Self-assembled Monolayer Coatings Research Articles

Molecular-scale topographic design of multicomponent self-assembled monolayers for environmentally adaptable, lyophobic coating

HypothesisThe water wettability of a hydrophilic surface is dictated by the hydration of the region less than 1 nm below the surface plane, yet it hitherto remains a topic rarely touched upon how a subtle change in surface structural features at molecular level affects the surface wettability and its response to the environmental nature. ExperimentsBinary self-assembled monolayers (SAMs) consisting of plain and functional thiols were constructed, where the surface fraction of the functional thiols was varied from 0.1 to 0.3, 0.6, 0.8 and 1.0 and the difference in CH2 unit number between two types of thiols – defined as height difference between surface polar (OH, NH2, COOH, or H2PO3) and non-polar (CH3) groups – from 0 to 1, 3, and 5. The surface wettability of as-prepared binary SAMs and their surface energy were assessed in both air/water/solid and oil/water/solid triphasic systems. FindingsWith the relative height of surface H2PO3 groups being deliberately set as 3 in particular, as-prepared binary SAMs gain unique environmentally adaptable surface lyophobicity, namely, they can be both oleophobic in water and hydrophobic in oil. Thanks to this new environmentally adaptable surface lyophobicity, such binary SAM coating enables copper meshes to realize selective and efficient oil/water separation without need of pre-wetting. Both the polar and dispersion interaction components of surface energy of the binary SAM coatings are found crucial to effective oil/water separation of the coated copper meshes; the former defines critical intrusion pressure and the latter must be larger than 22 nN/m.

Self-Assembled Monolayer Coatings on Gold and Silica Surfaces for Antifouling Applications: A Review

The resistance of surfaces to biomaterial adsorption/adhesion is paramount for advancing marine and biomedical industries. A variety of approaches that involve bioinert materials have been developed to modify surfaces. Self-assembled monolayers (SAMs) are powerful platforms in which the surface composition is easily fabricated and a well-defined structure is provided; thus, the molecular-level interaction between biomolecules/biofoulants and the surface can be understood. In this review, we describe a wide variety of SAM structures on gold and silica surfaces for antifouling applications and the corresponding mechanism of nonfouling surfaces. Our analysis divides the surface properties of films into the following types: (1) hydrophilic, (2) hydrophobic, and (3) amphiphilic films.

A mechanistic study of Pt particle electrodeposition and growth on a self-assembled monolayer as an active template

• Application of monolayers on gold as templates for deposition of Pt particles. • Evaluation of particle growth using cyclic voltammetry and chronoamperometry. • Evaluation of growth mechanism with SH model using chronoamperometric deposition. • Evaluation of growth mechanism using time dependant image J analysis of the SEM. • Comparing mechanism of growth evaluated using SH model with SEM analysis. Platinum is an essential component in many commercial catalytic processes. However, due to its cost and availability considerable effort is made to control the amount, location, and accessibility of the platinum species on an electrode surface. The current study investigates a low-cost, and versatile method for the deposition of Pt particles using a self-assembled monolayer (SAM) as an active assisted template on a gold electrode. Alkane (HSCH 2 (CH 2 ) 8 CH 3 ), alcohol (HSCH 2 (CH 2 ) 9 CH 2 OH) and carboxylate (HSCH 2 (CH 2 ) 8 CH 2 COOH) terminated monolayers were used to probe the impact of hydrophobicity on particle size, population density and growth mechanism. Cyclic voltammetry (CV) and chronoamperometry (CA) were employed to induce particle growth. From the CA data a dimensionless theoretical plot was calculated based on the Scharifker and Hills model to establish the mechanism as either progressive or instantaneous. The time-dependent study revealed a progressive nucleation mechanism for Pt particles synthesized on all three electrode surfaces, whereas microscopy and image analysis revealed an instantaneous nucleation mechanism for the alkane template only and progressive nucleation pathways for the alcohol and carboxylate SAM surfaces. The results indicated that SAM templating could be used to effectively control particle growth in a predictable fashion, however there is a limitation in application of the existing model due to a combination of factors including the migration of the SAM monomers on the electrode surface, the hydrophobicity of the terminal moieties and the occurrence of secondary and subsequent nucleation and growth phases. In addition, post-particle deposition CV was used to determine the impact of the SAM template on the active area of the Pt particles available for catalysis and showed that the hydrophobic SAM coatings gave significantly higher activity.

Robust silane self-assembled monolayer coatings on plasma-engineered copper surfaces promoting dropwise condensation

Dropwise condensation is well known to result in better heat transfer performance owing to efficient condensate/droplet removal, which can be harnessed in various industrial heat/mass transfer applications such as power generation and conversion, water harvesting/desalination, and electronics thermal management. The key to enhancing condensation via the dropwise mode is thin low surface energy coatings (<100 nm) with low contact angle hysteresis. Ultrathin (<5 nm) silane self-assembled monolayers (or SAMs) have been widely studied to promote dropwise condensation due to their minimal thermal resistance and scalable integration processes. Such thin coatings typically degrade within an hour during condensation of water vapor. After coating failure, water vapor condensation transitions to the inefficient filmwise mode with poor heat transfer performance. We enhance silane SAM quality and durability during water vapor condensation on copper compared to state-of-the-art silane coatings on metal surfaces. We achieve this via (i) surface polishing to sub-10 nm levels, (ii) pure oxygen plasma surface treatment, and (iii) silane coating integration with the copper substrate in an anhydrous/moisture-free environment. The resulting silane SAM has low contact angle hysteresis (≈20°) and promotes efficient dropwise condensation of water for >360 hours without any visible sign of coating failure/degradation in the absence of non-condensable gases. We further demonstrate enhanced heat transfer performance (≈5-7× increase over filmwise condensation) over an extended period of time. Surface characterization data post-condensation leads us to propose that in the absence of non-condensable gases in the vapor environment, the silane SAM degrades due to reduction and subsequent dissolution of copper oxide at the oligomer-substrate interface. The experiments also indicate that the magnitude of surface subcooling (or condensation rate) affects the rate of coating degradation. This work identifies a pathway to durable dropwise promoter coatings that will enable efficient heat transfer in industrial applications.

A Study on Functional Hydrophobic Stainless Steel 316L Using Single-Step Anodization and a Self-Assembled Monolayer Coating to Improve Corrosion Resistance

Stainless steel fabricated using chromium is widely being used in various industries due to its superior corrosion resistance compared to light metals such as aluminum, titanium, and magnesium. However, despite its excellent properties, a problem of poor corrosion resistance in harsh environments remains. In this study, an economical and environmentally friendly anodizing process was applied to the surface of stainless steel (SUS 316L) to create porous nanostructures to improve its water-repellent properties. In these experiments, voltages of 30, 50, 70, and 90 V were applied to stainless steel for 3 h to form an oxide film, prior to immersion in 0.1 M phosphoric acid for 10 min to expand the oxide pores. In addition, the change of the oxide structure was observed through field-emission scanning electron microscopy (FE-SEM). In terms of the contact angle, hydrophilicity was observed at applied voltages of 70 and 90 V, in which a porous film was formed; the best water repellency was observed at a 90 V applied voltage, after the application of an FDTS (1H,1H,2H,2H-perfluorodecyltrichlorosilane) coating, a self-assembled monolayer. Finally, the corrosion behavior of a hydrophobic specimen was tested using potentiodynamic polarization (PDP) experiments. The hydrophobic SUS 316L alloy subsequently displayed improved corrosion resistance in a 3.5 wt% NaCl solution.

Critical Packing Density of Water-Mediated Nonstick Self-Assembled Monolayer Coatings.

Nanoparticle-mineral surface interactions are relevant in many biological and geological applications. We have previously studied nanoparticle coatings based on closely packed bicomponent polyol-fluoroalkane self-assembled monolayers (SAMs) that can have tunable stickiness on calcite surfaces by changing the compositions of fluoroalkanes in SAMs, where the coatings show nonstick properties if fluoroalkanes can effectively perturb hydration layers on calcite surfaces. However, when applying coatings on nanoparticles, it can be challenging to predict the maximum achievable coating density. Here, we study how would water-mediated SAM-calcite interactions change with different SAM coating densities. Molecular dynamics simulations show that compositionally repulsive, closely packed polyol-fluoroalkane SAMs become adhesive to calcite surfaces with decreasing coating densities. Our modeling shows that this results from the collapsing of fluoroalkanes into the voids of SAMs, where fluoroalkanes can no longer perturb hydration layers on calcite surfaces. Interestingly, we find that the nonstick-stick transition occurs when the volume fractions of the voids on SAMs are greater than the volume fractions of hydrophilic coating molecules.

Elucidating the Mechanism of Condensation-Mediated Degradation of Organofunctional Silane Self-Assembled Monolayer Coatings.

Dropwise condensation is favorable for numerous industrial and heat/mass transfer applications due to the enhanced heat transfer performance that results from efficient condensate removal. Organofunctional silane self-assembled monolayer (SAM) coatings are one of the most common ultrathin low surface energy materials used to promote dropwise condensation of water vapors because of their minimal thermal resistance and scalable synthesis process. These SAM coatings typically degrade (i.e., condensation transitions from the efficient dropwise mode to the inefficient filmwise mode) rapidly during water vapor condensation. More importantly, the condensation-mediated coating degradation/failure mechanism(s) remain unknown and/or unproven. In this work, we develop a mechanistic understanding of water vapor condensation-mediated organofunctional silane SAM coating degradation and validate our hypothesis through controlled coating synthesis procedures on silicon/silicon dioxide substrates. We further demonstrate that a pristine organofunctional silane SAM coating resulting from a water/moisture-free coating environment exhibits superior long-term robustness during water vapor condensation. Our molecular/nanoscale surface characterizations, pre- and post-condensation heat transfer testing, indicate that the presence of moisture in the coating environment leads to uncoated regions of the substrate that act as nucleation sites for coating degradation. By elucidating the reasons for formation of these degradation nuclei and demonstrating a method to suppress such defects, this study provides new insight into why low surface energy silane SAM coatings degrade during water vapor condensation. The proposed approach addresses a key bottleneck (i.e., coating failure) preventing the adoption of efficient dropwise condensation methods in industry, and it will facilitate enhanced phase-change heat transfer technologies in industrial applications.

Controlling water-mediated interactions by designing self-assembled monolayer coatings

Engineered nanoparticles have been broadly used in biological and geological systems. Hydrophilic molecules such as polyols have been used as coatings on nanoparticle surfaces due to their good biocompatibility and solubility in saline water. However, polyol coatings can cause huge retention of nanoparticles when encountering mineral surfaces. Here, molecular dynamics simulations enlightened that the strong adhesion of hydrophilic coatings to mineral surfaces stemming from the partitioning of the hydroxy groups on the hydrophilic molecules to the well-defined bound hydration layers on the mineral surfaces. To mitigate the nanoparticle adhesion, we investigated introducing small percentages of omniphobic fluoroalkanes to form a bicomponent system of hydrophilic and fluoroalkanes, which greatly perturbed the hydration layers on mineral surfaces and resulted in nonstick surface coatings. Our results provide important insight for the design of tunable “stickiness” nanoparticle coatings in different mineralogies, such as applications in subsurface environments or targeted delivery in mineralized tissues.

Antifouling Coatings Generated from Unsymmetrical Partially Fluorinated Spiroalkanedithiols

Biofouling negatively impacts modern society on a daily basis, especially with regard to the important industries of medicine, oil, and shipping. This manuscript describes the preparation and study of model antifouling coatings generated from the adsorption of unsymmetrical partially fluorinated spiroalkanedithiols on gold. The antifouling properties of the self-assembled monolayers (SAMs) derived from the spiroalkanedithiols were compared to SAMs derived from analogous monodentate partially fluorinated and nonfluorinated alkanethiols. The antifouling properties were evaluated using in situ surface plasmon resonance spectroscopy (SPR), ex situ electrochemical quartz crystal microbalance (QCM) measurements, and ex situ ellipsometric thickness measurements. The resistance to nonspecific protein adsorption of the SAMs was evaluated with proteins having a wide range of properties and applications including protamine, lysozyme, bovine serum albumin, and fibrinogen. The results from the SPR and the QCM measurements demonstrated that in most cases, the SAM coatings derived from the partially fluorinated spiroalkanedithiols having mixed hydrocarbon and fluorocarbon tail groups exhibited better antifouling performance when compared to the SAMs derived from their single-component monodentate counterparts. The studies also revealed that while the SPR and the QCM measurements in most cases were able to distinguish the adsorption trends for the SAMs and proteins examined, the ellipsometric thickness measurements were markedly less discriminating. On the whole, these studies validate the use of unsymmetrical partially fluorinated spiroalkanedithiols for generating effective antifouling coatings on metal substrates.

Robust Silane Self-Assembled Monolayer Coatings on Plasma-Engineered Copper Surfaces Promoting Dropwise Condensation

Robust Silane Self-Assembled Monolayer Coatings on Plasma-Engineered Copper Surfaces Promoting Dropwise Condensation

Addition of Aloe Vera As Corrosion Inhibitor in Self-Assembled Monolayers Systems Designed to Improve Adhesion on Stainless Steel

Surface treatment by phosphatation is widely used in automotive industry and equipment manufacturing [1] because it’s an excellent adhesion primer before painting and due to its lubrication properties with low friction coefficient [2]. It is also known that this treatment offers a good protection against corrosion with a good suitability to be painted. Nevertheless, this process is detrimental for the environment, mostly because of the large amount of waste to be treated, with the presence of concentrated acids, nitrates and nitrites. Moreover, the required thickness is in the range of 2-30 μm, depending of the specifications, and there is a great interest to reduce this value, especially for the fasteners parts.The present work deals with an alternative to steel/phosphate systems by thin organic coating on 304 stainless steel, a low carbon content substrate free of Molybdenum available at affordable costs. To fit with the specification while being as attractive as possible, this alternative would be asked to present a thickness reduced to a few nanometres, environmentally compliant, non-toxic and easy to use. It must keep the main properties of phosphatation i.e. a good adherence or anti-wear functions.A novel process was design, with Self-assembled Monolayers (SAMs) [3] coatings in a first step, followed by a functionalization with Aloe Vera as ecological corrosion inhibitor [4][5]. SAMs are tunable by adjusting either their terminal group or the length of the carbon chain to obtain the desired surface properties such as adhesion or anti-wear. If the addition of Aloe Vera does not bother the tribological behavior of the coating, this is not the case for the systems designed for adherence.To obtain the best compromise, three different terminal groups combined with four carbon chain lengths have been tested. The latest were measured by a single lap joint and three-point bending tests as for result the value of the strength of the bonded joint as a function of the treatments performed. Adhesion results will be presented for different SAM coatings on 304 stainless steel, with a possible application of epoxy layer (polymer used for cataphoresis). The influence of the SAM alkyl chains length and of the terminal groups nature will be checked, as well as the grafting time.Finally, the effect of post-graft rinsing on adhesion as well as corrosion resistance in presence of Aloe Vera or not were evaluated. All findings were supported by surface analysis (SEM, EDS).[1] T. W. COSLETT, Treatment of iron or steel for preventing oxidation or rusting, British patent 8667 and US patent 870937, 1906et 1907[2] J. BOGI, Phosphate conversion coatings on steel, J Mater Sci (1977) 12, 2235-2240[3] A. ULMAN, Formation and structure of self-assembled monolayers, Chem. Rev. 1996, 96, 1533-1554[4] Devarayan, Kesavan, Green Inhibitors for Corrosion of Metals: A Review, Chem Sci Rev Letts, (2012)[5] M. MEHDIPOUR, Electrochemical noise investigation of Aloe plant extract as green inhibitor on the corrosion of stainless steelin 1M H2SO4, J. Ind. Eng. Chem, (2015), 21, 318-327

Comparative study of multiple thiol-based self-assembled monolayer coatings for the SERS detection of nitrite, nitrate, and perchlorate anions in water.

Raman-based sensors represent a promising solution to enable both detection and fingerprinting of anionic pollutants in the water distribution network. Due to the weak nature of Raman scattering, a signal intensity enhancement mechanism, such as surface enhanced Raman spectroscopy (SERS), is required. Given the combination of SERS being a first layer effect and the low affinity for metallic surfaces shown by anions, functionalization of the SERS substrates using positively charged self-assembled monolayers (SAMs) is required to guarantee a strong SERS signal. In this work, the performance of three thiol-based coatings, namely, 2-mercapto-4-methyl-pyrimidine, cysteamine, and 2-dimethyl-amino-ethanethiol, is systematically compared for the detection of nitrite, nitrate, and perchlorate ions in water. For each coating, the limit of detection of those analytes is studied in combination with commercial SERS substrates. Cysteamine-coated SERS substrates are shown to provide the lowest limit of detection for the three analytes of this study. Evaluation of this coating on real drinking water samples is reported.

Tuning hydrophobic surface with vertically polymerised microflake structures fabricated using heptadecafluoro-1,1,2,2-tetra-hydrodecyltrichlorosilane

This work presents a facile fabrication method for vertical polymerised structures with a tuned superhydrophobic surface on an aluminium alloy. This method includes vertically polymerising heptadecafluoro-1,1,2,2-tetra-hydrodecyltrichlorosilane (HDFS), which is used for self-assembled monolayer (SAM) coatings, wherein vertical polymerisation is normally considered a process defect. For fabricating the base structure, aluminium hydroxide was generated to form a nanostructure. HDFS was coated as an SAM and microflake fabrication was carried out on this surface for various lengths of time. Microflakes were fabricated by vertical polymerisation of HDFS through this process. Samples coated for 10–30 min showed the water contact angle (θC) of >168.3° and the lowest water sliding angle (θS) of <11.8°. After 30 min, the water droplet clung to the microstructure that increased in size beyond 1 μm, thereby increasing of θS. This micro/nano dual-scale-flake surface was achieved without any hazardous etching processes and can be facilely applied in various industrial fields that require superhydrophobic surfaces.

연마가공과 자기조립단분자막 코팅 기반 다종 금속의 극소수성 표면 제작기술

연마가공과 자기조립단분자막 코팅 기반 다종 금속의 극소수성 표면 제작기술

Room temperature thermally evaporated thin Au film on Si suitable for application of thiol self-assembled monolayers in micro/nano-electro-mechanical-systems sensors

Gold is a standard surface for attachment of thiol-based self-assembled monolayers (SAMs). To achieve uniform defect free SAM coatings, which are essential for bio/chemical sensing applications, the gold surface must have low roughness and be highly orientated. These requirements are normally achieved by either heating during Au deposition or postdeposition Au surface annealing. This paper shows that room temperature deposited gold can afford equivalent gold surfaces, if the gold deposition parameters are carefully controlled. This observation is an important result as heating (or annealing) of the deposited gold can have a detrimental effect on the mechanical properties of the silicon on which the gold is deposited used in microsensors. This paper presents the investigation of the morphology and crystalline structure of Au film prepared by thermal evaporation at room temperature on silicon. The effect of gold deposition rate is studied, and it is shown that by increasing the deposition rate from 0.02 to 0.14 nm s−1 the gold surface root-mean-square roughness decreases, whereas the grain size of the deposited gold is seen to follow a step function decreasing suddenly between 0.06 and 0.10 nm s−1. The x-ray diffraction intensity of the preferentially [111] orientated gold crystallites is also seen to increase as the deposition rate increases up to a deposition rate of 0.14 nm s−1. The formation and characterization of 1-dodecanethiol on these Au coated samples is also studied using the contact angle. It is shown that by increasing the Au deposition rate, the contact angle hysteresis (CAH) decreases until it plateaus, for a deposition rate greater than 0.14 nm s−1, where the CAH is smaller than 9° which is an indication of homogeneous SAM formation, on a smooth surface.

Effects of perfluoroalkylsilane molecular assembly on flow induced voltage generated by doped silicon wafers

We report the effects of surface modifications on (i) Seebeck coefficients and (ii) flow induced voltage generations of the n-type (n-Si) and p-type (p-Si) silicon wafers. The surfaces of n-Si and p-Si wafers were coated with 1H,1H,2H,2H- perfluorooctyltrichlorosilane (FOTS) molecules. The surface modified n-Si and p-Si of size 12 mm×4 mm were mounted on the π/4 angle inclined experimental mount, and nitrogen gas was flown over the inclined surface at the subsonic velocities, 5.3, 10.61, 15.91, 21.22, and 26.52 ms−1, and the voltage difference between the lead and rear ends of pristine and surface modified n-Si and p-Si was measured. The experimental results and theoretical relations are presented. The flow induced voltage generation is caused by the interplay between the Bernoulli flow and Seebeck effect. The flow-voltage response results show that the half coated and full coated n-Si and p-Si wafers generate more voltage than that of the uncoated at a given velocity. The band theory reveals that the flow of nitrogen gas accumulates charge carriers at the FOTS self assembled monolayer (SAM)—silicon interfaces, which resulted in the more voltage generation by full and half coated ni-Si and p-Si surfaces than that of the pristine surface. The enhanced voltage generations and high sensitivities are caused by an effective increase of the gradient of Fermi Energy (EF) (Seebeck coefficient) due to FOTS SAM coatings. Because of that the FOTS SAM modified n-Si and p-Si are become highly sensitive to nitrogen gas flow.

Slip-mediated dewetting of polymer microdroplets

Classical hydrodynamic models predict that infinite work is required to move a three-phase contact line, defined here as the line where a liquid/vapor interface intersects a solid surface. Assuming a slip boundary condition, in which the liquid slides against the solid, such an unphysical prediction is avoided. In this article, we present the results of experiments in which a contact line moves and where slip is a dominating and controllable factor. Spherical cap-shaped polystyrene microdroplets, with nonequilibrium contact angle, are placed on solid self-assembled monolayer coatings from which they dewet. The relaxation is monitored using in situ atomic force microscopy. We find that slip has a strong influence on the droplet evolutions, both on the transient nonspherical shapes and contact line dynamics. The observations are in agreement with scaling analysis and boundary element numerical integration of the governing Stokes equations, including a Navier slip boundary condition.

Hydrophobic and ice-phobic properties of self-assembled monolayers (SAMs) coatings on AA6061

The effects of alkyl chain length and immersion time (IT) on hydrophobic and ice-phobic properties were investigated through self-assembled monolayers of alkylsilane molecules, i.e. OT (trichloro-octylsilane) and OD (trichloro-octadecylsilane) on a flat aluminum alloy (AA6061) substrate. By increasing IT, the wetting behavior of the coatings, characterized by contact angle (CA) and contact angle hysteresis (CAH) measurements at room temperature, showed an enhancement of hydrophobic properties. However, the effect of IT on wetting properties was more significant in the case of the sample coated with longer alkyl chains (OD) than that with short alkyl chains (OT). Icing tests showed delayed ice formation and lower adhesion strength on samples with larger CA values and smaller CAH values. The scanning electron microscopy images of the coated surfaces showed the rough structure at micro/-nanometre scale on a polished aluminum surface. X-ray photoelectron spectroscopy (XPS) analysis of the coated samples showed the presence of more alkylsilane molecules when IT was increased from 15 min to 12 h.

Improvement of Surface Properties of Micro-Mold via SAM Coating for Fabrication of Microfluidic Device

N-octadecyltrichlorosilane [OTS, CH3(CH2)17SiCl3] self-assembled monolayer (SAM) coatings were deposited on Si micromolds by dip-coating. Chemical composition, surface roughness, friction coefficient, thermal stability and surface energy of coatings were investigated. OTS coated silicon (Si) micromolds were used to fabricate PMMA microfluidic devices by hot-embossed process. All OTS coatings were thermally stable up to 180 °C, which is higher than hot-embossing temperature of PMMA. OTS coated micromolds had low friction coefficient, adhesion and superior molding efficiency, which improved lifetime of a uncoated Si micromold from 3 to 27 times.

Dielectric Properties of Self-Assembled Monolayer Coatings on a (111) Silicon Surface

Novel nanomaterial systems as possible candidates for gate dielectric insulators play a key role in the fabrication of next-generation transistor devices in both metal-oxide-semiconductor (MOSFET) and organic thin-film transistors (OTFTs). We focus on one of these new alternative gate dielectric nanostructured materials: self-assembled monolayers (SAMs) of hydroxylated octadecyltrichlorosilane (OTS) chains deposited on a (111) Si substrate. Starting from the evaluation of the surface partial dipole of the SAM coating on the Si surface, we report a quantitative ab initio study of the static dielectric constant of the OTS thin-film coating at different coverage values of the hydrogenated (111) Si surface ranging from partial to full coverage. The main physical features of the OTS SAM films at different coverages have been studied with respect to their influence on the static dielectric constant, and a two-layer model is established. A linear dependence of the static dielectric constant versus the coverage is shown to hold, resulting from depolarization phenomena of the main contributors.