The toxicity and environmental impact of graphene oxide (GO) have been extensively studied due to its increasing use in various industrial and technological applications. Understanding its effects on biological systems and ecosystems is critical for assessing potential risks. This evaluation covers in vitro and in vivo studies, ecotoxicity, degradation pathways, dose-dependent effects, and the influence of surface chemistry, while contrasting GO with reduced graphene oxide (rGO).

In vitro studies have demonstrated that GO exhibits dose-dependent cytotoxicity in various cell lines. For example, exposure to concentrations exceeding 50 µg/mL of GO has been shown to reduce cell viability in human lung epithelial cells by over 20%, primarily due to oxidative stress and membrane damage. The sharp edges of GO sheets can physically disrupt cell membranes, leading to necrosis or apoptosis. Smaller GO sheets (lateral dimensions below 200 nm) exhibit higher cellular uptake and greater toxicity compared to larger sheets, which tend to aggregate on cell surfaces. Surface chemistry plays a significant role, as carboxylated or amine-functionalized GO shows altered interactions with cells, sometimes reducing cytotoxicity by minimizing membrane disruption.

In vivo studies in animal models reveal that GO's toxicity depends on the exposure route. Intravenous administration of GO at doses above 1 mg/kg in mice has been associated with inflammatory responses in the liver and spleen, where GO accumulates due to reticuloendothelial system uptake. Pulmonary exposure via inhalation or instillation induces dose-dependent lung inflammation, with higher doses (above 10 µg/mouse) causing granuloma formation. Oral exposure, however, shows lower acute toxicity, as most GO is excreted via the gastrointestinal tract without significant absorption. Chronic exposure studies indicate potential long-term effects, including fibrosis in organs where GO persists.

Ecotoxicity studies highlight the impact of GO on aquatic and terrestrial organisms. In aquatic environments, GO affects microorganisms, algae, and invertebrates. For instance, Daphnia magna exposed to GO at concentrations above 1 mg/L exhibit reduced mobility and increased mortality due to physical clogging of gills and oxidative stress. Algal growth inhibition occurs at similar concentrations, with GO disrupting photosynthesis by adhering to cell surfaces. Soil-dwelling organisms like earthworms show reduced growth and reproduction at GO concentrations exceeding 100 mg/kg soil, likely due to oxidative damage and impaired nutrient absorption.

The degradation of GO in the environment is a slow process influenced by abiotic and biotic factors. Abiotic degradation includes photochemical reduction under UV light, which breaks GO into smaller fragments but does not fully mineralize it. Biodegradation by bacteria and fungi is possible but inefficient; certain microbial enzymes, such as lignin peroxidases, can partially degrade GO over weeks or months. The persistence of GO in ecosystems raises concerns about long-term accumulation, particularly in sediments where it may interact with other pollutants.

Dose-dependent effects are evident across all studies. Low concentrations (below 10 µg/mL in vitro or 1 mg/kg in vivo) often show minimal adverse effects, while higher doses induce significant toxicity. The shape, size, and surface functional groups of GO modulate these effects. For example, highly oxidized GO with abundant carboxyl groups tends to disperse more readily in water, increasing bioavailability and potential toxicity. In contrast, GO with reduced oxygen content aggregates more easily, reducing its environmental mobility but increasing the risk of sedimentation and benthic organism exposure.

Reduced graphene oxide (rGO) differs from GO in toxicity profiles due to its lower oxygen content and altered surface properties. rGO generally exhibits lower cytotoxicity in vitro because its hydrophobic surface reduces cellular uptake. However, rGO's increased persistence in the environment raises concerns about long-term ecological effects. Unlike GO, rGO is less susceptible to biodegradation, leading to potential accumulation in ecosystems. Its stronger interaction with organic matter may also facilitate the transport of other contaminants.

Surface chemistry modifications can mitigate or exacerbate GO's toxicity. PEGylation or other polymer coatings reduce GO's interaction with biological membranes, lowering cytotoxicity. Conversely, covalent attachment of certain functional groups, such as sulfonates, may increase dispersion and bioavailability, enhancing toxicity. The environmental behavior of GO also depends on pH, ionic strength, and the presence of natural organic matter, which influence aggregation and sedimentation rates.

In summary, graphene oxide exhibits dose- and form-dependent toxicity in biological and environmental systems. Its effects are mediated by oxidative stress, physical disruption, and persistence in ecosystems. Reduced graphene oxide, while less immediately toxic, poses long-term risks due to its resistance to degradation. Surface modifications and environmental conditions play critical roles in modulating these impacts. Further research is needed to establish safe exposure limits and develop strategies for minimizing environmental risks.