Lysine as Size-Control Additive in a Bioinspired Synthesis of Pure Superparamagnetic Magnetite Nanoparticles

Magnetite nanoparticles (MNPs) are being used in a number of nanotechnological applications, especially biomedical, both in diagnosis and in therapeutics such as hyperthermia agents and as drug nanocarriers for targeted chemotherapy. However, the development of efficient methodologies to produce novel MNPs with the specific requirements needed for biomedical applications is still challenging. In this context, biomimetic approaches taking use of magnetosome proteins expressed as recombinant and/or polyamino acids are becoming of great interest. In fact, these protocols give rise to magnetite nanoparticles of adequate size, magnetic properties and surface functionalization that make them compatible for biomedical applications. In this respect, herein we show for the first time that lysine (Lys), unlike other amino acids like arginine (Arg), is able to exert a control over the size of MNPs produced in water and at room temperature. This control occurs through the stabilization of the magnetite nuclei by the lateral ammonium group of Lys. The strength of such stabilization allows a further release of these previously bonded nuclei to allow the further growth of the larger ones, thus resulting in larger crystals compared to those obtained by using Arg or no amino acids at all. MNPs obtained by the mediation of this amino acid are fairly large (30 nm) while being superparamagnetic at room temperature. They present an isoelectric point of 4, which may allow the coupling/release of these MNPs to other molecules based on electrostatic interaction, a large magnetic moment per particle and high magnetization saturation. This study highlights the effects that biological additives have in the process of magnetite biomineralization and goes along the line of previous reports using magnetosome proteins and polyamino acids.