# Doped Epoxy Resins as an Alternative to Luminescent Optical Sensors
## 1 Introduction  
Polymer materials (i.e., blends, composites) have found widespread use thanks to their favorable properties and wide modification possibilities. They owe their popularity to valuable properties such as low density, high mechanical strength, durability, resistance to atmospheric factors, thermostability and good processability [1, 2, 3, 4, 5, 6, 7, 8]. Conventionally, resins are cured using thermal, redox or, as described below, UV curing. Typically, in order to avoid a premature start of the polymerization process, the initiator is introduced just before the reaction begins. A more desirable curing process, however, would be a \"curve on demand\" system, in which the resin would not be cured until needed. Such a technique is photopolymerization, the reaction of which will not start until light in the range of wavelengths activating the added photoinitiator is supplied to the system. Photocuring technology has found many applications in various industrial sectors, where it outclasses other processing techniques, thanks to its advantages, namely, ultra-fast polymerization (for comparison, thermal polymerization can last from several minutes to several hours [9, 10, 11], and in the case of photopolymerization we are talking about seconds) of solvent-free preparations, which occurs selectively only in illuminated places [1, 12, 13, 14, 15, 16, 17, 18]. It is worth emphasizing the usefulness of this method in microelectronics, where UV-curable resins are used not only as photosensors in the imaging stage, but also as quick-drying adhesives, sealants and protective coatings [19, 20, 21]. Also, as protective coatings, derivatives of epoxy resins are used in optical fiber technology, and thanks to the ease of their modification, they can be used as well as a matrix for materials with dedicated properties used to produce special optical fibers. These fibers are used in sensors for detecting physicalparameters and biochemical compounds, but due to the increasing demand for more selective and sensitive sensors, this branch of the industry is still very interesting for researchers and is constantly being developed [6; 7; 8; 22; 23; 24; 25; 26]. The use of polymeric materials doped with luminophores poses a number of requirements regarding their composition and morphology. What has a major impact on the properties of the final product is the method of immobilizing the dopant in the polymer matrix. The dopant can connect to the matrix by a hydrogen bond, and then we are dealing with a physical connection, but when the dopant is connected by a covalent bond, we are dealing with a chemical connection. There are also materials exhibiting luminescent properties obtained by adsorption [27; 28; 29; 30; 31]. The luminescent composites obtained in this way are used to obtain thin films for sensing applications. These materials are characterized by quite a simple preparation methodology and, more importantly, their application on optical fibers or waveguides is equally simple. The method of obtaining the photosensitive element is that which distinguishes the work presented so far, from the research carried out at the Laboratory of Optical Fiber Technology, Maria Curie-Sklodowska University. Our goal is not to apply a photosensitive material to a fragment of a silica fiber, from which the protective coating has been removed, or to place this material on the tip of the fiber [32; 33; 34; 35; 36; 37] but to obtain a fiber for which outer jacket will be photosensitive throughout its entire volume. Thanks to this, we will obtain the best effect of the light interaction from the luminophore with the guiding part (core). According to our technology, in the case of polymer fibers, the best material compatibility is achieved by co-producing both the core and the cladding of the optical fiber. This is best illustrated by the example we describe in the article [38], where it was proven that during the preform fabrication styrene diffuses into poly(methyl methacrylate), creating a transition layer.