Gold Photonic Crystals and Photonic Quasi- Crystals for Reproducible Surface-Enhanced Raman Substrates

Summary In this paper we present efficient SERS substrates for plasmonic "label-free" nanobiosensors realized by electron beam lithograhy. We demonstrate that SERS enhancement factors of the order of ~107 can be reproducibly obtained using Au photonic quasi crystals arrays of nano-pillars. Introduction Since the initial discovery of surface-enhanced Raman scattering (SERS), an increased amount of work has been done on the research of substrates for highly efficient Raman scattering enhancement due to their extraordinary potential for trace analysis and biological tags. Recently, the plasmonic optical responses of metal nanoparticles based on localized surface plasmon resonances (LSPR) and significant fluorescence enhancement in the visible and near IR region, have been intensively researched. Many groups have demonstrated that the plasmon resonance is closely related to the size and shape of metal nanoparticles and the dielectric properties of the surrounding medium. The possibility of engineering complex metal nanoparticle arrays with distinctive plasmonic resonances extending across the entire visible spectrum can have a significant impact on the design and fabrication of novel nanodevices based on broadband plasmonic enhancement. In the present work we studied artificial electromagnetic (EM) nanomaterials such as photonic crystals (PCs) and photonic quasi-crystals (PQCs) to develop innovative plasmonic nanobiosensors based on SERS and working in the visible frequency band. With the use of PCs and PQCs, it is possible to synthesize novel artificial structures characterized by selective EM responses, which, in turn, undergo significant frequency shifts, in presence of biological material. Using a molecular monolayer of pMA (p-mercaptoaniline) as a Raman reporter, we show that higher values of SERS enhancement factors can be achieved in PQCs structures compared to their periodic counterparts. To demonstrate the feasibility of the fabricated nanostructures as efficient SERS substrates for biological applications, we devised a method to deposit single cells (human prostatic) on the photonic surfaces. Discussion In this work, our engineered substrates were manufactured using direct-write electron beam lithography (EBL). We fabricated 2D PQCs operating in the frequency band of visible based on resonant metallic structures in a Thue-Morse (ThMo) arrangement.1,2 The EBL process is essential to achieve the well-control of size, shape, composition, and configuration of plasmonic nanostructures. In fig. 1a, a SEM image of the 2D Au QC realized, with 60nm-Au nanopillars side size of 185nm in a ThMo sequence with a step of 270nm is shown. Fig. 1b shows the Stokes Raman spectra of pMA molecular monolayers deposited on top of the Au PQC. The three dominant Stokes modes (390, 1077, 1590 cm-1), arising from bending and stretching modes in the benzene rings of the pMA molecule, can clearly be distinguished in all the spectra. SERS spectra were collected with a 785 nm laser resulting in power at the sampling point of ~1.7 mW. To demonstrate the feasibility of the fabricated nanostructures as efficient SERS substrates for biological applications, we devised a method to deposit single cells (human prostatic) on the photonic surfaces. In Fig. 2a is represented the Raman image obtained by confocal sampling of one of the cells. Image reconstruction was performed by use of the peak intensity at 1320 cm-1, a signal characteristic of the wagging vibration of CH2 groups in the glycine unit. Fig.2b represents the spectra collected at position A. Spectra collected at different locations within the cell boundary display a widely variable scattering response. This ultrasensitive Raman probe may be advantageously used to monitor subtle molecular changes in t