Electrolyte-Doped Ice as a Platform for Atto- to Femtoliter Reactor Enabling Zeptomol Detection

Rapid freezing of an aqueous electrolyte in liquid nitrogen provides an effective way to fabricate uniform-sized liquid pores with the radius ranging from 0.15 to 3 μm (<1% rsd), corresponding to atto- to femtoliter volumes. The size of liquid pores depends on the temperature, and the concentration and type of a salt incorporated into an original aqueous solution. When the concentration of a salt is kept lower than 20 mM, liquid pores are discretely distributed in an ice matrix. Unlike usual small spaces accommodating liquid water, the pore size is tunable and continuously variable by changing the above experimental parameters. The liquid pore has been utilized as microreactors, in which the fluorescent complexation between Mg²⁺ and 8-hydroxyquinoline-5-sulfonic acid (HQS) is studied. Under the optimum condition, fluorescence from Mg²⁺ ions in the zeptomol level confined in a liquid pore is detected.