I am currently working on measuring the zeta potential of nanosheets, but I've encountered a challenge related to the unknown refractive index (RI) of my samples. Since the exact RI of the nanosheets is not known, is it valid to use the RI of the diluent instead? How critical is the accuracy of the RI in influencing the measurements of zeta potential? Can using an inaccurate RI significantly affect the reliability of the results?
My samples are gel-based (2D nanosheets), and while I am getting valid zeta potential readings, my dynamic light scattering (DLS) measurements are presenting a problem. The system reports a high polydispersity index (PDI) of 1, which is considered to indicate invalid or poor quality DLS data. Can these DLS data still be considered valid?
I am hesitant to apply sonication or centrifugation to improve the homogeneity of the sample, I feel these procedures might damage the structure of the nanosheets. Are there alternative methods or adjustments I can make to ensure more reliable Z potential and DLS readings without compromising the integrity of the nanosheets?
You don't need to know the RI of the particles for zeta potential measurement. But you do need to know the RI of the liquid.
Salma Younes Thank you for your question. There are a couple of points to discuss:
- Zeta potential (ZP) is a holistic property of a system (particle and the properties of the liquid surrounding it, especially pH). You do not mention the measurement conditions which are essential when reporting a ZP
- ZP is not measured directly, but inferred from mobility (electrical or acoustic) measurements
- As ZP is inferred from mobility measurements then the RI of the particle is completely irrelevant as pointed out by John Francis Miller
- The RI of the surrounding medium is needed as it affects the refraction of the laser beam as it passes through the liquid suspension. This is easily measured with an Abbe refractometer and is likely to be close to 1.33 for water
- Sheets of material will move in an unpredictable manner in an electrical field. This mobility measurement is converted to a spherical particle moving at the same rate of motion. It may be that you have some flexing type movement of the particles in the electrical or acoustic field
- The high PDI indicates a very wide distribution with potential sedimentation. I think an orthogonal light scattering technique is needed here - I think that size measurement by laser diffraction (not DLS) is appropriate in this case. Here you will require the RI of the particle plus the RI of the continuous phase
- Centrifuging or otherwise 'preparing the sample' is changing it. Obvious...
Salma Younes One further point. The charges on the edges of your plates in comparison to the faces will be very different. There is some literature from the 1930's, I believe, dealing with this concept. This will, again, affect how such particles will move/migrate under an electrical or acoustic field.