Proceeding Paper An Inkjet-Printed Amperometric H2S Sensor for Environmental Applications † Franc Paré 1,2 , Rebeca Castro 3 , Xavier Guimera 3 , Gemma Gabriel 4,5 and Mireia Baeza 1,2, * 1 Department of Chemistry, Faculty of Science, Edifici C-Nord, Universitat Autònoma de Barcelona, Carrer dels Til·lers, Bellaterra, 08193 Barcelona, Spain; franc.pare@uab.cat 2 GENOCOV Research Group, Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain 3 Department of Mining, Industrial and ICT Engineering, Universitat Politècnica de Catalunya, Avinguda de les Bases de Manresa 61-73, 08240 Manresa, Spain; rebeca.ignacia.castro@upc.edu (R.C.); xavier.guimera@upc.edu (X.G.) 4 Instituto de Microelectrónica de Barcelona, IMB-CNM (CSIC), Esfera UAB, Campus Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain; gemma.gabriel@imb-cnm.csic.es 5 CIBER, de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), 28029 Madrid, Spain * Correspondence: mariadelmar.baeza@uab.cat † Presented at the 1st International Electronic Conference on Chemical Sensors and Analytical Chemistry, 1–15 July 2021; Available online: https://csac2021.sciforum.net/. Abstract: Hydrogen sulfide (H2 S) is a highly toxic chemical capable of causing severe health issues. Due to its environmental impact, it is critical to create effective methods for its monitoring. Inkjet printing technology has become an alternative for sensor fabrication because it is an economic, fast, and reproducible method for mass producing micro-electrodes. Herein, a miniaturized 25 mm2 inkjet-printed amperometric sensor is presented. A gold electrode coupled with a silver track was modified with two inks: single-walled carbon nanotubes (SWCNTs) and a mixture of SCWCNTs and  poly(vinyl alcohol) (PVA). Morphological and electrochemical properties were studied, as well as  H2 S sensor performance. This approach is a suitable option for environmental H2 S tracking. Citation: Paré, F.; Castro, R.; Guimera, X.; Gabriel, G.; Baeza, M. Keywords: electrochemical sensor; amperometric sensor; H2 S sensor; single-walled carbon An Inkjet-Printed Amperometric H2 S nanotubes Sensor for Environmental Applications. Chem. Proc. 2021, 5, 4. https://doi.org/10.3390/ CSAC2021-10462 1. Introduction Environmental equilibrium is a hard-to-preserve resource, dangerously impacted by Academic Editor: Núria Serrano human heavy industrial activities [1,2]. It is naturally regulated through biogeochemical cy- Published: 30 June 2021 cles. Among these, the sulfur cycle is of crucial importance, since it is vital for maintaining the composition of both the atmosphere and soils, as well as most living beings. Nonethe- Publisher’s Note: MDPI stays neutral less, even more dangerous is an excess of highly toxic compounds such as water-soluble with regard to jurisdictional claims in hydrogen sulfide (H2 S) gas. This is a poisonous, inflammable, and corrosive chemical, published maps and institutional affil- hazardous to human health at concentrations as low as 20 ppm (1.1 µM) for prolonged iations. exposure [3]. Even though it generally appears as a gas, it has labile hydrogens, meaning it coexists as different species in aqueous media. Hydrogen sulfide can appear as different species depending on the pH of its medium, being capable of losing both its protons and transitioning from a gas to ions. It has a pka1 of approximately 7 and a pka2 of about 13.5, meaning that HS- predominates between pH 7.5 and 13. Due to the dangerous nature of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. H2 S and its frequent appearance in gas streams, a great need has recently arisen for many This article is an open access article biotechnological processes to remove it [4], which require adequate systems for quick and distributed under the terms and easy tracking. conditions of the Creative Commons Nonetheless, many of these removal processes occur in aqueous media, requiring Attribution (CC BY) license (https:// consideration of the environmental pH for adequate quantification. Thus, it is appropriate creativecommons.org/licenses/by/ to incorporate a simultaneous pH measurement with the H2 S tracking. 4.0/). Chem. Proc. 2021, 5, 4. https://doi.org/10.3390/CSAC2021-10462 https://www.mdpi.com/journal/chemproc Chem. Proc. 2021, 5, 4 2 of 6 In recent years, printed electronics have steadily replaced more traditional electrode fabrication methods. Among these, inkjet printing has the main advantage of not requiring any mask preparation, greatly reducing the time and cost of device fabrication. Moreover, its low drop volume and precision, added to its capacity for printing metal-based inks, allow for the fabrication of highly reproducible micro-electrodes. For biotechnological applications, microsensors can be printed on different substrates and their designs can be adapted to the shapes of bioreactors with little cost impact [5]. Therefore, inkjet printing technology is an interesting alternative for the development of a microsensors platform for H2 S and pH measurements. Moreover, electrochemical sensors have the advantages of high sensitivity, in situ application, and a broad range of applicable materials. However, H2 S determination using electrochemical sensors has many design and implementation challenges to solve, such as the pH influence on measurements [6] and electrode passivation by accumulation of S0 produced from H2 S oxidation [7]. Among the authors who have developed H2 S sensors, Yang et al. (2018) [8] fabricated a sensor using Nafion for H2 S measurement in gaseous samples. This membrane was added to carbon fibers modified with platinum and rhodium nanoparticles. The H2 S was adsorbed on the platinum, releasing protons that crossed the Nafion membrane while electrons moved through an external circuit. The H2 S concentration was proportional to the circulating charge. This sensor had a linear range from 2.9 µM to 5.9 mM, and the minimum detectable signal was 2.9 µM. Brown et al. (2019) [9] deposited a thin layer of S0 on a glassy carbon electrode (GCE) and covered it with electro-polymers to avoid passivation of the electrode via accumulation of S0 . The H2 S was measured by constant potential amperometry (CPA), using a potential of 0.3 V vs. Ag/AgCl. This sensor exhibited a high degree of selectivity and a linear range between 0 µM and 15 µM, with lowest and highest detection limits of (9 ± 6) nM and (79 ± 51) nM, respectively. Furthermore, new carbon materials with excellent electronic properties have become suitable options for electrode development. As an example, Lawrence et al. (2004) [10] modified a GCE with carbon nanotubes (CNTs) deposited on the surface by drop casting a solution of CNTs in dimethylformamide (DMF). The main advantage found was the catalytic capacity that decreased the oxidation potential from 0.4 V to −0.3 V (vs. Ag/AgCl). This allowed amperometric measurements at 0.1 V (vs. Ag/AgCl) in a range between 1.25 µM and 112.5 µM, with a detection limit of 0.3 µM. Li et al. (2017) [6] used cobalt to magnetically attach MoS2 monolayer sheets to CNTs, which were deposited on glassy carbon electrodes using a Nafion membrane as an adhesive. Analysis was performed by amperometry obtaining a linear range of application from 0.05 µM to 0.6 µM, with a detection limit of 7.6 nM. Despite authors applying carbon materials for H2 S sensor development, there are no reports of the use of SWCNTs ink for an inkjet-printed H2 S sensor. Moreover, several studies have reported poly(vinyl alcohol) (PVA) addition to conductive materials to improve its mechanical and adhesive properties [11–13]. In this study, the fabrication of a H2 S amperometric microsensor has been stud- ied using both a SWCNTs ink and a SWCNTs–PVA ink for Au electrode modification. Morphological and electrochemical characterizations were carried out for electrode perfor- mance analysis. 2. Materials and Methods 2.1. Inkjet-Printed H2 S Sensor Fabrication The printing process for the electrode was performed using Ag NPs ink (SI-J20x Nanosilver Inkjet Printing Ink from Agfa, Mortsel, Belgium), Au NPs ink (Drycure Au-JB 1010B from C-ink, Japan), and SWCNTs ink (Carbon nanotube, single-walled, conductive aqueous ink, SWCNT 1.00 mg/mL from Sigma-Aldrich, Spain). To passivate the electrode, SU8 ink (XP PriElex SU-8 1.0 Inkjettable Dielectric from Kayaku, Westborough, MA, USA) was used. All inks were printed over polyethylene teraphtalate (PET) sheets (Q65HA, Chem. Proc. 2021, 5, 4 3 of 6 Du Pont Teijin Films, Dumfries, UK), using a Dimatix printer (DMP-2831 from FUJIFILM Dimatix, Santa Clara, CA, USA). The SWCNTs–PVA composite ink was prepared by mixing a commercial SWCNTs ink, and a 5 wt.% PVA solution, both acquired from Sigma- Aldrich. For SWCNTs ink and SWCNTs–PVA ink deposition, the drop casting technique was performed on a thermal plate. The applied temperature was a studied parameter. The final electrode dimensions were a 1 mm diameter gold disk with a total length of 26 mm and surface of 25 mm2 , and an approximately 2 mm diameter SWCNTs disk. 2.2. Sensor Characterization Morphological characterization of the electrodes was carried out via optical mi- croscopy. Images were obtained with a digital microscope (USB microscope AM4815ZTL from DinoLite, Alemere, The Netherlands). Electrochemical characterization was performed with a PalmSens potentiostat– galvanostat (PalmSens4 from PalmSens, The Netherlands). A three-electrode configu- ration was used for the electrochemical cell. An Ag/AgCl (1 M KCl) reference electrode (reference electrode with Ag/AgCl in aqueous KCl from ItalSens, PalmSens, Houten, The Netherlands) and a platinum wire counter electrode (counter electrode made of platinum wire from ItalSens, PalmSens, Houten, The Netherlands) were used. The fabricated elec- trode functioned as the working electrode. Cyclic voltammetry (CV) measurements were carried out using the redox pair K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (0.01 M) at 0.01 V/s from −0.1 V to 0.5 V. Intensity peaks (Ip) resulting from redox reactions allowed for electrochemical characterization. To study the effect of deposition temperature in the SWCNTs–PVA ink, ten layers of PVA were drop casted over a gold inkjet-printed electrode at 90 ◦ C and at room temperature. Intensity-peak values were obtained. For H2 S calibration, a stock H2 S solution (0.1 M) was prepared by dissolving NaS·9H2 O and NaOH (both from Sigma-Aldrich, Spain) in deionized water (Milli-Q from Millipore Corporation, Burlington, MA, USA). Standardization was carried out according to standard methods [14]. The stock solution was diluted in phosphate-buffered saline solution (PBS) (from Sigma-Aldrich, Madrid, Spain) to obtain a 0.02 M H2 S solution. 3. Results and Discussion 3.1. Inkjet-Printed H2 S Miniaturized Sensor Fabrication Inkjet-printed micro-electrodes were fabricated by first printing the Ag tracks and pads, drying at 100 ◦ C in an oven, then printing the Au surfaces and connecting tracks, which were later dried at 120 ◦ C. Subsequently, inks were sintered at 150 ◦ C for 60 min to improve their conductivity. Finally, micro-electrodes were passivated by printing an SU8 layer over the tracks, preventing the short circuiting of the electrodes. Both SWCNTs and SWCNTs–PVA inks were drop casted on a gold electrode, covering Chem. Proc. 2021, 5, 4 4 of 6 it completely (Figure 1). Both inks showed good adhesion to gold, allowing morphological and electrochemical characterization of the microsensors. Figure 1. Photographs Photographs of of (A) (A) inkjet-printed inkjet-printed Au Au electrode, electrode, (B) (B) SWCNTs SWCNTs drop casted over Au electrode, electrode, and (C) (C) drop-casted drop-casted SWCNTs–PVA SWCNTs–PVA overover Au Au electrode. electrode. 3.2. Microsensor Electrochemical Characterization Cyclic voltammetry measurements of Au, SWCNTs, and SWCNTs–PVA electrodes were carried out. These allowed verification of whether the applied modifications yielded an electrode with properties of interest that could be used for an H2S sensor. The main interest was in fabricating a sensor capable of oxidizing H2S at low voltages and with a Chem. Proc. 2021, 5, 4 4 of 6 Figure 1. Photographs of (A) inkjet-printed Au electrode, (B) SWCNTs drop casted over Au electrode, and (C) drop-casted SWCNTs–PVA over Au electrode. 3.2.Microsensor 3.2. MicrosensorElectrochemical ElectrochemicalCharacterization Characterization Cyclicvoltammetry Cyclic voltammetrymeasurements measurementsof ofAu, Au,SWCNTs, SWCNTs,and andSWCNTs–PVA SWCNTs–PVAelectrodes electrodes were carried out. These allowed verification of whether the applied modifications were carried out. These allowed verification of whether the applied modifications yielded yielded an electrode with properties of interest that could be used for an H an electrode with properties of interest that could be used for an H2 S sensor. The main 2 S sensor. The main interestwas interest wasin in fabricating fabricating aa sensor sensor capable capable of of oxidizing oxidizing H H22SS at at low low voltages voltages andandwith witha ahigh hightolerance tolerancefor forsulfur sulfurpoisoning poisoningsince sinceititisiswell wellknown knownthat thatS0S0isishighly highlyinsoluble insolublein water in water [7][7] and hashas and a high a highaffinity forfor affinity AuAu atoms. atoms. Thus, the electrode Thus, the electrodewould gradually would graduallydete- riorate by accumulating non-conductive layers of sulfur deteriorate by accumulating non-conductive layers of sulfur atoms. atoms. Modifiedelectrodes Modified electrodespresented presentedaasimilar similarcurrent currentdensity densityand andaasmaller smallerpotential potentialgap gap thanbare than bareAu Auelectrodes electrodes (Figure (Figure 2).2). The The smaller smaller peakpeak separation, separation, specifically specifically duedue to reduc- to reducing ingpotential the the potential necessary necessary for ferrocyanide for ferrocyanide oxidation, oxidation, meant meant that that SWCNTs SWCNTs and and SWCNTs–PVA SWCNTs–PVA are both favorable arefor both usefavorable as an H2for use as an S sensor, withH2Sthe sensor, added with the added benefit benefitrates of lowered of low- of ered rates sulfur of sulfur deposition ondeposition on their their surfaces due surfaces due to the less-favorable to the less-favorable S–C interaction S–C compared interaction compared with S–Au. with S–Au. Figure2.2. Cyclic Figure Cyclic voltammetry voltammetry of of Au, Au, SWCNTs, SWCNTs, and and SWCNTs-PVA SWCNTs-PVA electrodes electrodes ininhexacyanofer- hexacyanofer- rate/hexacyanoferrate (0.01 M). rate/hexacyanoferrate (0.01 M). 3.3. 3.3.PV A Deposition PVA DepositionTemperature TemperatureStudy Study The Theeffect effectofofPVA PVAdeposition temperature deposition was temperature studied was through studied the the through drop casting drop of 10of casting layers of PVA on an Au inkjet-printed electrode at 90 ◦ C and at room temperature. Cyclic 10 layers of PVA on an Au inkjet-printed electrode at 90 °C and at room temperature. voltammetry was performed, Cyclic voltammetry and current-peak was performed, (Ip) values and current-peak were obtained. (Ip) values The results were obtained. The re- showed that at room temperature, PVA deposition had a smaller passivation sults showed that at room temperature, PVA deposition had a smaller passivation effect than at effect 90 ◦ C, in which case the electrode’s current values dropped significantly (Table 1). than at 90 °C, in which case the electrode’s current values dropped significantly (Table 1). Table 1. Effect of polymer deposition temperature on Au inkjet-printed electrodes. Ip Ip Temperature % Ip Reduction Au Electrode (µA) Au Electrode + PVA (µA) 25 ◦ C 8.12 7.46 8.1 90 ◦ C 3.92 1.22·10−5 100.0 3.4. Microsensor Calibration and Analytical Response Calibration of SWCNTs and SWCNTs–PVA microsensors was performed by adding different volumes of H2 S standard to a PBS solution, measuring a concentration range Ip Ip Temperature % Ip Reduction Au Electrode (µA) Au Electrode + PVA (µA) 25 °C 8.12 7.46 8.1 90 °C 3.92 1.22·10−5 100.0 Chem. Proc. 2021, 5, 4 5 of 6 3.4. Microsensor Calibration and Analytical Response Calibration of SWCNTs and SWCNTs–PVA microsensors was performed by adding different volumes of H2S standard to a PBS solution, measuring a concentration range between 0 and 600 µM. The chronoamperometry method was used for H2 S oxidation between 0(1)) (Equation andat600 µM. The chronoamperometry a polarization voltage of 50 mV, andmethod was used the resulting for was current H2S measured. oxidation (Equation (1)) at a polarization voltage of 50 mV, and the resulting current was measured. H→ H2 S S0 + 2e− + 2H+ 2S  S0 + 2e− + 2H+ (1) (1) performed for Calibrations were performed for both both SWCNTs and SWCNTs–PVA under under the the same same (Figure3). conditions (Figure 3).The Theresults resultsshowed showed good good correlation correlation between between thethe measured measured cur- current rent the and andanalyte the analyte concentration, concentration, with awith a similar similar slope slope for sensors for both both sensors but different but different linear linear ranges. ranges. Figure 3. Calibrations of two different sensors made of Au electrodes modified with SWCNTs and and SWCNTs-PVA by drop casting. Inset shows a reduced range of H S concentration for the SWCNTs-PVA by drop casting. Inset shows a reduced range of H2 S concentration for the SWCNTs 2 SWCNTs sensor sensor calibration. calibration. While the SWCNTs SWCNTs sensor sensorshowed showedaahigher highersensitivity (19.3±± 0.4 sensitivity(19.3 0.4mA/M) mA/M) compared to SWCNTs–PVA (9.4 SWCNTs–PVA (9.4 ± ± 0.2 mA/M), it also mA/M), it also hadhad a reduced working range. SWCNTs cancan measure H2SS concentrations concentrations from 8 µM µM to 60 µM,µM, with a limit of detection detection (LD) of 4.3 4.3 µM. µM. On the other hand, SWCNTs–PVA SWCNTs–PVA is is capable capable ofof measuring from 52 µM µM to 512 µM, µM, with a LD of LD of 34 34 µM. µM. 4. Conclusions 4. Conclusions We demonstrated We demonstrated aa novel novel miniaturized miniaturized inkjet-printed inkjet-printed amperometric amperometric H H22SS sensor sensor fab- fab- ricated by modification of a gold electrode with SWCNTs ink. The results showed that ricated by modification of a gold electrode with SWCNTs ink. The results showed that the the addition of a stabilizing polymer allowed for an increased range of H S concentration addition of a stabilizing polymer allowed for an increased range of H22S concentration measurements. Due to the low versatility of the SWCNTs sensor, the addition of PVA measurements. Due to the low versatility of the SWCNTs sensor, the addition of PVA was was considered, to improve the H2 S sensor performance. However, it was necessary to considered, to improve the H2S sensor performance. However, it was necessary to adapt adapt the fabrication temperature conditions to avoid electrode passivation, limiting the the fabrication temperature conditions to avoid electrode passivation, limiting the SWCNTs–PVA ink-deposition temperature to 25 ◦ C. This sensor offers an approach for H2 S tracking with environmental and biotechnological applications. Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/CSAC2021-10462/s1. Author Contributions: Conceptualization, M.B. and G.G.; methodology, F.P.; validation, M.B. and X.G.; formal analysis, F.P. and R.C.; investigation, F.P. and R.C.; resources, G.G. and M.B.; data curation, F.P. and R.C.; writing—original draft preparation, F.P. and R.C.; writing—review and editing, M.B., G.G., and X.G.; visualization, G.G. and M.B.; supervision, M.B., G.G., and X.G.; project administration, M.B.; funding acquisition, M.B. and G.G. All authors have read and agreed to the published version of the manuscript. Chem. Proc. 2021, 5, 4 6 of 6 Funding: This research was funded by the Spanish Government, Ministerio de Economía y Competi- tividad, through projects RTI2018-099362-B-C21 and RTI2018-099362-B-C22 MINECO/FEDER, EU. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the repository that is used to keep the data is a private one provided by the University. Acknowledgments: The authors wish to acknowledge David Gabriel and Xavier Gamisans for their coordinated direction of the ENSURE project. Conflicts of Interest: The authors declare no conflict of interest. References 1. Li, M.; Li, C.; Zhang, M. Exploring the spatial spillover effects of industrialization and urbanization factors on pollutants emissions in China’s Huang-Huai-Hai region. J. Clean. Prod. 2018, 195, 154–162. [CrossRef] 2. Aikawa, M.; Hiraki, T.; Suzuki, M.; Tamaki, M.; Kasahara, M. Separate chemical characterizations of fog water, aerosol, and gas before, during, and after fog events near an industrialized area in Japan. Atmos. Environ. 2007, 41, 1950–1959. [CrossRef] 3. Guidotti, T.L. Hydrogen Sulfide. Int. J. Toxicol. 2010, 29, 569–581. [CrossRef] [PubMed] 4. Syed, M.; Soreanu, G.; Falletta, P.; Béland, M. 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