New heater@luminescent thermometer nano-objects: Prussian blue core@silica shell loaded with a β-diketonate Tb³⁺/Eu³⁺ complex

Abstract

We report on the synthesis and investigation of new multifunctional Prussian blue (PB) nanoparticles coated by a mesoporous silica shell and loaded with a luminescent [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O complex. These multifunctional nano-objects work as efficient photothermal nano-heaters able to provide macroscopic temperature rises remotely triggered by light irradiation at 808 nm (ΔT = 20.4 °C under irradiation for 3 min with a laser power of 1.83 W cm⁻²). Their specific heat capacity, the primary parameter influencing the heating properties of nanoparticles, was determined by using the photothermal properties and the measured heat capacity of PB nanoparticles, yielding a value of 1.13 ± 0.03 J g⁻¹ K⁻¹. This moderate value indicates that once heated, the nanoparticles can retain heat effectively, making them suitable for applications requiring sustained and controlled thermal effects. On the other hand, these multifunctional nanoparticles exhibit the characteristic temperature-dependent luminescence of Tb³⁺ and Eu³⁺ with improved Tb³⁺-to-Eu³⁺ energy transfer, making them efficient as luminescent ratiometric thermometers. These nanothermometers operate in the 20–80 °C range exhibiting a maximal relative thermal sensitivity of 0.75% °C⁻¹ at 20 °C.

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1. Introduction

Photothermal nano-objects, renowned for their adeptness in efficiently converting light into heat and generating a significant temperature elevation at the macroscopic level, have garnered considerable interest over the past several decades.^1,2 This interest extends beyond fundamental exploration, encompassing a spectrum of technological applications. We can cite the most investigated biomedical area as hyperthermia therapy of cancer,^3–6 bacteria-related diseases,⁷ or thermally enhanced drug delivery.^8,9 On the other hand, these nano-objects are widely employed for photothermal catalysis,^10–12 water desalination and purification,^13–15 energy conversion,¹⁶ heat-mediated optical manipulation,^17,18 and optical devices,^19,20 among other applications. Over the years, various nano-objects have been revealed as efficient photothermal agents, including plasmonic metal nanoparticles,^21–24 carbon-based nanomaterials,^25,26 nanostructured semiconductors,²⁷ up-conversion nanoparticles,^28–30 metal–organic frameworks,^31–33 covalent organic frameworks^34–36 and more recently iron oxide nanoparticles.³⁷ Among these, Prussian blue (PB) nano-objects have emerged as promising alternatives, showcasing remarkable potential for photothermia-linked applications.

PB nanoparticles, characterized by the general formula A_1−xFe^III[Fe^II(CN)₆]_1−x (where A denotes an alkaline ion), represent a distinctive class of inorganic nanomaterials with versatile composition, well determined structure and promising physico-chemical properties.³⁸ PB nanoparticles present interesting advantages including: (i) the possibility to control both size and morphology, (ii) the possibility to dope the PB network with mono (Cs⁺, Tl⁺), divalent (Mn²⁺ or Zn²⁺) or lanthanide ions (Gd³⁺, Dy³⁺…) to endow or improve targeted properties, (iii) remarkable photo and thermal stability especially in aqueous solutions at low pH, and (v) biocompatibility, underscored by FDA approval for using bulk PB as an antidote for Cs⁺ and Tl⁺ poisoning. More specifically, PB nanoparticles are well-known for their strong light absorption with a high molar extinction coefficient in the near-infrared (NIR) spectral range (I window) spanning from 650 to 850 nm, due to the presence of a Fe²⁺-to-Fe³⁺ charge transfer band. Furthermore, they exhibit high efficiency in converting light energy into heat associated with important photothermal stability, making them highly attractive for nanoparticle-assisted photothermal heating. Photothermal properties of PB nanoparticles and PB analogues have been extensively proposed for applications in the medical field, particularly for cancer therapy,^39–44 tissue repair and wound healing,^45–48 as well as for antibacterial or anti-inflammatory applications,^49,50 related to Alzheimer's disease,⁵¹ thrombolytic management,⁵² or the development of biosensors.^53,54 It is also interesting to note that their photothermal effect has also been studied for other applications, such as photocatalysis,⁵⁵ deicing,⁵⁶ actuators,⁵⁷ self-healing materials⁵⁸ or coatings,⁵⁹ and crude oil adsorption.⁶⁰ Several investigations have been devoted to the determination of photothermal conversion efficiency (η) of PB nanoparticles. However, due to the fact that the latter significantly depends on nanoparticles’ parameters (size, surface agents, solvent, environment etc.), and the experimental conditions used, different values ranging from 50 to 80% have been provided.^61–64 Moreover, a comprehensive understanding of PB nanoparticles' heating properties has not been reached, particularly concerning their heating capacity, which has yet to be investigated. The latter is a crucial factor because it influences how quickly and efficiently the nano-heater can keep and transfer heat, impacting the overall performance and suitability for specific applications.

In the realm of photothermal heat generation by inorganic nanoparticles under light irradiation, a significant challenge lies in accurately regulating temperature not only at the macroscopic level but also in the immediate proximity of the surface of the nano-heater. In this context, considering that conventional temperature measurement instruments are ineffective at the nanoscale due to limitations in sensitivity, accuracy, and spatial resolution, precise temperature measurements are essential. They are necessary for understanding the behaviour of nanoparticles at this scale, where rapid fluctuations and nanoscale-related effects occur. Additionally, precise temperature measurements are crucial for assessing photothermal efficacy, ensuring safety, optimizing parameters, and maintaining quality control across various applications.

Luminescent thermometry is an emerging solution, which leverages the emission properties of various materials to provide highly accurate, non-contact, and localized temperature measurements, even at the micro and nanoscale. It offers several advantages, including high spatial precision (below 10 μm), temporal resolution (0.1 °C), short acquisition times (less than 10 μs), and high maximal relative sensitivity (S_rmax > 1% °C⁻¹).^65–67 Different types of luminescent thermometers including up-conversion nanoparticles or lanthanide ion-based coordination complexes, hybrid nanoparticles, organic dyes, quantum dots, and nano-diamonds have been widely investigated in recent decades. However, despite significant advancements in luminescent thermometry, examples of well-defined multifunctional nanoparticles that combine inorganic heaters with luminescent thermometers in close contact, capable of providing efficient heating triggered by external stimuli and cohesive temperature readouts, remain relatively scarce. Indeed, the development of multifunctional nano-systems combining nano-heaters and nanothermometers has evolved significantly over the past decade. Initially, research was conducted with the objective of measuring the temperature during the photo-thermal process using simple two-component systems comprising a separated nano-heater and thermometer dispersed together in a colloidal solution⁶⁸ or introduced within cells.⁶⁹ Plasmonic gold nanorods have been employed primarily as nano-heaters, and upconversion nanoparticles or quantum dots serving as nanothermometers. Similarly, gold or silver nanoparticles have been deposited on a surface and covered by a layer of silica or polymers containing Ln³⁺-based complexes as nanothermometers.^70,71 The next step in development involved more structured assemblies, where gold nano-heaters (mainly plasmonic Au nano-objects) were paired with nanothermometers through electrostatic interactions. These assemblies often featured core–satellite structures, with a central nano-heater core surrounded by thermometer satellites^72–76 or with a central thermometer core surrounded by nano-heater satellites,⁷⁷ or both components situated on the surface of a support.⁷⁸ However, these nano-objects often lack well-defined component structures, leading to aggregation and non-uniform distribution of satellites around the core or the components leaching, which can preclude the optimal functionality and reliability of the nano-heater/thermometer system. A significant advancement came with the creation of well-defined nano-systems where heating and temperature-sensing components were either integrated into a single nanoparticle^79,80 or assembled into core–shell structures.^81,82 These designs ensured precise, non-aggregated multifunctional nano-objects, with components positioned in close contact at exact distances, which is crucial for accurately determining the surface temperature of the nano-heaters. However, such sophisticated systems are relatively rare, highlighting an area of ongoing research and development.

In this article, we report on the design and investigation of new multifunctional heater/thermometer nano-objects containing a PB heater core enwrapped by a mesostructured silica shell loaded with a luminescent thermometer. Surprisingly, despite a few studies on the employment of luminescent species with PB nanoparticles (Ln³⁺-doped fluoride nanoparticles covered by a thin shell of PB in order to improve their heating ability⁸³ or PB nanoparticles with anchored/loaded fluorescent organic dyes for imaging^84–89), the combination of a PB heater with a luminescent thermometer has never been reported to date. The Tb³⁺/Eu³⁺ based nonanuclear luminescent coordination compound [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O (acac = acetylacetonate)^90,91 was selected as the emissive thermometer for several reasons: (i) it possesses nine lanthanide ions and may be considered as a “lanthanide molecular aggregate” recognized for its improved photophysical properties,⁹² (ii) the combination of Tb³⁺ and Eu³⁺ ions (Tb³⁺/Eu³⁺ ratio = 9 : 1) within the same molecule ensures self-referenced capabilities and promotes Tb³⁺-to-Eu³⁺ energy transfer which is beneficial for temperature-dependent emission, (iii) it already demonstrated excellent thermometric capacity when inserted into dendritic silica nanoparticles with a high maximum relative thermal sensitivity (1.4% °C⁻¹ at 43 °C), good photothermal stability and repeatability.⁹³ Therefore, PB@SiO₂ core@shell nanoparticles were functionalized with acac functions and then loaded with the luminescent [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O compound. The obtained hybrid nano-objects possess an excellent heating ability under irradiation at 808 nm, as well as a moderate specific heat capacity of 1.13 ± 0.03 J g⁻¹ K⁻¹, rendering them particularly compelling for applications requiring sustained and controlled thermal effects. Moreover, they exhibit bright luminescence in the visible region characteristic of Tb³⁺ and Eu³⁺ ions, which makes them multifunctional. The observed emission is temperature-dependent allowing the use of these nanoparticles as temperature nanoprobes in close proximity to the PB core with a satisfactory maximal relative sensitivity of 0.75% °C⁻¹ at 20 °C.

2. Materials and methods

All the chemicals were purchased commercially and used without further purification. Sodium hexacyanoferrate decahydrate (Na₄[Fe(CN)₆]·10H₂O, 99.99%), iron(III) chloride hexahydrate (FeCl₃·6H₂O, 97%), ammonium nitrate (NH₄NO₃, 99%) and cetyltrimethylammonium bromide (CTAB, 99%) were purchased from Sigma Aldrich (Steinheim, Germany); tetraethyl orthosilicate, Si(OEt)₄ (TEOS, 99%), was purchased from Abcr GmbH (Karlsruhe, Germany); sodium hydroxide (NaOH, 98%) was purchased from Honeywell (Charlotte, North Carolina, US); ammonia (NH₄OH, 28%) was purchased from VWR Chemicals (Radnor, Pensylvania, US), and ethanol (EtOH, 96%) was purchased from Merck (Darmstadt, Germany). The 3-[3-(triethoxysilyl)propyl]pentane-2,4-dione (acac-Si) was synthesized according to a previously published method.⁹⁴ The synthesis of the [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O complex (Tb³⁺/Eu³⁺ = 9/1) was performed following a previously published method.^90,95

2.1. Syntheses

Formation of pristine PB nanoparticles. Pristine cubic PB nanoparticles of ca. 90 nm were prepared by using a co-precipitation method as previously reported.^43,64,96,97 For this, aqueous solutions of FeCl₃·6H₂O (10.00 mM, 100 mL) and Na₄[Fe(CN)₆]·10H₂O (11.25 mM, 100 mL) were added simultaneously to 100 mL of ultrapure water at a rate of 4 mL h⁻¹, using a peristaltic pump (at 25 °C). Once the addition was complete, the NPs were collected by centrifugation (20 000 rpm, 10 min). The supernatant was removed and the NPs were washed 3 times with water and stored in water.

IR (KBr): ν(O–H) = 3635 cm⁻¹ (coordinated water), ν(O–H) = 3390 cm⁻¹ (crystallized water), ν(C≡N) = 2088 cm⁻¹ (Fe^III–C≡N–Fe^II), δ(O–H) = 1607 cm⁻¹ (crystallized water), ν(Fe^II–CN) = 605 cm⁻¹, δ(Fe^II–CN) = 503 cm⁻¹. EDS: 11.8/88.2 (Na/Fe). Formula found: Na_0.24Fe^III[Fe^II(CN)₆]_0.81·xH₂O. d_TEM = 90 ± 12 nm.

Synthesis of PB@SiO₂ nano-objects. The process for coating the PB nanoparticles with a mesoporous silica shell was adapted from a previously published procedure.⁹⁸ 700 mg of cetyltrimethylammonium bromide (CTAB) were dissolved in a mix of ethanol 96% (75 mL) and ultrapure water (450 mL), and then left under stirring (700 rpm) at 35 °C overnight. 80 mg of PB NPs were added to the CTAB solution before the addition of 2 mL of TEOS and 250 μl of ammonia (30%). The mixture was stirred for 2 hours at 35 °C, and 2 h at 80 °C to allow the silica shell growth. Nanoparticles were then collected by centrifugation (20 000 rpm, 10 min). The CTAB extraction was performed through two cycles of dispersion (sonication 30 min) and centrifugation (20 000 rpm, 10 min) using a 6 g L⁻¹ ammonia nitrate solution in EtOH. The nanoparticles were then washed with ultrapure water and centrifuged at 20 000 rpm for 10 min and redispersed in water and centrifuged at low speed (8000 rpm, 15 min) to remove side silica nanoparticles. Three cycles of dispersion and low speed centrifugation were performed. The obtained PB@SiO₂ nanoparticles were then stored in water.

IR (KBr): δ(Si–O–Si) = 476 cm⁻¹ (SiO₂), ν(Si–O–Si) = 800–1087 cm⁻¹ (SiO₂), ν(C–H) = 2800–3000 cm⁻¹ (residual CTAB), δ(CH₂,CH₃) = 1415 cm⁻¹ (CTAB), ν(C≡N) = 2088 cm⁻¹ (Fe^III–C≡N–Fe^II), δ(O–H) = 1607 cm⁻¹ (crystallized water), ν(Fe^II–CN) = 605 cm⁻¹, δ(Fe^II–CN) = 503 cm⁻¹. d_TEM = 132 ± 16 nm. EDS: Si/Fe = 62.5/37.5.

Synthesis of PB@SiO₂-acac nano-objects. The post-functionalization of the silica shell by the acetylacetonate (acac) moieties was performed by dispersing 10 mg of PB@SiO₂ nanoparticles in 4 mL of ethanol and 40 mL of toluene, and adding 100 μL of acac-Si (0.31 mmol). The suspension was then refluxed (110 °C) under magnetic stirring (700 rpm) overnight. The nanoparticles were then collected by centrifugation (20 000 rpm, 10 min) and washed three times in EtOH. The obtained nanoparticles were redispersed and stored in EtOH.

IR (KBr): δ(Si–O–Si) = 475 cm⁻¹ (SiO₂), ν(Si–O–Si) = 801–1092 cm⁻¹ (SiO₂), ν(C=C) = 1635 cm⁻¹ (acac-Si), ν(C=O) = 1695 cm⁻¹ (acac-Si, keto), ν(C–H) = 2800–3000 cm⁻¹ (acac-Si, residual CTAB), ν(C≡N) = 2088 cm⁻¹ (Fe^III–C≡N–Fe^II), δ(O–H) = 1607 cm⁻¹ (crystallized water), ν(Fe^II–CN) = 605 cm⁻¹, δ(Fe^II–CN) = 503 cm⁻¹. d_TEM = 139 ± 16 nm. EDS: Si/Fe = 65.1/34.9.

Synthesis of PB@SiO₂-acac/(Tb/Eu)₉ nano-objects. The encapsulation of the [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O (Tb³⁺/Eu³⁺ = 9/1) compound, symbolized here as (Tb/Eu)₉, was performed by adding 15.6 mg of the complex to the PB@SiO₂-acac nanoparticles (5 mg) dispersed in 20 mL of MeOH. The mixture was stirred at 65 °C under reflux for 2 h. The (Tb/Eu)₉ complex-containing nanoparticles, PB@SiO₂-acac/(Tb/Eu)₉, were washed twice with water (20 000 rpm, 10 min) and finally dried at 80 °C for 48 h.

IR (KBr): δ(Si–O–Si) = 477 cm⁻¹ (SiO₂), ν(Si–O–Si) = 800–1091 cm⁻¹ (SiO₂), ν(C=C) = 1655 cm⁻¹ (acac-Si), ν(C=O) = 1523 cm⁻¹, ν(C=O) = 1695 cm⁻¹ (acac-Si), ν(C–H) = 2800–3000 cm⁻¹ (acac-Si, residual CTAB), ν(C≡N) = 2088 cm⁻¹ (Fe^III–C≡N–Fe^II), δ(O–H) = 1607 cm⁻¹ (crystallized water), ν(Fe^II–CN) = 604 cm⁻¹, δ(Fe^II–CN) = 503 cm⁻¹. d_TEM = 138 ± 17 nm. EDS: Si/Fe = 71.0/28.9; Fe/Tb/Eu = 76.5/21.4/2.1.

2.2. Physical methods

Characterization. Transmission electron microscopy (TEM) images were recorded at 100 kV (JEOL 1200 EXII). Samples for TEM measurements were deposited from suspensions onto copper grids and allowed to dry before observation. The size distribution histograms were determined using enlarged TEM micrographs taken at a magnification of 100 K on a statistical sample of 200 to 300 nanoparticles. SEM/EDS microscopy was performed on an FEI Quanta FEG 200 instrument (Hillsboro, Oregon, US). The powders were deposited on an adhesive carbon film and analysed under high vacuum. The quantification of heavy elements was carried out using Oxford Instruments AZTEC software (Abingdon-on-Thames, UK), with a dwell time of 3 μs. HRTEM and STEM-EDS measurements were performed on a JEOL 2200 FS microscope (Tokyo, Japan) and an Oxford Instruments SDD EDX detector XMaxN 100 TLE (100 mm² – windowless) (Abingdon-on-Thames, UK). Infrared (IR) spectra using attenuated total reflectance (KBr-IR) were recorded using powdered samples with a PerkinElmer Spectrum Two FT-IR Spectrometer (Waltham, Massachusetts, US) with four acquisitions at a resolution of 4 cm⁻¹. Ultraviolet-visible spectroscopy (UV-vis) spectra were collected on a JASCO V-650 (Tokyo, Japan) spectrometer. X-Ray diffraction (XRD) powder patterns were recorded using a PANalycal X’pert MDP-Pro diffractometer in Bragg Brentano geometry with Ni filtered Cu-Kα radiation (Kα1 = 1.54059 Å, Kα2 = 1.54443 Å). Measurements were performed at room temperature in a 2θ range of 10°–60° using a step size of 0.033° (2θ) and a counting time per step of 1 s. The XRD patterns were analyzed by pattern matching using the Fullprof software.⁹⁹ The crystallite size was determined using the Williamson–Hall formula implemented in the same software using a pseudo-Voigt Cox–Thompson function while taking into consideration the instrumental broadening obtained using a reference (Y₂O₃ powder annealed at 1400 °C for 36 h). Nitrogen adsorption and desorption isotherms at 77 K were measured using a TriStar 3000 (V6.06 A) instrument. The samples were first dried under vacuum at 80 °C for 12 hours. The specific surface area (S_BET) was determined using the Brunauer–Emmett–Teller (BET) method, while the pore size distribution was derived from the desorption branch of the nitrogen isotherms based on the Barrett–Joyner–Halenda (BJH) equation.

Photoluminescence and thermometry measurements. Emission and excitation spectra were measured at room temperature (20 °C) in the solid state, using a Edinburgh FLS-1000 spectrofluorometer (Edinburgh, UK). The excitation source was a 450 W ozone free Xe arc lamp. The spectra were corrected for the detection and optical spectral response of the spectrofluorometer. Photoluminescence measurements as a function of temperature (luminescent thermometry) were performed by using the temperature setup incorporated into the Edinburgh spectrofluorometer. Emission spectra were recorded in the temperature range from 20 to 80 °C. At each temperature step, a period of 5 min was given to allow the temperature to stabilize, and then 1 emission spectrum was recorded with a dwell time of 0.2 s and a step of 1 nm.

Specific heat. Specific heat was determined under an argon atmosphere using a differential scanning calorimeter, DSC 404C Pegasus manufactured by NETZSCH. The samples were pelletized (6 mm diameter with a load of 1.5 T for 30 s) and loaded into an aluminium crucible covered with a lid. The lid is pierced to allow gas exchange during the experiment. The temperature profile is as follows: an isotherm at 40 °C is maintained for 10 minutes and is followed by an increase at a rate of 20 °C min⁻¹ to 250 °C. The setpoint is maintained for 10 min. At minimum 7 cycles are measured. The first cycle is not considered due to losses of water from the samples. The data are exploited following the ratio method on the linear temperature range of 60–150 °C to determine the specific heat.

Photothermia. Photothermia experiences were realized using a laser with a wavelength of 808 nm and laser powers of 0.80 W cm⁻², 1.00 W cm⁻², 1.33 W cm⁻² and 1.83 W cm⁻². A 100 μg mL⁻¹ PB core solution was deposited onto the glass surface as a thin film and left to dry before the measurements. The temperature was measured by using a thermal infrared camera with points taken every second during all thermal cycles.

Light-to-heat conversion coefficient (η). To determine the photothermal conversion efficiency 54 μg of PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles were deposited on a glass surface. The deposit had a circular shape with an area of 0.32 cm². It was irradiated with a power of 2.58 W cm⁻² at a wavelength of 808 nm. The temperature data obtained from the thermal camera was processed using a model of a glass slide with a thin layer on top, developed and solved in COMSOL software.¹⁰⁰ The resulting fit was used to extract η.

3. Results and discussion

3.1. Synthesis of multifunctional PB@SiO₂ core@shell nanoparticles loaded with a luminescent [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)] compound

The synthesis of multifunctional core@shell PB@SiO₂ nanoparticles loaded with the luminescent compound [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O inside the porosity of the silica shell was designed using a four-step approach as depicted in Scheme 1. It consists of: (i) the synthesis of the pristine cubic PB nanoparticles, (ii) their coating by a mesoporous silica shell to obtain PB@SiO₂ nano-objects, (iii) the functionalization of the silica pores with the acetylacetonate moiety, and (iv) the loading of the luminescent [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O compound inside the silica porosity. The mesoporous silica shell was used to enwrap the PB heater core and encapsulate the luminescent thermometer because: (i) its porosity permits loading of the luminescent complex inside the pores assuring its homogeneous distribution, (ii) the possibility of functionalizing the pores to render them less hydrophilic and ensure the retention of the lanthanide complex inside the pores, (iii) a good thermal shock resistance and a near zero thermal expansion, and (iv) the optical transparency required for both light activation and luminescent thermometry.^101–103

Scheme 1 Schematic representation of the four-step approach employed for the synthesis of multifunctional core@shell PB@SiO₂-acac nanoparticles loaded with luminescent [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)] compound.

Pristine cubic PB nanoparticles were prepared by using a co-precipitation method in water with controlled addition of Na₄[Fe(CN)₆]·10H₂O and FeCl₃·6H₂O precursors.^43,64,96,97 This method permits cubic nanoparticles of the formula Na_0.24Fe^III[Fe^II(CN)₆]_0.81·xH₂O to be obtained (see the Materials and methods section for details). Secondly, the mesoporous silica shell was grown on the surface of the PB nanoparticles by using a sol–gel process adapting a previously reported procedure.⁹⁸ The subsequent post-grafting with the acac moiety was performed by adding acac-siloxane to give PB@SiO₂-acac nanoparticles. The acac functionalization of the silica is necessary since the complex is not retained in the silica pores without functionalization. Then, the [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O complex with the Tb/Eu ratio = 9/1 (which is denoted as (Tb/Eu)₉) was loaded into the silica pores by its impregnation during 2 h in methanol at 65 °C to obtain PB@SiO₂-acac/(Tb/Eu)₉ nano-objects. They present the loading of 0.13 units of (Tb/Eu)₉ complex per SiO₂ unit (i.e. 0.11 of Ln³⁺ per unit of SiO₂).

Infrared (IR) spectroscopy was performed to characterize the samples at each step of the PB@SiO₂-acac/(Tb/Eu)₉ nano-objects’ preparation (Fig. 1). The typical stretching vibrations of the bridging cyanide group ν(Fe^II–CN–Fe^III) at 2090 cm⁻¹, along with the ν(Fe^II–C) and the δ(Fe^II–CN) bands at 605 cm⁻¹ and 503 cm⁻¹ are clearly present in the IR spectra of all samples indicating that the PB network was preserved after silica shell coating, acac functionalization and complex loading. The characteristic silica vibrations, ν(Si–O–Si) at 1090 and 800 cm⁻¹, δ(Si–O–Si) at 475 cm⁻¹ and ν(Si–OH) at 972 cm⁻¹ appear in the spectrum recorded for the PB@SiO₂ sample after silica coating. The bands from the grafted acac moieties (ν(C=O) at 1700, 1690 and 1630 cm⁻¹ in the keto and enol forms and ν(C=C) in the 1350–1400 cm⁻¹ range, as well as the C–H vibrations between 2900–3000 cm⁻¹ characterize their successful anchoring into the silica porosity. The IR spectrum of PB@SiO₂-acac/(Tb/Eu)₉ containing the incorporated luminescent [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O complex shows the decrease in the relative intensity of ν(C=O) vibrations of the free acac moiety at 1700 cm⁻¹ in comparison to the one of the band at 1630 cm⁻¹, as well as the appearance of the ν(C=O) at 1520 cm⁻¹ band (the coordinated enol form of acac in the complex) attesting the presence of the complex inside the pores (Fig. 1 and Fig. S1, ESI†). However, the presence of δ(H–O–H) situated in the 1620–1605 cm⁻¹ region renders the interpretation of the IR spectra related to the attribution of the acac form and conclusion on the coordination of the lanthanide of the complex to the acac groups of the silica difficult. Note that similar results have been obtained for stellate-like silica nanoparticles loaded with the same complex.⁹³

Fig. 1 IR spectra of different steps in the synthesis of PB@SiO₂-acac/(Tb/Eu)₉ nano-objects.

The powder X-ray diffraction (PXRD) pattern showed the typical fcc (space group Fm3̄m) structure indexed in the Fm3̄m (01-073-0689) space group with a lattice parameter of a = 10.1633(0) Å for the pristine PB, nano-objects (Fig. 2a).

Fig. 2 (a) Powder X-ray diffraction patterns for the pristine PB, PB@SiO₂ and PB@SiO₂-acac/(Tb/Eu)₉ nano-objects, and (b) absorption spectra in water for suspensions of PB, PB@SiO₂, PB@SiO₂-acac and PB@SiO₂-acac/(Tb/Eu)₉ nano-objects in the visible region.

This value did not drastically change after the silica coating, functionalization and complex loading: the value of 10.1625(2) Å is found for the PB@SiO₂-acac/(Tb/Eu)₉ sample (Table 1). This fact indicates that the cyano-bridged structure of the PB nanoparticles was preserved during the silica shell coating and the complex loading. The crystalline domains were calculated from the Williamson–Hall formula for the PB@SiO₂-acac/(Tb/Eu)₉ sample giving an average size value of ca. 90 nm. This size is in agreement with what is determined from the transmission electronic microscopy (TEM) measurements (see later).

Table 1 Main characteristics for PB, PB@SiO₂, PB@SiO₂-acac and PB@SiO₂-acac/(Tb/Eu)₉ nano-objects

Nano-objects	a (Å)	PB core size^b^c (nm)	SiO2 shell thickness^b (nm)	Total size^c (nm)
a Lattice a parameter obtained from PXRD. b The core size and shell thickness were measured directly from TEM images. c Obtained as the mean edge of the cube.
PB	10.1633(0)	90 ± 12	—	90 ± 12
PB@SiO2	10.1628(1)	91 ± 12	18 ± 4	132 ± 16
PB@SiO2-acac	—	90 ± 12	19 ± 4	139 ± 16
PB@SiO2-acac/(Tb/Eu)9	10.1625(2)	91 ± 12	18 ± 4	138 ± 17

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The absorption electronic spectra measured for all samples showed a classical broad band in the range 400–900 nm with a maximum at 723 nm for the pristine PB nanoparticles (Fig. 2b). This band is usually ascribed to the intervalence Fe²⁺-to-Fe³⁺ charge transfer through the cyanide bridge. Its maximum is shifted toward lower wavelengths for PB@SiO₂ after silica shell coating, which is in agreement with the previously reported results for the silica coated PB nanoparticles.⁹⁸ After the subsequential acac functionalization and complex loading, this band is red shifted (Fig. 2b).

TEM observations were carried out in order to evaluate the size and morphology of the obtained nanoparticles. TEM images obtained for the pristine PB nanoparticles show typical well defined cubic shaped nano-objects with the mean edge of the cube of 90 ± 12 nm (Fig. 3a and Fig. S2a, ESI† for size distribution histogram). Each single core was then coated by a mesoporous silica shell to provide core@shell PB@SiO₂ nanoparticles with a whole diameter of 132 ± 16 nm (see Fig. 3b and Fig. S2b, ESI† for size distribution histograms). The PB core size was almost not changed after the silica coating (91 ± 12 nm, Fig. S3a, ESI†) and the silica shell thickness obtained from the TEM images is equal to 18 ± 4 nm (Fig. S4a, ESI†). The TEM image for these PB@SiO₂ nanoparticles reveals the expected presence of the silica shell's mesoporosity. The grafting of the acac moiety (see Fig. 3c and Fig. S2c, S3b and S4b, ESI† for size distribution histograms) and the loading of the Tb³⁺/Eu³⁺ complex inside the silica pores (Fig. 3d and Fig. S2d, S3c and S4c, ESI† for size distribution histograms) do not modify the morphology and the size of the nano-objects. Indeed, the PB core size for the PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles is equal to 91 ± 12 nm and the silica shell thickness is equal to 18 ± 4 nm. The sizes of the cores and shells and the whole sizes of all obtained nanoparticles are gathered in Table 1.

Fig. 3 (a) TEM images for PB, (b) PB@SiO₂, (c) PB@SiO₂-acac and (d) PB@SiO₂-acac/(Tb/Eu)₉ nano-objects.

The obtained nanoparticles were characterized by HAADF-STEM with EDS mapping, which allowed us to visualize the presence of the silica shell (pink colour for Si atoms) around the PB core (Fe atoms are depicted in yellow), as well as the homogeneity of the atomic distribution of terbium (orange colour) and europium atoms (green colour) confirming the encapsulation of the Tb³⁺/Eu³⁺ complex within the silica shell (Fig. 4).

Fig. 4 (a) STEM-bright field image and STEM-EDS elemental mapping of PB@SiO₂-acac/(Tb/Eu)₉ nano-objects with topochemical distribution of (b) Fe (yellow), (c) Si (pink), (d) O (blue), (e) Eu (green) and (f) Tb (orange).

Nitrogen sorption measurements on the nano-objects provide an estimation of the effect of compound incorporation in the porosity of the silica shell. As observed through TEM, PB@SiO₂ appears to exhibit porosity, which is confirmed by the specific surface area of 371 m² g⁻¹ (Fig. S5, ESI†). After the complex loaded, the specific surface area decreased to 245 m² g⁻¹ for PB@SiO₂-acac/(Tb/Eu)₉, indicating that the complex was successfully incorporated into the pores of the silica shell.

3.2. Photothermal properties and determination of the specific heat capacity

The macroscopic photothermal properties were investigated for the PB@SiO₂-acac/(Tb/Eu)₉ nano-objects and compared with the ones of the pristine PB nanoparticles. Note that while the macroscopic photothermal properties of PB nanoparticles coated by silica shell have already been reported in the literature,^98,104,105 a clear comparison between PB and silica coated PB nano-objects has never been published before. The nanoparticles were deposited on a glass surface and submitted to laser irradiation under 808 nm with different laser powers ranging from 0.8 to 1.83 W cm⁻² and the macroscopic temperature rise was monitored by using a thermal camera.^106,107 An important temperature increase was observed for PB@SiO₂-acac/(Tb/Eu)₉ nano-objects under irradiation after only a few minutes. For instance, ΔT of 20.4 °C was observed under 3 min irradiation with the laser power of 1.83 W cm⁻², while no obvious heating effect was detected on the surface without nanoparticles (Fig. 5a). In comparison, PB nanoparticles without a silica shell provide higher macroscopic temperature elevation with ΔT of 40.1 °C under the same conditions. Of note, the absorption levels of both the bare PB nanoparticles and the silica coated ones were similar, 29% and 27%, respectively. This result is in agreement with the work of S. Merabia et al., which demonstrated that the thickness of the silica shell highly impacts the heat transfer of Au core silica shell nanoparticles at nano- and macroscales.¹⁰⁸ In this work, the authors showed that a very thin silica shell is most favourable for efficient heat transfer even in comparison with the pristine Au nanoparticles, while a thicker silica shell decreases macroscopic photothermal efficiency. As expected, both PB@SiO₂-acac/(Tb/Eu)₉ and PB nano-objects present a linear increase in the ΔT value as a function of laser power (Fig. 5b), which is rather common with other photothermal nano-objects. The photothermal conversion efficiency η of 69% was extracted using a deposit of 54 μg of PB@SiO₂-acac/(Tb/Eu)₉ nano-objects on a glass surface (see the Materials and methods section and Fig. S6, ESI†). Note that this efficiency depends on several factors, including the nature of the nanoparticles, their size, shape, interface (core/shell), environment, solvents, etc. For instance, in our previous studies, we showed that PB nanoparticles of ca. 81 nm in aqueous solution presented a light-to-heat conversion efficiency of 57%,⁶⁴ and those deposited on the surface demonstrated an efficiency of 50%.¹⁰⁹ Note that the value of 69% is comparable to what is found for Au nanocages, regarded as a benchmark for photothermal agents.¹¹⁰

Fig. 5 (a) Photothermal properties presented as ΔT vs. time curve for PB (blue dotted lines) and PB@SiO₂-acac/(Tb/Eu)₉ (green full lines) nano-objects deposited on the glass surface under irradiation at 808 nm (0.80 W cm⁻², 1.00 W cm⁻², 1.33 W cm⁻² and 1.83 W cm⁻²), measured over three cycles of irradiation/relaxation with the laser source, (b) ΔT measured for PB and PB@SiO₂-acac/(Tb/Eu)₉ nano-objects after 3 min of laser irradiation at 808 nm as a function of laser power, (c) experimental data for ΔT vs. time curve measured for PB and PB@SiO₂-acac/(Tb/Eu)₉ nano-objects under irradiation at 808 nm with the laser power of 1.83 W cm⁻² and its numerical solution of eqn (1) obtained from the fit of the thermal derivative curve fit (black lines), and (d) derivative of the temperature as a function of the time with its fit obtained using eqn (3)–(5).

In order to obtain a deeper understanding of the heat transfer of the PB and PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles, we determined their specific heat capacities. This property corresponds to the amount of heat energy required to raise the temperature of a unit mass of a substance by one degree celsius (°C) or one kelvin (K). It varies significantly between different materials and inversely affects the temperature change resulting from light absorption. Consequently, it influences the efficiency and dynamics of the light-to-heat transfer process and the heat retention. First, the specific heat capacity (C_p) of the PB nanoparticles was measured by using DSC in the linear temperature range 60–150 °C by using 5 heating/cooling cycles (Fig. S7, ESI†) (see the Methods and materials section for details). The obtained C_p value is equal to 0.92 ± 0.03 J g⁻¹ K⁻¹, which is close to the values usually given for bulk metal organic framework materials (0.75–0.85 J g⁻¹ K⁻¹). In comparison, metallic gold provides a much lower C_p value of 0.129 J g⁻¹ K⁻¹. It signifies that gold and PB nanoparticles behave differently regarding their photothermal properties. Indeed, without taking into account the interface thermal resistivity, gold nanoparticles very rapidly convert light to heat and transfer it to the macroscopic environment. In contrast, PB nano-objects require more energy to reach the same temperature as Au nanoparticles, but due to their higher heat capacity, once heated, they can store more thermal energy and release this stored heat at a slower pace compared to gold nanoparticles. Therefore, PB nanoparticles could have advantages for prolonged or localized heating phenomena, such as enhanced hot spot effect formation or maintaining stable local thermal conditions.

The specific heat capacity for PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles was determined from their photothermal properties by taking an approximation that the PB core keeps its initial C_p value unchanged after the silica coating and the complex loading. The absorption of the silica shell is neglected. It is also important to note that C_p and C_v are equivalent for a solid. We used the heat equation:

where C_p is the heat capacity of the sample, P is the heat source power and f is the heat exchange function which takes into account the exchange of energies between the system and the environment. In our case, the heat equation is applied to a thin layer of nano-objects in contact with 2 environments, the glass layer and the air. These two interfaces contribute to a high degree of complexity in the exchange function. If we consider that the exchange function is N times derivable, then it can be expressed as a Taylor series of the f function at the point T₀:The thermodynamic principles indicate that the first order of this series is zero. Then, this equation can be rewritten with unknown constants a_n:Fig. 5c shows experimental data of ΔT vs. time curve for heating of PB and PB@SiO₂-acac/(Tb/Eu)₉ nano-objects under laser irradiation with a laser power P of 1.83 W cm⁻². The temperature variation as a function of time for PB nanoparticles could be written as:and for PB@SiO₂-acac/(Tb/Eu)₉ nano-objects as:It is established that the f function is zero at the point t = 0 s. Furthermore, if the derivative of the temperature is taken (see Fig. 5d), the ratio P/C_p can be extracted from the initial point. Therefore, P₁ and P₂ depend on: (i) the source light power (1.83 W cm⁻²), (ii) the absorption of the layer and (iii) the light-to-heat conversion coefficient η. The absorption of a layer, designated as layer i, will be referred to as A_i. Therefore, P₁ and P₂ can be expressed as:

P₁ = A₁·P_laser·η₁ and P₂ = A₂·P_laser·η₂ (6)

In the case of PB, the total heat capacity depends on its mass and its mass-specific heat capacity:

C_p,1 = m_PB,1·C_p,PB (7)

In the case of PB@SiO₂-acac/(Tb/Eu)₉ nano-objects, the total heat capacity can take into account the mass of PB and SiO₂ and the mass-specific heat capacity of PB and SiO₂, while the contribution of organic function and luminescent compound loaded could be neglected:

C_p,2 = m_PB,2·C_p,PB + m_SiO2·C_p,SiO2 (8)

From the thermal derivative curves displayed in Fig. 5d we can extract the following ratio:If we assume that the light-to-heat conversion efficiency is the same for the PB nanoparticles and the PB core in the PB@SiO₂-acac/(Tb/Eu)₉ nano-objects, and that the mass of PB is proportional of the light absorption for both samples, we can write:The aforementioned equation can be employed in conjunction with the photothermal experiment and the EDS, which permits the extraction of the mass ratio between silica and PB, in order to ascertain the mass heat capacity of PB.With eqn (8) and (11) we obtain specific heat capacities of 1.31 ± 0.04 J g⁻¹ K⁻¹ and 1.13 ± 0.03 J g⁻¹ K⁻¹ for the silica shell and the PB@SiO₂-acac/(Tb/Eu)₉ nano-objects, respectively.

It is noteworthy that the derivative thermal curve can be fitted using eqn (3)–(5) (black lines in Fig. 5d). Utilizing the extracted f function from the fit, the solution to eqn (1) can be derived. Subsequently, the temperature as a function of time can be obtained (black lines in Fig. 5c). Note that the obtained specific heat capacity for the silica shell is in agreement with the values given for porous silica in the literature. Therefore, the increased C_p value of the PB@SiO₂-acac/(Tb/Eu)₉ nano-objects relative to bare PB nanoparticles is consistent with the enhanced heat retention offered by the silica shell, which explains the lower macroscopic heating of these nano-objects.

3.3. Luminescence and luminescent thermometry

The photoluminescence of PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles was investigated in the solid state first at room temperature in order to confirm the encapsulation of the luminescent compound within silica mesoporosity, and secondly in the 20–80 °C temperature range in order to demonstrate the thermometric ability of the nanoparticles. The solid-state excitation spectra at room temperature were recorded by monitoring the main ⁵D₀ → ⁷F₂ and ⁵D₄ → ⁷F₅ transitions of Eu³⁺ (at 615 nm) and Tb³⁺ (at 545 nm), respectively (Fig. 6). They exhibit a dominant broad band with a maximum at 308 nm, which can be ascribed to the acac ligand excited state. This fact attests the efficient excitation through the acac “antenna” sensitization. Besides, sharp low intensity bands can be observed, which can be attributed to the 4f transitions of both lanthanide ions. Note also that the Tb³⁺ characteristic transitions appear when the spectra are recorded by monitoring the main emission of Eu³⁺ at 615 nm. All these characteristics appear in the excitation spectrum of the free [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O compound as illustrated in Fig. S8, ESI.† However, for the latter, 4f transitions appear much more pronounced. Previously observed with the [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O compound encapsulated inside stellate silica nanoparticles,⁹³ this effect was interpreted as an antenna effect enhancement through the acetylacetonate-functionalized silica matrix sensitization. The solid-state emission spectra of PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles were recorded at room temperature upon excitation at 308 nm (in the acac-linked sensitization) since this excitation band is operational for both lanthanide ions (Fig. 6). The emission spectrum demonstrates a series of expected Tb^{3+ 5}D₄ → ⁷F_6-0 characteristic emission lines (labelled in green) with the most intense ⁵D₄ → ⁷F₅ transition, as well as Eu^{3+ 5}D₀ → ⁷F_0-4 characteristic transitions (labelled in red) with the most intense band situated at ca. 615 nm (⁵D₀ → ⁷F₂). In comparison with the solid-state emission spectrum of the free [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O compound, the relative intensities of the main Tb^{3+ 5}D₄ → ⁷F₅ transition decreases, while the intensities of two main Eu³⁺ characteristic transitions (⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄) increase. For instance, the relative intensities of the Tb³⁺ emissive transitions to the Eu³⁺ emissive ones at room temperature are much lower in PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles compared to the free complex: I₅₅₀/I₆₁₅ goes from 2.2 to 0.65 and I₅₅₀/I₇₀₀ goes from 9 to 1.5 for [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O and PB@SiO₂-acac/(Tb/Eu)₉ respectively (Fig. 6 and Fig. S8, ESI†). This means that the relative signals from the Eu³⁺ transitions have been multiplied by 3.4 at 615 nm and by 6 at 700 nm, compared to the one at 550 nm from the Tb³⁺. These modifications in the emission spectrum after complex loading can be attributed to the action of three potential effects occurring in the investigated system: (i) the expected boost of the Tb³⁺-to-Eu³⁺ energy transfer after insertion of the complex into the silica shell. This effect has previously been observed for acac-functionalized stellates silica nanoparticles loaded by [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O and explained by the slight modifications in the complex geometry occurred after encapsulation,⁹³ (ii) the possible energy transfer from Tb³⁺ to the silica host, which has previously been observed in the case of lanthanide-based complexes inserted into the silica matrix,¹¹¹ and (iii) the presence of PB nanoparticle absorption, occurring between 600 and 900 nm.

Fig. 6 (a) Excitation spectra of PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles monitored at λ_em = 545 (green) and λ_em = 615 nm (black) measured in the solid state at room temperature. Inset: Magnification of the excitation spectra between 350 and 480 nm showing the 4f-transitions; (b) emission spectra of PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles obtained at room temperature (red) in the solid state with excitation at λ_exc = 308 nm. Eu³⁺ and Tb³⁺ linked transitions are shown in red and green, respectively.

Secondly, the photoluminescence was investigated in the 20–80 °C temperature range to verify the potential of these nano-objects to act as luminescent temperature probes. Fig. 7a demonstrates the emission spectra of PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles measured under 308 nm excitation in the 20–80 °C range (Fig. 7a), where the intensities are temperature dependent. The corresponding temperature dependence of the luminescence intensity ratio (LIR) between ⁵D₄ → ⁷F₆ of Tb³⁺ (the integrated areas 480–505 nm) and ⁵D₀ → ⁷F₄ of Eu³⁺ (the integrated areas 603–635 nm) emissions providing the best results for thermometry is shown in Fig. 7b.

Fig. 7 (a) Emission spectra (λ_ex = 308 nm) of PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles in the 20 to 80 °C temperature range; (b) corresponding LIR between the ⁵D₄ → ⁷F₆ (Tb³⁺) and ⁵D₀ → ⁷F₂ (Eu³⁺) transitions. The solid line represents a second-degree polynomial fitting. Integrated areas: 480–505 nm (Tb³⁺: ⁵D₄ → ⁷F₆) and 603–635 nm (Eu³⁺: ⁵D₀ → ⁷F₂). Inset: Temperature dependence of S_r. The error bars correspond to the standard error of the mean determined from three consecutive temperature cycles.

It is important to note that the temperature dependent amplitude of ⁵D₄ → ⁴F₃ of Tb³⁺ is significantly less than that of the ⁵D₀ → ⁷F₄ of Eu³⁺. The error bars represent the standard deviation of average values obtained upon three consecutive temperature cycles. The temperature-dependent variation of the LIR parameters shows a second-degree polynomial correlation in the full temperature range 20–80 °C, while a linear correlation occurs between 20 and 40 °C. The relative thermal sensitivity value (S_r) is usually used as the characteristic, allowing a comparison of the different luminescent thermometer efficiencies.¹¹² The S_r parameter is the variation of LIR per degree of temperature, expressed by the equation:

S_r(T) = |∂LIR(T)/∂T|/LIR(T)

The temperature dependence of S_r is shown in Fig. 7b. The maximum on this curve obtained from the calibration data corresponds to the maximal relative sensitivity (S_rmax), which is equal to 0.75% °C⁻¹ at 20 °C. This value is rather close to the maximal relative thermal sensitivity of ∼1% °C⁻¹ frequently considered as the benchmark for efficient luminescent thermometers.⁶⁶

The thermal uncertainty was also calculated as the smallest temperature change that can be measured, i.e.:

δT = |δLIR(T)/LIR(T)|/S_r(T)

The measured thermal uncertainty is equal to 1.01 °C. The calibration parameters for PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles are gathered in Table 2.

Table 2 Calibration parameters for thermometry measurements for PB@SiO₂-acac/(Tb/Eu)₉ nanoparticles

	PB@SiO2-acac/(Tb/Eu)9
Temperature operating range	20–80 °C
Maximal relative sensitivity (Sr) with LIR490/615	0.75% °C^−1 at 20 °C
Maximal relative sensitivity (Sr) with LIR545/615	0.63% °C^−1 at 20 °C
Best uncertainty of LIR (δLIR)	0.0012
Best uncertainty of temperature (δT)	1.01 °C

---

4. Conclusion

This work presents the synthesis and investigation of new multifunctional heater@thermometer nano-objects obtained by coating PB nanoparticles with a mesoporous silica shell, followed by their covalent grafting with the acetylacetonate moieties and the loading of the luminescent compound [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O inside the pores. The functionalized silica shell plays a crucial role in hosting and protecting the luminescent compound and allowing its homogeneous distribution around the PB core.

The as-obtained multifunctional nanoparticles present a whole size of ca. 138 nm with a cubic PB core of ca. 91 nm and a well distinguished silica shell of ca. 18 nm. The PB nanoparticles retain their initial cubic morphology and size after the silica shell coating, its functionalization and luminescent compound loading. It is worth noting that the obtained hybrid nano-objects present a well-defined single core coated by a well-defined mesoporous silica shell. The encapsulation of [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)]·H₂O complexes ca. 13% (vs. SiO₂, i.e. 0.11 unit of Ln³⁺ per unit of SiO₂) was attested by an ensemble of characterizations, including IR spectroscopy, STEM-EDS atomic mapping, elemental and EDS analyses. The STEM-bright field images attest the homogenous distribution of the [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)] compound inside the silica shell. The investigation of the luminescence properties also proved the presence of the complex inside the silica porosity. Indeed, the nanoparticles exhibit similar Tb³⁺ and Eu³⁺ characteristic excitations and emissions in comparison with the spectra of the free compound. However, some modifications in the emission spectra of nanoparticles compared to the ones of the non-loaded complexes indicate the occurrence of a slight geometry modification of the latter, which can be induced by interactions with acetylacetonate functions of the silica. In particular, the encapsulation boosted the Tb³⁺-to-Eu³⁺ energy transfer already occurring in the compound. This effect has already been observed in the case of acac-functionalized stellate silica nanoparticles hosting this luminescent complex and explained thanks to theoretical calculations. Indeed, the encapsulation induces a slight shortening of intermolecular Tb³⁺–Eu³⁺ distances, which results in a drastic increase in the pairwise energy transfer rate.

Thanks to the presence of the PB heater core, the obtained multifunctional nano-objects exhibit photothermal properties allowing an important temperature rise under irradiation at 808 nm within a few minutes (for instance ΔT is 20.4 °C within 3 min of irradiation with a laser power of 1.83 W cm⁻²). The observed difference in the macroscopic temperature change (ΔT) between the bare PB nanoparticles and the multifunctional PB@SiO₂-acac/(Tb/Eu)₉ nano-objects (0.92 ± 0.03 vs. 1.13 ± 0.03 J g⁻¹ K⁻¹) can be attributed to an increased specific heat capacity due to the presence of the silica shell, which retains the transferred heat from the PB core. A more complex model should be developed in order to investigate the thermal resistivity at the PB/silica interface.

The PB@SiO₂-acac/(Tb/Eu)₉ nano-objects also exhibit temperature-dependent luminescence of the Tb³⁺/Eu³⁺ complex offering the function of a luminescent thermometer in the visible region in the 20–80 °C range displaying a maximal relative thermal sensitivity of 0.75% °C⁻¹ at 20 °C. This work opens up a route to new efficient heater/thermometer systems and investigations of the hot spot effect.

Author contributions

Conceptualization, S. S., G. F., J. L. and Y. G.; methodology, A. L., H. B., S. S., G. F., M. B., T. P., B. B., J. L., and Y. G.; validation, J. L., G. F., S. S.; formal analysis, A. L., H. B., M. B., S. S., Y. G., G. F.; investigation, A. L., H. B., G. F., S. S.; resources, S. S., J. L. and Y. G.; data curation, A. L., H. B., M. B., S. S., G. F., writing – original draft preparation, J. L., A. L., H. B., G. F.; writing – review and editing, S. S., G. F., J. L., and Y. G.; visualization, J. L., and Y. G.; supervision, S. S., G. F., J. L., and Y. G.; project administration, S. S., J. L. and Y. G.; funding acquisition, S. S. All authors have read and agreed to the published version of the manuscript.

Data availability

All data generated or analyzed during this study are included in the ESI† accompanying this article. Additional details and data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the University of Montpellier and CNRS and the ANR within the frame of the HotSpot project (ANR-23-CE09-0017) for funding. H. B. is grateful to the LabUM program, which funded her Master 2 internship (ANR-16-IDEX-0006). The authors are grateful to the Platform of Analysis and Characterization of ICGM and platform Electronic and Analysis microscopy at the University of Montpellier for analysis and measurements.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm00668b

‡ These authors contributed equally to this work.

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