Gold photonic crystals and photonics quasi-crystals for reproducible surface-enhanced raman substrates

Abstract

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 (Jarvis et al., Anal Chem 76:5198–202, 2004). The optical properties of noble metals with nanostructures have attracted enormous attention because of their potential application in optical sensing (Chen et al., Biosens Bioelectron 22:926–32, 2007), biosensor (Liu and Lu, J Am Chem Soc 125:6642–6643, 2003) and cell diagnostics (Huang et al., Nano Lett 7:1591– 1597, 2007). 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 (Huang et al. Adv Mater 21:4880–4910, 2009). 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 (Gopinath et al., Opt Express 17:3741, 2009). In the present work we studied artificial electromagnetic (EM) nanomaterials to develop innovative plasmonic nanobiosensors based on SERS and working in the visible frequency band. The last decade has been characterized by artificial EM materials, including photonic crystals (PCs) and photonic quasi-crystals (PQCs) (Matarazzo et al., J Opt 13:015602, 2011; Rippa et al., Nanoscale 5:331–336, 2013), making these very attractive given that there are new possibilities to control the EM field in innovative way. 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. Gold PQCs, in contrast to period structures, give rise to photonicplasmonic resonances which extend across the entire visible spectrum. Gold PQCs are proposed for the engineering of reproducible surface enhanced Raman scattering (SERS) substrates. 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. The resulting PQCs nanostructured films can serve as good SERS substrates, exhibit large electromagnetic field enhancement factors for pMA, and can readily be used in ultrasensitive, molecule-specific sensing utilizing vibrational signatures.