Citrullus Lanatus Peel Extract Templated Synthesis of ZnO/MnO2 Nanocomposites for Electrochemical Lead Detection and Quantification in Aqueous Solution

Abstract

Contamination of water by heavy toxic metals poses significant health risks, with lead being particularly harmful due to its severe impact on human health and ecosystems. Due to this, accurate detection and quantification of lead are crucial for ensuring safe drinking water and preventing lead poisoning. Among various detection methods, electrochemical techniques have gained prominence approach because of enhanced sensitivity, selectivity, and rapid response, particularly when electrode surfaces are modified with nanoparticles (NPs) and nanocomposites (NCs). This study explores the green synthesis of ZnO, MnO2, and ZnO/MnO2 NCs with different ratios using Citrullus lanatus peel extracts. Comprehensive characterization was performed using TGA/DTA, XRD, FTIR, SEM-EDX, UV-Vis DRS, BET, CV, and EIS. Qualitative phytochemical analysis of Citrullus lanatus peel extract revealed the presence of various bioactive compounds. XRD analysis confirmed the formation of NPs and NCs, revealing a crystallite size of 17.05, 22.6, 15.6, 17.33, and 17.4 nm for ZnO, α-MnO2, and ZnO/α-MnO2 NCs at ratios of 1:1, 1:2, and 2:1, respectively. FTIR confirmed the presence of functional groups responsible for reduction and stabilization during biosynthesis. SEM- EDX analysis showed spherical nanoparticles within the nanometer range and successful formation of stable and pure NPs and NCs. UV-Vis DRS confirmed the formation of NPs and NCs, with ZnO showing a sharp absorbance around 366 nm and α-MnO2 absorbing over the visible range, resulting in band gaps of 3.00 and 1.36 eV, respectively. ZnO/α-MnO2 NCs showed absorption edges around 380 nm, 450 nm, and 375 nm, with band gap values of 2.43, 2.35, and 2.29 eV for ratios of 1:1, 1:2, and 2:1, respectively. The specific surface area, pore size, and pore volume were determined using BET, indicating a higher surface area for the 1:2 ratios. Electrochemical characterization using CV and EIS showed enhanced electrode performance with ZnO/α-MnO2 NCs modified working electrode (MWE). Lead detection and quantification were performed in an acidic environment (0.1M HCl) using chronoamperometry at -1.1 V for 240 s, with further optimization using square wave anodic stripping voltammetry (SWASV). The study found that ZnO detected approximately 0.112 ppm of Pb2+, while α-MnO2 detected around 0.150 ppm. Among the NCs, 1:2 ZnO/α-MnO2 MWE detected 0.171 ppm of Pb2+. Under these optimized conditions, Pb2+ quantification was conducted with the ZnO/α-MnO2(1:2) MWE across concentrations ranging from 0.074 to 0.016 ppm, yielding a calibration curve with a correlation coefficient (R2) of 0.93577. The LOD for Pb2+ was determined to be 0.008 ppm. The NCs exhibit excellent sensitivity in lead detection and quantification, making it promising for accurately monitoring toxic heavy metals in water.