The selenium nanoparticles selectivity paradox: why the cellular metabolic landscape dictates the choice between cytotoxicity and protection.

Over the last few decades, SeNPs have attracted intense focus as promising biomedical agents, yet their therapeutic application is severely bottlenecked by a persistent toxicological paradox: the precise mechanisms governing their selective cytotoxicity toward malignancies versus cytoprotection in healthy tissues remain poorly understood. This review provides a comprehensive mechanistic assessment of this dual behavior in vitro. To isolate the intrinsic biological footprint of elemental selenium and eliminate confounding variables introduced by surface coatings or co-loaded drugs, we anchor our analysis strictly on unalloyed, bare spherical SeNPs with a standardized mean diameter of 100 nm. We critically evaluate how different fabrication pathways-specifically physical (pulsed laser ablation), chemical reduction, and eco-friendly green bio-synthesis-as well as morphological transitions into rod-like structures, modulate the activation of downstream cellular cascades. Through a detailed synthesis of recent literature, we dissect the intracellular pathways involved, including endoplasmic reticulum stress induction, disrupted calcium signaling, and the selective triggering of apoptotic over necrotic death. We demonstrate that SeNPs selectivity and dual-action therapeutic efficacy are fundamentally decoupled from nanoparticle geometry or manufacturing origin. Instead, the critical transition between cellular destruction and survival is strictly dictated by the intrinsic metabolic landscape, baseline oxidative stress, and unique biochemical wiring of the target biological specimen. These insights offer a clearer framework for predicting SeNPs behaviors in translational medicine.