| Carbon dots combine the common features of carbon nano materials, e.g., low toxicity to environment and human health, abundant suface functional groups which make it readily prepared by various approaches. Besides, CDs are fluorescent and so far the reported maximum quantum yield can be as high as 80%, which is comparable to those achieved by semiconductor quantum dots. Not surprisingly, C-dots are inspiring intensive research efforts in its own right.CDs prepared by use of different approaches and precursors find vast potentials in various fields. In this respect, it is highly desired to thoroughly investigate novel preparation procedures for CDs. With several favorable attributes, such as strong fluorescence and wavelength-dependent luminescence emission, as is known that so far the fluorescent mechanism of CDs is still under investigation. The suface functional groups are widely exploited for their modification to realize selective sensing and targeted imaging and delivery, while the reducing capability of their suface groups have not been reported yet. Optosensors based on the enhancement or queeching of CDs fluorescence have been investigated thoroughly, yet the application of reducing ability of CDs in constructing probes is rarely discussed. In this dissertation, we will discuss the new precursor to produce CDs, explore the reducing capability of CDs and its applications in the field of bio-sensing.Chapter 1 is a brief introduction for the classification of preparation methods, the feature of CDs, and their application in bioimaging, drug delivery, optical sensing and photocatalysis.In Chapter 2, a cationic branched polyelectrolyte of large molecular weight, i.e., polyethylenimine of 25 kDa, was used both as carbon source and passivating agent to prepare photoluminescent carbon dots in one step. After refluxing with HNO3, PEI is cut into small fragments, then carbonized and oxidized to form carbogenic core. Afterwards, amine groups from the unreacted small fragments bind to carboxylic groups on the surface of carbogenic core by means of amidation reaction. In the end, we get PEI-CDs with abundant amine and carboxylic groups. Owing to the surface groups, CDs are positively charged in acidic solution, while negatively charged in alkaline solution. These PEI-CDs have a distinct pH-sensitive feature that might be the results of protonation and deprotonation of amine and carboxylic groups. With the increase of pH, the fluorescence of CDs decreases significantly. The variation of fluorescence with pH is a reversible process, that is the fluorescence intensity is recovered to its original level after 10 cycles of increasing the pH value from 2 to 12, followed by decreasing from 12 to 2. The fluorescence of CDs is quite stable after 3 hours constant irradiation with 150 W Xe-lamp. Meanwhile, the fluorescence stays the same as in DI water even in high ionic strength solution, i.e.1 mol L-1 NaCl. When incubated with HeLa cells, the PEI-CDs could readily penetrate the cell membrane and exhibit low cytotoxicity and favorable biocompatibility. The PEI-CDs have been used for HeLa cell imaging and can emit blue and green light in the cells under the excitation of 340 and 495 nm respectively.Among the observation in Chapter 2, the abundant surface groups on CDs attract our attention, so the characterization of surface groups is discussed in Chapter 3. Carbon dots have been proven to show the capability for direct reduction of Ag+ to elemental silver (Ag0) without additional reducing agent or external photoirradiation by incubating with Ag+ for 5 min in water bath at 50?.The resulting Ag0 formed silver nanoparticles spontaneously with an average size of 3.1±1.5 nm. This process involves the oxidation of phenol hydroxyl or amine groups on the aromatic ring of CDs to quinine and azo. Meanwhile the as-prepared Ag-NPs grow on the surface of CDs, and form Ag-NPs/CDs composites. CDs protect and stabilize the Ag-NPs from aggregation in aqueous medium; that is, the Ag-NPs are stable at least for 45 days in aqueous medium. The formed Ag-NPs cause significant resonance light scattering, which correlates closely with the concentration of silver cation, and this facilitates quantitative detection of silver in environmental water samples with spiking recoveries of 103% and 106%. The reducing capability of CDs also finds itself application in catalysis growth of Au-NPs with an average diameter of 3.0±0.7 nm. The optimal synthesis medium for Ag-NPs and Au-NPs is alkaline and acidic solution respectively. The reason for the difference is that CDs are negatively charged in alkaline solution, while positively charged in acidic solution, which facilitates the binding of Ag+ and AuCl4- respectively.In the discussion of optimal conditions for the growth of Ag-NPs, we find excess CDs produce less Ag-NPs. Therefore, the mechanism and application of this interesting discovery is investigated in Chapter 4. The excessive CDs consume free Ag+ in the solution rapidly by binding Ag+ with functional groups on the CDs surface. With small amount of free Ag+ left in solution, the supply of Ag+ to CDs is blocked, and thus the growth of Ag-NPs is inhibited. Biothiols can coordinate with Ag+ through thiol groups, and afterward, the Ag+-biothiol complex gradually releases free Ag+ to ensure its reduction by CDs and thus facilitates the growth of Ag-NPs on CDs surface. A colorimetric assay procedure is thus developed for fast detection of biothiols based on Ag-NPs plasmon absorption. The method allows simple detection of Cys with a detection limit of 8.5 nM within a linear range of 20-400 nM in the presence of 0.2 mM Ag+ in the system. The linear calibration range can be regulated by controlling the concentration of Ag+. A lower level of Ag+, i.e.,0.1 mM, offers higher sensitivity for the assay of an ultratrace amount of biothiols with different linear range of 2.5-30 nM?5-40 nM and 2.5-30 nM for Cys, Hcy, and GSH, respectively. In addition, the sensing system exhibits good selectivity toward biothiols in the presence of other amino acids, the major metal cations, such as Ca2+ and Mg2+, and biomolecules, such as glucose, ascorbic acid and HSA in biologicalfluids. In the assay of Cys in human plasma, spiking recoveries of 94% to 108% are obtained at 100 ?M level. |
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