Abstract

Recently, the synthesis of low-toxic quantum dots (QDs) and their application in biomedicine have attracted considerable attention. ZnSe nanocrystals doped with Mn ions which do not contain any Class A element (Cd, Pb, and Hg) can be used as a new generation of luminescent nanocrystals due to their strong dopant emission. In 2001, Norris et al. presented an organometallic synthetic route for the preparation of Mn-doped ZnSe (Mn :ZnSe) nanocrystals and it was confirmed that the Mn impurities were embedded inside the nanocrystals. Peng et al. introduced nucleation-doping and growth-doping, two new synthetic strategies in high-temperature organometallic synthesis. Pure and strong dopant emission was observed due to the Mn T1( G) A1( S) transition. The Mn : ZnSe nanocrystals prepared by the organometallic method had high quantum yield (QY), high crystallinity and monodispersity. However, some organic reagents used in this procedure are environmentally unfriendly, and long reaction time and limited operation conditions were also necessary. Recently, an inorganic shell material with a wider band gap was used to passivate a cadmium chalcogenides quantum dot to reduce the bio-toxicity and improve the quantum yield. ZnS is a suitable shell material with a wide band gap (3.67 eV for bulk ZnS) for the formation of the core/shell nanostructure. To the best of our knowledge, no report has been published on the synthesis and optical properties of Mn : ZnSe/ZnS core/shell nanocrystals. Herein, a green and rapid route for the synthesis of low-toxic Mn :ZnSe/ZnS core/shell nanocrystals in the aqueous phase is presented. A sensing system for the detection of human IgG is established based on the FRET between the Mn :ZnSe/ZnS core/shell nanocrystals and AuNPs. Fig. 1 depicts the synthetic route for the Mn :ZnSe/ZnS core/shell nanocrystals. The Mn :ZnSe core nanocrystals with oleate capping ligands were firstly prepared via a microwaveassisted hydrothermal reaction for 40 min, and then the core nanocrystals reacted with mercaptopropionic acid (MPA). Zn ion is inclined to be a soft Lewis acid, and the RCOO group of oleate ligand is a hard Lewis base, while the RSH group in MPA is a soft Lewis base. The RSH groups prefer to bind to the Zn ions compared with RCOO because hard acids tend to associate with hard bases and soft acids with soft bases. So the surface ligands replacement of oleate capping ligands by MPA succeeded in the procedure, and the polar carboxylic groups renderd the nanocrystals water-soluble. An additional ZnS shell was deposited on the outer layer of the Mn :ZnSe to form the core/shell nanostructure. The detailed experiment for the preparation is elaborated in the Electronic Supplementary Information (ESI).w In a traditional aqueous synthesis, the growth rate of ZnSe QDs with MPA capping ligands was very slow by refluxing at 100 1C. However, microwave irradiation was fast and highly efficient for transferring energy into the reaction system and the temperature increased uniformly throughout the reactants. In our microwave irradiation reaction, high temperature (170 1C) to the advantage of doping Mn into the ZnSe nanocrystals lattice could be easily obtained in 5 min, and a fast and homogeneous nucleation process could be achieved, which improves the crystallinity of the Mn :ZnSe nanocrystals. The selection for using oleate capping ligands is appropriate at the high temperature, and conventional MPA can be partially decomposed at the temperature. In the procedure, the time required to attain good crystallinity and uniform size (about 5 nm) of Mn : ZnSe core nanocrystals was within one hour.

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