Preparation,Optical Properties And Application Of High Performance Fluorescent Carbon Dots

Carbon dots(CDs)are a class of fluorescent nanomaterials with excellent water solubility,good fluorescence stability and photobleaching resistance.The main component of the carbon dot is C element.CDs not only possess the advantages of good biocompatibility and low toxicity,but also have the characteristics of adjustable fluorescence wavelength,easy of fabrication,and abundant raw material sources.Since CDs were first discovered in 2004,they have attracted enormous attention from researchers and have been widely used in fluorescence detection and construction of optoelectronic devices.However,CDs still have problems such as low fluorescence quantum yield(QY),complicated preparation processes,poor anti-interference ability,and fluorescence quenching in solid state.This paper aims to develop simple preparation methods for high-performance CDs,explore general strategies for improving QY of CDs,develop effective methods for improving anti-interference ability of CDs,overcome fluorescence quenching of CDs in solid state and develop CDs-based selective detection methods for food and environmental analysis.The details are as follows:1.N,S-co-doped carbon dots(NSCDs)were prepared by microwave assisted method using ammonium citrate as C source and cysteine as N,S-co-dopant.The NSCDs exhibit blue light emission under ultraviolet light excitation with a narrow size distribution.The fluorescence excitation and emission wavelengths of the NSCDs are located at 353 nm and 426 nm.The QY of NSCDs(64%)is greatly improved by the incorporation of N and S elements.The preparation has the advantages of a short preparation time(2.5 min)and a high yield(62%).It was found that levofloxacin(LEV)can effectively quench the fluorescence of NSCDs,accompanied by an increased fluorescence peak at 499 nm,which is attributed to the intrinsic fluorescence signal of LEV.Based on variation of the fluorescence signal(I499/I426),a ratiometric fluorescent method for selective detecion of LEV was constructed.The method was successfully applied to the detection of LEV in environmental water samples with satisfactory results.2.Although CDs have been widely used in analytical detection,their anti-interference ability is insufficient in the analysis of complex samples.Honey is one kind of real sample with autofluorescence.The autofluorescence of honey matrix always overlaps with the fluorescence spectrum of CDs,making it difficult to achieve detection of tetracycline(TC)in honey samples based on CDs.Using TC as a template,single-hole hollow molecularly imprinted polymer(HMIP@CD)based on CDs was prepared via microwave-assisted method.Based on the HMIP@CD,a separation and redispersion method was successfully constructed,which has successfully avoided the autofluorescence interference of honey matrix.3.Solid phase pyrolysis is widely used in the preparation of CDs due to its advantages of simple operation,solvent saving and large-scale preparation.However,CDs are prone to agglomeration during solid phase pyrolysis,resulting in uneven particle size distribution and lower QY.We proposed a simple and effective solid phase dispersion surface pyrolysis method.Tiny carbon dots(T-CDs)with a high QY was synthesized using multi-walled carbon nanotubes(MWCNTs)as a support and 2-(N-morpholino)ethanesulfonic acid(MES)as a precursor.By simple grinding,MES can be uniformly dispersed on the surface of MWCNTs by hydrogen bonding and p-p interactions.Due to the excellent thermal conductivity and rigidity of MWCNTs,the precursors are effectively carbonized and aggregation between each other is avoided effectively.Compared with the CDs prepared by direct pyrolysis method without the participation of MWCNTs,the particle size of T-CDs is smaller and the particle size distribution is narrower with an increased QY of 42.8% from 17.8%.This solid phase dispersion surface pyrolysis method can effectively increase the QY of CDs.Subsequently,a “turn-off-on” detection system based on T-CDs was successfully constructed and successfully applied for the detection of rutin in human urine samples,human plasma samples and rutin tablets;and Al3+ in tap water samples.4.CDs generally face fluorescence quenching caused by aggregation in solid state,which limits their application.The sp3C-doped carbon dots(D-CDs)powder was synthesized via a one-step microwave-assisted pyrolysis method with a yield of 67.8%.Without any dispersion matrix,the D-CDs powder exhibites a single energy level and emit bright fluorescence in solid-state with a QY of 67.7%.Owing to the doping of sp3 C structure and a single emission level,fluorescence self-quenching in the D-CDs powder is greatly suppressed.Based on the D-CDs powder,bright blue LEDs were successfully constructed.A rapid fluorescence detection method for Fe2+ was also constructed based on the D-CDs.Under the optimal experimental conditions,the linear detection range of Fe2+ is 0-750 mM,and the detection limit is 0.6 mM.This method is applied for the detection of Fe2+ in environmental water samples.