Determination of the real electrochemical surface areas of screen printed electrodes

Abstract

A method is provided for determining a real electrochemical surface area of a working electrode (WE) of a screen printed sensor. A concentration of a mediator incorporated in a WE paste is determined. The diffusion coefficient of the mediator is then ascertained. A final real electrochemical surface area of the WE is then made.

1 . A method for determining a real electrochemical surface area of a working electrode (WE) of a screen printed sensor, comprising:
 determining a concentration of a mediator incorporated in a WE paste;   determining a diffusion coefficient of the mediator; and   determining a final real electrochemical surface area of the WE.   
   
   
       2 . The method of  claim 1 , wherein the screen printed sensor is a three electrode system that includes, two printed carbon electrodes acting as a WE and counter electrode, and a printed Ag/AgCl reference electrode. 
   
   
       3 . The method of  claim 1 , wherein the WE paste is a complex matrix structure containing both conducting elements and assay reagents, and wherein a concentration of the mediator incorporated in the WE paste is unknown. 
   
   
       4 . The method of  claim 1  wherein step (a) includes:
 (a) running cyclic voltammetry; and   (b) applying a standard addition method.   
   
   
       5 . The method of  claim 4 , wherein in step  4 (a), different solutions of mediator with different concentrations in PBS are prepared, cyclic voltammograms are run to observe a (Eox)of the WE after applying increased concentrations of mediator on the WE, and wherein a specific electrochemical response corresponds to an added concentration of the mediator as well as of an unknown concentration of the mediator present in the WE paste. 
   
   
       6 . The method of  claim 4  wherein, step  4 (b) includes, plotting oxidation peak currents observed in the CVs against a concentration of the added mediator, wherein after calculating a slope and an intercept of the oxidation peak currents the unknown concentration of the mediator is calculated by dividing the intercept with the slope. 
   
   
       7 . The method of  claim 1 , wherein calculation of the diffusion coefficient of the mediator includes:
 running cyclic voltammetry under three different scan rates to create electrode cyclic voltammograms; and   plotting oxidation peak currents against a square root of scan rates.   
   
   
       8 . The method of  claim 7 , wherein in step  7 (a) PBS is run on the electrode cyclic voltammograms under three different scan rates to obtain three different oxidation peak currents that correspond to the different scan rates. 
   
   
       9 . The method of  claim 7 , wherein in step ( 7 b), the slope of the plot of the three oxidation peak currents (Eox) against the square root of the scan rates is equal to a diffusion coefficient of the mediator. 
   
   
       10 . The method of  claim 1  wherein, said step ( 1 c) Chronocoulometry is used to determine the real electrochemical surface area of the WE. 
   
   
       11 . The method of  claim 10 , wherein Chronocoulometry is a measurement of charge as a function of time, wherein an analysis of chronocoulometric data is based on an Anson equation that defines a charge-time dependence for linear diffusion control:
     Q= 2 nFACD   1/2 π −1/2   t   −1/2     where:   Q is the charge (coulombs);   n is the number of electrons transferred;   A is the real electrochemical surface area of the WE (cm 2 );   F is Faraday's constant (96,485 coulombs/mole);   C is the concentration of the mediator;   D is a diffusion coefficient of the mediator (cm 2 /sec); and   t is time (sec).   
   
   
       12 . The method of  claim 11 , wherein the Anson plot is a plot of Q vs. t 1/2  and transforms data into a linear relationship whose slope (a) is equal to:
   2 nAFCD   1/2 /π 1/2     where:   n is the number of electrons transferred;   A is the real electrochemical surface area of the WE (cm 2 );   F is Faraday's constant (96,485 coulombs/mole);   C is the concentration of the mediator (moll/cm 3 ); and   D is a diffusion coefficient of the mediator (cm 2 /sec).