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Towards Full‐Length Accumulative Surface‐Enhanced Raman Scattering‐Active Photonic Crystal Fibers

The integration of microfluidics with photonics on a single platform using well-established planar device technology has led to the emergence of the exciting field of optofluidics. As both a light guide and a liquid/gas transmission cell, photonic crystal fiber (PCF, also termed microstructured or holey fiber), synergistically combines microfluidics and optics in a single fiber with unprecedented light path length not readily achievable using planar optofluidics. PCF, an inherent optofluics platform, offers excellent prospects for a multitude of scientific and technological applications. The accessibility to the air channels of PCF has also opened up the possibility for functionalization of the channel surfaces (silica in nature) at the molecular and the nanometer scales, in particular to impart the functionality of surface-enhanced Raman scattering (SERS) in PCF for sensing and detection. SERS, an ever advancing research field since its discovery in the 1970s, has tremendous potential for label-freemolecular identification at trace or even single-molecule levels due to up to 10 increase in the Raman scattering cross-section of a molecule in the presence of Ag or Au nanostructures. Seminal work has been reported on the development of 1D and 2D SERS substrates for a variety of sensing applications. The use of 3D geometry, i.e., substrates obtained by the deposition of noble nanoparticles onto porous silicon or porous aluminum membrane offered additional advantages of increased SERS intensity due to increased SERS probing area, as well as the membrane waveguiding properties. Specifically, several orders of magnitude higher SERS intensity, affording picoor zeptogram-level detection of explosives, has been demonstrated with porous alumina membranes containing 60-mm-long nanochannels, as compared to a solid planar substrate. SERS-active PCF optofluidics, as a special fiber optic SERS platform, offers easy system integration for in situ flow-through detection, and, more importantly, much longer light interaction length with analyte, thus promising to open a new vista in chemical/biological sensing, medical diagnosis, and process monitoring, especially in geometrically confined or sampling volume-limited systems. Various attempts have been made over the last several years to integrate SERS with the PCF platform by incorporating Ag or Au nanostructures albeit inside a very limited segment (typically a few centimeters) of the fiber air channels. Examples include deposition of Ag particulates and thin films by chemical vapor deposition at high pressure or coating of Ag and Au nanoparticles using colloidal solutions driven into the microscopic air channels via capillary force with backscattering as the typical data acquisition mode. Building uniform SERS functionality throughout the length of the PCF while preserving its light guide characteristics has remained elusive as measured Raman intensity is a combination of the accumulative gain from Raman scattering and the continuous loss from nanoparticle-induced light attenuation over the path length. As a result, we have recommended and recently described in a brief study forward scattering as a more suitable detection mode to unambiguously assess the SERS-active nature of PCF. To the best of our knowledge, this article is the first report of net accumulative SERS from the full-length Ag-nanoparticlefunctionalized PCFs. The finding has been enabled by a fine control of the coverage density of Ag nanoparticles and studies of a competitive interplay between SERS gain and light attenuation in the Raman intensity with PCFs of varying length. Using two different types of PCF, i.e., solid-core PCF and hollow-core PCF, we show that Raman gain in PCF prevails at relatively low nanoparticle coverage density (below 0.5 particlemm ), allowing full benefit of accumulation of Raman intensity along the fiber length for robust SERS sensing and enhanced measurement sensitivity. Light attenuation dominates at higher nanoparticle coverage density, however, diminishing the path-length benefit. Shown in Figure 1 are cross-sectional scanning electron microscopy (SEM) images of solid-core PCFand hollow-core PCF used in this work. Also depicted in the figure is the light-guide process in the corresponding PCFs that contain immobilized Ag nanoparticles and are filled with aqueous solution throughout the cladding air channels for solid-core PCFand in the center air core only for hollow-core PCF. Note that light is guided via total internal reflectance in both cases. The presence of the aqueous solution in the cladding air channels does not fundamentally change the contrast of the higher index silica core and the lower index liquid-silica cladding in the solid-core PCF. The selective