Citation: Novikova, A.; Katiyi, A.; Halstuch, A.; Karabchevsky, A. Green-Graphene Protective Overlayer on Optical Microfibers: Prolongs the Device Lifetime. Nanomaterials 2022, 12, 2915. https://doi.org/10.3390/nano 12172915 Academic Editors: Wugang Liao, Lin Wang and Filippo Giannazzo Received: 17 July 2022 Accepted: 22 August 2022 Published: 24 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Green-Graphene Protective Overlayer on Optical Microfibers: Prolongs the Device Lifetime Anastasia Novikova, Aviad Katiyi, Aviran Halstuch and Alina Karabchevsky * School of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel; novikoan@post.bgu.ac.il (A.N.); akatiyi@gmail.com (A.K.); ahalstuch@gmail.com (A.H.) * Correspondence: alinak@bgu.ac.il Abstract: Optical microfibers find new applications in various fields of industry, which in turn require wear resistance, environmental friendliness and ease of use. However, optical microfibers are fragile. Here we report a new method to prolong the microfiber lifetime by modifying its surface with green-extracted graphene overlayers. Graphene films were obtained by dispergation of shungite mineral samples in an aqueous medium. For this, we tapered optical fibers and sculptured them with graphene films mixed with gold nanoparticles. We observed that due to the surface modification the lifetime and survivability of the microfiber increased 5 times, as compared to the bare microfiber. The embedded gold nanoparticles can also be utilized for enhanced sensitivity and other applications. Keywords: green graphene; microfiber; gold nanoparticles 1. Introduction Optical fibers are dated back to 1880 when William Wheeler transmitted light through a glass pipe and referred to it as ’light piping’. It took until 1966 when the optical fibers with a higher guiding medium as compared to the cladding were proposed for light transmission [1]. This development was intended mainly for telecommunication due to low losses and ease in fabrication [2]. Furthermore, optical fibers can be tapered to microfiber dimensions for experiencing the novel properties [3]. Microfiber [4] can be used for a verity of applications such as sensing [5–8], determination of substances [9], human health monitoring [10] and many others. One of the applications in which microfibers are important is sensing. Due to the squeezing of the fiber diameter, the confinement of the mode decreases and the evanescent field penetration depth to the analyte increases which in turn improve the sensitvity [11,12]. This can be utilized for sensing with tapered fibers [11,13,14]. These days, optical microfibers are widely used for the determination of various substances with low concentrations. Namely, in biomedicine [15] and biology [16] (low concentrations of viruses [17,18], bacteria [19,20], proteins [21], nucleic acids [22], cancer cells [23], substances in body [24]), environmental protection [25] (pollutants in water and soil, components of biological pollution) [26–30], pharmacology and the chemical indus- try (pharmaceutical substances, new materials) [31,32], construction industry (concrete deformation/stress measurements [33], the use of fiber-optic sensors for detecting railway vehicles and monitoring the dynamic characteristics of the rock mass caused by railway rolling stock for the needs of civil construction [34,35], damage detection and characteriza- tion using long-gauge and distributed fiber optic sensors [36], a new tool for temperature measurements in boreholes [37,38]); in addition, optical sensors are used in the creation of smart fabrics [39], stratigraphy [40,41] (probe for rapid snow grain size, determination of layered, sedimentary and volcanogenic rocks). With such wide uses of microfibers, there are also different methods for modifying them, depending on the required parameters. One of the simplest methods is the method of Nanomaterials 2022, 12, 2915. https://doi.org/10.3390/nano12172915 https://www.mdpi.com/journal/nanomaterials