1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 DOI: 10.1002/elan.201800309 Optimized Potentiometric Assay for Non-Invasive Investigation of Skin Antioxidant Activity Khiena Z. Brainina,* [a, b] Maria G. Markina, [a] and Natalia Yu. Stozhko [a] Abstract: The paper presents a new variant of non- invasive potentiometric method for determination of human skin antioxidant activity (AOA). The use of a porous cellulose acetate membrane as the carrier for reagent solution (potassium hexacyanoferrate (III)) is presented. The increase of reagent shelf life, improvement the reproducibility of the results (during the day Sr  0.08, interday Sr  0.13) in comparison with previously de- scribed variant is demonstrated. It was shown that the components of the skin hydrolipid mantle and pH do not affect the potentiometric signal. The values of skin AOA determined by the developed method lie in the range 20– 4000 mM-eq. Recovery is R average = 99 %. The skin of healthy people was studied by the developed method. The obtained results indicate the prospects of using a new non-invasive approach in the study of the oxidation- reduction balance of human skin. Keywords: Non-invasive assay · antioxidant activity · antioxidants · human skin 1 Introduction In recent years, there has been a significant increase in the number of publications aimed at creating devices for non- invasive monitoring of people‘s health status [1–3]. Interest in wearable devices [1, 2], which allow to monitor the health status or the effectiveness of the therapy at home, is determined not only by the desire to optimize the system of rendering medical care in conditions of population demographic aging, but also by the need to develop simple and rapid methods of analysis using mobile technologies. The growing interest in the skin as an object of analysis is due to the fact that it serves as an indicator of the human health. At the same time, the skin is a convenient object of non-invasive analysis, which does not require complicated sample preparation or special laboratory conditions. Non-invasive devices for investigation the skin, and the secretion of the skin glands or for characterizing the work of internal organs (through the skin) may be divided into the following groups: * determination of the physical parameters (skin surface temperature, touch and pressure sensor [4], viscoelastic properties of skin [5], moisture level and strain sensor [6]), * monitoring of the internal organs functioning (ECG [1]), pulse [7], micro- and macro-blood flow [8], * determination of some indicators of a healthy lifestyle (pedometers, activity sensors and the number of caloric value of food eaten [9, 10]), * identification of pathogens, imbalance of skin micro- flora [11], * determination of the biochemical parameters of the skin and secretion of its glands. Increased interest to investigations, devoted to non- invasive chemical sensors, is observed this time [12]. Sweat and interstitial fluid are most often referred to as analytes [13]. In addition, some studies have shown that the stratum corneum of the human skin is a biochemically active and informative object of research [14–16]; the prospects of using washings from the human skin as an analyte have been noted [14, 15]. Attention is paid to development of wearable sensors that provide continuous and long-term monitoring of the biochemical parameter of interest [13]. Despite the achievements in this field, the developed sensors are far from being used in real conditions. Unresolved problems are associated with its instability, necessity to regenerate of the receptor layer of the sensor, combination of biocompatibility, miniaturization and ergonomics in one device, and a number of other problems. Current parameters of interest, concerning the skin and secretion of skin glands, include pH [17–19], concen- tration of cations Na + [20, 21], Ca 2 + [17], heavy metal ions [22]; glucose [13, 23, 24], lactate ions [23, 25–27], chloride ions [24, 28, 29], ethanol [30]. At the same time, the research of skin antioxidants has a great practical importance and scientific interest [31]. It is known that the constant negative impact of environ- [a] K.Z. Brainina, M. G. Markina, N. Y. Stozhko Ural State University of Economics, 8 March St., 62, Ekater- inburg, 620144, Russian Federation E-mail: baz@usue.ru [b] K. Z. Brainina Ural Federal University, Lenin Ave., 51, Ekaterinburg, 620000, Russian Federation Supporting information for this article is available on the WWW under https://doi.org/10.1002/elan.201800309 Full Paper www.electroanalysis.wiley-vch.de  2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2018, 30, 1 – 9 1 These are not the final page numbers! ÞÞ