Simultaneous determination of norepinephrine, uric acid, and ascorbic acid at a screen printed carbon electrode modified with polyacrylic acid-coated multi-wall carbon nanotubes.

A screen printed carbon electrode (SPCE) modified with polyacrylic acid-coated multi-wall carbon nanotubes (PAA-MWCNTs) has been prepared by the dispersion of MWCNTs in PAA aqueous solution via sonication and the followed drop-coating for the simultaneous determination of norepinephrine (NE), uric acid (UA), and ascorbic acid (AA). The presence of PAA inhibited the adsorption of AA owing to the electrostatic repulsion, but was favorable for the affinity adsorption of NE and UA via the ion exchange and hydrogen bonding mechanisms, respectively. This led to the decrease in the oxidation potential of AA and the significantly enhanced oxidation peak currents of NE and UA at the PAA-MWCNTs/SPCE. By cyclic voltammetry and differential pulse voltammetry, the oxidation potentials of NE, UA, and AA at the PAA-MWCNTs/SPCE in a ternary mixture were found to be well resolved so that their simultaneous determination could be achieved. Furthermore, the interaction of NE, UA, and AA with PAA accounted for their electrochemical responses at different pH values. Also, the effect of pH revealed that the oxidation of NE, UA, and AA at the PAA-MWCNTs/SPCE all involved the transfer of two electrons. In addition, by differential pulse voltammetry, the linear dependence of peak current on the concentration was obtained in the ranges of 0-10, 0-30, and 100-1000 microM with the lowest detection limits of 0.131, 0.458, and 49.8 microM for NE, UA, and AA, respectively.