Full Paper Novel Ni(II) Mixed Ligand Complex Modified Electrode: Catalytic Effect on Anodic Oxidation of Phenol Jegan Mohan, Seema Joshi, Rajiv Prakash,* a R.C.Srivastava Industrial Toxicology Research Centre, M.G. Marg, P.O. Box-80, Lucknow-226 001, India. a Present Address: Institut f¸r Physikalische Chemie, Jakob Welder-Weg 11, Johannes Gutenberg-Universit‰t Mainz, D 55128 Mainz, Germany *e-mail: rajivprakash12@yahoo.com Received: December 17, 2002 Final version: April 7, 2003 Abstract Cyclic voltammetry, using graphite paste electrode (GPE) and chemically modified GPE with Ni(II) mixed ligand complex (MGPE), is described for sensing and detoxification of phenol. A novel mixed ligand complex of nickel with 1,4,8,11-tetraaza cyclotetradecane (cyclam) and thiocyanate in the molar ratio of 1:2:5 is synthesized and characterized by spectroscopic and electrochemical studies. The Ni(II)-cyclam-thiocyanate complex behaves as a fast electron-transfer mediator, as nickel ions exist in higher oxidation state of Ni(III) on applying a potential of 1.25 V (vs. Ag/AgCl), which catalyses the oxidation of the target species. The mixed ligand complex, when incorporated in the graphite paste electrode, is sensitive to detect phenol as low as 10 mg/L in solution. The catalytic effect of the Ni(II)-cyclam-thiocyanate complex enhanced many folds the oxidation of phenol compared to GPE. The technique has potential for sensing/monitoring and detoxification of phenol released in the ecosystem from polluting industries. Keywords: Cyclic voltammetry, Chemically modified electrode, Mixed ligand complex, Phenol oxidation, Detoxification. 1. Introduction Phenol is human poison by inhalation, ingestion and skin absorption, and is a severe irritant to the eyes, skin and respiratory system [1, 2]. It is highly soluble in water. Many industrial processes generate wastewater flows with a high concentration of phenols and other related compounds. About 26.3% of phenol eventually ends up in air, approx- imately73.3%inwaterandabout0.2%interrestrialsoiland aquaticsediments[1].Theremoval/detoxificationofphenols from wastewaters is, therefore of great importance. Among the existing processes, photochemical destruction, chemical oxidation,wetoxidationhavebeenreported[3±4].However theseconventionaltreatmentprocessessufferduetocertain disadvantagesandlimitations[5].Electrochemicaloxidation techniquesoffertheprospectofrelativelysimpleequipment, environmentalfriendlinessandthepossibilityofhigh-energy efficiencybycomparisonwithothermethods[1,6±9]. Coordination compounds containing macrocyclic ligands viz. ™synthetic macrocycles∫, e.g., 1,4,8,11-tetraaza cyclo- tetradecane(cyclam),1,4,8,12-tetraazacyclopentadecane, phthalocyanines, metalloporphyrins and crown ethers etc., have been known and studied since the beginning of last century, which are now being used for the development of electrochemicalsensors[8,10±13].Cyclamcomplexeswith Cu,CoandNiwereverycommon,whichwerefirstreported in1960[14,15].Thesemetalcyclamcomplexeshavealsobeen reportedtoinhibitmetalinducedbiochemicalalterationsand intheameliorationofmetalinducedtoxicity[16±20]. Recently,modifiedcyclamcomplexesviz.polymerizationor substitution of the cyclam ring have been reported for the detoxificationofcarbondioxide[21,22]andnitricoxide(NO x ) [23]. However, the stability and electroactivity of the com- plexes are the major problem in their technological applica- tions [24]. In the present paper, we report the synthesis/ formation of mixed ligand complex of Ni-cyclam with thio- cyanate, their spectroscopic and electrochemical properties includingstability,electroactivepropertiesandredoxbehavior. The complex was further studied for its catalytic property especially for the electrochemical oxidation of phenol. 2. Experimental 2.1. Materials and Instruments 1,4,8,11-Tetraazacyclotetradecane(cyclam),graphitepow- der (2±4 mm) and Nujol were obtained from Aldrich Chemicals Company Inc., U.S.A. Potassium thiocyanate and NiCl 2 (AR) were obtained from Glaxo Chemicals, Mumbai, India. Tris (hydroxymethyl) aminomethane was obtainedfromSiscoResearchLaboratories,Mumbai,India. All the other chemicals used in this investigation were of analytical extrapure grade. Absorption spectra of complexes were recorded on a Cintra20UV/VisibleSpectrometer(GBC,Australia)inthe rangeof300±800nm. Electrochemical studies of mixed ligand complex of nickel in solution were performed using three-electrode 572 Electroanalysis 2004,16,No.7 ¹ 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200302841