The ICR (ionic current rectification) phenomenon in nanopores is influenced by both the shape and material of the nanopore, as well as the ion concentration within the electrolyte that permeates the nanopore. Specifically, nanopores composed of multiple materials can induce ICR due to variations in ion distribution along the nanopore-electrolyte interface across different sections of the material. In this study, we investigate the impact of configurations involving up to two layers of different materials (Si3N4 and Au) and six different electrolytes on solid-state nanoporous ICR. We aim to assess how different configurations of surface charges can influence ICR. The main advantage of nanopores made from gold and other metals may be their use as an ICR amplifier when illuminating nanopores with a wavelength of light lying in the plasmon resonance region of the metal used.
Due to their superior properties in single-molecule detection, plasmonic and nanopore-based sensors have attracted research interest. In recent times, they have been combined in a single device, resulting in plasmonic nanopores-based sensors. These solid-state devices featured unprecedented enhancements in single-molecule and nanoparticle detection, optical spectroscopies and trapping, control of local temperature. In this context, we have investigated two kinds of nanostructures: plasmonic nanopores and plasmonic nanoantennas, both of which were fabricated on free-standing Si3N4 membranes. As regards the nanopores, we were able to prove that their plasmonic coating enhanced their conductance when illuminated at 631 nm. On the other side, antenna-shaped nanopores (i.e., nanoantennas) were fabricated via plasmonic photochemical deposition. At this regard, we demonstrated that it was possible to fabricate nanoantennas with different internal diameters by different time of plasmon-induced photochemical deposition of metal precursors at the free tip of the nanoantenna. In conclusion, we proved that it was possible to use each nanoantenna (i.e., each decreasing internal diameter) to detect the translocation of nanoparticles with correspondingly decreasing diameters or of DNA.
Methods for sub 10 nm plasmonic nanopores fabrication are usually complex and require multi-step processes usually suitable for the preparation of single pores. Other processes to fabricate metallic nanopores involve pore shrinking by metal evaporation, which is applied to the whole substrate, increasing its thickness; therefore it is not localized and reduces spatial resolution. For that reason, a process in which metal deposition can be controlled at the nanoscale, is a key advancement in the field. Here, we report on a process for the fabrication of sub 10 nm solid-state plasmonic nanopores, via photocatalytic effect caused by the electromagnetic field enhancement in metallic rings on top of dielectric nanotubes. Under illumination, the areas of maximum field inside these structures trigger sites for metal nucleation and growth. Using this methodology, we fabricated Au-Ag and Au-Au nanopores, with consistent and reproducible shrinkage in pore diameter. Numerical simulations were performed in order to support the findings and to show how the obtained plasmonic structures can be used to confine the electromagnetic field, enhancing the intensity in a volume in the scale of sub 10 nm. The confinement of the field inside the final nanopore can be used for thermoplasmonic effects modifying ionic conductivity inside the pore under different illumination wavelengths.
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