Refractometric sensors utilizing surface plasmon resonance (SPR) should satisfy a series of performance metrics, bulk sensitivity, thin-film sensitivity, refractive-index resolution, and high-Q-factor resonance, as well as practical requirements such as manufacturability and the ability to separate optical and fluidic paths via reflection-mode sensing. While many geometries such as nanohole, nanoslit, and nanoparticles have been employed, it is nontrivial to engineer nanostructures to satisfy all of the aforementioned requirements. We combine gold nanohole arrays with a water-index-matched Cytop film to demonstrate reflection-mode, high-Q-factor (Qexp = 143) symmetric plasmonic sensor architecture. Using template stripping with a Cytop film, we can replicate a large number of index-symmetric nanohole arrays, which support sharp plasmonic resonances that can be probed by light reflected from their backside with a high extinction amplitude. The reflection geometry separates the optical and microfluidic paths without sacrificing sensor performance as is the case of standard (index-asymmetric) nanohole arrays. Furthermore, plasmon hybridization caused by the array refractive-index symmetry enables dual-mode detection that allows distinction of refractive-index changes occurring at different distances from the surface, making it possible to identify SPR response from differently sized particles or to distinguish binding events near the surface from bulk index changes. Due to the unique combination of a dual-mode reflection-configuration sensing, high-Q plasmonic modes, and template-stripping nanofabrication, this platform can extend the utility of nanohole SPR for sensing applications involving biomolecules, polymers, nanovesicles, and biomembranes.
Bibliographical noteFunding Information:
This work was supported by the Minnesota Partnership for Biotechnology and Medical Genomics (M.V., N.J.W., and S.-H.O.). C.T.E. was supported by the National Science Foundation Graduate Research Fellowship Program (NSF GRFP). N.J.W. acknowledges support from Lehigh University. C.T.E. and S.-H.O. acknowledge partial support provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR). S.-H.O. also acknowledges support from the McKnight Foundation and the Sanford P. Bordeau Endowed Chair in Electrical Engineering at the University of Minnesota. Device fabrication was performed in the Minnesota Nano Center, which is supported by the NSF through the National Nanotechnology Coordinated Infrastructure (NNCI) under Award Number ECCS-1542202. Parts of this work were carried out in the Characterization Facility at the University of Minnesota, a member of the NSF-funded Materials Research Facilities Network.
- long-range surface plasmon
- nanohole array
- surface plasmon resonance
- template stripping