Raghavendra Sai V V

SARS-CoV-2 nucleocapsid protein-based COVID-19 diagnosis is a promising alternative to the high-priced, time-consuming, and labor-intensive RT-PCR tests. Here, we developed a rapid, dip-type, wash-free plasmonic fiber optic absorbance biosensor (P-FAB) strategy for the point-of-care detection of SARS-CoV-2 N-protein, expressed abundantly during the infection. P-FAB involves a sandwich assay with plasmonic labels on the surface of a U-bent fiber optic sensor probe with a high evanescent wave absorbance (EWA) sensitivity. The SARS-CoV-2 N-protein is quantified in terms of the change in the intensity of the light propagating through the U-bent sensor probe coupled to a green LED and a photodetector. Firstly, the optical fiber material (silica vs. polymeric optical fiber), was evaluated to realize a sensitive sensor platform. The optimal size of AuNP labels (20, 40, and 60 nm) to achieve high sensitivity and a lower limit of detection (LoD) was investigated. Following the P-FAB strategy, fused silica/glass optical fiber (GOF) U-bent senor probe and citrate-capped AuNP labels (size 40 nm) gave rise to an LoD down to 2.5 ng/mL within 10 mins of read-out time. Further, studies on development and validation of a point of care (PoC) read-out device, and preclinical studies are in progress.

The refractive index (RI) sensitivity of a localized surface plasmon resonance (LSPR)-based fiber-optic probes is dependent on surface coverage of gold nanoparticles (GNP), fiber core diameter, and probe geometry. For U-bent LSPR fiber-optic probes, which demonstrated an order higher absorption sensitivity over straight probes, bend diameter and probe length may also have a significant influence on the sensitivity. This study on U-bent fiber-optic LSPR probes is aimed at optimizing these parameters to obtain highest possible RI sensitivity. RI sensitivity increases linearly as a function of surface coverage of GNP in the range of 2–22%. U-bent fiber-optic probes made of 200-, 400-, and 600-μm fiber core diameter show optimum bend diameter value as ∼1.4 mm. In addition, RI sensitivity is almost the same irrespective of fiber core diameter demonstrating flexibility in choice of the fiber and ease in optical coupling. The length of the probe preceding and succeeding the bend region has significantly less influence on RI sensitivity allowing miniaturization of these probes. In addition to these experimental studies, we present a theoretical analysis to understand the relative contribution of evanescent wave absorbance of GNP and refractive losses in the fiber due to GNP, towards the RI sensitivity.

Evanescent wave-based excitation of noble metal nanostructures on fiber core surface allows simultaneous excitation over a larger area in comparison with free space coupling, and hence highly suitable for development of surface-enhanced Raman scattering (SERS)-based sensor applications. This study describes the development of plasmonically active U-bent plastic optical fiber (POF) probes for SERS-based sensing. The SERS activity of gold nanostructures on the fiber probe surface obtained by means of electroless deposition, sputtering, and chemisorption of nanoparticles are thoroughly characterized using a confocal microscopic setup. Influence of excitation wavelength and laser excitation power on SERS activity and stability of the plasmonic U-bent probes is evaluated. Gold-sputtered POF probes and gold nanoparticle immobilized POF probes show higher enhancement factor in the order to ~108. A biosensor application of these POF-based SERS sensors is demonstrated by realizing a sandwich immunoassay with 4-MBA as Raman label.

This study presents design and development of evanescent wave absorbance based U-bent plastic optical fiber (POF) probe with high sensitivity and its applications to refractive index (RI) sensing. The probes were fabricated by a controlled decladding procedure to remove the fluorinated polymer without damaging the poly methyl methacrylate (PMMA) core and a simple and scalable fabrication technique to obtain POF U-bent probes of desired geometry. U-bent probes of fiber diameter from 250 to 1000 μm were fabricated and optimum bend diameter for each fiber diameter was investigated. The sensitivity was found to be maximum when the bend diameter of the probe varies from 2 to 3 times the fiber diameter. Probes with 500 μm core and 1.25 mm bend diameter showed highest sensitivity (5.57ΔA560 nm/ΔRIU) in the visible region to RI changes from 1.33 to 1.47 with a resolution better than 1 milli RI units. Furthermore, U-bent probes were amine functionalized and coated with gold nanoparticles to obtain a localized surface plasmon resonance (LSPR) based RI sensor that has an 8-fold improvement in RI sensitivity, hence extending their applicability to plasmonic biosensing.

Detecting the ultra-low abundance of analytes in real-life samples, such as biological fluids, water, soil, and food, requires the design and development of high-performance biosensing modalities. The breakthrough efforts from the scientific community have led to the realization of sensing technologies that measure the analyte's ultra-trace level, with relevant sensitivity, selectivity, response time, and sampling efficiency, referred to as Attomolar Analyte Sensing Techniques (AttoSens) in this review. In an AttoSens platform, 1 aM detection corresponds to the quantification of 60 target analyte molecules in 100 μL of sample volume. Herein, we review the approaches listed for various sensor probe design, and their sensing strategies that paved the way for the detection of attomolar (aM: 10-18 M) concentration of analytes. A summary of the technological advances made by the diverse AttoSens trends from the past decade is presented. This journal is

LSPR properties of bioconjugated gold nanoparticles are exploited in realizing evanescent wave absorbance based fiber optic sandwich immunosensor. In addition, a method to amplify the sensor response by about 2-5 times by using high refractive index medium around nanoparticle labels is demonstrated.

In this study, gold nanoparticles and thin film coated U-bent fiber optic probes were compared for SERS based immunobiosensing using direct and sandwich assays with the help of Raman labeled gold nanoparticles conjugated with antibodies.

Tuberculosis (TB) is a resurgent infectious disease affecting a large number of people in the developing countries. An on-site, affordable diagnostic screening at an early-stage for an immediate anti-TB treatment is known to tremendously minimize the high mortality rates. Lipoarabinomannan (LAM), a surface glycolipid, has been identified as a potential TB biomarker present in urine at ultra-low concentrations of a few fg/mL. Here, we report a plasmonic fiber optic absorbance biosensor (P-FAB) strategy for mannosylated LAM (Man-LAM or Mtb LAM) detection down to attomolar concentrations. It involves a plasmonic sandwich immunoassay on a U-bent fiber optic probe with gold plasmonic (AuNP) labels functionalized with anti-Mtb LAM immunoglobulin M (IgM) and anti-Mtb LAM IgG respectively. The Mtb LAM is quantified in terms of absorption of light passing through the fiber probe using a green LED and a photodetector. The choice of fiber optic probes (fused silica versus polymer), the optimum size (20, 40, 60 and 80 nm) and concentration (2 ×, 10 ×, and 20 × ) of AuNP labels were investigated to obtain high sensitivity and lower limits of analyte detection (LoD). P-FAB with a simple LED-photodetector pair, 200 μm fused silica U-bent fiber probe and 60 nm (20 × ) AuNP labels gave LoDs down to 1 fg/mL and 10 fg/mL in the buffer and synthetic urine respectively. Moreover, the anti-Mtb LAM IgM bound sensor probes and the AuNP reagent stored at 4 °C were stable up to 45 days. P-FAB based Mtb LAM sensor demonstrates its potential for an on-site TB diagnosis.

Milk fat content is an important parameter for milk quality assessment in the dairy industry. This study reports the development of a hand-held, highly sensitive fiber optic milk fat sensor using U-bent plastic optical fiber (POF) probes based on the refractive index (RI) of milk, an inherent physicochemical property of milk, which is significantly influenced by the milk fat content. The U-bent POF sensor probes facilitate dip type sensing and offer non-destructive assessment of milk quality in real-time. The sensor works on the principle of intensity modulation technique by exploiting evanescent wave absorbance and refractive losses. The fiber optic probes sense a change in RI of the medium as a result of attenuated total reflection by virtue of the evanescent wave interactions of light at the fiber core/cladding interface with the surrounding medium. A relative change in the refractive index property of milk as a result of variation in fat content and addition of water is investigated using U-bent POF probes for milk quality assessment. The sensor shows an optical absorbance sensitivity of 0.15 ΔA/Δ% fat for the variation of fat content in the milk. Besides, the response of the sensor to water addition in wide concentration range was studied. Furthermore, a low cost portable optoelectronic device is reported for instantaneous milk quality monitoring in farm fields. This sensor could be of huge potential in dairy industries, milk collection centers as well as end-user applications for real-time milk quality monitoring.

A methylene blue (MB) dye-based lipopolysaccharide (LPS) endotoxin quantification was performed, where the LPS was entrapped on a hydrophobic C-18 layer immobilized U-bent fiberoptic sensor (U-FOS) probe.