Making Nd(3+) a Sensitive Luminescent Thermometer for Physiological Temperatures—An Account of Pitfalls in Boltzmann Thermometry

Ratiometric luminescence thermometry employing luminescence within the biological transparency windows provides high potential for biothermal imaging. Nd(3+) 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 Nd(3+) almost unique among all lanthanides. Typically, emission from the two (4)F(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 (4)F(5/2) and (4)F(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%) Nd(3+)-doped LaPO(4) and LaPO(4): x% Nd(3+) (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 Nd(3+) concentration on their luminescence and performance as Boltzmann-based thermometers are analyzed. For low Nd(3+) concentrations, Boltzmann equilibrium starts just around 300 K. At higher Nd(3+) concentrations, cross-relaxation processes enhance the decay rates of the (4)F(3/2) and (4)F(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 Nd(3+) 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 (4)F(5/2) and (4)F(3/2) spin-orbit levels of Nd(3+) makes it possible to tailor the thermometric performance of Nd(3+) to enable physiological temperature sensing.