Synthesis Of Highly Biocompatible POSS-coated CdTe And CdSeTe Fluorescent Quantum Dots And Their Applications

Quantum dots (QDs) developed in recent years have attracted extensive attention since they offer substantial advantages over organic dyes, including size-tunable optical property, broad absorption, narrow emission, excellent photoluminescence efficiency, long fluorescence lifetime and high photochemical stability. These advantages make QDs an ideal fluorescent sensor for chemical or biological assays and real time, long term biomedical labeling. Ⅱ-Ⅵ group semiconductor QDs represent one of the most important types of nanomaterials. The aqueous synthesis route for QDs is relatively simpler, cheaper, less toxic and more environmentally friendly than the organometallic synthetic route, and the resulting QDs are water-soluble, also with high PLQYs, excellent photostability and biocompatibility, which make them convenient for the biological applications. QDs coated by capping ligands with their surrounding surface groups can obtain good biocompatibility since the release of Cd2+was effectively inhibited, and also retain their unique photophysical properties. The proper surface modification is needed to obtain desired functionality and improve photoluminescence response of QDs to some metal ions or molecules.A new and exciting research avenue for QDs is their applications as optical probes for imaging of cells and tissue. As to biological optical imaging, the near-infrared (NIR)-emitting window is appealing because of the low tissue absorption and scattering effects in the emission range (650-900nm). This allows penetration of excitation and fluorescence photons deep into biological samples with reduced interaction and photodamage to the surrounding tissues, allowing fluorescence imaging depths on the order of centimeters. The near-infrared imaging is much prior to the visible light for biological applications.In this paper, water soluble octa-mercaptopropyl POSS (OM-POSS) modified CdTe QDs have been successfully synthesized and employed as a novel fluorescent sensor for optical recognition of Cu2+in an aqueous medium. Their cytotoxicity and availability for cellular imaging in biomedical field were also studied. The near-infrared emitting CdSeTe QDs with octa-aminopropyl POSS (OA-POSS) capping were successfully synthesized and applied to SiHa cell imaging. The main contents are summarized as follows.1. OM-POSS coated CdTe QDs with high fluorescence QYs were successfully synthesized in aqueous solution. TEM images reveal that these nanoparticles appear as spherical with excellent monodispersity. The relative photoluminescence quantum yield of the POSS-CdTe QDs was estimated to be95%using Rhodamine6G (PLQY95%) as reference. Temporal evolution of PL emitting wavelength and QYs of CdTe QDs growth under varied pH values, Cd/Te molar ratios and NAC/Cd molar ratios were studied. The growth rate decreases with the modification of OM-POSS due to its numerous mercapto groups. The POSS-CdTe QDs exhibit significantly good performance after two hours of UV exposure, indicating its excellent photostability.2. The POSS-CdTe QDs have been employed as a novel fluorescent sensor for optical recognition of Cu2+in an aqueous medium. The fluorescence sensor for Cu2+is based on fluorescence quenching of the POSS-CdTe QDs. The selective quenching effect can be explained by the competitive binding of POSS capping layers between CdTe QDs and metal ions present in the solution. Fluorescence intensities of the POSS-CdTe QDs in PBS buffer solution at different pH values were studied. Stern-Volmer plots of different POSS-CdTe QDs concentrations were also investigated. Under optimum conditions, the detection limit for this sensing system is2.3×10-9mol L-1. The detection sensitivity for Cu2+is increased greatly with the capping of OM-POSS on CdTe QDs than the N AC-CdTe QDs. The Stern-Volmer plots at different temperatures show that POSS-CdTe QDs fluorescence was quenched by Cu2+through a static quenching mechanism. Furthermore, the practical utility of the POSS-CdTe QDs has been demonstrated by determination of trace Cu2+in water samples, obtaining satisfactory results with the standard addition method.3. It is demonstrated that the POSS coating on CdTe core QDs confers it good bio compatibility, which is an important feature for cell labeling in the field of biomedicine. The SiHa cells and mouse preosteoblast cells MC3T3-E1were used to evaluate cytotoxicity of the as-prepared QDs. In comparison with CdTe QDs, the POSS-CdTe QDs show lower cytotoxicity under the same conditions. The OM-POSS coating can effectively inhibit the release of Cd2+and protect the QDs from aggregating caused by photobleaching. Moreover, large number of mercapto groups on OM-POSS can provide protection against QD-induced ROS stress and effectively prevent the decrease in cell viability. Confocal fluorescence images demonstrate that the QDs were internalized into the cells via endocytosis. The subcellular localization of POSS-CdTe QDs was found to depend upon QDs size. POSS-CdTe QDs are more easily taken up by cells than CdTe QDs due to its surface coating with amphiphilic moieties, which allows rapid intracellular uptake across the lipophilic cell membranes. The POSS-CdTe QDs allows rapid intracellular uptake, enabling the use of lower concentrations of QDs for an overall reduced toxicity.4. The near-infrared emitting CdSeTe QDs with OA-POSS capping were successfully synthesized in aqueous solution. Temporal evolution of PL emitting wavelength and QYs of CdSeTe QDs growth under varied pH values, Cd/Te molar ratios and nonlinear composition effect were studied. TEM image reveals that these nanoparticles appear as spherical with excellent monodispersity. The absolute photoluminescence quantum yield of the POSS-CdTe QDs with681nm emitting wavelength was tested to be26.4%on Edinburgh FS-920fluorescence spectrometer. The SiHa cells were used to evaluate cytotoxicity of the as-prepared QDs. In comparison with CdSeTe QDs, the OA-POSS modified CdSeTe QDs show lower cytotoxicity under the same conditions. Confocal fluorescence images demonstrate that the near infrared emitting POSS-CdSeTe QDs were successfully internalized into the cells via endocytosis.