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 performed 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). The film formation was proven by recording the CV curves of the CMEs in ferrocene solution. The CMEs prepared at different charges or potentials were used for detection of heavy metal ions. Synthetic samples of heavy metal ions (Cd(II), Pb(II), Cu(II), Hg(II)) of concentrations between 10−8 and 10−4 M were analyzed. The most intense signal was obtained for Pb(II) ion (detection limit of about 10−8 M). Pb(II) ion can be detected by these CMEs in waters at such concentrations. The ability of the ligand L to form complexes with Pb(II) and Hg(II) ions was also tested by UV-Vis spectrometry. The obtained results showed the formation of Me(II)L2 complexes.

Chemically modified electrodes (CMEs) based on 2-phenyl-4-((4,6,8-trimethylazulen-1-yl)methylene)oxazol-5(4H)-one (M) were obtained by irreversible electrooxidation of M in millimolar solutions in 0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile. These CMEs were characterized by a ferrocene redox probe, electrochemical impedance spectroscopy (EIS), X-ray photon spectroscopy (XPS), and scanning electron microscopy (SEM). The influence of the preparation conditions (charge and potential) was examined. The CMEs were finally used for the analysis of synthetic samples of heavy metal (HM) ions. The paper highlights the importance of potential and electropolymerization charge on the film properties, with accent on recognition of HMs, in order to identify the best conditions for their detection in water. The observed findings are relevant for further design and development of advanced materials based on azulenyl-phenyloxazolone for the analysis of HMs in water.

Electrochemical and spectral studies of benzylidenerhodanine (BR) were performed in order to develop new sensors for heavy metals (HMs) based on chemically modified electrodes (CMEs). CMEs were obtained by cycling and by controlled potential electrolysis at different potentials and charges. Film formation was demonstrated by recording the CV curves of CMEs in transfer solutions containing ferrocene in 0.1 M TBAP/CH3CN. BR-CMEs were used for the analysis of HMs. Samples of Cd(II), Pb(II), Cu(II), and Hg(II), each possessing concentrations between 10−7 and 10−5 M, were analyzed by using CMEs prepared in different conditions. The most intense signal was obtained for the Pb(II) ion. These BR-CMEs can be used for the analysis of Pb(II) in monitored waters. An electrochemical study was performed at different concentrations of BR in 0.1 M TBAP/CH3CN on a glassy carbon electrode by differential pulse voltammetry, cyclic voltammetry, and rotating disk electrode voltammetry. The complexation ratio in the homogeneous solution has been established by the Mollard method in acetonitrile solutions.

Previous studies performed on 2-(azulen-1-yldiazenyl)-5-phenyl-1,3,4-thiadiazole (T) showed that T is a ligand with complexing properties towards heavy metals (HMs) in solution and can be attached to electrode surfaces. Films of T were deposited on glassy carbon to obtain chemically modified electrodes (T-CMEs), either through scanning or using controlled potential electrolysis in tetrabutylammonium perchlorate in acetonitrile. They were investigated using cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS), and spectroelectrochemistry. All methods provided evidence for showing the formation of insulating films, with properties depending on their electropolymerization potential. CV and EIS studies of T-CMEs in tetrabutylammonium perchlorate in acetonitrile as supporting electrolyte, and in the presence of a ferrocene redox probe resulted in a thickness of ~10 µm, an active surface area about 6 times higher than the geometrical one, and conductivity of about 10−6 S/cm. This characterization performed using voltammetric techniques reveals the symmetry of the reversible anodic and cathodic redox CV peaks for the polymer, while spectroelectrochemistry shows the intensification of the charge transport process through polarons, due to the anodic polarization of the film.

Rhodanine (R) is a heterocycle having complexing properties for heavy metal (HM) ions. Considering the similar electron-donating character of diethylaminobenzene and azulene, electrochemical characterization of (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-one (R1) and 5-(4 diethylamino-benzylidene)-2-thioxo-thiazolidin-4-one (R2) was performed to establish their common features. Chemically modified electrodes based on R1 and R2 were compared for HM recognition. Evidence for the formation of films was provided by scanning and controlled potential electrolysis, and HM recognition experiments were performed using their films. Parallel studies for analysis of HMs by complexation in solution were performed by UV-Vis. The analogy between R1 and R2 created the premise for easier selection of compounds for certain applications. The performance of the chemically modified electrodes was evaluated as detection limits for HMs. The azulene monomer (R1) proved to be the best candidate for Pb(II) detection, being about eight times more sensitive than R2. However, in solution, R2 proved to be a good choice for optical measurements, having a higher absorption coefficient. These results support the two ligands having different behaviors in homogeneous and heterogeneous systems.