Functionalized cyclodextrins as potentiometric sensors for onium ions

Abstract

Functionalized, lipophilic α-, β- and γ-cyclodextrins were synthesized and their suitability as onium ion-selective, potentiometric sensors investigated. Selective binding was apparent for NH₄ ⁺ NMe₄ ⁺ and NEt₄ ⁺ ions with peroctylated α-, β- and γ-cyclodextrin, respectively. The phenomenon of complexation was evidenced by ¹H and ¹⁴N nuclear magnetic resonance relaxation-time measurements and by electrospray mass spectrometry. Nernstian responses were also evident for acetylcholine chloride, dopamine hydrochloride and the surfactant myristyltrimethylammonium bromide. A particularly good sensor for NMe₄ ⁺ is peroctylated β-cyclodextrin with o-nitrophenyl octyl ether as plasticizer. A sensitivity of 60 mV decade^–1 was found at 310 K, with a limit of detection, –log a, of 5.8, and –log k^pot _ij for Na⁺, 3.8, for K⁺, 3.2, and for NH₄ ⁺, 3.5. The response of ‘blank’ membrane electrodes comprising poly(vinyl chloride), solvent mediator, and additive was compared with the equivalent electrode response for membranes containing functionalized cyclodextrins. In each instance, the electrode response was substantially enhanced and stabilized by the presence of the lipophilic cyclodextrin. The effect of using either 1.0 mmol dm^–3 NMe₄Cl or 0.01 mol dm^–3 analyte as inner filling solution was compared, and with 1.0 mmol dm^–3 NH₄Cl, was found to enhance electrode performance.