Dear all,
I've made some metal oxide-coated nanomaterials for biological applications and met several problems.
Well known commercial silanes were used in order to activate the surface of nanomaterials with -NH2 or -PEG. Then I dispersed them into D.W. water and other aqueous buffer solutions, each. (Thanks to the PEGylation, the nanoparticles were successfully dispersed in high conc. buffer solutions, right after the synthesis.)
It's been a month I made them, and now I figured out that the nanoparticles dispersed in D.W. waters are not stable in buffer solutions at all.
I doubt that the surface amine and PEG molecules were dettached in D.W. water, but were not in the PBS solution. Or regardless of the dispersion medium, every functional groups at surface were dissociated.
Thanks for read and please tell me your opinions and advices.
Sincerly.
Do you mean your silica suspension is not stable? Are you nanoparticles coming out of suspension? So I would suppose there may be two different routes for unstable suspensions. 1) As you suggested, the coating on your nanoparticles is coming off. This may be true, amino-silanes are relatively unstable. The amine portion on the silane aids in the hydrolysis of its own siloxane bond to the silica particle. As such, the amino coating over time comes off. 2) Your suspension may be somewhat unstable. If your nanoparticles stay in solution around a month then I would say it is a fairly stable suspension. To keep the suspension stable you may want to sonicate the fluid periodically.
Thanks for your kindness,
First of all, the word "not stable' means the particles aggregate in buffer solutions, but actually, they didn't right after the synthesis. The reason I adopted APTES was for further conjugations and PEG was for colloidal stability. Now seems the PEG has been dettached from the surface, thus I also doubt the stability of -NH2.
What is the size of the particles? In commercial silicas the larger the particle, more stable is the suspension.
I produced in the past silica using the Stober method (ammonia, ethanol and water). These particles were functionalized with amino propyl silane and where quite stable. You can see the procedure in: ”Effect of Silica-Supported Silver Nanoparticles on the Dihydrogen Yields from Irradiated Aqueous Solutions”.
The pH of the suspension will be ~10, I didn’t try other pHs. You can try reducing the pH using Ion exchange resin.
As mentioned above by many others, Si-O-M bonds may hydrolyze after being in water for a long time. I am not sure about the chemistry you used to bind PEG to NH2, was it perhaps PEG-COOH coupled via EDC to form amide? In any case, I suggest you wash your particles, collect them and do some analysis to double check whether the organics are present or not in them. I'd do elemental analysis (C,H,N), FTIR. If you suspect you have -NH2 the good old Kaiser test gives a quick result in matter of one or two minutes. If accessible you could also do zeta potential of a suspension or solid state NMR. Good luck!
Silane treatement is the most well established method for decorating particles whose surface have OH group(almost every oxide, nitride and metal also). During the treatment, particle and silane agent are covalently bonded. As you know the covalent bond is the strongest bonde in nature. I don't think bonds between particle and silane agent (Si-O)-Si-CH2-CH2-CH2-NH2) are broken or cleaved at all ( I assumed you might use the amonopropyltirethoxysilane, which is the most well-known amine-terminated silane agent.)
So, your unstability porblems might come from the other sides.
1.not enough silane coverage on Particle: Althought I dont know the exact coverage of your silane agent on particle surface, I don't think you have 100% of silane coverage. In that case particles can be agglomerated gradually. So, if you want the stable particles once again, you just need simple dispersion process with ultrasonic bath or probe type of ultrasonic once again. But the stability of your colloidal system is not changed at all. So, if you want 100% coverage of silane agent, put excessive amount of silane agent and wash out sevral times. Then, don't let them dry and slowly exchage the washing solvent with the solvent you finally want to use.
2. The ion adsorption on the functional groups: If you use the buffer solution such as simulated body fluid or artificial body plasma, your solution has lots of ion which can slowy adsorb on the your paritcle. The site for the ion adsorption can be the real silica surface whose silanol group is activated depnding on the local pH of solution or the functional groups of your silane agent, such as amine or carbonyl group which is also activated depending on the local pH of solution. These kinds of adsorption of ion can alter the surface charge of particles and induce the agglomeration of particles. (Check the zeta-potential of your fresh powder in fresh solution. then compare the result with thezeta pothential of particles in your unstable solution)
In this case, use fresh body fluid and do the mechanical dispersion process once again which can dissapate ions layer (different with electrical double layer in DLVO theory) on particles which might formed by the ion adsorption.
3. Check the size of your particle : If the size of your particle is big and the attractive force is enough to overcome the repulsive force formed by the silane agent (might be the combination of entropy and coulombic force) the particle will be agglomerated.
4. Check the solution pH. The charge of functiaonl group is altered gradually depending on the pH of solution. You solution should have it's own shelf life. So,if solution is changed by some reson, the pH can be changed, leading to the agglomeration of particles.
I am only experiencing the problem now. Can someone provide some literature on this hydrolytic cleavage? thank you
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