Generally nano particles or QDS posses no charge at all but a distinct static charge has been arisen at the surface of those particles due to their colloidal behavior. Those charge are purely static and tends to localized on the surface of the nano particles or QDs.and hence it shows purely non faradaic behavior and eventually we will not get any peak for the same in CV,DPV or SWV. For this purpose HNO3 will not be an suitable electrolyte but 0.1M KCl in PBS pH 7.0 buffer would be an excellent choice. It is known that mobility ratio of K+ and Cl- is exactly same and hence it posses a good electrical conductivity. Using the same will eventually boost up those static charge to delocalized and hopefully you will get a peak. Nonetheless, a control experiment with only the 0.1M KCl in buffer will also be performed for clear resolution.

Dear Anirban, 

Both Au NPs and CdS QDs are prepared as colloidal solutions. Furthermore, I've examined HClO4 , HCl , H2SO4 and HNO3 as electrolyte. The QDs showed considerable peak at negative potentials in all these electrolytes. But Au NPs just showed a weak peak at positive potentials in presence HNO3 and acetate buffer. Although, I could not find any paper to check the location of peak or optimize the parameters of electrochemical analysis. I'll check your proposed electrolyte. Thank you.

Surface oxidation of gold takes place at positive potential (more 1 V) in aqueous solution. A couple of redox peaks for oxidation of gold to gold oxide and the reduction of gold oxide to gold can be seen in a cyclic voltammogram at the positive potential range. Peak appeared at negative potential may be  due to the reduction of dissolved oxygen in the solution used, if the solution is not properly de-aerated.  In this regard, You may find several refs by following the works of Prof. Takeo Ohsaka, Tokyo Institute of Technology, Japan.

In our previous studies, we used colloidal Au NPs and DPV mode. The reduction peak of Au NPs in HCl 1M was located between 0.2-0.4 V. Here, I think it's the effect of CdS QD that interfere with the reaction of Au NP and acidic solutions.

Dear Maryam Daneshpour, Electrochemical impedance spectroscopic (EIS) method, if available, for Au NPs[1], CdS[2] , might help you.

1a. http://www.sciencedirect.com/science/article/pii/S1572665712004213  [Electrochemical impedance spectroscopic measurement of potential of zero charge of octanethiolate-protected Au and Pd nanoparticles]

1b. http://www.sciencedirect.com/science/article/pii/S1572665714001337 [Potential of zero charge of glassy carbon at elevated temperatures]

2. https://www.erpublication.org/admin/vol_issue1/upload%20Image/IJETR041812.pdf  [The electrochemical property of the CdS/CdSe co-sensitized ...]

Dear Dr Samaras

Thank you very much for these valuable propositions, however, unfortunately I have to use DPV, SWV or LSV for this purpose.