We developed a “Metal-Molecule-Metal Nanoparticle” (MMNP) configuration on a fiber-optic SPR sensor system in the present work. For this, a gold-coated optical fiber (with an exposed core in the central portion) was used, and this gold-coated surface was first functionalized with the Self-Assembled Monolayer (SAM) of 4-aminothiophenol (4-ATP) molecules. The 4-ATP SAMs were functionalized with biotin molecules through EDC/NHS coupling agents. Separately, citrate-capped gold nanoparticles (~20 nm) were synthesized and subsequently functionalized with streptavidin protein, which exhibits exceptional binding affinity towards biotin molecules. To establish the MMNP configuration, streptavidin-functionalized gold nanoparticles were flown over a biotin-functionalized SPR sensor surface. The Raman signal of 4-ATP molecules in the SPR sensor region was collected before and after interaction with streptavidin-functionalized gold nanoparticles. The Raman signals of the sensor region exhibited huge enhancement after interaction with gold nanoparticles, confirming the creation of the hypothesized MMNP configuration on successful biomolecular interaction between streptavidin-functionalized gold nanoparticles and biotin molecules on the SPR sensor surface. Concomitant with the Raman signal enhancement, a marked shift (~ 3-fold) in the SPR peak is also observed when interacting with the gold nanoparticles.
Calcium carbide has been long used as artificial fruit ripening agent. However, calcium carbide is not only highly toxic, but also carcinogenic in nature leading to its ban in the most of the countries. Furthermore, to prevent its use, various government regulatory bodies recommended adopting the natural ripening agent ethylene in artificial ripening centers. Generally, 10-150 ppm of ethylene is utilized in ripening centers. Gas sensing using optical fiber sensors is becoming increasingly important in many areas due to the advantage of fast response, high sensitivity, lower cost, possibility for remote sensing. Accordingly, in this work, we have integrated a fiber optic probe with sensing material for the detection of ethylene vapors in the permissible range. The ethylene gas is known to exhibit sensitivity towards coinage metals specifically copper (Cu). We therefore utilized Cu-salt impregnated fluorescent polymers and Cu-based metal-organicframeworks (MOFs) as sensing materials to detect ethylene in different sensing modes; fluorescence, evanescent wave absorption sensor.
Surface plasmon resonance (SPR) based sensors have emerged as an excellent analytical technique known for their ultra-high sensitivity capable of detecting even minute refractive index (RI) changes of the surrounding during biomolecular interactions. However, due to their exceptional sensitivity towards RI changes, they are also susceptible to variations to reaction parameters such as temperature, concentration and non-specific adsorption which contributes to its inherent non-specificity. To circumvent these problems, researchers have employed various downstream techniques like chromatographic separations linked with Mass spectroscopy (MS) to ascertain sensor specificity. In an attempt to resolve this issue, we have exploited Surface-enhanced Raman Spectroscopy (SERS) as a secondary tool to validate the specificity of SPR signals. For this purpose, we have utilized a fiber-optic SPR system and gold nanoparticles to create a “metal film-molecule- metal nanoparticle” configuration and have shown that the resultant enhancement in the Raman signal of the molecule can be utilized as a self-validating tool. Accordingly, we have functionalized our FO-SPR with the bifunctional Raman active molecules; 4- aminothiolphenol (4-ATP), which could interact with carboxylated gold nanoparticles to create a “metal film-moleculemetal nanoparticle” configuration. The successful binding of the gold nanoparticles was reflected in ~19 nm SPR shift along with the simultaneous increase in the Raman signals of the 4-ATP molecules sandwiched in-between metal film and gold nanoparticles.
Modified optical fiber tips with micrometer/sub micrometer dimensions are very promising in the field of Tip enhanced Raman Scattering (TERS), Surface Enhance Raman Scattering (SERS), optical trapping, plasmonic sensor etc. A unique axicon fabricated on the optical fiber tip by simple chemical etching in 48% Hydrofluoric acid (HF) followed by thin film gold (Au) coating by magnetron sputtering is demonstrated for anti-BSA and BSA binding. The dimension of conical axicon is ~2 μm apex at the core of fiber, opening diameter of ~70 μm at the tip and total length of ~300 μm. The inner surface of the hollow negative axicon is coated with thin film of gold. The dielectric tip experiences notable optical field enhancement due to the plasmonic coating. Further, such tip generates non-diffracting Bessel beam which has several advantages including collimation. To monitor the binding process of protein bovine serum albumin (BSA) and anti-BSA, the tip is processed for anti-BSA binding on the surface of the negative axicon. The tip is further tested to bind BSA of different concentrations. For every change in concentration of the BSA, time response curve with respect to variation in wavelength shift was recorded. The tip has shown sensitivity to protein binding process for BSA concentrations of range 10-12 mg/ml to 10-4 mg/ml. The sensor saturates for higher concentration of protein BSA around 10-4 mg/ml. The sensor tends to be very sensitive to proteins of different concentration. This deep seated negative axicon tip based probe can be used for sensing low volumetric bio-samples.
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