Tuning Water Density Fluctuations with Surface-Charged Colloidal Nanoparticles Probed by Luminescence

The anomalous properties of water are increasingly understood in terms of its structural fluctuations between high-density (HD) and low-density (LD) domains. However, the temperature at which these fluctuations vanish upon heating (the crossover temperature, <i>T</i>_c) and the fraction of LD domains under ambient conditions remain debated, particularly near interfaces. Here, we demonstrate that the surface charge density of colloidal upconverting nanoparticles (UCNPs) plays a key role in controlling <i>T</i>_c within the hydration layer, i.e., the water molecules forming the interfacial layer directly interacting with the nanoparticle surface. By combining the reanalysis of existing data with new experiments on compositionally similar UCNPs bearing different surface functionalizations, we demonstrate that <i>T</i>_c increases systematically with surface charge density. This trend holds regardless of particle size, surface chemistry, or pH. These results clarify previous conflicting findings and establish surface charge as a major factor governing the transition between LD and HD domains in hydration water. Beyond nanotechnology, these insights provide a framework for understanding how hydration water modulates biological processes, including protein stability and unfolding, where hydration shell dynamics play a determining role.