Biogenic Zinc nanoparticles: green approach to synthesis, characterization, and antimicrobial applications

Abstract

Background: Biogenic synthesis of zinc nanoparticles (ZnNPs) has attracted significant interest due to their unique properties and potential biological applications. Unlike chemical and physical methods, biogenic synthesis offers a greener and more eco-friendly alternative. This study explores the synthesis of zinc-based nanoparticles using two distinct bacterial strains.

Results: In this study, zinc nanoparticles were synthesized in two forms: single-phase zinc sulfide nanoparticles (ZnS NPs) and mixed-phase zinc sulfide-oxide nanoparticles (ZnS-ZnO NPs), using Achromobacter sp. S4 and Pseudomonas sp. S6. The synthesis conditions were optimized for each strain, with pH playing a crucial role: Achromobacter sp. S4 favored basic conditions (pH 8.0) for zinc nanoparticles production, while Pseudomonas sp. S6 preferred acidic conditions (pH 4.7). TEM analysis revealed that Zn NPs from Pseudomonas sp. S6 were rod-shaped, whereas those from Achromobacter sp. S4 were spherical. Further characterization using EDX, XRD, and FTIR confirmed the successful synthesis of single phase ZnS NPs and hybride phase ZnS-ZnO NPs. Antimicrobial dose-response testing showed that single-phase ZnS NPs inhibited Klebsiella pneumoniae, while mixed-phase ZnS-ZnO NPs were effective against Staphylococcus epidermidis at 100 µg/ml based on inhibition zone measurements.Furthermore, the mixed-phase ZnS-ZnO NPs at 25 µg/ml demonstrated superior inhibition of microbial growth in sludge samples, likely due to a synergistic effect.

Conclusion: The study demonstrates the successful biogenic synthesis of ZnS NPs, and ZnS-ZnO NPs using two bacterial strains, with distinct morphological and functional properties. The use of two bacterial species was to assess strain-specific differences in nanoparticle synthesis and performance. The synthesized nanoparticles exhibited promising antimicrobial and environmental remediation potential, highlighting their applicability in both biomedical and environmental fields.

Keywords: Achromobacter sp.; Pseudomonas sp.; Biosynthesis; Microbial proliferation; Sludge remediation.

Fig. 1

Schematic diagram for the work done in the study

Fig. 2

UV–Visible scan spectra of ZnNPs synthesized by strains S4 and S6, showing a maximum absorption peak at 310 nm

Fig. 3

Visible white-creamy color change in the positive sample containing cell-free supernatant reacted with 100 mM ZnSO₄. No color change was observed in the negative control (non-inoculated medium with 100 mM ZnSO₄)

Fig. 4

TEM micrographs at 500 nm scale for the two selected isolates: S4 (A) and S6 (B)

Fig. 5

Monitoring the maxium absorbance reading of reaction product at λ310 under different conditions for the selected strains (S4&S6). Culture age (A), reaction time (B), ZnSO₄ concentrations (C), supernatant of a cell (D), pH (E) and temperature (F)

Fig. 6

Transmission electron microscopy (TEM) images of biosynthesized ZnSNPs and ZnO–ZnSNPs displaying distinct morphologies: spherical nanoparticles produced by A. spanius strain S4 (A), and rod-shaped nanoparticles produced by P. resinovorans strain S6 (B). Scale bar = 100 nm. The particle sizes ranged from 14.1 to 29 nm for S4 and 3.93 to 43.9 nm for S6

Fig. 7

EDX spectra of ZnS nanoparticles synthesized by A. spanius strain S4 (A) and ZnO–ZnS nanoparticles synthesized by P. resinovorans strain S6 (B)

Fig. 8

XRD analysis of ZnS nanoparticles synthesized by A. spanius strain S4 (A) and ZnO–ZnS nanoparticles synthesized by P. resinovorans strain S6 (B)

Fig. 9

FTIR spectra of ZnS nanoparticles synthesized by A. spanius strain S4 (A) and ZnO–ZnS nanoparticles synthesized by P. resinovorans strain S6 (B)

Fig. 10

Antimicrobial activity of ZnSNPs from A. spanius S4 (A) and ZnS–ZnO NPs from P. resinovorans S6 (B) at 25, 50, and 100 µg/ml. Vancomycin, Gentamicin, and Fluconazole (100 µg/ml) served as positive controls against Gram-positive bacteria, Gram-negative bacteria, and Candida spp., respectively. Data represent mean ± SD of inhibition zones from three biological replicates. Different lowercase letters indicate significant differences (P ≤ 0.05) among treatments relative to the positive control

Fig. 11

Percentage growth reduction of sludge microorganisms treated with ZnS nanoparticles (NPs) from A. spanius S4 (A) and ZnO–ZnS NPs from P. resinovorans S6 (B) at concentrations of 12.5, 25, and 50 µg/ml. Growth reduction was assessed based on optical density measurements over two days and compared to the untreated control. Different lowercase letters indicate statistically significant differences among treatments (P ≤ 0.05)