and properties of biocompatible water-soluble silica ..

A reduction in the luminescent QY mediated by thiol binding can be mitigated by either crosslinking or adding layers to the surface []. Chan’s group [] first coated hydrophobic QDs with MPA and then crosslinked these ligands with lysine or diaminopimelic acid in the presence of dicyclohexylcarbodiimide. They later examined stability of these QDs in various biological conditions in order to optimize their behavior for various applications. This cost-effective, simple method was able to synthesize highly stable water-soluble QDs which maintained all of the hydrophobic QD optical properties. However, due to the relatively thick coating layer, the size was twice as big as the TOPO coated QDs.

Synthesis and properties of biocompatible water-soluble ..

and properties of biocompatible water-soluble silica-coated CdSe/ZnS ..

Journal of Nanoscience and Nanotechnology

High-quality QDs are typically prepared at elevated temperatures in organic solvents, such as tri-n-octylphosphine oxide and hexadecylamine (TOPO and HDA, both of which are high boiling-point solvents containing long alkyl chains). These hydrophobic organic molecules not only serve as the reaction media, but also coordinate with unsaturated metal atoms on the QD surface to prevent formation of bulk semiconductors. As a result, the nanoparticles are capped with a monolayer of the organic ligands and are soluble only in organic solvents such as chloroform and toluene. For biological applications, these hydrophobic dots are made water-soluble generally by three approaches, ligand exchange, silica shell capping, and the recently developed amphiphilic polymer coating. The ligand exchange approach is easy to perform, but the resulting water-soluble QDs are only stable for a short period and its quantum yield decreases significantly, because the original hydrophobic surface ligands are replaced by hydrophilic ligands such as mercaptoacetic acid. The newly discovered amphiphilic polymer coating approach solved these problems by retaining the coordinating organic ligands on the QD surface. Typically, amphiphilic polymers contain both a hydrophobic segment or side-chain (mostly hydrocarbons) and a hydrophilic segment or group (such as polyethylene glycol or multiple carboxylate groups). A number of polymers have been reported including octylamine-modified low molecular weight polyacrylic acid, polyethylene glycol (PEG) derivatized phospholipids, block copolymers, and polyanhydrides.- The hydrophobic domains strongly interact with TOPO on the QD surface, whereas the hydrophilic groups face outward and render QDs water soluble. Although the amphiphilic polymer coating represents the newest addition to the area of QD surface engineering and offers a number of advantages, silica shell capping remains as an attractive approach for QD solublization due to its stability, biocompatibility, and versatile surface chemistry. More importantly, the surface coating thickness can be precisely controlled in the range of 1-100s nm, which is difficult, if not impossible, to achieve based on the ligand exchange and amphiphilic polymer coating methods.

Surface modification, functionalization and …

SPION was synthesized by a co-precipitation method. 2.5 ml of a mixed iron solution in deionized water (2 mol/l FeCl2 and 1 mol/l FeCl3) was added to a 0.7 mol/l tetramethylammonium hydroxide (TMAOH) solution under vigorous stirring. The reaction was allowed to proceed open to the air at room temperature for 30 minutes while stirring. After 30 minutes, the black particles were separated from solution over a neodymium magnet and washed at least thrice with an equivalent volume of pH 12 TMAOH solution (so as to maintain the equivalent particle concentration as immediately after the reaction) until the particles were no longer magnetically separable. This colloidal suspension was sonicated for 10 minutes (Branson Digital Sonifier 450, Danbury, CT) and then 20 ml of the sonicated fluid was mixed with 20 ml pH 12 TMAOH and 160 ml ethanol. 7 ml tetraethylorthosilicate (TEOS) was then added to this suspension while stirring and allowed to react at room temperature while stirring for ~18 hours. The silica-coated SPION was then magnetically recovered from solution, washed thrice with ethanol and thrice with deionized water by magnetic decantation, and sonicated into deionized water for 10 minutes before further use. shows a Transmission electron microscopy (TEM) image of an aggregated silica-coated SPION: The size of a single nanoparticle (iron oxide core plus silica coating) is less than 20 nm. The size of the particles and the thickness of the silica shell were obtained using measurements from multiple TEM images of the sample, and the sizing results were then averaged to obtain the values presented here. The silica shell is visible as a thin layer of less-electron-dense material surrounding the more-electron-dense material of the SPION cores [].

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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.

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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.

Luminescent quantum dots for multiplexed biological detection ..