Preparation and Interfacial Properties of Phosphonic Acid-Terminated Self-Assembled Monolayers on Gold Surface

The preparation and interfacial properties of self-assembled monolayers (SAMs) of 3-mercaptopropylphosphonic acid [HS−(CH 2 ) 3 −PO 3 H 2, MPPA] adsorbed on Au surfaces have been systematically investigated with the electrochemical method, X-ray photoelectron spectroscopy (XPS), and surface-enhanced internal reflection infrared spectroscopy measurements. An improved rinse method (base−acid rinse protocol) is presented to prepare the MPPA monolayers free of unbound MPPA thiols. Experimental results show that the effect of the degree of dissociation of −PO 3 H 2 groups on the compactness of MPPA−SAMs is nonlinear possibly because of hydrogen bond interaction and the dibasic nature and hydrophilicity of the −PO 3 H 2 groups. The effects of pH and the electric field on the surface charge of MPPA−SAMs are investigated.