Preparation And Biological Application Of Polyoxometalates/Quantum Dots Composites With Florescent Switching Properties

The development of materials of which the photoluminescent properties could be changed by using an external stimuli, have been actively pursued in the past few years. The recent surge of interest in these switchable systems stems from the wide spread applications in displays, ultrahigh-density optical data storage, molecular switches, logic gates, molecular wires, optic/electronic devices, sensors, bioimaging and so on. Ideal photoluminescent switching materials for high-density optical data storage and bioimaging should have such characteristics as quick response, high on/off contrast, remarkable photostability, good cycling performance and multi-color/multi-state. This study aims to fabricate the color-tunable photoluminescence switching systems with quick response, high on/off contrast, remarkable photostability and good cycling performance by taking advantage of photochromic and electrochromic polyoxometalates and quantum dots（QDs）. Compared to the conventional organic products and photoactive fluorescent protein, the proposed system displays several advantages:（1） multi-color system in which the emission colors can be tuned by changing the reduction potentials or irradiation time;（2） high on/off contrast;（3） remarkable photostability and excellent resistance to fatigue. Furthermore, the proposed photoluminescence switching system has been be used for optical data storage and as photoswitchable probe for cell imaging and tissue imaging.Recently, multi-color fluorescent systems which emit multiple distinguishable fluorescence signals under a single wavelength excitation have attracted considerable attention in many fields including multiplexed bioassay, cell labelling, detection of cancer cells, monitoring of bacteria, ratiometric sensor, and emitting displays etc. However, application of the system is often hindered by inherent limitations of the photon-based modality, such as interferences with light absorption, scattering and autofluorescence from biological tissues. In this study, several nanocomposites with fluorescent switching properties were prepared by depositing polyoxometalates P₈W₄₈ and CdSe@Cd S QDs or CdTe QDs onto silica nanoparticles via layer-by-layer assembly technique. The photoluminescence of those composite nanoparticles can be switched on and off reversibly upon UV light or visible light, based on the fluorescent resonant energy transfer between reduced POMs and QDs. This system displays good cycling performance（no obvious changes after 50 cycles）, a quick response（off 5 min, on 10 min） and requires no physical contact thus avoiding potential risk of chemical contamination. Furthermore, the on/off contrast of P₈W₄₈/QDs fluorescence switching can be tuned by controlling the contents of P₈W₄₈, the highest on/off contrast could be as high as 80%. Finally, due to the fluorescent property of QDs, multi distinguishable fluorescence switchable systems under a single wavelength excitation can be realized.Another imperative challenge of photoluminescence switching devices toward real applications is to obtain multi-state systems with high on/off contrast and fast response time, in which the photoluminescence wavelength can be easily tuned in the visible region. In this study, we report the first inorganic example of a multistate photoluminescence switching system based on nanostructures of crown-type polyoxometalates(P₈W₄₈) and CdSe@Cd S core–shell QDs via the LBL technique. The photoluminescence of this system can be switched on and off reversibly upon application of step potentials, owing to the energy transfer between reduced POMs and QDs. This system displays a quick response（off 17 s, on 38 s）, high on/off contrast（91%）, good cycling performance（the modulation ratio is only decreased by 19% after 200 cycles） and also has the advantage of low power consumption. Furthermore, reversible four-state fluorescence switching is realized by integrating different-sized QDs in one system.The P₈W₄₈/CdTe@SiO₂ nanoparticles with unique fluorescence switchable property could be used as photoswitchable probe for cell imaging and tissue imaging, and exhibit high on-off contrast and good stability during in vitro and vivo imaging process. In vitro study demonstrated that this biocompatible fluorescent nanostructure could realize multicolor fluorescence imaging. Moreover, the P₈W₄₈/ CdTe@SiO₂ nanoparticles are suitable for in vivo imaging applications through intravenously injected with photoswitchable nanoparticles. Biotributation investigation demonstrated that tumor tissue, spleen, kidney and liver showed higher fluorescence than heart, brain and lung.