Thermoswitchable electrokinetic ion-enrichment/elution based on a poly(N-isopropylacrylamide) hydrogel plug in a microchannel.

This paper reports a novel protocol consisting of the thermomodulated electrokinetic enrichment, elution, and separation of charged species based upon a thermoswitchable swelling-shrinking property of a poly(N-isopropylacrylamide), PNIPAAm, hydrogel. A 0.2-1 mm long PNIPAAm hydrogel plug was photopolymerized inside a glass microfluidic channel to produce a composite device consisting of the PNIPAAm hydrogel plug and the glass microchannel (abbreviated as plug-in-channel). After voltage was applied to the composite device, anions, such as FITC, could be enriched at the cathodic end of the PNIPAAm plug when the temperature of the plug was kept below its lower critical solution temperature (LCST, ∼32 °C). The concentrated analytes could then be eluted by electroosmotic flow when the temperature of the plug was heated above the LCST. The mechanism of the thermoswitchable ion enrichment/elution process was studied with the results presented. The analytical potential of the composite device was demonstrated for the temperature-modulated preconcentration, elution, and separation of FITC-labeled amino acids.