Gold Plate Electrodes Functionalized by Multiwall Carbon Nanotube Film for Potentiometric Thallium(I) Detection

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 β_ML) 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 S^2- anions.

Keywords: ion-to-electron transducer; multiwall carbon nanotubes (MWCNTs); solidcontact ISEs; thallium.

Figure 1

Sensor construction using multiwalled carbon nanotubes (MWCNTs)

Figure 2

Time trace of the proposed sensors. Insert: Calibration curves of these ion sensors.

Figure 3

Effect of pH on the response of the sensor.

Figure 4

Selectivity coefficient (${\mathrm{Log}\mathrm{k}}_{\mathrm{Tl},\mathrm{J}}^{\mathrm{pot}}$) values of the proposed sensor using modified separate solution method (MSSM).

Figure 5

Water layer test for Tl membrane based sensor in the absence and presence of MWCNTs.

Figure 6

Chronopotentiograms for the solid-contact(SC/Tl⁺-ISE) (top) and SC/MWCNTs/Tl⁺-ISE (bottom) electrodes under the constant currents of ±1 nA in 1.5 × 10⁻⁵ M of Tl⁺ solution.

Figure 7

Electrochemical impedance spectroscopy (EIS)spectra of the SC/MWCNTs/Tl⁺-ISE with (circles) and SC/Tl⁺-ISE (squares) without the solid-contact layer of MWCNTs measured in 1.5 × 10⁻⁵ Tl (I) solution.

Figure 8

Potentiometric titration curves of NaI (a), Na₂S (b), NaBr (c), and a mixture of them (d) in solution (50 mL) using 0.01 M TINO₃ titrant (i.e, subfigure represents the 1^st derivative for the titration).