Investigation On The Adsorption And Nano-mechanical Properties Of Ss-DNA Film With The Surface Forces Apparatus

Biofilm(especially DNA film)has attracted much attention for its wide application in biomimetic materials,nano-devices,disease diagnosis,biological sensor and other fields.The rapid development of nanotechnology makes it possible to explore the structure,properties and application of biofilm.In this thesis,the adsorption and nano-mechanical properties of single-stranded DNA(ss-DNA)films on mica surface were explored systematically by cation bridging method and Surface Forces Apparatus(SFA).The main work is as follows:1.The effects of ion's valence,concentration and species on the structure of ss-DNA film were studied systematically,and the adsorption mechanism of ss-DNA was also analyzed.The results show that divalent cations can effectively promote the adsorption of ss-DNA,and DNA adsorption is dominated by the ion-relation effects.Bivalent transition metal ions are favorable for DNA adsorption,but non-transition metal have a weaker effect on adsorption.Besides,ss-DNA film is particularly sensitive to Co2+ concentration,but not sensitive to Zn2+ concentration,which is due to the Zn2+ diffusion and DNA rearrangement.2.The structure of ss-DNA film is different at different DNA concentrations.50 ng/?l is the lowest concentration that ss-DNA can construct a dense monolayer on mica surface,while ss-DNA form a bilayer or cross-link at 100 ng/?l.Besides,the thicker the ss-DNA film is,the weaker the adhesion force becomes,which is related to the charge screening effects or the steric effect of ss-DNA.3.The desorption behavior of ss-DNA film were observed by adding monovalent salt into the buffer solution.When the monovalent ion concentration increases to 100 times higher than that of divalent,ss-DNA molecules were completely desorbed from mica surface.It is indicated that ss-DNA desorption is due to the reduced ion-relation effects.4.The mechanical properties of ss-DNA films with different base were explored.The results show that there are significant differences in the initial distance of repulsion,the structure of the film and the adhesion strength between mica surfaces.5.The solid-liquid interface properties of high concentration monovalent salt and its effect on ss-DNA adsorption were studied.The results show that hydration of Na+ is not obvious at low concentration,and DLVO theory can predict the EDL structure.While,Na+ shows obvious hydration at high concentration.At 1 M NaCl,only sporadic ss-DNA molecules adsorb on mica surface,at higher concentrations,sodium salts show solidification-like crystalline particles on mica surface.6.The adsorption and mechanical properties of ss-DNA in biomimetic mixed solutions were investigated.At concentration ratio of 1:1,the mixed solution of Co,Na and Mg,Na,all can promote adsorption and formation of ss-DNA film;At concentration ratio of 1:10,only Co,Na mixture can promote ss-DNA adsorption.This thesis argues that the competitive effect between monovalent and divalent ions is the fundamental reason for ss-DNA adsorption in mixed buffer solutions.