Binary self-assembled monolayers modified Au nanoparticles as carriers in biological applications. 
Gold nanoparticles (AuNPs) are good nonviral carriers because of their ease of synthesis and conjugation in biochemistry, and self-assembled monolayers (SAMs) provide a tunable system to change their interfacial properties.  Using homogeneously mixed carboxylic acid and amine functional groups, a series of surface potentials and isoelectric points (IEPs) could be obtained and allow systematic study of the effect of surface potential.  In this work, the result of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay revealed that binary-SAM modified AuNPs have high biocompatibility with HEK293T cells.  The amount of AuNPs ingested by the cells was found to increase with increasing surface potential and the difference was also confirmed with a scanning transmission electron microscope.  The ability of binary-SAM modified AuNPs as carriers was examined, and the plasmid deoxyribose nucleic acid (DNA)-containing eGFP reporter gene was used as the model cargo.  Fluorescence imaging revealed that the transfection efficiency generally increased with increasing surface potential.  More importantly, when the IEP of the AuNPs was higher than that of the environment of the endosome but lower than that of the cytoplasm, the plasmid DNA can be protected better and released more easily during the endocytosis process hence higher efficiency is obtained with 60% NH2 and 40% COOH in the binary-SAM.