Ghazaleh Mazaheri Tehrani

This question has been asked and answered before:

https://www.researchgate.net/post/Which-equation-should-I-choose-for-correct-value-of-zeta-potential-in-the-software-zetasizer

https://www.researchgate.net/post/What_Fka_approximation_does_the_Malvern_ZetaSizer_nano_ZS_use_for_calculating_zeta_potential_values_in_low_conductivity

https://www.researchgate.net/post/Why_do_posters_of_questions_on_RG_not_search_the_platform_first

You can use either (or any value between 1.0 and 1.5) but in your case, Smoluchowski is preferred. Always state what you use in any documentation together with details of pH, ionic concentration etc. 50% difference isn't a great deal in this game as you may discover!

Be careful with your dilution. DI water is normally the worst diluent for nanomaterials. You want to keep the material in the same ionic and surfactant/stabilizer make up as the original when you dilute. A background conductivity approximating to 0.001 M NaCl will be adequate to supress the double layer and allow conductivity in the liquid.

There's a good interpretative document by Lowry et al. Guidance to improve the scientific value of zeta-potential measurements in nanoEHS

December 2015 Environmental science. Nano. 3(5)DOI: 10.1039/C6EN00136J

Ghazaleh Mazaheri Tehrani Do you mean you will have DPPC particles and gold particles existing as two different species in a single sample? If so, none of the calculation options will be valid. Furthermore, the DPPC particles will scatter light more or less strongly as the gold particles, so, unless you know the relative intensities from each particle type, you won't be able to make sense of the measurements. This is especially true for the "monomodal mode" that the instrument has.

As far as choosing between Smoluchowski and Huckel, it's easy to estimate. Read the instrument manual's discussion on ka. In your case, Huckel would be more appropriate from the gold particles but the DPPC particles would more likely require an intermediate ka value.

Other factors include the fact that gold is electrically conducting and so violates the assumptions of the double layer theories that the estimation of zeta potential from electrophoretic mobility relies on.

Ghazaleh Mazaheri Tehrani

John Francis Miller makes an excellent point with which I concur. You are not actually measuring zeta potential but inferring it in a situation which is ill-posed. The instrument actually measures movement (electrophoretic mobility). Thus, to avoid any ambiguity, you can simply plot and use/state the electrophoretic mobility thus avoiding any 'conversion' to zeta potential.

Just as John Francis Miller pointed out, depending on the concentration of each particle I am wondering if your dls can even see both particles or if it sees only one of them. Did you already try a size measurement?

Dear Ghazaleh Mazaheri Tehrani

The question is thoroughly answered in this section of the attached paper:

2.5.3. Evaluation of surface charge

In addition to morphology and size, the other important parameter to be considered in the characterization of nanoliposomal formulations is their overall electrostatic behavior as indicated by the zeta potential (ZP). Currently, there are a number of techniques available for measuring the quantity of ZP. These methods are generally based on one of the three electrokinetic effects: (i) electrophoresis, (ii) electroos- mosis, and (iii) the streaming potential [70, 71]. In the electrophoresis method, ZP is determined by placing the particles in an electric field and measuring their mobility, using an appropriate microscopic technique. The electrophoretic mobility (yE) is then related to the z-potential at the interface of the nanovesicles employing the “Smoluchowski equation” [72, 73] as described below ... (see attached manuscript) ...

... However, if ZP is measured in a non-polar solvent, f(Ka) should be set to 1.0 (Huckel’s approximation) [77]. Measurement of the ZP of nanoliposomal formulations can also be carried out using the laser Doppler velocimetry technique (LDV), named after the Austrian physicist Christian Doppler [78, 79]. ...

... There are some non-invasive laboratory instruments available, which make it possible to perform particle size, PDI and ZP analysis on the same sample. Generally called size and zeta potential analyzer instrument, they include NanoPlus (Micro- meritics), Zetasizer Nano (Malvern Panalytical), SZ-100 Nanopartica (Horiba), and NanoBrook 90Plus (Brookhaven) to name a few. These advanced analytical in- struments are equipped with softwares utilizing “electrophoretic mobility”, “Smolu- chowski equation”, “brownian motion”, “laser diffraction” and “diffusion barrier” principles to measure size, PDI and ZP with high precision. ...

From:

M. Danaei, M. Kalantari, M. Raji, H. Samareh Fekri, R. Saber, G. P. Asnani, S. M. Mortazavi, M. R. Mozafari, B. Rasti, A. Taheriazam. Probing nanoliposomes using single particle analytical techniques: effect of excipients, solvents, phase transition and zeta potential. Heliyon 4 (2018) e01088. doi: 10.1016/j.heliyon.2018.e01088

In general, the factor f(ka) can be calculated based on the ratio of particle size to screening length. There is a calculator option in the Zetasizer software, see https://www.materials-talks.com/blog/2020/01/21/protein-charge-determination-how-to/

Concerning a check whether there is interaction between the gold and the liposomes you may find some ideas in https://www.materials-talks.com/blog/2020/08/20/can-size-or-zeta-confirm-binding/

BOC Sciences provides a variety of customized liposome solutions and is always ready to help you with all your questions in liposome research.

https://liposomes.bocsci.com/solution/zeta-potential-measurement.html