Solvent-Induced Luminescence Variation of Upconversion Nanoparticles

Abstract

Solvent plays a vital role in the syntheses, purifications, and broad applications of upconversion nanoparticles (UCNPs). In this work, the effect of various dispersive solvents, including single solvents and mixed solvents, on the luminescence properties of NaYF:Yb, Er UCNPs was studied systematically. The differences in both upconversion luminescence (UCL) intensities and color outputs of the nanoparticles were observed when dispersing the UCNPs in deuterium oxide, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, or water. The attenuation of the excitation and emission light of the UCNPs caused by absorption of the solvents, as well as the high-frequency vibrational groups of the solvents, such as -OH, -CH, and -CH groups, are responsible for the decrease in UCL intensities and increase in the red to green emission intensity ratios (RGR). The changes in water or OH ion contents of ethanol/water mixed solvent triggered similar changes in UCL properties. Interestingly, the quenching of the solvents for the UCL cannot be fully eliminated by changing the dispersive solvents once the UCNPs have touched the solvents containing high-frequency vibrational groups. Our work will facilitate the comprehension of the solvent induced luminescence variations of the nanoparticles and provide guidance for their applications.

Amorphous-to-crystalline transition and photoluminescence switching in guest-absorbing metal–organic network thin films

“…In the case of the water-containing (humidity-treated) film, this could be explained by non-radiative relaxation processes due to highfrequency vibrations of the -OH groups. 38 However, since the removal of the crystalline water did not increase the luminescence intensity, but rather decreased it, it seems that the highfrequency vibrations of the -OH groups are not at least the main reason for the observed luminescence quenching. A more probable explanation is the decrease in film density upon the crystallization, which increases the inter-ionic distances and could thereby make the energy-transfer processes less efficient.…”

Upconversion nanoparticles for sensing pH

“…12 Upon excitation with a 980 nm laser, two luminescence bands in the green (~540 nm) and red (~650 nm) region are emitted. 13 The synthesis of these UCNPs is quite versatile: the size can be tuned down to 5 nm 14 and the surface can be modified to target certain types of cells. 15,16 Recently, studies have begun to explore the use of UCNPs for measurements of pH.…”

Optically Robust and Biocompatible Mechanosensitive Upconverting Nanoparticles

“…In general, LS nanoparticles dispersed in aqueous media have redder emission than AS nanoparticles in cyclohexane due to solvent interactions at the surface. 46,47,51,56 More specifically, OH stretching modes around 3500 cm –1 facilitate nonradiative transitions in Er 3+ that give rise to more probable pathways of populating the radiative red state. 46,47,54 In the SI, we compare the energetics of upconversion with and without OH bonds, based on models reported in the previous literature.…”

“…However, decreased intensity and changes in emission color have been reported for particles suspended in water compared to organic solvents (e.g., ethanol, dimethylformamide, cyclohexane). 46,47 Further, groups have shown evidence of fluoride leaching from the nanoparticle host (NaYF 4 ), resulting in complete emission quenching and the eventual disintegration of particles in water. 48,49 Such changes will convolute the optical signal (i.e., intensity and/or color) intended for detecting mechanical forces.…”