Abstract
Solid-contact potentiometric ion-selective electrodes (SC-ISEs) for thallium determination have been designed using multiwall carbon nanotubes (MWCNTs) as the ion-to-electron transducer. Dispersed MWCNTs were drop-casted over a gold plate electrode. Two different crown ethers were used in the sensing membrane for the recognition of thallium (I). Sensorsbased on dibenzo-18-crown-6 (DB18C6) as a neutral carrier and NaTPB as an anionic additive exhibited a near Nernstian response of 57.3 mV/decade towards Tl+ ions over the activity range 4.5 × 10−6–7.0 × 10−4 M, with a limit of detection of 3.2 × 10−7 M. The time required to achieve 95% of the steadyequilibrium potential was <10 s. The complex formation constant (log βML) between dibenzo-18-crown-6 and thallium (I) (i.e., 5.99) was measured using the sandwich membrane technique. The potential response was pH independent over the range 3.0–9.5. The introduction of MWCNTs as an electron-ion-transducer layer between gold plate and the sensing membrane lead to a smaller membrane resistance and a large double layer capacitance, which was proven using impedance spectra and chronopotentiometry (i.e., 114.9 ± 12 kΩ, 52.1 ± 3.3 pF, 200 ± 13.2 kΩ, and 50 ± 4.2 µF). Additionally, reduction ofthe water layer between the sensing membrane and the underlying conductor wastested. Thus, it is clear that MWCNTs can be used as a transducing layer in SC-ISEs. The proposed sensor was introduced as an indicator electrode for potentiometric titration of single and ternary mixtures of I−, Br−, and S2− anions.
Highlights
Natural processes and human activity increase the concentration of metals in the environment
Multiwall carbon nanotubes (MWCNTs) were used as an intermediate layer between the sensing membrane and Au, which acts as an electron conductor substrate
The potential responses revealed by the proposed sensors can be attributed to the selective interaction of Tl+ ions with DB18C6 and are affected by: (i) The ionic diameter of the crown ionophore; (ii) the ionic size of the thallium (I) ion; and (iii) the spatial configuration of this ionophore
Summary
Natural processes and human activity increase the concentration of metals in the environment. This poses a threat to human and other living organisms. One of the most significantpollutants that isintroduced and affects the environment is thallium (Tl), which is produced as waste from lead and coal combustion [1]. Thallium is the most dangerous pollutant, with a higher toxicity to the biosphere than Cd, Cu, Hg, and Pb [2]. It is used in the manufacture of low-temperature thermometers, optical lenses, semiconductors, jewelry, and as a catalyst in certain alloys [3]. Athalliumconcentration of 0.5 mg/100g of tissue can be regarded as thallium poisoning [5]
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