Biosensor based on laccase and an ionic liquid for determination of rosmarinic acid in plant extracts

Novel biosensors based on laccase from Aspergillus oryzae and the ionic liquids (ILs) 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) were constructed for determination of rosmarinic acid by square-wave voltammetry. The laccase catalyzes the oxidation of rosmarinic acid to the corresponding o-quinone, which is electrochemically reduced back to rosmarinic acid at +0.2 V vs. Ag/AgCl. The biosensor based on BMIPF6 showed a better performance than that based on BMIBF4. The best performance was obtained with 50:20:15:15% (w/w/w/w) of the graphite powder:laccase:Nujol:BMIPF6 composition in 0.1 mol L−1 acetate buffer solution (pH 5.0). The rosmarinic acid concentration was linear in the range of 9.99 × 10−7 to 6.54 × 10−5 mol L−1 (r = 0.9996) with a detection limit of 1.88 × 10−7 mol L−1. The recovery study for rosmarinic acid in plant extract samples gave values from 96.1 to 105.0% and the concentrations determined were in agreement with those obtained using capillary electrophoresis at the 95% confidence level. The BMIPF6-biosensor demonstrated long-term stability (300 days; 920 determinations) and reproducibility, with a relative standard deviation of 0.56%.