Nanoparticles induce cytotoxicity through multiple interconnected pathways, with oxidative stress, membrane disruption, and mitochondrial dysfunction being the most prominent mechanisms. These effects vary depending on the nanoparticle composition, size, surface charge, and exposure conditions. Metal oxides, carbon-based nanomaterials, and polymeric nanoparticles exhibit distinct cytotoxic behaviors, each triggering unique cellular responses while sharing common mechanistic features.

Oxidative stress is a primary mechanism of nanoparticle-induced cytotoxicity. Reactive oxygen species (ROS) generation occurs when nanoparticles interact with cellular components, leading to an imbalance between ROS production and antioxidant defenses. Metal oxide nanoparticles, such as TiO2, ZnO, and CuO, are particularly potent inducers of oxidative stress due to their redox-active surfaces. ZnO nanoparticles dissolve in acidic lysosomal compartments, releasing Zn2+ ions that catalyze Fenton-like reactions, producing hydroxyl radicals. In vitro studies with lung epithelial cells show a dose-dependent increase in ROS within hours of exposure to ZnO nanoparticles, accompanied by depletion of glutathione and activation of antioxidant enzymes like superoxide dismutase. Similarly, TiO2 nanoparticles generate ROS through photocatalytic activity, especially under UV light, causing DNA damage and lipid peroxidation in skin fibroblasts. Carbon-based nanomaterials, including graphene oxide and carbon nanotubes, induce oxidative stress through direct electron transfer from their surfaces or via interaction with mitochondrial electron transport chains. Multi-walled carbon nanotubes (MWCNTs) have been shown to increase intracellular ROS levels in macrophages, leading to NLRP3 inflammasome activation and subsequent interleukin-1β release. Polymeric nanoparticles, such as polystyrene and poly(lactic-co-glycolic acid) (PLGA), generally induce lower oxidative stress compared to metal oxides but can still trigger ROS production when surface-modified with cationic groups or when degradation products accumulate in cells. In vivo studies in rodent models reveal that repeated exposure to metal oxide nanoparticles results in persistent oxidative stress markers in lung, liver, and kidney tissues, correlating with histopathological changes like fibrosis and inflammation.

Membrane disruption is another critical cytotoxic mechanism, particularly for nanoparticles with high surface reactivity or sharp edges. Carbon-based nanomaterials like graphene sheets physically damage cell membranes through direct insertion and lipid extraction. Molecular dynamics simulations demonstrate that graphene nanosheets penetrate lipid bilayers, causing pore formation and increased membrane permeability. Experimental evidence in erythrocytes shows that graphene oxide induces hemolysis by extracting phospholipids from the cell membrane. Metal oxide nanoparticles, such as SiO2 and CeO2, adsorb to membrane surfaces, disrupting lipid packing and inducing curvature stress. This leads to ion imbalance and loss of membrane integrity, as observed in alveolar epithelial cells exposed to SiO2 nanoparticles. Polymeric nanoparticles with cationic surfaces, such as chitosan or polyethyleneimine-coated particles, interact electrostatically with negatively charged membrane components, causing lipid rearrangement and pore formation. In vivo studies demonstrate that cationic polymeric nanoparticles induce pulmonary surfactant dysfunction, leading to alveolar collapse and respiratory distress. Membrane disruption is often the initial event that precedes other cytotoxic effects, as compromised membranes allow uncontrolled nanoparticle uptake and organelle damage.

Mitochondrial dysfunction is a downstream consequence of nanoparticle-induced oxidative stress and membrane damage. Nanoparticles localize in mitochondria due to the organelle's negative membrane potential, where they interfere with electron transport chain (ETC) complexes. Metal oxide nanoparticles like CuO and Fe3O4 directly bind to ETC proteins, inhibiting ATP synthesis and increasing electron leakage, which further exacerbates ROS production. In vitro studies using hepatocytes reveal that CuO nanoparticles reduce mitochondrial membrane potential within 6 hours of exposure, leading to cytochrome c release and caspase activation. Carbon nanotubes accumulate in mitochondria due to their nanoneedle-like structure, causing physical disruption of cristae and uncoupling of oxidative phosphorylation. In vivo experiments in mice show that prolonged exposure to MWCNTs results in mitochondrial DNA deletions and reduced respiratory capacity in cardiac tissue. Polymeric nanoparticles, especially those with hydrophobic surfaces, integrate into mitochondrial membranes, altering their fluidity and permeability. PLGA nanoparticles have been shown to induce mitochondrial swelling in neuronal cells, accompanied by decreased ATP levels and increased lactate production. Mitochondrial dysfunction often culminates in apoptosis or necrosis, depending on the severity of damage and cellular energy status.

The interplay between these mechanisms determines the overall cytotoxic outcome. Oxidative stress initiates signaling cascades that exacerbate membrane and mitochondrial damage, while membrane disruption facilitates further nanoparticle uptake and organelle access. Metal oxides tend to dominate oxidative stress and mitochondrial dysfunction pathways due to their ionic dissolution and redox activity. Carbon-based nanomaterials excel in physical membrane disruption and direct mitochondrial interference, while polymeric nanoparticles primarily exert toxicity through surface chemistry-dependent interactions. In vivo, these mechanisms manifest as tissue-specific damage, with the liver, lungs, and kidneys being particularly vulnerable due to their roles in nanoparticle accumulation and clearance. Chronic exposure scenarios often show progressive toxicity as nanoparticles overwhelm cellular repair mechanisms, leading to fibrosis, chronic inflammation, or carcinogenesis in susceptible tissues. Understanding these mechanistic pathways is crucial for designing safer nanomaterials and developing targeted interventions to mitigate their adverse effects.