| Carbon dots(CDs)are promising fluorescent materials for a wide range of applications.The work about their synthesis,optical properties,photoluminescence(PL)mechanism and applications has grown exponentially over the past 15 years due to their unique optical properties and a wide range of applications.At present,the PL mechanism of CDs is still an open debate among researchers.Currently,the most accepted PL mechanism are the quantum confinement effect and the surface state.More recently,it has been confirmed that small molecular fluorophores are the PL origin of the citric acid and amine based CDs.CDs produced by different precusers and various approaches usually contain different complicated components and structures.Therefore,in this dissertation,we focuse on the products of citric acid and urea,aiming to find out the PL evolution and origin under different hydrothermal conditions,and discuss their applications according to the structural characteristics of the products in the molecular level.The miancontents and results are as follows:(1)Fluorescence origin evolution of CDs based on hydrothermal reaction of citric acid and urea under different reaction conditions.The products produced by hydrothermal reaction of citric acid with urea under different reaction conditions were systematically studied.The results showed that under mild hydrothermal conditions,a large number of solids precipitated after adding acid to the solution.The structure of the solid was identified as citrazinic acid by infrared spectroscopy(FTIR),powder X-ray diffraction(PXRD),nuclear magnetic resonance(NMR)and high resolution mass spectrometry(HR-MS).Additionally,there was no solid present in the solution with the increase reaction temperature and time,while the solution still possessed blue fluorescence.In order to further clarify the PL evolution and origin under different conditions,three typical samples prepared at 185°C for 2 h,185°C for 10 h and 230°C for 10 h were studied.The fluorescence photobleaching performances,excitation-dependence and lifetime were detaily compared with citrazinic acid and nitrogen doped graphene quantum dots(N-GQDs).It was found that as the reaction temperature increased and the reaction time prolonged,the fluorescence origin changed from the molecular citrazinic acid to polymer dots,and finally to highly carbonized CDs.Moreover,the nature of the products inside and outside the dialysis bag was quite different.In this work,for the first time we found citrazinic acid was the main PL origin of the CDs generated from mild hydrothermal reaction of citric acid and urea,which was easily separated by a one-step acid treatment.Meanwhile,the evolution process of the main product in CDs was clarified along with the increase in hydrothermal temperature and time.(2)Self-assembling citrazinic acid and cytosine for iodine capture.Based on the special molecular structure of hydroxyl,carboxyl groups and pyridine nitrogen in citrazinic acid,a new organic functional material(CACY)was developed through the intermolecular self-assembly of the synthesized citrazinic acid and cytosine by treatment of grinding and ultrasonication.The image of scanning electron microscopy(SEM)and transmission electron microscopy(TEM)shows CACY mainly comprised of nano-brick structure with a large number of stacked pores.Due to the unique structure of CACY,the results found that it is a great candidate for iodine capture.The measured saturated adsorption capacity of 48 h was 1.2-1.3 g g-1.FTIR,PXRD combined with density functional theory(DFT)calculation show that the organic functional materials are formed by noncovalent interactions between raw materials,like hydrogen bonds and ?-? stacking interactions.The recyclability of the adsorbent was also examined.The related adsorption mechanism was proposed after comparing the structure of CACY before and after iodine adsorption by Raman,FTIR and XRD techniques.Finally,the adsorption capability in hexane was also investigated.In this work,a simple intermolecular self-assembly method was used for the first time to investigate the application of citrazinic acid in the field of iodine adsorption,which provides a green and simple synthesis method of organic functional materials for solving nuclear waste pollution,and also sheds light on the further application of small organic molecules.(3)The fluorescence origin and its application in the hydrothermal system of citric acid and urea with sulfur doping.Citric acid,urea and thiourea were hydrothermally reacted under mild conditions.The resulting solution was found to be high blue fluorescent.XPS results showed that the product was doped with sulfur atoms.The Raman spectrum showed that the solution had carbogic structure with a G and D band.The fluorescence of the resulting solution is excitation dependent and high resistant to photobleaching.The obove results indicated that the main PL origin in the solution is nitrogen-sulfur co-doped CDs(N,S-CDs)with a higher degree of carbonization.In addition,the interaction of N,S-CDs with macromolecular protein was discussed.We found that the fluorescence of N,S-CDs was effectively quenched by bovine hemoglobin(BHb).The interaction mechanism between N,S-CDs and BHb,including the average lifetime of N,S-CDs under the different concentrations of BHb,binding constant and binding site was thoroughly investigated by a series of spectroscopic methods,such as UV spectrum,fluorescence spectrum,circular dichroism,etc.The results showed that the quenching is static quenching.The binding of hemoglobin and N,S-CDs is spontaneous,and hydrophobic interaction plays a major role.The results of circular dichroism showed that the conformation of BHb was changed after the interaction with N,S-CDs.In this work,we found that under mild reaction conditions the main PL origin of hydrothermal reaction of citric acid and urea after sulfur doping is carbonized CDs,which is helpful in the study of PL mechanism in related solutions and widens the application of carbonized CDs in the field of biological macromolecular recognition. |
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