S S symmetry Article Recognition of Heavy Metal Ions by Using E-5-((5-Isopropyl-3,8-Dimethylazulen-1-yl) Dyazenyl)-1H-Tetrazole Modified Electrodes Adina-Maria Păun, Ovidiu-Teodor Matica, Veronica Anăstăsoaie, Laura-Bianca Enache, Elena Diacu * and Eleonora-Mihaela Ungureanu Faculty of Applied Chemistry and Materials Science, University “Politehnica” of Bucharest, Gheorghe Polizu 1-7, Sector 1, 011061 Bucharest, Romania; [email protected] (A.-M.P.); [email protected] (O.-T.M.); [email protected] (V.A.); [email protected] (L.-B.E.); [email protected] (E.-M.U.) * Correspondence: [email protected]; Tel.: +40-722366378 Abstract: Chemically modified electrodes (CMEs) based on polymeric films of E-5-((5-isopropyl- 3,8-dimethylazulen-1-yl) diazenyl)-1H-tetrazole (L) deposited on the surface of the glassy carbon electrode have been used for the recognition of heavy metal (Me) ions. The electrochemical study of L was done by three methods: differential pulse voltammetry (DPV), cyclic voltammetry (CV), and rotating disk electrode voltammetry (RDE). The CV, DPV, and RDE studies for L were per- formed at different concentrations in 0.1 M tetrabutylammonium perchlorate solutions in acetonitrile. The polymeric films were formed by successive cycling or by controlled potential electrolysis (CPE). Citation: P˘aun,A.-M.; Matica, O.-T.; The film formation was proven by recording the CV curves of the CMEs in ferrocene solution. An˘ast˘asoaie,V.; Enache, L.-B.; Diacu, The CMEs prepared at different charges or potentials were used for detection of heavy metal ions. E.; Ungureanu, E.-M. Recognition of Synthetic samples of heavy metal ions (Cd(II), Pb(II), Cu(II), Hg(II)) of concentrations between 10−8 Heavy Metal Ions by Using E-5-((5- and 10−4 M were analyzed. The most intense signal was obtained for Pb(II) ion (detection limit of Isopropyl-3,8-Dimethylazulen-1-yl) about 10−8 M). Pb(II) ion can be detected by these CMEs in waters at such concentrations. The ability Dyazenyl)-1H-Tetrazole Modified of the ligand L to form complexes with Pb(II) and Hg(II) ions was also tested by UV-Vis spectrometry. Electrodes. Symmetry 2021, 13, 644. The obtained results showed the formation of Me(II)L complexes. https://doi.org/10.3390/ 2 sym13040644 Keywords: azulene derivative; chemically modified electrodes; voltammetric techniques; complexing Academic Editors: polymeric films; UV-Vis spectrometry; heavy metal ions Christophe Humbert and Enrico Bodo Received: 10 March 2021 1. Introduction Accepted: 9 April 2021 Chemically modified electrodes (CMEs) with complexing properties are alternative Published: 11 April 2021 tools for the recognition of heavy metal ions which can be done by using very sensitive methods such as atomic absorption spectroscopy [1], emission spectroscopy [2], cold vapor Publisher’s Note: MDPI stays neutral atomic fluorescence spectrometry [3] and inductively coupled mass spectrometry [4]. with regard to jurisdictional claims in The last techniques require laborious sample preparation and well-controlled experimental published maps and institutional affil- conditions being also expensive tools. That is why electrochemical detection, which uses iations. methods such as anodic stripping voltammetry, is a promising answer, its main advantage being the fact that it is a portable method at a low cost. Regarding to toxicity concerns, solid electrodes which can be further modified to increase the selectivity and sensitivity can replace the toxic mercury electrodes usually used for stripping [5]. The CMEs can Copyright: © 2021 by the authors. be prepared by physical absorption of several compounds or by electropolymerization Licensee MDPI, Basel, Switzerland. of specific complexing monomers on solid electrodes. For instance, Zhou [6] developed This article is an open access article a sensor for the specific detection of Cd(II) and Pb(II) by using amino acids that have distributed under the terms and cysteine as a functional side chain. The use of graphene oxide nanoparticles led to the conditions of the Creative Commons increase the electrochemical signal. Complexing CMEs can be obtained by covering the Attribution (CC BY) license (https:// electrode surface with polymer complexing films. The most efficient preparation is the creativecommons.org/licenses/by/ direct electropolymerization of a complexing monomer [7]. This allows for the obtaining of 4.0/). Symmetry 2021, 13, 644. https://doi.org/10.3390/sym13040644 https://www.mdpi.com/journal/symmetry Symmetry 2021, 13, 644 2 of 12 Symmetry 2021, 13, 644 the electrode surface with polymer complexing films. The most efficient preparation 2is of the 11 direct electropolymerization of a complexing monomer [7]. This allows for the obtaining of a film with a thickness that is easy to control [8] in a single step; the method is generally areliable film with and a reproducible. thickness that Different is easy to monomers control [8] have in a singlebeen used step; for the modification method is generally of elec- reliabletrode surfaces and reproducible. to detect heavy Different metals monomers (pyrrole, have thiophene). been used Compared for modification to them, of electrode azulene surfaceshas special to properties, detect heavy such metals as an (pyrrole,easy polyme thiophene).rization due Compared to its polar to them,character. azulene Its push- has specialpull structure properties, with suchseparate as an loads easy on polymerization the two cycles due (a cyclopentadien to its polar character.yl anion Its joined push-pull with structurea cycloheptatrienyl with separate cation loads having on the6 π twoelectrons cycles in (a each cyclopentadienyl ring [9]) allows anion for the joined use of with az- aulene cycloheptatrienyl derivatives in oxidation cation having and reduction 6 π electrons processes. in each Due ring to [their9]) allows particular for thechemistry, use of azulenesuch azulene derivatives derivatives in oxidation have been and reductionlittle used processes. in applications Due to such their as particular the electroanalysis chemistry, suchof heavy azulene metal derivatives ions. However, have beenthe CMEs little usedwith inpolymeric applications azulene such derivative as the electroanalysis films can be ofused heavy as sensing metal ions. tools However,for monitoring the CMEs the concen with polymerictrations of azulenedifferent derivative targets, such films as canheavy be usedmetals as in sensing wastewater tools forsamples monitoring [10]. the concentrations of different targets, such as heavy metalsThe in monomer wastewater used samples to prepare [10]. the complexing films in this paper is a tetrazole az- uleneThe (Figure monomer 1). Our used research to prepare led to the obtainin complexingg new films CMEs in based this paper on the is aazulene tetrazole derivative azulene (FigureL (L-CMEs)1). Our and research testing led their to obtainingability to new complex CMEs heavy based metals. on the azuleneThe electrooxidative derivative L (L-CMEs)polymerization and testing of L was their used ability to cover to complex the electrode heavy metals. surfaces The with electrooxidative complexing polymeric polymer- izationfilms by of whichL was the used recognition to cover the of electrodeheavy metal surfaces cations with (Hg(II), complexing Cu(II) polymeric, Pb(II), Cd(II)) films be- by whichcame possible. the recognition of heavy metal cations (Hg(II), Cu(II), Pb(II), Cd(II)) became possible. Figure 1. Structure of the azuleneazulene derivativederivative LL usedused asas monomer.monomer. 2. Materials and Methods 2. Materials and Methods The synthesis of azulene derivative L shown in Figure1 was performed accord- The synthesis of azulene derivative L shown in Figure 1 was performed according to ing to the described methodology [11]. Tetrabutylammonium perchlorate (TBAP, Fluka, the described methodology [11]. Tetrabutylammonium perchlorate (TBAP, Fluka, Mu- Munich, Germany, analytical purity ≥ 99.0%) and acetonitrile (CH3CN, Sigma Aldrich, nich, Germany, analytical purity ≥ 99.0%) and acetonitrile (CH3CN, Sigma Aldrich, elec- electronic grade 99.999% trace metals) were used as supporting electrolyte and solvent, tronic grade 99.999% trace metals) were used as supporting electrolyte and solvent, re- respectively. 0.1 M acetate buffer solution (pH = 4.5) was prepared from 0.2 M acetic acid spectively. 0.1 M acetate buffer solution (pH = 4.5) was prepared from 0.2 M acetic acid solutions (Fluka, Munich, Germany, >99.0%, trace select), 0.2 M sodium acetate (Riedel de solutions (Fluka, Munich, Germany, >99.0%, trace select), 0.2 M sodium acetate (Riedel de Haën, Seelze, Germany), and ultrapure water. Stock solutions of heavy metal salts were Haën, Seelze, Germany), and ultrapure water. Stock solutions of heavy metal salts were prepared by dissolving salts of mercury (II) acetate, cadmium acetate dihydrate (Fluka, prepared by dissolving salts of mercury (II) acetate, cadmium acetate dihydrate (Fluka, Munich, Germany, ≥98%), lead (II) acetate trihydrate (Fluka, Munich, Germany, ≥99.5%) Munich, Germany, ≥98%), lead (II) acetate trihydrate (Fluka, Munich, Germany, ≥99.5%) and copper (II) acetate monohydrate (Fluka, Munich, Germany, ≥98%) in ultrapure water. and copperFor the (II) electrochemical acetate monohydrate experiments (Fluka, cells Munich, with three Germany, electrodes ≥98%) were in ultrapure connected water. to an AUTOLABFor the potentiostat. electrochemical For experiments the preparation cells of with CMEs, three the electrodes working electrodewere connected