# Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanolay Materials
## 1 Introduction  
Clay minerals have been known since antiquity, when their applications were first explored to obtain products ranging from ceramics to health-related materials [1; 2]. Nowadays, these materials play important role in the research of diversified fields such as solar cell production [3; 4], environmentally friendly heterogeneous catalysts [5; 6], photo-catalysts [7], coatings [8; 9], cosmetics products [10] and biomedical applications (e.g., drug delivery, bioimaging, tissue engineering, regenerative medicine) [1]. This wide variety of applications is due to their convenient physicochemical features such as plasticity,swelling capacity, anisotropy of the layers, ion-exchange properties as well as the associated low cost of production [1; 2; 11]. Clay minerals are composed of two types of polymeric sheets. One type has tetrahedral (T) units as building blocks, and the second is formed from octahedral (O) units. The different types of clay minerals are classified into two groups based on the way these sheets connect to each other, i.e., T:O and T:O:T. The T:O type of clay minerals have rigid structures with the layers sequentially stacked to each other with a very small space between them. These types of clay minerals are thus used, e.g., as scaffolds for the production of ceramic materials [2], immobilization of proteins [12] and generation of nano-inorganic micelles to capture hydrocarbon and aromatic oils in vapor and liquid states [13]. On the other hand, T:O:T type materials have a much more flexible structure because the layers are connected to each other through a large interlayer space filled with hydrated cations in order to compensate for the negative charges originating from the aliovalent substitutions. Such negatively charged layers are regarded as one of the most important characteristics of the T:O:T type clay minerals due to the easy cation exchange and high absorption potential of the interlayer space. In fact, it is in the interlayer space where the most important technological functionalities of the T:O:T type clay minerals originate [2]. Different types of T:O:T materials have been employed in a broad range of applications. For instance, many smectite clays have been used for producing bionanocomposites for scaffolds and regenerative medicine [14]. Mechanochemically activated saponite can be used for the selective removal of Cu\({}^{2+}\) and Ni\({}^{2+}\) from aqueous solutions [15]. Fluorohectorite clay has proven to be an effective material for drug delivery of ciprofloxacin with a controlled release rate [16], and hectorite has been used to improve the abrasion resistance, water vapor barrier and anticorrosion properties of nanocomposite coatings [17]. Among this large group of T:O:T type clay minerals, the smectites represent the largest and most important class [11] because of their distinguished features such as high specific surface area, easily tunable structure, moderate layer charge and propensity for intercalating extraneous species of variable sizes [2]. Moreover, the ability of intercalating additional activator species into the interlayer space of smectites can produce a variety of optically active materials [18]. Although natural smectites can be found in abundance, they usually contain high concentrations of impurities (e.g., quartz, K, Na, Ca, Mg, Fe, and diverse organic impurities, depending on the sedimentation characteristic of the extracting mine) and structural defectAs (e.g., cis- or trans-vacancies, orientation of tetrahedra, localization of isomorphous substitutions, and stacking faults) [11; 19], which may deteriorate the performance of luminescent materials. Therefore, the current work is focused on the study of synthetic hectorite-like types of smectites, aiming for their application as host matrices for optically active species.