Hybrid materials obtained from kaolinite and phenylalanine complexed to Eu3+ ion as luminescent sensor of caffeine

With the advancement of economic and social development in recent decades, thousands of chemical compounds are dumped into natural effluents, posing a risk to both the environment and human health. The lack of knowledge about the behavior of these contaminants in the environment, as well as the emergence of new substances by mixing these pollutants, is an aggravating factor for this problem. Aiming to overcome this problem, a hybrid material to be used as a luminescent sensor for the contaminant caffeine was prepared from kaolinite functionalized with phenylalanine and used for complexing Eu3+ ions by (i) a conventional route already widely used in the literature, and (ii) a new route catalyzed by boric acid, for application. The lanthanide ion has unique optical properties such as thin emission bands with high lifetimes, bringing sensitivity and selectivity, essential properties for this application. The amino acid has a high molar absorptivity and offers greater structural rigidity to the complex, maximizing energy transfer to Eu3+, and increasing its radiative emission yield, while kaolinite offers thermal resistance and increases the dispersion of the complex, decreasing luminescence suppression. The secondary ligand tenoyltrifluoroacetonate was added to the complex to improve energy transfer to the Eu3+ ion and to remove water molecules in the first coordination sphere. The material was characterized by powder X-ray diffraction, infrared molecular spectroscopy, thermal analysis, scanning electron microscopy and luminescence spectroscopy. The presence of symmetric and antisymmetric stretch vibration bands of the COO- group of amino acids (1300 – 1600 cm-1) and modification of the kaolinite OH stretch bands was verified, showing the interaction and formation of the hybrid materials, also concluded from the increase in the kaolinite basal space from 0.7 nm to 7.9 nm evidenced by X-ray diffraction. The samples presented 0.2 mol of organic matter per kaolinite unit cell and pseudo-tubular morphology. The sample obtained by the boric acid-catalyzed route showed better optical properties with 1 water molecule complexed and 42 % internal radiative emission yield. The adsorption kinetic study of the sensor with the contaminant showed selectivity at concentrations below 25 mg/g and pseudo second order kinetics, attributed to materials with multiple adsorption sites with strong interactions between the contaminant and the hybrid material.