Soft electrostatic trapping in nanofluidics

Abstract

Trapping and manipulation of nano-objects in solution are of great interest and have emerged in a plethora of fields spanning from soft condensed matter to biophysics and medical diagnostics. We report on establishing a nanofluidic system for reliable and contact-free trapping as well as manipulation of charged nano-objects using elastic polydimethylsiloxane (PDMS)-based materials. This trapping principle is based on electrostatic repulsion between charged nanofluidic walls and confined charged objects, called geometry-induced electrostatic (GIE) trapping. With gold nanoparticles as probes, we study the performance of the devices by measuring the stiffness and potential depths of the implemented traps, and compare the results with numerical simulations. When trapping 100 nm particles, we observe potential depths of up to Q ≅24 k B T that provide stable trapping for many days. Taking advantage of the soft material properties of PDMS, we actively tune the trapping strength and potential depth by elastically reducing the device channel height, which boosts the potential depth up to Q ~200 k B T , providing practically permanent contact-free trapping. Due to a high-throughput and low-cost fabrication process, ease of use, and excellent trapping performance, our method provides a reliable platform for research and applications in study and manipulation of single nano-objects in fluids.

Pneumatically Controlled Nanofluidic Devices for Contact‐Free Trapping and Manipulation of Nanoparticles

“…The active tuning of the channel height enables not only control of the trap stiffness or releasing the particles eventually, but also provides the possibility of tuning the trapping stability. The obtained stiffness at a gauge pressure p g = 100 mbar of k r = 0.6 fN nm −1 in Figure corresponds to a mean trapping time (the average time a particles dwells in a trap) of only several seconds . At a gauge pressure of p g = 500 mbar, however, the mean trapping time is raised to several days and longer making long observations times of individually trapped nanoparticles possible.…”

“…The reservoirs were finally filled with a 0.1 × 10 −3 m monovalent buffer solution and the finished device was placed on a microscope holder. The particles were recorded using a custom‐built interferometric scattering (iSCAT) detection system . Teflon tubing was inserted into the inlet of the microfluidic air pressure channel of the device and connected to a pressure and pump system.…”

“…The particle finally escaped from the trap as the pressure was lowered to 50 mbar. By fabricating devices with a lower initial nanofluidic channel height h c,0 , stable trapping could be already achieved at a gauge pressure p g of 0 mbar . More accurate and direct optical measurements of the nanofluidic channel height h c might be performed independently of the stiffness k r by two elaborate methods using either fixed particles to the nanofluidic channel walls or from the interference signal of the cover glass and PDMS device …”

“…The nanofluidic channels contain small circular indentations (pockets), as sketched in Figure a, that result in local energy potential wells. Depending on the potential depth of these pockets, nano‐objects can be stably trapped from milliseconds to several hours and days . Electrostatic trapping was demonstrated for levitating negatively and positively charged gold nanoparticles (Au NPs), lipid vesicles, and even for angular depended trapping of silver nanorods .…”

“…Additionally, the required top‐down nanofabrication tools, such as electron beam (e‐beam) lithography and reactive ion etching needed for the fabrication of the devices, make the production time consuming and resource demanding and thereby limiting the wide‐spread usage of the GIE trapping devices. As an alternative, we recently introduced GIE trapping devices made by soft lithography with PDMS used for replica molding, which simplifies the fabrication and results in high throughput and low cost production processes …”