Dear all,
I prepared carbon quantum dots with surface functional groups of carboxylic acid, amine and alcohol. The pH of raw CQDs is 8.4 and while changing the pH to 1.5 also 13.0 with HCl and NaOH, the zeta potential also changed. The Zeta potential of CQDs at pH 1.5, 7.0, 13.0 is +0.34 mV, -23.4 mV, -12.2 mV respectively. From above results, the basic CQDs has more negative and acidic has nearly zero zeta potential value not more positive. My questions is, Shall I conclude all CQDs are not completely protonated? or Why zeta potential did not go for more positive, at-least +23.4 mV?
You need to look at the chemistry of your system and the fact that once monolayer coverage of H+ or OH- ions is achieved there may only be minor changes in zeta potential as the core gets shielded by more layers. I would worry more about untoward chemical compatibility issues in your system: at pH 1.5 stainless steel is probably attacked and at pH 13, glass is almost certainly slowly dissolved. How did you measure your zeta potentials? What materials were in contact with your suspensions at these pH's?
Dear Alan,
Thanks for reply and I measured Zeta potential value by Particulate Systems Nanoplus Zeta/nano particle analyser with Standard Zeta Potential Cell with Dome Electrode. Is this interfere with our material? then how can I measure zeta potential for my material. I have Low Conductivity Cell, Micro Volume Disposable Cell, High Concentration Cell also. which one is best for my case?
The zeta potential is very specific for each material type and the functional group / surface modification in addition to the type of buffer, salts, excipients, additives, and pH. Your 4 data points seem to indicate that the isoelectric point of your quantum dots is in the low pH range. Zeta potential is not necessarily "symmetric" in the sense that if it goes to -30mV on one side of the IEP then it does not mean that a condition must exist that it goes to +30mV on the other side of the IEP. The isoelectric point for CdS is likely somewhere between pH 2 to 4, so it is not surprising that your measured value is close to zero near the IEP.
http://www2.pitt.edu/~asher/homepage/colloid_pdf/Preparation%20and%20Properties%20of%20Tailored.pdf
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