Electrodeposition of palladium nanoparticles from a deep eutectic solvent and its application to uranyl electrocatalysis and sensing

Chemical and electrochemical methods have been reported to synthesize the metal nanoparticles. The chemical methods are generally time-consuming, multistep processes that require the use of both reducing agents and stabilizing molecules. In contrast, the electrochemical methods allow the nanostructures to be grown directly onto the final support in just one step, without the need for reducing and stabilizing agents or further sample preparation, thereby improving the electron pathway within the substrate, nanostructure and environment. Deep eutectic solvents (DESs) are a new generation of green solvents and have wide applications in nanoparticle synthesis, electrochemistry, and biochemistry owing to their remarkable physicochemical properties different from ionic liquids. We electrochemically synthesized palladium nanoparticles (PdNPs) utilizing a Pd salt on a gold (Au) electrode from a deep eutectic solvent formed by mixing choline chloride and urea at room temperature. PdNPs has emerged as an effective surface modifier for working electrodes in electrocatalysis and electroanalysis. However, the aqueous electrochemistry of the actinides on PdNPs is still unexplored. In this work, the aqueous electrochemistry of the redox couple [U^VIO₂(CO₃)₃]^4-/[U^VO₂(CO₃)₃]^5- was systematically investigated on a PdNPs/Au electrode in saturated Na₂CO₃ solution. It is known that the aqueous solution of UO²⁺ is very unstable as U(V) disproportionate to U(VI) and U(IV). The disproportionation reaction is catalyzed in the presence of acids. The carbonate forms a strong complex with uranium in the alkaline solution and thus stabilizes U(V) in aqueous solution. Due to long-term transport in the geosphere, CO₃^2- is abundant in groundwater. Therefore, it would be of interest to develop an efficient electrochemical sensor to detect uranium carbonate complexes migrating through the environment. In this direction, we have studied the electrochemistry of the U(VI)/U(V) redox couple on PdNPs in saturated Na₂CO₃ solution. The results showed that PdNPs can catalyze the redox chemistry of the U(VI)/U(V) redox couple by reducing both cathodic and anodic over potentials upto 50 mV compared to a bare Au electrode. (author)