Self-assembly of tetraphenylporphyrin monolayers on gold substrates

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.