Headspace-Sampling Paper-Based Analytical Device for Colorimetric

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Headspace-Sampling Paper-Based Analytical Device for Colorimetric/SERS Dual Sensing of Sulfur Dioxide in Wine Dan Li, Huazhen Duan, Yadan Ma, and Wei Deng Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00016 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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Analytical Chemistry

Headspace-Sampling Paper-Based Analytical Device for Colorimetric/SERS Dual Sensing of Sulfur Dioxide in Wine Dan Li, * Huazhen Duan, Yadan Ma, and Wei Deng* School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, P. R. CHINA *Corresponding author. E-mail: [email protected] (D. L.); [email protected] (W. D.). Fax: +86-21-6087 3241

ABSTRACT: This study demonstrates a novel strategy for colorimetric/surface enhanced Raman scattering (SERS) dual-mode sensing of sulfur dioxide (SO2) by coupling headspace sampling (HS) with paper-based analytical device (PAD). The smart and multifunctional PAD is fabricated with a vacuum filtration method in which 4-mercaptopyridine (Mpy)-modified gold nanorods (GNRs)-reduced graphene oxide (rGO) hybrids (rGO/MPy-GNRs), anhydrous methanol and starch-iodine complex are immobilized into cellulose-based filter papers. The resultant PAD exhibits a deep-blue color with a strong absorption peak at 600 nm due to the formation of an intermolecular charge-transfer complex between starch and iodine. However, the addition of SO 2 induces the Karl Fischer reaction, resulting in the decrease of color and increase of SERS signals. Therefore, the PAD can be used not only as a naked-eye indicator of SO2 changed from blue to colorless, but also as a highly sensitive SERS substrates because of the SO2triggered conversion of Mpy to pyridine methyl sulfate on the GNRs. A distinguishable change in the color was observed at a SO 2 concentration of 5 μM by naked eye and a detection limit as low as 1.45 μM was obtained by virtue of UV-vis spectroscopy. The PAD-based SERS method is effective over a wide range of concentrations (1 μM to 2000 μM) for SO2, and the detection limit for SO2 is found to be 1 μM. The HS-PAD based colorimetric/SERS method is applied for the determination of SO2 in wine, and the detection results match well with those obtained from the traditional Monier-Williams Method. This study not only offers a new method for on-site monitoring of SO2 but also provides a new strategy for designing of paper-based sensing platform for wide range of field-test applications.

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Sulfur dioxide (SO2) is widely used in the food and beverage industries as a preservative owing to its outstanding antioxidant and antimicrobial properties, which suppress undesirable bacterial growth and oxidation without causing discoloration. 1, 2 Moreover, high concentrations of SO2 can affect the lung function, worsen asthma attacks, and aggravate potential heart disease. 3, 4 Furthermore, the particulate matter derived from the adsorption of SO2 on the aerosol can migrate to the lungs and induce disagreeable health effects. 5 According to the European Union, the maximum allowable concentrations of SO2 in red wines and white wines are 160 mg/L and 210 mg/L, respectively. 6 The conventional methods such as ion chromatography, 7 HPLC, 8 surface acoustic wave 9 and quartz crystal microbalance 10 are widely used in the determination of SO2. However, many of these approaches are expensive, timeconsuming and labor intensive. Moreover, they usually require complicated sample pretreatment procedures and are therefore not readily amenable to use for the rapid and sensitive on-site detection of SO2 in complex matrix samples. Surface-enhanced Raman scattering (SERS) is emerging as a powerful technique for on-site monitoring of additives and pollutants directly, and provides “fingerprint” signatures of analytes. 11-14 However, the SERS substrates combining sensitivity with controllable nanostructures are still a challenge for the practical SERS applications. Paper-based analytical device (PAD) provides a revolutionary approach for conducting inexpensive and rapid on-site analysis. 15 PAD-based SERS substrates have gained considerable attention since they are flexible, inexpensive, efficient for sample collection, and readily

disposable. 16-18 However, only the target analytes that have a strong affinity toward the SERS substrates can yield strong Raman signals. Most small gaseous molecules possess low SERS cross-section and weak affinity toward metallic surfaces, limiting the application of PAD-based SERS detection of gaseous molecules. Therefore, integrating separation and preconcentration on PAD-based SERS substrates is highly desirable. With the introduction of headspace sampling (HS) techniques, it has become possible to promote the separation of volatile contents from complex matrix samples. 19 Considering the unique advantages of PAD as a promising platform for SERS analysis, we envision that the combination of HS-PAD and SERS might provide a more expedient strategy for analytical applications. For HS-PAD applications, the substrate for both preconcentration and SERS detection is beyond doubt of great importance. Reduced graphene oxide (rGO)–gold nanorods (GNRs) hybrids has attracted considerable attention because of its distinct properties and high affinity toward volatile organic compounds and sulfur-containing groups. 20, 21 The rGO–GNRs combines adequate Raman “hot spots” among the high-density GNRs and the excellent adsorption performance of rGO, can serve as an efficient SERS substrates for the detection of SO2. Moreover, rGO can generate weak SERS activity due to a chemical enhancement, 22 thus allowing for on-site fast screening and point-of-care application. In this study, we developed a novel HS-PAD for the colorimetric and SERS detection of the SO2 based on the Karl Fischer reaction, as illustrated in Scheme 1. In the proposed 1

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Scheme 1. Schematic illustration of the HS-PAD procedure and its dual-modal sensing of SO2 in wine. Stock solutions of Na2SO3 at different concentrations were system, the PAD is prepared by impregnating 4prepared by dissolving powdered Na2SO3 in pure water and mercaptopyridine (Mpy)-modified GNRs-rGO hybrids gradually diluting to the final concentration in the range of (rGO/MPy-GNRs), starch-iodine complex (SIC) and anhy0.1-5000 μM. All the stock solution and the standard solution drous methanol in a cellulose-based filter paper. After HSPAD extraction, the volatile SO2 is adsorbed and concentrated of Na2SO3 were freshly prepared prior to the experiments. Iodine (I2) solution (50 mM) for Karl Fischer reaction was by the PAD. Meanwhile, the Karl Fischer reaction is initiated prepared by adding 10 mL of 1 M KI to a mixture (25 mL) by absorption of SO2 on the PAD, resulting in the dissociation containing 0.02 M of KIO3 methanol solution and 0.1 M of SIC and decrease in color intensity. Moreover, SO2-induced H2SO4. conversion of MPy to pyridine methyl sulfate on the PAD can be used as a sensing method for SERS detection of SO2. This Synthesis of 4-Mercaptopyridine (Mpy)-Modified Gold new concept enables colorimetric and SERS detection of SO2 Nanorods (GNRs)-rGO Hybrids (rGO/MPy-GNRs). First, when the target component reaches the detection zone. ComGNRs were synthesized according to the modified seeded pared to existing PAD-based sensing systems, the application growth method. 23 Then, synthesis of the rGO was performed of HS-PAD has several advantages: (1) The discoloration of as reported previously. 24 The formation of rGO was confirmed by Raman and high-resolution X-ray photoelectron SIC and formation of pyridine methyl sulfate after the adsorpspectroscopies (XPS) (Figure S1 and Figure S2). The experition of SO2 result in the decrease of color and enhancement of mental details on their synthesis and purification were providSERS signals, which enables the colorimetric and SERS dualed in the supporting information. mode sensing of SO2. (2) The HS-PAD enables on-site fast The rGO/MPy-GNRs hybrid was synthesized by a selfscreening of SO2 with a high sensitivity and selectivity. The assembly process. Initially, MPy-modified GNRs (MPy-GNRs) distinct color intensity can be visualized by the naked eye in was prepared by mixing MPy (0.5 mL, 1 mM) with a disperthe higher concentration region (5-200 μM), whereas the UVsion of GNRs (5 mL, 1.8 nM). The mixture was equilibrated at vis spectroscopy is more suitable for sensing SO2 over a wide 20 °C for 24 h. Excess MPy in the mixture was removed by concentrations range (10-1000 μM). (3) The HS-PAD combined with a handheld Raman spectrometer facilitates the relicentrifugation at 9,000 rpm for 5 min and re-suspended in 5 able quantitative SERS detection of SO2 with different conmL of methanol. Then, 5 mL of 0.1 mg/mL rGO was added centrations ranging from 0.1 to 5000 μM. More importantly, into the resulting MPy-GNRs and the mixture was kept at the HS-PAD enables the on-site pre-concentration of gaseous room temperature for 24 h. The mixture was centrifugation at 8,000 rpm for 10 min to remove the excess rGO. After having analytes and efficient separation of the target component from been centrifuged, the obtained rGO/MPy-GNRs (1.2 nM) can complex matrix samples, which ensures the selectivity and sensitivity of sensing system and is more suitable for on-site be stored in methanol for more than three months before use. fast screening and point-of-care diagnosis. Fabrication of the Headspace-Sampling Paper-Based Analytical Device (HS-PAD). The rGO/MPy-GNRs and EXPERIMENTAL SECTION starch-iodine complex (SIC) (rGO/MPy-GNRs/SIC) celluloseChemicals. Chloroauric acid (HAuCl4•4H2O, 99.9%), based paper was used as the substrate of HS-PAD in this work, cetyltrimethylammonium bromide (CTAB, 99%), sodium and can be used as a test strips for colorimetric and SERS desulfite (Na2SO3, 98%), ascorbic acid (C6H8O6, 99%), sodium tection of SO2. The preparation of the rGO/MPy-GNRs/SIC borohydride (NaBH4, 99%), silver nitrite (AgNO3, 99.9%), paper was based on the vacuum filtration method. In brief, 2.5 sulfuric acid (H2SO4, 98 wt. %), acetic acid (CH3COOH, 99%), mg SIC were added into 1.5 mL of 1.2 nM rGO/MPy-GNRs 4-mercaptopyridine (MPy, 96%) and were purchased from colloid under stirring. The resulting solution was then disSigma-Aldrich (St. Louis, MO, USA). Potassium iodate (KIO3, persed in 10 mL methanol. The mixture was stirred for 10 min 99%) and potassium iodide (KI, 99%) were obtained from and sonicated for 20 min following a subsequent filtration on Aladdin-Reagent Co., Ltd. (Shanghai, China). Other reagents a cellulose-based filter paper (PC, Whatman) with a pore size were of analytical grade and used without further purification. of 0.22 μm, which were then dried for 30 min in a laminar air Ultrapure water with a conductivity of 18 MΩ•cm was used in flow cabinet. The resultant rGO/MPy-GNRs/SIC paper with a all experiments. thickness of around 2 mm was then stored at room temperature until use. 2


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HS-PAD procedure for SO2 was performed with a threeway T-shaped stopcock and two disposable syringes (Figure S3). Briefly, the rGO/MPy-GNRs/SIC paper was placed into a suitable filter paper holder and the filter paper holder was attached to one syringe. One milliliters of H2SO4 (0.1 M) was added to another syringe and then two syringes were connected to the inlet of three-way stopcock. Then the luer tip of three-way stopcock was plugged in the center of a glass vessel and sealed with 3M epoxy resin (Scotch-Weld CA40H) , thus preventing the leakage of volatile targets. After that, 100 μL H2SO4 (0.1 M) was added slowly into the glass vessel for the generation of SO2. Then SO2 was locked in the HS-PAD system by turning the three-way stopcock to a position in which SO2 is drawn through both the reaction vessel and the bypass arm. In the extraction process, the vessel temperature was kept constant at 50 °C using a temperature controller. Finally, the plunger was pulled out of the syringe barrel so that SO2 can be trapped on the paper. Following an 8 min extraction of the volatile SO2, the blue color of paper obviously decreases after exposure to SO2. After HS-PAD extraction, the rGO/MPyGNRs/SIC paper was removed from the filter paper holder and used for colorimetric and SERS analysis. Colorimetric and SERS Detection of SO2. Colorimetric and SERS sensing for SO2 were performed under the following procedures. Typically, 0.1 mL Na2SO3 with different concentrations was added into a mixture (1 mL) containing 1.2 nM rGO/MPy-GNRs (0.5 mL), 2.5 mg SIC and 0.5 mL of 10% methanol solution. Then, 100 μL H2SO4 (0.1 M) was added slowly into the mixture. The resulting mixtures were incubated for 8 min at 50 °C and then detected by the colorimetric and SERS channel. For colorimetric detection, the as-formed mixtures were used for colorimetric measurements by an UV-Vis spectrophotometer and photo graphed with a digital camera (Nikon D3200, Japan). The SERS measurements were performed on a portable Raman spectrometer (BWS415, B&W Tek Inc., USA) with a resolution of 5 cm-1 and a beam diameter of 10 mm. A 785 nm laser line was used for all the measurements. The laser power and acquisition time were 10 mW and 30 s, respectively. The instrument background was subtracted and baseline corrected with multiple point linear curve fitting. SERS signals from three different spots on each sample were measured. 10 μL of the mixture was dropped onto a silicon wafer under ambient conditions and dried in the ambient environment, and the SERS signal was collected. The rGO/MPy-GNRs/SIC paper enables the colorimetric and SERS dual-mode sensing of SO2. For visualization of SO2 in real samples, the filter holder covered with rGO/MPyGNRs/SIC paper was hung in the center of a glass vessel to ensure the headspace extraction of SO2 from the sample solution. The color intensities of the samples were compared with those of the calibration strips. The visual results can be quantified by measuring the color of the digital images with ImageJ software (NIH, USA). A rectangle section was used to select the analyzed region on the images. The size of the rectangle was set at 5 × 5 mm2 area, and the mean gray value of images was recorded. A background measurement was also taken for the same sized rectangle area closest to the analyzed region on the strips, which accounted for variations in color owing to differences in environmental illumination while the pictures were taken. The final colors of samples were corrected with respect to the background. For application in SERS detection of SO2, a laser power of 10 mW was selected and spectra were recorded with an integration time of 30 s.

Procedures for SO2 Sensing in Wine. A total of three commercially available wine samples were purchased from the supermarket. After HS-PAD extraction, the color intensities of rGO/MPy-GNRs/SIC based strips were compared with that of those of the calibration strips, allowing for colorimetric analysis of SO2. SERS spectra were collected from the test strips using the portable Raman spectrometer. The laser power and acquisition time were 10 mW and 30 s, respectively. The difference SERS spectra were obtained by subtraction of the SERS spectra of spiked samples from that of the blank samples. Characterization. UV-Vis spectra were obtained on a UV2100 spectrophotometer (Tokyo, Japan). The scanning electron microscopy (SEM) were carried out on an FEI-Sirion200 field-emission scanning electron microscope (FEI Co. with 20 kV operating voltage). All pH measurements were obtained using a pHS-25 pH meter (Shanghai, China). IR spectra were recorded using a Bruker Vertex 70 spectrometer. The XPS measurements were performed on an Axis Ultra DLD photoelectron spectrometer (Shimadzu Corp., Japan) with a monochromatic Al Kα radiation (1486.6 eV). The pressure in the analysis chamber was 10−9 mbar and the photoelectron takeoff angle relative to the surface was 45°. RESULTS AND DISCUSSION Characterization of rGO/MPy-GNRs. A convenient and facile two-step procedure is used to fabricate 4mercaptopyridine (Mpy)-modified gold nanorods (GNRs)rGO hybrids (rGO/MPy-GNRs) for the headspace-sampling paper-based analytical device (HS-PAD) (Scheme 1). After mixing rGO/MPy-GNRs with the starch-iodine complex (SIC), the rGO/MPy-GNRs/SIC is deposited on the cellulose-based filter paper by vacuum filtration method, resulting in a dense blue film on the paper, which can be confirmed by SEM characterization (Figure 1). The pure filter paper is composed of uniform cellulose fibers of ca. 12.5 μm width (Figure 1A), while it became a wrinkle-like structure consisting of bent sheets after assembling with rGO (Figure 1B). The rGO/MPyGNRs/SIC paper possesses a uniform and large specific surface, which made it an ideal extraction material for sour gases (Figure 1C). In addition, the density of GNRs on the surface of rGO could be easily controlled by changing the concentrations of Mpy during the assembly process (Figure 2). We also test four different batches of rGO/MPy-GNRs/SIC paper. The SEM images show that there is almost no change in size and shape of rGO/MPy-GNRs/SIC nanostructure in different batches (Figure S4), suggesting that the proposed synthetic method is high reproducibility and rGO/MPy-GNRs/SIC is highly uniformly adsorbed on the cellulose paper. The UV-Vis-NIR extinction spectrum of the GNRs has two characteristic peaks at 525 nm and 790 nm, corresponding to the transverse (TSPR) mode and the longitudinal (LSPR) mode, respectively (curve a in Figure 1D). 25 Moreover, the appearance of two broad bands around 248 nm and 337 nm confirms the presence of GO and Mpy, respectively (curve c in Figure 1D). 26, 27 Two absorption bands of rGO/MPy-GNRs located at 287 nm and 353 nm appear after interaction with SIC (curve d in Figure 1D), which are typical for tri-iodide negative ion. 28 The as-formed rGO/MPy-GNRs/SIC paper is in a deep blue color. The color of rGO/MPy-GNRs/SIC paper fades within minutes in the presence of SO2 (Inset of Figure 1C). The appearance of a broad band around 600 nm can be attributed to the formation of an intermolecular charge-transfer complex between starch and iodine (curve d of Figure1D). 29 3



Figure 1. Typical SEM images of (A) the pure cellulose paper, (B) the rGO and (C) the rGO/MPy-GNRs/SIC paper. Lower left insets in (A-C) show the photographs of the corresponding paper substrate under ambient light. (D) UV-Vis-NIR extinction spectra of (a) MPy-GNRs paper, (b) rGO paper, (c) rGO/MPy-GNRs paper, rGO/MPy-GNRs/SIC paper (d) before and (e) after the adsorption of SO2, respectively. (E) FT-IR spectra of (a) MPy-GNRs, (b) rGO/MPy-GNRs, rGO/MPyGNRs/SIC (c) before and (d) after the adsorption of SO2, respectively. (F) SERS spectra of (a) the pure cellulose paper, (b) the rGO paper, (c) the MPy-GNRs paper, rGO/MPyGNRs/SIC paper (d) before and (e) after adsorption of SO2, respectively. As can be seen from curve e in Figure 1D, two absorption bands of SIC located at 353 nm and 600 nm gradually disappear and shift to 335 nm because of the SO2-triggered Karl Fischer reaction. 29 The rGO/MPy-GNRs/SIC are characterized with the IR spectra (Figure 1E). The characteristic IR bands of MPy on the GNRs appear at 1633, 1425, and 1094 cm-1, which are in good agreement with previous report. 30 The weakly basic characteristic vibrational peak at 715 cm-1 can be assigned to trans C-S stretching, indicating that the presence of SO2 can trigger the conversion of MPy to pyridine methyl sulfate on GNRs. 31 Moreover, XPS data provides further information on the formation of pyridine methyl sulfate. As shown in Figure S5, the N 1s signal of rGO/MPy-GNRs/SIC at 400.6 eV shifts slightly to 399.2 eV after the addition of SO2. The observed red-shift can be attributed to the changes in the local environment of pyridyl nitrogen which may be caused by the SO2-triggered Karl Fischer reaction. 32 To evaluate the SERS performance of the rGO/MPyGNRs/SIC paper towards SO2, the SERS behavior of paper substrate through different modifications is investigated. The SERS spectra of rGO/MPy-GNRs/SIC paper show the distinct peak at 725, 992 and 1048 cm-1 (curve d in Figure 1F), which is typical for MPy. 33 Upon adsorption of SO2, a new peak appears at 518 cm-1 and an increase Raman intensity at 1000 cm-1 is observed (curve e in Figure 1F). These peaks can be assigned to SO2 bending and ring-breathing vibration of pyridine, respectively, and these bands correspond to the characteristic peaks of SO2-pyridine complex. 34 The large enhancement of rGO/MPy-GNRs/SIC paper can be attributed to the “hot spots” at junctions between GNRs and rGO. Meanwhile, the plasmonic properties of GNRs can be well tuned by controlling their aspect ratio (AR) of GNRs (Figure S6). The strongest SERS intensity occurs when the LSPR of the GNRs (AR=3.7) overlaps with the near-infrared Raman excitation source (785 nm) (Figure S7 and Figure S8). To investigate the SERS activity of rGO/MPy-GNRs/SIC quantitatively, the SERS spectrum of SO2 after HS-PAD-SERS analysis of Na2SO3 solution and normal Raman spectrum of bulk MPy

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Figure 2. SEM images of the as-prepared rGO/MPyGNRs/SIC with the different concentrations of Mpy during the assembly process: (A) 1×10-5 M, (B) 1×10-4 M, (C) 1×10-3 M, (D) 5×10-2 M. were recorded and the corresponding enhancement factor (EF) was calculated to be 3.9×104 (Figure S9). These results substantially demonstrate the successful preparation of rGO/MPyGNRs/SIC paper which combined the inherent properties of both the GNRs and the rGO, thus could be used both as colorimetric and SERS sensor for SO2, as demonstrated below. Colorimetric and SERS Detection for SO2. Considering the trace and ultra-trace amounts of SO2 in wine samples, it is necessary to optimize the HS-PAD parameters to obtain the optimal performance of the sensor. Therefore, a series of tests were performed to obtain the optimum conditions for the HSPAD procedure. An acidification process is necessary to effectively release SO2 from the sample solutions in the HS-PAD. As shown in Figure S10, there is no SERS signal obtained without the acid addition. Among the selected acids, H2SO4 is found to be the most efficient acid for producing SO2 because of its lower volatility. Moreover, temperature is a crucial factor affecting the HS-PAD velocity and efficiency. The effect of temperature in the range of 25-75 °C is studied, as shown in Figure 3A. I518 and I1000 increase when the temperature changes from 25 to 50 °C since the higher temperature accelerates the volatilization of SO2 from the solution, but the SERS signals show a slight decline when the temperature is higher than 60 °C (Figure 3B), which may be caused by the competing adsorption between SO2 and water vapour. The color intensities are almost the same from 50 °C to 65 °C. Taking the above results into consideration, an extraction temperature of 50 °C was selected. In addition, the effect of the extraction time is also studied. It can be observed that the SERS signals and color intensities increase when the incubation time increased up to 8 min and then remain constant, even when a longer extraction time is selected (Figure 3C). This result indicates that an equilibrium state is achieved after 8 min of incubation (Figure 3D). Therefore, for the following experiments, an extraction time of 8 min is chosen. The influence of the ratio of the volume of gas-liquid (VG/VL) on the HS-PAD extraction is also investigated as shown in Figure S11. With the increasing VG/VL, the colorimetric and SERS response remain stable in the range 4


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Figure 3. (A) The effect of temperature on the SERS spectra of rGO/MPy-GNRs/SIC paper with the presence of Na2SO3 at 100 μM. (B) SERS intensities at 518 cm−1 and 1000 cm-1 (left) and color intensity (right) with different temperature. (C) The effects of extraction time on the SERS spectra of rGO/MPyGNRs/SIC paper with the presence of Na2SO3 at 100 μM. (D) SERS intensities at 518 cm-1 and 1000 cm-1 (left) and color intensity (right) with different extraction time. The insets in (B) and (D) show the photograph of the corresponding rGO/MPyGNRs/SIC paper under ambient light. Each data point represents the average value from five measurements on the same samples. Error bars show the standard deviations. from 4:1 to 19:1, due to saturated adsorption of SO2 on the rGO/MPy-GNRs/SIC. In addition, the SERS intensities of SO2 at 518 cm-1 and 1000 cm-1 is almost the same between the standing and stirring mode (Figure S12). Therefore, the stirring is not necessary during the HS-PAD procedure. Under the optimal conditions, we firstly evaluate the sensitivity of our method. After the addition of different concentrations of Na2SO3 into the dispersion of rGO/MPy-GNRs/SIC at 50 oC for 8 min, the UV-vis and SERS spectra of the resulting mixture are investigated. With gradually increasing of Na2SO3 amount, the absorption intensity at 287 nm and 353 nm decrease (Figure 4A), while the intensity at 335 nm increased with increasing of the concentrations of Na2SO3 from 350 to 1000 μM (Figure 4C). This can be attributed to the discoloration of starch-iodine complexes (SIC) and the formation of pyridine methyl sulfate which results in color-fading of rGO/MPy-GNRs/SIC. 29 The over-all process involves the two-step reaction: 35 I2 + SO2 + H2O + Mpy → 2 Mpy·HI + Mpy·SO3 (1) Mpy·SO3 + CH3OH → Mpy·HSO4CH3 (2) The A287 values are linear with the Na2SO3 concentration within the range from 10 to 350 μM (Figure 4B) (A287 = 1.188-0.002 C Na2SO3 (μM), R2 =0.9997). Moreover, the A335 values increase linearly with the increase in the concentrations of Na2SO3 from 350 to 1000 μM (A335 = 0.1072+0.00036 CNa2SO3 (μM), R2 =0.9991) (Figure 4D). Based on the 3σ rule, the limit of detection (LOD) is calculated to be 1.45 μM by UV-Vis spectroscopy, which is much lower than the maximum allowable level of SO2 in red wines (2.5 mM) recommended by European Union. 6 The series of SERS spectra obtained from the different concentrations of Na2SO3 are shown in Figure 5A. The Raman peak at 518 cm-1 and 1000 cm-1 are used as a quantitative

Figure 4. (A) UV-vis spectra of the rGO/MPy-GNRs/SIC sensing system (1 mL) at the time point of 8 min after the addition of different concentration of Na2SO3 (0.1 mL, from 10 to 350 μM). (B) Plot of A287 versus concentration of Na2SO3 and photographs (inset) of the rGO/MPy-GNRs/SIC with different concentration of Na2SO3 (0.1 mL, from 10 to 350 μM). (C) UV-vis spectra of the rGO/MPy-GNRs/SIC sensing system (1 mL) at the time point of 8 min after the addition of different concentration of Na2SO3 (0.1 mL, from 350 to 1500 μM). (D) Plot of A335 versus concentration of Na2SO3 in the range of 350-1500 μM. evaluation of SERS sensitivity of rGO/MPy-GNRs/SIC toward SO2. The SERS signal of SO2 at 518 cm-1 can still be identified even when the concentration of Na2SO3 is down to 0.1 μM. As illustrated in Figure 5B, the SERS signal intensities at 518 cm-1 and 1000 cm-1 increase with the increasing concentrations of Na2SO3. The working curve for Na2SO3 concentrations from 0.1 to 5000 μM is fitted to Langmuir adsorption, and the limit of detection (LOD) of the SO2 assay based on rGO/MPy-GNRs/SIC is 0.086 μM, which is calculated from the three times the standard deviation of SERS intensities of blank sample at 518 cm-1 divided by the slope of working curve. More importantly, the concentrations of SO2 can be detected with the naked-eye observation under ambient light, and the visible LOD for SO2 is 5 μM by colorimetry (Figure 6A). A series of SERS spectra obtained from the different concentrations of Na2SO3 after HS-PAD extraction are shown in Figure 6C. The SERS assay can easily detect a concentration as low as 1 μM of SO2. The above results are consistent with the hypothetical process illustrated in Scheme 1. Moreover, the digital images of Figure 6A reveal that colorimetric and SERS signals are simultaneously applicable when SO2 is present in the paper strip, suggesting that it could allow dual-modality detection of SO2. The performance of the dual-mode sensing strategy for SO2 is comparable with those of the classical colorimetric assay and SERS method (Table S1). The sensitivity of the method is benefitted by the composite of the rGO and GNRs, which resulted in a high SERS effect contributed by the electromagnetic enhancement caused by the LSPR of the GNRs and the chemical enhancement caused by the charge transfer between the GNRs and the adjacent rGO. 34 We also synthesized three other different shapes of the carbonbased nanomaterials using a hydrothermal method and performed the same progress to detect SO2. As shown in the Figure S13, different morphologies of carbon-based nanomaterial, such as graphite, GO and single-walled carbon nanotubes 5



Figure 5. (A) SERS spectra of rGO/MPy-GNRs/SIC in the presence of different concentrations of Na2SO3 (0.1 mL, from 0.1 to 5000 μM). (B) SERS intensities at 518 cm-1 and 1000 cm-1 as a function of Na2SO3 concentration, respectively. Each data point represents the average value from five SERS spectra. Error bars show the standard deviations.

Figure 6. (A) Photographs of the paper strips (10 mm × 10 mm) impregnated with rGO/MPy-GNRs/SIC immediately after HS extraction of different concentration of Na2SO3 (from 0 to 200 μM). (B) The intensity of blue color developed on the rGO/MPy-GNRs/SIC sensing strips with different concentration of Na2SO3 ranging from 0 to 200 μM was measured using the software ImageJ upon digital image acquisition. (C) SERS spectra of SO2 after HS-PAD-SERS analysis of Na2SO3 standard solutions at different concentrations. The concentration of Na2SO3 in a-i were 0 μM, 1 μM, 10 μM, 50 μM, 100 μM, 500 μM, 1 mM, 1.5 mM and 2 mM, respectively. (D) SERS peak intensities at 518 cm-1 as a function of Na2SO3 concentration. Each data point represents the average value from five measurements on the same samples. Error bars show the standard deviations. are used as extraction materials to carry out the HS-PADSERS analysis. Although carbon-based nanomaterials with different sizes and shapes have the potential applications in SO2 detection, the adsorption capacity and the reproducibility of lamellar nanostructure of rGO have proved to be the best (Figure S14). The proposed method is of high sensitivity and good reproducibility, which can be assigned to the wrinklelike morphology of the lamellar rGO nanomaterial. These uniform wrinkle-like structures uninterruptedly amplified the surface plasmon excitation and greatly enhanced the electromagnetic field.

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Selectivity, Uniformity and Stability of HS-PAD. HS technique is a highly effective approach to reduce the interference in analytical applications, thus allowing the coexistence of many non-volatile components in the liquid matrix. To examine the specificity of the colorimetric and SERS assay for SO2, the rGO/MPy-GNRs/SIC paper without and with the addition of Na2SO3 upon the presence of potential interfering volatile components, including ethanol, ethyl acetate, acetaldehyde, n-butanol and acetic acid are measured by both the colorimetric and the SERS channels. As illustrated in Figure S15 and Figure S16, the separate addition of these interfering substances without presence of Na2SO3 does not exhibit obvious color changes and SERS signal intensities of SO2 at 518 cm-1. These results substantially suggest that these interfering substances do not interfere with the SO2 assay with the developed method in this study. Because the recognition mechanism of our colorimetric and SERS method is established on the specific Karl Fischer reaction between rGO/MPyGNRs/SIC and SO2, this method shows great selectivity for SO2 detection in complex matrix samples. Besides the selectivity of colorimetric and SERS assay, the uniformity of rGO/MPy-GNRs/SIC paper is very important. We collect SERS spectra of SO2 obtained from 10 randomly selected positions within the hybrid paper. As shown in Figure 7A and 7B, the SERS signal intensities of SO2 at 518 cm-1 and 1000 cm-1 are highly consistent, and the spot-to-spot variations of the SERS intensity are well below 15%. The good uniformity can be attributed to the uniform GNRs distribution and homogeneous rGO sheet, thus permitting the reproducible colorimetric and SERS signal from different spots on the rGO/MPy-GNRs/SIC paper. The long-term stability of the rGO/MPy-GNRs/SIC paper is also studied. The SERS spectra and color intensities of the

Figure 7. (A) SERS spectra of SO2 obtained at 10 randomly selected positions of the rGO/MPy-GNRs/SIC paper. The concentration of Na2SO3 was 100 μM. (B) SERS intensity distributions of the 518 cm-1 and 1000 cm-1 as shown in (A). (C) SERS spectra demonstrate the long-term stability of rGO/MPy-GNRs/SIC paper, monitored for 1-10 weeks. SERS spectra of the rGO/MPy-GNRs/SIC paper after extraction of SO2 using the HS-PAD. The concentration of Na2SO3 was 100 μM. (D) The SERS intensity distributions of the 518 and 1000 cm-1 as shown in (C). The average intensities of 10 spectra are indicated by the red dotted line, intensity variations of ±10% and ±15% are indicated by the green and yellow zones, respectively. 6


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digital images of hybrid paper are collected over a period of 10 weeks. As shown in Figure 7C, 7D and S17, the color intensities and profile of the SERS spectra of the hybrid substrates after 10 weeks is very similar to that of the freshly prepared substrates, neither a shift in the major Raman peaks nor a significant change in Raman intensity was observed, indicating the excellent stability of the proposed rGO/MPyGNRs/SIC paper and as a SERS substrate for potential application in the field of on-site monitoring. The performance of dual-mode sensing strategy for SO2 is compared with those of previously reported methods. 13, 14, 37-42 As shown in Table S1, the proposed method has a comparable or superior linear range and detection limit compared with other methods for SO2 detection. Colorimetric and SERS Dual-Mode Detection of SO2 in Wine. To demonstrate the validity of the dual-mode sensing assay for on-site detection of SO2, we investigate the performance of dual-mode assay for on-site detection of SO2 in wine samples. A total of three wine samples namely red wine, white wine and rose wine are purchased from supermarkets. In the determination of SO2, 1 mL wine samples are directly put into a 5 mL glass vessel for HS-PAD extraction without any other pre-treatment. After adding 100 μL of H2SO4 into the glass vessel, the rGO/MPy-GNRs/SIC paper is exposed to the reaction vessel. The whole vessel was then placed into a thermostat at 50 °C for 8 min. The color of rGO/MPy-GNRs/SIC paper can be visually detected with the naked eye. Meanwhile, the visual results can be quantified by measuring the color intensities of the digital images and SERS analysis. For comparison, the samples are also analyzed using classic MonierWilliams method. As shown in Table 1, the detection results obtained from colorimetric and SERS method match well with those from the Monier-Williams assays. The recoveries using colorimetric and SERS method are in the range of 87.12% 116.8% in spiked wine samples. The RSD values of the measurements are not greater than 9.5% and 14.2% for colorimetric method and SERS method, respectively. Moreover, the results obtained from SERS analysis are closed to those from the Monier-Williams assays (Figure S18), which demonstrate that the HS-PAD-SERS method can be used for on-site detection of different concentrations of SO2 in wine samples. These results further confirm the good accuracy and precision of the proposed method and indicate that the HS-PAD based colorimetric/SERS method is reliable in detection of ultra-trace amounts of SO2 in wine. More importantly, the colorimetric/SERS dual-mode sensing platform can not only enable visualization of SO2 simply with the naked eye, but also provide a reliable on-site method with stability and rapidity.

striking properties enable this dual-mode sensing assay more convenient and more readily adopted for on-site fast screening and identification of SO2 than existing systems, thus allowing for the conception of point-of-care diagnostic test and provide a promising approach for rapid analysis of volatile compounds in complex matrix samples. ASSOCIATED CONTENT Supporting Information Experimental materials and methods; SEM images and XPS spectra of the rGO/MPy-GNRs/SIC, optimization of the detection of SO2 based on colorimetric/SERS assay, and additional information as noted in the text. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (D. L.) * E-mail: [email protected] (D. W.)

ORCID Dan Li: 0000-0002-2191-8836

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research is supported by the Natural Science Foundation of China (No. 21507089), the Shanghai University Young Teacher Training Program (No. ZZyy15095), the Scientific Research Foundation for the Introduction of Talent of Shanghai Institute of Technology (YJ2015-6) and Shanghai Municipal Education Commission (Plateau Discipline Construction Program).

CONCLUSIONS In summary, we have developed an integrated rGO/MPyGNRs/SIC HS-PAD for the detection of SO2 based on Karl Fischer reaction. The HS-PAD exhibits decreased color and increased SERS signal intensities at 518 cm-1 of SO2. This economic, stable, selective and fast-response rGO/MPyGNRs/SIC based paper strip is used for a qualitative and semiquantitative detection of SO2 through the naked-eye observation. Compared with those commercially available techniques such as chromatography-based methods and distillationtitration method, the proposed HS-PAD sensing platform is much more operationally simplified and time-saving. The dual-mode sensing system has several advantages over conventional methods especially in high reliability, high selectivity and simplicity of the instrumentation and operation. These 7



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Table 1. Recoveries and precisions (RSD%, n=5) of the SO2 in wine samples by the HS-PAD and Monier-Williams method.

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Headspace-Sampling Paper-Based Analytical Device for Colorimetric

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