[show abstract][hide abstract] ABSTRACT: Biorepulsive oligo(ethylene glycol) substituted alkanethiolate (OEG-AT) monolayers on gold
can serve as primary templates for promoted (by electron irradiation) exchange reaction with
thiolated ssDNA species, resulting in the formation of mixed OEG-AT/ssDNA monolayers of
desired composition. Here we test the ability of alternative, disulfide precursors to serve as
substituents in such a reaction. Two representative molecules, based on adenine-based homooligonucleotide
(25-mer), were used, viz. asymmetric disulfide with a short second chain
(A25SSOH) and symmetric disulfide (A25SSA25). The results were compared to the
reference system of thiolated ssDNA (A25SH). Both disulfide precursors were found to be
suitable for the reaction, further extending the types of commercially available compounds
which can be used for this approach. A25SSOH exhibited quite high efficiency, similar to
A25SH, while the efficiency of A25SSA25 was noticeably lower, especially at low irradiation
doses (< 0.6 mC/cm2). Also, the single component, A25SSA25-based ssDNA monolayer was
of lower quality as compared to the films prepared from the A25SH and A25SSOH
precursors. The above observations were explained by the bulky character and conformational
flexibility of A25SSA25, which hinders the proper assembly and efficient exchange reaction.
The Journal of Physical Chemistry C 01/2014; · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nanoscopic metal-molecule-metal junctions consisting of Fe-bis(terpyridine)-based ordered nanostructures are grown in layer-by-layer fashion on a solid support. Hopping is demonstrated as the main charge-transport mechanism both experimentally and theoretically.
[show abstract][hide abstract] ABSTRACT: A series of self-assembled monolayers (SAMs) of partially fluorinated alkanethiols (PFAT) with variable length of the fluorocarbon segment, viz. F(CF2)n(CH2)11SH (FnH11SH, n = 6, 8, and 10) were prepared on GaAs(001) and characterized by several complementary spectroscopic techniques. The SAMs were found to be well ordered and densely packed, and thus able to protect the substrate from oxidation, with the highest quality F10H11SH monolayer being most effective in this regard. The packing density of the SAMs was governed by the bulky fluorocarbon segments having a helical conformation. With decreasing length of these segments, a slight decrease in the packing density accompanied by progressive deterioration of the orientational order and a slight disturbance of the conformational order in the fluorocarbon part of the films occurred. In contrast, the hydrocarbon segments of the FnH11SH SAMs exhibited similar average orientation in all studied monolayers, accompanied by a partial conformational disorder. This was explained by the effect of a strong bending potential favoring a substrate–S–C angle of 104° and predefining, thus, the orientation of the hydrocarbon segments.
The Journal of Physical Chemistry C 11/2013; 117(49):26166–26178. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Static charge transport (CT) properties of nitrile-substituted oligophenylenes and oligo(phenylene ethynylene)s (NC-OPh and NC-OPE, respectively) assembled via the thiolate anchor on gold substrates were measured by the mercury drop junction technique. The derived attenuation factors (β), viz. 0.53 ± 0.1 and 0.30 ± 0.08 Å–1 for the NC-OPh and NC-OPE monolayers, respectively, correlate well with the literature values for the analogous nonsubstituted systems, suggesting that the attachment of the nitrile group to the OPh or OPE backbone does not significantly affect their transport properties. This finding provides a basis for the use of the nitrile moiety as a resonantly addressable group and as specific charge injection site in the measurements of dynamic CT by resonant Auger electron spectroscopy. The comparison between the static and dynamic β values for the NC-OPh monolayers implies that the static value corresponds to nonresonant injection conditions. This suggests, potentially, that the static CT can be performed more efficiently by controlling the specific molecular orbitals into which charge carriers are injected.
The Journal of Physical Chemistry C 11/2013; 117(48):25556–25561. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: A series of self-assembled monolayers (SAMs) formed on Au(111) from partially fluorinated alkanethiols (PFAT) with variable length of the fluorocarbon chain, viz. F(CF2)n(CH2)11SH (FnH11SH, n = 6, 8, and 10), was studied by high resolution X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and near edge X-ray absorption fine structure spectroscopy. The SAMs were found to be highly ordered and densely packed. The packing density was governed by the bulky fluorocarbon segments which adopted a helical conformation typical of this entity. With decreasing length of the fluorocarbon segment, progressive deterioration of the orientational order accompanied by a slight decrease in the packing density was observed in the fluorocarbon part of the FnH11SH SAMs. The conformation of the fluorocarbon segment was persistent through the series, with probably only a minor deterioration for n = 8 and 6. The hydrocarbon segments of the monolayers were, however, unaffected by the deterioration of the orientational order in the fluorocarbon part. They persistently exhibited all trans planar conformation, typical of densely packed assemblies of these moieties, while their orientation, given by the characteristic tilt and twist angles, mimicked that of the nonsubstituted alkanethiolate SAMs on Au(111). This behavior is explained by the effect of the bending potential which predefines the orientation of the hydrocarbon segments even if they are separated beyond the equilibrium spacing by the bulky fluorocarbon moieties.
The Journal of Physical Chemistry C 08/2013; 117(37):18967-18979. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: We used a combination of synchrotron-based X-ray photoelectron spectroscopy (XPS) and angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the chemical integrity, purity and possible internal alignment of single-strand (ss) adenine deoxynucleotide (poly(A)) DNA brushes. The brushes were synthesized by surface-initiated enzymatic polymerization (SIEP) on a 25-mer of adenine self-assembled monolayer (SAM) on gold (A25-SH), wherein the terminal 3´-OH of the A25-SH serve as the initiation sites for SIEP of poly(A). XPS and NEXAFS spectra of poly(A) brushes were found to be almost identical to those of A25-SH initiator, with no unambiguous traces of contamination. Apart from the well-defined chemical integrity and contamination-free character, the brushes were found to have a high degree of orientational order, with an upright orientation of individual strands, despite their large thickness up to ~55 nm, that corresponds to a chain length of at least several hundreds nucleotides for individual ssDNA molecules. The orientational order exhibited by these poly(A) DNA brushes, mediated presumably by base stacking, was found to be independent of the brush thickness as long as the packing density was high enough. The well-defined character and orientational ordering of the ssDNA brushes make them a potentially promising system for different applications.
The Journal of Physical Chemistry B 07/2013; · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: Exchange reaction between the primary self-assembled monolayer (SAM) on gold and potential molecular substituents capable of forming a SAM on the same substrate can be promoted by electron irradiation. Here we demonstrate that such a promoted reaction can be performed not only with thiols but with disulfides as substituents as well. This extends significantly the assortments of the suitable compounds, resulting in a broader variety of mixed SAMs and chemical patterns which can be fabricated by this technique. The kinetics of the promoted exchange reaction was studied in detail. The feasibility and practical usefulness of the approach were demonstrated by the experiments with a disulfide substituent bearing a tail group which can serve as an initiator for surface-initiated polymerization. A variety of complex polymer brush patterns was prepared using several representative polymers, relevant for biomedical research and applications, as test systems.
The Journal of Physical Chemistry C 05/2013; · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Immobilization of single stranded DNA (ssDNA) on solid support is an
important issue for binding and detection of the complementary DNA
target as well as for the recognition of DNA binding proteins. Here we
present a universal two-step approach to fabricate mixed monomolecular
films comprising thiolated ssDNA and oligo(ethylene glycole) substituted
alkanethiolates (OEG-AT) which exhibit biocompatible properties
providing a suitable matrix for the ssDNA . At first, the primary
OEG-AT monolayer was irradiated with electrons to create defects in
a controlled fashion. In the second step, a defect-promoted exchange
between the molecules in the monolayer and ssDNA species in solution
occurred, with the extent depending on the irradiation dose. The
approach was combined with E-beam lithography, which allowed us
to fabricate versatile ssDNA patterns of any required shape imbedded
in the protein-repelling matrix. Further, applying surface initiated
enzymatic polymerization, we succeeded to amplify the above
ssDNA/OEG-AT patterns in the z-direction in a controlled fashion.
Both ssDNA pattern and related 3D nanostructures can be widely
used as versatile nanofabrication platform in such important fields as
bio-engineering, bio-technology, and sensor fabrication
DPG Spring Meetings 2013, University of Regensburg , Germany; 03/2013
[show abstract][hide abstract] ABSTRACT: Surface-enhanced Raman scattering (SERS) has become increasingly popular in the scientific and industrial communities because of its analytical capabilities and potential to study fundamentals in plasmonics. Although under certain conditions extremely high sensitivity is possible, the practical use of SERS is frequently limited by instability and poor reproducibility of the enhancement factor. For analytical applications or for comparative measurements to enable the distinction between electromagnetic and chemical enhancement, the development of standardized and recyclable SERS substrates, having uniform and persistent performance,is proposed. To this end, we have fabricated periodic nanoslit arrays using extreme ultraviolet lithography that provide average large (2*106) and homogeneous SERS enhancement factors with a spot-to-spot variability of less than 3%. In addition,they are reusable without any degradation or loss of enhancement. The fabrication of such arrays consists of two steps only, lithographic patterning followed by metal evaporation. Both processes may be performed over areas of several square mm on any planar substrate. The sensor capabilities were demonstrated by substrates with monomolecular films of several different thiols. The concept of reusable SERS substrates may open a powerful platform within an analytical tool and in particular for systematic SERS studies for the investigation of fundamental parameters such as chemical enhancement, surface selection rules, and molecular alignment.
Journal of Raman Spectroscopy 02/2013; 44:170-175. · 2.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Self-assembled monolayers (SAMs) of thiols on gold substrates are potentially important systems for future technologies such as molecular electronics and sensing. Especially in molecular electronics a strong interaction and coupling between the "device" molecules and substrate is crucial. In this context, we present here two series of novel SAM precursors, viz. bidentate oligophenylenes with either 1,3-phenylenedimethanethiol or pyridine-2,6-diyldimethanethiol anchoring group. Both series are shown to form densely packed monolayers with a low level of contamination and a high orientational order that are additionally promoted by the interaction between the terminal pyridine moiety and the substrate in the second series. At the same time, most of the SAM constituents do not exhibit a strictly bidentate bonding to the substrate - whereas one anchor group has a thiolate-type bonding, the other is weakly coordinated, unbound, or participating in a disulfide bridge with the adjacent molecules. We believe that such a bonding heterogeneity stems from the fundamental problems of molecular self-assembly in the given case.
Physical Chemistry Chemical Physics 01/2013; · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Creative design: An approach to preparing mixed monolayers of thiolated single-stranded DNA (ssDNA) and oligo(ethylene glycol)s (OEG-AT) in a broad range of compositions as well as ssDNA/OEG-AT patterns of any required shape (see top figure) has been shown. A combination of this approach with surface-initiated enzymatic polymerization allows complex 3D DNA nanostructures to be sculpted with high spatial precision (bottom).
Angewandte Chemie International Edition 09/2012; 51(41):10303-6. · 13.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: This article describes the molecular structure-function relationship for a series of biphenylthiol derivatives with varying torsional degree of freedom in their molecular backbone when self-assembled on gold electrodes. These biphenylthiol molecules chemisorbed on Au exhibit different tilt angles with respect to the surface normal and different packing densities. The charge transport through the biphenylthiol self-assembled monolayers (SAMs) showed a characteristic decay trend with the effective monolayer thickness. Based on parallel pathways model the tunneling decay factor β was estimated to be 0.27 Å(-1). The hole mobility of poly(3-hexylthiophene)-based thin-film transistors incorporating a biphenylthiol SAM coating the Au source and drain electrodes revealed a dependence on the injection barrier with the highest occupied molecular orbital (HOMO) level of the semiconductor. The possible role of the resistivity of the SAMs on transistor electrodes on the threshold voltage shift is discussed. The control over the chemical structure, electronic properties, and packing order of the SAMs provides a versatile platform to regulate the charge injection in organic electronic devices.
[show abstract][hide abstract] ABSTRACT: Self-assembled monolayers (SAMs) of isophthalic acid (IPA) and trimesic acid (TMA) formed on Cu modified Au(111) substrates by adsorption from aqueous solution were characterised by synchrotron-based X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and scanning tunneling microscopy (STM). Applying the layer-by-layer (LbL) method, the initial stages of growth of metal–organic coordination layers from Cu-acetate and TMA has been studied by STM. Both IPA and TMA SAMs exhibit a row structure with the aromatic units tilted by about 45° with respect to the surface normal. The average distance between rows is 3 × Au–Au distance (= 8.67 Å) with, however, slight variations between different rows. For IPA a very regular pattern is observed giving rise to a rectangular (9 × √3) unit cell containing three molecules. Combining all data a structure is suggested where the molecules are tilted and azimuthally rotated. The variation in the inter-row distance is explained by correlated tilts and rotations in opposite directions. STM studies of the first two LbL deposition cycles reveal predominantly disordered nucleation at domain boundaries. The observation of epitaxial features is explained to arise from a particular combination of molecular orientation in adjacent rows.
Chemical Science 05/2012; 3(6):1858-1865. · 8.31 Impact Factor
[show abstract][hide abstract] ABSTRACT: Cited By (since 1996):2, Export Date: 25 June 2013, Source: Scopus, References: Weissman, S.I., Intramolecular Electron Exchange in Anions of Paracyclophanes (1958) J. Am. Chem. Soc., 80, pp. 6462-6463;
[show abstract][hide abstract] ABSTRACT: Self-assembled monolayers (SAMs) of nitrile-substituted oligo(phenylene ethynylene) thiols (NC-OPEn) with a variable chain length n (n ranging from one to three structural units) on Au(111) were studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy and near-edge absorption fine-structure spectroscopy. The experimental data suggest that the NC-OPEn molecules form well-defined SAMs on Au(111), with all the molecules bound to the substrate through the gold-thiolate anchor and the nitrile tail groups located at the SAM-ambient interface. The packing density in these SAMs was found to be close to that of alkanethiolate monolayers on Au(111), independent of the chain length. Similar behavior was found for the molecular inclination, with an average tilt angle of ~33-36° for all the target systems. In contrast, the average twist of the OPEn backbone (planar conformation) was found to depend on the molecular length, being close to 45° for the films comprising the short OPE chains and ~53.5° for the long chains. Analysis of the data suggests that the attachment of the nitrile moiety, which served as a spectroscopic marker group, to the OPEn backbone did not significantly affect the molecular orientation in the SAMs.
Beilstein Journal of Nanotechnology 01/2012; 3:12-24. · 2.37 Impact Factor