Accessing crystal-crystal interaction forces with oriented nanocrystal atomic force microscopy probes.

Biominerals serve as critical structures of living systems and play important roles in biochemical processes. Understanding their crystallization mechanisms is therefore central to many areas of biology, biogeoscience, and biochemistry. Some biominerals, such as bone and dentin, are hierarchical nanocomposite structures constructed by sequential addition of individual oriented nanocrystals. The driving forces that enable this oriented assembly are still poorly understood, with advances in understanding limited in part by the availability of techniques that can precisely measure the delicate interactions between nanocrystals as a function of their separation distance and mutual orientation. Here, we provide a comprehensive protocol for (i) fabricating oriented single-nanocrystal atomic force microscopy (AFM) probes using focused ion beam (FIB) milling and (ii) performing oriented nanocrystal interaction force measurements using dynamic force spectroscopy (DFS)-based AFM and environmental transmission electron microscopy (ETEM)-AFM techniques. We illustrate how to fabricate oriented nanocrystal force probes using commercial bulk crystals or nano/microcrystals of calcite, zinc oxide, and rutile. The typical protocol for fabricating one AFM crystal probe takes 2-3 h. In addition, we illustrate how to quantify the direction-specific interaction forces for a given pair of interacting oriented nanocrystal faces. The methods are fully transferrable to other minerals of interest, such as the apatites constituting bone minerals. This allows researchers across many fields to measure and understand particle-based crystallization processes.