Self-assembly of tetraphenylporphyrin monolayers on gold substrates

Two tetraphenylporphyrins, TPP-P-disulfide and TPP-P-thiol, were synthesize d and used to form self-assembled monolayers (SAMs) on gold surfaces. The m onolayers were characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet/visible absorption spectroscopy (UV/vis), and scanning tunnelin g microscopy (STM). XPS binding energy shifts revealed that the porphyrins were chemisorbed to the surface through a sulfur-gold bond. A red shift wit hout a significant blue-shifted component of the Soret band in the absorpti on spectra demonstrated that the porphyrin molecules are aligned on the gol d surface in a side-by-side orientation. Round STM features, approximately 2 nm in diameter, correspond closely to the diameter of tetraphenylporphyri n (1.8 nm). Taken together, these data indicated the formation of monolayer s of uniformly spaced TPP-P-disulfide and TPP-P-thiol molecules, with the p orphyrin ring oriented parallel to the gold surface. Furthermore, porphyrin monolayers were stable for at least a week at ambient conditions. These mo nolayers 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.