Gold Surface Adsorption Properties of the Enzymatically Polymerized Amphiphilic Decyl Ester of L‐Tyrosine

In aqueous solution, both amphiphilic decyl esters of the amino acids D & L‐Tyrosine have been shown to self‐assemble at identical low mM range c.m.c. values to form similar rod or plate‐like fibers a few microns wide and with lengths ranging from tens to hundreds of microns [Mat. Sci. Eng. C11 2000 155–163]. These monomers have only slightly different rates of polymerization with horseradish peroxidase below their c.m.c. and have been shown to possess pH dependent self‐assembly properties, as well as significantly increased binding to the gold surface of a quartz crystal microbalance with increasing pH [Biotech. Progress 15 1999 522–528]. In the present study, we quantitate the pH dependence and gold surface adsorption kinetics of the decyl ester of L‐tyrosine using X‐ray photoelectron spectroscopy (XPS) and then correlate these data with monomer solution titration studies. These data support the notion of increasing gold surface adsorption with increasing pH in the range from 5–8. We present evidence that this involves the lowered solubility of the neutral charged species resulting from the deprotonation of the α‐NH₃ ⁺ in each monomer unit. At 24 h adsorption at pH 7.0, we observed XPS evidence for complete coverage of the Au surface. From these data for the Au 4f photoelectron signal attenuation, we estimate a film thickness of 5.89 nm from the attenuation of the Au4f photoelectron intensity by the growing film.

Keywords:

• amphiphilic monomer
• tyrosine decyl ester; pH titration
• X‐ray photoelectron spectroscopy
• gold surface
• enzymatic polymerization