Carbon nanohorns have emerged as promising candidates for biomedical imaging due to their unique structural and physicochemical properties. These nanostructures consist of conical graphene tubules aggregated into spherical assemblies, typically 80 to 100 nanometers in diameter, with individual horn-like projections extending outward. Their high surface area, porous structure, and ability to encapsulate or adsorb imaging agents make them particularly suitable for applications in magnetic resonance imaging (MRI), fluorescence imaging, and multimodal contrast enhancement.

One of the primary advantages of carbon nanohorns in biomedical imaging is their capacity to serve as carriers for contrast agents. For MRI applications, gadolinium-based compounds are commonly loaded onto or within the nanohorns to enhance proton relaxation rates. The large surface area of carbon nanohorns allows for high payloads of gadolinium chelates, significantly improving T1-weighted contrast. Studies have demonstrated that gadolinium-loaded carbon nanohorns can achieve relaxivity values exceeding 20 mM-1s-1, which is higher than many conventional small-molecule contrast agents. This enhancement is attributed to the restricted tumbling motion of gadolinium ions when bound to the nanohorn surface, leading to more efficient relaxation of surrounding water protons.

In fluorescence imaging, carbon nanohorns can be functionalized with near-infrared (NIR) dyes or quantum dots to enable deep-tissue imaging with minimal autofluorescence interference. The porous structure of nanohorns allows for stable encapsulation of hydrophobic fluorophores, preventing leakage and photobleaching. Additionally, the intrinsic optical properties of carbon nanohorns, such as their ability to absorb and scatter light, can be leveraged for photoacoustic imaging, where they act as contrast agents by converting absorbed laser energy into ultrasonic waves. This multimodal capability is particularly valuable for real-time imaging and image-guided therapies.

The loading of imaging probes onto carbon nanohorns can be achieved through several methods. Physical adsorption is the simplest approach, where contrast agents are non-covalently attached to the nanohorn surface via van der Waals interactions or π-π stacking. This method preserves the chemical integrity of both the nanohorns and the imaging agents but may result in variable loading efficiencies. Covalent conjugation, on the other hand, involves chemically linking contrast agents to functional groups on the nanohorn surface, such as carboxyl or hydroxyl moieties introduced through oxidation. This approach offers more controlled and stable loading but requires careful optimization to avoid compromising the imaging agent's efficacy. Encapsulation within the internal pores of nanohorns is another strategy, particularly for hydrophobic agents, providing protection from degradation and prolonged circulation times.

Biocompatibility is a critical consideration for any nanomaterial intended for biomedical applications. Carbon nanohorns exhibit favorable biocompatibility profiles compared to other carbon-based nanomaterials like carbon nanotubes, primarily due to their lack of metallic impurities and lower propensity for inducing oxidative stress. In vitro studies have shown that carbon nanohorns are well-tolerated by various cell lines, with minimal cytotoxicity observed at concentrations up to 100 µg/mL. Hemocompatibility assessments reveal no significant hemolysis or platelet aggregation, making them suitable for intravenous administration. However, long-term in vivo studies indicate that unmodified carbon nanohorns can accumulate in reticuloendothelial organs such as the liver and spleen, necessitating surface modifications to enhance clearance or reduce immunogenicity.

Surface functionalization with polyethylene glycol (PEG) is a common strategy to improve the pharmacokinetics and biocompatibility of carbon nanohorns. PEGylation reduces opsonization and subsequent macrophage uptake, prolonging circulation half-life and enhancing tumor accumulation through the enhanced permeability and retention (EPR) effect. Other biocompatible coatings, such as polysaccharides or peptides, can also be employed to tailor the interaction of nanohorns with biological systems. For instance, hyaluronic acid-functionalized carbon nanohorns have demonstrated selective targeting of CD44-overexpressing cancer cells, enabling tumor-specific imaging.

The biodistribution and excretion pathways of carbon nanohorns are influenced by their size, surface chemistry, and route of administration. Intravenously injected nanohorns predominantly accumulate in the liver and spleen, with smaller fractions reaching the lungs and kidneys. Renal clearance is limited due to their size, but partial degradation through enzymatic or oxidative processes can facilitate excretion over time. Modulating the surface charge and hydrodynamic diameter can optimize biodistribution, with neutral or slightly negative charges generally favoring reduced nonspecific uptake.

In addition to their role as contrast agents, carbon nanohorns can be engineered for theranostic applications, combining imaging and therapeutic functionalities. For example, doxorubicin-loaded carbon nanohorns enable simultaneous drug delivery and fluorescence tracking of drug release kinetics. Similarly, iron oxide-decorated nanohorns can serve as dual MRI contrast agents and magnetic hyperthermia mediators. The versatility of carbon nanohorns in accommodating diverse payloads underscores their potential as multifunctional nanoplatforms in precision medicine.

Despite these advantages, challenges remain in the clinical translation of carbon nanohorn-based imaging agents. Standardization of synthesis and functionalization protocols is necessary to ensure batch-to-batch reproducibility. Rigorous toxicological evaluations in larger animal models are needed to confirm long-term safety. Additionally, scalable production methods must be developed to meet potential clinical demand. Addressing these challenges will pave the way for carbon nanohorns to become a mainstream tool in biomedical imaging, offering high-performance alternatives to conventional contrast agents.

The unique combination of high loading capacity, biocompatibility, and multimodal imaging capabilities positions carbon nanohorns as a versatile platform for next-generation contrast agents. Ongoing research continues to explore novel functionalization strategies and applications, further expanding their utility in diagnostic and theranostic medicine. As understanding of their interactions with biological systems deepens, carbon nanohorns are poised to play an increasingly prominent role in advancing biomedical imaging technologies.