Sphurti Adigal Remember that zeta potential is a holistic property of the particle and the medium it resides in. Simply, in your case, that the value of the zeta potential at the slipping plane (where it is measured) is negative. The particles themselves could be positively charged and thus surrounded by a tightly bound negatively charged bound layer. Also, the pH could be at a point (basic/alkaline) where the surface of the particle has been modified (to negative) as well by charge neutralization.

Sphurti Adigal: The zeta potential measurements generally tell about the surface properties of gold particles and that can be conducted for the liquid solution containing nanoparticles as a suspension. A colloidal suspension is said to be stable if the zeta potential value attains any values except for the values between -30 mV to +30 mV (See the attached file AP023).

For gold NPs, the negative zeta potential value indicates the positively charged Au NPs. In addition, surfactant types(Anionic/cationic/nonionic) may increase or decrease the zeta potential values.

For more details, you may refer to the articles.

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Dear all, nothing is said on the preparation conditions and the medium where Au-NPs are. This crucial and the key to understand the surface chemistry/charge of NPs. Please have a look at the following document. My Regards

https://tinyurl.com/2s6n9svk

Thank you very much for all of your responses. I am able to follow your answers.

Thank you very much again for your effort and time.