Formation of high-quality self-assembled monolayers of conjugated dithiols on gold: base matters

This Article reports a systematic study on the formation of self-assembled monolayers (SAMs) of conjugated molecules for molecular electronic (ME) devices. We monitored the deprotection reaction of acetyl protected dithiols of oligophenylene ethynylenes (OPEs) in solution using two different bases and studied the quality of the resulting SAMs on gold. We found that the optimal conditions to reproducibly form dense, high-quality monolayers are 9-15% triethylamine (Et(3)N) in THF. The deprotection base tetrabutylammonium hydroxide (Bu(4)NOH) leads to less dense SAMs and the incorporation of Bu(4)N into the monolayer. Furthermore, our results show the importance of the equilibrium concentrations of (di)thiolate in solution on the quality of the SAM. To demonstrate the relevance of these results for molecular electronics applications, large-area molecular junctions were fabricated using no base, Et(3)N, and Bu(4)NOH. The magnitude of the current-densities in these devices is highly dependent on the base. A value of β=0.15 ?(-1) for the exponential decay of the current-density of OPEs of varying length formed using Et(3)N was obtained.     

Deprotecting thioacetyl-terminated terphenyldithiol for assembly on gallium arsenide

We characterize the assembly of terphenyldithiol (TPDT) on gallium arsenide (GaAs) from ethanol (EtOH) and tetrahydrofuran (THF) as a function of ammonium hydroxide (NH4OH) concentration. NH4OH facilitates the conversion of thioacetyl end groups of the TPDT precursor to thiolates in the assembly solution. The final structure of TPDT assembled on GaAs is sensitive not only to the assembly solvent but also to NH4OH concentration. In the presence of low concentrations of NH4OH (1 mM), TPDT assemblies from EtOH are oriented upright. The same assemblies are less upright when adsorption is carried out at higher NH4OH concentrations. In THF, TPDT does not adsorb significantly on GaAs at low NH4OH concentrations. The surface coverage and structural organization of these assemblies improve with increasing NH4OH concentrations, although these assemblies are never as organized as those from EtOH. The difference in the final structure of TPDT assemblies is attributed to differences in the thiolate fraction in the assembly solution at the point of substrate immersion.     

A new route to the formation of biomimetic phosphate assemblies on gold: synthesis and characterization

A biomimetic model system based on long-chain alkanethiols tailored with serine, threonine and tyrosine side-chain groups is created as a platform for the study of phosphorylated amino acids. The phosphorylated analogues are synthesized with protective tert-butyl groups that after assembly on thin polycrystalline gold films are removed in an acidic deprotection solution to form the corresponding phosphate self-assembled monolayers (SAMs). The SAMs are thoroughly characterized with null ellipsometry, contact angle goniometry, infrared reflection-absorption spectroscopy and X-ray photoelectron spectroscopy. The assembly and the subsequent deprotection process are optimized with respect to molecular orientation and chain conformation by varying the incubation time and the exposure time to the deprotection solution. The high quality of the generated SAMs suggests that the present assembly/deprotection approach is an attractive alternative when traditional synthetic routes become demanding because of solubility problems.     

Synthesis and physical properties of triptycene-based oligo(p-phenyleneethynylene)s

A series of symmetrical and unsymmetrical triptycene-based oligo(op-phenyleneethynylene)s were synthesized by deprotection of the acetone protected terminal alkynes, followed by Sonogashira coupling reactions. The photophysical properties of triptycene-based OPEs both in solution and solid state have been investigated by UV–Vis and fluorescence spectroscopy. Interestingly, the obtained compounds show strong fluorescence with partly high quantum yields in solid state, which suggested that triptycene moieties have not only prevented the intermolecular aggregation but also enforced the coplanarity of OPEs backbone in the solid state.     

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

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

Tail molecule dependence of thiolate adsorption on Au(111) surface: Theoretical study

The adsorption of thiolates with various tail molecules on the Au(111) surface has been investigated by first-principles calculations. We have considered six typical thiolate molecules, that is, methylthiolate, ethylthiolate, ethylenethiolate, acetylenethiolate, benzenethiolate, and thiophenethiolate. It is found that these thiolates exhibit little difference in their stable adsorption geometries. They are adsorbed at the bridge site with being significantly tilted from the surface normal. The adsorption energy of thiolate on Au, on the other hand, largely varies depending on the type of tail molecule, and is linearly proportional to the binding energy of thiolate with H. We discuss the tail molecule dependence in terms of the bonding environment around the C atom connected to the head S atom.     

Controlled, simultaneous assembly of polyethylenimine onto nanoparticle silica colloids

A novel precision-assembly methodology is described on the basis of the controlled, simultaneous assembly (CSA) of a core nanoparticle substrate and polyelectrolyte solutions. The method is capable of assembly rates at least as fast as 10(16) core particles s(-1) L(-1) and affords concentrated suspensions of stable colloids with an adsorbed polyelectrolyte. The resulting dispersions are highly homogeneous, have a low viscosity and narrow particle-size distribution, and are stable colloids, even at solid concentrations of at least 33 wt %. The adsorption isotherm and the saturation adsorption for polyethylenimine (PEI) assemblies onto a 15 nm silica colloid have been evaluated with 1H NMR spectroscopy. The saturation adsorption is highly dependent upon the pH at assembly and is given by the equation PEIa (micromol m(-2)) = 1.73pH - 1.89, R2 = 0.986, where micromoles refers to the concentration of the EI monomer. The saturation concentration increases from 6.8 micromol m(-2) at pH 5.0 to 13.7 micromol m(-2) at pH 9.0. The adsorbed polyelectrolyte may be cross-linked and thereby permanently fixed to the colloid surface to prepare nanoparticle-polyelectrolyte colloidal assemblies having enhanced colloid stability, high homogeneity, and a high fraction (>80%) of permanently adsorbed polyelectrolyte. These assemblies are stable at physiological pH and ionic strength and may represent ideal substrates for bioconjugation and, ultimately, the design of nanocarriers for in vivo applications.     

Coumarin-caged dG for improved wavelength-selective uncaging of DNA

Herein we report on diethylaminocoumarin (DEACM) as a new photoremovable protecting group for 2'-deoxyguanosine in oligonucleotides. An oligonucleotide with O(6)-DEACM-caged dG was synthesized and photochemically analyzed. The DEACM group shows superior photochemical properties at 405 nm with an uncaging efficiency (ε·φ) for deprotection that is 17 times higher than that for 2-(o-nitrophenyl)-propyl NPP caging groups in the same position. Wavelength-selective deprotection in the presence of NPP groups proceeds up to 80 times faster.     

Anion-induced adsorption of ferrocenated nanoparticles

Au nanoparticles fully coated with omega-ferrocenyl hexanethiolate ligands, with average composition Au225(omega-ferrocenyl hexanethiolate)43, exhibit a unique combination of adsorption properties on Pt electrodes. The adsorbed layer is so robust that electrodes bearing submonolayer, monolayer, and multilayer quantities of these nanoparticles can be transferred to fresh electrolyte solutions and there exhibit stable ferrocene voltammetry over long periods of time. The kinetics of forming the robustly adsorbed layer are slow; monolayer and submonolayer deposition can be described by a rate law that is first order in nanoparticle concentration and in available electrode surface. The adsorption mechanism is proposed to involve entropically enhanced (multiple) ion-pair bridges between oxidized (ferrocenium) sites and certain specifically adsorbed electrolyte anions on the electrode. Adsorption is promoted by scanning to positive potentials (through the ferrocene wave) and by high concentrations of Bu4N+ X- electrolyte (X- = ClO4(-), PF6(-)) in the CH2Cl2 solvent; there is no adsorption if X- = p-toluenesulfonate or if the electrode is coated with an alkanethiolate monolayer. The electrode double layer capacity is not appreciably diminished by the adsorbed ferrocenated nanoparticles, which are gradually desorbed by scanning to potentials more negative than the electrode's potential of zero charge. At very slow scan rates, voltammetric current peaks are symmetrical and nearly reversible, but exhibit E(fwhm) considerably narrower (typically 35 mV) than ideally expected (90.6 mV, at 298 K) for a one-electron transfer or for reactions of multiple, independent redox centers with identical formal potentials. The peak narrowing is qualitatively explicable by a surface-activity effect invoking large, attractive lateral interactions between nanoparticles and, or alternatively, by a model in which ferrocene sites react serially at formal potentials that become successively altered as ion-pair bridges are formed. At faster scan rates, both deltaE(peak) and E(fwhm) increase in a manner consistent with a combination of uncompensated ohmic resistance of the electrolyte solution and of the adsorbed film, as distinct from behavior produced by slow electron transfer.     

Adsorption modes of cysteine on Au(111): thiolate, amino-thiolate, disulfide

The adsorption of cysteine on the (111) surface of gold has been studied by means of periodic supercell density-functional theory calculations. A number of different adsorption modes are examined, including adsorption through the thiol group in either thiolate or disulfide form, and adsorption through both the thiol and amino functional groups. We find that at intermediate coverage densities the latter mode of adsorption is favored, followed by thiolate adsorption at the bridge (slightly displace toward fcc) site. The N-Au and S-Au bond strengths in the amino-thiolate adsorption are estimated to be of the order of 6 and 47 kcal/mol, respectively. The electronic structure of the different systems is analyzed, with focus on the total and projected density of states, as well as on the detailed character of the electronic states at the interface. States near the Fermi energy are found to have a metal-molecule antibonding character, whereas metal-molecule bonding states mostly occur near the lower edge of the Au-d band.     

Development of an ultra-high-performance liquid chromatography-tandem mass spectrometry method for high throughput determination of organophosphorus flame retardants in environmental water

Widely used as flame retardants, organophosphate esters (OPEs) are now broadly present in the indoor and outdoor environments. Currently available liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods share some drawbacks with gas chromatography (GC) methods, including time consuming, limited target OPEs, incomplete separation capability for some OPEs and low throughput. In this study, a fast and high throughput LC-MS/MS method was developed. For the first time, all the twelve OPEs that have been studied in literature, ranging from the very polar and volatile trimethyl phosphate to the very hydrophobic and non-volatile tris(2-ethylhexyl) phosphate, were separated within 11 min. Different from previous studies, we found that the blank contamination was mainly from organic mobile phase rather than the enrichment process, and it can be efficiently eliminated by using acetonitrile rather than methanol as the organic phase of the mobile phase. The signal to noise ratio (S/N) was significantly improved by using 0.1% formic acid as an organic modifier. The method exhibited high throughput and sensitivity and can baseline separate 11 of the 12 OPEs studied within 11 min with LOQs ranging from 2 to 6 ng/L. The relative standard deviations were in the range of 2-10%. For both reagent water and river water, the spiked recoveries of OPEs ranged from 70 to 110%, except for the very polar and volatile trimethyl phosphate that has recovery below 10%. The developed procedure was successfully applied to study the OPE contamination of the Songhua River, and it was found that all the target OPEs were detected with total concentrations of around 1 μg/L in the river waters.     

Improved and new syntheses of potential molecular electronics devices

New syntheses of ethyl and nitro substituted oligo(phenylene ethynylene)s (OPEs) have been developed. To further explore whether the presence of nitro functionality in OPEs leads to switching and memory capabilities, new nitro substituted OPEs have been designed and synthesized. An isatogen-based system, a structure that is isomeric to the nitro OPE, has been synthesized. Additionally, pyridine-based and chromium-based compounds have been synthesized. We surmise that redox reactions of these candidates may impart switching capabilities and electrochemical studies are shown. U-shaped OPEs were synthesized to inhibit leakage of metals deposited during formation of top contacts on self-assembled monolayers (SAMs). The OPEs contain either thiol-based moieties or isonitrile groups to enable formation of SAMs on metal substrates.     

Diphenyl disulfide and sodium in NMP as an efficient protocol for in situ generation of thiophenolate anion: selective deprotection of aryl alkyl ethers and alkyl/aryl esters under nonhydrolytic conditions

Aryl methyl ethers, methyl esters, aryl esters, and aryl sulfonates are chemoselectively deprotected under nonhydrolytic conditions by treatment with Ph(2)S(2) (0.6 equiv) and Na (1.6 equiv) in NMP under reflux or at 90 degrees C. Quantitative utilization of the 'PhS' moiety as the effective nucleophilic species represents conservation of atom economy. Other solvents such as HMPA, DMPU, DMEU, and DMF afforded comparable results. Chloro, nitro, aldehyde, alpha,alpha-diketone, and alpha,beta-unsaturated ketone functionalities remain unaffected. The deprotection was found to take place in the order aryl ester > alkyl ester > aryl alkyl ether. Substrates bearing strong electron-withdrawing groups react at a faster rate than those not having such substitution. The differences in rate of reaction has been exploited for selective deprotection for intramolecular competition. An aryl acetate/benzoate is deprotected selectively in preference to a methyl ester or aryl methyl ether. Selective deprotection of a methyl ester is observed in the presence of an aryl alkyl ether.     

Self‐Templated Assembly of AuI/AgI‐Thiolate Sheets with Central Holes

Composite crystalline sheets of Au^I/Ag^I‐thiolate with central holes are achieved by co‐assembly of Ag^I‐thiolate and Au^I‐thiolate in one‐pot without sacrificial template. Both Ag^I‐thiolate and Au^I‐thiolate can separately assemble to lamellar sheets with similar structures, which makes their co‐assembly possible, while the differences in their assembly pathways make the co‐assembly processes highly dynamic and complex. First, a core@shell structure with Ag^I‐thiolate at the core was formed upon the mixing of the two, then the core@shell structure transformed to a hole@shell structure by dissociation of the core. Finally, some instable hole@shell structures further dissociated and grew on stable ones to generate holed Au^I/Ag^I‐thiolate composite sheets, in which the two components neither have severe phase separation nor blend uniformly at atomic level. By tuning the feeding ratios, the average diameter of the holes can be controlled. Therefore, the work demonstrates the advantage of co‐assembly technique in obtaining complex structurers. The holed sheets can further assemble to porous macroscopic materials and transform to composite metal nanoparticles by pyrolysis.     

3- and 3,4-bis(2-cyanoethylsulfanyl)thiophenes as building blocks for functionalized thiophene-based pi-conjugated systems

The facile access to 3- and 3,4-bis(2-cyanoethylsulfanyl)thiophenes allows the straightforward synthesis of 3-alkylsulfanyl and symmetrical or unsymmetrical 3,4-dialkylsulfanylthiophenes by using a methodology based on a highly efficient sequence of deprotection/alkylation reactions of thiolate group(s).     

X-ray absorption spectroscopy to probe surface composition and surface deprotection in photoresist films

Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) is utilized to provide insight into surface chemical effects in model photoresist films. First, NEXAFS was used to examine the resist/air interface including surface segregation of a photoacid generator (PAG) and the extent of surface deprotection in the film. The concentration of PAG at the resist-air interface was higher than the bulk concentration, which led to a faster deprotection rate at that interface. Second, a NEXAFS depth profiling technique was utilized to probe for compositional gradients in model resist line edge regions. In the model line edge region, the surface composition profile for the developed line edge was dependent on the post exposure bake time.     

Reactions Coupled Self‐ and Co‐Assembly: A Highly Dynamic Process and the Resultant Spatially Inhomogeneous Structure

Reactions coupled self‐assembly represents a step forward towards biomimetic behavior in the field of supramolecular research. Here, two pH‐dependent reactions of thiol‐disulfide exchange and ligand exchange were used to couple with the self‐assembly of an Au^I‐thiolate coordination polymer consisting of two ligands. Thanks to the comparable rates between the reactions and self‐assembly, the compositions of the assemblies change continuously with time, resulting in a highly dynamic assembly process and spatially inhomogeneous structure that are very common in life systems but cannot be easily obtained with one‐pot artificial methods.     

液液提取-固相萃取-高效液相色谱-串联质谱测定人体血液中16种有机磷酸酯

人体体液中有机磷酸酯(OPEs)浓度的测定对于了解人体OPEs的暴露水平以及评估人体健康风险具有重要意义.然而,目前的研究大多数集中于尿液中OPEs代谢物含量的分析测定,将其作为人体OPEs暴露的生物标志物,而对人体血液中OPEs的分析研究较少,仅有的少量研究涉及的OPEs种类有限.该研究在优化前处理过程(固相萃取,S...     

Excess fibrinogen adsorption to monolayers of mixed lipids

Adsorption of fibrinogen to the monolayers of mixed lipids, dipalmitoyl phosphatidyl choline (DPPC) and eicosylamine (EA) was measured at a surface pressure of 20 mN/m by an in situ surface plasmon resonance technique. Pressure–area isotherms of DPPC + EA mixtures on water and buffer subphases indicated good lipid miscibility and some contraction of the monolayers at intermediate and higher surface pressures. Surface electric potential of the DPPC + EA monolayers showed excess values for intermediate DPPC:EA ratios. Fibrinogen adsorption and its adsorption rates from a dilute solution (0.03 mg/ml) were proportional to the fraction of EA in the monolayer indicating that protein binding was primarily driven by electrostatic interactions between positive EA charges in the monolayer and a net negative protein charge. At a higher protein concentration (0.06 mg/ml) both the fibrinogen adsorbed amount and its maximum adsorption rate showed excess values relative to the pure EA for 1:1, 2:1 and 3:1 DPPC + EA monolayers. This excess adsorption could be explained, in part, by the contraction of the monolayers with intermediate DPPC:EA ratios which resulted in an excess surface electric potential.     

Effect of polymer chain length on membrane perturbation activity of cationic phenylene ethynylene oligomers and polymers

The interactions of poly(phenylene ethynylene)- (PPE-) based cationic conjugated polyelectrolytes (CPEs) and oligo(phenylene ethynylene)s (OPEs) with different model lipid membrane systems were investigated to gain insight into the relationship between molecular structure and membrane perturbation ability. The CPE and OPE compounds exhibit broad-spectrum antimicrobial activity, and cell walls and membranes are believed to be their main targets. To better understand how the size, in terms of the number of repeat units, of the CPEs and OPEs affects their membrane disruption activities, a series of PPE-based CPEs and OPEs were synthesized and studied. A number of photophysical techniques were used to investigate the interactions of CPEs and OPEs with model membranes, including unilamellar vesicles and lipid monolayers at the air/water interface. CPE- or OPE-induced dye leakage from vesicles reveals that the CPEs and OPEs selectively perturb model bacterial membranes and that their membrane perturbation abilities are highly dependent on molecular size. Consistent with dye-leakage assay results, the CPEs and OPEs also exhibit chain-length-dependent ability to insert into 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG) monolayers. Our results suggest that, for PPE-based CPE and OPE antimicrobials, chain length can be tuned to optimize their membrane perturbation ability.