Reshma Bharadwaj

Evanescent wave absorbance based fiber optic biosensor for label-free detection of E. coli at 280 nm wavelength

A novel label-free technique for the detection of pathogens based on evanescent wave absorbance (EWA) changes at 280 nm from a U-bent optical fiber sensor is demonstrated. Bending a decladded fiber into a U-shaped structure enhances the penetration depth of evanescent waves and hence sensitivity of the probe. We show that the enhanced EWA response from such U-bent probes, caused by the inherent optical absorbance properties of bacterial cells or biomolecules specifically bound to the sensor surface, can be exploited for the detection of pathogens. A portable optical set-up with a UV light emitting diode, a spectrometer and U-bent fiber optic probe of 200 μm core diameter, 0.75 mm bend radius and effective probe length of 1cm demonstrated an ability to detect less than 1000 cfu/ml.

Study of localized surface-plasmon-resonance-based optical fiber sensor

A U-bent fiber-optic sensor based on the localized surface plasmon resonance (LSPR) of spherical silver nanoparticles has been studied. The redshift of the absorption maximum of the bound silver nanoparticles was observed due to the increment of nanoparticle density on the surface of the fiber. On the other hand, the blueshift was observed when the refractive index of the environment surrounding the nanoparticle was increased. These observations were analyzed in terms of a single nanoparticle theoretical framework. The departure from the spherical symmetry of the nanoparticle is attributed to the plasmonic coupling effect between the randomly distributed nanoparticles on the surface of the fiber core. This phenomenon can be cleverly exploited to develop different kinds of optical fiber sensors.

Emerging use of nanostructure films containing capped gold nanoparticles in biosensors

The localized surface plasmon resonance (LSPR) property of gold nanoparticles (GNP) has been exploited in a variety of optical sensor configurations including solution-based bioassays, paper-based colorimetric detection, surface-confined nanoparticle film/array-based sensing, etc. Amongst these, gold nanostructured films are of great interest because of their high stability, good reproducibility, robustness, and cost-effectiveness. The inherent optical characteristics of GNP, are attributed to parameters like size and shape (eg, nanospheres, nanorods, nanostars), eg, LSPR spectral location sensitivity to the local environment, composition (eg, gold-silver or silica-gold nanoshells), sensing volume, mesospacing, and multiplexing. These properties allow sensor tunability, enabling enhanced sensitivity and better performance of these biosensors. Ultrasensitive biosensor designs were realized using gold nanostructured films fabricated by bottom-up as well as top-down approaches. In this review, we describe the past, present, and future trends in the development of GNP-LSPR-based sensors, concentrating on both design (fabrication) and application. In the process, we have discussed various combinations of GNP size and shape, substrate, and application domains.

S-shaped SU-8 optical waveguide immobilized with gold nanoparticles for trace detection of explosives

In this study, we report a miniaturized optical sensor for direct detection of vapors of nitro-based explosives using gold nanoparticle (AuNP) coated SU-8 polymer optical waveguides. S-shaped waveguide geometry was chosen due its enhanced evanescent field sensitivity. Light was coupled into the waveguide structure to evanescently excite the localized surface plasmon resonance (LSPR) modes of the immobilized AuNP. The AuNP were functionalized with 4- mercaptobenzoic acid (4-MBA) which acts as the receptor for nitro-based explosives. The AuNP coated SU-8 optical waveguide sensor demonstrated an ability to detect 10 parts per billion (ppb) concentration of explosive analytes.

UV photodiode based portable fiber optic biosensor

The strong optical absorbance characteristics around 280 nm of several protein chromophores present in biomolecules like Immunoglobulin (IgG) and cell walls of bacteria can be exploited for direct detection of such biological analytes. In this report, we describe the development of a portable fiber optic biosensor, using a UV LED with peak emission at 280 nm and a UV photodiode to measure evanescent wave absorbance changes. A transimpedance amplifier is used as a (photo) current to voltage converter and for amplification of optical signals from the photodiode. The sensor is able to detect 50 µg/ml of Goat anti-Human Immunoglobulin (GaHIgG), used as a model protein and thus, could provide a suitable, inexpensive platform for label-free detection of water borne pathogen like E. coli cells.