Synthesis and Properties of Biocompatible Water-Soluble Silica-Coated CdSe/ZnS Semiconductor Quantum Dots

Abstract

We describe the synthesis of water-soluble semiconductor nanoparticles, and discuss and characterize their properties. Hydrophobic CdSe/ZnS core/shell nanocrystals with a core size between 2 and 5 nm are embedded in a siloxane shell and functionalized with thiol and/or amine groups. Structural characterization by AFM indicates that the siloxane shell is 1-5 nm thick yielding final particle sizes of 6-17 nm, depending on the initial CdSe core size. The silica coating does not significantly modify the optical properties of the nanocrystals. Their fluorescence emission is about 32-35 nm FWHM, and can be tuned from blue to red with quantum yields up to 18%, mainly determined by the quantum yield of the underlying CdSe/ZnS nanocrystals. Silanized nanocrystals exhibit enhanced photochemical stability over organic fluorophores. They also display high stability in buffers at physiological conditions (>150 mM NaCl). The introduction of functionalized groups onto the siloxane surface would permit the conjugation of the nanocrystals to biological entities. 2 IntroductionRecent progress in wet chemistry has established a robust route towards the synthesis of highly luminescent semiconductor nanocrystals having sizes ranging from 1.5 to 8 nm. 1,2 By judiciously controlling the growth conditions, the size, and even the shape of II-VI nanocrystals can be accurately tailored. 3,4 The ability to control these parameters has a profound impact in materials science since it can be harnessed for engineering assemblies of nanometer scale units with novel characteristics. [5][6][7][8] At this point, the prominent focus has been the optical properties of these semiconductor nanocrystals. They are governed by strong quantum confinement effects and, therefore, are size dependent. 2,9,10 The absorption onset and fluorescence emission shift to larger energy with decreasing size.Moreover, while the absorption spectrum is a continuum from the bandgap into the UV, the emission pattern is narrow, symmetric, and does not depend on the excitation frequency. Several different sizes of nanocrystals can thus be excited simultaneously with a single excitation source, resulting in well-resolved colors of emission. Further passivation of the nanocrystal surface by a thin shell of a higher band gap material does not significantly modify the absorption and emission features but increases the nanoparticle quantum yield up to 50-70%. [11][12][13] The passivation shell also imparts an efficient photochemical stability, so that the photobleaching is reduced, and the number of photons a single nanocrystal can emit dramatically increases. The ability to discriminate many different colors simultaneously under long-term excitation holds great promise for fluorescent labeling technologies, especially in biology. 14,15 In this respect, organic dye molecules suffer from several limiting factors.First, their narrow absorption bands make it difficult to excite several colors with a single 3 excitation source. In addition, due to the large spectral overlaps between t...

Silica-Coated CdTe Quantum Dots Functionalized with Thiols for Bioconjugation to IgG Proteins

“…In the first report of silica shell growth around CdSe/ZnS QDs, it was suggested that the silica shell thickness around 4 nm nanocrystals was 1-2 nm, while its thickness around 6-7 nm QDs was 3-4 nm. 6 This interpretation is consistent with further fluorescence correlation spectroscopy (FCS) measurements that indicate that the hydrodynamic radius of silanized CdSe/ZnS QDs is dependent on the initial size of the underlying quantum dots. The same trend could be observed in the case of CdTe quantum dots.…”

“…Gradual quenching of the silanized nanoparticles was observed while silanized CdSe/ZnS synthesized by known literature methods were the control. 6 Bioconjugates in that particular gel were of thin-shelled CdTe QDs and had not been subjected to ethanol precipitation. The quenching was highly reproducible from silanized sample to sample and showed the need to thicken the silica shell.…”

“…prepared nanocrystals. 6 Silica growth on the surface of CdTe in this instance was specifically done to decrease fluorescence quenching in biological buffers, create a platform to modify the surface, and enhance the biocompatibility of the nanocrystals.…”

Cellular Effect of High Doses of Silica-Coated Quantum Dot Profiled with High Throughput Gene Expression Analysis and High Content Cellomics Measurements

“…28 While the core/shell Qdots are only ~4-5 nm in size, the silane shell adds 2-3 nm in thickness and thus silanized Qdots are ~8-10 nm in diameter. 29 Such Qdot chemistry was observed to pose minimal toxicity to breast cancer cells when the cells were exposed to a solution containing 2-10 nM of PEG-silane-Qdots. 26 Human lung (IMR-90) and skin epithelial (HSF-42) cells were exposed for 48 h to a medium containing 8 or 80 nM Qdots or to an equivalent amount of 10 mM phosphate buffer as a control.…”

Photochemical Fine-Tuning of Luminescent Color of Cadmium Selenide Nanoparticles: Fabricating a Single-Source Multicolor Luminophore

“…This was dissolved in 50 mm 3 of 3-mercaptopropyltrimethoxysilane (MPTS) to modify the surface of CdSe nanoparticles (MPTS/ CdSe), where thiol groups of MPTS molecules were strongly bound to Cd 2+ sites of nanoparticles. 15 A colloidal solution of water-soluble SiO 2 -coated CdSe nanoparticles (SiO 2 /CdSe (col)) was prepared from MPTS/CdSe by using the same method as that reported by Alivisatos and co-workers, 5 with the outermost layer being prepared through the condensation of surface hydroxyl groups on the SiO 2 shell with (trihydroxysilyl)propyl methylphosphonate (TSPP) and hydrolysis ( Figure S1a in the Supporting Information). The resulting nanoparticles were dissolved in an aqueous solution and purified by dialysis.…”