Nanofluidic systems for individual and contact-free electrostatic trapping of charged objects

Contact-free trapping of nano-objects in solution is of broad interest for many applications, such as studying of polymer dynamics, detecting molecular reactions or investigating the structure and functionality of large biomolecules, to name a few. Although several trapping methods have been developed, stable and high-throughput trapping of individual nanometer-sized objects in a straightforward manner remain challenging. A powerful method of trapping charged objects smaller than 100 nm and without any external applied power is geometry-induced electrostatic (GIE) trapping. This method is based on altering the surface topography of nanofluidic channels that are charged when exposed to water. The topographically modified surfaces result in electrostatic potential wells, in which nano-objects can be trapped from milliseconds to several days, depending on the trap specification and the buffer solution. Various trapping geometries (e.g., circular pockets and rectangular slits or grids) can be realized using state-of-the-art nanofabrication tools.
This thesis explores the development and use of nanofluidic devices for electrostatic trapping and manipulation of nano-objects, such as gold nanoparticles (Au NPs) or DNA. For imaging the Au NPs, a home built interferometric scattering (iSCAT) detection system was used. iSCAT is a label free coherent optical microscopy technique that significantly increases the signal-to-noise ratio (SNR) in comparison to other imaging methods that are based on detecting only the signal scattered by a nano-object.
In detail, using standard silicon-based GIE trapping devices, Au NPs smaller than 60 nm become difficult to detect using iSCAT microscopy. To overcome this limitation, trapping devices made from glass substrate are introduced with a new developed fabrication process. These devices allow imaging of Au NPs with an increased contrast and SNR of an order of magnitude using iSCAT detection, enabling the detection of relatively smaller nanoparticles and thereby allowing the study of their trapping behavior.
Further, the GIE trapping method is integrated into a microfluidic system that comes with the key benefits of reduced sample volume, in situ change of solutions, precise control of solution delivery, and the feasibility to trap nano-objects along a gradient of e.g. salt or other reactants. Using this high-throughput screening device, the performance has been quantitatively analyzed by screening the electrostatic potential along a salt gradient using 60 nm Au NPs as probes in a single experiment. Additionally, the critical salt concentration for the stability of the colloidal dispersion could be observed. The advancement of this method sets the ground for a variety of new experiments. As an example, having the possibility to insert and flush the device with different solutions, functionalization of the nanofluidic channel walls with positively charged polyelectrolytes was achieved resulting in a reversal of the walls net charge and thus allowing the trapping of positively charged Au NPs.
One drawback that makes the development and application of GIE trapping devices made from rigid SiOx materials difficult, is the high cost and time-consuming nanofabrication in limiting infrastructures such as cleanroom facilities. Hence, new GIE trapping devices made from the soft material polydimethylsiloxane (PDMS) are introduced that are fabricated using a high-throughput and easy handling replica molding process. Stable trapping of Au NPs down to 60 nm in diameter is demonstrated and potential depths of up to Q ~ 24 kBT of circular pockets are experimentally observed that provide stable trapping for many days. In addition, by taking advantage of the feature that PDMS is a flexible material, the PDMS devices are elastically compressed, which results in a reduction of the device channel height and thus active tuning of trapping strengths and residence times. With this capability, extremely deep potentials of up to Q ~ 200 kBT are achieved, providing practically permanent contact-free trapping of individual nano-objects. Furthermore, the implementation of a 3D PDMS pneumatic valve system is demonstrated, which makes the devices capable of controlling the trap stiffnesses and residence times actively as well as trapping and releasing the nano-objects.
These devices will enable high-throughput trapping of nano-objects for studying their behavior and interactions in aqueous environment. The simple and low-cost fabrication process and the fact that the chip-based devices do not need externally applied fields or an elaborate build-up will make them equally available for research and commercial applications.