Metallic Nanohole Arrays on Fluoropolymer Substrates as Small Label-Free Real-Time Bioprobes

Abstract

We describe a nanoplasmonic probing platform that exploits small-dimension (≤ 20 μm 2 ) ordered arrays of subwavelength holes for multiplexed, high spatial resolution, and real-time analysis on biorecognition events. Nanohole arrays are perforated on a super smooth gold surface (roughness RMS < 2.7 Å) attached on a fluoropolymer (FEP) substrate fabricated by a replica technique. The smooth surface of gold provides a superb environment for fabricating nanometer features and uniform immobilization of biomolecules. The refractive index matching between FEP and biological solutions contributes to ∼ 20% improvement on the sensing performance. Spectral studies on a series of small-dimension nanohole arrays from 1 μm 2 to 20 μm 2 indicate that the plasmonic sensing sensitivity improves as the gold-solution contact area increases. Our results also demonstrate that nanohole arrays with dimension as small as 1 μm 2 can be used to effectively detect biomolecular binding events and analyze the binding kinetics. The future scientific opportunities opened by this nanohole platform include highly multiplexed analysis of ligand interactions with membrane proteins on high quality supported lipid bilayers. Nanoplasmonic sensing is an emerging technique that utilizes the interaction of light with surface plasmons -collective oscillations of free electrons in the conduction band -in metallic nanostructures to transduce chemical bindings to signal for remote optical readout. With receptive biological macromolecules immobilized on nanostructured metal surfaces, whose length scale is close to the wavelength of light, the binding of target biomolecules leads to an increase of the local refractive index, which is then transduced into the change of spectral features observed at the far-field. The greatest attraction of nanoplasmonic sensing is it does not require extrinsic fluorescent or radioactive labeling (i.e. "label-free"), which would potentially change the binding properties of biomolecules and cause other significant problems like background binding and autofluorescence. 1 The small sensing volume resulting from short electromagnetic field decay length leads to high sensitivity for detection biomolecular binding events in local environments.Significant improvement in nanofabrication and nanomaterials synthesis in the past decade has allowed a variety of nanostructures to be utilized as nanoplasmonic sensors. A wide range of nanostructure shapes such as triangles, cubes, rods, and rings 2, 3 have been reported to exhibit high sensitivity based on localized surface plasmon resonance (LSPR) or coupled plasmonic resonance. The discovery of extraordinary light transmission through metallic subwavelength- Another key point in developing a nanohole sensing system is the interface between the nanostructure surface and the chemical or biological systems. For lipid-membrane-mediated reactions, especially those associated with membrane proteins or membrane fusion processes, a smooth surface is greatly desired to help building a high...

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