Development And Analytical Applications Of Nitrogen Doped Grapheme Quantum Dots-Based Fluorescence Detection System

Graphene quantum dots(GQDs)is a new type of 0D carbon fluorescence materials,which are defined as the graphene sheets with lateral dimensions less than 100 nm in single-,double-and(3-10)layers.Because of their large specific surface area and the hydroxyl as well as carboxyl groups on the edge of the them,GQDs have the characteristic of easy to modify.Among the GQDs via chemical modification,nitrogen-doped graphene quantum dots(N-GQDs)have the excellent features of high quantum yield,high fluorescence intensity,good water solubility,low toxicity and good biocompatibility compared with GQDs.Therefore,N-GQDs has excellent prospects for development in the fields of fluorescence detection,environmental monitoring,photoelectric device and biological imaging.In this paper,we prepared N-GQDs with the one-step hydrothermal method on the basis of the preparation work of laboratory,using citric acid as carbon source and urea as nitrogen source,respectively.The optical properties of resultant N-GQDs were characterized using UV-visible spectroscopy,the transmission electron microscopy(TEM),x-ray diffraction(XRD),x-ray photoelectron spectroscopy(XPS)and fluorescence spectroscopy.And then we developed three fluorescence detection systems related with N-GQDs,based on which we carried out the quantitative analysis of tetracycline(TC)and ascorbic acid(AA)in real samples,as well as the speciation analysis of iron in environmental water samples.The main contents are as follows:1.The application of N-GQDs in the quantitative determination of TC.We synthesized N-GQDs of 4.4 ± 0.6 nm in average diameter by hydrothermal method,they have stronger fluorescence,they are more stable under pH and ionic strength.A novel fluorescence modulated sensor has been proposed based on the fluorescence quenching mechanism of TC on the fluorescence intensity of N-GQDs to detect TC on the basis of the optimization of experiment conditions,such as p H,reaction temperature and reaction time.The linear range of TC was within 0.20-20 ?mol·L-1,and the detection limit(3?,n=11)was 1.62 nmol·L-1.The proposed method showed excellent performance then other techniques,and can be successfully applied to the detection of TC in milk samples.2.The application of N-GQDs in the quantitative determination of AA.A sensitive sensor based on the Fe(?)modulated N-GQDs was developed and applied as fluorescence sensor for AA detection.The sensor was based on the different quenching effects of Fe(?)and Fe(II)on the fluorescence intensity of N-GQDs.The fluorescence of N-GQDs was quenched by Fe(?)via the non-radiation electron transfer.In the presence of AA,Fe(?)can be transformed to Fe(II)ascribed to the oxidation-reduction reaction,leading to the recovery of fluorescence,because Fe(II)has no obvious fluorescence quenching effects on N-GQDs.Under optimal conditions,the method showed a response to AA within a concentration range of 1.0–90 ?mol·L-1 with a good linear relationship and the detection limit(3?,n=11)for AA was 18 nmol·L-1.The developed sensor was successfully applied for the determination of AA in beverage samples.3.The application of N-GQDs in the speciation of iron.A novel method has been developed for the speciation of iron in natural waters based on the fluorescence quenching mechanism of Fe(?)on the fluorescence intensity of N-GQDs,and further detection of Fe(?)and Fe(II)in natural waters.The fluorescence of N-GQDs was quenched by Fe(?)via the non-radiation electron transfer,followed by the determination of total iron which also included the Fe(?)produced by the oxidation of Fe(II)by hydrogen peroxide.And then Fe(II)can be determined by subtracting the concentration of Fe(?)from Fe(tot).The method showed a linear range of 0.0100-200 ?mol·L-1 to Fe(?)under optimal conditions.The idetection limit(3?,n=11)was found to be 4.24 nmol·L-1 for Fe(?).The method is saved from complicated and troublesome configurations with high sensitivity and easy operation compared with previous reports,and the results of the speciation analysis of Fe(?)and Fe(II)reveal that this method is an alternative approach to the speciation analysis of dissolved iron in natural waters.