Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles†
We present here a new family of pH insensitive ion-selective optical sensors based on emulsified nanospheres containing densely functionalized 15-, 16-, 18- and 20-membered pyreneamide derivatives. These compounds were successfully synthesized by the reaction of α-diazo-β-ketoesters with cyclic ethers of the desired size in the presence of dirhodium complexes followed by a stereo-selective tandem amidation-transposition process and characterized by 1H-NMR, 13C-NMR, IR, HR-ESI-MS, UV-VIS and fluorescence spectroscopy and potentiometry. Their unique structure consisting of a crown ether ring linked to pyrene moieties through amide groups exhibits on–off switchable behavior upon binding of specific cations and allows one to incorporate these chemosensors as fluorescent ionophores into ion-exchange nanospheres. The nanosphere matrix is composed of bis(2-ethylhexyl)sebacate (DOS), poly(ethylene glycol) (PEG), sodium tetrakis 3,5-bis(trifluoromethyl)phenyl borate and pyreneamide functionalized 18-crown-6 ether (18C6). These optode nanoparticles exhibit a strong affinity to the potassium cation over other metal ions up to the millimolar concentration range in an exhaustive detection mode. The logarithmic complex formation constant was determined using the segmented sandwich membrane method and was found to be 6.5 ± 0.3 (SD) in PVC membrane plasticized with NPOE and 5.3 ± 0.3 (SD) in DOS with a 1 : 1 complex stoichiometry. The nanosensors were characterized in broad range of pH from 4 to 10 and the same linear calibration curves were obtained in the concentration range from 10−7 M to 10−5 M and thus the pH dependent response was largely overcome. These nanosensors are sufficiently stable, simple to prepare, exhibit a rapid response and their nanoscale size makes them suitable for sensing purposes in samples of limited dimensions.