Copper-modified poly(3,4-ethylenedioxythiophene) layers for selective determination of dopamine in the presence of ascorbic acid: II Role of the characteristics of the metal deposit

The present study focuses on the specific role of the number, size, and shape of copper crystals electrodeposited on poly(3,4-ethylenedioxythiophene) (PEDOT) layers for the electroanalytic determination of dopamine (DA). It is shown that the type of the metal deposit is significantly influenced by the thickness of the polymer layer and the type of copper ions plating solutions, e.g., copper sulfate or copper oxalate. By combining electrochemical measurements and scanning electron microscopic investigations, it is found that the loss of electroactivity for DA oxidation, established for Cu deposited on thick PEDOT layers (from copper sulfate solution), relates to the instability of small needle-like Cu crystals decorating the PEDOT surface in this case. Large (several micrometer-sized) copper crystals (as deposited on thin PEDOT layers from copper sulfate solution) are found to suffer surface destruction along the crystalline edges when exposed to electrochemical treatment in the presence of DA. Finally, smaller (similar to 0.5 A mu m) bulky crystals with a large surface coverage (up to 30%), as deposited from copper oxalate solution, are found to provide a stable and highly sensitive Cu-PEDOT composite material operating in the nanomolar DA concentration range. Differential pulse voltammetry measurements carried out in the 6 nM to 2 A mu M DA concentration range, in the presence of large excess (1 to 5 mM) of ascorbic acid, show two regions of linear response with different sensitivities, 9 nA nM(-1) and 0.313 A mu A A mu M-1, respectively.