Utilizing Energy Transfer in Mn²⁺/Ho³⁺/Yb³⁺ Tri-doped ZnAl₂O₄ Nanophosphors for Tunable Luminescence and Highly Sensitive Visual Cryogenic Thermometry

Abstract

Lanthanide (Ln 3+ )-doped upconversion (UC) phosphors converting near-infrared (NIR) light to visible light hold very high promise toward biomedical applications. The scientific findings on luminescent thermometers revealed their superiority for noninvasive thermal sensing. However, only few reports showcase their potential for applications in extreme conditions (temperatures below −70 °C) restricted by low thermal sensitivity. Here, we demonstrate the tailoring of luminescence properties via introducing Ho 3+ −Mn 2+ energy transfer (ET) routes with judicious codoping of Mn 2+ ions in ZnAl 2 O 4 /Ho 3+ ,Yb 3+ phosphor. Preferentially, a singular red UC emission is required to improve the bioimaging sensitivity and minimize tissue damage. We could attain UC emission with 94% red component by a twophoton UC process. Higher temperature annealing brings the color coordinates to the green domain, highlighting the potential for color-tunable luminescence switch. Moreover, this work investigates the thermometric properties of ZnAl 2 O 4 /Yb 3+ , Ho 3+ in the range of 80−300 K and influence of inducing extra ET pathways by Mn 2+ codoping. Interestingly, the luminescence intensities for nonthermally coupled ( 5 F 4 , 5 S 2 ) and the 5 F 5 radiative transitions of Ho 3+ ions display opposite behavior at 80 and 300 K, which revealed competition between temperature-sensitive decay pathways. The codoping of Mn 2+ ions is fruitful in causing a fourfold increase of absolute sensitivity. Notably, the color tunability from green through yellow to red is helpful in rough temperature estimation by naked eyes. The maximum relative (S r ) and absolute sensitivities (S a ) were estimated to be 1.89% K −1 (140 K) and 0.0734 K −1 (300 K), respectively. Even at 80 K, a S a of 0.00447 K −1 and S r of 0.6025% K −1 were achievable in our case, which are higher than most of the other Ln 3+ -based systems. The above-mentioned results demonstrate the potential of ZnAl 2 O 4 /Yb 3+ ,Ho 3+ for cryogenic optical thermometry and a strategy to design new Ln 3+ -based UC thermometers by taking advantage of ET routes.

Color tunable luminescence in ThO2:Er3+,Yb3+ nanocrystals: a promising new platform for upconversion

“…Upconversion nanophosphors (UCNPs) are regularly studied in the scientific community owing to their application in many areas encompassing health, energy and the environment. 1–7 Their applicability has been attributed to the conversion of low energy photons to high energy photons exhibiting anti Stoke’s shifts and displaying bright emission, lower autofluorescence, deeper penetration, negligible photobleaching, higher photo stability, etc. 5,8 The three basic requirements to achieve efficient upconversion luminescent materials are dopant, sensitizer and host matrix.…”

“…11 However, a fluoride based matrix is corrosive in nature and handling them in a normal laboratory set-up may be an issue. 2,12 So there is a huge demand within the scientific community working in the area of upconversion phosphors, to look for inorganic host matrices which stringently fulfil the important and necessary criteria: this includes that the host matrix should be thermally/structurally/chemically stable, should possess a wide band gap, easy to synthesize and exhibit low phonon frequency. In that context ThO 2 is considered an excellent luminescence host owing to its stable crystal structure, high chemical stability, single valence state of cation and low phonon frequency.…”

ZnAl2O4:Er3+ Upconversion Nanophosphor for SPECT Imaging and Luminescence Modulation via Defect Engineering

“…Another important prospect is that the design of UC nanomaterials for multimodal imaging and therapeutics needs to be advanced by exploiting the nanochemistry and rational choice of host and dopant ions. It is well known that the host material must have low phonon energy, a wide band gap, high chemical stability, low toxicity, and good photostability. − The selection of Ln 3+ ions (such as Er 3+ , Tm 3+ , and Ho 3+ ) is important to achieve efficient UC emission under NIR stimulation. These ions emit visible light due to their unique energy levels and 4f → 4f electronic transitions .…”