A continuous flow of PBS (buffer) was pumped into a flow cell (built in-house, Cole-Parmer, Vernon Hills, IL) at 1.81 mL/min using a syringe pump (Fisher Scientific, Waltham, MA, USA). Reference and working electrodes were attached to the flow cell accordingly. A 5-s bolus of Trp, Tyr, or both was introduced to the electrodes by using a 6-port injection valve (VICI, Houston, TX, USA).