At this wave numbers you will also see water on silica. I would try to look for the N-H and C-H bands (~3300 and ~2970 cm-1), but I guess they will also be difficult to see. If your surface area and loadings are low I really wouldn't expect to see anything by FTIR. Zeta potential is a lot better. Compare your pre and post treated material, potential should go from negative to positive at neutral pH. Another way is to do a chemical test: Kaiser or ninhydrin test (typically used in peptide synthesis to check for Fmoc deprotection): if you have amines your solid will turn from white to dark blue.

Hi Kim,

I haven't done APTES reaction on surface on my own. But this paper might be helpful. I know that it worked for them:

http://iopscience.iop.org/0957-4484/19/41/415605

You may want to have a look at their procedure and see if there is something different from your procedure. Hope that is helpful!

Shudipto

Hola Kim..

You have used always  highly basic conditions to coat silica nanoparticle with a shell silica.. Did you washed  the resulting materials,   After  the reaction of the silica, with APTES ,  the surface potential cannot be positively charged   excepted if the material have been   washed with a solution of HCl  5% 5mL  in mL EtOH..

Now you can passivate  the residual silanol groups of the resulting 3amino-propyl functionalized  material surface  by  washing it with a mixture of EtOH/water , then with  EtOH , then  with ethylether, then  after drying  at  60°C , dehydration under vacuum at 120°C ,  then reaction with Chlorotrimethylsilane...   then washing accordiing the previous protocole....without washing with HCl /EtOH ( the HCl formed during the trimethylsilylation  leading toi teh protonated amino groups ( ie Ammonium ones...)

As I remembered, the absorption peak at 1600 cm-1 represents the amide groups (CO-NH((1or2)), but the groups on APTES are amine groups (CH2-NH2). This is why you did not see any difference before and after functionalization. Like Igor mentioned, you may need to have a look at your FTIR spectra again to see if you can see the peak from CH2 at 2900 cm-1. The CH2 groups are introduced by the carbon chain from APTES. These peaks normally turns out as a group of 2 small peaks.

Zeta potential is more straightforward because your aim is to get the positively charged silica particles.

Another simple measurement is the TGA. From the weight loss between non-functionalized silica and amine functionalized silica, you can actually calculate the amount of amine groups grafting on the surface of silica particles. This is also the way to show the success of the modification.

After the measurement on zeta potential and TGA, you may find the problem. If the silica particles are not positively charged, then the grafting rate of APTES is low and the amount of amine groups calculated from TGA should be low. You may have problems with the synthesis process. If you get positively charged silica particles, then you need to find a correct way to characterize your samples and the results from TGA will tell you how many amine groups on the surface can achieve positively charged silica particles.

Hope this can help and wish you good luck.