Hello,
I would like to perform zeta potential measurments for amine-modified latex beads L2778-1ML with a buffer whichs allows positive zeta potential values. Indeed, I already performed measurements with miliQ water and MES buffer, which gave me negatives values. I think this is due to too high pH of miliQ water and to the counter ions of MES buffer which interacts with the beads to decrease their zeta potential. Do you have any suggestion of buffer to obtain positive values ?
Thank you for you help
Short answer: do not use a buffer, use a simple acid such as HCl(aq).
Why?
Buffers typically comprise multiple ionic species and, for commercial prepared buffers, the exact composition may not be known. Some of these species may selectively adsorb on your particles and change the interfacial properties. Many common buffers may have salt concentrations sufficiently high to create problems with commercial electrophoretic light scattering instruments. You want as much control and knowledge of what you are adding to your sample. When wanting to investigate the effect of pH on the zeta potential/electrophoretic mobility of particles, by far the most common and best way is to simply add either a simple strong acid (HCl) or a simple strong base (KOH). Preferably, you should look at a range of pHs. But, if you just want to be sure the particles are positively charged, you could use water adjusted to pH 3 with HCl(aq).
Side note, some manufacturers have a terrible habit of using the term "buffer" to refer to any liquid, not just true buffers.
Since these particles are fluorescent, be aware of possible issues with instrument's detector picking up any fluorescence.
Arthur Boldron
The NIST primary material SRM 1980 is a positive electrophoretic mobility (+µE) standard at the stated pH. This will verify your equipment's performance. Otherwise there are many other materials that can be used to generate positive values at pH's below the isoelectric point. Anything where a zeta-pH curve is constructed and intercepts the zeta/mobility axis. As John Francis Miller states use HCl and NaOH to lter pH - it's easier and doesn't have the buffering issues (they're resistant to pH change for obvious reasons). In my old zeta-pH titration days I could construct a plot of 7 or 9 points in less than 5 minutes by manual titration.
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