Abstract A simple biosensor constructed by bulk-modi- fication of carbon ink with manganese dioxide as a medi- ator was investigated for its ability to serve as ampero- metric detector for L-ascorbic acid in hydrodynamic mode. The sensor could be operated at pH 5.0 (0.05 M phosphate buffer) and exhibited excellent reproducibility and stability. Optimization of measurement parameters such as applied working potential and pH value were studied in detail. The screen printed electrode exhibited a linear amperometric increase with the concentration of L-ascorbic acid from 50 mg L –1 to 250 mg L –1 and gave a (LOD = 3σ) detection limit of 0.2 mg L –1 (1.172 μmol L –1 ). The manganese dioxide modified screen printed electrode shows long term stability. Introduction It is established that the oxidation of ascorbic acid (1) at low pH proceeds via two consecutive one-electron processes involving the participation of a radical anion in- termediate (2) (the latter being confirmed from electron paramagnetic resonance spectroscopy), to form dehydro- L-ascorbic acid (3). The latter species subsequently un- dergoes a hydration reaction characteristic of carbonyl groups to form the final electroinactive product (4), Scheme 1 [1]. Consequently the overall reaction can be classified as an EC process: AH 2 – 2e → A + 2H + E A + H 2 O → B C The electrochemical oxidation of L-ascorbic acid (H 2 A) at various electrode surfaces has been a subject of great in- terest because of the significance of this compound in bi- ological-biochemical (e.g., neurochemistry) and electro- chemical (e.g., electrocatalysis) settings [2, 3]. Extensive studies on electrochemical biosensors have been made during the last two decades, but as far as we know, the use of manganese dioxide as an efficient elec- trocatalyst for the determination of L-ascorbic acid con- centration has not been reported. We have recently devel- oped a biosensor for glucose [4] incorporating glucose ox- idase as a biocomponent and manganese dioxide as a me- diator in the carbon ink. The sensor response showed that Emir Turkusic · Vladimir Milicevic · Hamid Tahmiscija · Midhat Vehabovic · Sanela Basic · Vesna Amidzic Amperometric sensor for L-ascorbic acid determination based on MnO 2 bulk modified screen printed electrode Fresenius J Anal Chem (2000) 368 : 466–470 © Springer-Verlag 2000 Received: 30 December 1999 / Revised: 2 May 2000 / Accepted: 6 May 2000 ORIGINAL PAPER Dedicated to the memory of Linus Pauling, Nobel Prize winner E. Turkusic () · V. Milicevic · S. Basic · V. Amidzic Department of Chemistry, Faculty of Science, University of Sarajevo, Zmaja od Bosne 35, 71000 Sarajevo, Bosnia and Herzegovina H. Tahmiscija Dental Faculty, University of Sarajevo, Bolnicka 1, 71000 Sarajevo, Bosnia and Herzegovina M. Vehabovic Pharmaceutical company “Bosnalijek” Sarajevo, Jukiceva 55, 71000 Sarajevo, Bosnia and Herzegovina Scheme 1 Oxidation of ascorbic acid