Probing ice nucleation by bio-nanoparticles

Pure water droplets in clouds can remain liquid down to temperatures approaching -40 degrees C because creating an interface between the first region to solidify (the nucleus) and the remaining liquid has a free energy cost. However, even in very small water droplets, ice can also form at temperatures considerably closer to 0 degrees C, because of the presence of heterogeneous nucleating agents (HNAs) that lower the free energy barrier to nucleation. These ice HNAs are vitally important in contexts as varied as medicine, agriculture and environmental science. By modifying how water droplets freeze, they play a significant role in determining cloud reflectivity as a function of wavelength and therefore how different types of cloud contribute to global warming. In biology, HNAs for ice can prevent intracellular damage when freezing tissue for cryopreservation but can cause crop damage when produced by plant pathogens.

Despite their importance, ice HNAs are poorly understood. Understanding the many factors that contribute to how effective a particle is as an ice nucleating agent remains a significant challenge, as experimental data from which the separate influences of HNA size, surface chemistry and structure can be quantified are lacking. To date, the majority of ice nucleation studies have used naturally occurring micro- and nanoparticles, which are intrinsically heterogeneous, so that each sample necessarily contains HNAs with a wide range of efficiencies. Consequently, it is extremely difficult to relate HNA performance to specific structural and chemical properties.

To overcome this difficulty, we shall engineer bio-nanoparticles to act as model ice HNAs. We shall study two types of bio-nanoparticle having a well-controlled structure and chemical properties: quasi-spherical cage-like proteins and nanoparticles assembled using DNA origami. Quasi-spherical cage-like proteins are excellent model HNAs because they are highly symmetric and it is quite straightforward to modify their surface chemistry in a controlled way. For example, we shall investigate how changing the surface charge or introducing chemical patches that interact more and less strongly with water change the ice nucleating efficiency. DNA origami nanoparticles are a recent development, and are especially suited to study the strong effects of HNA geometry on ice nucleation efficiency. DNA origami makes use of the fact that DNA molecules incorporate a sequence of chemical units, known as bases, which can be programmed in such a way that the molecules assemble spontaneously into quite complex pre-determined shapes. In further experiments, we shall determine how the presence of solutes affects the ice nucleating efficiency of our engineered bio-nanoparticle HNAs, and investigate how they adsorb to pre-formed ice (which can inhibit further ice crystal growth).

This project will provide excellent interdisciplinary training. You will learn to use molecular biology techniques such as gel electrophoresis and protein assays, as well as gaining experience of modifying instrumentation for the ice nucleation experiments. Depending how the project evolves, there may also be an opportunity to use magnetic methods recently developed in our group to probe ice-nanoparticle interactions.

Enquiries
For enquiries about the application process contact [Email Address Removed]

How to apply
Please make an online application for this project at http://www.bris.ac.uk/pg-howtoapply. Please select Physics PhD on the Programme Choice page. You will be prompted to enter details of this specific project in the ‘Research Details’ section of the form.
Anticipated start date: September 2019

Candidate requirements
A first degree in physics or a related subject, normally at a level equivalent to at least UK upper second-class honours, or a relevant postgraduate master's qualification.
See international equivalent qualifications on the International Office website: http://www.bristol.ac.uk/international/countries/