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Upconverting nanoparticles (UCNPs) are well-known for their capacity to convert near-infrared light into UV/visible light, benefitting various applications where light triggering is required. At the nanoscale, loss of luminescence intensity is observed and thus, a decrease in photoluminescence quantum yield (PLQY), usually ascribed to surface quenching. We evaluate this by measuring the PLQY of NaGdF4:Er3+,Yb3+ UCNPs as a function of size (ca. 15 to 100 nm) and shape (spheres, cubes, hexagons). Our results show that the PLQY of α-phase NaGdF4 Er3+,Yb3+ surpasses that of β-NaGdF4 for sizes below 20 nm, an observation related to distortion of the crystal lattice when the UCNPs become smaller. The present study also underlines that particle shape must not be neglected as a relevant parameter for PLQY. In fact, based on a mathematical nucleus/hull volumetric model, shape was found to be particularly relevant in the 20 to 60 nm size range of the investigated UCNPs.

A “spray and measure” technology is provided that entails spraying a mixture of photoluminescent lanthanide (Eu3+ and Tb3+) complexes to form a film whose changes in luminescence contain information about the temperature of the substrate.

Magic-sized nanoclusters have gained tremendous attention from researchers as they offer applicability in a variety of fields ranging from chemistry and physics to electronics. The first report on magic-sized nanoclusters of CdSe gave impetus to deepen the understanding of the sub 2 nanometer (

Here, the synthesis of sub-micron MMnF3 (M = Na or K) particles by a rapid microwave-assisted approach is reported. Adjustment of the Na+-to-Mn2+ ratio in the reaction mixture yielded tuneable morphologies, i.e., rods, ribbons, and plates. Relaxometric results indicated that poly(acrylic acid)-capped MMnF3 particles exhibited characteristic magnetic properties, which endows them with potential T1-weighted contrast agent capabilities.

Lanthanide-based upconverting nanoparticles (UCNPs) are largely sought-after for biomedical applications ranging from bioimaging to therapy. A straightforward strategy is proposed here using the naturally sourced polymer phytoglycogen to coencapsulate UCNPs with hydrophobic photosensitizers as an optical imaging platform and light-induced therapeutic agents. The resulting multifunctional sub-micrometer-sized luminescent beads are shown to be cytocompatible as carrier materials, which encourages the assessment of their potential in biomedical applications. The loading of UCNPs of various elemental compositions enables multicolor hyperspectral imaging of the UCNP-loaded beads, endowing these materials with the potential to serve as luminescent tags for multiplexed imaging or simultaneous detection of different moieties under near-infrared (NIR) excitation. Coencapsulation of UCNPs and Rose Bengal opens the door for potential application of these microcarriers for collagen crosslinking. Alternatively, coloading UCNPs with Chlorin e6 enables NIR-light triggered generation of reactive oxygen species. Overall, the developed encapsulation methodology offers a straightforward and noncytotoxic strategy yielding water-dispersible UCNPs while preserving their bright and color-tunable upconversion emission that would allow them to fulfill their potential as multifunctional platforms for biomedical applications.

A low-cost template-free solution chemical route to highly porous nanocrystalline sponges of ZnO-EuO1.5 with 0-5 mol % Eu is presented. The process uses Zn- and Eu-acetate-nitrate and triethanolamine as precursors in methanol. After evaporation of the solvent and heating at 200 °C for 3 min, crystalline ZnO:Eu sponges with minor amounts of organic residues were obtained. Heating to 400 °C replaced the organics with carbonate, which in its turn was decomposed at temperatures below 600 °C, forming ZnO:Eu sponges. Samples heated to 200-1000 °C for 3 min were studied with XRD, SEM, TEM, TG, XPS, and IR spectroscopy. The ZnO:Eu crystallite sizes could be tuned from below 10 nm for sponges prepared at 200-500 °C, to over 100 nm range at 900 °C, without sintering of the overall microstructure. XRD showed the presence of hexagonal ZnO:Eu (or at 700-1000 °C, ZnO:Eu and cubic Eu2O3) as the only phases present. The ZnO:Eu had slightly larger unit cell dimensions than the literature value of ZnO for samples obtained at 200-600 °C, while the unit cells of samples obtained at higher temperatures were quite close to the value of undoped ZnO. XPS showed that Eu was mainly in its 3+ state and well-distributed within the sponges but segregated at the ZnO sponge surface upon heating at 700-1000 °C, in accordance with XRD studies showing Eu2O3 formation.

The chemical stability of oleate-capped sub-10 nm α- and β-NaREF4 NPs (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu for α- and RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy for β-phase NPs) was evaluated under the acidic conditions used for ligand removal towards water dispersibility. It was found that for such small NPs, a pH lower than 3 was necessary for the water transfer to be efficient and to yield well-dispersed ligand-free NPs. In stark contrast to the generally considered good chemical stability of NaREF4, these conditions were observed to pose a risk to phase transformation of the NaREF4 NPs into much larger, hexagonal- or orthorhombic-phase REF3, depending on the NP composition. A correlation between the thermodynamic stability of the α/β-NaREF4 and the hexagonal/orthorhombic REF3 phases – dictated by the RE ion choice – and the chemical stability of the NPs was found. For instance, β-NaGdF4 NPs remained stable, while α-NaGdF4 NPs underwent phase transformation into hexagonal GdF3. More general, NaREF4 NPs based on lighter RE ions were more prone towards phase transformation, while those based on heavier RE ions exhibited stability. Herein, within the RE series, the borderline for phase transformation was identified as Tb/Dy for α-NaREF4 NPs and Sm/Eu for β-NaREF4 NPs, respectively. Also, given the large interest in luminescent NPs for, e.g. biomedical applications, optically active Ln3+ ions (Ln = Nd, Eu, Tb, Er/Yb) were doped into α/β-NaGdF4 host NPs, and the dopant influence on the chemical stability was evaluated. Steady state and time-resolved spectroscopy unveiled spectral features characteristic for Ln3+ f–f transitions, i.e. downshifting and upconversion, before and after ligand removal. Overall, the results herein described emphasise the importance of minding the chemical procedure used for ligand removal of NaREF4 NPs of different crystalline phases and RE compositions.

Upconverting nanoparticles (UCNPs) are well-known for their capacity to convert near-infrared light into UV/visible light, benefitting various applications where light triggering is required. At the nanoscale, loss of luminescence intensity is observed and thus, a decrease in photoluminescence quantum yield (PLQY), usually ascribed to surface quenching. We evaluate this by measuring the PLQY of NaGdF

Nanocrystalline ZnO sponges doped with 5 mol% EuO1.5 are obtained by heating metal–salt complex based precursor pastes at 200–900 °C for 3 min. X-ray diffraction, transmission electron microscopy, and extended X-ray absorption fine structure (EXAFS) show that phase separation into ZnO:Eu and c-Eu2O3 takes place upon heating at 700 °C or higher. The unit cell of the clean oxide made at 600 °C shows only ≈0.4% volume increase versus undoped ZnO, and EXAFS shows a ZnO local structure that is little affected by the Eu-doping and an average Eu3+ ion coordination number of ≈5.2. Comparisons of 23 density functional theory-generated structures having differently sized Eu-oxide clusters embedded in ZnO identify three structures with four or eight Eu atoms as the most energetically favorable. These clusters exhibit the smallest volume increase compared to undoped ZnO and Eu coordination numbers of 5.2–5.5, all in excellent agreement with experimental data. ZnO defect states are crucial for efficient Eu3+ excitation, while c-Eu2O3 phase separation results in loss of the characteristic Eu3+ photoluminescence. The formation of molecule-like Eu-oxide clusters, entrapped in ZnO, proposed here, may help in understanding the nature of the unexpected high doping levels of lanthanide ions in ZnO that occur virtually without significant change in ZnO unit cell dimensions.

Here, the synthesis of sub-micron MMnF

The detailed structure of ZnO doped with 5 at.% (metal) of the large aliovalent Eu3+-ions was investigated using EXAFS to describe the local Eu and Zn coordination. The microstructure, crystalline phases, contents and ZnO unit-cell parameters for the ZnO:5%Eu sponges synthesised at 200 to 900 oC were obtained by XRD, SEM, and TEM analysis. XRD showed peaks solely of h-ZnO for the 600 oC sample, while heating at 700 oC and higher caused phase separation into h-ZnO:Eu and c-Eu2O3. XRD showed a close to zero increase in ZnO unit cell-volume of ca. 0.4 vol%, compared to un-doped ZnO for the non-phase separated, clean oxide made at 600 oC. The Zn EXAFS data showed an almost intact local ZnO structure. The Eu EXAFS showed an unusually low coordination number (CN) of ca. 5 for the 200-600 oC samples, while the CN increased for higher temperatures, in concert with the formation of c-Eu2O3. 23 DFT-generated theoretical ZnO structures containing Eu-clusters built from 1 to 4 Eu3+-Zn2+-vacancy- Eu3+ pairs were compared with the experimental data. The lowest formation energies and ZnO unit-cell volume increase versus un-doped ZnO (0.6-0.7 vol%), were obtained when combining two or four Eu3+-Zn2+-vacancy- Eu3+ pairs into Eu4 and Eu8 clusters showing an average Eu CN of ca. 5. These theoretically determined lowest energy structures were all in good agreement with the experimental results obtained by EXAFS and XRD. Photoluminescence excitation and emission spectra performed on the ZnO:5at%Eu sponges obtained at various temperatures, showed strong quenching of the characteristic Eu3+ transitions for samples obtained at 600 and 800 °C, most likely due to changes in the ZnO defect states, which are crucial for Eu3+ excitation, and due to self-quenching upon Eu clustering and c- Eu2O3 phase separation. Thus, the optical data further supported Eu clustering found by EXAFS, DFT and XRD techniques, corroborating structure-property relationships in these materials. Overall, as far as we can find, the findings reported herein point to a doping structure very different from those previously proposed in the literature. It demonstrates that the semiconductor ZnO can host molecular-sized clusters of metal-oxides, very dissimilar to ZnO.

We prepared a hybrid system composed of a continuous film of a dinuclear lanthanide complex [Ln2 bpm(tfaa)6] (Ln = Tb or Eu) and upconverting nanoparticles (UCNPs) using a straightforward drop-cast methodology. The system displayed visible emission under near-infrared (NIR) excitation, simultaneously stemming from sub-10 nm UCNPs and [Ln2] complexes, the latter species being otherwise directly excitable only using UV-blue radiation. In light of the results of steady-state—including power-dependent—and time-resolved optical measurements, we identified the radiative, primarily ligand-mediated nature of the energy transfer from Tm3+ ions in the UCNPs to Ln3+ ions in the complexes. Hyperspectral mapping and electron microscopy observations of the surface of the hybrid system confirmed the continuous and concomitant distribution of UCNPs and lanthanide complexes over the extensive composite films. Key features of the hybrid system are the simultaneous UV-blue and NIR light harvesting capabilities and their eas...

The optical features and photocatalytic activity of porous undoped and Eu3+-doped zinc oxide (ZnO) sponge-like structures, annealed at various temperatures, were assessed to establish the relations ...

Sodium gadolinium fluoride (NaGdF4) nanoparticles are promising candidates as T1 shortening magnetic resonance imaging (MRI) contrast agents due to the paramagnetic properties of the Gd3+ ion. Effects of size and surface modification of these nanoparticles on proton relaxation times have been widely studied. However, to date, there has been no report on how T1 relaxivity (r1) is affected by the different polymorphs in which NaGdF4 crystallizes: cubic (α) and hexagonal (β). Here, a microwave-assisted thermal decomposition method was developed that grants selective access to NaGdF4 nanoparticles of either phase in the same size range, allowing the influence of host crystallinity on r1 to be investigated. It was found that at 3 T cubic NaGdF4 nanoparticles exhibit larger r1 values than their hexagonal analogues. This result was interpreted based on Solomon–Bloembergen–Morgan theory, suggesting that the inner sphere contribution to r1 is more pronounced for cubic NaGdF4 nanoparticles as compared to their hexagonal counterparts. This holds true irrespective of the chosen surface modification, i.e. small citrate groups or longer chain poly(acrylic acid). Key aspects were found to be a polymorph-induced larger hydrodynamic diameter and the higher magnetization possessed by cubic nanoparticles.

Small and ultrasmall β-NaGdF4:Yb3+,Er3+ upconverting and near-infrared emitting nanoparticles were prepared by a rapid microwave-assisted strategy utilizing three different lanthanide (Ln) precursors: trifluoroacetates [Ln(TFA)3], oleates [Ln(OA)3], and acetates [Ln(Ac)3]. Assessment of their morphology, crystal phase, and photoluminescence revealed how the nature of the precursor has a great impact on the formation of these nanomaterials. In fact, our findings indicate a sizeable effect on the growth kinetics, which ultimately determined the nanoparticle size, crystal phase, and optical behavior. In a nutshell, the nanoparticles synthesized starting from [Ln(TFA)3] were the largest (5 to 9 nm) and featured the strongest emission. Those prepared from [Ln(OA)3] and [Ln(Ac)3], respectively, exhibited similar sizes in the ultrasmall size regime (ca. 3 nm). The different thermal stabilities of the respective lanthanide precursors – inducing different pressure conditions in the microwave reaction vessel – are suggested to affect the growth mechanism of the nanoparticles. Ultimately, this allows for size control within the sub-10 nm realm. The developed approach was further demonstrated suitable for the growth of core/shell architectures, which greatly improved the emission brightness, while maintaining the particle size below 20 nm.

The remarkable optomagnetic properties of the rare-earths (RE) make RE-based materials ideal for biomedical applications, including diagnostic (e.g., imaging, nanothermometry) and therapeutic (e.g., drug delivery, photodynamic therapy) approaches. This is due the unique electronic properties of the f-elements allowing for upconversion and near-infrared emission under near-infrared excitation as well as high magnetic moments. Yet, challenges remain; low emission intensity and efficiency of small nanoparticles (NPs), and reliable, fast synthesis routes. As material chemists, we tackle these challenges with new designs of RE-NPs by chemically controlled synthesis, application-oriented surface chemistry, and understanding of structure-property-relationships. Sodium rare-earth fluorides (NaREF4) are our favorite materials, and we developed a fast and reliable microwave-assisted synthesis approach allowing crystalline phase and size control in the sub 15nm realm. Such control is crucial for the understanding of fundamental structure-property relationships and to optimize their optical and magnetic properties, when aiming for the design of next-generation optical probes or contrast agents for magnetic resonance imaging. For instance, NaGdF4 NPs are gaining interest as alternative MRI contrast agent, while co-doping with RE3+ ions renders them excellent candidates for photoluminescent optical probes. The hexagonal crystalline phase of NaGdF4 is known as the more efficient host material for upconversion emission, yet interestingly, it was found that its cubic counterpart shows superior performance as MRI contrast agent. Having a fast and reliable synthesis route towards NaREF4 NPs on hand, we now explore various nanoparticle architectures and compositions with the goal to optimize their optomagnetic properties, ultimately resulting in the design of biocompatible multimodal bioprobes. This presentation will shed light on recent results and remaining challenges in the field of RE-based nanostructures with respect to their microwave-assisted synthesis as well as structural and optomagnetic properties, seeking biomedical application, while also touching on hyperspectral imaging as an emerging analytical tool offering spatio-spectral information about RE-based materials.

Growing attention toward optically active materials has prompted the development of novel synthesis methods for a more reliable and efficient access to these systems. In this regard, microwave-assisted approaches provide unique advantages over traditional solvothermal methods reliant on convectional heating: namely, significantly shorter reaction durations, more rigid reaction conditions, and thus a higher degree of reproducibility. Reported herein for the first time is a rapid synthesis of rare-earth (RE3+)-doped LiYF4 upconverting and downshifting microparticles with well-defined bipyramidal morphology and good size dispersion via a microwave-assisted solvothermal process. The suggested material growth mechanism identifies a suitable Li+ to RE3+ ion ratio, an abundance of pH-sensitive acetate surface-capping ligands, and an appropriate reaction temperature/time profile as crucial for enabling a phase transformation of an intermediary yttrium ammonium fluoride phase into LiYF4 and subsequent particle ripening. The versatility of the reported method is highlighted by its extension toward the synthesis of other state of the art M(RE)F4 (M = alkali metal) optical materials: RE3+-doped LiYbF4 microparticles and β-NaGdF4 and α-NaYF4 nanoparticles. All of the obtained Yb3+/Er3+- and Yb3+/Tm3+-codoped M(RE)F4 materials exhibited characteristic upconversion emission, while downshifting capabilities were induced through Ce3+/Tb3+ codoping of LiYF4. Further attention was devoted to single-particle optical characterization via hyperspectral imaging of Yb3+/Er3+- and Yb3+/Tm3+-codoped LiYF4 microparticles to explore the spatial variability of upconversion emission within individual particles.

Integrating sphere modifications enable novel absolute upconversion photoluminescence quantum yield measurements by heating and controlling sample temperature. Heating above 337K resulted in a 43% and 25% decrease in quantum yield for phosphor and nanoparticles, respectively.

The chemical stability of oleate-capped sub-10 nm α- and β-NaREF4 NPs (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu for α- and RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy for β-phase NPs) was evaluated under the acidic conditions used for ligand removal towards water dispersibility. It was found that for such small NPs, a pH lower than 3 was necessary for the water transfer to be efficient and to yield well-dispersed ligand-free NPs. In stark contrast to the generally considered good chemical stability of NaREF4, these conditions were observed to pose a risk to phase transformation of the NaREF4 NPs into much larger, hexagonal- or orthorhombic-phase REF3, depending on the NP composition. A correlation between the thermodynamic stability of the α/β-NaREF4 and the hexagonal/orthorhombic REF3 phases - dictated by the RE ion choice - and the chemical stability of the NPs was found. For instance, β-NaGdF4 NPs remained stable, while α-NaGdF4 NPs underwent phase transformation into hexagonal GdF3. More general, NaREF4 NPs based on lighter RE ions were more prone towards phase transformation, while those based on heavier RE ions exhibited stability. Herein, within the RE series, the borderline for phase transformation was identified as Tb/Dy for α-NaREF4 NPs and Sm/Eu for β-NaREF4 NPs, respectively. Also, given the large interest in luminescent NPs for, e.g. biomedical applications, optically active Ln3+ ions (Ln = Nd, Eu, Tb, Er/Yb) were doped into α/β-NaGdF4 host NPs, and the dopant influence on the chemical stability was evaluated. Steady state and time-resolved spectroscopy unveiled spectral features characteristic for Ln3+ f-f transitions, i.e. downshifting and upconversion, before and after ligand removal. Overall, the results herein described emphasise the importance of minding the chemical procedure used for ligand removal of NaREF4 NPs of different crystalline phases and RE compositions.

In this work, we describe a protocol for a novel application of hyperspectral imaging (HSI) in the analysis of luminescent lanthanide (Ln3+)-based molecular single crystals. As representative example, we chose a single crystal of the heterodinuclear Ln-based complex [TbEu(bpm)(tfaa)6] (bpm=2,2'-bipyrimidine, tfaa- =1,1,1-trifluoroacetylacetonate) exhibiting bright visible emission under UV excitation. HSI is an emerging technique that combines 2-dimensional spatial imaging of a luminescent structure with spectral information from each pixel of the obtained image. Specifically, HSI on single crystals of the [Tb-Eu] complex provided local spectral information unveiling variation of the luminescence intensity at different points along the studied crystals. These changes were attributed to the optical anisotropy present in the crystal, which results from the different molecular packing of Ln3+ ions in each one of the directions of the crystal structure. The HSI herein described is an example of the suitability of such technique for spectro-spatial investigations of molecular materials. Yet, importantly, this protocol can be easily extended for other types of luminescent materials (such as micron-sized molecular crystals, inorganic microparticles, nanoparticles in biological tissues, or labelled cells, among others), opening many possibilities for deeper investigation of structure-property relationships. Ultimately, such investigations will provide knowledge to be leveraged into the engineering of advanced materials for a wide range of applications from bioimaging to technological applications, such as waveguides or optoelectronic devices.

Lanthanide (Ln3+)-based nanoparticles (NPs) with light emission in the ultraviolet (UV), visible, and near-infrared (NIR) region under NIR excitation are emerging multifunctional players in the biomedical field. Excitation and emission wavelengths matching the NIR biological transparency windows (700-1870 nm) set the basis for deeper penetration depth into biological tissues due to less absorption, higher resolution as a result of reduced scattering, and improved imaging contrast due to tissue autofluorescence minimization; all being inevitable requirements for optical in vivo bioimaging. Moreover, frequency-conversion in Ln3+-based NPs is sensitive to their thermal environment, thus allowing for application as optical temperature sensors. Additional heating capability opens the pathway to self-monitored optical agents with potential for photothermal therapies (PTTs). UV-visible emitting upconverting nanoparticles (UCNPs) are of particular interest for applications in photodynamic therapies (PDTs) owing to their capability to trigger various photochemical processes upon NIR excitation. It is the aim of this review to highlight the most recent achievements and advances in the field of Ln3+-based NPs as frequency-converting materials for biomedical applications and to identify current trends as well as remaining challenges. Special focus will be set on the most recently presented Ln3+-based systems whose potential has been evaluated in vitro and in vivo as NIR-to-NIR bioimaging probes, NIR-operating nanothermometers and -heaters as well as photochemical agents.

With the goal of developing more accurate, efficient, non-invasive and fast diagnostic tools, the use of near-infrared (NIR) light in the range of the second and third biological windows (NIR-II: 1000–1350 nm, NIR-III: 1550–1870 nm) is growing remarkably as it provides the advantages of deeper penetration depth into biological tissues, better image contrast, reduced phototoxicity and photobleaching. Consequently, NIR-based bioimaging has become a quickly emerging field and manifold new NIR-emitting bioprobes have been reported. Classes of materials suggested as potential probes for NIR-to-NIR bioimaging (using NIR light for the excitation and emission) are quite diverse. These include rare-earth based nanoparticles, Group-IV nanostructures (single-walled carbon nanotubes, carbon nanoparticles and more recently Si- or Ge-based nanostructures) as well as Ag, In and Pb chalcogenide quantum dots. This review summarizes and discusses current trends, material merits, and latest developments in NIR-to-NIR bioimaging taking advantage of the region above 1000 nm (i.e. the second and third biological windows). Further consideration will be given to upcoming probe materials emitting in the NIR-I region (700–950 nm), thus do not possess emissions in these two windows, but have high expectations. Overall, the focus is placed on recent discussions concerning the optimal choice of excitation and emission wavelengths for deep-tissue high-resolution optical bioimaging and on fluorescent bioprobes that have successfully been implemented in in vitro and in vivo applications.

Two homodinuclear and one heterodinuclear lanthanide (Ln)-based complexes of the general formula [Ln2 (bpm)(tfaa)6 ] (Ln=Eu (1), Tb (2), Eu-Tb (3), bpm=2,2'-bipyrimidine, tfaa- =1,1,1-trifluoroacetylacetonate) were synthesized and characterized by single-crystal photoluminescence spectroscopy and hyperspectral imaging. Complexes 1 and 2 crystallize in two polymorphic structures, while three polymorphs were isolated for 3, namely having needle-, plate-, and block-like morphologies. Single-crystal photoluminescence spectroscopy and imaging on Eu3+ -containing 1 and 3 revealed polymorph-dependent J-splitting of the hypersensitive 5 D0 →7 F2 Eu3+ transition as well as electric-to-magnetic dipole emission intensity ratios. According to these observations, the lowest symmetry chemical environment was attributed to the Eu3+ ions present in the needle-like polymorph, also in agreement with single-crystal X-ray diffraction analysis. More importantly, hyperspectral imaging on all three single-crystal polymorphs of 3 exhibits optical anisotropy with photoluminescence enhancement at specific crystallographic faces. This behavior was ascribed to the distinct molecular packing of the Ln-Ln dimers in each polymorphic crystal as well as to face-specific local symmetry of the Eu3+ centers. Overall, opto-structural relationships of three Ln-Ln dimers and their single-crystal polymorphs were established as a particularly promising avenue for control of photoluminescence by chemical crystal engineering.

Owing to the alluring possibility of contactless temperature probing with microscopic spatial resolution, photoluminescence nanothermometry at the nanoscale is rapidly advancing towards its successful application in biomedical sciences. The emergence of near-infrared nanothermometers has paved the way for temperature sensing at the deep tissue level. However, water dispersibility, adequate size at the nanoscale, and the capability to efficiently operate in the second and third biological optical transparency windows are the requirements that still have to be fulfilled in a single nanoprobe. In this work, these requirements are addressed by rare-earth doped nanoparticles with core/shell-architecture, dispersed in water, whose excitation and emission wavelengths conveniently fall within the biological optical transparency windows. Under heating-free 800 nm excitation, double nanothermometry is realized either with Ho3+–Nd3+ (1.18–1.34 μm) or Er3+–Nd3+ (1.55–1.34 μm) NIR emission band ratios, both displaying equal thermal sensitivities around 1.1% °C−1. It is further demonstrated that, along with the interionic energy transfer processes, the thermometric properties of these nanoparticles are also governed by the temperature dependent energy transfer to the surrounding solvent (water) molecules. Overall, this work presents a novel water dispersible double ratiometric nanothermometer operating in the second and third biological optical transparency windows. The temperature dependent particle–solvent interaction is also presented, which is critical for e.g. future in vivo applications.

Sodium gadolinium fluoride (NaGdF4) nanoparticles are promising candidates as T1 shortening magnetic resonance imaging (MRI) contrast agents due to the paramagnetic properties of the Gd3+ ion. Effects of size and surface modification of these nanoparticles on proton relaxation times have been widely studied. However, to date, there has been no report on how T1 relaxivity (r1) is affected by the different polymorphs in which NaGdF4 crystallizes: cubic (α) and hexagonal (β). Here, a microwave-assisted thermal decomposition method was developed that grants selective access to NaGdF4 nanoparticles of either phase in the same size range, allowing the influence of host crystallinity on r1 to be investigated. It was found that at 3 T cubic NaGdF4 nanoparticles exhibit larger r1 values than their hexagonal analogues. This result was interpreted based on Solomon-Bloembergen-Morgan theory, suggesting that the inner sphere contribution to r1 is more pronounced for cubic NaGdF4 nanoparticles as compared to their hexagonal counterparts. This holds true irrespective of the chosen surface modification, i.e. small citrate groups or longer chain poly(acrylic acid). Key aspects were found to be a polymorph-induced larger hydrodynamic diameter and the higher magnetization possessed by cubic nanoparticles.

Lanthanide-doped upconverting nanoparticles (UCNPs) are well-known for their inherent ability to convert low energy near-infrared (NIR) excitation wavelengths into higher energy emission wavelengths covering the ultraviolet (UV) to NIR regions. This optical feature makes UCNPs highly attractive for a broad range of applications including (bio)imaging and the biomedical use of light-triggered processes such as drug release. In the quest for novel theranostic approaches, the combination of multiple modalities on a single nanoscale platform, for example, combining optical imaging and drug delivery, is very desirable. In this context, liposomes, artificially prepared constructs composed of a lamellar phase lipid bilayer, have been introduced as suitable nanocarriers for UCNPs. Here, we developed a hybrid nanocarrier consisting of Er(3+) and Yb(3+) co-doped NaGdF4 UCNPs that were encapsulated in the aqueous core of the liposomes and the potential of the obtained nanocarriers for drug delivery was shown by co-loading the model anticancer drug doxorubicin (DOX). Under 980 nm excitation, a decrease of the green upconversion emission of the NaGdF4:Er(3+), Yb(3+) UCNPs was observed when DOX was co-loaded with the UCNPs in the liposome nanocarrier. This quenching effect is assigned to the energy transfer between the donor UCNP and the acceptor DOX and is most significant, since it allows for the spectral monitoring of the DOX loading and release from the liposome nanocarriers. Thus, the drug loading, release, and spectral monitoring properties of the obtained liposome nanocarriers were thoroughly characterized allowing us to assess their future potential as theranostic nanocarriers.

Scattering affects excitation power density, penetration depth and upconversion emission self-absorption, resulting in particle size –dependent modifications of the external photoluminescence quantum yield (ePLQY) and net emission. Micron-size NaYF4:Yb3+, Er3+ encapsulated phosphors (∼4.2 µm) showed ePLQY enhancements of >402%, with particle-media refractive index disparity (Δn): 0.4969, and net emission increases of >70%. In sub-micron phosphor encapsulants (∼406 nm), self-absorption limited ePLQY and emission as particle concentration increases, while appearing negligible in nanoparticle dispersions (∼31.8 nm). These dependencies are important for standardising PLQY measurements and optimising UC devices, since the encapsulant can drastically enhance UC emission.

Based on their outstanding optical properties, lanthanide-based compounds have been suggested for a wide range of applications including the fields of biomedicine, optoelectronics, and solar energy conversion. For instance, the capability of lanthanide-based materials to emit visible and near-infrared (NIR) light under NIR excitation is highly sought after when aiming for biomedical applications. This is as NIR light penetrates deeper into biological tissue and is less phototoxic than UV light commonly used for optical bioprobes. Our favorite nanomaterials are lanthanide-based fluorides (MLnF4, M = alkali metal, Ln = lanthanides and Yttrium), and our research addresses challenges in their synthesis as well as the establishment of structure-property relationships. The growing attention toward such optically active materials has prompted the development of novel synthesis methods for a more reliable and efficient access to these systems. In this regard, microwave-assisted approaches provide unique advantages over traditional solvothermal methods reliant on convectional heating: namely, significantly shorter reaction durations, more rigid reaction conditions, and thus a higher degree of reproducibility. Upon control of multiple physico-chemical reaction parameters – including the choice of precursors, the reaction temperature/time profile, and the metal ion to Ln3+ ion ratio – MLnF4 materials of tailored crystalline phase and size at the sub-10 nm realm can be obtained. Such control is crucial for the understanding of fundamental structure-property relationships and when aiming for the design of design of next-generation bioprobes and energy converters. For instance, the upconverting properties of the obtained nanomaterials render them suitable energy donors when combined with lanthanide-based complexes acting as energy acceptors. Key features of the resulting hybrid system are the simultaneous UV-blue and NIR light harvesting capabilities. These traits render the presented hybrid system a formidable candidate for the development of photoactivated devices capable of operating under multiple excitation wavelengths and to transduce the absorbed light into narrow, well-defined spectral regions.

Two homodinuclear and one heterodinuclear lanthanide (Ln)-based complexes of the general formula [Ln

Four new praseodymium alkoxo and amido compounds ([Pr3(μ3-OtBu)2(μ2-OtBu)3(OtBu)4(HOtBu)2] (1), [Pr{OC(tBu)3}3(THF)] (2), [PrCl{N(SiMe3)2}2(THF)]2 (3), and [PrCl{OC(tBu)3}2(THF)]2 (4)) were synthesized and structurally characterized by single-crystal X-ray diffraction analysis. Application of these compounds in solvothermal synthesis of praseodymium oxide/hydroxide nanostructures showed their templating influence on the morphology and phase composition of the resulting solid-state materials. Differential reactivity of the chosen alkoxide ligands toward water and the different arrangements of metal-oxygen units in the studied precursor compounds strongly influenced the kinetics of hydrolysis and cross-condensation reactions as manifested in the morphological changes and phase composition of the final products. Thermal decomposition studies of 1-4 confirmed their conversion into the corresponding oxide phases. Activation of compounds 1, 2, and 4 by either a base or a stoichiometric amount of water showed the distinct influence of their chemical configuration on the obtained nanopowders: whereas 1 solely produced nanorods of Pr(OH)3, 2 predominantly formed a mixture of rod-shaped and spherical particles. The solvothermal decomposition of 4 resulted in Pr(OH)2Cl or PrOCl due to the presence of Cl ligands in the molecular precursor. The resultant materials were thoroughly characterized to demonstrate the relationship between precursor chemistry and the processing parameters that are clearly manifested in the morphology and phase of the final ceramics.

Lanthanide-doped upconverting nanoparticles (UCNPs) are highly promising candidates for bioimaging and for cellular nanothermometry as a novel diagnostic tool. Aiming for the diagnosis of diseases at very early stages in order to optimize therapy and recovery of the patient, it must be taken into account that thermal singularities are often one of the first indicators of a disease. It is therefore our goal to develop a nanothermometer based on UCNPs that is suitable to detect the temperature at a subcellular level in the physiological range. Thus, upconverting NaGdF4:Er3+,Yb3+ nanoparticles that convert near-infrared (NIR) into visible (VIS) light are synthesized by thermal decomposition. Appropriate surface modification with a thermoresponsive polymer pNIPAM (poly(N-isopropylacrylamide)) guarantees dispersibility in aqueous media required for biomedical applications. In a further step, the combination of the obtained UCNPs with an organic dye (FluoProbe532A) provides potential donor-acceptor-pairs allowi...

Fast and significant progress has been achieved in the development of new biomarkers in recent years providing promising approaches for the reliable detection of diseases at an early stage. Yet, the disadvantages of commonly used markers, including photobleaching, autofluorescence, phototoxicity, and scattering, when ultraviolet or visible light is used for excitation, need to be overcome. Lanthanide-doped host materials are well known for their excellent optical properties, such as their ability to (up)convert near-infrared excitation to higher energies spanning the ultraviolet, visible, and near-infrared regions or to undergo strong near-infrared luminescence following near-infrared excitation. Their application as biomarkers may overcome the aforementioned drawbacks of conventional dyes. Thus, lanthanide-based nanostructures are highly promising candidates for cellular and small animal imaging, while the assessment of their cytotoxicity remains a crucial issue. Recent developments in the field of upconversion and near-infrared bioimaging focusing on some of the latest results obtained in in vitro and in vivo studies assessing the toxicity of lanthanide-based nanophosphors are highlighted in this review.

Upconversion nanoparticles (UCNPs) are of high interest for biosensing because of their unique near-infrared-excitation and visible-emission features. An emerging field within UCNP biosensing is the detection of biological interactions through Forster resonance energy transfer (FRET). However, the relatively large size, the distribution of emitting lanthanide ions within the nanoparticle, the unknown photoluminescence (PL) quantum yields (QY) of these emitting ions, and the many available core–shell architectures make the interpretation of UCNP-based FRET data extremely difficult. Here, we present a detailed spectroscopic study of three types of NaGdF4:Er3+,Yb3+ UCNPs with and without shells and lanthanide-ion doping in the cores or the shells. The different architectures strongly influence the brightness and PL lifetimes of the UCNPs, which are important properties for FRET to Cy3.5 dyes attached to the UCNP surfaces through DNA. Analysis of the FRET-sensitized dye PL decays allows the determination of the FRET efficiencies, which, in turn, can be used to estimate donor–acceptor distances, Forster distances, and Er3+ donor QYs, all of which are difficult to assess by other methods.

Rare-earth-doped ceramic nanophosphor (RED-CNP) materials are promising near-infrared (NIR) fluorescence bioimaging (FBI) agents that can overcome problems of currently used organic dyes including photobleaching, phototoxicity, and light scattering. Here, we report a NIR–NIR bioimaging system by using NIR emission at 1550 nm under 980 nm excitation which can allow a deeper penetration depth into biological tissues than ultraviolet or visible light excitation. In this study, erbium-doped yttrium oxide nanoparticles (Er3+:Y2O3) with an average particle size of 100 and 500 nm were synthesized by surfactant-assisted homogeneous precipitation method. NIR emission properties of Er3+:Y2O3 were investigated under 980 nm excitation. The surface of Er3+:Y2O3 was electrostatically PEGylated using poly (ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAAc) block copolymer to improve the chemical durability and dispersion stability of Er3+:Y2O3 under physiological conditions. In vitro cytotoxic effects of bare and PEG-b-PAAc-modified Er3+:Y2O3 were investigated by incubation with mouse macrophage cells (J774). Microscopic and macroscopic FBI were demonstrated in vivo by injection of bare or PEG-b-PAAc-modified Er3+:Y2O3 into C57BL/6 mice. The NIR fluorescence images showed that PEG-b-PAAc modification significantly reduced the agglomeration of Er3+:Y2O3 in mice and enhanced the distribution of Er3+:Y2O3.

Using the molecular compound Zr(OBut)4 in a liquid-injection chemical vapor deposition process, microporous zirconia membranes were deposited on porous multi-layered alumina substrates consisting of a macroporous α-alumina base covered with a mesoporous γ-alumina layer. The structure of nanocrystalline zirconia (ZrO2), membranes formed by the thermal decomposition of the precursor varied depending on the decomposition temperature and the injection frequency. The deposited zirconia membranes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and atomic force microscopy (AFM). X-ray diffraction analysis patterns of the deposits obtained at 500 and 600 °C indicated a mixture of tetragonal and monoclinic ZrO2 phases, whereas the fraction of the monoclinic phase increased at higher substrate temperature and injection frequency. Morphology and surface roughness are strongly dependent on the process parameters. ZrO2 membranes prepared by the liquid-injection chemical vapor deposition had a microporous structure with less than 1 nm Kelvin diameter. The liquid-injection chemical vapor deposition is a unique method to prepare microporous membranes in one step without any post-treatment.

Near-infrared (NIR) triggered chemical delivery allows on-demand release with the advantage of external tissue stimulation. Bioresorbable polyester poly- l -lactic acid (PLLA) was compounded with photoadditives of neat zinc oxide (ZnO) nanoparticles and 980 → 365 nm LiYF 4 :Tm 3+ , Yb 3+ upconverting nanoparticles (UCNP). Subsequently, neat ZnO and UCNP blended PLLA films of sub-50 μm thickness were knife casted with a hydrophobic small molecule drug mimic, fluorescein diacetate. The PLLA films displayed a 500 times increase in fluorescein diacetate release from the 50 mW NIR irradiated PLLA/photoadditive film compared to non-irradiated PLLA control films. Larger ratios of UCNP/neat ZnO increased photocatalysis efficiency at low NIR duty cycles. The synergistic increase results from the self-assembled photoadditives of neat zinc oxide and upconverting nanoparticles (UCNPs), as seen in transmission electron microscopy. Colloidal ZnO, which does not self-assemble with UCNPs, had less than half the release kinetics of the self-assembled PLLA films under similar conditions, advocating Forster resonance energy transfer as the mechanism responsible for the synergistic increase. Alternative to intensity modulation, pulse width modulation (duty cycles from 0.1 to 1) of the low intensity 50 mW NIR laser diode allowed tailorable release rates from 0.01 to 1.4% per day. With the low intensity NIR activation, tailorable release rates, and favorable biocompatibility of the constituents, implanted PLLA photoadditive thin films could allow feedback mediated chemical delivery. Statement of Significance Upconverting nanoparticles and zinc oxide nanorods were found to spontaneously self-assemble into submicron particles in organic solvents. Exposure of the submicron particles to near-infrared light allows stop and go chemical release from biocompatible polymers. Sample preparation of thin films is done with ease through physical mixing of the photoadditives followed by air-dried knife casting. A colloidal ZnO variant that does not self-assemble with upconverting nanoparticles had slower chemical release, suggesting that synergistic chemical release is brought upon by highly efficient energy transfer mechanisms when the nanoparticles are less than 10 nm apart. Never before seen composite particles of UCNP/ZnO are displayed, which shows the close interaction of the photoadditives within the polymer matrix.

Yttrium hydroxyl carbonate (Y(OH)CO3) precursors were synthesized by the homogeneous co-precipitation method in the presence of polyacrylic acid (PAAc). Resultant precursor particle size is about 15–20 nm with narrow size distribution whereas the particle size is smaller than those acquired by the conventional homogeneous precipitation method. Effective decrease of Y(OH)CO3 particle size was found to be higher for the presence of weak polyanionic ionomer such as PAAc than the presence of strong polyanionic ionomer such as sodium polystyrene sulfonate (PSS). It was observed that the morphology and size of the precursors are almost unchanged after the calcination process. Er3+ doped Y2O3 nanoparticles were synthesized by PAAc assisted homogeneous co-precipitation method showed bright green (550 nm) and red (660 nm) upconversion (UC) as well as near-infrared (NIR) fluorescence (1550 nm) under 980-nm excitation. UC and NIR fluorescence bioimaging and in-vitro cytotoxicity assay of Er3+ doped Y2O3 nanoparticles were successfully attempted with commercially available macrophages and B-cell hybridomas. Cellular uptake of nanoparticles is evidenced from bright field, UC and NIR fluorescence images of macrophages.

Terbium alkoxides in homometallic ― [Tb3(μ3-OtBu)2(μ2-OtBu)3(OtBu)4 (HOtBu)2] (1), [Tb{OC(tBu)3}3(THF)] (2) ― and heterometallic configurations ― [TbAl(μ2-OiPr)3(OiPr)3(iPrOH)]2 (3), [TbAl3(μ2-OiPr)6(OiPr)6] (4) ― were synthesized and characterized by single-crystal X-ray diffraction. Decomposition of 1 and 2 under solvothermal conditions produced Tb(OH)3 nanorods, whereby the material formation and crystallization were influenced by the steric profile of the organic ligand, which controlled the hydrolysis and condensation reactions of the precursor molecules. Monophasic terbium aluminate in the perovskite phase (TbAlO3) was obtained by the sol–gel processing of 3. Heterometallic frameworks present in 3 and 4 were, however, unstable under solvothermal conditions and resulted in hydroxide–oxide composites [Tb(OH)3/Al2O3/Al(OH)3]. Compound 4 exhibited sufficient vapour pressure to be used in the chemical vapour deposition (CVD) process to grow Tb–O–Al thin films. Crystalline compositions obtained on MgAl2O4 and SiO2 (quartz) substrates were garnet and perovskite phases, and were ascribed to the crystallographic relationship between the substrate and CVD deposits. The optical properties of the powders obtained were studied by photoluminescence spectroscopy.

Nanocrystalline zirconia (ZrO 2 ) powders doped with 0.5 up to 5 mol% europium (Eu 3+) were prepared by hydrothermal process using ZrCl 4 and EuCl 3 .6H 2 O as starting materials. Optical emission and vibrational properties of as-prepared and calcined samples were investigated by phonon sideband (PSB), Fourier transform infrared (FT-IR) and photoluminescence spectroscopy as well as lifetime measurements. We found that the doping degree had a strong effect on the crystalline phase, m- and t-ZrO 2 , of the host material, which also influenced on its shape and intensity of the PSB spectra. Residual water in the as-prepared samples indicated by FT-IR spectra induced water relating vibrations in the PSB spectra. The obtained PSB were compared with the peaks in the recorded FT-IR spectra and related to the vibrational modes of m- and t-ZrO 2 .

Gadolinium hydroxide, Gd(OH)(3), nanostructures were examined for their possible use in imaging and tracking of cells and tissues by investigating their cellular interactions and cytotoxic behaviors. For this purpose, Gd(OH)(3) nanorods (length, several hundred nanometers; diameter, approximately 40 nm) and spherical nanoparticles (average diameter,10 nm) were synthesized by solvothermal decomposition of gadolinium containing molecular precursors. After comprehensive characterization of material properties, human colon adenocarcinoma (Caco2) and human lung epithelial (A549) cells were incubated with Gd(OH)(3) nanostructures in concentrations up to 900 microg/mL to perform cytotoxicity assays (BrdU (5-bromo-2'-deoxyuridine), WST-1 (4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzol-disulfonate)) and living/dead staining. As observed in all in vitro assays, the investigated Gd(OH)(3) nanostructures do not induce any significant cytotoxic effect, making them promising candidates for a new class of contrast agents, which may overcome the limitations of organic stains such as photobleaching and single usage.