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Bharti, Shivani, et al. Synthesis and characterization of CdSe/ZnS quantum dots conjugated with poly (ethylene glycol) diamine. AIP Conference Proceedings. Vol. 1675. No. 1. AIP Publishing, 2015.
CdSe/ZnS quantum dots have recently emerged as a useful and attractive bimolecular labeling material due to their unique properties, including narrow emission bands, photostability, broad excitation spectra, and accessibility for multifunctionalization that allows site-specific targeting. PEG is a nontoxic, nonimmunogenic, highly water-soluble, and FDA-approved drug, so it can improve biocompatibility and stability and reduce many of the challenges that limit the use of quantum dots in biological applications.
Experimental Details
The synthesis part is divided into two steps: the first step involves the synthesis of core/shell CdSe/ZnS QDs, while the second step describes the PEGylation of CdSe/ZnS QDs with polyethylene glycol diamine.
Step I: Synthesis of CdSe and CdSe/ZnS core/shell QDs by chemical precipitation using cadmium acetate dihydrate, a water-soluble cadmium precursor. Sodium selenite has been used as a source of selenide ions. The selenium source is prepared by adding elemental selenium to a hot aqueous solution of sodium bisulfite, the chemical reaction is as follows:
Na2SO3 + Se → Na2SeSO3
The mixture was stirred magnetically at 80 °C for 5-6 hours. The pH of a solution containing 10 ml of 0.05 M (CH3COO)2Cd.2H2O solution and 10 ml of thioglycolic acid (MAA) solution was adjusted to 9-10 by adding freshly prepared 1 M NaOH solution. A selenium source was then added to the above prepared solution. The mechanism of the synthesis involves the reaction of selenide ions with metal ions to form seeds (nucleation).
Cd2+ + SeSO32-+ 2OH- → CdSe + SO42- + H2O
After the synthesis of CdSe core, ZnS shell was grown on CdSe by chemical method using zinc acetate dihydrate [(CH3COO)2Zn.2H2O] as precursor of Zn and sodium sulfide as precursor of sulfur.
Step II: Derivatization of carboxylic acid coated NPs The most obvious method is to couple chemical and biological moieties containing amine functional groups to the QD surface using water-soluble activators 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and NHS (N-hydroxysuccinimide). Solid NHS was added to the core/shell QDs and dissolved completely. Solid NH2-PEG-NH2 was added to this mixture and the pH was adjusted to 7.0-8.0. Finally, 2 ml of EDC aqueous solution was added dropwise and the mixture was stirred at pH 7.0-8.0 for 5-6 h. UV spectra of the prepared PEGylated QDs were recorded on a photophotometer. FTIR measurements were performed using an infrared spectrometer.
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