Talanta 71 (2007) 1782–1787 Chromogenic radical based optical sensor membrane for screening of antioxidant activity Ivana Murkovi´ c Steinberg ∗ , Stjepan Milardovi´ c University of Zagreb, Faculty of Chemical Engineering and Technology, Maruli´ cev trg 19, HR-10000 Zagreb, Croatia Received 8 September 2005; received in revised form 16 August 2006; accepted 21 August 2006 Available online 26 September 2006 Abstract Solid-state optical sensor membranes based on immobilised chromogenic radicals for the assessment of antioxidant activity have been studied. Two stable lipophilic chromogenic radicals, DPPH • (2,2-diphenyl-1-picrylhydrazyl radical) and galvinoxyl radical, GV • , (2,6-di-tert-butyl-- (3,5-di-tert-butyl-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy radical), were immobilised in plasticised PVC films and screened for suitability as indicators of antioxidative activity. The spectrophotometric characterisation of the polymer films containing immobilised free radicals was performed, and the response of the immobilised free radicals toward standard antioxidants was studied. It has been demonstrated that the immobilised radicals retain their reactivity towards antioxidants and the results suggest that the reactivity of immobilised radicals is comparable to standard solution-based DPPH assays. Polymer films containing immobilised DPPH • radical respond to standard antioxidants in aqueous solutions by changing colour irreversibly from purple (absorption maximum at 520nm) to yellow. The initial slopes of the response curves to the phenolic antioxidant gallic acid, obtained in the 1–50 mM concentration range, gave a linear calibration plot in a 1 min exposure cuvette test. The polymer films were used to screen antioxidative activity of beverage and food samples known to contain antioxidants, such as black and green tea, coffee, red wine, fruit juice, olive oil and sunflower oil. It has been demonstrated that a rapid and simple qualitative screening test of untreated samples is possible using a test strip based on immobilised DPPH • radical. © 2006 Elsevier B.V. All rights reserved. Keywords: Antioxidative activity; Immobilised radicals; Polyphenolic antioxidants; DPPH • radical; Plasticised PVC; Screening test 1. Introduction Understanding the chemistry of free radicals and antioxi- dants is gaining importance, particularly in the areas of clini- cal medicine and nutritional science. Free radicals are known to cause damage to lipids, proteins and nucleic acids and are implicated in a number of pathological interactions and degener- ative diseases in living organisms, including cancer, Alzheimer’s and Parkinson’s disease [1]. Antioxidants act as free radical scavengers and can prevent the damage caused by oxidative reactions. Hence, the evaluation of the antioxidative activity of biomedical, cosmetic and food samples provides useful clinical and dietary information. A number of optical-based tests for the determination of antioxidant activity have been developed [2] based on differ- ∗ Corresponding author. Tel.: +385 1 45 97 287. E-mail address: ivana.murkovic@fkit.hr (I.M. Steinberg). ent chemical and biological mechanisms, including the Trolox Equivalent Antioxidant Capacity (TEAC) [3], Oxygen Radical Absorbance Capacity (ORAC) [4], Ferric Reducing Antioxi- dant Power (FRAP) [5] and Total Radical Antioxidant Poten- tial (TRAP) [6] assays. In general, the activity of antioxidants present in the sample is related to their free radical scavenging ability or to their reducing power, which is then coupled to chro- mogenic or fluorogenic detection. Some of these optical assays have been further adapted for use in commercial test kits and instruments using solution-based chemistries, such as the TEAC assay which has been commercialised by Randox Laboratories (San Francisco, CA, USA). Another optical method widely used to measure radical scav- enging capacity of food and plant compounds was introduced by Blois [7], and improved by Brand-Williams et al. [8]. The method is based on the decolourisation of the stable free radical DPPH • in the presence of antioxidants performed as a sim- ple solution-based test. A similar test in homogenous solutions and aqueous dispersions has been reported using the galvinoxyl 0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.08.015