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Laser-pyrolyzed electrochemical paper-based analytical sensor for sulphite analysis
Journal Pre-proof Laser-pyrolyzed electrochemical paper-based analytical sensor for sulphite analysis
Alisson Bezerra Martins, Alnilan Lobato, Nikola Tasic, Fernando J. Perez-Sanz, Pedro Vidinha, Thiago R.L.C. Paixão, Luís Moreira Gonçalves PII:
S1388-2481(19)30204-8
DOI:
https://doi.org/10.1016/j.elecom.2019.106541
Reference:
ELECOM 106541
To appear in:
Electrochemistry Communications
Received date:
22 July 2019
Revised date:
23 August 2019
Accepted date:
4 September 2019
Please cite this article as: A.B. Martins, A. Lobato, N. Tasic, et al., Laser-pyrolyzed electrochemical paper-based analytical sensor for sulphite analysis, Electrochemistry Communications (2019), https://doi.org/10.1016/j.elecom.2019.106541
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© 2019 Published by Elsevier.
Journal Pre-proof
Laser-pyrolyzed electrochemical paper‐based analytical sensor for sulphite analysis 1
1
1
Alisson Bezerra Martins, Alnilan Lobato, Nikola Tasic, Fernando J. Perez-Sanz, 1
#,1
Pedro Vidinha, Thiago R. L. C. Paixão,
and Luís Moreira Gonçalves
1
1,*
1
Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo (USP), São Paulo, SP, Brazil
#
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*[email protected]
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[email protected]
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Abstract
A CO2 laser can be used to pyrolyze the surface of common paperboard to
re
produce a conductive carbon material for subsequent use as an electrode for general electrochemical measurements. In this work a laser-pyrolyzed electrochemical paper‐
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based analytical device (LP-ePAD) of this type was combined with a gas-diffusion microextraction (GDME) unit for the square-wave voltammetric analysis of sulphites in
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commercial beverages. The fabricated LP-ePAD had a resistivity at room temperature of 11 ± 4 Ω per square electrical sheet (measured with a four-point probe system). The
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developed methodology showed a limit of detection of 1 mg L-1 for sulphite analysis, with good repeatability and reproducibility (coefficient of variation below 7%), as well having as the desirable qualities of portability and low price.
Keywords
Electroanalysis; Food quality; Sample preparation; Laser scribed paper-based analytical devices; Square-wave voltammetry; Volatile extraction
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Journal Pre-proof 1. Introduction Recently, one of the authors reported a new approach to the fabrication of electrochemical paper-based devices (ePADs) which does not require expensive equipment or controlled laboratory conditions [1]. The proposed approach used a CO2 laser to pyrolyze a paperboard surface in order to convert cellulose to carbon, creating a carbon surface for electroanalytical applications in a single low-cost step. ePADs were originally proposed in 2009 by Dungchai and co-authors using silk screen/screen printing procedures which could leave impurities on the paper after solvent evaporation [2]. Since then, a range of approaches to ePAD fabrication have been reported: casting
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[3], inkjet printing [4], use of graphite leads or direct pencil drawing, wire or fibre
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attachment [5], sputtering approaches under an inert atmosphere [6], as well as carbon pyrolysis using a tube furnace under oxygen [7]. However, all of these procedures have
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at least one of the following disadvantages: each ePAD has to be fabricated by hand, resulting in a lack of reproducibility, high cost, or the need for a controlled environment.
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By contrast, the laser pyrolysis procedure [8] can be used to fabricate reproducible lowcost devices in a single step [1].
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Sulphites have been used as food additives for many years, due to their many applications (prevention of the microbial growth, browning reactions and flavour
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stabilizing properties) and their outstanding efficiency [9,10]. However, their use should be controlled because of possible adverse clinical effects, particularly to sensitive individuals, ranging from dermatitis, urticaria, abdominal pain and diarrhoea to some
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rare yet dangerous anaphylactic responses [11]. Typically, the term ‘sulphites’ refers to a variety of compounds: sulphur dioxide, sodium sulphite, sodium bisulfite, sodium metabisulfite, potassium metabisulfite, and potassium sulphite (INS 220 to INS 225) [12]. However, in practice, in the pH range 2– 6 most common in beverages, sulphur is present in the form of hydrogen sulphite (HSO3) [10]. The electrochemical response of sulphites has been studied for over a decade [13], using several types of electrodes, including: mercury [14–23], gold [24– 26], platinum [12,27,28], copper [29], glassy carbon electrode (GCE) [30–32], multiwalled carbon nanotubes [33,34], screen-printed carbon electrode (SPCE) [35,36], and modified working electrodes [37–43]. To the best of the authors’ knowledge, however, sulphites have never been investigated using paper-based analytical devices. Electroanalysis has several advantages for testing food samples, primarily speed, sensitivity and portability, but in general this approach is susceptible to interference from other compounds present in these complex matrices. In order to circumvent common interference problems, a recognition element, such as an antibody
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Journal Pre-proof [44,45], aptamer or a molecular imprinted polymer [46–50], may be attached to the electrode surface. However, that can be expensive and sometimes technically challenging. Alternatively, one can make use of a sample preparation step, which may result in a cheaper and often more robust analytical methodology [51,52]. Taking all of this into account, this work’s rationale was, as a proof-of-concept, to combine inexpensive and portable paper electrodes with a light, low-cost sample preparation system, namely gas-diffusion microextraction (GDME) [35,36,53], for sulphite analysis in food matrices.
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2. Materials and methods
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2.1. Chemicals and samples
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All reagents were of analytical grade and were used as received. Aqueous solutions were prepared using ultrapure water with a resistivity not less than 18.2 MΩ
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cm at 298 K.
Sodium sulphite (Merck, Darmstadt, Germany) was used as a source of
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sulphites; hydrochloric acid (HCl) solutions were prepared from the concentrated acid (CAQ, Diadema, Brazil); an acetate buffer (pH 4.8, 0.01 mol L-1) was prepared with
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sodium acetate and acetic acid (both from CAQ). Regarding the Ripper method: sulphuric acid solution (25 %) was prepared from the concentrated acid (CAQ);
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potassium iodate (KI) was acquired from CAAL (Santa Cecília, Brazil). Starch and potassium iodide (KIO3) were bought from Synth (Diadema, Brazil). Beverage samples were obtained from an ordinary supermarket.
2.2. Extraction
The GDME extraction system has been described before (Figure 1A) [53–56]. The membrane was a Mitex PTFE (Millipore) with a porosity of 5.0 μm. The extraction procedure was carried out as follows: (1) 10 mL of sample were placed inside the sampling flask; (2) 2 mL of 1 mol L-1 HCl were added to the sample just before the extraction; (3) extraction occurred in a thermostatic water bath at 55 °C in the course of 15 minutes; (4) the acceptor solution was 1.00 mL of acetate buffer; (5) 80 µL of 1 mol L-1 HCl was added to 80 µL of the extract just before the electroanalysis to convert the HSO3- species into voltammetry-sensitive SO2.
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Journal Pre-proof 2.3. Fabrication of electrodes and resistance measurements
The electrodes were fabricated via room-temperature laser pyrolysis, using a WorkSpecial CO2 laser cutting system (model WS 9060C). The optimized fabrication parameters were as follows: (a) the laser wavelength was 10.6 µm with a pulse duration of approx. 14 µs; (b) the laser scan rate was ca. 16 mm s-1; (c) the working distance between laser and the target material was ca. 13 mm. RDWorks software was used to design the LP-ePADs templates on 665 g m2 paperboard (WestRock Company, Atlanta, USA) and control the fabrication procedure. Prior to the electrochemical measurements, silver grids were printed on the connection pads of the
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electrodes to improve their electrical conductivity. Furthermore, these silver grids were protected from direct contact with the electrolyte by printing a barrier of fast-drying inert
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glue (Cascola from Henkel, Düsseldorf, Germany). Finally, to overcome the common
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problem of reference electrode instability in paper-based devices, silver ink was painted on top of the reference electrode area. More details can be found in the
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literature [1].
The optimization procedure was based on the feedback results obtained from
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the four-point probe resistivity measurements, which were carried out using a Keysight DAQ970A (Santa Rosa, USA). Furthermore, the reproducibility of the proposed
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fabrication protocol was investigated by measuring the resistivity of 15 identical electrodes prepared using the parameters specified above.
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2.4. Electrochemical measurements
Voltammetric measurements were performed using a Metrohm AUT86702 system with Nova v 2.1.4 software. Electrochemical measurements were performed at room temperature without removing oxygen with the nitrogen flow. The square-wave voltammetry (SWV) parameters for sulphite analysis had been optimized previously [35], and the chosen conditions were as follows: the square-wave frequency was 50 Hz, the potential step 5 mV and the wave amplitude 25 mV (resulting in a scan rate of 250 mV s-1).
2.5. Comparison of Methods The ‘Ripper’ methodology [57] is an old simplification of the OIV (Organisation Internationale de la Vigne et du Vin) method [58] based on iodometric titration of the acidified sample (the titrating triiodine was formed from KIO3 with an excess of KI).
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Journal Pre-proof Specifically, 5 mL of 1:4 concentrated H2SO4 were added to 25 mL of the sample immediately before the titration, and starch was used as the endpoint indicator [22].
3. Results and discussion The voltammetric reduction of SO2 is far from being trivial [59]. Moreover, it has long been studied with electrodes such as mercury [14–18] or copper [29]. Kolthoff and Miller first suggested that, at a low pH, sulphur dioxide undergoes a two-electron, twoproton reduction forming sulphoxylic acid [15]:
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SO2 + 2e− + 2H + → H2 SO2
This was more recently corroborated by Streeter et al. via electrochemical
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electron spin resonance (ESR) [29].
The matrix effect is a common issue encountered when analysing complex
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mixtures by electrochemical methods. Sample preparation is often the best way to solve this problem – and not only can it solve the selectivity issue but it can also
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enhance the sensitivity [52]. GDME is a simple and portable technique that combines the advantages of microextraction with membrane-aided volatile and semi-volatile
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extraction [52,53]. In this study, the sample was acidified prior to extraction, changing the pH to a value such that all hydrogen sulphite is converted to sulphite. Then, when gaseous SO2 fills the headspace, an aliquot diffuses through the membrane and is
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entrapped in the acceptor solution, changing back to non-gaseous hydrogen sulphite with a pH change. In this way, sulphite can easily be determined without requiring an antibody, aptamer or any other recognizing element. The resistance of the electrodes was measured using a four-point probe system (Figure 1B) [60]. Typically, in these kinds of measurement it is necessary to apply a correction factor that depends on the geometry (G) of the surface. In this study, the distance between the probes was specified (i.e. 7 mm) so that G = 1, and no further correction is required. This is due to the fact that the width of the active surface (4 mm) is less than three times the distance between the probes [61,62]. The following resistances were obtained: working electrode – (20 ± 6) Ω; counter-electrode – (20 ± 8) Ω; and reference electrode before the addition of silver ink – (16 ± 4) Ω (i.e. the same within the uncertainty range). This resulted in an average of (19 ± 6) Ω, and considering a width of 4 mm, a sheet resistance of 11 ± 4 Ω per square. The expression ‘sheet resistance’ is often found in the literature concerning conductive surfaces, and values
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Journal Pre-proof are expressed in the unit of Ω per square [63,64]. Sheet resistance is the equivalent of resistivity for a thin conductive surface:
𝑅=ρ
𝑙 𝐴
where 𝑅 is the resistance of a conductor (Ω), ρ is the resistivity of the material of the conductor (Ω m-1), 𝐴 is the surface of the conductor perpendicular to the current direction (m2), and 𝑙 is the length. Furthermore, the surface resistivity of a
of
perpendicular section of the conductor is:
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𝐴=𝑤𝑡
where 𝑤 is the width (m) and 𝑡 is the thickness (m). By combining both
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equations:
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𝑙 𝑤𝑡
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𝑅=ρ
It is common in studies of conductive surfaces to use sheet resistance (𝑅SH )
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the equation to:
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instead of resistivity to avoid determination of the surface thickness, which simplifies
𝑅 = 𝑅SH
𝑙 𝑤
𝑅SH is the resistance of a rectangular surface for which length and width have the same value, ergo the sheet resistance is the resistance of a geometrical square of a thin surface. Measurements with the four-point probe system not only gave rise to more reproducible results but also, in general, 60–80% lower resistance when compared with data from a common 2-wire multimeter. The analytical parameters obtained from the calibration curve (Figure 2) are as follows: the square of the sample correlation coefficient (r2) is 0.9987, while the limits of detection and of quantification (LOD and LOQ) are1.2 mg L-1 and 3.9 mg L-1, respectively. The intercept was above zero, probably due to the presence of sulphites in the electrodes (sulphites are a common additive in pulping during the paper manufacturing process). The LOD and LOQ were calculated as three and ten times the standard deviation of the intercept/slope, respectively. Although there are lower LODs
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Journal Pre-proof in the literature (around 0.01 mg L-1 [23,30]), these limits are suitable for liquid samples like wine or juices and can be decreased with longer extraction times. They are also appropriate bearing in mind that many countries have regulatory limits of 10 mg L-1. Repeatability was evaluated by measuring five different solutions of 30 mg L-1, and was (176 ± 11) µA, i.e. the coefficient of variation (C.V.) was 6%. Reproducibility was evaluated in the same manner but on three different days, and was (183 ± 5) µA, i.e. the C.V. is 3%. The free sulphite content was determined in several different samples by the proposed methodology and by the simplified Ripper methodology. Quantification by GDME-LP-ePAD was performed by a single standard addition. As shown in Table 1, the results are similar within error margins. No dark-coloured samples were analysed
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because the endpoint of the titration using starch (in the Ripper method) is a dark violet shade that would be difficult to observe. No interference studies were performed since
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there is no suspicion that any other compounds volatile enough to be extracted at the
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temperatures used (ethanol, carbon dioxide, acetaldehyde, etc.), have similar
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behaviour regarding pH changes, or might have the same peak potential in the SWV.
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4. Conclusions
For the first time LP-ePADs were used in an analytical assay. Assisted by a
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simple GDME sample preparation step, a LOQ lower than the 10 mg L-1 regulatory limit was attained, along with suitable reproducibility. The developed analytical method is particularly competitive in terms of price and portability, allowing measurements at the
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point of production. It may therefore be of interest to drinks producers who use sulphites as an additive.
Acknowledgements
Authors acknowledge the São Paulo Research Foundation (FAPESP) for financial support (2018/14425-7, 2018/20745-4, 2019/11214-8 and 2018/13922-7) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES). We acknowledge the FAPESP and Shell Brazil through the ‘Research Centre for Gas Innovation – RCGI’ (2014/50279-4), hosted by USP and support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. the Research Center for Gas Innovation (RCGI). We would like to also acknowledge professor Josef Wilhelm Baader for the kind gift of some of the reagents.
All authors declare no conflicts of interest.
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[64]
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Journal Pre-proof Table and figure captions Table 1. Comparison of analytical results obtained using the proposed method (GDMELP-ePADs) and the simplified Ripper method. All sulphite values are in mg L-1 and analyses were done in duplicate.
Figure 1 (A) Simple scheme showing the GDME; (B) photo of the resistance being measured.
Figure 2 Square wave voltammograms of several extracts with different sulphite
of
concentrations (from 2.5 to 25 mg L-1). Inset: the corresponding calibration curve (peak
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na
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re
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current vs. sulphite concentration).
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Journal Pre-proof Table 1 GDME-LP-ePAD
Ripper method
Wine 1 ABV 11,5%, white wine
11.8 ± 1.1
13.2 ± 1.0
Wine 2 ABV 7,5%, sparkling white wine
28.1 ± 2.3
26.8 ± 2.1
Wine 3 ABV 10.0%, white wine
19.7 ± 1.6
20.7 ± 1.7
Beer 1 ABV 4.5%, Pilsen
13.4 ± 1.2
16.7 ± 1.4
Beer 2 ABV 4.5%, Pilsen
16.3 ± 1.3
14.7 ± 1.2
Non-alcoholic beer
10.9 ± 0.9
Vinegar
15.5 ± 1.2
Coconut water
31.5 ± 2.5
re
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ro
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Sample
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na
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ABV – alcohol by volume
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9.6 ± 0.8 15.9 ± 1.3 31.9 ± 2.5
Journal Pre-proof
Highlights
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Electroanalysis using low-cost pyrolyzed electrochemical paper-based analytical sensor. Paper electrodes are fabricated using a CO2 laser in a single reagentless step. The paper electrodes had a resistance of ca. 11 ohm/square.
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Figure 1
Figure 2
Alisson Bezerra Martins, Alnilan Lobato, Nikola Tasic, Fernando J. Perez-Sanz, Pedro Vidinha, Thiago R.L.C. Paixão, Luís Moreira Gonçalves PII:
S1388-2481(19)30204-8
DOI:
https://doi.org/10.1016/j.elecom.2019.106541
Reference:
ELECOM 106541
To appear in:
Electrochemistry Communications
Received date:
22 July 2019
Revised date:
23 August 2019
Accepted date:
4 September 2019
Please cite this article as: A.B. Martins, A. Lobato, N. Tasic, et al., Laser-pyrolyzed electrochemical paper-based analytical sensor for sulphite analysis, Electrochemistry Communications (2019), https://doi.org/10.1016/j.elecom.2019.106541
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© 2019 Published by Elsevier.
Journal Pre-proof
Laser-pyrolyzed electrochemical paper‐based analytical sensor for sulphite analysis 1
1
1
Alisson Bezerra Martins, Alnilan Lobato, Nikola Tasic, Fernando J. Perez-Sanz, 1
#,1
Pedro Vidinha, Thiago R. L. C. Paixão,
and Luís Moreira Gonçalves
1
1,*
1
Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo (USP), São Paulo, SP, Brazil
#
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*[email protected]
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[email protected]
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Abstract
A CO2 laser can be used to pyrolyze the surface of common paperboard to
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produce a conductive carbon material for subsequent use as an electrode for general electrochemical measurements. In this work a laser-pyrolyzed electrochemical paper‐
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based analytical device (LP-ePAD) of this type was combined with a gas-diffusion microextraction (GDME) unit for the square-wave voltammetric analysis of sulphites in
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commercial beverages. The fabricated LP-ePAD had a resistivity at room temperature of 11 ± 4 Ω per square electrical sheet (measured with a four-point probe system). The
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developed methodology showed a limit of detection of 1 mg L-1 for sulphite analysis, with good repeatability and reproducibility (coefficient of variation below 7%), as well having as the desirable qualities of portability and low price.
Keywords
Electroanalysis; Food quality; Sample preparation; Laser scribed paper-based analytical devices; Square-wave voltammetry; Volatile extraction
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Journal Pre-proof 1. Introduction Recently, one of the authors reported a new approach to the fabrication of electrochemical paper-based devices (ePADs) which does not require expensive equipment or controlled laboratory conditions [1]. The proposed approach used a CO2 laser to pyrolyze a paperboard surface in order to convert cellulose to carbon, creating a carbon surface for electroanalytical applications in a single low-cost step. ePADs were originally proposed in 2009 by Dungchai and co-authors using silk screen/screen printing procedures which could leave impurities on the paper after solvent evaporation [2]. Since then, a range of approaches to ePAD fabrication have been reported: casting
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[3], inkjet printing [4], use of graphite leads or direct pencil drawing, wire or fibre
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attachment [5], sputtering approaches under an inert atmosphere [6], as well as carbon pyrolysis using a tube furnace under oxygen [7]. However, all of these procedures have
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at least one of the following disadvantages: each ePAD has to be fabricated by hand, resulting in a lack of reproducibility, high cost, or the need for a controlled environment.
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By contrast, the laser pyrolysis procedure [8] can be used to fabricate reproducible lowcost devices in a single step [1].
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Sulphites have been used as food additives for many years, due to their many applications (prevention of the microbial growth, browning reactions and flavour
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stabilizing properties) and their outstanding efficiency [9,10]. However, their use should be controlled because of possible adverse clinical effects, particularly to sensitive individuals, ranging from dermatitis, urticaria, abdominal pain and diarrhoea to some
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rare yet dangerous anaphylactic responses [11]. Typically, the term ‘sulphites’ refers to a variety of compounds: sulphur dioxide, sodium sulphite, sodium bisulfite, sodium metabisulfite, potassium metabisulfite, and potassium sulphite (INS 220 to INS 225) [12]. However, in practice, in the pH range 2– 6 most common in beverages, sulphur is present in the form of hydrogen sulphite (HSO3) [10]. The electrochemical response of sulphites has been studied for over a decade [13], using several types of electrodes, including: mercury [14–23], gold [24– 26], platinum [12,27,28], copper [29], glassy carbon electrode (GCE) [30–32], multiwalled carbon nanotubes [33,34], screen-printed carbon electrode (SPCE) [35,36], and modified working electrodes [37–43]. To the best of the authors’ knowledge, however, sulphites have never been investigated using paper-based analytical devices. Electroanalysis has several advantages for testing food samples, primarily speed, sensitivity and portability, but in general this approach is susceptible to interference from other compounds present in these complex matrices. In order to circumvent common interference problems, a recognition element, such as an antibody
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Journal Pre-proof [44,45], aptamer or a molecular imprinted polymer [46–50], may be attached to the electrode surface. However, that can be expensive and sometimes technically challenging. Alternatively, one can make use of a sample preparation step, which may result in a cheaper and often more robust analytical methodology [51,52]. Taking all of this into account, this work’s rationale was, as a proof-of-concept, to combine inexpensive and portable paper electrodes with a light, low-cost sample preparation system, namely gas-diffusion microextraction (GDME) [35,36,53], for sulphite analysis in food matrices.
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2. Materials and methods
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2.1. Chemicals and samples
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All reagents were of analytical grade and were used as received. Aqueous solutions were prepared using ultrapure water with a resistivity not less than 18.2 MΩ
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cm at 298 K.
Sodium sulphite (Merck, Darmstadt, Germany) was used as a source of
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sulphites; hydrochloric acid (HCl) solutions were prepared from the concentrated acid (CAQ, Diadema, Brazil); an acetate buffer (pH 4.8, 0.01 mol L-1) was prepared with
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sodium acetate and acetic acid (both from CAQ). Regarding the Ripper method: sulphuric acid solution (25 %) was prepared from the concentrated acid (CAQ);
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potassium iodate (KI) was acquired from CAAL (Santa Cecília, Brazil). Starch and potassium iodide (KIO3) were bought from Synth (Diadema, Brazil). Beverage samples were obtained from an ordinary supermarket.
2.2. Extraction
The GDME extraction system has been described before (Figure 1A) [53–56]. The membrane was a Mitex PTFE (Millipore) with a porosity of 5.0 μm. The extraction procedure was carried out as follows: (1) 10 mL of sample were placed inside the sampling flask; (2) 2 mL of 1 mol L-1 HCl were added to the sample just before the extraction; (3) extraction occurred in a thermostatic water bath at 55 °C in the course of 15 minutes; (4) the acceptor solution was 1.00 mL of acetate buffer; (5) 80 µL of 1 mol L-1 HCl was added to 80 µL of the extract just before the electroanalysis to convert the HSO3- species into voltammetry-sensitive SO2.
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Journal Pre-proof 2.3. Fabrication of electrodes and resistance measurements
The electrodes were fabricated via room-temperature laser pyrolysis, using a WorkSpecial CO2 laser cutting system (model WS 9060C). The optimized fabrication parameters were as follows: (a) the laser wavelength was 10.6 µm with a pulse duration of approx. 14 µs; (b) the laser scan rate was ca. 16 mm s-1; (c) the working distance between laser and the target material was ca. 13 mm. RDWorks software was used to design the LP-ePADs templates on 665 g m2 paperboard (WestRock Company, Atlanta, USA) and control the fabrication procedure. Prior to the electrochemical measurements, silver grids were printed on the connection pads of the
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electrodes to improve their electrical conductivity. Furthermore, these silver grids were protected from direct contact with the electrolyte by printing a barrier of fast-drying inert
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glue (Cascola from Henkel, Düsseldorf, Germany). Finally, to overcome the common
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problem of reference electrode instability in paper-based devices, silver ink was painted on top of the reference electrode area. More details can be found in the
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literature [1].
The optimization procedure was based on the feedback results obtained from
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the four-point probe resistivity measurements, which were carried out using a Keysight DAQ970A (Santa Rosa, USA). Furthermore, the reproducibility of the proposed
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fabrication protocol was investigated by measuring the resistivity of 15 identical electrodes prepared using the parameters specified above.
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2.4. Electrochemical measurements
Voltammetric measurements were performed using a Metrohm AUT86702 system with Nova v 2.1.4 software. Electrochemical measurements were performed at room temperature without removing oxygen with the nitrogen flow. The square-wave voltammetry (SWV) parameters for sulphite analysis had been optimized previously [35], and the chosen conditions were as follows: the square-wave frequency was 50 Hz, the potential step 5 mV and the wave amplitude 25 mV (resulting in a scan rate of 250 mV s-1).
2.5. Comparison of Methods The ‘Ripper’ methodology [57] is an old simplification of the OIV (Organisation Internationale de la Vigne et du Vin) method [58] based on iodometric titration of the acidified sample (the titrating triiodine was formed from KIO3 with an excess of KI).
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Journal Pre-proof Specifically, 5 mL of 1:4 concentrated H2SO4 were added to 25 mL of the sample immediately before the titration, and starch was used as the endpoint indicator [22].
3. Results and discussion The voltammetric reduction of SO2 is far from being trivial [59]. Moreover, it has long been studied with electrodes such as mercury [14–18] or copper [29]. Kolthoff and Miller first suggested that, at a low pH, sulphur dioxide undergoes a two-electron, twoproton reduction forming sulphoxylic acid [15]:
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SO2 + 2e− + 2H + → H2 SO2
This was more recently corroborated by Streeter et al. via electrochemical
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electron spin resonance (ESR) [29].
The matrix effect is a common issue encountered when analysing complex
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mixtures by electrochemical methods. Sample preparation is often the best way to solve this problem – and not only can it solve the selectivity issue but it can also
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enhance the sensitivity [52]. GDME is a simple and portable technique that combines the advantages of microextraction with membrane-aided volatile and semi-volatile
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extraction [52,53]. In this study, the sample was acidified prior to extraction, changing the pH to a value such that all hydrogen sulphite is converted to sulphite. Then, when gaseous SO2 fills the headspace, an aliquot diffuses through the membrane and is
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entrapped in the acceptor solution, changing back to non-gaseous hydrogen sulphite with a pH change. In this way, sulphite can easily be determined without requiring an antibody, aptamer or any other recognizing element. The resistance of the electrodes was measured using a four-point probe system (Figure 1B) [60]. Typically, in these kinds of measurement it is necessary to apply a correction factor that depends on the geometry (G) of the surface. In this study, the distance between the probes was specified (i.e. 7 mm) so that G = 1, and no further correction is required. This is due to the fact that the width of the active surface (4 mm) is less than three times the distance between the probes [61,62]. The following resistances were obtained: working electrode – (20 ± 6) Ω; counter-electrode – (20 ± 8) Ω; and reference electrode before the addition of silver ink – (16 ± 4) Ω (i.e. the same within the uncertainty range). This resulted in an average of (19 ± 6) Ω, and considering a width of 4 mm, a sheet resistance of 11 ± 4 Ω per square. The expression ‘sheet resistance’ is often found in the literature concerning conductive surfaces, and values
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Journal Pre-proof are expressed in the unit of Ω per square [63,64]. Sheet resistance is the equivalent of resistivity for a thin conductive surface:
𝑅=ρ
𝑙 𝐴
where 𝑅 is the resistance of a conductor (Ω), ρ is the resistivity of the material of the conductor (Ω m-1), 𝐴 is the surface of the conductor perpendicular to the current direction (m2), and 𝑙 is the length. Furthermore, the surface resistivity of a
of
perpendicular section of the conductor is:
ro
𝐴=𝑤𝑡
where 𝑤 is the width (m) and 𝑡 is the thickness (m). By combining both
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equations:
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𝑙 𝑤𝑡
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𝑅=ρ
It is common in studies of conductive surfaces to use sheet resistance (𝑅SH )
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the equation to:
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instead of resistivity to avoid determination of the surface thickness, which simplifies
𝑅 = 𝑅SH
𝑙 𝑤
𝑅SH is the resistance of a rectangular surface for which length and width have the same value, ergo the sheet resistance is the resistance of a geometrical square of a thin surface. Measurements with the four-point probe system not only gave rise to more reproducible results but also, in general, 60–80% lower resistance when compared with data from a common 2-wire multimeter. The analytical parameters obtained from the calibration curve (Figure 2) are as follows: the square of the sample correlation coefficient (r2) is 0.9987, while the limits of detection and of quantification (LOD and LOQ) are1.2 mg L-1 and 3.9 mg L-1, respectively. The intercept was above zero, probably due to the presence of sulphites in the electrodes (sulphites are a common additive in pulping during the paper manufacturing process). The LOD and LOQ were calculated as three and ten times the standard deviation of the intercept/slope, respectively. Although there are lower LODs
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Journal Pre-proof in the literature (around 0.01 mg L-1 [23,30]), these limits are suitable for liquid samples like wine or juices and can be decreased with longer extraction times. They are also appropriate bearing in mind that many countries have regulatory limits of 10 mg L-1. Repeatability was evaluated by measuring five different solutions of 30 mg L-1, and was (176 ± 11) µA, i.e. the coefficient of variation (C.V.) was 6%. Reproducibility was evaluated in the same manner but on three different days, and was (183 ± 5) µA, i.e. the C.V. is 3%. The free sulphite content was determined in several different samples by the proposed methodology and by the simplified Ripper methodology. Quantification by GDME-LP-ePAD was performed by a single standard addition. As shown in Table 1, the results are similar within error margins. No dark-coloured samples were analysed
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because the endpoint of the titration using starch (in the Ripper method) is a dark violet shade that would be difficult to observe. No interference studies were performed since
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there is no suspicion that any other compounds volatile enough to be extracted at the
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temperatures used (ethanol, carbon dioxide, acetaldehyde, etc.), have similar
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behaviour regarding pH changes, or might have the same peak potential in the SWV.
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4. Conclusions
For the first time LP-ePADs were used in an analytical assay. Assisted by a
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simple GDME sample preparation step, a LOQ lower than the 10 mg L-1 regulatory limit was attained, along with suitable reproducibility. The developed analytical method is particularly competitive in terms of price and portability, allowing measurements at the
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point of production. It may therefore be of interest to drinks producers who use sulphites as an additive.
Acknowledgements
Authors acknowledge the São Paulo Research Foundation (FAPESP) for financial support (2018/14425-7, 2018/20745-4, 2019/11214-8 and 2018/13922-7) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES). We acknowledge the FAPESP and Shell Brazil through the ‘Research Centre for Gas Innovation – RCGI’ (2014/50279-4), hosted by USP and support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. the Research Center for Gas Innovation (RCGI). We would like to also acknowledge professor Josef Wilhelm Baader for the kind gift of some of the reagents.
All authors declare no conflicts of interest.
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Journal Pre-proof Table and figure captions Table 1. Comparison of analytical results obtained using the proposed method (GDMELP-ePADs) and the simplified Ripper method. All sulphite values are in mg L-1 and analyses were done in duplicate.
Figure 1 (A) Simple scheme showing the GDME; (B) photo of the resistance being measured.
Figure 2 Square wave voltammograms of several extracts with different sulphite
of
concentrations (from 2.5 to 25 mg L-1). Inset: the corresponding calibration curve (peak
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re
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current vs. sulphite concentration).
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Journal Pre-proof Table 1 GDME-LP-ePAD
Ripper method
Wine 1 ABV 11,5%, white wine
11.8 ± 1.1
13.2 ± 1.0
Wine 2 ABV 7,5%, sparkling white wine
28.1 ± 2.3
26.8 ± 2.1
Wine 3 ABV 10.0%, white wine
19.7 ± 1.6
20.7 ± 1.7
Beer 1 ABV 4.5%, Pilsen
13.4 ± 1.2
16.7 ± 1.4
Beer 2 ABV 4.5%, Pilsen
16.3 ± 1.3
14.7 ± 1.2
Non-alcoholic beer
10.9 ± 0.9
Vinegar
15.5 ± 1.2
Coconut water
31.5 ± 2.5
re
-p
ro
of
Sample
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na
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ABV – alcohol by volume
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9.6 ± 0.8 15.9 ± 1.3 31.9 ± 2.5
Journal Pre-proof
Highlights
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Electroanalysis using low-cost pyrolyzed electrochemical paper-based analytical sensor. Paper electrodes are fabricated using a CO2 laser in a single reagentless step. The paper electrodes had a resistance of ca. 11 ohm/square.
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Figure 1
Figure 2