Quenching of the upconversion luminescence of NaYF₄:Yb³⁺,Er³⁺and NaYF₄:Yb³⁺,Tm³⁺nanophosphors by water: the role of the sensitizer Yb³⁺in non-radiative relaxation

Abstract

We have studied the mechanisms of water-based quenching of the upconversion photoluminescence of upconverting nanophosphors (UCNPs) via luminescence decay measurements for a better understanding of the non-radiative deactivation pathways responsible for the relatively low upconversion luminescence efficiency in aqueous solutions. This included both upconversion luminescence measurements and the direct excitation of emissive energy states of Er(3+) and Yb(3+) dopants in NaYF4:Yb(3+),Er(3+) UCNPs by measuring the decays at 550 and 655 nm upon 380 nm excitation and at 980 nm upon 930 nm excitation, respectively. The luminescence intensities and decays were measured from both bare and silanized NaYF4:Yb(3+),Er(3+) and NaYF4:Yb(3+),Tm(3+) UCNPs in H2O and D2O. The measurements revealed up to 99.9% quenching of the upconversion photoluminescence intensity of both Er(3+) and Tm(3+) doped bare nanophosphors by water. Instead of the multiphonon relaxation of excited energy levels of the activators, the main mechanism of quenching was found to be the multiphonon deactivation of the Yb(3+) sensitizer ion caused by OH-vibrations on the surface of the nanophosphor. Due to the nonlinear nature of upconversion, the quenching of Yb(3+) has a higher order effect on the upconversion emission intensity with the efficient Yb-Yb energy migration in the ∼35 nm nanocrystals making the whole nanophosphor volume susceptible to surface quenching effects. The study underlines the need of efficient surface passivation for the use of UCNPs as labels in bioanalytical applications performed in aqueous solutions.

Revisiting the classification of NIR-absorbing/emitting nanomaterials for in vivo bioapplications

“…163 This is especially true for NPs where both the sensitizer and activator ions are susceptible to surface quenching. 164 Fortunately, lanthanide materials also have valuable Stokes emissions in the NIR spectral regions (Figures 12a-c), 165 which have been used for imaging in the NIR biological transmission window NIR, 166,167 in intentional light-to-heat and heat-totemperature [168][169][170][171] readout features in single (Figure 12d) or hybrid NP-based 'devices'. There are numerous comprehensive review articles of the physics, spectroscopy and biomedical applications or nanotoxicity of lanthanide-doped NPs, and thus the readers are referred to these articles for further information.…”

Quenching Pathways in NaYF4:Er3+,Yb3+ Upconversion Nanocrystals

“…Meanwhile, the efficiency is limited by undesired competing processes such as cross-relaxation, back-transfer, nonradiative decay, and, in the case of NCs, quenching by energy transfer to high-energy vibrations on the surface and/or in the environment of the NCs. 10,11,16,18,22,30 To understand UC luminescence from NCs and minimize quenching requires detailed and quantitative studies that unravel the competition between the various processes at play.…”

Phase angle encoded upconversion luminescent nanocrystals for multiplexing applications

“…In view of this, we developed a simple method to generate phaseangle encoded UCNPs while maintaining high luminescence intensity, i.e., nanoparticle size adjustment regulated by opticallyinert-ion doping. This method takes advantages of the inherent surface quenching effect for UCNPs, i.e., that the lifetimes of involved states of both the sensitizers and activators are susceptible to environments 43,53,54 . (Fig.…”