Self-assembled Monolayers Samples Research Articles

Structural Stability and Phase Transitions of Octanethiol Self-Assembled Monolayers on Au(111) in Ultrahigh Vacuum.

To understand the structural stability of as-prepared octanethiol (OT) self-assembled monolayers (SAMs) with a fully covered c(4 x 2) phase on Au(111) in ultrahigh vacuum (UHV) conditions of 3 x 10(-7) Pa at room temperature, we examined OT SAM samples obtained as a function of storage period using scanning tunneling microscopy (STM). STM imaging revealed that phase transition of OT SAMs after storage in UHV for 3 days occurs from the c(4 x 2) phase to the mixed phase containing ordered c(4 x 2) and disordered phases. It was also observed that the disordered phase was mainly located at around vacancy islands and near step edges of Au(111) terraces, implying that desorption of OT molecules chemisorbed on Au(111) in UHV occurs more quickly in these regions compared with in the closely packed and ordered domains. After a longer storage in UHV for 6 days, OT SAMs with the c(4 x 2) phase were changed to the disordered phase containing a partially ordered domain with a row structure. From this study, we clearly demonstrated that OT molecules in SAMs on Au(111) are desorbed spontaneously in UHV at room temperature, resulting in the formation of disordered and row phases.

Quantum-Dot-Sensitized Solar Cell Using a Photoanode Prepared by in Situ Photodeposition of CdS on Nanocrystalline TiO2 Films

CdS quantum dots (QDs) have been incorporated into mesoporous TiO2 nanocrystalline films by a photodeposition (PD) technique we have recently developed [CdS(PD)/mp-TiO2], and for comparison, the conventional successive ionic layer adsorption and reaction (SILAR) and self-assembled monolayer (SAM) methods have also been used for preparing the coupling system. The most important characterstic of the PD technique is that the efficicent interfacial charge transfer between the semiconductors is guaranteed because the photocatalytic redox property of TiO2 is taken advatage of to form the heteronanojunction. The N2 adsorption−desorption data analysis by the Barret−Joyner−Halenda method and the elemental depth profile by electron probe microanalysis showed that CdS QDs are distributed in the mesopores of the film without pore-blocking in the PD sample and with partial pore-blocking in the SILAR sample, whereas only the upper part of the film is covered with CdS QDs in the SAM sample. The PD technique enables one ...

The effects of humidity and surface free energy on adhesion force between atomic force microscopy tip and a silane self-assembled monolayer film

The relationship between atomic force microscopy probe-sample adhesion force and relative humidity (RH) at five different levels of surface free energy (γs) of an organic self-assembled monolayer (SAM) has been investigated. Different γs levels were achieved by exposing a patterned SiO2/CH3-terminated octyldimethylchlorosilane SAM sample to an ultraviolet (UV)/ozone atmosphere. A model consisting of the Laplace-Kelvin theory for capillary condensation for nanosized probe and probe-sample molecular interaction was derived to describe the adhesion force as a function of RH from 25 to 90% for different SAM γs values. The equations were solved analytically by using an equivalent curvature of the probe tip shape. Experimental results show that the adhesion force increases slightly with RH for nonpolar SAM. However, for polar SAM surfaces, it increases at first, reaches a maximum, and then decreases. Both the rate of increase and the maximum of the adhesion force with humidity are γs-dependent, which is in good agreement with theoretical prediction. The large rise in the adhesion force in this RH range is due to the capillary force.

Scanning probe anodization patterning of Si substrates covered with a self-assembled monolayer dependent on surface hydrophilicity

Contact-mode atomic force microscopy (AFM)-based anodization patterning was performed on silicon (Si) substrates covered with a self-assembled monolayer (SAM) in order to investigate effects of relative humidity (RH), surface wetability of the SAM, and probe-tip material on widths and heights of drawn lines. Three types of SAMs, that is, methyl-terminated, ester-terminated, and carboxyl-terminated SAMs, were prepared on hydrogen-terminated Si substrates by a thermal activation method. These SAMs were covalently fixed on the Si substrates through Si–C bonds without an interfacial oxide layer between the SAM and Si. Rh-coated and boron-doped conductive diamond-coated AFM probe tips were used for patterning with a positive sample bias of 10V. Consequently, the region scanned with the AFM probe became protruded due to degradation of the SAM and anodization of Si. When the Rh-coated Si tip was used, the width of the protruded line increased with increasing RH on each SAM sample. The linewidth on the most hydrophobic methyl-terminated SAM was narrowest, while that on the most hydrophilic carboxylic SAM was widest. However there was no distinct difference in the pattern width at the patterning under low RH of 10%. In the case of patterning with the boron-doped conductive diamond-coated tip on the ester SAM, the width of the patterned line showed no clear increase with increasing RH. These pattern width changes were discussed in terms of the size of adsorbed water meniscus at the AFM-tip/sample junction as confirmed by force curve measurements; in the case of the boron-doped conductive diamond-coated tip which is rather more hydrophobic than the Rh-coated Si tip, the size of water meniscus hardly affected RH.

Study of the Hyperthermal Proton Bombardment Effects on Self-Assembled Monolayers of Dodecanethiol on Au(111)

Proton bombardment on self-assembled monolayers (SAMs) of dodecanethiol formed on a gold surface is performed and its effect is studied by scanning tunneling microscope (STM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). STM gives clear evidence that a relatively uniform layer of organic nanoclusters with different sizes and densities can be created on the surface. Analysis of the mass spectra collected from the SAM samples reveals that large hydrocarbon fragments from the bombarded sample area show higher intensity than those from the virgin area, which is a strong indication that selective C-H bond cleavage and cross-linking are induced by the proton beam bombardment. We demonstrate that the size and density of the clusters can be harnessed by controlling the proton beam energy and fluence. The clusters grow bigger, at the expense of a decreasing number density, with increasing proton energy or fluence.

Rapid degradation of alkanethiol-based self-assembled monolayers on gold in ambient laboratory conditions

Self-assembled monolayers (SAMs) consisting of alkanethiols and similar sulfur-containing molecules on noble metal substrates are extensively used and explored for various chemical and biological surface-functionalization in the scientific community. SAMs consisting of thiol- or disulfide-containing molecules adsorbed on gold are commonly used due to their ease of preparation and stability. However, the gold–thiolate bond is easily and rapidly oxidized under ambient conditions, adversely affecting SAM quality and structure. Here, the oxidation of dodecanethiol on gold is explored for various 12-h exposures to ambient laboratory air and light. SAM samples are freshly prepared, air-exposed, and stored in small, capped vials. X-ray photoelectron spectroscopy (XPS) reveals nearly complete oxidation of the thiolate in air-exposed samples, and a decrease in carbon signal on the surface. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the carbon K-edge shows a loss of upright orientational order upon air exposure. Alternatively, the oxidation of the thiolate is minor when SAMs are stored in limited-air-containing small 15 ml vials. Thus, care must be taken to avoid SAM degradation by ensuring alkanethiolates on gold have sufficient durability for each intended environment and application.

Nanoscopic surface architecture based on molecular self-assembly and scanning probe lithography

Scanning probe lithography and molecular self-assembly have been integrated for constructing surface nanostructures. In the first part of this paper, organosilane monolayers grown through molecular self-assembly, i.e., self-assembled monolayers (SAMs) are applied as patterning media in scanning probe lithography based on scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) with an electrical conductive probe. The SAMs are successfully applied to resist films patternable in scanning probe lithography and etching masks for nanostructuring Si. Some of advanced techniques, namely, current-regulated AFM lithography, multilayer resist system and integration with photolithography, are presented as well. In the latter part, chemical methods to fabricate surface nanostructures are shown. Using SAM samples, on which nanopatterns formed by scanning probe lithography, as templates, minute objects including, organic molecules, nanoparticles and biomolecules are spatially arranged.

Molecular dynamics simulation of non-contact atomic force microscopy of self-assembledmonolayers on Au(111)

We attempted molecular dynamics (MD) simulation of non-contact atomic forcemicroscopy (nc-AFM) of a self-assembled monolayer (SAM) on Au(111) by modifying apreviously developed MD simulation method. In the previous simulation, the interactionbetween a single gold atom tip and a SAM sample consisting of the united atoms(CH3) with amass (m0) and a continuum gold substrate was treated explicitly in terms of microscopicpotentials. On the other hand, the probe tip with an artificial reduced mass(msl) was bound to a cantilever spring in order to describe its macroscopic nc-AFM behaviour. Inthe present study, we treated explicitly the interaction between a gold probe tip (theradius = R) having a single gold atom at the apex and the same SAM on gold. By using this new MDmethod, we succeeded in interpreting our recent experimental nc-AFM observation.Namely, the local atom–atom interactions in terms of Lennard-Jones potentials togetherwith the long-range sphere–substrate interaction are responsible for inversion andmolecular-dependent imaging of van der Waals (i.e. chemically inert) molecules bound ongold substrates.

Long-Term Stability of Self-Assembled Monolayers in Biological Media

Undecanethiol (C11H23SH) and tri(ethylene glycol)-terminated undecanethiol (HO(C2H4O)3C11H22SH) self-assembled monolayers (SAMs) on clean gold surfaces were prepared and characterized. The SAMs were then immersed into either phosphate-buffered saline or calf serum. The SAM samples were investigated using several analytical techniques at numerous points over the next 35 days. Contact angles and current densities in voltammetry changed dramatically for the PBS samples over the time period, particularly after 21 days. Results indicate substantial loss of the integrity of the SAM. Similar alterations with time were observed for the calf serum samples in both contact angle and voltammetry measurements. X-ray photoelectron spectroscopy indicates that the likely origin is desorption of the alkanethiol moiety as evidenced by appreciable loss of the S 2p signal after 35 days.

Photoisomerisation process of self-assembled monolayers of some novel azobenzenes

Self-assembled monolayers (SAMs) of four azobenzene derivatives containing a sulphonamide linkage and a thiol group were deposited on both vacuum-evaporated and sputtered gold (Au) thin films. The optically induced switching effect on the surface potential Us upon alternating irradiation with UV (centred at 360 nm) and visible (centred at 450 nm) light was investigated by the Kelvin probe technique. Changes in surface potential were in the range ΔUs=4–70 mV for SAM samples, depending on the structure of the molecule. The dependence of surface potential change and decay time upon irradiation on the structure of the Au film was observed. Copyright © 2000 John Wiley & Sons, Ltd.

Photoisomerisation process of self‐assembled monolayers of some novel azobenzenes

Self-assembled monolayers (SAMs) of four azobenzene derivatives containing a sulphonamide linkage and a thiol group were deposited on both vacuum-evaporated and sputtered gold (Au) thin films. The optically induced switching effect on the surface potential Us upon alternating irradiation with UV (centred at 360 nm) and visible (centred at 450 nm) light was investigated by the Kelvin probe technique. Changes in surface potential were in the range ΔUs=4–70 mV for SAM samples, depending on the structure of the molecule. The dependence of surface potential change and decay time upon irradiation on the structure of the Au film was observed. Copyright © 2000 John Wiley & Sons, Ltd.

Surface Modification of Polyanhydride Microspheres

Polyanhydrides are emerging as a new class of biodegradable polymers for drug delivery.1-3 The degradation of polyanhydride delivery systems is largely through surface erosion, potentially leading to zero-order release of encapsulated drugs.4,5 Recently, the Food and Drug Administration has approved the use of the polyanhydride poly[sebacic acid-co-1,3-bis(p-carboxyphenoxy)propane] to deliver drugs for treatment of brain cancer.6 This is one of the few examples where an implantable synthetic degradable polymer has been approved for human use. The use of polyanhydride polymers in oral delivery of insulin and genes further broadens the scope of their applications in drug delivery.7 Despite numerous studies on polyanhydrides, the compositions thus far developed do not have the capability to target specific organs or cell types. To achieve this goal, it would be important to have an approach that could enable the chemical functionalization of microspheres to allow the attachment of targeting ligands (e.g. antibodies, etc). The recognition of ligands on the surface of delivery systems by organ-specific or cellspecific receptors may result in organ-targeted drug delivery8,9 or in improved drug uptake through ligand-induced endocytosis.10,11 Here we describe a one-step procedure to covalently modify the surface of poly(sebacic acid) (pSA)12 microspheres with ligands containing amino groups. Amino groups react with the anhydride groups on the surface of microspheres, which results in covalent attachment through an amide bond (Scheme 1). Experimental SectionsMaterialsspSA was synthesized from sebacic acid following a published procedure.12 L-Argininamide dihydrochloride and guanidine hydrochloride were purchased from Sigma. N-(5-Aminopentyl)biotinamide, 6-((biotinoyl)amino)hexanoic acid, and fluorescein-conjugated avidin were purchased from Molecular Probes (Eugene, Oregon). The pSA (Mw ) 34 kDa) microspheres (mean diameter ) 24 ( 1 μm) were formed by solvent evaporation.13 Surface Modification of Microspheres by ArgininamidesThe pSA microspheres (30 mg for each concentration of argininamide) were suspended in solutions containing different concentrations of argininamide (0, 0.40, 1.2, 4.0, 12, and 40 mM, 2 mL each) in borate buffer (0.20 M, pH 9.0) at room temperature. After 15 min, the surface modification was stopped by acidifying the solution to ∼pH 6 by addition of acetate buffer (0.20 M, pH 5.0, 5 mL).14 The microspheres were collected after centrifugation (10 000 rpm for 10 min) at 4 °C, washed with 2 M aqueous NaCl, distilled water (twice), and ethanol, and dried in vacuo. The surface density of argininamide was analyzed by X-ray photoelectron spectroscopy (XPS), and its bulk concentration was analyzed by 1H NMR (400 MHz) of microspheres dissolved in CDCl3. The size of the microspheres was measured by a Coulter Multisizer (Coulter Electronic Limited, Luton, U.K.). Guanidine Hydrochloride ExperimentsThe pSA microspheres (20 mg) were suspended in 30 mM guanidine hydrochloride solution in borate buffer (0.20 M, pH 9.0, 2 mL) at rt. After 15 min, acetate buffer (0.20 M, pH 5.0, 5 mL) was added. The microspheres were collected, washed with 2 M NaCl, distilled water (twice), and ethanol, and dried in vacuo. Self-Assembled Monolayer (SAM) ExperimentssThe gold substrates were prepared by e-beam evaporation of 5 nm of titanium and 200 nm of gold to a single-crystal silicon wafer in a deposition chamber. The gold-coated wafers were cut into ca. 1 cm × 2 cm pieces and self-assembled monolayers (SAMs) were formed by immersing the gold substrates in an ethanolic solution of 16-mercaptohexadecanoic acid (2 mM, 10 mL) at room-temperature overnight.15 One sample was used as an unmodified control. Two other samples were soaked in 10 mM argininamide solution in borate buffer (0.20 M, pH 9.0) at rt. After 15 min, one sample was taken out and blown dry by Dust-Off (Falcon Safety Products, Inc., NJ). The other sample was washed by 2 M aqueous NaCl and distilled water (twice) and finally blown dry. XPS analyses showed nitrogen signals in the unwashed SAM sample but not in the washed SAM sample nor the control sample (data not shown). The carboxylic acid groups on the SAMs were further transformed to interchain anhydrides using trifluoroacetic anhydride following a literature procedure.15 The resulting SAM sample was soaked in 10 mM argininamide solution in borate buffer (0.20 M, pH 9.0) at rt for 15 min. The sample was washed by 2 M aqueous NaCl and distilled water (twice) and blown dry. XPS analyses showed the appearance of nitrogen signals. Avidin Binding to the Surface of Biotin-Attached pSA MicrospheressThe pSA microspheres (10 mg) were suspended in a solution ofN-(5-aminopentyl)biotinamide (10.0 * To whom correspondence should be addressed. Telephone: (617) 253-3107. Fax: (617) 258-8827. Email: rlanger@mit.edu. † Massachusetts Institute of Technology. ‡ Massachusetts General Hospital. § Harvard University. Scheme 1sSurface modification of pSA microspheres with ligands containing amino groups via amide formation.