CH3-terminated Self-assembled Monolayers Research Articles

Chemical Bath Deposition of Copper Sulfide on Functionalized SAMs: An Unusual Selectivity Mechanism.

We have investigated the chemical bath deposition (CBD) of CuS using thioacetamide on functionalized self-assembled monolayers (SAMs) using scanning electron and optical microscopies, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. For all SAMs studied, the amount of CuS deposited is strongly dependent on the bath pH and can be attributed to the interaction of the SAM terminal groups with the chalcogenide ions present in solution. For -CH3-terminated SAMs, there is a steady increase in the amount of CuS deposited with an increase in the bath pH because there is an increase in the concentration of chalcogenide ion. However, for -OH- and -COOH-terminated SAMs, we observe that the maximum amount of CuS is deposited at pH 10. We attribute this behavior to a competition between the repulsion of the chalcogenide ions by the negatively charged SAM terminal groups and an increase in the chalcogenide ion concentration with an increase in the bath pH. Using the interaction of the chalcogenide ions with the different SAM terminal functional groups, we demonstrate that CuS can be selectively deposited on the -CH3-terminated areas of patterned -OH/-CH3- and -COOH/-CH3-terminated SAMs.

Effect of Ethanolamines on the Electroless Deposition of Copper on Functionalized Organic Surfaces.

Electroless deposition (ELD) is widely used in industry to deposit metals because it is inexpensive and compatible with organic materials. The deposition rate and deposited film properties critically depend on the reducing agent, complexing agent, and bath pH and temperature as well as bath additives. We have investigated the role of ethanolamine additives in the ELD of copper using the reducing agent dimethylamine borane on -CH3- and -OH-terminated self-assembled monolayers (SAMs) adsorbed on gold. Three additives were studied: ethanolamine (EOA), diethanolamine (DEOA), and triethanolamine (TEOA). Both the chemical identity and concentration of the ethanolamine significantly affect the deposition process. We show that the Cu deposition rate is faster on -CH3-terminated surfaces than on -OH-terminated SAMs because of the stronger interaction of the ethanolamines with the hydroxyl terminal group. In contrast to physical vapor deposition and other ELD processes, Cu deposits atop methyl-terminated SAMs using TEOA. However, using EOA and DEOA, copper penetrates through -CH3-terminated SAMs to the Au/S interface. For -OH-terminated SAMs, copper is observed to penetrate through the SAM for all ethanolamines investigated. The amount of copper penetration through the SAM to the Au/S interface increases with ethanolamine concentration. These effects are attributed to an adsorption-inhibition mechanism and differences in the chelation of Cu2+ in the deposition bath.

Reaction Mechanism of Area-Selective Atomic Layer Deposition for Al2O3 Nanopatterns.

The reaction mechanism of area-selective atomic layer deposition (AS-ALD) of Al2O3 thin films using self-assembled monolayers (SAMs) was systematically investigated by theoretical and experimental studies. Trimethylaluminum (TMA) and H2O were used as the precursor and oxidant, respectively, with octadecylphosphonic acid (ODPA) as an SAM to block Al2O3 film formation. However, Al2O3 layers began to form on the ODPA SAMs after several cycles, despite reports that CH3-terminated SAMs cannot react with TMA. We showed that TMA does not react chemically with the SAM but is physically adsorbed, acting as a nucleation site for Al2O3 film growth. Moreover, the amount of physisorbed TMA was affected by the partial pressure. By controlling it, we developed a new AS-ALD Al2O3 process with high selectivity, which produces films of ∼60 nm thickness over 370 cycles. The successful deposition of Al2O3 thin film patterns using this process is a breakthrough technique in the field of nanotechnology.

Adsorption and Orientation of 3,4-Dihydroxy-l-phenylalanine onto Tunable Monolayer Films

3,4-Dihydroxy-l-phenylalanine (l-DOPA) is considered to be responsible for the mussel adhesion to a variety of surfaces. A molecular level understanding of the interactions between DOPA molecules and surfaces with different wettability and chemistry, however, posts significant challenges to control marine antifouling. Here, different self-assembled monolayers (SAMs) on gold surfaces were fabricated: (i) OH-, (ii) COOH-, and (iii) CH3-terminations. The effect of surface wettability and chemistry on the adsorption of DOPA upon the series of surfaces was investigated in situ, showing that the adsorbed mass was lower and the water content of DOPA layer was higher on hydrophilic surfaces (including OH- and COOH-terminated SAMs) than that on hydrophobic ones (including CH3-terminated SAMs and gold surface). Direct evidence regarding the DOPA orientation and the interaction between DOPA and film surfaces were obtained: on the OH-terminated surface a flexible and loose structure formed via coordinate hydrogen bonds of the hydroxyl end groups of the surface interacting with carboxyl groups of DOPA, while for the CH3-terminated surface, DOPA molecules mainly adopt a flat conformation due to the formation of hydrophobic “bonds” between the hydrophobic functional groups of alkyl chains on surface and aromatic rings of DOPA molecules. This study led a new insight into the adsorption mechanisms based on the adsorption processes and layer structures, and it proposed novel concepts for the design of antifouling and adhesive surfaces.

Effect of Amino-, Methyl- and Epoxy-Silane Coupling as a Molecular Bridge for Formatting a Biomimetic Hydroxyapatite Coating on Titanium by Electrochemical Deposition

The objective of this study was to determine the role of functional groups of silane coupling on bioactive titanium (Ti) surface by electrochemical deposition, and calcium phosphate (CaP) coating, as well as bone cell adhesion and proliferation. Methyl group ( CH3), amino group ( NH2), and epoxy group ( glyph name C(O)C) were introduced onto the bioactive Ti surface using self-assembled monolayers (SAMs) with different silane coupling agents as molecular bridges. The effect of the surface functional groups on the growth features of the CaP crystals was analyzed (including chemical compositions, element content, minerals morphology and crystal structure etc.). CH3-terminated SAMs showed a hydrophobic surface and others were hydrophilic by contact angle measurement; NH2-terminated SAMs showed a positive charge and others were negatively charged using zeta-potential measurement. Scanning electron microscopy results confirmed that flower-like structure coatings consisting of various pinpoint-like crystals were formatted by different functional groups of silane coupling, and the CaP coatings were multicrystalline consisting of hydroxyapatite (HA) and precursors. CaP coating of CH3-terminated SAMs exhibited more excellent crystallization property as compared to coatings of NH2 and C(O)C groups. In vitro MC3T3-E1 cells adhesion and proliferation were performed. The results showed that CaP coatings on silane coupling functionalized surfaces supported cell adhesion and proliferation. Thus, these functional groups of silane coupling on Ti can form homogeneous and oriented nano-CaP coatings and provide a more biocompatible surface for bone regeneration and biomedical applications.

Surface chemistry regulates valvular interstitial cell differentiation in vitro

The primary driver for valvular calcification is the differentiation of valvular interstitial cells (VICs) into a diseased phenotype. However, the factors leading to the onset of osteoblastic-like VICs (obVICs) and resulting calcification are not fully understood. This study isolates the effect of substrate surface chemistry on in vitro VIC differentiation and calcified tissue formation. Using ω-functionalized alkanethiol self-assembled monolayers (SAMs) on gold [CH3 (hydrophobic), OH (hydrophilic), COOH (COO−, negative at physiological pH), and NH2 (NH3+, positive at physiological pH)], we have demonstrated that surface chemistry modulates VIC phenotype and calcified tissue deposition independent of osteoblastic-inducing media additives. Over seven days VICs exhibited surface-dependent differences in cell proliferation (COO−=NH3+>OH>CH3), morphology, and osteoblastic potential. Both NH3+and CH3-terminated SAMs promoted calcified tissue formation while COO−-terminated SAMs showed no calcification. VICs on NH3+-SAMs exhibited the most osteoblastic phenotypic markers through robust nodule formation, up-regulated osteocalcin and α-smooth muscle actin expression, and adoption of a round/rhomboid morphology indicative of osteoblastic differentiation. With the slowest proliferation, VICs on CH3-SAMs promoted calcified aggregate formation through cell detachment and increased cell death indicative of dystrophic calcification. Furthermore, induction of calcified tissue deposition on NH3+ and CH3-SAMs was distinctly different than that of media induced osteoblastic VICs. These results demonstrate that substrate surface chemistry alters VIC behavior and plays an important role in calcified tissue formation. In addition, we have identified two novel methods of calcified VIC induction in vitro. Further study of these environments may yield new models for in vitro testing of therapeutics for calcified valve stenosis, although additional studies need to be conducted to correlate results to in vivo models. Statement of SignificanceValvular interstitial cell (VIC) differentiation and aortic valve calcification is associated with increased risk of mortality and onset of other cardiovascular disorders. This research examines effects of in vitro substrate surface chemistry on VIC differentiation and has led to the identification of two materials-based initiation mechanisms of osteoblastic-like calcified tissue formation independent of soluble signaling methods. Such findings are important for their potential to study signaling cascades responsible for valvular heart disease initiation and progression as well providing in vitro disease models for drug development. We have also identified a VIC activating in vitro environment that does not exhibit confluence induced nodule formation with promise for the development of tissue regenerating scaffolds.

Morphological control of PbS grown on functionalized self-assembled monolayers by chemical bath deposition.

We have investigated the chemical bath deposition (CBD) of PbS on functionalized alkanethiolate self-assembled monolayers (SAMs) using time-of-flight secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The deposition mechanism involves both cluster-by-cluster and ion-by-ion growth. The dominant reaction pathway and the chemical composition and morphology of the deposited layer are dependent on both the SAM terminal group and the experimental conditions. On -COOH-terminated SAMs, three types of crystallites are observed: nanocrystals formed by heterogeneous ion-by-ion growth, larger needle-like particles, and ~2 μm particles deposited by homogeneous cluster-by-cluster deposition. The nanocrystals nucleate at Pb(2+)-carboxylate surface complexes, and so strongly adhere to the substrate. On -OH- and -CH3-terminated SAMs, only the micrometer-sized particles are formed by a cluster-by-cluster deposition mechanism. These particles do not adhere strongly to the SAM surface and can be easily removed. SIMS and XPS analyses indicate that the larger needle-like crystals and micrometer-sized particles are composed of oxidized lead sulfide and lead oxides, while the nanocrystals are composed of ≥85% PbS. Using sonication-assisted CBD, we demonstrate that PbS is deposited by ion-by-ion growth alone on -COOH-terminated SAMs. The deposited film is more compact with a smaller grain size and is >90% PbS.

Control of molecular orientations of poly(3-hexylthiophene) on self-assembled monolayers: molecular dynamics simulations

We theoretically investigate the energetically favorable orientation of poly(3-hexylthiophene) (P3HT) on self-assembled monolayers (SAMs) using molecular dynamics simulations. The effects of different kinds of SAMs are studied by examining a CH3-terminated SAM with a hydrophobic surface and an NH2-terminated SAM with a hydrophilic surface. We also investigate dynamic behavior of the systems with limited numbers of P3HT molecules on the SAM surfaces. The important factors in controlling the molecular orientation are elucidated from these results. We demonstrate that the edge-on orientation is more energetically favorable than the face-on orientation on both SAMs. On the other hand, the face-on orientation gains more intermolecular interaction energy between the P3HT molecules and the SAMs. This energy gain is larger in the NH2-terminated SAM than the CH3-terminated SAM. A limited number of P3HT molecules prefer to take the face-on orientation rather than the edge-on orientation. Our theoretical results suggest that the molecular orientation of P3HT is controllable by tuning the conditions of the film formation process and the intermolecular interactions between the P3HT molecules and SAMs.

Work-Function Engineering of Graphene Electrodes by Self-Assembled Monolayers for High-Performance Organic Field-Effect Transistors.

We have devised a method to optimize the performance of organic field-effect transistors (OFETs) by controlling the work functions of graphene electrodes by functionalizing the surface of SiO2 substrates with self-assembled monolayers (SAMs). The electron-donating NH2-terminated SAMs induce strong n-doping in graphene, whereas the CH3-terminated SAMs neutralize the p-doping induced by SiO2 substrates, resulting in considerable changes in the work functions of graphene electrodes. This approach was successfully utilized to optimize electrical properties of graphene field-effect transistors and organic electronic devices using graphene electrodes. Considering the patternability and robustness of SAMs, this method would find numerous applications in graphene-based organic electronics and optoelectronic devices such as organic light-emitting diodes and organic photovoltaic devices.

Long-term stability of self-assembled monolayers on 316L stainless steel

316L stainless steel (316L SS) has been extensively used for making orthopedic, dental and cardiovascular implants. The use of phosphonic acid self-assembled monolayers (SAMs) on 316L SS has been previously explored for potential biomedical applications. In this study, we have investigated the long-term stability of methyl (−CH3) and carboxylic acid (−COOH)-terminated phosphonic acid SAMs on 316L under physiological conditions. The stability of SAMs on mechanically polished and electropolished 316L SS was also investigated as a part of this study. Well-ordered and uniform −CH3- and −COOH-terminated SAMs were coated on mechanically polished and electropolished 316L SS surfaces. The long-term stability of SAMs on 316L SS was investigated for up to 28 days in Tris-buffered saline (TBS) at 37 °C using x-ray photoelectron spectroscopy, atomic force microscopy and contact angle goniometry. A significant amount of phosphonic acid molecules was desorbed from the 316L SS surfaces within 1 to 7 days of TBS immersion followed by a slow desorption of molecules over the remaining days. The −COOH-terminated SAM was found to be more stable than the −CH3-terminated SAM on both mechanically and electropolished surfaces. No significant differences in the desorption behavior of SAMs were observed between mechanically and electropolished 316L SS surfaces.

Making Nanoflowerbeds: Reaction Pathways Involved in the Selective Chemical Bath Deposition of ZnS on Functionalized Alkanethiolate Self-Assembled Monolayers

We have investigated the chemical bath deposition (CBD) of ZnS on functionalized alkanethiolate self-assembled monolayers (SAMs) using time-of-flight secondary ion mass spectrometry and scanning electron microscopy. The reaction mechanism involves both cluster-by-cluster and ion-by-ion growth. The dominant reaction pathway is dependent on both the SAM terminal group and the experimental conditions. On -COOH-terminated SAMs, two types of crystallites are observed: approximately 500 nm nanoflowers formed by ion-by-ion growth, and larger approximately 2 mum crystallites formed by cluster-by-cluster deposition. The nanoflowers nucleate at Zn(2+)- carboxylate surface complexes. On -OH- and -CH(3)-terminated SAMs, only the larger crystallites are formed. These do not adhere strongly to the SAM surface and can be easily removed. Finally, we demonstrate that under appropriate experimental conditions ZnS selectively deposits on the -COOH-terminated SAM regions of -COOH/-CH(3)-patterned SAM surfaces, forming nanoscale "flowerbeds".

Behavior of Hydroxide Ions in Vicinity of Self-Assembled Monolayers of Alkanethiols on Metals

Interactions between self-assembled monolayers (SAMs) with various non-dissociative terminal groups were explored with an atomic force microscope (AFM). In the pure water, hydrophobic CH3-terminated SAMs showed long-range repulsion (> 50 nm) indicating that hydroxide ions are concentrated at the water-SAM interface. On the other hand, hydrophilic OH-terminated SAMs exhibited only short-range attraction and no repulsion originated from ions accumulated at the SAM-water interface. The interaction between SAMs of oligo(ethylene glycol)-terminated alkanethiols strongly depend on the substrate (Au and Ag). That is, the hydroxide ions were concentrated near the Au-supported SAM, whereas no accumulation of the ions near the Ag-supported SAM was observed. Our results demonstrated that not only the terminal groups but also the density of the molecules constituting the SAM critically govern the redistributions of ions at the SAM-water interfaces. [DOI: 10.1380/ejssnt.2009.601]

Monolayer-Directed Electrodeposition of Oxide Thin Films: Surface Morphology versus Chemical Modification

In this paper we describe the general role that surface morphology and chemical nature play in guiding the cathodic deposition of oxide thin films onto electrode surfaces. By use of a “featureless” stamp for microcontact printing (μCP), the pregrooved microfeatures (“mountains” and “valleys” representing the track trails) of recordable compact disc (CD-R) gold substrates can be selectively modified with OH- or CH3-terminated self-assembled monolayers (SAMs). For comparison, “flat” gold substrates were patterned with the above SAMs in parallel “microstrips” that are analogous to the CD-R substrate (but no height differences). Electrochemical deposition of zirconia thin films showed that, on the CD-R substrates, surface morphology (height difference) dominates over the blocking effects of the SAMs; that is, deposition occurred primarily on the mountains despite these sites being modified with organic monolayers. For flat gold substrates it was found that n-alkanethiolate SAMs block deposition in modified ar...

Biomaterial-mediated retroviral gene transfer using self-assembled monolayers

Biomaterial-mediated gene delivery has recently emerged as a promising alternative to conventional gene transfer technologies that focus on direct delivery of viral vectors or DNA-polymer/matrix complexes. However, biomaterial-based strategies have primarily targeted transient gene expression vehicles, including plasmid DNA and adenovirus particles. This study expands on this work by characterizing biomaterial properties conducive to the surface immobilization of retroviral particles and subsequent transduction of mammalian cells at the cell-material interface. Self-assembled monolayers (SAMs) of functionally-terminated alkanethiols on gold were used to establish biomaterial surfaces of defined chemical composition. Gene transfer was observed to be greater than 90% on NH 2-terminated surfaces, approximately 50% on COOH-functionalized surfaces, and undetectable on CH 3-terminated SAMs, similar to controls of tissue culture-treated polystyrene. Gene delivery via the NH 2-SAM was further characterized as a function of retrovirus coating time, virus concentration, and cell seeding density. Finally, SAM-mediated gene delivery was comparable to fibronectin- and poly- l-lysine-based methods for gene transfer. This work is significant to establishing safe and effective gene therapy strategies, developing efficient methods for gene delivery, and supporting recent progress in the field of biomaterial-mediated gene transfer.

Quantitative and morphological analysis of biofilm formation on self-assembled monolayers

In spite of intensive studies over the past two decades, the influence of surface properties on bacterial adhesion and biofilm formation remains unclear, particularly on late steps. In order to contribute to the elucidation of this point, we compared the impact of two different substrates on the formation of bacterial biofilm, by analysing bacterial amount and biofilm structure on hydrophilic and hydrophobic surfaces. The surfaces were constituted by NH 2- and CH 3-terminated self-assembled monolayers (SAMs) on silicon wafers, allowing to consider only the surface chemistry influence because wafers low roughness. A strain of Escherichia coli K12, able to produce biofilm on abiotic surfaces, was grown with culture durations varying from 4 h to 336 h on both types of substrates. The amount of adhered bacteria was determined after detachment by both photometry at 630 nm and direct counting under light microscope, while the spatial distribution of adhered bacteria was observed by fluorescence microscopy. A general view of our results suggests a little influence of the surface chemistry on adherent bacteria amount, but a clear impact on dynamics of biofilm growth as well as on biofilm structure. This work points out how surface chemistry of substrates can influence the bacterial adhesion and the biofilm formation.

Reactive metal contact at indium–tin–oxide/self-assembled monolayer interfaces

With the aim of improving the electrical and adhesion properties of the indium–tin–oxide (ITO) electrode/organic interface, we tested Cl- and CF3-terminated self-assembled monolayers (SAMs), which react with the indium atoms of the electrode, and compared the results to those obtained using a CH3-terminated SAM. The contact resistance of the interface between the Cl-terminated surface and the ITO electrode (1.5kΩ) was found to be much lower than that of the interface between the ITO and the CF3-terminated surface (21.3kΩ), which can be attributed to the higher dipole moment of the In–Cl complex compared to the In–F complex. In the ITO films deposited on the CH3-terminated surface, the contact resistance (138.0kΩ) was much higher than those of the reactive metal contacts because the ITO thin film deposited on the CH3-terminated surface does not react with the SAM.

Molecular dynamics simulation of the enhancement of cobra cardiotoxin and E6 proteinbinding on mixed self-assembled monolayer molecules

Molecular dynamics simulations are performed onn-alkinethiol self-assembled monolayers (SAMs) and their mixture on a gold surface so thatthe orientations of the binding of cobra cardiotoxin and E6 protein molecules can beselected using the mixing ratio of CH3-terminated SAMs with different chain lengths.The simulations suggest that a SAM surface with different mixing ratios mayprovide a possible platform for aligning protein molecules with a desired orientationand for enhancing the binding energy of the protein on the designed surface.

Surface design of monolayer-template for reproducible microfabrication of metal oxide film

This paper presents the optimum chemical template to microfabricate reproducibly the metal oxide film, i.e., tin oxide (SnO x ), on the substrate from aqueous solution of tin chloride utilizing “monolayer-template-patterning (MTP)” technique. Silicon substrates covered with self-assembled monolayers (SAMs) were photolithographically micropatterned to prepare the chemical templates of SAMs on SiO x /Si substrate. Three types of SAM-covered substrates, that is, CH 3-, CHO-, and COOH-terminated substrates, were investigated to find the most appropriate chemical template for the MTP of SnO x . On the CH 3-terminated SAM template, well-defined microstructures of SnO x were observed, while neither of others, i.e., CHO-SAM and COOH-SAM templates, has reproducibly resulted in micropatterned SnO x .

Local Packing Environment Strongly Influences the Frictional Properties of Mixed CH3- and CF3-Terminated Alkanethiol SAMs on Au(111)

Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).

Adsorption of polyacrylic acid on self-assembled monolayers investigated by single-molecule force spectroscopy

Single-molecule force spectroscopy was used to study the adsorption and desorption processes of individual polyacrylic acid (PAA) chains from self-assembled monolayers (SAMs) with different terminal groups (CH3, OH, COOH). PAA was grafted to the tip of an AFM cantilever and all measurements were recorded with the same tip. For the OH- and the COOH-SAM, the desorption force was found to be independent of the salt concentration for NaCl, as well as for CaSO4. On CH3-SAM, a repulsive electrostatic interaction was found with NaCl in solution whereas a slightly attractive electrostatic interaction occurred with CaSO4 in solution. On the OH- and the COOH-terminated SAM, the non-electrostatic contributions to the desorption forces are of comparable magnitude (∼50–65 pN) whereas the non-electrostatic contribution for the CH3-terminated SAM is significantly higher (∼80–90 pN). Additional experimental findings concerning the length, the total number of adsorption events and the number of multiple polymer strands adsorbed in parallel, support the assumption that this difference in adsorption force is due to a different quality of the solvent for the polyacrylic acid chains at the SAM–water interface. Comparative surface pressure measurements at the air–water interface revealed a surface energy for polyacrylic acid of 30 mJ m2 which would correspond to a force contribution of ∼10 pN to the desorption force. This is in good agreement with the experimental results on hydrophobic SAMs. For further characterization of the surfaces the pH was varied from 3 to 11 at constant salt concentration (for a NaCl concentration of 100 mM and a CaSO4 concentration of 5 mM, respectively). In the presence of NaCl, the desorption force was found to be independent of the pH for all three substrates. For CaSO4, the desorption force was pH-independent for the OH- and the CH3-SAM. In the case of the COOH-SAM, a jump from 60 to 90 pN was recorded at pH 8, which we attribute to the formation of salt bridges between the polyacrylic acid chain and the COOH-SAM at higher pH.