Citation: Carrillo-Betancourt, R.A.; López-Camero, A.D.; Hernández-Cordero, J. Luminescent Polymer Composites for Optical Fiber Sensors. Polymers 2023, 15, 505. https://doi.org/10.3390/ polym15030505 Academic Editor: Carmen Rial Tubio Received: 29 November 2022 Revised: 6 January 2023 Accepted: 9 January 2023 Published: 18 January 2023 Copyright: © 2023 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/). polymers Article Luminescent Polymer Composites for Optical Fiber Sensors Rodolfo A. Carrillo-Betancourt, A. DaríoLópez-Camero and Juan Hernández-Cordero * Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, A.P. 70-360, Ciudad de México 04510, Mexico * Correspondence: jhcordero@iim.unam.mx Abstract: Optical fiber sensors incorporating luminescent materials are useful for detecting physical parameters and biochemical species. Fluorescent materials integrated on the tips of optical fibers, for example, provide a means to perform fluorescence thermometry while monitoring the intensity or the spectral variations of the fluorescence signal. Similarly, certain molecules can be tracked by monitoring their characteristic emission in the UV wavelength range. A key element for these sensing approaches is the luminescent composite, which may be obtained upon allocating luminescent nanomaterials in glass or polymer hosts. In this work, we explore the fluorescence features of two composites incorporating lanthanide-doped fluorescent powders using polydimethylsiloxane (PDMS) as a host. The composites are obtained by a simple mixing procedure and can be subsequently deposited onto the end faces of optical fibers via dip coating or molding. Whereas one of the composites has shown to be useful for the fabrication of fiber optic temperature sensors, the other shows promising result for detection of UV radiation. The performance of both composites is first evaluated for the fabrication of membranes by examining features such as fluorescent stability. We further explore the influence of parameters such as particle concentration and density on the fluorescence features of the polymer blends. Finally, we demonstrate the incorporation of these PDMS fluorescent composites onto optical fibers and evaluate their sensing capabilities. Keywords: PDMS fluorescent composites; optical fiber sensors; up-conversion; down-conversion 1. Introduction Detection and quantification of biological and chemical species are required in several areas, such as environmental science, clinical diagnostics, biology and chemistry. Among the available options for performing these tasks, optical methods offer several advantages over other sensing approaches, the most important of which are their high sensitivity and selectivity, both of which are highly desired features in biochemical sensing systems [1,2]. Although several techniques for optical sensing have been demonstrated, fluorescence- based measurements are recognized as among the most powerful tools for biochemical sensing [1–3]. Aside from providing remote monitoring capabilities, fluorescence signals can be analyzed in different ways, allowing for the realization of a wide variety of sensing schemes. Spectral features, lifetime and intensity are perhaps the most common examples of the different aspects that can be analyzed from a fluorescent signal. The advent of compact and rugged optical instruments such as solid-state spectrometers, as well as the availability of optical fibers, light sources and detectors, has enabled the development of compact and portable fluorescent sensing platforms, exploiting the versatility offered by fluorescence sensing. Although some analytes provide suitable fluorescence emission for sensing, external fluorescent indicators (i.e., exogenous fluorophores) may be added to the samples if the target molecule lacks endogenous fluorophores [3]. Nanoparticles of different kinds, gold surfaces, carbon and quantum dots have been combined with chemical receptors, yield- ing fluorescent materials and enabling fluorescence detection [4]. Fluorescent polymers obtained either by incorporation of fluorophores in the polymer chain or by aggregation Polymers 2023, 15, 505. https://doi.org/10.3390/polym15030505 https://www.mdpi.com/journal/polymers