Disulfide Self-assembled Monolayers Research Articles

Study of the helium cross-section of unsymmetric disulfide self-assembled monolayers on Au(111)

We have investigated the formation of self-assembled monolayers (SAMs) of 11-hydroxyundecyl decyl disulfide (CH3-(CH2)9-S-S-(CH2)11-OH, HDD) and 11-hydroxyundecyl octadecyl disulfide (CH3-(CH2)17-S-S-(CH2)11-OH, HOD) produced by supersonic molecular beam deposition (SMBD). The study has been carried out by means of helium diffraction at very low film coverage. In this regime helium single molecule cross sections have been estimated in a temperature range between 100K and 450K. The results show a different behavior above 300K that has been interpreted as the starting of mobility with the formation of two thiolate moieties either linked by a gold adatom or distant enough to prevent cross section overlapping. Finally, helium diffraction patterns measured at 80K for the SAMs grown at 200K are discussed and the results support the proposed hypothesis of molecular dissociation based on the cross section data.

Investigation of the deposition and thermal behavior of striped phases of unsymmetric disulfide self-assembled monolayers on Au(111): the case of 11-hydroxyundecyl decyl disulfide.

Self-assembled monolayers (SAMs) of unsymmetric disulfides on Au(111) are used to form mixed SAMs that can be utilized in many applications. Here, we have studied 11-hydroxyundecyl decyl disulfide (CH3-(CH2)9-S-S-(CH2)11-OH, HDD) SAMs produced by supersonic molecular beam deposition and characterized by He diffraction. The film growth was monitored at different temperatures up to a coverage which corresponds to a full lying down phase and the diffraction analysis shows that below 250 K the phase is different from the phase measured above 300 K. During the annealing of the film, two phase transitions were observed, at 250 K and 350 K. The overall data suggest that the former is related to an irreversible phase separation of HDD above 250 K to decanethiolate (-S-(CH2)9-CH3, DTT) and hydroxyundecylthiolate (-S-(CH2)11-OH, MUDT), while the latter to a reversible melting of the film. Above 450 K, the specular intensity shows an increase related to film desorption and different chemisorbed states were observed with energies in the same range as observed for decanethiol (H-S-(CH2)9-CH3, DT) and mercaptoundecanol (H-S-(CH2)11-OH, MUD) SAMs.

11-Hydroxyundecyl octadecyl disulfide self-assembled monolayers on Au(111)

Here, we report a helium atom diffraction study of 11-hydroxyundecyl octadecyl disulfide (CH3(CH2)17SS(CH2)11OH, HOD) self-assembled monolayers (SAMs) produced by supersonic molecular beam deposition (SMBD). Two different lying down monolayer phases were observed depending on the substrate temperature. At low temperatures a poorly ordered phase was observed, while the diffraction patterns of the film grown at high temperatures were similar to that of mercaptoundecanol (MUD) SAMs reported previously in the literature. The transition from the low temperature phase to the high temperature phase is due to SS bond cleavage at the surface. Desorption from several different chemisorbed and physisorbed states were observed with energies in the same range as observed for MUD and octadecanelthiol (ODT) SAMs.

Molecular-Scale Investigation of Tolane Disulfide Self-Assembled Monolayers on Au(111) Using Scanning Tunneling Microscopy

Molecular-scale surface structures of self-assembled monolayers (SAMs) formed by the spontaneous adsorption of tolane disulfides (TDS) on Au(111) in a 1 mM mixed solution of ethanol/N, N'-dimethylformamide (9:1) were examined using scanning tunneling microscopy (STM). The STM study revealed that TDS SAMs formed after a 24 h immersion at room temperature were composed of two-dimensional (2D) ordered phases with inhomogeneous surface morphologies and no clear domain boundaries. However, after 2 h immersion at 50 degrees C, uniform 2D ordered domains with clear domain boundaries were observed, which could be described as c(3 x square root of 3) structures with centered rectangular unit cell. Interestingly, a unique intermediate ordered phase with a low surface coverage was also observed. After a longer immersion for 24 h at 50 degrees C, only the uniform c(3 x square root of 3) domains were observed with a corrugation that may have reflected surface reconstruction of the Au(111) surfaces. From this study, we found that 2D ordered TDS SAMs with large and uniform domains on Au(111) can be obtained by controlling the solution temperature and immersion time.

Estimate of the potential difference across metal/water interfaces and across the lipid bilayer moiety of biomimetic membranes: an approach

Mercury is a particularly advantageous support of lipid, thiol and disulfide self-assembled monolayers (SAMs), because it provides them with a perfectly smooth and fluid surface and allows their gradual expansion in aqueous solution with progressive tilt of the adsorbed molecules, without water incorporation. These unique advantageous features permit an estimate of the extrathermodynamic absolute potential difference, ϕ, across the Hg|water interface and of the surface dipole potential of mercury-supported SAMs. Two different models of metal-supported tethered bilayer lipid membranes (tBLMs) incorporating a cation-selective channel predict that the ϕ value at the inflection point of a plot of the in-phase component, Y′, of the electrochemical admittance against the applied potential E is almost coincident with the surface dipole potential, χs, of the hydrophilic spacer moiety of the tBLM. This prediction allows an estimate of the absolute potential difference, ϕ, across the interface between any metal capable of supporting tBLMs and the bulk aqueous phase, provided the χs value of the tBLM is known. Moreover, the potential difference across the lipid bilayer moiety of the tBLM (i.e., the transmembrane potential) is shown to be practically equal to χs at the inflection point of the corresponding Y′ vs.E plot. This approach is applied to polycrystalline Au and Ag(111).

A New Method to Evaluate the Surface Dipole Potential of Thiol and Disulfide Self-Assembled Monolayers and Its Application to a Disulfidated Tetraoxyethylene Glycol

A procedure to evaluate the surface dipole potential chi of thiol and disulfide self-assembled monolayers (SAMs) is described. The procedure consists of self-assembling the monolayers on a hanging mercury drop electrode and of measuring the charge involved in a progressive expansion of the mercury drop. This measurement is then combined with an estimate of the charge density q experienced by diffuse layer ions, obtained by measuring the diffuse layer capacitance of the SAM at different electrolyte concentrations by electrochemical impedance spectroscopy. These chi measurements, combined with chronocoulometric measurements of the total charge density sigma(M) against potential, indicate that SAMs of tetraoxyethylene glycol-D,L-alpha-lipoic acid ester (TEGL), 2,3-di-O-phytanyl-sn-glycerol-1-tetraoxyethylene glycol-D,L-alpha-lipoic ester (DPTL), and trioxyethyleneoxythiol (EO3) on mercury may undergo a reversal in the surface dipole potential of their polyoxyethylene chain with a change in the interfacial electric field. Moreover, TEGL and EO3 form stable SAMs without electron transfer to the metal, while no such conclusion can be drawn for DPTL.

Catalytic probe lithography: catalyst-functionalized scanning probes as nanopens for nanofabrication on self-assembled monolayers.

This article describes the use of scanning catalytic probe lithography for nanofabrication of patterns on self-assembled monolayers (SAMs) of reactive adsorbates. Catalytic writing was carried out by scanning over bis(omega-tert-butyldimethyl-siloxyundecyl)disulfide SAMs using 2-mercapto-5-benzimidazole sulfonic acid-functionalized gold-coated AFM tips. The acidic tips induced local hydrolysis of the silyl ether moieties in the contacted areas, and thus patterned surfaces were created. Diffusion effects arising from the use of an ink were excluded in these type of experiments, and therefore structures with well-defined shapes and sizes were produced. The smallest lines drawn by this technique were about 25 nm wide, corresponding to the actual contact area of the tip. Lateral force microscopy studies performed on different SAMs helped to clarify the nature and cause of the friction contrasts observed by AFM. Dendritic wedges with thiol functions inserted into the catalytically written areas, thus enhancing the height contrast. The created patterns open possibilities to build 3D nanostructures.

Alignment Behavior of Discotic Nematic and Rectangular Columnar Mesophases on Self-Assembled Monolayers of Alkanethiol and Asymmetrical Alkyl Disulfide

Alignment behavior of a discotic liquid crystalline triphenylene (C8OBTP) on substrates coated with self-assembled monolayers (SAMs) of alkanethiol and asymmetrical alkyl disulfide was investigated by polarized microscopy. Discotic nematic (ND) and rectangular columnar (Colr) phases of C8OBTP shows a different alignment behavior depending on the element of SAMs. C8OBTP exhibits a fan-shaped texture on an alkanethiol SAM in Colr phase, while it has a tendency to show a planar alignment on asymmetrical alkyl disulfide SAM. The different elements of the SAM could influence the alignment behavior of discotic liquid crystal distinctly even in columnar mesophase which has highly self-cohesive forces of systems.

Photoisomerization Reaction of Unsymmetrical Azobenzene Disulfide Self-Assembled Monolayers Studied by Surface Plasmon Spectroscopy: Influences of Side Chain Length and Contacting Medium

We report the characteristics of azobenzene-containing self-assembled monolayers (SAMs) which are designed and synthesized for surface photoisomerization reactions. The photoreactive SAMs were composed of unsymmetrical azobenzene disulfides, in which the free volumes for photoreaction of azobenzene moieties are guaranteed by 50% dilution of dye functions at the molecular level on the surface. The photoswitching reaction was monitored in situ through the change of optical film thickness by means of surface plasmon spectroscopy. The influences of alkyl side chain length and contacting medium on the photoreaction were also investigated. The profile of the trans to cis photoisomerization reaction by UV light irradiation exhibited a clear deviation from first-order kinetics, suggesting a steric hindrance effect on the photoreaction process.

Novel self-assembled monolayers of disulfides with bicyclo[2.2.2]octane moieties of Au(111).

A series of disulfides containing bicyclo[2.2.2]octane moieties have been synthesised and their self-assembled monolayers (SAMs) on Au(111) have been characterized using scanning tunnelling microscopy (STM).

Nanoscopic Evidence for Dissociative Adsorption of Asymmetric Disulfide Self-Assembled Monolayers on Au(111)

We have monitored the adsorption process of 11-hydroxyundecyl octadecyl disulfide (CH3(CH2)17SS(CH2)11OH, HUOD) self-assembled monolayers (SAMs) on the Au(111) surface during the initial SAM growth stage using scanning tunneling microscopy (STM). STM imaging clearly exhibits two types of phase-separated domains having different corrugation periodicities which are consistent with the lengths of CH3(CH2)17S and HO(CH2)11S molecules, respectively. This is the first direct observation of the dissociative adsorption of disulfides on the nanometer scale. The self-assembly process of HUOD molecules physisorbed on graphite, on the other hand, is mainly governed by a hydrogen bond with a hydroxyl group facing another hydroxyl group of adjacent molecules without any S−S bond cleavage of the disulfide group unlike the adsorption process of HUOD molecules on gold. We have demonstrated a new and simple insightful method of comparing monolayers chemisorbed on Au(111) with those physisorbed on graphite.