Molecular Gradients of ω-Substituted Alkanethiols on Gold Studied by X-ray Photoelectron Spectroscopy

Abstract

Molecular gradients of ω-substituted alkanethiols HS(CH2)15X (X:CH3, CH2OH, CO2CH3, CH2C⋮N) were prepared on sputtered gold films by using a novel cross-diffusion methodology. Ellipsometry, contact angle measurements, infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to monitor the coverage, chain conformation and gradient profiles. The XPS analysis were performed with improved lateral resolution as compared to our earlier infrared study (Langmuir 1995, 11, 3821) by imaging a 160 μm wide segment of the gradient surface on the slit of the XPS analyzer and by using a step size of 0.5 mm. The so-obtained gradient profiles are expected to be more representative of the true composition profiles and were for the three gradients investigated found to be 5−6 mm, about 2−3 mm shorter than the corresponding profiles obtained by scanning infrared spectroscopy. The coverages (alkyl chain densities) were found to be constant for the gradients prepared from thiols with relatively small tail groups OH, C⋮N and CH3, whereas a minor reduction in coverage, ≈ 3−4%, was observed in a narrow range for the gradient containing the CO2CH3-terminated alkanethiol. This change is, however, too small to be detected by ellipsometry and infrared spectroscopy and supports a model where the gradient assemblies approach the limiting structure of densely packed all trans polymethylenes.

Order/disorder gradients of n-alkanethiols on gold

“…Fig. 3(a-d) shows the advancing and receding contact angles with hexadecane for the following four diffusion pairs: (a) HS -(CH 2 ) 15 [1,2]. One reason for this difference could be that we for the first time use contact angles as a mean to characterize the whole gradient.…”

“…With an existing methodology to prepare twocomponent molecular gradients of self-assembled monolayers (SAMs) of alkanethiolates on gold [1,2] we want to address a number of critical issues of relevance for the development of novel biomaterials [3,4] and biosensing interfaces [5][6][7]. We are also interested in gaining a deeper fundamental knowledge about the interfacial properties of these assemblies [8].…”

Solution and Surface Composition Gradients via Microfluidic Confinement: Fabrication of a Statistical‐Copolymer‐Brush Composition Gradient

“…[9,10] Several methods have been developed to make chemicalcomposition gradients, including the diffusion of molecules in gases or gels. [1,11] Gradients have also been prepared by ion/ metal deposition or ultraviolet-ozone irradiation. [10,12] The use of scanning probe microscopy to form gradients at the nanometer scale has been explored.…”

Comparison between wettability gradients made on gold and on Si/SiO2 substrates

“…whether it is dependent upon the substrate hydrophilicity and charge, or solely upon surface hydrophilicity. With the technique devel-oped by Liedberg and Tengvall [8,9] it is possible to create gradients with a large number of chemical groups, among them alcohol groups. Their technique uses alkanethiols with different chemical head groups, where the thiols react strongly with the gold surface [10,11].…”