نانوشیمی و نانوتکنولوژی وکاربرد

Various surface modifications of QDs have been explored, among which the inorganic coating is one of the most popular strategies to provide dense surface protection. Passivation of the nanocrystals surface by a thin semiconductor shell didn’t significantly modify the absorption and emission wavelengths, but increased the quantum yield up to 50-70%, e.g., CdSe/ZnS,26 CdSe/CdS27 and ZnSe/ZnS28 core/shell structure. With higher band gap energy than that of the core, the ZnS or CdS shell didn’t absorb the emission from the core and helped eliminate the broadband emission of the QDs.29 The core-shell-shell nanocrystals were also developed, i.e., CdSe/CdS/ZnS, CdSe/ZnSe/ZnS,30 and CdSe/ZnTe/ZnS31 QDs. The middle shell allows considerable strain relaxation between the inner core and the outer shell, and helps to further improve the photoluminescence efficiency and photostability exceeding those of CdSe/ZnS nanocrystals.

Depending on the relative position of band gaps of the core and shell materials, there are two types of core/shell QDs, as illustrated by the schematics in Fig. 2.3. When the valence band (VB) edge of core is higher than that of shell and the conduction band (CB) edge of core is lower than that of shell, the electron (solid circle) and hole (open circle) are localized in the same region and can recombine very quickly. This is the so-called type I core/shell structure, as shown in Fig. 2.3 (A). On the contrary, if both the valence and conduction band edges of core are higher or lower than those of shell, the electron and hole are prone to being spatially separated between the core and shell for lower energy states, resulting in longer lifetime of excitons before recombination. This is the type II core/shell structure, as shown in Fig. 2.3 (B) and (C). Based on the relative band edge position of the semiconductor compounds,32 the CdS/ZnS core/shell QDs are type I, while the CdSe/CdS, CdTe/CdSe,33, 34 CdSe/ZnTe31 QDs are type II. Interestingly, the core/shell QDs can be tuned between type I and type II by continuously changing the shell thickness.35

Besides the semiconductor coating, many other methods for surface modification of QDs were proposed and studied. For example, the silica coated CdSe QDs were obtained by using 3-mercaptopropyl trimethoxysilane in a weak alkaline solution, to achieve the enhanced PL intensity and stability.36 The hydroxide shell can be generated under basic condition too, to cover the nanoparticles and enhance their emission intensity by passivation of surface state.37 The biocompatible modification and functionalization of QDs was realized via the direct reaction of organic silanes in toluene.38 Multidentate organic ligands consisting of various lengths of polyethylene glycol (PEG) and thioctic acid provided a method to achieve the aqueous solubility of QDs over a broad pH range based on the dihydrolipoic acid (DHLA) ligand motif.39 The homogeneous CdS and CdSe/polymer nanocomposites were prepared via in situ radical polymerization in miniemulsion, which maintained the optical properties of encapsulated QDs.40 It was also reported that the water-soluble QD micelles were synthesized through encapsulation of hydrophobic QDs into surfactant/lipid micelles, showing good monodisperse and biocompatibility.17 Conjugates of bovine serum albumin and L-cysteine capped CdTe nanoparticles were produced with a significant increase of the emission.41

Sometimes the QDs surface is modified with multiple layers in the sequence of inorganic coating, functional biomolecule or amphiphilic polymer, and linker molecule or receptor protein. As an example, Fig. 2.4 shows the QD core covered by a ZnS shell and DHLA capping molecules, and then conjugated with avidin which can specifically link with any biotinylated targets.42 The MBP-zb was used to regulate the number of avidin molecules present on each QD. In this case, the enhanced emission, improved stability and bioconjugation flexibility of QDs are all achieved. However, the ultra small size of QDs may be sacrificed as the complex become considerably big.