Selenium Nanoparticles Functionalized with Methyl Aminolevulinic Acid as a Dual-Function Platform for Broad-Spectrum Antimicrobial Efficacy and Potential for Sustainable Seed Biofortification

• Broad-Spectrum Activity: MALASe exhibits potent antimicrobial activity, achieving 97.72% growth inhibition of Staphylococcus aureus, 95.43% for Escherichia coli, and 81.88% for Candida albicans at a concentration of 64 µg/mL. • Dual-Mode Antibacterial Activity: Against Escherichia coli ATCC 25922, MALASe (64 µg/mL) demonstrates dual-mode activity, combining significant intrinsic ‘dark toxicity’ with a light activated antimicrobial photodynamic therapy (aPDT) effect that boosted efficacy by approximately 27 percentage points (p < 0.005). • A Self-Amplifying Mechanism: We demonstrate for the first time that MALASe triggers the intracellular biosynthesis of the potent photosensitizer protoporphyrin IX (PpIX) in Escherichia coli. This effectively turns target bacteria into factories for their own destruction, creating a robust, self-amplifying photodynamic response upon light exposure. • Agricultural Applications: In the agricultural sector, a low concentration of 0.160 µg/mL inhibited the growth of the seven most relevant phytopathogens (Alternaria grandis, Cercospora kikuchii, Colletotrichum truncatum, Corynespora cassiicola, Fusarium oxysporum, Macrophomina phaseolina and Rhizoctonia solani) at levels similar to or higher than the commercial fungicide Mitraconazole (MCZ) . This work presents a multifunctional crop management platform based on photoreduced, methyl aminolevulinic acid-functionalized selenium nanoparticles (MALASeNPs). The spherical, 92 nm crystalline particles address nutrient deficiency and pathogen pressure through two primary mechanisms: selenium biofortification and intrinsic antimicrobial activity. The platform demonstrated potent broad-spectrum efficacy in the dark, achieving > 95% growth inhibition of Staphylococcus aureus and Escherichia coli, and 81.88% for Candida albicans at 64 µg/mL. Importantly, beyond this inherent "dark toxicity," the MALA functionalization enables an additional, light-activated mode of action via antimicrobial photodynamic therapy (aPDT). When activated with 460 nm light, this aPDT effect provided a significant and quantifiable boost, enhancing inhibition against Escherichia coli by approximately 27 percentage points (p < 0.005). In standard phytosanitary trials (conducted in the dark), a remarkably low concentration of 0.160 µg/mL outperformed or matched the commercial fungicide miconazole against seven key phytopathogens, highlighting its intrinsic activity. For crop enhancement, nanopriming wheat ( Triticum aestivum L.) seeds with 51.2 µg/mL optimally delivered selenium for biofortification without phytotoxicity. Hyperspectral imaging confirmed rapid in planta metabolism, with minimal residual nanoparticle coverage (0.015%) three days post-priming. These results position MALASeNPs as a versatile and sustainable tool, combining reliable dark antimicrobial action, an optional and potent light-activated boost for targeted bacterial control, and effective crop biofortification. .