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 limited only by the fiber length. We report the strategy and methods in realizing full-length
surface-enhanced Raman scattering (SERS) PCF optofluidics by immobilization of negatively
charged Ag nanoparticles (NP) through polyelectrolyte-mediated approach or direct deposition of
positively charged Ag NP on the PCF air channels. Through forward propagating Raman measurements,
we demonstrate the full-length SERS-active PCF optofluidics with accumulative Raman signal gain along
the entire fiber length. We show SERS measurements of 1x10<sup>-7</sup> M (~48 ppb) Rhodamine 6G and 1x10<sup>-8</sup> M
(~0.8 ppb) sodium thiocyanate in a minute volume of ~10<sup>-7</sup>-10<sup>-8</sup> liter aqueous solution using PCF with
immobilized Ag NP over ~20 cm in length. The combination of high detection sensitivity and small
sampling volume renders the SERS-active PCF optofluidic platform excellent potential for a multitude of
applications ranging from label-free chemical and biological sensing to process monitoring in
geometrically confined systems.
A new procedure was used for the preparation of stable silver colloids by reduction of silver nitrate with (N (2 hydroxyethyl) piperazine N'-2 ethanesulfonic acid (HEPES). The nanoparticle size and the surface charge could be tuned by changing the initial pH of a HEPES solution. Rhodamine 6G and NaSCN were used respectively as model cationic and anionic analytes to study the effect of surface charge of the silver colloids on detection sensitivity. The silver colloids exhibit SERS activity comparable to those obtained by the popular Lee-Meisel approach. The combination of the high SERS sensitivity and the ability to control the nature of surface charge renders HEPES-reduced polyampholytic silver colloids a potentially powerful platform for sensing and detection of both cations and anions in aqueous solutions.