Multi Functional Fluorescence Sensing Systems Based On Carbon-dots And Polymers

Fluorescence sensing technology is always a hot research point. The development of molecular fluorescent probe is the earliest and most matures, because of its good response properties and widely application in environmental monitoring, ion recognition, fluorescence imaging and other fields. However, these kind of small molecules have some disadvantages, such as single fluorescent emission, poor water solubility, lack of stability and so on. It limits the application of the small molecule probe in the actual environment and biological system. In recent years, the polymer sensors which containing the small molecular probe have showed good potential and value. On the other hand, the fluorescent material based on carbon dots is a new type of fluorescent material, which is similar to the inorganic quantum dots. Because of the excellent luminescent properties of carbon dots, the development of these materials is very rapid in recent years. In this article, in order to construct the multi- responsive sensing system, we built a series of small organic molecules – carbon dots- polymer hybrid materials, and focuse on studing their application in the fluorescence emission, identification, environmental sensing and so on.Firstly, a copolymer poly(NIPAM-co-RD) consisting of N-isopropylacrylamide and N-tripodal rhodamine 6G units as the thermoresponsive and sensing parts respectively has been prepared. At room temperature, the copolymer is fully dissolved in water and it could be used as a highly sensitive and selective fluorescent sensor for Hg2+ in a wide range of p H from 5.0 to 10.0. The non-fluorescent copolymer showed a significant fluorescence enhancement in the presence of Hg2+ over other competitive metal ions. It works based on the Hg2+ induced ring-opening of the rhodamine units. The detection limit was as low as 2.25×10-8 M. Upon heating above the lower critical solution temperature(LCST), the copolymer may collapse and form aggregates, therefore the copolymer containing Hg2+ could be separated from water by centrifugation at high temperature. In addition, the separated copolymer with Hg2+ could be treated by sodium sulfide, and the original copolymer was successfully regenerated and then reused for further Hg2+ sensing.Secondly, we have designed and synthesized a rhodamine 6G functionalized carbon dots as a novel nanosensor for ratiometric fluorescent sensing of Fe3+ in aqueous solution. This proposed R6G-CD nanosensor works based on a Fe3+-triggered FRET process: the spirolactam ring of rhodamine can be converted into its open-ring form in the presence of Fe3+, while the emission spectrum of CDs shows a large overlap with the absorbance of the ring-opened rhodamine 6G, then a FRET process is occurred, which allowing a ratiometric fluorescence response for the detection of Fe3+. In addition, it can be applied to the test paper to achieve detecting Fe3+ in solid phase. Moreover, the R6G-CD nanosensor exhibits high selectivity and sensitivity both in solution and solid phase. The FRET system on the basis of this platform has a great potential for development of a wide variety of ratiometric fluorescent sensors due to its excellent properties.Thirdly, the carbon dots and the p H sensor fluorescein were copolymerized to the temperature sensitive PNIPAM microgel particles. In the microgel particles, the carbon dots were the FRET donor and fluorescein was the FRET receptor. With the increase of p H, the fluorescence of the microgel changed from blue to green because fluorescein group in the system gradually opened ring and the energy of carbon dots was transferred to the ring-opened fluorescein group. At the same time, the microgels exists temperature induced phase transition behavior. When the temperature is higher than LCST, the microgel collapsed, with the size decreasing from 326 nm to 86 nm, and the FRET transmission efficiency increased.At last, the nanoparticles based PMMA containing CDs as FRET donor and spiropyran as acceptor were successfully synthesized. The nanoparticles retained the fluorescent properties of the CDs which emitted green light at excitation 420 nm. The fluoresence of the nanoparticles could be reversibly converted from green to red with UV/vis irradiation. FRET efficiency could be controlled by the excitation wavelength and the ratio of acceptor/donor. In addition, the nanoparticles can be applied in the photoswitchable fluorescence patterning and cell dual-clor imaging.In sumary, with the small organic molecular fluorescent probe as the sensing group, in order to build the multi- responsive fluorescent materials, in this article we prepared several organic- carbon dots- polymer hybrid materials and realized their application in p H detection, ion identification and reversible light modulation fluorescence.