Thiol-terminated monolayers on oxide-free Si: assembly of semiconductor-alkyl-S-metal junctions

Self-assembled monolayers formed by thermal hydrosilylation of a trifluoroacetyl-protected alkenylthiol on Si-H surfaces, followed by removal of the protecting groups, yield essentially oxide-free monolayers suitable for the formation of Si-C11H22-S-Hg and Si-C11H22-S-Au junctions in which the alkyl chains are chemically bound to the silicon surface (via Si-C bonds) and the metal electrode (via Hg-S or Au-S bonds). Two barriers to charge transport are present in the system: at low bias the current is temperature activated and hence limited by thermionic emission over the Schottky barrier in the silicon, whereas as at high bias transport is limited by tunneling through the organic monolayer. The thiol-terminated monolayer on oxide-free silicon provides a well-characterized system allowing a careful study of the importance of the interfacial bond to the metal electrode for current transport through saturated molecules.     

Alkanethiol monolayers at reduced and oxidized zinc surfaces with corrosion protection: a sum frequency generation and electrochemistry investigation

In this work, octadecanethiol (ODT) was demonstrated to form ordered monolayers at either electrochemically reduced or oxidized Zn surfaces, by means of sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG spectra of ODT-modified Zn electrodes featured three methyl group resonances in the C-H vibrational region (2800-3100 cm(-1)). A significant decrease in interfacial capacitance and an increase in charge-transfer resistance were observed in EIS measurement after ODT modification. The alkane chain tilt angle of ODT within a monolayer at the Zn surface was estimated as 0 degrees with respect to the surface normal by interfacial capacitance measurement via EIS. CV and SFG investigation revealed that ODT monolayers undergo reductive desorption from the Zn electrode in 0.5 M NaOH at -1.66 V (vs SCE) and in 0.5 M NaClO4 at -1.62 V. The integrated charge consumed to the desorption of ODT is determined as 87 mC/cm2 from the reductive peak on CV curve, resulting in a coverage of 9.0 x 10(-10) mol/cm2 (5.4 x 10(14) molecules/cm2) if assuming the reduction follows a one-electron process. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt, and acid media. However, the protection efficiency was reduced with immersion time due to the presence of defects within the monolayers.     

Rapid assembly of photosystem I monolayers on gold electrodes

Photosystem I (PSI) has drawn widespread interest for use in biomimetically inspired energy conversion devices upon extracting it from plants or cyanobacteria and assembling it at surfaces. Here, we demonstrate that a critically dense monolayer of spinach-derived PSI must be formed on an electrode surface to achieve optimal photocurrents, and we introduce a new method for preparing these dense PSI monolayers that reduces the time required for assembly by approximately 80-fold in comparison to that for adsorption from solution. This method consists of applying a vacuum above the aqueous PSI solution during assembly to concentrate PSI and precipitate it into a thick layer onto the surface of various self-assembled monolayers or directly onto the electrode surface. Rinsing with water yields a dense monolayer of PSI that draws approximately 100 nA/cm2 of light-induced current from the gold electrode in the presence of appropriate mediators.     

Toposelective electrochemical desorption of thiol SAMs from neighboring polycrystalline gold surfaces

We describe a method for the selective desorption of thiol self-assembled monolayers from gold surfaces having micrometer-scale separations on a substrate. In an electrolyte solution, the electrical resistance between the adjacent areas can be much lower than the resistance between a surface and the counter electrode. Also, both reductive and oxidative thiol desorption may occur. Therefore, the potentials of the surfaces must be independently controlled with a multichannel potentiostat and operating windows for a given thiol/electrolyte system must be established. In this study operating windows were established for 1-dodecanethiol-based SAMs in phosphate buffer, phosphate-buffered saline, and sodium hydroxide solution, and selective SAM removal was successfully performed in a four-electrode configuration.     

Latex piezoelectric immunoassay: Effect of interfacial properties

Latex piezoelectric immunoassay is a technique for detecting agglutination of antibody- or antigen-bearing latex by an immunoreaction using a piezoelectric quartz crystal; the agglutination decreases the oscillation frequency of the crystal. This is advantageous in that immobilization of antibody or antigen on the crystal surface is unnecessary. In this report, different kinds of chemical functional groups were immobilized on the electrode surface, allowing us to consider the effect of interfacial structure on the frequency change. Electrode modifications such as self-assembly of alkanethiol and aminoalkoxysilane monolayers, and polyethylenimine-glutaraldehyde coating as well as plasma treatment were examined. The sensitivity of the system was found to imitate the interfacial properties so that modification of the electrode surface could improve the response. Among the methods examined for this electrode surface modification, the polyethylenimine-glutaraldehyde modification had the advantages of high reproducibility, fast operation and simplicity. It was also suggested that the frequency change originated primarily from the immunoreaction at the interface.     

Examining the frictional forces between mixed hydrophobic-hydrophilic alkylsilane monolayers

Monolayers presenting methyl-terminated (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces on silica have been studied by molecular dynamics simulation and the effects of hydrogen bonding, chain length, and chain mixing on the frictional properties determined. The hydroxyl-terminated monolayers were found to show large adhesion zones as a result of strong interfacial interlayer hydrogen bonds; the interfacial sliding forces observed in the hydroxyl-terminated monolayers being one order of magnitude higher than the interfacial forces for the hydrophobic surfaces at the characteristic point of zero-load. Mixed hydroxyl- and methyl-terminated monolayers of equal length were found to exhibit intermediate shear stress values between those observed for pure monolayers, with the magnitude of the shear stress depending on the surface content of the hydroxyl-terminated chains. For mixed monolayers of unequal chain lengths, at high loads a maximum in the magnitude of the shear stress as a function of the length of the methyl-terminated chain was observed due to the creation of a buffer zone between the hydroxyl-terminated chains that produces strong hydrogen-bonding interactions. The effect of a constant normal load or constant separation simulation ensemble on the results has also been studied and in general found to have minimal influence on the observed behavior, although some differences are observed for the shear stress at intermediate normal loads due to the formation of stronger hydrogen bond networks at constant load compared to constant separation.     

Self-assembled monolayers on electrode surfaces: a probe for redox kinetics

The formation and characterization of self-assembled monolayers of organosulfur compounds like alkanethiols and dialkyl (di)sulfides on metal surfaces such as gold are areas of current research interest. The presence of an aromatic ring in a thiol molecule can enhance the binding between Au and the thiol, resulting in the formation of compact and impervious self-assembled monolayers. The formation of a monolayer of 2-mercaptobenzothiazole (MBT), containing an aromatic group with a fused thiazole ring but no long alkyl chain, is achieved on a gold electrode surface. Voltammetric investigations of ferro/ferricyanide and ferrous/ferric redox systems carried out on this Au|MBT electrode are reported. Further, the possibility of using such an Au|MBT electrode to distinguish between inner and outer sphere electron transfer reactions is indicated. Received: 2 January 1998 / Accepted: 14 May 1998     

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     

Modeling Organic Surfaces with Self-Assembled Monolayers

The interfacial properties of organic materials are of critical importance in many applications, especially the control of wettability, adhesion, tribology, and corrosion. The relationships between the microscopic structure of an organic surface and its macroscopic physical properties are, however, only poorly understood. This short review presents a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena. Long-chain thiols, HS(CH₂)_nX, adsorb from solution onto gold and form densely packed, oriented monolayers. By varying the terminal functional group, X, of the thiol, organic surfaces can be created having a wide range of structures and properties. More complex systems can be constructed by coadsorbing two or more thiols with different terminal functional groups or with different chain lengths onto a common gold substrate. By these techniques, controlled degrees of disorder can be introduced into model surfaces. We have used these systems to explore the relationships between the microscopic structure of the monolayers on a molecular and supramolecutar scale and their macroscopic properties. Wettability is a macroscopic interfacial property that has proven of particular interest.     

Small-signal ac response theory for systems exhibiting diffusion and trapping of an electroactive species

A theoretical expression is derived for the interfacial admittance of an electrochemical system in which a product species diffuses into (or along internal surfaces of) an electrode subject to reversible trapping at one or more types of stationary sites in the electrode. The admittance obtained in a generalization of earlier results for diffusion without trapping and is exactly representable by an equivalent electrical circuit involving a lossy transmission line. Representative numerical data are given for the impedance of this distributed element. Trapping effects are found to be significant primarily when the number of trapped atoms is comparable to the number of mobile atoms. The results obtained may therefore be of greatest interest in the case of polycrystalline or amorphous electrodes.     

Probing Biomolecular Interactions at Conductive and Semiconductive Surfaces by Impedance Spectroscopy: Routes to Impedimetric Immunosensors,DNA‐Sensors,and Enzyme Biosensors

Impedance spectroscopy is a rapidly developing electrochemical technique for the characterization of biomaterial‐functionalized electrodes and biocatalytic transformations at electrode surfaces, and specifically for the transduction of biosensing events at electrodes or field‐effect transistor devices. The immobilization of biomaterials, e.g., enzymes, antigens/antibodies or DNA on electrodes or semiconductor surfaces alters the capacitance and interfacial electron transfer resistance of the conductive or semiconductive electrodes. Impedance spectroscopy allows analysis of interfacial changes originating from biorecognition events at electrode surfaces. Kinetics and mechanisms of electron transfer processes corresponding to biocatalytic reactions occurring at modified electrodes can be also derived from Faradaic impedance spectroscopy. Different immunosensors that use impedance measurements for the transduction of antigen‐antibody complex formation on electronic transducers were developed. Similarly, DNA biosensors using impedance measurements as readout signals were developed. Amplified detection of the analyte DNA using Faradaic impedance spectroscopy was accomplished by the coupling of functionalized liposomes or by the association of biocatalytic conjugates to the sensing interface providing biocatalyzed precipitation of an insoluble product on the electrodes. The amplified detections of viral DNA and single‐base mismatches in DNA were accomplished by similar methods. The changes of interfacial features of gate surfaces of field‐effect transistors (FET) upon the formation of antigen‐antibody complexes or assembly of protein arrays were probed by impedance measurements and specifically by transconductance measurements. Impedance spectroscopy was also applied to characterize enzyme‐based biosensors. The reconstitution of apo‐enzymes on cofactor‐functionalized electrodes and the formation of cofactor‐enzyme affinity complexes on electrodes were probed by Faradaic impedance spectroscopy. Also biocatalyzed reactions occurring on electrode surfaces were analyzed by impedance spectroscopy. The theoretical background of the different methods and their practical applications in analytical procedures were outlined in this article.     

Electron-transfer pathways between redox enzymes and electrode surfaces: reagentless biosensors based on thiol-monolayer-bound and polypyrrole-entrapped enzymes

Based on previous results which showed that quinohemo-protein alcohol dehydrogenase (QH-ADH) entrapped within polypyrrole is able to directly transfer electrons via the conducting polymer to the electrode surface, the electron-transfer properties of this multi-cofactor enzyme adsorbed and covalently-bound to self-assembled thiol monolayers and bare electrode surfaces has been investigated more closely. While the dissolved enzyme is able to transfer electrons to the electrode via heme c as well as via the more deeply buried PQQ (fast adsorption-chemical reaction-desorption mechanism), an orientation of adsorbed QH-ADH on hydrophobic electrode surfaces, as well as of adsorbed and covalently bound QH-ADH on negatively-charged thiol monolayers could be observed. In these cases the heme c units are pointing towards the electrode surfaces resulting in an optimised direct ET rate.     

Interfacial rheology of polyelectrolytes and polymer monolayers at the air–water interface

In this review, we describe interfacial rheology studies of polymer monolayers at the air–water interface. Since polyelectrolytes are usually soluble in water, the formation of surface monolayers requires the presence of a surfactant of opposite charge. The first part of the review is dedicated to these mixed monolayers. The second part is related to neutral monolayers that can be either adsorbed or deposited at the interface. Interfacial rheology studies of these systems are still scarce, despite a considerable interest: insoluble polymer monolayers in two dimensions are suitable model systems for the tests of polymer theories in two dimensions, such as and glass transition. The rheology of soluble polymer monolayers has important connections with the dynamic properties of dispersions stabilized with these polymers.     

Dense monolayers of metal-chelating ligands covalently attached to carbon electrodes electrochemically and their useful application in affinity binding of histidine-tagged proteins

In this work, monolayers of metal complexes were covalently attached to the surface of carbon electrodes with the goal of binding monolayers of histidine-tagged proteins with a controlled molecular orientation and a maintained biological activity. In this novel method, which is simple, versatile, and efficient, the covalent attachment was accomplished in a single step by the electrochemical reduction of aryl diazonium ions that were substituted with a nitrilotriacetic (NTA) or an imminodiacetic (IDA) ligand at the para position. The transient aryl radicals that were generated in the reduction were grafted to the surfaces of glassy carbon, highly oriented pyrolitic graphite, and graphite-based screen-printed electrodes, producing dense monolayers of the ligands. The NTA- and IDA-modified electrodes were shown to efficiently chelate Cu(II) and Ni(II) ions. The presence of the metal was established using X-ray photoelectron spectroscopy and electrochemistry. Surface coverages of the ligands were indirectly determined from the electroactivity of the copper(II) complex formed on the electrode surface. Studies on the effect of electrodeposition time and potential showed that, at sufficiently negative potentials, the surface coverage reached a saturating value in less than 2 min of electrodeposition time, which corresponds to the formation of a close-packed monolayer of ligand on the electrode surface. Once loaded with a metal ion, the modified electrode was able to bind specifically to histidine-tagged proteins such as the horseradish peroxidase (His-HRP) or to an enhanced, recombinant green-fluorescent protein via its N-terminal hexahistidine tail. In the case of His-HRP, the amount of active enzyme specifically immobilized by metal-chelating binding was determined from the analysis of electrocatalytic currents using cyclic voltammetry. The electrochemical grafting makes it possible to accurately controlled and electronically address the amount of deposited ligand on the conductive surfaces of carbon electrodes with any size and shape.     

Preparation of microscopic and planar oil-water interfaces that are decorated with prescribed densities of insoluble amphiphiles

Langmuir monolayers (monolayers of insoluble molecules formed at the surface of water), and associated Langmuir-Blodgett/Schaefer monolayers prepared by transfer of Langmuir films to the surfaces of solids, are widely used in studies aimed at understanding the physicochemical properties of biological and synthetic molecules at interfaces. In this article, we report a general and facile procedure that permits transfer of Langmuir monolayers from the surface of water onto microscopic and planar interfaces between oil and aqueous phases. In these experiments, a metallic grid supported on a hydrophobic solid is used to form oil films with thicknesses of 20 mum and interfacial areas of 280 mum x 280 mum. Passage of the supported oil films through a Langmuir monolayer is shown to lead to quantitative transfer of insoluble amphiphiles onto the oil-water interfaces. The amphiphile-decorated oil-water interfaces hosted within the metallic grids (i) are approximately planar, (ii) are sufficiently robust mechanically so as to permit further characterization of the interfaces outside of the Langmuir trough, (iii) can be prepared with prescribed and well-defined densities of amphiphiles, and (iv) require only approximately 200 nL of oil to prepare. The utility of this method is illustrated for the case of the liquid crystalline oil 4-pentyl-4'-cyanobiphenyl (5CB). Transfer of monolayers of either dilauroyl- or dipalmitoylphosphatidylcholine (DLPC and DPPC, respectively) to the nematic 5CB-aqueous interface is demonstrated by epifluorescence imaging of fluorescently labeled lipid and polarized light imaging of the orientational order within the thin film of nematic 5CB. Interfaces prepared in this manner are used to reveal key differences between the density-dependent phase properties of DLPC and DPPC monolayers formed at air-water as compared to that of nematic 5CB-aqueous interfaces. The methodology described in this article should be broadly useful in advancing studies of the interfacial behavior of synthetic and biological molecules at liquid-liquid interfaces.     

The Use of Light in the Study of Interfacial Electrochemical Reactions

Spectroscopic analysis techniques have become particularly important in the study of interfacial electrochemical processes since it has become possible to increase their sensitivity to the extent where changes occurring in the absorption of light by fractions of monolayers can be detected. At the same time the development of new techniques in surface physics, though most of these can only be used in ultra-high vacuum, has also had a considerable influence on electrochemistry since it has enabled studies to be carried out on electrodes after transfer to UHV. The availability of well-defined semiconductors in which photocurrents can be generated by the absorption of light has led to the development of photoelectrochemical methods that can be used for the study of electrode reactions at semiconductors or at metal surfaces which are covered with a semiconducting film. Finally, it has been observed during the study of semiconductor electrodes that redox reactions at an electrode can result in the generation of luminescence. This, in turn, can lead to important information on the mechanism of the charge transfer process. These recent developments are reviewed in the present article.     

Selective reductive desorption of a SAM-coated gold electrode revealed using fluorescence microscopy

The reductive desorption of a self-assembled monolayer (SAM) of a fluorescent thiol molecule (BodipyC10SH) from Au was characterized using electrochemistry and epi-fluorescence microscopy. Molecular luminescence is quenched near a metal surface, so fluorescence was only observed for molecules reductively desorbed and then separated from the electrode surface. Fluorescence imaging showed that reductive desorption was selective, with desorption occurring from different regions of the Au electrode depending on the extent of the negative potential excursion. When desorbed, the molecules were sufficiently mobile, diffusing away from the electrode surface, thereby preventing oxidative readsorption. At sufficiently negative desorption potentials, all of the thiol was desorbed from the electrode surface, resulting in fluorescence at the air/solution interface. The selective removal of the thiol monolayer from distinct regions was correlated to features on the electrode surface and was explained through potential-dependent interfacial energies. This in situ electrofluorescence microscopy technique may be useful in sensor development.     

菁染料单分子膜覆盖的铂电极的电化学特性

L-B膜作为一种单分子膜技术已经得到了广泛的应用。最近很多人用电化学方法研究覆盖有L-B膜电极的特性^[1,2]。许多工作^[3-5]证明了脂肪酸L-B膜具有很强的抑制离子透过和抑制电子隧道电流的能力。     

Switching the electrochemical impedance of low-density self-assembled monolayers

Because the active remodeling of biointerfaces is a paramount feature of nature, it is very likely that future, advanced biomaterials will be required to mimic at least certain aspects of the dynamic properties of natural interfaces. This need has fueled a quest for model surfaces that can undergo reversible switching upon application of external stimuli. Herein, we report the synthesis and characterization of a model system for studying reversibly switching surfaces based on low-density monolayers of mercaptohexadecanoic acid and mercaptoundecanoic acid. These monolayers were assembled on both gold and silver electrodes. When conducting electrochemical impedance spectroscopy under physiological conditions, these monolayers exhibit significant changes in their electrochemical barrier properties upon application of electrical DC potentials below +400 mV with respect to a standard calomel electrode. We further found the impedance switching to be reversible under physiological conditions. Moreover, the impedance can be fine-tuned by changing the magnitude of the applied electrical potential. Before and during impedance switching at pH 7.4 in aqueous buffer solutions, the low-density monolayers showed good stability according to grazing angle infrared spectroscopy data. We anticipate low-density monolayers to be potentially useful model surfaces when designing active biointerfaces for cell-based studies or rechargeable biosensors.     

Monolayer-derivative functionalization of non-oxidized silicon surfaces

This article describes a variety of monolayers anchored directly onto silicon surfaces without an oxide interlayer, their formation mechanisms, their technological applications, and our personal views on the future prospects for this field. The chemical modification of non-oxidized silicon surfaces utilizing monolayers was first reported in 1993. The basic finding that a non-oxidized silicon surface could be neutralized with alkyl chains through direct covalent linkage, i.e., silicon-carbon, has offered chemical scientists ease of handling even in an ambient environment and, thus, research has been predictably focused on forming anti-stiction coating films for nano- and micro-electromechanical systems (NEMS/MEMS). Such surface reforming has also been achieved by using other monolayers, which form interfacial bonds, e.g., silicon-nitrogen and silicon-oxygen. The resultant monolayer surfaces are useful for silicon-based applications including molecular electron transfer films, monolayer templates, molecular insulators, capsulators, and bioderivatives. Such monolayers are applicable not only for surface modification, but also for manipulating individual nanomaterials. By modifying the terminal groups of monolayers with nanomaterials including nanocrystals and biomolecules, the nanomaterials can remarkably be immobilized directly onto non-oxidized silicon surfaces based on the formation mechanisms of the monolayer. Such immobilizations will revolutionize the analysis of the specific features and capabilities of individual nanomaterials. Furthermore, the path will be opened for the development of more advanced monolayer-derived chip technology. To achieve this goal, it is extremely important to thoroughly understand the functionalization processes on silicon, since the resultant internal structures and properties of monolayer-derivative silicon may strongly depend on their course of formation.