Interpretation of DNA adsorption on silanized surfaces by measuring interaction forces at various pHs using atomic force microscopy

We investigated the interaction between DNA and n-alkylsilanized glass surfaces at pH 8 using fluorescence microscopy and atomic force microscopy (AFM). The surface densities of DNA molecules deposited on the glass surfaces with or without n-alkylsilanes were measured by fluorescence microscopy. In order to clarify the interaction between the DNA molecule and the substrate surface, forces in various buffer solutions (pH 2.0–9.0) between a cleaned Si3N4 tip and each substrate surface were measured by AFM. The repulsive forces were measured at pH 8 between the cleaned Si3N4 tip and all of the substrate surfaces, when they were separated in the buffer solution. The repulsive forces can be attributed to observation that the Si3N4 tip and all of the substrate surfaces were charged negatively at pH 8. The repulsive electrical double layer (EDL) forces against the bare glass surface and the surface silanized with n-propyltrimethoxysilane (C3), however, were higher than that against the surface with n-hexadecyltrimethoxysilane (C16). The adhesive forces, which are partially due to van der Waals forces, between the tip and the substrate surface were much higher on the surface with C16 than on those with C3 and of the bare glass substrate. It is important to note that both DNA and the Si3N4 tip at pH 8 were charged negatively. The higher repulsive EDL and the lower adhesive forces observed on the surfaces with C3 and of the bare glass substrate at pH 8 rationalized the fluorescence microscopic observation that the DNA molecules hardly adsorbed on these two surfaces. On the contrary, the higher adhesive force as well as the lower repulsive EDL force on the surface with C16 at pH 8 rationalized why the DNA molecules adsorbed well on this surface.