Magnetic control of particle trapping in a hybrid plasmonic nanopore

Plasmonic nanopores are extensively investigated as single molecules detectors. The main limitations in plasmonic nanopore technology are the too fast translocation velocity of the molecule through the pore and the consequent very short analysis times, as well as the possible instabilities due to local heating. An interesting approach to enable longer acquisition times is represented by the ability to stably trap the nanoparticles used to tag molecules close to the nanopore. Here, we theoretically investigate the performance of a magneto-plasmonic nanopore prepared with a thin layer of cobalt sandwiched between two gold layers. A nanopore is then coupled with a bifunctional (magnetic and plasmonic) core-shell nanoparticle made of magnetite (core) covered with a thin layer of gold (shell). By setting the magnetic configuration of the cobalt layer around the pore by an external magnetic field, it is possible to generate a nanoscale magnetic tweezer to trap the nanoparticle at a specific point. Considering a similar to 10 nm diameter magnetite nanoparticle, we calculate a trapping force up to 28 pN, an order of magnitude above the force that can be obtained with standard optical or plasmonic trapping approaches. Moreover, the magnetic force pulls the nanoparticle in close contact with the plasmonic nanopore's wall, thus enabling the formation of a nanocavity enclosing a sub-10 nm(3) confined electromagnetic field with an average field intensity enhancement up to 230 at near-infrared wavelengths. The presented hybrid magneto-plasmonic system points toward a strategy to improve nanopore-based biosensors for single-molecule detection and potentially for the analysis of various biomolecules.