3 hydroxytyramine hydrochloride

Lactate oxidase (Pediococcus species), L-glutamic salt, and 3-hydroxytyramine hydrochloride were used (Sigma-Aldrich, St. Louis, MO, USA). L-lactic acid (lithium salt) 99% was obtained from Fisher Scientific. 1.3 mg LO_x was dissolved in 500 µL 1× phosphate buffer (PBS) solution to form the Lactate oxidase enzyme stock solution [40 (link)]. Slight agitation was needed to accelerate the dissolution. Then the stock solution was aliquoted to 20 µL and stored at −20 °C. 960 mg L-lactic acid (lithium salt) was dissolved in 10 ml 10× PBS to form 1 M L-lactate stock solution. The stock solution was stored at room temperature. Thirty-eight milligrams of 3-hydroxytyramine hydrochloride was dissolved in 200 mL deionized (DI) water to form the 1 mM dopamine (DA) stock solution. L-glutamic salt (37.428 mg) was dissolved in 200 mL DI water to form the 1-mM glutamate (Glu) stock solution.

All chemicals were used as received without further purification. Melamine (C₃H₆N₆, 99%) was purchased from Alfa Aesar. Sucrose (C₁₂H₂₂O₁₁, BioXtra, ≥99.5%), aluminum chloride hexahydrate (AlCl₃^.6H₂O, 99%), Ferron (8-hydroxy-7-iodo-5-quinolinesulfonic acid, ≥98.5%), 3-hydroxytyramine hydrochloride (99%), tris-base (C₄H₁₁NO₃, ≥ 99%), and concentrated hydrochloride (HCl, 37 wt%) were purchased from Sigma–Aldrich. Sodium hydroxide (NaOH, 98.6%) were purchased from VWR Chemicals. Silver nitrate (AgNO₃, ≥98.5%), sodium chloride (NaCl, ≥99.5%), lithium chloride anhydrous (LiCl, ≥99.5%), potassium chloride (KCl, ≥99%), calcium chloride dihydrate (CaCl₂^.2H₂O, ≥99%), magnesium chloride (MgCl₂^.6H₂O, 99%), acetone and methanol were purchased from Fischer Chemical. Sodium acetate (CH₃COONa), 1,10-phenanthroline (C₁₂H₈N₂, 99 + %), and hydroxylamine hydrochloride (NH₂OH^.HCl) were purchased from Fluka Analytical. Milli-Q water (conductivity = 0.055 µS/cm, Q3 level) with pH of 5.7 was used throughout the experiments. Characterizations are listed in Supplementary Methods.