On-chip handling of solutions and electrochemiluminescence detection of amino acids

On-chip determination of amino acids was conducted by electrochemiluminescence (ECL) following microfluidic transport, mixing of solutions, and generation of ECL. The fabricated system consisted of a polydimethylsilixane (PDMS) substrate with a flow channel structure and three-electrode systems formed on a glass substrate that resulted in the microfluidic transport and the ECL. An amino acid solution and a reagent solution containing tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+) were transported by electrowetting by applying a negative potential to gold working electrodes formed along the flow channels. The two solutions were then mixed in the mixing channel using an additional gold working electrode formed between the electrodes for transport. ECL was generated by applying a potential to a platinum working electrode formed in the mixing channel. The ECL from l-proline, l-lysine, l-leucine, l-valine, and l-histidine was examined. Clear ECL was observed, and its intensity depended on the concentration of the amino acids. By increasing the pH by mixing solutions and generating the oxidized form, Ru(bpy)33+, on the chip, the detection sensitivity could be improved. l-proline showed the strongest ECL, and concentrations of the nM order could be detected.