Silane Self-assembled Monolayers Research Articles

The Effect of Hydrophobic Alkyl Silane Self-Assembled Monolayers on Adult Barnacle Adhesion

AbstractAlthough there exists a wide range of nonbiocidal and environmentally friendly surface coatings to reduce biofouling on marine structures, there is still not a fundamental understanding of barnacle adhesion upon reattachment. The purpose of this study is to assess the effect of hydrophobicity on adhesion in the barnacle Amphibalanus amphitrite, an abundant and widespread biofouler. Self-assembled monolayers were made on glass slides from alkyl silanes with methylated and fluorinated terminal groups to produce hydrophobic surfaces. Coated and uncoated glass slides underwent a 2-week barnacle reattachment assay. Barnacles were removed using a force gauge, and critical shear stress was calculated for each substrate. Following reattachment assays, a Coomassie Blue G250 protein stain was used to quantify the amount of glue remaining on substrates by measuring pixel density with ImageJ software on glue scans. Critical shear stress was found to be significantly higher for both hydrophobic surfaces as compared to the hydrophilic uncoated glass, and correspondingly, the density of residual glue was higher on hydrophobic surfaces. Given that hydrophobic substrates can exclude water from the surface, they may provide a protected environment for glue release that is favorable for adhesive bond formation with the substrate as well as inter- and intramolecular bonding within the glue layer. Critical shear stress showed a strong positive correlation with residual glue density, suggesting that barnacle release occurs primarily via cohesive failure. Scanning Electron Microscope (SEM) micrographs confirm morphological differences in the glue remnants, depending on the substrate coating. Among the hydrophobic substrates tested, results suggest that contact angle alone is not enough to predict the critical shear stress of barnacles. The chemical and physical properties of the coating become important parameters to consider in antifouling coating design.

Fluorinated benzalkylsilane molecular rectifiers.

We report on the synthesis and electrical properties of nine new alkylated silane self-assembled monolayers (SAMs) – (EtO)3Si(CH2)nN = CHPhX where n = 3 or 11 and X = 4-CF3, 3,5-CF3, 3-F-4-CF3, 4-F, or 2,3,4,5,6-F, and explore their rectification behavior in relation to their molecular structure. The electrical properties of the films were examined in a metal/insulator/metal configuration, with a highly-doped silicon bottom contact and a eutectic gallium-indium liquid metal (EGaIn) top contact. The junctions exhibit high yields (>90%), a remarkable resistance to bias stress, and current rectification ratios (R) between 20 and 200 depending on the structure, degree of order, and internal dipole of each molecule. We found that the rectification ratio correlates positively with the strength of the molecular dipole moment and it is reduced with increasing molecular length.

Cobalt(III)-Mediated Permanent and Stable Immobilization of Histidine-Tagged Proteins on NTA-Functionalized Surfaces.

We present the cobalt(III)-mediated interaction between polyhistidine (His)-tagged proteins and nitrilotriacetic acid (NTA)-modified surfaces as a general approach for a permanent, oriented, and specific protein immobilization. In this approach, we first form the well-established Co(2+) -mediated interaction between NTA and His-tagged proteins and subsequently oxidize the Co(2+) center in the complex to Co(3+) . Unlike conventionally used Ni(2+) - or Co(2+) -mediated immobilization, the resulting Co(3+) -mediated immobilization is resistant toward strong ligands, such as imidazole and ethylenediaminetetraacetic acid (EDTA), and washing off over time because of the high thermodynamic and kinetic stability of the Co(3+) complex. This immobilization method is compatible with a wide variety of surface coatings, including silane self-assembled monolayers (SAMs) on glass, thiol SAMs on gold surfaces, and supported lipid bilayers. Furthermore, once the cobalt center has been oxidized, it becomes inert toward reducing agents, specific and unspecific interactions, so that it can be used to orthogonally functionalize surfaces with multiple proteins. Overall, the large number of available His-tagged proteins and materials with NTA groups make the Co(3+) -mediated interaction an attractive and widely applicable platform for protein immobilization.

Nanoscale patterning of self-assembled monolayers using DNA nanostructure templates.

We describe a method to pattern arbitrary-shaped silane self-assembled monolayers (SAMs) with nm scale resolution using DNA nanostructures as templates. The DNA nanostructures assembled on a silicon substrate act as a soft-mask to negatively pattern SAMs. Mixed SAMs can be prepared by back filling the negative tone patterns with a different silane.

Photocurrent generation of a single-gate graphene p–n junction fabricated by interfacial modification

A back-gate graphene p–n junction was achieved by selective interfacial modification of a chemical vapor deposition (CVD)-grown graphene field effect transistor (FET). Silane self-assembled monolayer (SAM) patterns were used to fabricate uniform p- and n-doped regions and a sharp p–n junction in the graphene FET channel. A gate-dependent photocurrent response was observed at the graphene p–n junction, and exhibited a maximum signal between two Dirac point voltages of SAM-doped graphene regions. A spatial photocurrent map shows that the photocurrent generated at the junction region was much larger than that from graphene/electrode junctions under the same incident laser power. This single-peak characteristic photocurrent in CVD graphene is dominated by the photothermoelectric contribution, and is highly sensitive to the power of incident laser. The SAM interfacial modification method provides a feasible route for the fabrication of efficient graphene-based photodetectors.

Stabilization of Insulin by Adsorption on a Hydrophobic Silane Self-Assembled Monolayer.

The interaction between many proteins and hydrophobic functionalized surfaces is known to induce β-sheet and amyloid fibril formation. In particular, insulin has served as a model peptide to understand such fibrillation, but the early stages of insulin misfolding and the influence of the surface have not been followed in detail under the acidic conditions relevant to the synthesis and purification of insulin. Here we compare the adsorption of human insulin on a hydrophobic (-CH3-terminated) silane self-assembled monolayer to a hydrophilic (-NH3(+)-terminated) layer. We monitor the secondary structure of insulin with Fourier transform infrared attenuated total reflection and side-chain orientation with sum frequency spectroscopy. Adsorbed insulin retains a close-to-native secondary structure on both hydrophobic and hydrophilic surfaces for extended periods at room temperature and converts to a β-sheet-rich structure only at elevated temperature. We propose that the known acid stabilization of human insulin and the protection of the aggregation-prone hydrophobic domains on the insulin monomer by adsorption on the hydrophobic surface work together to inhibit fibril formation at room temperature.

Self‐assembled silane monolayers: an efficient step‐by‐step recipe for high‐quality, low energy surfaces

Organosilane self‐assembled monolayers (SAMs) are commonly used for modifying a wide range of substrates. Depending on the end group, highly hydrophobic or hydrophilic surfaces can be achieved. Silanization bases on the adsorption, self‐assembly and covalent binding of silane molecules onto surfaces and results in a densely packed, SAM. Following wet chemical routines, the quality of the monolayer is often variable and, therefore, unsatisfactory. The process of self‐assembly is not only affected by the chemicals involved and their purity but is also extremely sensitive to ambient parameters such as humidity or temperature and to contaminants. Here, a reliable and efficient wet‐chemical recipe is presented for the preparation of ultra‐smooth, highly ordered alkyl‐terminated silane SAMs on Si wafers. The resulting surfaces are characterized by means of atomic force microscopy, X‐ray reflectometry and contact angle measurements. Copyright © 2015 John Wiley & Sons, Ltd.

Effect of Surface Hydrophobicity on Critical Pinning Concentration of Nanoparticles To Trigger the Coffee Ring Formation during the Evaporation Process of Sessile Drops of Nanofluids

The evaporation process of sessile drops of water/nanofluid on hydrophilic/hydrophobic surfaces was observed experimentally. The evaporation process of nanofluid drops generally consists of four stages in the following sequence: (1) the constant contact radius mode, (2) the constant contact angle mode, (3) the mixed mode, and (4) the second pinned mode. The contact angle of nanofluid drops at the occurrence of the second pinning is consistently lower than the receding contact angle of the surface. The critical pinning concentration, at which the contact line is pinned again to initiate the second pinned mode, is independent of the initial concentration of silica nanoparticles. The critical pinning concentration increases along with the surface hydrophobicity, which can be identified by the receding contact angle of the surface. The critical pinning concentration is linearly dependent on the receding contact angle. It is interesting to find out that there exists a critical receding contact angle of 33° in this study. A droplet of nanofluid containing silica nanoparticles placed on self-assembled silane monolayer coated silicon wafer with its receding contact angle smaller than the critical receding contact angle (33°) would always pin immediately to trigger the coffee ring formation.

Fully printable mesoscopic perovskite solar cells with organic silane self-assembled monolayer.

By the introduction of an organic silane self-assembled monolayer, an interface-engineering approach is demonstrated for hole-conductor-free, fully printable mesoscopic perovskite solar cells based on a carbon counter electrode. The self-assembled silane monolayer is incorporated between the TiO2 and CH3NH3PbI3, resulting in optimized interface band alignments and enhanced charge lifetime. The average power conversion efficiency is improved from 9.6% to 11.7%, with a highest efficiency of 12.7%, for this low-cost perovskite solar cell.

Polypyrrole films prepared on self-assembled silane monolayers and applications

Polypyrrole (PPy) films were successfully prepared on the self-assembled silane monolayers (SAMs) for electromagnetic interference (EMI) shielding. X-ray photoelectron spectroscopy was used to estimate the surface properties of SAMs and hydrolytic silane. The deposition process of PPy films on silane SAMs surface was discussed. The PPy films synthesised on SAMs were characterised by scanning electron microscopy, Fourier transform infrared, thermogravimetric analysis and differential scanning calorimetry. The results showed that the uniformity and density of PPy films were improved effectively by using SAMs modification and adding dopants, which could further increase the conductivity and EMI shielding effectiveness (SE) of PPy films. A maximum conductivity of 51·4 S cm–1 and SE value of 37·03 dB for I2 doped PPy films on SAMs has been achieved. The doped PPy films can be used for the practical EMI shielding application for commercial electronic appliances.

Near-field photochemical and radiation-induced chemical fabrication of nanopatterns of a self-assembled silane monolayer.

A general concept for parallel near-field photochemical and radiation-induced chemical processes for the fabrication of nanopatterns of a self-assembled monolayer (SAM) of (3-aminopropyl)triethoxysilane (APTES) is explored with three different processes: 1) a near-field photochemical process by photochemical bleaching of a monomolecular layer of dye molecules chemically bound to an APTES SAM, 2) a chemical process induced by oxygen plasma etching as well as 3) a combined near-field UV-photochemical and ozone-induced chemical process, which is applied directly to an APTES SAM. All approaches employ a sandwich configuration of the surface-supported SAM, and a lithographic mask in form of gold nanostructures fabricated through colloidal sphere lithography (CL), which is either exposed to visible light, oxygen plasma or an UV–ozone atmosphere. The gold mask has the function to inhibit the photochemical reactions by highly localized near-field interactions between metal mask and SAM and to inhibit the radiation-induced chemical reactions by casting a highly localized shadow. The removal of the gold mask reveals the SAM nanopattern.

Multiscale modeling of the electrostatic impact of self-assembled monolayers used as gate dielectric treatment in organic thin-film transistors.

This study sheds light on the microscopic mechanisms by which self-assembled monolayers (SAMs) determine the onset voltage in organic thin-film transistors (OTFTs). Experiments and modeling are combined to investigate the self-assembly and electrostatic interaction processes in prototypical OTFT structures (SiO2/SAM/pentacene), where alkylated and fluoroalkylated silane SAMs are compared. The results highlight the coverage-dependent impact of the SAM on the density of semiconductor states and enable the rationalization and the control of the OTFT characteristics.

Selective charge doping of chemical vapor deposition-grown graphene by interface modification

The doping and scattering effect of substrate on the electronic properties of chemical vapor deposition (CVD)-grown graphene are revealed. Wet etching the underlying SiO2 of graphene and depositing self-assembled monolayers (SAMs) of organosilane between graphene and SiO2 are used to modify various substrates for CVD graphene transistors. Comparing with the bare SiO2 substrate, the carrier mobility of CVD graphene on modified substrate is enhanced by almost 5-fold; consistently the residual carrier concentration is reduced down to 1011 cm−2. Moreover, scalable and reliable p- and n-type graphene and graphene p-n junction are achieved on various silane SAMs with different functional groups.

High-performance organic transistors with high-k dielectrics: A comparative study on solution-processed single crystals and vacuum-deposited polycrystalline films of 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene

High carrier-mobility organic field-effect transistors are developed employing high-k gate dielectrics so that unprecedentedly high transconductance is realized. 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) solution-crystallized films are coated on hybrid gate insulators of silane self-assembled monolayers and high-k Al2O3 formed by atomic-layer-deposition. Intrinsically high carrier mobility exceeding 10 cm2/Vs in the crystalline C10-DNTT is preserved even on the high-k gate insulators because of suppressed coupling of the field-induced carriers to the polarization of the dielectrics.

Preparation of polypyrrole films on insulating substrates by self-assembled monolayers

The objective of this study is to develop an effective method for the preparation of polypyrrole (PPy) films on insulating substrates modified by self-assembled silane monolayers (SAMs). The morphology of silane SAMs and PPy films was investigated using scanning electron microscopy (SEM). The SEM pattern of SAMs showed that silane coupling agent molecules were arranged uniformly, orderly and tightly on the surface of substrate. The as-prepared PPy films exhibited the compact and uniform morphology when the substrates were dipped in silane coupling agent for about 10min. The hydrolysis of silane and deposition process scheme of SAMs and PPy films were described. Chemical analysis of PPy films was performed by X-ray photoelectron spectroscopy (XPS). The studied PPy films showed potential applications in electromagnetic interference shielding and solid electrolytic capacitor.

Organozinc Compounds as Effective Dielectric Modification Layers for Polymer Field‐Effect Transistors

AbstractThe interface between the organic semiconductor and dielectric plays an important role in determining the device performance of organic field‐effect transistors (OFETs). Although self‐assembled monolayers (SAMs) made from organosilanes have been widely used for dielectric modification to improve the device performance of OFETs, they suffer from incontinuous and lack uniform coverage of the dielectric layer. Here, it is reported that by introduction of a solution‐processed organozinc compound as a dielectric modification layer between the dielectric and the silane SAM, improved surface morphology and reduced surface polarity can be achieved. The organozinc compound originates from the reaction between diethylzinc and the cyclohexanone solvent, which leads to formation of zinc carboxylates. Being annealed at different temperatures, organozinc compound exists in various forms in the solid films. With organozinc modification, p‐type polymer FETs show a high charge carrier mobility that is about two‐fold larger than a control device that does not contain the organozinc compound, both for devices with a positive threshold voltage and for those with a negative one. After organozinc compound modification, the threshold voltage of polymer FETs can either be altered to approach zero or remain unchanged depending on positive or negative threshold voltage they have.

Preparation and wettability examinations of transparent SiO2binder-added MgF2nanoparticle coatings covered with fluoro-alkyl silane self-assembled monolayer

SiO2-added MgF2 nanoparticle coatings with various surface roughness properties were formed on fused silica-glass substrates from autoclaved sols prepared at 100-180 °C. To give it hydrophobicity, we treated the samples with fluoro-alkyl silane (FAS) vapor to form self-assembled monolayers on the nanoparticle coating and we examined the wettability of the samples. The samples preserved good transparency even after the FAS treatment. The wettability examination revealed that higher autoclave temperatures produced a larger average MgF2 nanoparticle particle size, a larger surface roughness, and a higher contact angle and the roll-off angle.

Aptamer based electrochemical sensor for detection of human lung adenocarcinoma A549 cells

We report results of the studies relating to development of an aptamer-based electrochemical biosensor for detection of human lung adenocarcinoma A549 cells. The aminated 85-mer DNA aptamer probe specific for the A549 cells has been covalently immobilized onto silane self assembled monolayer (SAM) onto ITO surface using glutaraldehyde as the crosslinker. The results of cyclic voltammetry and differential pulse voltammetry studies reveal that the aptamer functionalized bioelectrode can specifically detect lung cancer cells in the concentration range of 103 to 107 cells/ml with detection limit of 103 cells/ml within 60 s. The specificity studies of the bioelectrode have been carried out with control KB cells. No significant change in response is observed for control KB cells as compared to that of the A549 target cells.

Molecular Tilting and Its Impact on Frictional Properties of n-Alkane Self-Assembled Monolayers

Hydrophobic, methyl-terminated self-assembled monolayer (SAM) surfaces can be used to reduce friction. Among methyl-terminated SAMs, the frictional properties of alkanethiol SAMs and silane SAMs have been well-studied. In this research, we investigated friction of methyl-terminated n-hexatriacontane (C36) SAM and compared its friction properties with the alkanethiol and silane SAMs. Alkane SAM does not have an anchoring group. The alkane molecules stand on the surface by physical adsorption, which leads to a higher surface mobility of alkane molecules. We found that C36 SAM has a higher coefficient of friction than that of octadecyltrichlorosilane (OTS) silane. When an atomic force microscope (AFM) tip was swiped across the alkane SAM with a loading force, we found that the alkane SAM can withstand the tip loading pressure up to 0.48 GPa. Between 0.48 and 0.49Ga, the AFM tip partially penetrated the SAM. When the tip moved away, the deformed SAM healed and maintained the structural integrity. When the loading pressure was higher than 0.49 GPa, the alkane SAM was shaved into small pieces by the tip. In addition, we found that the molecular tilting of C36 molecules interacted with the tribological properties of the alkane SAM surface. On one hand, a higher loading force can push the rod-like alkane molecules to a higher tilting angle; on the other hand, a higher molecular tilting leads to a lower friction surface.

Quantitative analysis of the surface composition of mixed self‐assembled monolayers using ToF‐SIMS and FT‐IR analyses

AbstractMixed self‐assembled monolayers (SAMs) consisting of different molecules have been used to modify surface wettability, functionalize end groups for chemical binding, and control adsorption of proteins, DNA, or cells. In these applications, control over the quantification of the surface composition of a mixed SAM is especially important when designing biochips and biosensors. Previously, we presented a method by which ToF‐SIMS can be used to quantify the composition of two adsorbates with acetylene and propene functional groups from their secondary ion yield ratio in mixed SAMs on gold substrates. We concluded that although the ion yield ratio of the two adsorbates in mixed SAM was not equal to the mole fraction of Diyne (‐acetylene) and Diene (‐propene) in a solution, this was due to a different secondary ion formation, and not to the difference in the amount of adsorbates on the surface. In this study, we revisit the same sample system, this time applying a FT‐IR technique to test the accuracy of our previous findings. We found that the absorbances of the acetylene and propene functional groups neatly and linearly correlated with the mole fraction in the solution, which were consistent with our previous conclusions. We also used this methodology to quantify mixed silane SAMs on a silica surface. Our study shows that the ToF‐SIMS and FT‐IR techniques can complement one another and be useful to quantify mixed SAMs on various substrates. Copyright © 2010 John Wiley & Sons, Ltd.