Ultrasensitive optical thermometer based on abnormal thermal quenching Stark transitions operating beyond 1500 nm

Abstract

Light with wavelength longer than 1500 nm has great potential to afford deep biotissue penetration due to its extremely weak photon scattering and undetectable autofluorescence in vivo. Here, in order to satisfy the requirements for thermometry during the tumor hyperthermia process, an ultrasensitive optical thermometer operating beyond 1500 nm is developed by employing the thermally coupled Stark sublevels of Er 3+ : 4 I 13/2 → 4 I 15/2 transition based on fluorescence intensity ratio (FIR) technology in Yb 3+ and Er 3+ codoped BaY 2 O 4 . Compared with the typical upconversion (UC) material β-NaYF 4 : Yb 3+ /Er 3+ and Y 2 O 3 : Yb 3+ /Er 3+ , BaY 2 O 4 : Yb 3+ /Er 3+shows more intense red Er 3+ : 4 F 9/2 → 4 I 15/2 transition and 1.5 μm near-infrared (NIR) Er 3+ : 4 I 13/2 → 4 I 15/2 transition induced by its larger phonon energy and higher quenching concentration of Er 3+ . An equivalent four-level model is proposed to investigate the temperature characteristics of the NIR emission, from which four Stark transitions are separated from the raw spectra, named α, β, γ, and δ respectively. Then, the NIR thermal sensing performance have been developed by utilizing the FIR of I β to I α and I γ to I α . More importantly, an ultra-high sensitivity for optical thermometry has been obtained through the combination of transition β and γ, especially in the physiological temperature region. Furthermore, the detection depth of NIR light in bio-tissues is assessed by an ex vivo test, demonstrating that the maximal detection depth of NIR emission can reach to 8 mm without any influence on optical thermometry. These findings indicate that Yb 3+ and Er 3+ codoped BaY 2 O 4 is a remarkable contender for optical thermometry in deep tissue with ultra-high sensitivity. K E Y W O R D S1.5 μm emission, energy transfer, Stark sublevel, temperature sensing, upconversion | 5785 XIANG et Al.

Achieving ultrasensitive temperature sensing through non-thermally coupled energy levels to overcome energy gap constraints

“…[6][7][8] Generally speaking, the thermometric sensitivity of TCLbased optical thermometers is proportional to the corresponding energy gaps (ΔE), which are confined within the range of 200 cm −1 -2000 cm −1 to avoid the occurrence of the decoupling effect. [9][10][11] This constraint of the ΔE value makes it difficult for the relative sensitivity (S R ) of TCL-based optical thermometers to exceed 2875/T 2 , which has seriously hindered their performance improvement. Fortunately, abundant excited states exist in rare earth ions and exhibit different temperature dependent behaviors, which enable temperature sensing based on the FIR between two non-thermally coupled energy levels (NTCLs).…”

“…Therefore, the excitation and emission wavelengths of the optical thermometers are preferably located in the biological windows (BWs, 650 nm-1700 nm) to minimize the absorption and scattering effect of the biological tissues. 9,18 Theoretically speaking, the Yb 3+ /Er 3+ / Nd 3+ tridoped upconversion (UC) luminescence system is a suitable candidate for satisfying these technical requirements. Under the sensitization of Yb 3+ ions, thermally quenched red UC emission originating from the Er 3+ : 4 F 9/2 → 4 I 15/2 transition around 660 nm and thermally enhanced near-infrared (NIR) light assigned to the Nd 3+ : 4 F 5/2 → 4 I 9/2 transition around 800 nm can be simultaneously obtained.…”

Upconversion luminescence color modulation and temperature sensing of Na0.5Bi2.5Nb2−xTaxO9:Er3+/Yb3+ phosphors

“…Er 3+ /Yb 3+ co‐doped upconversion luminescence (UL) phosphors will enable many applications from temperature sensing, 1–3 mapping temperature images, 4 photovoltaic solar cell, 5,6 theranostics, 7 anti‐counterfeit labels, 8,9 and upconversion lasing 10 due to the special properties, that is to say that low‐energy long electromagnetic wavelengths can be converted into high‐energy short wavelengths through multiphoton processes 11 . However, UL intensity, UL color modulation, and optical temperature sensing performances need to be further improved for the previous applications.…”