Biogenic single-crystal composites, such as sea urchin spines and calcitic prisms from mollusk shells, contain organic macromolecules inside of inorganic single-crystal matrices. The nanoscale internal structure of these materials, however, is poorly understood, especially how the biomacromolecules are distributed within the crystals without significantly disrupting the crystalline lattice. Here, annular dark-field scanning transmission electron microscopy and electron tomography reveal, in three dimensions, how biomacromolecules are distributed within the calcitic prisms from Atrina rigida shells. Disk-like nanopatches, whose scattering intensity is consistent with organic inclusions, are observed to be anisotropically arranged within a continuous, single-crystalline calcite matrix. These nanopatches are preferentially aligned with the (000l) planes of calcite. Along the crystallographic c-axis, there are alternating organic-rich and -poor regions on a length scale of tens of nanometers, while, in the ab plane, the distribution of nanopatches is more random and uniform. The structural features elucidated in this work have relevance to understanding the structure–property relationships and formation mechanisms of biominerals, as well as to the development of bio-inspired strategies to extrinsically tune the properties of single-crystals.

Calcium tartrate tetrahydrate and α-glycine single crystals grown in agarose hydrogels incorporate agarose polymer networks, resulting in polymer/single-crystal composites.

Calcite crystals grown in agarose hydrogels incorporate the gel network while retaining their single-crystal nature. The amount of gel network that is incorporated is determined by two competing factors: crystallization pressure promotes the exclusion of the gel network, while faster growth rates favor its incorporation.

This paper describes the control of the nucleation and growth of calcite crystals by a matrix composed of an agarose hydrogel on top of a carboxylate-terminated self-assembled monolayer (SAM). The design of this matrix is based upon examples from biomineralization in which hydrogels are coupled with functionalized, organic surfaces to control, simultaneously, crystal morphology and orientation. In the synthetic system, calcite crystals nucleate from the (012) plane (the same plane that is observed in solution growth). The aspect ratio (length/width) of the crystals decreases from 2.1 ± 0.22 in solution to 1.2 ± 0.04 in a 3 w/v % agarose gel. One possible explanation for the change in morphology is the incorporation of gel fibers inside of the crystals during the growth process. Etching of the gel-grown crystals with deionized water reveals an interpenetrating network of gel fibers and crystalline material. This work begins to provide insight into why organisms use hydrogels to control the growth of crystals.

This paper describes the internal structure of calcite (CaCO3) crystals grown in an agarose hydrogel and demonstrates that the gel-grown calcite crystals, like biogenic calcite crystals, incorporate the organic matrix, resulting in internal structures with pores on the order of 100's of nanometers.