Portable GO Quantum Dot Fluorescence Sensor For The Detection Of Iron Ions In Water

Compared with conventional quantum dots,graphene quantum dots(GOQDs)have excellent biocompatibility,low cytotoxicity,and good water solubility.Besides,GOQDs have electrical and mechanical properties similar to graphene,as well as excellent optical properties due to their small size and abundant oxygen-containing functional groups on the surface,which have attracted the attention of researchers.Larger and nonuniform size have always been the main drawbacks of GOQDs prepared by top-down methods because the non-uniformity in size affected the luminescence performance of GOQDs significantly.Besides,the luminescence and fluorescent sensing mechanism of GOQDs are still not clear,which hindered the further optimization and application of GOQDs.To address these issues,a fabrication method was developed to obtain GOQDs with uniform size in this thesis,and it was found the prepared GOQDs have a specific recognition effect on iron ions.Accordingly a portable GOQDs fluorescence sensor was built to realize in-field detection of iron ion in water.Last,the luminescence mechanism and fluorescence sensing mechanism of GOQDs were discussed.The followings were reported in details in this thesis:(1)GOQDs containing large amounts of oxygen-functional groups were prepared using an improved Hummers methods that adoped 900℃ expanded graphite as the carbon source.Next,GOQDs with different size were separated using gradually centrifugal method after strong ultrasonic dissection.Here GOQDs with smaller size were obtained by centrifugal treatment with higher speed.The GOQDs were characterized by transmission electron microscopy(TEM),X-ray diffraction(XRD),Fourier infrared spectroscopy(FT-IR),X-ray photoelectron spectroscopy(XPS),Raman,Ultraviolet-Visible absorption spectroscopy(UV-Vis)and Fluorescence spectroscopy(PL),and the optimum preparation conditions were investigated.It was found that the prepared GOQDs had small size(2.32 nm)and size distribution range(1.2-3.9 nm),good stability,and excellent fluorescence properties(orange fluorescence,537 nm emission by 450 nm excitation).(2)Combining the excellent optical properties of GOQDs,a fluorescence sensor was built to detect the concentration of iron ions in solution.The sensor has a fast response speed,strong anti-interference ability,and a good linear relationship with the concentration of iron ions.It has a wide detection concentration range(9.9 nM238.58 μM),and has been successfully applied to the detection of tap water.The detection value is 4.77 μM,which is 2.36%different from the result in standard laboratory detection.A portable fluorescent GOQD detector was developed for the in-field detection of iron ion in water.The advantages of the detector included:(1)Simple to operate.The testing can be completed by sending instructions from smart phone after injecting the GOQDs solution and water sample into the micro fluidic chip in sequence.(2)Compact size.The size of the whole detector is approximately 10 cm×6 cm×8 cm.(3)Low cost.The PDMS microfluidic chip,the driving and photocurrent detection ASIC was home-made and in low cost.The light source adopt the commercial LED.Thus the total cost was reduced;(4)Rapid detection.The incubation time of adding water sample to GOQDs solution is 2.5 min,and the whole detection process can be finished in 4 min.The detector showed a good linearity in the range from 0.091 μM to 551.77 μM.Iron ion in tap water was detected to verify the validity of the portable detector.In result,the detection value is 4.39 μM,which is 5.8%different from the results obtained by the standard laboratory method.According to the World Health Organization,the upper limit standard of iron ion in drinking water is 5.4 μM,which is far within the detection range of the detector,so the detector is able to realize the on-site rapid detection of environmental water.(3)The changing fluorescence performance with different pH values was investigated.By analyzing the functional groups according to their FT-IR spectrum and fluorescence intensity at different pH values,it was found that the fluorescence generation mainly depended on the oxygen-containing functional groups on the surface of GOQDs.This hypothesis was proved by annealing of GOQDs,combined with before and after annealing fluorescence spectrum and FT-IR spectrum analysis.Based on the luminescence mechanism,the fluorescence intensity ratio of the detection of bivalent and trivalent iron under neutral conditions and the fluorescence life curve before and after the addition of iron ion,which showed no significant difference,indicating that the sensing mechanism mainly depended on the complexation reaction with iron ions.In conclusion,the thesis presented the preparation of GOQDs with good uniformity,the fluorescence mechanism of GOQDs and the fluorescence quenching mechanism for iron ion sensing.Accordingly,a portable GOQDs fluorescent sensor was built to detect iron ions in environmental water successfully.