The chemicals used were analytical grade and were purchased from MilliporeSigma (St. Louis, MO). Phosphate buffered saline (PBS) tablets were purchased from Fisher Bioreagents (Waltham, MA, USA). 1x PBS (pH 7.40) was prepared by dissolving five PBS tablets in 1.0 L nanopure water (Milli-Q IQ7000, MilliporeSigma, St. Louis, MO). 0.5 mM Trp and Tyr stock solutions were prepared in PBS and frozen into small aliquots until use. On the day of the experiment, stock solutions were thawed at room temperature and standard solutions of 0.5 μM, 1 μM, 3 μM, and 5 μM tryptophan and tyrosine solutions were prepared from the stock. Dopamine and serotonin stocks (100 µM each) were made in PBS and frozen into small aliquots until use. On the day of the experiment, stock solutions were thawed at room temperature and 1 µM dopamine and serotonin standards were made via dilution using PBS. As these compounds are sensitive to air oxidation, we used solutions within the hr.