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Some examples of this approach include the work of Niezabitowska et al. [102] that employed sodium dodecyl sulfate as anionic surfactant to disperse carbon nanotubes and polycaprolactone. Negri et al. [103] used SWCNT oxidized with 2% SDBS suspensions to generate nanotubular paramagnetic probes, with potential utility as contrast agents (CAs) in magnetic resonance imaging (MRI). Cerpa et al. [104] used a SDBS solution to disperse single-walled carbon nanotubes to obtain anisotropic relaxation probes for di usion MRI studies. Bharti et al. [105] compared the role of a cationic an anionic surfactants in the synthesis and resulting properties of MWCNTs decorated with bi-metallic Pt–Pd nanoparticles. Yasujima [106] used Triton X-100® to disperse CNTs in a bioanode preparation process in a multienzyme immobilized carbon-felt electrode. Martinez-Paz et al. [107] employed 0.015% Pluronic F68 culture medium solution to obtain a homogenous suspension of oxidized MWCNTs in order to determine the possible toxic e ects in invertebrate Chironomus riparius caused by CNTs environmental dispersion.

2.2.3 Biomolecules

The non-covalent functionalization by attaching biomolecules to the CNTs surfaces is attracting great attention in biomedical research because of their promising preclinical possibilities. With this aim, di erent biological molecules and macromolecules can be bound to the nanotubes, like polypeptides, DNA bases, DNA oligonucleotides, amino acids, phospholipids, etc.

2.2.3.1  Proteins  Carbon nanotubes can interact directly with proteins through – stacking of their aromatic residues (Trp, Phe, and Tyr) enhancing their absorptivity and biocompatibility [108, 109] and so increasing the possibilities of being used in preclinical evaluations. A di erent approach uses a bridge to anchor proteins to CNTs, either through covalent modifications of nanotubes [110], or by a non-covalent method employing pyrene derivatives [90].

2.2.3.2  DNA Derivatives  DNA can bind to carbon nanotubes, forming helices around them [111] or can form non-covalent conjugates through the – staking with the aromatic bases [30]. DNA-functionalized CNTs can be used as biological transporters and also as biosensors [111, 112].

2.2.3.3  Phospholipids  Due to their amphiphilic nature, phospholipids can be used as surfactants to solubilize CNTs. Lysophospholipids, or single-chained phospholipids, are very e cient in this task [113]. In this case, the lipid part wraps the nanotubes as striations, whereas the hydrophilic part provides CNT solubility and biocompatibility. Using this approach, phospholipid-polyethylene glycol (PEG) is employed to functionalize SWCNT for a range of di erent biomedical applications [114] (Fig. 3). In a di erent approach, lipid bilayers can encapsulate CNTs, creating a model to study di erent biological process occurring at the cell membranes [115].

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