<p>Oxygen Functional Groups on MWCNT Surface as Critical Factor Boosting T2 Relaxation Rate of Water Protons: Towards Improved CNT-Based Contrast Agents</p>

Abstract

Purpose: Salicyl (Sal)-among other oxygen functionalities-multi-walled carbon nanotubes (MWCNTs) and their nanohybrids are investigated as promising contrast agents (CA) in magnetic resonance imaging (MRI) or drug delivery platforms, due to their unique properties. The preliminary results and the literature reports were the motivation to endow high r 2 relaxivities, excellent dispersibility in water, and biocompatibility to superparamagnetic MWCNTs nanohybrids. It was hypothesized that these goals could be achieved by, not described in the literature yet, two-stage oxygen functionalization of MWCNTs. Results: Two structurally different MWCNT materials differing in diameters (44 and 12 nm) and the iron content (4.7% and 0.5%) are studied toward the functionalization effect on the T 2 relaxometric properties. MWCNT oxidation is typically the first step of functionalization resulting in "first generation" oxygen functional groups (OFGs) on the surface. Until now, the impact of OFGs on the relaxivity of MWCNT was not truly recognized, but this study sheds light on this issue. By follow-up functionalization of oxidized MWCNT with 4-azidosalicylic acid through [2+1] cycloaddition of the corresponding nitrene, "second generation" of oxygen functional groups is grafted onto the nanohybrid, ie, Sal functionality. Conclusion: The introduced OFGs are responsible for an almost 30% increase in the relaxivity, which leads to remarkable r 2 relaxivity of 951 mM −1 s −1 (419 (mg/mL) −1 s −1), the unprecedented value reported to date for this class of CAs. Also, the resulting nanohybrids express low cytotoxicity and superb diffusion after subcutaneous injection to a mouse.

A Comprehensive Study of Pristine and Calcined f-MWCNTs Functionalized by Nitrogen-Containing Functional Groups

“…Indeed, the maximum occurrence in ZFC curves can be associated with the blocking process of some Fe-complexes as nanoparticle catalyst residues. The observed M(T) variations are relatively consistent with previously published results [ 23 , 34 ]. Jamrozik et al [ 34 ] revealed that the M(T) dependence at a lower temperature for all studied nanotubes is different than expected for pure cementite, suggesting the influence of additional iron-based species, which lead to a slight increase in magnetization with decreasing temperature.…”

“…As the authors claim, the magnetization results demonstrate a predominant amount of iron in the ferromagnetically ordered (or in the form of superparamagnetic particles) cementite phase. Smaller magnetization values for MWCNTs samples than that of bulk Fe 3 C can also be caused by some additional diamagnetic elements such as oxygen present in the samples [ 23 , 34 ]. Indeed, as shown by XRD and XPS analysis, the diamagnetic elements are observed in all studied samples, but their relative contribution is reduced after calcination.…”

“…Nevertheless, CNTs, usually in their pristine form, tend to aggregate by forming agglomerates combined with van der Waals interactions [ 16 ]. Because pristine CNTs are hydrophobic and chemically inert, their functionalization with various suitable organic groups leads to new activities on the CNT surface for nanoparticle grafting, which enhances their hydrophilicity and dispersibility in several solvents [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. Understanding the impact of the functionalization type on the various properties of CNTs is one of the key factors for their potential future applications.…”

Effect of Nitrogen Atoms in the CNT Structure on the Gas Sensing Properties of PANI/CNT Composite

“…A further increase in the concentration of nitrogen atoms in CNTs makes no sense since this leads to a critical decrease in the length of nanotubes with a slight change in the electronic characteristics [26]. The presence of oxygen in the XPS spectra of all synthesized nanotubes is due to its adsorption from the atmosphere, as well as the presence of chemically bonded functional groups of various types, which are always present on the surface [27]. The high-energy C5 component of the spectrum corresponds to the state of carbon atoms in the composition of -COOH groups [25].…”

Applications of metal–phenolic networks in nanomedicine: a review

“…Its signal intensity is related to factors such as proton density, longitudinal (T 1 ) and transversal (T 2 ) relaxivity times and the cellular microenvironment and can be enhanced by the use of CAs. [131][132][133] Common Gd complexes, such as Magnevist, are widely used as CAs due to their long electron spin relaxation time but have disadvantages such as short cycle time in the body, low longitudinal relaxation (R 1 ) and single function. Using plant polyphenols as organic ligands to synthesize Gd/ Fe-bimetallic-phenolic coordination polymer NPs can effectively enhance the R 1 , and the molar ratio of R 1 to Gd/Fe is positively correlated.…”