Preparation Of High-performance Nitrogen Doped Carbon Dots And Their Properties And Applications

Carbon dots?CDs?is a new type of 0D carbon-based fluorescence nanostructures with diameters below 10 nm that that are composed mainly of carbon as the basic structural unit,combined with the elements such as H,O,N.Compared with other fluorescent material,CDs show many excellent physical and chemical properties,such as excellent biocompatibility,good water dispersibility,strong light stability,easy modification,low toxicity and facile synthesis,etc.Great efforts have been paid to their potential applications in bio-sensing,bio-imaging,ion detection,energy,environment,light emitting device,catalysis,and other aspects.However,the research of CDs is still on preliminary stage,there are still many problems to be solved urgently.Differently colored CDs have been synthesized with different approaches,but most commonly blue and green;scarcely any CDs undertake selective sub-cellular tissue fluorescence imaging;no mechanism fully understands the luminescence process of CDs.Furthermore,the research on preparation and application of multifunctional materials based on CDs compositing with other functional nanomaterials should also be further studied.Based on these problems,in my thesis,we prepared series of high performance CDs and CDs based nanocomposites through simple hydrothermal method.By studying their structures and physicochemical properties,we make a deep exploration of the luminescence mechanism of the CDs and their potential applications in bio-imaging,MRI,and sensing,etc.The main contents are as follows:?1?One-pot synthesis polyamines improved magnetism and fluorescence Fe₃O₄-carbon dots nano-particles?NPs?for dual modal imaging.In our synthetic strategy ferric ammonium citrate?FAC?as a cheap and nontoxic iron precursor and the carbon source and triethylenetetramine?TETA?present in the reaction medium as reducing agent and nitrogen source,such that Fe³⁺ was reduced to form Fe₃O₄ NPs byTETA,meanwhile,citrate and TETA carbonized into nitrogen doped CDs.Finally hybrid NPs based on magnetic iron oxide and fluorescent CDs can be obtained in one-step conveniently.The obtained Fe₃O₄-CDs NPs not only show the superparamagnetic properties of the Fe₃O₄,but also manifest the compelling photoluminescent properties of the CDs.The advantages of our preparation strategy are eliminating the tedious preparation of magnetic–fluorescent NPs.In addition TETA is proved as major factor for the excellent properties of Fe₃O₄-CDs NPs by controlled trial.These developed Fe₃O₄-CDs NPs are non-toxic and low cost,which should be ideal candidates for combined dual-modal imaging diagnosis.?2?Either excitation–dependent luminescence?EDE?or excitation–independent luminescence?EIE?property of CDs is one of the hot topics.For the first time,we find that unpassivated surfaces of CDs doped by nitrogen possess adjustable EDE and EIE properties by changing the corresponding environmental pH values for photoluminescence properties.Structural characterizations and property tests demonstrate that the unique photoluminescence properties of CDs can be attributed to synergistic effect from PL centers in carbon core and on the surface.Doping nitrogen in carbon core improves the emission efficiency of PL centers;these enhanced PL sites can even dominate fluorescence emission.Passivated surfaces of CDs modified with amino groups not only futher boost the emission efficiency,but also make energy levels of PL centers more uniform.In addition,the mechanism of synergistic effect opens up the possibility of designing CDs with desirable PL characteristics.?3?We develop a simple and low-cost synthesis strategy to fabricate nitrogen doped CDs?N-CDs?by hydrothermal treatment of soluble starchand ethylenediamine.The as-prepared products emit two strong emission peaks at about 450 nm and 540 nm under a single-wavelength excitation.The intensity ratios of the two emission peaks are influenced by pH values,leading to the change of fluorescent colours.Surprisingly,N-CDs can serve as very effective fluorescence probes for fluorescence imaging of cellular nucleus.To the best of our knowledge,the aforementioned properties are novel and seldom reported.In addition,the fluorescence intensity of N-CDs can be effectively quenched by Fe³⁺ ion and restored by ascorbic acid?AA?.Therefore,N-CDs can be a promising “on-off-on” probe for Fe³⁺ and AA.