# Photothermal Heating and Real-Time In-situ Luminescent Thermometry with Iron Oxide Core-Silica Shell Nano-objects

Farah Abdel Sater, Gautier Félix, Saad Sene, Udara Bimendra Gunatilake, Basile Bouvet, Tristan Pelluau, Erwan Oliviero, Albano Neto Carneiro Neto, Luís Dias Carlos, Belén Albela, et al.

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Farah Abdel Sater, Gautier Félix, Saad Sene, Udara Bimendra Gunatilake, Basile Bouvet, et al. Photothermal Heating and Real-Time In-situ Luminescent Thermometry with Iron Oxide Core-Silica Shell Nano-objects. Small, 2025, 21 (49), pp.e08497. 10.1002/smll.202508497. hal-05458133

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Submitted on 14 Jan 2026

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# Photothermal Heating and Real-Time *In-situ* Luminescent Thermometry with Iron Oxide Core-Silica Shell Nano-objects

Farah Abdel Sater,[a] Gautier Félix,[a] Saad Sene,[a] Udara Bimendra Gunatilake,[a] Basile Bouvet,[a] Tristan Pelluau,[a] Erwan Oliviero,[a] Albano N. Carneiro Neto,[b,c] Luís D. Carlos,[b] Belén Albela,[d] Laurent Bonneviot,[d] Yannick Guari*[a] and Joulia Larionova*[a]

**Abstract:** In a world increasingly focused on precision and efficiency, achieving reliable nanoscale thermal feedback in nanoparticle-assisted heating presents significant challenges. It requires meticulous control over the morphology of nano-objects and the precise spatial arrangement of the heater and the local temperature probe, both of which are critical for accurate surface temperature readouts. In this work, we present real-time nanoscale temperature measurements during the nanoparticle-assisted photothermal heating by using new nano-objects consisting of single iron oxide nanoparticles encased in stellate-like silica shells, loaded with a luminescent coordination compound, namely [(Tb/Eu)₅(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)]. These multifunctional nano-objects act as efficient light-triggered nano-heaters and as ratiometric luminescent thermometers operating between 20 – 65 °C in water with excellent cyclability and good maximum relative thermal sensitivity of 0.75 ± 0.02 %·°C⁻¹ at 65.0 °C and thermal uncertainty of 1 °C. Real-time *in-situ* temperature monitoring *via* the Tb³⁺/Eu³⁺ luminescence intensity ratio during photothermal heating under 808 nm irradiation demonstrates reproducibility and reliable thermal feedback, highlighting its potential for advanced temperature-responsive applications.

## Introduction

Inorganic nano-objects capable of generating significant heat when remotely activated by external stimuli have garnered considerable attention over the past several decades. This increasing interest is attributed to their promising applications across various fields, including hyperthermia treatment of cancer[1-4] and bacteria-related diseases,[5-11] drug and gene delivery,[12-17] low-temperature catalysis,[18] regulation of enzymatic reactions,[19] and others. For this reason, numerous nano-heaters activated either by light irradiation (photothermal agents) or by the application of an alternating current magnetic field (magnetothermal agents), have been extensively reported.

A major challenge in nanoparticle-assisted heat generation is achieving precise temperature control, particularly at the nanoscale surface/interface. This is crucial not only for understanding nanoscale heat transfer but also for improving current applications and enabling new ones. However, most reported studies rely on macroscopic temperature measurements using conventional probes, which fail to capture nanoscale variations. Recent findings suggest that heat transfer at the nanoscale deviates from macroscopic laws, yet experimental limitations make it difficult to verify.[20] In this context, the concept of “hot nano-spots” or “thermal inhomogeneity” in nanoparticle-assisted heating has emerged.[14] This idea proposes that a significant temperature gradient may exist between nanoparticles activated by external stimuli, and their surrounding macroscopic environment.[18,21-24] While still debated,[25-27] this concept challenges conventional heat diffusion theories and enables breakthroughs, especially when bulk heating is ineffective.

This promise has boosted the development of various nanoscale temperature sensing methods able to work in combination with magnetothermal or photothermal heating.[28-33] Among them, the most embraced approach consists of employing photoluminescent thermometry based on the temperature-dependent emission of different luminescent species.[34-43] Early luminescent thermometry during

<footnote>[a] F. Abdel Sater, B. Bouvet, Dr. S. Sene, Dr. G. Félix, Dr. T. Pelluau, Dr. Erwan Oliviero, Dr. Y. Guari and Pr. J. Larionova ICGM, Univ. Montpellier, CNRS, ENSCM, Montpellier, France.</footnote>

<footnote>[b] Dr. A. N. Carneiro Neto, L. D. Carlos
Department of Physics and CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193, Aveiro, Portugal.</footnote>

<footnote>[c] Dr. A. N. Carneiro Neto
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany.</footnote>

<footnote>[d] Dr. Belén Albela, Pr. L. Bonneviot
Laboratoire de Chimie, ENS de Lyon, Université de Lyon, Lyon, France.</footnote>

<footnote>* E-mail: joulia.larionova@umontpellier.fr, yannick.guari@umontpellier.fr</footnote>
photothermal heating mainly used plasmonic gold heaters with electrostatically attached sensors,[44-46],[47-51] an approach that suffered from components leaching and limited control over sensor-heater positioning. The drawbacks have been partially addressed using lanthanide fluorides,[52-55] oxifluoride[56] or Ag₂S[41,57] nanoparticles, which enable temperature elevation under near-infrared (NIR) irradiation, while exhibiting self-monitored luminescence for temperature sensing. Despite good size and shape control, these nanostructures still suffer from low photothermal or photoluminescence efficiency, stability issues, broad emission, and potential photodegradation.

Alternatively, iron oxide nanoparticles (IONPs) have recently been reported as efficient photothermal agents under NIR irradiation, leveraging their absorption and significant light-to-heat conversion to generate macroscopic heat.[58-65] Moreover, IONPs offer size and shape tunability, easy surface functionalization, and biocompatibility.[2,66-68] Surprisingly, nanoscale heating around IONPs during photothermia remains largely unexplored, though one study reported IONP core temperature elevation relative to the macroscopic temperature, inferred from enhanced IONP degradation under light irradiation.[33] Note that IONP nano-systems developed for temperature monitoring during magnetothermia are not well suited for simultaneous photothermal heating and temperature sensing due to the inherent limitations of their components. First, some systems incorporate organic fluorophores,[22,25] or proteins,[23] which suffer from low photothermal stability and broad overlapping emission bands, limiting the reliability of luminescence intensity ratio (LIR)-based thermometry. Moreover, polymer shells enwrapping luminescent temperature probes, such as organic dyes[24,38,69] or Ln³⁺ complexes,[34,70] exhibit temperature-dependent softening, viscosity changes, and low thermal conductivity, which reduce the accuracy of temperature readouts. Beyond material limitations, achieving reliable temperature measurement demands precise morphological control of the nanostructure. This entails a well-defined, single heater core in close proximity to a uniformly distributed luminescent thermometer, while minimizing the risk of component leaching.

In this article, we report real-time in-situ temperature detection during a photothermal process using new well-defined nano-objects composed of a single IONP heater core enwrapped by a controlled stellate-like silica shell containing a lanthanide-based luminescent thermometer. In contrast to polymers, the silica shell provides a rigid, thermally stable, and optically transparent matrix, ensuring precise spatial arrangement of the luminescent thermometer and uniform heat distribution suitable for emissive temperature measurements during photothermal heating. Despite an effort in the synthesis of IONPs enwrapped by silica shells,[71-76] they have scarcely been employed for the design of heater/thermometer nanoparticles.[42,77] We selected in this work a stellate-like silica shell for its tunable large-pore structure enabling loading of bulky luminescent complexes, open centre-radial pore morphology allowing optimized diffusion/mass transport, good thermal shock resistance with minimal expansion, and optical transparency for light activation and thermometry.[75,78-81] We selected the [(Tb/Eu)₉(acac)₁₆(□₃-OH)₈(□₄-O)(□₄-OH)] (acac = acetylacetonate) complex as a ratiometric luminescent thermometer based on its excellent temperature-sensing performance after encapsulation.[82,83] Therefore, we coated the IONP core with functionalized stellate-like silica shell and loaded it with the luminescent [(Tb/Eu)₉(acac)₁₆(□₃-OH)₈(□₄-O)(□₄-OH)]·H₂O complex. The resulting nano-objects present a well-defined morphology with a single IONP core surrounded by a silica shell with a uniform complex distribution, functioning as efficient nano-heaters triggered by 808 nm irradiation. They exhibit temperature-dependent Tb³⁺ and Eu³⁺ luminescence, with an exceptionally high Tb³⁺→Eu³⁺ energy transfer rate in the order of 10¹¹ s⁻¹.[84-86] The best energy transfer-driven nanothermometer operates in the 20-65 °C range in water with good cyclability and maximum relative thermal sensitivity of 0.75 ± 0.02 %·°C⁻¹ at 65.0 °C and thermal uncertainty of 1 °C. The thermal feedback reliability from these nano-objects was confirmed by real-time temperature in-situ monitoring during the photothermal heating via the Tb³⁺/Eu³⁺ LIR.
# Results and Discussion

## Design of IONP core@SiO₂ shell nano-objects incorporated luminescent Tb³⁺/Eu³⁺ complex

The synthesis of multifunctional core@shell IONP@SiO₂ nanoparticles, loaded with a luminescent [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)] compound within the silica shell, was performed using a four-step approach (Scheme 1). It involves: (i) synthesizing the pristine IONP nanoparticles stabilized by oleate and oleylamine, (ii) growing a mesoporous silica shell functionalized with acetylacetonate on the surface of IONP, (iii) incorporating the luminescent compound into the shell's porosity, and (iv) sealing the pores.

The synthesis of pristine IONPs was performed through a thermal decomposition method, followed by an oxidation step to convert the majority of FeO into Fe₃O₄, as previously reported.[87] The functionalized mesoporous silica shell was grown on the surface of the IONP using an adapted sol-gel procedure.[39] The silica pores functionalization with acac was carried out *in situ* through a co-condensation reaction during the sol-gel reaction by adding an acac-siloxane derivative and TEOS. The acac functionalization of the silica pores is essential, as complex encapsulation is inefficient with a non-functionalized silica.

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Representation of the synthesis of multifunctional core@shell IONP@SiO₂-acac nanoparticles loaded with the luminescent [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)] complex.</figcaption></figure>

<table><caption>**Table 1.** Some characteristics of samples 1A, B – 3A, B.</caption><thead><tr><th></th><th>Composition*</th><th>d<sub>core</sub> (nm)</th><th>d<sub>whole</sub> (nm)</th><th>d<sub>shell</sub> (nm)</th></tr></thead><tbody><tr><td>1A</td><td>IONP<sub>0.109</sub>@(SiO<sub>2</sub>)<sub>1</sub>(CTA)<sub>0.0123</sub>(acac)<sub>0.0296</sub></td><td>25.6±0.8</td><td>103.7±4.7</td><td>38.9±2.4</td></tr><tr><td>1B</td><td>IONP<sub>0.123</sub>@(SiO<sub>2</sub>)<sub>1</sub>(CTA)<sub>0.0127</sub>(acac)<sub>0.0302</sub></td><td>25.8±0.9</td><td>93.1±4.6</td><td>33.6±2.4</td></tr><tr><td>2A</td><td>IONP<sub>0.109</sub>@(SiO<sub>2</sub>)<sub>1</sub>(CTA)<sub>0.0123</sub>(acac)<sub>0.0296</sub>/(Tb/Eu)<sub>0.0041</sub></td><td>25.7±0.8</td><td>103.1±1.8</td><td>38.6±1.1</td></tr><tr><td>2B</td><td>IONP<sub>0.123</sub>@(SiO<sub>2</sub>)<sub>1</sub>(CTA)<sub>0.0127</sub>(acac)<sub>0.0302</sub>/(Tb/Eu)<sub>0.0227</sub></td><td>25.5±0.9</td><td>93.8±3.7</td><td>33.9±1.9</td></tr><tr><td>3A</td><td>IONP<sub>0.109</sub>@(SiO<sub>2</sub>)<sub>0.963</sub>(CTA)<sub>0.0123</sub>(acac)<sub>0.0296</sub>/(Tb/Eu)<sub>0.0023</sub>@SiO<sub>0.037</sub></td><td>25.6±0.8</td><td>109.8±4.3</td><td>41.9±2.2</td></tr><tr><td>3B</td><td>IONP<sub>0.123</sub>@(SiO<sub>2</sub>)<sub>0.965</sub>(CTA)<sub>0.0127</sub>(acac-Si)<sub>0.0302</sub>/(Tb/Eu)<sub>0.0083</sub>@SiO<sub>0.035</sub></td><td>25.6±0.8</td><td>107.3±5.2</td><td>40.7±2.1</td></tr></tbody></table>

The pH variation during the silica shell formation allowed two stellate-like morphologies of IONP@SiO₂-acac nanoparticles with different pores opening, (1A) at pH = 7 and (1B) at pH = 5.6 (see ESI). The morphological change can be attributed to the competitive adsorption of the tosylate anion from the surfactant and silicate oligomers on micelles.[80,88] After nanoparticles' formation, the surfactant was removed through washings, which opened up the silica porosity and allowed access to the acetylacetonate groups (see Table 1 for main characteristics of 1A and 1B). Next, the [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)]·H₂O compound with a Tb/Eu ratio of 9/1 was inserted into the pores of the silica shells resulting in samples 2A and 2B. Finally, the pores were sealed using TEOS to

<footnote>* Calculated from ICP and EDS.</footnote>
trap the inserted complex inside them, preventing its leaching. The obtained samples were labelled as **3A** and **3B** for respectively less and more open stellate-like porosity of the silica shell. EDS (Fe, Si, Tb, Eu) and ICP (Fe, Tb) analyses were performed to ascertain the chemical formula of samples **1A**, **B** – **3A**, **B** (see Experimental Part (ESI) and **Table 1**). Samples **3A** and **3B** presented a loading capacity of 0.0023 and 0.0083 units of complex per SiO₂ unit, equivalent to 0.021 and 0.075 Tb/Eu units per unit of SiO₂, respectively. Note that the more open morphology of the silica shell in **3B** enhances complex insertion, increasing the loading capacity by more than 3.6 times. Note that a comparable loading was observed in our previous work.[82] Infrared (IR) spectroscopy confirms the successful grafting of the acac functions into the silica pores for samples **1A** and **1B**, as well as the [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)] complex loading for **3A** and **3B** (**Figure S1**, ESI).

Transmission Electron Microscopy (TEM) images reveal that IONP@SiO₂-acac nanoparticles **1A** and **1B** exhibit a core-shell morphology with a single spherical IONP core of 25.6±0.8 nm coated with a stellate-like silica shell (**Figure 1**). The spherical morphology and size of the IONP core are preserved following shell growth, as pristine IONP have the size of 25.9±0.9 nm (**Figure S2**, **S3**, ESI). Both silica shells possess open radial porosity with less or more pronounced openings observed for **1A** and **1B**, respectively. The overall nanoparticle sizes are 103.7±4.7 nm for **1A** and 93.1±4.6 nm for **1B** (**Figures 1 & S3**, ESI, **Table 1**). Therefore, the silica shell thicknesses, calculated as the difference between the overall size and the IONP core size, are equal to 41.9±2.2 nm and 40.7±2.1 nm for **1A** and **1B**, respectively (**Table 1**). Nitrogen sorption isotherms confirm the open porosity of the silica shell in **1A** and the successful pore clogging for **3B** (**Figure S4**, ESI). The obtained specific surfaces decrease from 418.22 m²/g to 71 m²/g for **1A** and **3B**, respectively.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** TEM images of: core@shell IONP@SiO₂-acac nanoparticles **1A** (a) and **1B** (b); IONP@SiO₂-acac/(Tb/Eu)₉ nanoparticles **2A** (c) and **2B** (d); IONP@SiO₂-acac/(Tb/Eu)₉@SiO₂ nanoparticles **3A** (e) and **3B** (f). Scale bars = 100 nm.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** STEM-bright field images (a) and (e) and STEM-EDS elemental mapping of IONP@SiO₂-acac/(Tb/Eu)₉@SiO₂ nanoparticles **3A** (top) and **3B** (bottom) with topochemical distribution of Fe (yellow) (b) and (f), Si (pink) (c).</figcaption></figure>

Incorporation of the [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)] complex into the silica shell porosity did not significantly alter the nanoparticle size (**Figures S1 & S3**, ESI, **Table 1**). On the other hand, silica pore clogging increased the overall nanoparticle size (109.8±4.3 and 107.3±5.2 nm for **3A** and **3B**, respectively), while IONP core size remained unchanged (**Figures S1 & S3**, ESI, **Table 1**). This fact, along with a slight modification of porosity, particularly in the more obstructed stellate morphology (**3A**), suggests successful pore clogging. Dynamic light scattering (DLS) measurements in water indicate that the nanoparticles are well dispersed and non-aggregated (**Figure S5**, ESI).

The IONP@SiO₂-acac/(Tb/Eu)₉@SiO₂ nanoparticles were characterized using HAADF-STEM with EDS mapping, which confirmed their core@shell morphology (**Figure 2**). The iron atoms from IONP are depicted in yellow, surrounded by a silicon shell from SiO₂ shown in pink. The terbium atoms (in green) are homogeneously distributed within the silica shell, confirming the successful encapsulation of the complex. Note that the europium atoms could not be clearly visualized due to their very low amount, a limitation previously reported by another group[70] and in our previous work.[82]

## Photothermal properties

While IONPs have long been recognised as effective magnetothermal heaters, their photothermal properties have only attracted significant attention over the past decade.[58–63,90–92] However, the mechanisms underlying their radiation-to-heat conversion are not yet fully understood.[62,93,94] It has been demonstrated that, unlike magnetothermia, photothermal heating primarily depends on laser power and solvent properties and is only minimally influenced by the size and shape of IONP.[95] While several studies have explored the photothermal properties of IONP core-silica shell nanoparticles with either dense[72,96] or dendritic silica shells,[39,75] comparison between them and uncoated IONPs with the same core remains rather controversial. The photothermal effect of nanoparticles **3A** and **3B** in water was investigated under 808 nm laser irradiation at a power of 2.58 W·cm⁻². A significant temperature increase dependent on nanoparticle concentration was observed for both samples after 10 min irradiation, while no noticeable heating was detected in water (ΔT = 0.1 °C) (**Figure 3a**, **Figure S6**, ESI). **Figure 3a** shows ΔT vs time measurements for colloidal solutions of sample **3A** at various concentrations from 2.8 to 27.5 mg mL⁻¹ of IONP (taken as Fe₃O₄) (or from 0.03 to 0.35 mol L⁻¹ of [Fe]), where ΔT represents the temperature difference between the nanoparticles’ solution and water. The ΔT value increases linearly with time during the first two minutes and then tends to saturation. The curve ΔTmax (reached after ten minutes) vs IONP concentration plotted in **Figure 3b** indicates that macroscopic heating varies linearly at low concentrations and then deviates from linearity for the higher ones due to the limited penetration depth of NIR light, affected by absorption and diffusion phenomena. The macroscopic photothermal properties of these nanoparticles are comparable to those of IONPs coated with gallic acid, which share the same IONP core but lack a silica shell, obtained under similar conditions (**Figure S7**, ESI). This result suggests that the stellate-like silica shell with open porosity does not significantly impede heat
transfer from the IONP core to the surrounding water. Moreover, the preservation of photothermal efficiency indicates that the shell is thermally permeable at the nanoscale, acting as a protective and functional scaffold without compromising the core's heating capability. Note also that the obtained heating for our **3A** and **3B** nanoparticles falls in the range of results previously obtained for IONP core stellate-like shell nanoparticles, although those studies involved different IONP composition and were conducted under different conditions (irradiation at 1064 with the laser power of 1 – 2 W cm⁻¹).[³⁹]

The photothermal conversion efficiency, η, was calculated by fitting the T vs time curves with the COMSOL software[⁹⁷] (see **Figure S6** for the fits and Section 2.4 of the SI for details). The obtained η value stands at 89±1 and 84±1 % for **3A** and **3B**, respectively. Although these values should be interpreted with caution since different estimation methods and experimental conditions can provide varying results,[⁹⁸] they are nonetheless among the highest reported for IONP-based systems.[⁶⁵]

<figure><img src="image_5.png" /><figcaption>**Figure 3.** a) Δ*T* vs time curves performed for aqueous colloidal solutions of **3A** with different concentrations of IONP (taken as Fe₃O₄) performed under irradiation at 808 nm (2.58 W cm⁻²) with Δ*T* representing the difference between the temperatures of colloidal solutions of **3A** and water; b) Maximal temperature achieved at 10 min as a function of IONP concentration for **3A**.</figcaption></figure>

## Photoluminescence and luminescence thermometry

While the [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)] complex is not water-soluble, its encapsulation within the silica shell of IONP@SiO₂-acac/(Tb/Eu)₉@SiO₂ nanoparticles enables the observation of luminescence in aqueous colloidal solutions. First, the luminescent properties of samples **3A** and **3B** were investigated in water at pH = 7.4 to confirm the successful incorporation of the complex within the silica shell's porosity. The photophysical behaviour and energy transfer dynamics of **3A** and **3B** in aqueous suspension in the 20–65 °C temperature range are demonstrated in **Figure 4** and **Figure S8**
<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) Room temperature excitation spectra of **3A** in water, monitored at λ<sub>em</sub> = 545 nm (green) and 615 nm (red); the asterisk denotes the Raman scattering peak of water). (b) Temperature-dependent emission spectra (20-65 °C) of **3A** in water under 315 nm excitation, showing quenching of the Tb<sup>3+</sup> <sup>5</sup>D<sub>4</sub>→<sup>7</sup>F<sub>J</sub> transitions. (c) CIE chromaticity coordinates illustrating the red shift of emission colour with increasing temperature; (d) Energy level diagram indicating forward and backward energy transfer; (e-h) Temperature dependence of the calculated energy transfer rates ⟨*W*⟩<sub>i→j</sub>: forward rates (i odd → j even) and backward rates (i even → j odd).</figcaption></figure>

(ESI). The excitation spectra monitoring the Tb<sup>3+</sup> <sup>5</sup>D<sub>4</sub>→<sup>7</sup>F<sub>5</sub> (green curve) and Eu<sup>3+</sup> <sup>5</sup>D<sub>0</sub>→<sup>7</sup>F<sub>2</sub> (red curve) show broad bands between 250 and 350 nm, attributed to the sensitisation of the Ln<sup>3+</sup> from the acac ligand (Figure 4a, Figure 8a, ESI). The inset of Figure 4a magnifies the region from 350 to 410 nm, revealing characteristic Tb<sup>3+</sup> transitions (e.g. <sup>7</sup>F<sub>6</sub>→<sup>5</sup>G<sub>5</sub>, <sup>7</sup>F<sub>6</sub>→<sup>5</sup>L<sub>10</sub>, and <sup>7</sup>F<sub>6</sub>→<sup>5</sup>G<sub>6</sub>) when the emission of the Eu<sup>3+</sup> is monitored (red curve), thereby confirming efficient Tb<sup>3+</sup> → Eu<sup>3+</sup> energy transfer. The energy transfer is further evidenced in the excitation spectrum of **3B**, monitored at 700 nm (Eu<sup>3+</sup> <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>4</sub> transition), where a band at 490 nm, assigned to the Tb<sup>3+</sup> <sup>7</sup>F<sub>6</sub>→<sup>5</sup>D<sub>4</sub> transition, and another at 380 nm, corresponding to the Tb<sup>3+</sup> <sup>7</sup>F<sub>6</sub>→<sup>5</sup>G<sub>6</sub>, <sup>5</sup>D<sub>3</sub> transitions, are observed (Figure 8a, ESI). The emission spectra of **3A** and **3B** under 315 nm excitation exhibit a series of characteristic transitions for Tb<sup>3+</sup>
$^5D_4 \to ^7F_{6-0}$ and $Eu^{3+}$, $^5D_0 \to ^7F_{0-4}$ (Figures 4b and S8b, in green and red labels, respectively). Similar transitions were observed in the spectra of [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-OH)]·H₂O (Figures S9, ESI) proving its successful encapsulation. However, several modifications can be noted in the relative intensities of the $^5D_4 \to ^7F_{5/5}D_0 \to ^7F_2$ and the $^5D_4 \to ^7F_{5/5}D_0 \to ^7F_4$ emissions when the spectra of 3A and 3B are compared with the free complex. In fact, for 3A, the Tb³⁺/Eu³⁺ intensity ratio of the $^5D_4 \to ^7F_{5/5}D_0 \to ^7F_2$ and the $^5D_4 \to ^7F_{5/5}D_0 \to ^7F_4$ transitions went from 3.8 and 22 to 0.72 and 0.47 after encapsulation. A similar tendency was observed in the case of 3B (Figure S8), for which the intensity ratios $^5D_4 \to ^7F_{5/5}D_0 \to ^7F_2$ and $^5D_4 \to ^7F_{5/5}D_0 \to ^7F_4$ were found to be equal to 0.83 and 0.63 after encapsulation. This dramatic change in the relative intensities of the Ln³⁺ transitions was previously observed in dendritic silica nanoparticles containing the same complex and explained by theoretical modelling.³² Encapsulation of the complex within the functionalised silica pores was shown to induce slight symmetry modifications, shorten intermetallic Ln³⁺-Ln³⁺ distances, and thereby enhance Tb³⁺ to Eu³⁺ energy transfer, which is responsible for the thermometric performance, as we will show from simulations. Moreover, the absorption from the IONP core can also impact the emission spectrum in the visible region.

Second, the temperature-dependent luminescence of nanoparticles 3A and 3B was investigated to evaluate their employment as luminescent thermometer in water. While the peak positions in the emission spectra of both samples remain fixed, indicating that there is no energy shifting, the intensity of the Tb³⁺ $^5D_4 \to ^7F_J$ (J = 6, 5, and 4) transitions steadily reduces as temperature rises, reflecting thermally enhanced Tb³⁺→Eu³⁺ energy transfer (Figure 4b, Figure S10, ESI). The CIE chromaticity diagram for 3A shown in Figure 4c exhibits a continuous red shift with increasing temperature, highlighting the growing Eu³⁺ participation to the overall emission due to the enhanced energy transfer, despite a Tb³⁺/Eu³⁺ ratio of 9:1 in the compound. Figure 4d presents the adopted energy level diagram for the (Tb/Eu)₉ cluster, with arrows denoting the forward energy transfer rates $\langle W \rangle_{odd \to even}$ and corresponding backward rates, $\langle W \rangle_{even \to odd}$. The temperature-dependent energy transfer rates, computed via state-of-the-art theoretical models (see Supporting Information for further detail),[84,85,99] are plotted in Figures 4e-4h. In Figure 4e, the forward transfer involving transitions from the Tb³⁺ $^5D_4$ and Eu³⁺ $^5D_0$ emitting levels ($\langle W \rangle_{1 \to 2}$), besides being more than three orders of magnitude higher than its back transfer ($\langle W \rangle_{2 \to 1}$), it steadily increases with temperature, whereas its reverse remains almost invariant. This is direct evidence of the quenching of the Tb³⁺ $^5D_4 \to ^7F_J$ emissions observed in Figure 4b. A similar trend appears for the higher-lying channel $\langle W \rangle_{1 \to 4}$ and $\langle W \rangle_{3 \to 4}$ in Figures 4f and 4h, contributing to the overall enhanced Tb³⁺ to Eu³⁺ energy transfer. Figure 4g shows negligible rates (< 1 s⁻¹) that involve high energy gaps (non-resonant conditions) that will not affect the overall Tb³⁺→Eu³⁺ energy transfer process.

Using these rates, together with the radiative rates ($A_{rad}$, Eq. S4) and the multiphonon decay ($W_{mp}$, Eq. S1) rates for Eu³⁺ $^5D_0 \to ^7F_6$ and Tb³⁺ $^5D_4 \to ^7F_0$ (Table S1), we constructed the rate equation model (Eqs. S15-S20) to estimate the populations of each emitting level ($P_1$ and $P_2$) and, consequently, the theoretical LIR = $P_1/P_2$.

The unusually higher intensity of the Eu³⁺ $^5D_0 \to ^7F_4$ transition compared to $^5D_0 \to ^7F_2$ (Figure 4b) is due to the sensitivity of the Eu³⁺ ion to changes in its local symmetry.[100,101] These transitions are governed by the Judd-Ofelt intensity parameters: $^5D_0 \to ^7F_2$ depends only on $\Omega_2$ while $^5D_0 \to ^7F_4$ depends only on $\Omega_4$. Each $\Omega_\lambda$ reflects the influence of the surrounding ligand field on Eu³⁺ and can be calculated as:

$$ \Omega_{\lambda} = (2\lambda + 1) \sum_{t,p} \frac{|B_{\lambda,t,p}|^2}{2t + 1} \qquad 1 $$

where $B_{\lambda,t,p}$ are odd-rank ligand field parameters and are determined mainly by the Forced Electric Dipole (FED) and Dynamic Coupling (DC) mechanisms in a non-centrosymmetric system.[102,103] Both mechanisms depend on the local symmetry, expressed through spherical harmonics:

$$ B_{\lambda,t,p} \propto \sum_{i} \frac{Y_{p}^{t*}(\theta_{i}, \phi_{i})}{R_{i}^{t+1}} \qquad 2 $$
where $Y_p^{t*}(\theta, \phi)$ are complex conjugate of spherical harmonics of rank $t$ (odd values, $t = \lambda-1$ and $\lambda+1$) and component $p$ (integer values from $-t$ to $+t$). $R_i$ is the distance from $Ln^{3+}$ to the $i$-th ligating, and $\theta$ and $\phi$ are the polar and azimuthal angles, respectively.

From these equations, $\Omega_2$ involves lower-rank ligand field terms ($t = 1$ and $3$), whereas $\Omega_4$ involves higher-rank terms ($t = 3$ and $5$). When the local environment of $Eu^{3+}$ approaches a more symmetric geometry (such as the $[(Tb/Eu)_9(acac)_{16}(\mu_3-OH)_8(\mu_4-O)(\mu_4-OH)]$ complex when encapsulated in silica), contributions from opposite ligands (i.e. the sum in Eq. 2) tend to cancel out. This cancellation effect is significantly stronger on the lower-rank terms governing $\Omega_2$. Consequently, $\Omega_2$, and thus the $^5D_0 \rightarrow ^7F_2$ transition, weakens much faster than $\Omega_4$ (and $^5D_0 \rightarrow ^7F_4$), which can be visualized by the simulation in Supporting Video 1 (through the JOYSpectra web platform),[104] showing that the $\Omega_2$ decreased faster than $\Omega_4$ under a $C_{4v}$ to $O_h$ symmetry change.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Experimental (black circles) and theoretical (blue lines) thermometric performance of **3A** in water over the 20–65 °C range. (a) LIR ($^5D_4 \rightarrow ^7F_5$/$^5D_0 \rightarrow ^7F_4$) and (b) $S_r$ from 20 to 65 °C temperature range. Error bars correspond to the standard deviation from three consecutive temperature cycles and the theoretical curves are normalised.</figcaption></figure>

The measured LIR as a function of temperature for **3A**, calculated from the integrated emission of the $Tb^{3+}~^5D_4 \rightarrow ^7F_5$ (530–560 nm) and the $Eu^{3+}~^5D_0 \rightarrow ^7F_4$ (675–715 nm) transitions is demonstrated in **Figure 5a**. Error bars represent the standard deviation obtained from three consecutive temperature cycles. Over the 20–65 °C range, LIR decreases monotonically with increasing temperature, confirming its suitability as a self-referencing sensor. Comparable results were observed for sample **3B** (Figure S10b, ESI). The relative thermal sensitivity, $S_r$ (Eq. S21), defined as the percentage change in LIR per degree Celsius,[105] is plotted in **Figure 5b** and **Figure S10**, ESI. The maximum $S_r$ reaches 0.75 %·°C⁻¹ at 65.0 °C for **3A** and 0.70 %·°C⁻¹ at 65.0 °C for **3B**, values similar to the peak sensitivities (~1 %·°C⁻¹) typically reported for ratiometric lanthanide thermometers in aqueous media.[106] The theoretical $S_r$, shown as a blue line in **Figure 5b** for **3A**, is normalised because the rate equation simulations use normalised populations. Nevertheless, the simulated temperature dependencies of both LIR and $S_r$ agree closely with the experimental data. The good relative standard deviation (RSD) indicating measurement
repeatability, defined as the maximum RSD among measurement values, is below 5% for **3A** and 11% for **3B**. The measured thermal uncertainty (δT, Eq. S23) is very satisfactory for **3A**, with a value of 1 °C, while it is higher for **3B**, reaching 4 °C.

The absence of Ln³⁺ complex leaching was confirmed by analysing the luminescence of the remaining solution after 3A removal. Its emission spectrum, measured strictly in the same conditions, did not present the characteristic Ln³⁺ transitions (**Figure S11**, ESI).

All luminescent and thermometric measurements in this study were carried out in water at pH = 7.4; comparable emission spectra were also observed at pH = 5.7, while more detailed investigations on aging, stability, and pH-dependent thermometry, particularly relevant for biomedical applications, will be addressed in future work.

## Real-time in-situ temperature monitoring during photothermal heating

The temperature detection by luminescence signal during the photothermal heating was performed with aqueous colloidal solutions of IONP@SiO₂-acac/(Tb/Eu)₉@SiO₂ **3A**, since these nanoparticles present better S<sub>r</sub>. To simultaneously monitor the eigen temperature fluctuation in this colloidal solution during photothermal heating, the Edinburgh FLS1000 spectrofluorimeter was coupled with an 808 nm laser (laser power of 2.58 W·cm⁻²), as shown in **Figure S12** (ESI). An optical fibre was introduced into the solution to probe the macroscopic temperature (see ESI).

<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) Temperature dependences of the emission intensities for ⁵D₄→⁷F₅ (Tb³⁺) (green) and ⁵D₀→⁷F₂ (Eu³⁺) (orange) transitions of **3A** during the heating up to 60 °C and cooling to 37 °C with thermalisation by the temperature controller at 37 °C (the same scale is used for Tb³⁺ and Eu³⁺ related intensities); b) Temperature dependence of LIR (between intensities of ⁵D₄→⁷F₅ (Tb³⁺) and ⁵D₀→⁷F₂ (Eu³⁺) transitions) during the heating up to 60 °C and cooling to 37 °C regulated by the temperature controller; c) The corresponding calibration curve represented as LIR (I<sub>545/615</sub>) vs temperature (blue) and the macroscopic temperature monitored by optical fibre (orange); d) The fluctuation of the emission intensities of ⁵D₄→⁷F₅ (Tb³⁺) (green) and ⁵D₀→⁷F₂ (Eu³⁺) (red) transitions under irradiation at 808 nm (power 2.58 W·cm⁻²) in the on/off mode; e) Fluctuation of corresponding LIR under irradiation in on/off mode; f) Temperature variation obtained by using the calibration curve under irradiation at 808 nm in on/off mode (blue) and the macroscopic temperature monitored by optical fibre (orange).</figcaption></figure>
First, the local temperature detection during the photothermal heating was performed with the thermalisation of the whole colloidal solution at 37 °C. The calibration curves were obtained by using the emission intensities of the main ⁵D₄→⁷F₅ (for Tb³⁺) and ⁵D₀→⁷F₂ (for Eu³⁺) transitions upon heating/cooling cycles from 37 to 65 °C and back to 37 °C using a thermal controller. **Figure 6a** shows that the intensity of the Tb³⁺ ⁵D₄→⁷F₅ transition decreases (19 %) as the temperature increases, while the Eu³⁺ ⁵D₀→⁷F₂ transition remains almost unaffected. The variation of the LIR between ⁵D₄→⁷F₅ (for Tb³⁺) and ⁵D₀→⁷F₂ (for Eu³⁺) transitions during the heating from 37 to 65 °C and subsequent cooling from 65 to 37 °C is depicted in **Figure 6b**. The superposition of the LIR variations during heating/cooling provides the calibration curves (**Figure 6c**).

Second, the intensity variations for the same ⁵D₄→⁷F₅ (for Tb³⁺) and ⁵D₀→⁷F₂ (for Eu³⁺) transitions were monitored under 808 nm irradiation (laser power 2.58 W·cm⁻²) in on/off mode with 10-minute segments (**Figure 6d**). As expected, the Tb³⁺ ⁵D₄→⁷F₅ intensity is sensitive to irradiation, decreasing when the laser is on and returning to its initial value when the laser is off, while the Eu³⁺ ⁵D₀→⁷F₂ intensity does not show significant fluctuation. The corresponding LIR modulation under on/off irradiation is shown in **Figure 6e**. Note that the intensities of the Tb³⁺ ⁵D₄→⁷F₅ and Eu³⁺ ⁵D₀→⁷F₂ transitions for the IONP-free control SiO₂-acac/(Tb/Eu)₉@SiO₂ nanoparticles were not sensitive to irradiation, and the corresponding LIR remained constant regardless of whether the laser was on or off (**Figure S13**, ESI). Based on the calibration curve depicted in **Figure 6c**, the temperature dependence relative to irradiation time is illustrated in **Figure 6f** with blue lines, while the macroscopic temperature of the solution is indicated by the orange line. The heating up to 47 °C and the subsequent cooling back to 37 °C are monitored during each 10-min segment of the laser on/off mode. The temperature measured using our emission probe during photothermal heating aligns perfectly with the macroscopic temperature variation monitored by the optical fibre, indicating the reliability of our nano-system.

Third, the experiments were conducted in the same conditions but with colloidal solution thermalised at 20 °C instead of 37 °C (**Figure S14**, ESI). Our results reveal noticeable thermal inhomogeneity under laser irradiation, with local temperatures recorded by luminescence reaching up to 35 °C, while the bulk solution remained at 31 °C (**Figure S14f**, ESI). Since the calibration curve was well-validated, these discrepancies suggest nanoscale thermal gradients, likely due to reduced convection, decreased Brownian motion, localized heat accumulation, and temperature-dependent heat dissipation.[46] These findings imply that macroscopic measurements may underestimate local temperatures under certain conditions, highlighting the need for further research into nanoscale heat transfer dynamics. This promising result opens the door for further exploration of the “thermal inhomogeneity” effect under photothermal heating, with future research aimed at optimizing the system’s effectiveness in various conditions.

## Conclusion

In summary, this study introduces a new multifunctional system for real-time in-situ temperature monitoring at the nanoscale via ratiometric luminescence thermometry during photothermal heating. The system features challenging, well-defined heater/thermometer nano-objects, consisting of IONP core stellate-like silica shell nanoparticles, with pores initially loaded and later sealed to trap and protect the luminescent complex [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-O)] inside. These nanoobjects were synthesized by a multistep approach involving the synthesis of IONP@SiO₂ shell nanoparticles functionalized with an acetylacetonate moiety, followed by loading with the complex and clogging the pores with silica to protect the complex and prevent its leaching. The obtained multifunctional IONP@SiO₂-acac/(Tb/Eu)₉@SiO₂ nanoparticles exhibit a size of approximately 100 nm and retain their morphology after complex loading and pore sealing. Structurally well-defined, these nanoparticles consist of a single IONP core encased within a precisely engineered stellate-like silica shell, available with either smaller or larger pore openings depending on the pH.

Solid-state photoluminescence demonstrated the successful loading of [(Tb/Eu)₉(acac)₁₆(µ₃-OH)₈(µ₄-O)(µ₄-O)] into the silica shells. Moreover, while the excitation and emission transitions exhibit similar positions before and after loading the complex, variations in the intensity ratio between specific transitions are evident. They have been attributed to a slight alteration in the complex's geometry during
the encapsulation, involving a shortening of intramolecular Ln³⁺-Ln³⁺ distances, providing the Tb³⁺ to Eu³⁺ energy transfer enhancement. The theoretical modelling demonstrated an exceptionally high Tb³⁺→Eu³⁺ energy transfer rate, which may reach on the order of 10¹¹ s⁻¹. The energy transfer dynamics and thermometric performance were also simulated through state-of-the-art models in the literature.
The macroscopic heating of IONP@SiO₂-acac/(Tb/Eu)₉@SiO₂ nanoparticles in aqueous colloidal solutions was investigated under 808 nm light irradiation and demonstrated expected concentration-dependent macroscopic heat. This latter is comparable with the macroscopic temperature elevation obtained for IONP nanoparticles without silica shell measured in the same conditions, suggesting that the open porosity of the stellate-like silica shell is favourable for rapid water exchange and heat dissipation. Therefore, the stellate like silica shell serves additional roles such as enhancing colloidal stability, providing functionality for luminescent complex insertion, while maintaining good photothermal response.

The temperature dependence of the photoluminescence realized in aqueous colloidal solutions in the 20–65 °C range indicates that nanoparticles can be used as efficient nanothermometers with good cyclability and a maximum relative sensitivity of 0.75 %·°C⁻¹ at 65 °C, enabling precise temperature detection compared to other emissive temperature probes. Employing the Tb³⁺ to Eu³⁺ energy transfer driven thermometer, we successfully modelled the Tb³⁺→Eu³⁺ energy transfer and demonstrated real-time in-situ temperature monitoring during photothermal heating under continuous 808 nm irradiation. These latter revealed noticeable nanoscale thermal gradients between the nanoparticles surface and environment, depending on thermal conditions. These findings highlight the importance of considering local temperature variations, as macroscopic measurements may systematically underestimate the true local temperatures and open the door for further exploration into the mechanisms of thermal inhomogeneity.

## Supporting Information

The authors have cited additional references within the Supporting Information.[39,82,84,85,87,89,97,107–125]

## Acknowledgements

The authors thank the University of Montpellier, CNRS for financial support. L. C. thanks the projects CICECO-Aveiro Institute of Materials, UIDB/50011/2020 (DOI 10.54499/UIDB/50011/2020), UIDP/50011/2020 (DOI 10.54499/UIDP/50011/2020) & LA/P/0006/2020 (DOI 10.54499/LA/P/0006/2020) financed by Portuguese funds through the FCT/MCTES (PIDDAC). A. C. N. and J. L. acknowledge support from the Portugal–France 2024 PESSOA programme. The work was developed within the scope of the ANR project Killer (ANR-22-CE09-0026), and the authors are grateful for the funding. B.A. and L.B. thank the French “Ministère de l'Enseignement Supérieur et de la Recherche (MESR)” as well as the ENS de Lyon for the PhD funding (CDSN). Authors are grateful to UAR2041 PAC Chimie Balard for measurements.

## Keywords

Multifunctional nanoparticles • thermometry • photothermia • iron oxide nanoparticles • real-time temperature detection

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