Differential pulse voltammetry detection of Pb(ii) using nitrogen-doped activated nanoporous carbon from almond shells

Abstract

Almond shell-based charcoal was prepared by carbonizing almond shells in a nitrogen atmosphere. Nanoporous carbon (NPC) was formed via activating the obtained charcoal using potassium hydroxide as an activating agent, followed by the synthesis of nitrogen-doped nanoporous carbon (N-NPC) via a hydrothermal reaction using urea as the nitrogen source. The obtained N-NPC possessed a large surface area (1075 m² g^-1), narrow pore-size distribution (1-2 nm) and nitrogen content reaching 2.23 wt%. Using N-NPC with Nafion to modify a glassy carbon electrode, a highly sensitive electrochemical sensor was fabricated for the determination of Pb(ii) in aqueous solutions with differential pulse anodic stripping voltammetry (DPASV). The peak current of Pb(ii) showed linearity over concentrations from 2.0 to 120 μg L^-1 and the detection limit (S/N = 3) was estimated to be 0.7 μg L^-1 for Pb(ii), which was 15-fold lower than the guideline value of drinking water given by the World Health Organization (WHO). The experimental data indicated that this easy and low-cost method is an accurate and fast method for the detection of trace Pb(ii).

Fig. 1. (a and b) SEM images of N-NPC.

Fig. 2. (a) Pore-size distribution, (b) N₂ sorption isotherm, (c) XRD spectrum and (d) Raman spectrum of N-NPC.

Fig. 3. XPS spectra of (a) NPC and (b) N-NPC.

Fig. 4. High-resolution spectra of (a) C1s, (b) N1s and (c) O1s.

Fig. 5. DPASV curves at different electrodes in 0.1 M HAC–NaAc buffer (pH = 5.0) containing 100 μg L⁻¹ of Pb(ii).

Fig. 6. Influence of various experimental parameters on stripping response of 50 μg L⁻¹ of Pb(ii): (a) load amount of N-NPC on GCE, (b) pH value, (c) deposition potential and (d) deposition time.

Fig. 7. (a) DPASV and (b) calibration plots of stripping peak current were tested in 0.1 M HAc–NaAc buffer solution by changing the concentration of Pb(ii) from 2 to 120 μg L⁻¹.

Fig. 8. (a) Three electrodes (every single one was tested six times) were used to evaluate the repeatability of N-NPC/Nafion/GCE; (b) six independent electrodes (each one was triplicated) were used to evaluate reproducibility. The experiment parameters were as follows: optimal load: 7.5 μg; optimal pH value: 5.0; optimized deposition potential: −1.3 V (E vs. Ag/AgCl); optimal deposition time: 330 s. DPASV was obtained at the scan rate of 0.1 V s⁻¹.