Construction Of Three Kinds Of Carbon Nanodots-based Fluorescence Probes And Their Application In The Rapid Detection Of Carmine In Food Samples

Carmine(CRM)is an azo synthetic pigment with red rhombic crystals or reddish-brown powder in appearance.It is often used as a food coloring agent for dried fruits,beverages,pickles,candies,etc.to enhance the attractiveness of their appearance.The toxicology studies showed that CRM has certain carcinogenic and mutagenic effects and genotoxic potential.Therefore,long-term excessive consumption of CRM has a huge potential threat to the health of organisms.The GB2760—2014 "Standards for the Use of Food Additives" issued by the National Health and Family Planning Commission of China stipulates that CRM is allowed to be added into fruit juice beverages,dried fruit products,candies and pickles with the maximum addition amount not exceed 50.0 mg/kg or 50.0 mg/L.Therefore,the development of a fast,convenient and accurate analytical method to detect the content of CRM in foodstuffs is highly important to protect human health.In recent years,the rapid development of fluorescent carbon dots(CDs)-based nanoprobes,especially the doped CDs-based nanoprobes,has opened up new prospects for efficient and rapid detection of CRM due to its simplicity,rapidity,low cost,high selectivity and high sensitivity.In this paper,three kinds of doped CDs-based nanoprobes,namely N-CDs,N,Cl-CDs and N,Cl,S-CDs,were prepared by using different precursors.Based on the quenching effect of CRM on CDs,the fluorescence detection methods based on three kinds of doped CDs were established and used for the rapid detection of CRM in real food samples.(1)The first part of this thesis is the preparation and characterization of physical and chemical properties of the as-prepared doped CDs.First,nitrogen-doped carbon dots(N-CDs)were prepared by a one-step hydrothermal synthesis method(180?,12h)using m-phenylenediamine as raw material.The N-CDs had optimal excitation/emission wavelengths of 350/440 nm and a quantum yield of 57.6%.Secondly,using glucose as carbon source and HCl as Cl atom donor,nitrogen and chlorine co-doped carbon dots(N,Cl-CDs)were prepared via acid-base neutralization heat.The N,Cl-CDs had optimal excitation/emission wavelength of 370/445 nm and a quantum yield of 32.7%.Finally,using glucose as carbon source and HCl and H2SO4 as S and Cl atom donors,nitrogen,chloride and sulfur co-doped carbon dots(N,Cl,S-CDs)were prepared by acid-base neutralization heat.The N,Cl,S-CDs had optimal excitation/emission wavelengths of 380/455 nm and a quantum yield of 29.1%.In this work,transmission electron microscopy(TEM),elemental analysis,fourier transform infrared spectroscopy(FTIR),ultraviolet-visible(UV-vis)absorption and fluorescence spectroscopy were used to characterize the particle size,chemical structure,UV-vis and fluorescence spectral properties.(2)The second part of this paper is based on the establishment of fluorescence detection methods based on doped CDs.Based on the quenching effect of CRM on CDs fluorescence,three fluorescence detection methods based on N-CDs,N,Cl-CDs and N,Cl,S-CDs were proposed.The main working parameters(including pH,CDs concentration and reaction time)were optimized in this work.The results showed that the optimal concentration of the three doped CDs were 0.03 mg/mL,0.03 mg/mL and 0.05 mg/mL,respectively.The optimal pH and the best reaction time for the three detection work were was 6.6 and 1.0 min,respectively.During the establishment and validation process of the analytical method,it was found that the three doped CDs probes had high selectivity and good anti-interference for CRM detection with detection limits of 11.20,9.84 and 5.68 nM,respectively,and linear ranges of 0.01-10.0,0.01-10.0 and 0.01-15.0 ?M,respectively.The results show that the N,Cl,S-CDs probe had the highest sensitivity and the widest linear range.Comparing the detection performance of the three fluorescence assays with those of the other reported assays,it was found that the detection limits(5.68-11.20 nM)of the three assays proposed in this work were all much lower than previously reported CDs-based fluorescence methods.Speficially,the sensitivity of N,Cl,S-CDs probe is 2 orders of magnitude higher than the reported CDs probes,and it is even 1-2 orders of magnitude higher than some modern instruments-based analytical methods such as capillary electrophoresis and HPLC.The fluorescence quenching mechanism of CDs by CRM was investigated by UV-vis absorption spectroscopy and fluorescence spectroscopy.It was found that the fluorescence quenching effects of CRM on the three kinds of doped CDs were all attributed to the inner filter effect.The fluorescence lifetime analysis revealed that the interactions between CRM and three CDs were static quenching,dynamic quenching and dynamic quenching,respectively.(3)The third part of this paper is to explore the feasibility of the new fluorescence detection method for real sample analysis.The N-CDs,N,Cl-CDs and N,Cl,S-CDs probes were used for quantitative CRM detection in real food samples(watermelon frost lozenges,dried strawberries,dried bayberry,hawthorn slices and energy drinks)purchased from the local market in Yangzhou,respectively.The results showed that the contents of CRM in the tested samples were all within the permitted concentration range allowed by China government and therefor they were safe to eat.The spiked recoveries of the three detection methods were 98.6-105.4%,96.3-105.1%,and 96.8-104.6%,and the relative standard deviations(RSDs)were 0.69-4.07%,0.97-3.85%,and 0.66-2.52%,respectively.Meanwhile,HPLC method(national standard method)was used to detect the contents of CRM in real food samples,and it was found that the three detection methods proposed in this work had good consistency with the detection results of HPLC method(national standard method),indicating that the detection method proposed in this work has high accuracy.