Luminescence nanothermometry requires standardization for reliable and quantitative evaluation.

A new type of near infrared absorbing near infrared emitting (NANE) luminescent nanothermometer is presented, whose physical background relies on efficient and temperature dependent Nd³⁺ to Yb³⁺ energy transfer under 808 nm photo-excitation.

The chemical architecture of lanthanide doped core–shell up-converting nanoparticles can be engineered to purposely design the properties of luminescent nanomaterials, which are typically inaccessible to their homogeneous counterparts. Such an approach allowed to shift the up-conversion excitation wavelength from ∼980 to the more relevant ∼808 nm or enable Tb or Eu up-conversion emission, which was previously impossible to obtain or inefficient. Here, we address the issue of limited temperature sensitivity range of optical lanthanide based nano-thermometers. By covering Yb–Er co-doped core nanoparticles with the Yb–Nd co-doped shell, we have intentionally combined temperature dependent Er up-conversion together with temperature dependent Nd → Yb energy transfer, and thus have expanded the temperature response range ΔT of a single nanoparticle based optical nano-thermometer under single ∼808 nm wavelength photo-excitation from around ΔT = 150 K to over ΔT = 300 K (150–450 K). Such engineered nanocrystals are suitable for remote optical temperature measurements in technology and biotechnology at the sub-micron scale.

A new type of luminescent thermometer based on highly temperature dependent d–d Cr ³⁺ transitions related to barely temperature dependent f–f Nd ³⁺ transitions for bio-application is reported for the first time, showing exceptionally high sensitivity.

The current frontier in nanomaterials engineering is to intentionally design and fabricate heterogeneous nanoparticles with desirable morphology and composition, and to integrate multiple functionalities through highly controlled epitaxial growth. Here we show that heterogeneous doping of Nd³⁺ ions following a core–shell design already allows three optical functions, namely efficient (η > 72%) light-to-heat conversion, bright NIR emission, and sensitive (S_R > 0.1% K⁻¹) localized temperature quantification, to be built within a single ca. 25 nm nanoparticle. Importantly, all these optical functions operate within the transparent biological window of the NIR spectral region (λ_exc ∼ 800 nm, λ_emi ∼ 860 nm), in which light scattering and absorption by tissues and water are minimal. We find NaNdF₄ as a core is efficient in absorbing and converting 808 nm light to heat, while NaYF₄:1%Nd³⁺ as a shell is a temperature sensor based on the ratio-metric luminescence reading but an intermediate inert spacer shell, e.g. NaYF₄, is necessary to insulate the heat convertor and thermometer by preventing the possible Nd–Nd energy relaxation. Moreover, we notice that while temperature sensitivity and luminescence intensity are optically stable, increased excitation intensity to generate heat above room temperature may saturate the sensing capacity of temperature feedback. We therefore propose a dual beam photoexcitation scheme as a solution for possible light-induced hyperthermia treatment.