Thèse de doctorat
|Anion recognition is a topical area of research warranted by the potential applications of anion receptors in environmental and biological monitoring. Anions are indeed widespread in nature, are involved in many biochemical processes and are also major aqueous pollutants. Molecular receptors able to recognize anions with high affinity and selectivity in organic solvents are well documented in the literature but only few systems are efficient in aqueous media. Water is undeniably a particularly challenging solvent to work with due to the competition of water in the recognition process. Moreover, most of the receptors that are known to bind anions with high affinity and selectivity in organic solvents are not soluble in water. One strategy used to make hydrophobic molecular receptors “water-compatible” is micellar incorporation. This strategy is straightforward as no synthetic modifications of the receptor are required and has furthermore been seen to enhance the apparent properties, in particular binding properties, of the receptors. Following previous work undertaken in the laboratory, this thesis was devoted to the study of the micellar incorporation of different anion receptors. The first part of this thesis focused on the potential of Nuclear Magnetic Resonance (NMR) and more precisely Paramagnetic Relaxation Enhancement (PRE) experiments to provide robust information on the localisation of receptors within micelles. We studied the effect of various parameters and were able to rationalize the effect of the nature and concentration of the counterion and of the surfactant concentration on the PRE values obtained with cationic cetyltrimetylammonium (CTAX) micelles. By applying a normalization procedure we were then able to compare different receptor/micelle systems. This work has been reported in the Journal of Physical Chemistry (“Paramagnetic Relaxation Enhancement Experiments: a Valuable Tool for the Characterization of Micellar Nanodevices”, F. Keymeulen, P. De Bernardin, A. Dalla Cort, and K. Bartik, J. Phys. Chem. B, 2013, 117, 11654–11659).The second part of our work consisted in the study of the role played by the surfactant on the efficiency of the supramolecular system formed. Using UV-vis and/or NMR titrations, we studied the impact of the nature of the surfactant (cationic, zwitterionic or neutral) as well as its concentration on the apparent binding affinity for fluoride of two uranyl-salophen receptors. We showed that the supramolecular systems formed with cationic micelles are the most efficient, due to the favourable electrostatic interaction between the positively charged micelle and the fluoride despite the competition of the surfactant counter-ion. The concentration of the cationic surfactant does however have an impact as the apparent affinity decreases with surfactant concentration as a consequence of this non-specific interaction of the guest with the micelles. PRE and DLS (Dynamic Light Scattering) experiments allowed us to better understand the differences between the different types of micelles. This work has been reported in Organic & Biomolecular Chemistry (“Fluoride binding in water with the use of micellar nanodevices based on salophen complexes”, F. Keymeulen, P. De Bernardin, I. Giannicchi, L. Galantini, K. Bartik, and A. Dalla Cort, Org. Biomol. Chem., 2015, 13, 2437–2443).The final part of our study was devoted to the investigation of the applicability of micellar incorporation to other receptors. Two other uranyl-based receptors were studied in cationic CTAX and neutral Triton X-100 micelles. One suffered from chemical stability issues and the other receptor did not perform any better than the ones previously studied. We also studied trimesitylborane which can bind fluoride via Lewis acid-base interactions. This system, which is highly efficient in organic solvents, was shown to be ineffective once incorporated into micelles, probably because the change in the hybridization of the boron atom upon fluoride binding is not favourable in the confined micellar environment. Indolocarbazole-based anion receptors, which recognize acetate and benzoate via hydrogen bonding, were successfully incorporated into DPC micelles, albeit at low concentrations, and were observed to be efficient as the apparent binding affinity measured in water is of the same order of magnitude or higher as the one observed in organic solvents.