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In this study, peroxidase from alfalfa sprout extract (Medicago sativa) was immobilized on a gold electrode, using a solution of the zwitterionic surfactant 3-(1-tetradecyl-3-imidazolio)propanesulfonate (ImS3-14) containing well-dispersed gold nanoparticles. In the presence of hydrogen peroxide, the peroxidase enzyme catalyzes the oxidation of hydroquinone to o-quinone, which is electrochemically reduced at a potential of +0.09 V vs. Ag/AgCl. The hydroquinone biosensor exhibited excellent electrocatalytic activity under the optimized experimental conditions ([peroxidase] = 97 units mL−1, [H2O2] = 7.4 × 10−5 M, pH 7.0, frequency of 100 Hz, pulse amplitude of 60 mV and scan increment of 4.0 mV). Peak current linearity was observed in the range of 6.96 × 10−7 to 8.54 × 10−6 M, and the detection limit was estimated to be 1.88 × 10−7 M. This biosensor demonstrated good repeatability and reproducibility with relative standard deviations of 3.6% and 6.5%, respectively, and an acceptable stability (at least 100 determinations). The method has been applied to the determination of hydroquinone in skin-brightening creams and the recoveries were 98.3–104.5%.

A new bean sprout peroxidase was immobilized on a gold electrode modified with self-assembled monolayers (SAM) of l -cysteine for the determination of dopamine in pharmaceutical samples using square wave voltammetry. In the bean sprout–(SAM)–Au electrode, the peroxidase, in the presence of hydrogen peroxide, catalyzes the oxidation of dopamine to the corresponding quinone, which is electrochemically reduced back to dopamine at +0.15 V vs. Ag/AgCl. The performance and the factors influencing the response of this biosensor were studied in detail. The best performance was obtained using 0.1 mol L −1 phosphate buffer solution (pH 6.0), 6.0 × 10 −5 mol L −1 hydrogen peroxide, frequency of 100 Hz, pulse amplitude of 80 mV and scan increment of 4.0 mV. The analytical curve was linear for dopamine concentrations from 9.91 × 10 −6 to 2.21 × 10 −4 mol L −1 and the detection limit was 4.78 × 10 −7 mol L −1 . The recovery of dopamine ranged from 98.0 to 111.8% and the relative standard deviation was 3.1% for a solution containing 1.30 × 10 −5 mol L −1 dopamine ( n = 6). The lifetime of this biosensor was 15 days (at least 300 determinations). The results obtained for dopamine determination in pharmaceutical formulations using the proposed bean sprout–SAM–Au electrode were in agreement with those obtained with the standard method at the 95% confidence level.

A novel biosensor based on laccase immobilized on microspheres of chitosan crosslinked with tripolyphosphate was developed for rutin determination in pharmaceutical formulations by square wave voltammetry. In the biosensor, laccase catalyzes the oxidation of rutin to the corresponding o-quinone, which is electrochemically reduced back to rutin at +0.35 V vs. Ag/AgCl. The resultant reduction current is used as the analytical response. Under the optimum conditions established (laccase 0.55 units mg−1; pH 4.0; frequency 30 Hz; pulse amplitude 30 mV; and scan increment 2.0 mV), the analytical curve showed two linear ranges: 5.99 × 10−7 to 3.92 × 10−6 mol L−1 and 5.82 × 10−6 to 1.31 × 10−5 mol L−1 rutin with detection limits of 6.23 × 10−8 and 7.12 × 10−7 mol L−1, respectively. The recovery of rutin ranged from 90.7 to 105.0% and the relative standard deviation was 3.1% for a solution containing 2.0 × 10−6 mol L−1 rutin (n = 9). The lifetime of this biosensor was 320 days (at least 930 determinations). The results obtained for rutin in pharmaceutical formulations using the proposed biosensor were in agreement with those obtained with the standard method at the 95% confidence level.