  The sensitive and reliable in-droplet chemical analysis benefits from the
enhanced partition of an analyte into the droplets. This work, we will show
that chemical reactions in surface nanodroplets can shift the partition of
analytes from a highly diluted solution to the droplets. Seven types of organic
acids with partition coefficients (LgP) ranging from -0.7 to 1.87 are used as
model analytes dissolved in an oil solution that are extracted from the flow
into aqueous nanodroplets immobilized on a substrate. The timescale of
integrated extraction and reaction in droplets was represented by the
decoloration time of the droplets. Our results show that the effective
distribution coefficient of the analyte can be decreased by 3 to 11 times of
the distribution coefficient of the analyte in the bulk liquids. The principle
behind the significantly shifted partition is proposed to be enhanced the
transfer of the analyte across the droplet surface. The chemical reaction in
the droplets enhances the partition of the analyte from a highly diluted
solution. Our results show that the interfacial behavior of the analyte may be
advantageous as it may improve extraction and partition. Such enhanced
extraction may be leveraged for sensitive chemical detection using reactive
droplets.
