(Almost) Stationary Isotachophoretic Concentration Boundary in a Nanofluidic Channel Using Charge Inversion

The present work is an experimental study of a new means to induce a quasi-stationary boundary for concentration or separation in a nanochannel induced by charge inversion. Instead of using pressure-driven counter-flow to keep the front stationary, we exploit charge inversion by a highly charged electrolyte, Ru­(bpy)₃Cl₂, that changes the sign of the zeta potential in part of the channel from negative to positive. Having a non-charge inverting electrolyte (MgCl₂) in the other part of the channel and applying an electric field can create a standing front at the interface between them without added dispersion due to an externally applied pressure-driven counterflow. The resulting slow moving front position can be easily imaged optically since Ru­(bpy)₃Cl₂ is fluorescent. A simple analytical model for the velocity field and front axial position that reproduces the experimental location of the front shows that the location can be tuned by changing the concentration of the electrolytes (and thus local zeta potential). Both of these give the charge inversion-mediated boundary significant advantages over current methods of concentration and separation and the method is, therefore, of particular importance to chemical and biochemical analysis systems such as chromatography and separations and for enhancing the stacking performance of field amplified sample injection and isotachophoresis. By choosing a non-charge inverting electrolyte other than MgCl₂, either this electrolyte or the Ru­(bpy)₃Cl₂ solution can be made to be the leading or trailing electrolyte

Stable Single-Walled Carbon Nanotube-Streptavidin Complex for Biorecognition

A novel method is described for preparing single walled carbon nanotube (SWNT)-streptavidin complexes via the biotin-streptavidin recognition. The complex shows stability in 18 days, strong biotin recognition capability, and excellent loading capacity (about I streptavidin tetramer per 20 nm of SWNT). capturing biotinylated DNA, fluorophores, and Au nanoparticles (NPs) on the SWNT-streptavidin complexes demonstrates their usefulness as a docking matrix, for instance for electron microscopy Studies, a technique requiring if virtually electron transparent Support

Numerical investigation of micro- and nanochannel deformation due to discontinuous electroosmotic flow

Large pressures can induce detrimental deformation in micro- and nanofluidic channels. Although this has been extensively studied for systems driven by pressure and/or capillary forces, deflection in electrokinetic systems due to internal pressure gradients caused by non-uniform electric fields has not been widely explored. For example, applying an axial electric field in a channel with a step change in conductivity and/or surface charge can lead to internally generated pressures large enough to cause cavitation, debonding, and/or channel collapse. Finite electric double layers within nanofluidic channels can further complicate the physics involved in the deformation process. In order to design devices and experimental procedures that avoid issues resulting from such deformation, it is imperative to be able to predict deformation for given system parameters. In this work, we numerically investigate pressures resulting from a step change in conductivity and/or surface charge in micro- and nanofluidic channels with both thin and thick double layers. We show an explicit relation of pressure dependence on concentration ratio and electric double layer thickness. Furthermore, we develop a numerical model to predict deformation in such systems and use the model to unearth trends in deformation for various electric double layer thicknesses and both glass and PDMS on glass channels. Our work is particularly impactful for the development and design of micro- and nanofluidic-based devices with gradients in surface charge and/or conductivity, fundamental study of electrokinetic-based cavitation, and other systems that exploit non-uniform electric fields