Interfacial Partitioning Enhances Microextraction by Multicomponent Nanodroplets

Sensitive and reliable in-droplet chemical analysis benefits from enhanced partition of an analyte into the droplets. In this work, we will show that colorimetric chemical reactions in surface nanodroplets can shift the partition of acid analytes from a highly diluted solution to the droplets. As a result, acids appear to be more acidic in the droplets. In our experiments, seven types of organic acids with partition coefficients (LgP) ranging from -0.7 to 1.87 dissolved in an oil solution are extracted into aqueous nanodroplets on a substrate. The timescale of coupled extraction and colorimetric reaction in the droplets was revealed by the decoloration time of the droplets. Our results show that the distribution coefficient of the acid can be shifted by 3-11 times of that in bulk. Such significantly shifted partition is attributed to enhanced transfer of the acid across the droplet surface from a highly diluted solution. The interfacial activity of the analyte may be advantageous as it may also improve extraction and partition. Such enhanced extraction by chemical reaction in droplets may be leveraged for ultrasensitive chemical detection using reactive droplets.