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Table 2 The commonly used materials for MNPs functionalization

From: Pharmacokinetics of magnetic iron oxide nanoparticles for medical applications

Organic compounds

Coating type

Advantages/applications

Monolayers and small molecules

Citrates

Stabilizers MNPs; the free carboxylic groups render a sufficient negative charge on the surface of particles making them hydrophilic

Folic acid

Effective tumor targeting agent

Phosphates

Surfactant and stabilizer for nanoparticle dispersion; affords efficient binding ligands on the surface of MNPs

Amines and aminosilanes

Stabilizing agents in the fabrication of various functionalized MNPs; provide -NH2 groups for subsequent functionalization and attaching drugs; used for drug delivery applications

Thiols

Very high binding affinity towards MNPs; used to functionalize MNPs for targeted drug delivery

Macromolecules

Synthetic polymers

Polyethylene glycol (PEG)

Enhances the hydrophilicity and water solubility; provides stability and prevents MNPs agglomeration; PEGylated nanoparticles show a lower cell uptake rate by macrophages, which increases the circulation time in blood; provides -OH groups; used as T1 MRI contrast agents and drug delivery systems

Poli vinyl alcohol (PVA)

Prevents agglomeration, giving rise to monodispersibility

Alginate

Improves the stability and biocompatibility

Natural polymers

Dextran

Enables optimum polar interactions with iron oxide surfaces, improves the blood circulation time, stability and biocompatibility

Chitosan

Enhances the biocompatibility and stability; provides functional groups: -NH2 and -OH for subsequent functionalization and attaching drugs; hydrophilic; good for drug delivery applications; widely used as non-viral gene delivery system

Starch

Improves the biocompatibility; good for MRI and drug target delivery

Inorganic compounds

Silica

Enhances the biocompatibility and stability of the nanoparticles; the mesoporous silica coating is biocompatible and offers high controlled porosity; good for drug delivery applications; useful in the fabrication of multifunctional MNPs

Metals

The most popular approach in this category is the conjugation of Fe3O4 with gold because of its biocompatibility and multifunctionality; the final applications are numerous: medical imaging (MRI, CT, PA), radiosensitiation, radiofrequency ablation, biosensing, cell sorting

Metal oxides

Metal oxide (ZnO, TiO2) functionalization has photocatalytic applications