Er Upconversion Nanoparticles Research Articles

Dye-sensitized lanthanide-doped upconversion nanoprobe for enhanced sensitive detection of Fe3+ in human serum and tap water

Iron ion (Fe3+) detection is crucial for human health since it plays a crucial role in many physiological activities. In this work, a novel Schiff-base functionalized cyanine derivative (CyPy) was synthesized, which was successfully assembled on the surface of upconversion nanoparticles (UCNPs) through an amphiphilic polymer encapsulation method. In the as-designed nanoprobe, CyPy, a recognizer of Fe3+, is served as energy donor and β-NaYF4:Yb,Er upconversion nanoparticles are adopted as energy acceptor. As a result, a 93-fold enhancement of upconversion luminescence is achieved. The efficient energy transfer from CyPy to β-NaYF4:Yb,Er endows the nanoprobe a high sensitivity for Fe3+ in water with a low detection limit of 0.21 μM. Moreover, the nanoprobe has been successfully applied for Fe3+ determination in human serum and tap water samples with recovery ranges of 95 %-105 % and 97 %-106 %, respectively. Moreover, their relative standard deviations are all below 3.72 %. This work provides a sensitive and efficient methodology for Fe3+ detection in clinical and environmental testing.

Synthesis of NaYbF4:Er nanocrystals with controllable size, morphology, and multicolor upconversion luminescence for Anticounterfeiting applications

The dopant concentration of lanthanide ions plays one of the critical roles to manipulate size/morphology, photon population pathway, or luminescence color/intensity in upconversion nanoparticles (UCNCs), which have a significant effect on their application developments. Herein, a strategy is demonstrated for simultaneous size, morphology and multicolor luminescence tuning in the single doped lanthanide activator Er3+ ion system through simply tuning the concentration of Er3+ ions. The doping of different Er3+ ion amounts in NaYbF4:Er nanocrystals can control nanoparticle size (from several hundreds to tens of nanometers) and morphology. Moreover, multicolor upconversion luminescence involving green, yellow, orange and red can be obtained in the NaYbF4:Er UCNCs by changing the excitation power density of 980 nm laser. Mechanistic studies reveal the size and morphology evolution process as well as saturation/cross relaxation effects induced tunable multicolor emissions. These nanocrystals have demonstrated promising potential applications in anticounterfeiting. Our results not only provide a simple and feasible route for simultaneous size/morphology control and multicolor upconversion luminescence tuning, but also can further expand the utility of UCNCs in the frontier applications ranging from optical encoding to information security.

Methylene blue loaded K0.3Bi0.7F2.4:Yb,Er upconversion nanoparticles for near-infrared activated photodynamic therapy

Bismuth-based fluoride nanoparticles have received a lot of interest as host materials for upconversion and have been recommended for lighting devices and biological applications.

NaYF4: Yb,Er Upconversion Nanoparticles for Imaging: Effect on Red Blood Cells

(1) Background: Upconversion nanoparticles (UCNPs) are a promising tool for biological tissue structure visualization and photodynamic therapy (PDT). The luminescence of such NPs is excited in the spectrum’s near-infrared (NIR) region, while the NPs luminesce in the visible region. Conjugating such NPs with photodynamic dyes that absorb their luminescence makes it possible to increase the depth at which PDT is performed. (2) Methods: We conducted a comprehensive study on the possibility of using NaYF4:Er:Yb UCNPs in vivo for imaging and PDT. The NPs were synthesized by a hydrothermal method. The synthesis of NPs with a size of 80 nm and hexagonal structure was demonstrated. (3) Results: The accumulation of NPs in organs after their intravenous injection into rats was studied. The effect of NPs on the shape, size, and degree of aggregation of red blood cells (RBC) was also investigated. (4) Conclusions: The possibility of luminescent visualization of NPs in histological sections and their subcutaneous distribution is demonstrated. All investigated particles showed moderate toxicity, causing mostly reversible changes.

Hydrothermal synthesis of upconversion nanoparticles NaYF<sub>4</sub>:Yb,Er and their functionalization for use as biosensors

Using hydrothermal synthesis NaYF4:Yb,Er upconversion nanoparticles in the form of rods were obtained. To give the nanoparticles hydrophilic properties, their surface was modified by replacing the oleate shell with L-cysteine molecules without significant changes in their photophysical properties. Visible fading of the luminescence brightness of the modified upconversion nanoparticles wasn’t observed for months.

A Novel and Rapid Smear Cytomorphology Detection Strategy Based on Upconversion Nanoparticles Immunolabeling Integrated with Wright's Staining for Accurate Diagnosis of Leukemia.

Accurate, sensitive, and rapid identification of leukemia cells in blood and bone marrow is of paramount significance for clinical diagnosis. An integrative technique combining traditional cytomorphology with immunophenotyping was proposed to improve the diagnostic efficiency in leukemia. On account of high photostability, biocompatibility, and signal-to-background ratio, upconversion nanoparticles (UCNPs) as luminescent labels have drawn substantial research scrutiny in immunolabeling. To achieve simultaneous determination, NaYF4:Yb,Er UCNPs were coupled with CD38 antibodies to construct immunofluorescence probes that were developed to bind to diffuse large B cell lymphoma (DLBCL) cells, followed by Wright's staining that has been widely used in clinical work for morphological diagnosis. Further, the experimental conditions were optimized, such as medium, slice-making method, antibody dosage, incubation time, etc. The cell morphology and immunolabeling could be observed simultaneously, and its simple operation rendered it a possibility for clinical diagnosis. The developed immunolabeling assay could achieve DLBCL cell counting with high reproducibility and stability, and the detection limit was as low as 1.54 cell/slice (>3 σ/s). Moreover, the proposed method also realized real blood and bone marrow sample analysis, and the results were consistent with the clinical diagnosis. Overall, this strategy can be carried out after simple laboratory training and has prospective biomedical applications in leukemia classification, diagnosis validation, and differential diagnostics.

In situ growth of lanthanides-doped nanoparticles inside zeolites with enhanced upconversion emission for gallic acid detection

The combination of upconversion nanoparticles (UCNPs) with porous zeolites could enable the development of multifunctional composites and extend their optical applications in sensing, detection and biomedical monitoring. Herein, a series of high luminescent UCNPs@Zeolites nano-micro composites were constructed via the in situ growth strategy, by taking the low phonon-energy fluoride nanoparticles of NaLnF4 (Ln = Y, Gd) as doping hosts for Er3+/Yb3+, desilicated FAUY and ZSM-5 as the target zeolites. Benefiting from the formation of tightly combined interfaces between the UCNPs and the target zeolites that effectively passive the surface defects of UCNPs, three orders of magnitude of improved upconversion emission in maximum was obtained under 980 nm excitation through afterward heat treatment at 400 ℃. Moreover, the pre-exchange of Yb3+ into target zeolites before the in situ growth of UCNPs is another feasible approach to drastically improve the upconversion emission intensity of the UCNPs@Zeolites nano-micro composites. By taking NaGdF4:Yb,Er@DSZSM-5/HT as an example probe, the detection of GA was demonstrated and the detection ability of which is super than that of the corresponding bare NaGdF4:Yb,Er UCNPs. This research provided a universal approach to construct the UCNPs@Zeolites nano-micro composites with varied upconversion emission colors simply by choosing activator ions, which therefore indicates wide potential applications.

Fluorescence immunoassay for simultaneous detection typical β-agonists in animal derived food using blue-green upconversion nanoparticles as labels

Common typical β-agonists mainly include ractopamine (RAC), salbutamol (SAL), and clenbuterol (CLB). In view of the harm to human health causes by the ingestion of animal derived food containing β-agonists, and a series of regulations have been issued to restrict the usage of β-agonists as growth promoters. In this work, a fluorescence immunoassay is developed for the simultaneous detection of typical β-agonists based on blue-green upconversion nanoparticles (UCNPs) combine with magnetic separation. Here, blue-green UCNPs act as a signal amplification source, and magnetic polystyrene microspheres (MPMs) act as an ideal separation medium. Based on a competitive form, capture probe competes (RAC-OVA@MPMs and SAL-OVA@MPMs) with targets to bind corresponding signal probe (anti-RAC antibody@NaYF4:Yb, Tm UCNPs and anti-SAL antibody@NaYF4:Yb, Er UCNPs). The fluorescence difference values of the competitive immune-complex obtained via magnetic separation at 483 nm and 550 nm are proportional to concentrations of RAC and SAL, respectively. The immunoassay has the wide detection linear range from 0.001 to 100 μg L−1, and the low limit of detection (LOD) is 5.04 × 10−4 μg L−1 for RAC, 1.97 × 10−4 μg L−1 for SAL, respectively. Meanwhile, use of antibody with same recognition ability for SAL and CLB makes that the fluorescence immunoassay can achieve simultaneous detection of three typical β-agonists (RAC, SAL, and CLB). This fluorescence immunoassay has good application value and practicability for simultaneous detection of typical β-agonists in animal derived food.

NIR-responsive photoelectrochemical sensing platform for the simultaneous determination of tetrodotoxin and okadaic acid in Nassariidae

Simultaneous detection of tetrodotoxin (TTX) and okadaic acid (OA) is important for seafood safety. In this work, a novel paper electrode-based near-infrared (NIR) light-responsive photoelectrochemical (PEC) immunosensor was constructed using Ag2S quantum dots (QDs) and NaYF4: Yb, Er upconversion nanoparticles (UCNPs) matched with BiOI for the simultaneous detection of TTX and OA in aquatic products. A low-cost, easily prepared gold nanoparticle-functionalized paper-based screen-printed electrode with six channels was designed to immobilize OA and Ab1 of TTX. Correspondingly, PEC signal immunoprobes (BiOI@UCNPs-Ab and Ab2-Ag2S QDs) with NIR-light response were introduced to construct competitive-type and sandwich-type PEC immunosensors for OA and TTX, respectively. Under optimal conditions, the linear ranges for TTX and OA were 0.001–100 and 0.001–80 ng mL−1, respectively, and the detection limits were 5 and 7 pg mL−1, respectively. The proposed sensor was successfully used for the simultaneous analysis of TTX and OA in Nassariidae samples.

Transient energy trapping as a size-conserving surface passivation strategy for producing bright ultrasmall upconversion nanoprobes

Lanthanide-doped upconversion nanoparticles (UCNPs) have been widely exploited as nanoprobes or energy transducers in traditional as well as emerging biological applications, such as bioimaging, photodynamic therapy, optogenetics, gene editing. However, the breadth and depth of their utility in the biomedical areas are still not comparable to conventional luminescent probes, such as fluorescent dyes and semiconductor quantum dots. Their application is largely limited by their large size, typically > 20 nm, to ensure a sufficient luminescence brightness. In order to enhance the brightness of UCNPs without exceeding the critical size limitations for biomedical applications, we employ here a transient energy trapping effect as a nanoprobe surface passivation strategy to prevent deleterious distant energy migration in the host lattice, which is particularly prevalent in ultrasmall UCNPs and leads to luminescence quenching. We demonstrate this strategy by incorporating Tm3+ ions as energy trapping centers near the surface of sub-10 nm NaYF4: Yb, Er UCNPs and obtain an emission enhancement by almost one order of magnitude without any increment on the nanoparticle size. Our work presents a promising strategy for the preparation of ultrasmall and bright upconversion nanoprobes that are less vulnerable to surface quenching and that potentially minimize the interference with the object. This facilitates their biomedical applications as here demonstrated by unprecedented high-quality cell labeling and imaging, featured with very uniform nanoparticle distribution in the outer nuclear region.

Upconversion photoluminescence enhancement by Gd-doped NaYF4:Yb,Er@SiO2 nanoparticles and their application in dye-sensitized solar cells

Introducing upconversion nanoparticles (UCNPs) onto photoanode films is an effective way to enhance the photovoltaic performance of dye-sensitized solar cells (DSSCs). In this contribution, the β-NaYGdF4:Yb, Er UCNPs and β-NaYGdF4:Yb,Er@SiO2 nanoparticles (NPs) were prepared by tuning the dopant contents of the Gd ions and the β-NaYGdF4:Yb,Er@SiO2 NPs were applied as an upconverting layer in DSSCs. Upconversion photoluminescence indicates that the UCNPs doped with 5% Gd ions exhibit the strongest green and red fluorescence emissions. Moreover, the DSSCs device incorporating the NPs with 5% Gd ion dopant can achieve an optimal photovoltaic efficiency of 5.89%, which is 20.20% enhancement in comparison to the device without NPs and 16.17% increasement with respect to the device containing β-NaYF4:Yb,Er@SiO2 nanocrystals, thus highlighting the effect of the doping of the Gd ions in nanoparticles on their upconversion photoluminescence and photoelectric performance. Accordingly, the current results offer a feasible strategy for further regulating the photoluminescence property of UCNPs and optimizing the photovoltaic performance of UCNPs-based DSSCs devices.

Upconversion luminescence-based aptasensor for the detection of thyroid-stimulating hormone in serum.

Thyroid-stimulating hormone (TSH) plays a crucial physiological and pathological role in humans, and a timely and sensitive detection of TSH is critical for early diagnosis and prevention of thyroid-related diseases. Herein, we developed a simple wash-free biological aptasensor based on luminescence resonance energy transfer (LRET) between NaYF4:Yb,Er upconversion nanoparticles (UCNPs) and tetramethylrhodamine (TAMRA) for the detection of TSH with high sensitivity. In this LRET system, UCNPs as donors and TAMRA as receptors were modified with nucleic acid aptamers Apt-1 and Apt-2, respectively. When TSH was present, the two aptamer strands both specifically recognized TSH to form a hairpin-like structure, thereby shortening the space between UCNPs and TAMRA. The LRET occurred under radiation of 980-nm light. By detecting the change of upconversion luminescence (UCL) intensity (I545nm), the activity of TSH was quantified. The resulting detection dynamic range and the limit of detection were 0.1-5.0 mIU·L-1 and 0.065 mIU·L-1, respectively. The aptasensor using UCNPs as LRET donors was capable of effectively eliminating the background interference of a complicated biological environment, and showed good specificity because of the excellent recognition function of aptamers. Due to high sensitivity, easiness of fabrication, operational convenience, and selectivity, the UCL-based aptasensor is a promising candidate for clinical TSH determination. Based on nucleic acid aptamer and the mechanism of luminescence resonance energy transfer (LRET) between upconversion nanoparticles (UCNPs) donor and tetramethylrhodamine (TAMRA) receptor, an aptasensor was constructed for the quantitative analysis of TSH activity in serum by testing the change of I545nm.

Stability, dissolution, and cytotoxicity of NaYF4-upconversion nanoparticles with different coatings

Upconversion nanoparticles (UCNPs) have attracted considerable attention owing to their unique photophysical properties. Their utilization in biomedical applications depends on the understanding of their transformations under physiological conditions and their potential toxicity. In this study, NaYF4:Yb,Er UCNPs, widely used for luminescence and photophysical studies, were modified with a set of four different coordinatively bound surface ligands, i.e., citrate, alendronate (AA), ethylendiamine tetra(methylene phosphonate) (EDTMP), and poly(maleic anhydride-alt-1-octadecene) (PMAO), as well as silica coatings with two different thicknesses. Subsequently, the aging-induced release of fluoride ions in water and cell culture media and their cytotoxic profile to human keratinocytes were assessed in parallel to the cytotoxic evaluation of the ligands, sodium fluoride and the lanthanide ions. The cytotoxicity studies of UCNPs with different surface modifications demonstrated the good biocompatibility of EDTMP-UCNPs and PMAO-UCNPs, which is in line with the low amount of fluoride ions released from these samples. An efficient prevention of UCNP dissolution and release of cytotoxic ions, as well as low cytotoxicity was also observed for UCNPs with a sufficiently thick silica shell. Overall, our results provide new insights into the understanding of the contribution of surface chemistry to the stability, dissolution behavior, and cytotoxicity of UCNPs. Altogether, the results obtained are highly important for future applications of UCNPs in the life sciences and bioimaging studies.

Low-temperature open-air synthesis of PVP-coated NaYF4:Yb,Er,Mn upconversion nanoparticles with strong red emission.

Cubic (α-phase) NaYF4:Yb,Er upconversion nanoparticles (UCNPs) are uniquely suited to biophotonics and biosensing applications due to their near-infrared excitation and visible red emission (λex approx. 660 nm), enabling detection via thick overlying tissue with no bio-autofluorescence. However, UCNP synthesis typically requires high temperatures in combination with either high pressure reaction vessels or an inert atmosphere. Here, we report synthesis of α-phase NaYF4:Yb,Er,Mn UCNPs via the considerably more convenient PVP40-mediated route; a strategy that requires modest temperatures and relatively short reaction time (160°C, 2 h) in open air, with Mn2+ co-doping serving to greatly enhance red emission. The optimal Mn2+ co-doping level was found to be 35 mol %, which decreased the average maximum UCNP Feret diameter from 42 ± 11 to 36 ± 15 nm; reduced the crystal lattice parameter, a, from 5.52 to 5.45 Å; and greatly enhanced UCNP red/green emission ratio in EtOH by a factor of 5.6. The PVP40 coating enabled dispersal in water and organic solvents and can be exploited for further surface modification (e.g. silica shell formation). We anticipate that this straightforward UCNP synthesis method for producing strongly red-emitting UCNPs will be particularly beneficial for deep tissue biophotonics and biosensing applications.

Upconversion Nanoparticles-Based Fluorescence Immunoassay for the Sensitive Detection of 2-Amino-3-methylimidazo [4,5-f] Quinoline (IQ) in Heat Processed Meat.

A competitive fluorescence immunoassay for the quantitative detection of 2-amino-3-methylimidazo [4,5-f] quinoline (IQ) in pan-fried meat patties was developed, using magnetic nanoparticles coupled with coating antigen as the capture probe and anti-IQ antibody coupled with NaYF4: Yb, Er upconversion nanoparticles as the signal probe. Under optimal conditionals, the wide detection range for IQ in phosphate buffer saline is from 0.01 to 100 μg·L−1 (R2 = 0.991) with a detection limit of 0.007 μg·L−1. This proposed method has been applied to detect IQ in two different types of pan-fried meat patties at varying frying times, and the IQ content in chicken patties and fish patties are 2.11–3.47 μg·kg−1 and 1.35–2.85 μg·kg−1, respectively. These results are consistent with that of the ultraperformance liquid chromatography-tandem mass spectrometry. In summary, this method can serve as a sensitive and specific test tool for the determination of IQ in processed meat.

Emitter-Active Shell in NaYF4:Yb,Er/NaYF4:Er Upconversion Nanoparticles for Enhanced Energy Transfer in Photodynamic Therapy

To realize the potential of near-infrared (NIR) upconversion nanosensitizers for photodynamic therapy of cancer, upconversion luminescence and energy transfer (ET) efficiency from emitter donors to photosensitizer acceptors need to be improved. In the current work, upconversion nanoparticles (UCNPs) with a core/emitter-active shell structure were constructed to enhance not only the upconversion emission but also the ET from the nanoparticles to surface-anchored photosensitizers. The emitter was doped into the shell to bridge the migration of upconverted energy to the surface. NaYF4:Yb,Er/NaYF4:Er UCNPs and rose bengal (RB) photosensitizer were employed as an example. The upconversion emission was lifted by up to ∼81 times of the core counterpart. The bridge effect of the emitter-doped shell was obvious for the constructed nanophotosensitizer. The emission of the RB photosensitizer was up to ∼36 times that of the core counterpart. The NaYF4:Yb,Er/NaYF4:Er UCNPs also endow the RB photosensitizers with the most efficient reactive oxygen species production capability under NIR irradiation. In vitro photodynamic tests on glioma cells were conducted to validate the efficacy of the NaYF4:Yb,Er/NaYF4:Er/RB agent. Therefore, this work can facilitate the development of ET-based upconversion nanosystems.

Biocompatible NaYF4:Yb,Er upconversion nanoparticles: Colloidal stability and optical properties

Currently, highly luminescent colloidal upconversion nanoparticles (UCNPs) have expanded an increasing interest of researchers because of their facilitating lability in the biomedical/clinical field. In this study, NaYF4:Yb,Er UCNPs are prepared by eco-friendly metal complexation-based thermal decomposition method at a lower temperature in aqueous media. The phase structure, crystallinity, phase purity, morphology, colloidal dispersibility, surface structure, surface charge, and optical and luminescent properties were evaluated carefully by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive x-ray analysis (EDX), Thermogravimetric analysis (TGA), zeta potential, Fourier transform infrared (FTIR), UV/visible and photoluminescent spectroscopic techniques. XRD pattern shows a pure single-phase cubic structure with an average grain size of 30–35 nm. TEM and SEM micrographs exhibited irregularly shaped spherical morphologies, porous surface structures highly aggregated UCNPs with the narrow-size distribution. Positive zeta potential has shown value signifying high absorption in the visible region which indicates particle's good colloidal stability in aqueous media. Under NIR-laser light excitation, the UCNPs emit strong UC emission transitions in the visible region. A broad infrared absorption peak of hydroxyl groups (–OH) in FTIR spectrum and mass loss at a lower temperature in TGA verified the surface functionality of UCNPs, with high colloidal stability, and excellent biocompatibility in aqueous media. In terms of their surface characteristics and high luminescent properties, the NaYF4:Yb,Er UCNPs could be interestingly applied in tagging of biomolecules, drug delivery, proteins labeling, and therapeutic and thermostats applications.

Competitive immune-nanoplatforms with positive readout for the rapid detection of imidacloprid using gold nanoparticles.

Twohigh-sensitivity competitive immune-nanoplatforms based on the inner filter effect (IFE-IN) and magnetic separation (MS-IN) with a positive readout were developed to rapidly detect imidacloprid (IMI) using gold nanoparticles (AuNPs). For IFE-IN, IMI competes with AuNPs-labeled IMI antigens (IMI-BSA-AuNPs) to bind with anti-IMI monoclonal antibody (mAb)-conjugated NaYF4:Yb,Er upconversion nanoparticles, which changes the fluorescence signal at excitation/emission wavelength of 980/544nm. For MS-IN, the immunocomplex of IMI-BSA-AuNPs and magnetic-nanoparticles-labeled mAb (mAb-MNPs) dissociates in the presence of IMI, and the optical density of IMI-BSA-AuNPs at 525nm increases with the IMI concentration after magnetic separation. Under the optimal conditions, the IMI concentration producing a 50% saturation of the signal (SC50) and linear range (SC10- SC90) were found to be 4.30ngmL-1 and 0.47 - 21.37ngmL-1 for IFE-IN, while 1.21ngmL-1 and 0.07 - 10.21ngmL-1 for MS-IN, respectively. Both IFE-IN and MS-IN achieved excellent accuracy for the detection of IMI in different matrices. The quantities of IMI in apple samples detected by IFE-IN and MS-IN were consistent with the high-performance liquid chromatography results. For IFE-IN, analyte competes with AuNPs-labeled-antigen to bind with the mAb-conjugated-UCNPs, which changes the fluorescence signal at 544nm. For MS-IN, the immunocomplex of AuNPs-labeled-antigen and mAb-conjugated-MNPs dissociates in the presence of analyte, and the optical density of AuNPs-labeled-antigen at 525nm increases with increasing analyte concentration after separation.

Photostable and Small YVO4:Yb,Er Upconversion Nanoparticles in Water

In this work, we report a simple method of silica coating of upconversion nanoparticles (UCNPs) to obtain well-crystalline particles that remain small and not agglomerated after high-temperature post-annealing, and produce bright visible emission when pumped with near-infrared light. This enables many interesting biological applications, including high-contrast and deep tissue imaging, quantum sensing and super-resolution microscopy. These VO4-based UNCPs are an attractive alternative to fluoride-based crystals for water-based biosensing applications.

Multiband emission from single \u03b2-NaYF4(Yb,Er) nanoparticles at high excitation power densities and comparison to ensemble studies

Ensemble and single particle studies of the excitation power density (P)-dependent upconversion luminescence (UCL) of core and core-shell β-NaYF4:Yb,Er upconversion nanoparticles (UCNPs) doped with 20% Yb3+ and 1% or 3% Er3+ performed over a P regime of 6 orders of magnitude reveal an increasing contribution of the emission from high energy Er3+ levels at P > 1 kW/cm2. This changes the overall emission color from initially green over yellow to white. While initially the green and with increasing P the red emission dominate in ensemble measurements at P < 1 kW/cm2, the increasing population of higher Er3+ energy levels by multiphotonic processes at higher P in single particle studies results in a multitude of emission bands in the ultraviolet/visible/near infrared (UV/vis/NIR) accompanied by a decreased contribution of the red luminescence. Based upon a thorough analysis of the P-dependence of UCL, the emission bands activated at high P were grouped and assigned to 2–3, 3–4, and 4 photonic processes involving energy transfer (ET), excited-state absorption (ESA), cross-relaxation (CR), back energy transfer (BET), and non-radiative relaxation processes (nRP). This underlines the P-tunability of UCNP brightness and color and highlights the potential of P-dependent measurements for mechanistic studies required to manifest the population pathways of the different Er3+ levels.