Using the modified electrode, you may measure the voltammogram for a standard redox couple, ferrocene or Fe2+/Fe3+ or ruthenium complex in a solution in which the diffusion coefficient and concentration are known. From the measured peak current you can estimate the value of active surface area.

Dear Md. Mominul Islam Sir,

Then The calculated surface belongs to the redox coupe we used, but not to the target molecule. Isn't it Sir?

And one more thing we need remember is, Mr Nadar has Nafion on his electrode, Hence the calculation of active surface area with -vely charged Fe couple would not be reliable as there will be greater repulsion between the -vely charged Fe and the Sulfate functionality present on the Nafion.

If he go for Ru couple that is also leads to the surface area with respect Ru only and there the story is reverse of Fe couple. 

 I see, it is a great question about nafion, since the surface of nafion is a superacid. I am not sure, but you may try with ferrocene couple. As we know, ferrocene is sparingly soluble is aqueous solution, so non-aqueous solution may be chosen. Otherwise  ferrocene carboxylic acid derivative may be chosen for aqueous solution. However this may not be suitable due to the effect of nafion with carboxylic group!

In my onion, the redox activity of ferrocene  would not be hampered by the activity of nafion present on the surface. However, it could be fundamentally interesting work to be carried out!!.