Synthesis and properties of biocompatible water-soluble ..
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 .