Abstract
The localised surface plasmon resonance (LSPR) at the surface of metal nanostructures can induce a highly intense electromagnetic (EM) field, which is confined to the edges with big curvature or at narrow gaps between nanostructures. Therefore, the localisation of target molecules at these sites is crucial to achieve high sensitivity in LSPR-based biosensors. To this end, we fabricated a 40 nm high gold nano-truncated cone (GNTC) array using thermal nanoimprint lithography. As the EM field is most intense at the side surface and relatively weak at the top surface of GNTC, we improved the detection sensitivity by blocking the top surface with oxides to limit adsorption of antibodies and antigens to the top surface. We observed the difference in sensitivity by detecting α-fetoprotein (AFP) on the oxide-capped and uncapped GNTC arrays through sandwich immunoassay and enzymatic precipitation. The capped GNTC array exhibited higher detection sensitivity than the uncapped one. Particularly, six-fold enhancement of sensitivity was achieved in the serum sample. We used atomic force microscopy and electron microscopy to validate that the deposition of the oxides on the top surface of GNTC effectively blocked the adsorption of the biomolecules and the target molecules were preferentially adsorbed on the side surfaces.
Highlights
The localised surface plasmon resonance (LSPR) at the surface of metal nanostructures can induce a highly intense electromagnetic (EM) field, which is confined to the edges with big curvature or at narrow gaps between nanostructures
The array of nanostructures realised through imprinting has high levels of reproducibility and fidelity similar to those obtained with electron beam lithography (EBL) or focused ion beam (FIB) lithography
The EM field originating from LSPR in metal nanostructures is unevenly distributed on the metal surface and localised on the edge with the largest surface curvature or at the narrow gap between metal surfaces facing each other
Summary
The localised surface plasmon resonance (LSPR) at the surface of metal nanostructures can induce a highly intense electromagnetic (EM) field, which is confined to the edges with big curvature or at narrow gaps between nanostructures. The detection sensitivity in LSPR-based biosensors can be improved by selectively immobilising the receptor molecules on the regions of high field density, so that the adsorption of the analyte is limited to these regions. This fact was addressed in a few recent studies[18,19,20]. Fabrication of nanostructures using top-down nanolithography has several advantages, such as size uniformity, easy adjustment of inter-structural spacing, and high reproducibility[21,22,23,24] Unlike colloidal nanoparticles, these structures are free from undesired signal interference caused by plasmon coupling due to aggregation. There are no reports on the utilisation of selective adsorption of molecules to enhance the detection sensitivity of LSPR-based biosensors realised with such nanostructures
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