Sensor platform development

Highly sensitive fiber optic refractive index and evanescent wave absorbance based sensors are developed by fabricating U-bent optical fiber with optimum bend diameters. A simple technique for Plastic optical fiber bending and decladding has been developed. Optimization studies have been carried out theoritically and experimentally and these probes have been utilized for physical, chemical and biological sensing applications.

U-bent Silica optical fiber sensor fabrication

Bending: 30 cm long optical fiber with polished ends is decladded over a length of 1 cm in the middle portion using a sharp blade followed by acetone wiping. A simple candle flame or oxy-butane flame was used to bend the fibers at the decladded region. The probes having a bend diameter of 1.5 mm

Decladding: The fiber probe region was cleaned by dipping it in acetone (5 min) followed by MeOH + HCl treatment for 5 min to remove traces of any polymer cladding or other organic contamination on the surface.

Surface functionalization & Gold nanoparticle immobilization: These fiber probes were immersed in sulphochromic solution (100 ml conc. H2SO4 + 1 ml of 0.5 mg/ml K2Cr2O7) for 10 min, washed in DI water and dehydrated for 2 h at 115 ◦C to generate silanol sites). Fibers were silanized by dipping in 1% aminosilane in ethanol: acetic acid (10:4 v/v) mixture for 5 min and washed thrice in ethanol to generate amine functional groups. The pH of silane solution was maintained at 3.4 in order to minimize adsorption of protonated amine groups with silanol sites. These fiber probes were placed in a hot-air oven at 110 ◦C for 20 min for condensation. Gold nanoparticles could be immobilized on amine functionalized probes.

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U-bent Plastic (PMMA) optical fiber sensor fabrication

Bending: POF comprises of polymethylmethacrylate (PMMA) and fluorinated polymer as core and cladding having refractive indices of 1.49and 1.41 respectively. The fiber wasbent in order to bring both ends closer and pushed inside a 15 cm long glass capillary tube of varying inner diameter. The capillary tube was placed in a hot air oven for 10 min at 100◦C. Here, fibers are bent by placing them in the hot ambience without subjecting them to a blow of hot air directly, to reduce thermal stresses.

Decladding: The U-bent probes were prepared by precisely decladding the fluorinated polymer layer over the U-bent region by chemical etching to sensitize the core surface for evanescent wave based sensing.These probes were decladded by dipping in a vertical flow cell of smaller cross sectional area which was completely filled with ethylacetate for 2 min.

Surface functionalization & Gold nanoparticle immobilization: The surface of PMMA fiber probe was amine functionalized using Hexamethylenediamine as described by Fixe et al. Gold nanoparticles are immobilized onto the amine functionalized probe to generate an plasmonically active U-bent RI sensor.

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Optimization studies

The sensitivity of the U-bend probes, depends greatly on the bend diameter of the probe. Experimental and theoretical investigations have been carried out to understand light propagation in U-bent optical fiber and to estimate the bend diameter for achieving maximum sensitivity.


These probes are effective low-cost refractive index sensors. They could also be used for force, pressure and level sensing. There is a pressing need for a simple and portable milk quality detection, since food adulteration is one of the major problems in India. These probes are being tested for determination of fat percentage and presence of adulterants such as urea or sucrose.