Thiol-terminated Self-assembled Monolayers Research Articles

Vapor-deposition of naphthalene diimide derivatives and interface control between aluminum cathode

Two types of naphthalenediimide derivatives, N,N′-bis(benzyl)naphthalenediimide (Bzl-NDI) and N,N′-bis(p-vinylbenzyl)naphthalenediimide (PVB-NDI), were vapor-deposited either on a bare aluminum or an aluminum modified with a self-assembled monolayer (SAM) of (3-mercaptopropyl)trimethoxysilane. The deposition on the SAM-modified surface was achieved under UV irradiation to enhance thiol-ene reaction of the SAM with vinyl groups of the deposited molecules. The films of Bzl-NDI gradually crystallized, while PVB-NDI formed stable amorphous films. Electron-only devices were prepared with combinations of Bzl- and PVB-NDIs with bare and the SAM-modified aluminum cathodes. PVB-NDI was less conductive than Bzl-NDI due to the amorphous nature of the film. The SAM modification on the cathode was effective to increase the electron current of the PVB-NDI device, whereas the SAM modification did not increase the current of the Bzl-NDI device. The implication is that the thiol-terminated SAM can bind with PVB-NDI that has the vinyl group, leading to an improvement of the organic/cathode interface.

Role of Hydrogen Bonds in Thermal Transport across Hard/Soft Material Interfaces.

The nature of the bond is a dominant factor in determining the thermal transport across interfaces. In this paper, we study the role of the hydrogen bond in thermal transport across interfaces between hard and soft materials with different surface functionalizations around room temperature using molecular dynamics simulations. Gold (Au) is studied as the hard material, and four different types of organic liquids with different polarizations, including hexane (C5H11CH3), hexanamine (C6H13NH2), hexanol (C6H13OH), and hexanoic acid (C5H11COOH), are used to represent the soft materials. To study the hydrogen bonds at the Au/organic liquid interface, three types of thiol-terminated self-assembled monolayer (SAM) molecules, including 1-hexanethiol [HS(CH2)5CH3], 6-mercapto-1-hexanol [HS(CH2)6OH], and 6-mercaptohexanoic acid [HS(CH2)5COOH], are used to functionalize the Au surface. These SAM molecules form hydrogen bonds with the studied organic liquids with varying strengths, which are found to significantly improve efficient interfacial thermal transport. Detailed analyses on the molecular-level details reveal that such efficient thermal transport originates from the collaborative effects of the electrostatic and van der Waals portions in the hydrogen bonds. It is found that stronger hydrogen bonds will pull the organic molecules closer to the interface. This shorter intermolecular distance leads to increased interatomic forces across the interfaces, which result in larger interfacial heat flux and thus higher thermal conductance. These results can provide important insight into the design of hard/soft materials or structures for a wide range of applications.

Developing the Surface Chemistry of Transparent Butyl Rubber for Impermeable Stretchable Electronics.

Transparent butyl rubber is a new elastomer that has the potential to revolutionize stretchable electronics due to its intrinsically low gas permeability. Encapsulating organic electronic materials and devices with transparent butyl rubber protects them from problematic degradation due to oxygen and moisture, preventing premature device failure and enabling the fabrication of stretchable organic electronic devices with practical lifetimes. Here, we report a methodology to alter the surface chemistry of transparent butyl rubber to advance this material from acting as a simple device encapsulant to functioning as a substrate primed for direct device fabrication on its surface. We demonstrate a combination of plasma and chemical treatment to deposit a hydrophilic silicate layer on the transparent butyl rubber surface to create a new layered composite that combines Si-OH surface chemistry with the favorable gas-barrier properties of bulk transparent butyl rubber. We demonstrate that these surface Si-OH groups react with organosilanes to form self-assembled monolayers necessary for the deposition of electronic materials, and furthermore demonstrate the fabrication of stretchable gold wires using nanotransfer printing of gold films onto transparent butyl rubber modified with a thiol-terminated self-assembled monolayer. The surface modification of transparent butyl rubber establishes this material as an important new elastomer for stretchable electronics and opens the way to robust, stretchable devices.

Probing redox reaction of azurin protein immobilized on hydroxyl-terminated self-assembled monolayers with different lengths

Immobilization of proteins with controlled electron transfer properties is an important task for future biomedical applications. In this work, immobilization of azurin on gold electrodes modified with 6-mercaptohexanol (MCH), and 11-mercaptoundecanol (MUD) self-assembled monolayers (SAMs) was studied by electrochemical techniques. The immobilized azurin showed a quasi-reversible redox response on MCH/Au and MUD/Au electrodes, with E1/2=0.11V vs. Ag/AgCl and ∆Ep=10mV and 40mV, respectively. Cyclic voltammograms (CVs) of azurin at different pHs revealed that proton-coupled redox reaction of azurin is a one-electron, three-proton process. The tunneling electron transfer rate constant (k0) of immobilized azurin on MCH/Au electrode was estimated 1.35 (±0.05)×103s−1 using a fast scan CV method, and that on MUD/Au electrode was measured as 120±10s−1 by scanning electrochemical microscopy (SECM). In SECM studies, the approach curves were recorded at different substrate overpotentials with different surface coverages of azurin. For extraction of kinetic parameters, azurin was reduced on MUD/Au electrode via tunneling electron transfer across the SAM, and oxidized back via solution-phase tip-generated ferrocyanide. The bimolecular electron transfer rate constant (kBI) between immobilized azurin and [Fe(CN)6]4− mediator was estimated as 3×108mol−1cm3s−1. Based on distribution of hydrophilic and hydrophobic amino acids on azurin structure, and the distinct voltammetric behavior of azurin on MCH and MUD SAMs as hydroxyl-terminated SAMs vs. methyl- and thiol-terminated SAMs, possible orientations of azurin on different SAMs were also assessed.

Crystal growth of barium nitrate on thiol-terminated self-assembled monolayers and a Raman spectroscopic investigation of the crystal facets

Self-assembled monolayers (SAMs) of (3-mercaptopropyl)trimethoxysilane (MPTS) were used as templates to induce the nucleation and growth of Ba(NO3)2 microcrystals. X-ray diffraction (XRD) patterns and optical microscopy images were employed to investigate the crystallinity and morphology of the microcrystals. It is interesting that the soaking direction of the templates plays an important role in the crystal growth process. When the MPTS-modified substrates were soaked in a vertical manner, square-like microcrystals with the (100) orientation were synthesized; however, when the substrates were soaked in a horizontal manner, triangular-like microcrystals with the (111) orientation were obtained. These differences may be due to the different regulatory mechanisms involved in the modulation of the crystal growth by the SAMs. These mechanistic discoveries pave a new way to control and design new crystals. Another interesting phenomenon is that the Raman intensity of the (111) facet of the triangular Ba(NO3)2 microcrystals is five times that of the (100) facet. By using this difference, the crystal facet index can be rapidly determined.

Chalcogen Atom Interaction with Palladium and the Complex Molecule–Metal Interface in Thiol Self Assembly

In the case of reactive metals, on adsorption of organic chalcogenide molecules like thiols, chalcogenide-C bond scission can occur. Thus, the high reactivity of Pd leads to initial thiol dissociation and formation of a complex PdS interface layer on which thereafter thiol self-assembled monolayers (SAM) can form. In this context we investigate in detail the adsorption of S, Se, alkanethiols, and aromatic dithiols on Pd by photoemission with synchrotron radiation. The nature of the PdS and PdSe layers formed is studied, and thiol adsorption on Pd(111), PdS, and PdSe surfaces is investigated, along with interface characteristics. After initial strong sulfidation (selenization) in Na2S(Se) solutions, a well-ordered surface PdS (PdSe) layer can be obtained by annealing. For S, annealing leads to formation of a PdS (√7 × √7)R19.1° layer, whereas for Se, large domains of this structure are formed. Experiments suggest that in thiol adsorption the Pd–sulfide interface is not simply similar to the (√7 × √7)R19.1° PdS layer but that modifications in this surface sulfide layer are induced. A similar effect is observed on the selenide interface layer. In addition, 1,4-benzenedimethanethiol adsorption on Pd is investigated with the aim of creation of thiol-terminated dithiol molecular layers. Unlike the case of surfaces like Au, no clear indication of a standing-up, thiol-terminated SAM was found. X-ray radiation damage effects are reported.

Bi-ligand surfaces with oriented and patterned protein for real-time tracking of cell migration

A bioactive platform for the quantitative observation of cell migration is presented by (1) presenting migration factors in a well-defined manner on 2-D substrates, and (2) enabling continuous cell tracking. Well-defined substrate presentation is achieved by correctly orienting immobilized proteins (chemokines and cell adhesion molecules), such that the active site is accessible to cell surface receptors. A thiol-terminated self-assembled monolayer on a silica slide was used as a base substrate for subsequent chemistry. The thiol-terminated surface was converted to an immobilized metal ion surface using a maleimido-nitrilotriacetic acid (NTA) cross-linker that bound Histidine-tagged recombinant proteins on the surface with uniform distribution and specific orientation. This platform was used to study the influence of surface-immobilized chemokine SDF-1α and cell adhesion molecule ICAM-1 on murine splenic B lymphocyte migration. While soluble SDF-1α induced trans-migration in a Boyden Chamber type chemotaxis assay, immobilized SDF-1α alone did not elicit significant surface-migration on our test-platform surface. Surface-immobilized cell adhesion protein, ICAM-1, in conjunction with activation enabled migration of this cell type on our surface. Controlled exposure to UV light was used to produce stable linear gradients of His-tagged recombinant SDF-1α co-immobilized with ICAM-1 following our surface chemistry approach. XPS and antibody staining showed defined gradients of outwardly oriented SDF-1α active sites. This test platform can be especially valuable for investigators interested in studying the influence of surface-immobilized factors on cell behavior and may also be used as a cell migration enabling platform for testing the effects of various diffusible agents.

Photochemical reactions of thiol-terminated self-assembled monolayers (SAMs) for micropatterning of gold nanoparticles and controlled surface functionality

This paper reported a facile method for the patterning of gold nanoparticles (AuNPs) on SiO2/Si by combining photochemical reaction and self-assembly techniques, and the conversion of surface functionality through thiol-ene click chemistry. The oxidation of terminal thiols in self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane upon exposure to 254nm UV light under ambient atmosphere was investigated. Chemically well-defined microstructures were obtained by UV irradiation through a mask, and subsequent immersion of the substrate into a dispersion of AuNPs resulted in site-specific assembly of AuNPs via Au–S covalent bond in the unexposed area. Thiol-ene “click” reaction between surface thiol-group and alkene-containing molecules under illumination of 365nm UV light was also demonstrated. X-ray photoelectron spectroscopy study indicated the successful conversion of surface functionality.

A Novel Technique for Preparation of the Fluorescence Sensor Based on Covalent Immobilization of 1-Aminopyrene

A novel technique to covalently immobilize indicator dyes with terminal amino groups for preparing optical sensors is investigated. Au nanoparticles are used as bridges and carriers for anchoring indicator dyes on the surface of a quartz glass slide. 1-Aminopyrene (AP) was employed as an example of indicator dyes and covalently immobilized onto the outmost surface of the glass slide. First, the glass slide was functionalized by (3-mercaptopropyl) trimethoxysilane (MPS) to form a thiol-terminated self-assembled monolayer, where Au nanoparticles were strongly anchored via covalent link. Then, 16-mercaptohexadecanoic acid (MHDA) was self-assembled to bring carboxylic groups onto the surfaces of Au nanoparticles. A further activation by using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) converted the carboxylic group into succinimide ester. Finally, the active succinimide ester was reacted with 1-aminopyrene (AP). Thus, AP was covalently immobilized to the glass slide and an AP-immobilized sensor was obtained. The resulting sensor was used to determine rutin based on fluorescence quenching. It showed a linear response toward rutin (R) from 5.0 × 10-7 to 6.0 × 10-4 mol L-1 with a detection limit of 2.0× 10-7 mol L-1. This AP-immobilized sensor has very satisfactory reproducibility, reversibility, rapid response and no dye-leaching.

A novel fluorescence sensor based on covalent immobilization of 3-amino-9-ethylcarbazole by using silver nanoparticles as bridges and carriers

A novel technique of covalent immobilization of indicator dyes in the preparation of fluorescence sensors is developed. Silver nanoparticles are used as bridges and carriers for anchoring indicator dyes. 3-amino-9-ethylcarbazole (AEC) was employed as an example of indicator dyes with terminal amino groups and covalently immobilized onto the outmost surface of a quartz glass slide. First, the glass slide was functionalized by (3-mercaptopropyl) trimethoxysilane (MPS) to form a thiol-terminated self-assembled monolayer, where silver nanoparticles were strongly bound to the surface through covalent bonding. Then, 16-mercaptohexadecanoic acid (MHDA) was self-assembled to bring carboxylic groups onto the surface of silver nanoparticles. A further activation by using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) converted the carboxylic groups into succinimide esters. Finally, the active succinimide esters on the surface of silver nanoparticles were reacted with AEC. Thus, AEC was covalently bound to the glass slide and an AEC-immobilized sensor was obtained. The sensor exhibited very satisfactory reproducibility and reversibility, rapid response and no dye-leaching. Rutin can quench the fluorescence intensity of the sensor and be measured by using the sensor. The linear response of the sensor to rutin covers the range from 2.0 × 10 −6 to 1.5 × 10 −4 mol L −1 with a detection limit of 8.0 × 10 −7 mol L −1. The proposed technique may be feasible to the covalent immobilization of other dyes with primary amino groups.

A novel DNA biosensor based on ellipsometry

In this study, we have firstly investigated the effects of dipping time, solution concentration and solvent types on the formation of self-assembled monolayers (SAMs) with 3-mercaptopropyltrimethoxysilane (MPTS) molecules on the Si(0 0 1) surfaces. Ellipsometric measurements showed that monolayers with a thickness of about 0.73 nm were formed when the dipping time is about 1 h, while more profound agglomerations were observed for longer time periods and MPTS solutions with higher concentrations. Monolayers were formed with solvents having larger dielectric constants. Contact angle measurements were in good correlation with the ellipsometric data. 5′-Thiolated oligodeoxynucleotides (ODNs) probes were immobilized onto these thiol-terminated SAMs by disulfide bond formation. The thickness of the ODN-layer on the MPTS modified surfaces reached almost a constant value of about 2.5 nm in 1 h. Target ODNs were detected by monitoring hybridization onto the surfaces by ellipsometry. The analytical signal (the delta ( Δ) value) measured was correlated with the target ODNs concentration.

Surface Modification Using Thiol−Acrylate Conjugate Addition Reactions

A novel method for forming surface-bound polymer films using amine-catalyzed thiol−acrylate reactions is investigated. The surface is modified with a thiol-terminated self-assembled monolayer (SAM) and treated with acrylate monomer in the presence of an amine catalyst. A conjugate addition reaction takes place at the surface, attaching an acrylate molecule to the surface. The fraction of attached acrylate on the surface was controlled by the reaction time, forming a gradient of acrylate surface density, and the acrylate fraction of the surface was changed from 0 to 0.6. An orthogonal gradient comprised of either the same acrylate functionality or multiple functionalities was also obtained on the same surface. The technique was further modified to yield acrylate surface density gradient on a polymer surface. Thicker films were grown in a controlled manner by polymerizing a dithiol−diacrylate mixture on a thiol-terminated surface, and the thicknesses of the films were controlled from 0.1 to 6 nm by changing...

Thiol−Ene Photopolymer Grafts on Functionalized Glass and Silicon Surfaces

A simple and straightforward method for forming polymer grafts on glass and silicon surfaces using thiol−ene photopolymerizations is presented. A linear thiol−ene system composed of 1,6-hexanedithiol and triethylene glycol divinyl ether was photopolymerized over a silicon surface functionalized with a thiol-terminated self-assembled monolayer (SAM). Although the grafted thiol−ene films are thin, ranging from 0.1 to 9.6 nm, the thickness is linearly related to the molecular weight of the polymer formed in the bulk. Thus, the thickness is tunable simply by controlling the polymerization conversion or the stoichiometric excess of one component. By lithographically patterning the SAM chemistry on the surface prior to polymerization, 5 μm wide lines of grafted thiol−ene polymer were created. Thin, grafted acrylic films, from 0.5 to 1.7 nm in thickness, were formed by photopolymerizing tert-butyl acrylate with a small amount of thiol chain transfer agent to control chain length and facilitate radical transfer t...

Micropatterning organosilane self-assembled monolayers with plasma etching and backfilling techniques

Organosilane self-assembled monolayers (SAMs) were micropatterned using standard photoresist-based lithographic and plasma etching techniques. Etched regions were backfilled with a second SAM containing a different organic substituent. By repeating the patterning process, a complex, multifunctional patterned surface composed of three different SAMs (amino-, methyl-, and thiol-terminated) was formed. Since two distinct etching steps are used, the backfilled SAMs can be deposited in trenches of different depths. Multifunctional, micropatterned SAMs were visualized by friction mode scanning probe microscopy (SPM). Gold-coated SPM tips yielded high contrast friction images from surfaces patterned with thiol-terminated SAMs.

Abrupt change in the structure of self-assembled monolayers upon metal evaporation

We have studied the interaction of vapor-deposited nickel with thiol-terminated self-assembled monolayers (SAMs) of oligophenylthiolates on Au, which can be considered as model systems for molecular electronics. Instead of usually observed disordering of the molecular layer, a drastic decrease in molecular tilt was observed at the initial stage of the deposition. This was attributed to the formation of Ni-thiol complexes at the SAM–ambient interface. The reorientation resulted in the appearance of transient channels for the metal diffusion into the SAM and to the SAM–substrate interface.

Pendant thiol groups-attached Pd(II) for initiating metal deposition

A new activation method has been developed for initiating electroless metal deposition on silicon substrates without SnCl 2 sensitization and roughening condition. Silicon wafers are first coated with thiol-terminated self-assembled monolayers (SAMs), and then catalyzed with a stable tin-free Pd(II)-based colloidal solution. Atomic force microscopy (AFM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to characterize the step-by-step surfaces and study the binding mechanism of Pd(II) with SAMs onto surfaces. Results show that Pd(II) oligomer particles are chemisorbed on pendant thiol surfaces through SPd bonds. This process involves fewer steps than the conventional Sn/Pd combined activation one. Furthermore, the chemical bound initiator possesses longevity and can be stored for a long time before metallization.

Use of the laser-desorption technique for the preparation of a mixed-thiol monolayer on a gold electrode

A thiol-terminated self-assembled monolayer on a gold surface provides a useful model system for the study of both the transmembrane electron exchange process and the ion-permeation process into well-organized molecular assemblies [l-3]. On the other hand, recent efforts in the study of a self-assembled monolayer have been concentrated on the physicochemical and electrochemical properties of mixed-thiol monolayer assemblies containing terminal functional groups [4-111. Although the method of co-adsorption of two different thiols has usually been used for the preparation of mixed-thiol monolayers, it is difficult to control the morphology of two adsorbed thiols in this method. Furthermore, this method cannot be used if both the thiols are insoluble at the same time in the same solvent. In this study, we will describe a novel method for the preparation of a mixed-thiol monolayer using the technique of laser desorption and subsequent adsorption. Two types of laser desorption were examined to investigate the adsorption morphology. Glutathione (GSH, y-L-glutamyl-L-cysteinyl-glycine) and n-dodecyl mercaptan (DDM, CH,(CH2)i1SH) were used as the test thiols, because GSH is very soluble in water but insoluble in organic solvent, and DDM is vice versa [12,13].