As both a waveguide and a gas/liquid transmission cell, photonic crystal fiber (PCF) allows synergistic integration of
optics and microfluidics to form an unconventional optofluidic platform with long interaction path. In this paper, we
report our strategy to achieve surface-enhanced Raman scattering (SERS) PCF optofluidics by polyelectrolyte-mediated
immobilization of Ag nanoparticles (NPs) inside the fiber air channels. Through forward propagating Raman
measurements and hyperspectral Raman imaging, we demonstrate the realization of SERS-active PCF optofluidics with
accumulative Raman signal gain along the entire fiber length using both solid-core PCF (SC PCF) and hollow-core PCF
(HC PCF). By numerical simulation and Raman measurements, we show that suspended-core PCF (SP PCF) consisting
of a silica core surrounded by three large air channels conjoined by a thin silica web is the most robust platform of the
three SC PCF microstructures investigated for evanescent-field SERS spectroscopy.
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 1x10-7 M (~48 ppb) Rhodamine 6G and 1x10-8 M
(~0.8 ppb) sodium thiocyanate in a minute volume of ~10-7-10-8 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.
We have explored the use of index-guiding liquid-core (LC) photonic crystal fiber (PCF) as a robust platform for measurements of solutions of trace volume using normal and surface-enhanced Raman scattering (SERS). The LC PCF was fabricated by selectively sealing the cladding air channels at the distal ends of a hollow-core PCF while leaving the center core open, using a fusion splicer. Utilizing a 30-cm-long LC PCF with the entire center core filled with the ~0.1-µL solution of interest, we have obtained normal Raman spectra of water, ethanol, and 1 vol% ethanol in water. Sensitive and reproducible SERS detection of 1.7×10−7 M thiocyanate anions (14 ppb of NaSCN) in water has also been achieved.
We have explored the use of index-guiding liquid-core photonic crystal fiber (LC-PCF) as a platform for sensing and
measurements of analyte solutions of minute volume by normal and surface-enhanced Raman scattering (SERS). The
index-guiding LC-PCF was fabricated by selectively sealing via fusion splicing the cladding air channels of a hollow-core
PCF (HC-PCF) while leaving the center core open at both ends of the fiber. The center core of the resultant fiber
was subsequently filled with water-ethanol solution mixtures at various ethanol concentrations for normal Raman
scattering measurements and with water-thiocynate solutions containing Ag nanoparticle aggregates for SERS detection
of thiocynate at trace concentrations. The light-guiding nature in the solution phase inside the LC-PCF allows direct and
strong light-field overlap with the solution phase over the entire length of the PCF (~30 cm). This detection scheme also
dramatically reduces the contribution of silica to Raman spectral background, compared with the solid-core counterpart,
thus its potential interference in spectral analysis. These features attribute to ready normal Raman measurements of
water, ethanol, and water (99 vol.%)-ethanol (1 vol.%) solutions as well as sensitive and reproducible SERS detection of
~10 ppb thiocynate in water, all at a volume of ~0.1 μL.
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