I am not too sure if this is appropriate for biological tests but I have seen people stabilize TiO2 with humic acid. Humic acid is present in natural water systems so it is probably not way off. See the paper referenced below:

Horst, Allison M., Ji. Z., Holden, P.A. 2012. Nanoparticle dispersion in environmentally relevant culture media: a TiO2 case study and considerations for a general approach. Journal of Nanoparticle Research 14(8), 1404.

Dear Adeyemi,

thanks for your answer. I'll check if I can apply this approach in my assays. 

in a stable nanoparticle solution you will lose your particles after they are dissolved.

Problem you should be aware of is that nanoparticles in biological fluid will be covered by a protein corona, which will determine their behaviour. But if you want to measure the biological effect then destabilization will be an important part of the real world behaviour.

Sonication after binding Tio2 to DMEM may works.