Carbon nanohorns (CNHs) are a unique class of carbon-based nanomaterials characterized by their horn-like tubular structures, typically aggregated into spherical assemblies. These nanostructures exhibit promising applications in drug delivery, imaging, and energy storage due to their high surface area, porosity, and tunable surface chemistry. However, their potential toxicity and biocompatibility must be thoroughly evaluated to ensure safe use in biomedical and environmental applications. This article examines the current understanding of CNH toxicity and biocompatibility, focusing on in vitro and in vivo studies, while comparing them with other carbon nanomaterials like carbon nanotubes (CNTs) and graphene.

The toxicity of carbon nanohorns is influenced by several factors, including size, surface chemistry, aggregation state, and purity. Studies have shown that pristine CNHs, which are typically aggregated into larger spherical structures with diameters ranging from 50 to 100 nm, exhibit lower cytotoxicity compared to smaller, dispersed nanoparticles. This is attributed to their reduced cellular uptake and minimal membrane disruption. In contrast, individualized CNHs or smaller aggregates may penetrate cells more readily, leading to increased oxidative stress and inflammatory responses. Surface modifications, such as oxidation or functionalization with biocompatible polymers, can further alter their interactions with biological systems. For instance, oxidized CNHs demonstrate improved dispersibility in aqueous media but may also introduce reactive oxygen species (ROS)-generating surface groups.

In vitro studies have provided insights into the cellular responses to CNHs. Research using human lung epithelial cells, macrophages, and fibroblasts has shown that CNHs induce dose-dependent cytotoxicity, with higher concentrations leading to reduced cell viability and increased ROS production. However, compared to multi-walled carbon nanotubes (MWCNTs), CNHs generally exhibit lower cytotoxicity at equivalent mass concentrations. This difference is partly due to the absence of metallic impurities in CNHs, which are often present in CNTs and contribute to their toxicity. Additionally, the curved, closed structure of CNHs reduces the likelihood of physical piercing of cell membranes, a phenomenon observed with needle-like CNTs.

The aggregation state of CNHs plays a critical role in their biological interactions. While aggregated CNHs are less likely to be internalized by cells, they may still trigger inflammatory responses if they accumulate in tissues. In vitro experiments with immune cells have shown that CNHs can activate macrophages, leading to the release of pro-inflammatory cytokines such as TNF-α and IL-6. However, these responses are typically milder than those induced by CNTs or graphene oxide. Surface functionalization with polyethylene glycol (PEG) or other biocompatible coatings has been shown to mitigate these effects by reducing protein adsorption and cellular recognition.

In vivo studies have further elucidated the biocompatibility and long-term effects of CNHs. Animal models, primarily mice and rats, have been used to assess the distribution, accumulation, and toxicity of CNHs following various routes of administration, including intravenous, pulmonary, and oral exposure. Intravenously injected CNHs tend to accumulate in the liver and spleen due to uptake by the reticuloendothelial system. While no significant acute toxicity has been reported at moderate doses, chronic exposure studies indicate potential liver and kidney inflammation at high concentrations. Pulmonary exposure studies, which are relevant for occupational safety, have shown that inhaled CNHs are less fibrogenic than CNTs but can still cause mild granulomatous inflammation at high doses.

One notable advantage of CNHs over other carbon nanomaterials is their biodegradability. Unlike CNTs, which persist in tissues for extended periods, CNHs have been shown to undergo gradual degradation under physiological conditions. This property reduces the risk of long-term accumulation and associated chronic toxicity. Enzymatic degradation by myeloperoxidase and other oxidative enzymes contributes to the breakdown of CNHs into smaller, less harmful fragments. However, the rate and extent of degradation depend on the degree of surface functionalization and the local inflammatory environment.

The surface chemistry of CNHs is a key determinant of their biocompatibility. Pristine CNHs, which are hydrophobic, tend to aggregate in biological fluids and adsorb proteins, leading to opsonization and rapid clearance by the immune system. In contrast, functionalized CNHs with hydrophilic groups (e.g., carboxyl or amine groups) exhibit improved stability in physiological media and reduced protein corona formation. These modifications also enhance their compatibility with blood components, making them more suitable for intravenous applications. For example, PEGylated CNHs have demonstrated prolonged circulation times and reduced immunogenicity in animal models.

Compared to other carbon nanomaterials, CNHs offer a balance between functionality and biocompatibility. While graphene-based materials exhibit superior electrical and mechanical properties, their sharp edges and high aspect ratio can cause significant membrane damage and inflammation. CNTs, on the other hand, share some structural similarities with CNHs but are often more toxic due to their needle-like morphology and residual metal catalysts. Carbon quantum dots and fullerenes are generally more biocompatible but lack the high surface area and porosity of CNHs, limiting their utility in applications like drug delivery and catalysis.

Despite their favorable properties, challenges remain in the safe deployment of CNHs. Standardized protocols for toxicity assessment are needed to account for variations in synthesis methods, purity, and functionalization. Additionally, long-term studies are required to evaluate the potential for delayed immune responses or unforeseen organ damage. Regulatory frameworks must also adapt to address the unique characteristics of CNHs and other emerging nanomaterials.

In summary, carbon nanohorns exhibit promising biocompatibility compared to other carbon nanomaterials, with lower cytotoxicity, reduced inflammatory potential, and gradual biodegradability. Their toxicity profile is highly dependent on factors such as size, aggregation state, and surface chemistry, which can be tailored to optimize their performance in biomedical applications. While further research is needed to fully understand their long-term effects, CNHs represent a versatile and relatively safe option for nanotechnology-based solutions in medicine and beyond.