Volumetric Temperature Mapping Using Light-Sheet Microscopy and Upconversion Fluorescence from Micro- and Nano-Rare Earth Composites

We present a combination of light-sheet excitation and two-dimensional fluorescence intensity ratio (FIR) measurements as a simple and promising technique for three-dimensional temperature mapping. The feasibility of this approach is demonstrated with samples fabricated with sodium yttrium fluoride nanoparticles co-doped with rare-earth ytterbium and erbium ions (NaYF4:Yb3+/Er3+) incorporated into polydimethylsiloxane (PDMS) as a host material. In addition, we also evaluate the technique using lipid-coated NaYF4:Yb3+/Er3+ nanoparticles immersed in agar. The composite materials show upconverted (UC) fluorescence bands when excited by a 980 nm near-infrared laser light-sheet. Using a single CMOS camera and a pair of interferometric optical filters to specifically image the two thermally-coupled bands (at 525 and 550 nm), the two-dimensional FIR and, hence, the temperature map can be readily obtained. The proposed method can take optically sectioned (confocal-like) images with good optical resolution over relatively large samples (up to the millimetric scale) for further 3D temperature reconstruction.

Impact of crystal structure on optical properties and temperature sensing behavior of NaYF4:Yb3+/Er3+ nanoparticles

We report a comprehensive study of the structural, morphological, and optical properties, and UC-based ratiometric temperature sensing behavior of (α) cubic and (β) hexagonal phases of NaYF₄:Yb³⁺/Er³⁺ nanoparticles. The α-NaYF₄:Yb³⁺/Er³⁺ and β-NaYF₄:Yb³⁺/Er³⁺ nanoparticles were synthesized using co-precipitation and hydrothermal methods, respectively. Powder X-ray diffraction studies confirmed the phase purity of the samples. The morphological studies show uniform particle sizes of both phases; the average particle size of α-NaYF₄:Yb³⁺/Er³⁺ and β-NaYF₄:Yb³⁺/Er³⁺ was 9.2 nm and 29 nm, respectively. The Raman spectra reveal five sharp peaks at 253 cm^-1, 307 cm^-1, 359 cm^-1, 485 cm^-1, and 628 cm^-1 for β-NaYF₄:Yb³⁺/Er³⁺, whereas α-NaYF₄:Yb³⁺/Er³⁺ shows two broad peaks centred at 272 cm^-1 and 721 cm^-1. The optical property measurements show that α- and β-NaYF₄:Yb³⁺/Er³⁺ phases have distinct upconversion emission and temperature sensing behavior. The upconversion emission measurements show that β-NaYF₄:Yb³⁺/Er³⁺ has higher overall emission intensities and green/red emission intensity ratio. The temperature-dependent upconversion emission measurements show that α-NaYF₄:Yb³⁺/Er³⁺ has higher energy separation between ²H_11/2 and ⁴S_3/2 energy states. The temperature sensing performed utilizing these thermally coupled energy levels shows a maximum sensitivity of 0.0069 K^-1 at 543 K and 0.016 K^-1 at 422 K for β-NaYF₄:Yb³⁺/Er³⁺ and α-NaYF₄:Yb³⁺/Er³⁺, respectively.

Upconversion nanoparticle–Ag@C@Ag composite films for rapid temperature sensing

The response rate of optical temperature sensing of upconversion nanoparticles is significantly improved by coupling with Ag@C@Ag nanoparticles which have excellent thermal conductivity.

Fabrication and characterization of up-converting β-NaYF4:Er3+,Yb3+@NaYF4 core-shell nanoparticles for temperature sensing applications

This paper presents the use of soft template method to synthesize core and core-shell up-converting nanoparticles usefull for temperature sensing applications. Based on the stock solutions of core β-NaYF4:Er3+,Yb3+ nanoparticles and involving soft template method without any additional process of surface functionalization, it is possible to directly design the core-shell β-NaYF4:Er3+,Yb3+@NaYF4 nanoparticles, which can be perfectly dispersed in cyclohexane and surfactants like oleic acid (OA), triethanolamine (TEA) or Cetyltrimethylammonium bromide (CTAB). The morphological, crystalline and elemental characteristics of samples were investigated by Field Emission Scanning Electron Microscopy, X-Ray Diffraction, High Resolution Transmission Electron Microscopy, Selected Area Electron Diffraction patterns and Energy-Dispersive X-Ray Spectroscopy (EDX) measurements. The results showed that the synthesized NaYF4:Er3+,Yb3+@NaYF4 core-shell nanoparticles have roughly spherical shape, pure hexagonal β phase with core size of about 35 ± 5 nm and shell thickness of about 40 ± 5 nm. It has been shown that the coating of the β-NaYF4:Er3+,Yb3+ core with NaYF4 shell layer enables to enhance the green upconversion (UC) emission intensities in respect to red one. Under 976 nm excitation, the synthesized β-NaYF4:2%Er3+,19%Yb3+@NaYF4 core-shell nanoparticles revealed three strong emission bands at 520 nm, 545 nm and 650 nm corresponding to 2H11/2, 4S3/2 and 4F9/2 to 4I15/2 transitions of Er3+ ions with the lifetimes of 215, 193 and 474 µs, respectively. The calculated CIE chromaticity coordinates proved that the emission colour of core-shell nanoparticles was changed from red into yellowish green upon increasing the power density of the 976 nm laser from 0.73 to 9.95 W/cm2. The calculated slopes indicated that in the β-NaYF4:2%Er3+,19%Yb3+@NaYF4 core-shell nanoparticles, two-photon and three-photon UC processes took place simultaneously. Although the former one is similar as in the case of β-NaYF4:Er3+,Yb3+ bare core nanoparticles, the latter one, three-photon UC process for green emission occurs, due to cross relaxation processes of two Er3+ ions only within nanoparticles with core-shell architecture. Moreover, the energy difference between the 2H11/2 and 4S3/2 levels and associated constant of NaYF4@NaYF4 host lattice were determined and they reached ~ 813 cm-1 and 14.27 (r2 = 0.998), respectively. In order to investigate the suitability of nanoparticles for optical temperature sensing, the emission spectra were measured in a wide temperature range from 158 to 298 K. An exceptionally high value of relative sensitivity was obtained at 158 K and it amounted to 4.25% K-1. Further temperature increase resulted in gradual decrease of relative sensitivity, however, it maintained a high value > 1% K-1 in the entire analyzed temperature range.