  A combinative approach for chemical analysis makes it possible to distinguish
a mixture of a large number of compounds from other mixtures in a single step.
This work demonstrates a combinative analysis approach by using surface
nanodroplets for integrating nanoextraction and colorimetric reactions for the
identification of multi-component mixtures. The model analytes are acidic
compounds dissolved in oil that are extracted into aqueous droplets on a solid
substrate. The proton from acid dissociation reacts with the halochromic
chemical compounds inside the droplets, leading to the color change of the
droplets. The rate of the colorimetric reaction exhibits certain specificity
for the acid type, distinguishing acid mixtures with the same pH value. The
underlying principle is that the acid transport rate is associated with the
partition coefficient and the dissociation constant of the acid, in addition to
the concentration in oil. As a demonstration, we showed that droplet-based
combinative analysis can be applied for anti-counterfeiting of various
alcoholic spirits by comparing decolor time of organic acid mixtures in the
spirits. The readout can be done by using a common hand-hold mobile phone.
