Modulating the red-to-green emission ratio in NaYF4:Yb3+,Er3+ upconverting nanoparticles by adjusting the reaction pH

The modulation of the red-to-green (R/G) emission ratio in NaYF₄:Yb³⁺,Er³⁺ upconverting nanoparticles (UCNPs) is essential for various applications, including deep-tissue optical imaging, photodynamic therapy, photonic nanosensors, solar cells, and plasmonic coupling. This study demonstrates the improvement of the R/G emission ratio of UCNPs by increasing the pH of the reaction medium, within the range from <2 to 11. In this solvothermal method, ethylene glycol (EG) and dithiodipropionic acid (DTDPA) were strategically employed as the solvent and diprotic ligand, respectively. The degree of DTDPA protonation, along with its esterification reaction with EG, directly influenced its interaction with lanthanides (Ln³⁺), affecting the R/G emission ratio, size distribution and other structural properties of the synthesized UCNPs. The intensification of red signals observed at higher pH levels in both NaY 0.80 F 4 :Yb 3+ 0.18 ,Er 3+ 0.02 and NaY 0.38 F 4 :Yb 3+ 0.60 ,Er 3+ 0.02 configurations is attributed to the incorporation of hydroxyl groups (-OH) and hydroxide ions (OH⁻) in the UCNPs structure. Those groups lead to non-radiative transitions 4 I 11/2 → 4 I 13/2 and 2 H 11/2 + 4 S 3/2 → 4 F 9/2 . These transitions occur due to the resonance between the vibrational energy of -OH and OH⁻ and the related energy gaps, thereby increasing the probability of populating the 4 F 9/2 energy level, from which the red emission originates. Indeed, the R/G emission ratio for NaY 0.38 F 4 :Yb 3+ 0.60 ,Er 3+ 0.02 samples increases from 2.99 to 7.80 for reactions performed at pH < 2 and pH 11, respectively. This research is significant for the design and synthesis of tunable upconversion nanomaterials and their multiple applications.