Temporal evolution of benzenethiolate SAMs on Cu(100)

The structure and thermal stability of self-assembled monolayers (SAMs) of benzenethiolate (BT) on Cu(100) have been studied by means of thermal desorption spectroscopy (TDS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray adsorption fine structure spectroscopy (NEXAFS). Vapor deposition at room temperature yields a well-ordered, densely packed c(6 × 2) saturation structure. At room temperature, this film is, however, metastable and transforms via partial decomposition by cleavage of the S-C bond into a less densely packed layer that reveals a coexisting p(2 × 2) phase. Such a transition occurs on a time scale of several days and is accompanied by a reduction of the work function change with respect to the bare Cu(100) surface from Δ? = -0.9 eV for a freshly prepared saturated layer to -0.5 eV for an aged film. TDS experiments exhibit the presence of two distinct desorption channels (dissociative and intact desorption) occurring at different temperatures that reflects a variation of the local Cu-S interaction strength of BT at differently coordinated adsorption sites. Heating to above room temperature causes a rapid degradation and continuous thinning of BT films whereas above 500 K all thiolate species have desorbed or dissociated, leaving a sulfide overlayer behind that is accompanied by a substrate reconstruction. Interestingly, the upright orientation of BT adopted in the saturated monolayer remains almost identical upon heating and demonstrates the absence of downward tilting upon thermally induced thinning of the film.     

Electron transfer of plurimodified DNA SAMs

An STM-based current-voltage (I/V) investigation of deoxyribonucleic acid (DNA) 18 base pair (bp) oligonucleotide monolayers on gold is presented. Three bases of each of the immobilized and complementary strands were modified with either iodine or phenylethylene moieties. The oligonucleotides were immobilized on template stripped gold (tsg) surfaces and characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). AFM imaging showed that monolayers of the expected height were formed. A comparative study of normal, halogenated, and phenyl-modified DNA was made with the STM in tunneling spectroscopy (TS) mode. I/V spectroscopic measurements in the range +/-250 mV on both single- and double-stranded (ds) DNA monolayers (modified and unmodified) showed that for negative substrate bias (U(sub)) electron transfer is more efficient through a phenyl-modified monolayer than through normal or halogenated DNA. This effect was particularly clear below a threshold bias of -100 mV. For positive U(sub), unmodified ds DNA was found to conduct slightly better than the modified strands. This is presumably caused by greater order in the unmodified versus modified DNA monolayers. Modifications on the immobilized (thiolated) strand seem to improve electron transport through the DNA monolayer more than modifications on the complementary (not surface-bound) strand.     

Controlling DNA orientation on mixed ssDNA/OEG SAMs

We present and characterize a mixed self-assembled monolayer (SAM) consisting of single-stranded oligonucleotide (ssDNA)- and oligo(ethylene glycol) (OEG)-terminated thiols. The ssDNA/OEG SAMs are prepared by simultaneous coadsorption from a common thiol solution over a broad range of compositions. Electron spectroscopy for chemical analysis (ESCA) is used to measure the surface coverage of ssDNA, whereas surface plasmon resonance (SPR) sensor is used to measure the hybridization of complementary ssDNA and protein resistance. Through the complementary use of these techniques, we find that the composition of OEG in the assembly solution controls a key parameter: the surface coverage of ssDNA on the surface. There is evidence that it influences the orientation of the immobilized ssDNA probes. Lower OEG concentrations yield a surface with higher ssDNA coverage and less favorable orientation, whereas higher OEG concentrations produce a surface with lower DNA coverage and more favorable orientation. Competition between these two effects controls the hybridization efficiency of the ssDNA surface. Compared to ssDNA surfaces prepared with other diluent thiols, the use of OEG improves the protein resistance of the surface, making it more broadly applicable.     

Deposition of cationic polymer micelles on planar SiO2 surfaces

The deposition of PS-PVPH+ polymer micelles from a pH 1 aqueous solution onto Si wafers has been studied using a simple dip-coating technique. It has been found that the rate of evaporation of the solvent and the rate of withdrawal have a considerable influence on the density and ordering of the adsorbed micelles. The highest density and degree of ordering (as judged by the 2D pair correlation function) is achieved when solvent evaporation dominates the deposition process, but a fairly homogeneous distribution of polymer micelles can be achieved over a distance of at least 3-4 mm by controlling the solvent evaporation rate and the rate of substrate withdrawal. We did not observe any significant effect of added KCl (up to 0.1 M) during the deposition process or soaking in 1 M KCl after deposition. The attachment of these micelles is quite robust, as they cannot be washed off in pH 1 water (with or without KCl) without significant mechanical assistance. However, we did find that the micelles are rather easily caused to dewet and partially aggregate under the influence of 65 degrees C water vapor.     

Gold nanoparticle decoration of DNA on silicon

Electrostatic assembly of cationic nanoparticles onto the negatively charged backbone of double-stranded DNA has been shown to produce one-dimensional chains with potential use as nanoelectronic components. In this paper, micron long DNA templates stretched on aminosilane- and hexamethyldisilazane-modified silicon surfaces are used to assemble 3.5 nm gold nanoparticles passivated with cationic thiocholine. Atomic force microscopy is used to analyze the density and defects along the approximately 5 nm high structures, with comparison between positively charged and neutral surfaces. Low background adsorption of nanoparticles is facilitated by both these surface chemistries, while the neutral surface yields a more densely packed assembly.     

Deposition behaviors and patterning of TiO2 thin films on different SAMs surfaces from titanium sulfate aqueous solution

Patterning of TiO₂ thin films was successfully obtained on different self-assembled monolayers (SAMs) in aqueous solution by micro-contact printing (μCP) method. The substrates were immersed in an aqueous solution containing titanium sulfate (Ti(SO₄)₂) and hydrogen peroxide for deposition at 80 °C. The growth rates on various surfaces were as follows: sulfonic (–SO₃H) > amino (–NH₂) > methyl (–CH₃) > hydroxyl (–OH). According to the XPS results, SAMs with the terminal groups of –SO₃H and –NH₂ were favorable for the deposition. The TiO₂ film deposited on the SAM with the terminal group of –CH₃ could be easily peeled off. Clearly, TiO₂ patterns were obtained on the prepatterned surfaces of –SO₃H/–CH₃ and –NH₂/–CH₃ SAMs. The deposition mechanism might be relevant to electrostatic interaction, Stern layer, lone pair electrons and Van der Vaals forces. The TiO₂ film was anatase after annealing at 500 °C and comprised particles with an average diameter of ca. 10 nm.     

Ion permeability of SAMs on nanoparticle surfaces

This article reports on the ion permeability of self-assembled monolayers (SAMs) formed on the surface of charged alkanethiol-protected gold nanoparticles, so-called monolayer-protected clusters (MPCs). The capacitance and thus the charging energy required to add/remove an electron from the metal core are extremely sensitive to ions entering the monolayer, and the extent of ion penetration can be tuned by the charge and size of the ions and the permittivity of the solvent. Experimentally, this effect is comparable to ion association with conventional redox molecules, indicating that MPCs despite their large size and the fundamentally differing nature of the electron transfer process can be treated analogously to redox molecules.     

Density control of dodecamanganese clusters anchored on silicon(100)

A synthetic strategy to control the density of Mn12 clusters anchored on silicon(100) was investigated. Diluted monolayers suitable for Mn12 anchoring were prepared by Si-grafting mixtures of the methyl 10-undecylenoate precursor ligand with 1-decene spectator spacers. Different ratios of these mixtures were tested. The grafted surfaces were hydrolyzed to reveal the carboxylic groups available for the subsequent exchange with the [Mn12O12(OAc)16(H2O)4]4 H2O2 AcOH cluster. Modified surfaces were analyzed by attenuated total reflection (ATR)-FTIR spectroscopy, X-ray photoemission spectroscopy (XPS), and AFM imaging. Results of XPS and ATR-FTIR spectroscopy show that the surface mole ratio between grafted ester and decene is higher than in the source solution. The surface density of the Mn12 cluster is, in turn, strictly proportional to the ester mole fraction. Well-resolved and isolated clusters were observed by AFM, using a diluted ester/decene 1:1 solution.     

Pd在p型单晶硅(100)表面自催化化学沉积

研究了Pd在氢终止的p型单晶硅(100)表面的自催化化学沉积(AED). 在室温下将刻蚀过的硅片浸入常规的HF-PdCl2-HCl溶液制备了Pd膜. 将沉积了Pd的基底作为工作电极, 用循环伏安法(CV)、原子力显微镜(AFM)和X射线光电子能谱(XPS)研究了Pd膜的阳极溶出行为和形貌. 结果表明, Pd的生长遵循Volmer-Weber (VW)生长模式, Pd膜给出了很好的支持.     

DNA microarrays on silicon surfaces through thiol-ene chemistry

The potential of thiol-ene chemistry as a selective strategy to functionalize silicon materials for DNA microarraying is demonstrated and applied to discriminate genetic variations.     

Encapsulation of DNA by cationic diblock copolymer vesicles

Encapsulation of dsDNA fragments (contour length 54 nm) by the cationic diblock copolymer poly(butadiene-b-N-methyl-4-vinyl pyridinium) [PBd-b-P4VPQ] has been studied with phase contrast, polarized light, and fluorescence microscopies, as well as scanning electron microscopy. Encapsulation was achieved with a single emulsion technique. For this purpose, an aqueous DNA solution is emulsified in an organic solvent (toluene) and stabilized by the amphiphilic diblock copolymer. The PBd block forms an interfacial brush, whereas the cationic P4VPQ block complexes with DNA. A subsequent change of the quality of the organic solvent results in a collapse of the PBd brush and the formation of a capsule. Inside the capsules, the DNA is compacted as shown by the appearance of birefringent textures under crossed polarizers and the increase in fluorescence intensity of labeled DNA. The capsules can also be dispersed in an aqueous medium to form vesicles, provided they are stabilized with an osmotic agent [poly(ethylene glycol)] in the external phase. It is shown that the DNA is released from the vesicles once the osmotic pressure drops below 10(5) N/m(2) or if the ionic strength of the supporting medium exceeds 0.1 M. The method has also proven to be efficient to encapsulate pUC18 plasmid in submicrometer-sized vesicles, and the general applicability of the method has been demonstrated by the preparation of the charge inverse system: cationic poly(ethylene imine) encapsulated by the anionic diblock poly(styrene-b-acrylic acid).     

Surface complexation of DNA with a cationic surfactant

We have studied the surface complexation of DNA with a cationic surfactant (DTAB) using a combination of methods: dynamic surface tension, ellipsometry and Brewster angle microscopy. Below the surfactant critical aggregation concentration (cac), complexation occurs only at the surface, and the results are consistent with neutralization of the surfactant charges by the free polymer ions. Above the cac, surfactant starts to bind cooperatively to DNA in the bulk, and adsorption of the preformed hydrophobic surfactant DNA aggregate is now possible, leading to thick surface layers. At still higher concentrations of surfactant (still below saturation of binding in the bulk), there is decrease in adsorption due to competition with bulk aggregates. Finally, as surfactant concentration is increased still further, bulk aggregates become less soluble and large amounts are adsorbed, forming a surface layer, which is solid-like and brittle.     

Electrostatics of DNA complexes with cationic lipid membranes

We present the exact solutions of the linear Poisson-Boltzmann equation for several problems relevant to electrostatics of DNA complexes with cationic lipids. We calculate the electrostatic potential and electrostatic energy for lamellar and inverted hexagonal phases, concentrating on the effects of dielectric boundaries. We compare our results for the complex energy with the known results of numerical solution of the nonlinear Poisson-Boltzmann equation. Using the solution for the lamellar phase, we calculate the compressibility modulus and compare our findings with the experimental data available. Also, we treat charge-charge interactions across, along, and between two low-dielectric membranes. We obtain an estimate for the strength of electrostatic interactions of one-dimensional DNA smectic layers across the lipid membrane. We discuss in the end some aspects of two-dimensional DNA condensation and DNA-DNA attraction in the DNA-lipid lamellar phase in the presence of di- and trivalent cations. We analyze the equilibrium DNA-DNA separations in lamellar complexes using the recently developed theory of electrostatic interactions of DNA helical charge motifs.     

The first observation of four-electron reduction in [60]fullerene-metal cluster self-assembled monolayers (SAMs)

Self-assembled monolayers (SAMs) of a mu 3-eta 2:eta 2:eta 2-C60 triosmium cluster complex Os3(CO)8(CN(CH2)3Si(OEt)3)(mu 3-eta 2:eta 2:eta 2-C60) (2) on ITO or Au surface exhibit ideal, well-defined electrochemical responses and remarkable electrochemical stability being reducible up to tetranionic species in their cyclic voltammograms.     

Oxidation of the (100) face of crystalline silicon

The density functional theory was used to calculate the equilibrium structures of clusters serving as models of the sequence of reaction steps in the oxidation of the (100) face of crystalline silicon and their relative heat effects. The formation of all the intermediates on the (100) face proceeds without activation energy, suggesting the feasibility of avalanche-like formation of an SiO₂ film. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 4, pp. 257–261, July–August, 2007.     

Amphipathic and membrane-destabilizing properties of the cationic acrylate polymer Eudragit E100

The cationic acrylate polymer Eudragit E100 (E100) produces a biphasic effect on the stability of casein micelles disrupting their internal structure. These results suggested that this polymer could have some amphipathic character. Therefore, in this study the polymer was characterized with respect to its interaction with different amphipathic systems (bile-acid micelles, lipoproteins and liposomes), cell membranes (red blood cells) and virus membranes (Herpes simplex type 2 virus). As with caseins, a biphasic effect was observed with bile acids with a precipitation phase at low polymer/bile acid ratio and a solubilization phase when the polymer concentration was increased. Upon interaction with human plasma, an important reduction in cholesterol and triglycerides was observed upon remotion of E100 by a rise in pH to 8.5 and centrifugation. In agreement with this finding, an important reduction in plasma lipoproteins was observed upon its treatment with E100 and further remotion by pH rise and centrifugation. However, the amount of the major protein components of human plasma and the activity of several enzymes and antibodies were not affected by their treatment with E100. The membrane-destabilizing properties of E100 were confirmed by its lytic activity on liposomes and red blood cells and by an important antiviral effect of E100 on Herpes simplex virus type 2. Altogether, these results show that, despite its water solubility and cationic character, E100 displays a significative amphipathic and membrane-destabilizing character with potential biotechnological applications. [diagram in text].     

Assessing the influence of pH and cationic strength on i-motif DNA structure

The i-motif is a biologically relevant non-canonical DNA structure formed by cytosine-rich sequences. Despite the importance of the factors affecting the formation/stability of such a structure, like pH, cation type and concentration, no systematic study that simultaneously analysed their effect on the i-motif in vitro has been carried out so far. Therefore, here we report a systematic study that aims to evaluate the effect of these factors, and their possible interaction, on the formation of an i-motif structure. Our results confirm that pH plays the main role in i-motif formation. However, we demonstrate that the effect of the cation concentration on the i-motif is strictly dependent on the pH, while no significant differences are observed among the investigated cation types.

Graphical abstract

     

Virus-sized DNA nanoparticles for gene delivery based on micelles of cationic calixarenes

Macrocyclic amphiphilic molecules based on calix[4]arenes are highly attractive for controlled supramolecular assembly of DNA into small nanoparticles, since they present a unique conical architecture and can bear multiple charged groups. In the present work, we synthesized new amphiphilic calixarenes bearing cationic groups at the upper rim and alkyl chains at the lower rim. Their self-assembly in aqueous solution was characterized by fluorescent probes, fluorescence correlation spectroscopy, dynamic light scattering, gel electrophoresis and atomic force microscopy. We found that calixarenes bearing long alkyl chains (octyl) self-assemble into micelles of 6 nm diameter at low critical micellar concentration and present the unique ability to condense DNA into small nanoparticles of about 50 nm diameter. In contrast, the short-chain (propyl) analogues that cannot form micelles at low concentrations failed to condense DNA, giving large polydisperse DNA complexes. Thus, formation of small DNA nanoparticles is hierarchical, requiring assembly of calixarenes into micellar building blocks that further co-assemble with DNA into small virus-sized particles. The latter showed much better gene transfection efficiency in cell cultures relative to the large DNA complexes with the short-chain analogues, which indicates that gene delivery of calixarene/DNA complexes depends strongly on their structure. Moreover, all cationic calixarenes studied showed low cytotoxicity. Thus, this work presents a two-step hierarchical assembly of small DNA nanoparticles for gene delivery based on amphiphilic cone-shaped cationic calixarenes.     

Influence of applied potential on the impedance of alkanethiol SAMs

Self-assembled monolayers are generally considered to behave as dielectric layers with a capacitance that is dependent on the monolayer thickness and the relative permittivity that is determined by the hydrocarbon tail. We show that the impedance response of alkanethiol-modified gold surfaces can be modeled as a parallel network consisting of the capacitance and resistance of the monolayer over a wide potential range. At potentials positive to -0.3 V (Ag/AgCl), the monolayer resistance is greater than 10(6) Omega cm2; however, at more negative potentials, the monolayer resistance decreases exponentially with potential with an inverse slope of about 250 mV. Over the same potential range, the monolayer capacitance is independent of potential. Although the same behavior is observed on ultrasmooth, template-stripped gold, the resistance at any potential is larger than for evaporated gold. The progressive increase in permeability of the monolayer is associated with an increase in electric field at potentials negative to the potential of zero charge.     

Modulation of photo-oxidative DNA damage by cationic surfactant complexation

The natural packaging of DNA in the cell by histones provides a particular environment affecting its sensitivity to oxidative damage. In this work, we used the complexation of DNA by cationic surfactants to modulate the conformation, the dynamics, and the environment of the double helix. Photo-oxidative damage initiated by benzophenone as the photosensitizer on a plasmid DNA complexed by dodecyltrimethylammonium chloride (DTAC), tetradecyltrimethylammonium chloride (TTAC), cetyltrimethyammonium chloride (CTAC) and bromide (CTAB) was detected by agarose gel electrophoresis. By fluorescent titration in the presence of ethidium bromide (EB) and agarose gel electrophoresis, we experimentally confirmed the complexation diagrams with a critical aggregation concentration on DNA matrix (CAC DNA) delimiting two regions of complexation, according to the DNA-phosphate concentration. The study of the photo-oxidative damage shows, for the first time, a direct correlation between the DNA complexation by these surfactants and the efficiency of DNA cleavage, with a maximum corresponding to the CAC DNA for DTAC and CTAC, and to DNA neutralization for CTAC and CTAB. The localization of a photosensitizer having low water solubility, such as benzophenone, inside the hydrophobic domains formed by the surfactant aggregated on DNA, locally increases the photoinduced cleavage by the free radical oxygen species generated. The inefficiency of a water-soluble quencher of hydroxyl radicals, such as mannitol, confirmed this phenomenon. The detection of photo-oxidative damage constitutes a new tool for investigating DNA complexation by cationic surfactants. Moreover, highlighting the drastically increased sensitivity of a complexed DNA to photo-oxidative damage is of crucial importance for the biological use of surfactants as nonviral gene delivery systems.