XPS and static SIMS have been employed to characterize
the structures and the photooxidation of self-assembled monolayers (SAMs) of alkylthiols on silver. Samples were
prepared by immersing silver substrates
in thiol solutions for a range of times from 30 min to 40 h, but no
changes were observed in either the
compositions of the monolayers determined by XPS or the static SIMS
spectra. Static SIMS spectra exhibited
a range of characteristic molecular species that facilitated
quantification of changes in the SAM structure
following exposure to UV light. Photooxidation led to the
formation of alkylsulfonate species at a rate that
depended on the length of the adsorbate alkyl chain. However,
evaluation of XPS compositional data suggested
that significant amounts of unoxidized sulfur remain even when SIMS
data indicate the absence of intact,
unoxidized thiolate species in the monolayer. In agreement with
the results of other workers, we propose
that photoirradiation of SAMs on silver leads to two processes:
photooxidation of thiol adsorbates to
alkylsulfonates and sulfur−carbon bond scission. The latter
process, not observed in an earlier study of
SAMs on Au, leads to the formation of significant quantities of
inorganic sulfur that are subsequently slowly
oxidized to yield inorganic oxides of sulfur.
The rates of photooxidation of self-assembled monolayers (SAMs) of alkanethiols containing methyl,
hydroxyl, and carboxylic acid terminal groups have been studied using static secondary ion mass spectrometry
(SIMS). SIMS has revealed a wealth of structure-specific peaks that enable the progress of photochemical
reactions within the SAM to be monitored accurately and effectively. The addition of eight methylene
groups to the alkyl chain leads to a reduction in the rate of reaction by a factor of approximately 4. The
nature of the adsorbate terminal group exerts a profound influence over the reactivity of the SAM, with
rate constants for the photooxidation reaction being in the ratio 4:2:1 for CH3:OH:COOH, for both long
and short chain alkanethiols. The reduction in the rate constant observed for SAMs with polar terminal
groups is attributed to the existence of strong hydrogen bonding interactions that stabilize the SAM, either
by increasing adsorbate order or by reducing the rate of diffusion of active oxygen species to the S−Au
interface. Tail group interactions may be more generally important in determining the structures and
reactivities of SAMs.
Protein adsorption and growth of primary human osteoblasts on self-assembled monolayers of alkylthiols on gold (SAMs) with carboxylic acid and hydroxyl and methyl termini were investigated. Single-component SAMs and SAMs patterned by photolithographic techniques were used. Cell growth on patterned SAMs demonstrated preferences for one pattern region in all combinations of alkylthiols, with the hierarchical preference COOH > OH > CH(3). Patterned SAMs and immunochemistry were used to investigate adsorption of fibronectin and albumin with respect to different alkylthiol termini. Fibronectin adsorption from both pure solution and serum containing cell culture medium (SDMEM) followed the sequence COOH > OH > CH(3). Albumin adsorption from pure solution followed the sequence OH > COOH > CH(3); from SDMEM the sequence was CH(3) > OH > COOH. Cell attachment to SAMs with the above termini, after preadsorption with fibronectin, albumin, or mixtures of fibronectin and albumin, was measured. Attachment was maximal on COOH-terminated SAMs precoated with fibronectin. Attachment to COOH was significantly reduced only when fibronectin was omitted from the protein preadsorption solution. On OH and CH(3) SAMs increasing the proportion of albumin in the solution was sufficient to significantly reduce cell attachment. The distribution vinculin and the integrins alpha(5)beta(1) and alpha(v)beta(3) indicated that focal contact formation by cells varied with alkylthiol termini in the following sequence: COOH > OH > CH(3).
Primary human osteoblasts were cultured on self-assembled monolayers (SAMs) of alkylthiols on gold with carboxylic acid and methyl termini, and the kinetics of cell attachment and proliferation were measured. Over 90 min approximately twice as many cells attached to carboxylic-acid-terminated monolayers as attached to methyl-terminated monolayers. After 24 h the number of cells attached to carboxylic-acid-terminated monolayers was ten times that attached to the methyl-terminated monolayers. Cell morphology and cytoskeletal actin organization also were found to be different. Osteoblasts were cultured on SAMs that were patterned by photolithographic techniques. Cells attached almost exclusively to carboxylic-acid-functionalized areas of the patterned surfaces, leaving methyl-functionalized regions bare. The patterns strongly influenced the morphology of the attached cells. After 24 h, cells were observed to bridge between carboxylic-acid-terminated regions separated by 75 microns, methyl-terminated regions but not those separated by 150 microns methyl-terminated regions. After 6 days in culture osteoblasts formed multilayers on the carboxylic-acid-terminated regions of the pattern.
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