The surface coverage of thiolated tetraphenylporphyrin derivatives assembled on gold has been determined using ultraviolet/visible absorption spectroscopy (UV/Vis) and x-ray photoelectron spectroscopy (XPS). Ultraviolet/visible absorption spectroscopy was used to calculate the surface concentration of porphyrin molecules through two independent approaches: the first measured the porphyrin concentration in solution after the displacement of a porphyrin monolayer from the gold surface; and the second directly exploited the absorptivity of the monolayers. Furthermore, we determined experimentally the extinction coefficients for the porphyrin monolayers and verified that the extinction coefficients are in agreement with a simple theoretical model that is frequently applied to porphyrin monolayers. Two separate models, both based on a uniform overlayer, were applied to the XPS data to determine the surface concentration of the porphyrin monolayers. Finally, for comparison of the UV/Vis and XPS results, a model of an ordered tetraphenylporphyrin arrangement on a surface was utilized to calculate coverage. The results of these two spectroscopic techniques differed by as much as a factor of 4, but they were generally comparable to those for related porphyrin systems. Determination of the surface coverage of monolayers is often challenging, thus the comparison of two independent experimental techniques allows for a more accurate estimation of monolayer coverage, and demonstrates the often unrealized potential to calculate the coverage of organic monolayers using XPS.
Two tetraphenylporphyrins, TPP-P-disulfide and TPP-P-thiol, were synthesized and used to form self-assembled monolayers (SAMs) on gold surfaces. The monolayers were characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet/visible absorption spectroscopy (UV/vis), and scanning tunneling microscopy (STM). XPS binding energy shifts revealed that the porphyrins were chemisorbed to the surface through a sulfur−gold bond. A red shift without a significant blue-shifted component of the Soret band in the absorption spectra demonstrated that the porphyrin molecules are aligned on the gold surface in a side-by-side orientation. Round STM features, approximately 2 nm in diameter, correspond closely to the diameter of tetraphenylporphyrin (1.8 nm). Taken together, these data indicated the formation of monolayers of uniformly spaced TPP-P-disulfide and TPP-P-thiol molecules, with the porphyrin ring oriented parallel to the gold surface. Furthermore, porphyrin monolayers were stable for at least a week at ambient conditions. These monolayers have the potential to anchor biorecognition molecules in an ideal spacing for protein and cell attachment, making them appropriate models for the development of new biorecognition surfaces.
Protein layers are deposited on the surface of implanted biomaterials. Better understanding of the interaction between the surface protein layers and the biological system would lead to the development of future biomaterials with superior biocompatibilities. Well‐organized biorecognizable surfaces can be formed with various template molecules that provide an appropriate spacing for the attachment of recognition groups to the modified surface. Silane coupling reagents, porphyrin thiols, and cyclic peptides are being used as templates to introduce nano‐scale patterns on solid surfaces. Synthesis of these templates and characterization of the modified surface are described. © 1998 John Wiley & Sons, Inc. Biopoly 47: 185–193, 1998
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