Revealing the dynamics of nanoparticle formation

The motivation – The Gates lab at Simon Fraser University is studying fundamental processes by which nanoparticles are formed through synthesis in solution. The ability to control the shape, size, composition, and purity of the product is critical for many applications, specifically those that rely on the nanoscale dimensions of these materials. Many syntheses can also yield unwanted by-products that include larger particles. Some syntheses also use reagents that are difficult to obtain or purify. Dr. Gates and his graduate student, Mr. Rana Faryad Ali sought to develop a simpler approach, while also establishing a detailed understanding of how nanoparticle shape, size, and composition evolve over time during the reaction.

The discovery – For this study, they focused on preparing nanoparticles of a man-made compound called lithium niobate. This material has important applications in modern technologies – from optical devices to serving as contrast agents for cells – because it has the ability to convert longer wavelengths into shorter wavelengths through a process called “second harmonic generation”. In their new synthesis of these nanoparticles, the particles underwent an initial process of nucleation and agglomeration. One of the most interesting findings from this project was that these agglomerates were larger than the final particle size achieved from this synthesis. The particles continue to grow, yielding a product at a particular time point that is single-crystalline and uniform in size. Further growth did, however, lead to an increase in the polydispersity of the product. The nanoparticles were also confirmed to be optically active for second harmonic generation.

Its significance – This interesting process of forming nanoparticles could apply to particles beyond this system. The final growth of the lithium niobate nanoparticles proceeds through a process called “Ostwald ripening” where material is added to stable nuclei that grow into larger particles. These particles could enable further development of second harmonic generation based microscopy techniques, such as contrast agents or markers for bioimaging applications with improved photostability over molecular reagents and a lower elemental toxicity than many quantum dots.

Read the paper – “Synthesis of Lithium Niobate Nanocrystals with Size Focusing through an Ostwald Ripening Process” by Rana Faryad Ali and Byron D. Gates. Chemistry of Materials 30(6): 2028–2035 (2018). DOI: 10.1021/acs.chemmater.7b05282