KINETIC AND THERMODYNAMIC DRIVERS OF AMORPHOUS SILICA NUCLEATION ON ORGANIC SURFACES: TOWARDS AN UNDERSTANDING OF BIOSILICIFICATION PROCESSES

Biochemical investigations have begun to yield information about structural and chemical properties of organic macromolecules involved in biosilicification processes. However, the mechanisms by which these molecules mediate biosilica formation remain unclear. The formation of mineralized structures in organisms is rooted in processes taking place at the nanoscale, and therefore, molecular level investigative probes are required. Insights into how mineral formation occurs within living organisms can be gained by conducting experimental studies with simple model systems that emulate key features of biological systems. Our approach utilizes a novel AFM-based approach to measure the dependence of amorphous silica nucleation kinetics on the chemical and structural nature of the underlying substrate.

Model biological surfaces terminated with carboxyl, hydroxyl, and amine moieties were generated through the spontaneous adsorption of Ω-alkanethiol self-assembled monolayers onto ultra-flat (111) surfaces of gold. Silica nucleation experiments used supersaturated solutions of silicic acid that were produced by the acid catalyzed hydrolysis of tetramethyl orthosilicate. Measurements of the surface nucleation rate were conducted under conditions that simulate current views of conditions within silica deposition vesicles of major diatom species, (e.g. ambient temperature, pH = 5.0, NaCl = 0.1 mol/kg). Aqueous silicate levels were varied to examine dependencies on saturation state.

Analysis of the kinetic data within the framework of nucleation theory quantifies the height of the kinetic barrier to silica formation, and the net energy of the silica-substrate\solution interfaces. By conducting experiments for COOH, NH₃⁺, and OH-functionalized substrates, we determine the kinetic and thermodynamic controls of functional group chemistry on the heterogeneous nucleation of amorphous silica. These findings are providing new insights into how biochemical interfaces mediate the onset of silica formation.