Fabrication and Optimization of a Nanoplasmonic Chip for Diagnostics

Abstract: To increase the survival rate from infectious- and noncommunicable diseases, reliable diagnostic during the preliminary stages of a disease onset is of vital importance. This is not trivial to achieve, a highly sensitive and selective detection system is needed for measuring the low concentrations of biomarkers available. One possible route to achieve this is through biosensing based on plasmonic nanostructures, which during the last decade have demonstrated impressive diagnostic capabilities. These nanoplasmonic surfaces have the ability to significantly enhance fluorescence- and Raman signals through localized hotspots, where a stronger then normal electric field is present. By further utilizing a periodic sub-wavelength nanohole array the extraordinary optical transmission phenomena is supported, which open up new ways for miniaturization. In this study a nanoplasmonic chip (NPC) composed of a nanohole array —with lateral size on the order of hundreds of nanometer— covered in a thin layer of gold is created. The nanohole array is fabricated using soft nanoimprint lithography on two resists, hydroxypropyl cellulose (HPC) and polymethyl methacrylate (PMMA). An in depth analysis of the effect of thickness is done, where the transmittance and Raman scattering (using rhodamine 6G) are measured for varying gold layers from 5 to 21 nm. The thickness was proved to be of great importance for optimizing the Raman enhancement, where a maximum was found at 13 nm. The nanohole array were also in general found beneficial for additionally enhancing the Raman signal. A transmittance minima and maxima were found in the region 200-1000 nm for the NPCs, where the minima redshifted as the thickness increased. The extraordinary transmission phenomena was however not observed at these thin gold layers. Oxygen plasma treatment further proved an effective treatment method to reduce the hydrophobic properties of the NPCs. Care needs be taken when using thin layers of gold with a PMMA base, as the PMMA structure could get severely damaged by the plasma. HPC also proved inadequate for this projects purpose, as water-based fluids easily damaged the surface despite a deposited gold layer on top.