Preparation And Smartphone-based Sensing Applications Of Novel Fluorescent Carbon Nanodots

With the rapid development of fluorescence microscopy and nanotechnology,fluorescent materials have become the focus of researchers from various fields.In recent years,fluorescent carbon nanodots(CNDs)as a new member of carbon nanomaterials,have been widely investigated.Compared with conventional inorganic semiconductor quantum dots and organic small molecule fluorescent dyes,CNDs possess many advantages,such as low preparation cost,good water solubility,excellent fluorescence performance,good biocompatibility,excellent photobleaching resistance,chemical inertness,easy surface functionalization and low toxicity.Because of these advantages,CNDs have been used in chemical sensing,cell imaging,photoelectric devices,anti-counterfeiting,biomedicine,catalysis and many others.Among them,chemical sensing is a typical application,however,there are still some problems to be solved or optimized for better use of CNDs:1)the photoluminescence mechanism of CNDs is complicated,further elucidation of the luminescent composition with separation technologies is needed;2)tuning the optical properties by efficient selection of carbon sources or rational heteroatoms doping still needs more efforts;3)for sensing applications,use of widely available 3D printing to construct low-cost portable multi-mode smartphone-based sensing platforms using CNDs is an important direction of the development.Based on these facts and previous works,this dissertation is devoted to the development of fluorescent CNDs with various emission,electrophoretic separation and characterization of CNDs composition,and development of portable smartphone-based sensing device.The work mainly includes:(1)The effect of reaction conditions on fluorescent compositions and yields of a kind of blue fluorescent CNDs(?em=410 nm)was explored.The CNDs were successfully used for Cr(VI)detection in environment samples.(2)Two smartphone-based sensing devices with portability were constructed.Based on this,the fluorescence spots brightness and size dual-mode detection method for Cr(VI)was proposed.The visual estimation of Cr(VI)content in disposable centrifuge tubes was attempted.(3)CNDs(?em=426 nm)-catalyzed and MnO2 nanosheets-mediated redox reaction was adopted for smartphone-based detection of drug molecules based on the fluorescence of the CNDs.(4)A smartphone-based tri-mode detection device was constructed.Using fluorescent CNDs(?em=491 nm)as a probe,a novel method for acidic phosphatase(ACP)activity evaluation was developed based on fluorescence spots brightness,size and absorption spots brightness,with sensitivities comparable to the normal spectrometric methods.Screening of ACP enzyme inhibitors was achieved on the prepared device.(5)A novel method was proposed to improve the selectivity of CNDs for 2,4,6-tri-nitrophenylphenol(TNP)detection by pH modulation,which will provide a reference for improving the selectivity of the IFE-based methods.The dissertation contains six chapters:Chapter 1 briefly introduces the background of fluorescent carbon dots(CDs),systematically summarizes the structure,properties and synthesis method of fluorescent CDs,and elaborates the application progress of fluorescent CDs in various research fields.The smartphone devices and their practical application as detectors are described.Chapter 2,with the assistance of CE,a series of nitrogen-doped carbon nanodots(N-CNDs)with high fluorescence was prepared by hydrothermal method using guanidine triaminohydrochloride(TGcl)and citric acid(CA)as starting materials.The effects of different synthesis conditions,including molar ratio,reaction temperature and reaction time on fluorescence spectra and fluorescence intensity were studied.Because fluorescence spectroscopy merely provides the summation or average fluorescence properties of all components,through CE with a fluorescence detector and a UV detector,the effects of reaction conditions on the composition and yields of the products were clearly determined.The CNDs synthesized under the optimum reaction conditions could be used as a turn-off chemical sensor for rapid determination of Cr(VI)content.Under the selected detection conditions,there was a good linear relationship between fluorescence quenching efficiency and Cr(VI)concentration in a range of 0.2-80 ?M with a limit of detection(LOD)of 0.11 ?M.The proposed method was successfully used for the measurement of Cr(VI)in water and soil samples.Chapter 3,two types of low-cost and portable smartphone-based devices used for fluorescence spots brightness and size dual-mode sensitive detection of Cr(VI)were constructed with the aid of the 3D printing.The 3D printed devices can effectively eliminate the problems from the ambient light and difference in images taken by different operators.The devices mainly include a smartphone,a UV LED,power supply and other small parts.The camera parameters of the smartphone-based devices were optimized systematically.With the first smartphone-based device,Cr(VI)can be quantified through fluorescence spots brightness and size,respectively.Based on the brightness reflected by the blue(B)channel value from the images,a linear relationship between quenching efficiency and concentration of Cr(VI)in a range of 0.2-150 ?M with a LOD of 0.058 ?M was attained,which is comparable to that from fluorescence spectrometers.According to the size of fluorescence spots,there is a linear relationship between the distance expressed in pixels and Cr(VI)concentration in the range of 10-350 ?M.With the second device,the Cr(VI)concentration observed by naked eye could be low to 3 ?M.The applicability of smartphone-based detector in environmental samples was verified.Chapter 4,a water-soluble fluorescent CNDs was prepared with hydrothermal treatment using TGcl and glucose as starting materials.CE with fluorescence detector demonstrated that CNDs are mixtures composed of multiple fluorescent components.A novel fluorescence enhancement method for the determination of tiopronin using CNDs-MnO2 nano sheets composite was developed.MnO2 nano sheets not only quenched the fluorescence of CNDs,but also acted as an important medium for response of tiopronin.The roles of CNDs as the fluorescence signal source and also as a catalyst to accelerate the redox reaction between MnO2 nanosheets and tiopronin,were systematically confirmed.Under the selected conditions,the linear range for tiopronin measured with fluorescence spectroscopy was 0.5-70 ?M and LOD was 0.25 ?M.While with the smartphone-based visualized sensing device,the lower LOD of 0.20 ?M was obtained.The proposed method was successfully applied to the determination of tiopronin content in tiopronin enteric-coated tablets.Chapter 5,a low-cost and portable smartphone-based triple mode device was constructed and used for the evaluation of ACP activity.The device only contains two 400 nm LED,a smartphone,quartz cuvette and 3D-printed holder and several small parts.Since p-nitrophenol(p-NP)could quench the fluorescence of green CNDs,companied with the absorbance enhancement at 400 nm,the availability of tri-mode detection device was investigated.The catalytic hydrolysis of p-nitrophenol phosphate(pNPP)to p-NP in the presence of ACP was used for tri-mode evaluation of ACP activity on smartphone.Based on the fluorescence spots brightness captured with smartphone,the quenching efficiency reflected by B channel value exhibited a linear relationship with ACP activity in the range of 0.02-1 mU/mL with LOD of 0.008 mU/mL.Based on the fluorescence spots size,the distance at the dark side of the spot was linearly correlated with ACP activity in the range of 0.05-2 mU/mL.While based on the absorbance spots brightness,a linear relationship was obtained for the absorption and ACP activity in a range of 0.03-1.5 mU/mL.A new approach for screening of ACP inhibitors was developed.The performance obtained with smartphone-based tri-mode detection device was comparable with spectrometric methods,while it has the advantages of low cost,good portability and easy visual detection.Chapter 6,water-soluble CNDs with bright yellow-green fluorescence were prepared with hydrothermal treatment using isoproterenol hydrochloride and m-phenylenediamine(m-PDA)as starting materials.With the material as a probe,a highly selective method by pH modulation for the detection of 2,4,6-trinitrophenol(TNP)was established.Based on IFE,the method could successfully differentiate two electron-deficient compounds TNT and TNP,thus eliminating the interference from TNT.By simply adjusting the pH of analytical solution to strongly acidic,the interference of other nitrophenol compounds such as o-nitrophenol,m-nitrophenol,p-nitrophenol and 2,4-dinitrophenol were effectively eliminated,so highly selective detection of TNP was realized.The strategy was applied to the smartphone-based sensing device for successful visual detection of TNP.