Paper BI-ThP13
Magnetic Tweezer Sensor for Ensemble Binding Events of Nonmagnetic Particles

Thursday, October 23, 2008, 6:00 pm, Room Hall D

Current methods to measure molecular binding strengths include force pulling methods such as atomic force microscopy and optical tweezing. These methods are greatly limited in their throughput, requiring molecular fishing and individual particle targeting that produces binding data on the order of minutes or hours. To overcome these deficiencies, the authors have developed a magnetic system that allows for fast ensemble measurements of thousands of single particle-substrate bonds simultaneously using a High Gradient Magnetic Separation (HGMS) system. The described system is a dense array of micron-size ferromagnetic thin islands on a substrate. These magnetic arrays offer the ability to apply very strong particle forces that can be in the range of tens of nanonewtons, orders of magnitude higher than most optical or electrical systems. Additionally, this system can be used to apply forces on nonmagnetic particles by submerging them in biocompatible magnetic fluids, a technique known as “inverse” magnetophoresis. The ferrofluid causes the nonmagnetic particles to behave as magnetic holes allowing for particle-substrate bonds to be broken through the islands’ applied magnetic force. The authors have extended this system onto the surface of a quartz crystal microbalance (QCM) sensor, which allows for the accurate sensing of ensemble particle movement. To test this system, the authors use a mixed self-assembled monolayer of biotin and oligoethylene glycol and selectively bind streptavidin coated particles to the magnetic islands. Using the magnetic islands and an external magnetic field, streptavidin particles are attracted to a preprogrammed edge of the islands and are allowed to undergo molecular binding with the surface. As an opposite external field is applied, the particles will be pulled en masse to the opposite side of the islands, a movement that can be sensed and analyzed by the QCM. Through knowing the applied magnetic forces, this system allows for the ensemble quantification of bond dissociation between any chemically active particle and substrate.