Self-assembling process of alkanethiol monolayers on gold surface via underpotential deposition

It was demonstrated feasible that underpotential deposition(UPD) of copper on a monolayer-modified gold substrate can be used to determine the gold electrode area. The deposition and stripping of a Cu adlayer can take Place reversibly and stably at a bared or a self-assembled monolayer modified gold electrode. The growth kinetics of decanethiol/Au was also investigated via Cu UPD. The difference between the assembling kinetics determined by UPD and that by quartz crystal microbalance measurements reveals the configuration transmutation of the assembled molecules from a disordered arrangement to an ordered arrangement during the self-assembling processes.

Electrochemical growth and characterization of polyoxometalate-containing monolayers and multilayers on alkanethiol monolayers self-assembled on gold electrodes

A general strategy has been developed for fabrication of ultrathin monolayer and multilayer composite films composed of nearly all kinds of polyoxometalates (POMs), including isopolyanions (IPAs), and heteropolyanions (HPAs). It involves stepwise adsorption between the anionic POMs and a cationic polymer on alkanethiol (cysteamine and 3-mercaptopropionic acid) self-assembled monolayers (SAMs) based on electrostatic interaction. Here a Keggin-type HPA SiMo11VO405- was chosen as a main representative to elucidate, in detail, the fabrication and characterization of the as-prepared composite films. A novel electrochemical growth method we developed for film formation involves cyclic potential sweeps over a suitable potential range in modifier solutions. It was comparatively studied with a commonly used method of immersion growth, i.e., alternately dipping a substrate into modifier solutions. Growth processes and structural characteristics of the composite films are characterized in detail by cyclic voltammetry, UV-vis spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS), micro-Fourier transform infrared reflection-absorption spectroscopy (FTIR-RA), and electrochemical quartz crystal microbalance (EQCM). The electrochemical growth is proven to be more advantageous than the immersion growth. The composite films exhibit well-defined surface waves characteristic of the HPAs' redox reactions. In addition, the composite films by the electrochemical growth show a uniform structure and an excellent stability. Ion motions accompanying the redox processes of SiMo11VO405- in multilayer films are examined by in situ time-resolved EQCM and some results are first reported. The strategy used here has been successfully popularized to IPAs as well as other HPAs no matter what structure and composition they have.

Electrochemical characterization of self-assembled Ferrocene-terminated alkanethiol monolayers on low-index gold single crystal electrodes

We present a voltammetric and in situ STM study of 11-ferrocenyl-1-undecanethiol (FcC11) assembled on low-index single crystal and polycrystalline gold electrodes. The influence of electrode surface structure as well as of structure defects in the self-assembled FcC11 monolayers on the electrochemical response during the oxidation and reduction of the terminal ferrocene group is explored. The nature of the redox peaks is discussed in detail. We identified the coexistence of disordered FcC11 regions with 2D patches of “locally ordered” FcC11 species. We demonstrate that close-packed domains are preferentially formed at atomically flat terraces. Increasing the defect density of the substrate surface leads to a decreasing amount of locally ordered FcC11 molecules.

Self Assembled Monolayers of Alkanethiolson Silver Surfaces

This short review compiles the studies on self assembled alkanethiol monolayers formed on silver surfaces with respect to their structure and stability. Alkanethiol-based assemblies on silver surfaces are poor cousins of thiol monolayers on gold. The formation of well-ordered monolayers on silver surfaces is relatively more difficult than the corresponding systems on gold since the inherent oxide film on silver interferes with the formation and stability of the assembly. There are contradictory reports on the nature and physicochemical characteristics of alkanethiol monolayers on silver surfaces. This review attempts to highlight various studies in the literature including our efforts in this area.

Electrochemistry of heteropolyanions in coulombically linked self-assembled monolayers

A composite film containing heteropolyanion was fabricated on gold by attaching the Keggin-type heteropolyanion, PMo12O403- on a 4-aminothiophenol SAM via Au-S bonding. Reflection FTIR, cyclic voltammetry and XPS were used for the characterization of the composite film. Reflection FTIR studies indicate that there is some Coulombic interaction between PMo12O403- and the surface amino group in the composite film, which greatly improves the film stability and prevents effectively the destructive intermolecular aggregation. The composite him shows three reversible redox couples within the pH range pH less than or equal to 7.0, attributed to three two-electron and two-proton electrochemical reduction-oxidation processes of PMo12O403-. Compared with PMo12O403- in the solution, the PMo12O403- of the composite film electrode can exist in a larger pH range, and shows smaller peak-to-peak separation, and more reversible reaction kinetics. Moreover, the composite him obtained shows a good catalytic activity for the reduction of BrO3-. (C) 1998 Elsevier Science S.A. All rights reserved.

K+ sensors based on supported alkanethiol/phospholipid bilayers

A novel kind of K+ sensor with valinomycin-incorporated bilayers supported on a gold electrode consisting of self-assembled alkanethiol monolayers (SAMs) and a lipid monolayer has been fabricated successfully. The lipid monolayer is deposited on the alkylated surface of the first alkanethiol monolayer through three different methods, such as the Langmuir-Blodgett (LB) technique, painted method and painted-frozen method. The response of K + sensors produced by a painted or painted-frozen lipid monolayer on an alkanethiol alkylated gold electrode is larger than that by the LB method, which is due to the difference in fluidity of the three kinds of bilayers. Selectivity coefficients KK+, Na+, KK+, Li+, KK+, Ca2+ and KK+, Mg2+ are 10(-4), 10(-4), 2 x 10(-5) and 3 x 10(-5) respectively, and there is no obvious difference among different fabricating methods. A linear response toward the potassium ion was found in the range from 10(-1) M to 10(-5) M with the detection limit of 10(-6) M. The sensor has a slope of 60 mV per decade. Meanwhile, the longevity of the sensor was improved obviously for at least two months at about -10 degrees C. The higher stability shows the possibility to fabricate a practical biosensor.

Electroless-plated gold films for sensitive surface plasmon resonance detection of white spot syndrome virus

The paper describes the rapid and label-free detection of the white spot syndrome virus (WSSV) using a surface plasmon resonance (SPR) device based on gold films prepared by electroless plating. The plating condition for obtaining films suitable for SPR measurements was optimized. Gold nanoparticles adsorbed on glass slides were characterized by transmission electron microscopy (TEM). Detection of the WSSV was performed through the binding between WSSV in solution and the anti-WSSV single chain variable fragment (scFv antibody) preimmobilized onto the sensor surface. Morphologies of the as-prepared gold films, gold films modified with self-assembled alkanethiol monolayers, and films covered with antibody were examined using an atomic force microscope (AFM). To demonstrate the viability of the method for real sample analysis, WSSV of different concentrations present in a shrimp hemolymph matrix was determined upon optimizing the surface density of the antibody molecules. The SPR device based on the electroless-plated gold films is capable of detecting concentration of WSSV as low as 2.5 ng/mL in 2% shrimp hemolymph, which is one to two orders of magnitude lower than the level measurable by enzyme-linked immunosorbant assays. (c) 2007 Elsevier B.V. All rights reserved.

Length dependence of electron conduction for oligo(1,4-phenylene ethynylene)s: A conductive probe-atomic force microscopy investigation

The dependence of electron conduction of oligo(1,4-phenylene ethynylene)s (OPEs) on length, terminal group, and main chain structure was examined by conductive probe-atomic force microscopy (CP-AFM) via a metal substrate-molecular wire monolayer-conductive probe junction. The electron transport in the molecular junction was a highest occupied molecule orbital (HOMO)-mediated process following a coherent, non-resonant tunneling mechanism represented by the Simmons equation.

In Situ Scanning Tunneling Microscopy on the Dynamic Process of the Replacement of Coronene on Au (111) by 6-Mercapto-1-Hexanol

The replacement of coronene monolayer on Au (111) by 6-mercapto-1-hexanol (MHO) was studied by in situ scanning tunneling microscopy (STM) in solutions. It was found that the rate of replacement depends strongly on the concentration of MHO. The replacement finished within a second at a higher concentration of MHO. At a lower concentration, the slow replacement could be followed by in situ STM. The replacement occurred initially near the elbow position of reconstructed Au (111) with the formation of pits in a single or several missing molecules. With the proceeding of replacement, these small pits expanded, and the surrounding coronene molecules were gradually substituted by MHO, which developed into ordered domains within a spatial confined environment. Meanwhile, the reconstruction of Au (111) was lifted. The replacement expanded fast along the reconstruction lines in the domain. For the fast replacement, a (root 3 x root 3) R30 degrees adlattice was observed, while a c(4 x 2) superlattice was observed for the slow replacement.

Molecular sensing with the tunnel junction of an Au nanogap in solution

The tunnel junction of a gold nanogap was fabricated electrochemically for a molecular sensing device in solution. The tunnel junction was sensitive enough to detect the variation of a potential barrier within the nanogap, such as the chemical adsorption of molecules. By monitoring the variation of the tunneling current, which represents the change of a potential barrier due to molecular adsorption, the molecules could be detected.

Electrochemical Study of the Self-exchange between Viologen-thiol Self assembled Monolayers and Alkanethiol Gold Electrodes

The preparation and cyclic voltammetric behaviors of self assembled monolayers (SAMs) containing electroactive viologen group have been investigated. Treatment of this viologen SAM with solutions of alkanethiols remits in replacing the electroactive third, shifting negatively its formal potentials and decreasing its heterogeneous elixtron transfer constants along with the immersion time. The aim of the work is to understand the exchange regularity of the mixed SANK on gold electrode surface.

Potential and kinetic sputtering of alkanethiol self-assembled monolayers by impact of highly charged ions

Highly charged ions have been used to study the sputtering of positive molecular fragments from mercaptoundecanoic acid and dodecanethiol self-assembled monolayers on gold surfaces. The samples were bombarded with Arq+ (42n⁺, and Cn+1O2H_{2n + 1}⁺ from mercaptoundecanoic and H+, CnH_2n⁺, and Cn+1H_{2n + 3}⁺ from dodecanethiol. The proton yields were increased with larger charge state q of the highly charged ion (HCI) in both samples, scaling as qgamma, with gamma~5. The charge state dependence is discussed in terms of electron transfer to the HCI. The final yield of protons depends on molecular functional group characteristics, orientation on the surface, and reneutralization phenomena.

Microelectrode array in mixed alkanethiol self-assembled monolayers: Electrochemical studies

We report an efficient alternative to obtain recessed microelectrodes device on gold electrode surface, in which mixed self-assembled monolayer of long and short carbon alkanethiol chains was used for this purpose. Development of the modified electrodes included the chemical adsorption of 11-mercaptoundecanoic acid and 2-mercaptoethanol solution, as well as their mixtures, on gold surface, resulting in the final mixed self-assembled monolayer configuration. For comparison, the electrochemical performance of self-assembled monolayer of 11-mercaptoundecanoic acid. 3-mercaptopropionic acid, 4-mercapto-1-butanol and 6-mercapto-1-hexanol modified electrodes was also investigated. It was verified that, in the mixed self-assembled monolayer, the 11-mercaptoundecanoic acid acts as a barrier for electron transfer while the short alkanethiol chair is deposited in an island-like shape through which electrons can be freely transferred to ions in solution, allowing electrochemical reactions to occur. The performance of the modified electrodes toward microelectrode behavior was investigated via cyclic voltammetry and electrochemical impedance spectroscopy measurements using [Fe(CN)(6)](3-/4-) redox couple as a probe. In this case, sigmoidal voltammetric responses were obtained, very similar to those observed for microelectrodes. Such behavior reinforces the proposition of electron transfer through the short alkanethiol chain layer and surface blockage by the long chain one. Electrochemical impedance results allowed calculated the mean radius value of each microelectrode disks of 3.8 mu m with about 22 mu m interval between them. The microelectrode environment provided by the mixed self-assembled monolayer can be conveniently used to provide an efficient catalytic conversion in biosensing applications. (C) 2012 Elsevier Ltd. All rights reserved.

Ferrocene-terminated alkanethiol self-assembled monolayers: An electrochemical and in situ surface-enhanced infra-red absorption spectroscopy study

The effect of anions on the redox behavior and structure of 11-ferrocenyl-1-undecanethiol (FcC11) monolayers (SAM) on Au(1 1 1) single crystal and Au(1 1 1-25 nm) thin film electrodes was investigated in 0.1 M solutions of HPF6, HClO4, HBF4, HNO3, and H2SO4 by cyclic voltammetry (CV) and in situ surface-enhanced infrared reflection-absorption spectroscopy (SEIRAS). We demonstrate that the FcC11 redox peaks shift toward positive potentials and broaden with increasing hydrophilicity of the anions. In situ surface-enhanced IR-spectroscopy (SEIRAS) provided direct access for the incorporation of anions into the oxidized adlayer. The coadsorption of anions is accompanied by the penetration of water molecules. The latter effect is particularly pronounced in aqueous HNO3 and H2SO4 electrolytes. The adlayer permeability increases with increasing hydrophilicity of the anions. We also found that even the neutral (reduced) FcC11 SAM is permeable for water molecules. Based on the property of interfacial water to reorient upon charge inversion, we propose a spectroscopic approach for estimating the potential of zero total charge of the FcC11-modified Au(1 1 1) electrodes in aqueous electrolytes.

Advancing atomic nanolithography: cold atomic Cs beam exposure of alkanethiol self-assembled monolayers

We report the results of a study into the quality of functionalized surfaces for nanolithographic imaging. Self-assembled monolayer (SAM) coverage, subsequent post-etch pattern definition and minimum feature size all depend on the quality of the Au substrate used in atomic nanolithographic experiments. We find sputtered Au substrates yield much smoother surfaces and a higher density of {111} oriented grains than evaporated Au surfaces. A detailed study of the self-assembly mechanism using molecular resolution AFM and STM has shown that the monolayer is composed of domains with sizes typically of 5-25 nm, and multiple molecular domains can exist within one Au grain. Exposure of the SAM to an optically-cooled atomic Cs beam traversing a two-dimensional array of submicron material masks ans also standing wave optical masks allowed determination of the minimum average Cs dose (2 Cs atoms per SAM molecule) and the realization of < 50 nm structures. The SAM monolayer contains many non-uniformities such as pin-holes, domain boundaries and monoatomic depressions which are present in the Au surface prior to SAM adsorption. These imperfections limit the use of alkanethiols as a resist in atomic nanolithography experiments. These studies have allowed us to realize an Atom Pencil suitable for deposition of precision quantities of material at the microand nanoscale to an active surface.