Nanocomposite Sensor Improves Heavy Metal Detection in Milk

Milk Contamination Challenge

Given the widespread consumption of milk and its vulnerability to environmental contamination, ensuring milk safety by monitoring trace heavy metals like lead (Pb²⁺) and cadmium (Cd²⁺) is essential. The proposed sensor aims to provide a sensitive, selective, and rapid approach to detect these contaminants, addressing limitations of conventional analytical techniques.

Milk acts as a critical dietary source but is increasingly susceptible to contamination by toxic heavy metals such as Pb²⁺ and Cd²⁺. These metals pose serious health risks due to their persistence, bioaccumulation, and interference with biological processes. Traditional detection methods such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) provide high accuracy but are costly, require complex instrumentation, and require laborious sample preparation, limiting their practicality for field use.

Nanocomposite Design & Electrode Fabrication

The MgO and ZnO nanoparticles were synthesized separately via precipitation and alkali precipitation, followed by hydrothermal treatment to obtain the MgO/ZnO nanocomposite. The particles were characterized structurally and morphologically using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-visible spectroscopy (UV-Vis), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), confirming successful synthesis and appropriate elemental composition.

To fabricate the sensor, the MgO/ZnO nanocomposite was integrated into a carbon paste matrix, forming the modified electrode. Electrochemical performance was evaluated using cyclic voltammetry (CV) with potassium ferrocyanide as a redox probe, assessing parameters such as reversibility and electroactive surface area, compared with electrodes modified only with ZnO or MgO. Optimization experiments fine-tuned variables including solution pH, deposition potential, metal ion concentration, and deposition time to maximize sensor response.

Heavy metal detection was conducted using anodic stripping voltammetry (ASV), whereby target metal ions first accumulated on the electrode surface, followed by electrochemical reduction and stripping to produce measurable current peaks. Real samples of cow milk from local farms and a university dairy were collected and analyzed using the sensor via a standard addition method to account for matrix effects. Validation of results was performed against conventional flame atomic absorption spectroscopy (FAAS).

Electrochemical Performance & Real Sample Analysis

The structural analyses demonstrated that the MgO/ZnO nanocomposite possessed a porous architecture and high surface area suitable for electrochemical applications. The formation of p–n heterojunctions between MgO and ZnO enhanced electron transfer by facilitating charge redistribution at the interface, as supported by EDX elemental analysis, which showed successful incorporation of ZnO into the MgO matrix.

Electrochemical characterization via cyclic voltammetry indicated that the MgO/ZnO modified electrode displayed higher peak currents and better reversibility than single-metal oxide modified electrodes, confirming increased electroactive surface area and improved charge transfer kinetics. The sensor showed enhanced sensitivity attributable to the synergistic effect of the nanocomposite, which provides abundant adsorption sites and facilitates rapid electron transfer.

The detection mechanism involves selective adsorption of Pb²⁺ and Cd²⁺ ions onto the MgO/ZnO surface through electrostatic attraction and complexation with oxygen-containing functional groups. During ASV, the adsorbed metal ions are reduced to their metallic forms, with stripping peaks corresponding to individual metals, allowing for simultaneous detection. The sensor exhibited well-defined anodic peaks with excellent reproducibility in voltammograms.

Analytical performance revealed a linear response for Pb²⁺ and Cd²⁺ ions in the concentration range of 100–8000 μg/L, with detection limits of 0.142 μg/L for Pb²⁺ and 0.45 μg L^-¹ for Cd²⁺. Sensitivities of 0.710 μA·μg^-¹ L^-¹  for Pb²⁺ and 0.787 μA·μg^-¹·L^-¹ for Cd²⁺ were reported. Comparative studies showed that this sensor outperforms many previously reported electrodes, highlighting its practical utility.

The sensor demonstrated excellent selectivity by effectively discriminating Pb²⁺ and Cd²⁺ ions over common interfering ions such as Cu²⁺, Zn²⁺, Co²⁺, and Fe³⁺. Furthermore, the electrode maintained stability, retaining over 95% of its initial response after three weeks of storage. Repeatability tests showed low relative standard deviations (2.7% for repeatability and 1.98% for reproducibility), underscoring the consistent fabrication and robust electron transfer properties.

Sensor Potential & Future Directions

This work successfully demonstrates the synthesis and utilization of a MgO/ZnO nanocomposite-modified carbon paste electrode as an efficient electrochemical sensor for detecting toxic heavy metal ions in milk. The nanocomposite imparts enhanced electrochemical properties, including increased surface area, improved electron transfer, and selective adsorption, all of which contribute to superior sensitivity, stability, and specificity.

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The study underscores the potential to further develop MgO/ZnO/CPE platforms into portable, miniaturized devices for on-site food safety and environmental monitoring. Overall, this electrochemical sensor holds promise for broad applications in public health protection and environmental surveillance, providing a cost-effective and reliable tool for trace heavy metal detection.

Journal Reference

Kebede M.T., Tsegaye A.A., et al. (2026). An electrochemical sensor based on MgO/ZnO nanocomposite modified carbon paste electrode for toxic heavy metal ions in milk. Scientific Reports. DOI: 10.1038/s41598-026-49835-y, https://www.nature.com/articles/s41598-026-49835-y