.

Lanthanide-Oligomeric Brush Films: From Luminescence Properties to Structure Resolution.

Lanthanide (Ln) based luminescent materials are experiencing an increasing interest in their applications in several fields. In this study, we report a series of new lanthanide-oligomeric brush films, supported on quartz substrates and prepared using a layer-by-layer method (LbL). Oligomeric brush films are composed of small oligomers from our previously reported coordination polymers [x-EuL] and [x-TbL] (with x = 1, 3, and 5 generations of Ln complexes), which are grown perpendicularly from a carboxylate self-assembled monolayer. Oligomers composed of our previously described helical lanthanide complex LnL (Ln: Eu and Tb) as a luminescent moiety and benzene-1,4-dicarboxylate acid (bdc) used as a linker. Mixed films having the fifth-generation Ln complexes composed of equimolar mixture of Eu and Tb ions were prepared. Oligomeric brush films are highly transparent and exhibited a colored emission under UV irradiation. Pure Ln (Eu or Tb) films showed a strong luminescence from the Ln ions. Their luminescent properties depended on the number of lanthanide layers in the films composed of the first to third generations of lanthanide complexes. Then, the increase of the complex layers induced no difference in the luminescent properties. An energy transfer from Tb to Eu ions in the mixed films indicated a short distance between lanthanide ions of a fifth layer. The structural analysis together with the observed luminescent properties and some previous studies allowed to clarify the disposition of the oligomers in the films.

Real-time monitoring of calcium carbonate and cationic peptide deposition on carboxylate-SAM using a microfluidic SAW biosensor.

A microfluidic biosensor with surface acoustic wave technology was used in this study to monitor the interaction of calcium carbonate with standard carboxylate self-assembled monolayer sensor chips. Different fluids, with and without biomolecular components, were investigated. The pH-dependent surface interactions of two bio-inspired cationic peptides, AS8 and ES9, which are similar to an extracellular domain of the chitin synthase involved in mollusc shell formation, were also investigated in a biological buffer system. A range of experimental conditions are described that are suitable to study non-covalent molecular interactions in the presence of ionic substances, such as, mineral precursors below the solubility equilibrium. The peptide ES9, equal to the mollusc chitin synthase epitope, is less sensitive to changes in pH than its counterpart AS8 with a penta-lysine core, which lacks the flanking acidic residues. This study demonstrates the extraordinary potential of microfluidic surface acoustic wave biosensors to significantly expand our experimental capabilities for studying the principles underlying biomineralization in vitro.

Kinetics of leucine-lysine peptide adsorption and desorption at -CH3 and -COOH terminated alkylthiolate monolayers

The kinetics of adsorption and desorption of two highly asymmetrical model peptides were studied at methyl- and carboxylic acid-terminated alkylthiolate self-assembled monolayer (SAM) surfaces on gold. The model peptides were leucine-lysine (LK), α-helical (LKα14), and β-strand (LKβ15) peptides that have a well-defined secondary structure with the leucines localized on one side and the lysines on the other side. These secondary structures were previously shown to be maintained after adsorption and to control LK peptide orientation on these surfaces. The kinetics of peptide adsorption were analyzed by surface plasmon resonance as a function of peptide solution concentrations at pH 7.4. Peptide desorption was measured by rinsing with a buffer at various times along the adsorption curve. Both peptides had a saturation coverage of approximately 1 ML (monolayer) on the carboxyl SAM. Both peptides exhibited mostly irreversible binding on both surfaces, but the LKα14 peptide showed some limited reversible binding. Reversibly bound peptides could be in the second adlayer interacting only with peptides in the first layer or peptides interacting with a partially covered adsorption site and therefore not able to fully bind to the SAM surface. The near complete lack of reversible binding for LKβ15 is possibly due to strong peptide-peptide hydrogen bonding in β-sheet structures within the adsorbed layer. For a given dose of either peptide, much less peptide adsorbed on the methyl SAMs. The adsorption rate of irreversibly bound LKα14 on carboxylic acid SAMs was first-order with respect to solution concentration. Both peptides showed nucleation-like adsorption kinetics on the carboxylic acid SAM, indicating that peptide-peptide bonding is needed to stabilize the adsorbed layer. Adsorption on the methyl SAM was much lower in quantity for both peptides and seemed to require prior aggregation of the proteins in solution, at least for LKβ15.

Study on chemical vapor deposited copper films on cyano and carboxylic self-assembled monolayer diffusion barriers

Thin copper films were produced by chemical vapor deposition using the precursor Cu IIbis-hexafluoroacetylacetonate on the SiO 2/Si substrate modified with cyano and carboxylic self-assembled monolayers (SAMs) as diffusion barriers. The characterizations of the deposited copper films were measured by various thin film analysis techniques, i.e., scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The comparison between copper deposited on SiO 2 and on the SAM-modified SiO 2 substrates indicates that the copper films tend to be deposited onto the SAM-modified substrate, which is further proved by the calculation results of the interaction energies of copper and the SAMs with density functional theory method.

An ultrathin polymer coating of carboxylate self-assembled monolayer adsorbed on passivated iron to prevent iron corrosion in 0.1 M Na 2SO 4

For preparing an ultrathin two-dimensional polymer coating adsorbed on passivated iron, a 16-hydroxyhexadecanoate ion HO(CH 2) 15CO 2 − self-assembled monolayer (SAM) was modified with 1,2-bis(triethoxysilyl)ethane (C 2H 5O) 3Si(CH 2) 2Si(OC 2H 5) 3 and octadecyltriethoxysilane C 18H 37Si(OC 2H 5) 3. Protection of passivated iron against passive film breakdown and corrosion of iron was investigated by monitoring of the open-circuit potential and repeated polarization measurements in an aerated 0.1 M Na 2SO 4 solution during immersion for many hours. The time required for passive film breakdown of the polymer-coated electrode was markedly higher in this solution than that of the passivated one, indicating protection of the passive film from breakdown by coverage with the polymer coating. The protective efficiencies of the passive film covered with the coating were extremely high, more than 99.9% in 0.1 M Na 2SO 4 before the passive film was broken down, showing prominent cooperative suppression of iron corrosion in the solution by coverage with the passive film and polymer coating. The polymer-coated surface was characterized by contact angle measurement and electron-probe microanalysis (EPMA). Prevention of passive film breakdown and iron corrosion for the polymer-coated electrode healed in 0.1 M NaNO 3 was also examined in 0.1 M Na 2SO 4.

Impact ignition of nano and micron composite energetic materials

Nanoscale reactive materials demonstrate three orders of magnitude reduced laser ignition sensitivity compared to their micron scale counterparts. Reduced ignition delay times have been reported for a variety of nano composites including aluminum combined with another metal (e.g., Ni), metallic oxides (e.g., MoO 3, Fe 2O 3, CuO, WO 3), or polymer binders such as polytetrafloroethylene (Teflon™). A new reported melt dispersion mechanism associated with nano-aluminum particle ignition explains the enhanced thermal ignition sensitivity associated with nano-aluminum composites. As a comparison to laser thermal ignition behaviors, this study focuses on impact induced ignition of nano- and micron-aluminum particle composites that also include Ni, MoO 3, or Teflon. A modified type-12 impact tester is used to measure ignition of the energetic composites to drop weight impact. Ignition is determined using high-speed thermal imaging. To assess the influence of the alumina passivation shell on the ignition dynamics, oxide-free nano-aluminum passivated with a perfluoroalkyl carboxylate self-assembled monolayer has also been investigated. Results show that the difference in ignition sensitivity between nano and micron composites is significantly greater with laser than with mechanical ignition and the melt dispersion mechanism does not describe impact ignition.

Protection of zinc from corrosion by coverage with a hydrated cerium(III) oxide layer and ultrathin polymer films of a carboxylate self-assembled monolayer modified with alkyltriethoxysilanes

Ultrathin films of one- and two-dimensional polymers were prepared on a zinc electrode coated with a hydrated cerium(III) oxide Ce 2O 3 layer by modification of a 16-hydroxyhexadecanoate ion HO ( CH 2 ) 15 CO 2 - ( HOC 16 A - ) self-assembled monolayer (SAM) with alkyltriethoxysilanes. The protective efficiency, P of the film was determined by polarization measurements of the electrodes bare and covered with the polymer films in an oxygenated 0.5 M NaCl solution. The respective P values for the one-dimensional polymers of the HOC 16A − SAM modified with octyltriethoxysilane C 8H 17Si(OC 2H 5) 3 (C 8TES) and octadecyltriethoxysilane C 18H 37Si(OC 2H 5) 3 (C 18TES) were 94.4 and 95.1%. The values for the two-dimensional polymers of the HOC 16A − SAM modified with 1,2-bis(triethoxysilyl)ethane (C 2H 5) 3Si(CH 2) 2Si(C 2H 5) 3 plus C 8TES and C 18TES were 95.4 and 96.5%, respectively, indicating higher than those of the one-dimensional polymers. Structures of the polymer films were characterized by contact angle measurement with a drop of water and X-ray photoelectron and FTIR reflection spectroscopies.