Core-Shell nanoparticle models for in-situ SERS measurements of carbonate dissolution under environmentally realistic conditions

Our aim is to develop an in-situ analytical probe of dissolution processes that can address, at a molecular level, the mechanistic questions raised by uncertainties in the rates of biogenic carbonate dissolution. Surface enhanced Raman spectroscopy (SERS) will be used to monitor the behaviour of carbonate shells grown on custom made gold nanoparticles ('Core-Shell' structures). By manipulating the structure and habit of the carbonate in these systems we will explore the individual effects of chemical composition, mineral structure and lattice imperfection on carbonate solubility with an in-situ, molecularly specific probe. The particular issue we are addressing is that of the dissolution of marine carbonates, which represents a fundamental component of the global carbon cycle. The production of biogenic carbonate in the surface ocean greatly outweighs the supply of its ingredients from rivers and without dissolution of much of this carbonate prior to its burial at the seafloor, the system could not sustain itself. However, anthropogenic emission of CO2 leading to ocean acidification (OA), is effectively lowering the saturation state of the oceans with respect to carbonate and raising considerable concerns regarding the future carbon balance in the oceans. Despite decades of study there remains great uncertainty concerning some of the most fundamental aspects of carbonate dissolution: even the rate order for carbonate dissolution has an uncertainty factor of 4. Furthermore, the partial dissolution of carbonate shells is empirically known to affect their chemical composition, which itself is often used to proxy ancient oceanic conditions. The current uncertainty as to how dissolution precisely affects composition presents critical limitations on the applicability of these tools. Many of these uncertainties have arisen from the complexity (in mineralogy and form) of 'typical' biogenic carbonates (e.g. foraminifera and coccoliths, that comprise most of the global production of marine carbonates). The proposed studentship aims to develop core-shell nanoparticle SERS as an analytical technique capable of providing mechanistic surface information under realistic conditions. Nanoparticle core-shell synthesis techniques for carbonates will be developed and rates of dissolution determined and compared against bulk rates and shell thickness measurements from TEM (FENAC) and XPS. Raman can distinguish between all the different structural polymorphs of calcium carbonates and thus will enable the project to progress to an investigation of the different carbonate structures. Specific gold core shapes (spheres, rods, cubes etc) will be synthesised to investigate the role of structural stress on the carbonate shell structure, growth habit and subsequent dissolution. The project will lay the ground work for the development of a high pressure system which will be the subject of a future application. The project offers a unique opportunity for a PhD student to gain extensive training in nanoscience techniques and nanoscale synthesis combined with experience in the field of analytical chemistry applied to environmental problems.