Novel solvents to stabilise nanoparticles
Nanoparticles and large molecules are everywhere in advanced materials and pharmaceutical products. From touchscreens to protein drugs, the stabilisation of these particles is central to their ability to perform their required function. For instance, vaccine formulations, which use proteins or nucleic acids, often require precise conditions like extremely cold temperatures to ensure they remain effective. It is these requirements that makes the supply and distribution of vaccines difficult – a prominent challenge in the COVID-19 pandemic. Finding an effective way to stabilise and store these materials will underpin our ability to develop better technological products in the future.
A solvent (a liquid substance that can dissolve other substances) is typically used to disperse and stabilise these tiny particles. For the last century, scientists have traditionally used water for this. That is because we know water very well – it’s readily available, we understand its strengths and weaknesses, and its long-term behaviour. However, it is in no way ideal as it often promotes the degradation of unstable materials, such as proteins. Organic solvents also present several limitations, as they have a significant environmental effect, with the volatile and toxic compounds difficult to make and dispose of safely. As such, everyone from material scientists to pharmaceutical companies, have a keen interest in finding alternative solvents to keep these important materials safe.
Alternatives to water have been explored for many years. Recently, an exciting class of solvents, called Deep Eutectic Solvents (DES), have emerged as potential alternatives to traditional solvents for many applications, such as the synthesis of advanced materials and drug delivery systems. These could hold the answer to many of the current challenges we face finding alternatives for the stabilisation and preservation of nanoparticles. DES are liquids that are simply prepared by a mixture of two solid materials at a specific ratio. For example, two natural sugars – glucose and fructose – which are both white powders, become a transparent liquid when mixed in equal proportions. Synthesised by mixing readily available substances, such as fatty acids, sugars, and alcohols, DES are safer, cheaper, and more sustainable than classical organic solvents. The variety of combinations of compounds to make DES also means they can be tailored to perfectly match a specific function, for instance the requirements for the storage of a protein. As such, DES could be designed to mimic the natural biological environment of the protein, and prevent it from degrading.
Unlike water, however, there’s a lot we don’t know about these solvents. To fully understand their potential, it is important to study them in as much detail as possible. An international collaboration between scientists at Institut Laue-Langevin, Lund University (Sweden), European Spallation Source (Sweden), and University of Bath (UK) have explored the ability of DES to stabilise nanoparticles. The team of researchers have looked closely at the interactions between the molecules, so that they can model, and in turn predict, the behaviour of interacting particles in DES. With the wide range of possibilities of DES, this study helps academia and industry to identify the fundamental aspects of the behaviour of these solvents for preserving easily broken-down materials.
The best possible technique for studying a solution like DES is Small Angle Neutron Scattering (SANS). This technique allows the user to view how the particles are distributed in space, and observe how all components interact. For example, we can see how particles repel each other (a pre-requisite for the stability of the dispersion) or conversely, how they aggregate (eventually leading to a phase separation: the solvent does not disperse well the compound). With neutrons, you also have the capability to employ a technique called contrast variation. This allows scientists to make chosen parts of their analysis appear or disappear, by varying the amount of deuterium – a heavy form of hydrogen – introduced to the sample. By probing different sets of dispersed particles in DES with the powerful neutron beams at ILL, the researchers produced a scattering pattern that represented the interactions occurring between particles in the DES solution. By mapping it using theoretical models, they were able to establish how the electrostatic landscape controls the distribution of particles in space, and how that impact the stability and behaviour of the system.
This study marks a step forward in our understanding of these promising solvents, which brings us to the next generation of more sustainably designed solvents for technological applications. This is an important foundation for building a new framework that can effectively share and enable access to new technologies. For example, DES may one day contribute to the holy grail of vaccine storage – with vital medicines stored at room temperature thanks to their perfectly preserving solvents.
Re.: “Long-Range Electrostatic Colloidal Interactions and Specific Ion Effects in Deep Eutectic Solvents", by Adrian Sanchez-Fernandez et al. , Journal of the American Chemical Society (2021).
ILL instrument : Lowest momentum transfer & lowest background small-angle neutron scattering instrument D11
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