Abstract
Ratiometric luminescence thermometry employing luminescence within the biological transparency windows provides high potential for biothermal imaging. Nd3+ is a promising candidate for that purpose due to its intense radiative transitions within biological windows (BWs) I and II and the simultaneous efficient excitability within BW I. This makes Nd3+ almost unique among all lanthanides. Typically, emission from the two 4F3/2 crystal field levels is used for thermometry but the small ~100 cm−1 energy separation limits the sensitivity. A higher sensitivity for physiological temperatures is possible using the luminescence intensity ratio (LIR) of the emissive transitions from the 4F5/2 and 4F3/2 excited spin-orbit levels. Herein, we demonstrate and discuss various pitfalls that can occur in Boltzmann thermometry if this particular LIR is used for physiological temperature sensing. Both microcrystalline, dilute (0.1%) Nd3+-doped LaPO4 and LaPO4: x% Nd3+ (x = 2, 5, 10, 25, 100) nanocrystals serve as an illustrative example. Besides structural and optical characterization of those luminescent thermometers, the impact and consequences of the Nd3+ concentration on their luminescence and performance as Boltzmann-based thermometers are analyzed. For low Nd3+ concentrations, Boltzmann equilibrium starts just around 300 K. At higher Nd3+ concentrations, cross-relaxation processes enhance the decay rates of the 4F3/2 and 4F5/2 levels making the decay faster than the equilibration rates between the levels. It is shown that the onset of the useful temperature sensing range shifts to higher temperatures, even above ~ 450 K for Nd concentrations over 5%. A microscopic explanation for pitfalls in Boltzmann thermometry with Nd3+ is finally given and guidelines for the usability of this lanthanide ion in the field of physiological temperature sensing are elaborated. Insight in competition between thermal coupling through non-radiative transitions and population decay through cross-relaxation of the 4F5/2 and 4F3/2 spin-orbit levels of Nd3+ makes it possible to tailor the thermometric performance of Nd3+ to enable physiological temperature sensing.
Highlights
Temperature is an important control parameter that governs, e.g., the rate of chemical reactions, and the optimum working efficiency of electronic devices or dynamics and viability of biological systems
Among the various temperature-dependent optical parameters that can be used for temperature calibration, the luminescence intensity ratio (LIR) of two emissive transitions from thermally coupled excited states has emerged as an especially robust and yet measurable representative to extract information about the local temperature of a medium in contact with luminescent nanocrystals
In order to investigate the feasibility of temperature sensing based on the temperature dependent emission intensity ratio of 4F3/2- and 4F5/2-related emission, both microcrystalline, dilute La0.999Nd0.001PO4 and nanocrystalline La1−xNdxPO4 (x = 0.02, 0.05, 0.10, 0.25, 1.00) were synthesized by means of a co-precipitation approach and structurally, morphologically and optically characterized
Summary
Temperature is an important control parameter that governs, e.g., the rate of chemical reactions, and the optimum working efficiency of electronic devices or dynamics and viability of biological systems. Non-invasive, sensitive and remote temperature measurement techniques with the capability to spatially resolve temperature variations down to the micrometer range are becoming increasingly relevant [1,2,3,4,5,6,7]. Luminescence nanothermometry is an appealing and rapidly emerging technique that meets those requirements and constantly improves [8,9,10,11,12,13,14]. Among the various temperature-dependent optical parameters that can be used for temperature calibration, the luminescence intensity ratio (LIR) of two emissive transitions from thermally coupled excited states has emerged as an especially robust and yet measurable representative to extract information about the local temperature of a medium in contact with luminescent nanocrystals. Other methodological approaches such as lifetime thermometry [15,16,17,18,19,20,21], or the concept of excited state absorption (ESA) [22,23,24] have been developed for that purpose while organic framework-based thermometers increasingly attract attention [25,26]
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