Essential Role of Methyl Groups in Ice Recrystallization Inhibition of Antifreeze Silver Nanoparticles Studied by Surface-Enhanced Raman Scattering

Controlling ice recrystallization is of fundamental importance in a variety of applications, including the preservation of food, cells, and tissue samples used in regenerative medicine, to prevent damage from freezing. Despite the high demand for ice recrystallization inhibition (IRI) materials, their rational design remains challenging due to a lack of understanding of the IRI mechanism at the molecular level. In this study, the antifreeze behavior of silver nanoparticles (AgNPs) modified with mercaptopyrimidine derivatives is studied using surface-enhanced Raman scattering (SERS). AgNPs exhibit IRI activity when modified with molecules that have outward-facing methyl groups. In contrast, pyrimidine derivative monomers do not exhibit IRI activity, suggesting that immobilization of the molecules on the AgNP surface is important. SERS spectra indicate that the interaction between outward-facing methyl groups and liquid water molecules is enhanced upon freezing. No direct binding between methyl groups and ice is confirmed. Thus, the IRI activity of antifreeze AgNPs can be reasonably understood by assuming the formation of a clathrate-like hydration shell in the vicinity of the methyl groups. Liquid water is stabilized by hydrophobic hydration, resulting in insufficient water available for ice crystal growth and higher IRI activity. This work provides fundamental insights into the design of IRI materials that have high potential in cell cryopreservation and other IRI applications.