自组装膜技术在电分析化学中的研究与应用

本文对自组膜（ＳＡＭｓ）在电分析化学中的研究和应用进行了比较全面的综述。ＳＡＭｓ是单分子膜化学修饰电极发展的最高形式，本文着重阐述了硫醇／金单分子层自组膜在微电极、生物电化学和生物传感器、液相色谱电化学、电催化、光谱电化学等电分析化学研究领域中的应用，并进行了展望。     

High-affinity chelator thiols for switchable and oriented immobilization of histidine-tagged proteins: a generic platform for protein chip technologies

Protein micro-/nanoarrays are becoming increasingly important in systematic approaches for the exploration of protein-protein interactions and dynamic protein networks, so there is a high demand for specific, generic, stable, uniform, and locally addressable protein immobilization on solid supports. Here we present multivalent metal-chelating thiols that are suitable for stable binding of histidine-tagged proteins on biocompatible self-assembled monolayers (SAMs). The architectures and physicochemical properties of these SAMs have been probed by various surface-sensitive techniques such as contact angle goniometry, ellipsometry, and infrared reflection-absorption spectroscopy. The specific molecular organization of proteins and protein complexes was demonstrated by surface plasmon resonance, confocal laser scanning, and atomic force microscopy. In contrast to the mono-NTA/His6 tag interaction, which has major drawbacks because of its low affinity and fast dissociation, drastically improved stability of protein binding by these multivalent chelator surfaces was observed. The immobilized histidine-tagged proteins are uniformly oriented and retain their function. At the same time, proteins can be removed from the chip surface under mild conditions (switchability). This new platform for switchable and oriented immobilization should assist proteome-wide wide analyses of protein-protein interactions as well as structural and single-molecule studies.     

Comparison of the anchoring of nematic liquid crystals on self-assembled monolayers formed from semifluorinated thiols and alkanethiols

The anchoring of nematic liquid crystals on self-assembled monolayers (SAMs) formed by the chemisorption of semifluorinated thiols or alkanethiols on gold is compared and contrasted. The planar anchoring of 4-n-pentyl-4-cyanobiphenyl (5CB) observed in the past on SAMs formed from alkanethiols is also observed on SAMs formed from semifluorinated thiols. The azimuthal anchoring of 5CB, however, differs on these two types of surfaces: nematic 5CB anchored on SAMs formed from alkanethiols has a grainy appearance due to the formation of domains with sizes 10 mum whereas 5CB forms large domains ( 100 mum) with diffuse branches emerging from defects of strength 1/2 when anchored on SAMs formed from semifluorinated thiols. Mixed (two-component) SAMs formed from either short and long semifluorinated thiols or short and long alkanethiols cause homeotropic anchoring of 5CB. We discuss these results in light of the known differences in the structure of SAMs formed from alkanethiols and semifluorinated thiols, i.e. the tilt of the chains and conformational freedom (flexibility) of the chains within these SAMs.     

Preparation of high quality electrical insulator self-assembled monolayers on gold. Experimental investigation of the conduction mechanism through organic thin films

Self-assembled monolayers (SAMs) form highly ordered, stable dielectrics on conductive surfaces. Being able to attach larger-area contacts in a MIM (metal-insulator-metal) diode, their electrical properties can be determined. In this paper, the electrical conduction through thiolate SAMs of different alkyl chain lengths formed on gold surfaces were studied and discussed. The influence of the headgroup with respect to the surface quality and prevention of short circuits is investigated. Phenoxy terminated alkanethiols were found to form high quality SAMs with perfect insulating properties. Synthesis of the required terminally substituted long chain thiols have been developed. The I(V) characteristics of MIM structures formed with these SAMs are measured and simulated according to theoretical tunneling models for electrical conductivity through thin organic layers. SAM based electronic devices will become especially important for future nanoscale applications, where they can serve as insulators, gate dielectric of FETs, resistors, and capacitor structures.     

Thiol-mediated anchoring of ligands to self-assembled monolayers for studies of biospecific interactions

We report a method to immobilize thiol-containing ligands onto self-assembled monolayers (SAMs) of alkanethiolates presenting chloracetylated hexa(ethylene glycol) groups. The chloroacetyl groups react with thiols under mild basic conditions, enabling the stable immobilization of biologically active ligands in a well-defined orientation. These SAMs on gold are well suited for studies of biospecific interactions of immobilized ligands with proteins and cells. As a demonstration, we functionalized these SAMs with thiol-containing derivatives of biotin and benzene sulfonamide and observed the specific binding of neutravidin and carbonic anhydrase, respectively. We also used this method to generate mixed SAMs presenting the Arg-Gly-Asp (RGD) peptide sequence and demonstrated the integrin-mediated adhesion of fibroblast cells to these SAMs. This approach would allow the immobilization of proteins and other sensitive biomolecules and ligands for a wide variety of applications in biotechnology.     

Strong resistance of oligo(phosphorylcholine) self-assembled monolayers to protein adsorption

In this work, we demonstrate the strong resistance of oligo(phosphorylcholine) (OPC) self-assembled monolayers (SAMs) to protein adsorption and cell adhesion. OPC SAMs were characterized using X-ray photoelectron spectroscopy (XPS), and protein adsorption was measured using a surface plasmon resonance (SPR) sensor. Results are compared with those of phosphorylcholine (PC) SAMs. Despite the existence of negative charge on OPC SAMs and the simple synthesis procedure of OPC thiols, OPC SAMs resist protein adsorption as effectively as or better than PC SAMs formed from highly purified PC thiols. The ease of their preparation and the effectiveness of their function make OPC SAMs an attractive alternative for creating nonfouling surfaces.     

The Influence of Bromine Adsorption on Copper Electrodeposition on Polycrystalline Gold Electrodes Modified with Self-Assembled Monolayers

The influence of bromine adsorption on copper electrodeposition on a polycrystalline gold electrode modified with self-assembled monolayers (SAMs) has been investigated by chronoamperometry and cyclic voltammetry. It was found that the deposition potential of copper was shifted negatively due to the SAMs. The hydrogen bond interaction between omega-carboxyl thiols decreased the defect density of the SAMs and significantly retarded the deposition of copper. The presence of bromide anions also shifted the potential more negatively through adsorption into the defects of SAMs. Copyright 2001 Academic Press.     

Self-assembled monolayers containing terminal mono-, bis-, and tris-nitrilotriacetic acid groups: characterization and application

We have undertaken a structural and functional study of self-assembled monolayers (SAMs) formed on gold from a series of alkylthiol compounds containing terminal multivalent chelators (MCHs) composed of mono-, bis-, and tris-nitrilotriacetic acid (NTA) moieties. SAMs were formed from single-component solutions of the mono-, bis-, and tris-NTA compounds, as well as from mixtures with a tri(ethylene glycol)-terminated alkylthiol (EG(3)). Contact angle goniometry, null ellipsometry, and infrared spectroscopy were used to explore the structural characteristics of the MCH SAMs. Ellipsometric measurements show that the amount of the MCH groups on surfaces increases with increasing mol % of the MCH thiols in the loading solution up to about 80 mol %. We also conclude that mixed SAMs, prepared in the solution composition regime 0-30 mol % of the MCH thiols, consist of a densely packed alkyl layer, an amorphous ethylene glycol layer, and an outermost layer of MCH groups exposed toward the ambient. Above 30 mol %, a significant degree of disorder is observed in the SAMs. Finally, functional evaluation of the three MCH SAMs prepared at 0-30 mol% reveals a consistent increase in binding strength with increasing multivalency. The tris-NTA SAM, in particular, is enabled for stable and functional immobilization of a His6-tagged extracellular receptor subunit, even at low chelator surface concentrations, which makes it suitable for applications when a low surface density of capturing sites is desirable, e.g., in kinetic analyses.     

Nitroxyl radical self assembled monolayers: Ion pairing investigation in organic and aqueous media

Self assembled monolayers (SAMs) formed from TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) derivative thiols have been studied by electrochemical quartz crystal microbalance (EQCM) in both aqueous and non-aqueous solvents. The in situ study of the mass transport occurring during the oxidation of TEMPO provides evidence of a ion pair formation without incorporation of solvent in densely packed nitroxyl radical SAMs. For SAMs having a low nitroxyl radical surface coverage, the effect of mixed SAMs is evidenced and seems to avoid the solvent incorporation.     

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.     

Functionalized fullerenes in self-assembled monolayers

Anisotropy of intermolecular and molecule-substrate interactions holds the key to controlling the arrangement of fullerenes into 2D self-assembled monolayers (SAMs). The chemical reactivity of fullerenes allows functionalization of the carbon cages with sulfur-containing groups, thiols and thioethers, which facilitates the reliable adsorption of these molecules on gold substrates. A series of structurally related molecules, eight of which are new fullerene compounds, allows systematic investigation of the structural and functional parameters defining the geometry of fullerene SAMs. Scanning tunnelling microscopy (STM) measurements reveal that the chemical nature of the anchoring group appears to be crucial for the long-range order in fullerenes: the assembly of thiol-functionalized fullerenes is governed by strong molecule-surface interactions, which prohibit formation of ordered molecular arrays, while thioether-functionalized fullerenes, which have a weaker interaction with the surface than the thiols, form a variety of ordered 2D molecular arrays owing to noncovalent intermolecular interactions. A linear row of fullerene molecules is a recurring structural feature of the ordered SAMs, but the relative alignment and the spacing between the fullerene rows is strongly dependent on the size and shape of the spacer group linking the fullerene cage and the anchoring group. Careful control of the chemical functionality on the carbon cages enables positioning of fullerenes into at least four different packing arrangements, none of which have been observed before. Our new strategy for the controlled arrangement of fullerenes on surfaces at the molecular level will advance the development of practical applications for these nanomaterials.     

二维碳纳米薄膜制备的新途径——由自组装膜到石墨烯

本文描述了芳香族分子作为自组装单分子膜(SAMs)前驱体在电子辐照下引发芳香基团交联,在真空或惰性气氛中转化为具有较高热稳定性的碳纳米薄膜(CNMs)。CNMs具有足够的机械强度,可从其基底表面转移作为独立的薄膜材料,经高温淬火后转化为石墨烯。根据制备条件,如芳香分子前驱体的化学结构、电子辐照和淬火参数等,可以调整所制得的石墨烯的形状、结晶度、厚度、孔径等各种性能。各种芳香族硫醇,如低环及多环芳烃碳氢化合物,获得的CNMs的结构和功能由其单分子膜的结构所决定。本文详细讨论了电子辐射诱导SAMs芳香分子交联反应的机理。CNMs/石墨烯异质结构的非破坏性化学功能化组装为CNMs/石墨烯在电子、光子器件以及生物膜中的应用开辟了一条灵活的途径。     

Metallic contact formation for molecular electronics: interactions between vapor-deposited metals and self-assembled monolayers of conjugated mono- and dithiols

We present grazing-incidence Fourier transform infrared and AFM data of Au, Al, and Ti vapor-deposited onto self-assembled monolayers (SAMs) of conjugated mono- and dithiols. SAMs of 4,4'-dimercapto-p-quaterphenyl, 4,4"-dimercapto-p-terphenyl, and 4,4'-dimercapto-p-biphenyl have reactive thiols at the SAM/vacuum interface that interact with vapor-deposited Au or Al atoms, preventing metal penetration. Conjugated monothiols lack such metal blocking groups, and metals (Au, Al) can penetrate into their SAMs. Vapor deposition of Ti onto conjugated mono- and dithiol SAMs and onto hexadecanethiol SAMs destroys the monolayers.     

Electrochemical investigations on stability and protonation behavior of pyridine-terminated aromatic self-assembled monolayers

Based on electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), Au(111) electrodes modified by self-assembled monolayers (SAMs) of a homologous series of pyridine-terminated thiols with aromatic backbones have been investigated. An important correlation between the chain structure and film integrity in electrolytic media was found. Monolayers with odd numbers of methylene spacers in the molecular chain showed superior barrier properties compared to even numbered counterparts. A positive influence of an increase in the number of attached phenyl rings on the integrity of SAMs was observed. Furthermore, cathodic desorption of the investigated SAMs is characterized by multiwave desorption peaks and extraordinarily large cathodic charges indicating an unusual desorption process. Moreover, protonation behavior of the SAMs has been investigated by X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Protonation has been found to be reversible and surface pK(a) values have been determined to be around 5 for all investigated monolayers.     

Characterization of self-assembled monolayers of fullerene derivatives on gold surfaces: implications for device evaluations

The widely employed approach to self-assembly of fullerene derivatives on gold can be complicated due to multilayer formations and head-to-tail assemblies resulting from the strong fullerene-fullerene and fullerene-gold interactions. These anomalies were not examined in detail in previous studies on fullerene self-assembled monolayers (SAMs) but were clearly detected in the present work using surface characterization techniques including ellipsometry, cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). This is the first time that SAMs prepared from fullerene derivatives of thiols/thiol esters/disulfides have been analyzed in detail, and the complications due to multilayer formations and head-to-tail assemblies were revealed. Specifically, we designed and synthesized several fullerene derivatives based on thiols, thiol acetates, and disulfides to address the characterization requirements, and these are described and delineated. These studies specifically address the need to properly characterize and control fullerene-thiol assemblies on gold before evaluating subsequent device performances.     

Stability of mixed PEO-thiol SAMs for biosensing applications

The secret of a successful affinity biosensor partially hides in the chemical interface layer between the transducer system and the biological receptor molecules. Over the past decade, several methodologies for the construction of such interface layers have been developed on the basis of the deposition of self-assembled monolayers (SAMs) of alkanethiols on gold. Moreover, mixed SAMs of polyethylene oxide (PEO) containing thiols have been applied for the immobilization of biological receptors. Despite the intense research in the field of thiol SAMs, relatively little is known about their biosensing properties in correlation with their long-term stability. Especially the impact of the storage conditions on their biosensing characteristics has not been reported before to our knowledge. To address these issues, we prepared mixed PEO SAMs and tested their stability and biosensing performance in several storage conditions, i.e., air, N2, ethanol, phosphate buffer, and H2O. The quality of the SAMs was monitored as a function of time using various characterization techniques such as cyclic voltammetry, contact angle, grazing angle Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. In addition, the impact of the different storage conditions on the biosensor properties was investigated using surface plasmon resonance. Via the latter technique, the receptor immobilization, the analyte recognition, and the nonspecific binding were extensively studied using the prostate specific antigen as a model system. Our experiments showed that very small structural differences in the SAM can have a great impact in their final biosensing properties. In addition it was shown that the mixed SAMs stored in air or N2 are very stable and retain their biosensor properties for at least 30 days, while ethanol appeared to be the worst storage medium due to partial oxidation of the thiol headgroup. In conclusion, care must be taken to avoid SAM degradation during storage to retain typical SAM characteristics, which is very important for their general use in many proposed applications.     

Edge spreading lithography and its application to the fabrication of mesoscopic gold and silver rings

A new form of edge lithography, edge spreading lithography (ESL), has been demonstrated and applied to the formation of coinage metal rings. In this process, alkanethiols are delivered from a flat PDMS stamp to the surface of a metal film through a two-dimensional array of spherical silica colloids. The thiols further spread on the metal surface, forming highly ordered SAMs in the form of a ring pattern. Following lift-off of beads, the pattern in the SAMs can be transferred into the metal film through wet chemical etching, with SAMs serving as the resist. The dimensions of the rings can be readily controlled by several parameters such as the beads diameter, the concentration of the thiol solution, and the contact time between the stamp and the silica beads.     

Synthesis of fluorescent long-chain thiols/disulfides as building-blocks for self-assembled monolayers preparation

New symmetrical disulfides together with the corresponding thiols bearing fluorescent end-groups have been synthesized as building-blocks for self-assembled monolayers (SAMs). The synthesis has been accomplished starting from aromatic nitrogen heterocycles in three steps. The conversion of the tosylated intermediate into the final disulfide is accomplished by use of sodium hydrogen sulfide (NaSH). Both products (thiols and disulfides) were isolated and characterized.      

Fabrication of biomolecular nanostructures by scanning near-field photolithography of oligo(ethylene glycol)-terminated self-assembled monolayers

The UV photo-oxidation of oligo(ethylene glycol) (OEG)-terminated self-assembled monolayers (SAMs) has been studied using static secondary ion mass spectrometry, X-ray photoelectron spectroscopy, contact angle measurement, and friction force microscopy. OEG-terminated SAMs are oxidized to yield sulfonates, but photodegradation of the OEG chain also occurs on a more rapid time scale, yielding degradation products that remain bound to the surface via gold-sulfur bonds. The oxidation of these degradation products is the rate-limiting step in the process. Photopatterning of OEG-terminated SAMs may be accomplished by using a mask and suitable light source or by using scanning near-field photolithography (SNP) in which the mask is replaced by a scanning near-field optical microscope coupled to a UV laser. Using SNP, it is possible to fabricate patterns in SAMs with a full width at half-maximum height (fwhm) as small as 9 nm, which is approximately 15 times smaller than the conventional diffraction limit. SNP-patterned OEG-terminated SAMs may be used to fabricate protein nanopatterns. By adsorbing carboxylic acid-terminated thiols into oxidized regions and converting these to active ester intermediates, it has been possible to fabricate lines of protein molecules with widths of only a few tens of nanometers.     

In situ STM evidence for the adsorption geometry of three N-heteroaromatic thiols on Au(111)

The electrochemical behavior of three heteroaromatic thiols (MBs) (2-mercaptobenzimidazole (MBI), 2-mercaptobenzothiazole (MBT), and 2-mercaptobenzoxazole (MBO)) on a Au(111) surface has been investigated by electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry (CV) in 0.1 M HClO(4) solution. All three thiols form oriented molecular cluster lines along the reconstruction line direction at 0.55 V. With the electrode potential shifting negatively, the molecules undergo a disordered-ordered structural transition. Molecularly resolved STM images show that all three molecules form striped adlayers in the desorption region on the Au(111) surface. The different heteroatoms in the heteroaromatic rings result in different electrochemical behavior of the MB self-assembled monolayers (SAMs). MBI, MBT, and MBO are proposed to interact with the substrate via the S-Au bonds from thiol group and the coordination interaction of N, S, and O with the substrate from the heteroaromatic ring, respectively. These results provide direct evidence of the electrochemical behavior and the adlayer structures of MB SAMs on the Au electrode.