Copper has been known as an essential bioelement for some time but its biological role(s) has been recognized only in recent decades due to the rapid development of bioinorganic chemistry, a successful interaction between model complexes and protein biochemistry [1], [2], [3], [4] and [5]. Copper complexes have been widely studied as structural and functional models for the active centers of copper containing redox enzymes [6], [7], [8], [9], [10] and [11]. The construction of a mimetic enzymatic sensor is the most promising area in the development of analytical procedures for the determination of various analytes [12], [13] and [14].
Biomimetic catechol oxidase catalysts have been reported in the literature and have been shown to be efficient catalysts for the oxidation of phenolic substrates to quinones [12] and [13]. Catechol oxidase is a copper-containing enzyme which catalyzes the oxidation of diphenols by molecular oxygen with the production of o-quinones. The o-quinones formed in these reactions subsequently undergo non-enzymatic reactions to yield brown-black melanin pigments [15].
Since the construction of the first carbon paste electrode by Adams [16] several modified carbon paste electrodes have been successfully developed and applied in the determination of various species. They generally consist of electrically conducting graphite powder and an organic liquid that is immiscible with the contacting aqueous solution. Most of these electrodes are based on the ion-exchange mechanism of the active component incorporated into the carbon paste matrix [17].
Hydroquinone (1,4-benzenediol) introduced for clinical use in 1961, is the most popular depigmenting agent. It may interfere with pigmentation through: (a) the covalent binding to histidine or interaction with coppers at the active site of tyrosinase, (b) the inhibition of DNA and RNA synthesis and (c) the alteration of melanosome formation and melanin. However, because of the hazards of long-term treatments, the European Committee has banned the use of hydroquinone in cosmetics and formulations and it is available only through prescription by physicians and dermatologists [18] and [19]. Several analytical methods have been used for the determination of hydroquinone in samples of cosmetic creams. These include electrochemistry [19] and [20], high performance liquid chromatography [21] and [22] and spectrophotometry [23] and [24] procedures. Vieira and Fatibello-Filho [19] developed an organic-phase biosensor, based on paraffin/graphite modified with sweet potato (Ipomoea batatas (L.) Lam.) tissue as the source of peroxidase, for determining hydroquinone in cosmetic creams in the concentration ranges of 7.5 × 10−5 to 1.6 × 10−3 mol L−1. An electroanalytical method has been proposed [20] for the simultaneous determination of hydroquinone ethers using a carbon paste electrode. The samples are preconcentrated at the electrode surface prior to voltammetric determination.
This paper describes the synthesis and characterization of a new dinuclear copper(II) complex [Cu2(HL)(OAc)](ClO4)2 containing a novel ligand, N,N′,N′-[tris-(2-pyridylmethyl)]-N-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,3-propanediamine-2-ol (H2L) as a catechol oxidase catalyst, for use in the construction of a biomimetic sensor for the determination of hydroquinone concentrations in cosmetics by square wave voltammetry. The results obtained were compared with the official method [19]. The influence of different experimental parameters including carbon paste/Cu(II) complex composition, pH, frequency, pulse height and Epc (cathodic peak potential) were investigated to optimize this proposed biomimetic sensor.