An STM study on nonionic fluorosurfactant zonyl FSN self-assembly on Au(111): large domains, few defects, and good stability

Self-assembled monolayer formation of pentamers-like molecules onto FCC(111) surfaces: the case of curcuminoids onto Au(111) surface

The adsorption of rigid straight electrically polarized pentamers over a FCC(111) surface is studied. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and simulated under the frame of the density functional theory. We now obtain and report the charge distribution of the molecule which allows to propose a deposition model followed by Monte Carlo simulations over an ad-hoc lattice gas model. We show that for a certain value of the chemical potential there exists an isotropic-nematic phase transition which can explain the formation of a self-assembled monolayer like the one observed in the transmission electron microscopy images. An order parameter is defined to characterize the transition which presents a step-like behavior at a critical chemical potential value. The possible nature of the nematic transition in conjunction with an ergodicity breakdown is discussed as future work by means of statistical physics techniques.

Coalescence Dynamics of Mobile and Immobile Fluid Interfaces

Coalescence dynamics between deformable bubbles and droplets can be dramatically affected by the mobility of the interfaces with fully tangentially mobile bubble-liquid or droplet-liquid interfaces expected to accelerate the coalescence by orders of magnitude. However, there is a lack of systematic experimental investigations that quantify this effect. By using high speed camera imaging we examine the free rise and coalescence of small air-bubbles (100 to 1300 μm in diameter) with a liquid interface. A perfluorocarbon liquid, PP11, is used as a model liquid to investigate coalescence dynamics between fully mobile and immobile deformable interfaces. The mobility of the bubble surface was determined by measuring the terminal rise velocity of small bubbles rising at Reynolds numbers, Re, less than 0.1 and the mobility of free PP11 surface by measuring the deceleration kinetics of the small bubble toward the interface. Induction or film drainage times of a bubble at the mobile PP11-air surface were found to be more than 2 orders of magnitude shorter compared to the case of bubble and an immobile PP11-water interface. A theoretical model is used to illustrate the effect of hydrodynamics and interfacial mobility on the induction time or film drainage time. The results of this study are expected to stimulate the development of a comprehensive theoretical model for coalescence dynamics between two fully or partially mobile fluid interfaces.

Electrochemical Oxidation of Hydrogen in Bis(trifluoromethylsulfonyl)imide Ionic Liquids under Anaerobic and Aerobic Conditions

The electrochemical behavior of hydrogen oxidation on a platinum electrode in two aprotic room temperature ionic liquids (RTILs)-1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][NTf₂] and 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide [Bmpy][NTf₂]-was investigated in both anaerobic and aerobic conditions. At platinum electrode in the ILs, the first step of hydrogen oxidation is the formation of Pt-H(ad) (the Tafel step), which is similar to those observed in the aqueous electrolytes. However, there are differences in the oxidation steps (the Heyrovsky and Volmer steps). In ILs, the oxidation of Pt-H(ad) forms a hydrogen radical and a proton rather than a proton or a water in aqueous acid or alkaline electrolytes, respectively. This difference is significant as it results in a completely different following reaction pathway in the anaerobic vs aerobic conditions. A coupled chemical reaction between oxygen and hydrogen oxidation intermediates was observed in aerobic conditions which has a correlation with hydrogen concentrations. Furthermore, the overall rate of hydrogen oxidation is shown to be much higher in [Bmpy][NTf₂] than that of [Bmim][NTf₂], which is rationalized as the result of both higher solubility of hydrogen and the unique IL-electrode interface structure which promotes the hydrogen adsorption in [Bmpy][NTf₂] than that of [Bmim][NTf₂]. This study is the first example showing that hydrogen oxidation mechanism in aprotic ILs follows two different oxidation mechanisms in anaerobic and aerobic conditions.

Facet effects of palladium nanocrystals for oxygen reduction in ionic liquids and for sensing applications

Palladium nanocrystals enclosed by {100} and {110} crystal facets, were successfully synthesized through an aqueous one-pot synthesis method. A new thermal annealing approach was developed for fabricating these palladium nanocrystals as a working electrode on a gas permeable membrane to study the facet effects of the oxygen reduction process in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpy][NTf2]). Results were compared with the same processes at a conventional platinum electrode. Our study shows that the structural difference between the two facets of Pd nanocrystals has little effect on the oxygen reduction process but significantly affects the oxidation process of the superoxide. It is found that the Pd{110}/IL interface can better stabilize superoxide radicals revealed by a more positive oxidation potential compared to that of Pd{100}. In addition, the analytical characteristic of utilizing both palladium nanocrystals as electrodes for oxygen sensing is comparable with a polycrystal platinum oxygen sensor, in which Pd{110} presents the best sensitivity and lowest detection limit. Our results demonstrate the facet-dependence of oxygen reduction in an ionic liquid medium and provide the fundamental information needed to guide the applications of palladium nanocrystals in electrochemical gas sensor and fuel cell research.

Exploitation of desilylation chemistry in tailor-made functionalization on diverse surfaces

Interface engineering to attain a uniform and compact self-assembled monolayer at atomically flat surfaces plays a crucial role in the bottom-up fabrication of organic molecular devices. Here we report a promising and operationally simple approach for modification/functionalization not only at ultraflat single-crystal metal surfaces, M(111) (M=Au, Pt, Pd, Rh and Ir) but also at the highly oriented pyrolytic graphite surface, upon efficient in situ cleavage of trimethylsilyl end groups of the molecules. The obtained self-assembled monolayers are ultrastable within a wide potential window. The carbon-surface bonding on various substrates is confirmed by shell-isolated nanoparticle-enhanced Raman spectroscopy. Application of this strategy in tuning surface wettability is also demonstrated. The most valuable finding is that a combination of the desilylation with the click chemistry represents an efficient method for covalent and tailor-made functionalization of diverse surfaces.

Nonionic fluorosurfactant as an ideal candidate for one-step modification of gold nanorods

In this work, a novel protocol was developed for size tuning and cetyltrimethylammonium bromide (CTAB) removal of gold nanorods using commercially available nonionic fluorosurfactants (FSN), an excellent candidate for PEG and other modification reagents. The tunable gold nanorods can easily be obtained by stopping the ligand replacement reaction at different time intervals. The FSN-coated gold nanorods are stable in the presence of high salt concentrations and over a wide range of pH values. Additionally, the cellular uptake experiments demonstrate that the FSN-coated gold nanorods have superior features in comparison to the widely used PEG-coated gold nanorods, such as high uptake amount, tunable uptake and excellent stability. Our findings suggest that FSN ligands are an ideal candidate for modifying gold nanorods with tunable aspect ratios, excellent biocompatibility, nontoxicity, and high stability, enabling conjugation to biomolecules for specific targeting, uptake, and delivery.

High coating of Ru(II) complexes on gold nanoparticles for single particle luminescence imaging in cells

Gold nanoparticles are efficiently labelled with a luminescent ruthenium complex, producing 13 and 100 nm diameter, monodisperse red-emissive imaging probes with luminescence lifetimes prolonged over the molecular unit. Single, 100 nm particles are observed in whole cell luminescence imaging which reveals their biomolecular association with chromatin in the nucleus of cancer cells.

Sequence-selective recognition of nucleic acids under extremely low salt conditions using nanoparticle probes

Extensive secondary structures in nucleic acid targets seriously impede the binding of complementary oligonucleotide probes. We report here a method to conduct the detection under extremely low salt conditions where the secondary structures are less stable and more accessible. A new type of nanoparticle probes prepared by functionalizing gold nanoparticles with nonionic morpholino oligos is employed. Because of the salt-independent hybridization of the probes with nucleic acid targets, nanoparticle assemblies can be formed in 2 mM Tris buffer solutions containing 0-5 mM NaCl, leading to the colorimetric target recognition. The sharp melting transitions of the target-probe hybrids allow discrimination of single-base imperfection, including substitution, deletion, and insertion. The method works effectively in detecting sequences that are likely to form secondary structure. In addition, the study provides direct evidence of the relationship between the aggregate structure and the melting behavior of the DNA-linked nanoparticles.

Comparative electrochemical scanning tunneling microscopy study of nonionic fluorosurfactant zonyl FSN self-assembled monolayers on Au(111) and Au(100): a potential-induced structural transition

We investigate the structure of nonionic fluorosurfactant zonyl FSN self-assembled monolayers on Au(111) and Au(100) in 0.05 M H(2)SO(4) as a function of the electrode potential by electrochemical scanning tunneling microscopy (ECSTM). On Au(111), a (3(1/2) × 3(1/2))R30° arrangement of the FSN SAMs is observed, which remains unchanged in the potential range where the redox reaction of FSN molecules does not occur. On Au(100), some parallel corrugations of the FSN SAMs are observed, which originate from the smaller distance and the repulsive interaction between FSN molecules to make the FSN molecules deviate from the bridging sites, and ECSTM reveals a potential-induced structural transition of the FSN SAMs. The experimental observations are rationalized by the effect of the intermolecular interaction. The smaller distance between molecules on Au(100) results in the repulsive force, which increases the probability of structural change induced by external factors (i.e., the electrode potential). The appropriate distance and interactions of FSN molecules account for the stable structure of FSN SAMs on Au(111). Surface crystallography may influence the intermolecular interaction through changing the molecular arrangements of the SAMs. The results benefit the molecular-scale understanding of the behavior of the FSN SAMs under electrochemical potential control.

Fluorosurfactant-capped gold nanoparticles-enhanced chemiluminescence from hydrogen peroxide-hydroxide and hydrogen peroxide-bicarbonate in presence of cobalt(II)

Nonionic fluorosurfactant (FSN)-capped gold nanoparticles (GNPs) remain excellently stable at a wider pH range and high ionic strength, which is useful to investigate some CL systems involved in high salt and a strict pH range. In this study, we utilized FSN-capped GNPs of different sizes to distinguish the emitting species from H2O2-Co2+-NaOH and H2O2-Co2+-NaHCO3 systems. When the pH of FSN-capped gold colloidal solution was adjusted to 10.2 by dropwise addition of 0.05 M NaOH, the CL intensity of H2O2-Co2+-NaHCO3 system was enhanced 6-fold or 60-fold respectively in the presence of FSN-capped 14 nm or 69 nm GNPs with comparison to H2O2-Co2+-NaOH. The variation of CL spectra and UV-vis spectra, as well as the quenching effect of reactive oxygen species scavengers were studied in detail to understand the CL enhancement mechanisms of FSN-capped GNPs on the two systems. For H2O2-Co2+-NaOH system, the gold(I) complexes intermediate and singlet oxygen dimol species were proposed as the emitting species. The excited states of the carbon dioxide dimers and singlet oxygen dimol species were considered responsible for the light emission of H2O2-Co2+-NaHCO3 system. To our knowledge, this work is the first time to study the two CL systems simultaneously using nanoparticles.

STM study on nonionic fluorosurfactant zonyl FSN Self-Assembly on Au(100): [array: see text] molecular lattice, corrugations, and adsorbate-enhanced mobility

Nonionic fluorosurfactant zonyl FSN self-assembly on Au(100) is investigated by using scanning tunneling microscopy under ambient conditions. High-resolution STM images reveal that a [array: see text] arrangement of the FSN SAMs is formed on Au(100). Different from the uniform structure of FSN SAMs on Au(111), the adsorption sites of FSN molecules on Au(100) change gradually and form a kind of corrugated structure. The change in the adsorption sites probably originates from the repulsive force among FSN molecules because the nearest-neighbor distance of FSN molecules is 0.41 nm, which is smaller than 0.50 nm on Au(111). The mobility of surface atoms on the Au substrate is enhanced by the interaction between FSN molecules and the Au substrate; therefore, no Au island is observed on the FSN-SAM-covered Au(100).

TiO2 thin films self-assembled with a partly fluorinated surfactant template

New TiO(2) films have been self-assembled on solid substrate by dip-coating using TiCl(4) as the titanium source and the partly fluorinated surfactant F(CF(2))(8)C(2)H(4)(OC(2)H(4))(9)OH as the liquid crystal template. By control over the dip-withdrawal speed, film thicknesses from a minimum of 43 nm were produced with rms roughnesses of 0.5-0.7 nm. The films were characterized by X-ray reflectivity, grazing incidence small-angle X-ray scattering, atomic force microscopy, contact angle measurements, and Raman spectroscopy. Their GI-SAXS patterns are characteristic of a 2-D hexagonal structure in which tubular rods of the fluorinated surfactant are packed hexagonally and aligned parallel to the substrate. Reflectivity and contact angle measurements of the as-prepared film indicate that a low-density hydrophilic TiO(2) surface presents to the air.