Abstract
Carboxylate sensing solid-contact ion-selective electrodes (ISEs) were created to provide a proof-of-concept ISE development process covering all aspects from in silico ionophore design to functional sensor characterization. The biscarbazolylurea moiety was used to synthesize methylene-bridged macrocycles of different ring size aiming to fine tune selectivity towards different carboxylates. Cyclization was achieved with two separate strategies, using either amide synthesis to access up to –[CH2]10– macrocycles or acyl halides to access up to –[CH2]14– macrocycles. Seventy-five receptor–anion complexes were modelled and studied with COSMO-RS, in addition to all free host molecules. In order to predict initial selectivity towards carboxylates, 1H NMR relative titrations were used to quantify binding in DMSO-d6/H2O solvent systems of two proportions – 99.5%:0.5% m/m and 90.0%:10.0% m/m, suggesting initial selectivity towards acetate. Three ionophores were selected for successful sensor prototype development and characterization. The constructed ion-selective electrodes showed higher selectivity towards benzoate than acetate, i.e., the selectivity patterns of the final sensors deviated from that predicted by the classic titration experiments. While the binding constants obtained by NMR titration in DMSO-d6/H2O solvent systems provided important guidance for sensor development, the results obtained in this work emphasize the importance of evaluating the binding behavior of receptors in real sensor membranes.
Highlights
We aimed to investigate the effects of ring size on the binding of carboxylates and the biscarbazolylurea moiety was selected
MC002 was of sufficient size to accommodate smaller carboxylate ions
The sensors with the ionophore MC012 had the best performance in this study and those sensors compared quite favourably to some of the previously published ion-selective electrodes (ISEs) for acetate by exhibiting a wider linear range, slope closer to theory, and a lower detection limit [36]
Summary
These types of sensors are easier to prepare but their life span is shorter due to the possibility of the receptor molecule leaking from the polymeric membrane into the analyte solution. This was because for the macrocycles MC003–MC005, the computational geometries suggested that the ring is too small to bind any of the studied anions. According to the computation results, neither benzoate nor pivalate were able to fit into the ring of these receptors because these anions were too large.
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