Short-chain Alkanethiols Research Articles

Effects of chain-chain interaction on the configuration of short-chain alkanethiol self-assembled monolayers on a metal surface.

Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2-6) on the Au surface. For monolayers with n≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even-odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°-79°) and a smaller CH2 tilt angle (56°-68°). Such a unique configurational transition from n = 2 to n≥ 3 discloses the steric effect owing to chain-chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain-chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications.

Metastable Metal-Monolayer-Semiconductor Junctions: Diverse Chain-Length-Dependent and Ultraslow Electrical Progression

Molecularly tunable metal-semiconductor (MS) junctions have been fabricated by modifying mercury drop electrodes with n-alkanethiols, 1-CH3(CH2)n–1SH (n = 10, 11, 12, 14, 16, and 18) prior to the formation of intimate contact with hydrogen-terminated silicon (H-Si≡) and characterized by both solid-state electrical and electrochemical measurements. We have demonstrated that the current-voltage properties of these molecular junctions change with time and that diverse time-dependent progression “patterns” were observed between the systems of long-chain alkanethiols (C14SH, C16SH, and C18SH) and short-chain alkanethiols (C10SH, C11SH, and C12SH). It is remarkable that for mercury contact electrodes modified with long-chain alkanethiolate self-assembled monolayers (SAMs), the junctions became more rectifying and stabilized over a prolonged period (∼2 h). For short-chain counterparts, surprisingly, they initially became more rectifying, then changed to ohmic over a similar period. It was proposed that at the MS interface, short-chain alkanethiolate SAMs first reorganize to be more ordered before eventually collapse. Long-chain alkanethiolate SAMs, on the other hand, achieve a more uniform, oriented, and stable packing as time goes by. These novel findings shed light on “long-term” intermolecular interactions that drive molecular systems to undergo ultraslow reorganizations, which can be readily modulated by simply varying the precursor structures (e.g., chain length of n-alkanethiols).

Evaluation of Different Monolayer Chemistries on Carbon Electrodes for the Fabrication of Electrochemical, Aptamer-Based Sensors

Electrochemical, DNA-based sensors represent a powerful tool to selectively detect and quantify molecular targets in highly complex media such as whole blood in living animals. These sensors consist of a biointerface containing redox reporter-modified DNA molecules attached to the surface of (typically) gold electrodes via sulfur-gold bonds. They also employ short chain alkanethiols for the purpose of passivating the electrode surface, thus preventing undesired electrochemical reactions and achieving antifouling characteristics to, for example, prevent non-specific protein binding. Despite the huge success and promising applications of electrochemical, DNA-based sensors and the development of this platform for over 15 years, the DNA sensing architecture has been almost exclusively limited to the use of thiol-based monolayers on gold electrodes. This limits the performance of such sensors by the surface stability of alkanethiols, which are labile, and excludes the use of electrode materials other than gold, considerably reducing potential fields of application for DNA-based sensors. Motivated to expand the diversity of monolayer chemistries and electrode materials that can be used to fabricate DNA-based sensors, our group is currently exploring different approaches to chemically functionalize the surface of carbon-based electrodes. In this work, I will discuss the different chemical approaches we are pursuing in this regard and their performance in terms of monolayer packing, extent of surface passivation, and long-term stability in physiological buffers and biological fluids. I will also discuss how these chemistries compare with benchmark thiol-on-gold monolayers.

Electroreductive Desorption of Alkanethiols on Gold and UPD Copper/Gold Surfaces Using In Situ Second Harmonic Generation

The reductive desorption of three alkanethiols: 1-decanethiol (DT), 1-dodecanethiol (DDT), and 1-octadecanethiol (ODT) was studied. Real-time second harmonic generation (SHG) was used to monitor changes at the surface as the self-assembled monolayer was electrodesorbed from a gold and UPD Cu/Au electrode through cyclic voltammetry (CV). Before and after reductive desorption, samples are examined ex situ with sum frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle (CA). These techniques confirmed the electrodesorption of short-chain alkanethiols, while long-chain alkanethiols were proven to remain near the surface of the electrode. However, a difference was observed for a DDT monolayer on the UPD Cu/Au electrode. While DDT was easily desorbed from a gold substrate, DDT on the UPD Cu/Au electrode behaved similarly to a long-chain alkanethiol on gold. Significantly, SHG was able to determine the relative alkanethiol coverage even while the desorption-features in the CV were obscured by the hydrogen evolution current.

Highly sensitive and fast perovskite photodetector functionalized by plasmonic Au nanoparticles-alkanethiol assembly

We present a perovskite photodetector functionalized with Au NPs assembled with 3-mercaptopropionic acid (MPA), a short-chain alkanethiol (HS(CH2)nCOOH, n = 2), where the assembled Au NPs are embedded in the PEDOT:PSS layer of the hole transport layer. The incorporation of the Au NPs-MPA assembly not only enhances the responsivity strongly, but also improves the detectivity due to a reduction in dark current and enhances the response speed of the device. In particular, the responsivity, noise equivalent power (NEP), and response speed of the Au NPs-MPA assembly device significantly improved by approximately 4.1, 15.6, and 2.8 folds, respectively, as compared with those of the pristine device under visible light illumination. Our device exhibits a responsivity of ~2063 A W−1 and a NEP of ~1.93 × 10-14 W Hz−1/2 due to the incorporation of the Au NPs-MPA assembly, and a response time of ~300 ns is achieved by the fabrication of a short channel in the device. The improved charge extraction/transport and optical absorption by the introduction of Au NPs-MPA assembly lead to the overall improved device performance.

MoS2 nanosheets via electrochemical lithium-ion intercalation under ambient conditions

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are continuously attracting attention for both fundamental studies and technological applications. The physical and chemical properties of ultrathin TMD sheets are extraordinarily different from those of the corresponding bulk materials and for this reason their production is a stimulating topic, especially when the preparation method enables to obtain a remarkable yield of nanosheets with large area and high quality. Herein, we present a fast (<1 h) electrochemical exfoliation of molybdenum disulfide (MoS2) via lithium-ion intercalation, by using a solution of lithium chloride in dimethyl sulfoxide (DMSO). Unlike the conventional intercalation methods based on dangerous organolithium compounds, our approach leads to the possibility to obtain mono-, bi- and tri-layer thick MoS2 nanosheets with a large fraction of the semiconducting 2H phase (∼60%), as estimated by X-ray photoelectron spectroscopy (XPS). The electrical properties of the exfoliated material were investigated through the fabrication and characterization of back-gated field-effect transistors (FETs) based on individual MoS2 nanosheets. As-fabricated devices displayed unipolar semiconducting behavior (n-type) with field-effect mobility µFE ≤ 10−3 cm2 V−1 s−1 and switching ratio Ion/Ioff ≤ 10, likely limited by 1T/2H polymorphism and defects (e.g. sulfur vacancies) induced during the intercalation/exfoliation process. A significant enhancement of the electrical performances could be achieved through a combination of vacuum annealing (150 °C) and sulfur-vacancy healing with vapors of short-chain alkanethiols, resulting in µFE up to 2 × 10−2 cm2 V−1 s−1 and Ion/Ioff ≈ 100. Our results pave the way towards the fast preparation – under ambient conditions – of semiconducting MoS2 nanosheets, suitable for application in low cost (opto-)electronic devices.

Nuclear Magnetic Resonance Spectroscopy

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Short-Chain Alkanethiol Coating for Small-Size Gold Nanoparticles Supporting Protein Stability

The application of gold nanoparticles (AuNPs) is emerging in many fields, raising the need for a systematic investigation on their safety. In particular, for biomedical purposes, a relevant issue are certainly AuNP interactions with biomolecules, among which proteins are the most abundant ones. Elucidating the effects of those interactions on protein structure and on nanoparticle stability is a major task towards understanding their mechanisms at a molecular level. We investigated the interaction of the 3-mercaptopropionic acid coating of AuNPs (MPA-AuNPs) with β2-microglobulin (β2m), which is a paradigmatic amyloidogenic protein. To this aim, we prepared and characterized MPA-AuNPs with an average diameter of 3.6 nm and we employed NMR spectroscopy and fluorescence spectroscopy to probe protein structure perturbations. We found that β2m interacts with MPA-AuNPs through a highly localized patch maintaining its overall native structure with minor conformational changes. The interaction causes the reversible precipitation of clusters that can be easily re-dispersed through brief sonication.

Insights into the Key Compounds of Durian (Durio zibethinus L. 'Monthong') Pulp Odor by Odorant Quantitation and Aroma Simulation Experiments.

Sixteen compounds, previously identified as potent odorants by application of an aroma extract dilution analysis and the gas chromatography-olfactometry analysis of static headspace samples, were quantitated in the pulp of durians, variety Monthong, and odor activity values (OAVs) were calculated by dividing the concentrations obtained by the odor thresholds of the compounds in water. In combination with data recently reported for hydrogen sulfide and short-chain alkanethiols, OAVs > 1 were obtained for 19 compounds, among which ethyl (2S)-2-methylbutanoate (fruity; OAV 1700000), ethanethiol (rotten onion; OAV 480000), and 1-(ethylsulfanyl)ethane-1-thiol (roasted onion; OAV 250000) were the most potent, followed by methanethiol (rotten, cabbage; OAV 45000), ethane-1,1-dithiol (sulfury, durian; OAV 23000), and ethyl 2-methylpropanoate (fruity; OAV 22000). Aroma simulation and omission experiments revealed that the overall odor of durian pulp could be mimicked by only two compounds, namely, ethyl (2S)-2-methylbutanoate and 1-(ethylsulfanyl)ethane-1-thiol, when combined in their natural concentrations.

Enhancement of self-assembly of large (>10 nm) gold nanoparticles on an ITO substrate

Densely arraying gold nanoparticles (AuNPs) immobilized on substrates is still a challenge. For the arraying, surface modification of AuNPs with alkanethiols is a key technology. However, if the particle size is larger than 20 nm, short-chain alkanethiols have very weak interparticle interaction such that self-assembly is not realized, whereas long-chain alkanethiols have very strong interaction such that self-aggregation is induced in solution. One solution is their combination. Eventually, a mixture gave a dense array of 55 nm AuNPs on an ITO substrate with a coverage of approximately 80%. Our approach provides high-performance plasmonic optics with a resonance wavelength of more than 700 nm.

Self-assembled alkanethiol monolayers on gold surfaces: resolving the complex structure at the interface by STM.

The surface properties of metals and metal oxides can be modified by adding a single layer of organic molecules. A most popular route for depositing such a molecular layer is via the formation of self-assembled monolayers (SAMs). The molecules that form SAMs have a functionality which binds to the surface and the adsorption is self-regulated to terminate at exactly one single molecular layer. The very first example, which has become the most widely studied system, of SAMs on metal surfaces consists of chemisorbed alkylthiolate on gold. Despite the simplicity in the preparation of alkanethiol SAMs and the seemingly straightforward structure of such SAMs, the detailed bonding between the sulfur head group and gold is still subject to debate. Experimental and theoretical effort in the last six years has led to a much improved understanding of this classical system of SAMs. In this review, we will highlight the most recent progress in the study of the interfacial structure of alkanethiol SAMs on gold. We focus on the important phenomenon of phase transition that occurs from n-propanethiol to n-butanethiol, and propose a unified structural model to explain how the (3 × 4) phase for short chain alkanethiol monolayers (methyl-, ethyl- and propylthiolate monolayers) changes into the (3 × 2√3)-rect./c(4 × 2) phase for long chain molecular monolayers.

Dynamical Simulation of Electron Transfer Processes in Alkanethiolate Self-Assembled Monolayers at the Au(111) Surface

Electron transfer is investigated in a series of self-assembled monolayers (SAMs) consisting of nitrile-substituted short chain alkanethiolate molecules adsorbed at the Au(111) surface. Using first-principles methods and a model electron transfer Hamiltonian, we analyze the main factors controlling, at the molecular level, the electron injection times from donor states localized at the tail group of the SAM into the Au(111) substrate. We show that the donor–acceptor electronic couplings depend significantly on the orbital symmetry of the donor state and the length of the aliphatic spacer chain of the SAM. The dependence on the donor state symmetry and on the molecular structure of the linker can be used to control the electron injection times even in situations where the energy separation between the donor states is smaller than their width.

Anodic Passivation of Tin by Alkanethiol Self-Assembled Monolayers Examined by Cyclic Voltammetry and Coulometry

The self-assembly of medium chain length alkanethiol monolayers on polycrystalline Sn electrodes has been investigated by cyclic voltammetry and coulometry. These studies have been performed in order to ascertain the conditions under which their oxidative deposition can be achieved directly on the oxide-free Sn surface, and the extent to which these electrochemically prepared self-assembled monolayers (SAMs) act as barriers to surface oxide growth. This work has shown that the potentials for their oxidative deposition are more cathodic (by 100-200 mV) than those for Sn surface oxidation and that the passivating abilities of these SAMs improve with increasing film thickness (or chain length). Oxidative desorption potentials for these films have been observed to shift more positively, and in a highly linear fashion, with increasing film thickness (~75 mV/CH2). Although reductive desorption potentials for the SAMs are in close proximity to those for reduction of the surface oxide (SnOx), little or no SnOx formation occurs unless the potential is made sufficiently anodic that the monolayers start to be removed oxidatively. Our coulometric data indicate that the charge involved with alkanethiol reductive desorption or oxidative deposition is consistent with the formation of a close-packed monolayer, given uncertainties attributable to surface roughness and heterogeneity phenomena. These experiments also reveal that the quantity of charge passed during oxidative desorption is significantly larger than what would be predicted for simple alkylsulfinate or alkylsulfonate formation, suggesting that oxidative removal involves a more complex oxidation mechanism. Analogous chronocoulometric experiments for short-chain alkanethiols on polycrystalline Au electrodes have evidenced similar oxidative charge densities. This implies that the mechanism for oxidative desorption on both surfaces may be very similar, despite the significant differences in the inherent dissolution characteristics of the two materials at the anodic potentials employed.

Ab initio DFT study of 6-mercapto-hexane SAMs: effect of Au surface defects on the monolayer assembly

This paper reports a novel ab initio density functional theory (DFT) simulation approach to the study of surface defects in self-assembled monolayers (SAMs) on gold (0 0 1) surfaces. Surfaces defects can cause changes in gold atom spacing and the ligand footprint. DFT calculations of 6-mercapto-hexane on Au(0 0 1) surfaces have been performed as a function of ligand footprints of 18, 20, 22, 24 and 26 Å2, using a slab geometry and periodic boundary conditions with varying gold atom spacing. The simulation results indicate that as the ligand packing density increases, the position of ideal binding sites changes from fcc to bridge sites. The packing density of 24 Å2 corresponds to the highest binding energy and to a 28.85° cant angle. These results are in good agreement with the experiments from ligand-capped gold nanoparticles. The simulation results also prove that the SAM structure of short-chain alkanethiols on gold is dominated by surface interactions rather than ligand interactions. The electron density mapping study confirms this observation, where only the top two layers of gold atoms contributed charge transfer to sulphur atom and no charge transfer was observed from the alkane chain.

Multifunctional modified silver nanoparticles as ion and pH sensors in aqueous solution

Silver nanoparticles capped with mercaptoacetic acid and 2-aminoethanethiol short-chain alkanethiols were prepared by a one-step method in aqueous solution for monitoring pH and a range of heavy metal ions. The mode of transduction is optical, based on the change in aggregation of the nanoparticles in solution. Because of the different ionic interactions between the modified nanoparticles, these nanoparticle sensors can rapidly detect Pb(2+), Cu(2+) and Fe(2+), with detection limits as low as 1 × 10(-5) M, 5 × 10(-7) M and 5 × 10(-5) M respectively, as well as having the ability to detect Cu(2+) ions from Pb(2+) and Fe(2+). Furthermore, the same functionalised nanoparticles are also sensitive to pH; exhibiting a good linear dynamic response between pH 1 and 10.

Chain-Length-Dependent Branching of Irradiation-Induced Processes in Alkanethiolate Self-Assembled Monolayers

The effect of X-ray and low-energy (50 eV) electron irradiation on short-chain alkanethiolate (AT) self-assembled monolayers (SAMs) on Au(111) was studied by synchrotron-based high resolution X-ray...

Electrochemical and X-ray Photoelectron Spectroscopy Characterization of Alkanethiols Adsorbed on Palladium Surfaces

The self-assembly of alkanethiolate monolayers on palladium substrates and their stability in aqueous solutions have been studied by electrochemical techniques and X-ray photoelectron spectroscopy (XPS). Alkanethiols adsorb on Pd from ethanolic solutions forming a complex interface which consists of thiolates onto a diluted palladium sulfide interphase, with surface coverages θSulfide ≈ 0.4 and θthiolate ≈ 0.30, respectively. These complex adlayers exhibit hydrocarbon chain-length dependent barrier properties like those formed on Au and Ag substrates. For short chain alkanethiols, thiolates are more stable against reductive desorption than self-assembled monolayers (SAMs) on Au, Ag, and Ni. The increased stability of the organic species seems to be related to the presence of the diluted sulfide layer that also explains the chain length independence of the stability potential range. The stability of these monolayers in aqueous solutions indicates that Pd is a suitable platform for thiolate-based devices such as sensors and biosensors. These results also suggest that electrochemical cleaning should be a simple way to prepare metallic nanoparticles from thiol-capped Pd nanoparticles adsorbed on conducting substrates.

Adsorption and thermal reaction of short-chain alkanethiols on GaAs(1 0 0)

By means of temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation, we investigated the adsorption and thermal decomposition of alkanethiols (RSH, R = CH 3, C 2H 5, and C 4H 9) on a GaAs(1 0 0) surface. All chemisorbed alkanethiols can deprotonate to form thiolates below 300 K via dissociation of the sulfhydryl hydrogen (–SH). Two types of thiolates species are observed on GaAs(1 0 0), according to adsorption on surface Ga and As sites. The thiolates adsorbed on a Ga site preferentially recombine with surface hydrogen to desorb as a molecular thiol at 350–385 K. The thiolate on the As site exhibits greater thermal stability and undergoes mainly dissociation of the C–S bond at ∼520 K, independent of the alkyl chain length. The decomposition of CH 3S either directly desorbs CH 3 or transfers the CH 3 moiety onto the surface. The surface CH 3 further evolves directly from the surface at 665 K. The dissociations of C 2H 5S and C 4H 9S yield surface C 2H 5 and C 4H 9, which further decompose to desorb C 2H 4 and C 4H 8, respectively, via β-hydride elimination. The complete decomposition of alkanethiol leads to the formation of surface S without deposition of carbon. Adsorption of CH 3SSCH 3 results in the formation of surface CH 3S at initial exposures via scission of the S−S bond. Compared with the adsorption of CH 3SH, the CH 3S on the Ga site exhibits greater thermal stability because surface hydrogen is absent. At a high exposure, CH 3SSCH 3 can absorb molecularly on the surface and decompose to desorb CH 3SCH 3 via formation of a CH 3SS intermediate.

Fluorescence decay of quasimonolayered porphyrins near a metal surface separated by short-chain alkanethiols

We investigated the spectral feature and fluorescence lifetime of quasimonolayered porphyrins at very short distance to metal substrates (1–2.5nm) through fine-tuning the length of alkanethiols. The ordered self-assembled monolayers of alkanethiols on Au(111) act as a uniform electronic decoupling layer and suppress the interface quenching via charge transfer. However, the fluorescence quenching via nonradiative energy transfer to the metal still prevails in the porphyrin-alkanethiol-metal sandwich structures. The decay rates are found to follow a 1∕d3 dependency on spacer thickness, which suggests that the classical electromagnetic theory appears still valid at distance down to 1nm through volume damping.

Odd-Even Oscillation in the Rate of Heterogeneous Electron Transfer for Solution-Based Redox Couples at Short-Chain Alkanethiolate Monolayers on Gold

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