A simple on-plastic/paper inkjet-printed solid-state Ag/AgCl pseudoreference

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Beatrice Shaw
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1 Supporting Information A simple on-plastic/paper inkjet-printed solid-state Ag/AgCl pseudoreference electrode Everson Thiago Santos Gerôncio da Silva,, Sandrine Miserere, Lauro Tatsuo Kubota and Arben Merkoçi, * Nanobioelectronics & Biosensors Group, Catalan Institute of Nanotechnology, 08193, Bellaterra, Barcelona, Spain. Catalan Institution for Research and Advanced Studies (ICREA), 08010, Barcelona, Spain. Department of Analytical Chemistry, Institute of Chemistry UNICAMP, P.O. Box 6154, , Campinas-SP, Brazil. Table of contents - Materials, equipment and chemicals - Inkjet printing parameters - Characterization of the AgNP inkjet printed on paper - Fabrication of pre on PET and paper with bleach - Morphological characterization of the paper-based p-re - Electrochemical Formation of AgCl layer - Inkjet printing resolution S-1

2 Materials, equipment and chemicals All chemical were of analytical grade. The potassium chloride (KCl) was acquired from Sigma-Aldrich and deionized water (MilliQ Millipore system) was used to prepare the 3.0 mol L -1 solution. Silver nanoparticles (AgNP) ink for the inkjet printer (< 50nm, 20 wt.% in ethyleneglycol) was purchased from Sigma-Aldrich. The substrates polyethylene terephthalate (PET) and Whatman #1 chromatographic paper were acquired from McDermid Autotype and GE Healthcare Europe GMBH, respectively. A piezoelectric Dimatix Materials Printer DMP-2800, FUJIFILM Dimatix, Inc., Santa Clara, USA (Figure S1), was used to print the AgNP ink on the substrates. A commercial bleach solution (approximately 40 mg ml -1 NaClO) was used to modify the silver electrode and obtain a pseudo-reference (Ag/AgCl). Electrochemical experiments were performed with a potentiostat PGSTAT-12 Model from Autolab Eco Chemie (Utrecht, The Netherlands) connected to a PC (Software GPES 4.9). For these experiments, a two-electrode system was used with a commercial Ag/AgCl reference electrode against the inkjet printed pseudo-reference electrodes produced in this work. (a) (b) Figure S1. Image of the (a) Inkjet printer DIMATIX and (b) its 10 pl cartridge with 16 nozzles. S-2

3 Inkjet printing parameters Before printing, 1.5 ml of the AgNP ink was filtered using a syringe nitrocellulose filter with 0.2 µm pore and injected inside the 10pL DMC cartridge (Figure S1b). For both substrates, the waveform used to print was Dimatix Model Fluid 2 Waveform with the voltage between 18 and 22 V. To ensure a good and fast printing, five nozzles were used with the jetting velocity approximately 5.6 m/s. The printing process was carried out at room temperature and cleaning cycle (Purge 1.0 second Blot) was used only before printing. Different drop spacing (ds) had to be used for each substrate due to its differences regarding the interactions between the ink and the surface of the substrate, besides its porosity. When PET was used as substrate, 20 µm of ds was chosen and just one layer of ink was necessary to obtain homogeneous and conductive silver film. However, 30 µm of ds and 4 layers of ink were necessary to obtain a conductive silver film on the chromatographic paper, mainly due to its high surface area. Characterization of the AgNP inkjet printed on paper After print 4 layers of AgNP ink on paper and cure at 120 C for 20 min (each layer), the area of the electrode that had to be modified was delimitated by inkjet wax printing using a Color Qube 8570 XEROX printer. As can be observed in Figure S2, after printed wax on paper, it was heated at 120 C for 1 min to allow the wax penetrate into the paper pores and avoid the liquids flow through the electrical contacts. Images obtained from scanning electron microscopy (SEM) are presented in Figure S3, where is possible to observe the paper fibers surrounded for AgNP. S-3

4 Figure S2. Schematic representation of wax printing step using Whatman #1 paper as substrate. Figure S3. Images from SEM of the AgNP ink inkjet printed on paper (4 layers), with different maximizations. Fabrication of pre on PET and paper with bleach The Ag/AgCl formation was carried out by immersing the silver electrodes in a reservoir containing 40 mg ml -1 bleach solution for a short time, as was explained before. For each substrate used, different times were necessary to obtain a good and stable pseudo-reference electrode due to its different porosities. S-4

As was also mentioned on the main text, after a few seconds of immersing in bleach solution (2 or 3 s) it was possible to observe the color change due to the formation of AgCl layer in both

5 As was also mentioned on the main text, after a few seconds of immersing in bleach solution (2 or 3 s) it was possible to observe the color change due to the formation of AgCl layer in both substrates. However, when PET was used as substrate, 60 s was necessary to obtain a p-re that could be stable for at least 10 minutes of measurement (Figure S4). When reaction time was increased to 300 s (5 min) the electrode becomes even more stable, keeping the potential for at least 30 min (Figura S4). This improvement in the electrode stability could be explained by the formation of a thicker layer of AgCl, which protects the silver layer at the Ag/AgCl electrode, preventing further diffusion of chlorine ion and variations of potential. On the other hand, after 600 s (10 min) of reaction, the electrode loses all the conductivity and leach, thus destroying the p-re. (a) (b) After 10 min Figure S4. (a) Chronopotentiometry analysis of the p-re inkjet printed on PET followed by chlorination with bleach solution (1 and 5 min) against a commercial Ag/AgCl 3.0 mol L -1. (b) A picture of the p-re after 10 minutes of reaction with bleach. The time of immersing in bleach solution was also optimized for the p-re printed on paper. Due to its high porosity and consequently high surface area, less time of reaction was necessary to obtain a stable p-re, as can be observed in Figure S5. S-5

6 Figure S5. Chronopotentiometry analysis of the p-re inkjet printed on paper followed by chlorination with bleach solution (60 and 30 s) against a commercial Ag/AgCl 3.0 mol L -1. Inside: A picture of the paper-based Ag/AgCl p-re after the chemical treatment with bleach. After 60 s of immersing in bleach solution, the p-re inkjet printed on paper shown a small lack of stability after the first minutes of measurement. This lack of stability can be attributed to the decrease of the silver layer in the electrode with high surface area, which may difficult the charge transfer and conceal its properties as electrode. However, when 30 s was used as the time of reaction with bleach, this problem was solved and the electrode becomes stable during all the measurement, 30 min (Figure S5). Considering a complete electrochemical cell inkjet printed on a substrate, the poison of the other electrodes by the addition of bleach solution can be avoided by isolating the bleaching step. This step can be isolated by different ways. For example by using paper as substrate, we can print first the reference electrode modify it, and fold the paper with the other electrodes to make contact with the modified RE; or we can modify the reference electrode before printing the WE. Another way, normally applied in hydrophobic and non-porous substrates, is depositing a drop of bleach solution just on top of the RE. S-6

Morphological characterization of the paper-based p-re To confirm that the chemical treatment used in the AgNP film inkjet printed on paper generate the same results

As can be observed in Figure S6a, after the chemical treatment with bleach the nanoparticles have become bigger than without the treatment (Figure S3), as demonstrated

To confirm the formation of AgCl layer, an EDX analysis was carried out and similar results as using the PET as substrate were observed (Figure S6b), indicating that

(a) SEM images obtained from the Ag/AgCl electrode inkjet printed on paper and (b) the EDX analysis, before and after the chemical chlorination.

7 Morphological characterization of the paper-based p-re To confirm that the chemical treatment used in the AgNP film inkjet printed on paper generate the same results as on the PET, a morphological characterization with SEM and EDX analysis were carried out and the results are presented in Figure S6. As can be observed in Figure S6a, after the chemical treatment with bleach the nanoparticles have become bigger than without the treatment (Figure S3), as demonstrated when PET was used as substrate. To confirm the formation of AgCl layer, an EDX analysis was carried out and similar results as using the PET as substrate were observed (Figure S6b), indicating that this treatment can be also performed successfully in microfluidic paper platforms to obtain an Ag/AgCl p-re. (b) S2. Morfological (paper electrode) 2 µm (a) Figure S6. (a) SEM images obtained from the Ag/AgCl electrode inkjet printed on paper and (b) the EDX analysis, before and after the chemical chlorination. Electrochemical Formation of AgCl layer To compare the pseudo-reference electrodes obtained in this work with the conventional obtained using the electrochemical procedure, the inkjet-printed AgNP films were also electrochemically treated to produce AgCl layer. For the electrochemical formation of AgCl, a chronoamperometry was carried out in a 0.5 mol S-7

8 L -1 NaCl solution and applying 0.5V. In this experiment the inkjet-printed silver electrode was used as WE and a platinum wire was used as CE-RE in a two electrodes system. The potential was applied during different times and the results are presented in Figure S7. Figure S7. Electrochemical formation of AgCl layer on top of the silver electrode (inkjet-printed on PET) by chronoamperometry in 0.5 mol L -1 NaCl solution, and applying 0.5V vs Pt wire electrode for different times of applied potential. As can be observed in Figure S7, the curves showed the same behavior when different reaction times were used. The current measured for all the electrodes tested become stable after 30 seconds of electrochemical reaction, and this time of reaction was used as standard to compare the results with the p-re proposed in this work. Before evaluate the performance of the p-re prepared electrochemically, a morphological characterization with SEM and EDX analysis were carried out and the results are presented in Figure S8. S-8

As it was expected, after the electrochemical treatment the nanoparticles have become bigger than without the treatment (Figure S8a), as observed when bleach was used (Figure 2).

presence of Cl when the treatment was performed (Figure S8b).

(a) SEM images obtained from the Ag/AgCl electrode inkjet printed on PET and (b) the EDX analysis, before and after the electrochemical treatment.

9 As it was expected, after the electrochemical treatment the nanoparticles have become bigger than without the treatment (Figure S8a), as observed when bleach was used (Figure 2). To confirm the formation of AgCl layer also with this conventional method, an EDX analysis was carried out and similar results were observed, with the appearance of a signal related to the presence of Cl when the treatment was performed (Figure S8b). These results confirm that the conventional electrochemical treatment can produce the same results as the low cost and simple method proposed in this work. (b) 2 µm (a) Figure S8. (a) SEM images obtained from the Ag/AgCl electrode inkjet printed on PET and (b) the EDX analysis, before and after the electrochemical treatment. The electrochemical behavior of the p-re treated electrochemically was demonstrated by chronoamperometry in 3.0 mol L -1 KCl solution against a commercial Ag/AgCl reference electrode and the result was compared with the p-re proposed in this work (obtained by chemical chlorination in bleach), with the SPE and also with the silver electrode. The results are presented in Figure S9. S-9

10 Figure S9. Chronopotentiometric analysis comparing the behavior of the silver electrode inkjet printed on PET and the Ag/AgCl p-re produced chemical and electrochemically, besides the SPRE used as standard, against a commercial Ag/AgCl 3.0 mol L -1. Inside: A magnified view of the graphic, highlighting the stabilization of the potential for the chemically and electrochemically prepared p-re. The silver electrode showed no stability against a commercial Ag/AgCl reference electrode as expected. However, after the electrochemical treatment for the formation of an AgCl layer, the p-re demonstrated a great stability with the E approximately V (Figure S9). As can be observed in Figure S9, the p-res prepared electrochemically, chemically (with bleach) and the SPRE showed similar results with great potential stability and with E almost zero, which indicated that the p-re obtained with the proposed method as well as the electrode obtained electrochemically are reliable and can be used as reference for the most different point-of-care devices. In Figure S9inside is also possible to observe that the electrodes produced using both methods are stable since the beginning, with no need to conditioning time, which is very attractive for point-of-care rapid diagnostics. S-10

Inkjet printing resolution The size of the electrodes used for this work was

length with a contact pad of 15 mm 2 (silver part).

only by changing ink properties, such as viscosity and concentration, and

The size limit depends also on the printer resolution and the drop space used

For PET substrate, for example, and using a silver ink (AgNPs 20%wt, in

11 Inkjet printing resolution The size of the electrodes used for this work was about 500 µm width and 4 mm length (the Ag/AgCl part); 500 µm width and 20 mm length with a contact pad of 15 mm 2 (silver part). However, smaller reference electrodes can be produced using this technique only by changing ink properties, such as viscosity and concentration, and modifying substrate surface, considering the interaction between both. The size limit depends also on the printer resolution and the drop space used to print the pattern. For PET substrate, for example, and using a silver ink (AgNPs 20%wt, in ethylene glycol), it was possible to print electrodes of 100 µm width with an interspace of 100 µm as can be observed in Figure S10. Figure S10. Silver electrodes inkjet printed on PET. S-11

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