Compared with florescent protein and organic dyes, quantum dots (QDs) possesses several advantages in bioimaging such as high Quantum Yield (QY),tunable color emission from UV to NIR, low photobleaching and multicolor emission under single excitation wavelength [1].However, there are still several challenging problems in the biological application field of QDs. First, how to achieve highly bright and stable water soluble QDs just as their organically synthesized counterparts with high quantum yield and hydrophobic surface. Second, how to enhance the florescent brightness in terms of the single QD that usually used. Third, how to
prepare efficient QDs assemblies with nano-carriers for signal amplification and ultra sensitive labeling detection.
In current work, we demonstrated an effective and versatile silica coating strategy on hydrophobic
QDs based on organosilane micellization and silicate deposition. This method skipped the
conventional water solubilization step for oil-soluble QDs and greatly favored the fluorescence
preservation by confining the QDs in a lipophilic interior of a silica bead. The facile ultrasonic
fragmentation of organosilane precursor enabled an easy scaling over silica bead size and their
loading amount of hydrophobic contents. By this manner, we created a serious of CdSe/ZnS@SiO2 nano-spheres ranging from 16 nm to 38 nm with multi-core structure (enhance the brightness and reduce the blinking) and a tunable silica shell thickness [2]. These nano-spheres exhibited a quantum yield of 45% (90% relative to the oil sample), robust photochemical and colloidal stability in biological medium. We applied the transferrin labeled fluorescent beads for
targeted HeLa cell imaging (Fig. 1).
The coating strategy not only works for II-VI semiconductor QDs but also fulfill the water
solubilization of different surface capped QDs including the most promising and challenging
I-III-VI QDs [3]. In this case we incorporated single or multiple hydrophobic CuInS2/ZnS directly
into silica beads with a relative small of 17-25 nm. The silica coating maintained the emission
properties of CuInS2/ZnS regarding the photolumilescent spectrum, quantum yield above 60% in aqueous solution and the PL lifetime. The obtained CuInS2/ZnS@SiO2 nanoparticles exhibited
advanced PL stability in aqueous phase along with the low cytotoxicity (MTT test) which favored
their biomedical fluorescent imaging. More interestingly, the strategy is applicable for
hydrophobic assemblies such as silica@QDs nano-spheres with high packing density. This allows
the fluorescent nano-spheres water soluble with high quantum yield. One can use the assembly-silication-assembly circle to prepare fluorescent encoded nano-spheres, noble metal and magnetic particles encapsulated QDs@SiO2 multifunctional nanocomposites |
