Physics of Complex Fluids and MESA + Institute for Nanotechnology, Department of Science and Technology, University of Twente, Enschede, The Netherlands.
Nanotechnology. 2011 Jul 29;22(30):305706. doi: 10.1088/0957-4484/22/30/305706. Epub 2011 Jul 1.
We use atomic force microscopy (AFM) to determine electrostatic interactions between Si tips and Si wafers in aqueous electrolytes of variable composition. We demonstrate that dynamic force spectroscopy (DFS) in the frequency modulation (FM) mode with stiff cantilevers and sharp tips allows for a continuous detection of the tip-sample interactions without mechanical jump-to-contact instability and with substantially higher lateral resolution than standard colloidal probe measurements. For four different species of salt (NaCl, KCl, MgCl(2), CaCl(2)) we find repulsive electrostatic forces at the lowest salt concentrations (1 mM) that become progressively screened until they are dominated by attractive van der Waals forces at the highest concentration (100 mM). For the divalent cations the crossover from repulsive to attractive forces occurs at lower concentrations than for monovalent cations. Surface charges extracted from fits to standard Poisson-Boltzmann double layer theory indicate a rather weak dependence of the surface charge on the ion concentration. The high lateral resolution of our approach is illustrated by a 2D force field measurement over a patterned surface of a supported lipid bilayer on a mica substrate.
我们使用原子力显微镜（AFM）来确定 Si 尖端和 Si 晶片在不同组成的水溶液电解质之间的静电相互作用。我们证明，使用具有刚性悬臂和锋利尖端的调频（FM）模式的动态力谱（DFS）可以连续检测尖端-样品相互作用，而不会出现机械跳跃接触不稳定性，并且具有比标准胶体探针测量高得多的横向分辨率。对于四种不同的盐（NaCl、KCl、MgCl2、CaCl2），我们在最低盐浓度（1 mM）下发现了排斥静电力，这些力逐渐被屏蔽，直到在最高浓度（100 mM）下被吸引力范德华力主导。对于二价阳离子，从排斥力到吸引力的转变发生在比一价阳离子更低的浓度下。从标准泊松-玻尔兹曼双电层理论拟合中提取的表面电荷表明表面电荷对离子浓度的依赖性相当弱。我们的方法的高横向分辨率通过在云母衬底上的支撑脂质双层的图案化表面上进行二维力场测量得到了说明。
