Antibodies are proteins with special Y- shapes. They can specifically recognize and bind to antigens in both the recognition phase (cellular receptors) and during the effectors phase (synthesis and secretion) of humoral immunity. Active targeting by monoclonal antibodies (mAbs) combined with SPION is a promising technology for magnetic resonance imaging.
Ionic liquids are considered as organic solvents with high polarity and a pre-organized solvent structure. Crystalline NPs can be synthesized by using ionic liquid at ambient temperature . Warner and coworkers have developed magnetic NPs coated with a group of uniform materials based on organic salts by using ion exchange water-in-oil (w/o) microemulsion approach . These MNPs could potentially be applied in drug delivery, MRI and protein separations.
In addition to organic coatings, core-shell structures, such as biocompatible silica- or gold-covered magnetic nanoparticles, have provided an attractive approach to developing stealth nanoparticles. Silica shells serve as protective stable nanoparticle coatings under aqueous conditions. The ability to encapsulate functional molecules within the nanoparticle matrix is a unique feature of these nanostructures. Hyeon and Moon developed Fe3O4 nanocrystal-embedded, core-shell mesoporous silica nanoparticles, and they demonstrated their multifunctional application to simultaneous MR/optical imaging and drug delivery . This study suggested a precise method for controlling the size of the silica nanoparticles smaller than 100 nm. The surfactant cetyltrimethylammonium bromide (CTAB) provided an organic template for the formation of a mesoporous silica shell and stabilized the hydrophobic Fe3O4 nanocrystals in an aqueous solution. The sol-gel process occurred through the template by using tetraethylorthosilicate (TEOS) and rhodamine B isothiocyanate (RITC)-labeled aminopropyltriethoxysilane (APS), and generated amine groups containing silica shell, to which PEG was covalently conjugated via succinimidyl end group to render further biocompatibility. Dox molecules loaded onto the as-synthesized Fe3O4@mSiO2(R)-PEG NPs to convey therapeutic properties. The core-shell structure exhibited magnetic and fluorescent properties, as well as a therapeutic index, suggesting the utility of the nanostructure in biomedical theranostic applications. On the other hand, gold provides several advantages as a coating material due to its inertness and its unique ability to absorb near-IR radiation. Hyeon and Cho described magnetic gold nanoshells (Mag-GNS) consisting of gold nanoshells encapsulating magnetic Fe3O4 nanoparticles as a novel nanomedical platform for simultaneous diagnostic imaging and thermal therapy . Monodisperse 7 nm Fe3O4 nanoparticles stabilized with 2-bromo-2-methylpropionic acid (BMPA) were covalently attached to amino-modified silica spheres through a direct nucleophilic substitution reaction between the bromo groups and the amino groups. Gold seed nanoparticles were then attached to the residual amino groups of the silica spheres. Finally, a complete 15 nm thick gold shell embedded with Fe3O4 nanoparticles formed around the silica spheres to generate Mag-GNS. To target breast cancer, an anti-HER2/neu antibody was conjugated onto the surfaces of the Mag-GNS. SKBR3 breast cancer cells treated with Mag-GNS could be detected using a clinical MRI system, followed by selective destruction by near-IR radiation.
In general, nanoparticles tend to aggregate through hydrophobic interactions or attractive van der Waals forces in an effort to minimize the surface energy. In the blood stream, such aggregates can trigger opsonization, the process by which a particle becomes covered with opsonin proteins, thereby making it more visible to the mononuclear phagocytic system (MPS), such as RES. The phagocytic mechanisms render nanoparticles ineffective as theranostic devices by removing them from the bloodstream . Therefore, evading uptake by RES and increasing the blood circulation half-life are major challenges for developing theranostic nanoparticles in clinical applications . Several methods of camouflaging nanoparticles have been developed to yield 'stealth' nanoparticles, which are invisible to MPS. These approaches interfere with the binding of opsonin proteins to the nanoparticle surfaces in support of a long circulation half-life, thereby increasing the chance that the nanoparticles can effectively target tumor sites. In order to impart stealth properties to the nanoparticles, one of the most promising molecules is the FDA-approved PEG. Natural or synthetic polymers, small organic molecules, and core-shell structures have also been utilized for nanoparticle surface coatings. However, a high surface coverage can decrease binding to and uptake by target cancer cells. This section describes the use of several coating molecules as shielding materials. The optimal surface densities of the coating materials and the targeted ligands will be discussed.
Recently, Chen and Sun used the Mannich reaction to couple biomolecules to nanoparticles . In this work, iron oxide nanoparticles functionalized with active hydrogen groups were reacted with amine group-containing cyclic RGD peptides to develop ultrasmall c(RGDyK)-coated Fe3O4 nanoparticles (with an 8.4 ± 1.0 nm hydrodynamic diameter) as tumor-targeted imaging agents. Nonspecific uptake of the iron oxide nanoparticles by RES in the blood stream complicates the development of small biocompatible nanoparticles with targeting capabilities. Initially, they synthesized Fe3O4 nanoparticles via the thermal decomposition of Fe(CO)5 in benzyl ether in the presence of 4-methylcatecol, as a surfactant, followed by air oxidation. The 4-methylcatecol formed a tight thin coating layer over the nanoparticle surface via formation of a strong chelating bond between the iron and the catechol unit. The aromatic ring of the 4-methylcatecol on the nanoparticles was directly coupled with the amine group of a lysine residue in the cyclic RGD peptide, c(RGDyK) (Figure ). High-resolution transmission electron microscopy (HRTEM) images of the nanoparticles indicated an iron oxide core size of 4.5 nm and a coating layer containing the c(RGDyK) peptide 2 nm in thickness, close to the size in water.
Riboflavin is an essential vitamin for cellular metabolism, and the riboflavin carrier protein (RCP) is highly upregulated in metabolically active cells [,]. Thus, flavin mononucleotide (FMN), an endogenous RCP ligand, was used as a small molecule targeting ligand for metabolically active cancer or endothelial cells. Kiessling and co-workers synthesized FMN-coated ultrasmall superparamagnetic iron oxide nanoparticles (FLUSPIO) as MRI/optical dual probes for cancer detection . USPIO was coated with FMN through the phosphate groups of FMN, and guanosine monophosphate was added to stabilize the colloid. The hydrodynamic radius of FLUSPIO was 97 ± 3 nm, and an intense fluorescence emission band was observed at 530 nm due to FMN. cellular uptake of FLUSPIO was investigated by MRI (3T), TEM, and fluorescence microscopy of PC3 cells and HUVEC cells. Both PC3 cells and HUVEC cells showed a significantly higher R2 relaxation rate after 1 h incubation with FLUSPIO than with nontargeted USPIO. Such an uptake was considerably reduced by competitive blocking of RCP with free FMN. A strong green fluorescence in the cells was observed after FLUSPIO incubation. The perinuclear fluorescence signal suggested endosomal localization of the nanoparticles, consistent with TEM results, suggesting that FMN could serve as a versatile building block for generating tumor-targeted imaging and therapeutic modalities.
The research on superparamagnetic iron oxide nanoparticles (SPIONs) has been growing exponentially over the last several years. The field continues to drive in the direction of biomedical applications, especially molecular therapeutics by exploiting the immense qualities of SPIONs . This includes the distinctive controllable properties such as size, shape, magnetism, crystallinity and flexibility in fabricating multifunctional SPIONs with fluorescence, targeting ligands, drugs etc, thanks to the advancements in the syntheses and functionalization techniques developed hitherto. There are some excellent synthetic methods in prior arts on the formation of superparamagnetic magnetite (Fe3O4) and maghemite (γ-Fe2O3) SPIONs, with size control, narrow distribution, water solubility and surface functionalization [-]. The co-precipitation method is a conventional synthetic paradigm where Fe(II) and Fe(III) salts are co-precipitated in a basic solution in the presence of coating materials such as polymer or dextran (or its derivatives). Although the resulted iron oxide nanoparticles (NPs) are larger in size (ca. 100 nm) and partially crystalline, the particles are readily water soluble where their surfaces are directly functionalized. Alternatively, thermal decomposition method using precursors such as Fe(CO)5, Fe(Stearate)2, with high boiling solvents (octadecene, benzyl ether) and surfactants/ligands (oleic acid, oleylamine) can be used to synthesize smaller sized hydrophobic SPIONs (5-10 nm). In order to impart the SPIONs with water solubility for biomedical applications, water-oil microemulsion method can be employed as a reaction medium for coating a hydrophilic ligand (e.g. silica, peptides) on the hydrophobic surface.
Folate-conjugated SPIONs were used for solid tumor targeting in specific magnetic hyperthermia mediators . The folate surface of maghemite NPs could recognize the folate receptor and was proven by folate receptor expressing cell lines or by radio-labeled folic acid in competitive binding experiments. Water-dispersible sugar-coated SPIONs were designed as magnetic fluid hyperthermia heat mediators and 2 negative contrast agents for MRI . SPIONs of ca. 16-18 nm had good transverse relaxivity and large heat release upon application of radio frequency (RF) electromagnetic radiation with amplitude and frequency close to the human tolerance limit. The authors claimed that these particles could be used as an efficient bifunctional targeting system for theranostic applications. Initially, the hydrophobic oleic acid stabilized SPIONs synthesized via thermal decomposition method were made water soluble by the displacement of the stabilizing agents, and by covalent grafting of the carbohydrate derivatives via the phosphonate function.