Sachin K. Srivastava

Simulation of a localized surface-plasmon- resonance-based fiber optic temperature sensor

A fiber optic temperature sensor based on localized surface plasmon resonance of spherical gold nanoparticles embedded in a dielectric layer around the unclad core of a small portion of the fiber has been analyzed. Simulations have been carried out for a number of dielectric materials that show considerable changes in their refractive indices due to a change in the temperature in addition to having refractive indices higher than that of the fiber core. The analysis is based on the spectral interrogation method. The surface plasmons in metal nanoparticles have been excited by the light refracted through the core and the dielectric interface. The sensitivity of the sensor has been determined for each dielectric material used, and it is found to be the maximum for CdGeP(2) as a sensing medium. The temperature sensing range of the present sensor is also wide because the melting points of the metal and the fiber core, as well as the sensing medium, are large. The proposed fiber optic temperature sensor is compact, light weight, and highly sensitive with a wide temperature sensing range.

Theoretical modeling of a localized surface plasmon resonance based intensity modulated fiber optic refractive index sensor

A localized surface plasmon resonance based fiber optic sensor for refractive index sensing has been analyzed theoretically. The effects of size of the spherical metal nanoparticle as well as the light sources on the performance of the sensor have been studied rigorously. It is observed that a diffuse light source along with an intensity modulation method gives better performance in terms of sensing range. In addition, the use of a diffuse source makes the sensing device very cheap and compact, which is an important issue for the commercial applications. The refractive index range of the sensor is larger than the ranges reported for various types of fiber optic sensors utilizing intensity modulation.

Surface-Plasmon-Resonance-Based Fiber-Optic Sensor for the Detection of Low-Density Lipoprotein

We report the fabrication and characterization of a surface plasmon resonance (SPR)-based fiber-optic sensor for the detection of low-density lipoprotein (LDL). The probe is prepared by first coating a 50-nm-thick gold film on the unclad portion of the optical fiber core and then immobilizing 4-aminothiophenol (4-ATP), followed by antiapolipoprotein B, over the 4-ATP/Au surface. Spectral interrogation technique is used for the characterization of the probe. SPR spectra are recorded for sample solutions of LDL with concentrations in the range of 0-190 mg/dl. Sensitivity of the sensor is found to be 0.18387 nm per mg/dl. Furthermore, the response time of the sensor is very small (around 2 min). The sensor can be utilized in online monitoring as well. This paper may result in the commercialization of a miniaturized low-cost reusable fast and accurate optical fiber sensor for the detection of LDL level in human blood.

A Multitapered Fiber-Optic SPR Sensor With Enhanced Sensitivity

A multitapered fiber-optic surface plasmon resonance (SPR) sensor for refractive index sensing has been fabricated and characterized. The sensor has been analyzed for different taper periods in a given sensing length. The sensing probe with the smallest taper period is found to be the most sensitive. In addition, the sensor is less fragile than previously studied tapered fiber-optic SPR sensors, as the core waist is not very thin in the present probe.

Fiber Optic Plasmonic Sensors: Past, Present and Future

We review various fiber optic sensors utilizing both the propagating and localized surface plasmon resonance techniques. The utilization of optical fibers in plasmon based sensing has provided several advantages in sensing of various physical, chemical and biochemical parameters. The article starts with a brief introduction of the propagating surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR), presents the review of some of the past studies on SPR and LSPR based state-of-the-art fiber optic sensors and finally ends with the future scope of the plasmonics based fiber optic sensors. In addition, we discuss some latest results on the plasmon enhanced whispering gallery mode sensors, which require tapered optical fibers for excitation. The present review may provide the researchers a rigorous and organized literature for the understanding of the basics, utility and trends of fiber optic plasmonic sensor in a chronological order.

Surface plasmon resonance based fiber optic glucose biosensor

A surface plasmon resonance (SPR) based fiber optic biosensor has been fabricated and characterized for the detection of blood glucose. Optical fiber sensor was fabricated by first coating a 50 nm thick gold film on the bare core of optical fiber and then immobilizing glucose oxidase (GOx) over it. Aqueous glucose solutions of different concentrations were prepared. To mimic the blood glucose levels, the concentration of glucose solutions were kept equal to that in human blood. The refractive indices of these sample solutions were equal to that of water up to third decimal place. SPR spectra for the sensor were recorded for these glucose solutions. When the glucose comes in contact to glucose oxidase, chemical reactions take place and as a result, the refractive index of the immobilized GOx film changes, giving rise to a shift in the resonance wavelength. Unlike electrochemical sensors, the present sensor is based on optics and can be miniaturized because of optical fiber. The present study provides a different approach for blood glucose sensing and may be commercialized after optimization of certain parameters.

Self-referenced sensor utilizing extra-ordinary optical transmission from metal nanoslits array

We report the first self-referenced sensor based on the extraordinary optical transmission (EOT) of metal-nanoslits array in the near-infra-red (NIR) telecommunication window of the electromagnetic spectrum. The nanoslits array shows two enhanced transmission peaks, out of which one shows a red shift with an increase in the refractive index of the analyte medium, while the other remains fixed. We demonstrate the detection of small amounts of water in ethanol using the nanoslits array chip. The present study might be useful in developing ultra-small biosensor chips integrated to optical fibers for online monitoring and remote sensing applications.

SPR Based Fiber Optic Sensor for the Detection of Vitellogenin: AnEndocrine Disruption Biomarker in Aquatic Environments

We have fabricated a fiber optic SPR biosensor for the detection of vitellogenin (Vg), an endocrine disruption biomarker in aquatic environments. The sensor was fabricated by immobilizing anti-Vg antibodies on the sensor surface. Control experiments performed on another similar protein, fetuin and on a sensing probe without the anti-Vg antibody receptor confirmed the specificity of the sensor. The limit of detection of the sensor was found to be as small as 1 ng/ml in our experimental window. The sensitivity of the sensor was 0.48 nm/(ng/ml). This sensor can be utilized for remote monitoring of the water bodies for any endocrine disruption phenomena.

Surface plasmon resonance based multi-channel and multi-analyte fiber optic sensor

Surface plasmon resonance (SPR) based fiber optic sensor has been studied for multichannel and multianalyte sensing. Simulations have been carried out for a fiber optic sensor having two sensing regions coated with silver and gold for multichannel and multianalyte sensing. The simulated results have been obtained for different combinations of refractive indices of the samples around the probes. To support simulations optical fiber SPR probes with two sensing regions coated with silver and gold have been fabricated. SPR spectra for these sensors have been recorded for aqueous sucrose solutions of varying refractive indices. The refractive index of the liquid samples around both the gold and silver coated regions was kept the same to see the potential of SPR based fiber optic multichannel sensing, while it was kept different for studies related to multianalyte sensing. Both the theoretical and experimental results match qualitatively. The SPR resonance wavelengths for gold and silver being different, these sensors can be utilized for both multichannel and multianalyte sensing.

Quantitative assessment of paraoxon adsorption to amphiphilic β-sheet peptides presenting the catalytic triad of esterases

Organophosphate compounds that are used as pesticides affect the nervous system by binding irreversibly to the active site of the enzyme acetylcholine esterase (AChE) and disrupting neuro-signaling nerve cells. In this study we characterized adsorption of paraoxon to a set of designed peptides that present different arrangements of the three amino acids of the AChE catalytic site: histidine, glutamic-acid and serine. The peptides set included two β-strands with no net charge and three β-hairpins that differ in their net charge. Circular dichroism, Thioflavin T assays and TEM images provided only qualitative insights on paraoxon binding to the different peptides. Paraoxon binding to the different peptides was measured with dialysis membrane tubes filled with the peptide solutions and suspended in a reservoir of paraoxon solution. Among all the tested peptides, the single strand peptide, denoted ssESH exhibited at 100 μM in random conformation prefibrillar state, the maximum paraoxon adsorption, with a binding mol ratio of one paraoxon per two peptides and an estimated equilibrium binding constant 5 ∗ 104 M-1. The three β-hairpin peptides demonstrated that a net negative charge is unfavorable for paraoxon adsorption. Surface enhanced Raman spectroscopy measurements with ssESH enabled the detection of nanomolar adsorbed concentrations of paraoxon.