Polarization-independent SERS substrates for trace detection of chemical and biological molecules

We propose plasmonic nanostructures - a 2D array of circular 'nanopillars inside square nanoholes' - as polarization independent SERS substrates for portable detection of chemical and biological molecules. These substrates were fabricated in a reproducible and controllable manner on a wafer-scale using a combination of deep-UV lithography, reactive ion etching (RIE) and E-Beam evaporation. The SERS spectra were measured using a portable Raman spectrometer to demonstrate portable SERS based sensing and the limit of detection was found to be similar to 13.14 femtogram for the detection of 2,4-DNT. Furthermore, numerical modeling of the proposed substrates was carried out using Finite Difference Time Domain (FDTD) modeling to study the effect of structural parameters on the electromagnetic enhancement factor and the resonance wavelength. Moreover, based on numerical simulations and experimental results, it was found that the SERS signal from these SERS substrates is only slightly dependent on polarization. Thus, the proposed SERS substrate can be employed for polarization independent SERS based trace detection of chemical and biological molecules in real-time field settings using a portable Raman spectrometer.