Abstract
The solution-phase self-assembly of bidentate 16-[3,5-bis(mercapto-methyl)phenoxy]hexadecanoic acid (BMPHA) on Au(111) was studied using nano-fabrication protocols with scanning probe nanolithography and immersion particle lithography. Molecularly thin films of BMPHA prepared by surface self-assembly have potential application as spatially selective layers in sensor designs. Either monolayer or bilayer films of BMPHA can be formed under ambient conditions, depending on the parameters of concentration and immersion intervals. Experiments with scanning probe-based lithography (nanoshaving and nanografting) were applied to measure the thickness of BMPHA films. The thickness of a monolayer and bilayer film of BMPHA on Au(111) were measured in situ with atomic force microscopy using n-octadecanethiol as an internal reference. Scanning probe-based nanofabrication provides a way to insert nanopatterns of a reference molecule of known dimensions within a matrix film of unknown thickness to enable a direct comparison of heights and surface morphology. Immersion particle lithography was used to prepare a periodic arrangement of nanoholes within films of BMPHA. The nanoholes could be backfilled by immersion in a SAM solution to produce nanodots of n-octadecanethiol surrounded by a film of BMPHA. Test platforms prepared by immersion particle lithography enables control of the dimensions of surface sites to construct supramolecular assemblies.
Highlights
Considerable effort has been directed toward studies of monothiol-based self-assembled monolayers on gold; the surface self-assembly of multidentate thiol adsorbates has received far less attention, at the molecular level
The bidentate molecule selected for this study is 16-[3,5-bis(mercaptomethyl)phenoxy]hexadecanoic acid (BMPHA), shown in Figure 1 [15]
Either monolayer or bilayer films of BMPHA can be formed on Au(111) by controlling the parameters of solution concentration and the duration of immersion in ethanolic solution
Summary
Considerable effort has been directed toward studies of monothiol-based self-assembled monolayers on gold; the surface self-assembly of multidentate thiol adsorbates has received far less attention, at the molecular level. The stability of organosulfur-based adsorbates to oxidation, heat, and exposure to light can limit the durability of surface films [1,2,3,4,5,6,7,8,9,10]. Having additional thiol moieties in the headgroup of these adsorbates enables a chelate effect, which improves the stability of the films [12,13]. The adsorption of tridentate adsorbates proceeds via a more complex assembly pathway than that of monothiols since the multidentate molecules require successive steps to form S–Au bonds to the surface. Electrostatic interactions at the interface can produce head-to-head linkages of acid endgroups to form bilayer films [19]
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