Highly Sensitive AgNP/MWCNT/Nafion Modified GCE-Based Sensor for the Determination of Heavy Metals in Organic and Non-organic Vegetables

Abstract

Silver nanoparticles/multi-walled carbon nanotubes/Nafion modified glassy carbon electrodes (AgNPs/MWCNTs/Nafion-GCE) were fabricated and were used as working electrode in anodic stripping voltammetry (ASV) for trace level determination of lead (Pb²⁺) and cadmium (Cd²⁺). The fabricated electrodes were characterized using scanning electron microscopy and cyclic voltammetry. The amounts of the electrode modifiers and the ASV parameters were optimized. It was found that the electrode modified with 1 mg AgNPs and 2 mg MWCNTs exhibited the best analytical response towards the determination of Pb²⁺ and Cd²⁺. The optimized ASV parameters were 60 s for the deposition time, 90 s for the accumulation time, and 100 mV/s for the scan rate. The electrode exhibited linearity from 0.493 ppb to 157.2 ppb for Pb²⁺ and 1.864 ppb to 155.1 ppb for Cd²⁺. The limit of detection was found to be 0.216 ppb for Pb²⁺ and 0.481 ppb for Cd²⁺. Real sampling analysis was carried out using organic vegetables from Sitio San Ysiro, Antipolo and Daraitan, Rizal and commercially available vegetables from Divisoria, all in Luzon, Philippines. Trace amounts of lead, cadmium, and copper were detected in the samples. Unwashed vegetables contained more heavy metal concentration compared to the washed vegetables. Atomic absorption spectroscopy was performed to validate the presence of the heavy metals in the vegetables.

Figure 1

Voltammograms for varying MWCNT and AgNP for the simultaneous detection of Cd²⁺ and Pb²⁺.

Figure 2

Anodic current peaks for varying AgNPs and MWCNTs for the simultaneous detection of Cd²⁺ and Pb²⁺.

Figure 3

Voltammograms for the optimization of deposition time.

Figure 4

Anodic peak currents for varying deposition time for the simultaneous detection of Cd²⁺ and Pb²⁺.

Figure 5

Voltammograms for the optimization of accumulation time.

Figure 6

Anodic peak currents of Cd²⁺ and Pb²⁺ for varying accumulation time.

Figure 7

Voltammograms for the optimization of scan rate.

Figure 8

Anodic peak currents of Cd²⁺ and Pb²⁺for varying scan rate.

Figure 9

Cyclic voltammograms in 0.1 M NaCl at bare and modified electrodes at a scan rate of 0.09 V/s.

Figure 10

Successive voltammetric scans of GCE modified with AgNPs/MWCNTs/Nafion in 0.1 M NaCl solution at a scan rate of 0.09 V/s.

Figure 11

SEM images of (a) MWCNTs/Nafion modified GCE and (b) AgNPs/Nafion modified GCE.

Figure 12

FESEM image of AgNP/MWCNT/Nafion modified GCE.

Figure 13

FESEM with EDX point analysis of modified electrode surface.

Figure 14

Voltammograms of varying Pb²⁺ and Cd²⁺ concentrations from 0.9 ppb to 200 ppb.

Figure 15

Calibration curves for Pb²⁺ and Cd²⁺.

Figure 16

Voltammograms of (a) organic camote tops and (b) organic squash.