Upconverting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows: β-NaYF₄:Yb³⁺– Er³⁺@SiO₂ Temperature Sensor

Abstract

Upconverting core@shell type β-NaYF4:Yb3+–Er3+@SiO2 nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λex = 975 nm), i.e., upconversion luminescence of Er3+ and down-shifting emission of Yb3+, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range, respectively. The intensity ratios of the emission bands of Er3+ and Yb3+ observed in the vis–near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er3+ levels (2H11/2 → 4I15/2/4S3/2 → 4I15/2) and the nonthermally coupled Yb3+/Er3+ levels (2F5/2 → 2F7/2/4I9/2 → 4I15/2 or 4F9/2 → 4I15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and second-order polynomial fits for temperature-sensing purposes, i.e., Er3+ 525/545 nm (max S r = 1.31% K–1) and Yb3+/Er3+ 1010/810 nm (1.64% K–1) or 1010/660 nm (0.96% K–1). Additionally, a fresh chicken breast was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb3+/Er3+ bands, vs. the typically used Er3+ 525/545 nm band ratio, i.e., better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range vis–NIR optical, contactless sensors of temperature.

Optical Vacuum Sensor Based on Lanthanide Upconversion—Luminescence Thermometry as a Tool for Ultralow Pressure Sensing

“…A great number of optically active functional materials is based on the Ln 2+/3+ , because of their unique spectroscopic properties, such as multicolor photoluminescence induced by UV or near‐infrared (NIR) (energy up‐conversion) irradiation, narrow absorption/emission bands, large spectral shift of the emission bands in relation to the absorption ones, long emission lifetimes, etc . Matrices hosting Ln 3+ ions are usually fluorides, oxides, vanadates, phosphates, and borates . This is mainly because of their resistance to photobleaching and high temperature treatment, as well as relatively low phonon energy in contrast to organic compounds .…”

“…Matrices hosting Ln 3+ ions are usually fluorides, oxides, vanadates, phosphates, and borates . This is mainly because of their resistance to photobleaching and high temperature treatment, as well as relatively low phonon energy in contrast to organic compounds . Moreover, the Ln 3+ ‐doped inorganic materials may exhibit up‐conversion (UC) phenomena, i.e., anti‐Stokes emission of higher‐energy photons, generated by the absorption of two or more lower‐energy photons …”

“…In the case of pressure, the measured spectroscopic parameter is usually a pressure‐induced line shift, i.e., spectral shift of the emission bands of Cr 3+ (ruby) or Sm 2+ . Whereas, in the case of temperature the commonly measured parameter is the luminescence intensity ratio (LIR), i.e., band ratio of two thermally coupled levels (TCLs; separated by ≈200–2000 cm −1 ) of, e.g., Nd 3+ , Er 3+ , or Tm 3+ , which is directly related to the local temperature of the system (probe), and conforms Boltzmann distribution …”

Optical Nanoscale Thermometry: From Fundamental Mechanisms to Emerging Practical Applications

“…Rare-earth-doped nanoparticles are advantageous in this regard as their excitation can be spectrally optimized via doping with various metal ions in different matrices, making them effective nanosensor candidates for thermosensing applications in biological tissues. [213][214][215][216][217][218][219] A main drive for the development of in vivo thermometry techniques is the promise of selective and controlled heating, e.g., for hyperthermia applications. Current methods are magnetic-and optic-based heat treatments, which are often limited in resolution.…”

Lanthanide-doped mesoporous MCM-41 nanoparticles as a novel optical–magnetic multifunctional nanobioprobe

“…Therefore, silica-coated Ln 3+ -doped nanoparticles have successfully used in biomedical applications, such as biomarkers, biosensors, and in anti-cancer therapy. [26][27][28] Moreover, the silica coating may be also useful as drug delivery vehicle, e.g. for mesoporous-silica (m-SiO 2 ), active biological compounds, like anticancer drug or photosensitizers can be adsorbed, due to its unique porous ordered structure.…”