Headspace Thin-Film Microextraction Coupled with Surface

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Headspace Thin-film Microextraction Coupled with SurfaceEnhanced Raman Scattering as a Facile Method for Reproducible and Specific Detection of Sulfur Dioxide in Wine Zhuo Deng, Xuexu Chen, Yiru Wang, Enhua Fang, Zhigang Zhang, and Xi Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac503341g • Publication Date (Web): 21 Nov 2014 Downloaded from http://pubs.acs.org on November 29, 2014

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Analytical Chemistry

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Headspace Thin-film Microextraction Coupled with

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Surface-Enhanced Raman Scattering as a Facile Method for

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Reproducible and Specific Detection of Sulfur Dioxide in Wine

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Zhuo Deng1, Xuexu Chen1, Yiru Wang1*, Enhua Fang3, Zhigang Zhang3, Xi

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Chen1,2**

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1

Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis &

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Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University,

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Xiamen 361005, China

9 10 11

2

State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005,

China 3

Inspection and Quarantine Technology Center, Xiamen Entry-Exit Inspection and Quarantine

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Bureau of the People`s Republic of China, 2165 Jian`gang Road, Xiamen 361026, China

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Corresponding Author

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* E-mail: [email protected]; Fax: +86 592 2184530; Tel:+86 13599510908

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** E-mail: [email protected]; Fax: +86 592 2184530; Tel:+86 5922184530

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1

ACS Paragon Plus Environment


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Abstract

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By coupling thin-film microextraction (TFME) with surface enhanced Raman

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scattering (SERS), a facile method was developed for the determination of sulfur

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dioxide (SO2), the most effective food additive in winemaking technology. The TFME

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substrate was made by free settling of sea urchin-like ZnO nanomaterials on a glass

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sheet. The headspace sampling (HS) procedure for SO2 was performed in a simple

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home-made device, and then the SO2 was determined using SERS after uniformly

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dropping or spraying a SERS-active substrate (AuNPs) onto the surface of the TFME

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substrate. A reproducible and strong SERS response of the SO2 absorbed onto the

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ZnO substrate was obtained. After condition optimization, the SERS signal intensity

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at a shift of 600 cm-1 and the SO2 concentration showed a good linearity in the range 1

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to 200 µg/mL, and the linear correlation coefficient was 99.2 %. The detection limit

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for SO2 was found to be 0.1 µg/mL. The HS-TFME-SERS method was applied for the

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determination of SO2 in wine, and the results obtained agreed very well with those

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obtained using the traditional distillation and titration method. Analysis of variance

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and student's t test show that there is no significant difference between the two

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methods, indicating that the newly developed method is fast, convenient and sensitive,

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and has selective characteristics in the determination of SO2 in wine.

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Keywords: thin-film microextraction, SERS, sulfur dioxide, wine

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Introduction

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In recent years, surface-enhanced Raman scattering (SERS) has emerged as a

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powerful molecular spectroscopic technique because of its nondestructive and

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ultrasensitive characteristics.1 SERS-active substrates, which generally cause

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excitation of the localized surface plasmon resonance using a nanostructured surface

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or nanoparticle,2,3 play an important role in SERS measurement. The conventional

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SERS-active substrates are metallic colloids due to their ease of preparation and large

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SERS signal enhancement ability.4 However, metallic colloids tend to be unstable and

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susceptible to matrix effects,5,6 resulting in the reduction or loss of SERS signals,7 and

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this has severely limited the practical application of SERS. Appropriate improvements

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based on novel SERS-active materials and technologies have been developed and, for

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example, attempts have been made to realize the in situ application of SERS using the

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specific interaction between Ag SERS-active materials and analyte, but the Ag SERS

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signals are highly reactive, and it is difficult to obtain good results in complex

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matrices.8,9 Recently, an ultrathin coating shell-isolated nanoparticle-enhanced Raman

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spectroscopy was developed,10 in which the nanoparticles could be kept from

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agglomerating. In application, the ultrathin coating nanoparticles are not easy to

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prepare, which limits its widespread application. Therefore, a simple, cheap and quick

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sample pretreatment is necessary for SERS application.

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Since a highly effective sample pretreatment would benefit the overall performance

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of an analytical method,11 sample pretreatment has been recognized as the main

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bottleneck of analytical processes, especially in the trace analysis of a complex

3



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matrix.12 Solid-phase microextraction (SPME), first reported in 1990,13 has become a

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popularly used technique in the pre-concentration of trace analytes due to its

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solvent-free and easy-to-automate properties, but the dozens of microns diameter of

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the SPME fiber is not suitable for SERS applications, in which a millimetre-sized

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laser spot is used. However, a developed extraction approach, thin-film

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microextraction (TFME),14 has revealed more advantages for SERS applications, such

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as a larger surface area-to-volume ratio, small thickness of the extraction phase,

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higher extraction rate and shorter equilibration time.15 Although TFME has been

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coupled to an analytical instrument including a home-made thermal desorption unit

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coupled with gas chromatography16 and membrane introduction mass spectrometry17,

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these approaches are neither simple nor fast in their application. Therefore, the

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combination of SERS and TFME may provide a more convenient strategy for

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analytical applications.

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For TFME-SERS applications, the extraction substrate is beyond doubt of great

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importance. ZnO has received special attention in recent years in view of its excellent

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performance

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sulfur-containing groups.18,19 ZnO nanomaterials with high thermal stability can be

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easily synthesized in various shapes and sizes using different chemical methods,20

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which thus provide a good choice in the TFME substrate. More interestingly,

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semiconductor ZnO can directly generate weak SERS activity by supporting chemical

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enhancement,21,22 which brings a much higher SERS effect to ultra-sensitive SERS

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detection.

and

high

affinity

towards

volatile

organic

4


compounds

and

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In our study, we developed a new approach combining TFME with SERS to

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perform a simple, selective and easy-operation for the determination of SO2, a most

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effective food additive in winemaking technology.23 In the preparation of the

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extraction substrate, sea urchin-like ZnO nanomaterials were deposited on a glass

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support sheet for TFME. After headspace sampling (HS), the highly volatile SO2 was

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adsorbed and concentrated by the nano ZnO, and the content of SO2 was then

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measured using SERS. Compared to such reported methods as ion chromatography24

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and spectrophotometry25, the TFME-SERS provided a simple, rapid and selective

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approach for the determination of SO2 in wine.

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Experimental Section

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Materials

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Analytical grade zinc nitrate hexahydrate (Zn(NO3)2·6H2O) was bought from the

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Xilong Chemical Co., Ltd (Guangdong, China), and anhydrous sodium sulfite Na2SO3

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(97%) and the other chemicals used in the study were purchased from the Sinopharm

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Chemical Reagent Co., Ltd. (Shanghai, China). Nerolidol was obtained from

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Sigma–Aldrich (Shanghai, China). All acids were diluted with pure water (V water:

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Vacid=3:1) before use. Other chemical reagents were all of analytical grade.

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Different concentration stock solutions of Na2SO3 were prepared by dissolving

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powdered Na2SO3 in pure water and gradually diluting to the final concentration in the

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range 0.1 to 200 µg/mL. All the stock solution and the standard solution of Na2SO3

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were freshly prepared prior to the experiments. Pure water was obtained from a

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Millipore Autopure WR600A system (Millipore, Ltd., USA). Wine samples were 5



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obtained from local supermarkets.

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Citrated-reduced gold hydrosol was purchased from the Push Nanotechnology Co.,

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Ltd. (Xiamen, China). The concentration of gold colloid was 0.3 mmol/L and the

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average size of the AuNPs was about 55 nm which proved to be efficient for SERS

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under near-infrared (785 nm) excitation. The commercial gold hydrosol was qualified

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by the comparison of scanning electron microscopy and UV-Vis absorbance spectra

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between different colloid batches (Figure S1 of the Supporting Information). These

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gold hydrosol can be in stable condition for 6 month before opening when storage at

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the 4-10 °C. Gold nanoparticles from the commercial gold hydrosol were

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preconcentrated before use. 45.0 mL gold hydrosol was reduced to 1.0 mL by

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removing the supernatant with centrifugation at 4000 r/min for 15 min.

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Preparation of sea urchin-like nano ZnO for thin-film microextraction

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A sea urchin-like nanostructure of ZnO was prepared for TFME based on a

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previous report with minor modification.26 Specifically, 15 mL zinc nitrate solution (5

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mM) was slowly added to 15 mL aqueous potassium hydroxide (40 mM) in a beaker

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under stirring. Then, a glass sheet 0.8 × 0.8 cm was placed evenly on the beaker

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bottom. After the reaction solution had been kept at room temperature for 12 h, a

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dense layer of nano ZnO substrate was formed on the glass sheet surface. The

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obtained glass sheet was finally washed with pure water three times and then dried.

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HS-TFME and SERS measurement of SO2

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In the HS-TFME of SO2, as described in Scheme 1, the glass sheet covered with

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nano ZnO film was hung in the center of a glass vessel to ensure the headspace

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extraction of SO2 from the sample solution. The sample volume in the glass vessel

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could be selected based on analytical demands. In the extraction process, the vessel

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temperature was kept constant at 55 °C using a temperature controller and, after an

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excess of acid was added, the glass vessel was immediately sealed. After extraction,

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the glass sheet covered with the nano ZnO layer was removed from the vessel, and

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then 10 µL concentrated AuNPs was dropped onto its surface for SERS analysis. All

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SERS measurements were carried out in triplicate. Each SERS signal was collected

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3~4 times from randomly selected positions within the SERS-active substrate area,

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and the data were then averaged.

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Instrumentation

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Scanning electron microscope (SEM) images of the nano ZnO were acquired using

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an S4800 field-emission SEM (Hitachi, Tokyo, Japan). X-ray diffraction (XRD)

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analysis was conducted with a Rigaku Ultima IV XRD using Cu Kα radiation (35 kV,

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15 mA, λ = 1.54051 Å). SERS spectra were recorded using a commercial portable

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spectrometer (DeltaNu Inspector Raman, USA). The laser wavelength was selected at

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785 nm with a spot size of approximately 3 mm2. The system resolution was 8 cm-1

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and the Raman signal acquisition integration time was set as 1 s. All measurements

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were performed at room temperature.

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Results and discussion

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Preparation and characterization of nano ZnO

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A simple and practicable one-step reaction procedure was used to prepare nano

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ZnO for TFME. After mixing zinc nitrate solution with potassium hydroxide aqueous

7



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solution, nano ZnO was deposited layer by layer on the glass sheet, resulting in a

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dense white flat film on the sheet. As shown in Figure 1a, the flat surface was

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convenient for SERS measurements since the SERS-active substrate (AuNPs) could

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be well dispersed on it.

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The XRD pattern of the nano ZnO (Figure 1b) indicates that all the diffraction

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peaks could be indexed to the hexagonal ZnO, and no additional peaks corresponding

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to other impurities were observed. These results showed that the resulting nano ZnO

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was highly pure. In Figure 1c and Figure 1d, SEM images reveal that most of the sea

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urchin-like nano ZnO was rather loose and uniform, and the needling length was

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about 4 µm. In addition, owing to the preparation in an alkaline medium, the sea

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urchin-like nano ZnO possessed an alkaline and large specific surface, which made it

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an ideal extraction material for sour gases. We also test different batches of the sea

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urchin-like nanostructure of ZnO, the SEM images showed that there is almost no

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change in size and shape of sea urchin-like ZnO nanostructure in different batches

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(Figure S2 of the Supporting Information), which means the proposed synthetic

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method is high reproducibility.

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The effect of different size of Au nanoparticles

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The effect of using different size of Au nanoparticles was also investigated.

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Typically, different size gold nanoparticles were selected to perform the SERS

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progress of parallel samples, the Na2SO3 standard solution was selected as 100 µg/mL.

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All these gold nanoparticles of average diameter of 30 nm, 55 nm, 70 nm, 90 nm, 120

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nm are obtained from the Push Nanotechnology Co., Ltd. (Xiamen, China). The

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UV-Vis absorbance spectra and SEM images between different colloids are shown in

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the Figure S3 and Figure S4 (Supporting Information). We concentrated the different

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size gold nanoparticles and kept them the same concentration before used. From the

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SERS spectra, as shown in Figure S5 (Supporting Information), no obvious change of

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the SO2 shift (600 cm-1) could be found using different size nanoparticles, but

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different SERS intensity could be obtained (Figure S6 of the Supporting Information).

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The SERS intensity reached its maximum value when 55 nm gold nanoparticles was

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used in addition to its lowest background signal (Figure S5 of the Supporting

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Information). Furthermore, based on the experimental results, 55 nm gold

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nanoparticles was easier to store, thus we chose 55 nm gold nanoparticles as the SERS

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substrate.

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Reproducibility of the SERS signal using the nano ZnO

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The reproducibility of the SERS signal is an important factor for evaluating the

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applicability of a new method. In the HS-TFME, 10 mL 200 µg/mL Na2SO3 standard

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solution was selected. After extraction, 10 µL SERS-active substrate (AuNPs) was

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dropped on the nano ZnO substrate, and then SERS spectra were collected and

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recorded from 15 randomly selected positions within the SERS-active substrate area.

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As shown in Figure 2, the SERS signal intensities of SO2 at 600 cm-1 were highly

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uniform, and the relative standard deviations of the SERS signal intensity were found

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to be 5.6%, suggesting identical capabilities for the different sites in the extraction of

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SO2 and Raman signal enhancement by the SERS-active substrate. The good 9



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reproducibility of the SERS signals were attributable to the loose structure of the nano

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ZnO substrate, which caused the AuNPs to be homogeneously distributed on the nano

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ZnO surface (Figure S7 of the Supporting Information).

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Generally, a slow chemical deposition will be helpful to obtain a homogeneous

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layer, and the control of the speed of chemical deposition, the reagent concentrations

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and reaction temperature need to be well controlled. In the experiments, under the

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selected conditions as listed in the experimental section, the SERS signal

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reproducibility of SO2 was measured using the different nano ZnO substrates prepared

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in the same batch. As indicated in Figure 3, for the different concentrations of SO2,

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the relative standard deviations of the SERS signal intensity were found to be in the

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range 15.8to 3.2% for the different concentrations of Na2SO3 (0.5, 1, 5, 10, 50 and

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100 µg/mL), indicating that the preparation process of the nano ZnO was suitable for

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application in the determination of SO2.

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Optimization of souring progress

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In order to release SO2 from the sample solutions effectively, an acidifying process

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is necessary in the HS. Since different acids have different effects on the quantity of

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SO2 volatilized and the HS-TFME procedure, several acids were chosen to acidify the

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Na2SO3 solution. The SERS signal intensities of SO2 at 600 cm-1 using different acids

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are shown in Figure 4. Obviously, there is no SERS signal obtained without the acid

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addition. Among the selected acids, the best SERS signal was achieved using H2SO4

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due to its lower volatility. In the acidifying process, volatile acids such as CH3COOH

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and HCl would be adsorbed on the nano ZnO surface, which would change the basic 10


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properties of the nano ZnO and impede the interaction between SO2 and the nano ZnO

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in the headspace procedure. Because of the reducibility of Na2SO3, an oxidation acid

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such as HNO3 was not suitable for use. Comparison of the results from the SERS

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signals indicated that the stronger acid H2SO4 was more suitable than H3PO4 in the

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acidifying process. Therefore, an excess of H2SO4 was added into the sample solution

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for the HS-TFME procedure.

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Optimization of the extraction conditions

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Several experimental factors were optimized in order to obtain the best extraction

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efficiency. Generally, a stirring mode was helpful to increase the extraction efficiency,

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but as shown in Figure 5a, the SERS signal of SO2 at 600 cm-1 was the same under the

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standing mode in the HS-TFME procedure owing to the high diffusion rate of SO2 in

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the headspace. This result indicated that the agitation mode did not affect the

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HS-TFME results obviously, and so in the HS-TFME procedure, agitation was not

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necessary.

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Extraction temperature is a crucial factor affecting extraction velocity and

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efficiency, and so the effect of an extraction temperature in the range 25 to 65 °C was

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studied. The Raman signal increased when the extraction temperature changed from

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25 to 55°C (Figure 5b) since the higher temperature accelerated the volatilization of

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analyte from the solution, but the SERS signal showed a slight decline when the

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temperature was higher than 55 °C, caused by the competing adsorption between SO2

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and water vapor, as well as the balance of adsorption and desorption of the SO2 on the

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nano ZnO film. Taking the above results into consideration, an extraction temperature 11



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of 55 °C was selected. In addition, the heating mode was also investigated by heating

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the device with a block heater or water bath. There was a minor difference between

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the two heating modes (Figure 5c), but because of the easy operating characteristic

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and less risk of contamination compared to a water bath, a block heater was used in

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the HS-TFME procedure.

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The effect of the extraction time was also studied, and the SERS signal intensity of

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SO2 increased when the extraction time increased to 10 min and then remained

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constant, even when a longer extraction time was selected (Figure 5d). The result

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indicated an equilibrium state was achieved after 10 min extraction, and in the

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following experiments, an extraction time of 10 min was therefore chosen.

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Analytical performance of the HS-TFME-SERS for SO2

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HS-TFME was performed as described in the experimental section. 2.0 mL of

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Na2SO3 standard solution with different concentrations ranging from 0.1 to 200

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µg/mL was added into a 15 mL HS-TFME glass vessel. The series of SERS spectra

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obtained from the different concentrations of Na2SO3 after HS-TFME are shown in

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Figure 6a.The primary peak located at 600 cm-1 was attributed to the symmetric

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bending vibration of O-S-O. The broad band in 930 cm-1 was attributed to the

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symmetric and asymmetric S–O stretching vibration.27,28 The may observed Raman

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shift and literature values along with band assignments are listed in Table S1. Since

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the sulfate is physisorbed on the dry ZnO surface under our experimental conditions,

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the slight discrepancies of Raman shifts may by reasonable. The SERS signal of SO2

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at 600 cm-1 could still be identified even when the concentration of SO2 became 0.1 12


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µg/mL, suggesting the sensitivity of the method. The calibration curve in Figure 6b

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showed a good linear relationship from 1 to 200 µg/mL of Na2SO3, and the linear

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correlation coefficient was found to be 99.2 %. These results illustrated that the

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HS-TFME-SERS approach could achieve high determination sensitivity to sulfite.

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The sensitivity of the method was benefitted by a composite of the semiconductor

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ZnO and AuNPs, which resulted in a high SERS effect contributed to by the

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electromagnetic enhancement caused by the localized surface plasmon resonance of

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the AuNPs and the chemical enhancement caused by the charge transfer between the

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AuNPs and the adjacent ZnO semiconductor.29 We also synthesis other three different

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size and shape of the ZnO nanoparticles using hydrothermal method and perform the

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same progress to detect SO2. As shown in the Figure S8 (Supporting Information),

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different morphology of ZnO nanomaterials, such as the sea urchin-like ZnO

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nanostructures, ZnO nanospheres composed by ZnO nanosheets, ZnO nanoparticles

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about 1-2 µm and ZnO nanorods were used as extraction materials to carry out the

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HS-TFME-SERS analysis. In the experiments, 100 µg/mL Na2SO3 standard solutions

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were used. The SERS intensities of SO2 (600 cm-1) using the different size and shape

272

of the ZnO nanomaterials are shown in Figure S9 (Supporting Information). The

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results reveal that SERS intensity of SO2 by using sea urchin-like nanostructures,

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nanospheres, nanoparticles and nanorods of ZnO was about 100, 77, 65 and 13 (taking

275

the SERs signal of sea urchin-like ZnO nanostructures as 100), respectively. Although

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ZnO nanomaterials with different size and shape have the potential applications in

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SO2 detection, the adsorption capacity and the reproducibility of sea urchin-like

13



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nanostructure of ZnO proved to be the best one. The method was of high sensitivity

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and good reproducibility, which could be contributed by the radial needlelike

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morphology of the sea urchin-like ZnO nanomaterial. These multiple horn-like

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structures uninterruptedly amplified the surface plasmon excitation, and enhanced the

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electromagnetic field greatly. In addition, the slow chemical deposition is helpful to

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obtain a homogeneous layer, resulting more satisfactory precision than those of other

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different size and shape of ZnO nanomaterials

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A matrix non-contacting extraction is a highly effective approach to reducing the

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interference in analytical applications, as a result of which many non-volatile

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components in the liquid matrix cannot be extracted. In the extraction of SO2 from

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wine samples, the big advantage of HS-TFME is that less substrate disturbance was

289

caused by the sample. According to previous reports,30-32 the main co-existing volatile

290

components in wine include ethanol, isopropanol, ethyl acetate, nerolidol and

291

acetaldehyde. The interference by these compounds was estimated following the same

292

procedure as the sulfite analysis, except that the Na2SO3 solution was replaced by the

293

different above-mentioned volatile components at a concentration of 1000 µg/mL.

294

The experimental results are shown in Figure 7. Although some volatile components

295

such as acetaldehyde were easily adsorbed on the nano ZnO substrate as reported,33

296

there was no SERS signal in the 600 cm-1 shift, and the same results were obtained for

297

the other compounds. The results revealed that the co-existing volatile components in

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wine samples did not interfere with the determination of SO2. In addition, since the

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nano ZnO possessed alkaline surface, volatile acids that also can impede the

14


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interaction between SO2 and nano ZnO in the headspace procedure. Thus we examine

301

interferences caused by different concentrations of acetic acid which is the main

302

component of the volatile acidity of wines. The result showed the high concentration

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volatile acids do have influence on adsorption behavior for the ZnO film, but there is

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no obvious decrease of the peak intensities of SO2 when concentration of acetic acid

305

is lower than 1000 µg/mL (Figure S10 of the Supporting Information). Considering

306

the general concentration of acetic acid in wines is approximately 500 µg/mL,34 we

307

can ignore the interferences caused by acetic acid when detect SO2 in wine. These

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results confirmed the good selectivity and free of interference performance of

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HS-TFME-SERS in the sulfite analysis of wine samples.

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In the application of the HS-TFME, the content of SO2 was analyzed in several

311

wine samples collected from local markets. In the determination of SO2, a 2 mL wine

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sample was directly put into a 15 mL glass vessel for HS-TFME without any other

313

pretreatment. After the nano ZnO substrate was placed into the glass vessel, 200 µL

314

H2SO4 was added to the wine sample and the vessel was immediately sealed. The

315

whole vessel was then placed into a thermostat at 55 °C for 10 min. The amount of

316

sulfite in the wine was calculated after SERS analysis. In order to prove that the

317

HS-TFME method was reliable, the official method of the Association of Official

318

Analytical Chemists and the U.S. Food and Drug Administration24,35 was used as a

319

method for comparison of the results. Using the official method, any SO2 in the

320

samples was distilled under an acidic condition, and then oxidized using hydrogen

321

peroxide. The transformation product, H2SO4, was finally titrated against a sodium

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

322

hydroxide solution. A few typical SERS spectra of these wine samples are shown in

323

Figure S11 (Supporting Information), and the results of the comparison, shown in

324

Table 1. To evaluate the precision and accuracy of the proposed method, we made

325

analysis of variance and student's t test of 10 samples detection results of the SERS

326

and Monier-Williams analyses. The results were shown as Table S2 (Supporting

327

Information). The variance analysis reveal that there is no significant difference of

328

precision between the two methods (All Fcalculating < Fstandard), showing the precision

329

of the proposed method should meet with approval. The student's t test showed that

330

there is no significant difference of average amount of detected SO2 results of the two

331

methods. That is to say, the accuracy of the proposed method is credible. There was

332

only obvious difference for the results of No. 10 sample (concentrate of SO2 is nearly

333

or lower than 10 µg/mL) between the standard method and the proposed method.

334

Since the detection limit of the standard Monier-Williams method is about 10

335

µg/mL,36 the near or lower concentrate of SO2 caused large deviation. In addition, a

336

statistical test (p = 0.965) showed no significant difference between the two methods

337

(p>0.05, indicated there is no significant difference). These results confirmed the

338

capability of the proposed HS-TMFE-SERS method for SO2 quantification in wine.

339

Compared to the traditional methods that generally consist of time-consuming

340

distillation and titration steps, this method is simpler and more time-saving since a

341

single HS-TFME-SERS run takes only about 10 min and the data acquisition takes

342

only 1 second. Furthermore, in the SO2 analysis, only one sample at a time can be

343

processed using the traditional methods, but multi-samples can be simultaneously

16


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344

analyzed in the HS-TFME procedure, which greatly reduces the overall analysis time.

345

In addition, only a 2 mL sample was used, insuring a more economical approach in

346

practical applications for routine analysis. It is also worth mentioning that the high

347

sensitivity, selectivity and noninvasive nature of the method provides an alternative

348

tool for the fast analysis of sulfite in various sample matrices such as acid rain, white

349

sugar, and fresh or dried vegetables.

350

Conclusions

351

In summary, we developed a simple and reproducible method combining TFME

352

with SERS to analyze sulfites, a widely used and the most versatile food additive in

353

wine. Sea urchin-like ZnO nanomaterials were synthesized and used as the substrate

354

for HS-TFME. Coupled with SERS, the method was successfully applied in the

355

sensitive and selective determination of SO2 in wine samples. As a result of the

356

significant enrichment superiority of the nano ZnO toward SO2 and the characteristics

357

of the SO2 SERS spectra, SO2 could be detected in wine samples in the range 1 to 200

358

µg/mL, and the results were closely consistent with the official method. The

359

characteristics of simpler operation, shorter analysis time and less sample

360

consumption make it a fast, convenient and more economical approach in routine

361

applications. Moreover, since SERS provides an alternative coupling detection tool

362

for the TFME, HS-TFME-SERS may provide a promising approach for the fast

363

analysis of volatile compounds.

364

Acknowledgements 17



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Page 18 of 30

365

The research was financially supported by the National Nature Scientific Foundation

366

of

367

(2011YQ030124), which are gratefully acknowledged. We thank Professor John

368

Hodgkiss of The University of Hong Kong for assistance with the English in the

369

paper.

370

Supporting Information Available

371

Figures about characterization of the materials, sample analysis, table about Raman

372

frequency and assignment for sulfite, and additional information as noted in the text.

373

This information is available free of charge via the Internet at http://pubs.acs.org/.

374

Notes and references

375 376

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(No.

21105084)

and

the

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Instrumentation

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Chem. 2008, 1, 601-626. (4) Lin, X.-M.; Cui, Y.; Xu, Y.-H.; Ren, B.; Tian, Z.-Q. Anal. Bioanal. Chem. 2009, 394, 1729-1745.

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435

Figures

436 437

Scheme 1. Graphical sketch of the HS-TFME process and SERS analysis of SO2 in

438

wine.

439

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

440

441

442

Figure 1. (a) Photograph of the nano ZnO for TFME; (b)XRD pattern of the nano

443

ZnO; (c) SEM image of the nano ZnO; and (d) magnified view of the SEM image of

444

the nano ZnO

445

22


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446 447

Figure 2. SERS spectra of SO2 obtained at 15 randomly selected positions within the

448

SERS- active substrate area on the nano ZnO substrate. The concentration of Na2SO3

449

was 200 µg/mL.

450

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

451 452 453 454 455

Figure 3. SERS signal of SO2 at 600 cm-1 with different concentrations of Na2SO3 (0.5, 1, 5, 10, 50, and 100 µg/mL). Data were obtained at 3~4 randomly-chosen positions on each different nano ZnO substrate.

24


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456 457

Figure 4. Different SERS peak intensities of SO2 at 600 cm-1 when different acids

458

were added before the HS-TFME procedure.

459

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

460 461

Figure 5. Optimization of the HS-TFME extraction efficiency: (a) SERS signals of

462

SO2 at 600 cm-1 using standing or stirring mode in the HS-TFME procedure; (b)

463

effects of extraction temperature; (c) SERS signals using different heating modes at

464

different temperatures; and (d) effects of extraction time. The HS-TFME conditions:

465

Na2SO3 concentration: 100 µg/mL, sample volume: 2 mL, extraction temperature:

466

55°C, extraction time: 10 min.)

467

26


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468 469

Figure 6. (a) SERS spectra of SO2 after HS-TFME-SERS analysis of Na2SO3 standard

470

solutions at different concentrations and (b) simulation curve of the SERS peak

471

intensities at 600 cm-1 using different concentrations of Na2SO3 standard solutions

472

(sample volume: 2 mL).

473

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

474 475

Figure 7. SERS spectra of the major volatile components found in wine after

476

HS-TFME-SERS analysis.

477

28


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478 479

Table 1. The amounts of sulfite in different wine samples analyzed using the Monier-Williams method36 and HS-TFME-SERS Monier-Williams

HS-TFME-SERS

Sample Detected （µg/mL）

RSD (n=2) （%）

Detected （µg/mL）

RSD (n=9) （%）

Sample 1

114.2±3.1

2.7

115.1±6.9

6.0

Sample 2

47.9±2.6

7.7

49.3±3.4

7.0

Sample 3

59.0±1.6

2.8

64.3±7.7

11.9

Sample 4

102.7±1.6

1.5

97.1±10.4

10.7

Sample 5

92.3±2.4

2.6

90.0±10.9

12.1

Sample 6

60.8±1.1

1.7

68.5±5.0

7.3

Sample 7

59.3±2.1

3.5

51.1±2.6

5.2

Sample 8

115.8±8.0

6.9

115.3±7.0

6.0

Sample 9

14.5±0.4

2.8

7.9±1.2

14.7

Sample 10

N.D.a

—

N.D.a

—

a

N.D.: not detected

480 481

29



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482

For TOC only

483

30


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Headspace Thin-Film Microextraction Coupled with Surface

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