The use of energy looping between Tm3+^{3+} and Er3+^{3+} ions to obtain an intense upconversion under the 1208 nm radiation and its use in temperature sensing

The upconversion phenomenon allows for the emission of nanoparticles (NPs) under excitation with near-infrared (NIR) light. Such property is demanded in biology and medicine to detect or treat diseases such as tumours. The transparency of biological systems for NIR light is limited to three spectral ranges, called biological windows. However, the most frequently used excitation laser to obtain upconversion is out of these ranges, with a wavelength of around 975 nm. In this article, we show an alternative – Tm$^{3+}$/Er$^{3+}$-doped NPs that can convert 1208 nm excitation radiation, which is in the range of the 2$^{nd}$ biological window, to visible light within the 1$^{st}$ biological window. The spectroscopic properties of the core@shell NaYF$_4$:Tm$^{3+}$@NaYF$_4$ and NaYF$_4$:Er$^{3+}$,Tm$^{3+}$@NaYF$_4$ NPs revealed a complex mechanism responsible for the observed upconversion. To explain emission in the studied NPs, we propose an energy looping mechanism: a sequence of ground state absorption, energy transfers and cross-relaxation (CR) processes between Tm$^{3+}$ ions. Next, the excited Tm$^{3+}$ ions transfer the absorbed energy to Er3+ ions, which results in green, red and NIR emission at 526, 546, 660, 698, 802 and 982 nm. The ratio between these bands is temperature-dependent and can be used in remote optical thermometers with high relative temperature sensitivity, up to 2.37%/°C at 57 °C. The excitation and emission properties of the studied NPs fall within 1$^{st}$ and 2$^{nd}$ biological windows, making them promising candidates for studies in biological systems

Room-temperature liquid-phase synthesis of aluminium nanoparticles

Aluminium nanoparticles, Al(0), are obtained via liquid-phase synthesis at 25 °C. Accordingly, AlBr₃ is reduced by lithium naphthalenide ([LiNaph]) in toluene in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA). The Al(0) nanoparticles are small (5.6 ± 1.5 nm) and highly crystalline. A light yellow colour and absorption at 250–350 nm are related to the plasmon-resonance absorption. Due to TMEDA functionalization, the Al(0) nanoparticles are colloidally and chemically stable, but show high reactivity after TMEDA removal

Ionic-liquid-based synthesis of GaN nanoparticles

GaN nanoparticles, 3–8 nm in diameter, are prepared by a microwave-assisted reaction of GaCl3 and KNH2 in ionic liquids. Instantaneously after the liquid-phase synthesis, the β-GaN nanoparticles are single-crystalline. The band gap is blue-shifted by 0.6 eV in comparison to bulk-GaN indicating quantum confinement effects. The GaN nanoparticles show intense green emission with a quantum yield of 55 ± 3%

Real-time direct transmission electron microscopy imaging of phase and morphology transformation from solid indium oxide hydroxide to hollow corundum-type indium oxide nanocrystallites

A time-resolved series of high-resolution transmission electron microscopy (HRTEM) images are used to monitor phase and morphology transformation of rod-like and spherical particles with the initial ortho-rhombic In OOH phase in situ under continuous illumination with high-energy electrons in a transmission electron microscope. For both particle types, the electron-beam irradiation induces a fast InOOH to rh-In₂O₃ decomposition accompanied by the formation of voids within the particle/rod center. After illu-mination time intervals of about 1–2 min (i.e.electron dose 6.3–12.6 × 10⁷enm¯²) for particles and8 min (4.3 × 108enm¯²) for rods, respectively, several small empty cavities become visible in the particle/rod center. The cavities coalesce and form a large hollow space/canal after further illumination. Time-resolved in situ HRTEM unambiguously shows that the formation of internal voids in both nano particle types is a consequence of the structural InOOH-to-rh-In₂O₃ phase transition that starts at the surface of the corresponding particle. The as-formed oxide phase encapsulates the untransformed hydroxylated phase. Its decomposition does not follow the Kirkendall mechanism; the matter transferred outwards is removed in the form of water, leading to void formation inside without an increase of the particle size

Aqueous Conversion of Fructose Phosphate Precursor Nanoparticles into Emissive C-Dot Composite Nanoparticles

[ZrO]$^{2+}$[F6P]$^{2}$− and [Eu(OH)]$^{2}$+[F6P]$^{2}$− precursor nanoparticles (F6P: D-fructose-6-phosphate) are converted in a one-pot, aqueous approach to C-dot@[ZrO]$^{2+}$+[HPO$_{4}$]$^{2-}$ and C-dot@[Eu(OH)]$^{2+}$[HPO$_{4}$]$^{2-}$ composite nanoparticles. Herein, the C-dots (2–3 nm) are embedded in a dense zirconyl/europium phosphate matrix. The resulting composite nanoparticles (40–50 nm) are well-dispersible in water and show blue and red emission. A one-pot, fully water-based synthesis of blue- and red-emitting C-dots is presented. To this concern, [ZrO]$^{2+}$[F6P]$^{2}$− and [Eu(OH)]$^{2}$+[F6P]$^{2}$− precursor nanoparticles (F6P: D-fructose-6-phosphate) are prepared in water and converted to C-dot@[ZrO]$^{2+}$+[HPO$_{4}$]$^{2-}$ and C-dot@[Eu(OH)]$^{2+}$[HPO$_{4}$]$^{2-}$ composite nanoparticles in boiling water (100 °C) via microwave heating. Composition, structure, and fluorescence of the composite nanoparticles are validated by different analytical methods (e. g., FT-IR, EA, TG, DLS, SEM, TEM, EDXS). The resulting aqueous suspensions are characterized by high colloidal stability and intense emission. Specifically, C-dot@[Eu(OH)]$^{2+}$[HPO$_{4}$]$^{2-}$ exhibits Eu$^{3+}$-type red emission in water. The one-pot water-based synthesis with fructose-containing precursor nanoparticles and the structure of the phosphate-stabilized C-dot composite nanoparticles are reported for the first time

A quantitative microscopic view on the gas-phase-dependent phase transformation from tetragonal to monoclinic ZrO2

ZrO2 is a versatile material with diverse applications, including structural ceramics, sensors, and catalysts. The properties of ZrO2 are largely determined by its crystal structure, which is temperature- and atmosphere dependent. Thus, this work focuses on a quantitative analysis of the temperature- and gas atmosphere-dependent phase transformation of tetragonal t-ZrO2 into monoclinic m-ZrO2 during heating–cooling cycles from room temperature to 1273 K. Synchrotron-based in situ X-ray diffraction (XRD) studies in gas atmospheres of different reduction strengths, namely, 5 vol% H2/Ar, He, CO2, and air, revealed a stabilizing effect of inert and reductive environments, directly yielding different temperature onsets in the phase transformation during cooling (i.e., 435, 510, 710, and 793 K for 5 vol% H2/Ar, He, CO2, and air, respectively). Rietveld refinement shows a direct influence of the atmosphere on grain size, unit cell, and weight fraction of both polymorphs in the product composite matrix. The tetragonal-to-monoclinic (t–m) phase transformation is suppressed in the sample heated only up to ∼850 K, independent of the gas atmosphere. The results of ex situ XRD, transmission electron microscopic, electron paramagnetic resonance, and oxygen titration experiments confirmed that the phase transformation is accompanied by a change in the crystallite/particle size and the amount of lattice defects (i.e., oxygen vacancy). Due to the different onset temperatures, a complex interplay between kinetic limitations of phase transformation and grain sintering yields different pathways of the phase transformation and, eventually, very different final crystallite sizes of both t-ZrO2 and m-ZrO2

Coordination mechanism of cyanine dyes on the surface of core@active shell β-NaGdF4:Yb3+,Er3+ nanocrystals and its role in enhancing upconversion luminescence

The sensitization of lanthanide-doped upconversion nanocrystals (UCNCs) using organic dyes with a broad and intense optical absorption is an interesting approach for efficient excitation-energy harvesting and enhancing the upconversion luminescence of such UCNCs. In this work, an ultrasmall (similar to 6.5 nm in diameter) beta-NaGdF4:Yb3+,Er3+ core and related core@shell UCNCs were sensitized using six NIR-excitable cyanine dyes with a wide range of functional groups and optical properties. The greatest UC enhancement of 680-times was observed for the conjugate between the Cy 754 dye and NaGdF4:Yb3+,Er3+@NaGdF4:10%Yb3+,30%Nd3+ core@shell UCNCs excited using a 754 nm laser. The enhancement was estimated relative to NaGdF4:Yb3+,Er3+@NaGdF4:10%Yb3+,30%Nd3+ core@shell UCNCs capped with oleic acid and excited using a similar intensity (75 W cm(-2)) of a 980 nm laser. UC intensity measurements for identical dye-sensitized UCNCs carried out in methanol and in deuterated methanol under argon, as well as in air, allowed us to reveal the connection of the dye triplet states with UCNC sensitization as well as of the hydroxyl groups with quenching of the excited states of lanthanide ions. For UCNCs dispersed in methanol, the strong quenching UC luminescence was always observed, including core@shell UCNCs (with a shell of similar to 2 nm). A strong influence of the triplet states of the dyes was observed for the two dyes Cy 754 and Cy 792 that bind firmly to UCNCs and allow the distances between the dye and the UCNC to be reduced, whereas the contribution of this sensitization pathway is very insignificant for Cy 740 and Cy 784 dyes that bind weakly to UCNCs

Nanosized Gadolinium and Uranium—Two Representatives of High-Reactivity Lanthanide and Actinide Metal Nanoparticles

Gadolinium (Gd0) and uranium (U0) nanoparticles are prepared via lithium naphthalenide ([LiNaph])-driven reduction in tetrahydrofuran (THF) using GdCl3 and UCl4, respectively, as low-cost starting materials. The as-prepared Gd0 and U0 suspensions are colloidally stable and contain metal nanoparticles with diameters of 2.5 ± 0.7 nm (Gd0) and 2.0 ± 0.5 nm (U0). Whereas THF suspensions are chemically stable under inert conditions (Ar and vacuum), nanoparticulate powder samples show high reactivity in contact with, for example, oxygen, moisture, alcohols, or halogens. Such small and highly reactive Gd0 and U0 nanoparticles are first prepared via a dependable liquid-phase synthesis and stand as representatives for further nanosized lanthanides and actinides

Coordination mechanism of cyanine dyes on the surface of core@active shell β-NaGdF4_{4}:Yb3+^{3+},Er3+^{3+} nanocrystals and its role in enhancing upconversion luminescence

The sensitization of lanthanide-doped upconversion nanocrystals (UCNCs) using organic dyes with a broad and intense optical absorption is an interesting approach for efficient excitation-energy harvesting and enhancing the upconversion luminescence of such UCNCs. In this work, an ultrasmall (∼6.5 nm in diameter) β-NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$ core and related core@shell UCNCs were sensitized using six NIR-excitable cyanine dyes with a wide range of functional groups and optical properties. The greatest UC enhancement of 680-times was observed for the conjugate between the Cy 754 dye and β-NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$@NaGdF$_{4}$:10%Yb$^{3+},30$^{3+} core@shell UCNCs excited using a 754 nm laser. The enhancement was estimated relative to NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$@NaGdF$_{4}$:10%Yb$^{3+},30$^{3+} core@shell UCNCs capped with oleic acid and excited using a similar intensity (75 W cm$^{-2}$) of a 980 nm laser. UC intensity measurements for identical dye-sensitized UCNCs carried out in methanol and in deuterated methanol under argon, as well as in air, allowed us to reveal the connection of the dye triplet states with UCNC sensitization as well as of the hydroxyl groups with quenching of the excited states of lanthanide ions. For UCNCs dispersed in methanol, the strong quenching UC luminescence was always observed, including core@shell UCNCs (with a shell of ∼2 nm). A strong influence of the triplet states of the dyes was observed for the two dyes Cy 754 and Cy 792 that bind firmly to UCNCs and allow the distances between the dye and the UCNC to be reduced, whereas the contribution of this sensitization pathway is very insignificant for Cy 740 and Cy 784 dyes that bind weakly to UCNCs