A solution study of silica condensation and speciation with relevance to in vitro investigations of biosilicification

Abstract

Requiring mild synthesis conditions and possessing a high level of organization and functionality, biosilicas constitute a source of wonder and inspiration for both materials scientists and biologists. In order to understand how such biomaterials are formed and to apply this knowledge to the generation of novel bioinspired materials, a detailed study of the materials, as formed under biologically relevant conditions, is required. In this contribution, data from a detailed study of silica speciation and condensation using a model bioinspired silica precursor (silicon catechol complex, SCC) is presented. The silicon complex quickly and controllably dissociates under neutral pH conditions to well-defined, metastable solutions of orthosilicic acid. The formation of silicomolybdous (blue) complexes was used to monitor and study different stages of silicic acid condensation. In parallel, the rates of silicomolybdic (yellow) complex formation, with mathematical modeling of the species present, was used to follow the solution speciation of polysilicic acids. The results obtained from the two assays correlate well. Monomeric silicic acid, trimeric silicic acids, and different classes of oligomeric polysilicic acids and silica nuclei can be identified and their periods of stability during the early stages of silica condensation measured. For experiments performed at a range of temperatures (273-323 K), an activation energy of 77 kJ.mol(-1) was obtained for the formation of trimers. The activation energies for the forward and reverse condensation reactions for addition of monomers to polysilicic acids (273-293 +/- 1 K) were 55.0 and 58.6 kJ.mol(-1), respectively. For temperatures above 293 K, these energies were reduced to 6.1 and 7.3 kJ.mol(-1), indicating a probable change in the prevailing condensation mechanism. The impact of pH on the rates of condensation were measured. There was a direct correlation between the apparent third-order rate constant for trimer formation and pH (4.7-6.9 +/- 0.1) while values for the reversible first-order rates reached a plateau at circumneutral pH. These different behaviors are discussed with reference to the generally accepted mechanism for silica condensation in which anionic silicate solution species are central to the condensation process. The results presented in this paper support the use of precursors such as silicon catecholate complexes in the study of biosilicification in vitro. Further detailed experimentation is needed to increase our understanding of specific biomolecule silica interactions that ultimately generate the complex, finely detailed siliceous structures we observe in the world around us.

Figure 1

(A) Results of titrimetry with hydrochloric acid with markers representing the levels of molar ratio additions used in the ¹H NMR study. (B) Typical ¹H NMR spectra of partially and fully dissociated complex (singlet at 6.65ppm from complex, multiplet centred at 6.85 from dissociated 1,2 dihydroxybenzene). X:Y represents the mole ratios of Si:H⁺. (C) Plot of residual complex for samples treated with increasing amounts of acid. (D) Acid dissociation of silicon catecholate complex precluding the formation of partially hydrolysed species in the time frame of the NMR experiment.

Figure 2

(A) Plot of residual complex with dissociation time for samples treated with increasing amounts of acid. (B) Relationship between pH and residual complex concentration. The pH values 7.2, 7.1, 6.9, 6.4 and 3.0 correspond to molar equivalents of acid to complex of 1.4, 1.6, 1.8, 1.9 and 2.0:1 respectively.

Figure 3

(A) The decrease in silicomolybdous acid (molybdenum blue) complex with time following pH reduction for 30 mM initial [Si(OH)₄] at pH 6.8, and (B) the formation of silicomolybdic acid yellow complex after increasing condensation times. The vertical lines in (A) represent a selection of the sampling times used to obtain the molybdenum yellow data presented in (B). The vertical line on (B) denotes the time at which the blue silicomolybdic acid complex is formed from species available in solution.

Figure 4

Silicate speciation during the condensation process for 30mM initial [Si(OH)₄] at pH 6.8, with 3 (A), 4 (B) and 5 (C) species fits.

Figure 5

(A) Formation of silicomolybdic acid complex after increasing condensation times for 10mM initial [Si(OH)₄] at pH 6.8 and (B) Silicate speciation during the condensation process for 10mM initial [Si(OH)₄] at pH 6.8–3 species. The vertical line denotes the time at which corresponding molybdenum blue data was collected for the reactions.

Figure 6

(A) Apparent third order regions for condensation measured by the molybdenum blue assay. (B) Plot of the log of the apparent third order rate constants against pH and (C) Log of the forward and reverse of the first order rate constant variation with pH.

Figure 7

(A) apparent third order rate domains at 273 – 323K (B) Arrhenius plot of the apparent third order rates and (C) Arrhenius plot for the forward and reverse first order rate constants.

Scheme 1

Silicate species isolated by curve fitting of molybdenum yellow (silicomolybdic acid) data during the condensation of orthosilicic acid from an example 30mM condensing system