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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.

Temperature touches all aspects of our daily life, including climate, production plants, food storage, transportation, metrology, microelectronics, and medicine, and is a major factor dictating performance of nanotechnologies.1-4 However, while the heat transfer is well understood in bulk, neither experimental nor theoretical models provide a complete picture of the thermal dynamics at the nanoscale.5-7 Here, in situ luminescence thermometry is used to probe the heat propagation taking place within lanthanide (Ln3+)-doped upconverting nanoparticles (UCNPs). We have designed UCNPs with Er3+ and Tm3+ thermometric layers positioned at different locations relative to their surface, varying the distance a heat wave travels before encountering the layers. Despite being separated only by a few tens of nanometers, the thermometric layer closer to the surface of UCNPs detects temperature increase much earlier than the one located at the center – yielding the heat propagation speed in UCNPs ~1.3 nm/s. The UCNPs featuring the two thermometric layers in a single nanostructure confirmed the above result and allowed us to uncover diffusive and non-diffusive (ballistic) heat transport regimes, as well as their interplay and complex heat exchange dynamics taking place in colloidal nanoparticles (nanofluids) at a room temperature.

In this work, we have established a one-pot synthesis strategy to develop a new theranostic nanoplatform by simultaneously encapsulating Er3+, Yb3+ doped NaGdF4 upconverting nanoparticles (UCNPs) and photosensitizer zinc phthalocyanine (ZnPc) into polymeric micelle/silica nanocapsules. This approach consisted of interfacial templating condensation, using triblock copolymers, ethylene oxide)106(propylene oxide)70(ethylene oxide)106 (PEO-PPO-PEO), as the templating and protecting agent. The encapsulation followed a straightforward microemulsion mechanism in an aqueous environment of a near neutral pH. The surface hydrophobic nature of UCNPs is crucial for the success of encapsulation. To prevent the interaction between the hydrophobic OA ligands of UCNPs and the silanol groups of hydrated tetramethoxysilane (TMOS), we modified the previous procedure by tuning the addition sequence of TMOS. It allowed first to encapsulate UCNPs in PEO-PPO-PEO micelles, and then grow the silica shell within the micellar PPO core and PEO corona interface. The silica shell is incorporated for its chemical and mechanical stability, while the PEO corona confers additional steric stability to the nanocapsule. Using the modified strategy we successfully co-encapsulated UCNPs and ZnPc in one-pot, and minimized the distance between the two payloads to facilitate the energy transfer from UCNPs to ZnPc, as compared to conventional PS loading in the mesoporous silica coating. The integrated nanocapsule has an average hydrodynamic size of 85 nm with a low polydispersity index of 0.1, and demonstrates excellent colloidal stability, biocompatibility, as well as enhanced negative contrast for T2-weighted imaging and photodynamic therapy. The latter is obtained through indirect excitation of co-encapsulated ZnPc by UCNPs, resulting in singlet oxygen generation and in vitro eradication of BT474 breast cancer cells. Overall, the presented one-pot approach shined light on the co-encapsulation of OA capped inorganic UCNPs with hydrophobic photosensitizers, constituting an important step forward in the surface engineering of UCNPs, as well as upconversion based photodynamic therapy delivery systems.

Carbon nanodots (CNDs) have attracted substantial scientific curiosity because of their intriguing stimuli-responsive optical properties. However, one obstacle to the more widespread use of CNDs as transducers for e.g., biodetection systems is incomplete knowledge regarding the underlying chemical changes responsible for this responsiveness, and how these chemical features can be engineered via the precursors chosen for CND synthesis. This study demonstrates that the precursor's functional groups play a key role in directing N/S/Se heteroatom dopants either towards the surface of the CNDs, towards the aromatic core, or towards small organic fluorophores in the core. Divergent optical properties, which were consistent amongst groups of CNDs prepared with similar precursors, were obtained including either a decrease or increase of fluorescence intensity in the presence of hydrogen peroxide. Moreover, CNDs were identified with orthogonal responsiveness to radical (hydroxyl radicals, ˙OH; down to 2.5 μM) vs. non-radical oxidants (H2O2; down to 50 μM), which suggests that control of the chemistry of CNDs via the choice of precursor could yield probes that are specific to certain sub-species of reactive oxygen species or entirely different molecules altogether, based on the way they chemically-modify the surface (respond faster) and core functional groups (respond slower) associated with chromophores/fluorophores of which the CNDs are composed.

Luminescent nanomaterials that can be excited, as well as emit, in the near-infrared (NIR) have been investigated for use in a plethora of applications including nanomedicine, nanoelectronics, biosensing, bioimaging, photovoltaics, photocatalysis, etc. The use of NIR light for excitation mitigates some of the drawbacks associated with high-energy (UV or blue) excitation, for example, little to no background autofluorescence from the specimen under investigation as well as no incurred photodamage. Moreover, one of the biggest limitations is of course, that of penetration. As such, NIR light can penetrate tissues much better than high-energy light especially when these wavelengths lie within the three biological windows (BW-I: 700-950, BW-II: 1000-1350, BW-III: 1550-1870 nm) where tissues are optically transparent. At the forefront of NIR excited nanomaterials are rare earth doped nanoparticles, which due to their 4f electronic energy states can undergo conventional (Stokes) luminescence and emit in the three NIR biological windows. However, unlike other classes of nanoparticles, they can also undergo a multiphoton process (known as upconversion) where the NIR excitation light is converted to higher energies resulting in anti-Stokes luminescence spanning the UV-visible-NIR regions. Perhaps the biggest impact of such materials would be in the field of disease diagnostics and therapeutics, now commonly referred to as theranostics. Due to the versatility of their optical properties, it now becomes possible to generate high-energy light (UV or blue) in situ to trigger other light activated therapeutic modalities (i.e. drug release) while using the NIR emission for diagnostics (i.e. bioimaging, nanothermometry). Here, we present the synthesis of various NIR excited (and emitting) rare earth doped core/shell (and multishell) nanoparticles and demonstrate how their luminescence properties can be exploited for potential use in diverse biomedical applications.

Fluorophores with multifunctional properties known as rare-earth-doped nanoparticles (RENPs) are promising candidates for bioimaging, therapy, and drug delivery. When applied in vivo, these nanoparticles (NPs) have to retain long blood-circulation time, bypass elimination by phagocytic cells, and successfully arrive at the target area. Usually, NPs in a biological medium are exposed to proteins, which form the so-called "protein corona" (PC) around the NPs and influence their targeted delivery and accumulation in cells and tissues. Different surface coatings change the PC size and composition, subsequently deciding the fate of the NPs. Thus, detailed studies on the PC are of utmost importance to determine the most suitable NP surface modification for biomedical use. When it comes to RENPs, these studies are particularly scarce. Here, we investigate the PC composition and its impact on the cellular uptake of citrate-, SiO2-, and phospholipid micelle-coated RENPs (LiYF4:Yb3+,Tm3+). We observed that the PC of citrate- and phospholipid-coated RENPs is relatively stable and similar in the adsorbed protein composition, while the PC of SiO2-coated RENPs is larger and highly dynamic. Moreover, biocompatibility, accumulation, and cytotoxicity of various RENPs in cancer cells have been evaluated. On the basis of the cellular imaging, supported by the inhibition studies, it was revealed that RENPs are internalized by endocytosis and that specific endocytic routes are PC composition dependent. Overall, these results are essential to fill the gaps in the fundamental understanding of the nano-biointeractions of RENPs, pertinent for their envisioned application in biomedicine.

Upconversion nanothermometry combines the possibility of optically sensing temperatures in very small areas, such as microfluidic channels or on microelectronic chips, with a simple detection setup in the visible spectral range and reduced heat transfer after near-infrared (NIR) excitation. We propose a ratiometric strategy based on Eu3+ ion luminescence activated through upconversion processes. Yb3+ ions act as a sensitizer in the NIR region (980 nm), and energy is transferred to Tm3+ ions that in turn excite Eu3+ ions whose luminescence is shown to be thermally sensitive. Tridoped SrF2:Yb3+,Tm3+,Eu3+ nanoparticles (average size of 17 nm) show a relative thermal sensitivity of 1.1% K−1 at 25.0 °C, in the range of the best ones reported to date for Ln3+-based nanothermometers based on upconversion emission. The present nanoparticle design allows us to exploit upconversion of lanthanide ions that otherwise cannot be directly excited upon NIR excitation and that may provide operational wavelengths with a highly stable read out to fill the spectral gaps currently existing in upconversion-based nanothermometry.

Light triggered theranostic (therapy and diagnostic) nanoplatforms have gained a considerable attention in recent years. In theranostics, light as an external trigger stands out due to its non-invasiveness, high local precision and temporal resolution. Many such nanoplatforms employ high-energy (visible or UV) light to initiate the individual therapeutic and diagnostic modalities. However, light at these wavelengths suffers from inherent drawbacks such as having little to no penetration in living tissue, inducing autofluorescence from inherent fluorophores or chromophores in tissues and causing photodamage. The use of near-infrared (NIR) light for excitation mitigates such drawbacks associated with high-energy excitation, for example, little to no background autofluorescence from the specimen under investigation as well as no incurred photodamage. Moreover, one of the biggest limitations is that of penetration and NIR light can penetrate tissues much better than high-energy light especially when these wavelengths lie within the three biological windows where tissues are optically transparent. At the forefront of NIR excited nanomaterials are lanthanide doped nanoparticles, which can undergo conventional (Stokes) luminescence and emit in the NIR biological windows. However, unlike other classes of nanoparticles, they can also undergo a multiphoton excitation process where the NIR excitation light is converted to higher energies resulting in anti-Stokes luminescence spanning the UV-visible-NIR regions (known as upconversion). Thus, it now becomes possible to generate upconverted high-energy light (UV or visible) in situ to trigger other light activated therapeutic modalities (i.e. drug release) while using the NIR emission for diagnostics (i.e. bioimaging, nanothermometry). Here, we demonstrate how the luminescence properties (upconversion and NIR) of various lanthanide doped core/shell (and multishell) nanoparticles can be exploited for potential use in theranostics.

Made available in DSpace on 2021-06-25T11:05:02Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-12-01 Near-infrared (NIR) nanothermometers are sought after in biomedicine when it comes to measuring temperatures subcutaneously. Yet, temperature sensing within the third biological imaging window (BW-III), where the highest contrast images can be obtained, remains relatively unexplored. Here, LiErF4/LiYF4 rare-earth nanoparticles (RENPs) are studied as NIR nanothermometers in the BW-III. Under 793 nm excitation, LiErF4/LiYF4RENPs emit around 1540 nm, corresponding to the 4I13/2 → 4I15/2radiative transition of Er3+. The fine Stark structure of this transition allows to delineate intensity regions within the emission band that can be used for single-band ratiometric nanothermometry. These nanothermometers have a relative temperature sensitivity of ≈0.40% °C−1. The temperature-dependent energy transfer to the surrounding solvent molecules plays a significant role in the thermometric properties of the RENPs. In addition, Ce3+ions are doped in the core of the RENPs to examine whether it affects the NIR emission and temperature sensitivity. Ce3+ at 1 mol% marginally influences the downshifting emission intensity of the RENPs, yet increases the relative thermal sensitivity to ≈0.45% °C−1. Furthermore, Ce3+ quenches the visible upconversion emission of the RENPs. Together, LiErF4:Ce3+/LiYF4RENPs enable single-band photoluminescence nanothermometry in the BW-III, with the future possibility of its integration within multifunctional decoupled theranostic nanostructures. Institut National de la Recherche Scientifique Centre Énergie Matériaux et Télécommunications Université du Québec, 1650 Boul. Lionel-Boulet Institute of Chemistry São Paulo State University—UNESP Institute of Chemistry São Paulo State University—UNESP

In this work, rare-earth-based nanoparticles based on LiYbF4 and doped with Er3+, Tm3+ and Ho3+ were investigated. The effect of the concentration of the dopant ions was thoroughly studiedfrom a spectroscopic standpoint under 980 nm excitation. The upconversion emission that originates from the absorption of the optical energy from the Yb3+ ions in the crystal lattice and the subsequent energy transfer to the other rare earth in the lattice (i.e., Er3+, Tm3+ or Ho3+) highly depends on the concentration of the latter ion. Overall, the findings in this manuscripts provide important insights into the properties and rational design of upconverting nanoparticles.

In view of minimally-invasive clinical interventions, laser tissue soldering assisted by plasmonic nanoparticles is emerging as an appealing concept in surgical medicine, holding the promise of surgeries without sutures. Rigorous monitoring of the plasmonically-heated solder and the underlying tissue is crucial for optimizing the soldering bonding strength and minimizing the photothermal damage. To this end, we propose a non-invasive, non-contact, and non-ionizing modality for monitoring nanoparticle-assisted laser-tissue interaction and visualizing the localized photothermal damage, by taking advantage of the unique sensitivity of terahertz radiation to the hydration level of biological tissue. We demonstrate that terahertz radiation can be employed as a versatile tool to reveal the thermally-affected evolution in tissue, and to quantitatively characterize the photothermal damage induced by nanoparticle-assisted laser tissue soldering in three dimensions. Our approach can be easily extended and applied across a broad range of clinical applications involving laser-tissue interaction, such as laser ablation and photothermal therapies.

Neodymium-doped yttrium aluminum garnet (YAG:Nd3+) has been widely developed during roughly the last sixty years and has been an outstanding fluorescent material. It has been considered as the gold standard among multipurpose solid-state lasers. Yet, the successful downsizing of this system into the nano regimen has been elusive, so far. Indeed, the synthesis of a garnet structure at the nanoscale, with enough crystalline quality for optical applications was found to be quite challenging. Here, we present an improved solvothermal synthesis method producing YAG:Nd3+ nanocrystals of remarkably good structural quality. Adequate surface functionalization using asymmetric double-hydrophilic block copolymers, constituted of a metal-binding block and a neutral water soluble block, provides stabilized YAG:Nd3+ nanocrystals with a long term colloidal stability in aqueous suspensions. These newly stabilized nanoprobes keep the spectroscopic quality (long lifetimes, narrow emission lines, and large Stokes shift) characteristic of bulk YAG:Nd3+. The narrow emission lines of YAG:Nd3+ nanocrystals are exploited by differential infrared fluorescence imaging, thus achieving an autofluorescence-free in vivo readout. In addition, nanothermometry measurements, based on the ratiometric fluorescence of the stabilized YAG:Nd3+ nanocrystals, are demonstrated. The progress here reported paves the way for the implementation of this new stabilized YAG:Nd3+ system in the preclinical arena.

We propose two effective approaches to enhance the Förster resonance energy transfer (FRET) efficiency from near-infrared excited upconverting nanoparticles (UCNPs, namely, LiYF4:Yb3+,Tm3+) to CuInS2 quantum dots (QDs) upon engineering of the donor’s architecture. The study of the particles’ interaction highlighted a radiative nature of the energy transfer among the moieties under investigation when in solution. However, analyses performed on dry powders allowed observing clear evidence of a FRET mechanism. In particular, photoluminescence lifetime measurements showed that FRET efficiency could be effectively increased by both reducing the size of the UCNPs and directly controlling the distribution of the active ions throughout the donor’s volume, i.e., doping them only in the outer shell of a core/shell system. Both strategies resulted at least in a more than doubled FRET efficiency compared to larger core-only UCNPs. Obtained experimental values were compatible with those predicted from geometrical considerations on the active ions’ distribution over the UCNP volume. These results provide a concrete proof of the potential of a UCNP–QD FRET pair when the system is properly designed, hence setting a solid base for the development of robust and efficient all-inorganic probes for FRET-based assays.

Lanthanide-activated SrF2 nanoparticles with a multishell architecture were investigated as optical thermometers in the biological windows. A ratiometric approach based on the relative changes in the intensities of different lanthanide (Nd3+ and Yb3+) NIR emissions was applied to investigate the thermometric properties of the nanoparticles. It was found that an appropriate doping with Er3+ ions can increase the thermometric properties of the Nd3+-Yb3+ coupled systems. In addition, a core containing Yb3+ and Tm3+ can generate light in the visible and UV regions upon near-infrared (NIR) laser excitation at 980 nm. The multishell structure combined with the rational choice of dopants proves to be particularly important to control and enhance the performance of nanoparticles as NIR nanothermometers.

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.

Lanthanide-doped upconverting nanoparticles (UCNPs) have emerged as excellent nanotransducers for converting longer wavelength near-infrared (NIR) light to shorter wavelengths spanning the ultraviolet (UV) to the visible (Vis) regions of the spectrum via a multiphoton absorption process, known as upconversion. Here, we report the development of NIR to UV-Vis-NIR UCNPs consisting of LiYF4:Yb(3+)/Tm(3+)@SiO2 individually coated with a 10 ± 2 nm layer of chitosan (CH) hydrogel cross-linked with a photocleavable cross-linker (PhL). We encapsulated fluorescent-bovine serum albumin (FITC-BSA) inside the gel. Under 980 nm excitation, the upconverted UV emission cleaves the PhL cross-links and instantaneously liberates the FITC-BSA under 2 cm thick tissue. The release is immediately arrested if the excitation source is switched off. The upconverted NIR light allows for the tracking of particles under the tissue. Nucleus pulposus (NP) cells cultured with UCNPs are viable both in the presence and in the absence of laser irradiation. Controlled drug delivery of large biomolecules and deep tissue imaging make this system an excellent theranostic platform for tissue engineering, biomapping, and cellular imaging applications.

In the biomedical field, research has recently focused on nanoparticle-based “theranostic” agents for the treatment of diverse diseases, including cancer. This new paradigm in personalized medicine intends to exploit nanoplatforms that carry both therapeutic and diagnostic (theranostic) modalities. Compared with delivering drugs or imaging agents separately, theranostic agents can simultaneously deliver them to specific sites, enabling detection and treatment of disease in a single procedure. Many theranostic nanoplatforms are triggered by light, however, the vast majority of these are limited in that the ultraviolet or visible excitation light used has minimal applicability in biological applications. Rare earth doped nanoparticles, on the other can be excited with biologically friendly near-infrared light (in the biological windows) and can emit in the ultraviolet, visible or near-infrared regions through a multiphoton upconversion process while simultaneously emitting in the near-infrared region (also in the 3 biological windows) through a Stokes or downshifted process. Hence, the upconversion luminescence can be used to trigger the therapeutic application (drug delivery, photodynamic therapy, etc.) while the near-infrared luminescence can be used for the diagnostic modality (bioimaging, nanothermometry/temperature sensing, etc). In this presentation, we will introduce rare earth doped nanoparticles and demonstrate their usefulness in theranostics. In particular, we will show that complex nanoparticle architectures can endow further functionality to these nanoparticles and discuss how they can be used for temperature sensing (nanothermometry) in biological systems.

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.

In recent years, a significant amount of research has focused on nanoparticle-based “theranostic” agents for the treatment of a wide array of diseases, including cancer. This new paradigm in personalized medicine intends to exploit nanoplatforms that carry both therapeutic and diagnostic (theranostic) modalities. Compared with delivering drugs or imaging agents separately, theranostic agents can simultaneously deliver them to specific sites, enabling detection and treatment of disease in a single procedure. Many theranostic nanoplatforms are triggered by light, however, the vast majority of these are limited in that the ultraviolet or visible excitation light used has minimal applicability in biological applications. Lanthanide doped nanoparticles, on the other can be excited with biologically friendly near-infrared light (in the biological windows) and can emit in the ultraviolet, visible or near-infrared regions through a multiphoton upconversion process while simultaneously emitting in the near-infrared region through a Stokes or downshifted process. Hence, the upconversion luminescence can be used to trigger the therapeutic application (drug delivery, photodynamic therapy, etc.) while the near-infrared luminescence can be used for the diagnostic modality (bioimaging, nanothermometry, etc). In this presentation, we will introduce lanthanide doped nanoparticles and demonstrate their usefulness in theranostics. In particular, we will show complex nanoparticle architectures can endow further functionality to these nanoparticles including the ability to decouple the theranostic processes that are conventionally delivered simultaneously.

Among the foreseeable therapeutic approaches at the cellular level, nanoplatform-driven photothermal therapy is a thriving tool for the selective eradication of malignant tissues with minimal side effects to healthy ones. Hence, chemically versatile, near-infrared absorbing plasmonic nanoparticles are distinctly appealing and most sought after as efficient photothermal agents. In this work, a straightforward method to synthesize monodisperse PEGylated copper sulfide nanoparticles of pure covellite (CuS) phase, featuring strong localized surface plasmonic resonance absorption in the near-infrared and flexible surface chemistry, imparted by monomethyl ether polyethylene glycol molecules, is developed and optimized. These nanoparticles show a remarkable photothermal heat conversion efficiency (HCE) of 71.4%, which is among the highest for CuS systems and rivals that of plasmonic noble metal nanostructures. Moreover, through critical evaluation and mathematical modeling of the material’s properties and measurement methodology, it is assessed that the calculated HCE values drastically depend on experimental conditions such as wavelength-dependent solvent absorption properties, sol concentration, and optical path. These findings are of paramount relevance to the photothermal community, since they call for a standardization of the procedure for the evaluation of the HCE of proposed photothermal agents, in order to make the reported values universally and reliably comparable.

Upconversion nanothermometry combines the possibility of optically sensing temperatures in very small areas, such as microfluidic channels or on microelectronic chips, with a simple detection setup in the visible spectral range and reduced heat transfer after near-infrared (NIR) excitation. We propose a ratiometric strategy based on Eu

Eu3+ doped CaF2 and SrF2 nanoparticles were synthesized through a facile hydrothermal technique, using citrate ions as capping agents and Na+ or K+ as charge compensator ions. A proper tuning of the reaction time can modulate the nanoparticle size, from few to several tens of nanometers. Analysis of EXAFS spectra indicate that the Eu3+ ions enter into the fluorite CaF2 and SrF2 structure as substitutional defects on the metal site. Laser site selective spectroscopy demonstrates that the Eu3+ ions are mainly accommodated in two sites with different symmetries. The relative site distribution for lanthanide ions depends on the nanoparticle size, and higher symmetry Eu3+ sites are prevalent for bigger nanoparticles. Eu3+ ions in high symmetry sites present lifetimes of the 5D0 level around 27 ms, among the longest lifetimes found in the literature for Eu3+ activated materials. As a proof of concept of possible use of the Eu3+ activated alkaline-earth fluoride nanoparticles in nanomedicine, the red luminescenc...

Upconverting materials are currently explored in the field of solar energy conversion in order to extend the light harvesting properties of semiconductors to the near-infrared (NIR) region, their absorption being generally limited to the visible region of the solar spectrum. Here, we propose to photosensitize nanostructured films of tungsten oxide (WO3), a semiconductor widely investigated in photoelectrochemistry, photocatalysis and electrochromics, with NaGdF4:Er3+, Yb3+ upconverting nanoparticles (UCNPs). In order to do so, we fabricate nanocomposite films of WO3 and NaGdF4:Er3+, Yb3+ UCNPs (indicated as UCNP/WO3 films). Current–time measurements show that, under irradiation at λ = 980 nm, a relative increase in current of about 3% with respect to the dark current is observed in the UCNP/WO3 films. The UCNP/WO3 mol% ratio and the temperature of the thermal treatment of the nanocomposite films are both critical to simultaneously achieve photosensitization and charge carrier transport in the UCNP/WO3 films.

Lanthanide-doped upconverting nanoparticles (UCNPs) have been studied for diverse biomedical applications due to their inherent ability to convert near-infrared (NIR) excitation light to higher energies (spanning the ultraviolet, visible, and NIR regions). To explore additional functionalities, rational combination with other optically active nanostructures may lead to the development of new multimodal nanoplatforms with theranostic (therapy and diagnostic) capabilities. Here, we develop a nanocomposite consisting of NaGdF4:Er3+, Yb3+ UCNPs, mesoporous silica (SiO2), gold nanorods (GNRs) and a photosensitizer, with integrated functionalities including luminescence imaging, photothermal generation, nanothermometry and photodynamic effects. Under 980 nm irradiation, GNRs and UCNPs are simultaneously excited due to the overlap between the surface plasmon resonance of the GNRs and the absorption of the UCNPs leading to plasmonic enhancement of the upconverted luminescence, while concomitantly creating a temperature gradient. The temperature increase can be determined from the intensity ratio of the upconverted green emission of the UCNPs. Finally, a photosensitizer, zinc phthalocyanine, was loaded into the mesoporous SiO2. Upon laser irradiation, the upconverted visible light subsequently activates the photosensitizer to release reactive oxygen species. The multifunctional GNR@SiO2@UCNPs nanocomposites showed strong luminescence signal when incubated in HeLa cervical cancer cells, making them ideal bioprobes for future theranostic applications.

Strontium fluoride nanoparticles activated with Nd 3+ ions were studied as nanosized optical thermometers in the first (800–950 nm) and second (1000–1300 nm) biological windows, by monitoring the Nd 3+ emission as a function of the temperature. The variations of the Nd 3+ emissions were correlated with the temperature changes and the thermometric properties of SrF 2 nanoparticles were evaluated using a ratiometric method. The best thermal sensitivity was found to be 0.60% K −1 , a value which is the highest found for water dispersible colloidal nanothermometers based on the near-infrared luminescence of Nd 3+ ions only in single phase materials. The excellent thermometric performance is also demonstrated by a thermal sensitivity of 1.2 °C, that can be achieved in temperature evaluation of real systems. Temperature measurements using a phantom tissue confirmed that the samples serve as efficient ratiometric nanothermometers that can work in two biological windows.

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.

CaF 2 nanoparticles activated with Nd 3+ ions have been investigated as potential optical nanothermometers in the biological windows. The variation of the Nd 3+ emission around 850 and 1060 nm as a function of the temperature has been measured. A ratiometric approach, based on the relative changes in the intensities of different emission bands of the Nd 3+ has been considered to investigate the thermometric properties of the NPs. The evaluated thermal sensitivity is around 0.12% K −1 , a value that is similar to those found for other water dispersible nanothermometers based on the near-infrared luminescence of the Nd 3+ lanthanide ion. The temperature uncertainty is around 1.8 °C, small enough to monitor the local temperature during a photothermal treatment.

The accurate determination of the photothermal response of nanomaterials represents an essential aspect in many fields, such as nanomedicine. Specifically, photothermal cancer therapies rely on the precise knowledge of the light-to-heat transfer properties of plasmonic nanoparticles to achieve the desired temperature-induced effects in biological tissues. In this work, we present a novel method for the quantification of the photothermal effect exhibited by nanoparticles in aqueous dispersions. By combining the spatial and temporal thermal dynamics acquired at terahertz frequencies, the photothermal conversion efficiency associated with the geometry of the plasmonic nanoparticles can be retrieved in a noncontact and noninvasive manner. The proposed technique can be extended to the characterization of all those nanomaterials which feature a temperature-dependent variation of the refractive index in the terahertz regime.

Light triggered theranostic (therapy and diagnostic) platforms have gained a considerable attention in recent years. In theranostics, light as an external trigger stands out due to its non-invasiveness, high local precision and temporal resolution. Many such theranostic nanoplatforms employ high-energy (visible or UV) light to initiate the individual therapeutic and diagnostic modalities. However, light at these wavelengths suffers from inherent drawbacks such as having little to no penetration in living tissue, induces autofluorescence from inherent fluorophores or chromophores in tissues and can cause photodamage. The use of NIR light for excitation mitigates such drawbacks associated with high-energy excitation, for example, little to no background autofluorescence from the specimen under investigation as well as no incurred photodamage. Moreover, one of the biggest limitations is of course, that of penetration. As such, NIR light can penetrate tissues much better than high-energy light especially when these wavelengths lie within the three biological windows (BW-I: 700-950, BW-II: 1000-1350, BW-III: 1550-1870 nm) where tissues are optically transparent. At the forefront of NIR excited nanomaterials are rare earth doped nanoparticles, which due to their 4f electronic energy states can undergo conventional (Stokes) luminescence and emit in the three NIR biological windows. However, unlike other classes of nanoparticles, they can also undergo a multiphoton excitation process where the NIR excitation light is converted to higher energies resulting in anti-Stokes luminescence spanning the UV-visible-NIR regions (commonly known as upconversion). Due to the versatility of their optical properties, it now becomes possible to generate upconverted high-energy light (UV or blue) in situ to trigger other light activated therapeutic modalities (i.e. drug release) while using the NIR emission for diagnostics (i.e. bioimaging, nanothermometry). Here, we present the synthesis of various NIR excited (and emitting) rare earth doped core/shell (and multishell) nanoparticles and demonstrate how their luminescence properties can be exploited for potential use in theranostics.

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.

Nowadays, one of the most exciting applications of nanotechnology in biomedicine is the development of localized, noninvasive therapies for diverse diseases, such as cancer. Among them, nanoparticle-based photothermal therapy (PTT), which destroys malignant cells by delivering heat upon optical excitation of nanoprobes injected into a living specimen, is emerging with great potential. Two main milestones that must be reached for PTT to become a viable clinical treatment are deep penetration of the triggering optical excitation and real-time accurate temperature monitoring of the ongoing therapy, which constitutes a critical factor to minimize collateral damage. In this work, a yet unexplored capability of near-infrared emitting semiconductor nanocrystals (quantum dots, QDs) is demonstrated. Temperature self-monitored ­QD-based PTT is presented for the first time using PbS/CdS/ZnS QDs emitting in the second biological window. These QDs are capable of acting, simultaneously, as photothermal agents (heaters) and high-resolution fluorescent thermal sensors, making it possible to achieve full control over the intratumoral temperature increment during PTT. The differences observed between intratumoral and surface temperatures in this comprehensive investigation, through different irradiation conditions, highlight the need for real-time control of the intratumoral temperature that allows for a dynamic adjustment of the treatment conditions in order to maximize the efficacy of the therapy., This project was supported by the Spanish Ministerio de Economía y Competitividad under project and MAT2013-47395-C4-1-R. B.d.R. thanks Universidad Autónoma de Madrid for an FPI grant. F.R. acknowledges scholarship support from the Fonds de recherche du Québec—Nature et technologies (FRQNT) under the Programme de Bourses d’Excellence (Merit Scholarship Program for Foreign Students). A.B. is thankful to the Canadian Institutes of Health Research and the Breast Cancer Society of Canada (CIHR-BCSC), for postdoctoral funding granted to him through an Eileen Iwanicki Fellowship in Breast Cancer Imaging. F.V. is grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada, FRQNT, and the Canada Foundation for Innovation (CFI) for supporting his research program. F.V. also gratefully acknowledges the Fondation Sibylla Hesse. D.M. is grateful for the financial support from NSERC, FRQNT, CFI, and the Centre Québécois sur les Matériaux Fonctionnels (CQMF), Canada

During the last decade inorganic luminescent nanoparticles that emit visible light under near infrared (NIR) excitation (in the biological window) have played a relevant role for high resolution imaging of cancer. Indeed, semiconductor quantum dots (QDs) and metal nanoparticles, mostly gold nanorods (GNRs), are already commercially available for this purpose. In this work we review the role which is being played by a relatively new class of nanoparticles, based on lanthanide ion doped nanocrystals, to target and image cancer cells using upconversion fluorescence microscopy. These nanoparticles are insulating nanocrystals that are usually doped with small percentages of two different rare earth (lanthanide) ions: The excited donor ions (usually Yb3+ ion) that absorb the NIR excitation and the acceptor ions (usually Er3+, Ho3+ or Tm3+), that are responsible for the emitted visible (or also near infrared) radiation. The higher conversion efficiency of these nanoparticles in respect to those based on QDs and GNRs, as well as the almost independent excitation/emission properties from the particle size, make them particularly promising for fluorescence imaging. The different approaches of these novel nanoparticles devoted to "in vitro" and "in vivo" cancer imaging, selective targeting and treatment are examined in this review, J.A.C. is a Concordia University Research Chair in Nanoscience and is grateful to Concordia University for financial support of his research. J.A.C. and F.V. are grateful for financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. R.N. is grateful for NSERC financial support through the Alexander Graham Bell Graduate Scholarship Program. B.F.Z. is grateful to les Fonds Québécois de Recherche sur la Nature et les Technologies (FQRNT) for financial support through the Graduate Scholarship Program. This work was also supported in part by the Universidad Autónoma de Madrid and Comunidad Autónoma de Madrid (Projects CCG087-UAM/MAT-4434 and S2009/MAT-1756), by the Spanish Ministerio de Educación y Ciencia (MAT 2010–16161). EMR acknowledges financial support from Fundación Alfonso Martín Escudero and Marie Curie IOF Fellowship Program (project 274404 LUNAMED)

In recent years, rare earth doped nanoparticles have been proposed for a number of exciting applications in a wide-range of fields including nanomedicine, nanoelectronics, biosensing, bioimaging, photovoltaics, photocatalysis, etc. This is due, primarily, to their interesting and versatile optical properties including their inherent ability to convert low-energy near-infrared (NIR) light to higher energies spanning the UV, visible, and NIR regions of the spectrum via a process known as upconversion. Upconversion, inherent to the rare earths, results from the multitude of 4f electronic energy states, many of which are spaced equally and long-lived, that facilitate the absorption of multiple low energy photons to populate the higher energy emitting states. Thus, upconversion is a multiphoton process, but unlike other two-photon excited materials, the need for expensive ultrafast lasers is eliminated since the simultaneous absorption of multiple photons is not required; due to the long lifetimes of the rare earth ion excited states, sequential absorption occurs efficiently. Moreover, following NIR excitation, these nanoparticles can also undergo conventional luminescence and emit in the three NIR biological windows where tissues are optically transparent. Here, we will discuss the luminescence properties of rare earth doped nanoparticles and present a perspective on both their applicability as well as drawbacks for use in various applications. Finally, we will demonstrate that the intelligent combination of diverse materials, with different properties, will allow for the engineering of novel multifunctional nanostructures, which can usher in a new era for rare earth doped nanoparticles.

We report the synthesis of Tm3+/Yb3+/Er3+-doped Lu3Ga5O12 nanocrystals prepared by a simple sol−gel method exhibiting bright white light following excitation with lower energy near-infrared light (λexc = 980 nm) via an upconversion process. The combination of upconverted blue (from Tm3+), green, and red (from Er3+) emissions resulted in the white luminescence, which is intense and visible to the naked eye at a laser power less than 30 mW (3.4 W/cm2). The calculated Commission internationale de l′eclairage (CIE) color coordinates, which is the standard reference for defining colors, fall well within the white region and shift only slightly with the incident laser powers indicating that the material might be promising for the development of devices such as white light lasers and LEDs.

We report the fabrication and testing of dye sensitized solar cells (DSSC) based on tin oxide (SnO2) particles of average size ~20 nm. Fluorine-doped tin oxide (FTO) conducting glass substrates were treated with TiOx or TiCl4 precursor solutions to create a blocking layer before tape casting the SnO2 mesoporous anode. In addition, SnO2 photoelectrodes were treated with the same precursor solutions to deposit a TiO2 passivating layer covering the SnO2 particles. We found that the modification enhances the short circuit current, open-circuit voltage and fill factor, leading to nearly 2-fold increase in power conversion efficiency, from 1.48% without any treatment, to 2.85% achieved with TiCl4 treatment. The superior photovoltaic performance of the DSSCs assembled with modified photoanode is attributed to enhanced electron lifetime and suppression of electron recombination to the electrolyte, as confirmed by electrochemical impedance spectroscopy (EIS) carried out under dark condition. These results indicate that modification of the FTO and SnO2 anode by titania can play a major role in maximizing the photo conversion efficiency.

Water dispersible SrF2:Tm3+,Yb3+ upconverting nanoparticles (9 nm diameter) have been synthesized to elucidate their potential as ultraviolet emitters. High intensity upconversion ultraviolet emission bands centered at 350 and 360 nm were detected following excitation at 980 nm in the near-infrared region for H2O and D2O colloidal dispersions of the upconverting nanoparticles, corresponding to transitions from the 1I6 and 1D2 energy levels of thulium (Tm3+) ions. Both emission intensities were strongly dependent on the temperature and the H2O/D2O molar ratio of the media, two important criteria that define the different environments in which the nanoparticles can be applied. Since the variations observed were different for each emitting level, the relevance of each band is discussed in relation to these two criteria.

Over the past decade, near-infrared (NIR)-emitting nanoparticles have increasingly been investigated in biomedical research for use as fluorescent imaging probes. Here, high-quality water-dispersible core/shell/shell PbS/CdS/ZnS quantum dots (hereafter QDs) as NIR imaging probes fabricated through a rapid, cost-effective microwave-assisted cation exchange procedure are reported. These QDs have proven to be water dispersible, stable, and are expected to be nontoxic, resulting from the growth of an outer ZnS shell and the simultaneous surface functionalization with mercaptopropionic acid ligands. Care is taken to design the emission wavelength of the QDs probe lying within the second biological window (1000-1350 nm), which leads to higher penetration depths because of the low extinction coefficient of biological tissues in this spectral range. Furthermore, their intense fluorescence emission enables to follow the real-time evolution of QD biodistribution among different organs of living mice, after low-dose intravenous administration. In this paper, QD platform has proven to be capable (ex vivo and in vitro) of high-resolution thermal sensing in the physiological temperature range. The investigation, together with the lack of noticeable toxicity from these PbS/CdS/ZnS QDs after preliminary studies, paves the way for their use as outstanding multifunctional probes both for in vitro and in vivo applications in biomedicine. Low-dose in vivo near-infrared (NIR) fluorescence imaging is achieved by using carefully designed PbS/CdS/ZnS quantum dots (QDs), intensely emitting within the second biological window (1000-1350 nm). Moreover, preliminary studies both in vitro and in vivo have proven the lack of noticeable toxicity of these QDs. As an additional advantage, this NIR-fluorescence imaging platform has demonstrated useful multifunctionality, thus being capable, both ex vivo and in vitro, of high-resolution thermal sensing in the physiological temperature range., A.B., F.R., and E.C. contributed equally to this work. A.B. thanks the Fonds de recherche du Québec-Nature et Technologies (FRQNT) and the Canadian Institutes of Health Research–Breast Cancer Society of Canada (CIHR-BCSC) for postdoctoral funding through the Programme de Bourses d’Excellence and an Eileen Iwanicki Fellowship in Breast Cancer Imaging, respectively. F.R. greatly appreciates financial support from the Merit Scholarship Program for Foreign Students from the Ministére de L’Education, du Loisir et du Sport du Québec (MELS). F.V. and D.M. are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) and FRQNT for funding. F.V. wholeheartedly thanks the Foundation Sibylla Hesse for supporting his research. D.M. also thanks the Quebec Center for Functional Materials for support. This work was also supported by Spanish Ministerio de Economia y Competitividad under projects MAT2010–16161 and MAT2013–47395-C4–1-R.

Novel approaches for high contrast, deep tissue, in vivo fluorescence biomedical imaging are based on infrared-emitting nanoparticles working in the so-called second biological window (1,000–1,400 nm). This allows for the acquisition of high resolution, deep tissue images due to the partial transparency of tissues in this particular spectral range. In addition, the optical excitation with low energy (infrared) photons also leads to a drastic reduction in the contribution of autofluorescence to the in vivo image. Nevertheless, as is demonstrated here, working solely in this biological window does not ensure a complete removal of autofluorescence as the specimen’s diet shows a remarkable infrared fluorescence that extends up to 1,100 nm. In this work, we show how the 1,340 nm emission band of Nd3+ ions embedded in SrF2 nanoparticles can be used to produce autofluorescence free, high contrast in vivo fluorescence images. It is also demonstrated that the complete removal of the food-related infrared autofluorescence is imperative for the development of reliable biodistribution studies.

The luminescent properties of 1 mol % Eu3+-doped cubic Lu2O3 nanocrystals prepared by a combustion synthesis route were investigated. The visible emission spectrum of the europium doped Lu2O3 nanocrystals indicate that the structural environment surrounding the dopant Eu3+ ion is distorted when compared to a bulk sample with a micrometer particle size. From the resulting emission spectra, the Ω2 and Ω4 Judd-Ofelt intensity parameters were calculated. The lifetimes of the 5D0 excited state for both the C2 and C3i sites were found to be nearly double that found for a similarly doped sample with larger particle size (bulk sample). This behavior is attributed to a change in the refractive index of the nanocrystalline material that in turn modifies the oscillator strength of the 4f ↔ 4f transitions.

The luminescence properties of nanocrystalline Y2O3:Er3+ prepared via wet chemical synthesis were investigated. A broadening of both the reflectance spectrum and 77 K luminescence spectrum (λexc = 488 nm) was observed in the nanocrystalline material compared to bulk Y2O3:Er3+. The spectral broadening was attributed to the presence of Er3+ ions on the surface of the particle, which experienced different crystal fields than the ions buried in the core of the particle. Upconversion was observed in both the bulk and nanocrystal material following excitation with 650-nm or 800-nm radiation. Following excitation with 800-nm radiation, an enhancement of the red (4F9/2 → 4I15/2) upconverted emission was observed and occurred as a result of the (4I9/2, 4I11/2) → (4I13/2, 4F9/2) ion-pair process that directly populated the 4F9/2 state. The magnitude of the red enhancement in the nanocrystalline material prepared via wet chemical synthesis was less than that of the identically doped bulk sample and less still than Y2O3:Er3+ nanocrystals prepared via a combustion synthesis technique. An explanation is proposed to account for the drastic difference in the red upconverted luminescence intensity.

The optical properties of five different nanocrystalline Y2O3:Er3+, Yb3+ samples are presented and discussed. Green and red emission was observed following excitation with 488 nm and attributed to 2H11/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions, respectively. Striking red enhancement was observed in the upconversion spectra when exciting the Y2O3:Er3+, Yb3+ samples with 978 nm, and it became more pronounced with an increase in Yb3+ concentration. A cross relaxation mechanism (4F7/2→4F9/2 and 4F9/2←4I11/2) was responsible for directly populating the 4F9/2 state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline Y2O3:Er3+, Yb3+ samples. The 4F9/2 level was populated via a nonresonant mechanism that involved the 4F9/2←4I13/2 transition that is more prevalent in the nanocrystals, which is due to the high energy phonons inherent in this type of material. In nanocrystalline Y2O3:Er3+, Yb3+, we observe a change in the upconversion mechanis...

The visible and upconversion luminescence of an erbium doped calcium sodium aluminosilicate (CSAS) glass was investigated at room temperature and 77 K. Upconverted visible luminescence was observed upon excitation in the red (λexc=650 nm) and NIR (λexc=800 nm) regions of the spectrum. The intensity of the upconverted visible luminescence was relatively weak compared to that obtained upon direct excitation (λexc=488 nm) owing to efficient multiphonon relaxation, a result of the high phonon energies of the aluminosilicate host. A two-photon upconversion process was proposed via two independent pathways: excited state absorption (ESA) and energy transfer upconversion (ETU).

Intense upconversion luminescence is reported for the first time in a Ho3+-doped nanocrystalline material. Anti-Stokes blue and green luminescence was noted in nanocrystalline Gd3Ga5O12:Ho3+ after excitation into the 5F5 level with 647 nm radiation. Excitation at 756 nm into the 5I4 level resulted in green and red anti-Stokes emission. In both cases, an excited state adsorption mechanism is thought responsible for the observed upconversion luminescence. The temporal dependence of the upconverted luminescence for the two excitation wavelengths revealed that the 5I7 level was the intermediate level in both two-photon processes.