\ 15	Page 4 of 41	Topics in Current Chemistry	(2020) 378:15

Fig. 2  Carbon nanotube classifications. a According to the number of sheets, there are single- (SWCNTs) and multi-wall carbon nanotubes (MWCNTs); b depending on the rolling up of the sheets, they can be armchair, zig-zag, or chiral

In CNTs, each carbon atom is bonded with sp2 hybridization, stronger than the sp3 bonds in diamond, which provide these compounds with exceptional mechanical [18–20], electrical [21–24], optical [25], and thermal properties [26]. Other unique features owing to their large surface, needle shape, and residual metal impurities content placed them among the most promising nanomaterials for potential applications that range from nanomedicine to nanoelectronics, including the production of quantum dots by introducing fullerenes [27] and nanowires by filling CNT with pure elements for molecular electronics [28].

Regarding the use of carbon nanotubes in biomedicine and preclinical research, and due to their ability to cross the cell membrane, these materials has been tested as transporters for di erent drugs [29], biomolecules as enzymes, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) [30], as well as to form ion transport channels [31]. CNTs have also been used as nanoreactors, taking advantage of their reduced inner size and the special electronic characteristics due to the CNT wall curvature [32]. Nevertheless, despite their wide range of biomedical applications, carbon nanotubes have two deficiencies to solve: their inherent toxicity, due to the metal catalyst residue remaining from the synthesis process, and their low solubility in water.

On these grounds, we have focused this review on the presentation of the main functionalization processes, characterization methodologies, and biomedical applications of CNTs, identifying also the most relevant strategies to overcome the inherent problems that currently hinder their preclinical use.

2  Functionalization of Carbon Nanotubes

Carbon nanotubes are produced using several synthetic methods [33], but most of them left metallic impurities in the sample and render a mixture of CNTs with di erent diameters, lengths, and chirality properties. Additional problems are the

1 3