How can I acheive positively charged mesoporous silica nanoparticles (MSNs) with APTES?

10 October 2013 9 9K Report

I tried to use APTES to modify the surface of mesoporous silica nanoparticles. The solvent I used was ethanol and the reaction was kept under room temperature for 24 hours. I tried different amount of APTES to modify 40mg MSNs. The highest dosage of APTES was 400ul in 40ml absolute ethanol. I expected to introduce positive charge onto the surface. But after modification, the zeta potential measured by DLS was still negative (about -1.2 mV). I was really struggling with this modification.

Alexandre Mantion

... .... ... in what solvent do you measure?

APTES is neutral, only at low pH it starts to be charged. You have to quaternize the amine to get it pH insensitive...

Zhifeng Yi

Thanks for your answer. I measured the zeta potential in PBS (pH=7.4). Several papers have stated that they can obtain positively charged MSNs in PBS (pH=7.4). Because PBS is slightly basic, is it possible?

Majid Vafaeezadeh

I am agree with alex. I suggest to quaternize with acid. Trifluoromethanesulfonic acid is suitable. meanwhile, I am not sure about the temperature condition (r.t.).

Please see the following two articles:

J. Am. Chem. Soc. 2008, 130, 218-228.

Angew. Chem. Int. Ed. 2007, 46, 2879-2882.

Gary Dunderdale

Zhifeng, I modified silica particles using APTES in ethanol at room temperature also. Zeta potential measurements showed in increase in potential from about -50mV to -10mV at around pH 7. But as Alexandre has said, it is pH dependent.

At pH values above the isoelectric point (pH 4) the APTES functionalized particles are zwitterionic. The positive charge being from the protonated amine, and the negative charge deprotonation of silanol groups. But the net charge (as measured by zeta potentials) is negative. At pH values below the isoelectric point, the particles have a net positive charge - +36mV at pH 1. But unmodified silica particles also have a positive charge of +9mV at this pH value. Hope this helps.

Also what is the reason for the surface modification? If you want to introduce amine functional groups, it doesnt matter what the zeta potential is. Whereas if you want positively charged particles, functionalization with APTES will not give you a positive charge unless at low pH or quaternised using methyl iodide or something. I think functionalization with poly(allyl amine) is a much easier way to create positively charged silica particles.

Zhifeng Yi

Thank you guys. You are really nice. Your recommended papers are really helpful and give me a better understanding about the surface functionalization of nanoparticles.

I have actually modified the surface of MSNs with different concentration of APTES in ethanol. The zeta potential increases and tends to be zero when increasing the concentration of APTES. The highest one I got is -1.22 mV in PBS at r.t.. Raw MSNs have a zeta potential of -19. Positive charge is important for my experiments to absorb negatively charged molecules. So I will try several methods you guys suggested, such as conducting the experiments in Toluene under high temperature and poly(allylamine).

Ira Bhatnagar

I have modified the surface of my MSNps with APTES. I used 10mg MSNp in 100 microlitre of APTES with 2 mL of deionised water and stirred at 50 degrees for 12 hrs. The zeta pot. of amine functionalized MSNps was +7.5mV as compared to -15.8mV of unmodified MSNPs. Hope this helps..