Polypyrrole-Wrapped Carbon Nanotube Composite Films Coated on Diazonium-Modified Flexible ITO Sheets for the Electroanalysis of Heavy Metal Ions

Abstract

Highly sensitive multicomponent materials designed for the recognition of hazardous compounds request control over interfacial chemistry. The latter is a key parameter in the construction of the sensing (macro) molecular architectures. In this work, multi-walled carbon nanotubes (CNTs) were deposited on diazonium-modified, flexible indium tin oxide (ITO) electrodes prior to the electropolymerization of pyrrole. This three-step process, including diazonium electroreduction, the deposition of CNTs and electropolymerization, provided adhesively-bonded, polypyrrole-wrapped CNT composite coatings on aminophenyl-modified flexible ITO sheets. The aminophenyl (AP) groups were attached to ITO by electroreduction of the in-situ generated aminobenzenediazonium compound in aqueous, acidic medium. For the first time, polypyrrole (PPy) was electrodeposited in the presence of both benzenesulfonic acid (dopant) and ethylene glycol-bis(2-aminoethylether)-tetraacetic acid (EGTA), which acts as a chelator. The flexible electrodes were characterized by XPS, Raman and scanning electron microscopy (SEM), which provided strong supporting evidence for the wrapping of CNTs by the electrodeposited PPy. Indeed, the CNT average diameter increased from 18 ± 2.6 nm to 27 ± 4.8, 35.6 ± 5.9 and 175 ± 20.1 after 1, 5 and 10 of electropolymerization of pyrrole, respectively. The PPy/CNT/NH₂-ITO films generated by this strategy exhibit significantly improved stability and higher conductivity compared to a similar PPy coating without any embedded CNTs, as assessed by from electrochemical impedance spectroscopy measurements. The potentiometric response was linear in the 10^-8-3 × 10^-7 mol L^-1 Pb(II) concentration range, and the detection limit was 2.9 × 10^-9 mol L^-1 at S/N = 3. The EGTA was found to drastically improve selectivity for Pb(II) over Cu(II). To account for this improvement, the density functional theory (DFT) was employed to calculate the EGTA-metal ion interaction energy, which was found to be -374.6 and -116.4 kJ/mol for Pb(II) and Cu(II), respectively, considering solvation effects. This work demonstrates the power of a subtle combination of diazonium coupling agent, CNTs, chelators and conductive polymers to design high-performance electrochemical sensors for environmental applications.

Keywords: chelator; diazonium; electrochemical sensors; heavy metal ions; multiwalled carbon nanotubes; polypyrrole.

Scheme 1

(a) Procedure for electrodeposition of polypyrrole on bare ITO (Route 1), NH₂-ITO (Route 2), and CNT/NH₂-ITO in the presence of ethylene glycol-bis(2-aminoethylether)-tetraacetic acid (EGTA) (Route 3), the structure of which is shown in (b).

Figure 1

Electropolymerization of PPy on (A) bare ITO, (B) NH₂-ITO and (C) CNTs/NH₂-ITO.

Figure 2

(A) Cyclic voltammograms, and (B) Nyquist plots of electrochemical impedance spectra obtained for PPy-ITO, PPy/NH₂-ITO, PPy/CNTs/NH₂-ITO. For CV: scan rate of 50 mV s⁻¹; for EIS: within the frequency range of 100 mHz–10 kHz. Inset of Figure 2B displays the equivalent circuit. Cd: double layer capacitance; Rct: Charger transfer resistance; Zw: Warburg resistance; Re: resistance of the solution.

Figure 3

Raman spectra of NH₂-ITO, CNT/NH₂-ITO, PPy/CNT/NH₂-ITO.

Figure 4

CNT size distribution. Histogram of the relative frequency versus the CNT diameters determined. (A) CNT/NH₂-ITO, (B) PPy/CNT/NH₂-ITO formed by CV for one cycle, (C) PPy/CNT/NH₂-ITO formed by CV for five cycles and (D) PPy/CNT/NH₂-ITO formed by CV for 10 cycles.

Figure 5

Scanning electron microscopy (SEM) images of (a) the interface between CNTs and NH₂-ITO at high magnification (scale bar 200 nm) and (b) low magnification (scale bar 2 µm), (c) Interface between PPy-wrapped CNTs and NH₂-ITO at high magnification (scale bar 200 nm). Inset of (b) shows front view of the cross-section at high magnification (scale bar 200 nm).

Figure 6

X-ray photoelectron spectroscopy (XPS) spectra of CNT/NH₂-ITO (a,c,e,g) and PPy/CNT/NH₂-ITO (b,d,f,h): Survey regions (a,b), C1s (c,d), N1s (e,f) and S2p (g,h).

Figure 7

(A) Differential pulsed voltammetry (DPV) of Pb (II) at pH 5 containing Pb(II) 10⁻⁷ mol L⁻¹ for PPy/CNT/NH₂-ITO and PPy/NH₂-ITO, (B) Time effect of pre-concentration metal ion on PPy/CNT/NH₂-ITO, (C) Pb(II) stripping peak vs. Pb(II) concentration in the 10⁻⁸–2.5 × 10⁻⁷ mol L⁻¹ range. (D) Calibration curve for the detection of Pb(II) using PPy/CNT/NH₂-ITO electrode.

Figure 8

DPV simultaneous determination of concentration 2.510⁻⁶ M of a Pb²⁺ and Cu²⁺ mixture with initial Cu²⁺/Pb²⁺ molar ratio = 1. Inset shows that for [Cu²⁺]_i/[Pb²⁺]_i = 1, 10, 40, I_s/I₀ = 90, 50 and 45, respectively, without EGTA; whilst I_s/I₀ = 98, 95 and 90, respectively, in the presence of EGTA.

Scheme 2

Thermodynamic cycles used to calculate the complexation energy in gas (a) and in solvent (b).

Figure 9

Structures of stable conformers of EGTA and, [EGTA-Pb]²⁺ and [EGTA-Cu]²⁺ complexes.