Susanta Bera

Sol–Gel-Based Titania–Silica Thin Film Overlay for Long Period Fiber Grating-Based Biosensors

An evanescent wave optical fiber biosensor based on titania-silica-coated long period grating (LPG) is presented. The chemical overlay, which increases the refractive index (RI) sensitivity of the sensor, consists of a sol-gel-based titania-silica thin film, deposited along the sensing portion of the fiber by means of the dip-coating technique. Changing both the sol viscosity and the withdrawal speed during the dip-coating made it possible to adjust the thickness of the film overlay, which is a crucial parameter for the sensor performance. After the functionalization of the fiber surface using a methacrylic acid/methacrylate copolymer, an antibody/antigen (IgG/anti-IgG) assay was carried out to assess the performance of sol-gel based titania-silica-coated LPGs as biosensors. The analyte concentration was determined from the wavelength shift at the end of the binding process and from the initial binding rate. This is the first time that a sol-gel based titania-silica-coated LPG is proposed as an effective and feasible label-free biosensor. The specificity of the sensor was validated by performing the same model assay after spiking anti-IgG into human serum. With this structured LPG, detection limits of the order of tens of micrograms per liter (10(-11) M) are attained.

Influence of Al doping on microstructural, optical and photocatalytic properties of sol–gel based nanostructured zinc oxide films on glass

Al doped nanostructured zinc oxide thin films (thickness, 165 ± 5 nm) on silica glass were prepared from zinc acetate based solutions of varying dopant content (0 to 10 at% with respect to Zn). X-ray diffraction confirmed the presence of nanocrystalline hexagonal ZnO. On increasing the doping level, we observed a change in ZnO morphology (spherical, hexagon and plate-shaped) under field emission scanning and transmission electron microscopes and a gradual decrease of ZnO crystallite size (14.0 to ∼10 nm) vis-a-vis an enhancement of direct band gap energy of the films. Root mean square film surface roughness and chemical state of elements were studied by atomic force microscopy and X-ray photoelectron spectroscopy respectively. In addition to common intrinsic defects in ZnO, a defect (designated as D˙ZA, appearing as a paramagnetic singly negatively charged oxygen vacancy) was identified up to 4% doping from the appearance of photoluminescence emission at 398 nm and measurement of paramagnetic property of the films. Film photocatalytic activity towards Rhodamine 6G dye decomposition was performed under UV (254 nm) irradiation and the film with 4% doping (A4ZO) showed the highest value of first order decomposition rate constant. On increasing Al content, the trend of change of defect concentration (oxygen vacancies) analyzed by Raman spectra was found to be identical with the dye photodecomposition activity (PA) of the films. A major role of DZA˙ towards the PA was explored under visible light. We proposed the reaction mechanism of PA based on the experimental results. The A4ZO would be expected to decompose micro-organisms under visible light.

Synthesis, characterization and antibacterial activity of Ag incorporated ZnO–graphene nanocomposites

The present work reports on the successful one-pot surfactant-free in situ synthesis of silver incorporated ZnO–chemically converted graphene (CCG) nanocomposites (AZG) by adopting a low temperature solution technique from zinc acetate dihydrate, silver nitrate and graphene oxide, and the varying silver content (up to 20% Ag with respect to Zn) in the precursors. X-ray diffraction and transmission electron microscopy studies confirmed the presence of Ag and ZnO nanoparticles (NPs), distributed uniformly with CCG. FTIR, Raman, UV-visible and X-ray photoelectron spectroscopic analyses confirmed the existence of interaction between CCG with the inorganic moieties (ZnO/Zn2+ and Ag NPs) of the AZG samples. In vitro cytotoxicity and quantitative cell viability of the human ovarian teratocarcinoma cell line (PA1) was studied up to a maximum sample concentration of 200 μg ml−1. Antibacterial activity was also measured on E. coli and S. aureus to confirm the efficiency of the nanocomposite, especially for killing bacterial cells without any major effect on the surrounding cells. Among the nanocomposites, the 10% Ag incorporated sample at a 6.25 μg ml−1 dose showed excellent antibacterial activity with negligible cytotoxicity. This simple strategy could be applied in the synthesis of Ag incorporated different metal oxide–CCG nanohybrids for antibacterial applications.

Low temperature synthesis of graphene hybridized surface defective hierarchical core–shell structured ZnO hollow microspheres with long-term stable and enhanced photoelectrochemical activity

The present work reports on successful in situ synthesis of chemically converted graphene (CCG) hybridized, surface defective core–shell structured ZnO hollow microspheres (ZG-CSHM) from a surfactant/template free precursor by adopting a low temperature solution method. This special architecture has been synthesized as an intermediate product between solid and hollow microspheres via Ostwald ripening process by optimizing the reaction time, as observed by field emission scanning and transmission electron microscopic studies. The samples have also been characterized by X-ray photoelectron, FTIR and Raman spectral as well as X-ray diffraction analyses. From textural property measurement by BET N2 adsorption–desorption isotherms, it is seen that the ZG-CSHM possesses an enhanced specific surface area with narrow distribution of mesopores. Relatively higher photoelectrochemical activity with long term stability of ZG-CSHM is found compare to pristine core–shell structured ZnO hollow microspheres. The synergic effect of graphene hybridization and the presence of surface defects of ZnO nanoparticles in the mesoporous sample can play the key roles in advancing its photoelectrochemical activity. The surface defects can prolong the recombination rate of photogenerated charge carriers and the high surface area with narrow sized mesopore distribution can provide large number of active sites, make electrolyte diffusion and mass transportation easier. The ZG-CSHM sample also shows an improved photocurrent density compare to solid and hollow microspheres. Moreover, the existence of chemically interacted CCG with ZnO inhibits the photocorrosion, resulted long-term stable photoelectrochemical activity of ZG-CSHM. This facile process can create an avenue for synthesis of core–shell structured microspheres from different metal oxide semiconductors for improving their photoelectrochemical activity.

Sol–gel based simonkolleite nanopetals with SnO2 nanoparticles in graphite-like amorphous carbon as an efficient and reusable photocatalyst

We report a new sol–gel nanocomposite (STC) having simonkolleite nanopetals (SC) and quasi-spherical tin oxide (SO) nanoparticles embedded in graphite-like amorphous carbon (C) as an efficient and reusable photocatalyst for the degradation of rhodamine 6G dye under UV (254 nm) illumination. The STC was synthesized using vacuum curing (450 °C) of precursor gel derived from a sol (Zn : Sn, 2 : 1) in 2-methoxyethanol with acetylacetone. The presence of tetragonal SO well decorated on rhombohedral SC forming nanoheterostructures in the carbon matrix was identified by X-ray diffraction, micro-Raman and X-ray photoelectron spectroscopy and electron microscopes (field emission scanning electron and transmission electron) studies. Carbon content and thermal weight loss behaviour in STC were studied by carbon determinator and thermogravimetry. The nanocomposite showed high photocatalytic activity (10−5 M dye solution degraded completely in 32 min). Reusability test of the photocatalyst exhibited about 95% of dye degradation after five successive recycles. In addition to accelerating photo-induced charge carrier separation and electron transport in the nanoheterostructures as revealed from electrochemical impedance spectroscopy response of the UV-exposed nanocomposite, an active role of the carbon at an optimum content (∼18%) was found for generating high BET specific surface area (∼143 m2 g−1). This simple synthesis strategy could open a new avenue to the development of sol–gel nanocomposites as efficient and reusable photocatalysts from various simonkolleite-based metal oxide semiconductors embedded in graphite-like amorphous carbon.

Hierarchically Structured Macro with Nested Mesoporous Zinc Indium Oxide Conducting Film

Fabrication of homogeneously distributed (HD) macropores by breath figure process is an active research area. Adopting the process, for the first time, we report the fabrication of HD macro with nested meso (hierarchical) porous nanocrystalline zinc indium oxide conducting sol-gel thin film on glass by dip-coating at 45-50% room relative humidity (RH) from a solution in ethanol-2-butanol (1:1, w/w) medium with a 1:1, Zn:In ratio. In this process, solution composition and RH are found to play key roles on HD macropore generation. The film is highly promising toward visible-light-driven photoelectrochemical water splitting.

ZnO–graphene–polyaniline nanoflowers: solution synthesis, formation mechanism and electrochemical activity

Three dimensional hierarchical inorganic–organic hybrid nanoflowers of conducting ZnO–chemically converted graphene–polyaniline nanocomposite have been successfully synthesized at low temperature from surfactant-free precursor solution and studied their formation mechanism. The nanocomposite with hierarchical architecture shows an enhanced electrochemical activity, illustrating a promise for application in electrochemical devices.

High refractive index and temperature sensitivity LPGs for high temperature operation

A fiber optic sensor for high sensitivity refractive index and temperature measurement able to withstand temperature up to 450 °C is reported. Two identical LPG gratings were fabricated, whereas one was coated with a high refractive index (~1.78) sol-gel thin film in order to increase its sensitivity to the external refractive index. The two sensors were characterized and compared in refractive index and temperature. Sensitivities of 1063 nm/RIU (1.338 – 1.348) and 260 pm/°C were achieved for refractive index and temperature, respectively.

Label-free IgG/anti-IgG biosensing based on long period fiber gratings: a comprehensive feasibility study

Long period fiber gratings (LPGs) have recently been proposed as label-free biosensors. A biochemical interaction occurring along the grating region can be evaluated as a refractive index (RI) change, which modifies the transmission spectrum of the fiber. This is an emergent, alternative choice with respect to other label-free optical systems, such as surface plasmon resonance, interferometric and in-fiber configurations, and resonating structures. In this work, various types of not-coated LPGs, in which the coupling occurs with increasing cladding mode orders, were manufactured for increasing the RI sensitivity of these sensors. After the functionalization of the fiber surface using Eudragit L100 copolymer, a label-free IgG/anti-IgG bioassay was realized for analyzing the antigen/antibody interaction following the same model assay. A comprehensive feasibility study was carried out among the different LPGs in order to assess and compare the biosensor performance, highlighting the advantages and the disadvantages of each type. Experimental results proved an improvement in the RI sensitivity and in the biosensor performance in the case of high-order cladding mode LPGs, with values of detection limit lower than 50 ng mL-1 (330 pM). The performance enhancement can be explained with the increase in the penetration depth of the evanescent field due to the increase of the cladding mode order. The sensor response was also studied using complex matrices made up of human serum.

Manufacturing and optimization of sol-gel-based TiO 2 -SiO 2 thin Films as high refractive index overlays for long period grating-based biosensing

The manufacturing procedure and the optimization of high refractive index overlays for long period grating-based sensors are reported. The overlay consists of a sol-gel-based TiO2-SiO2 thin film. By carefully tuning the overlay thickness and refractive index, it is possible to bring the LPG in the so-called transition mode working region, and to optimize and maximize the LPG sensing performances. LPGs are here characterized as optical refractometers, and, after a suitable functionalization of the sol-gel coated fiber surface, as biosensors performing an IgG/anti-IgG bioassay.