Devices and methods for capturing biological materials using a potential well. An electrical signal is applied across a nanopipette having one end in a back-fill chamber and another end in a collection chamber containing a suspending medium including one or more types of particles. The collection end of the nanopipette includes a tip having an opening. The electrical signal applied across the nanopipette is configured to generate the potential well proximate to the tip in which the electrokinetic forces acting on the particles are balanced. The potential well may be configured to selectively trap one or the other types of particles suspended in the suspending medium. The particles may be transferred to a sample collection medium by immersing the tip in the sample collection medium and reversing the polarity of the electrical signal.