Formation and Structure of Self-Assembled Monolayers

Article

The immobilization of biomolecules on substrate surfaces for biosensor development requires linking molecules that must meet a specific set of criteria. Two such agents based on bifunctional alkyltrichlorosilane structures, 1-bromo-11-(trichlorosilyl)-undecane and 1-(thiotrifluoroacetato)-11-(trichlorosilyl)-undecane, are employed to generate thiol-functionalized surfaces either by nucleophilic substitution followed by reduction (bromine-containing derivative) or deprotection (fluorine-containing compound). Both molecules have been attached to the surfaces of silicon wafers in conjunction with the diluent silane, octyltrichlorosilane. X-ray photoelectron spectroscopic analysis in the conventional and angle-resolved modes confirms that both silanization reactions were successful. The alkyl-bromine surfaces were subjected to treatment with a set of nucleophilic reactants followed by reduction and derivatization with trifluoroacetic anhydride. The latter in conjunction with surface analysis was used to estimate the level of thiol functionalization achieved. The fluorine-containing undecane surface has been studied by surface analysis both before and after deprotection of the thiol group by hydroxylamine solution. The results indicate that a high coverage of the surface was found for the protected moiety, with approximately 10% of the trifluoro acetate groups remaining after the deprotection procedure.Key words: bifunctional silanes, thiol surface functionalization, biomolecule immobilization, X-ray photoelectron spectroscopy.

Charge-Transfer Complexation of C(60) with Diphenyltetrathiafulvalene (DPTTF) Capped Gold Clusters

4,4'-Diphenyltetrathiafulvalene (DPTTF) capped gold nanoparticles of 3- to 5-nm diameter have been prepared and their thermal stability has been investigated by variable temperature FT-IR spectroscopy. The DPTTF molecules at the cluster surface undergo charge-transfer complexation with C(60) to form Au/DPTTF.C(60). Direct evidence of complexation is observed in the x-ray photoelectron and infrared spectra of the clusters. The orientational ordering transition of C(60) in Au/DPTTF.C(60) clusters occur at a higher temperature than pure C(60) indicating increased resistance to orientational disorder due to charge transfer. Variable-temperature infrared spectroscopic studies yield complementary information. Both FT-IR and mass spectra show that some of the sites on the gold surface are occupied by the phase-transfer reagent used in the cluster preparation. Copyright 1999 Academic Press.

Investigation of membrane dryers and evaluation of a new ozone scrubbing material for the sampling of organosulphur compounds in air

The applicability of two different types of Nafion membrane dryers (based on counter-current flow and desiccant drying) and of a new ozone scrubbing material, polyphenylene sulphide wool (noXon-S), to adsorptive sampling of selected volatile sulphur compounds (methanethiol, dimethyl sulphide, isopropanethiol and isobutanethiol) is investigated at the low ppb (v/v) level (1-5 ppb). No analyte losses occur with either type of dryer at relative humidities (RH) of < or = 50%, while at higher RH values particularly the thiols tend to be lost (between 6 and 32%) even after conditioning. The actual losses depend more on the state of the individual permeation membrane rather than on the type of dryer. NoXon-S is a highly suitable ozone scrubber material for sulphur compounds since it efficiently removes ozone without retainment of the analytes and without the formation of blanks or artefacts from the scrubber material. The combined use of a Nafion membrane dryer and a noXon-S ozone scrubber is thus recommended for artifact-free sampling of sulphur compounds.

3D Monolayers of 1,4-Benzenedimethanethiol on Au and Ag Clusters: Distinct Difference in Adsorption Geometry with the Corresponding 2D Monolayers

Au and Ag clusters of 3-nm mean diameter were prepared with 1,4-benzenedimethanethiol (BDMT) as the capping molecule. In both the monolayer-capped clusters, BDMT adsorbs in the dithiolate form in contrast to in the 2D monolayers, where the dithiolate form exists only in the case of Ag. As a result, the molecule lies flat on the cluster surface in both Au and Ag, whereas the molecular plane is perpendicular to the surface in the 2D monolayer of Au. This difference in adsorbate structure is attributed to the evolution in the cluster surface as adsorption occurs. Transmission electron microscopy and X-ray powder diffraction suggest that no superlattice of the clusters exists. Mass spectrometry suggests the presence of the phase transfer reagent at the surface. X-ray photoelectron spectroscopy confirms the chemical similarity of the two surfaces and suggests that the thiolate at the surface is susceptible to X-ray beam induced damage. Copyright 1999 Academic Press.