Ho R, Yuan J Y, Shao Z
Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville, Virginia 22908 USA.
Biophys J. 1998 Aug;75(2):1076-83. doi: 10.1016/S0006-3495(98)77597-6.
Using a hard sphere model and numerical calculations, the effect of the hydration force between a conical tip and a flat surface in the atomic force microscope (AFM) is examined. The numerical results show that the hydration force remains oscillatory, even down to a tip apex of a single water molecule, but its lateral extent is limited to a size of a few water molecules. In general, the contribution of the hydration force is relatively small, but, given the small imaging force ( approximately 0.1 nN) typically used for biological specimens, a layer of water molecules is likely to remain "bound" to the specimen surface. This water layer, between the tip and specimen, could act as a "lubricant" to reduce lateral force, and thus could be one of the reasons for the remarkably high resolution achieved with contact-mode AFM. To disrupt this layer, and to have a true tip-sample contact, a probe force of several nanonewtons would be required. The numerical results also show that the ultimate apex of the tip will determine the magnitude of the hydration force, but that the averaged hydration pressure is independent of the radius of curvature. This latter conclusion suggests that there should be no penalty for the use of sharper tips if hydration force is the dominant interaction between the tip and the specimen, which might be realizable under certain conditions. Furthermore, the calculated hydration energy near the specimen surface compares well with experimentally determined values with an atomic force microscope, providing further support to the validity of these calculations.
利用硬球模型和数值计算方法，研究了原子力显微镜（AFM）中锥形针尖与平面之间的水化力效应。数值结果表明，即使对于单个水分子的针尖顶端，水化力仍保持振荡，但它的横向范围限制在几个水分子的大小。一般来说，水化力的贡献相对较小，但是，考虑到通常用于生物标本的较小成像力（约0.1 nN），一层水分子可能会“附着”在标本表面。针尖与标本之间的这层水层可以起到“润滑剂”的作用以减小横向力，因此这可能是接触模式原子力显微镜能够实现超高分辨率的原因之一。要破坏这层水并实现真正的针尖 - 样品接触，需要几纳牛顿的探针力。数值结果还表明，针尖的最终顶端将决定水化力的大小，但平均水化压力与曲率半径无关。后一个结论表明，如果水化力是针尖与标本之间的主要相互作用，那么使用更尖锐的针尖不会有不利影响，这在某些条件下可能是可行的。此外，计算得到的标本表面附近的水化能与原子力显微镜实验测定的值吻合良好，为这些计算的有效性提供了进一步支持。
