Three-dimensional AFM imaging of hydration and flexible surface structures at solid-liquid interfaces*

At a solid-liquid interface, water interacts with a surface to present a non-uniform distribution referred to as hydration structure. In addition, if the surface consists of soft organic or biological molecules, the flexible surface structures often exhibit thermal fluctuations to present a three-dimensional (3D) distribution. These 3D hydration and flexible surface structures play critical roles in various interfacial phenomena such as protein folding, crystal growth, self-assembly, molecular recognition, metal corrosion, friction and anti-fouling. However, these 3D structures are hard to visualize by conventional imaging techniques. Here, we aim to solve this problem by 3D-AFM [1]. In 3D-AFM, a tip is scanned in vertical and lateral directions to cover the 3D interfacial space. During the scan, the tip interacts with the surrounding molecules so that the measured 3D force distribution shows molecular-scale contrasts reflecting the molecular distributions. So far, several groups have shown that the method can visualize 3D hydration structures on minerals (e.g. mica, calcite and fluorite) and biological molecules (e.g. bR, GroEL and DNA). In addition, it was shown that 3D-AFM can visualize 3D distribution of fluctuating molecules such as lipid headgroups at a membrane surface [2] and surfactants on an HOPG substrate. More recently, the possibility of visualizing 3D hydrations above heterogeneous structures such as point defects and complex minerals have been explored. Furthermore, there has been an attempt to visualize dynamic changes of the 3D hydration structures by improving the operation speed of 3D-AFM. In this study, we present an overview of these advanced 3D-AFM techniques and their applications.

[1] T. Fukuma, Y. Ueda, S. Yoshioka, H. Asakawa, Phys. Rev. Lett. 104 (2010) 016101.
[2] H. Asakawa, S. Yoshioka, K. Nishimura, T. Fukuma, ACS Nano 6 (2012) 9013.