NaPiDro - Nanoelectrodes in Picoleter Droplets: Towards Single Molecule Chemistry
Final Report Abstract
This project explored the potential of using aqueous droplets within a carrier liquid as microreactors for electrochemical reactions. There are several potential advantages of conducting experiments in this way: use of minute amounts of chemicals, potentially fast conversion, efficient distribution of flux of energy and the possibility to transport the reaction mixtures to specific location where they are causing an intended action. In this project, electrochemical reactions were in the focus. For the intended concepts they bring some new possibilities and some challenges. We built on procedures of generating very small droplets by exploiting the breakup of a neck between an orifice of a dispensing unit and a mother droplet to form extremely small droplets. Those droplets can be easily charged because the process is performed with an applied voltage. The charged aqueous droplets can then be guided between the orifice, at which they are formed, towards a microelectrode on which they shall land for a further conversion. Different small droplets can be merged on a sessile droplet on a microelectrode surface, similar to dosing reagents to a laboratory reactor. Furthermore, by manipulating the voltage between the orifice and the microelectrode, the number of drops that land on the microelectrode can also be restricted to a small number and further droplets are repelled. There are also challenges: The landing event of droplets on the microelectrode surface does not form an electrochemical cell immediately. Apparently, a lamella of the carrier liquid remains between the aqueous droplet. This lamella ruptures on a timescale of seconds, after which an electrolytic current sets in. This electrolysis current can be well described by established equation while the complete explanation and description of the rupture event was unexpected and has not been completed within this project. A second challenges concerns the hydrophobic, viscous and nonvolatile carrier liquid, in which the droplets are generated. The electrolysis of the solutes within the aqueous droplets can be conducted if two microelectrodes contact the aqueous phase at the same time. This can be challenging to achieve if the droplet reactors are very small. Alternatively, the counter electrode and a reference electrode for better adjustment of the electrode potential are placed in the carrier liquid, outside the aqueous droplet. This requires the carrier liquid to be conductive. We devised composition of such conductive carrier liquids that enable such cell setups.
Publications
- Local studies of photoelectrochemical reactions at nanostructured oxides. Curr. Opin. Electrochem. 2019, 13, 25-32
G. Wittstock, S. Rastgar, S. Scarabino
(See online at https://doi.org/10.1016/j.coelec.2018.10.007) - Catalytic activity of alkali metal cations for chemical oxygen reduction reaction in a biphasic liquid system probed by scanning electrochemical microscopy. Chem. Eur. J. 2020, 26, 10882- 10890
S. Rastgar, K. Teixeira Santos, C. A. Angelucci, G. Wittstock
(See online at https://doi.org/10.1002/chem.202001967) - Pneumatic conveying printing based on super hydrophobic surfaces. Adv. Mater. Interfaces 2020, 7, 1902131/1-8
D. Li, Y. Cao, H. Dong, X. Wu, Q. Sun, C. Ma, B. Huang, S. Rastgar, G. Wittstock, Y. Liu, Y. Zhang
(See online at https://doi.org/10.1002/admi.201902131)
