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In the present investigation, we demonstrated a simple, rapid, and highly sensitive cladding-modified optical fiber urea sensor based on the evanescent wave absorption (EWA) technique. Cladding modification was performed over a 2-cm unclad portion of optical fiber using a polyaniline-zinc oxide (PANI-ZnO) matrix with enzyme-Urease (Urs) cross-linked to it using glutarldehyde as a cross-linking agent. The PANI-ZnO matrix was characterized by X-ray diffraction, scanning electron microscopy, ultraviolet-visible, and Fourier transform-infrared spectroscopy to explore its various properties. The developed sensor shows a linear response to urea concentration in the range 10 nM−1 M in the form of the absorption spectrum at a wavelength of ∼250 nm with specific selectivity. Under the proper conditions (storage at temperature 4 • C after each measurement), it shows 40-day stability without any decrement in the intensity of the absorption spectrum. Thus, the developed sensor is highly sensitive, stable, and specific, with a lower detection limit of a urea concentration of 10 nM.

Present study is specially designed for the recent advances in biosensors to detect and quantify urea concentration. Urea (carbamide) is an organic compound made up of the carbonyl (C=O) functional group with two −NH2 groups having chemical formula CO (NH2)2. In nature, urea is found everywhere as the result of various processes, and in the human body, urea is an end product of nitrogen metabolism. An excessive concentration of urea in the human body is responsible for different critical diseases such as indigestion, acidity, ulcers, cancer, malfunctioning of kidneys, renal failure, urinary tract obstruction, dehydration, shock, burns, gastrointestinal bleeding, and so on. Moreover, below the normal level may cause hepatic failure, nephritic syndrome, cachexia, and so on. As well as in various fields such as fishery, dairy, food preservation, agriculture, and so on, urea is normally found and its detection is necessary. In urea biosensors, enzyme urease (Urs) is used as a bioreceptor element and retains its long last activity is the critical issue in front of the researcher. During recent decades, different nanoparticles (zinc oxide, nickel oxide, iron oxide, titanium dioxide, tin(IV) oxide, etc.), conducting polymer (polyaniline, polypyrrole, etc.), conducting polymer–nanoparticles composites, carbon materials (carbon nanotubes, graphene oxide, reduced graphene oxide graphene), and so on are used in urea biosensors. The main emphasis of the present study is to provide cumulative and comprehensive information about the sensing parameters of urea biosensors based on the materials used for enzyme immobilization. Besides this special task, this review provides a fruitful discussion on the basics of biosensors briefly for new and upcoming researchers. Thus, the present study may act as a gift for a large audience that come from different fields and are working in biosensors research.

The present investigation deals with the NO2 gas detection via nanostructured zinc oxide (ZnO) thin films harvested over a glass substrate via a simple and facile successive ion layer adsorption and reaction (SILAR) approach. ZnO nanostructured thin films are developed by different SILAR cycles (ZnO:15, ZnO:30, ZnO:45, ZnO:60). The physicochemical, optical, and morphological properties of prepared thin films are explored by various characterization techniques. The phase purity and grains size are confirmed by X‐ray diffraction (XRD) and Scherrer formula, which reveal the hexagonal phase structure with variant grain size (≈16–21 nm). The surface morphology of deposited films shows the well‐dispersion of ZnO nanoparticles, which is picturized by field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), attenuated total reflectance (ATR). UV–Vis and FTIR spectroscopy elaborates on the optical properties and functional groups of ZnO films, respectively. Further, the sensing ability of ZnO thin films is studied with different concentrations (50–250 ppm) of NO2 gas in the span of 50 ppm by recording the resistance transient. The ZnO:30 thin‐film displays an enhanced gas sensing response of 1.42 GΩ at 100 °C compared to other synthesized films and at different temperature ranges. The ZnO:30 film shows the highly selective response toward the NO2 gas compared to other interference gas species. Thus, the present study highly recommends the investigated method for the ZnO thin film formation to be employed toward NO2 gas sensing.