Attachment of Amine‐ and Maleimide‐Containing Ferrocene Derivatives onto Self‐Assembled Alkanethiol and Alkanedithiol Monolayers: Voltammetric Evaluation of Cross‐Linking Efficiencies and Surface Coverage of Electroactive Groups

Ferrocene derivatives containing primary amines and maleimide groups were attached covalently onto N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol self‐assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross‐linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol SAM. The covalent attachment of β‐ferrocenylethylamine onto a 11,11′‐dithio‐bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross‐linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael‐type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross‐linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross‐linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups.     

Orientation modulation of a synthetic polypeptide in self-assembled monolayers: a TOF-SIMS study

Structure and orientation of molecules are key properties of functionalized surfaces. Using time-of-flight secondary ion mass spectrometry (TOF-SIMS), here we investigate how to modulate these parameters upon the immobilization process varying the conditions of self-assembly. The molecule of interest, a template-assembled synthetic protein (TASP), consists of a central peptide ring with orthogonally arranged residues. Thioalkane chains allow the directed self-assembly of the molecule on a gold surface; four serine residues on the opposite side of the ring can be used as anchoring sites for various functional sensing molecules. The TASP conformation and its orientation in self-assembled monolayers (SAMs) play a central role for the accessibility of these serine residues. To study the influence of the self-assembly conditions, two series of samples were prepared. Pure TASP monolayers of different surface densities are compared to mixed TASP/alkanethiol monolayers prepared by sequential adsorption varying sequence and particular incubation times as well as by coadsorption modifying incubation times and TASP/alkanethiol mass ratios. Switching the TASP orientation from a state where the molecules are lying flat on the surface to an upright orientation turned out to be possible by inserting the TASP into a preformed alkanethiol monolayer of an appropriate surface density. This study demonstrates that TOF-SIMS is an excellent tool not only to investigate the surface composition, but also the molecular structure of functionalized surfaces.     

Electrochemical Fabrication of Nanostructured Surfaces for Enhanced Response

The objective of this work is to explore approaches to enhance electrochemical signals through sequential deposition and capping of gold particles. Gold nanoparticles are electrodeposited from KAuCl₄ solution under potentiostatic conditions on glassy carbon substrates. The number density of the nanoparticles is increased by multiple deposition steps. To prevent secondary nucleation processes, the nanoparticles are isolated after each potentiostatic deposition step by self‐assembled monolayers (SAMs) of decanethiol or mercaptoethanol. The increasing number of particles during five deposition/protection rounds is monitored by assembling electroactive SAMs using a ferrocene‐labeled alkanethiol. A precise estimation of the surface area of the gold nanoparticles by formation of an oxide layer on gold is difficult due to oxidation of the glassy carbon surface. As an alternative approach, the charge flow of the electroactive SAM is used for surface measurement of the gold surface area. A sixfold increase in the redox signal in comparison to a bulk gold surface is observed, and this increase in redox signal is particularly notable given that the surface area of the deposited nanoparticles is only a fraction of the bulk gold surface. After five rounds of deposition there is a gold loading of 1.94 μg cm^?2 of the deposited nanoparticles as compared to 23.68 μg cm^?2 for the bulk gold surface. Remarkably, however, the surface coverage of the ferrocene alkanethiol on the bulk material is only 10 % of that achieved on the deposited nanoparticles. This enhancement in signal of the nanoparticle‐modified surface in comparison to bulk gold is thus demonstrated not to be attributable to an increase in surface area, but rather to the inherent properties of the surface atoms of the nanoparticles, which are more reactive than the surface atoms of the bulk material.     

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.     

One-step immobilization of alkanethiol/glycolipid vesicles onto gold electrode: amperometric detection of Concanavalin A

Functionalized vesicles composed of glycolipid and alkanethiol lipids have been immobilized onto gold surface through one-step self-assembly to construct an electrochemical biosensor for Concanavalin A (Con A) detection. Incorporation of alkanethiol lipid molecules into the vesicles allows for firm attachment of the vesicles onto a gold surface to form a sensing interface. At the same time, the introduction of alkanethiol lipid avoids cumbersome organic syntheses of sulfur-containing compound, making the biosensor greater applied prospect. Through the recognition of Con A by glycolipid which was immobilized on the surface of electrode, a decrease of electrochemical signal was observed. This decrease was restored when the electrode was immersed in a stronger binding solution such as glucose. The repeated usability of the novel sensor is excellent.     

Synthesis of Ferrocene Derivatives Allowing Linear Free Energy Studies of Redox Potentials

A series of ferrocene derivatives, which have diverse redox potentials modulated by functional groups, have been synthesized as potential ‘multi‐potential’ probes. A Hammett constant analysis revealed a linear free energy correlation between the redox potentials and the electron density of the ferrocene derivatives as determined by the choice of functional group used to modify the ferrocene core.     

Ferrocene embedded in an electrode-supported hybrid lipid bilayer membrane: a model system for electrocatalysis in a biomimetic environment

An electrode-supported system in which ferrocene molecules are embedded in a hybrid bilayer membrane (HBM) has been prepared and characterized. The redox properties of the ferrocene molecules were studied by varying the lipid and alkanethiol building blocks of the HBM. The midpoint potential and electron transfer rate of the embedded ferrocene were found to be dependent on the hydrophobic nature of the electrolyte and the distance at which the ferrocene was positioned in the HBM relative to the electrode and the solution. Additionally, the ability of the lipid-embedded ferrocenium ions to oxidize solution phase ascorbic acid was evaluated and found to be dependent on the nature of the counterion.     

Thiolate chemistry: a powerful and versatile synthetic tool for immobilization/functionalization of oligothiophenes on a gold surface

The synthesis and characterization of a series of quaterthiophenes (4Ts) with thiolate groups protected with 2-cyanoethyl (CNE), 2-trimethylsilylethyl (TMSE), and acetyl (Ac) groups are described. Sequential cleavage of these different protecting groups allows for the preparation of 4Ts derivatized with ferrocene and/or alkanethiol chains. The electrochemical behavior of these compounds has been analyzed in solution by cyclic voltammetry (CV). A ferrocene-derivatized dithiol 4T 14 and a dithiol 4T 15 with two TMSE-protected thiolate groups have been immobilized on a gold surface as monolayers that have been characterized by CV, ellipsometry, contact-angle measurement, and X-ray photoelectron spectroscopy (XPS). The results show that molecules 14 and 15 are doubly grafted with a horizontal orientation of the conjugated system relative to the surface. Furthermore, application of the deprotection/alkylation sequence of the remaining protected thiolate groups on a monolayer of 15 allows for efficient post-functionalization.     

Electrical Control of Alkanethiols Self-Assembly on a Gold Surface as an Approach for Preparation of Microelectrode Arrays

 It is shown by capacitive monitoring that the self-assembly of alkanethiols on gold electrodes and desorption of these self-assembled monolayers from the electrodes are controlled by the electrode potential. At neutral pH, chemical adsorption of alkanethiols was observed at an electrode potential of +300 mV vs SCE, but only physical adsorption was detected when the electrode potential was −1400 mV vs SCE. At electrode potentials between these values (−300 mV, −600 mV), chemical adsorption of alkanethiols occurred, but the alkanethiol monolayers were not stable in the absence of the alkanethiol in the bulk solution and were desorbed from the gold electrode. The desorption rate was higher at more negative electrode potentials. These results can be used in designing methods for electrically addressable immobilization of different receptors on (micro)electrode arrays. This has been demonstrated by deposition of two different types of alkanethiols onto a two-electrode array. Received June 24, 1998. Revision October 19, 1998.     

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.     

Electron transfer at self-assembled monolayers measured by scanning electrochemical microscopy

New approaches have been developed for measuring the rates of electron transfer (ET) across self-assembled molecular monolayers by scanning electrochemical microscopy (SECM). The developed models can be used to independently measure the rates of ET mediated by monolayer-attached redox moieties and direct ET through the film as well as the rate of a bimolecular ET reaction between the attached and dissolved redox species. By using a high concentration of redox mediator in solution, very fast heterogeneous (10(8) s(-1)) and bimolecular (10(11) mol(-1) cm(3) s(-1)) ET rate constants can be measured. The ET rate constants measured for ferrocene/alkanethiol on gold were in agreement with previously published data. The rates of bimolecular heterogeneous electron transfer between the monolayer-bound ferrocene and water-soluble redox species were measured. SECM was also used to measure the rate of ET through nonelectroactive alkanethiol molecules between substrate gold electrodes and a redox probe (Ru(NH(3))(6)(3+)) freely diffusing in the solution, yielding a tunneling decay constant, beta, of 1.0 per methylene group.     

Nanografting versus solution self-assembly of alpha,omega-alkanedithiols on Au(111) investigated by AFM

The solution self-assembly of alpha,omega-alkanedithiols onto Au(111) was investigated using atomic force microscopy (AFM). A heterogeneous surface morphology is apparent for 1,8-octanedithiol and for 1,9-nonanedithiol self-assembled monolayers (SAMs) prepared by solution immersion as compared to methyl-terminated n-alkanethiols. Local views from AFM images reveal a layer of mixed molecular orientations for alpha,omega-alkanedithiols, which evidence surface structures with heights corresponding to both lying-down and standing-up orientations. For dithiol SAMs prepared by solution self-assembly, the majority of alpha,omega-alkanedithiol molecules chemisorb with both thiol end groups bound to the Au(111) surface with the backbone of the alkane chain aligned parallel to the surface. However, AFM images disclose that there are also islands of standing molecules scattered throughout the surface. To measure the thickness of alpha,omega-alkanedithiol SAMs with angstrom sensitivity, methyl-terminated n-alkanethiols with known dimensions were used as molecular rulers. Under conditions of spatially constrained self-assembly, nanopatterns of alpha,omega-alkanedithiols written by nanografting formed monolayers with heights corresponding to an upright configuration.     

Measurement of Ibuprofen Binding to Mixed Monolayers Containing β-Cyclodextrin Active Sites

A molecular imprinting method involving a three-step sequential self-assembly procedure was applied to prepare gold electrodes responsive towards ibuprofen. The electrode modified with the cyclodextrin derivative binds ferrocene to form an electroactive complex with the ferrocene oxidation current decreasing in the presence of ibuprofen in the solution. The competition of ferrocene and ibuprofen for the cyclodextrin cavities in the monolayer provided a means for the determination of the binding constants of ibuprofen with two derivatives of lipoylamide β-cyclodextrin of different hydrophobicity.     

Chirality organization of ferrocenes bearing podand dipeptide chains: synthesis and structural characterization

A variety of ferrocenes bearing podand dipeptide chains have been synthesized to form an ordered structure in both solid and solution states and have been investigated by 1H NMR, FT-IR, CD, and X-ray crystallographic analyses. Conformational enantiomerization through chirality organization was achieved by the intramolecular hydrogen bondings between the podand dipeptide chains. The single-crystal X-ray structure determination of the ferrocene 2 bearing the podand dipeptide chains (-D-Ala-D-Pro-OEt) revealed two C2-symmetric intramolecular hydrogen bondings between CO (Ala) and NH (another Ala) of each podand dipeptide chain to induce the chirality-organized structure. The molecular structures of the ferrocene 1 composed of the podand L-dipeptide chains (-L-Ala-L-Pro-OEt) and 2 are in a good mirror image relationship, indicating that they are conformational enantiomers. An opposite helically ordered molecular arrangement was formed in the crystal packing of 2 as compared with 1. The ferrocene 2 exhibited induced circular dichroism (CD), which appeared at the absorbance of the ferrocene moiety. The mirror image of the CD signals between 1 and 2 was observed, suggesting that the chirality-organized structure via intramolecular hydrogen bondings is present even in solution. The ferrocene 4 bearing the podand dipeptide chains (-Gly-L-Leu-OEt) also showed an ordered structure in the crystal based on two intramolecular hydrogen bondings between CO (Gly) and NH (another Gly) of each podand dipeptide chain, together with intermolecular hydrogen bondings between CO adjacent to the ferrocene unit and NH (neighboring Leu) to create the highly organized self-assembly. A different self-assembly was observed in the crystal of the ferrocene 5 composed of the podand dipeptide chains (-Gly-L-Phe-OEt), wherein each molecule is bonded to two neighboring molecules through two pairs of symmetrical intermolecular hydrogen bonds to form a 14-membered intermolecularly hydrogen-bonded ring. These ordered structures based on the intramolecular hydrogen bondings in the solution state are also confirmed by 1H NMR and FT-IR.     

Discotic hexa-peri-hexabenzocoronenes with strong dipole: synthesis, self-assembly and dynamic studies

Strong dipole moments have been built into two hexa-peri-hexabenzocoronene (HBC) derivatives (1 and 2) originating from the push-pull structure of the molecules with one electron-donating and one electron-withdrawing substituent. The influence of dipole moment on the self-assembly of HBCs in solution and in bulk has been investigated.     

Thermotropic properties of ferrocene derivatives bearing a cholesteryl unit: structure–properties correlations

The synthesis and structural characterization of new liquid‐crystalline compounds containing ferrocene, azo‐aromatic and cholesteryl groups are reported. Taking into account the advantage brought by chirality, ferrocene and azo units, these structures could be good precursors for obtaining materials capable of responding to magnetic and electric fields or to UV‐light exposure. The influence of each structural unit (ferrocene, cholesterol, azo aromatic core and flexible chain length) has been studied by comparing analogous compounds possessing the same structure but without the element being analyzed. Ferrocene is a three‐dimensional bulky unit, so that, regardless of the substituents' nature, this unit could cause steric repulsions with neighboring molecules. These interactions could lead to a decrease of the transition temperature domain. Surprisingly, a decrease in the clearing point was not observed for the compounds discussed. This behavior was possible because ferrocene is connected to the mesogen via a flexible unit. As a consequence, both phenyl analogues and ferrocene derivatives presented liquid‐crystalline properties with similarly high clearing points, but above the thermal stability of derivatives with azo groups. Their melting points depend on the way the molecules are packed, with different crystalline states being detected in the case of ferrocene derivatives. In order to explain the liquid‐crystalline behavior of the compounds synthesized, molecular simulations were performed using the Hyperchem program. Copyright © 2005 John Wiley & Sons, Ltd.     

Open tubular capillary electrochromatography using capillaries coated with films of alkanethiol-self-assembled gold nanoparticle layers

We have used alkanethiol self-assembly and dithiol layer-by-layer (LBL) self-assembly processes to prepare an Au nanoparticle (NP)-coated open tubular capillary electrochromatography (OTCEC) column for the separation of three neutral steroid drugs (testosterone, progesterone, and testosterone propionate). The CEC column was fabricated through LBL self-assembly of Au NPs on a 3-aminopropyltrimethoxysilane (APTMS)-modified fused-silica capillary and subsequent surface functionalization of the Au NPs through self-assembly of alkanethiols. We investigated the electrochromatographic properties of the resulting Au NP-coated CEC column using a "reversed phase" test mixture of three steroid drugs. We found that the key factors affecting the separation performance were the number of Au NP layers, the length of the carbon-atom chain of the alkanethiol self-assembled on the Au NPs, the percentage of organic modifier, and the pH of the running electrolyte. This study reveals that the self-assembly of alkanethiols and dithiols onto Au NPs provides stationary phases for CEC separation that are easy to prepare and whose retention behavior is highly controllable and reproducible. We believe that our findings will contribute to further studies of the application of nanotechnology to separation science.     

Voltammetric studies of through-space and through-bond electrostatic interactions in alkyl linked ferrocene and benzoaza-15-crown-5 receptor molecules in acetonitrile

Six new ferrocene alkyl-benzoaza-15-crown-5 molecules with different alkyl spacer lengths were synthesized and investigated by voltammetry in acetonitrile. Their mean potentials (E(o)') were more negative than that of ferrocene. The changes were greater for the bis-substituted ligands than for the monosubstituted ones. Increasing the alkyl spacer length shifted E(o)' negatively from -CH2- to -(CH2)2- but positively from -(CH2)2- to -(CH2)4-. This unusual variation is attributed to the combined electron donating and withdrawing influences and also the steric effect from the substituents on the ferrocene moiety. Analyses of the potentials of the molecules and their fully protonated forms suggested intramolecular electrostatic signaling through not only the space but also the alkyl chain which is usually considered to be an insulator for through-bond communication. Diffusion coefficients with insignificant differences between the receptors and their fully protonated forms were derived from cyclic voltammograms, suggesting insignificant further conformational variation upon protonation.     

Metal-organic frameworks post-synthetically modified with ferrocenyl groups: framework effects on redox processes and surface conduction

Metal-organic framework (MOF) materials based on zinc(II) and aluminium(III) dicarboxylate frameworks with covalently attached ferrocene functional redox groups were synthesised by post-synthetic modification and investigated by voltammetry in aqueous and non-aqueous media. In the voltammetry experiments, ferrocene oxidation occurs in all cases, but chemically reversible and stable ferrocene oxidation without decay of the voltammetric response requires a "mild" dichloroethane solvent environment. The voltammetric response in this case is identified as "surface-confined" with fast surface-hopping of electrons and without affecting the bulk of MOF microcrystals. In aqueous media a more complex pH-dependent multi-stage redox process is observed associated with chemically irreversible bulk oxidation and disintegration of the MOF framework. A characteristic 30 mV per pH unit dependence of redox potentials is observed attributed to a "framework effect": the hydroxide-driven MOF framework dissolution.     

Effect of chain length of self-assembled monolayers in dip-pen nanolithography using molecular dynamics simulations

The pattern transfer mechanism of an alkanethiol self-assembled monolayer (SAM) with different chain lengths during the dip-pen nanolithography (DPN) process and pattern characterizations are studied using molecular dynamics (MD) simulations. The mechanisms of molecular transference, alkanethiol meniscus characteristics, surface adsorbed energy, transfer number, and pattern formation are evaluated during the DPN process at room temperature. The simulation results clearly show that the molecular transfer ability in DPN is strongly dependent on the chain length. Shorter molecules have significantly better transport and diffusion abilities between the meniscus and substrate surface, and the transport period can be maintained longer. The magnitude of adsorbed energy increases with chain length, so many more molecules can be transferred to the surface when shorter molecules are used. After deposition, the magnitude of the adsorbed area and pattern height decrease with increasing chain length.