Gel-network/single-crystal composites formed in gel media

Abstract

Gel-grown method has been adopted to grow single crystals for over a century, and has triggered increasing interest with a large number of single crystals obtained. Some organisms construct their hard tissues where foreign macromolecules have been incorporated into the brittle crystals to enhance their mechanical properties. In 1969, the researchers found out that gel-network could be incorporated into the single-crystalline matrix in the similar way. The unique property of the gel-incorporated single-crystals is the combination of long-range order (single-crystallinity) and composite structures, which has enlightened the research on not only understanding biomineralization, but also exploring possible applications of these unique composites. This review will briefly introduce recent progress in gel-network/single-crystal composites formed in gel media in five aspects. Firstly, we will discuss the growth process of the composites. The growth procedures fall into four categories: (1) The reaction method, (2) the chemical reduction method, (3) the solubility reduction method, (4) the decomplexing method, and various types of crystals have been testified the successful inclusion of gel media including soluble and insoluble compounds, metal, etc. Secondly, we will briefly introduce several characterizations to demonstrate the distinctive structure of the composites. Incorporated gel fibers can be seen clearly inside the etched pits of the composite under scanning electron microscope. The fibrous structure can be further demonstrated using scanning transmission electron microscope and electron tomography by imaging a thin section cut from a gel-grown crystal. For example, tomographic reconstruction of an agarose network inside of a section of the crystal proves the three-dimensional random-interpenetrating structure between the gel guest and the crystal host while selected-area (area contains both crystal and fibers) electron diffraction pattern shows that the crystal remains its single-crystallinity. Thirdly, we will discuss several factors, which influence the effective absorption of gel inside the crystals. By measuring the amount of incorporated gel-network using thermal gravimetric analyzer, researchers have concluded that gel-network with higher strength tend to be embedded inside the crystal while weaker gel is more likely to be pushed away. Besides, faster growth rate as well as stronger interaction between the crystal host and the gel guest favors the gel incorporation. Fourthly, we will discuss a new approach to functionalize single crystals. The reinforced mechanical properties of gel-incorporated crystals inspire scientists to introduce other functional components inside the crystals to achieve multiple nonintrinsic properties. The gel-mediating strategy offers a facile way to realize this idea by trapping functional nanoparticles within gel media and inducing inclusion of nanomaterials into crystals along with gel-networks. Finally, we will put forward some promising research directions in this area including intensive study of the mechanism involved, exploring more crystal-gel combinations, putting heroic efforts on the functionalization of the composites and most importantly, digging up potential application prospects.