Toward full length SERS-active photonic crystal fiber as a optofluidic sensing platform

The unique feature of photonic crystal fiber (PCF) both as a light guide and a liquid transmission cell allows synergistic integration of optics and microfluidics to form an unconventional optofluidic platform of long interaction path. This work aims to develop full-length SERS-active PCF optofluidic platform using two representative PCF types, i.e., solid-core PCF and hollow-core PCF. A polyelectrolyte-mediated approach to Ag nanoparticle immobilization enabled a fine control of the coverage density of Ag nanoparticles inside the fiber air channels. Through forward propagating Raman measurements, we have shown the competitive interplay between SERS gain and light attenuation as the optical path length increases for a PCF containing immobilized Ag nanoparticles, with low particle coverage density (typically < ∼0.5 particle/mum 2) being essential for a net accumulative Raman gain along the fiber length. Our forward hyperspectral Raman imaging measurements at the fiber distal end revealed that waveguiding properties of PCFs were well preserved after controlled immobilization of Ag nanoparticles at relatively low coverage density. SERS detection of 1x10-7 M or ∼48 ppb Rhodamine 6G in ∼10-7--10-8 liter aqueous solution was achieved using ∼20 cm PCFs. Numerical simulation and Raman/SERS measurements of three solid-core PCFs of different air-cladding microstructures were carried out to assess their respective potential for evanescent-field SERS sensing. Suspended-core PCF consisting of a silica core surrounded by three large air channels conjoined by a thin silica web is the most robust of the three SERS-active PCFs, with a demonstrated detection sensitivity of 1x10-10 M Rhodamine 6G in an aqueous solution of only ∼7.3 microL. In a related effort, we have shown that SERS activity of Ag nanoparticles should not be taken for granted in the presence of oxidizing agents. Our study indicated that Ag oxidation can lead to a drastic decrease in the SERS enhancement. Through ozone exposure under UV irradiation in controlled circumstances, we were able to quantify the effect of oxidation on SERS activity of Ag nanoparticles. Importantly, we determined that formation of only submonolayer Ag2O is sufficient to substantially degrade SERS enhancement of Ag nanoparticles. This knowledge is crucial for the SERS community to develop robust and reproducible sensing platform for a host of applications.