Driving Biomineralisation Using Soft Templation
Authors:   , Lendrum Conrad Douglas 
Published: November 2021

<p>Sea shells, bones and teeth are three examples of Nature's unrivalled ability to produce complex hierarchical structures from simple inorganic materials. Unlike the synthetic approach of using 'exotic' materials to introduce functionality, Nature has employed structural control to maximise properties. Key to this control is the use of an organic framework to guide inorganic nucleation and growth. The question of how structural information is transferred from the organic framework to the inorganic crystal has inspired many studies in the field of biomineralisation, yet our understanding remains limited. One aspect that has received considerable attention is the molecular recognition process that occurs at the organic/inorganic interface. Unlocking the mysteries of the intermolecular interactions associated with molecular recognition using a model Langmuir monolayer system is the aim of this research. Elucidation of the molecular recognition process requires an understanding of host/guest chemistry, double layer theory, Langmuir monolayer chemistry, and crystallisation theory, with the added complexity that both the host and guest are dynamic and constantly changing. This level of complexity demands a holistic approach to accommodate the many interacting parameters, therefore this study consists of a comparative analysis of calcium carbonate crystallisation under twelve subtly altered surfactant monolayer systems. Based around the acid and alcohol moieties, commonly explored in biomineralisation studies, these monolayer systems involve: mixtures of octadecanoic acid and octadecanol, hydroxyl-, carboxyl-, bromine- and methyl- substituted octadecanoic acids. By making minor chemical modifications to the membrane molecules we can subtly alter the electronic landscape presented to the supersaturated subphase and probe how the mix of intermolecular forces changes the interfacial interaction. In order to understand the monolayer/subphase interaction and therefore build up a picture of the crystallising system each monolayer was probed on pure water, calcium chloride and sodium bicarbonate subphases. The understanding gleaned from these experiments fed into the elucidation of the significantly more complex calcium carbonate crystallising subphase/monolayer interaction. Information about monolayer and subphase behaviour was obtained from surface pressure isotherms, surface potential measurements, Brewster Angle Microscopy, grazing incidence Xray diffraction (GIXD) and X-ray reflectivity (XRR). This information was correlated with crystal properties such as the nucleation face and gross morphology to develop a picture of the interfacial interaction. Results show that monolayer surface charge and ion-ion electrostatic interactions are important but do not dictate crystal orientation. The manipulation of the head group chemistry highlighted the influence of head group spacing and therefore lattice matching in crystal orientation. Further it was found that a high degree of interfacial matching not only facilitated face-selective nucleation but also has a significant impact of crystal morphology. GIXD results show the rearrangement of the monolayer structure upon nucleation for the first time. Combined with X-ray reflectivity generated electron density profiles this has lead to a significant improvement in our understanding of the interfacial interaction. As such this body of work has culminated in the proposition of a cation-mediated hydrogen-bonded soap network facilitated by the presence of the bicarbonate anion as an intermediate entity for crystal nucleation under Langmuir monolayers. Such a network accounts for the influence of electrostatics, lattice, symmetry and spatial geometry matching that contribute to face-selective nucleation and more generally the molecular recognition process in biomineralisation. However the evidence presented here for a monolayer/subphase network is largely qualitative and the hypothesis requires more direct validation.</p>