Strategies and methods in the development of SERS-active planar substrate and photonic crystal fiber sensing platforms

This work aims to develop the strategy and the knowledge base for molecular- and nano-scale modification of planar substrates and the air holes of silica-based solid-core photonic crystal fibers (SC-PCF) for chemical detection using surface-enhanced Raman scattering (SERS). Specifically, negatively charged Ag nanoparticles (Ag [-]) were synthesized via citrate or 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) for sensing of positively charged analytes. A self-assembled polyallylamine hydrochloride (PAH) monolayer at controlled pH enabled immobilization of Ag [-] on silica surface via electrostatic interactions. Planar substrates with 5 nanoparticles/mum 2 coverage density exhibited a SERS sensitivity of ∼1 nM (0.5 ppb) for rhodamine 6G (R6G). Positively charged Ag nanoparticles (Ag [+]) were prepared by UV-assisted reduction of silver nitrate using branched polyethyleneimine (BPEI) and HEPES solutions for detection of negatively charged analytes. Planar substrates immobilized with Ag [+] at 30 particles/mum2 coverage density showed SERS sensitivity of single digit ppb for perchlorate, cyanide and thiocyanate anions in aqueous solutions. The ultra-high SERS sensitivity was attributed to synergistic binding of analyte anions to primary amino and amide groups of BPEI chains adsorbed on the Ag [+] nanoparticle surface. Finally, the strategies developed via planar substrates were applied to the SC-PCF for immobilization of Ag [-] and Ag [+] with controlled coverage density ranging from 0.1 to 5 particles/mum2 on the air hole surface for SERS sensing, with theoretical calculations of evanescent field interaction with Ag nanoparticles as a guidance for attenuation prediction. The SERS-active PCFs up to 20 cm in length with nanoparticle coverage density at the lower end showed a detection sensitivity of 5 ppb and 1 ppb for R6G and thiocyanate, respectively. More importantly, the SERS intensity increased with fiber length. PCFs with nanoparticle coverage at the higher end, however, experienced significant propagation loss in both the excitation laser light and the evanescent-field induced SERS signals. The dependence of the performance of the SERS-active PCF platforms on the nanoparticle coverage density resulted from the interplay between the accumulative gain in SERS signal and its continued attenuation along the fiber length.