Nanofluidic lab-on-a-chip trapping devices for screening electrostatics in concentration gradients

•Microfluidic devices with an integrated field free electrostatic trapping system•Contact-free trapping of gold nanoparticles along a salt gradient•Study on critical concentration for trapping and colloidal dispersion stability

Geometry-induced electrostatic (GIE) trapping is a novel contact-free method of stably confining charged nano-objects in solution. This method has proven to be very effective in trapping sub-100 nm objects and is based only on the electrostatic repulsion between the charged object and the device surfaces, without requiring an external control or power. We report on fabricating a GIE trapping device integrated into a microfluidic system and demonstrate its performance in screening the behaviour of individually trapped nano-objects along a NaCl salt concentration gradient. We use 60 nm gold particles as probes to analyze the trapping stiffness and residence time of the particles along the salt gradient. We show that in our devices a critical concentration for the reliable trapping of the particles in the order of seconds is reached at an ionic concentration of 0.3 mM. By analyzing the trap stiffness and residence times, we determine a smooth gradient of the salt concentration, as expected from Fick's first law. Furthermore, we find that the instability of the colloidal dispersion is reached at 0.8 mM NaCl.

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