Synthesis Of Multi-emission Carbon Dots And Their Analytical Application In Visual Sensing

Carbon dots(CDs),that are expected to replace inorganic quantum dots,are used widely in many fields such as bioimaging,cancer diagnosis,photocatalysis,material synthesis and environmental science.Due to their superior luminescent properties,photobleaching resistance,low toxicity,good water solubility and adjustable excitation emission wavelength,carbon dots have attracted significant attention.To further improve their optical properties and expand their application in more fields,such as optoelectronic devices,sensing analysis and imaging,high fluorescence quantum yields(FLQYs),long-wavelength emission and multi-emissions under one-wavelength excitation are recent research trends.Nowadays,most carbon dots in literature only have one emission under one-wavelength excitation(S-CDs),so the fluorescence probes developed with S-CDs only produce monochromatic change.Compared with S-CDs,the D/M-CDs which have two or more fluorescence peaks under one-wavelength excitation have many advantages.Multi-peak response signals can effectively surmount interference from analyte-independent factors such as environmental change and background signal.On the other hand,we can use the D/M-CDs to develop simple sensitive visualization assay by adjusting peak intensities of dual/multi-emissions and changing the hue of system.Last,taking advantage of M-CDs and other fluorescence materials into a system based on three primary colors(Red-Green-Blue,RGB),a common strategy for white-light-emitting diodes or hydrogels assembly could be designed.So,it is very significant to prepare D/M-CDs.Up to date,a few researches about D/M-CDs were reported,especially for the mechanism of dual/multi-emission optical property.The further investigation still remains a significant challenge.Therefore,the following three tasks have been carried out based on synthesis and visualization of D/M-CDs and their multicolor-sensing:(1)Synthesis of the Cu-doped dual-emission fluorescent carbon dots and its analytical application.A new Cu-doped dual-emission carbon dots was synthesizedrapidly and simply via one-pot solvothermal method and its special photoluminescence mechanism was studied.D-CDs have two FL emission peaks(426 nm and 488 nm)under one-wavelength excitation(350 nm)and can be used as dual-signal sensor which is usually designed with two or more substances.The characterizations of carbon dots including transmission electron microscopy(TEM),X-ray photoelectron spectra(XPS),inductively coupled plasma(ICP)spectroscopic analysis,X-ray diffraction(XRD)assignation of phases,thermogravimetric analysis and differential scanning calorimetry(TGA-DSC),Fourier transform infrared spectroscopy(FTIR),fluorescence spectrum and ultraviolet-visible spectrum(UV-vis)were conducted.The mechanism investigation indicates that the unique dual-emission property is mainly caused by the energy level gaps generated by the surface defects of D-CDs.The prepared D-CDs show excellent water solubility,photostability,salt tolerance,oxidation resistance and special optical properties.They have good potential in dual-signal analysis and visualization sensing.To demonstrate the practical application,ferric ions,vitamin A acetate and pH,respectively,have been determined successfully.(2)Preparation of efficient multi-emission carbon dots and their luminescence mechanism.Although previous work successfully synthesized D-CDs,the visualization is not obvious resulted from the low FLQY.So,we synthesized multi-emission CDs via one-pot hydrothermal method utilizing 2,4-diaminotoluene,ethylenediamine and phosphoric acid.Strikingly,the M-CD fluorescence quantum yield is up to 50.68%.The prepared M-CDs exhibit three fluorescence emissions(370,425,505 nm)under one-wavelength excitation of 320 nm.What is more,various emission origins dominate the unique multi-emission property,which is illuminated by means of radiation decay process,CD internal graphite domains and abundant functional groups characterization.The characterization results fully indicate that there are three energy states: band-gap state(carbon-core state),surface-defect state and molecular state.And the peculiar concentration-dependent and polarity-modulated properties further support the explanation of CD multi-energy states.Our findings mainly account for this luminescence mechanism of CDs.This work utilizes time-resolved emission spectra for the first time to distinguish and identify multi-emission origins,which would provide scientific inspiration for finding effective ways to regulate or control these luminescence processes.In addition,since M-CDs can show bright green fluorescence excited over 400 nm,the color of concentrated M-CD solution incredibly turns from maroon to green in visible light observed from different angles.To demonstrate itspractical application,the detection method of PFOS has been established in the concentration range of PFOS from 4 ?mol/L to 100 ?mol/L with limit of detection(LOD)2.93 ?mol/L and has been applied for the determination of PFOS in Jialing River water of the Three Gorges Reservoir Area and the tap water.(3)A multicolor pH sensor developed with dual-emission carbon dots.Previous two probes only perform dual-emissions in the visible light region.In order to achieve multi-color visualization,we mixed the fluorescent dye,Rhodamine B(RhB),and D-CDs into a system to detect pH.And the new D-CDs were prepared by calcein as the carbon source and anhydrous ethanol as the solvent.There are three FL peaks: 440 nm,520 nm and 580 nm in visible light region.With the increase of pH in the acidic medium,the orange and yellow-green peak intensities gradually increase resulted from the deprotonation on the surface of CDs-RhB fluorescent probes is enhanced,however,the blue peak remains constant.Eventually the FL color changes from blue to cyan to green to yellow and to orange.And the method has been applied in the pH determination of commercial drinks and the results are consistent with that detected by the Mettler FE20 pH meter.In summary,for future research on D/M-CDs,it is essential to pay close attention to the precursors and chemical reaction process.However,controllable synthesis of M-CDs via selecting and controlling the amount of raw materials and adjusting synthesis condition is a huge challenge.So in this study D/M-CDs are synthesized and applied to the visual multicolor sensing,in which the D/M-CDs acted as the core,the luminescence mechanism as the emphasis and the visual sensing is the purpose.