Horseradish peroxidase

Sigma-Aldrich (St. Louis, MO, USA) was the supplier of horseradish peroxidase (HRP), and 3-(3,4-dihydroxyphenyl)-2-propenoic acid (also known as caffeic acid; CA) was obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Low endotoxin egg white ovalbumin (OVA) (less than 1 EU/mg) and cholera toxin were procured from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). Additionally, Thermo Fisher Scientific (Waltham, MA, USA) was the source of Alexa Fluor^TM 488-conjugated OVA (AF488-OVA).

Pyrrole, ammonium persulfate, horseradish peroxidase (HRP, EC1.11.1.17, 190 unit·mg⁻¹) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). 1-Ethyl-3-methylimidazolium bromide ([C₂mim]Br, 99%), 1-hexyl-3-methylimidazolium bromide ([C₆mim]Br, 99%), and 1-dodecyl-3-methylimidazolium bromide ([C₁₂mim]Br, 99%) were provided by Monils Chem. Eng. Sci. & Tech. (Shanghai) Co. Ltd. (Shanghai, China). K₄Fe[(CN)₆], K₃Fe[(CN)₆], Na₂HPO₄, NaH₂PO₄, KCl, citric acid and 30% hydrogen peroxide (H₂O₂) solution were obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Hydrogen tetrachloroaurate hydrate (HAuCl₄, Au 28.6%) was provided by Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China). Ascorbic acid (AA), D-(+)-glucose and uric acid (UA) were obtained from Aladdin Industrial Co., Ltd. (Shanghai, China). Sodium deoxycholate (NaDC, 98%) and aspartic acid (Asp, 98%) were purchased from Shanghai J&K Scientific (Shanghai, China). Ultrapure water with a resistivity of 18.2 MΩ·cm from a Simplicity water purification system (Millipore, Boston, MA, USA) was used for preparing buffer solutions with different pHs. All other chemicals were of analytical reagent grade and used without further purification.