Hey Jahowa.

Your message is a bit confused, so it would be difficult to answer precisely. So, you make a mixture of copper nanoparticles and dopamine and afterwards you drop the mixture on the glassy carbon electrode. Did you dry your mixture or not? Are you using another electrolyte or the dopamine acting both as a reacting agent and as a electrolyte?

Decreasing of current density is usually caused by losses of the catalytic surface of nanoparticles. The reasons could be:

1) Not stable conditions for copper or wrong potential range --> lead to dissolution of Cu and therefore low current densities, Another possibility is that the Cu nanoparticles does not have well contact to GC-elektrode.

2) Depending on your potential and conditions your copper nanoparticles could grow and again you will observe lower current densities, You could measure TEM to check if that is true for your case.

3) Some of the organic or non-organic molecules could block the surface of the copper via adsorption and therefore with further reaction time you would observe smaller current densities. In your case i would possibly exclude this point cause usually you are "cleaning" the surface of the electrodes with cyclic voltammetry.

If you can explain you exact reaction conditions: potential range, electrolyte, current densities and so on, then it would be easier to identify your problem.

Best,

Stefan 

Hello Professor Stefan,

Thank you very much for your valuable time and opinion. I pardoned that I couldn’t explain properly.

In the beginning, I made copper nano particle from ascorbic acid. Then I mixed copper nanoparticles and dopamine in 1:1 ratio. Then, measured cv in Pbs buffer-7 but it didn’t work. In this case, when I dropped the mixture of cunp+dopamine on the surface of electrode, the mixture didn’t dry completely.

So, I did two trials by using nafion. In first trial, I dropped 5µL cuNp on glassy electrode surface and after that I dropped 10µL dopamine. When the surface of electrode was dried completely then I again dropped 5µL nafion on that. After two hours when the surface was completely dried that time I took single cycle(initial potential -0.5, vertex potential 0.8 and final potential -0.5, scan rate 0.05) in PBS buffer-7. Then I got two oxidized peak(one is for dopamine and another is for cunp) and two reduction peak(one is for dopamine and another is for cunp) as well. After that I again took multicycle cyclic volatammgram(15 cycle). Then I found, on each cycle both oxidized and reduced peak become decreasing. After 15 cycles, I again took 1 cycle that time I found dopamine’s (oxidized and reduced) peaks becomes so small and there were no oxidized&reduced peak of copper nano particles.

In second trial, I first dropped 5µL dopamine first then 10µL cunp on that. After that same procedure followed what I had done for first trial. In this case, when I took single cycle after 15 cycles that time I found both oxidized peak (one is for dopamine and another is for cunp) and both reduced peak (one is for dopamine and another is for cunp) were observable though it were very small compared to the initial single cycle (before taking 15 cycle).Here, after one day, I again check cv but still it was decreasing.

So it means my copper nanoparticle was not stable. So is there any way that I can stable my copper nano particle peak on glassy carbon electrode? or is there any way where I can get stable copper nanoparticale and dopamine peak together?

I will highly appreciate if you could share your experiences on this topic.

If possible please kindly provide relevant materials or web link

Hello Jahowa. First of all, i am not a professor yet :)

Thank you for the detailed explanation. I have still two questions. What is your reference electrode? or what are the potentials you referring to? to reversible hydrogen electrode, to Ag/AgCl electrode or to standard hydrogen electrode? That would be good to see, in which real potential range you treat your nanoparticles.

Second question, why i set your potential so high? If you see the oxidation of copper, then you should be aware, that you might dissolve it into your electrolyte, in this case in pbs-buffer. Before you plan the experiment you have to check which copper product (Cu2+, CuO or Cu(OH)2) you will have. Just google "Poirbaix Diagramm Cu" or go to the link "http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/021/27021617.pdf". This information will bring you to my first question of the potential you referring to.

In case you have Cu2+, the copper ions would definetely dissolve into buffer solution and you will observe smaller current. In case of Cu-oxides and Cu-hydroxides the should remain on your electrode.

For you it would be easier to reduce your potential until you see only one dopamine redox-wave. Copper should stay at one state: reduced or oxidized. If you current or peak remain unchanged during the cyclic voltammetry, then you found your problem: it will be the copper oxidation/reduction.

If the dopamine peak still shifts during the cyclic voltammetry, then the whole system is not perfectly stable. In this case i would set the potentials only in copper range (means that you check only the copper oxidation and reduction during cyclic voltammetry, may be without the dopamine) and see whether your cyclic voltammetries are reproducible and stable! Once you got the stable system, then you could introduce more parameters, such as including dopamine, widen the potential range and so on.

In my opinion it is good to dry your electrode, otherwise your Cu-NP and dopamine could easy drop away from the GC-electrode.

At pH 7 many things can happen, including dimerization, cyclization and ultimately polymerization of phenols like dopamine. Clearly the electrode surface composition is changing vs. time. This is quite normal for the circumstances.  If you would like to see very reproducible cyclic voltammograms of dopamine try a solution in 0.1M perchloric acid -- of course this is not relevant to most applications.   Coupling reactions are much less of a problem at lower pH. If you go to pH 8 or 9, everything will be much worse!

It is not clear WHY you are doing these experiments.  Stefan makes good points...