Upconverting Nanoparticles Working As Primary Thermometers In Different Media

In the past decade, noninvasive luminescent thermometry has become popular due to the limitations of traditional contact thermometers to operate at scales below 100 μm, as required by current demands in disparate areas. Generally, the calibration procedure requires an independent measurement of the temperature to convert the thermometric parameter (usually an intensity ratio) to temperature. A new calibration procedure is necessary whenever the thermometer operates in a different medium. However, recording multiple calibrations is a time-consuming task, and not always possible to perform, e.g., in living cells and in electronic devices. Typically, a unique calibration relation is assumed to be valid, independent of the medium, which is a bottleneck of the secondary luminescent thermometers developed up to now. Here we report a straightforward method to predict the temperature calibration curve of any upconverting thermometer based on two thermally coupled electronic levels independently of the medium, demonstrating that these systems are intrinsically primary thermometers. SrF₂:Yb/Er powder and water suspended nanoparticles were used as an illustrative example

Photoluminescent Thermometer Based on a Phase-Transition Lanthanide Silicate with Unusual Structural Disorder

The hydrothermal synthesis of the novel Na­[LnSiO₄] (Ln = Gd, Eu, Tb) disordered orthorhombic system is reported. At 100 K, and above, these materials are best described in the centrosymmetric orthorhombic <i>Pnma</i> space group. At lower temperatures (structure solved at 30 K) the unit cell changes to body-centered with <i>Imma</i> symmetry. The materials exhibit unique photophysical properties, arising from both, this phase transformation, and the disorder of the Ln³⁺ ions, located at a site with <i>D</i>_<i>2d</i> point symmetry. Na­[(Gd_0.8Eu_0.1Tb_0.1)­SiO₄] is an unprecedented case of a luminescent ratiometric thermometer based on a very stable silicate matrix. Moreover, it is the first example of an optical thermometer whose performance (viz., excellent sensitivity at cryogenic temperatures <100 K) is determined mainly by a structural transition, opening up new opportunities for designing such devices

Photoluminescent Lanthanide-Organic Framework Based on a Tetraphosphonic Acid Linker

A new metal–organic framework based on the highly flexible tetraphosphonic acid linker hexamethylenediamine-<i>N,N,N</i>′<i>,N</i>′-tetrakis­(methylphosphonic acid) (H₈htp) is reported. [Ln₂(SO₄)₂(H₆htp)­(H₂O)₄]·10H₂O [Ln³⁺= Eu³⁺ (<b>1</b>), Sm³⁺ (<b>2</b>), and Gd³⁺ (<b>3</b>)] was readily obtained by microwave heating at moderate temperatures (80 °C) and low reaction time (15 min). The reaction was carried out in aqueous medium and, because of the high flexibility of the organic linker, sulfuric acid was added in small quantities. This acid delays the coordination process and blocks access of the phosphonic acid groups by coordinating the sulfate anion to the metal center, leading to the formation of a compact 3D network. Sulfuric acid further proved to be crucial for the formation of the materials because the use of different acids led to either no precipitation or amorphous compounds. When compared to the only known and reported material based on the same building blocks, this approach allowed us to significantly reduce the reaction time to just 15 min with an immediate crystal formation (compared to the 2 months reported). Crystals were obtained with sizes suitable for single-crystal X-ray diffraction analysis for <b>1</b>. Materials consist of a 3D network with the metal centers forming a close packed layer, being interconnected by the organic linker, forming cavities which are filled with solvent water molecules. Topologically, <b>1</b>–<b>3</b> are binodal networks with a 4,8-connectivity and a Schäfli point symbol of {4¹²·6¹²·8⁴}­{4⁶}₂. This topology is unusual for MOFs, especially for phosphonic acid based linkers, resembling the known mineral fluorite. The photoluminescence properties of <b>1</b> were studied showing an emission lifetime of 0.43 ± 0.01 ms and 0.57 ± 0.01 at 297 and 13 K, respectively

Metal-Free Highly Luminescent Silica Nanoparticles

Stable, cost-effective, brightly luminescent, and metal-free organosilica nanoparticles (NPs) were prepared using the Stöber method without any thermal treatment above 318 K. The white-light photoluminescence results from a convolution of the emission originated in the NH₂ groups of the organosilane and oxygen defects in the silica network. The time-resolved emission spectra are red-shifted, relative to those acquired in the steady-state regime, pointing out that the NPs emission is governed by donor–acceptor (D<i>–</i>A) recombination mechanisms. Moreover, the increase of the corresponding lifetime values with the monitored wavelength further supports that the emission is governed by a recombination mechanism typical of a D<i>–</i>A pair attributed to an exceptionally broad inhomogeneous distribution of the emitting centers peculiar to silica-based NPs. These NPs exhibit the highest emission quantum yield value (0.15 ± 0.02) reported so far for organosilica biolabels without activator metals. Moreover, the emission spectra and the quantum yield values are quite stable over time showing no significant aging effects after exposure to the ambient environment for more than 1 year, stressing the potential of these NPs as metal-free biolabels

Lamellar Salt-Doped Hybrids with Two Reversible Order/Disorder Phase Transitions

A lamellar bilayer hierarchically structured amide cross-linked alkyl/siloxane hybrid matrix (mono-amidosil, m-A(14)) was doped with a wide concentration range of potassium triflate (KCF₃SO₃), magnesium triflate (Mg­(CF₃SO₃)₂), and europium triflate (Eu­(CF₃SO₃)₃). In the K⁺-, Mg²⁺-, and Eu³⁺-based samples with <i>n</i> ≥ 5, 20, and 60 (where <i>n</i> is the molar ratio of amide CO groups per cation), respectively, the original lamellar structure of m-A(14) coexists with a new lamellar phase with lower interlamellar distance. The texture of the mono-amidosils doped with K⁺, Mg²⁺, and Eu³⁺ ions mimics cabbage leaves, foliated schist, and sea sponges, respectively. In the three series of materials, the cations bond to the oxygen atoms of the amide carbonyl groups. The amide–amide hydrogen-bonded array of m-A(14) is less perturbed by the inclusion of KCF₃SO₃ and Mg­(CF₃SO₃)₂ than by the incorporation of Eu­(CF₃SO₃)₃. The degree of ionic association is low for <i>n</i> ≥ 20. The cations coordinate to the oxygen atoms of the triflate ions, forming contact ion pairs at higher salt content. In the Mg­(CF₃SO₃)₂- and Eu­(CF₃SO₃)₃-containing materials with <i>n</i> = 5 and 10, respectively, crystalline salt is formed. The structural changes undergone by the alkyl chains of selected mono-amidosils in a heating/cooling cycle are reversible, are time-independent, and exhibit two distinct hysteresis domains, one associated with the order/disorder phase transition of the original lamellar bilayer structure of m-A(14) and the second one associated with the order/disorder phase transition of the new lamellar bilayer structure formed in the presence of the salts

Ratiometric Nanothermometer Based on an Emissive Ln³⁺-Organic Framework

Luminescent thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. Lanthanide-based materials are among the most versatile thermal probes used in luminescent nanothermometers. Here, nanorods of metal organic framework Tb_0.99Eu_0.01(BDC)_1.5(H₂O)₂ (BDC = 1-4-benzendicarboxylate) have been prepared by the reverse microemulsion technique and characterized and their photoluminescence properties studied from room temperature to 318 K. Aqueous suspensions of these nanoparticles display an excellent performance as ratiometric luminescent nanothermometers in the physiological temperature (300–320 K) range

Deciphering Density Fluctuations in the Hydration Water of Brownian Nanoparticles via Upconversion Thermometry

We investigate the intricate relationship among temperature, pH, and Brownian velocity in a range of differently sized upconversion nanoparticles (UCNPs) dispersed in water. These UCNPs, acting as nanorulers, offer insights into assessing the relative proportion of high-density and low-density liquid in the surrounding hydration water. The study reveals a size-dependent reduction in the onset temperature of liquid-water fluctuations, indicating an augmented presence of high-density liquid domains at the nanoparticle surfaces. The observed upper-temperature threshold is consistent with a hypothetical phase diagram of water, validating the two-state model. Moreover, an increase in pH disrupts the organization of water molecules, similar to external pressure effects, allowing simulation of the effects of temperature and pressure on hydrogen bonding networks. The findings underscore the significance of the surface of suspended nanoparticles for understanding high- to low-density liquid fluctuations and water behavior at charged interfaces

Deciphering Density Fluctuations in the Hydration Water of Brownian Nanoparticles via Upconversion Thermometry

We investigate the intricate relationship among temperature, pH, and Brownian velocity in a range of differently sized upconversion nanoparticles (UCNPs) dispersed in water. These UCNPs, acting as nanorulers, offer insights into assessing the relative proportion of high-density and low-density liquid in the surrounding hydration water. The study reveals a size-dependent reduction in the onset temperature of liquid-water fluctuations, indicating an augmented presence of high-density liquid domains at the nanoparticle surfaces. The observed upper-temperature threshold is consistent with a hypothetical phase diagram of water, validating the two-state model. Moreover, an increase in pH disrupts the organization of water molecules, similar to external pressure effects, allowing simulation of the effects of temperature and pressure on hydrogen bonding networks. The findings underscore the significance of the surface of suspended nanoparticles for understanding high- to low-density liquid fluctuations and water behavior at charged interfaces

Excimer Formation in a Terbium Metal–Organic Framework Assists Luminescence Thermometry

Photoluminescent isotypical phosphonate-based metal–organic frameworks, [Ln­(H₅btp)]·2H₂O [where Ln³⁺ = Tb³⁺ (<b>1</b>) or Gd³⁺ (<b>2</b>) and H₈btp = [1,1′-biphenyl]-3,3′,5,5′-tetrayltetrakis­(phosphonic acid)] prepared under solvo­(hydro)­thermal conditions exhibit a three-dimensional supramolecular structure built up from phosphonate linkers bridging Ln³⁺ ions. Single-crystal X-ray diffraction of <b>1</b> revealed hydrogen bonds along the <i>a</i>-axis between the crystallization water molecules and the phosphonate groups. The network features lozenge-shaped tubular channels with intermolecular interactions between adjacent organic ligand molecules. Crystal packing forces bring together two ligand biphenyl groups in a parallel arrangement, forming an excimer. A ratiometric luminescent thermometer operative close to room temperature was built based on the excimer and the ⁵D₄ → ⁷F₅ Tb³⁺ emissions, exhibiting a maximum relative thermal sensitivity of 1.26%·K^–1 and a minimum temperature uncertainty of 0.75 K at 319 K. This performance is similar to that of a thermometer based on the ligand singlet <i>S</i>_2,1 → <i>S</i>₀ and metal ⁵D₄ → ⁷F₅ transitions. Thus, the same MOF material provides two separate temperature data sets. This work shows the possibility of using excimer emissions in ratiometric luminescence thermometry

Modulating the Photoluminescence of Bridged Silsesquioxanes Incorporating Eu³⁺-Complexed <i>n</i>,<i>n</i>′-Diureido-2,2′-bipyridine Isomers: Application for Luminescent Solar Concentrators

Two new urea-bipyridine derived bridged organosilanes (<b>P5</b> and <b>P6</b>) have been synthesized and their hydrolysis–condensation under nucleophilic catalysis in the presence of Eu³⁺ salts led to luminescent bridged silsesquioxanes (<b>M5-Eu</b> and <b>M6-Eu</b>). An important loading of Eu³⁺ (up to 11%_w) can be obtained for the material based on the 6,6′-isomer. Indeed the photoluminescence properties of these materials, that have been investigated in depth (photoluminescence (PL), quantum yield, lifetimes), show a significantly different complexation mode of the Eu³⁺ ions for <b>M6-Eu</b>, compared with <b>M4-Eu</b> (obtained from the already-reported 4,4′-isomer) and <b>M5-Eu</b>. Moreover, <b>M6-Eu</b> exhibits the highest absolute emission quantum yield value (0.18 ± 0.02) among these three materials. The modification of the sol composition upon the addition of a malonamide derivative led to similar luminescent features but with an increased quantum yield (0.26 ± 0.03). In addition, <b>M6-Eu</b> can be processed as thin films by spin-coating on glass substrates, leading to plates coated by a thin layer (∼54 nm) of Eu³⁺-containing hybrid silica exhibiting one of the highest emission quantum yields reported so far for films of Eu³⁺-containing hybrids (0.34 ± 0.03) and an interesting potential as new luminescent solar concentrators (LSCs) with an optical conversion efficiency of ∼4%. The ratio between the light guided to the film edges and the one emitted by the surface of the film was quantified through the mapping of the intensity of the red pixels (in the RGB color model) from a film image. This quantification enabled a more accurate estimation of the transport losses due to the scattering of the emitted light in the film (0.40), thereby correcting the initial optical conversion efficiency to a value of 1.7%