Iron 2 sulfate

L-p-tyrosine and 3,4-dihydroxy-L-phenylalanine (L–DOPA) and nitrilotriacetic acid were purchased from Sigma-Aldrich (St. Louis, MO, USA). 3-L-nitrotyrosine, DL-m-tyrosine, DL-o-tyrosine, DL-phenylalanine, 4-hydroxy-L-proline, copper(II) sulfate, SDS, sodium acetate, acetic acid, ammonium hydroxide, hydrogen peroxide, isopropyl alcohol and iron(II) sulfate were obtained from Fisher Scientific (Pittsburgh, PA, USA). 3,3’-Dityrosine was synthesized by the Synthetic Chemical Biology Core Laboratory at the University of Kansas (Lawrence, KS, USA). The following materials were purchased and used for microchip and electrode fabrication: quartz glass plates (4 in × 2.5 in × 0.085 in, Glass Fab, Rochester, NY, USA); AZ 1518 positive photoresist and AZ 300 MIF developer (AZ Electronic Materials, Somerville, NJ, USA); SU-8 10 negative photoresist and SU-8 10 developer (Micro-Chem, Newton, MA, USA); polydimethylsiloxane (Sylgard^™ 184) silicon elastomer base and curing agent (Dow Corning Corp., Midland, MI, USA); copper wire (22-gauge, ACE Hardware, Lawrence, KS, USA); epoxy glue (J–B Weld, Sulphur Springs, TX, USA), and colloidal silver liquid (Ted Pella, Inc., Redding, CA, USA). All the solutions were prepared in 18.2 MΩ.cm Milli-Q water (Millipore, Kansas City, MO, USA).