Molecular organization in SAMs used for neuronal cell growth

The attachment of cells onto solid supports is fundamental in the development of advanced biosensors or biochips. In this work, we characterize cortical neuron adhesion, growth, and distribution of an adhesive layer, depending on the molecular structure and composition . Neuronal networks are successfully grown on amino-terminated alkanethiol self-assembled monolayer (SAM) on a gold substrate without adhesion protein interfaces. Neuron adhesion efficiency was studied for amino-terminated, carboxy-terminated, and 1:1 mixed alkanethiol SAMs deposited on gold substrates. Atomic force microscopy and X-ray photoelectron spectroscopy were used to measure the roughness of gold substrate and thickness of SAM monolayers. Conformational ordering and ionic content of SAMs were characterized by vibrational sum frequency generation (VSFG) spectroscopy. Only pure amino-terminated SAMs provide efficient neuronal cell attachment. Ordering of the terminal amino groups does not affect efficiency of neuron adhesion. VSFG analysis shows that ordering of the terminal groups improves with decreasing surface roughness; however the number of gauche defects in alkane chains is independent of surface roughness. We monitor partial dissociation of carboxy groups in mixed SAMs that implies formation of NH3+ neighbors and appearance of catanionic structure. Such catanionic environment proved inefficient for neuron adhesion. Surface roughness of metal within the 0.7-2 nm range has little effect on the efficiency of neuron adhesion. This approach can be used to create new methods that help map structure-property relationships of biohybrid systems.     

Assembled monolayers of Mo3S4(4+) clusters on well-defined surfaces

A class of inorganic monolayers formed by assembling the molybdenum-sulfur cluster, Mo3S4(4+), onto a well-defined Au(111) surface is presented. The monolayers have been comprehensively characterized by electrochemistry, X-ray photoelectron spectroscopy (XPS), and in situ scanning tunneling microscopy (in situ STM). The voltammetric data show strong reductive and oxidative desorption signals from Au-S interactions, supported by the presence of both S and Mo signals in XPS. In situ STM shows many small pits in the dense adlayers uniformly spread over the surface, which is a typical feature of self-assembled monolayers (SAMs) of alkanethiols. The density of the pits is ca. 23 (+/-5)% and is significantly higher than for straight-chain alkanethiol SAMs with a single -SH group. The pit shapes are irregular, suggesting multiple Au-S interactions from Mo3S4(4+). High resolution images disclose bright round spots of ca. 8 A diameter representing individual molecules in the SAM. This is the first example of in situ monolayer formation by a metal-chalcogenide cluster directly anchored onto the gold surface through core ligands and offers a simple way to prepare a new class of functionalized inorganic monolayers.     

金表面不同链长烷基硫醇自组装单分子膜表面电势的变化规律

从Helmholtz模型出发,对生长在金表面不同链长烷基硫醇自组装单分子膜(SAM)表面电势的变化规律进行了理论研究.利用量子化学软件Gaussian03和MOPAC,讨论了分子偶极矩、相对介电常数以及分子的倾斜角对SAM表面电势的影响.研究表明,不同链长烷基硫醇SAM中分子的倾斜角随烷基链长度的规律性变化是引起SAM表面电势变化的主要原因.从SAM形成机制出发,对金表面不同链长烷基硫醇SAM表面电势的变化规律及其成因提出了新的解释.     

Electrochemically deposited palladium as a substrate for self-assembled monolayers

The vast majority of reports of self-assembled monolayers (SAMs) on metals focus on the use of gold. However, other metals, such as palladium, platinum, and silver offer advantages over gold as a substrate. In this work, palladium is electrochemically deposited from PdCl2 solutions on glassy carbon electrodes to form a substrate for alkanethiol SAMs. The conditions for deposition are optimized with respect to the electrolyte, pH, and electrochemical parameters. The palladium surfaces have been characterized by scanning electron microscopy (SEM) and the surface roughness has been estimated by chronocoulometry. SAMs of alkane thiols have been formed on the palladium surfaces, and their ability to suppress a Faradaic process is used as an indication for palladium coverage on the glassy carbon. The morphology of the Pd deposit as characterized by SEM and the blocking behavior of the SAM formed on deposited Pd delivers a consistent picture of the Pd surface. It has been clearly demonstrated that, via selection of experimental conditions for the electrochemical deposition, the morphology of the palladium surface and its ability to support SAMs can be controlled. The work will be applied to create a mixed monolayer of metals, which can subsequently be used to create a mixed SAM of a biocomponent and an alkanethiol for biosensing applications.     

Investigation into pH-responsive self-assembled monolayers of acylated anthranilate-terminated alkanethiol on a gold surface

This article describes the preparation of pH-responsive self-assembled monolayers (SAMs) of acylated anthranilate-terminated alkanethiol. These monolayers are formed by chemisorption of the alkanethiol molecules onto a gold surface, resulting in different wetting properties of the surfaces depending upon the pH. By using various characterization techniques (e.g., infrared spectroscopy, cyclic voltammetry, contact angle measurements, and surface energy analysis), we have found that the changes in the wetting properties originate from the different surface structures of the monolayers in different pH environments. From surface energy analysis, we found that the disperse components of the surface energy on such SAMs predominate after treatment with pH 1 water, whereas the polar components of the surface energy on such SAMs predominate after treatment with pH 13 water. It is greatly anticipated that this line of research will provide new insight into the mechanism behind pH-responsive properties, facilitating the design and synthesis of new surface-active molecules for the fabrication of pH-responsive functional surfaces.     

Dynamic and collective electrochemical responses of tetrathiafulvalene derivative self-assembled monolayers

Electroactive tetrathiafulvalene (TTF)-containing alkanethiol self-assembled monolayers (SAMs) were designed and synthesized to elucidate the relationship between electrochemical responses and film structures. Two TTF derivative molecules having one alkanethiol chain (1) and two alkanethiol chains (2) were utilized to modulate the molecular packing arrangements in the SAMs, and the formation and structure of the SAMs were characterized by surface plasmon resonance spectroscopy (SPR). SPR measurements in various contacting media demonstrated loose packing of SAM 1 and close packing of SAM 2 due to the different space fillings of the molecules. Two successive one-electron redox waves were observed for both SAMs by cyclic voltammetry. The peak widths of the redox waves were strongly dependent on the oxidation states of the TTF moieties, the packing arrangement of the SAMs, and the contacting medium. We found that TTF-based SAMs exhibited collective electrochemical responses induced by dynamic structural changes, depending on the degree of freedom for the component molecules in the SAMs. These results imply that the molecular design, taking into account the electrochemical responses, extends the available range of molecular-based functionalities in TTF-based SAMs.     

New dialkyldithiophosphinic acid self-assembled monolayers (SAMs): influence of gold substrate morphology on adsorbate binding and SAM structure

We report the fabrication and characterization of new self-assembled monolayers (SAMs) formed from dihexadecyldithiophosphinic acid [(C(16))(2)DTPA] molecules on gold substrates. In these SAMs, the ability of the (C(16))(2)DTPA headgroup to chelate to the gold surface depends on the morphology of the gold substrate. Gold substrates fabricated by electron-beam evaporation (As-Dep gold) consist of ～50-nm grains separated by deep grain boundaries (～10 nm). These grain boundaries inhibit the chelation of (C(16))(2)DTPA adsorbates to the surface, producing SAMs in which there is a mixture of monodentate and bidentate adsorbates. In contrast, gold substrates produced by template stripping (TS gold) consist of larger grains (～200-500 nm) with shallower grain boundaries (<2 nm). On these substrates, the low density of shallow grain boundaries allows (C(16))(2)DTPA molecules to chelate to the surface, producing SAMs in which all molecules are bidentate. The content of bidentate adsorbates in (C(16))(2)DTPA SAMs formed on As-Dep and TS gold substrates strongly affects the SAM properties: Alkyl chain organization, wettability, frictional response, barrier properties, thickness, and thermal stability all depend on whether a SAM has been formed on As-Dep or TS gold. This study demonstrates that substrate morphology has an important influence on the structure of SAMs formed from these chelating adsorbates.     

Surface plasmon microscopy of biotin-streptavidin binding reactions on UV-photopatterned alkanethiol self-assembled monolayers

We report surface plasmon imaging of streptavidin binding to photopatterned biotinylated alkanethiol self-assembled monolayers (SAMs) on gold. Micrometer-scale patterns of a mixed biotin- and hydroxyl-terminated monolayer were formed in an inert, hydroxy-terminated alkanethiol monolayer using a UV-photopatterning procedure. Using surface plasmon microscopy, contrast is readily observed between the mixed biotin- and hydroxy-terminated SAM region after specific binding of streptavidin has occurred and the pure hydroxy-terminated region where nonspecific binding of streptavidin is negligible. Surface plasmon microscopy was also able to monitor in situ and in real time the binding of streptavidin to the patterned SAMs. The ability of surface plasmon microscopy to detect and spatially resolve 2-dimensional monolayer binding events may prove useful in diagnostic applications involving the parallel interrogation at surface biomolecular arrays.     

Immobilization of acetylcholinesterase in lipid membranes deposited on self-assembled monolayers

Human red blood cell acetylcholinesterase was incorporated into planar lipid membranes deposited on alkanethiol self-assembled monolayers (SAMs) on gold substrates. Activity of the protein in the membrane was detected with a standard photometric assay and was determined to be similar to the protein in detergent solution or incorporated in lipid vesicles. Monolayer and bilayer lipid membranes were generated by fusing liposomes to hydrophobic and hydrophilic SAMs, respectively. Liposomes were formed by the injection method using the lipid dimyristoylphosphatidylcholine (DMPC). The formation of alkanethiol SAMs and lipid monolayers on SAMs was confirmed by sessile drop goniometry, ellipsometry, and electrochemical impedance spectroscopy. In this work, we report acetylcholinesterase immobilization in lipid membranes deposited on SAMs formed on the gold surface and compare its activity to enzyme in solution.     

Photochemical patterning of a self-assembled monolayer of 7-diazomethylcarbonyl-2,4,9-trithiaadmantane on gold films via Wolff rearrangement

Photolithographic attachment of functional organic molecules via ester or amide linkages to self-assembled monolayers (SAMs) on gold thin films was achieved by employing a novel photoreactive surface anchor, 7-diazomethylcarbonyl-2,4,9-trithiaadmantane. The photoreactive SAM was prepared by the spontaneous physical adsorption of the photoreactive surface anchor onto gold surfaces. The alpha-diazo ketone moiety of the SAM was found to display the classical Wolff rearrangement reactivity to produce a ketene intermediate on the exposed area. Organic molecules such as alcohols and amines can thus be attached to the gold surfaces selectively by the facile in situ formation of ester or amide linkages. The structure and reactivity of the photoreactive surface anchor were characterized by real-time FT-IR, fluorescence, and polarization modulation infrared reflectance absorption spectroscopy (PM-IRRAS). The Wolff rearrangement reactivity of the SAM suggested that a "surface-isolated" carbonylcarbene may be generated when the SAM was exposed to 255-nm irradiation.     

Polymerization of diacetylenes by hydrogen bond templated adlayer formation

Adlayers were formed on self-assembled monolayers (SAMs) formed by alkanethiols on gold. Base SAMs exposing amide functional groups at the SAM surface were formed with 12-mercaptododecanamide. Adlayers of diacetylene-containing monomers were then formed via amide hydrogen bonding in decalin and decalin/toluene mixtures. Grazing angle FTIR, contact angle measurements, and ellipsometry suggest that these adlayer films exhibit ordering and packing similar to that of SAMs on gold. Resonance Raman spectroscopy showed that these diacetylene adlayers could be readily polymerized by exposure to UV light.     

Immobilization of peroxidase glycoprotein on gold electrodes modified with mixed epoxy-boronic Acid monolayers

The development of bioelectronic enzyme applications requires the immobilization of active proteins onto solid or colloidal substrates such as gold. Coverage of the gold surface with alkanethiol self-assembled monolayers (SAMs) reduces nonspecific adsorption of proteins and also allows the incorporation onto the surface of ligands with affinity for complementary binding sites on native proteins. We present in this work a strategy for the covalent immobilization of glycosylated proteins previously adsorbed through weak, reversible interactions, on tailored SAMs. Boronic acids, which form cyclic esters with saccharides, are incorporated into SAMs to weakly adsorb the glycoprotein onto the electrode surface through their carbohydrate moiety. To prevent protein release from the electrode surface, we combine the affinity motif of boronates with the reactivity of epoxy groups to covalently link the protein to heterofunctional boronate-epoxy SAMs. The principle underlying our strategy is the increased immobilization rate achieved by the weak interaction-induced proximity effect between slow reacting oxyrane groups in the SAM and nucleophilic residues from adsorbed proteins, which allows the formation of very stable covalent bonds. This approach is exemplified by the use of phenylboronates-oxyrane mixed monolayers as a reactive support and redox-enzyme horseradish peroxidase as glycoprotein for the preparation of peroxidase electrodes. Quartz crystal microbalance, atomic force microscopy, and electrochemical measurements are used to characterize these enzymatic electrodes. These epoxy-boronate functional monolayers are versatile, stable interfaces, ready to incorporate glycoproteins by incubation under mild conditions.     

Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces

The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM). In this work, we performed systematic analysis with SAMs having various terminal groups (-OEG, -OH, -COOH, -NH(2), and -CH(3)). The results of the protein adsorption experiment by the quartz crystal microbalance (QCM) method suggested that having one EG unit and the neutrality of total charges of the terminal groups are essential for protein-resistance. In particular, QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM via a very weak interaction compared with other SAMs. Contrary to the protein resistance, at least three EG units as well as the charge neutrality of the SAM are found to be required for anti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate, our force measurements revealed that only the OEG-SAMs possessing more than two EG units showed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs with other terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs was always observed independent of solution conditions [NaCl concentration (between 0 and 1 M) and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disrupts the three-dimensional network of the water molecules. We therefore concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation between the above results, we propose that the layer of the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and platelets from adsorption or adhesion.     

Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers

An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)--a SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercury-rectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH(2))(n-1)CH(3) (n = 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 +/- 2 when the SAM on mercury was derived from HS(CH(2))(15)CH(3). The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.     

Molecular-level approach to inhibit degradations of alkanethiol self-assembled monolayers in aqueous media

A molecular-level approach is developed to prevent or inhibit the degradation processes of alkanethiol self-assembled monolayers (SAMs). Previous studies revealed two degradation pathways: direct desorption and oxidation-desorption. By use of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in situ and time-dependent imaging reveals and confirms that degradations of alkanethiol SAMs on gold mainly initiate at defect sites, such as domain boundaries and vacancy islands, and then propagate into the ordered domains. Our approach targets at attaching small molecules with preferred adhesion to the defects. The best candidates are aqueous media containing a small amount of amphiphilic surfactant molecules, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). High-resolution studies demonstrate that DMSO and DMF molecules attach to SAM surfaces and more favorably at defect sites, forming relatively stable adsorbates. This attachment increases the activation energy sufficiently to inhibit both degradation pathways. The robustness of this approach has been investigated as a function of surfactant concentration, solution temperature, and the stirring condition. Molecular-level mechanisms and energetics for degradation inhibition of SAMs are also discussed in detail.     

Blocking behavior of self-assembled monolayers on gold electrodes

Self-assembled monolayers (SAMs) with metal electrodes, especially thiols on gold, are the subject of this investigation because of the unique properties of SAM-modified surfaces. Normal alkanethiols are used to modify the surface of a conventional gold electrode to block certain ions such as Pb(II) and Cu(II) from the surface of the electrode. Normal alkanethiols are also used to study the SAM-gold interfacial adsorption-desorption behavior of the self-assembled monolayer. The effects of varying chain length of SAMs, varying concentration of the alkanethiol solutions, immersion time of the pure gold electrode in the SAM solution, and the stability of a SAM-modified gold electrode in fresh chloroform are investigated using the oxidation-reduction peaks of gold. Conditions that optimize the surface coverage and the uniformity of the SAMs have been determined. Normal alkanethiols proved to be a good insulator on the electrode surface. Received: 16 January 1997 / Accepted: 4 March 1997     

Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(1 1 1)

Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.     

Construction of mixed mercaptopropionic acid/alkanethiol monolayers of controlled composition by structural control of a gold substrate with underpotentially deposited lead atoms.

Mixed monolayers of 3-mercaptopropionic acid (MPA) and alkanethiols of various chain lengths have been constructed on Au based on a novel concept, namely, control of the composition of the component thiols in mixed monolayers by controlling the surface structure of the substrate. The Au substrate surface was first modified with underpotentially deposited Pb (UPD Pb) atoms, followed by the formation of a self-assembled monolayer (SAM) of alkanethiol. The UPD Pb atoms were then oxidatively stripped from the surface to create vacant site, on which MPA was adsorbed to finally form the mixed monolayers. The surface coverages of Pb, alkanethiol and MPA, and the total numbers of thiols were determined using an electrochemical quartz crystal microbalance, X-ray photoelectron spectroscopy, and reductive desorption voltammetry. These results demonstrate that the surface coverage of MPA in the mixed monolayers is determined by the initial coverage of UPD Pb. Fourier transform infrared spectra also support this conclusion. The observed single peak in the cyclic voltammogram for the reductive desorption shows that MPA and alkanethiol do not form their single-component domains. Scanning tunneling microscopy revealed the single-row pinstripe structure for all the thiol adlayers formed during each step of the preparation. This shows that the surface structure of the mixed monolayers is determined by the structure of the initially formed SAM on Au partially covered with UPD Pb.     

Structure of thiol self-assembled monolayers commensurate with the GaAs (001) surface

Observed properties of thiol self-assembled monolayers (SAMs) on GaAs (001) surfaces can be explained by the presence of surface reconstructions, but their exact form is generally unknown. We propose a new approach to modeling the SAM-surface interface based on using alkanethiol dense packing structures as a starting point and adjusting the surface reconstruction to accommodate them. Obtained in such a way, model SAMs adsorb along the trenches in the [110] direction and exhibit a 19 degrees tilt and +/- 45 degrees twist angles, in agreement with available experimental data. The molecules of the SAM bind to both Ga and As, and cover only 50% of the available surface sites. The requirements for the SAM formation process to achieve the proposed structures are discussed.     

Unimolecular rectification of monolayers of CH3C(O)S-C14H28Q(+)-3CNQ(-) and CH3C(O)S-C16H32Q(+)-3CNQ(-) organized by self-assembly, Langmuir-Blodgett, and Langmuir-Schaefer techniques

The advantage of "self-assembly" (strong covalent binding to substrates) was combined with the advantage of Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS) transfer to a solid substrate (quantitative transfer of monolayers to the substrate). The electrical rectification (asymmetric conduction) by a monolayer of thioacetylalkylquinolinium tricyanoquinodimethanide was critically compared when these molecules had been transferred, by such competing techniques, onto gold electrodes, and then covered by a "cold gold" pad electrode. Unimolecular rectification was observed in the expected directions in the LB and LS monolayers. The Self-Assembled Monolayers (SAMs) were disordered; macroscopic measurements of rectification were unsuccessful for the SAMs, but successful for the down-stroke LB and LS monolayers, whose orientation and potential bonding to the Au surface should be identical to that of an ideal SAM.