Fluorescence Derivation Analysis Of Folic Acid And Spectrophotometry Of COD Based On Nano-TiO₂ Photocatalysis

As an advanced oxidation technology,photocatalytic technology has the advantages of safety,high efficiency and non-pollution.It has been widely applied in fields of pollutant degradation,fuel cell,photoelectric chemical sensor,etc.The combination of photocatalysis and various analytical techniques has become a trend in the field of analytical chemistry.Spectral analysis is an important method for environmental and biological analysis.However,the research and application of combining photocatalytic technology with spectral analysis are relatively limited.Therefore,in order to broaden the application range of photocatalysis and maximize the double advantages of photocatalysis and spectral analysis,photocatalysis was combined with fluorescence analysis and spectrophotometry respectively in this research.Based on the photocatalytic redox action of nano-TiO₂to change the redox status of the substrate,the fluorescence analysis method for determination of folic acid and the spectrophotometry for chemical oxygen demand（COD）were established respectively.The main contents of this dissertation are shown below.（1）Under the irradiation of UV,the efficient photocatalytic oxidation of the non-fluorescent substance folic acid by nano-TiO₂was carried out to oxidize folic acid to a highly fluorescent substance 6-formylteropterin（6-FOP）,then,the fluorescence detection method of folic acid was established.Under optimized experimental conditions,the linear range of the folic acid detection method was from 0.0010 to 1.30 mg L^-1,and the detection limit was 0.35μg L^-1.The method could be used for the determination of folic acid in real samples.The photocatalytic oxidation mechanism of folic acid was studied by absorption spectra and liquid chromatography-tandem mass spectrometry.The results showed that the photocatalytic oxidation process was similar to the photodegradation of folic acid.Hydroxyl radicals generated by nano-TiO₂oxidized the C-N bond in folic acid,resulting in the disappearance of intramolecular photoinduced electron transfer effect and enhanced molecular fluorescence.Compared with chemical fluorescence derivatization,the photocatalytic fluorescence derivatization process is simple and efficient,without the use of strong oxidant.This method has advantages of photocatalysis（economic,efficient and clean）and fluorescence analysis（high sensitivity,low cost and rapid detection）,providing a new idea for the combination of photocatalysis and fluorescence method for quantitative analysis.（2）The ultraviolet-visible spectrophotometry for the determination of COD was established based on the reduction chromogenic of Mo（Ⅵ）in molybdophosphate heteropoly acid and the oxidation of organics by TiO₂photocatalysis.Under acidic conditions,ammonium molybdate would react with sodium dihydrogen phosphate to form molybdophosphate heteropoly acid.Using nano-TiO₂as photocatalyst,under UV irradiation,Mo（Ⅵ）in molybdophosphate heteropoly acid could be reduced to blue reduction state Mo（V）by photo-induced electron,and potassium phthalate monobasic（KHP,COD standard）can be oxidized by photo-generated holes or free radicals.Under optimized experimental conditions,there was a good linear relationship between the concentration of COD(0.50-100 mg L^-1)and the absorbance of the photocatalytic solution,and the detection limit was 0.17 mg L^-1.Compared direct photoinduced reduction with photocatalytic reduction,the results showed that the solution of photocatalytic reduction was more stable.The reason might be that the photoinduced reduction was chromogenic due to the charge transfer between KHP and Mo（Ⅵ）,it was a reversible discoloration process.In the presence of TiO₂,KHP was oxidized by photo-generated holes or free radicals,Mo（Ⅵ）was mainly reduced by photo-induced electron to produce color,which was relatively difficult to fade.As a chromogenic agent,molybdophosphate heteropoly acid could also be used as an electron trapping agent to inhibit the recombination of photo-generated electrons and holes,then improved the efficiency of photocatalytic oxidation.Besides,the method combined photocatalytic technology with chromogenic agent for the determination of substance spectrometry.It is mild and easy to operate,and had good development potential.