Amalgamation of complex iron(III) ions and iron nanoclusters with MWCNTs as a route to potential T(2) MRI contrast agents

Iron-filled multiwall carbon nanotubes (Fe@MWCNTs) were functionalized toward a variety of potential magnetic resonance imaging contrast agents. Oxidized Fe@MWNCTs were covered with PEG5000 via direct esterification or using acyl chloride derivatives. Alternatively, the latter were functionalized with an aminophenol ligand (Fe@O-MWCNT-L). Moreover, pristine Fe@MWCNTs were functionalized with N-phenylaziridine groups (Fe@f-MWCNT) via [2+1] cycloaddition of nitrene. All of these chemically modified nanotubes served as a vehicle for anchoring Fe(3+) ions. The new hybrids – Fe(III)/Fe@(f-/O-)MWCNTs – containing 6%–14% of the “tethered” Fe(3+)ions were studied in terms of the acceleration of relaxation of water protons in nuclear magnetic resonance. The highest transverse relaxivity r(2)=63.9±0.9 mL mg(−1) s(−1) was recorded for Fe(III)/Fe@O-MWCNT-L, while for Fe(III)/Fe@f-MWCNT, with r(2)=57.9±2.9 mL mg(−1) s(−1), the highest impact of the anchored Fe(III) ions was observed. The T(1)/T(2) ratio of 30–100 found for all of the nanotube hybrids presented in this work is a very important factor for their potential application as T(2) contrast agents. Increased stability of the hybrids was confirmed by ultraviolet–visible spectrophotometry.