Short Alkanethiol Research Articles

Shedding Light on the Interfacial Structure of Low-Coverage Alkanethiol Lattices

A comprehensive description of the self-assembly process of alkanethiols on Au(111) is presented, focused on the initial formation of the lying down phases. Low-coverage monolayers are prepared by the disintegration of Au144(RS)60 nanoclusters on the reconstructed (22 × ?3)-Au(111) surface. The method provides a limited number of thiols together with a large excess of gold adatoms. Scanning tunneling microscopy and density functional theory calculations were employed to study the transition between low to high thiolate coverage phases. The process involves different lattices and surface transformations, including thiyl radicals on the herringbone reconstruction, radical-induced herringbone lifting, and the formation of energetically similar metastable phases formed by RS-Au-RS moieties. Results also show that the transition is slow, and different surface structures can coexist on the same sample. Along the process, the first source of Au adatoms to form the RS-Au-SR moieties is the lifting of the herringbone reconstruction because of the lower energetic cost to extract the extra Au atom. However, for hexanethiol (and shorter alkanethiols) at low coverage, additional Au adatoms must be taken from terraces leading to vacancy islands. This process can be entirely suppressed by growing the lying down phases in the presence of an excess of Au adatoms. Taken together, our results shed light on the elusive initial steps of thiol adsorption on clean reconstructed Au, showing that the RS-Au-SR staple motif is also present at the interface of low-coverage self-assembled monolayers.

Protection of silver and gold LSPR biosensors in corrosive NaCl environment by short alkanethiol molecules; characterized by extinction spectrum, helium ion microscopy and SERS.

We investigated the utility of localized surface plasmon resonance sensors in a biologically relevant environment containing NaCl. Our sensors are fabricated by depositing gold or silver on a monolayer of adsorbed monodisperse SiO2 nanospheres. While silver nanostructures are rather unstable in air and water as assessed by drifts in the extinction peak, even gold nanostructures have been found to drift at elevated NaCl concentrations. In an attempt to protect these nanostructures against NaCl, we modified them with alkanethiols with different lengths in the vapor phase and found that shorter chain alkanethiols such as 1-butanethiol are particularly effective against even 250 mM NaCl, rather than longer-chain alkanethiols more suitable for robust SAM formation. A vapor phase treatment method, in contrast to widely used solution phase treatment methods, was selected with the intention of reducing the solvent effect, i.e. destruction of intricate nanostructures upon contact with a solvent when nanostructures have been prepared in a vacuum system. Moreover, the treatment with 1-butanethiol led to an enhanced sensitivity as expressed by peak shift in nm per refractive index unit, nm per RIU. We show the results of evaluating alkanethiol-protected silver and gold nanostructures by extinction spectroscopy, helium ion microscopy and surface-enhanced Raman spectroscopy. The vapor phase treatment method with short chain alkanethiols is an effective way to protect intricate gold and silver nanostructures prepared in a vacuum system.

Mixed‐monolayer of N‐hydroxysuccinimide‐terminated cross‐linker and short alkanethiol to improve the efficiency of biomolecule binding for biosensing

The goal of this study was to use a novel surface chemistry for modifying gold surfaces to decrease the steric hindrance, minimize the nonspecific bindings while providing directed immobilization of proteins for advancing the transducer property and to provide a biosensing platform for surface plasmon resonance (SPR) applications. Mixed self‐assembled monolayers (mSAMs) were prepared using 3,3′‐Dithiodipropionic acid di (N‐hydroxysuccinimide ester) (DSP) and 6‐mercapto‐1‐hexanol (MCH) and the selected model proteins bovine serum albumin (BSA) and lysozyme were tested for binding efficiency. First, binding of these two proteins at constant concentration to different DSP:MCH mSAMs were compared to deduce the best molar ratio for forming mSAM using a continuous flow system coupled to SPR. Coincidently the maximum protein binding DSP:MCH mSAM were the same for both proteins. The change in Response Unit (∆RU) signal due to protein binding between DSP SAM and maximum protein binding DSP:MCH mSAM for lysozyme binding was more in comparison to BSA binding. Second, the effect of BSA and lysozyme concentration on binding efficiency to maximum protein binding DSP:MCH mSAM were compared and discussed. Lysozyme and BSA were shown to reach saturations on the same monolayer at concentrations of 5.7x10−5 and 8.96x10−6 [M] respectively, hence the molar ratio for limit concentrations is 6:1. The DSP SAM, MCH SAM, and DSP:MCH mSAMs where maximum and minimum protein binding occurs were also characterized with XPS and Attenuated total reflectance‐Fourier transform infrared (ATR‐FTIR) spectroscopy. Blank gold surface, maximum protein binding DSP:MCH mSAM and BSA immobilized DSP:MCH mSAM on gold surface were also investigated utilizing tapping mode AFM.

Modification of a Mercury Electrode with Different Thioalkanes: Structure‐Sensitive Bovine Serum Albumin Analysis

AbstractChronopotentiometric stripping (CPS) in combination with mercury electrodes is convenient for structure‐sensitive protein analysis. By using this method, we tested the discrimination between native and denatured bovine serum albumin (BSA) at various alkanethiol self‐assembled monolayers (SAMs). Our results showed that alkanethiol SAMs with different headgroups or different chain lengths had different influences on the efficiency of CPS in protecting BSA from the electric field‐induced denaturation. SAMs with longer alkanethiol chain lengths were more efficient in protecting BSA from denaturation at negatively charged electrode surface than those formed by short‐chain alkanethiols. Short alkanethiol SAMs could be prepared by co‐adsorption with the protein on the electrode surface, which was not possible with the long‐chain alkanethiols. CPS data were supported by impedance analysis. We believe that our work will be useful in the electrochemical sensing of nucleic acid‐protein and protein‐protein binding.

Acid-enhanced conformation changes of yeast cytochrome c coated onto gold nanoparticles, a FT-IR spectroscopic analysis

Under conditions with or without linker molecules, the effects of acidic pH on the conformation of yeast iso-1-cytochrome c coated onto gold nanoparticles (AuNPs) in correlation with color changes of a Cyt c-coated AuNPs solution/suspension were examined by Fourier transform infrared (FT-IR) spectroscopy and correlated to color change. The results of detailed secondary structural analysis revealed that although the color changes coincide with acid-induced conformational changes in Cyt c coated onto AuNPs, the pH-related conformational unfolding of Cyt c coated onto AuNPs differed dramatically from that of its counterpart in solution. For Cyt c free in solution, the acid-induced unfolding did not occur until the pH was below 3.0, whereas for Cyt c coated onto AuNPs via C102 coordination near the C-terminal, a partial unfolding was observed even at near neutral pH which continuously intensified as pH decreased. Insertion of a short alkanethiol (3-mercaptoproprionic acid, 3-MPA) molecule between Cyt c and AuNP, which changes the interaction mode from a thiol coordination between Cyt c and AuNP to an electrostatic interaction between Cyt c and 3-MPA, which stabilized the conformation of Cyt c significantly, but did not prevent the acid-induced aggregation of Cyt c-3MPA-AuNPs.

Synthesis of oligo(lactose)-based thiols and their self-assembly onto gold surfaces

The ability to produce monomolecular coatings with well-defined structural and functional properties is of key importance in biosensing, drug delivery, and many recently developed applications of nanotechnology. Organic chemistry has proven to be a powerful tool to achieve this in many research areas. Herein, we present the synthesis of three oligo(lactosides) glycosylated in a (1→3) manner, and which are further functionalized with amide-linked short alkanethiol spacers. The oligosaccharides (di-, tetra-, and hexasaccharide) originate from the inexpensive and readily available lactose disaccharide. These thiolated derivatives were immobilized onto gold surfaces, and the thus formed self-assembled monolayers (SAMs) on planar gold were characterized by wettability, ellipsometry and infrared reflection–absorption spectroscopy. Further, the ability of these SAMs to stabilize gold nanoparticles in saline solutions was also demonstrated, indicating that the oligosaccharides may be used as stabilizing agents in gold nanoparticle-based assays.

Pickering Emulsions Stabilized by Nanoparticle Surfactants

Amphiphilic gold nanoparticles are demonstrated to effectively stabilize emulsions of hexadecane in water. Nanoparticle surfactants are synthesized using a simple and scalable one-pot method that involves the sequential functionalization of particle surfaces with thiol-terminated polyethylene glycol (PEG) chains and short alkane-thiol molecules. The resulting nanoparticles are shown to be highly effective emulsifying agents due to their strong adsorption at oil-water and air-water interfaces. The original nonfunctionalized gold nanoparticles are unable to effectively stabilize oil-water emulsions due to their small size and low adsorption energy. Small-angle X-ray scattering and electron microscopy are used to demonstrate the formation of nanoparticle-stabilized colloidosomes that are stable against coalescence and show significant shifts in plasmon resonance enhancing the near-infrared optical absorption.

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 3.8μm with about 22μm 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.

Synthesis of Silver Nanoparticles with Monovalently Functionalized Self-Assembled Monolayers

The importance of having nanoparticles that are soluble, stable, and that have no non-specific binding is often overlooked, but essential for their use in biology. This is particularly prominent with silver nanoparticles that are susceptible to the effects of aggregation and metal-surface reactivity. Here we use a combination of several small peptidols and short alkanethiol ethylene glycol ligands to develop a ligand shell that is reasonably resistant to ligand exchange and non-specific binding to groups common in biological molecules. The stability of the nanoparticles is not affected by the inclusion of a functional ligand, which is done in the same preparative step. The stoichiometry of the nanoparticles is controlled, such that monofunctional silver nanoparticles can be obtained. Two different sets of nanoparticles, functionalized with either Tris-nitrilotriacetic acid or a hexa-histidine peptide sequence, readily form dimers/oligomers, depending on their stoichiometry of functionalization.

X-ray photoelectron spectroscopy characterization of gold nanoparticles functionalized with amine-terminated alkanethiols

Gold nanoparticles (AuNPs) functionalized with a short chain amine-terminated alkanethiol (HS-(CH(2))(2)NH(2) or C2 NH(2)-thiol) are prepared via a direct synthesis method and then ligand-exchanged with a long chain amine-terminated alkanethiol (HS-(CH(2))(11)NH(2) or C11 NH(2)-thiol). Transmission electron microscopy analysis showed the AuNPs were relatively spherical with a median diameter of 24.2 ± 4.3 nm. X-ray photoelectron spectroscopy was used to determine surface chemistry of the functionalized and purified AuNPs. The ligand-exchange process was monitored within the time range from 30 min to 61 days. By the fourth day of exchange all the C2 NH(2)-thiol molecules had been replaced by C11 NH(2)-thiol molecules. C11 NH(2)-thiol molecules continued to be incorporated into the C11 NH(2) self-assembled monolayer between days 4 and 14 of ligand-exchange. As the length of the exchange time increased, the functionalized AuNPs became more stable against aggregation. The samples were purified by a centrifugation and resuspension method. The C2 NH(2) covered AuNPs aggregated immediately when purification was attempted. The C11 NH(2) covered AuNPs could be purified with minimal or no aggregation. Small amounts of unbound thiol (∼15%) and oxidized sulfur (∼20%) species were detected on the ligand-exchanged AuNPs. Some of the unbound thiol and all of the oxidized sulfur could be removed by treating the functionalized AuNPs with HCl.

Bifunctional Tri(ethylene glycol) Alkanethiol Monolayer Modified Gold Electrode for On-Chip Electrochemical Immunoassay of pg Level Leptin

An on-chip enzyme-linked immunosorbent assay combined with an electrochemical detection method (EC-ELISA) was employed to detect a leptin, one of the most important adipose derived hormones, using gold electrodes modified with a tri(ethylene glycol) terminated short alkanethiol (TEGCnSH, Cn = (CH(2))n, n = 2, 4, 6, and 8) monolayer. These TEGCnSH monolayers on gold electrodes can suppress non-specific protein adsorption without affecting the electrochemical activity required for detecting p-aminophenol (PAP), which is an alkaline phosphatase (ALP) product. We measured leptin with a highly sensitive detection range (100 pg mL(-1) to 10 ng mL(-1) level) and with the desired detection limit (13.6 pg mL(-1)) by using electrochemical detection. For detecting leptin, the EC-ELISA method using TEGC4SH modified gold electrode with a poly(dimethylsiloxane) based microchannel was superior to the conventional ELISA method. With the EC-ELISA method, we were able to measure leptin with a satisfactory detection range and a pg level detection limit within 30 min, which is a much lower detection level than that obtained with conventional plate based ELISA.

Adsorption and thermal stability of alkanethiol films on GaAs(110): A comparative study by TOF-DRS and TOF-SIMS

We present an ion beam study of the adsorption and the thermal stability of short alkanethiol molecules adsorbed on GaAs(110). Direct recoiling spectroscopy shows that the adsorption of ethanethiol and hexanethiol proceeds directly towards a dense standing up phase without passing through a stable phase of lying down molecules as is the case for Au(111). Measurements along specific azimuths suggest that both Ga and As rows are covered by the organic molecules. Short adsorption times from the vapor phase result in films having two desorption peaks near 300 and 500 K. On the other hand, leaving the sample in a thiol atmosphere for several hours produces more stable films, similar to those produced by immersion in the corresponding thiol-ethanol solution. TOF-SIMS results confirm the C–S scission mechanism during the thermal desorption.

Determination of low levels of cadmium ions by the under potential deposition on a self-assembled monolayer on gold electrode

The electrochemical determination of low levels of Cd using a self-assembled monolayer (SAM) modified Au electrode is reported. Determination was based on the stripping of Cd, which was deposited by under potential deposition (UPD). A series of short alkanethiol SAMs bearing different end groups, i.e., sulfonate, carboxylate and ammonium, were examined. Lowest level of detection (ca. 50 ng L −1) was achieved with a 3-mercaptopropionic acid (MPA) monolayer using subtractive anodic square wave voltammetry (SASV). Additional surface methods, namely, reductive desorption and X-ray photoelectron spectroscopy, were applied to determine the interfacial structure of the electrodeposited Cd on the modified electrodes. We conclude that the deposited Cd forms a monoatomic layer, which bridges between the gold surface and the alkanethiol monolayer associating with both the gold and the sulfur atoms.

Confinement and compression of an oligomer brush

Self-assembled monolayers and oligomer brushes confined between two parallel plates show compressional forces that are nonmonotonic as a function of plate separation. In a realistic model of short alkanethiols, based on the rotationally isomeric state model with parameters from ab initio calculations, the authors show that nonmonotonic forces arise from the elimination of longer conformers as the distance between the plates is reduced. This nonmonotonicity is a size effect that disappears when the length of the polymer molecule is sufficiently increased. An analytical model is developed that allows experimentalists to extract energy-averaged brush height distributions from compressional force curves.

Methylene Blue Incorporation into Alkanethiol SAMs on Au(111): Effect of Hydrocarbon Chain Ordering

A detailed polarization modulation infrared reflection absorption spectroscopy, scanning tunneling microscopy, and electrochemical study on methylene blue (MB) incorporation into alkanethiolate self-assembled monolayers (SAMs) on Au(111) is reported. Results show that the amount of MB incorporated in the SAMs reaches a maximum for intermediate hydrocarbon chain lengths (C10-C12). Well-ordered SAMs of long alkanethiols (C > C12) hinder the incorporation of the MB molecules into the SAM. On the other hand, less ordered SAMs of short alkanethiols (C < or = C6) are not efficient to retain the MB incorporated through the defects. For C12 the amount of incorporated MB increases as the SAM disorder is increased. This information is essential to the design of efficient thiol-based Au vectors for transport and delivery of molecules as well as thiol-based Au devices for molecular sensing.

One-Step Detection of Galectins on Hybrid Monolayer Surface with Protruding Lactoside

Galectins, or beta-galactoside binding lectins, are detected deep in tumor tissue and are recognized as diagnostic and prognostic markers of cancer and other serious diseases. There is a need to develop a faster, easier, and simpler method for detecting galectins. We have succeeded in forming a mixed self-assembled monolayer (SAM) interface consisting of beta-galactoside terminated alkanethiol (lactoside protuberant dodecanethiol) and tri(ethylene glycol) (TEG) terminated short alkanethiol, which proved to be a superior protein resistant material, to enable us to develop a label-free, one-step, and highly sensitive system for detecting the expected biomarker, galectin. We successfully detected nanomolar level (~ 1 nM) galectin-4 and -8 on a 4% lactoside protrusive surface, even though the affinity between the galectins and lactoside was very weak (KD = 1 x 10(-3)~1 x 10(-6)). The combination of the suppression of background noise by filling with TEG terminated short alkanethiol and control of the ligand ratio in the interface contributed to the highly sensitive detection of galectin. We also detected galectin-4 at subten nanomolar levels even in a solution containing much higher concentrations of serum proteins (1800 times larger than the galectin concentration) without using molecule labeling or an immunological method.

Study of the interaction between short alkanethiols from ab initio calculations

We have developed a force field to describe the interaction of alkanethiols HS(CH(2))(n-1) CH(3) (C(n) for short) by fitting a set of approximately 220 interaction energies for dimers of C(n) (with n = 1,2,...6) and CH(4) molecules obtained from second-order Møller-Plesset perturbation theory calculations. The derived force field, based on a sum of so-called exp-6 pairwise potentials and effective Coulomb interaction potential between the HS- heads, predicts very well the interaction energies for dimers and trimers of alkanethiols not included in the input database for the fit. Also the calculated minimum energy tilt angle of the alkyl chains for a hexagonal arrangement of alkanethiolates with a nearest neighbor distance of 5 A is in good agreement with the available experimental data for a sqrt [3] x sqrt [3] self-assembled monolayer (SAM) on Au(111). Thus, the derived force field might be suitable for large-scale molecular dynamics and/or Monte Carlo simulations to predict the structure, stability and/or kinetics of SAMs on other surfaces.

Electrochemcial characterisation and hybridisation efficiency of co-assembled monolayers of PEGylated ssDNA and mercaptohexanol on planar gold electrodes

The realisation of efficient genosensors relies crucially on the surface chemistry strategy selected for the immobilisation of DNA probes. We report on the performance of surfaces prepared via the direct co-immobilisation of a 20-nucleotide (nt) thiolated single stranded DNA probe (ssDNA) in the presence of the short alkanethiol mercaptohexanol (MCH) at varying ratio. Electrochemical impedance spectroscopy was used for the physical characterisation of the formed monolayers. Functional characterisation of the surfaces was achieved by means of differential pulse voltammetry and calibration curves in the range 6.25–100 nM of the complementary sequence 104 nt in length were obtained for each surface studied, using the guanine specific redox marker methylene blue (MB) as hybridisation indicator. The best sensor performance was obtained for a ratio of 1:100, as previously suggested by electrochemical impedance spectroscopy. No hybridisation could be measured at 1:1 ratio and significantly lower hybridisation were recorded for surfaces prepared at 1:10 and 1:1000 ratio. For comparison, sensors prepared via initial thiolated ssDNA immobilisation followed by the backfilling of the surface with MCH, as widely reported, were also assessed. Our results suggested that the performances of such sensors were sub-optimum and comparable to that obtained at 1:10 co-immobilisation. We concluded that the co-immobilisation approach offers several advantages, with highly reliable surfaces being prepared in a single step.

Dithioesters and trithiocarbonates monolayers on gold

We show that dithioester and trithiocarbonate compounds, used as chain transfer agents in reversible addition fragmentation chain transfer polymerization, form monolayers on gold. We evidence that the monolayers are slightly less dense than those formed by long alkanethiols on gold, but show a surface coverage that is close to the one obtained for short alkanethiols. Since the grafting density of polymer brushes is limited by the steric hindrance which inhibits the diffusion of large free polymer chains to open-surface reactive sites, the relatively high grafting density of dithioesters and trithiocarbonates anchoring groups is not expected to limit the density of the brushes.

Electrochemical surface nanopatterning by selective reductive desorption from mixed metal surfaces

We report on a novel strategy to the functionalisation of electrode surfaces based on the preparation and patterning of mixed metal electrodes using metal selective electrodesorption of a sacrificial alkanethiol. Plain palladium (Pd) and plain polycrystalline gold (poly-Au) electrodes were used initially to determine metal specific potential windows within which electrodesorption of the short alkanethiol mercaptoethanol could be achieved. We found that stripping of mercaptoethanol from gold was achieved at potentials lower than −0.800 V, whilst stripping from palladium was achieved at more positive potentials i.e. around −0.650 V. Mixed metal electrodes were prepared by electroplating for short period of times palladium onto poly-Au electrodes. The resulting surfaces were characterised electrochemically in 1 M H 2SO 4 and clearly exhibited reduction peaks for both gold and palladium oxide formation. The mixed metal electrodes were coated with mercaptoethanol, which was further selectively removed from Pd by cyclic voltammetry in NaOH in the Pd-specific potential window. The presence of bare Pd domains revealed following electrodesorption was confirmed by subsequently adsorbing the electroactive alkanethiol 6-ferrocenylhexanethiol onto the freshly revealed Pd. Cyclic voltamogramms exhibited sharp redox peaks that could only be attributed to the successful immobilisation of 6-ferrocenylhexanethiol onto fresh Pd domains. Control surfaces, i.e. MCE fully coated Pd/Poly-Au electrode, exposed to 6-ferrocenylhexanethiol did not exhibit significant voltammetric features, attesting to the efficient patterning of the mixed metal electrode by employing metal specific reductive desorption of short alkanethiols. The possibility to pattern electrode surfaces in such way will find application in the field of diagnostics, and also in heterogeneous catalysis where Pd–Au alloys have received an increased interest in the recent years.