You are cooking the samples by contact with the electrodes.  Many organic materials, especially those is living systems, are designed to break down under voltage.  You can try reducing the voltage but the best route is the diffusion barrier method where the material does not contact the electrodes.  See:

Protein zeta potential measurements using Malvern’s new diffusion barrier method

https://www.brainshark.com/malvern/vu?pi=698451780&b=1&tx=120486&c1=22

and the attached application notes.  Note that surface charge and zeta potential are not the same thing.  There's been discussion on ResearchGate on this topic that should help you.

You are cooking the samples by contact with the electrodes.  Many organic materials, especially those is living systems, are designed to break down under voltage.  You can try reducing the voltage but the best route is the diffusion barrier method where the material does not contact the electrodes.  See:

Protein zeta potential measurements using Malvern’s new diffusion barrier method

https://www.brainshark.com/malvern/vu?pi=698451780&b=1&tx=120486&c1=22

and the attached application notes.  Note that surface charge and zeta potential are not the same thing.  There's been discussion on ResearchGate on this topic that should help you.

Surface and zeta potential:

What is surface zeta potential and how do I measure it?

https://www.brainshark.com/malvern/vu?pi=892751704&b=1&tx=120486&c1=22

Loss of sample isn't an issue for me. I want to ask if I use 154mM NaCl, the electrode in the cuvette won't turn black? If I decrease the voltage it would be okay? Another thing I had noticed was formation of bubbles when the samples incubated in serum were used for analysis and migration of the nanoparticles towards one electrode in the aggregated form. The instrument I had used was from Micromeritics not Malvern. This week I'm gonna use Malvern instrument for the analysis.

It isn't about low amounts of sample - the diffusion barrier is about preventing material accessing the electrodes while still maintaining conductivity.  Your electrode blackening and bubble formation is an indication that you have material decomposing on the electrodes.  Reducing the voltage only helps a little - Brownian motion is also increased in this scenario broadening the peak. 154 mM NaCl is very high.  If you can reduce this (ideally by a factor of 100) this would help more and reduce Joule heating.  With the barrier method (have you read the attachments?) then this is less of an issue as material cannot access the electrodes in a reasonable amount of time.

Yes, I've read those attachments. I've used different concentrations of NaCl to check the stability of the nanoparticles I've synthesized and by increasing the concentration of NaCl the size increased. I wanted to see the change in zeta potential readings of the samples upon changing the concentration of NaCl. One of the attachments says we can use the barrier method with concentration as high as 200mM by adjusting the voltage and number of runs. In your opinion, using the salt concentration of 200mM alone wouldn't damage the electrode?

The salt concentration doesn't damage the electrodes - it's the cooking of your sample that does.

Hi, I also tried to study electroosmotic velocity using NaCl salt concentration of 100 mM, and applied voltage of about 20 V, then I found my electrodes corroding. But when I used DI water, there was no corrosion at all. What is exactly happening? I used Alternating Voltage at high frequency.  I wanted to know the exact cause and method to prevent this. Some literature links would be highly appreciated. Thank you.

You need conductivity for a mobility measurement.  A measurement in DI water is meaningless.

Hello Sanam  Seems you have not read all the words and paragraphs that lie above your question.  When you have done this, you can delete your question and I will delete this post too.

I know this is an old question but science doesn't stop(!)

ANY evidence of electrode damage/bubbles/discoloration invalidates a measurement. Period. Unless you can somehow quantify and compensate for the electrochemical phenomena occurring (polarization, electrolysis) and Joule heating, you cannot get a meaningful result from ANY commercial instrument. Ignore what the vendors will tell you. You can get a number but you cannot have confidence in it.

Vendors could lessen this problem by stopping the use of gold electrodes (or palladium, too) and use platinum black (which has its downsides, though).

There is one non-commercial instrument (literally one) that can make measurements at 0.15M (in fact, up to 4M KCl). I developed it last year and is the first real step in ELS since I invented PALS in 1988.

I gave a presentation at the ACS National Meeting in Boston in 2018 that describes it.

http://enlightenscientific.com/2018/08/17/next-generation-els/

There's a very important message: all PALS measurements are meaningless unless you know for certain that you have a single mode of electrophoretic mobilities under the conditions you are measuring. Which is impossible to know at high salt concentration with commercial instruments. The new system measures a distribution using classic LDE and an average using PALS even beyond 1M. To do this, the electrode signal frequency has to be many 100s of Hz - far beyond commercial capability.

Hi John,

The diffusion barrier method as described by Alan above has been shown to give reliable zeta potential measurements at high salt concentrations, and whilst I share your concern over electrode damage, the data in the attached application note demonstrates that no electrode damage effects are observed when using this method.

I have looked at your ACS presentation with some interest. Do you have an example of data showing a multi-modal range of mobilities using your system?

Hi Alex,

Congratulations on your adaptive correlation patent. A nice piece of work indeed.

I humbly disagree with Alan (sorry, Alan). Blackening of gold electrodes occurs even in just electrolyte solution (very easy to demonstrate). Even if you prevent the particles from reaching the electrode, you will get electrode polarization and electrolysis. Polarization occurs at voltages less than that required to see it happening. There's a balance to make - you can raise the voltage to minimize the contribution of polarization to the strength of the field but you risk electrolysis. Or you can lower the voltage to minimize electrolysis but you increase the contribution of polarization. If you don't quantitatively measure either phenomena (my system does) then you can't get an accurate reading. You can get a reliable reading.

I'll need to dig through my data to find a multimodal result. I had a difficult time deliberately making one!

BTW, I'm surprised Anna and/or Ulf haven't spoken with you. I demonstrated an early version to them at Pittcon 2018. My progress with high salt was after we met.

John.

A couple of useful items regarding electrolysis (blackening) of gold electrodes in potassium chloride solution (no particles etc)

Approx. 10mM, ramping up to 10V at 10Hz across 4mm for 90sec. Brookhaven dip electrodes.