| Objective:To construct a heterogeneous nanoassembly consisting of upconversion nanoparticles(UCNPs)and quantum dots(QDs)by using DNA as a medium.The assembly has both unique optical properties(such as conversion of near infrared(NIR)excitation light into shorter wavelength luminescence)from UCNPs and strong fluorescence characteristics of quantum dots,which lays a good foundation for dual-mode imaging.In addition,due to the programmability and biocompatibility of DNA,it can be designed as an aptamer that recognizes nucleolin overexpressed on the surface of tumor cell membrane,thereby achieving targeted imaging of tumor cells.Methods:After synthesizing quantum dots using DNA as a template,QDs and UCNPs are successfully assembled into a heterogeneous nanostructure by strong coordination interaction between the exposed DNA on the surface of the quantum dots and the lanthanide metal ions.The morphometry,optical property,particle size and zeta potential of the superastructure were characterized by means of transmission electron microscope(TEM),high-angle annular dark-field scanning transmission electron microscopy-energy dispersive X-ray spectroscopy(HAADF-STEM-EDS)elemental mapping),ultraviolet-visible spectrophotometer,fluorescence spectrophotometer,ultra-fast lifetime spectrofluorometer and Zetasizer Nano ZS nano-particle size and zeta potential.The DNA was replaced with an aptamer that specifically recognizes the nucleolin overexpressed on the surface of the tumor cell membrane,and then explored specific biorecognition capabilities of hetero-nanostructures through flow cytometer and confocal laser scanning microscopy(CLSM),providing scientific basis for targeted dual-modality imaging live cancer cells.Results:The cadmium telluride(CdTe)quantum dots were successfully synthesized with DNA as template.The particle size is about 3 nm.It has a narrow and symmetrical fluorescence emission peak with a maximum emission wavelength of 548.6 nm and a broad absorption peak.Upconversion nanoparticles(UCNPs)with uniform particle size(around 40 nm)and regular morphology were synthesized by conventional solvothermal method.The assembled nanocomposites(DNA-QD/UCNPs)were observed by transmission electron microscopy.The surface of the upconversion nanoparticles was uniformly coated with a quantum dots shell,which was consistent with the hydrated particle size test results(the hydration particle size of pure upconversion nanoparticles was 106.3 nm,while the nanocomposite was 128.7 nm).At the same time,the Zeta potential test results further proved that the heterogeneous nanostructure formed successfully.The Zeta potential of the pure upconversion nanoparticles is +28.23 mV,while the heterogeneous nanostructures are became negatively charged.Owing to the presence of DNA(-13.93 mV).In addition,after collecting the fluorescence spectrum and the ultraviolet-visible absorption spectrum,the assembly was found to have both near-infrared excitation luminescence properties of the upconversion nanoparticles,and exhibiting emission peaks of the doping element Tm at 360,450 and 475 nm;the strong fluorescence property of CdTe quantum dots with a narrow emission peak at 548.6 nm was also observed,providing a good practical basis for dual-modality imaging.Finally,we explored the use of the strategy to endow hetero-nanostructures with biological application in targeted dual-modality imaging of cancer cells by introducing DNA aptamers.CLSM revealed that MDA-MB231 cells treated with the resulted assembilies displayed both strong upconversion luminescence originated from UCNPs and green fluorescence signals from QDs.The good overlap of the two imaging signals confirms the cellular uptake of the hetero-nanostructures.In contrast,hetero-nanostructures modified with control DNA showed much weaker internalization by the MDA-MB231 cells,confirming the targeting role of the aptamer sequence.Furthermore,flow cytometry analysis showed that the intracellular fluorescence intensity of cells incubated with Apt-QD/UCNPs was much higher than that of cells treated with Rdm-QD/UCNPs(1.25 folds).Conclusions:We designed a simple and versatile method to control the synthesis of multifunctional heterogeneous nanostructures by DNA-mediated assembly of QDs and UCNPs.The complex simultaneously has the luminescent properties of two assembled components,providing an opportunity for multi-modality bioimaging.Furthermore,assembly using a DNA aptamer,an aptamer specifically recognizes the nucleolin overexpressed on the surface of the tumor cell membrane,targeting cancer cells and dual-modality imaging were successfully achieved. |
