Adhesion of chemically and electrostatically bound gold nanoparticles to a self-assembled silane monolayer investigated by atomic force volume spectroscopy

The adhesion of gold nanoparticles either electrostatically or chemically attached to a substrate has been probed using AFM operating in force spectroscopy mode. A monolayer of –NH₂ terminated 3-aminopropyltriethoxysilane or –SH terminated 3-mercaptopropyltrimethoxysilane was self-assembled onto a p-type silicon (100) substrate. Each silane monolayer provided the point of attachment for citrate stabilised gold colloid nanoparticles. In the case of the –NH₂ terminated layer gold colloid assembly was driven by the electrostatic attraction between the negative, citrate-capped, gold nanoparticles and a partially protonated amine layer. In the case of the –SH terminated regions, well-known gold–thiol chemistry was used to chemically attach the nanoparticles. An atomic force microscope tip was chemically modified with 3-mercaptopropyltrimethoxysilane and scanned across each surface, where the cantilever deflection was measured at each x, y pixel of the image to create an array of adhesion force curves. This has allowed an unprecedented nanoscale characterisation of the adhesion force central to two common surface attachment methods of gold colloid nanoparticles, providing useful insights into the stability of nanoscale constructs.