2 nitro 5 thiobenzoic acid

All chemicals were of analytical grade and used without further purification. Sodium citrate and ethanol were obtained from Merck (Darmstadt, Germany). Tyrosinase (from mushrooms, EC1.14.18.1) was purchased from Sigma-Aldrich (St. Louis, MO, USA). FeDC, chloroauric acid (HAuCl₄), ammonium hydroxide (NH₄OH; 25% NH₃ basis), catechol, GSH, 2-nitro-5-thiobenzoic acid (TNB), 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB), meta-phosphoric acid (MPA), bovine serum albumin (BSA), doxorubicin, dibasic sodium phosphate, monobasic sodium phosphate, glutaraldehyde, and potassium chloride were obtained from Sigma-Aldrich. For the preparation of the biosensor, the following solutions were used 100 U μL⁻¹ tyrosinase, 2.5% glutaraldehyde solution and 1% BSA solution were prepared in 50 mM phosphate buffer at pH 6.5. Stock 100 mM solution of catechol (99%; Sigma-Aldrich) and all the working solutions were prepared daily by dilution in phosphate buffer. Five mM FeDC dissolved in 95% ethanol was used as the mediator applied in the tyrosinase biosensor system. Water was obtained by passing twice-distilled water through a Milli-Q system (18 MΩ cm; Millipore, Billerica, MA, USA).