Food Chemistry 159 (2014) 428–432
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Determination of the wine preservative sulphur dioxide with cyclic voltammetry using inkjet printed electrodes Marion Schneider a,⇑, Alexander Türke a, Wolf-Joachim Fischer a, Paul A. Kilmartin b a b
Institute of Semiconductors and Microsystems, Technische Universität Dresden, Nöthnitzer Str. 64, 01187 Dresden, Germany Polymer Electronics Research Centre, School of Chemical Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand
a r t i c l e
i n f o
Article history: Received 5 December 2013 Received in revised form 6 March 2014 Accepted 9 March 2014 Available online 18 March 2014 Keywords: Cyclic voltammetry Inkjet printing Polyphenols Sulphur dioxide Wine
a b s t r a c t During winemaking sulphur dioxide is added to prevent undesirable reactions. However, concerns over the harmful effects of sulphites have led to legal limits being placed upon such additives. There is thus a need for simple and selective determinations of sulphur dioxide in wine, especially during winemaking. The simultaneous detection of polyphenols and sulphur dioxide, using cyclic voltammetry at inert electrodes is challenging due to close oxidation potentials. In the present study, inkjet printed electrodes were developed with a suitable voltammetric signal on which the polyphenol oxidation is suppressed and the oxidation peak height for sulphur dioxide corresponds linearly to the concentration. Different types of working electrodes were printed. Electrodes consisting of gold nanoparticles mixed with silver showed the highest sensitivity towards sulphur dioxide. Low cost production of the sensor elements and ultra fast determination of sulphur dioxide by cyclic voltammetry makes this technique very promising for the wine industry. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Winemaking is a complex process requiring the addition of several substances to achieve high quality products and to prevent undesirable effects like browning. Among these substances, sulphur dioxide (SO2) is widely used due to its antioxidant and preservative properties. In white wines, SO2 is typically added to achieve concentrations of 15–40 mg L1 free SO2 at bottling (Jackson, 2008). However, the amount of added SO2 is limited by law (for white wines 210 mg L1) (Council of the European Union, 1999), because high concentrations can cause noxious effects, such as allergic reactions or headaches in sensitive individuals (Freedman, 1980). Therefore, the determination of a wine additive like SO2 is motivated by the legal limits upon such substances. Cyclic voltammetry is an easy-to-use and selective electrochemical characterisation method for determining the content of small electroactive molecules. Cyclic voltammetry is well suited to wine analysis and a few reports exist concerning the determination of SO2 in wines (Makhotkina and Kilmartin, 2010; Makhotkina and Kilmartin, 2013). The present study is based upon the work of Türke et al. (2012), who investigated the electrochemistry of SO2, ascorbic acid and polyphenols in white wines and in a model wine solution using ⇑ Corresponding author. Tel.: +49 351 463 32653; fax: +49 351 463 37021. E-mail address: [email protected] (M. Schneider). http://dx.doi.org/10.1016/j.foodchem.2014.03.049 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.
modified electrodes. However, the determination of these substances in wines using inkjet printed electrodes has not been reported to date. In recent years, inkjet printing has become a significant printing technology due to the variety of applications to which it is suited. Using inkjet printing, single-use, cost competitive and small sensors with fast response times and low detection limits can be fabricated (Singh et al., 2010; Crowley et al., 2008). Additionally, inkjet printing has many advantages compared to other printing techniques. Due to the non-contact and mask-less deposition of ink droplets, the potential cross-contamination and damage that may arise by printing several layers is avoided (Morrin, 2012). Inkjet printing is independent of the substrate employed, and allows printing on flexible materials such as polymer foils (Lin and Bai, 2004). In the present study, electrodes were fully inkjet printed on a PET foil to develop a single-use sensor for determination of SO2 during transportation and storage of wine without the need of a three dimensional cell. Such an inkjet printed electrochemical sensor has the advantage of small sample volumes, because a single drop could be deposited on the planar electrodes to record the electrochemical signals. Printing electrodes on a flexible material benefits in small, portable sensor systems. By cyclic voltammetry, SO2 and common polyphenols were investigated. The determination of SO2 in the presence of polyphenols is challenging, because polyphenols distort cyclic
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voltammetry measurements due to their similar oxidation potentials (Türke et al., 2012; Makhotkina and Kilmartin, 2009; Makhotkina and Kilmartin, 2013). Different approaches to lessen these effects were tested in order to develop an inkjet printed sensor that can easily measure the content of free SO2 in white wines, without the need of further modification of the electrode. The contributions of this paper are (a) the synthesis of a suitable ink consisting of gold nanoparticels mixed with silver, (b) the development of inkjet printed electrodes as a sensing element, and (c) the quantification of sulphur dioxide, even in the presence of polyphenols.
Hostetler et al., 1998). A solution of TOAB (2.5 eq) in 40 mL toluene was prepared. After this, an aqueous solution of HAuCl4 (1 eq) in 12.5 mL water was added. Following discolouration of the organic phase, 1-dodecanethiol (4 eq) was added to the organic phase and the solution was stirred for 15 min. The reductant sodium borohydride (10 eq) in 12.5 mL water was added and the solution was stirred for 3 h. The black organic phase was collected, the solvent was removed and the solid was washed with ethanol and acetone before drying. The dodecanethiol-protected gold nanoparticles had a diameter of 10 nm, which was determined by scanning electron microscopy (Hitachi S-4700). To prepare the inks for printing, the solid silver precursor and gold nanoparticles were dissolved in xylene. Before printing, the inks were filtered. Inkjet printing was performed using a Dimatix-DMP 2830 inkjet printer (Fujifilm Dimatix, Inc.) on PET foils. Different approaches were tested to increase the sensitivity as well as the selectivity towards SO2. For this reason different types of working electrodes were inkjet printed:
2. Experimental 2.1. Materials Silver nitrate (99%), gold (III) chloride hydrate (HAuCl4 aq, 99.9%), 1-dodecanethiol (P98%), caffeic acid (98%), quercetin dihydrate (P98%), (+)-catechin hydrate (P96%), sodium borohydride (P96%), tetraoctylammonium bromide (TOAB, P98%), methanol and xylene (P99%) were purchased from Sigma–Aldrich. Ammonia (28%) was purchased from BASF, acetaldehyde from Fluka, toluene (P99.8%) from AppliChem, sodium metabisulfite (Na2 S2 O5 , P98%) from Riedel-de Haën, L(+)-tartaric acid (P99.5%) from Lachema, neodecanoic acid from ABCR and sodium hydroxide (99%) from J.T.Baker. All chemicals were used without further purification. Solutions were prepared using ultra pure water. A white wine (Müller-Thurgau, 2011, Saale-Unstrut) from Germany was investigated.
(a) Bare silver electrodes were printed to compare with the gold nanoparticle modified electrodes. (b) A gold nanoparticle ink was printed above silver working electrodes (‘‘gold on silver electrode’’). Note that the PET foil with the silver electrodes was sintered in an oven before printing the gold nanoparticle layer. (c) Gold nanoparticles were mixed with the silver ink and printed (‘‘mixed electrode’’). After printing, the PET foils were sintered in an oven for 30 min at 200 °C to remove residual organics and the solvent. In Fig. 1 SEM images of the inkjet printed electrodes are presented. The inkjet printed silver electrodes had a comparable appearance to the inkjet printed silver structures described in (Dearden et al., 2005; Perelaer et al., 2009). Densely packed silver clusters with some voids were obtained. The ‘‘gold on silver’’ electrode had a more smooth surface, because of printing a layer above the silver layer. But larger voids were obtained. Therefore, the ‘‘gold on silver’’ electrodes had a higher surface roughness (determined by atomic force microscopy AIST-NT, non-contact mode). The highest surface roughness was observed for the ‘‘mixed electrodes’’. Smaller clusters were obtained due to a possible diffusion of gold nanoparticles through silver colloids. This diffusion depressed the formation of large silver clusters.
2.2. Preparation of inks and inkjet printing Silver neodecanoate is a common silver precursor for inkjet printing (Dearden et al., 2005; Lee et al., 2007). Gold nanoparticles are well known for their catalytic properties and support electrochemical reactions, as was shown in Yu et al. (2003). Therefore, inks prepared with these materials were used in the present study. Two different precursor inks were synthesised: a silver precursor ink (silver neodecanoate) was synthesised in a modified two step procedure according to (Teng and Vest, 1988): Briefly, ammonia (1 eq, 28%) was added slowly to neodecanoic acid (1 eq, 20 wt.%) in methanol to synthesise ammonium neodecanoate. Then, silver nitrate (1 eq) was dissolved in 70 mL water and was added to the ammonium neodecanoate solution. The solution was stirred for 1.5 h. The precipitate was filtered and washed with methanol and water. It was dried under vacuum and a white solid was obtained. A second ink was prepared with dodecanethiol-protected gold nanoparticles. These gold nanoparticles were synthesised in a phase transition reaction followed by a reduction step (after
(a)
2.3. Preparation of the wine solution; SO2 and polyphenol solutions To prepare the model wine solution, 0.033 M L (+)-tartaric acid was dissolved in a 12 vol% ethanol–water mixture. The pH of this solution was adjusted to 3.3 using NaOH to match the typical pH of white wines. At this pH the dominate species present is the
(b)
(c)
Fig. 1. SEM images of the inkjet printed (a) silver electrode (b) ‘‘gold on silver electrode’’ and (c) ‘‘mixed electrode’’ at high resolution.
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bisulphate ion, according to the pH dependent equilibrium of SO2 species (OBrien et al., 2010). It should be noted, that 1 ppm Na2 S2 O5 is equivalent to 0.68 ppm SO2:
Na2 S2 O5 þ H2 O ! 2Naþ þ 2HSO3 HSO3 þ H3 Oþ SO2 þ 2H2 O
ð1Þ
Stock solutions of Na2 S2 O5 and polyphenols were prepared by dissolving the substances in the model wine solution. A wine sample (Müller-Thurgau, 2011, Saale-Unstrut) was used without dilution or pretreatment except for adjusting the pH to 3.3 using L (+)-tartaric acid in order to compare with the measurements performed in the model wine solution. Stock solutions of SO2 were prepared by dissolving Na2 S2 O5 in the wine sample. 2.4. Cyclic voltammetry measurements Cyclic voltammetry measurements were performed with PAR (263A) potentiostat from Princeton Applied Research. For background correction purposes, scans of the model wine solution were first recorded. Different concentrations of the stock solutions of Na2 S2 O5 and polyphenols were then added, and were well mixed by stirring. The potential was cycled between 200 mV and 500 mV against a Ag/AgCl reference electrode at a scan rate of 100 mV s1 . Due to the damage of the printed working electrode at higher potentials, a maximum value of 500 mV was used. A three electrode setup consisting of the printed working electrode and an external reference electrode (Ag/AgCl, from BASi) in conjunction with a platinum counter electrode was used. Three measurements were performed for each concentration.
Table 1 Slopes of the linear fits of the anodic peak maxima and oxidation potentials of the printed electrodes.
Slope of linear fit/103 lA L mol1 Maximum oxidation potential/mV
Silver
Gold on silver
Mixed
0.133 410
0.281 450
0.477 440
Modifying the silver ink with gold nanoparticles enhanced the sensitivity, compared to the silver electrode; the slope of the ‘‘mixed electrode’’ was more than three times higher. In Fig. 3a) voltammograms for the anodic current that resulted with different concentrations of SO2 at the mixed electrode are shown. The voltammograms are presented after subtracting the response of a blank run taken in the model wine solution prior to the addition of SO2. The background curve includes a rising response about 400 mV for both gold and silver containing electrodes due to oxidation of the ethanol containing electrolyte. Compared with the other inkjet printed electrodes, the peaks were more well-defined with maxima at around 440 mV for SO2 oxidation. The gold nanoparticles used in this study had an average diameter of 10 nm, and melting is unlikely due to their small size. It is well known that the melting point of particles decreases with decreasing size (Takagi, 1954). This behaviour was also reported for gold nanoparticles (Buffat and Borel, 1976). However, at 200 °C, gold nanoparticles can undergo coalescence, because the organic protecting shell has been removed. The coalesced gold
3. Results and discussion 3.1. Inkjet printing By inkjet printing, working electrodes were printed successfully on PET foils (Fig. 2). 3.2. Cyclic voltammetry measurements 3.2.1. Cyclic voltammetry of SO2 To estimate the sensitivity for the oxidation of SO2, the slope of the linear fit of the current peak maximum versus the SO2 concentration was calculated. A steep slope presents a high sensitivity. The slope was determined from five concentrations, taking three measurements at each concentration, as presented in Table 1.
(a)
(b) Fig. 2. Inkjet printed silver working electrode on a PET foil.
Fig. 3. (a) Voltammograms (current with background corrections) for inkjet printed ‘‘mixed electrodes’’ with different concentrations of sulphur dioxide and (b) maxima of anodic peak current for all electrodes.
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nanoparticles can diffuse through the silver colloids as explained above and cause structures with high surface areas. Printing gold nanoparticles above a silver layer increased only slightly the surface area, because the ink was only printed as a thin layer above the silver layer. Therefore, gold was only present in a catalytic amount at the surface. The standard deviation of the anodic peak current of five ‘‘mixed electrodes’’ were calculated. For SO2 the standard deviation was 7.4 mV. These results showed the reproducibility of the measurements with the inkjet printed electrodes. Deviations of graphs of the voltammograms are a result of the printing method, because the electrochemical active surface area of the inkjet printed electrodes may differ also inkjet printing is a relatively accurate deposition method. 3.2.2. Cyclic voltammetry of polyphenols To investigate the influence of polyphenols on the electrochemical behaviour of SO2 at the inkjet printed electrodes, three polyphenols were chosen: caffeic acid, (+)-catechin hydrate and quercetin dihydrate. These polyphenols represent examples for the main groups of polyphenols found in wines (Moreno and Peinado, 2012). The typical concentrations of polyphenols found in white wines are in the range of 1 ppm up to 260 ppm (Jackson, 2008). Cyclic voltammetry experiments were run separately in the model wine solution with a concentration range of polyphenols from 2 ppm up to 100 ppm for each polyphenol. By comparison, the three inkjet printed electrodes (silver electrode, ‘‘gold on silver electrode’’ and the ‘‘mixed electrode’’) showed a similar electrochemical behaviour. These electrodes showed similar voltammograms with no oxidation or reduction peaks as the concentration of polyphenols was increased, indicating no sensitivity of the printed electrodes to polyphenols. To confirm this assumption, simultaneous measurements of polyphenols and SO2 were performed. All inkjet printed electrodes were tested and showed similar results. In Fig. 4a), voltammograms for different concentrations of SO2 in the presence of 20 ppm caffeic acid are presented. Compared to Fig. 3a) without caffeic acid, both sets of voltammograms showed very similar curves and similar peak intensities. Similar results were obtained for the other polyphenols, indicating that the polyphenols did not influence the measurements of SO2 concentration. Interestingly, the voltammograms for SO2 showed large reduction peaks. This effect was observed for all inkjet printed electrodes. Therefore, the calculation of the slopes can be performed additionally using the cathodic peaks. The mentioned effect could be a result of the reduction of sulphur species, that were generated during oxidation followed by a chemical reaction. This was proved by adding acetaldehyde to a model wine solution containing 100 ppm SO2 (Fig. 4b)). Acetaldehyde binds up free SO2:
HSO3 þ CH3 CHO HSO3 CH3 CHO SO3 CH3 CHOH
(a)
(b) Fig. 4. (a) Voltammograms (current with background corrections) of inkjet printed ‘‘mixed electrodes’’ in the model wine solution with different concentrations of SO2 in the presence of 20 ppm caffeic acid. (b) Addition of acetaldehyde to 100 ppm SO2 in the model wine solution.
value was observed for sulphur dioxide towards higher potentials (460 mV for the solid silver electrode and 440 mV for inkjet printed silver electrode). Furthermore, the solid silver electrode did not show any electrochemical response towards the polyphenols tested above. In contrast, at solid gold electrodes, polyphenols typically stick on the electrode surface distorting the electrochemical response (Türke et al., 2012). Additionally, polyphenols have a similar oxidation potential to SO2, making a distinction between these substances difficult at gold electrodes.
ð2Þ
The large reduction peak (and the oxidation peak as well) was removed and the voltammograms showed equal signals to the model wine solution before adding SO2. Therefore, all displayed current peaks were a result of oxidation or reduction of SO2 and its byproducts. 3.2.3. Comparison with a solid silver electrode At a solid silver electrode cyclic voltammetry experiments of SO2 and polyphenols were performed to compare the response with the inkjet printed electrodes. The solid silver electrode was prepared by moulding a silver wire (10 mm in diameter) in epoxy resin. The voltammograms were similar to those obtained with the printed silver electrodes. Only a slight shift of the oxidation peak
Fig. 5. Voltammograms of inkjet printed ‘‘mixed electrodes’’ in a wine sample with added concentrations of SO2.
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3.2.4. Measuring in a wine sample In order to prove the suitability of the inkjet printed electrodes for analysis of SO2, cyclic voltammetry experiments were performed in a wine sample. In Fig. 5, voltammograms of the ‘‘mixed electrodes’’ are presented. Different concentrations of SO2 were added to an undiluted wine sample and anodic current peaks were observed at 470 mV and cathodic current peaks at 170 mV. The voltammograms were in accordance to the results obtained in the model wine solution (oxidation peak at 440 mV and reduction peak at 310 mV). The voltammograms show boarder curves, that may indicate the influence of other wine ingredients such as sugars or acids. Nevertheless, the determination of SO2 in wines using inkjet printed electrodes was successful. 4. Conclusions Wine components such as sulphur dioxide and polyphenols were investigated by cyclic voltammetry at fully inkjet printed electrodes. Different approaches were tested in order to increase the sensitivity. It was shown that modification of the silver ink with gold nanoparticles enhanced the sensitivity towards sulphur dioxide oxidation in a model wine solution. Additionally, concentration dependent peak current responses were observed. In contrast, all of the inkjet printed electrodes were insensitive towards polyphenols in the potential range used to determine sulphur dioxide concentrations. Therefore, using the inkjet printed electrodes, determination of sulphur dioxide can be performed even in the presence of polyphenols, which has not been reported to date. Different concentrations of SO2 were determined successfully in a wine sample. For these reasons, the inkjet printed silver electrodes modified with gold nanoparticles show excellent suitability for SO2 analysis in white wines. Further investigations will focus on experiments in white wines to investigate the influence of other wine components on the electrochemical behaviour of SO2 and on measurements using in addition to the inkjet printed working electrode also inkjet printed reference and counter electrodes. Acknowledgements The research was supported by funding from Federal Office for Agriculture and Food (BLE Project 7/11-12) and the German Research Foundation (DFG project 1401/1).
References Buffat, P., & Borel, J.-P. (1976). Size effect on the melting temperature of gold particles. Physical Reviews A, 13, 2287–2298. Crowley, K., Morrin, A., Smyth, M., Killard, A., Shepherd, R., in het Panhuis, M., & Wallace, G. (2008). Fabrication of chemical sensors using inkjet printing and application to gas detection. In Sensors, 2008 IEEE (pp. 13–16). Dearden, A. L., Smith, P. J., Shin, D.-Y., Reis, N., Derby, B., & O’Brien, P. (2005). A low curing temperature silver ink for use in ink-jet printing and subsequent production of conductive tracks. Macromolecular Rapid Communications, 26, 315–318. Freedman, B. J. (1980). Sulphur dioxide in foods and beverages: Its use as a preservative and its effect on asthma. British Journal of Diseases of the Chest, 74, 128–134. Hostetler, M. J., Wingate, J. E., Zhong, C.-J., Harris, J. E., Vachet, R. W., Clark, M. R., et al. (1998). Alkanethiolate gold cluster molecules with core diameters from 1.5 to 5.2 nm: Core and monolayer properties as a function of core size. Langmuir, 14, 17–30. Jackson, R. S. (2008). 6 – Chemical constituents of grapes and wine (3rd ed.). San Diego: Academic Press. Lee, J. J., Park, J. C., Tae Sun Chang, M. H. K., Kim, S. T., Kooa, S. M., You, Y. C., et al. (2007). Silver complex inks for ink-jet printing: The synthesis and conversion to a metallic particulate ink. Journal of Ceramic Processing Research, 8, 219–223. Lin, L., & Bai, X. (2004). Ink-jet technology: Status quo and future relevance to surface coatings. Pigment & Resin Technology, 33, 238–244. Makhotkina, O., & Kilmartin, P. A. (2009). Uncovering the influence of antioxidants on polyphenol oxidation in wines using an electrochemical method: Cyclic voltammetry. Journal of Electroanalytical Chemistry, 633, 165–174. Makhotkina, O., & Kilmartin, P. A. (2010). The use of cyclic voltammetry for wine analysis: Determination of polyphenols and free sulfur dioxide. Analytica Chimica Acta, 668, 155–165. Makhotkina, O., & Kilmartin, P. A. (2013). Electrochemical oxidation of wine polyphenols in the presence of sulfur dioxide. Journal of Agricultural and Food Chemistry, 61, 5573–5581. Moreno, J., & Peinado, R. (2012). Chapter 5 – Polyphenols. In Enological chemistry (pp. 53–76). San Diego: Academic Press. Morrin, A. (2012). Inkjet printed electrochemical sensors. In Inkjet-based micromanufacturing (pp. 295–311). Wiley–VCH Verlag GmbH & Co. KGaA. OBrien, J., Hinkley, J., Donne, S., & Lindquist, S.-E. (2010). The electrochemical oxidation of aqueous sulfur dioxide: A critical review of work with respect to the hybrid sulfur cycle. Electrochimica Acta, 55, 573–591. Perelaer, J., Hendriks, C. E., de Laat, A. W. M., & Schubert, U. S. (2009). One-step inkjet printing of conductive silver tracks on polymer substrates. Nanotechnology, 20, 165303. Singh, M., Haverinen, H. M., Dhagat, P., & Jabbour, G. E. (2010). Inkjet printing: Process and its applications. Adv. Mater., 22, 673–685. Takagi, M. (1954). Electron-diffraction study of liquid-solid transition of thin metal films. Journal of the Physical Society of Japan, 9, 359–363. Teng, K., & Vest, R. (1988). Metallization of solar cells with ink jet printing and silver metallo-organic inks. Components, Hybrids, and Manufacturing Technology, IEEE Transactions on, 11, 291–297. Türke, A., Fischer, W.-J., Beaumont, N., & Kilmartin, P. (2012). Electrochemistry of sulfur dioxide, polyphenols and ascorbic acid at poly(3,4ethylenedioxythiophene) modified electrodes. Electrochimica Acta, 60, 184–192. Council of the European Union (1999). Council regulation (EC) of 17 May 1999 on the common organisation of the market in wine, annex V: Limits and conditions for certain oenological practices, a: Sulphur dioxide content. Official Journal of the European, Communities, No. 1493/1999, 179/1–179/784. Yu, A., Liang, Z., Cho, J., & Caruso, F. (2003). Nanostructured electrochemical sensor based on dense gold nanoparticle films. Nano Letters, 3, 1203–1207.
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Determination of the wine preservative sulphur dioxide with cyclic voltammetry using inkjet printed electrodes Marion Schneider a,⇑, Alexander Türke a, Wolf-Joachim Fischer a, Paul A. Kilmartin b a b
Institute of Semiconductors and Microsystems, Technische Universität Dresden, Nöthnitzer Str. 64, 01187 Dresden, Germany Polymer Electronics Research Centre, School of Chemical Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand
a r t i c l e
i n f o
Article history: Received 5 December 2013 Received in revised form 6 March 2014 Accepted 9 March 2014 Available online 18 March 2014 Keywords: Cyclic voltammetry Inkjet printing Polyphenols Sulphur dioxide Wine
a b s t r a c t During winemaking sulphur dioxide is added to prevent undesirable reactions. However, concerns over the harmful effects of sulphites have led to legal limits being placed upon such additives. There is thus a need for simple and selective determinations of sulphur dioxide in wine, especially during winemaking. The simultaneous detection of polyphenols and sulphur dioxide, using cyclic voltammetry at inert electrodes is challenging due to close oxidation potentials. In the present study, inkjet printed electrodes were developed with a suitable voltammetric signal on which the polyphenol oxidation is suppressed and the oxidation peak height for sulphur dioxide corresponds linearly to the concentration. Different types of working electrodes were printed. Electrodes consisting of gold nanoparticles mixed with silver showed the highest sensitivity towards sulphur dioxide. Low cost production of the sensor elements and ultra fast determination of sulphur dioxide by cyclic voltammetry makes this technique very promising for the wine industry. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Winemaking is a complex process requiring the addition of several substances to achieve high quality products and to prevent undesirable effects like browning. Among these substances, sulphur dioxide (SO2) is widely used due to its antioxidant and preservative properties. In white wines, SO2 is typically added to achieve concentrations of 15–40 mg L1 free SO2 at bottling (Jackson, 2008). However, the amount of added SO2 is limited by law (for white wines 210 mg L1) (Council of the European Union, 1999), because high concentrations can cause noxious effects, such as allergic reactions or headaches in sensitive individuals (Freedman, 1980). Therefore, the determination of a wine additive like SO2 is motivated by the legal limits upon such substances. Cyclic voltammetry is an easy-to-use and selective electrochemical characterisation method for determining the content of small electroactive molecules. Cyclic voltammetry is well suited to wine analysis and a few reports exist concerning the determination of SO2 in wines (Makhotkina and Kilmartin, 2010; Makhotkina and Kilmartin, 2013). The present study is based upon the work of Türke et al. (2012), who investigated the electrochemistry of SO2, ascorbic acid and polyphenols in white wines and in a model wine solution using ⇑ Corresponding author. Tel.: +49 351 463 32653; fax: +49 351 463 37021. E-mail address: [email protected] (M. Schneider). http://dx.doi.org/10.1016/j.foodchem.2014.03.049 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.
modified electrodes. However, the determination of these substances in wines using inkjet printed electrodes has not been reported to date. In recent years, inkjet printing has become a significant printing technology due to the variety of applications to which it is suited. Using inkjet printing, single-use, cost competitive and small sensors with fast response times and low detection limits can be fabricated (Singh et al., 2010; Crowley et al., 2008). Additionally, inkjet printing has many advantages compared to other printing techniques. Due to the non-contact and mask-less deposition of ink droplets, the potential cross-contamination and damage that may arise by printing several layers is avoided (Morrin, 2012). Inkjet printing is independent of the substrate employed, and allows printing on flexible materials such as polymer foils (Lin and Bai, 2004). In the present study, electrodes were fully inkjet printed on a PET foil to develop a single-use sensor for determination of SO2 during transportation and storage of wine without the need of a three dimensional cell. Such an inkjet printed electrochemical sensor has the advantage of small sample volumes, because a single drop could be deposited on the planar electrodes to record the electrochemical signals. Printing electrodes on a flexible material benefits in small, portable sensor systems. By cyclic voltammetry, SO2 and common polyphenols were investigated. The determination of SO2 in the presence of polyphenols is challenging, because polyphenols distort cyclic
429
M. Schneider et al. / Food Chemistry 159 (2014) 428–432
voltammetry measurements due to their similar oxidation potentials (Türke et al., 2012; Makhotkina and Kilmartin, 2009; Makhotkina and Kilmartin, 2013). Different approaches to lessen these effects were tested in order to develop an inkjet printed sensor that can easily measure the content of free SO2 in white wines, without the need of further modification of the electrode. The contributions of this paper are (a) the synthesis of a suitable ink consisting of gold nanoparticels mixed with silver, (b) the development of inkjet printed electrodes as a sensing element, and (c) the quantification of sulphur dioxide, even in the presence of polyphenols.
Hostetler et al., 1998). A solution of TOAB (2.5 eq) in 40 mL toluene was prepared. After this, an aqueous solution of HAuCl4 (1 eq) in 12.5 mL water was added. Following discolouration of the organic phase, 1-dodecanethiol (4 eq) was added to the organic phase and the solution was stirred for 15 min. The reductant sodium borohydride (10 eq) in 12.5 mL water was added and the solution was stirred for 3 h. The black organic phase was collected, the solvent was removed and the solid was washed with ethanol and acetone before drying. The dodecanethiol-protected gold nanoparticles had a diameter of 10 nm, which was determined by scanning electron microscopy (Hitachi S-4700). To prepare the inks for printing, the solid silver precursor and gold nanoparticles were dissolved in xylene. Before printing, the inks were filtered. Inkjet printing was performed using a Dimatix-DMP 2830 inkjet printer (Fujifilm Dimatix, Inc.) on PET foils. Different approaches were tested to increase the sensitivity as well as the selectivity towards SO2. For this reason different types of working electrodes were inkjet printed:
2. Experimental 2.1. Materials Silver nitrate (99%), gold (III) chloride hydrate (HAuCl4 aq, 99.9%), 1-dodecanethiol (P98%), caffeic acid (98%), quercetin dihydrate (P98%), (+)-catechin hydrate (P96%), sodium borohydride (P96%), tetraoctylammonium bromide (TOAB, P98%), methanol and xylene (P99%) were purchased from Sigma–Aldrich. Ammonia (28%) was purchased from BASF, acetaldehyde from Fluka, toluene (P99.8%) from AppliChem, sodium metabisulfite (Na2 S2 O5 , P98%) from Riedel-de Haën, L(+)-tartaric acid (P99.5%) from Lachema, neodecanoic acid from ABCR and sodium hydroxide (99%) from J.T.Baker. All chemicals were used without further purification. Solutions were prepared using ultra pure water. A white wine (Müller-Thurgau, 2011, Saale-Unstrut) from Germany was investigated.
(a) Bare silver electrodes were printed to compare with the gold nanoparticle modified electrodes. (b) A gold nanoparticle ink was printed above silver working electrodes (‘‘gold on silver electrode’’). Note that the PET foil with the silver electrodes was sintered in an oven before printing the gold nanoparticle layer. (c) Gold nanoparticles were mixed with the silver ink and printed (‘‘mixed electrode’’). After printing, the PET foils were sintered in an oven for 30 min at 200 °C to remove residual organics and the solvent. In Fig. 1 SEM images of the inkjet printed electrodes are presented. The inkjet printed silver electrodes had a comparable appearance to the inkjet printed silver structures described in (Dearden et al., 2005; Perelaer et al., 2009). Densely packed silver clusters with some voids were obtained. The ‘‘gold on silver’’ electrode had a more smooth surface, because of printing a layer above the silver layer. But larger voids were obtained. Therefore, the ‘‘gold on silver’’ electrodes had a higher surface roughness (determined by atomic force microscopy AIST-NT, non-contact mode). The highest surface roughness was observed for the ‘‘mixed electrodes’’. Smaller clusters were obtained due to a possible diffusion of gold nanoparticles through silver colloids. This diffusion depressed the formation of large silver clusters.
2.2. Preparation of inks and inkjet printing Silver neodecanoate is a common silver precursor for inkjet printing (Dearden et al., 2005; Lee et al., 2007). Gold nanoparticles are well known for their catalytic properties and support electrochemical reactions, as was shown in Yu et al. (2003). Therefore, inks prepared with these materials were used in the present study. Two different precursor inks were synthesised: a silver precursor ink (silver neodecanoate) was synthesised in a modified two step procedure according to (Teng and Vest, 1988): Briefly, ammonia (1 eq, 28%) was added slowly to neodecanoic acid (1 eq, 20 wt.%) in methanol to synthesise ammonium neodecanoate. Then, silver nitrate (1 eq) was dissolved in 70 mL water and was added to the ammonium neodecanoate solution. The solution was stirred for 1.5 h. The precipitate was filtered and washed with methanol and water. It was dried under vacuum and a white solid was obtained. A second ink was prepared with dodecanethiol-protected gold nanoparticles. These gold nanoparticles were synthesised in a phase transition reaction followed by a reduction step (after
(a)
2.3. Preparation of the wine solution; SO2 and polyphenol solutions To prepare the model wine solution, 0.033 M L (+)-tartaric acid was dissolved in a 12 vol% ethanol–water mixture. The pH of this solution was adjusted to 3.3 using NaOH to match the typical pH of white wines. At this pH the dominate species present is the
(b)
(c)
Fig. 1. SEM images of the inkjet printed (a) silver electrode (b) ‘‘gold on silver electrode’’ and (c) ‘‘mixed electrode’’ at high resolution.
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bisulphate ion, according to the pH dependent equilibrium of SO2 species (OBrien et al., 2010). It should be noted, that 1 ppm Na2 S2 O5 is equivalent to 0.68 ppm SO2:
Na2 S2 O5 þ H2 O ! 2Naþ þ 2HSO3 HSO3 þ H3 Oþ SO2 þ 2H2 O
ð1Þ
Stock solutions of Na2 S2 O5 and polyphenols were prepared by dissolving the substances in the model wine solution. A wine sample (Müller-Thurgau, 2011, Saale-Unstrut) was used without dilution or pretreatment except for adjusting the pH to 3.3 using L (+)-tartaric acid in order to compare with the measurements performed in the model wine solution. Stock solutions of SO2 were prepared by dissolving Na2 S2 O5 in the wine sample. 2.4. Cyclic voltammetry measurements Cyclic voltammetry measurements were performed with PAR (263A) potentiostat from Princeton Applied Research. For background correction purposes, scans of the model wine solution were first recorded. Different concentrations of the stock solutions of Na2 S2 O5 and polyphenols were then added, and were well mixed by stirring. The potential was cycled between 200 mV and 500 mV against a Ag/AgCl reference electrode at a scan rate of 100 mV s1 . Due to the damage of the printed working electrode at higher potentials, a maximum value of 500 mV was used. A three electrode setup consisting of the printed working electrode and an external reference electrode (Ag/AgCl, from BASi) in conjunction with a platinum counter electrode was used. Three measurements were performed for each concentration.
Table 1 Slopes of the linear fits of the anodic peak maxima and oxidation potentials of the printed electrodes.
Slope of linear fit/103 lA L mol1 Maximum oxidation potential/mV
Silver
Gold on silver
Mixed
0.133 410
0.281 450
0.477 440
Modifying the silver ink with gold nanoparticles enhanced the sensitivity, compared to the silver electrode; the slope of the ‘‘mixed electrode’’ was more than three times higher. In Fig. 3a) voltammograms for the anodic current that resulted with different concentrations of SO2 at the mixed electrode are shown. The voltammograms are presented after subtracting the response of a blank run taken in the model wine solution prior to the addition of SO2. The background curve includes a rising response about 400 mV for both gold and silver containing electrodes due to oxidation of the ethanol containing electrolyte. Compared with the other inkjet printed electrodes, the peaks were more well-defined with maxima at around 440 mV for SO2 oxidation. The gold nanoparticles used in this study had an average diameter of 10 nm, and melting is unlikely due to their small size. It is well known that the melting point of particles decreases with decreasing size (Takagi, 1954). This behaviour was also reported for gold nanoparticles (Buffat and Borel, 1976). However, at 200 °C, gold nanoparticles can undergo coalescence, because the organic protecting shell has been removed. The coalesced gold
3. Results and discussion 3.1. Inkjet printing By inkjet printing, working electrodes were printed successfully on PET foils (Fig. 2). 3.2. Cyclic voltammetry measurements 3.2.1. Cyclic voltammetry of SO2 To estimate the sensitivity for the oxidation of SO2, the slope of the linear fit of the current peak maximum versus the SO2 concentration was calculated. A steep slope presents a high sensitivity. The slope was determined from five concentrations, taking three measurements at each concentration, as presented in Table 1.
(a)
(b) Fig. 2. Inkjet printed silver working electrode on a PET foil.
Fig. 3. (a) Voltammograms (current with background corrections) for inkjet printed ‘‘mixed electrodes’’ with different concentrations of sulphur dioxide and (b) maxima of anodic peak current for all electrodes.
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nanoparticles can diffuse through the silver colloids as explained above and cause structures with high surface areas. Printing gold nanoparticles above a silver layer increased only slightly the surface area, because the ink was only printed as a thin layer above the silver layer. Therefore, gold was only present in a catalytic amount at the surface. The standard deviation of the anodic peak current of five ‘‘mixed electrodes’’ were calculated. For SO2 the standard deviation was 7.4 mV. These results showed the reproducibility of the measurements with the inkjet printed electrodes. Deviations of graphs of the voltammograms are a result of the printing method, because the electrochemical active surface area of the inkjet printed electrodes may differ also inkjet printing is a relatively accurate deposition method. 3.2.2. Cyclic voltammetry of polyphenols To investigate the influence of polyphenols on the electrochemical behaviour of SO2 at the inkjet printed electrodes, three polyphenols were chosen: caffeic acid, (+)-catechin hydrate and quercetin dihydrate. These polyphenols represent examples for the main groups of polyphenols found in wines (Moreno and Peinado, 2012). The typical concentrations of polyphenols found in white wines are in the range of 1 ppm up to 260 ppm (Jackson, 2008). Cyclic voltammetry experiments were run separately in the model wine solution with a concentration range of polyphenols from 2 ppm up to 100 ppm for each polyphenol. By comparison, the three inkjet printed electrodes (silver electrode, ‘‘gold on silver electrode’’ and the ‘‘mixed electrode’’) showed a similar electrochemical behaviour. These electrodes showed similar voltammograms with no oxidation or reduction peaks as the concentration of polyphenols was increased, indicating no sensitivity of the printed electrodes to polyphenols. To confirm this assumption, simultaneous measurements of polyphenols and SO2 were performed. All inkjet printed electrodes were tested and showed similar results. In Fig. 4a), voltammograms for different concentrations of SO2 in the presence of 20 ppm caffeic acid are presented. Compared to Fig. 3a) without caffeic acid, both sets of voltammograms showed very similar curves and similar peak intensities. Similar results were obtained for the other polyphenols, indicating that the polyphenols did not influence the measurements of SO2 concentration. Interestingly, the voltammograms for SO2 showed large reduction peaks. This effect was observed for all inkjet printed electrodes. Therefore, the calculation of the slopes can be performed additionally using the cathodic peaks. The mentioned effect could be a result of the reduction of sulphur species, that were generated during oxidation followed by a chemical reaction. This was proved by adding acetaldehyde to a model wine solution containing 100 ppm SO2 (Fig. 4b)). Acetaldehyde binds up free SO2:
HSO3 þ CH3 CHO HSO3 CH3 CHO SO3 CH3 CHOH
(a)
(b) Fig. 4. (a) Voltammograms (current with background corrections) of inkjet printed ‘‘mixed electrodes’’ in the model wine solution with different concentrations of SO2 in the presence of 20 ppm caffeic acid. (b) Addition of acetaldehyde to 100 ppm SO2 in the model wine solution.
value was observed for sulphur dioxide towards higher potentials (460 mV for the solid silver electrode and 440 mV for inkjet printed silver electrode). Furthermore, the solid silver electrode did not show any electrochemical response towards the polyphenols tested above. In contrast, at solid gold electrodes, polyphenols typically stick on the electrode surface distorting the electrochemical response (Türke et al., 2012). Additionally, polyphenols have a similar oxidation potential to SO2, making a distinction between these substances difficult at gold electrodes.
ð2Þ
The large reduction peak (and the oxidation peak as well) was removed and the voltammograms showed equal signals to the model wine solution before adding SO2. Therefore, all displayed current peaks were a result of oxidation or reduction of SO2 and its byproducts. 3.2.3. Comparison with a solid silver electrode At a solid silver electrode cyclic voltammetry experiments of SO2 and polyphenols were performed to compare the response with the inkjet printed electrodes. The solid silver electrode was prepared by moulding a silver wire (10 mm in diameter) in epoxy resin. The voltammograms were similar to those obtained with the printed silver electrodes. Only a slight shift of the oxidation peak
Fig. 5. Voltammograms of inkjet printed ‘‘mixed electrodes’’ in a wine sample with added concentrations of SO2.
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3.2.4. Measuring in a wine sample In order to prove the suitability of the inkjet printed electrodes for analysis of SO2, cyclic voltammetry experiments were performed in a wine sample. In Fig. 5, voltammograms of the ‘‘mixed electrodes’’ are presented. Different concentrations of SO2 were added to an undiluted wine sample and anodic current peaks were observed at 470 mV and cathodic current peaks at 170 mV. The voltammograms were in accordance to the results obtained in the model wine solution (oxidation peak at 440 mV and reduction peak at 310 mV). The voltammograms show boarder curves, that may indicate the influence of other wine ingredients such as sugars or acids. Nevertheless, the determination of SO2 in wines using inkjet printed electrodes was successful. 4. Conclusions Wine components such as sulphur dioxide and polyphenols were investigated by cyclic voltammetry at fully inkjet printed electrodes. Different approaches were tested in order to increase the sensitivity. It was shown that modification of the silver ink with gold nanoparticles enhanced the sensitivity towards sulphur dioxide oxidation in a model wine solution. Additionally, concentration dependent peak current responses were observed. In contrast, all of the inkjet printed electrodes were insensitive towards polyphenols in the potential range used to determine sulphur dioxide concentrations. Therefore, using the inkjet printed electrodes, determination of sulphur dioxide can be performed even in the presence of polyphenols, which has not been reported to date. Different concentrations of SO2 were determined successfully in a wine sample. For these reasons, the inkjet printed silver electrodes modified with gold nanoparticles show excellent suitability for SO2 analysis in white wines. Further investigations will focus on experiments in white wines to investigate the influence of other wine components on the electrochemical behaviour of SO2 and on measurements using in addition to the inkjet printed working electrode also inkjet printed reference and counter electrodes. Acknowledgements The research was supported by funding from Federal Office for Agriculture and Food (BLE Project 7/11-12) and the German Research Foundation (DFG project 1401/1).
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