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Elastomeric microvalves in poly(dimethylsiloxane) (PDMS) devices are today's paradigm for massively parallel microfluidic operations. Here, we report that such valves can act as nanochannels upon closure. When tuning nanospace heights between ~55 nm and ~7 nm, the nanofluidic phenomenon of concentration polarization could be induced. A wide range of concentration polarization regimes (anodic and cathodic analyte focusing and stacking) was achieved simply by valve pressure actuation. Electro-osmotic flow generated a counterpressure which also could be used to actuate between concentration polarization regimes. 1000-fold preconcentration of fluorescein was achieved in just 100 s in the anodic focusing regime. After valve opening, a concentrated sample plug could be transported through the valve, though at the cost of some defocusing. Reversible nanochannels open new avenues for integrating electrokinetic operations and assays in large scale integrated microfluidics. [ABSTRACT FROM AUTHOR]

Isotachophoretic separations are triggered at the border of a nanochannel-induced ion-depleted zone. This depletion zone acts as a terminating electrolyte and is created by concentration polarization over the nanochannel. We show both continuous and discrete sample injections as well as separation of up to four analytes. Continuous injection of a spacer compound was used for selective analyte elution. Zones were kept focused for over one hour, while shifting less than 700 μm. Moreover, zones could be deliberately positioned in the separation channel and focusing strength could be precisely tuned employing a three-point voltage actuation scheme. This makes depletion zone isotachophoresis (dzITP) a fully controllable single-electrolyte focusing and separation technique. For on-chip electrokinetic methods, dzITP sets a new standard in terms of versatility and operational simplicity. [ABSTRACT FROM AUTHOR]