Ion Concentration Polarization Based Devices or 'What can ions do for me that electrons have not already done?'
Nanoscale solid-state devices that shuttle electrons around (for e.g.: the transistors firing away to bring this text to your screen) have been actively developed for decades. Nanofluidic structures that harness ion transport (for e.g.. the ion channels firing away as you see the screen) are common in Nature but we have only relatively recently learned to fabricate nanostructures and devices robust enough to explore and harness such mechanisms. Nanofluidic devices can be developed with direct conceptual parallels to their solid-state cousins (diodes, field-effect transistors..) and beyond. Interesting unexplored phenomena abound and unexpected yet impactful applications continue to arise.
I am exploring one such set of phenomena and applications along with other members of the Han lab where a nanofluidic concentrator was developed that traps molecules in an electric field gradient arising at the interface of microscale and nanoscale regions when a voltage is applied across them. Nanochannels or nanopores with surface fixed charge allow counter-ions to pass through while blocking or impeding co-ion transport. This leads to the formation of a depletion region and an ion concentration (majority carrier) gradient. The resultant electric field gradient traps molecules (minority carriers) of the right mobility and concentrates them unto into a plug as they flow in but do not flow past. This rapid concentration increase (up to a million-fold in less than an hour) leads to a plethora of applications in dramatically increasing sensitivity of a variety of sensors in a plug-and-play manner without any fundamental changes to the the sensing mechanisms.
[From Lab Chip 2011] A nanofluidic concentrator device traps and rapidly concentrates molecules in an ion depletion zone with high electric field. A reacting molecule pair (an enzyme and substrate) and the fluorescent reaction product are trapped and concentrated here. Local fluid flow instabilities in the ion depletion region are seen as fluctuations here.