Digitally Controlled Procedure for Assembling Fully Drawn Paper

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Digitally Controlled Procedure for Assembling Fully Drawn PaperBased Electroanalytical Platforms Nicolò Dossi,*,† Stefano Petrazzi,† Rosanna Toniolo,† Franco Tubaro,† Fabio Terzi,‡ Evandro Piccin,§ Rossella Svigelj,† and Gino Bontempelli† †

Department of Agrifood, Environmental and Animal Science, University of Udine, via Cotonificio 108, I-33100 Udine, Italy Department of Chemical and Geological Science, University of Modena and Reggio Emilia, via Campi 183, I-41125 Modena, Italy § Department of Chemistry, Federal University of Minas Gerais, 31270-901 Belo Horizonte, Brazil ‡

ABSTRACT: A simple, reliable, and low-cost fabrication method is proposed here for assembling paper-based electrochemical devices (PEDs) using a commercial desktop digitally controlled plotter/cutter, together with ordinary writing tools. Permanent markers (tips of 1 mm) were used to create effective hydrophobic barriers on paper, while micromechanical pencils (mounting 4B graphite leads, 0.5 mm in diameter) were adopted for automatically drawn precise reference, counter, and working carbon electrodes. Fabrication parameters, such as writing pressure and speed, were first optimized, and the electrochemical performance of these devices was then evaluated by using potassium hexacyanoferrate(II) as redox probe. The good interdevice reproducibility (4.8%) displayed by the relevant voltammetric responses confirmed that this strategy can be profitably adopted to easily assemble paper-based electrochemical devices in a highly flexible manner. The simplicity of the instrumentation used and the low cost of each single device (about $0.04), together with the speed of fabrication (about 2 min), are other important features of the proposed strategy. Finally, to confirm the effectiveness of this prototyping method for the analysis of real samples and rapid controls, PEDs assembled by this simple approach were successfully exploited for the analysis of vitamin B6 in food supplements.

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to be coupled with paper-based analytical devices (μPADs).13,14 Micropaper-based electroanalytical devices (μPEDs) were successfully proposed for clinical, food, and environmental analysis, displaying promising performance in terms of sensitivity and selectivity.15−18 The choice of materials and procedures for assembling conductive electrodes is however critical for electrochemical sensing. The use of noble metals, such as gold and platinum, deposited as thin films by sputtering,19 or in the form of wires and meshes, was proposed.16,20 However, the deposition of conducting materials in the form of inks, consisting of particles and/or nanomaterials suspended in solvent mixtures, dispersants, or polymeric binders, is frequently preferred. The deposition of these materials is commonly achieved via inkjet printing and screen printing.21−23 However, these procedures present some limitations, such as the use of quite sophisticated instruments (sputtering deposition) or the need to formulate suitable inks (inkjet printing). The use of dedicated masks or templates, containing the circuit to be patterned, as well as the quite large waste of material occurring during deposition (screen-printing technology), could represent further limiting aspects when a proper fabrication technology for assembling electrodes for simple and

uring the last two decades, interest in the development of miniaturized, cost-effective, and simple analytical devices has increased considerably. In fact, lab-on-a-chip (LOC) systems, achieved by miniaturization of conventional instrumentations to microscale, make possible on-site precise and accurate analysis, characterized by short analysis time and low reagent consumption.1−3 With the aim of producing disposable, low-cost and easy-toconstruct analytical devices, paper substrates have revealed attractive features such as ability to transport liquids by capillarity, disposability, and biocompatibility.4−6 Moreover, paper is compatible with a wide range of printing techniques, such as inkjet printing, roll-to-roll printing, stamp printing, and wax printing. In particular, these techniques make possible the rapid patterning of hydrophobic barriers, enabling the easy and rapid assembly of hydrophilic miniaturized channels, without requiring sophisticated technologies commonly required for microfabrication.7−9 In addition, by profiting from the intrinsic writable properties and flexibility of paper, as well as its low cost, simple fabrication procedures have been proposed for assembling paper-based fluidic systems by simply drawing hydrophobic lines with wax pens or markers filled with waterproof inks.10−12 In view of the low cost, portability, and low-power requirements of instrumentations, together with the possibility of miniaturizing also sensing elements, which are proper for electrochemical detection, this appears to be the ideal candidate © XXXX American Chemical Society

Received: June 29, 2017 Accepted: September 1, 2017 Published: September 1, 2017 A

DOI: 10.1021/acs.analchem.7b02521 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

Figure 1. Schematic representation of the sequence adopted for assembling PEDs by the plotter/cutter: (a) patterning of ring-shaped paper-based electrochemical cells onto filter paper with a permanent black wax-based ink from a Lumocolor marker; (b) drawing of graphite electrodes by micropencil leads; (c) assembled paper-based electrochemical device. W, R, and C are working, reference, and counter pencil-drawn electrodes, respectively.

low-cost electrochemical devices is planned. Moreover, it is worth underlining that carbon-based materials are frequently used because of their low cost, good electron transfer kinetics, good chemical stability, and biocompatibility.24,25 Leads from commercial pencils, consisting of compact rods obtained by extrusion of a mixture of graphite powders and clay or polymeric binders, were extensively used as solid carbonbased working electrodes either for conventional cells or coupled with paper-based devices.26−30 Above all, pencils are known as day-to-day tools to draw graphitic tracks on writable surfaces. Mechanical abrasion of pencil leads on rough enough surfaces, such as paper, causes exfoliation of pencil leads and consequent entrapment of graphite particles into cellulose fibers. Thus, by profiting from the intrinsic electrical conductivity of graphite, simple handwriting can produce quite uniform, well-adhered, and electrically conductive carbon tracks on porous materials. The pencilon-paper approach was profitably adopted for easily drawing carbon electrodes (pencil-drawn electrodes, PDEs) with the desired geometry, suitable for even quite complex electrochemical circuits on porous and rough substrates such as paper.31−35 Conversely, the performance achieved when pencil electrodes are instead drawn on smooth surfaces, such as unmodified polymeric materials, suffers from the formation of a scarcely adherent and poorly conductive, pattern. For this reason, modification of these smooth surfaces is needed, in order to facilitate abrasive deposition of carbon particles.36 However, it is worth noting that the use of hand-drawing procedures leads to width and thickness of patterned lines quite sensitive to pressure and writing speed, as well as to a pencil lead tip-size increase during abrasion. Consequently, scarce repeatability and reproducibility are expected for electrochemical processes investigated by using drawn lines as electrodes. Thus, while this method is ideal for rapid

prototyping and small productions, the development of novel cost-efficient strategies suitable for higher productions with high precision is a key element for bringing these devices closer to real world applicability. Micromechanical pencils, consisting of pencil leads with diameters ranging from 0.3 to 1 mm, offer many advantages over typical wood pencils. In fact, they enable thin lines to be drawn with high precision, without the need to sharpen during layer-by-layer deposition.34 In this article we describe for the first time a simple procedure for assembling fully drawn paper-based electrochemical devices by computer-controlled mechanical drawing. Marker pens containing a permanent hydrophobic ink and micromechanical pencils were installed on a low-cost desktop digital craft plotter/cutter for patterning hydrophobic barriers and assembling graphite miniaturized three electrode cells, respectively. This X−Y approach enables more precise control of the pressure during writing, thus achieving a marked increase of the device-to-device reproducibility with respect to the hand-drawn approach. After the preliminary evaluation of their performance by using hexacyanoferrate(II) as the redox prototype species, they were applied to the detection of vitamin B6 in dietary supplement tablets. We report here the procedure adopted to assemble these fully drawn paper-based electrochemical devices (PEDs), together with the good performance achieved by the use of this simple approach.

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EXPERIMENTAL SECTION Chemicals and Instrumentation. Analytical reagent grade potassium chloride, sodium acetate, Patent Blue, acetic acid, pyridoxine hydrochloride (vitamin B6), ascorbic acid, and potassium hexacyanoferrate(II) trihydrate were purchased from B


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1b), working, reference, and counter graphite electrodes were drawn onto paper. Electrodes were drawn according to the digital layout created with a surface area of ca. 7 mm2 for the working electrodes, with a width of 1 mm and at a distance of 3 mm from one another. The layout of PED cells is shown in Figure 1c. Finally, after installing the cutting blade onto the plotter, patterned paper foils were cut into pieces to obtain single PED devices. Miniature crocodile connectors located outside circular hydrophobic wax barriers surrounding hydrophilic cells were used for electrical connections to all electrodes. Voltammetric measurements in PED cells were conducted by laying these cells horizontally on a closed box containing water to minimize the electrolyte evaporation during measurements. A controlled volume (10 μL) of electrolyte solution containing known concentrations of assayed analytes was laid on a corner of the cell in order to soak paper channels, without covering the upper surface of electrodes.

Sigma-Aldrich (St. Louis, MO) and were employed without further purification. Aqueous 1 M KCl and 0.1 M acetic acid + sodium acetate (pH 5) solutions were prepared in high-purity deionized water (purified with an Elgastat UHQ-PS system, Elga, High Wycombe, UK) and used as supporting electrolytes in voltammetric measurements. Stock standard solutions (10 mM) of all analytes were prepared by adding weighed amounts to known volumes of 1 M KCl or 0.1 M sodium acetate supporting electrolytes. Controlled amounts of these stock solutions were then suitably diluted to the desired concentrations with the convenient supporting electrolyte prior to each experiment. All voltammetric and amperometric measurements were performed by a CH 732 potentiostat (CH Instruments, Austin, TX) driven by the relevant CHI software (version 12.04). Commercial marker pens Lumocolor 317-9, purchased by Staedler (Nuernberg, D), were used for patterning paper-based electrochemical cells, while 4B grade Super HI-Polymer micropencil leads (Ø = 0.5 mm), from Pentel (Swindon, UK), were adopted to draw graphite electrodes. These last items were chosen intentionally because they proved to be wellsuited for tracing lines characterized by good conductivity and porosity, thus allowing high signal-to-noise ratios to be attained.37 Moreover, these very thin micropencils allowed lines to be traced repeatedly in all parts of paper sheets, without requiring renewal of their tip, since it was not appreciably worn by continuous drawing. Sample Preparation. Commercial food-supplement tablets containing magnesium oxide, magnesium citrate, magnesium lattate, silicium dioxide, magnesium stearate, calcium pantothenate (vitamin B5), pyridoxine hydrochloride (vitamin B6), and thiamine chloridrate (vitamin B1), were purchased from a local supermarket. These tablets were first finely ground in a mortar. Then, weighed amounts of the resulting powdered samples were dissolved in the acetate buffer and filtered on a 0.45 μm Minisart purchased from Sigma-Aldrich. Finally, the pH of the obtained solution (10 mL) was adjusted with acetic acid to reach a pH of 5. Fabrication of PEDs. A series of ring-shaped paper-based electrochemical cells (PED cells) were patterned onto filter paper with a permanent black wax-based ink, according to a previously reported method.11,12,38,39 With this purpose, a Lumocolor marker filled by a permanent hydrophobic ink was first inserted into the specific pen holder of a Silhouette Studio plotter/cutter (Silhouette CAMEO, Micronet Italia, Milano, Italy) as schematically shown in Figure 1a. This plotter/cutter can act as an X−Y automatic writing device allowing the vertical pressure to be regulated on a linear scale ranging from 1 to 32, up to a maximum value of 2.1 N, controlled by the relevant software. Such software also makes it possible to regulate the writing speed on a linear scale ranging from 1 to 10 cm s−1, as well as to program the digital layout of the circuit to be patterned. Filter paper foils (20 cm × 20 cm, grade 1) from Whatman (Maidstone, UK) were positioned and fixed with small pieces of transparent adhesive tape onto a reusable plastic carrier sheet which is moved by the X−Y device. A series of rings were first drawn by marker pens containing a permanent hydrophobic black ink and left at room temperature to allow the ink solvent to evaporate, thus obtaining circular pads displaying a hydrophilic area of ca. 113 mm2, defined by a hydrophobic barrier (ca. 1.8 mm in width). Subsequently, after replacing the marker pen with a commercial 4B grade mechanical micropencil lead (see Figure

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RESULTS AND DISCUSSION Design and Control of the Effectiveness of DrawnHydrophobic Barriers. Permanent writing on different surfaces and materials is conventionally made by indelible permanent markers. They contain a liquid ink consisting usually of a hydrophobic resin, a solvent, and a colorant contained in a reservoir connected to a nib of variable dimensions. To draw precise and well-defined but at the same time not too thin traces on paper, we selected a medium nib (1 mm). Contact of the nib onto porous enough materials, such as filter paper, enables ink flowing until it penetrates completely into paper pores, thus creating hydrophobic lines extending through the entire thickness of paper. These barriers prevent water slipping out, so that they are particularly suitable for surrounding hydrophilic paper channels or zones where aqueous solutions have to be confined. During the writing process, a sufficient enough ink delivery must be ensured, avoiding at the same time an excessive broadening of written patterns as a consequence of an excessive impregnation of paper fibers. Consequently, both duration and pressure of the contact between the pen nib and paper must be carefully controlled. Thus, to identify the best operative conditions, the X−Y plotter was used to draw on paper different rings by changing both the writing speed from 1 to 10 cm s−1 and the writing pressure on the mentioned numerical scale ranging from 1 to 32. The most well-defined and precise lines, piercing through paper pores to the back face, were achieved by resorting to two steps of deposition, each consisting of a double passage of the nib onto the paper substrate, with a speed of 1 cm s−1 and a pressure value of 7 (likely equivalent to 0.46 N). These parameters were thus adopted for the fabrication of circular hydrophilic cells surrounded by hydrophobic barriers (Figure 1a). The use of higher pressures led to rapid and excessive degradation of the pen nib, while lower pressures and higher writing rates did not allow sufficient ink amounts to be deposited, so as to create barriers extending through the entire thickness of paper. The effective capacity of the generated barriers to dam aqueous solutions was tested by using a 1 M KCl solution containing Patent Blue, which is a commonly adopted food dye, in order to allow possible leakages to be identified easily. Amounts of this solution up to 13 μL, covering completely all the available hydrophilic area, were laid on a corner of the cell C


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Figure 2. Images of assembled circular PADs after contact with 10, 11, 12, and 13 μL of the Patent Blue aqueous solution (a−d) for 60 min, together with an optical microscope image at 8× magnification of the device containing pencil-drawn electrodes (e).

assembled, as shown in Figures 1c and 2e, this last reporting a magnified image of drawn lines. The electrochemical performance of these PEDs was subsequently tested by recording cyclic voltammograms on 1 mM hexacyanoferrate(II) solutions in 1 M KCl, with this compound being chosen as a redox prototype species in view of its well-known electrochemical behavior. This performance was evaluated as a function of deposited graphite layers. Thus, different devices were assembled using one, two, three, and four steps of deposition, each consisting of a double passage of pencil leads onto the paper substrate. The results obtained in these experiments increased the evidence supporting that the increase of the number of deposited layers led to an increase of recorded peak currents (a ca. 56% increase was found on passing from 2 to 8 layers). Concomitantly, a decrease of the corresponding peak-to-peak separation was observed. As a matter of fact, these findings were expected because the increase of deposition steps caused the increase of deposited graphite with the consequent increase not only of the electrode surface, but also of the interconnection between graphite particles, thus reducing parasite electrical resistances affecting the electron transfer kinetics. Conversely, further deposition steps above the fourth did not lead to further improvement of signals, whereas paper fibers turned out to be damaged and scratched. Cyclic voltammograms for the recorded redox probe potassium hexacyanoferrate(II) at PEDs assembled with four deposition steps displayed the best results consisting of current−potential curves that are proper for a quite reversible

which was placed horizontally on a closed container partially filled with water to avoid the rapid evaporation of water from paper. No colorant leakage was observed for up to more than 60 min, as shown in Figure 2a−d, pointing out that the dye aqueous solution was unable to penetrate or dissolve the hydrophobic barrier, whatever the volume of the dye solution introduced in the cell. On the contrary, some leakages occurred when circular barriers were drawn by using a single step of ink deposition, thus increasing the evidence of their scarce efficiency. Design and Electrochemical Characterization of PEDs. Once the capacity of drawn hydrophobic barriers to dam water was verified, carbon-based electrodes were drawn on these circular paper-based cells. With this purpose, mechanical pencils with a lead-advance mechanism, pushing leads by simple pressure, were installed on the X−Y plotter, as shown in Figure 1b. Graphite-based working, reference, and counter electrodes were hence drawn onto paper surfaces inside hydrophobic barriers by profiting from the digital file suitably created previously. To optimize this step, different values for both writing speed and writing pressure were applied, selected on the scales mentioned in the previous section. In order to deposit abundant enough amounts of graphite particles, so as to achieve a good electrical conductivity for drawn tracks, pressure values of 30 (likely equivalent to 1.9 N) were adopted (the usually applied force during hand writing is about 0.6−1.6 N34). As to the writing speed, a value of 3 cm s−1 ensured a rapid enough drawing, without affecting the integrity of both pencil lead tips and paper surfaces. Under these writing conditions, precise and well-defined graphite electrodes were D


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that the performance of PEDs was unaffected by the sheet region where they were drawn, conceivably thanks to the use of such thin micropencils that could allow lines to be traced repeatedly in all parts of paper sheets, without requiring renewal of their tip. With the aim of evaluating the performance of assembled devices under more severe conditions, such as low pH, voltammetric experiments were also performed on PEDs soaked with solutions of 1 mM of ascorbic acid dissolved in an aqueous 1 M KCl electrolyte to which 0.1 M hydrochloric acid was added. Figure 4 shows the recorded voltammetric

process with a fast electron transfer, typically expected at graphite-based electrodes, as shown in Figure 3.

Figure 3. Cyclic voltammograms recorded at a scan rate of 50 mV s−1 for 1 mM potassium hexacyanoferrate(II) in 1 M KCl () and for the sole electrolyte (···), together with the plot of peak current vs the square root of the scan rate (inset).

Figure 4. Cyclic voltammograms recorded at a scan rate of 50 mV s−1 for 1 mM ascorbic acid in 0.1 M HCl + 1 M KCl () and for the sole electrolyte (···).

It is worth noting that no voltammetric peak was detected when voltammetric experiments were performed on PEDs soaked with the sole supporting electrolyte (1 M KCl), thus pointing out that no electroactive interfering species was appreciably released by pencil leads. Consequently, all further investigations were performed by using PEDs drawn with four deposition steps, in order to achieve the best electrochemical performance. In order to ascertain whether high scan rates were able to affect voltammetric profiles, cyclic voltammograms were recorded at scan rates between 10 and 150 mV s−1, for 1 mM potassium hexacyanoferrate(II) solutions in 1 M KCl. No peak distortion was observed, and peak current increased linearly with the square root of the scan rate (y (μA) = 0.210v1/2 (mV s−1)1/2 + 0.267; R2 = 0.998), thus proving that no disturbance is caused by PEDs for this diffusion-controlled process. Moreover, repeatable voltammograms in terms of peak heights (RSD of 2.1%) were recorded for 10 different analyses performed on 1 mM potassium hexacyanoferrate(II) solutions in 1 M KCl, without observing any passivation of the electrode surface. Finally, very small device-to-device differences were found for peak currents recorded on 1 mM potassium hexacyanoferrate(II) in 1 M KCl applied to 10 different PED cells assembled using the same procedure. In fact, a reproducibility characterized by an RSD value of 4.8% was found for peak currents, which is better than that usually achieved by simple hand-drawing (about 8−10%).31−33 Higher variations were instead observed for peak potentials, due conceivably to the use of the pseudoreference carbon electrode. It is worth noting that a quite similar performance was also displayed by using different pencils taken from different batches. To thoroughly examine reproducibility, different PED cells were also assembled, by adopting once more the procedure above, in different regions of a single sheet (i.e., printed on top, bottom, middle, left, or right sides). Of these PED cells, 20 (four printed on each region) were then tested by recording voltammograms for 1 mM potassium hexacyanoferrate(II) solutions in 1 M KCl. RSD characterizing peak currents recorded turned out to be once again 4.8%, thus pointing out

profile displaying a sole anodic peak in the forward scan at a potential of ca. 0.6 V and the absence of any associated return peak in the reverse scan, in agreement with the results of previously reported investigations concerning this analyte at graphite electrodes.40 Also, in this case, repeatable voltammograms, confirming very small device-to-device differences, were found. However, it is worth underlining that the possibility of using PEDs (consisting of ring-shaped paper-based cells patterned with a wax-based ink onto which electrodes are drawn by suitable pencil leads) for the detection in aqueous samples at different pH values was previously proven under both static and flow conditions just by us for several analytes, such as ferrocene, food dyes, ortho-diphenols, paracetamol, dopamine, and cysteine.31−33,37,41 On the contrary, the use of hydrophobic barriers drawn by commercial inks is problematic enough for analyzing organic media, in that these barriers are easily enough dissolved or penetrated in fairly short times by almost all organic media. However, to dam solvents such as methanol and acetonitrile, barriers assembled by the use of particularly solvent-resistant inks containing polymeric materials such as polydimethylsiloxane can be profitably used.41 Application of Fully Drawn PEDs to Pyridoxine Determination in Real Sample. With the aim of verifying whether these fully drawn PEDs can be employed as effective analytical tools, they were tested for the analysis of vitamin B6 in food supplements. Preliminary cyclic voltammetric experiments were performed at PEDs on synthetic samples of pyridoxine (vitamin B6) in acetate buffer (pH = 5), in order to evaluate its redox behavior. In agreement with previous studies conducted on carbon-based electrodes,42,43 the observed voltammetric profile displayed a sole anodic peak in the forward scan at quite high potentials (ca. 0.9 V) and the absence of any associated return peak in the reverse scan, thus pointing out the irreversibility of the electrochemical process involved, as shown in Figure 5, inset. A progressive lowering of E


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proposed here did not suffer from interference from other components present in samples.

the anodic peak height was observed in the subsequent scans, due reasonably to a partial passivation of the electrode surface.

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CONCLUSIONS This paper describes the use of a simple and office-available apparatus (digital plotter/cutter) and day-to-day writing tools (permanent markers and pencil leads) for the fast and very reproducible fabrication of paper-based electrochemical devices. The method is rapid and inexpensive and can be performed by untrained personnel. High flexibility is another interesting feature characterizing the proposed approach. In fact, it is possible to assemble devices with new design for both fluidic and electrochemical circuits, by simply changing the digital layout. This direct-writing approach, resorting to readily available tools and easily accessible instrumentation, makes possible the rapid dry deposition of conductive tracks on rough surfaces as paper supports, without tackling problems such as solvent evaporation and waste of hazardous chemicals instead encountered when inks or paste are used. This procedure involves a direct maskless deposition without further steps between output design and circuit fabrication, differently from other deposition procedures. For instance, screen printing methods instead require the use of blades or squeegees to spread conductive materials over masks or screens containing the layout design to be replicated onto the substrate. Circuit layout and electrode geometry can be rapidly modified on demand with the procedure proposed here by easily changing the computer-aided design (CAD) software. Moreover, it permits the avoidance of problems related to nozzle dimension, ink viscosity, surface tension, and particle size, which are critical parameters to be controlled when other maskless approaches are used (such as inkjet printing). Finally, it is worth noting that the possibility of using even pencil leads modified with electrocatalysts33 could increase the application range of this pencil-drawing approach. The use of this rapid and simple fabrication method is particularly promising and advantageous for the development of low-cost devices for in-field applications. In particular, it could be very profitable for providing efficient analytical tools in remote regions where modern technologies are not available, as well as for introducing chemistry students to microfluidics. In fact, this simple procedure, requiring the sole change of writing tools between the step of paper-based cell fabrication and the subsequent step of electrochemical circuit drawing, offers many advantages in terms of production speed for assembling simple and low-cost portable and do-it-yourself electrochemical devices. It must also be highlighted that the manufacturing cost of these devices is very low. About 50 cells can be drawn on a paper sheet whose cost can be estimated to be ca. $0.5, and a permanent marker (ca. $2) permits the preparation of ca. 200 cells. In addition, about 15 PEDs can be prepared by a single micropencil lead (ca. $0.2). Consequently, a cost of about $0.04 can be estimated for each PED.

Figure 5. Differential pulse voltammogramms recorded at PEDs for pyridoxine solutions at the indicated concentrations in acetate buffer (pH 5) as supporting electrolyte, together with the DPV voltammogram of a real sample (···). Inset: cyclic voltammogram for 1 mM pyridoxine at 50 mV s−1 recorded in the same medium.

In order to minimize the detection limit in the subsequent determination tests, differential pulse voltammetric (DPV) measurements were performed. Detection parameters, pulse amplitude, step potential, and scan rate were optimized by evaluating their effect on peak currents recorded for 1 mM pyridoxine solutions in 0.1 M aqueous acetate electrolyte (pH 5.0). The results obtained made it possible to conclude that the best performance was achieved by using a potential increment of 5 mV, a pulse amplitude of 0.2 V, a pulse width of 0.1 s, a sampling width of 0.05 s, and a pulse period of 0.5 s. In spite of this optimization, the irreversible behavior of pyridoxine strongly affected the shape and height of DPV profiles. In fact, markedly lower and broader peaks were observed for this species with respect to those recorded for similar concentrations of potassium hexacyanoferrate(II), which is instead characterized by a reversible process. This notwithstanding, peak currents linearly dependent on pyridoxine concentration were recorded, as shown in Figure 5 in the range 50−800 μM (y (μA) = 0.0033CVitB (μM) + 0.3230; R2 = 0.983). The limit of detection (LOD), evaluated for a signal-to-noise ratio (S/N) of 3, turned out to be as low as 7 μM, and the limit of quantization (LOQ), inferred for S/N of 10, proved to be 23 μM. Finally, this detection procedure was applied to the determination of pyridoxine in real samples consisting of food-supplement tablets purchased from local supermarkets. These samples (ca. 1.3 g) were subjected to a minimal pretreatment consisting of their grinding in a mortar, subsequent suspension with acetic buffer, and filtration on a 0.45 μm filter. Finally, before analysis the pH was adjusted to 5 and the sample volume to 10 mL. The effect of possible interferences caused by thiamine hydrochloride (vitamin B1) and pantothenic acid (vitamin B5) on voltammetric responses of vitamin B6 was previously evaluated by reaching the conclusion that these species were not electroactive within the explored potential range. A voltammetric response typically recorded for these real samples is reported in Figure 5 (···). Quantitative determinations were performed by exploiting the calibration plot constructed for synthetic samples. A good agreement (±5%) with values declared by manufacturers was observed, thus confirming that electrochemical measurements on PEDs

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+39) 0432 558835. Fax: (+39) 0432 558803. E-mail: [email protected]. ORCID

Nicolò Dossi: 0000-0002-2136-3260 F


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(35) Li, W.; Qian, D.; Wang, Q.; Li, Y.; Bao, N.; Gu, H.; Yu, C. Sens. Actuators, B 2016, 231, 230−238. (36) Frazier, K. M.; Mirica, K. A.; Walish, J. J.; Swager, T. M. Lab Chip 2014, 14, 4059−4066. (37) Dossi, N.; Toniolo, R.; Impellizzieri, F.; Tubaro, F.; Bontempelli, G.; Terzi, F.; Piccin, E. Anal. Chim. Acta 2017, 950, 41−48. (38) Gallibu, C.; Gallibu, C.; Avoundjian, A.; Gomez, F. A. Micromachines 2016, 7, 6. (39) Curto, V. F.; Lopez-Ruiz, N.; Capitan-Vallvey, L. F.; Palma, A. J.; Benito-Lopez, F.; Diamond, D. RSC Adv. 2013, 3, 18811−18816. (40) Hu, I.-F.; Kuwana, T. Anal. Chem. 1986, 58, 3235−3239. (41) Dornelas, K. L.; Dossi, N.; Piccin, E. Anal. Chim. Acta 2015, 858, 82−90. (42) Hernandez, S. R.; Ribero, G. G.; Goicoechea, H. C. Talanta 2003, 61, 743−753. (43) Brunetti, B.; Desimoni, E. J. Food Compos. Anal. 2014, 33, 155− 160.

The authors declare no competing financial interest.

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