Biomineralization Proteins Mediate The Synthesis Of Magnetic Nanoparticles For MR Imaging

Iron oxide nanoparticles have been widely used in biomedical field,especially in magnetic resonance imaging(MRI)due to their high specific surface area,unique magnetic properties,good biocompatibility and metabolizability.MRI contrast agents are mainly divided into T1 contrast agents and T2 contrast agents.T1 contrast agents with positive enhancement are widely used in clinical high-resolution imaging.It is generally believed that iron oxide nanoparticles(IONPs)with ultra-small size(?5 nm)have T1 contrast enhancement effect.However,the process for the production of small-sized IONPs usually involves the use of organic solvents,resulting in IONPs with hydrophobic domains which may cause serious nonspecific effects.Exploring ligands on IONPs can confer hydrophilicity and biocompatibility,but undoubtedly increase the complexity of hydrophilic ligands replacement and reduce the synthesis efficiency.Therefore,it is still a great challenge to develop IONPs with better biocompatibility and T1 enhancement effect in a simple method.The magnetosomes formed through biomineralization give us inspiration.Magnetosome,magnetite(Fe3O4)nanocrystals enveloped by phospholipid bilayer membrane,is a special biomineralization product of magnetotactic bacteria.Compared to chemically synthesized magnetic nanoparticles(MNPs),magnetosome MNPs have the superiorities of unfaulted crystal,good size uniformity and biocompatibility.The biomineralization process of magnetosomes is strictly controlled by a various of mineralizing proteins,resulting in the formation of mineral particles with uniform crystalline and size.These biomineralizing proteins have specific spatial structure and sequence,good biocompatibility,monodispersity and stability,which make them ideal bio-templates for the synthesis of MNPs.The use of biomineralizing proteins to direct the "bottom up" synthesis is thus a good strategy to get high-performance nanomaterials with good biocompatibility and bioactivity.Moreover,this kind of biomimetic synthesis is repeatable and environmentally friendly.In this work,we selected two representative biomineralizing proteins,which named Mms6(Magnetosome membrane specific 6)and BSA(Bovine serum albumin).After exploring the reaction process,we successfully synthesized high-performance IONPs for T1 MRI via two different synthesis concepts.The amphiphilic Mms6 protein is a key protein controlling the magnetsome formation in magnetotactic bacteria.And BSA protein is rich in carboxyl,hydroxyl and sulfhydryl groups,which provide a large number of binding sites for inorganic nanoparticles,with the synthetic products of good stability and biocompatibility.In the first part,we described a novel approach to produce magnetosome-like magnetic nanoparticles by combining Mms6 protein with the reverse micelle system to construct a nanoreactor similar to the magnetosome vesicle.This nanoreactor system can effectively improve Mms6 protein regulating crystal formation function.Magnetosome-like MNPs show the same crystalline form as natural magnetosomes and high-performance magnetic properties that rapidly respond to external magnetic field.Moreover,magnetosome-like nanoparticles coated with DSPE-mPEG,which avoid the disadvantages of natural magnetosome MNPs,possess excellent monodispersity,good water solubility and small hydrodynamic diameters.During MRI scans,the magnetosome-like MNPs with strong magnetic targeting ability were enriched in large quantities in the lesion area,significantly improving the T1-weighted MR imaging effect.All these advantages make the magnetosome-like MNPs highly promising for biomedical applications.In the second part,we successfully synthesized ultra-small sized(0.35 nm)Fe2O3@BSA nanoparticles using BSA nanocage,exhibiting highly uniform and monodispersity.Interestingly,we also found and confirmed that a well-defined spherical cage-like structure were self-assembled by approximately 6-7 BSA subunits.The Fe2O3@BSA nanoparticles showed a high r1 value of 6.8 mM-1s-1 and a low r2/r1 ratio of 10.6 at 3.0 T magnetic field.Compared with the commercially available Gd-DTPA,Fe2O3@BSA nanoparticles had brighter signal and longer angiographic effect,which is beneficial to steady-state and high-resolution MR imaging.In vitro and in vivo biostability,toxicity and renal clearance evaluations revealed that,the Fe2O3@BSA exhibit high biocompatibility,and can be easily eliminated without any kidney damage and obvious side effects at the right dosage,showing the great potential as a secure T1 contrast agent.This work provides an unprecedented avenue for high-quality magnetic nanomaterials and sheds lights on understanding the role of proteins at the organic/inorganic interfaces in the biomineralization process.