Mercury adsorption on gold surfaces employed in the sampling and determination of vaporous mercury: a scanning tunneling microscopy study

To clarify the mechanism of mercury adsorption on gold surfaces thin epitaxial gold films have been exposed to trace amounts of gaseous mercury under laboratory conditions for different periods of time. The changes in the surface morphology of the thin films caused by the exposure have been studied by scanning tunneling microscopy (STM). The evolution of the surface structures with time has been also investigated, in the course of a few days after the exposure. The adsorption of mercury on the gold surfaces has caused drastic changes in the morphology of the surfaces. Pits and islands of 2 to 30 nm in diameter have appeared on the surface, their size and density per unit area depending on the amount of exposure to mercury. The formation of pits and islands followed a certain path of events.     

A spectroscopic study of mercury vapor adsorption on gold nanoparticle films

We have modified the surfaces of glass and Si(100) with 3-aminopropyltrimethoxy silane, a fourth generation amine-terminated poly(amidoamine) dendrimer, and poly(diallydimethyl ammonium chloride) to facilitate adsorption onto colloidal gold particles (average diameter 3, 5, 12, and 22 nm). UV-vis absorption spectroscopy and atomic force microscopy monitored the adsorption process, which is governed by particle diffusion to the surface. The differences in adsorption to the three adhesion layers as a function of pH are discussed. Mercury vapor was exposed to the gold particle films and quantified by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. The surface plasmon oscillation of 5-, 12-, and 22-nm particles blue-shifts after exposure to parts-per-million levels of mercury vapor in air. Particle films prepared from the 3-nm gold particles develop a broad peak centered near 530 nm after exposure to mercury vapor. The results demonstrate a novel "litmus" film for mercury vapor.     

Adsorption of mercury on gold and silver surfaces

In order to study the adsorption mechanism of Hg on Au and Ag substrates, thin film Au(111) and Ag(111) substrates were exposed to gaseous metallic mercury, while the mercury concentration, substrate temperature, and exposure length were varied. The resulting changes in the surface morphology of the substrates were studied with scanning tunneling microscopy (STM). The amount of adsorbed Hg required to cause saturation, i.e. a decrease in the adsorption rate was found to be dependent on the mercury concentration and substrate temperature. The observations lead to the conclusion that the adsorption includes place exchange processes and concerted adsorption of more than one Hg atom in one process. The results show that the collection efficiency of single-crystalline surfaces is a function of both mercury concentration and temperature. Therefore, results from measurements performed at different conditions using single-crystalline surfaces may not be comparable. Received: 3 February 1999 / Revised: 7 June 1999 / Accepted: 9 June 1999     

Evaporation induced self-assembly of templated silica and organosilica thin films on various electrode surfaces

A new one-step method is reported for the deposition of hybrid mesoporous thin films on various electrode surfaces (gold, platinum, glassy carbon). Deposition was achieved by spin-coating sol–gel mixtures in the presence of a surfactant template to get mesostructured thin layers on the various conducting substrates. Film formation occurred by evaporation induced self-assembly (EISA) involving the hydrolysis and (co)condensation of silane and/or organosilane precursors on the electrode surface. Extraction of the surfactant from the ordered mesoporous films led to a large increase of mass transport rates into the materials and imparted high accessibility to the organic moieties in case of functionalized mesoporous overlayers. The electrochemical properties of the film-modified electrodes have been studied by cyclic voltammetry (CV), and also via the chemical accumulation of mercury ions prior to their stripping analysis by differential pulse voltammetry (i.e. for thiol-functionalized thin films). Some evidences to support the formation of self-assembled monolayers (SAMs) on electrodes, have been also discussed. The formation of well-adhering mesoporous thin films on solid electrode surfaces is expected to have a high impact on the development of new electrochemical sensors.     

水溶性聚合物和两亲聚合物——10.两亲聚合物PAMC_(16)S的自组装有序膜

用自组装技术在金(纯金和经阳极氧化的金)表面上获得了新型两亲聚合物PAMC_(16)S的有序膜。用接触角测试,XPS谱和电化学分析等方法对自组装膜进行了表征。根据膜表面的润湿性,金表面的自组装膜是疏水的,亲水的磺酸基团连于金表面,而疏水的碳氢链从表面伸展出。XPS实验结果支持金表面上单层膜的疏水结构。聚合物单层膜复盖的金电极起到含有针孔缺陷的阻膈型电极的作用。单层膜在法拉第反应中显示很强的吸附稳定性,说明聚合物LB膜在潜在应用中有其特有的特点。     

Surfactant adsorption onto cellulose surfaces

The adsorption of the cationic surfactant, hexadecyl trimethyl ammonium bromide, C16TAB, onto model cellulose surfaces, prepared by Langmuir-Blodgett deposition as thin films, has been investigated by neutron reflectivity. Comparison between the adsorption of C16TAB onto hydrophilic silica, a hydrophobic cellulose surface, and a regenerated (hydrophilic) cellulose surface is made. Adsorption onto the hydrophilic silica and onto the hydrophilic cellulose surfaces is similar, and is in the form of surface aggregates. In contrast, the adsorption onto the hydrophobic cellulose surface is lower and in the form of a monolayer. The impact of the surfactant adsorption and the in situ surface regeneration on the structure of the cellulose thin films and the nature of solvent penetration into the cellulose films are also investigated. For the hydrophobic cellulose surface, intermixing between the cellulose and surfactant occurs, whereas there is little penetration of surfactant into the hydrophilic cellulose surface. Measurements show that solvent exchange between the partially hydrated cellulose film and the solution is slow on the time scale of the measurements.     

Development of a biologically relevant calcium phosphate substrate for sum frequency generation vibrational spectroscopy

A novel biologically relevant composite substrate has been prepared consisting of a calcium phosphate (CaP) layer formed by magnetron sputter-coating from a hydroxyapatite (HA) target onto a gold-coated silicon substrate. The CaP layer is intended to mimic tooth and bone surfaces and allows polymers used in oral care to be deposited in a procedure analogous to that used for dental surfaces. The polymer cetyl dimethicone copolyol (CDC) was deposited onto the CaP surface of the substrate by Langmuir Blodgett deposition, and the structure of the adsorbed layer was investigated by the surface specific technique of sum frequency generation (SFG) vibrational spectroscopy. The gold sublayer provides enhancement of the SFG signal arising from the polymer but plays no part in the adsorption of the polymer. The surface morphology of the substrate was investigated using SEM and AFM. The surface roughness was commensurate with that of the thermally evaporated gold sublayer and uniform over areas of at least 36 mum(2). The chemical composition of the CaP-coated surface was determined by FTIR and TOF-SIMS. It was concluded that the surface is primarily calcium phosphate present as a mixture of amorphous, non-hydroxylated phases rather than solely stoichiometric hydroxyapatite. The SFG spectra from CDC on CaP were closely similar, both in resonance wavenumbers and in their relative intensities, with spectra of thin films of CDC recorded directly on gold. Application of previous analysis of the spectra of CDC on gold therefore enabled interpretation of the polymer orientation and conformation on the CaP substrate.     

Roughening of gold atomic steps induced by interaction with tetrahydrofuran

Exposure of a clean gold surface to tetrahydrofuran (THF) under ambient conditions was observed to cause roughening of atomic step edges. This change was followed in situ using a scanning tunneling microscope during the exposure of a gold surface to a controlled stream of THF vapor. THF is a common solvent used in depositing molecules, self-assembled monolayers, and polymer films on surfaces, in electrochemistry, and in chemical reactions. Unlike other solvents, such as methanol, ethanol and diethyl ether, however, we found that THF itself has a profound effect on the surface morphology that needs to be taken into account when reporting on the interactions of solutes with a gold surface. At the same time, this finding may present new opportunities in catalysis or nanostructuring of surfaces.     

Electron scattering cross section on the surface of thin ag and au films induced by atomic deuterium adsorption

The electron scattering cross section on the surface of thin silver and gold films induced by adsorption of atomic deuterium under conditions when a single adsorption state is formed was determined. Adsorption of atomic deuterium carried out at 78 K on sintered thin silver (gold) films deposited on Pyrex glass under ultrahigh vacuum conditions was studied measuring the resistance changes DeltaR "in situ". The adsorption runs performed at various exposures were followed by thermal desorption. This allowed establishment of a correlation between DeltaR and the uptake of the adsorbate. BET measurements were performed to determine the real area of the thin films and calculate the density of the adsorbate on their surface. It was found that in agreement with Wissmann's equation1 a linear dependence of DeltaR on the density of the adsorbate nads exists within a large interval of the population (nads < or = 1 x 10(15) D adatoms/cm2 on silver and 7 x 10(14) D adatoms/cm2 on gold) available under our experimental conditions. On the basis of this equation the electron scattering cross section Aads induced by adsorption of atomic deuterium on sintered thin silver and gold films was calculated as reaching 4.75 x 10(-16) and 4.46 x 10(-16) cm2, respectively. A small isotope effect in the electron scattering cross section for adsorption of hydrogen on silver was observed: Aads = 5.48 x 10(-16) cm2.     

Chain-length dependence of metastable striped structures of alkanethiols on Au111

The chain-length dependence of metastable striped phases of alkanethiols films partially covering the gold surface has been determined by means of atomic force microscopy. These structures are obtained from solution and consist of molecules adsorbed with their carbon chains flat on the surface. The stripes run parallel to the next-nearest-neighbor direction of Au(111) and have been found to always coexist with islands of upright molecules. The stripe spacing changes linearly with molecular length differently than twice the chain length. This dependence is discussed in terms of both interdigitation and herringbone-like lamella models. With time and under ambient conditions, these phases transform, without increasing coverage, by aggregation of the lying flat molecules to the preexisting islands with upright configuration.     

Infrared spectroscopic studies of monothiol ligand adsorption on CdS nanocrystal films in aqueous solutions

Probing the surface chemistry of thiol ligand binding to cadmium chalcogenide nanocrystals is important to clarify factors involved in quantum dot stability and surface functionalization. Deposited CdS nanocrystal films have been used in this work as model quantum dot surfaces for ligand adsorption studies. The adsorption of mercaptoacetic acid, mercaptopropionic acid, and mercaptoethanol, from aqueous solution to CdS thin films, has been studied by in situ infrared spectroscopy. The absence of a S-H stretch absorption for the adsorbed species shows that adsorption occurs via the deprotonated thiol group, and the spectrum of the adsorbed carboxylic acid species closely resembles those of the solution ligands. Adsorption of mercaptoacetic acid and of mercaptopropionic acid resulted in pKa(COOH) decreases of 1.5 and 0.5, respectively. Significant changes in the spectrum of mercaptoethanol upon adsorption have been observed, but the present uncertainty in mercaptoethanol spectral interpretation does not provide structural inferences. Adsorption isotherms determined from the spectral data indicate strong thiol adsorption to CdS. The adsorption isotherms have been fitted to both Langmuir and Freundlich equations, with the latter providing a better fit. This may be attributed to a change in the probability of adsorption to vacant surface sites due to the increased CdS surface negative charge as the surface coverage increases.     

Effect of pH and applied potential on the adsorption of DNA on highly oriented pyrolytic graphite electrodes. Atomic force microscopy surface characterisation

Single-stranded and double-stranded DNA electrochemical biosensors prepared by adsorption during 3 min on HOPG, with or without an applied potential, at pH 5.3 and 7.0, were characterised by MAC mode AFM. During adsorption DNA condenses on the substrate forming complex network films with pores exposing the HOPG surface. The thin films formed in pH 5.3 acetate buffer always presented a better coverage of the HOPG surface with DNA molecules than films formed in pH 7.0 phosphate buffer. The application of a positive potential of 300 mV during adsorption enhanced the robustness and stability of the DNA films with the formation of bigger network holes and a more condensed and compact self-assembled DNA lattice. The knowledge of the morphology of adsorbed DNA on electrode surfaces explains non-specific adsorption on the electrode surface and can be used to improve and develop DNA-electrochemical biosensors.     

Using ferroelectric poling to change adsorption on oxide surfaces

Adsorption has been invoked to explain many phenomena in ferroelectric materials including the unanticipated stability of ultrathin ferroelectric films; however, the intrinsic surface properties of ferroelectric oxides have been largely unexplored. Therefore, the effect of ferroelectric poling on the adsorption/desorption of two polar molecules, acetic acid and 2-propanol, and one nonpolar molecule, dodecane, on LiNbO3(0001) was compared. The two polar molecules were found to adsorb significantly more strongly on the positive surface. Temperature-programmed desorption (TPD) data yielded desorption pre-exponentials of the two polar molecules more than 11 orders of magnitude lower than expected. Ferroelectric materials are also intrinsically pyroelectric, and it is shown that the unusually low desorption pre-exponentials can be explained by temperature dependent heats of adsorption that result from changes in the surface dipole as the samples are heated. This conclusion was supported by dodecane adsorption/desorption, which was independent of polarity with normal desorption pre-exponentials. The differences between the polar and nonpolar molecules indicate that interactions between polar molecules and ferroelectric surfaces are dominated by electrostatics. It is shown that adsorption energy differences between positive and negative surfaces are large enough to switch the polarity of ferroelectric thin films.     

Collection and Determination of Mercury in Air

A method has been developed for the determination of low concentrations of mercury in air (nanograms/m³), i.e. in the range of the believed natural levels of mercury in the atmosphere (20 ng/m³). Mercury vapour has been collected from up to 200 1 of air in glass tubes containing thin films of gold on sieved ceramic powder. In the laboratory the absorbed mercury was then released into a quartz-window cell by heating the tube in an oven at 500°C.

In this paper it is demonstrated that, by using extremely thin films of precipitated gold, quantitative recovery is obtained and memory effects, which result from the use of thicker films, are avoided.     

Stability and Morphology of Gold Nanoisland Arrays Generated from Layer-by-Layer Assembled Nanoparticle Multilayer Films: Effects of Heating Temperature and Particle Size

This article reports the effects of heating temperature and composition of nanoparticle multilayer films on the morphology, stability, and optical property of gold nanoisland films prepared by nanoparticle self-assembly/heating method. First, nanoparticle-polymer multilayer films are prepared by the layer-by-layer assembly. Nanoparticle multilayer films are then heated at temperature ranging from 500 °C to 625 °C in air to induce an evaporation of organic matters from the films. During the heating process, the nanoparticles on the solid surface undergo coalescence, resulting in the formation of nanostructured gold island arrays. Characterization of nanoisland films using atomic force microscopy and UV-vis spectroscopy suggests that the morphology and stability of gold island films change when different heating temperatures are applied. Stable gold nanoisland thin film arrays can only be obtained after heat treatments at or above 575 °C. In addition, the results show that the use of nanoparticles with different sizes produces nanoisland films with different morphologies. Multilayer films containing smaller gold nanoparticles tend to produce more monodisperse and smaller island nanostructures. Other variables such as capping ligands around nanoparticles and molecular weight of polymer linkers are found to have only minimal effects on the structure of island films. The adsorption of streptavidin on the biotin-functionalized nanoisland films is studied for examining the biosensing capability of nanoisland arrays.     

Photochemical patterning of a self-assembled monolayer of 7-diazomethylcarbonyl-2,4,9-trithiaadmantane on gold films via Wolff rearrangement

Photolithographic attachment of functional organic molecules via ester or amide linkages to self-assembled monolayers (SAMs) on gold thin films was achieved by employing a novel photoreactive surface anchor, 7-diazomethylcarbonyl-2,4,9-trithiaadmantane. The photoreactive SAM was prepared by the spontaneous physical adsorption of the photoreactive surface anchor onto gold surfaces. The alpha-diazo ketone moiety of the SAM was found to display the classical Wolff rearrangement reactivity to produce a ketene intermediate on the exposed area. Organic molecules such as alcohols and amines can thus be attached to the gold surfaces selectively by the facile in situ formation of ester or amide linkages. The structure and reactivity of the photoreactive surface anchor were characterized by real-time FT-IR, fluorescence, and polarization modulation infrared reflectance absorption spectroscopy (PM-IRRAS). The Wolff rearrangement reactivity of the SAM suggested that a "surface-isolated" carbonylcarbene may be generated when the SAM was exposed to 255-nm irradiation.     

The adsorption of quaternary ammonium salts at a mercury solution interface: Part II. Non-symmetrical quaternary ammonium salts

The adsorption of triethyl alkyl (and allyl) ammonium bromides on a mercury electrode has been investigated. The isotherms and values of the free energy of adsorption yield information on the mode of adsorption of the cations. The formation of partial bimolecular films on the surface of mercury by the cations studied has been described and a simple explanation put forward concerning the mechanism of destruction of these films.     

Silica-stabilized gold island films for transmission localized surface plasmon sensing

Ultrathin gold films prepared by evaporation of sub-percolation layers (typically up to 10 nm nominal thickness) onto transparent substrates form arrays of well-defined metal islands. Such films display a characteristic surface plasmon (SP) absorption band, conveniently measured by transmission spectroscopy. The SP band intensity and position are sensitive to the film morphology (island shape and inter-island separation) and the effective dielectric constant of the surrounding medium. The latter has been exploited for chemical and biological sensing in the transmission localized surface plasmon resonance (T-LSPR) mode. A major concern in the development of T-LSPR sensors based on Au island films is instability, manifested as change in the SP absorbance following immersion in organic solvents and aqueous solutions. The latter may present a problem in the use of Au island-based transducers for biological sensing, usually carried out in aqueous media. Here, we describe a facile method for stabilizing Au island films while maintaining a high sensitivity of the SP absorbance to analyte binding. Stabilization is achieved by coating the Au islands with an ultrathin silica layer, ca. 1.5 nm thick, deposited by a sol-gel procedure on an intermediate mercaptosilane monolayer. The silica coating is prepared using a modified literature procedure, where a change in the reaction conditions from room temperature to 90 degrees C shortened the deposition time from days to hours. The system was characterized by UV-vis spectroscopy, ellipsometry, XPS, HRSEM, AFM, and cyclic voltammetry. The ultrathin silica coating stabilizes the optical properties of the Au island films toward immersion in water, phosphate buffer saline (PBS), and various organic solvents, thus providing proper conditions where the optical response is sensitive only to changes in the effective dielectric constant of the immediate environment. The silica layer is thin enough to afford high T-LSPR sensitivity, while the hydroxyl groups on its surface enable chemical modification for binding of receptor molecules. The use of silica-encapsulated Au island films as a stable and effective platform for T-LSPR sensing is demonstrated.     

Structural evolution of perfluoro-pentacene films on Ag(111): transition from 2D to 3D growth

The structural evolution and thermal stability of perfluoro-pentacene (PF-PEN) thin films on Ag(111) have been studied by means of low-temperature scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Well-defined monolayer films can be prepared by utilizing the different adsorption energy of mono- and multilayer films and selectively desorbing multilayers upon careful heating at 380 K, whereas at temperatures above 400 K, a dissociation occurs. In the first monolayer, the molecules adopt a planar adsorption geometry and form a well-ordered commensurate (6 × 3) superstructure where molecules are uniformly oriented with their long axis along the <110> azimuth. This molecular orientation is also maintained in the second layer, where molecules exhibit a staggered packing motif, whereas further deposition leads to the formation of isolated, tall islands. Moreover, on smooth silver surfaces with extended terraces, growth of PF-PEN onto beforehand prepared long-range ordered monolayer films at elevated temperature leads to needle-like islands that are uniformly aligned at substrate steps along <110> azimuth directions.     

Scanning probe microscopy of adsorption layers of sodium polystyrenesulfonate/dodecyltrimethylammonium bromide complexes

Scanning probe microscopy is used to study adsorption films of sodium polystyrenesulfonate/dodecyltrimethylammonium bromide complexes, which are transferred from aqueous solution surfaces to the mica surface by the Langmuir-Blodgett method. It is established that the results obtained at surfactant concentrations below critical micelle concentration agree with the conclusions on the structure of the films dased on the data of the dilatational surface rheology (Noskov, B.A., Loglio, G., Miller, R., J. Phys. Chem. B, 2004, vol. 108, p. 18615). At low concentrations of the low-molecular-mass surfactant, a thin adsorption film containing a large number of holes is formed on the surface of an aqueous solution. As the surfactant concentration is increased to reach the region of a dramatic reduction in the dynamic surface elasticity, the morphology of the film drastically changes; i.e., dense three-dimensional aggregates are formed on the solution surface.