Improving temperature resolution of luminescent nanothermometers working in the near-infrared range using non-thermally coupled levels of Yb³⁺ & Tm³⁺

Authors:

• Natalia Stopikowska,
• Marcin Runowski,
• Przemysław Woźny,
• Szymon Goderski,
• Stefan Lis

Abstract

Up-converting nanothermometer YF3: 20% Yb3+, 0.5% Tm3+ was obtained via hydrothermal synthesis and a subsequent post-synthesis thermal treatment. The nanomaterial formed a stable colloid and showed (under 975 nm excitation) blue upconversion luminescence visible to the naked eye. The emission spectrum consisted of five thulium ion bands (450–870 nm) and one ytterbium ion band (above ~900 nm). The determined band intensity ratios of the non-thermally coupled levels (non-TCLs), i.e. emission bands of Tm3+ and Yb3+ observed in the visible − NIR ranges, monotonously change with temperature. Using linear fits, the determined band intensity ratios were correlated with temperature, and the relative temperature sensitivities and temperature resolutions were calculated for four of them, associated with the non-TCLs of Yb3+ and Tm3+. Utilizing the 2F5/2 → 2F7/2/3H4 → 3H6 band intensity ratio of Yb3+/Tm3+ (940/800 nm), located within the NIR spectral range of the first biological window, and exhibiting very high signal intensity, the developed nanothermometer has a decent relative sensitivity (Sr ≈ 1% K−1) and excellent temperature resolution (δT ≈ 0.2 K) at 313 K. The obtained material can be used to construct new nanothermometers, useful both in biological experiments and in industrial research.