Self-assembled monolayer (SAM)-modified nano-materials are a new technology to deliver
drug molecules. While the majority of these depend on covalently immobilizing molecules on the surface, it is proposed that electrostatic interactions may be used to deliver drugs. By tuning the surface potential of solid substrates with
SAMs,
drug molecules could be either absorbed on or desorbed from substrates through the difference in electrostatic interactions around the selected iso-electric point (IEP). In this work, the surface of
silicon substrates was tailored with various ratios of
3-aminopropyltrimethoxysilane (APTMS) and
3-mercaptopropyltrimethoxysilane (MPTMS), which form
amine- and
thiol-bearing
SAMs, respectively. The ratio of the functional groups on the
silicon surface was quantified by X-ray photoelectron spectrometry (XPS); in general, the deposition kinetics of APTMS were found to be faster than those of MPTMS. Furthermore, for solutions with high MPTMS concentrations, the relative deposition rate of APTMS increased dramatically due to the
acid-base reaction in the
solution and subsequent electrostatic interactions between the molecules and the substrate. The zeta potential in aqueous
electrolytes was determined with an electro-kinetic analyzer. By depositing
SAMs of binary functional groups in varied ratios, the surface potential and IEP of
silicon substrates could be fine-tuned. For <50%
amine concentration in
SAMs, the IEP changed linearly with the chemical composition from <2 to 7.18. For higher
amine concentrations, the IEP slowly increased with concentration to 7.94 because the formation of hydrogen-bonding suppressed the subsequent protonation of
amines.