Silica-based U-bent fiber optic sensor (U-FOS) probes exhibit excellent absorbance and refractive index sensitivity. They have been typically fabricated by manual means with the help of a butane flame, which is plagued by high probe-to-probe variations in the geometry, leading to rejection rates as high as 50% - 70%. In particular, fibers with 200 μm core and bend diameter as small as 1 mm pose a severe challenge. To overcome these limitations, we have developed an automated fiber bending machine (FBM) that consists of a CO2 laser as heating source with a mechanism to automate laser beam deflection for precise control of heating zones on fiber and an automated articulating arm mechanism that holds both the ends of fiber and bends them after reaching glass transition temperature of about 1200 °C. FBM is capable of fabrication of U-FOS probes as many as 60-80 probes in an hour with bend diameter down to 0.55 mm and minimal geometric deviations. The proposed design is highly rugged, and more than ten thousand probes have been fabricated with this FBM so far.
This proof-of-concept study demonstrates a seed-mediated growth technique to synthesize gold film on a U-bent fiber optic sensor probe for plasmonic sensing applications. Here, gold seed nanoparticles were physisorbed on the surface of a U-bent silica probe. Later the seed-immobilized probe was incubated in growth solution for gold film growth. The newly fabricated gold film-coated probe exhibited a surface plasmon peak at 655 nm wavelength and sensitivity of 2271 nm/RIU and a figure of merit of 22.
This study investigates the use of U-shaped cladded plastic optical fiber (POF) and glass optical fiber (GOF) probes for refractive index (RI) sensing as they allow a simple one-step fabrication process. The RI sensitivity of the U-shaped cladded POF probes were evaluated in narrow (1.333 to 1.348, increment of 0.003 RI) and intermediate RI ranges (1.34 to 1.39, increment of 0.01 RI). No considerable improvement or drop was observed. However, the U-shaped silica cladded GOF showed 1.3 and 1.54 -fold improvement in the RI sensitivity in comparison to the decladded probes in the narrow and intermediate RI range respectively. The highest sensitivities for cladded POF and GOF probes in the intermediate RI range were 4.7 and 10.8 ΔA530 nm/ΔRIU respectively.
This study presents the development of low cost, rapid and highly sensitive plasmonic sandwich DNA biosensor using U-bent plastic optical fiber (POF) probes with high evanescent wave absorbance sensitivity and gold nanoparticles (AuNP) as labels. Plastic optical fiber (PMMA core and fluorinated polymer as cladding) offer ease in machinability and handling due to which optimum U-bent geometry (with fiber and bend diameter of 0.5 and 1.5 mm respectively) for high sensitivity could be achieved. A sensitive fiber optic DNA biosensor is realized by (i) modifying the PMMA surface using ethylenediamine (EDA) in order to maximize the immobilization of capture oligonucleotides (ONs) and (ii) conjugating probe ONs to AuNP labels of optimum size (~ 35 nm) with high extinction coefficient and optimal ON surface density. The sandwich hybridization assay on U-bent POF probes results in increase in optical absorbance through the probe with increase in target ON concentration due to the presence of increased number of AuNPs. The absorbance of light passing through the U-bent probe due to the presence of AuNP labels on its surface as result of sandwich DNA hybridization is measured using a halogen lamp and a fiber optic spectrometer. A picomolar limit of detection of target ON (0.2 pM or 1 pg/ml or 5 attomol in 25 μL) is achieved with this biosensing scheme, indicating its potential for the development of a highly sensitive DNA biosensor.
In this work we report characterization of bacterial biofilm using gold sputtered optical fiber probe as substrates for confocal Raman spectroscopy measurements. The chemical composition and the heterogeneity of biofilms in the extracellular polymeric substances (EPS) was evaluated. The spatial distribution of bacterial biofilm on the substrates during their growth phase was studied using Raman imaging. Further, the influence of substrate’s surface on bacterial adhesion was investigated by studying growth of biofilms on surfaces with hydrophilic and hydrophobic coatings. This study validates the use of gold sputtered optical fiber probes as SERS substrates in confocal microscopic configuration to identify and characterize clinically relevant biofilms.
An evanescent wave absorbance (EWA) based U-bent fiberoptic sandwich immunobiosensor with human IgG detection limits of 6.67 fM (1 pg/ml of human IgG) and 66.7 aM (10 fg/ml of HIgG) is demonstrated by exploiting immunogold labels and subsequently silver enhancement respectively. Such very low detection limits were achieved with the help of enhanced evanescent filed at the bend region of U-bent optical fiber probe that allows efficient interaction of light with 40 nm immunogold labels on the probe surface resulting in measurable optical absorbance changes. The other significant advantages of the demonstrated sensing scheme are low cost optoelectronic instrumentation consisting of an commercial green LED and a photodetector (S150C, Thorlabs Inc.), small volumes of sample and immunogold reagent each of 25 μl and rapid detection in 20 min. These results from the plasmonic fiberoptic biosensor demonstrate its huge potential for development of point-of-care diagnostic devices for sensitive and rapid detection of analytes.
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