Rapid, accurate, and real-time measurements of ocean salinity are of great importance for a host of scientific, commercial and defense applications. We demonstrate a highly sensitive, fast-responding fiber-optic salinity sensor that integrates long-period fiber gratings (LPFG) with ionic strength-responsive hydrogel. Submicron-thick hydrogels were synthesized via layer-by-layer (LbL) assembly of partially quaternized poly(4-vinyl pyridine) (qP4VP) and poly(acrylic acid) (PAA), followed by chemical crosslinking of qP4VP and removal of PAA. Spectroscopic ellipsometry studies of hydrogels with 37% quaternized qP4VP revealed robust and reversible swelling/deswelling behavior of the coatings in solutions with different salt concentrations at pH 7.5. The performance of hydrogel-coated LPFG for the monitoring of sodium chloride solution in the salinity relevant range of 0.4 to 0.8 M was investigated. The swelling/deswelling process induced remarkable changes in the refractive index of the coating, resulting in robust shift in the resonance wavelength of LPFG. The hydrogel-coated LPFG exhibited a sensitivity of 7 nm/M with a response time less than 1 second. There is a linear correlation between the resonance wavelength shift and the salt concentration, making quantification of measured salinity straightforward.
Solution pH is one of the most critical parameters for chemical reactions which are important for clinical, environmental or manufacturing applications. The integration of pH-responsive polyelectrolyte via layer-by-layer assembly (LbL) with long period grating (LPG) has enabled the monitoring of the pH in real time with high sensitivity at minimized volume. We show that the pH-responsive profile of the LbL/LPG platform can be tailored to specific needs depending on the deposition parameters during the LbL process. By choosing different deposition pH, weak polyelectrolytes showed either linear responsiveness or responsiveness with a reversal point at pH 4.5 over the pH range of 2-11. The most sensitive area was related to both the pKa of the chosen polyelectrolyte in the coating and the deposition pH during the LbL process. The study provides important guide to optimize and improve the sensitivity in specific pH range by selection of polyelectrolytes and/or deposition parameters. The LbL/LPG platform affords a powerful tool to study the mechanisms of the responsiveness and physiochemical properties of the polyelectrolytes.
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.
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.
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.
Conference Committee Involvement (6)
Fiber Optic Sensors and Applications VIII
28 April 2011 | Orlando, Florida, United States
Fiber Optic Sensors and Applications VII
7 April 2010 | Orlando, Florida, United States
Fiber Optic Sensors and Applications VI
15 April 2009 | Orlando, Florida, United States
Photonic Crystals and Photonic Crystal Fibers for Sensing Applications III
9 September 2007 | Boston, MA, United States
Photonic Crystals and Photonic Crystal Fibers for Sensing Applications II
3 October 2006 | Boston, Massachusetts, United States
Photonic Crystals and Photonic Crystal Fibers for Sensing Applications
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