Rapid Detection Of Nitrite By Surface-Enhanced Raman Spectroscopy Based On Plasmonic Photocatalysis

Nitrite has severe toxicity and possible carcinogenicity to humans that poses a serious threat to human health,which is widely existed in pickled vegetables and processed meats.Thus,rapid detection of nitrite is of great importance to food safety monitoring.p-Aminothiophenol(PATP),one of the most important probe molecules in surface-enhanced Raman spectroscopy(SERS)study,has very unique SERS signal.This project introduced another new selective enhancement phenomenon of PATP that the three characteristic Raman bands(i.e.1138,1388,1433 cm-1)will be selectively enhanced in SERS spectrum after the addition of nitrite ion(NO2-)in acid solution.Based on surface-enhanced Raman spectroscopy,density functional theory and mass spectrometry,the effect of NO2-on the SERS spectra of PATP-modified GNPs was investigated.It was confirmed that NO2-is the fundamental reason for the generation of the three selectively enhanced Raman bands,which were actually belong to p,p '-dimercaptoazobenzene(DMAB),the reaction product of plasma-assisted catalytic reaction of PATP on the surface of GNPs.On the basis of these results,two mechanisms of the nitrite-triggered surface plasmon-assisted catalytic conversion of PATP on GNPs were proposed,by exploring the effect of laser power,ambient conditions and other experimental conditions on the conversion of PATP to DMAB.Nitrite participated in this conversion as electron acceptor or "hot" electron quencher.After several steps of electron/proton transferring process,an intermediate of p-nitrothiophenol(PNTP)was generated.Then,after the nucleophilic addition of PATP to PNTP or through the dimerization of the PNTP,the DMAB was therefore generated.Finally,inspired by the high selectivity of the above nitrite-triggered reaction,a simple and fast nitrite screening method based on surface-enhanced Raman spectroscopy of PATP-modified GNPs probe was also developed.After optimizing the relative parameters,the optimal size of GNPs(?60 nm),optimal particle concentration(1.8 ?M or 0.18 ?M)and the pH range(pH?2.5)were obtained.This method was suitable for detecting nitrite below 100 ?M with a minimum detection limit of 1.68 ?M.This SERS method was also used to determine sodium nitrite in diluted water with a linear range of 11.25?100 ?M.The R2 values of 1138,1388 and 1433 cm-1 were 0.9584,0.9395 and 0.9535,respectively.The lowest detectable concentration is 11.25 ?M,which meets the national standards for drinking water requirements(71.4 ?M).Then the nitrite concentration in tap water,saliva and pickled radish were tested to verify the feasibility of this method in real samples.The results indicated that this method is suitable for rapid detection of nitrite in low-salt real samples.In this project,a rapid detection method for nitrite by SERS technique based on plasmonic photocatalysis of PATP absorbed on large size GNPs was introduced.The proposed method exhibited several advantages of good selectivity and sensitivity,simple and fast,sample pretreatment-free,etc,so that it is expected to become on-site fast screening or point-of-care testing method for nitrite.