Conjugation of polyethylenimine and its derivatives to carbon-encapsulated iron nanoparticles

Carbon-based nanomaterials functionalized by cationic polymers are interesting starting materials for the development of nanotheranostic systems. In this study, polyethylenimine (PEI) and its pre-synthesized derivatives were conjugated to carbon-encapsulated iron nanoparticles (CEINs). Branched PEIs of various molecular weight were derivatized. The aim of the polymer modification was to introduce carboxylic functionality to the PEI structure. Two different synthetic pathways were proposed: the amide-type reaction with succinic acid anhydride and reductive amination using p-formylbenzoic acid. The polyethylenimine derivatives were analyzed by means of spectroscopic methods (NMR and FT-IR). In order to determine the ratio of primary, secondary and tertiary amine groups in the modified polymers, inverted-gate C-13 NMR spectroscopy was applied. Next, CEINs modified with two different surface carboxylic linkers were functionalized using pristine PEI and its derivatives. The conjugation of the polymer to the surface-modified nanoparticles was carried out using the carbodiimide-amine type reaction. The success of the conjugation process was confirmed by thermogravimetry and infrared spectroscopy. The morphological details were analyzed using transmission electron microscopy, whilst the surface zeta potential and the average particle size were determined by dynamic light scattering. It was found that the molecular weight of the polymer and the type of the surface linker were the key factors which crucially influenced the functionalization yield and the physicochemical features of the synthesized nanoplatforms. The best dispersion stability in aqueous media and the smallest mean hydrodynamic particle size was found for CEINs with the longer carboxylic linker.