J. Cosmet. Sci. J 56, 79-90 (March/April 2005) Evaluation of antioxidant properties of dermocosmetic creams by direct electrochemical measurements C. GUITTON, P. GROS, and M. COMTAT, Laboratoire de Genie ChimiqueJ UMR 5503J Universite Paul SabatierJ 118 Route de Narbonne, 31062 Toulouse, France, and R. TARROUX and P. BORDAT, Centre de Recherche Dermo-cosmetique Pierre Fabre, Allee Camille Soula, BP 7 4-Vigoulet, 31322 Castanet Tolosan, France. Accepted for publication February 16, 2005. Synopsis Cyclic voltammetry and linear sweep voltammetry were preliminarily used in order to evaluate the global antioxidant properties of dermocosmetic creams. Experiments were performed by introducing electrodes directly into the creams without any pretreatment of the samples. Current-potential curves showed significant anodic current depending on the antioxidant-containing cream studied. In comparison, little amperometric response was recorded with an antioxidant-free cream base. Aqueous solutions of the corresponding anti­ oxidants showed analogous anodic waves and similar peak potentials. A correlation between the global anodic peak and the presence of the antioxidant species in the cream was made with eleven skin creams, attesting to the reliability of the method. Among the tested electrode materials, platinum gave the best results in terms of electrochemical kinetics and measurement precision (current peak standard deviation less than 5%). Exposure of a depilatory cream to oxidizing agents (e.g., hydrogen peroxide, air, or light) caused a decrease in peak current as expected. This methodology enabled us to evaluate the evolution of the total antioxidant capacity under oxidative stress and gives encouragement to further development of a voltam­ metric method to quantify cream antioxidant power. INTRODUCTION Among body tissues particularly involved in aging, skin constitutes one of the major targets, since it is the most exposed to external oxidant agents: UV light, chemical pollutants, oxygen, etc. The symptoms are quite different, ranging from the early appearance of wrinkles to possible skin cancer caused by excessive exposure to sunlight, and including numerous diseases such as psoriasis (1-3). This process and its conse­ quences represent a crucial health problem and also an exponentially grown commercial Abbreviations: ABTS: 2,2 '-azinobis(3-ethylbenzothiazolinesulfonate). BHA: tert-butyl-4-methoxyphenol. BHT: butyl-hydroxy-toluene. EDTA: ethylenediamine-tetraacetic acid. FRAP: ferric-reducing ability of plasma. MSE: mercurous sulphate electrode. PEG: polyethyleneglycol. ROS: reactive oxygen species. TEAC: Trolox-equivalent antioxidant capacity. 79

80 JOURNAL OF COSMETIC SCIENCE market for the cosmetic industry, since an increasingly aging population takes care of a healthy appearance, trying to "turn back the clock": more than 30% of the personal care market in 1999 concerned the skin care segment, with almost $20 billion worldwide (4). One of the main causes of aging is a deregulation of the cell respiratory metabolism involving incomplete oxygen reduction with production of superoxide anion 0 2 ·-, hy­ droxyl radical OH·, hydrogen peroxide H 2 0 2 , etc. (5-7). Skin has different antiradical defense systems to regulate these reactive species, namely enzymes and low-molecular­ weight antioxidants (8-10). Defection of these preventive mechanisms or excessive production of reactive oxygen species induces so-called oxidative stress ( 11, 12). Numerous methods are available to determine total antioxidant capacity, based on inhibition reactions in solution involving a specific oxidant reagent like Fe3+ (FRAP assay) or ABTS (TEAC assay) (13-16). In this field, electrochemistry appears to be a convenient approach to evaluate the overall antioxidant properties of cosmetics. Recently cyclic voltammetry has been shown to be a suitable electrochemical method to evaluate the global antioxidant capacity of real samples like edible plants, plasma, or wine (17-19): the oxidation signal traduces the ability of the medium to give electrons, i.e., to scavenge the ROS by reducing them. This paper presents preliminary work allowing the evaluation of the total antioxidant properties of a dermocosmetic product by means of electrochemical techniques per­ formed directly on the bulk of the cream, without any pretreatment of the sample. Cyclic and linear sweep voltammetry were used to show the overall antioxidant capacity of the product. Comparison of the voltammograms with those recorded with antioxidants containing aqueous solutions allowed us to correlate the electrochemical characteristics of the samples with the properties of redox species. Finally, the influence of irradiation and oxygen, as well as the addition of hydrogen peroxide as an oxidant on the global redox status of a depilatory cream, was highlighted. PRINCIPLES OF VOLTAMMETRY (20) Cyclic voltammetry has been one of the most frequently used electrochemical methods for more than three decades. The reason is its relative simplicity and its high information content. Cyclic voltammetry is performed using potentiostatic equipment in experi­ mental conditions such as that the only mass transport phenomenon to be taken into account is semi-infinite diffusion. Generally, the working electrode on which the oxi­ dation and reduction reactions are studied is flat so that the mass transport may be considered as unidirectional. The solution contains an electrolyte in large excess com­ pared to the concentration of the electroactive species the ions of the electrolyte are not involved in the electron transfer reaction at the electrode. This electrolyte decreases the internal cell resistance and enables us to neglect the migration phenomena of the charged electroactive species. There is no forced convection because the electrodes are fixed and the solution is not stirred. Furthermore, the relatively short experimental time scales allow the natural convection to be neglected. The electrode surface area, the volume of the solution, and the concentrations of the electroactive species are such that the ex­ perimental determination of the oxidation and reduction current (proportional to the heterogeneous electron transfer rate) change the electroactive species concentration in a negligible way. The waveform of the voltage applied to the working electrode versus the