Vol.:(0123456789) 1 3 Applied Physics B ( 2019) 125:161 https://doi.org/10.1007/s00340-019-7272-2 REGULAR PAPER Detection of saline‑based refractive index changes via bilayer ZnO/ Ag‑coated glass optical fber sensor Zahra Samavati 1  · Alireza Samavati 1  · A. F. Ismail 1  · Mukhlis A. Rahman 1  · Mohd Hafz Dzarfan Othman 1 Received: 24 October 2018 / Accepted: 25 July 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The combination of sensitive nanostructure thin flms and optical fber ofers the great prospective for understanding the novel sensor concepts. The partially unclad and bilayer zinc oxide (ZnO)/silver (Ag)-coated multimode glass fber as a simple and reliable probe is proposed in this work to detect the ambient refractive index changes using two broadband sources of IR and UV–Vis. The wavelength and intensity of propagating light both are modulated when the saline concentration is varied. The appropriate etching time for partially removing the cladding suitable for both IR and UV–Vis sources is 53 min. The fabri- cated ZnO/Ag/fber sensor exhibits excellent repeatability and high sensitivity to saline with diferent concentrations from 0 to 30% at room temperature. Among the sensors probe, higher sensitivity is observed for ZnO/Ag/fber sample when IR is used as a light source. In this sensor by changing the refractive index of the media from ~ 1 to ~ 1.38, the normalized inten- sity drop to 0.6 of its maximum value and corresponding wavelength shifted from ~ 1559 to ~ 1585 nm. The high sensitivity of fabricated probe is attributed to two phenomena related to bilayer structure of sensing probe: frst, the uncontinuous Ag coating which makes the optical tunneling to the outer layer be possible and, second, altering the optical properties of ZnO by oxygen absorbance through interaction of saline by ZnO nanostructure and changing the refractive index of the deposited layer. The wavelength and intensity are found to be less sensitive for both partially unclad and ZnO/Ag/fber once UV–Vis is used as a light source, which is due to slighter penetration of evanescent wave in the cladding part compared to IR source. 1 Introduction Precise determining of the refractive index (RI) changes by optical fber sensors is highly demanded for extensive range of applications such as chemical and biochemical analysis, vapor detection, controlling the food quality, bacteria rec- ognition and platform for DNA/Aptamer detection [1–6]. On one hand, ZnO is a wide band gap semiconductor hav- ing band gap energy of about 3.4 eV at room temperature with alteration possibility due to oxygen vacancies and elec- tron exchanging. It is regularly used as nontoxic transpar- ent conductive layer [7]. Furthermore, it displays multiple piezoelectric, semiconducting and piezoelectric properties. Because of these distinctive nanostructure features, ZnO is probably the richest family of nanostructures among all materials [8]. On the other hand, Ag nanostructure shows surface plasmons (SPs) efects once irradiated with light. It is a collective oscillation of electrons at the boundaries between materials. Surface plasmons are classifed into (1) localized surface plasmons (LSPR) for nanoparticles and uncontinuous thin flm and (2) propagating surface plasmons (PSPs) in the case of continuous thin flm. The localized surface plasmon resonance (LSPR) is the interaction of light with single metal nanoparticle when the nanoparticle is greatly smaller than the light wavelength. Therefore, con- sidering the electrostatic approximation, the nanoparticle is polarized in the electrostatic feld and dipole is induced inside it. The resonance wavelength depends on the nano- particle size [9]. These optical properties of Ag material together with its high refectance in UV–Vis range can be utilized to develop nanoscale optical, chemical and biologi- cal sensors [10–14]. These materials are employed to chemi- cally deposit as a frst and second layer on partially unclad fber optic to detect the refractive index changes. Optical fber sensors based on evanescent absorption and refection have attracted a wide attention in the last few years * Alireza Samavati alireza.samavati@yahoo.com * A. F. Ismail afauzi@utm.my 1 Advanced Membrane Technology Research Centre (AMTEC), Universiti Technologi Malaysia (UTM), 81310 Johor Bahru, Malaysia