Surface Enhanced Raman Spectroscopy Combined With Microextraction Methods For On-site Analysis Organic Pollutants

Surface-enhanced Raman spectroscopy(SERS),as a vibrational spectroscopic technique,could provide rich molecular structural information.It has the advantage of detection of trace amounts of analytes rapidly,nondestructive and no interference from water in the common area.Therefore,SERS have been widely used in the detection of inorganic or organic pollutants and biological components.Electromagnetic and chemical enhancement mechanisms have been generally used to explain the origination of SERS.Physical and chemical ways have been used to synthesize high effect SERS substrates based on the two mechanisms.The commonly used SERS substrates are classified into two types according to the state of matterials:noble metal nanostructure colloids and silver or gold solid substrates.It is possible for on-site,fast and high throughput detection using the portable Raman spectrometer.With the development of analytical technology,sample pretreatment step makes great contrabutions to analytical process including the separation and enrichment of analytes from complex sample matrices.So far,a number of sample preparation methods were developed and standardized for analysis,such as Soxhlet extraction(SE),accelerated solvent extraction(ASE),supercritical fluid extraction(SFE),and pressurized liquid extraction(PLE).Resulting materials were further analyzed using chromatographic techniques coupled with mass spectrometry(GC-MS,LC-MS)and spectroscopic methods,such as fluorescence spectrometry,infrared spectrometry,UV-Vis spectrometry and so on.Most of them have been used in Chinese standard.Although the development of high precision analytical methods is important,the development of on-site,rapid,in situ,high-throughput detection methods is also of great practical significance.The influence of organic pollutants on environment and health has been paid much more attention nowadays.Therefore,several conbined SERS-pretreatment methods have been developed in this paper and described in detail listed as following:1.Analysis of solid samples by SERS combined with an analytical method:Polycyclic aromatic hydrocarbons(PAHs)have attracted much attention for their carcinogenic,teratogenic and easy to enrichment in organisms' characteristics.Therefore,PAHs have been listed in the relevant laws.and regulations.PAHs could migrate into the body and do harm for health through diet and skin from contamidated food contact materials(toys,bottles,bags and handle etc.).The traditional detection method for solid sample analytes includes pretreatment(seperation and concentration)and analytical methods.We integrated the sample pretreatment and sample analysis process by developing the surface microextraction-SERS hyphenated method for in-situ detection of PAHs in food contact materials.Methanol was used as extractant for extraction of trace analytes.Sliver nanoparticles functionalized by 1-propanethiol were used as SERS-active substrate for enhancing Raman signal of the measured object.This integration method simplified the sample pretreatment procedure,not only save time,but also avoid the loss of analytse during transfer process and the accumulated error.The SERS signal was collected from a portable Raman spectrometer acquisition through the detection.The spectrum collection time for each sample was only 1 s.Trace amount of analytes could be detected by this fast,convenient and high efficient method.The uniformity and stability of this method have been evaluated.The principle of normalization used propanethiol as internal standard during the quantitative detection of PAHs has been clarified.LOD of fluoranthene in A1 foil was 0.27 ng cm-2.LOD of fluoranthene and pyrene in actual sample was 0.194 ?g cm-2 and 28.3 ng cm-2,respectively.Three kinds of PAHs(fluoranthene,pyrene and fluorene)in five kinds of actual food contact materials(French fries plastic boxes,food boxes,paper cups,sweet wrappers and plastic food bags)and their mixture in plastic food bags were detected.Isomers in the mixture can be identified by naked eyes or through principal components analysis techinique from the SERS spectra.This integration Microextraction-SERS method could be further used for rapid in-situ analysis for more pollutants.2.Analysis of aqueous phase samples by SERS combined with an analytical method:Dispersed Liquid-Liquid Microextraction(DLLME),which was reported in 2006,was widely used as a sample pretreatment for multiple organic and metal ion analysis.Combined with the characteristics of DLLME,such as simple,fast,cheap and environmentally friendly,and the characteristics of SERS,such as rapid,high throughput and trace detection,the method of DLLME-SERS hypatheted detection of PAHs in water was carried out by using a portable kit.The kit includes three tubes(tube 1 containing a mixture of extractant and dispersant,tube 2 containing methanol,and tube 3 containing SERS-active sol),a portable Raman spectrometer and a self-made 96-well plates which was used as microreactor.DLLME is a kind of balanced extraction method:the mixture of the extractant and the dispersant is rapidly injected into the sample,and the extractant is divided into a number of fine droplets under the action of the dispersant,and the sample solution suddenly turned cloudy.The fine extract droplets have a large contact area with the sample solution,and the analytes moves quickly between the two phases and reaches the distribution equilibrium.After 1 min low speed centrifugation,the extraction phase is separated from the sample solution phase.The extraction phase which has larger density than the sample solution reached to the bottom of the centrifuge tube.The extraction phase was transferred from the centrifuge tube to the self-made 96-well plate,and then methanol and SERS-active sol were added to the same hole.After drying,the SERS spectra were collected by a portable Raman spectrometer.We examined the quantitative determination of pyrene by this method and fitted it linearly with a LOD of 0.50 ?g L-1.We also investigated the effect of this method for three real samples(Black tiger spring,Darning Lake water and bottled drinking water)with simulated PAHs,and evaluated the homogeneity and stability of this method and the detection of PAHs mixture.This method uses 1 min low speed centrifugation as a sample pretreatment step and self-made 96-hole microreactor.The collection time of SERS spectral for each sample was 1 s,so the SERS peak height information of 1000 samples could be collected only in 17 min theoretically.Their concentration could be quantified quickly refer to the regression curve.This DLLME-SERS method has some significance for fast and high throughput detection.3.Analysis of oil liquid phase samples by SERS combined with an analytical method:Single-Drop Liquid Phase Microextraction(SDLPME)is a sample pretreatment method based on the equilibrium extraction principle.The extraction phase is a single droplet(1 ?L-10 ?L).The analytes are separated from the sample phase and enriched by the extraction phase until reached equilibrium between phases.We developed SDLPME-SERS method for the determination of contaminants in oil samples using a 2 ?L SERS-active silver sol single droplet as microextraction agent.We detected the concentration of propanethiol in n-octane simulated gasoline using the SDLPME-SERS method.Analysis of the contaminant content in oil samples usually requires sample pretreatment techniques,including the removal of organic matter and the enrichment of contaminants with specific reagents.In the SDLPME-SERS method,2 ?L of silver sol was added as a single droplet microextraction agent to 1 mL oil phase sample,stand for a while and then moved to the self-made 96-well plate.The spectral data of the extracted microextraction agent were collected by a portable Raman spectrometer to achieve on-site,rapid and high-throughput quantitative analysis.We optimized the SDLPME-SERS extraction time and the extractant volume.The homogeneity and time stability were investigated,and quantitative detection and high throughput detection were carried out.This method can not only be used for the detection of mercaptan in gasoline,but also has broad prospects for the analysis of other oil samples,such as pesticide residues in edible oil and impurities in silicone oil.