Influence of Calcination Temperature on the Structure and Antimicrobial Properties of Arthrospira platensis-Mediated Zinc Oxide Nanoparticles

<b>Background/Objectives</b>: <i>Arthrospira platensis</i> (<i>A. platensis</i>) is a cyanobacterium rich in bioactive compounds with proven antioxidant, antimicrobial, and stabilizing properties, making it an ideal candidate for the green synthesis of zinc oxide nanoparticles (ZnO NPs). This study aimed to synthesize ZnO NPs using <i>A. platensis</i> extract and to evaluate the influence of post-synthesis temperature on their physicochemical and antimicrobial properties. <b>Methods</b>: ZnO NPs were synthesized via a co-precipitation method using <i>A. platensis</i> extract, followed by post-synthesis treatments at 80 °C and 400 °C. Comprehensive characterization was performed using Ultraviolet-Visible Spectroscopy (UV-Vis), Fourier Transform Infrared Spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM), and Energy Dispersive X-ray Spectroscopy (EDX) to assess optical, structural, and compositional features. Antioxidant activity (DPPH assay) and antimicrobial properties against <i>Staphylococcus aureus</i>, <i>Escherichia coli</i>, and Candida albicans were also evaluated. <b>Results</b>: FE-SEM analysis confirmed a temperature-dependent effect, with ZnO NPs synthesized at 80 °C appearing as polydispersed, irregular aggregates (45.2 ± 8.6 nm), while calcination at 400 °C yielded compact, angular nanoparticles (37.1 ± 6.3 nm). In contrast, pure ZnO NPs were smaller (26.4 ± 4.1 nm), and <i>A. platensis</i> extract alone showed amorphous, irregular structures. FTIR spectra demonstrated the involvement of biomolecules in nanoparticle capping and stabilization, whereas EDX analysis revealed that higher calcination reduced organic residues and increased zinc purity. Antioxidant assays indicated a decrease in phenolic and flavonoid content with increasing temperature, leading to reduced DPPH radical scavenging activity. Antimicrobial evaluation showed superior inhibition zones (17.8-26.0 mm) for <i>A. platensis</i>-ZnO NPs compared to the crude extract, with <i>S. aureus</i> being most susceptible, particularly to the 400 °C nanoparticles. <b>Conclusions</b>: The study demonstrates that <i>A. platensis</i> extract provides a sustainable and efficient route for ZnO NP biosynthesis. Calcination temperature significantly affects nanoparticle morphology, biochemical composition, and antimicrobial performance. These findings highlight the potential of <i>A. platensis</i>-ZnO NPs as eco-friendly antimicrobial agents for biomedical, pharmaceutical, and food preservation applications.