Surface functionalization is a critical step in tailoring the properties of carbon nanohorns for specific applications. Carbon nanohorns, consisting of conical graphene tubules aggregated into spherical structures, possess unique physical and chemical characteristics. However, their inherent hydrophobicity and tendency to aggregate limit their utility in biological and catalytic systems. Functionalization addresses these challenges by introducing chemical groups that enhance solubility, biocompatibility, and reactivity. Two primary approaches are employed: covalent and non-covalent modification.

Covalent functionalization involves the formation of chemical bonds between functional groups and the carbon nanohorn surface. This method provides stable and controllable modifications. One common strategy is oxidation, which introduces carboxyl, hydroxyl, and carbonyl groups onto the nanohorn surface. These oxygen-containing groups improve hydrophilicity, enabling dispersion in aqueous solutions. For instance, nitric acid treatment generates carboxyl groups, which can further react with amines or alcohols to form amide or ester linkages. This stepwise modification is particularly useful for attaching biomolecules, such as proteins or drugs, for targeted delivery systems.

Another covalent approach involves cycloaddition reactions, such as the 1,3-dipolar cycloaddition of azomethine ylides, which introduces pyrrolidine rings onto the nanohorn surface. This method preserves the sp2 carbon network while adding functional handles for further derivatization. Additionally, radical reactions using diazonium salts can graft aryl groups onto nanohorns, enhancing their electronic properties for catalytic applications. The choice of covalent method depends on the desired functionality and the need to maintain the structural integrity of the nanohorns.

Non-covalent functionalization relies on physical interactions, such as van der Waals forces, π-π stacking, or hydrophobic effects, to adsorb molecules onto the nanohorn surface. This approach avoids disrupting the carbon lattice, preserving the electronic and mechanical properties of the nanohorns. Surfactants, such as sodium dodecyl sulfate or pluronic polymers, are often used to stabilize nanohorn dispersions in water. These amphiphilic molecules adsorb onto the hydrophobic surface, with their hydrophilic tails extending into the solvent, preventing aggregation.

Polymer wrapping is another non-covalent strategy, where conjugated polymers, such as polyethylene glycol or polyvinylpyrrolidone, interact with the nanohorn surface through π-π stacking. This method not only improves solubility but also provides a platform for further functionalization. For example, PEGylated nanohorns exhibit prolonged circulation times in biological systems, making them suitable for drug delivery. Similarly, aromatic molecules, like pyrene derivatives, can anchor functional groups to the nanohorn surface through π-π interactions, enabling the attachment of targeting ligands or catalytic moieties.

The introduction of functional groups plays a pivotal role in enhancing the performance of carbon nanohorns in various applications. Carboxyl and hydroxyl groups improve biocompatibility by reducing nonspecific protein adsorption and cellular toxicity. These groups also serve as anchoring sites for covalent conjugation of drugs or imaging agents. For instance, doxorubicin, a chemotherapeutic agent, can be attached to carboxylated nanohorns via amide bonds, enabling controlled release in tumor environments. The presence of hydrophilic groups also facilitates the formation of stable colloidal suspensions, essential for intravenous administration.

In catalysis, functionalized nanohorns act as supports for metal nanoparticles or organocatalysts. The oxygen-containing groups on oxidized nanohorns can stabilize metal ions, which are subsequently reduced to form nanoparticles. For example, platinum nanoparticles supported on functionalized nanohorns exhibit high activity in hydrogenation reactions due to the uniform dispersion and strong metal-support interaction. Similarly, palladium-loaded nanohorns demonstrate excellent performance in cross-coupling reactions, with the functional groups preventing nanoparticle aggregation and leaching.

The unique structure of carbon nanohorns, with their high surface area and porous network, further enhances their utility in drug delivery and catalysis. The conical tips and interstitial spaces provide numerous sites for functionalization and guest molecule incorporation. In drug delivery, this allows for high loading capacities and sustained release profiles. For example, functionalized nanohorns loaded with anticancer drugs show enhanced uptake in tumor cells, attributed to the targeting ligands and improved solubility. The drugs can be released in response to stimuli such as pH or temperature, enabling site-specific therapy.

In catalytic applications, the porous structure of nanohorns facilitates mass transport, while the functional groups ensure active site accessibility. The combination of covalent and non-covalent functionalization can be employed to design multifunctional catalysts. For instance, a hybrid system with covalently attached organocatalysts and non-covalently adsorbed metal nanoparticles can perform tandem reactions, leveraging the strengths of both components. The functional groups also influence the electronic environment of the active sites, modulating reactivity and selectivity.

Surface functionalization also addresses challenges related to nanohorn aggregation and stability. Covalent modification introduces repulsive electrostatic or steric interactions that prevent aggregation in aqueous or organic solvents. Non-covalent methods, such as polymer wrapping or surfactant adsorption, provide dynamic stabilization, allowing reversible disassembly and reassembly under different conditions. This tunability is crucial for applications requiring precise control over nanohorn dispersion and interaction with other components.

The choice between covalent and non-covalent functionalization depends on the application requirements. Covalent methods offer permanent and well-defined modifications but may alter the intrinsic properties of the nanohorns. Non-covalent methods preserve the carbon lattice but may lack the stability needed for harsh conditions. A combination of both approaches can be used to achieve the desired balance between functionality and performance.

In summary, surface functionalization of carbon nanohorns is a versatile tool for enhancing their solubility, biocompatibility, and reactivity. Covalent methods provide stable and specific modifications, while non-covalent approaches preserve the nanohorn structure and enable dynamic interactions. The introduced functional groups play a critical role in drug delivery and catalytic applications, enabling targeted therapies and efficient chemical transformations. By carefully selecting the functionalization strategy, carbon nanohorns can be tailored to meet the demands of diverse technological and biomedical applications.