Emulsification-induced selective concentration in microdroplets

The selective concentration method of the contents in a microdroplet using spontaneous emulsification (Selective Concentration method by using Spontaneous Emulsification, SESC method) was reviewed. Water-in-oil microdroplets formed in microfluidic devices have been studied with eagerness in the past decade because they can be used as picolitter-sized chemical containers and applied to analysis for high-throughput microanalyses, such as single cell assay. However, quantification of the microdroplet contents was difficult because of its short optical path lengths. In order to overcome this difficulty, we developed the selective condensation method for microdroplet contents by utilizing spontaneous emulsification at the interface of the microdroplets. When microdroplets having a 40-μm diameter were formed in octane containing Span 80, they shrank to 10 μm within 10 min with nanodroplet formation at the interface of the microdroplets. The microdroplets’ contents either stayed in the microdroplet or partitioned into the nanodroplets, depending on their properties. In this review, we describe our basic understandings about the SESC method. In addition, applications of the SESC method to the bioanalytical processes, such as protein quantification and protein crystallization, are summarized. We expect that the SESC method will enhance the flexibility of the design of droplet analytical processes and widen their applicability.