Waste-Derived Green Synthesis of Mn-Doped ZnO Nanoparticles using Bos indicus Cow Dung Extract for Antimicrobial and Anticancer Applications

Technical Summary: Waste-Derived Green Synthesis of Mn-Doped ZnO Nanoparticles using Bos indicus Cow Dung Extract

Problem Statement
The synthesis of nanomaterials often relies on physical or chemical methods that can be environmentally hazardous, expensive, or toxic. While biological (green) synthesis offers an eco-friendly alternative, there is a need to utilize abundant, renewable waste resources to create functional nanomaterials. Furthermore, pure Zinc Oxide (ZnO) nanoparticles, though promising for biomedical applications, suffer from limitations such as a wide band gap and restricted visible-light absorption. Doping ZnO with transition metals like Manganese (Mn) is a known strategy to tailor these properties, but the development of a sustainable, waste-derived synthesis route for Mn-doped ZnO with verified antimicrobial and anticancer efficacy remains an area of exploration.

Methodology
The study employed a green synthesis approach using aqueous extract from Bos indicus (Indian cow) dung as both a reducing and stabilizing agent.

• Synthesis: Pure ZnO and Mn-doped ZnO nanoparticles were synthesized by reacting zinc nitrate hexahydrate (and manganese chloride tetrahydrate for doping) with the cow dung extract. The reaction was conducted under controlled pH (8–12), temperature (60–90°C), and stirring conditions, followed by calcination at 500°C.
• Characterization: The synthesized nanomaterials were systematically characterized using:
  • Powder X-ray Diffraction (XRD) for crystal structure and crystallite size.
  • UV-Visible Diffuse Reflectance Spectroscopy (DRS) for optical properties and band gap determination.
  • Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) for morphology and particle size.
  • Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) for elemental composition and oxidation states.
• Biological Evaluation:
  • Antimicrobial Activity: Evaluated against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus, Bacillus subtilis) bacteria using the disk diffusion method.
  • Anticancer Activity: Assessed against the MCF-7 human breast cancer cell line using the MTT assay to determine cell viability and IC50 values.

Key Results

• Structural and Optical Properties: XRD analysis confirmed the hexagonal wurtzite structure for both pure and Mn-doped samples, with no secondary phases detected. Mn doping resulted in a reduction in crystallite size (from 66.84 nm for pure ZnO to 37.67 nm for Mn-doped ZnO) and a slight broadening of diffraction peaks. UV-Vis DRS revealed a blue shift in the absorption edge upon Mn incorporation, increasing the optical band gap from 3.10 eV (pure ZnO) to 3.20 eV (Mn-doped), attributed to sp–d exchange interactions.
• Morphology and Composition: SEM and TEM analyses indicated that pure ZnO consisted of quasi-spherical, agglomerated particles (approx. 46.81 nm average), while Mn-doped ZnO exhibited a modified morphology with rod-like and needle-shaped structures and a reduced average particle size (approx. 34.27 nm). EDS and XPS confirmed the successful incorporation of Mn²⁺ ions into the ZnO lattice, with Mn present in the +2 oxidation state. Trace elements from the cow dung extract (C, K) were detected, acting as capping agents.
• Antimicrobial Activity: The nanoparticles demonstrated concentration-dependent antibacterial activity. Mn-doped ZnO showed enhanced inhibition zones compared to pure ZnO, particularly against S. aureus and E. coli. The mechanism is attributed to the generation of Reactive Oxygen Species (ROS) and the disruption of bacterial cell membranes by Zn²⁺ and Mn²⁺ ions.
• Anticancer Activity: Both pure and Mn-doped ZnO nanoparticles exhibited cytotoxic effects on MCF-7 cells in a dose-dependent manner. Pure ZnO nanoparticles showed an IC50 value of 107.5 μg/mL, while Mn-doped ZnO showed an IC50 of 219.56 μg/mL. The study notes that pure ZnO demonstrated slightly higher cytotoxic potency against this specific cell line compared to the doped variant.

Significance and Claims
The paper claims that the utilization of Bos indicus cow dung extract provides a sustainable, cost-effective, and eco-friendly route for synthesizing Mn-doped ZnO nanoparticles, aligning with circular bioeconomy principles. The study demonstrates that this green synthesis method successfully produces nanomaterials with controlled size and modified electronic properties.

The authors assert that the synthesized nanoparticles possess significant potential for biomedical applications, specifically as antimicrobial agents against pathogenic bacteria and as candidates for anticancer therapy. While the Mn-doped variant showed enhanced antibacterial performance, the study modestly reports that pure ZnO retained slightly higher efficacy against the MCF-7 breast cancer cell line in this specific experimental setup. The work concludes that these biosynthesized nanoparticles are promising "smart and encouraging agents" for biomedical fields, though the authors acknowledge that further mechanistic and in vivo studies are required to fully establish their therapeutic potential.