This presentation reports our findings in the fabrication and evaluation of nanostructured sapphire optical fiber (NSOF) for surface-enhanced Raman spectroscopy (SERS) sensing at elevated temperatures. Specifically, we systematically investigated the morphological stability and the mechanical properties of the anodized aluminum oxide (AAO) cladding of NSOF after cyclic thermal treatment at temperatures ranging from 1000°-1500°C with the pore diameter, interpore distance, as well as the cladding thickness as parameters. The cladding/sapphire fiber interface integrity due to possible mismatch in the coefficient of thermal expansion between AAO and sapphire fiber was also examined. We also immobilized Ag nanoparticles in the pore channels of AAO cladding for in-situ SERS measurements in a hot furnace. We will show that Ag nanoparticles confined in the nanoscopic pore channels of AAO exhibit much better thermal stability, compared with those on a planar substrate, making high-temperature harsh environment SERS sensing possible with NSOF.
Current industrial technologies for selective oxidation of propene via a single-stage oxidation process in H2/O2 catalyzed by Au holds excellent prospect of green production of C3H6O. Fundamentals of the molecular mechanisms between catalytic Au and the oxidant remain unclear for decades, however, impeding the development of its rational design and implementation. We explore a multifunctional, highly organized nanoporous anodized aluminum oxide (AAO) substrate with immobilized Au nanoparticles (Au NPs) both as a catalytic reactor and an ultra-sensitive SERS probe to investigate the molecular level details during Au-catalyzed oxidation of propene in situ. Nanoporous AAO offers excellent thermal stability and enhanced particle coverage density for the immobilized Au NPs within to enable high temperature SERS interrogation, opening up new opportunities in the study of the catalytic reactions. Different size of Au NPs and pores of AAO are explored for improved SERS sensitivity and catalytic activity.
We describe an innovative and scalable strategy of transforming a commercial unclad sapphire optical fiber to an allalumina nanostructured sapphire optical fiber (NSOF) that overcomes decades-long challenges faced in the field of sapphire fiber optics. The strategy entails fiber coating with metal Al followed by subsequent anodization to form anodized alumina oxide (AAO) cladding of highly organized pore channel structure. We show that Ag nanoparticles entrapped in AAO show excellent structural and morphological stability and less susceptibility to oxidation for potential high-temperature surface-enhanced Raman Scattering (SERS). We reveal, with aid of numerical simulations, that the AAO cladding greatly increases the evanescent-field overlap both in power and extent and that lower porosity of AAO results in higher evanescent-field overlap. This work has opened the door to new sapphire fiber-based sensor design and sensor architecture.
An improvement in light confinement in sapphire fiber is obtained by employing nanoporous alumina as a cladding. The
fabrication strategy entails freeze-coating metal Al on sapphire fiber and its subsequent anodization to form alumina
cladding with highly organized nanopore channels vertically aligned to the fiber axis with dimensions of ~20 nm. We
investigated the confinement dependence on the porosity of the cladding, showing an improvement in comparison to
unclad sapphire fibers. The versatility of anodized alumina cladding with tunable structural and optical characteristics
has the potential to enable a new class of specialty sapphire optical fibers by engineering the light propagation for new
sensor development.
We present a study on surface-enhanced Raman scattering (SERS) utilizing unclad single crystal sapphire fiber with Ag nanoparticles (NPs) immobilized either at the fiber distal end for direct excitation or on the fiber surface for evanescent-field interaction. The dependence of SERS intensity on the coverage density of Ag NPs was investigated. We demonstrated robust SERS sensitivity in both cases. For direct excitation-based sensing, we found that a sensitivity maximum exists with increased particle coverage beyond which the sensitivity starts to decline. More importantly though, for evanescent-field based measurements, we revealed that multimode sapphire fiber can accommodate Ag NPs at a far higher particle coverage density than single-mode fiber while maintaining the dominance of SERS gain despite competitive absorption and scattering loss by Ag NPs with a limit of detection of 10-9 M Rhodamine 6G solution.
We have explored the use of a fiber-optic probe with surface-enhanced Raman scattering (SERS) sensing modality for
early, noninvasive and, rapid diagnosis of potential renal acute rejection (AR) and other renal graft dysfunction of kidney
transplant patients. Multimode silica optical fiber immobilized with colloidal Ag nanoparticles at the distal end was used
for SERS measurements of as-collected urine samples at 632.8 nm excitation wavelength. All patients with abnormal
renal graft function (3 AR episodes and 2 graft failure episodes) who were clinically diagnosed independently show
common unique SERS spectral features in the urines collected just one day after transplant. SERS-based fiber-optic
probe has excellent potential to be a bedside tool for early diagnosis of kidney transplant patients for timely medical
intervention of patients at high risk of transplant dysfunction.
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