Complete protection of NIR-luminescent molecular rubies from oxygen quenching in air by L-arginine-mediated silica nanoparticles

The application of emerging luminophores such as near-infrared (NIR) emissive complexes based on earth-abundant chromium as central ion and triplet-triplet annihilation upconversion (TTA-UC) systems in air as optical reporters for bioimaging or photonic materials for energy conversion requires simple and efficient strategies for their complete protection from luminescence quenching by oxygen. Therefore, we explored the influence of sol–gel synthesis routes on the oxygen protection efficiency of the resulting core and core/shell silica nanoparticles (SiO₂ NPs), utilizing the molecular ruby-type luminophores CrPF₆ ([Cr(ddpd)₂](PF₆)₃; ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridin-2,6-diamine) and CrBF₄ ([Cr(ddpd)₂](BF₄)₃) with their oxygen-dependent, but polarity-, proticity-, viscosity-, and concentration-independent luminescence as optical probes for oxygen permeability. The sol–gel chemistry routes we assessed include the classical Stöber method and the underexplored L-arginine approach, which relies on the controlled hydrolysis of tetraethoxysilane (TEOS) in a biphasic cyclohexane/water system with the catalyst L-arginine. As demonstrated by luminescence measurements of air- and argon-saturated dispersions of CrPF₆- and CrBF₄-stained SiO₂ NPs of different size and particle architecture, utilizing the luminescence decay kinetics of argon-saturated solutions of CrPF₆ and CrBF₄ in acetonitrile (ACN) as benchmarks, only SiO₂ NPs or shells synthesized by the L-arginine approach provided complete oxygen protection of the Cr^III complexes under ambient conditions. We ascribe the different oxygen shielding efficiencies of the silica networks explored to differences in density and surface chemistry of the resulting nanomaterials and coatings, leading to different oxygen permeabilities. Our L-arginine based silica encapsulation strategy can open the door for the efficient usage of oxygen-sensitive luminophores and TTA-UC systems as optical reporters and spectral shifters in air in the future.