Freeze-thaw stability of aluminum oxide nanoparticles

The use of inorganic nanoparticles (NPs) gains interest for pharmaceutical applications, e.g. as adjuvants or drug delivery vehicles. Colloidal stability of NPs in aqueous suspensions is a major development challenge. Both frozen and lyophilized liquids are alternative presentations to liquid dispersion. To improve the basic understanding, we investigated the freeze-thawing stability of model α-Al₂O₃ NPs. Freeze-thawing was conducted in three different buffer types at pH5 and 8 without and with additives to determine fundamental formulation principles. Before freeze-thawing, α-Al₂O₃ NPs could be stabilized in sodium citrate buffer at pH5 and 8, and in sodium or potassium phosphate at pH8. Particles revealed low zeta potential values in phosphate buffers at pH5 indicating insufficient electrostatic stabilization. After freeze-thawing, an increase in NP size was strongly reduced in potassium phosphate and sodium citrate buffers. Subsequent pH measurements upon freezing revealed a drastic acidic pH shift in sodium phosphate which was further demonstrated to destabilize NPs. The ionic stabilizers gelatin A/B, Na-CMC, and SDS, were suitable to improve colloidal stability in phosphate buffers at pH5 highlighting the importance of charge stabilization. Freeze-thawing stability was best in presence of gelatin A/B, followed by PVA, mannitol, or sucrose. Depletion and steric stabilization were insufficient using PEG and surfactants respectively. Thus, we could identify the fundamental formulation principles to preserve inorganic NPs upon freezing: i) sufficient charge stabilization, ii) a maintained pH during freezing, and iii) the addition of a suitable stabilizer, preferably gelatin, not necessarily surfactants. This forms the basis for future studies, e.g. on lyophilization.