Novel Functionalized Nanopores for Plasmonic Sensing

Abstract: Nanoplasmonic sensors offer a label-free platform for real-time monitoring of biomolecular interactions by tracking changes in refractive index through optical spectroscopy. However, other surface sensitive techniques such as conventional surface plasmon resonance offer similar capabilities with equal or even better resolution in terms of surface coverage. Still, plasmonic nanosensors provide unique possibilities when the nanoscale geometry itself is of interest. By taking the advantage of nanofabrication technology, it is possible to engineer nanostructures with controllable dimensions, curvature and optical properties to study nanometer-sized biomolecules, such as proteins.   This thesis presents different types of nanoplasmonic sensing platforms, each with a unique geometry in which biomolecules can be probed through plasmonic read out. Two of the structures were fabricated by short-range ordered colloidal self-assembly and etching of the solid support underneath a nanohole array. The structures are denoted as ‘nanocaves’ and ‘nanowells’ depending on the degree of anisotropy. By suitable surface functionalization, the geometry of the plasmonic nanocavities was utilized for location-specific detection of proteins. In addition, nanowells were employed to investigate curvature-dependent biomolecular interactions.   We have also developed subwavelength apertures in optically thin silicon nitride membranes covered with continuous metal films, referred to as solid-state ‘nanopores’. Plasmonic nanopores in suspended metal-insulator-metal films were fabricated in short-range ordered and long-range ordered arrays using short-range ordered colloidal self-assembly and electron beam lithography, respectively. Both methods prevent the metal from ending up on the silicon nitride walls which is of paramount importance for plasmonic properties and selective chemical functionalization. Preparing nanopores with identical structure but different aperture ordering provides the opportunity to understand how aperture ordering influences plasmonic response, particularly with respect to the nature of far field spectral features. Long-range ordered vs. short-range ordered nanopores exhibit similar optical properties with only a few differences in the plasmonic response.   By appropriate material-specific modification (thiol and trietoxysilane chemistry) plasmonic nanopores were employed for detection of average sized proteins (~ 60 kg/mol) inside the pores. In addition to nanoplasmonic sensing, we could also construct a credible mimic (in terms of geometry) of nuclear pore complexes, when the size of the solid state nanopores approaches ~ 50 nm. The unique geometry and size of nanopores (diameter ~ 50 nm) opens up the possibility to mimic the geometry of biological nanomachines with integrated optical sensing capabilities.