Carbon nanohorns, a lesser-known but highly promising member of the carbon nanomaterial family, exhibit remarkable antibacterial properties that make them attractive for biomedical and environmental applications. These conical nanostructures, typically 2–5 nm in diameter and 40–50 nm in length, aggregate into spherical assemblies resembling dahlia flowers. Their unique structural and electronic properties contribute to potent antimicrobial activity through multiple mechanisms, while their biocompatibility and functionalization potential further enhance their utility.

The antibacterial action of carbon nanohorns primarily stems from two key mechanisms: physical disruption of bacterial membranes and induction of oxidative stress. The sharp, horn-like tips of these nanostructures can mechanically puncture bacterial cell walls and membranes, leading to leakage of cellular contents and eventual cell death. This physical damage is particularly effective against Gram-negative bacteria, whose outer membranes are vulnerable to nanoscale piercing. Additionally, the high surface area and electron-rich carbon framework of nanohorns facilitate the generation of reactive oxygen species (ROS) upon interaction with bacterial cells. ROS, including superoxide radicals and hydrogen peroxide, cause oxidative damage to lipids, proteins, and DNA, disrupting critical cellular functions.

Functionalization of carbon nanohorns further enhances their antibacterial efficacy and selectivity. Covalent modification with hydrophilic groups, such as carboxyl or hydroxyl moieties, improves dispersion in aqueous environments, increasing contact with bacterial cells. Non-covalent functionalization with antimicrobial agents, such as silver nanoparticles or antibiotics like ciprofloxacin, creates synergistic effects. For example, silver-decorated nanohorns combine the membrane-penetrating ability of the carbon framework with the ion-release toxicity of silver, resulting in broad-spectrum activity against both Gram-positive and Gram-negative strains. Similarly, polyethylenimine (PEI)-functionalized nanohorns exhibit enhanced binding to bacterial surfaces due to electrostatic interactions, followed by membrane disruption and intracellular ROS generation.

Biocompatibility is a critical consideration for biomedical applications of carbon nanohorns. Studies indicate that surface oxidation or PEGylation reduces potential cytotoxicity to mammalian cells while retaining antibacterial properties. Oxidized nanohorns, bearing carboxylic acid groups, show lower hemolytic activity compared to pristine structures, making them suitable for blood-contacting applications. The balance between antibacterial activity and biocompatibility can be finely tuned by controlling the degree of functionalization. For instance, moderate oxidation preserves antimicrobial effects while minimizing adverse immune responses, as demonstrated in macrophage viability assays.

The antibacterial performance of carbon nanohorns is influenced by their aggregation state and dispersion. Individual nanohorns or small aggregates exhibit higher activity due to increased surface area and sharper protrusions. Surfactants or biomolecular coatings, such as polysorbate 80 or albumin, can stabilize dispersions and prevent aggregation, ensuring consistent antimicrobial effects. The concentration-dependent activity follows a threshold behavior, with significant bacterial reduction observed at concentrations above 10 µg/mL for most strains, while lower doses may inhibit growth without causing complete cell death.

Comparative studies reveal that carbon nanohorns outperform other carbon nanomaterials like graphene oxide in certain scenarios, particularly where a combination of mechanical and oxidative mechanisms is advantageous. Their tubular structure allows deeper penetration into bacterial biofilms compared to planar graphene sheets, enhancing biofilm disruption. Additionally, the absence of metal catalysts in their synthesis avoids residual toxicity issues associated with some carbon nanotubes.

Environmental applications leverage the stability and reusability of carbon nanohorns. In water purification systems, nanohorn-embedded filters exhibit sustained antibacterial activity over multiple cycles, with minimal leaching of functional agents. The photothermal effect, where nanohorns convert light energy into heat, further augments their antimicrobial utility. Near-infrared irradiation of nanohorn-treated bacterial suspensions leads to localized heating, achieving near-complete eradication of pathogens within minutes.

Challenges remain in scaling up production and ensuring uniform functionalization for reproducible effects. Batch-to-batch variations in tip sharpness and aggregation can influence antibacterial performance. Advanced characterization techniques, including high-resolution TEM and Raman mapping, are essential for quality control. Future directions include the development of stimulus-responsive nanohorns, where antibacterial activity is triggered by pH, enzymes, or light, enabling targeted therapy.

In summary, carbon nanohorns represent a versatile platform for antibacterial applications, combining inherent structural properties with tunable functionalization. Their dual-action mechanism, biocompatibility, and adaptability position them as promising candidates for combating antibiotic-resistant infections and contamination in both medical and environmental settings. Continued research into precise surface engineering and large-scale fabrication will further unlock their potential in antimicrobial nanotechnology.