Synthesis Of Non-conjugated Polymer Dots And Their Applications In Cell Imaging And Magnetic Resonance Imaging

The non-conjugated polymer dots(NCPDs)are emerging as a new member of the carbon dots family.The typical structure of NCPDs is composed of an amorphous carbon core and a polymer shell.Generally,NCPDs can be synthesized by carbonization of a kind of non-conjugated polymers,or via polymerization of organic monomers and then being carbonized.Compared with the conjugated polymer dots(CPDs),NCPDs not only have a good photo-stability while showing an excitation-dependent multi-colored fluorescence,but also possess favorable biocompatibility and are easy to be modified and processed into target products.Therefore,NCPDs have potential applications in biological imaging and labeling,and become one of the recent hot topics in the field of bio-nanomaterials.Although carbon dots were first found in 2004,the study of NCPDs is just beginning now,and there are still a lot of prospects to be investigated.In this paper,we adopted two different routes to synthesize NCPDs:one involved dehydration and then carbonization of a certain nitrogen-containing linear polymer,the other referred to a thermal-initiation and polymerization of a vinyl monomer and a subsequent carbonization.Furthermore,the synthesis methods,structures,physical and chemical properties,fluorescence properties of the as-prepared NCPDs,as well as their applications in fluorescence imaging and nuclear magnetic resonance imaging(MRI)at a cellular level were studied in detail.The main research works in this paper are summarized as follows:(1)Using five kinds of polyvinylpyrrolidone(PVP)with different molecular weights as carbon sources,a series samples of nitrogen-doped NCPDs(named as PVP dots)were synthesized by one-pot hydrothermal method.The morphology,size and fluorescent properties of PVP dots were modulated by changing the hydrothermal reaction temperature,time and PVP concentration.Each sample structure was characterized by means of 1H-NMR,FT-IR and UV-Vis absorption spectroscopies.It was found that there were a large number of PVP molecules with different chain lengths remaining on the surface,which retained five-membered aromatic heterocyclic structure in the pendant groups.The morphology,particle size,hydrodynamics radius and fluorescence properties of the obtained PVP dots were studied by means of transmission electron microscopy,dynamic light scattering and fluorescence spectroscopy.The results showed that all of PVP dots were quasi-spherical with amorphous carbon cores.However,along with the increase of the molecular weight of PVP in feed,the hydrodynamic radius of the obtained PVP dots decreased linearly,which might be due to a more thorough degradation of PVP long chains during the hydrothermal reaction,and then shorter PVP chains being left on the surface.Among them,the optimum hydrothermal reaction conditions are determined as 230 ?,9 hours while using PVP with a molecular weight of 8000.The as-synthesized PVP dots have an average particle size of 13.85 nm,and its aqueous solution show a bright blue fluorescence(?em = 410 nm)under 365 nm excitation light,in which the fluorescence quantum yield(QY)can reach as high as 84.1%.Moreover,this fluorescence behavior could be affected by the pH value of the solution,the polarity of the solvent and the temperature of the solution.In addition,cytotoxicity assay result indicated that PVP dots had an extremely low cytotoxicity,which showed good fluorescence imaging effect in Chinese hamster ovary cells(CHO).(2)A novel kind of highly fluorescent NCPDs(QY = 58%)was synthesized by one-pot hydrothermal synthesis from vinyl imidazole monomer,and named as PVIm dots.They also showed a blue fluorescence(?em=402 nm)at 365 nm excitation.The influences of different reaction conditions(including the hydrothermal temperature,time and the concentration of vinyl imidazole monomer)on the morphology,particle size and fluorescent property of the obtained PVIm dots were investigated.Among them,the hydrothermal temperature was found to have the greatest impulse on the fluorescent properties of PVIm dots.Besides,the effects of external stimuli such as pH value of solution,solvent temperature and different solvents,on the fluorescent properties of PVIm dots were also investigated.It was found that PVIm dots showed the most stable fluorescence in an aqueous solution at pH = 3-13 and with a high ionic strength.When a certain amount of Cu2+ was added into PVIm dots aqueous solution,the fluorescence intensity of PVIm dots decreased linearly and finally even quenched with the increase of Cu2+ concentration.The detection limit for Cu2+ is 2.34 ?M.Furthermore,cytotoxicity test result demonstrated that PVIm dots possessed low cytotoxicity.Next,we measured the longitudinal and transverse magnetic relaxation time(T1,T2)of Cu2+-PVIm dots complex.New findings included that the T1 value of the complex is much shorter than that of the free Cu2+,and the magnetic relaxation rate of the Cu2+-PVIm dots complex(r1=1.05 mM-1 s-1)is more than two times higher than that of the free Cu2+ ions(r1 = 0.43 mM-1 s-1),and three times higher than that of the complex formed between Cu2+ and VIm monomer(r1 = 0.32 mM-1 s-1),indicating that the Cu2+-PVIm dots complex could be used as a T1 weighted MRI contrast agent.Last,we chose MCF-7 cell as a model cell line for in vitro test.It was found that the Cu2+-PVIm dots complex could stain MCF-7 cells effectively,and showed a good MRI effect in vitro.All of the results have indicated that Cu2+-PVIm dots complex might be a safer Cu2+-based MRI contrast agent than the traditional Gd3+-based contrast agent.