Influence of amino acids on crystallisation of CaCO3

In an investigation of the influence of amino acids on the crystallisation of CaCO3 Borukhin et al. investigated samples prepared in the presence of each of the 20 naturally occurring amino acids at diffferent concentrations in aqueous solution [1]. Important factors seen to be controlling the incorporation of the amino acids include size, rigidity, and the relative pKa of the carboxyl and amino groups of the acid. More-acidic amino acids (such as aspartic acid) are richly incorporated, as was cystene (probably because of a thiol-calcium interaction).

Figure 1 shows the calcite 104 reflection for samples prepared in the presence of aspartic acid indicating the development of a bulk lattice strain along the c axis on incorporation of the molecule. Annealing at temperatures of 200ºC and above leads to relaxation of the strain on degradation of the incorporated molecules, accompanied by a marked increase in the peak width which indicates a sharp drop in crystallite size and an increase in microstrain. This behaviour very much replicates what was seen in earlier studies on samples of calcite of natural biogenic origin (sea shells). The insights from this study shed new light on the biomineralisation process via the incorporation of organic molecules into an inorganic host.

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Figure 1. (left) 104 reflection from calcite samples prepared with aspartic acid showing a shift to a larger d-spacing with the incorporation of the molecule into the structure. A hint of remnant material with no incorporated acid is visible at the two highest concentrations. (right) Annealing at 200ºC and above leads to relaxation of the lattice strain as the incorporated molecules are removed, and a reduction in crystallite size and an increase of microstrain, [1].

 

[1] Screening the incorporation of amino acids into an inorganic crystalline host: The case of calcite. Borukhin S., Bloch L., Radlauer T., Hill A.H., Fitch A.N., Pokroy B. Adv. Funct. Mater. 22, 4216, (2012).

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Incorporation of a copolymer in calcite

There is also interest in the creation of artificial materials via incorporation of molecules into mineral hosts, in an attempt to mimic natural processes and produce materials with specific properties. Remarkably, it was shown that single crystals of calcite can be produced with 13% of occludedmicelles of an anionic diblock copolymer [2]. The material was investigated by a wide range of techniques, including high resolution powder diffraction, where highly asymmetric peaks were apparent, Figure 2, indicative of a compressive-strain gradient in the calcite lattice, decreasing from a maximum at the interface beteween calcite and the micelles.

 

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Figure 2. (a) 006 reflection of calcite showing marked asymmetry towards lower d spacings; (b) model used to evaluate the effect of the particles on the crystal lattice: a particle of radius a causes a strain gradient in the lattice to a distance b from its centre (strain-affected zone) also showing the distribution of compressive strain; (c-e) variation of peak width components for the 104 reflection on heating, [2].

The influence of this strain on the calcite lattice was evaluated using a model derived for metal-particle ceramic composites, with Vp = (a/b)3, where Vp is the micelle volume fraction, a is the micelle radius, and b is the radius of the calcite strain-affected zone. Assuming a particle radius of 10 nm and that the nanocomposites comprise 29 vol% copolymer, the strain-affected zone in the crystal lattice is calculated to extend 15 nm from the particle centre, suggesting that 40 vol% of the crystal lattice is included within the strain-affected zone.

Samples were annealed at temperatures between 100 and 600°C for 30 min, and spectra were recorded after each heating cycle. Analysis of the line profile showed that two phenomena occurred on heating, namely a reduction in the peak anisotropy up to 300°C, followed by a general broadening of the peaks with further heat treatment. The decrease in peak anisotropy can be attributed to softening and then decomposition of the particles on heating, with overall broadening compatible with a reduction in particle size, as seen with biogenic materials.

[2] An artificial biomineral formed by incorporation of copolymer micelles in calcite crystals. Kim Y.Y., Ganesan K., Yang P., Kulak A.N., Borukhin S., Pechook S., Rosenbrier Ribeiro L., Kröger R., Eichhorn S.J., Armes S.P., Pokroy B., Meldrum F.C. Nature Mater. 10, 890, (2011).

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References:

[1] Screening the incorporation of amino acids into an inorganic crystalline host: The case of calcite. Borukhin S., Bloch L., Radlauer T., Hill A.H., Fitch A.N., Pokroy B. Adv. Funct. Mater. 22, 4216, (2012).

[2] An artificial biomineral formed by incorporation of copolymer micelles in calcite crystals. Kim Y.Y., Ganesan K., Yang P., Kulak A.N., Borukhin S., Pechook S., Rosenbrier Ribeiro L., Kröger R., Eichhorn S.J., Armes S.P., Pokroy B., Meldrum F.C. Nature Mater. 10, 890, (2011).