1 Department of Chemistry and Biochemistry, Centre for NanoScience Research, Concordia University 7141 Sherbrooke Street West Montreal QC H4B 1R6 Canada john.capobianco@concordia.ca.

Dye sensitization is a promising route to enhance luminescence from lanthanide-doped upconverting nanoparticles (LnUCNPs) by improving the photon harvesting capability of LnUCNPs through the use of dye molecules, characterized by higher absorption coefficients. The literature does not fully address the poor photostability of NIR dyes, hindering solution-based applications. The improvements achieved by dye-sensitized LnUCNPs are usually obtained by comparison with non-dye sensitized LnUCNPs. This comparison results in exciting the LnUCNPs at different wavelengths with respect to the dye-sensitized LnUCNPs or at the same wavelengths, where, however, the non-dye sensitized LnUCNPs do not absorb. Both these comparisons are hardly conclusive for a quantification of the improvements achieved by dye-sensitization. Both shortcomings were addressed by studying the photodegradation via thorough spectroscopic evaluations of a 4-nitrothiophenol-modified and unmodified IR820-LnUCNP system. The modified IR820 dye system exhibits a 200% enhancement in the emission of NaGdF₄:Er³⁺,Yb³⁺/NaGdF₄:Yb³⁺ nanoparticles relative to the unmodified IR820-sensitized LnUCNPs and emits for over twice the duration, demonstrating a substantial improvement over previous dye-LnUCNP systems. Upconversion dynamics between the dyes and Er³⁺ establish the importance of back-transfer dynamics in modulating the dye-LnUCNP luminescence. Quantum yield measurements further illustrate the mechanism of sensitization and increased efficiency of this new nanosystem.