Synthesis and sensor appplication of reusable fluorescent uranyl-imprinted polymer gels

The sol-gel process, also called as chemical solution deposition, is a technique at room temperature commonly used in ceramic engineering and materials science such as electronics, nanotechnology, optics, medicine etc. In generally, sol-gel describes the synthesis and processing of ceramic and glasses. In addition the sol-gel process, also referred to as the "chimie douce", is based on the preparation of a solid solution of solids at lower temperatures than conventional preparation methods, using either a left or gel medium step. This is a simple technology in theory but in practice, it requires a significant amount of effort. Many scientists have experienced about this phenomenon for many years. Thanks to this process (stability, optical transparency, chemical) porosity and chemical resistance, etc.), homogeneous inorganic oxide materials can be obtained at room temperature without the need for the high melting temperature required for conversion to inorganic glasses. The ability of the sol- gel process to be achieved under extreme moderate conditions (often at room temperature) and the availability of products in various shapes, sizes and formats has allowed this technology to become increasingly used in various scientific and engineering fields. In general, sol-gel chemistry, hydrolysis and condensation reactions, oxide are based on the transformation of molecular network precursors in the network. Alkoxide groups in the alcohol-water solution are gradually removed by hydrolysis in the presence of basic or acidic catalyst and hydroxyl- groups. Gelling occurs when the growing polymer networks come together to form a network covering the entire solution volume. At this point of gelation, both the viscosity and the elastic modulus rise steadily. The gel can be dried by evaporation to form xerogels, or by supercritical fluid extraction to obtain aerogels. The hydrolysis and condensation reactions depends on many different parameters such as pH, temperature, solvent, the kind of catalyst, nature and concentration of the initiator, the type of solvent and the acidity of the medium, type and concentration of additives, aging time of the mixture, component concentration in the initial mixture and ratio of the alkoxy with respect to water. One of the most important aspects of sol-gel science is molecular imprint polymers. Molecular Imprint Technology (MIT) is a synthetic approach to designing powerful molecular recognition materials that can mimic natural recognition agents such as antibodies and biological receptors. Although the origin of molecular imprint polymers is based on the early 1930s, the interest of researchers has only attracted attention in the 1970s, and since 1970, the interest in this field has been increasing and has become an important and active research area in recent years. These polymers are seen as versatile and future bright polymers capable of recognizing both biological and chemical molecules, including amino acids, proteins, nucleotide derivatives, drugs and foods. Because of the unique properties of MIPs, they are used in different applications such as separation, purification, sensor and biosensor construction and drug delivery. As can be seen from many recent articles published in recent years, chemical sensors and biosensors are receiving increasing attention in the field of modern analytical chemistry. This increase in the number of studies is mainly due to the growing demand in basic fields such as medicine, food and the environment. The identification and removal of toxic and radioactive heavy metals is very important because of their biological, environmental and industrial effects. The development of selective and differential chemosensors for the determination of toxic heavy metal ions is of interest and this interest is increasing. Uranium is a radioactive and toxic element that occurs in the environment as a result of many processes such as nuclear power generation, construction of nuclear weapons, processing and destruction of nuclear waste, nuclear accidents and burning of fossil fuels. Nuclear fuel production causes pollution in water and soil. Under environmental conditions, the uranyl ion in water forms in hexagonal form, moving on the earth around uranium, nuclear waste zones and processing facilities. The human body may be exposed to airborne dust, or to uranium-contaminated food and water. Although soluble uranium compounds can be rapidly extirpated during the digestion process, they are confused with the bloodstream, especially by the ingestion of powders into the lungs, creating a serious hazard for human health. Uranium detection is crucial for applications related to nuclear work such as monitoring environmental cleanliness, producing and reprocessing fuel. One uranium species of interest, uranyl ion, has accumulated over many years in the bone, liver, kidney and reproductive tissues. For this reason, the monitoring of the small amount of uranium in the forest is an important issue. Numerous methods are used to determine uranium. However, these methods require limited capacity and high operating costs. In this study, four different kinds of polymeric materials were synthesized, two of which were obtained as fluorescent sol-gel particles and two of which were obtained as fluorescent stimuli-responsive hydrogels. In the first part of study, the fluorescent functional precursor (QP) was synthesized by the reaction between 3-aminoquinoline and 3-isocyanatopropyltrimethoxysilane, and then the sol-gel material was synthesized by using QP as fluorescent probe, TMOS as cross-linker and TEA as catalyst in the presence of uranyl ion. The obtained sol-gel particles in powder form was designated as U-imp1 and the sol-gel particles which was synthesized in the absence of uranyl ion was designated as N-imp1. The characterizations of both sol-gel particles were made by FT-IR, BET analysis, optic microscopy, SEM and TGA. The optimum solution conditions such as composition of solvent mixture and pH for the determination of uranyl ions by using both N-imp1 and U-imp1 hydrogels were investigated and compared by fluorescence emission exhibited by the particles in solution. In the second part of the study, fluorescent non-imprinted (N-imp2) and uranyl ion-imprinted (U-imp2) hydrogels were synthesized and characterized. U-imp2 hydrogel was prepared in dioxane by free radical polymerization of N-isopropyl acrylamide as a main component, methacrylic acid as a functional monomer, 9-vinylcarbazole as a fluorescent monomer and N,N-methylenebis acrylamide as a cross-linker in the presence of uranyl ion. Imprinted hydrogel, which was prepared in the presence of uranyl ion, contains negatively charged monomer units to interact with positively charged uranyl ion. Upon interaction with uranyl ion, fluorescent gel exhibits a change in fluorescence intensity. As uranyl ion concentration increases in solution, the quenching of fluorescence of imprinted polymer increases. When compared with N-imp2 hydrogel that was prepared in the absence of uranyl ion, the amplified fluorescence quenching was observed in the case of imprinted hydrogel, showing that certain cavity was created for selective binding of uranyl ion. Swelling behavior of polyelectrolyte hydrogel was also investigated in solvent mixture and collapsing behavior of the hydrogels was evaluated depending on temperature. Furthermore, swelling kinetic of stimuli-responsive hydrogel was investigated in ethanol-water mixtures with different composition. The sensor behavior of fluorescent hydrogels for the determination of uranyl ion was evaluated both in swollen and shrunken state by monitoring of the fluorescence intensity of the carbazole moiety in hydrogels.