A biosensor based on the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI center dot Tf2N) and a novel source of peroxidase (tissue from the pine nuts of Araucaria angustifolia) was constructed. This enzyme was immobilized on chitosan crosslinked with citrate and the biosensor used for the determination of rosmarinic acid by square-wave voltammetry. The peroxidase in the presence of hydrogen peroxide catalyzes the oxidation of rosmarinc acid to quinone and the electrochemical reduction of the product was obtained at a potential of +0.15 V vs. Ag/AgCl. Different analytical parameters influencing the biosensor response, that is, peroxidase units, pH, hydrogen peroxide concentration and parameters for the square-wave voltammetry (frequency, pulse amplitude and scan increment), were investigated. The best performance was observed for the biosensor under the following conditions: 1000 units mL(-1) peroxidase, pH 7.0 and 8.3 x 10(-4) mol L-1 hydrogen peroxide with a frequency of 30 Hz, pulse amplitude of 100 mV and scan increment of 5.0 mV. The biosensor gave a linear response to rosmarinic acid over the concentration range of 9.07 x 10(-7) to 4.46 x 10(-6) mol L-1 with a detection limit of 7.25 x 10-8 mol L-1. The recovery of rosmarinic acid in plant extracts ranged from 97.0% to 109.6% and the determination of this substance in these samples using the biosensor compared favorably with that using the capillary electrophoresis method. (C) 2009 Elsevier Inc. All rights reserved.