Carbon and its numerous allotropes, e.g. graphite, diamond, and fullerene, are materials of many facettes. Due to their characteristics in structure and properties, carbon materials show significant application potential ranging from material science to medical research.

The most common carbon compound graphene is formed by a regular infinite hexagonal mesh, as all carbon atoms are sp² hybridized, so that each carbon has three covalent bonds directed to three other carbons in the same plane. The fourth valence electron is delocalized. Multiple layers of graphene are forming graphite with the adjacent layers being weakly bonded through van der Waals forces. Single-wall carbon nanotubes (SWCNT) can be understood as cutouts of graphene, rolled-up to form a hollow cylinder. Multi-walled nanotubes (MWNT) are formed by multiple concentric tubes of nested single-wall nanotubes of different diameter.

Fullerene typically consists of 60 carbon atoms forming the shape of a hollow ball. The addition of further carbon atoms to the equator results in rather oblong structures.

 → Discover our portfolio on fullerenes!

The hydrophobic nature of these materials can be exploited, e.g. to block hydrophobic binding pockets of enzymes and proteins. Besides, all these systems are “benzene”-like, thus forming a two-dimensional π-double bond electron rich system, which explains the capability to scavenge radicals and act as antioxidants.

Carbon polymers range from flat common graphene over nanotubes consisting of a single wall (SWNT) and multiple walls (MWNT) to Buckminster fullerenes.

Despite multiple applications, the all-carbon structure of any carbon compound is hardly reactive and cannot be addressed by conventional chemical reactions. However, by plasma treatment technologies, formerly inert surfaces can be decorated with reactive chemical functionalities.

Carbon materials can be treated with ammonia plasma introducing amino functions on the surface. The degree of functionalization can be monitored via X-ray photoelectron spectroscopy (XPS). Attachment of any self-immolative or permanent linker or linker-cargo conjugate can be carried out by amide bond formation.

→ For more details about surface modification, see our previous blog “Functionalization of (Inert) Polymeric Surfaces”! 

Another option is represented by photochemical surface modification. Aryl azides are well-known precursors of nitrenes and have been introduced by Fleet et al. as versatile photoaffinity labeling agents to probe biological receptors. Upon photolysis, dinitrogen is liberated and a highly reactive singlet phenylnitrene is being formed in situ, which reacts rather randomly with neighboring molecules. In contrast, perfluorophenyl azides stabilize the nitrene intermediate which then undergoes rather controlled insertion and addition reactions in moderate to good yields instead of intermolecular rearrangements. This type of compounds has been used as photo cross-linker (λ_max = 258 nm) in estrogen receptor studies and for direct surface coating of carbon and organic-based polymers. Subsequent attachment of any kind of cargo molecule with or without linker can easily be realized by forming a stable amide bond. The cargo can either be directly conjugated to the surface or combined with a linear, permanent PEG spacer, which can balance the hydrophobicity of the hydrophobic carbon carrier.

Fluorinated aryl azides release dinitrogen after irradiation with UV light yielding a stabilized nitrene. This undergoes addition to double bonds forming a stable conjugation to the fullerene surface. Subsequent amide bond formation with any amino terminated (permanent or self-immolative) linker molecule or linker-cargo conjugate delivers a fullerene-cargo conjugate.

→ For more details about linker attachment to carbon materials, click here! 

→ Interested in our linker technologies? Download our brochure Linkerology® 

References:

Fullerene-based delivery systems; H. Kazemzadeh, M. Mozafari; Drug Discov Today 2019; 24: 898-905. https://doi.org/10.1016/j.drudis.2019.01.013

Water-soluble fullerenes for medical applications; I. Rašović; Materials Science and Technology 2016; 33: 777-794. https://doi.org/10.1080/02670836.2016.1198114

Anti-influenza activity of c60 fullerene derivatives; M. Shoji, E. Takahashi, D. Hatakeyama, Y. Iwai, Y. Morita, R. Shirayama, N. Echigo, H. Kido, S. Nakamura, T. Mashino, T. Okutani, T. Kuzuhara; PLoS One 2013; 8: e66337. https://doi.org/10.1371/journal.pone.0066337

Fullerenol Nanoparticles: Toxicity and Antioxidant Activity; R. Injac, M. Prijatelj and B. Strukelj; Oxidative Stress and Nanotechnology: Methods and Protocols D. Armstrong, D. J. Bharali 2013: 75-100. https://doi.org/10.1007/978-1-62703-475-3_5

Medicinal chemistry and pharmacological potential of fullerenes and carbon nanotubes; F. Cataldo, T. Da Ros; Springer Science & Business Media, 2008; 1.

Medicinal applications of fullerenes; R. Bakry, R. M. Vallant, M. Najam-ul-Haq, M. Rainer, Z. Szabo, C. W. Huck, G. K. Bonn; International journal of nanomedicine 2007; 2: 639-49. https://doi.org/10.2147/IJN.S2.4.639

Comparison of Atmospheric-Pressure Plasma versus Low-Pressure RF Plasma for Surface Functionalization of PTFE for Biomedical Applications; C. Sarra-Bournet, S. Turgeon, D. Mantovani, G. Laroche; Plasma Processes and Polymers 2006; 3: 506-515. https://doi.org/10.1002/ppap.200600012

Ammonia RF−Plasma on PTFE Surfaces: Chemical Characterization of the Species Created on the Surface by Vapor−Phase Chemical Derivatization; P. Chevallier, M. Castonguay, S. Turgeon, N. Dubrulle, D. Mantovani, P. H. McBreen, J. C. Wittmann, G. Laroche; The Journal of Physical Chemistry B 2001; 105: 12490-12497. https://doi.org/10.1021/jp011607k

XPS and ToF-SIMS investigation of alpha-helical and beta-strand peptide adsorption onto SAMs; J. S. Apte, G. Collier, R. A. Latour, L. J. Gamble and D. G. Castner; Langmuir 2010; 26: 3423-32. https://doi.org/10.1021/la902888y

N-Hydroxysuccinimide Ester Functionalized Perfluorophenyl Azides as Novel Photoactive Heterobifunctional Crosslinking Reagents. The Covalent Immobilization of Biomolecules to Polymer Surfaces; M. Yan, S. X. Cai, M. N. Wybourne and J. F. W. Keana; Bioconjugate Chemistry 1994; 5: 151-157. https://doi.org/10.1021/bc00026a007

Perfluorophenyl azides: new applications in surface functionalization and nanomaterial synthesis; L. H. Liu and M. Yan; Acc Chem Res 2010; 43: 1434-43. https://doi.org/10.1021/ar100066t

Affinity Labelling of Antibodies with Aryl Nitrene as Reactive Group; G. W. J. Fleet, R. R. Porter, J. R. Knowles; Nature 1969; 224: 511-512. https://doi.org/10.1038/224511a0