Alkanethiol anchored self-assembled monolayers (SAMs) on gold are widely used to immobilize and detect molecules including DNA and proteins. Most of these molecules are covalently bonded with the SAM on the Au surface and cannot be released easily. By using different functional groups, the interfacial charge of SAMs can be selected, and thus, they can be considered as adaptors for immobilizing and releasing materials selectively through electrostatic interaction under given conditions. In this work, as an additional factor to control the surface charge, SAMs with mixed functional groups are presented, and it is demonstrated that the isoelectric point (IEP) can be tailored by the ratio of functional groups. Using carboxylic acid- and amine-SAM on gold substrates as an example, isoelectric points (IEPs) from 3.5 to 6.5 can be obtained arbitrarily. The ratio between the functional groups on the surface was quantified by X-ray photoelectron spectrometry (XPS) and was found to be slightly different from the deposition solution. The homogeneous spatial distribution of the functional groups was determined with scanning electrical potential microscopy (SEPM). The interfacial charge of SAMs with mixed functional groups on gold was investigated by electrokinetic analysis in aqueous electrolyte solutions.

Amine-terminated self-assembled monolayers (SAMs) on Au surfaces are commonly used to immobilize various types of molecules, including DNA and proteins. However, little is known about the stability of these types of surfaces. In this work, it was observed that the surface potential (as well as the isoelectric point) of amine-bearing SAMs on flat gold substrates changed significantly with time, indicating that the surface functional group is not stable under ambient conditions (standard temperature and pressure). Contact angle analysis indicated that after degradation, the polar component of the interfacial force decreased and the dispersion component increased. These results indicate that the surface has undergone a chemical transformation. X-ray photoelectron spectroscopy (XPS) was used to detect changes in the chemical state of the surface nitrogen atoms. The chemical shift of the binding energy indicates that the nitrogen is partially oxidized. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), the oxidation of the amino groups to nitroso groups was evident, as was the previously reported oxidation of the thiol groups to sulfonate groups. Two methods for retarding the oxidation of the amine functional group are presented in this work. By isolating the SAM from either light or air, the oxidation is suppressed and the surface properties are preserved. In other words, the shelf life of the amine-modified gold substrates is prolonged. This result suggests that the oxidation is either photocatalyzed or photoinduced.