Heiko J. van der Linden

Single-Electrolyte Isotachophoresis Using a Nanochannel-Induced Depletion Zone

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.

Tunable ionic mobility filter for depletion zone isotachophoresis.

We present a novel concept of filtering based on depletion zone isotachophoresis (dzITP). In the micro/nanofluidic filter, compounds are separated according to isotachophoretic principles and simultaneously released selectively along a nanochannel-induced depletion zone. Thus, a tunable low-pass ionic mobility filter is realized. We demonstrate quantitative control of the release of fluorescent compounds through the filter using current and voltage actuation. Two modes of operation are presented. In continuous mode, supply, focusing, and separation are synchronized with continuous compound release, resulting in trapping of specific compounds. In pulsed mode, voltage pulses result in release of discrete zones. The dzITP filter was used to enhance detection of 6-carboxyfluorescein 4-fold over fluorescein, even though it had 250× lower starting concentration. Moreover, specific high-mobility analytes were extracted and enriched from diluted raw urine, using fluorescein as an ionic mobility cutoff marker and as a tracer for indirect detection. Tunable ionic filtering is a simple but essential addition to the capabilities of dzITP as a versatile toolkit for biochemical assays.

Solvent exchange module for LC-NMR hyphenation using machine vision-controlled droplet evaporation

We report the use of pendant droplet evaporation for exchange of eluents for (1)H nuclear magnetic resonance ((1)H NMR) purposes. Analytes are fed and retained in 500 nL droplets, which are concentrated by evaporation and subsequently redissolved in deuterated solvent. Droplet size is monitored by machine vision (MV), and heating rates are adjusted concordingly to maintain a stable droplet volume. Evaporation control is independent of solvent properties, and the setup handles feed rates up to 7 μL min(-1). The interface is capable of exchanging up to 90% of solvent for deuterated solvent, with a good recovery and repeatability for tomato extracts (Solanum lycopersicum). The system was capable of handling both polar and nonpolar analytes in one run. Volatiles such as formate, acetate, and lactate and the thermosensitive compound epigallocatechin gallate were recovered without significant losses. Ethanol and propionate were recovered with significant losses due to the formation of a minimum boiling azeotrope. The current setup is ideally suited for on- and off-line hyphenation of liquid chromatography and NMR, as it is comprehensive, fully automated, and easy to operate.

A Temperature Controlled Micro Valve for Biomedical Applications Using a Temperature Sensitive Hydrogel

The fabrication and characterization results of a temperature sensitive hydrogel micro valve based on the polymer N-isopropylacrylamide is presented. The hydrogel actuator was fabricated by in-situ polymerization using standard UV photolithography. The valve was opened and closed by applying a temperature change from 16 to 30°C. Potential uses for the valves include skin triggered valves for biomedical micro devices.