The environmental pollution by phenol-based aromatic nitro compounds in water samples is a major concern worldwide . These nitrophenols are mostly widespread within surrounding environment from industrial wastes, as they are extremely used in the production of pharmaceuticals, pesticides, insecticides, explosives and dyes . According to USA Environmental Protection Agency (EPA), 4-nitrophenol (4-NP) is the most toxic, hazardous and persistent organic pollutant, which can cause significant damages to the health and environment, even at low-level concentrations . Hence, there is need for highly-stable, efficient, robust and reliable sensors that can detect traces of 4-NP, in a rapid and ultrasensitive manner . Until now, several techniques such as capillary electrophoresis, fluorescence, high-performance liquid chromatography (HPLC), mass spectrometry combined with liquid chromatography (LC-MS) or with gas chromatography (GC-MS), and surface enhanced Raman spectroscopy (SERS) have been widely employed for hazardous chemical pollutants sensing, including 4-NP [5, 6]. However, these classical analytical techniques have limitations of being time-consuming, and they typically require sophisticated and expensive instrumentation, trained personnel, as well as multistep sample preparation protocols, being thus quite expensive techniques to be commonly used in daily life and industry . In addition, electrochemical approaches such as cyclic voltammetry, linear sweep voltammetry, differential pulse voltammetry and chronoamperometry have similarly shown their potential for the detection of 4-NP [4, 8]. Despite the fact that electrochemical methods are generally cost-effective, highly-sensitive and selective, their performance strongly depends on the electrode modifiers, and more important, they are not so stable at temperature and pH fluctuations .
Different from the above-mentioned detection techniques, fiber optic – surface plasmon resonance (FO-SPR) sensing is a relatively novel biochemical method with the advantages of featuring a compact footprint, label-free detection and real-time monitoring capabilities, as well as offering the possibility to perform rapid and non-invasive measurements [10, 11, 12]. Such a reflection-type FO-SPR sensor is commonly prepared by uncladding first a small portion at one end of the FO, and then coating the exposed FO core by a plasmonic metal layer, typically gold (Au) or silver (Ag) [13, 14]. The state of surface plasmons excited with light (guided by total internal reflection through the FO) at the metal/dielectric surface interface changes when the coated-FO core is immersed within the environment solution containing the target analyte. Thus, a SPR dip at a particular wavelength is then obtained in the reflection spectrum, which strongly depends on the refractive index (RI) of the sensing medium around the metallic layer [10, 13]. Owing to that, FO-SPR sensors have been widely used in medical diagnostics and environmental monitoring applications, for studying molecular interactions and their binding specificity [15, 16, 17]. For example, Singh et al. reported the development of a transmission-type FO-SPR biosensor for the detection of phenolic compounds (catechol, m-cresol, phenol and 4-chlorophenol) in aqueous solutions . The sensing probe was prepared by depositing Ag film onto FO core via a thermal evaporation method followed by the immobilization of enzyme tyrosinase, using a gel entrapment technique. In this case, the authors claimed a limit of detection (LOD) for all analyzed phenolic compounds in the low µM concentrations range. Alternatively, Cennamo et al. presented a detection scheme of another nitrophenol compound (TNT – 2,4,6-trinitrotoluene) based on the combined approach of FO-SPR and molecular imprinting technique . The SPR device was obtained by coating a 60 nm thick Au film over the FO core using a sputtering method. The sensing method demonstrated a detection limit of 51 µM with a sensitivity of 27 µm/M. However, one year later the authors have shown further improvements of the TNT sensor by designing a localized SPR (LSPR) device, through the incorporation of branched Au nanostars dispersed into a molecular imprinted polymer initially coated on the FO core. In this way, the authors obtained better LOD and sensitivity values, of 2.4 µM and 84 µm/M, respectively . Noteworthy, to the best of our knowledge, yet there is no evidence in literature of employing a reflection-type FO-SPR sensor for 4-NP detection.
In this work, results on the fabrication and characterization of an innovative FO-SPR sensor, based on a polyaniline (PANI) / platinum (Pt) bilayer coated over an unclad FO core, and used for 4-NP detection, were reported for the first time. The Pt thin film was deposited by DC magnetron sputtering and it replaced the conventional Au layer commonly preferred with a traditional reflection-type FO-SPR sensor . So far, only a limited number of theoretical studies were reported with Pt-coated FO-SPR sensors operating in transmission mode [17, 18]. Herein, using Pt as a plasmonic material within reflection-type FO-SPR sensors is a novel approach and the excellent catalytic properties of Pt are essential for subsequent PANI synthesis steps [19, 20]. Complementarily, PANI is an organic polymer with excellent stability and physico-chemical properties in terms of high electrical conductivity, large electro-active surface, and unique combination of RedOx states and proton doping profiles [4, 21]. These particular features render PANI as an extremely responsive polymer to several molecular species, being so far successfully used in energy storage applications , pH monitoring , gas sensing  and pollutants detection including nitrophenol compounds . In this work, PANI was synthetized using a cost-effective electroless polymerization approach, in an attempt to uniformly deposit thin sensitive PANI films on the curved Pt-coated FO core three-dimensional (3D) geometry. The PANI/Pt-based FO-SPR sensor was then morphologically characterized and evaluated for highly-sensitive 4-NP pesticide detection in water samples, demonstrating a sensor’s LOD in the low pM concentrations range. This work represents thus a step forward in the fabrication of reliable FO-SPR sensors, not only with improved performance, but also with extended functionality.