Formation of Vaterite Mesocrystals in Biomineral-like Structures and Implication for Biomineralization

Abstract

Vaterite mesocrystals with a hexagonal prism structure were successfully achieved in the presence of sodium citrate (SC) and sodium dodecyl benzenesulfonate (SDBS) by use of a gas-diffusion method at room temperature. X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), selected area electron diffraction (SAED), thermogravimetric analysis (TGA), nitrogen physisorption analysis, and fieldemission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray (EDX) were used to characterize the hexagonal prisms. XRD and FESEM results reveal that the superstructures are composed of hundreds of well-stacked nanoflakes, which construct the laminated hexagonal prism of vaterite. TEM and SAED analyses show that the hexagonal prism has the same crystallographic symmetry as single-crystal vaterite, confirming that the hexagonal prismatic architectures are orientationally aligned mesocrystals of vatreite. However, no hexagonal prism structures can be produced only with SC or SDBS, indicating that the cooperation of SC and SDBS is indispensable to the formation of hexagonal prismatic vaterite mesocrystals. The hexagonal prism mesocrystals of vaterite exhibit remarkable similarity to the nacreous layers of vaterite in freshwater cultured pearls from mussels and the columns/lamellae of vaterite in bivalve in architectures. Therefore, the current study on vaterite mesocrystals will be helpful for us to mimic and learn from nature and may provide another pathway toward full insight into biomineralization mechanism.

Polymorphs, Proteins, and Nucleation Theory: A Critical Analysis

“…These studies were in vitro in nature and utilized corresponding calcium carbonate mineralization assays as described for aragonite studies [48,52,53]. Again, as per the aragonite studies, these studies confirmed the formation of metastable vaterite crystals in the presence of the proteins under investigation [48,52,53].…”

“…In comparison to the aragonite studies, there have been fewer published studies that examine the role of proteins in metastable vaterite formation [48,52,53], most likely because of fewer organisms utilizing this polymorph over calcite or aragonite [52,53,57]. These studies were in vitro in nature and utilized corresponding calcium carbonate mineralization assays as described for aragonite studies [48,52,53].…”

“…The major reason for this is the lack of standardization: even though conditions for studying non-classical nucleation schemes were published nearly 8 years ago [30] there are few examples in the protein literature that utilize these methods of generating ion supersaturation and subsequent nucleation. Further, many studies relied on different ion concentration levels, control conditions, time periods, temperatures, pH values, and most importantly, protein quantities, in their assay experiments [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]. Typically, in most cases protein quantities have been reported as mass/volume values [34][35][36][37][38][39][40][41][42][43][44][45]52,53] rather than mole values [46][47][48][49][50][51], and this lack of standardization prevents cross-comparisons between protein studies.…”

“…The other important issue regarding past protein-polymorph studies is the intent: These studies focused exclusively on a "yes or no" outcome-i.e., did aragonite or vaterite form [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]?-rather than monitor the stepwise or temporal process of polymorph formation that is a prerequisite for mechanism development. These studies confirm that certain calcium carbonate polymorphs do form when proteins are present [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53], but tell us nothing about how this process occurs, particularly within non-classical nucleation [29][30][31][32][33] or CPA [15] schemes.…”

“…Second, and perhaps most importantly, genomic and proteomic studies of biomineralization proteins have uncovered a vast repertory of proteins [6][7][8]13,14,[26][27][28] that could potentially manage many aspects of proposed nucleation processes, including polymorphism. However, although many protein studies have confirmed that polymorphs do form when proteins are present [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53], these studies have not provided information that is needed for detailing the mechanisms of polymorph formation, or, have utilized a variety of testing methods that prevent cross-comparisons of datasets necessary for mechanism building [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]…”