Analysis of force distance curve for biomolecule imaging

We have examined the interaction force between the Si(1 0 0) surface and the probe tip both in air and in liquid using AFM. The silicon substrates were treated by diluted hydrofluoric acid (HF) and by aminopropyltriethoxysilane (APTES) solution, and by degreasing process only. After each sample treatment procedure, the force-distance measurements were carried out immediately by a bio-cantilever with ∼0.06 N/m spring constant. The sample surface treated with HF solution exhibits greater adhesion force than the surface treated with APTES. And, the surface treated with degreasing process only presents highest adhesion from hydrophilic characteristics of the native oxide on the silicon surface. The adhesion force in liquid was measured to be significantly smaller than that in air for all sample surfaces due to the reduction of the capillary force in liquid. In addition, the small repulsive interaction in the non-contact region was exhibited from double layer repulsion of charged surfaces across liquid solutions. Non-contact imaging of DNA molecules on the APTES treated Si surface is presented.