Balancing the Rate of Cluster Growth and Etching for Gram‐Scale Synthesis of Thiolate‐Protected Au25 Nanoclusters with Atomic Precision

We report a NaOH‐mediated NaBH₄ reduction method for the synthesis of mono‐, bi‐, and tri‐thiolate‐protected Au₂₅ nanoclusters (NCs) with precise control of both the Au core and thiolate ligand surface. The key strategy is to use NaOH to tune the formation kinetics of Au NCs, i.e., reduce the reduction ability of NaBH₄ and accelerate the etching ability of free thiolate ligands, leading to a well‐balanced reversible reaction for rapid formation of thermodynamically favorable Au₂₅ NCs. This protocol is facile, rapid (≤3 h), versatile (applicable for various thiolate ligands), and highly scalable (>1 g Au NCs). In addition, bi‐ and tri‐thiolate‐protected Au₂₅ NCs with adjustable ratios of hetero‐thiolate ligands were easily obtained. Such ligand precision in molecular ratios, spatial distribution and uniformity resulted in richly diverse surface landscapes on the Au NCs consisting of multiple functional groups such as carboxyl, amine, and hydroxy. Analysis based on NMR spectroscopy revealed that the hetero‐ligands on the NCs are well distributed with no ligand segregation. The unprecedented synthesis of multi‐thiolate‐protected Au₂₅ NCs may further promote the practical applications of functional metal NCs.     

Thiol and thiolate bond formation of ferrocene-1,1-dithiol to a Ag(111) surface

Using density functional calculations, we show that the adsorption of ferrocene dithiol on the Ag(111) surface is remarkably flexible, i.e., a large number of different configurations have binding energies that differ by less than 0.1 eV per molecule. The thiolate bond is slightly favored over the thiol bond (by less than 0.1 eV) but may not be formed due to considerable activation barriers. Electronically, we found that the thiolate bound molecule is conducting, whereas thiol bonds turn it into semiconducting.     

Substituent effects on Ni-S bond dissociation energies and kinetic stability of nickel arylthiolate complexes supported by a bis(phosphinite)-based pincer ligand

Pincer complexes of the type [2,6-(R(2)PO)(2)C(6)H(3)]NiSC(6)H(4)Z (R = Ph and i-Pr; Z = p-OCH(3), p-CH(3), H, p-Cl, and p-CF(3)) have been synthesized from [2,6-(R(2)PO)(2)C(6)H(3)]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni-S bond length (approx. 2.20 ?) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with "[2,6-(Ph(2)PO)(2)C(6)H(3)]Ni" rather than "[2,6-(i-Pr(2)PO)(2)C(6)H(3)]Ni", and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH(2)Br, CH(3)I, CCl(4), and Ph(3)CCl have been examined and several mechanistic pathways have been explored.     

Mechanism of the reactions of synthetic Fe-S-based clusters with PhCOCl: parallel pathways involving free and coordinated thiolate as nucleophiles

Terminal thiolate ligands on the synthetic Fe-S-based clusters [Fe4S4(SR)4]2- (R = Et or SPh) or [{MoFe3S4(SPh)3}2(mu-SPh)3]3- are replaced by chloride in a reaction with PhCOCl to produce [Fe4S4Cl4]2- and [{MoFe3S4Cl3}2(mu-SPh)3]3-, respectively. Kinetic studies using stopped-flow spectrophotometry show that, in general, the mechanisms of these reactions in MeCN occur by two pathways. One pathway is independent of the concentration of PhCOCl and involves rate-limiting dissociation of the thiolate ligand. The free thiolate subsequently reacts with PhCOCl to produce PhCOSR and the Cl- which binds to the vacant site on the cluster. The second pathway exhibits a nonlinear dependence on the concentration of PhCOCl and involves initial, rapid binding of PhCOCl to the cluster followed by intramolecular thiolate ligand attack on the coordinated acid chloride. The intermediate in which PhCOCl is bound to the cluster has been detected spectrophotometrically. The ways in which the rates of the reactions between PhCOCl and Fe-S-based clusters are affected by changes of the terminal thiolate, the metal composition of the cluster core, and the protonation state of the cluster have been investigated and are compared with the effect these same changes have on the rates of nucleophilic substitution.     

Determination of the DeltapKa between the active site cysteines of thioredoxin and DsbA

Thioredoxin superfamily members share a considerable degree of structural similarity, with a conserved CX(i)X(j)C motif at the active site, where C stand for two cysteines that alternate between a reduced thiol and oxidized disulfide states, and X(i)and X(j) are two amino acids different in each family member. Despite these similarities, they display very different redox potentials and pKas for the active site dithiol, and fulfill different physiological roles. Thioredoxin, for example, promotes the reduction of disulfide bonds, while DsbA promotes their oxidation in prokaryotic cells. The factors that promote these differences are still not fully understood. However, it is generally accepted that the different stabilities of the redox active disulfide bond depends on the degree of stabilization, in the reduced state, of the thiolate of one of the active site cysteines (nucleophilic cysteine). In this work we have used QM/MM methods to compare and characterize the active site dithiols of both enzymes, and to shed some light on the structural features responsible for the large differences in pKa and redox potential between two homologous enzymes, thioredoxin and DsbA. We have also analyzed the main factors pointed out in the literature as responsible for their different properties. We obtained the value of 4.5 for pKa difference (DeltapKa) between the nucleophilic cysteines of both enzymes, which is in excellent agreement with most of the experimental values. Additionally, we found that the principal differentiating factor responsible for this observed DeltapKa are the alpha2-alpha helices, which greatly contribute to the mentioned value, by stabilizing the DsbA thiolate in a much greater extend than the thioredoxin thiolate. A double mutation of the conserved residues Asp26 and Lys57, in thioredoxin, and Glu24 Lys58, in DsbA, by alanines did not change the DeltapKa value; this supports the hypothesis that these residues are not involved in the differentiation of the properties of the active centre dithiol. However, we found out that these residues are important for the stabilization of the nucleophilic thiolate. The X(i) and X(j) residues also do not seem to promote the stabilization of the thiolates. In fact, the corresponding double alanine mutants are more stable than the wild-type enzymes. However, these residues are involved in the differentiation between thioredoxin and DsbA, stabilizing the DsbA thiolate by a larger extent than the thioredoxin thiolate.     

Synthesis and characterization of gold nanoparticles stabilized by palladium(II) phosphine thiol

This work is focused on the synthesis of innovative hybrids made by linking gold nanoparticles to protected organometallic Pd(II) thiolate. The organometallic protected Pd(II) thiolate, i.e. trans-thioacetate-ethynylphenyl-bis(tributylphosphine)palladium(II) has been synthesized, in situ deprotected and linked to Au nanoparticles. In this way new hybrid, with a direct link between Pd(II) and Au nanoparticles through a single S bridge, has been isolated. The combination of the organometallic Pd(II) thiol with gold nanoparticles allows the enhancement and tailoring of electronic and optical properties of the new organic-inorganic nano-compound. Single-crystal gold nanoparticles, uniform in shape and size were obtained by applying a modified two-phase method (improved Brust-Schiffrin reaction). In addition, the chemical environment of the Au nanoparticles was investigated and a covalent bonding between Au nanoparticles and the organometallic thiols was observed.     

Modification of Electrode Surfaces by Self‐Assembled Monolayers of Thiol‐Terminated Oligo(Phenyleneethynylene)s

The wire‐like properties of four S‐(4‐{2‐[4‐(2‐phenylethynyl)phenyl]ethynyl}phenyl) thioacetate derivatives, PhC≡CC₆H₄C≡CC₆H₄SAc ( 1 ), H₂NC₆H₄C≡CC₆H₄C≡CC₆H₄SAc ( 2 ), PhC≡CC₆H₂(OMe)₂C≡CC₆H₄SAc ( 3 ) and AcSC₆H₄C≡CC₆H₄C≡CC₆H₄SAc ( 4 ) (Figure 1 ), all of which possess a high degree of conjugation along the oligo(phenyleneethynylene) (OPE) backbone, were investigated as self‐assembled monolayers (SAMs) on gold and platinum electrodes by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The redox probe [Fe(CN)₆]₄^? was used in both the CV and impedance experiments. The results indicate that the thiolates derived from thioacetate‐protected precursor molecules 1 and 2 form well‐ordered monolayers on a gold electrode, whereas SAMs derived from 3 and 4 exhibit randomly distributed pinholes. The electron tunnelling resistance and fractional coverage of SAMs of all four compounds were examined using electron tunnelling theory. The analysis of the results reveal that the well‐ordered SAMs of 1 and 2 exhibit higher charge‐transfer resistance in comparison to the defect‐ridden SAMs of 3 and 4 . The additional steric bulk offered by the methoxy groups in 3 is likely to prevent efficient packing within the SAM, leading to a microelectrode behaviour, when assembled on a gold electrode surface. The protected dithiol derivative 4 probably binds to the surface through both terminal groups which prevents dense packing and leads to the formation of a monolayer with randomly distributed pinholes. Atomic force microscopy (AFM) was used to examine the morphology of the monolayers, and height images gave root‐mean‐square (RMS) roughness′s which are in agreement with the proposed SAM structures.     

Voltammetric and spectroscopic analysis of interactions of pyridine mono-carboxylic acid isomers with cysteine at physiological pH

The interactions of pyridine mono-carboxylic acid isomers (PCAIs) (nicotinic acid (NA), isonicotinic acid (INA) and picolinic acid (PA)) with cysteine (RSH) at physiological pH (7.40) have been investigated by square-wave and cyclic voltammetry (SWV and CV), and UV-Vis and infrared spectroscopy. By the addition of isomeric pyridine mono-carboxylic acids, the reduction peak current of mercurous cysteine thiolate could be decreased and also its peak potential E _p was shifted to more positive values. Also, the significant changes in formal potential E ⁰′, electron transfer coefficient α and electrode reaction standard rate constant k _s of mercurous cysteine thiolate (Hg₂(SR)₂) in the presence and absence of PCAIs were observed. The results of voltammetric measurements indicated that binding reactions were occurred between PCAIs and RSH and new electroactive molecular complexes were formed, which resulted in the decrease of free cysteine concentration and the decrease of the reduction peak current of mercurous cysteine thiolate. The logarithmic values of binding constants of NA, INA and PA were calculated as 13.4, 17.7 and 18.9, respectively. The binding ratios for NA-RSH, INA-RSH and PA-RSH complexes were determined as 1: 3, 1: 4 and 1: 4, respectively. Both UV-Vis and FTIR studies also confirmed these interaction reactions.     

A SERS-active system based on silver nanoparticles tethered to a deposited silver film

A series of SERS-active nanostructures were produced by exposing a freshly deposited silver film (fabricated to be as free from roughness as practicable) to a solution containing a mixture of 1-decanethiol (m) and 1,9-nonanedithiol (d) of varying concentrations of m to d, then allowing colloidal silver nanoparticles to interact with the surface. Silver nanoparticles were found to bind exclusively to films which were prepared from solutions with a nonzero concentration of the dithiol implying that the nanoparticles were tethered to the silver surface by the dithiol with one of the thiolate groups bound to the nanoparticle and the other to the silver film. Intense SERS spectra were observed even from samples in which the m/d concentration ratio was so large that the adsorbed molecules in the vicinity of only approximately 8 +/- 3 nanoparticles were illuminated by the diffraction-limited focused laser beam. At such high dilution, the molecules (numbering at most approximately 330) residing in the SERS "hot spots" associated with the approximately 8 nanoparticles consisted primarily of m (although, of course, for each nanoparticle, at least one molecule in the hot spot had to be d to serve as the linker). This was corroborated by the SERS spectra. An analysis is presented, which accounts for the fact that as the concentration ratio of m/d increases, the SERS intensity associated with bands belonging to m first increases to a maximum then decreases. The nanoparticle-metal film system presented here is a simple embodiment of a more general range of SERS-active sensing platforms in which a molecular tether is used to create a SERS hot spot that (although nanosized) is large enough to accommodate analyte molecules that cannot themselves function as linkers, which are subsequently detected by SERS at the few-molecule level.     

Polycationic Pillar[5]arene Derivatives: Interaction with DNA and Biological Applications

Dendritic pillar[5]arene derivatives have been efficiently prepared by grafting dendrons with peripheral Boc‐protected amine subunits onto a preconstructed pillar[5]arene scaffold. Upon cleavage of the Boc‐protected groups, water‐soluble pillar[5]arene derivatives with 20 ( 13 ) and 40 ( 14 ) peripheral ammonium groups have been obtained. The capability of these compounds to form stable nanoparticles with plasmid DNA has been demonstrated by gel electrophoresis, transmission electron microscopy (TEM), and dynamic light scattering (DLS) investigations. Transfection efficiencies of the self‐assembled 13 /pCMV‐Luc and 14 /pCMV‐Luc polyplexes have been evaluated in vitro with HeLa cells. The transfection efficiencies found for both compounds are good, and pillar[5]arenes 13 and 14 show very low toxicity if any.     

Mercury(II) 2-aminoethanethiolate clusters: intramolecular transformations and mechanisms

The combination of HgF2 and 2-aminoethanethiol (AET, with some AET.HCl present) yielded a cyclic tetranuclear thiolate, [Hg4Cl4(SCH2CH2NH2)4] (1), with alternating Hg and S atoms. The Cl from the reaction mixture led to the formation of Hg-Cl bonds with no Hg-F in the final product. In contrast, a similar reaction with HgBr2 yielded a nonanuclear cluster, [Hg9Br15(SCH2CH2NH3)15]3+ (2), and the disulfide salt {[HgBr4][(NH3CH2CH2S-)2]} (3). Despite similar reactions, the AET groups in 2 are protonated compared to the nonprotonated amine groups in 1, which allows the ligand to chelate the Hg atom in the latter compound. The reaction with HgI2 yielded a cyclic tetranuclear compound, [Hg4I6(SCH2CH2NH2)2(SCH2CH2NH3)2](H2O/EtOH) (4), containing protonated and nonprotonated AET groups. Compound 4 at room temperature irreversibly rearranges to [Hg4I4(SCH2CH2NH2)4] (5), which is isostructural to 1. A systematic pathway for the formation of 1 along with the intramolecular conversion of 4 to 5 is proposed. These compounds demonstrate that very diverse Hg-S compounds form under similar reaction conditions.     

Dithiol-mediated incorporation of CdS nanoparticles from reverse micellar system into Zn-doped SBA-15 mesoporous silica and their photocatalytic properties

CdS nanoparticles, as prepared in reverse micellar systems, were incorporated into alkanedithiol-modified Zn-doped SBA-15 mesoporous silica (dtz.sbnd;ZnSBA-15; pore diameter, ca. 4 nm), which were themselves prepared via hydrolysis of tetraethylorthosilicate (TEOS) in the presence of Zn(NO(3))(2) and triblock copolymer, as a nonsurfactant template and pore-forming agent, followed by contact with dithiol molecules. A particle-sieving effect for the dtz.sbnd;ZnSBA-15 was observed, in that the incorporation of the nanoparticles was remarkably decreased with increasing the nanoparticle size. The resulting CdSz.sbnd;ZnSBA-15 composite was then used as photocatalysts for the generation of H(2) from 2-propanol aqueous solution. Under UV irradiation (lambda>300 nm), a high photocatalytic activity was observed for this composite material. This is effected by electron transfer from the photoexcited ZnS (dithiol-bonded Zn on SBA-15) to CdS nanoparticles. The photocatalytic activity is increased with a decrease in the number of methylene groups in the dithiol molecules, according to the rank order 1,10-decanedithiol < 1,6-hexanedithiol < 1,2-ethanedithiol.     

Building layer-by-layer a bis(dithiocarbamato)copper(II) complex on Au[111] surfaces

A bis(dithiocarbamato)copper(II) complex (CuDTC2) was built on Au{111} surfaces (sheets and electrode beads) using different building blocks in a layer-by-layer (LbL) procedure. The process was followed by AFM and cyclic voltammetry. Initially 4-piperidinemethanethiol, which was synthesized here for the first time, was self-assembled on a gold surface and a highly organized array was obtained. The resulting monolayer was treated with CS2 and NH3 to transform the NH groups of piperidine into dithiocarbamate groups (DTC) with the formation of an amphiphilic ligand (DTCpipS) with thiolate and DTC terminal anionic groups. Two reductive desorption peaks were observed in the cyclic voltammogram of self-assembled DTCpipS, a more intense peak at -0.87 V (thiolate group) and a broader, less intense peak at -0.68 V, corresponding to the desorption of the DTC group bound to the gold surface after the ligand made a approximately 180 degrees flip on the surface. Copper(II) and the morpholyldithiocarbamate anion were associated with self-assembled DTCpipS in order to complete the formation of the CuDTC2 complex on the gold surface. In the voltammogram of the LbL self-assembled CuDTC2 complex the reductive desorption peak at -0.68 V disappeared and one single peak was observed at -0.85 V. This corresponds to the reorientation of all of the DTCpipS dianions in order to coordinate to copper(II) through the DTC groups, leaving the self-assembly only through the thiolate groups. The complete formation of the LbL self-assembled CuDTC2 complex was confirmed by XPS and ToF SIMS, with a detected fragment corresponding to the whole complex.     

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 30

The protected G*[s ]C*[s ]U*[s ]A*[s ]U*[s ]A*[s ]G*[s ]C* octanucleoside 24 was prepared by S‐alkylation of the thiolate derived from tetranucleoside 23 with the methanesulfonate 22 , and transformed to the silylated and isopropylidenated 25 , and further into the fully deprotected octanucleoside 26 . Compound 22 was derived from the methoxytrityl‐protected tetranucleoside 21 , and 21 was obtained by S‐alkylation of the thiolate derived from the dinucleoside 19 with methanesulfonate 17 derived from 16 by detritylation and mesylation. Similarly, tetranucleoside 23 resulted from S‐alkylation of the thiolate derived from 18 with the methanesulfonate 20 derived from 19 . Dinucleosides 16 and 18 resulted from S‐alkylation of the thiolate derived from the known cytidine‐derived thioacetate 15 with the C(8)‐substituted guanosine‐derived methanesulfonates 12 and 14 , respectively, that were synthesized from the protected precursors 4 and 7 by formylation, reduction, protection, and mesylation. The structures of the duplexes of 25 and 26 were calculated using AMBER* modelling and based on the known structure of the core tetranucleoside U*[s ]A*[s ]U*[s ]A*. The former shows a helix with a bent helix axis and strong buckle and propeller twists, whereas the latter is a regular, right‐handed, and apparently strain‐free helix. In agreement with modelling, the silylated and isopropylidenated octanucleoside 25 in (CDCl₂)₂ solution led to a mixture of associated species possessing at most four Watson? Crick base pairs, while the fully deprotected octanucleoside 26 in aqueous medium forms a duplex, as evidenced by a decreasing CD absorption upon increasing the temperature and by a UV‐melting curve with a melting temperature of ca. 10° below the one of the corresponding RNA octamer, indicating cooperativity between base pairing and base‐pair stacking.     

Dependence of product formation from decomposition of nitroso-dithiols on the degree of nitrosation. Evidence that dinitroso-dithiothreitol acts solely as an nitric oxide releasing compound

Hitherto, the decay mechanisms of nitrosated dithiols as well as formation of related products have not been conclusively elucidated. In this paper, we demonstrate that nitrosated dl-dithiothreitol (DTT) decays via two independent pathways, that is, one producing exclusively nitric oxide and one producing (initially) nitroxyl (HNO/3NO-). The importance of the two decomposition pathways depends on the degree of nitrosation of DTT. Dinitroso-dithiothreitol (NODTTNO) generates quantitatively nitric oxide, whereas mononitroso-dithiothreitol (NODTT) yields initially nitroxyl. Since NODTT and DTT are both targets for nitroxyl, their availability governs the HNO-derived formation of nitric oxide (with NODTT as reactant) or hydroxyl amine and ammonium ion (with DTT as reactant). The formation of NH4+ from the HNO-DTT reaction probably proceeds by a stepwise, NH2OH-independent mechanism, because DTT-derived sulfinamide was identified by N-15 NMR spectrometry as an intermediate. Our data are in line with the assumption that triplet nitroxyl (3NO-) is formed by a unimolecular decay of the deprotonated (thiolate) form of NODTT, because CBS-QB3 calculations predict the existence of a low-lying triplet state of the latter species. The identified pathways are proposed to be of general importance for physiological systems because control experiments showed that the physiological dithiol thioredoxin reacts in a similar manner.     

Synthesis and characterization of a new Tiara Pd(II) thiolate complex, [Pd(SC12H25)2]6, and its solution-phase thermolysis to prepare nearly monodisperse palladium sulfide nanoparticles

A new tiara Pd(II) thiolate complex, [Pd(SC12H25)2]6, has been synthesized and fully characterized by X-ray single crystal analysis, elemental analysis, MALDI, 1H NMR, powder XRD, IR, Raman, and UV/vis. It was found that, in each complex cluster, the six palladium atoms form a nearly planar hexagonal ring and the adjacent palladium atoms are bridged by sulfur atoms from both sides. Then the complex was further used as a single-source precursor to prepare nearly monodisperse palladium sulfide (PdS) nanoparticles through the high-temperature-induced decomposition in diphenyl ether. The obtained nanoparticles are 2.87 +/- 0.51 nm in diameter and protected by a layer of thiolate species on the surface.     

In situ scanning tunneling microscopy of 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol adsorbed on pt(111) electrodes

Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were used to examine four dithiol molecules, including 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol, adsorbed on well-ordered Pt(111) electrodes in 0.1 M HClO(4). The open-circuit potential (OCP) of Pt(111) electrodes decreased substantially from 0.95 to 0.3 V (versus reversible hydrogen electrode) upon the adsorption of dithol molecules, which indicates that these adsorbates injected electrons into the Pt electrode. For all dithiol molecules, ordered adlattices of p(2 x 2) and (square root 3 x square root 3)R30 degrees were formed when the dosing concentration was lower than 150 microM and the potential of Pt(111) was more negative than 0.5 V. Raising the potential of Pt(111) from 0.1 to 0.4 V or more positive values could transform p(2 x 2) to (square root 3 x square root 3)R30 degrees before it turned disarray. The insensitivity of the structure of dithiol adlayers with their chemical structures was explained by upright molecular orientation with the formation of one Pt-S bond per dithiol molecule. This molecular orientation was independent of the coverage of dithiol molecules, as nucleation seeds produced at the beginning of adsorption were also constructed with p(2 x 2). The triangular-shaped STM molecular resolution suggested 3-fold binding of sulfur headgroup on Pt(111). All dithiols were adsorbed so strongly on Pt(111) electrodes that switching the potential negatively to the onset of hydrogen evolution in 0.1 M HClO(4) or water reduction in 1 M KOH could not displace dithiol admolecules.     

Maerocyelic polyether sulfide syntheses. The preparation of thia-crown-3,4, and 5 compounds

Macrocyelic polyether sulfides have been prepared by reacting an oligoethylene glycol dichloride with a dithiol or sodium sulfide in ethanol. The trivial naming system for these compounds is an extension of the trivial crown nomenclature (5). A thia prefix is used to show that sulfur atoms have replaced ether oxygen atoms in the polyether ring. The following new compounds were prepared: 1-thia-(15-crown-5) (II) 1,4-dithia-(15-crown-5) (III) 1,7-dithia-(15-crown-5) (IV) 1,4-dithia-(12-crown-4) (VII) 1,4,7-trithia-(12-crown-4) (VIII) 1-thia-(9-crown-3) (IX) and 1,4-dithia-(9-crown-3) (X). Four other previously reported maerocyelic polyether sulfides were also prepared. The symmetry of the nmr spectra of these compounds gives added evidence for the proposed ring structures.     

A spectroscopic study of self-assembled monolayer of porphyrin-functionalized oligo(phenyleneethynylene)s on gold: the influence of the anchor moiety

Porphyrin-functionalized oligo(phenyleneethynylene)s (OPE) are promising molecules for molecular electronics applications. Three such molecules () with the common structure P-OPE-AG (P and AG are a porphyrin and anchor group, respectively) and different anchor groups, viz. an acetyl protected thiol, -S-COCH(3) (), an acetyl protected thiol with methylene linker, -CH(2)-S-COCH(3) (), and a trimethylsilylethynyl group, -C[triple bond, length as m-dash]C-Si(CH(3))(3) () have been synthesized and the corresponding self-assembled monolayers (SAMs) on Au(111) substrates have been prepared. The integrity and structural properties of these films were studied by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The results suggest that the films formed from have a high orientational order with an almost upright orientation and dense packing of the molecular constituents, i.e. represent a high quality SAM. In contrast, molecule formed disordered molecular layers on Au, even though the molecule-surface bonding (thiolate) is the same as in the case of molecule . This suggests that the methylene linker in molecule has a strong impact on the quality of the resulting film, so that a well-ordered SAM cannot be formed. The silane system, , is also able to bind to the gold surface but the resulting SAM has a poor quality, being significantly disordered and/or comprised of strongly inclined molecules. The above results suggest that the nature of the anchor group along with a possible linker is an important parameter which, to a high extent, predetermines the entire quality of OPE-based molecular layers.     

Determination of accessible amino groups on surfaces by chemical derivatization with 3,5-bis(trifluoromethyl)phenyl isothiocyanate and XPS/NEXAFS analysis

The determination of amino groups on surfaces capable of binding biomolecules is important for the understanding and optimization of technologically relevant coupling processes. In this study, three different types of amino-functionalized model surfaces, amino thiolate on Au, amino siloxane on Si, and polyethylene (PE) foils and films reacted with 1,2-diaminoethane (DAE) were derivatized with 3,5-bis(trifluoromethyl)phenyl isothiocyanate. Subsequently, these samples were analyzed by chemical derivatization X-ray photoelectron spectroscopy (CD-XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The determination of amino groups by this analytical approach allows gaining insight into the availability of groups on surfaces that can actually serve as attachment sites for biomolecules in technical applications. In the case of the amino thiolate on Au, almost 90% of the expected amino groups were detected by CD-XPS. Investigation of the amino siloxane films revealed lower yields for the derivatization reaction in the order of 30%. The lowered reaction yields are thought to be due to interactions between the amino siloxane’s amino and silanol groups or the underlying substrate, making them inaccessible to the derivatization agent. The aminated PE samples are characterized by a complex surface chemistry and structure, and reaction yields of the derivatization reaction cannot be unequivocally derived. However, 1–3% of the total carbon atoms in the surface layer were found to be bound to amino groups accessible to the derivatization agent. It can be concluded that, depending on the detailed character of the investigated amino-terminated surface, the amount of amino groups accessible to CD-XPS can be substantially lower than the total amount of amino groups present at the surface.