Analysis of Chlorogenic Acids in Coffee with a Multi-walled Carbon Nanotube Electrode

An electrochemical analysis using a multi-walled carbon nanotube (MWCNT) electrode is applied to examine chlorogenic acids (3-cafeoylquinic (3CQ), 4-cafeoylquinic (4CQ), 5-cafeoylquinic (5CQ), 3,4-dicafeoylquinic (34dCQ), 3,5-dicafeoylquinic (35dCQ), 4,5-dicafeoylquinic (45dCQ), 3-feruloylquinic (3FQ), 4-feruloylquinic (4FQ), and 5-feruloylquinic (5FQ) acid), which are the main ingredients of coffee polyphenols. The MWCNT electrode shows an excellent performance in the electrochemical quantification of chlorogenic acids. Cyclic voltammetry (CV) of all the chlorogenic acids shows redox peak pairs ranging between + 0.27 and + 0.31 V that are assigned to the redox reaction of the catechol group. Changes in the current due to individual chlorogenic acids at the peak potentials are proportional to their concentrations (3CQ, 4CQ, and 5CQ: linear range 0.4–194 μM, sensitivity 1.8 μA μM⁻¹ cm⁻²; 34dCQ, 35dCQ, and 45dCQ: linear range 9.5–194 μM, sensitivity 4.3 μA μM⁻¹ cm⁻²; and 3FQ, 4FQ, and 5FQ: linear range 5–194 μM, sensitivity 0.094 μA μM⁻¹ cm⁻²). High-performance liquid chromatography (HPLC) analysis of coffee infusions shows that chlorogenic acids are almost entirely composed of 3CQ, 4CQ, and 5CQ. The total amounts of 3CQ, 4CQ, and 5CQ in coffee infusions obtained by HPLC show good agreement with those obtained by CV using the MWCNT electrode. This result indicates that the redox peak current can quantify the total amount of chlorogenic acid in a coffee infusion.