From Upconversion Nanoparticles to Proteins: Probing Hydration-Water Density Fluctuations by Luminescence Thermometry

Area of Science:

• Physical Chemistry
• Materials Science
• Biophysics

Background:

• Water exhibits anomalous behavior attributed to fluctuations between low-density (LD) and high-density (HD) structural motifs.
• Understanding these fluctuations in hydration layers at interfaces is crucial for colloidal stability, biomolecular function, and reactivity.
• Conventional bulk techniques struggle to probe the microscopic organization of interfacial water.

Purpose of the Study:

• To establish luminescence nanothermometry as a versatile tool for investigating hydration layer density fluctuations.
• To explore how probe properties, specifically effective surface charge density, influence water structure.
• To investigate the role of nuclear quantum effects, such as isotopic substitution, on hydration water structure.

Main Methods:

• Utilized lanthanide-doped upconversion nanoparticles (UCNPs) and enhanced green fluorescent protein (EGFP) as luminescent probes.
• Monitored temperature-dependent optical and Brownian motion of probes to infer local water density.
• Systematically varied nanoparticle size, surface chemistry, pH, and isotopic composition (H2O vs. D2O).

Main Results:

• Identified a crossover temperature (Tc) where hydration water observables change behavior, linked to LD motif depletion.
• Demonstrated that Tc collapses onto a master curve when plotted against effective surface charge density, unifying diverse probe data.
• Showed that D2O shifts Tc upward and enhances protein stability, highlighting the role of hydrogen in quantum effects.

Conclusions:

• Luminescent nanoprobes are sensitive reporters of hydration water density fluctuations.
• Interfacial electrostatics and nuclear quantum effects significantly sculpt nanoscale water structure.
• This approach offers new avenues for studying protein stability and designing functional nanomaterials.