i suggest iron oxide could be first coated with TEOS to obtain higher surface density of OH， beneftting that the more APTES will be grafted on material.

For iron oxide coating, it is better to use quantitative mixture of TEOS plus APTES to get amine functionalized IONPs.

Thanks for reply. Can you give any reference to TEOS coating + APTES coating is better than APTES coating alone.

Dear Viji N

Please go through the attached article. I think it contains a bundle of helpful knowledge that you need.

Best regards

https://pdfs.semanticscholar.org/1ea4/d022051d9f77d50c814f2d77b1d297db320b.pdf