Rapid Detection Of Nitrite Content In Pickled Vegetables By Molybdenum Disulfide Fluorescence Probe

Pickled vegetables are popular with consumers due to their unique flavor and good taste.However,the microorganisms attached to the vegetable surface will reduce nitrate in vegetables to nitrite in the process of pickling,resulting in the risk of nitrite content exceeding the standard in pickled vegetables.As known,nitrite has strong toxicity and potential carcinogenicity to human body.Therefore,it is necessary to rapidly detect nitrite content in pickled vegetables.As far,the methods for nitrite detection include spectrophotometry,electrochemistry,chromatography,fluorescence spectrometry and so on.Among them,fluorescence spectrometry has attracted much attention for its simplicity,high sensitivity and low detection limit.How to design and fabricate fluorescent nanoprobes with good selectivity and high sensitivity is the key to the determination of nitrite by fluorescence method.Molybdenum disulfide quantum dots?MoS₂ QDs?,as new type of graphene-like nanomaterials,have been widely used in the fields of bioimaging,sensing and detection due to their good physical,optical and biological properties.In this article,two new strategies for nitrite detection have been developed using MoS₂ QDs based on fluorescence spectroscopy.Moreover,the fluorescence colorimetric detection of nitrite was also realized by combining smart phones and microfluidic chips.The main study contents are as follows:?1?Nitrite was detected by fluorescence resonance energy transfer?FRET?effect between MoS₂ QDs and neutral red?NR?.Firstly,MoS₂ QDs were prepared by ultrasonic shearing method.Then,the morphology and structure of MoS₂ QDs were characterized by transmission electron microscopy and Raman spectroscopy.The fluorescence spectra show that MoS₂ QDs have the largest fluorescence emission peak at 530 nm when the excitation wavelength is 460 nm.When NR was introduced into MoS₂ QDs solution,the FRET behavior occurred between them,which made the fluorescence of MoS₂ QDs quenched.When nitrite was added to MoS₂ QDs-NR system,NR reacted with nitrite to form diazo salt,blocking the FRET process and causing the fluorescence recovery of MoS₂ QDs.Based on this phenomenon,this study achieved high sensitivity detection of nitrite in the range of 0.01-0.3 mg·L^-1,and the detection limit is 0.38 nM according to the formula 3?/S.Finally,the MoS₂ QDs-NR system was applied to the determination of nitrite in pickled garlic samples with the satisfactory recoveries of 94.0%-102.7%,indicating the great potential of the developed methods for nitrite detection in pickled garlic.?2?A ratiometric fluorescent probe based on molybdenum disulfide-gold nanoclusters?MoS₂ QDs-AuNCs?was designed for nitrite detection in pickled cabbage.Firstly,MoS₂ QDs and AuNCs emitting blue and red fluorescence were prepared respectively by hydrothermal method and chloroauric acid reduction method,and MoS₂QDs-AuNCs complex was obtained by EDC/NHS reagent.Then the prepared materials were characterized by transmission electron microscopy,X-ray diffraction spectroscopy,Fourier transform infrared spectroscopy,fluorescence spectroscopy and ultraviolet-visible absorption spectroscopy,and the detection mechanism was analyzed in detail.The fluorescence spectra show that when the excitation wavelength is 365 nm,the MoS₂ QDs-AuNCs complex exhibits double emission fluorescence at 430 and 615 nm,corresponding to the fluorescence peaks of MoS₂ QDs and AuNCs,respectively.When nitrite was added to MoS₂ QDs-AuNCs system,the ratio of fluorescence intensity(I₆₁₅/I₄₃₀)to nitrite concentration in the range of 0.5-40 mg·L^-1 had a good correlation?R²=0.991?,and the detection limit was 0.67 nM.Finally,the MoS₂ QDs-AuNCs system was applied to the determination of nitrite in pickled cabbage samples with the satisfactory recoveries of 92.5%-102.5%,which indicates the proposed method could be used to detect nitrite in pickled cabbage.?3?Nitrite was detected by fluorescence colorimetry based on molybdenum disulfide nanocomposite system.Based on the previous two chapters,according to the change of fluorescence color of MoS₂ QDs-NR and MoS₂ QDs-AuNCs system with nitrite concentration,microfluidic chips based on MoS₂ QDs-NR and MoS₂ QDs-AuNCs complex were constructed respectively,and a self-assembled image acquisition device was used to collect the corresponding fluorescent color.Then,based on Android smart phone and computer vision development platform?OpenCV?,an Android application for detecting nitrite was developed by processing fluorescent image data,which achieved the quantitative determination of nitrite in the range of 0.01-0.3 mg·L^-1 and 1.0-20 mg·L^-1.Finally,the nitrite content in six kinds of pickled vegetables was determined by the strategy proposed here and the national standard method-Griess reagent,respectively.The results showed that the order of nitrite content from high to low was pickled radish>pickled mustard>pickled cucumber>pickled cabbage>pickled garlic cloves>pickled soybean sauce.Moreover,compared with the national standard method,the error rate of nitrite determined by fluorescence colorimetry developed in this study is 2.06%-16.67%,indicating that this method can be used to detect nitrite content in pickled vegetables.In this study,two fluorescence systems based on MoS₂ QDs were employed to the rapid nitrite detection in pickled vegetables,and image processing technology and microfluidic chip were used to realize the fluorescence visualization of nitrite.This paper provides new technical support for real-time and rapid detection of nitrite content in pickled vegetables.