Highly monodisperse sodium citrate-coated spherical silver nanoparticles (Ag NPs) with controlled sizes ranging from 10 to 200 nm have been synthesized by following a kinetically controlled seeded-growth approach via the reduction of silver nitrate by the combination of two chemical reducing agents: sodium citrate and tannic acid. The use of traces of tannic acid is fundamental in the synthesis of silver seeds, with an unprecedented (nanometric resolution) narrow size distribution that becomes even narrower, by size focusing, during the growth process. The homogeneous growth of Ag seeds is kinetically controlled by adjusting reaction parameters: concentrations of reducing agents, temperature, silver precursor to seed ratio, and pH. This method produces long-term stable aqueous colloidal dispersions of Ag NPs with narrow size distributions, relatively high concentrations (up to 6 × 1012 NPs/mL), and, more important, readily accessible surfaces. This was proved by studying the catalytic properties of as-synthesized Ag NPs using the reduction of Rhodamine B (RhB) by sodium borohydride as a model reaction system. As a result, we show the ability of citrate-stabilized Ag NPs to act as very efficient catalysts for the degradation of RhB while the coating with a polyvinylpyrrolidone (PVP) layer dramatically decreased the reaction rate.
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.
Surface modification, functionalization and …
Considerable effort has been made toward the research and development of multifunctional nanoparticle systems for cancer targeted imaging and therapy. Among the nanoparticle platforms, magnetic nanoparticle-based theranostic systems show particular promise. The intrinsic magnetic properties lend themselves to diagnostic MRI applications, and the surfaces may be easily modified using a variety of targeting moieties, such as antibodies, peptides, small molecules, aptamers, or therapeutic agents through a number of conjugation strategies. Furthermore, magnetic nanoparticles unlike other inorganic nanoparticles such as gold- and carbon-based ones can be degraded to Fe ions in the body particularly in the acidic compartments of cells (e.g. lysosomes), alleviating the potential toxicity of long-term residence of nanoparticles. Therefore, magnetic nanoparticle formulations are considered valuable tool for disease diagnosis with anatomical details at an early stage, delivery of therapeutic agents to the target tumor sites, real-time monitoring of therapeutic responses, thereby currently attracting increasing interest for clinical utility. For the preparation of nanoparticles in potential clinical uses, it is critical to be aware of the design parameters discussed in this review. Better characterization methodologies are also needed, including pharmacokinetics and long-term toxicity studies. The costs required to integrate multiple components is an additional consideration for commercial viability. Although all such considerations are to be resolved in the near future, one should consider the regulatory hurdles encountered during clinical trials. The required regulatory processes become more complexed with the multifunctionalities of the nanoparticles because they are consisted of multiple additional components and also claim multiple indications with single nanoparticle. Nontheless, there is high probability that cancer-specific nanoprobes arming with therapeutic capabilities are to be used in clinic in the near future because they are able to meet unmet medical needs for effective cancer treatments with minimal adverse effects.