A novel application of a highly sensitive biosensor based on long-period fiber gratings (LPFG) coated with microporous polyelectrolyte coating for Gram-negative bacteria detection was investigated. The uniform microporous coating with large surface area was fabricated with weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) via layer-by-layer (LbL) assembly, followed by a brief exposure to acidic aqueous solutions at pH=4. The diameter of the micropores could be adjusted in a wide range by thermal treatment and ionic etching. The effect of pore size and surface topography on bacteria adhesion was examinged. Optofluidic LPFG platform for real-time monitoring of the bacteria binding/ adhesion in a flowing condition was investigated by measuring the spectral shift in the resonance wavelength. The coated LPFG platform was further functionalized with covalently immobilized bacteria antibody for specific bacterial detection with a concentration as low as 104 CFU/ml. Comparing with the widely used biosensors based on surface plasmon resonance (SPR), no moving part or metal deposition is required in our biosensor, making it highly sensitive, accurate, compact and cost effective.
Surface-enhanced Raman scattering (SERS) integrates high levels of sensitivity and spectroscopic precision with tremendous potential for chemical and biomolecular sensing. On the other hand, metal nanoparticles have been employed in catalysis with great promise for future energy technologies. Interactions between oxygen and gold surfaces are of fundamental importance in catalysis and other multiple and diverse areas of science and technology. We propose to synergistically integrate the two aspects of metal nanoparticles in dual-functional Ag@Au core-shell nanostructures to take advantage of high SERS enhancement factor of Ag and unique catalytic property of Au. Pure Au nanoparticles with specific size are also prepared to be a comparison with Ag@Au core-shell structures in terms of SERS enhancement and catalytic properties by in situ Raman detection during the decomposition of hydrogen peroxide.
Manganese is an important heavy metal element that influences nervous system. Detection of manganese in various mediums has thus attracted lots of attentions. Here we report a study on silver nanoparticles functionalized long-period fiber grating (LPFG) for manganese sensing. Silver nanoparticles (AgNPs) with a size in the range of 70nm10nm were synthesized with polyvinyl pyrrolidone (PVP)-glycol. The interplay between arginine, an agent that can cause aggregation of AgNPs, and Mn2+ leads to refractive index change in the AgNPs colloidal solution, thus a shift in the resonance wavelength of LPFG that is surrounded by the colloidal solution. A sensitivity of 0.2nm shift/10-6M was achieved using such strategy. We believe the integration of nanoparticles with LPFG represents a promising sensing strategy for more advanced applications important for not only environmental but also health science.
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.
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.
Hospital acquired infections in indwelling device have become a life-threatening issue accompanied by the wide use of medical devices and implants. The infection process typically involves the attachment, growth and eventual assemblage of microbial cells into biofilms, with the latter exhibiting extremely higher antibiotic tolerance than planktonic bacteria. Surface constructed antimicrobial coatings offer a viable solution for bacteria responsive antibiotic strategy in medical devices such as catheter and stents. Therapeutic peptide has pioneered the field for their attractive pharmacological profile with broad antibacterial spectrum, great efficacy and long life-span. It has been a common practice to separately assess bacteria responses through commercially available activity assay kits after their exposure to antibiotic coatings, limiting the assessment of their activity in vitro with a discontinuous fashion. We developed and demonstrated an innovative all-optical lab-on-fiber optofluidic platform (LOFOP) to fill in this technical gap by allowing in situ measurement of the bacteria attachment in a continuous manner. This LOFOP allows for evaluation of drug release and resultant bacterial response by integrating glass capillary with lytic peptide-containing LbL-coated long period graing (LPG) as its core. S. aureus suspension is introduced through the assembled optofluidic platform with the capillary and the peptide-coated LPG. The efficacy of the peptide-containing coating is evaluated in situ by monitoring the attachment of bacteria and the ensuing development of biofilms using the LPG. LPG without antimicrobial coatings will be explored and compared as control.
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.
Dielectric nanoparticle in integration with the long-period grating (LPG) is explored and its effect on the sensitivity is evaluated in the in situ monitoring of the deposition of drug delivery thin film. SiNPs were immobilized on the LPG via layer-by-layer self-assembly using poly allylamine hydrochloride (PAH). Theoretical calculation reveals that the SiNPs coating increases the evanescent field overlap in the surrounding of the LPG thus enhances its sensitivity. The increased total surface for the following thin film deposition also contributes to the enhancement of the sensitivity. Its unique capability for the in-situ monitoring of drug delivery thin film [chitosan (CHI) / Poly arylic acid (PAA) / Gentamicin sulfate (GS) /PAA]n through layer-by-layer assembly (LbL) was demonstrated with a sensitivity of 8.1 nm shift/tetralayer for LPG with 1 layer of SiNPs with 50 nm in diameter. The sensitivity enhancement of the LPG also depends heavily on the layer numbers and sizes of the SiNPs. The LPG with SiNPs of 8 layer numbers exhibits a sensitivity of only 1.2 nm shift/tetralayer. Control experiment of LPG without the SiNPs for the monitoring of [CHI/PAA/GS/PAA]n shows a sensitivity of 2.4 nm shift/tetralayer. This investigation suggests that SiNPs are effective in fine tune the optical property of the LPG. SiNPs coating thick enough can be used as an effective insulation for LPG from outer species. This investigation sets up the foundation for the development of SiNPs enabled optical fiber LPG sensor for the in-situ study of drug delivery LbL thin film.
The release profiles of gentamicin sulfate (GS) from [chitosan (CHI)/poly(acrylic acid) (PAA)/GS/PAA]n polyelectrolyte multilayers were investigated in situ using an innovative lab-on-fiber (LOF) optofluidic platform that mimics physiologically relevant fluid flow in a microenvironment. The LOF was constructed by enclosing in a flow-enabled and optically coupled glass capillary a long-period fiber grating both as a substrate for LbL growth of [CHI/PAA/GS/PAA]n and a measurement probe for GS release. We show that the LOF is very robust in monitoring the construction of the [CHI/PAA/GS/PAA]n multilayers at monolayer resolution as well as evaluating the rate of GS release with high sensitivity. The release processes in the LOF under static and a range of dynamic conditions are evaluated, showing a faster release under dynamic condition than that under static condition due to the varying circumstance of GS concentration gradient and the effect of flow-induced shear at the medium-multilayer interface. The LOF platform has the potential to be a powerful test bed to facilitate the design and evaluation of drug-eluting polyelectrolyte thin films for their clinical insertion as part of patient care strategy.
Layer-by-layer (LbL) self-assembly via hydrogen bonding is one of the primary mechanisms to achieve stimuliresponsive
polyelectrolyte thin films for a variety of applications. The ability to monitor the individual LbL steps in situ
is of great significance in the development and understanding of hydrogen-bonded LbL systems of new design functions
and properties. Long-period gratings in single-mode fiber (SMF-LPG) has the potential for such application by virtue of
its high index sensitivity. We report a theoretical and experimental investigation to sort out the coupled cladding mode
that is most sensitive to molecular adsorption events during LbL deposition. We have undertaken a combined theoretical
and experimental investigation to illustrate and demonstrate the strong correlation between the order of the coupled
cladding modes in long-period gratings (LPG) in conventional single mode fiber and their sensitivity to the process of
layer-by-layer (LbL) assembly of polyelectrolyte multi-layers. We show that high-order cladding modes such as LP0,10 in
LPG are significantly more sensitive than their lower-order counterparts such as LP0,2, with LP0,10 yielding a shift of
1.575 nm in resonance wavelength per polyelectrolyte repeating unit of poly(vinyl pyrrolidone)/poly(methacrylic acid)
bi-layer. We illustrate the potential utility of SMF-LPG integrated with stimuli-responsive LbL thin films as sensors. The
integrated LPG/LbL strategy as a robust LbL test-bed has broad ramifications in exploring and exploiting sensors and
devices enabled by versatile, stimuli-responsive polyelectrolyte thin films.
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.
Long-period gratings (LPG) are inscribed in endlessly single mode (ESM) photonic crystal fiber (PCF) and conventional single mode fiber (SMF) with symmetric and asymmetric CO2 laser irradiation. Transmission measurements and near field imaging indicate that symmetric index perturbation induced by laser irradiation with the aid of a 120° gold-coated reflecting mirror results in LP0n symmetric mode coupling, while asymmetric irradiation without using the mirror leads to LP1n asymmetric mode coupling for both fiber types. Symmetric irradiation yields far more reproducible LPG in PCF than asymmetric irradiation. On the other hand, the irradiation symmetry has little effect on the reproducibility of LPG inscribed in SMF due to the isotropy of its all-solid cladding structure. The removal of the outer solid cladding of PCFLPG results in a significant change in its transmission characteristics, which provides rich information on the refractive index perturbation and mode field distribution in the outer solid cladding of PCF-LPG.
The influence of backside illumination and temperature on the fabrication of large and high aspect ratio silicon microchannel plates (MCPs) by photoelectrochemical (PEC) process is described. Backside illumination is provided by three 150-W tungsten halogen lamps with a feedback loop, keeping a constant current density. The etching temperature is maintained by a circulation system. Proper backside illumination and the lower temperature can provide better integrated etching conditions compared to that without illumination and temperature control. Etching under the improved conditions results in smoother undercutting and better surface topography for large (effective diameter of about 80 mm for 4-inch silicon substrates) silicon microchannel plates. Enhancing the backside illumination within the etching temperature range ensures that the aspect ratio is more than 40, boding well for applications of silicon microchannel plates.
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