We explore the use of tetraethoxysilane (TEOS) as a silica source for the formation of carbonate-silica composite materials known as 'biomorphs'. The basic hydrolysis of TEOS furnishes silica in a controllable fashion, allowing a significantly higher reproducibility of the obtained silica-barium and silica-strontium carbonate co-precipitates compared to commercial water glass silica used so far. We further discuss the influence of ethanol used as a co-solvent on the morphologies of biomorphs, which are examined by optical microscopy, field emission scanning electron microscopy (FESEM) and energy dispersive X-ray analysis (EDX).

ABSTRACT:The precipitation of calcium carbonate in alkaline silica solutions results in the formation of complex curvilinear forms if aragonite formation is encouraged by growth at an elevated temperature (80°C). The resulting coralline self-assembled silica-calcium carbonate particles are "biomorphs", bearing a striking resemblance to natural coral forms. These materials, comprised of calcium carbonate nanocrystals and an amorphous silica matrix, have a complex ultrastructure, made of clusters of gathered sheets of variable curvatures formed by successive curling. The nanocrystals within these "ruled surfaces" are thin, elongated, densely packed needles of aragonite. These clusters are outgrowths from central saddlelike cores that resemble developable petaloid surfaces. The size, shape, crystallography, and chemical composition of the resulting biomorphs were examined by optical microscopy, field emission scanning electron microscopy (FE-SEM), powder X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM and HRTEM), and energy dispersive X-ray analysis (EDX).

The influence of egg white lysozyme on the size, shape, crystallography, and chemical composition of amorphous calcium carbonate (ACC) particles obtained from aqueous CaCl2-dimethyl carbonate (DMC)-NaOH solutions was studied. At the onset of precipitation, the presence of lysozyme led to much smaller particles (50-400 nm spherical amorphous lysozyme-calcium carbonate particles (Ly-ACC)) than those obtained from lysozyme-free solution. The nanospheres were in some cases aggregated and in addition embedded in a faint network. Their size and interconnection depended on the concentration of egg white lysozyme. When the Ly-ACC particles were left in contact with the mother liquor (CaCl2/DMC/NaOH/lysozyme solution) for 24 h, they transformed directly and exclusively into crystalline calcite. The observed results may be of relevance for a better understanding of the role of lysozyme in the process of eggshell mineralization.