Thiol Coverage Research Articles

Study of microheterogeneity of silatrane-based silica surface bonding chemistry and its optimization for the synthesis of chiral stationary phases for enantioselective liquid chromatography

The present work systematically investigates the chemical microheterogeneity as part of the optimization of a single-step surface bonding chemistry of 3-mercaptopropylsilatrane (MPS) on mesoporous silica gel in comparison to the state-of-the-art silane chemistry with 3-mercaptopropyltrimethoxysilane (MPTMS). MPS functionalization turns out to be a favourable chemistry for the further use in thiol-ene click reactions such as the immobilization of chiral selectors, herein tert-butylcarbamoylquinine (tBuCQN), for the synthesis of chiral stationary phases (CSPs). MPS has higher reactivity than MPTMS and prefers the formation of trifunctional siloxane bondings unlike MPTMS which favours difunctional siloxane bonds to silica, as investigated by solid-state cross-polarization/magic angle spinning (CP/MAS) NMR (29Si and 13C nuclei). Reaction conditions (ternary mixtures of methanol, water and toluene; with and without acid; prewetting of silica; HCl pretreatment of silica) were evaluated with the aim to find conditions which promote the formation of a horizontal siloxane polymer layer on top of the silica surface. Silanization reaction times could be reduced to 2 h. The 29Si NMR signal corresponding to trifunctional siloxane bonding could be increased to 60% with no T1 signal that refers to monofunctional siloxane bonding in spite of water in the ternary reaction mixture. Furthermore, no significant disulfide bridges were formed in this approach, leading to high selector loadings. The thiol and selector coverage reached up to 4.6 and 1.4 µmol/m2, respectively. With the preferred CSP, the enantioselectivity could be increased for a chiral probe (FMOC-Phe) and the mass transfer resistance (C-term) bisected compared to the corresponding CSP prepared from benchmark MPTMS-modified silica (2.54 vs 5.72 ms). It is demonstrated that the fine-tuning of the microstructure on the silica surface can have a significant influence on enantioselectivity and mass transfer kinetics of the resultant CSPs.

Size and surface coverage density are major factors in determining thiol modified gold nanoparticles characteristics

Gold nanoparticles (GNPs) have prominent advantages due to their ultrasmall size and high surface-area-to-volume ratio, with multiple and broad applications in medical, agricultural and pharmaceutical industries. GNPs can be functionalized by various molecules including polysaccharides, proteins, peptides, fatty acids, plasmids, or oligonucleotides. GNP surface represents one of the most easily functionalized platforms, the complex level of surface decoration being used to obtain efficacy and to limit off-target toxicity. In the present study, molecular dynamics simulations of thiol modified GNPs were performed to provide insights at the molecular level for the design of conjugated nanoparticles. Our study shows that due to the significant enhancement in the conformational order and packing of the thiol chains, both nanoparticle size and thiol coverage density can be used to modify the structural properties of thiol-modified GNPs (e.g. end-to-end distance, shape factor, tilt angle of thiol chains). This strategy has a lot of potential in designing any (thiol-modified gold) nanoparticle-based carriers (such as those that are used to find and modify the genes that are associated with diseases).

Simulation studies on structural and thermal properties of alkane thiol capped gold nanoparticles

The structural and thermal properties of the passivated gold nanoparticles were explored employing molecular dynamics simulation for the different surface coverage densities of the self-assembled monolayer (SAM) of alkane thiol. The structural properties of the monolayer protected gold nanoparticles such us overall shape, organization and conformation of the capping alkane thiol chains were found to be influenced by the capping density. The structural order of the thiol capped gold nanoparticles enhances with the increase in the surface coverage density. The specific heat capacity of the alkane thiol capped gold nanoparticles was found to increase linearly with the thiol coverage density. This may be attributed to the enhancement in the lattice vibrational energy. The present simulation results suggest, that the structural and thermal properties of the alkane thiol capped gold nanoparticles may be modified by the suitable selection of the SAM coverage density.

Polymethacrylate monoliths with immobilized poly-3-mercaptopropyl methylsiloxane film for high-coverage surface functionalization by thiol-ene click reaction

In this work, new polythiol-functionalized macroporous monolithic polymethacrylate-polysiloxane composite materials are presented which can be useful substrates for highly efficient immobilization of (chiral) catalysts, chromatographic ligands, and other functional moieties by thiol-ene click reaction. Poly(glycidyl methacrylate-co-ethylene dimethacrylate) (poly(GMA-co-EDMA)) monoliths were coated with a poly-3-mercaptopropyl methylsiloxane (PMPMS) film and subsequently the polymer was covalently immobilized by formation of crosslinks via nucleophilic substitution reaction with pendent 2,3-epoxypropyl groups on the monolith surface. This monolith, though, showed similar levels of surface coverage as a reference monolith obtained by opening of the epoxide groups with sodium hydrogen sulfide. However, a 3-step functionalization by amination of the epoxy monolith, followed by its vinylation with allylglycidyl ether and subsequent thiolation by coating of a thin polythiol (PMPMS) film and crosslinking by click reaction furnished a monolith with more than 2-fold elevated thiol coverage. Its further functionalization with a clickable chiral quinine carbamate selector clearly documented the benefit of highly dense thiol surfaces for such reactions and synthesis of functional materials with proper ligand loadings. The new monoliths were chromatographically tested in capillary electrochromatography mode using N-3,5-dinitrobenzoyl-leucine as chiral probe and the capillary column with the monolith having the highest selector coverage, produced from the precursor with the most thiols on the surface, showed the largest separation factor. By performic acid oxidation the surface characteristic could be tuned and strongly altered due to a delicate balance of enantioselective and non-specific interactions.

Influence of Organic Amino and Thiol Ligands on the Geometric and Electronic Surface Properties of Colloidally Prepared Platinum Nanoparticles

The influence of organic ligands on the geometric and electronic surface properties of colloidally prepared Pt nanoparticles (Pt NPs) was investigated by means of diffuse reflectance infrared spectroscopy (DRIFTS) and using CO as probe molecule. Dodecylamine (DDA) and dodecylthiol (DDT) were used as surface functionalizing ligands that exhibit the same hydrocarbon tail but different functional groups. While for DDA an electronic donor effect as well as a geometric effect was found, the effect of DDT was mainly geometric in nature. For high DDA and DDT coverages, on-top sites were blocked for CO adsorption to a higher extent than bridge and threefold hollow sites. Whereas DDA is only weakly adsorbed and displaced by strongly binding adsorbates (e.g., CO), DDT is strongly attached to the particle surface forming an evenly distributed capping layer. With decreasing thiol coverage, blocking of bridge and threefold hollow sites became more pronounced than on-top site blocking. The influence of both ligands on the selectivity of the hydrogenation of crotonaldehyde catalyzed by Pt NPs was investigated. For amino-functionalized NPs the catalytic properties did not differ from that of “unprotected” Pt NPs. In contrast, an increased selectivity can be found for thiol-functionalized particles, which, according to the IR-spectroscopic investigations, was attributed to a geometric modification of the surface by the ligand.

Aggregation of thiol coated gold nanoparticles: A simulation study on the effect of polymer coverage density and solvent

Molecular dynamics simulations have been carried out to investigate the effect of alkane thiol coverage density, terminal group and water solvent on the aggregation of thiol coated gold nanoparticles. The increase in polymer coverage density enhances the size of the aggregate as the van der Waals force of attraction dominates with the increase in the number of thiol chains. The change in the terminal group from methyl (hydrophobic) to hydroxy (hydrophilic) group modifies the size of the aggregate as the extent of interdigitation between thiol chains is reduced due to the hydrogen bond interactions between the hydroxy terminal groups. The water solvent does not much alter the aggregation of methyl terminated gold nanoparticles. It weakens the aggregation of hydroxy terminated gold nanoparticles due to the hydrophilic interaction between the water molecules and the hydroxy terminal groups.

Solvent Influence on the Role of Thiols in Growth of Thiols-Capped Au Nanocrystals

Solvent has a key role in the controllable synthesis of nanocrystals (NCs). Here, using X-ray absorption spectroscopy, we demonstrate that solvent can significantly influence the adsorption of thiols on Au NCs, and thereby affects their growth. It is shown that increasing the solvent polarities leads to the higher thiol coverage on the NC surface. The high coverage of thiols then retards the growth of particles, and as a result, the NCs’ sizes decrease with the increase in the solvents’ polarities. The underlying reason for the solvent dependence is proposed to be the synergistic effects of different hydrogen bond and hydrophobic interactions between the sulfhydryl and alkyl groups of thiols with the solvents molecules. This work addresses the important role of the solvent environments in the size-controlled synthesis of NCs.

A first principles study of thiol-capped Au nanoparticles: Structural, electronic, and magnetic properties as a function of thiol coverage

We have studied the stability of thiolated Au38 nanoparticles (NPs) via density functional theory based calculations varying the coverage from 0 up to 32 molecules. Three different initial core arrangements were considered for the cluster, spherical, tubular, and bi-icosahedral, while thiol groups were attached to the cluster via the sulfur atom either as single molecules or forming more complex staple motifs. After molecular dynamics runs several metastable configurations are found at each coverage thus allowing to analyze the properties of the NPs in the form of ensemble averages. In particular, we address the structural and electronic properties as a function of the number of thiols. The study emphasizes the strong influence of the core structure on the stability of the NPs, and its interplay with the thiol coverage and adsorption geometries. The magnetic properties of the NPs have also been explored via spin-polarized calculations including spin-orbit coupling. No evidence for the existence of a robust intrinsic ferromagnetism is found in any of the structures.

Adsorption of Mixed Peptide/Thiol Adlayers on InAs: Assessment of Different Functionalization Strategies Using X-ray Photoelectron Spectroscopy

Mixed adlayers are commonly used to create surfaces with multiple functional groups or molecular gradients. The resulting surfaces are designed to have the combined effects of each adsorbed functional group. In this work, we combine the passivation ability of a thiol with the functionality of a peptide by assembling mixed peptide/thiol adlayers on InAs. Both sequential (insertion) and simultaneous (co-assembly) methods are used to functionalize InAs and are analyzed by contact angle measurements and X-ray photoelectron spectroscopy (XPS). Peptide coverage, thiol coverage, and oxide layer thicknesses are estimated from XPS data. For each method, oxide layer thicknesses were significantly lower than the oxide thicknesses for as-received samples. Surfaces functionalized using the insertion methods had lower oxide layer thicknesses than peptide only samples, indicating that the addition of the thiol helped block oxide regrowth. Functionalization with the thiol followed by the peptide yielded surfaces with the...

Activation of Thiols at a Silver Nanoparticle Surface

Adsorption of organosulfur (thiol) molecules on noble metal surfaces has been widely used to fabricate self-assembled monolayers or modify surface properties in a broad range of novel technological applications. Adsorption of thiols on the surface of noble metals, such as gold or silver, has been intensively studied to gain fundamental understandings on the adsorption process, including its mechanism and rates. It is generally accepted that thiol adsorption onto the gold/ silver surfaces consists of many steps, starting with physical adsorption, followed by the sulfur–metal bonding reaction, the reorientation of the adsorbed thiol molecules, and the formation of a compact self-assembled layer. On the other hand, both diffusion-limited and non-diffusion-limited adsorptions have been reported, though the conditions for the two scenarios have not been consistently defined. 13–16] Thiol adsorption has been widely used to influence the properties of metallic nanoparticles, such as controlling their biomedical functions, tuning optical properties, changing the attached chromophores, and the synthesis of nanoparticles/nanocages of different shapes and size. The adsorption of thiol molecules on metallic nanoparticles have been studied mostly using optical techniques. For example, a sum frequency vibrational spectroscopy study has revealed that long-chain thiol molecules on gold nanoparticles of varying sizes have different conformations. The adsorption free energies of several thiol molecules on gold and silver nanoparticles have been determined using fluorescence or second harmonic generation (SHG) measurements. More than a decade ago, hyper Rayleigh scattering from gold nanoparticles was pursued and first reported as an incoherent second harmonic method for examining nanoparticles, as it was thought that there is an inherent size restriction in SHG such that the method could not be detected from particles with diameters much smaller than the optical wavelength. We have subsequently demonstrated that SHG from the surface of nanometer size particles can be detected at specific scattering angles where phase matching conditions are satisfied. This advancement has led to the detection of SHG from silver nanoparticles (AgNPs). In principle, the SHG signal may arise from the surface and/or bulk of the nanoparticles. In the case of AgNPs, the surface contribution is identified by the response in the SHG signal to the formation of surface bonding that would diminish the polarizability of the free electrons at the surface. The surface portion of the SHG signal can then be used for probing processes occurring at the surface. It was illustrated in our work that the adsorption of thiols onto the AgNP surface can be directly monitored with time using SHG. The SHG intensity generated at the surface layer of the silver particle is sensitive to the formation of the S Ag bonds at the nanoparticle surface. It was found that the majority of the SHG signal from AgNPs of 80 nm diameter can be quenched by the adsorption of thiol molecules as the formation of the S Ag bonds localizes the Ag electrons that are responsible for the nonlinear susceptibility. The change of the SHG intensity can be quantitatively related to the thiol coverage on the surface, and used for the determination of the adsorption and desorption rates as well as the free energy change of the adsorptive reaction process. If there is a barrier associated with the ratelimiting process in the adsorption, temperature dependence of the adsorption rate should enable the determination of the activation energy and reveal which is the activation process. Herein we present experimental observations indicating that the thiol reactions at the silver nanoparticle surface is an activated process and that temperature can be used to control the reaction rates. For the case of 1,2-benzenedithiol adsorption onto a silver nanoparticle, the activation energy was determined as 8.4 kcalmol . This energy barrier is likely associated with the formation of the transition state during the formation of sulfur–silver bond and not from the diffusion-limited process. Figure 1 shows that as 1,2-benzenedithiol at different concentrations was added into Ag colloids at 293 K, the intensity of the SHG light scattered from silver nanoparticles decreased. The decrease is resulted from the reduction of the nonlinear susceptibility of the Ag surface atoms as S Ag bonds form. As indicated by the rate of the SHG intensity decay, which is proportional to the square of the thiol coverage on the surface, the rate of adsorption of thiol molecules is faster when thiol concentration is higher. Figure 1 shows three of the five experimental measurements conducted with added thiol concentrations varying from 0.1 to 0.5 mm. The Langmuir model can be used to describe the adsorption kinetics with adsorption and desorption rate constants ka and kd:

The Complex Thiol−Palladium Interface: A Theoretical and Experimental Study

This paper presents a theoretical study of the surface structures and thermodynamic stability of different thiol and sulfide structures present on the palladium surface as a function of the chemical potential of the thiol species. It has been found that as the chemical potential of the thiol is increased, the initially clean palladium surface is covered by a (√3 × √3)R30° sulfur lattice. Further increase in the thiol pressure or concentration leads to the formation of a denser (√7 × √7)R19.1° sulfur lattice, which finally undergoes a phase transition to form a complex (√7 × √7)R19.1° sulfur + thiol adlayer (3/7 sulfur + 2/7 thiol coverage). This transition is accompanied by a strong reconstruction of the Pd(111) surface. The formation of these surface structures has been explained in terms of the catalytic properties of the palladium surface. These results have been compared with X-ray photoelectron spectroscopy results obtained for thiols adsorbed on different palladium surfaces.

Quantitatively Modeling the Equilibrium Properties of Thiol-Decorated Gold Nanoparticles

We employ a molecular mean-field theory to quantitatively understand the sizes, surfactant surface coverage, and size fluctuations of gold nanocrystals decorated with thiol surfactants of different chain lengths. Our model assumes that surfactant-coated nanoparticles are equilibrium structures. We find that packing constraints experienced by the surfactant tails are less significant for more curved (smaller) particles. This effect enables us to rationalize the experimental observations/deductions that the thiol coverage per unit area increases with decreasing particle size. The reduction of surface coverage with increasing size also explains the fact that size polydispersity increases with increasing nanoparticle size. We find that increasing the length of the surfactants results in larger nanoparticles.

On the Interpretation of Multiple Waves in Cyclic Voltammograms of Self-Assembled Monolayers of n-Alkane Thiols on Gold

A homologue series of n-alkane thiols (CH3(CH2) m SH, MCm, m between 8 and 22) adsorbed on Au/mica substrates was studied in an alkaline environment using cyclic voltammetry (CV) and second harmonic generation (SHG). The known split of the reductive desorption signal in the cyclic voltammogram for thiols with chain length of m > 12 disappears again for m = 22. Simultaneous recording of SHG and CV data allowed correlation of thiol coverage with charge which was found to be linear, irrespective of the shape of the desorption signal. This evidences that the double peak structure of the desorption signal does not arise from the separation of faradaic and capacitive processes. The same is found for the double wave of the readsorption signal. The transient appearance of a split desorption signal for the one electron process of thiol desorption in the range of 12 < m < 22 is suggested to originate from intermolecular interactions.

Odd–even effects in the cyclic voltammetry of self-assembled monolayers of biphenyl based thiols

Monolayers of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH 3C 6H 4C 6H 4(CH 2) m SH, m=1–6, BP m) adsorbed on polycrystalline gold were investigated by cyclic voltammetry. In sharp contrast to alkane thiols, the BP m monolayers show an electrochemical stability which alternates with the length of the alkane spacer. For m=even, reductive desorption takes place at potentials 83 mV more positive compared with m=odd. This odd–even effect is detected only for native layers and is lost after the first voltammetric cycle due to incomplete readsorption. In addition to the desorption potential the charge associated with the desorption also exhibits an odd–even behaviour. Beyond odd–even variations of intermolecular interactions and the thiol coverage which affect capacitive and Faradaic contributions to the desorption peak, the influence of an m-dependent charge distribution at the sulphur ∣ gold interface is discussed with respect to its influence on the Faradaic component and the assumption of a one electron transfer process in the reductive desorption of thiols.

Formation of organic thin films at the liquid-solid interface studied by second harmonic spectroscopy

Self-assembly of ω-(p-nitroanilino) dodecane thiol (p-O 2N-C 6H 4-NH-(CH 2) 12SH) (NAT) on polycrystalline gold films by adsorption from solution was studied in situ with second harmonic generation (SHG). Depending on the wavelength different parts of the system are probed. At a fundamental wavelength of 1 064 nm thiol adsorption is monitored in real time by the formation of the sulfur-substrate bond. A linear dependence of the non-linear susceptibility from the thiol coverage describes the time-dependent SH signal quantitatively. In contrast, the nitroaniline (NA) end group measured at a SH wavelength of 350 nm exhibits a very different behavior. Phase-sensitive measurements carried out in situ reveal that an increase of the density of the film causes the phase of the SH signal from the NA moiety to advance. This phase shift which corresponds to a blue-shift of the resonance becomes effective for coverages higher than 0.4 and is explained by the repulsive interaction of the NAT molecules.

Surface chemical activation of quartz crystal microbalance gold electrodes — analysis by frequency changes, contact angle measurements and grazing angle FTIR

A hydroxyalkanethiol, HS(CH 2) 12OH was deposited onto the gold electrodes of quartz crystal microbalances and gold-coated quartz microscope slides in order to aid in subsequent immobilization of antibodies. FTIR and contact angle measurements confirm the presence of thiol on the fresh gold substrates. In contrast, the change in frequency of the quartz crystal during thiol adsorption indicates an apparent decrease in mass rather than the expected increase due to thiol coverage of the surface. This anomalous behavior appears to be partly due to the removal of organic contaminants from the “aged” gold surface by solvent action. In addition, there may be some dissolution of gold by the thiol solution. QCM frequency changes and FTIR have also been used to monitor subsequent carbodiimide activation of the thiol, as well as antibody binding to the activated surface. The thiolated gold surface was used to fabricate an ordered layer of IgG molecules for probing various immuno-reactions. This was demonstrated by the binding of anti-IgG to IgG in the ratio of ca. 1:1.

Wavelength Dependent Second Harmonic Generation: A New Spectroscopic Tool for the Study of Interfaces

AbstractThe adsorption of n‐hexadecane thiol on gold in ethanol has been examined by wavelength dependent second harmonic generation (SHG). In addition to the amplitude, the phase of the second harmonic signal has been measured. At fundamental wavelengths of 1064 nm and 532 nm the temporal dependence of the SH signal reveals that the second order susceptibility is linearly related to the thiol coverage. Between 610 nm and 660 nm a dramatic variation of the time dependent second harmonic signal both in magnitude and in phase is observed due to a coverage dependent resonance. The results demonstrate the potential of second harmonic generation as a spectroscopy for monitoring the effect of adsorbates on the electronic properties of surfaces.

Adsorption kinetics of n-alkyl thiols on gold studied by second harmonic generation and x-ray photoelectron spectroscopy

Optical second harmonic generation (SHG) was used to study the adsorption kinetics of self-organizing n-alkyl thiol films on polycrystalline gold substrates in situ and in real time. Thiols with carbon chains ranging from 1 to 22 were studied. SHG results showed that the average thiol coverage is constant for thiols longer than hexanethiol. The relation between the SHG signal change and the degree of thiol coverage was established by comparison with ex situ x-ray photoelectron spectroscopy measurements. The temporal dependence of the adsorption is well described by Langmuir adsorption kinetics and the source of the SHG signal is found to be linearly related to the nonlinear susceptibility of the adsorbate–substrate interaction allowing a straightforward in situ determination of the coverage.

The adsorption of alkanethiols on gold studied quantitatively by XPS inelastic background analysis

Organic films of alkanethiols, CH 3 (CH 2) n-1 SH ( n = 2, 6, 8, 11, 16, 18, 22), adsorbed from solution onto gold surfaces are investigated by quantitative XPS. Application of a recent formalism based on inelastic background analysis (the Tougaard method) to the XPS spectra allows information on the thiol film in-depth distribution to be extracted. For the analysis, a knowledge of the product of the inelastic mean free path λ and the inelastic scattering cross-section K( T) in the thiol film is needed. To examine the influence of the thiol λ K( T) function on the result of the analysis, the “universal cross-section” as well as the λ K( T) value for polyethylene evaluated from REELS spectra are applied in the present analysis. Our results show that the polyethylene λ K( T) function is superior to the universal function in describing the thiol λ K( T) function. In the present analysis of the thiol/gold system an in-depth distribution model assuming uniform overlayer growth with the thickness Δ x depending linearly on n was found to match the actual profile. For n = 22 the fractional thiol coverage of the gold surface is shown to be at least 90%.

Investigation of self-organizing thiol films by optical second harmonic generation and X-ray photoelectron spectroscopy

The adsorption of self-organizing thiol films on polycrystalline gold substrates was investigated in situ by optical second harmonic generation. Growth of the films could be detected at submonolayer coverage. For a comparison with classical surface analytical methods the thiol coverage was measured ex situ by X-ray photoelectron spectroscopy. We find that formation of ordered thiol films up to monolayer coverage can be described by Langmuir adsorption kinetics.