PLASMONIC NANOPORES: exploring new possibilities in DNA sensing and trapping

Abstract: In this work we explore the new DNA sensing and trapping possibilities offered by a plasmonic nanopore. This new device combines the sensing capability of solid state nanopores with the effects arising from the collective oscillation of electrons in gold nanoparticles (plasmons). The device consists in a nanopore drilled in a silicon nitride membrane and triangular gold nanoparticles in a bowtie configuration co-aligned with the pore. We exploit plasmonic heating to characterise the metallic nanostructures and we study its effect on pore stability. These observations allow us to optimise the sample characteristics and the experimental conditions for DNA translocation experiments. In this regard, we focus on how plasmonic excitation influences the depth of a current blockade, evaluating the contribution of the shift of the resonance spectrum due to the presence of a molecule in the surroundings of the particles. We also forwarded the hypothesis of a contribution of thermophoretic forces, due to high local heating induced by the plasmonic structures. Heating and thermophoretic forces could also influence the increase in translocation frequency, observed in our experiments upon plasmonic excitation. This increase is probably due to changes in the buffer viscosity which affects the magnitude of the pore’s capture radius of the DNA molecules. We also explore the possibility of controlling the motion of DNA inside the pore by means of optical trapping. This principle is based on utilising optical forces from the strong gradient of the evanescent electric field that is created, upon illumination, from the surface of the metallic nanoparticles to few nanometers away. From preliminary molecular dynamics simulations, we expect the optical force to be able to overcome the driving electric force at experimentally relevant excitation powers. Indeed, for plasmonic excitation around laser power 10 mW and above, we observed long (> 1s) and multilevel events, which indicate successful plasmonic trapping of DNA in nanopore. However we note that such long events were often present also after turning off the plasmonic excitation, likely due to permanent sticking of to the pore/bowtie. Although we need to solve some issues of sticking of molecules and to perform more systematic experiments before drawing any conclusion regarding trapping, the results are promising and indicate that plasmonic nanopores may enable light-controlled trapping of DNA in nanopores.