Making Nd³⁺ a Sensitive Luminescent Thermometer for Physiological Temperatures-An Account of Pitfalls in Boltzmann...

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 ⁴F_3/2 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 ⁴F_5/2 and ⁴F_3/2 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 ⁴F_3/2 and ⁴F_5/2 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 ⁴F_5/2 and ⁴F_3/2 spin-orbit levels of Nd³⁺ makes it possible to tailor the thermometric performance of Nd³⁺ to enable physiological temperature sensing.