Doped Graphene Quantum Dots As Fluorescent Probes For The Determination Of Iron (Ⅲ) And Marine Shelltoxin

Graphene quantum dots（GQDs）are new carbonaceous materials with a horizontal size of less than 100 nm and a vertical size of less than a few nanometers.Due to the large number of defects on the surface of GQDs,their fluorescence quantum yield is low,which limits the wide application of GQDs in the sensing field.In this paper,by selecting suitable doping sources,heteroatom（P,Cl;S,P）co-doped GQDs were successfully prepared.As the heteroatoms change the charge density distribution and band gap of GQDs,their fluorescence quantum yield were improved.For the applications,first,P,Cl-GQDs were used as fluorescent probes to the detection of Fe³⁺with high selectivity and high sensitivity;second,S,P-GQDs were used as the fluorescent immunoprobe of okada acid（OA）,combining with competitive fluorescent immunosorbent assay（cFLISA）,a rapid and sensitive detection method for OA was established;third,S,P-GQDs and fluorescent gold nanoparticles（AuNPs）were used to label OA and saxitoxin（STX）,respectively,and cFLISA was established to achieve simultaneous detection of OA and STX,and the method was successfully applied to the detection of shellfish samples.1.The preparation strategy of P,Cl-GQDs was investigated and the resulting P,Cl-GQDs were used as a fluorescent probe to achieve highly selective and highly sensitive detection of Fe³⁺.The P,Cl-GQDs were prepared via hydrothermal treatment of maltose,phosphoric and hydrochloric acids as C,P and Cl sources,respectively.The structure,morphology and optical properties of P,Cl-GQDs were investigated.The results show that the maximum excitation wavelength of P,Cl-GQDs is 390 nm,the maximum emission wavelength is 590 nm,the average size is 4.83 nm,and the fluorescence quantum yield is 14.91%.Under optimal conditions,the concentration of Fe³⁺in the range of 0.1-8μmol/L is in a good linear relationship with the decrease of fluorescence intensity,the correlation coefficient was 0.9988,and the detection limit was as low as 0.06μmol/L（S/N=3）.Finally,the P,Cl-GQDs was successfully utilized for the detection of Fe³⁺in spiked water and human serum samples.2.The S,P-GQDs with good fluorescence performance were successfully prepared by hydrothermal method using citric acid,sodium phytate and sodium sulfate as precursors.S,P-GQDs were used as fluorescent markers by covalently coupling with monoclonal antibody of OA（Anti-OA-MAb）.Based on the cFLISA results,in PBS containing 1%BSA,the 50%inhibitory concentration(IC₅₀)of OA was 14.20 ng/mL,and the linear range was 2.04-98.92ng/mL with a detection limit of 1.27 ng/mL.Finally,S,P-GQDs were successfully utilized for the detection of OA in three kinds of shellfish sampleswith satisfactory recoveries between88.0-116.0%.These results indicated the probability of the proposed method in the detection of OA in actual samples.3.Synthesis of OVA-AuNPs by the reduction of chloroauric acid under alkaline conditions using ovalbumin as a reducing agent.Using OVA-AuNPs as fluorescent markers for monoclonal antibody of STX（Anti-STX-MAb）,the cFLISA method of STX was established.Based on these observations,S,P-GQDs（blue fluorescence）and OVA-AuNPs（red fluorescence）which produced two-color fluorescence,was labeled on Anti-OA-MAb and Anti-STX-MAb respectively.cFLISA was established for simultaneous detection of OA and STX.The results showed that the IC₅₀ values for OA and STX in the PBS buffer containing 1%BSA were 17.94ng/mL and 8.48 ng/mL,respectively,the linear ranges were 2.51-128.04 ng/mL and 2.44-29.50ng/mL,and the detection limits were 1.87 ng/mL and 0.63 ng/mL.Finally,the cFLISA was successfully applied in the spiked recovery detection of OA and STX in three kinds of shellfish samples.