An electrocatalytic, cobalt-phthalocyanine screen-printed carbon voltammetric sensor for the measurement of the antioxidant gallic acid in instant coffee

A disposable, low-cost, screen-printed carbon electrode (SPCE), modified with cobalt phthalocyanine (CoPC), has been successfully employed as a voltammetric sensor for the determination of gallic acid (GA) in coffee. The cyclic voltammetric (CV) behaviour of GA indicates that this antioxidant undergoes electrocatalytic oxidation at the CoPC-SPCE in a supporting electrolyte of 0.1 M PBS (pH 3.0); the oxidation product was identified as a quinone species. Calibration studies were performed with the electrocatalytic sensor in conjunction with differential pulse voltammetry (DPV). Well-defined analytical responses with good reproducibility were achieved for GA concentrations between 0.25 and 10.00 μM; this range was suitable for measurements in coffee. A simple pretreatment step, consisting only of dissolving a known mass of instant coffee powder in deionised water, was required. Quantification was then performed by the standard-addition method. The original GA content of instant coffee was 1.855% (m/m). Further validation of the new voltammetric procedure was performed by spiking a known quantity of GA into the same coffee sample and determining recovery and precision of 99.909 and 7.2%, respectively. Clearly, the electrocatalytic sensor, in conjunction with DPV, provides a simple, accurate, and low-cost method for determining GA in instant coffee with good precision.