Stability Of Self-assembled Monolayers Research Articles

Noncovalent Self-Assembled Monolayers on Graphene as a Highly Stable Platform for Molecular Tunnel Junctions.

Monolayer graphene is used as the bottom electrode to fabricate stable and high-quality self-assembled monolayer (SAM)-based tunnel junctions. The SAMs are formed on graphene via noncovalent bonds without altering the structure of the graphene. This work paves the way to new types of molecular electronic junctions based on 2D materials.

1-Dodecanethiol based highly stable self-assembled monolayers for germanium passivation

As a typical semiconductor material, germanium has the potential to replace silicon for future-generation microelectronics, due to its better electrical properties. However, the lack of stable surface state has limited its extensive use for several decades. In this work, we demonstrated highly stable self-assembled monolayers (SAMs) on Ge surface to prevent oxidization for further applications. After the pretreatment in hydrochloric acid, the oxide-free and Cl-terminated Ge could be further coated with 1-dodecanethiol (NDM) SAMs. The influence factors including reaction time, solvent component and reaction temperature were optimized to obtain stable passivated monolayer for oxidation resistance. Contact angle analysis, atomic force microscopy, ellipsometer and X-ray photoelectron spectroscopy were performed to characterize the functionalized Ge surface respectively. Meanwhile, the reaction mechanism and stability of thiols SAMs on Ge (111) surface were investigated. Finally, highly stable passivated NDM SAMs on Ge surface could be formed through immersing oxide-free Ge in mixture solvent (water/ethanol, v/v=1:1) at appropriately elevated temperature (∼80°C) for 24h. And the corresponding optimized passivated Ge surface was stable for more than 10 days even in water condition, which was much longer than the data reported and paved the way for the future practical applications of Ge.

Formation of highly stable self-assembled alkyl phosphonic acid monolayers for the functionalization of titanium surfaces and protein patterning.

A protocol for the preparation of improved phosphonate monolayers on a titanium (Ti) substrate is presented. Zirconium ions were used to enhance the bonding between the phosphonate headgroup and the pretreated Ti surface. Contact angle and X-ray photoelectron spectroscopy were used to characterize self-assembled monolayers (SAMs) of alkylphosphonic acid that formed spontaneously on Zr-mediated Ti (Zr/Ti) surfaces. The surfaces that were treated with an aqueous solution of zirconium oxychloride showed significantly enhanced stability in buffer compared with those formed directly on the native oxidized Ti. A bifunctional molecule, 10-mercaptodecanyl phosphonic acid (MDPA), was also used to form SAMs on Zr/Ti surfaces using an identical method, which enabled us to regulate the surface functionality through the terminal functional group. Protein patterning on the surface was carried out using UV irradiation through a mask to selectively degrade regions of the MDPA molecules. The surface was then backfilled with a protein-resistant molecule in the exposed regions followed by selective immobilization of proteins to the unexposed areas using a heterobifunctional linker molecule. The presented strategy significantly improved the stability of the phosphonate SAMs on oxidized Ti surfaces, which provided an ideal approach foundation for biomolecular immobilization and patterning onto the Ti surfaces. Thus, this method provided a versatile platform to activate the surfaces of biomedical Ti implants.

Erratum: Corrigendum: Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold

Nature Chemistry 6, 409–414 (2014); published online 23 March 2014; corrected after print 21 May 2014.10.1038/nchem.1891 In the version of this Article originally published, ref. 40 was incorrect, it should have read: Rodriguez-Castillo, M. et al. Reactivity of gold nanoparticles towards N-heterocyclic carbenes.

Construction of octadecyltrichlorosilane self-assembled monolayer on stainless steel 316L surface

Alkyltrichlorosilane self-assembled monolayer (SAM) was regarded as the most stable SAM on silicon/silica substrate, but it was rarely coated on stainless steel 316L (SS316L) substrate surface due to the serious surface corrosion caused by the hydrochloric acid released during the hydrolysis process of alkyltrichlorosilane molecules. In this paper, a novel technical route aiming at constructing solid octadecyltrichlorosilane (OTS) SAM on SS316L surface was put forward. By prolonging the storage time of OTS–toluene reaction solution and adding a moderate amount of alkali into the reaction solution, a well-ordered and integrated OTS SAM was successfully coated on SS316L surface. The obtained OTS SAM was characterized by contact angle, atomic force microscopy (AFM), scanning electron microscope (SEM) and attenuated total reflectance Fourier transform infrared (ATR-FTIR). The long-term in vitro stability study of OTS SAM on SS316L showed that, even immersed in physiological saline or phosphate buffered saline (PBS) solution at 37°C for up to 60 days, the sample surface still kept a typical hydrophobicity.

Erratum: Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold

Erratum: Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold

Stable self-assembled monolayers

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Formation, Characterization, and Stability of Methaneselenolate Monolayers on Au(111): An Electrochemical High-Resolution Photoemission Spectroscopy and DFT Study

We investigated the mechanism of formation and stability of self-assembled monolayers (SAMs) of methaneselenolate on Au(111) prepared by the immersion method in ethanolic solutions of dimethyl diselenide (DMDSe). The adsorbed species were characterized by electrochemical measurements and high-resolution photoelectron spectroscopy (HR-XPS). The importance of the headgroup on formation mechanism and the stability of the SAMs was addressed by comparatively studying methaneselenolate (MSe) and methanethiolate (MT) monolayers. Density Functional Theory (DFT) calculations were performed to identify the elementary reaction steps in the mechanisms of formation and decomposition of the monolayers. Reductive desorption and HR-XPS measurements indicated that a MSe monolayer is formed at short immersion times by the cleavage of the Se-Se bond of DMDSe. However, the monolayer decomposes at long immersion times at room temperature, as evidenced by the appearance of atomic Se on the surface. The decomposition is more pronounced for MSe than for MT monolayers. The MSe monolayer stability can be greatly improved by two modifications in the preparation method: immersion at low temperatures (-20 °C) and the addition of a reducing agent to the forming solution.

Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold.

Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold-carbon bond relative to that of a gold-sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.

Tetra-anionic porphyrin loading onto ZnO nanoneedles: A hybrid covalent/non covalent approach

A stepwise surface functionalization procedure, based on hybrid covalent and non-covalent approach is herein proposed to anchor tetra-anionic meso-tetrakis(4-sulfonatophenyl)porphyrin on ZnO nanorods. Carboxyalkylphosphonic acids have been proven effective to form stable self-assembled monolayers through the surface grafting of –PO3H2 headgroups. The exposed carboxylic functionalities are suitable for the successful grafting of cationic poly-l-lysine that drives, in water, the non-covalent anchoring of the anionic porphyrin. A stepwise surface characterization, provided by X-ray photoelectron spectroscopy, elucidates the multilayers deposition and surface composition after each process step, thus, giving interesting insights on the chemical speciation of the exposed functionalities. UV–vis spectroscopy confirms the role of ZnO morphology to increase the porphyrin loading onto the investigated surfaces. The proposed approach is effective to achieve deposition of anionic porphyrins on ZnO nanostructures and combines the robustness of covalent functionalization with the versatility and full reversibility of the non-covalent strategies.

Comparative study of stability of phosphonate self-assembled monolayers on indium–tin oxide electrodes prepared using different methods

It is essential to obtain stable self-assembled monolayers (SAMs) for their applications to electrochemical (bio)sensors. Two commonly used methods for preparing phosphonate SAMs were compared in detail for the first time. SAMs of phosphonates with different alkyl chains were formed on indium–tin oxide (ITO) electrodes using the dipping method and the T-BAG method (tethering by aggregation and growth). In addition, two different post-assembly washing methods were assessed. The stability of the SAMs measured by their charge-transfer blocking abilities were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The SAMs were tested to assess their stability against ultrasonic washing and their long-term stability in phosphate-buffered saline. Only the phosphonate with the longest alkyl chain (octadecylphosphonic acid, ODPA) was stable to the ultrasonic washing, with the charge-transfer blocking ability of SAMs prepared from decyl- and hexadecylphosphonic acid (DPA and HDPA) being significantly reduced after the process owing to damage of the monolayer. Moreover, the ODPA SAMs gave similar X-ray photoelectron spectroscopic data, irrespective of the method of preparation and washing process used, providing further evidence of the stability of this monolayer. An increase in the length of the alkyl chain of the phosphonate (i.e., the length of the dielectric SAMs) decreased the double-layer capacitance and increased the charge-transfer resistance (blocking ability) against a redox reaction of Fe(CN)63-/Fe(CN)64-. After 7days of immersion in phosphate-buffered saline, the ODPA SAMs prepared by the dipping method maintained their blocking ability to a greater extent than those prepared using the T-BAG method.

Investigating phosphonate monolayer stability on ALD oxide surfaces

We report a series of studies aimed at investigating the stability of phosphonate self-assembled monolayers (SAMs) made from octadecylphosphonic acid (ODPA) or a perfluorinated phosphonic acid (PFPA) on hafnium and aluminum oxide surfaces deposited by atomic layer deposition (ALD). The monolayers were deposited by a series of techniques including self-assembly from solution, tethering by aggregation and growth, and the Langmuir–Blodgett (LB) method. SAMs prepared by LB method were primarily used in our stability investigations because they were found to be the most uniform and reproducible. All films deposited on ALD oxide-coated substrates were characterized by means of water contact angle measurements, spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). XPS data conclusively showed covalent phosphonate formation on both substrates. SAMs formed on both Al2O3 and HfO2 were stable upon exposure to water. PFPA SAMs on HfO2 were found to be the most stable SAMs studied here in either water or phosphate buffer (PBS) at room temperature. We also show that similar silane-based SAMs made from octadecyltrichlorosilane (OTS) on silicon oxide (SiO2) are less stable in PBS than phosphonate SAMs on atomic layer deposited HfO2 substrates. These data suggest that phosphonate SAMs should be considered for use in (bio)molecular sensing and actuator devices that utilize ALD and require longer-term stability under aqueous conditions.

Ideal Redox Behavior of the High-Density Self-Assembled Monolayer of a Molecular Tripod on a Au(111) Surface with a Terminal Ferrocene Group

A dyad consisting of a tripod-shaped trithiol with an adamantane core and a terminal ferrocenyl group linked through ap-phenyleneethynylene bridge was synthesized. The trithiol formed a stable self-assembled monolayer (SAM) on Au(111), wherein each molecule is bound to the surface by three-point adsorption using all sulfur atoms, with confirmation by PM-IRRAS and XPS analyses. Cyclic voltammetry of the SAM showed a line shape typical of an ideal adsorbed system, that is, a monolayer with negligible electrostatic interaction among the terminal ferrocenyl groups. Thus, a rare SAM was achieved, in which the component molecules were isolated from adjacent molecules without the coadsorption of nonelectroactive molecules.

Nanometer-Wide Lines of Self-Assembled Monolayer: A Molecular Dynamics Simulation Study

A molecular dynamics simulation was performed to study the nanometer-wide lines carved out of a self-assembled monolayer (SAM) of octadecanethiol on gold. This simulation is relevant to the nanoscale SAM patterns created by nanolithography. The molecular packing structure in the SAM line was compared with that in a bulk SAM. A stable SAM line must be at least 1.7 nm wide, and two lines can merge if they are less than 3.0 nm apart. This presumably sets the ultimate resolution of SAM patterns. A finite length of the SAM line or the crossing of two lines further destabilizes the SAM line.

Extremely Strong Self-Assembly of a Bimetallic Salen Complex Visualized at the Single-Molecule Level

A bis-Zn(salphen) structure shows extremely strong self-assembly both in solution as well as at the solid-liquid interface as evidenced by scanning tunneling microscopy, competitive UV-vis and fluorescence titrations, dynamic light scattering, and transmission electron microscopy. Density functional theory analysis on the Zn(2) complex rationalizes the very high stability of the self-assembled structures provoked by unusual oligomeric (Zn-O)(n) coordination motifs within the assembly. This coordination mode is strikingly different when compared with mononuclear Zn(salphen) analogues that form dimeric structures having a typical Zn(2)O(2) central unit. The high stability of the multinuclear structure therefore holds great promise for the development of stable self-assembled monolayers with potential for new opto-electronic materials.

Study of Alkylthiolate Self-assembled Monolayers on Au(111) Using a Semilocal meta-GGA Density Functional

We present a density functional theory study of the structure and stability of self-assembled monolayers (SAMs) of alkylthiolate on Au(111) as a function of the alkyl chain length. The most favorable structure of the SAMs involves an RS–Au–SR complex (S being sulfur, R being an alkyl chain) formed through sandwiching one Au adatom by two alkylthiolates (RSs). Comparing a generalized gradient (GGA-PBE) and a meta-GGA (MGGA-M06-L) exchange-correlation functional we find that only the meta-GGA functional predicts the experimentally observed attractive intermolecular interactions within the SAMs. In particular, the use of M06-L yields an increased stability of the SAMs with increasing alkyl chain length and an increased attractive interaction between RS–Au–SR complexes at shorter distances.

Controlled Thicknesses of Vaporized Self-Assembled Multilayers on Copper Nanopowders Under Ultra-High Vacuum (UHV)

Copper nanoparticles were coated with 1-octanethiol self-assembled monolayers (SAMs) using the dry-coating method for oxidation prevention. In this study, thicknesses of 1-octanethiol SAMs were successfully controlled, and the stability of SAMs as a passivation layer on copper nanoparticles was examined. Thicknesses of 1-octanethiol SAMs varied with vacuum levels and coating cycles. Under low-vacuum conditions, the thickness was 10 nm, regardless of the coating conditions. In contrast, various thicknesses resulted under ultra-high vacuum (UHV) and ranged from 4 nm to 10 nm. SAMs that were nearly a monolayer thick (4 nm) resulted from two coating cycles of 1.5 min, and the oxidation inhibition period was 15 days. Thus, the dry-coating method successfully controlled the thicknesses of SAMs with satisfactory oxidation inhibition properties under ultra-high vacuum.

Fabrication of Self-Assembled Monolayer Based on Phenylethynylbenzene and Fluorescent Probe Pyrene and Detecting for Nitroanilines

Self-Assembled monolayers (SAMs), a new type of organic ultra-thin films, show a lot of interesting properties in chemistry, physics, optics and electronic transport, and have potential application in biomaterials, nanoelectro-mechanical systems, sensors, and so on. In this paper, a full-conjugated unit-containing SAM ended with a functional amino group (NH2) was fabricated by self-assembly and using a light-sensitive diazonium-containing compound 4-(4-amino phenylene ethynyl) benzenic diazonium salt (APEB- 2 N + ). The APEB- 2 N + was easily deposited on a pre-treated quartz substrate with negative charges by electrostatic interaction and then grafted on the surface of a quartz substrate through covalent bond after the de- composition of diazonium group under UV irradiation, and a stable SAM based on phenylethynylbenzene was obtained. Fur- thermore, based on the reactivity of the functional group NH2, the fluorescent probe pyrene is anchored on the surface of the full-conjugated SAM through two approaches: amidation reaction between amino group on the SAM and carboxyl group of 1-pyrenebutyric acid, and self-assembly of 2 N + , which is transferred from NH2 on SAM, and electronegative COO - of 1-pyrenebutyric acid in basic aqueous solution. The obtained ultra-thin films with fluorescence probe pyrene exhibit lumi- nescent property because of the emission of excimer of pyrene chromophore, which is mainly due to the formation of the g-g conformation of pyrene for the induction effect of the full-conjugated SAM. The electron-acceptor nitroanilines (NAs) can quench the fluorescence of the obtained ultra-thin film because of electron and energy transfer between the electron-rich pyrene and NAs. The fluorescence quenching varies obviously for the different isomers of NA, and the quenching efficiency decreases according to the sequence of p-NA, o-NA, m-NA, because the electronic effect varies obviously when the relative position of electron withdrawing group NO2 and donating group NH2 of NA are different. The fluorescence of the ultra-thin film decreases with the increasing of the NA concentration. Keywords conjugated self-assembled monolayer; pyrene; photochemistry of diazonium salts; luminescent property; detec- tion of nitroaniline

Thiol-functionalized anthraquinones: mass spectrometry and electrochemical studies.

In this work, the synthesis of various thiol-functionalized anthraquinone compounds is presented. The studied compounds were characterized by mass spectrometry and the main fragmentation pathways are discussed. The compounds studied formed stable self-assembled monolayers (SAMs) in the gold surface. The parameters for the reduction processes in the gold surface of the studied new anthraquinones were determined by cyclic voltamperometry tests.Graphical abstract

Self-Assembly of Protein Monolayers Engineered for Improved Monoclonal Immunoglobulin G Binding

Bacterial outer membrane proteins, along with a filling lipid molecule can be modified to form stable self-assembled monolayers on gold. The transmembrane domain of Escherichia coli outer membrane protein A has been engineered to create a scaffold protein to which functional motifs can be fused. In earlier work we described the assembly and structure of an antibody-binding array where the Z domain of Staphylococcus aureus protein A was fused to the scaffold protein. Whilst the binding of rabbit polyclonal immunoglobulin G (IgG) to the array is very strong, mouse monoclonal IgG dissociates from the array easily. This is a problem since many immunodiagnostic tests rely upon the use of mouse monoclonal antibodies. Here we describe a strategy to develop an antibody-binding array that will bind mouse monoclonal IgG with lowered dissociation from the array. A novel protein consisting of the scaffold protein fused to two pairs of Z domains separated by a long flexible linker was manufactured. Using surface plasmon resonance the self-assembly of the new protein on gold and the improved binding of mouse monoclonal IgG were demonstrated.