Preparation And Multifunctional Application Of Long Wavelength Emission Fluorescent Carbon Dots

As a shining“star”in carbon-based material system,carbon dots（CDs）have been widely used in various fields,such as disease diagnosis,optoelectronic devices,analysis detection,biological imaging and catalysis due to unique photoluminescence,low toxicity,excellent stability and exceptional biocompatibility.However,at present,most of fluorescent CDs mainly focus on the short-wavelength region（blue and green）,and they are limited to single-function as well as single-mode detection,which impedes their further application.To solve the above problems,in this paper,four new types of long-wavelength fluorescent CDs were developed by screening suitable precursors using a simple one-step hydrothermal method.Further,we studied the structure and properties of as-synthesized CDs in detail and explored their applications in the fields of analysis detection and biological imaging.The main contents of this thesis are as follows:Chapter 1:Firstly,we briefly introduce the structure,synthesis method and optical properties of CDs.In next part,we focus on the luminescence mechanism,photoluminescence regulation and application of CDs.Finally,based on the main advantages and disadvantages of CDs and the problems to be solved,this thesis puts forward the design ideas and research significance.Chapter 2:Aiming at the problem of short-emission wavelength of CDs,raw materials were provided by neutral red and malic acid for the preparation of red-fluorescent nitrogen-doped carbon dots（RCDs）using a one-pot approach.Owing to aggregation induced emission mechanism of RCDs and permanganate,as-prepared RCDs can act as a turn-on fluorescence sensor for quantitative analysis of permanganate in the scope of 0–115μM with a detection limit of 14.5 n M.Most importantly,the photoluminescence of RCDs exhibits a p H-sensitive feature,in which a good linearity in the p H range of 6.2–8.2 can be readily realized.Meanwhile,the color changes of RCDs solution under sunlight can be visualized at different p H,which indicates that RCDs can identify p H by naked eyes and are more desirable for the fluorescence and colorimetric bimodal sensing of p H.More importantly,it was observed that RCDs can successfully target lysosome in living cells.And as-developed RCDs can be applied for the label-free and real-time fluorescence imaging of permanganate and p H in living cells and zebrafish,providing an outstanding candidate matrix material for further application in biological system.Chapter 3:In the construction of fluorescent CDs,ratiometric fluorescent CDs can greatly improve the detection accuracy through endogenous self-calibration ability.Thanks to the success of the early work,a novel yellow/blue-emitting nitrogen-doped carbon dots（CDs）was simply synthesized by hydrothermal treatment of neutral red and urea,resulting in discrete dual-emission bands（440 nm/542 nm）under single-wavelength excitation of 380nm.It was discovered that the fluorescence（FL）signal of CDs at 440 nm enhances gradually with the addition of L-Lys,whereas the FL intensity at 542 nm remains invariable.Surprisingly,no significant luminescence response is observed after the addition of D-Lys,implying that as-prepared CDs can serve as a ratiometric FL probe for enantioselective sensing of L-Lys.And,the corresponding sensing mechanism was also studied.Excitingly,it was discovered that as-synthesized CDs have also excellent p H sensitivity in the scope of p H 2.0–8.0,achieving the ratiometric FL detection of p H.Confocal FL imaging experiments further verify that the CDs can be also implemented as a visual sensing platform to monitor L-Lys and p H in living cells and zebrafish using ratiometric mode.This work provides a new idea for the construction of multifunctional ratiometric FL sensors.Chapter 4:In order to further optimize the luminous properties of CDs,we obtained red/blue-emitting nitrogen and sulfur co-doped carbon dots（NSCDs）by a one-pot approach using neutral red and p-aminobenzenesulfonic acid.Studies have shown that Hg²⁺can induce NSCDs nanoparticle aggregation,while the high-affinity binding of GSH and Hg²⁺can reverse the process from the aggregation to dispersion state upon adding GSH into the NSCDs system.As a result,as-prepared NSCDs can be developed as optical dual-mode ratiometric probe for sequential quantification of Hg²⁺and GSH.Interestingly,NSCDs also show obvious fluorescence color changes at different concentrations of Hg²⁺and GSH.Combined with the mobile app of smartphone,a simple,label-free and fast smartphone imaging platform for visual quantification of Hg²⁺and GSH is established.Besides,NSCDs@Hg²⁺was implemented for cancer cell recognition and visual FL imaging of GSH in vivo and in vitro,having prospect in biomedical field.Chapter 5:On the basis of previous work,red/blue-emitting nitrogen-doped carbon dots（NCDs）were prepared using neutral red and citric acid by a simple hydrothermal method.Based on the strong interaction between Hg²⁺and NCDs surface groups,an optical dual-mode ratiometric probe is designed to detect Hg²⁺.With the increase of Hg²⁺concentration,the fluorescence of NCDs-based test strips shows a significant change.In view of this phenomenon,a smartphone-based pocket handheld device is developed through3D printing technology,which integrates with NCDs-based test strips,and a portable handheld sensing platform was constructed for Hg²⁺detection.In addition,the developed portable device was applied for convenient,real-time and rapid on-site detection of Hg²⁺in real water samples.Confocal fluorescence imaging results show that as-prepared NCDs can be extended to visualize Hg²⁺in living cells and zebrafish.This strategy provides a simple and convenient means for on-site detection of Hg²⁺,indicating the potential application in environmental monitoring.Chapter 6:We summarize and prospect the four research works of this thesis.