Introducing Monoalkyl Phosphinic Acids as a New Ligand Platform for Zinc Sulfide Quantum Dots

Quantum Dots (QDs) stand at the forefront of modern technology. Their applications are integrated in optoelectronic devices such as biological imaging, biosensors, and even in the computer screen you are likely reading from. Due to their relatively small sizes, a large number of atoms are present on the particle's surface. This results in a high surface energy that can be passivated through the attachment of surface ligands. Organic ligands consist of carbon chains and hydrophilic functional groups, which vary in their degree of saturation and the length of their carbon chains. The properties of QDs, such as colloidal stability, can be manipulated by altering the organic ligand-capped surface, leading to their numerous applications. Carboxylic acid and phosphonic acid ligands are commonly used organic ligands to passivate the surface of QDs. Recently, Dhaene et al. proposed monoalkyl phosphinic acids as a new alternative ligand to the conventional carboxylic acid ligands for the effective passivation of the nanocrystal surface. Monoalkyl phosphinic acid ligands are great candidates for surface functionalization since their binding affinity is close to that of phosphonic acids, while having a similar binding mode to carboxylic acids. This project aims to build upon Dhaene et al.'s study by synthesizing two alkyl phosphinic acids-octadecylphosphinic acid and hexylphosphinic acid-characterizing the binding affinities of the phosphinic acids on the ZnS QD surface, and analyzing the surface ligand's colloidal stability based on chain length (18 carbons vs. 6 carbons). This project reports the syntheses of two monoalkyl phosphinic acid ligands, their characterization, and their attachment to ZnS QDs.