Proceeding Paper Tunable Electrochemical Sensors Based on Carbon Nanocomposite Materials towards Enhanced Determination of Cadmium, Lead and Copper in Water † Laia L. Fernández 1,2 , Julio Bastos-Arrieta 3 , Cristina Palet 1 and Mireia Baeza 2, * 1 Grup de Tècniques de Separació en Química (GTS), Departament de Química, Universitat Autònoma de Barcelona, Carrer dels Til·lers, 08193 Bellaterra, Catalunya, Spain; Laia.Lopez@uab.cat (L.L.F.); cristina.palet@uab.cat (C.P.) 2 Group of Biological Treatment and of Liquid and Gaseous Effluents, Nutrient Removal, and Odors and Volatile Organic Compounds (GENOCOV), Departament Química, Universitat Autònoma de Barcelona, Carrer dels Til·lers, 08193 Bellaterra, Catalunya, Spain 3 Grup de Biotecnologia Molecular i Industrial, Departament d’Enginyeria Química, Universitat Politècnica de Catalunya, Rambla Sant Nebridi, 22, 08222 Terrassa, Catalunya, Spain; julio.bastos@upc.edu * 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: Many carbon materials are well-known conductive materials, widely used in the fabrication of composite electrodes. In this work, diverse allotropic forms of carbon such as graphite, MWCNTs and rGO were tested. Furthermore, these materials allow the construction of cheaper, smaller, portable, reliable and easy-to-use devices, which can be easily modified. The above-mentioned   composite electrodes were developed for metal analysis in water such as Cu, Cd and Pb that, at a high concentration, can have consequences on human health. SWASV is the selected technique. It would Citation: Fernández, L.L.; Bastos-Arrieta, J.; Palet, C.; Baeza, M. be ideal to exploit the potential properties of mercury for metal detection by tuning the electrode’s Tunable Electrochemical Sensors surface. Due to mercury’s hazardous properties and to reduce the amount of this substance used in Based on Carbon Nanocomposite polarography, the use of nanoparticles is a good option due to their properties. Mercury nanoparticles Materials towards Enhanced were used to modify the surface of the composite electrodes to improve electroanalytical sensor Determination of Cadmium, Lead response. For this reason, using these modified composite electrodes can lower detection limits and Copper in Water. Chem. Proc. and widen the linear range that can be achieved for Cd (0.05–1 mg·L−1 ) and Pb (0.045–1 mg·L−1 ). 2021, 5, 8. https://doi.org/ However, for Cu (0.114–1.14 mg·L−1 ), meaningful variations were not observed compared to the 10.3390/CSAC2021-10456 bare electrode. Academic Editor: Núria Serrano Keywords: electrochemistry; Hg nanoparticles; graphite; composite electrodes; metal analysis; SWASV Published: 30 June 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Water is fundamental for all Earth’s living forms, and a key issue for social and iations. economic development. Currently, water analysis is a vital topic, for because monitoring some parameters is important to prevent some health problems. One of the parameters that has become important involves determining the concentration of heavy metals in water. To do this, several analysis techniques are used, such as atomic absorption spectroscopy (AAS) [1], inductively coupled plasma (ICP) [2], high-performance liquid chromatography Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. (HPLC) [3], etc. Some of the metals that can be found in water are Cu, Cd and Pb and, at This article is an open access article high concentrations, can have consequences on human health [4–6]. distributed under the terms and In this work, a voltametric technique has been chosen, known as square-wave anodic conditions of the Creative Commons stripping voltammetry (SWASV) [7,8]. SWASV consists of two steps: first, applying a Attribution (CC BY) license (https:// potential to preconcentrate the analyte on the surface of the electrode; second, taking a creativecommons.org/licenses/by/ measurement by applying staircase potential to record the current generated. 4.0/). Chem. Proc. 2021, 5, 8. https://doi.org/10.3390/CSAC2021-10456 https://www.mdpi.com/journal/chemproc Chem. Proc. 2021, 5, 8 2 of 7 To use this technique, composite electrodes were construct using different carbon ma- terials and a non-conductor epoxy. The behavior of graphite, reduced graphene oxide (rGO) and carbon nanotubes (CNTs) were tested in the detection of Cd, Pb and Cu. However, we work with the bare electrode; the modification of their surface with mercury nanoparticles (Hg-NPs) was also tested [9]. Mercury was used, a long time ago, in polarography, and it is well-known for its ability to form amalgams with some metals, reducing the potential where they appear [10,11]. Hence, taking advantage of these properties, the aim of this work is to reduce the amount of mercury used in polarography for the determination of Cd, Pb, and Cu. 2. Composite Electrodes Construction, Characterization, and Modification 2.1. Composite Electrode Construction Composites were constructed using three different carbon materials: graphite, CNTs and rGO. The first step is to weld a copper sheet to a commercial connector; after that, it is placed in a PVC tube. A mixture of one of the carbon materials and Epotek H77 is prepared, and the PVC tube (2.1 cm, ∅6 mm) is filled with this mixture. Then, it is cured for 2 days at 80 ◦ C. Then, the surface must be polished. The percentages tested of carbon materials are shown in Table 1. These percent- ages were optimized previously, and they are related to their respective improvement in the electroanalytical properties of developed sensors, in terms of detection limit and sensitivity [12]. Table 1. Percentages used in the construction of each electrode. Material % Carbon Material % Epotek H77 15 85 Graphite 20 80 CNTs 10 90 rGO 15 85 2.2. Composite Electrode Chacaterization Electrodes were characterized using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) using a computer-controlled Multi-AUTOLAB M101 (Eco Chemie, Utrecht, The Netherlands) with a three-electrode cell: a platinum-based electrode 53–671 (Crison Instruments, Alella, Barcelona, Spain) as a counter electrode, an Ag/AgCl handmade electrode as a reference electrode and the constructed composite electrodes as working electrodes. The characterization was performed in solution composed of 0.01 M K4 Fe(CN6 ), 0.01 M K3 Fe(CN6 ) and 0.1 M KCl. For CV, the scan rate was 10 mV·s−1 and the rate of frequencies used in EIS was 0.01 to 104 Hz. The behavior of the 15% rGO electrode was unusual, possibly related to the orientation of the layers in the Epotek H77 matrix, and its characterization using CV and EIS was not successful. In Figure 1, the characterization of the rest of the carbon electrodes, with graphite or CNTs, can be observed. The most notable difference is showed in EIS, where the 20% graphite presents the lower charge transference resistance. Thus, a highly conductive surface is then available for the preconcentration of cationic metals. Chem. Chem.Proc. Proc.2021, 5,5, 2021, 8 3 3ofof 7 Chem. Proc. 2021, 5, 8 8 3 of 77 Figure Figure1. Figure 1.1.CV CV(a) CV (a)and (a) andEIS and EIS(b) EIS (b)characterization (b) characterizationof characterization ofofthe thedifferent the differentelectrodes. different electrodes. electrodes. 2.3. 2.3.Composite 2.3. CompositeElectrode Composite ElectrodeModification Electrode Modificationwith Modification withHg-Nps with Hg-Nps Hg-Nps After After electrode characterization, After electrode characterization, thesurface electrode characterization, the the surface surfaceof of the ofthe electrode electrodeisisismodified theelectrode modified modifiedwith with mer- withmer- mer- cury cury nanoparticles (Hg-NPs) following the synthesis cury nanoparticles (Hg-NPs) following the synthesis from [9]. In the synthesis, 78mg nanoparticles (Hg-NPs) following the synthesis from from [9]. [9]. In In the the synthesis, synthesis, 78 78 mg mg Hg HgHg (NO (NO 222(NO ) · 3)332)2·2 22H · 2HH O is used, 1 mL 22 2O is used, 1 mL1M 1 M 1MHNOHNO HNO 33isis added added 3 is addedandand then andthen 0.5 then0.5 0.5mLmL of a solution mLofofa asolution of solutionofof3.53.5 3.5ggg of of PVA ofPVA (Polyvinyl PVA(Polyvinyl (PolyvinylAlcohol) Alcohol) added Alcohol)addedaddedto toto16 16 mL 16mLmLof ofofMilli-Q Milli-Q water. Milli-Qwater. All water.All the Allthe steps thesteps ofofthe stepsof the synthesis thesynthesis synthesis were performed at 25 ◦ C and under stirring conditions. were performed at 25 °C and under were performed at 25 °C and under stirring conditions. stirring conditions. AAAtotal total totalof of 20 of20 µL 20µL µLofofofthe the nanoparticle thenanoparticle solution nanoparticlesolution solutionis isisdrop drop casted dropcasted castedon on the onthe electrode theelectrode surface electrodesurface surface and dried in the oven ◦C 2 h. The modified electrodes were characterized using and and dried in the oven at 80 °C for 2h. The modified electrodes were characterizedusing dried in the oven at 80 °C for 2h. The modified electrodes were characterized using scanning scanning electron scanningelectron microscopy electronmicroscopy (SEM) microscopy(SEM) (MerlinFe-SEM, (SEM)(MerlinFe-SEM, (MerlinFe-SEM,Carl Carl Zeiss, CarlZeiss,Germany) Zeiss,Germany) and Germany)and the Hg-NPs andthetheHg- Hg- were NPs NPswerecharacterized werecharacterized characterized using transmission using using transmission transmissionelectron microscopy electron electron microscopy (TEM) microscopy (TEM)(JEM-2011 (TEM) (JEM-2011 200 kV, (JEM-2011 200Jeol, 200 kV, kV, Peabody, Jeol, Peabody, MA, MA, USA)USA) (see Figure (see 2). 2). Figure Jeol, Peabody, MA, USA) (see Figure 2). Figure Figure2. (a)(a)Retrodispersive Retrodispersive(left) (left)and andsecondary secondaryelectron electron(right) (right)SEM SEMimages; images;(b) (b)20% 20%graphite Figure 2.2.(a) Retrodispersive (left) and secondary electron (right) SEM images; (b) 20% graphite graphite electrode electrodedrop dropcasted castedwith withHg-NPs Hg-NPsimage; image;(c)(c)TEM TEMimage imageofofthe theHg-NPs. Hg-NPs. electrode drop casted with Hg-NPs image; (c) TEM image of the Hg-NPs. 2.4. 2.4.Metal 2.4. MetalSolution Metal SolutionPreparation Solution Preparationand Preparation andDetermination and Determination Determination The Themetal The metalsolutions metal solutionswere solutions wereprepared were preparedusing prepared using certified usingcertified stock certifiedstock standards stockstandards standardsof ofof37 3737mg mg·L mg·L−1 ·L−1−1 Pb(NO Pb(NO Pb(NO 3)32))2(≥99%, ≥99%,supplied 3 2 ((≥99%, suppliedfrom fromSigma-Aldrich), from Sigma-Aldrich),11,438 Sigma-Aldrich), 11,438mg·L 11,438 ·L−−11Cu(NO mg·L mg −1 Cu(NO Cu(NO 3)32)2(99.5 3 2 (99.5 %, %,%,pur- pur- pur- chased from chasedfrom chased Merck) from Merck) and 1000 Merck) and 1000 mg·L 1000 mg·L Cd(NO − −1−1 mg·L Cd(NO1 Cd(NO 3) (99 3)23(99 2 %, obtained %, %, )2 (99 obtained from obtainedfrom Panreac). Panreac). from They Panreac). were TheyThey were added added were totoa ato added 0.1aM 0.1 Macetic 0.1 Macetic acid acetic acid (CH acid (CH (CH3COOH, 3COOH, 99.9% 3 COOH,99.9% 99.9% acquired acquiredfrom acquired fromJ.T.Baker, J.T.Baker,HPLC J.T.Baker, HPLC HPLC Chem. Proc. 2021, 5, 8 4 of 7 reagent)/0.1 M Ammonium acetate (NH4CH3COO, 97 % purchased from Panreac) buffer Chem. Proc. 2021, 5, 8 4 of 7 with Milli-Q water at pH 4.6 [13]. 2.5. Bare Composite Electrodes reagent)/0.1 M Ammonium acetate (NH4 CH3 COO, 97 % purchased from Panreac) buffer For metal determination, the technique chosen was SWASV. This consists of apply- with Milli-Q water at pH 4.6 [13]. ing a potential (−1.4 V) for 7 min that reduces the metal ions on the electrode surface; then, staircase 2.5. Barepotential Composite is applied and the current generated is recorded. This process is per- Electrodes formedForunder metalN 2 bubbling. Moreover, determination, a modification the technique in theThis chosen was SWASV. electrochemical cell is used. consists of applying Instead of using a handmade reference electrode, the one used for the measurements a potential (−1.4 V) for 7 min that reduces the metal ions on the electrode surface; then, is Orion 900 potential staircase electrodeis(Thermo Scientific, applied and Beverly, the current MA, USA). generated is recorded. This process is per- formed under Firstly, theNbare 2 bubbling. Moreover, electrodes a modification were used in the electrochemical for the electrochemical cellof detection is Cd, used.Pb and Instead of using a handmade reference electrode, the one Cu. The results for all electrodes studied are shown in Figure 3. used for the measurements is Orion 900 electrode (Thermo Scientific, Beverly, MA, USA). Firstly, the bare electrodes were used for the electrochemical detection of Cd, Pb and Cu. The results for all electrodes studied are shown in Figure 3. Figure 3. Calibration curves for Cd (a), Pb (b) and Cu (c) for each raw material. Figure 3. Calibration curves for Cd (a), Pb (b) and Cu (c) for each raw material. As can be seen, 20% graphite electrodes showed the best response, as it has a better As can compared sensitivity be seen, 20% withgraphite electrodes 15% graphite showed and 10% CNTs the best response, composite electrodesasfor it has a better three sensitivity compared with metal cations analyzed. 15% graphite and 10% CNTs composite electrodes for three metal cations analyzed. 2.6. Hg-NPs Drop Casted Electrodes The next 2.6. Hg-NPs stepCasted Drop is to modify the surface of the 20% graphite electrode with Hg-NPs, as Electrodes mentioned above. Once the surface is modified, the electrode is tested for Cd, Pb and Cu The next step is to modify the surface of the 20% graphite electrode with Hg-NPs, as determination using SWASV. The corresponding results are shown in Figure 4. mentioned above. Once the surface is modified, the electrode is tested for Cd, Pb and Cu determination using SWASV. The corresponding results are shown in Figure 4. Chem. Proc. 2021, 5, 8 5 of 7 Chem. Proc. 2021, 5, 8 5 of 7 Figure 4. Calibration Figure curves 4. Calibration forfor curves CdCd (a),(a), Pb Pb (b)(b) andand CuCu (c) (c) forfor 20% graphite 20% (black) graphite andand (black) 20% graphite 20% plus graphite Hg-NPs plus (blue). Hg-NPs (blue). With thisthis With modified 20% graphite modified electrode, 20% graphite lower quantification electrode, limits can be lower quantification achieved. limits can be In achieved. Table 2, allInthe parameters Table 2, all theofparameters the calibration of thecurves are summarized. calibration curves are summarized. Table 2. Feature Table parameters: 2. Feature sensitivity, parameters: r2 and sensitivity, linear r2 and range linear of each range cationic of each metal cationic detected metal separately. detected separately. 2+ ] 2+ [Cd[Cd ] Electrode Electrode (20% (20% graphite) graphite) Sensitivity Sensitivity [A·(mg ·L−1 )−1−1])−1] [A·(mg·L r2 (n) r2 (n) Linear Linear Range Range ·L−1 ) −1) (mg(mg·L Bare Bare (1.6(1.6 ± 0.1) ± 0.1) × 10×−10 4 −4 0.995 0.995 (n =(n5)= 5) 0.1–1 0.1–1 plus plus Hg-NPs Hg-NPs (3.4(3.4 ± 0.2) ± 0.2) × 10×−10 5 −5 0.98 0.98 (n =(n6)= 6) 0.05–1 0.05–1 [Pb2+] [Pb2+ ] Electrode (20% graphite) Sensitivity [A·(mg·L ) ] −1 −1 r2 (n) Linear Range (mg·L−1) Electrode (20% graphite) Sensitivity [A·(mg·L−1 )−4−1 ] r2 (n) Linear Range (mg·L−1 ) Bare (1.9 ± 0.2) × 10 0.95 (n = 4) 0.09–0.45 Bare plus Hg-NPs (1.9 ± 0.2) (6.4 × 10×−10 ± 0.3) 4 −5 0.95 (n =(n4)= 7) 0.98 0.09–0.45 0.045–1 plus Hg-NPs (6.4 ± 0.3) × 10 − 5 0.98 (n = 7) 0.045–1 [Cu2+] Electrode (20% graphite) Sensitivity [A·(mg·L [Cu −1 ) ] 2+ ] −1 r (n) 2 Linear Range (mg·L−1) Bare Electrode (20% graphite) (9.7[A Sensitivity ± ·0.9) (mg·×L−101 )−5−1 ] 0.95 r (n)(n = 7) 2 0.057–1.14 Linear Range (mg·L−1 ) plus Hg-NPs (7 ± 1) × 10−5 −6 0.90 (n = 5) 0.114–1.14 Bare (9.7 ± 0.9) × 10 0.95 (n = 7) 0.057–1.14 plus Hg-NPs (7 ± 1) × 10 −6 0.90 (n = 5) 0.114–1.14 3. Conclusions Carbon composite electrodes are very versatile, robust, and reliable electrodes to 3. Conclusions work with for Cd, Pb and Cu detection. The well-known properties of mercury to form an amalgam Carbon with other electrodes composite metals canare be very takenversatile, advantage of to and robust, modify the surface reliable of the electrodes carbon to work composite with electrode for Cd, Pb and Cuindetection. order to decrease the limitproperties The well-known detection of mercury the bare to electrode. form anTo Chem. Proc. 2021, 5, 8 6 of 7 amalgam with other metals can be taken advantage of to modify the surface of the carbon composite electrode in order to decrease the limit detection of the bare electrode. To emulate the polarography, the use of Hg-NPs reduces the amount of mercury used without losing its properties. In this case, Cd and Pb form an amalgam with Hg, reducing the detection limit (Cd = 0.05 mg·L−1 ; Pb = 0.045 mg·L−1 ) in comparison with the bare electrode. The Cu metallic cation does not exhibit this behavior. Although the bare electrode has higher sensitivity because its electroactive area is not modified, when the electrode was modified with Hg-NPs, its electroactive area decreases. We added a polymer (from the synthesis of the NPs) over the electrode’s surface that is not as good as a conductor as graphite. On the other hand, we improved the detection limit due to the specific interaction of mercury with metals cations. Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/CSAC2021-10456/s1. Author Contributions: Conceptualization, M.B. and C.P.; methodology, L.L.F.; validation, M.B. and C.P.; formal analysis, L.L.F.; investigation, L.L.F.; resources, M.B.; data curation, L.L.F. and C.P.; writing—original draft preparation, L.L.F.; writing—review and editing, M.B., C.P. and J.B.-A.; visualization, M.B and C.P.; supervision, M.B., C.P. and J.B.-A.; project administration, M.B.; funding acquisition, M.B. All authors have read and agreed to the published version of the manuscript. Funding: Authors are thankful for the financial support from the RTI2018-099362-B-C21 research project from the Spanish Ministerio de Economía y Competitividad y Fondo Europeo de Desarrollo Regional (MINECO/FEDER, UE). 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: Laia L. Fernández acknowledges Universitat Autònoma de Barcelona (UAB) for the PIF grant. She thanks Servei de Microscopia from UAB for the assistance in electron microscopy characterization. Conflicts of Interest: The authors declare no conflict of interest. References 1. Hashim, R.; Song, T.H.; Muslim, N.Z.M.; Yen, T.P. Determination of heavy metal levels in fishes from the lower reach of the kelantan river, Kelantan, Malaysia. Trop. Life Sci. Res. 2014, 25, 21–39. [PubMed] 2. Pérez-Ràfols, C.; Trechera, P.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M. Determination of Pd(II) using an antimony film coated on a screen-printed electrode by adsorptive stripping voltammetry. Talanta 2017, 167, 1–7. [CrossRef] [PubMed] 3. Okano, G.; Igarashi, S.; Yamamoto, Y.; Saito, S.; Takagai, Y.; Ohtomo, T.; Kimura, S.; Ohno, O.; Oka, Y. 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