Deciphering Density Fluctuations in the Hydration Water of Brownian Nanoparticles via Upconversion Thermometry


We investigate the intricate relationship among temperature, pH, and Brownian velocity in a range of differently sized upconversion nanoparticles (UCNPs) dispersed in water. These UCNPs, acting as nanorulers, offer insights into assessing the relative proportion of high-density and low-density liquid in the surrounding hydration water. The study reveals a size-dependent reduction in the onset temperature of liquid-water fluctuations, indicating an augmented presence of high-density liquid domains at the nanoparticle surfaces. The observed upper-temperature threshold is consistent with a hypothetical phase diagram of water, validating the two-state model. Moreover, an increase in pH disrupts the organization of water molecules, similar to external pressure effects, allowing simulation of the effects of temperature and pressure on hydrogen bonding networks. The findings underscore the significance of the surface of suspended nanoparticles for understanding high- to low-density liquid fluctuations and water behavior at charged interfaces.


Fernando Eduardo Maturi

our authors


This work was partially developed within the scope of Project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 (DOI 10.54499/UIDB/50011/2020), UIDP/50011/2020 (DOI 10.54499/UIDP/50011/2020), and LA/P/0006/2020 (DOI 10.54499/LA/P/0006/2020), financed by national funds through the FCT/MCTES (PIDDAC). This work was also financially supported by the project Shape of Water (PTDC/NAN-PRO/3881/2020) through Portuguese funds. F.E.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under Marie Sklodowska-Curie Grant Agreement 823941 (FUNGLASS) and financial support from the FCT through Grant UI/BD/151445/2021. The authors thank Ricardo Longo (Federal University of Pernambuco), Christoph Rose-Petruck (Brown University), and Artemov Vasily (EPFL) for fruitful discussions.

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