The impact of shell host (NaYF₄/CaF₂) and shell deposition methods on the up-conversion enhancement in Tb³⁺, Yb³⁺ codoped colloidal α-NaYF₄ core-shell nanoparticles

Lanthanide-Sensitized Upconversion Iridium Complex via Triplet Energy Transfer

Cyclometalated iridium (Ir) complexes demonstrate impressive capabilities across a range of fields, including biology and photocatalysis, due to their tunable optical characteristics and structure flexibility. However, generating upconversion luminescence of Ir complexes under near-infrared light excitation is challenging. Herein, by employing lanthanide-doped upconversion nanoparticles (UCNPs) as the sensitizer, a new strategy is demonstrated to gain upconversion luminescence of Ir complexes via triplet energy transfer. This design relies on a rationally designed hybrid of core-shell structured NaYbF4:Tb@NaTbF4 UCNPs and new Ir phosphonate complexes, in which UCNPs can migrate upconverted energy to the surface of nanoparticles through Tb3+-mediated energy migration and then sensitize the upconversion luminescence of Ir complexes upon 980 nm excitation. Both experimental and theoretical investigations highlight the significance of triplet energy transfer from excited Tb3+ ions to the triplet state of Ir complexes in the sensitization of upconversion luminescence of Ir complexes. These findings may open exciting avenues for fabricating hybrid Ir materials with new functions and driving the development of UCNP-based nanomaterials.

Photophysics and its application in photon upconversion

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interaction, where the energy of the emitted photons is higher than that of the incident photons. PL upconversion has promising applications in optoelectronic devices, displays, photovoltaics, imaging, diagnosis and treatment. In this review, we summarize the mechanism of PL upconversion and ultrafast PL physical processes. In particular, we highlight the advances in laser cooling, biological imaging, volumetric displays and photonics.

Multichannel Control of PersL/Upconversion/Down-Shifting Luminescence in a Single Core-Shell Nanoparticle for Information Encryption

Persistent luminescence (PersL) has been attracting substantial attention in diverse frontier applications such as optical information security and in vivo bioimaging. However, most of the reported PersL emissions are based on the dopants instead of the host matrix, which also plays an important role. In addition, there are few works on the PersL-based multifunctional nanoplatform in nanosized materials. Here, we report a class of novel nanostructure designs with PersL, upconversion, and down-shifting luminescence to realize the fine-tuning of emission colors under different excitation modes including steady-state irradiation, time-gating, and PersL generation. Blue, orange, and green emissions were easily achieved in such a single nanoparticle under suitable excitation modes. Moreover, the physical origin of the PersL of the CaF₂ matrix was discussed by simulating the energy band structure with Ca_xF_y defects. Our results provide new opportunities for the design of a new class of multifunctional materials, showing great promise in the field of information encryption security and multilevel anticounterfeiting.

Engineering the Compositional Architecture of Core-Shell Upconverting Lanthanide-Doped Nanoparticles for Optimal Luminescent Donor in Resonance Energy Transfer: The Effects of Energy Migration and Storage

Förster Resonance Energy Transfer (FRET) between single molecule donor (D) and acceptor (A) is well understood from a fundamental perspective and is widely applied in biology, biotechnology, medical diagnostics, and bio-imaging. Lanthanide doped upconverting nanoparticles (UCNPs) have demonstrated their suitability as alternative donor species. Nevertheless, while they solve most disadvantageous features of organic donor molecules, such as photo-bleaching, spectral cross-excitation, and emission bleed-through, the fundamental understanding and practical realizations of bioassays with UCNP donors remain challenging. Among others, the interaction between many donor ions (in donor UCNP) and many acceptors anchored on the NP surface and the upconversion itself within UCNPs, complicate the decay-based analysis of D-A interaction. In this work, the assessment of designed virtual core-shell NP (VNP) models leads to the new designs of UCNPs, such as …@Er, Yb@Er, Yb@YbEr, which are experimentally evaluated as donor NPs and compared to the simulations. Moreover, the luminescence rise and decay kinetics in UCNP donors upon RET is discussed in newly proposed disparity measurements. The presented studies help to understand the role of energy-transfer and energy migration between lanthanide ion dopants and how the architecture of core-shell UCNPs affects their performance as FRET donors to organic acceptor dyes.

Photon upconversion through triplet exciton-mediated energy relay

Exploration of upconversion luminescence from lanthanide emitters through energy migration has profound implications for fundamental research and technology development. However, energy migration-mediated upconversion requires stringent experimental conditions, such as high power excitation and special migratory ions in the host lattice, imposing selection constraints on lanthanide emitters. Here we demonstrate photon upconversion of diverse lanthanide emitters by harnessing triplet exciton-mediated energy relay. Compared with gadolinium-based systems, this energy relay is less dependent on excitation power and enhances the emission intensity of Tb3+ by 158-fold. Mechanistic investigations reveal that emission enhancement is attributable to strong coupling between lanthanides and surface molecules, which enables fast triplet generation (<100 ps) and subsequent near-unity triplet transfer efficiency from surface ligands to lanthanides. Moreover, the energy relay approach supports long-distance energy transfer and allows upconversion modulation in microstructures. These findings enhance fundamental understanding of energy transfer at molecule-nanoparticle interfaces and open exciting avenues for developing hybrid, high-performance optical materials.

Single-molecule photoreaction quantitation through intraparticle-surface energy transfer (i-SET) spectroscopy

Quantification of nanoparticle-molecule interaction at a single-molecule level remains a daunting challenge, mainly due to ultra-weak emission from single molecules and the perturbation of the local environment. Here we report the rational design of an intraparticle-surface energy transfer (i-SET) process, analogous to high doping concentration-induced surface quenching effects, to realize single-molecule sensing by nanoparticle probes. This design, based on a Tb³⁺-activator-rich core-shell upconversion nanoparticle, enables a much-improved spectral response to fluorescent molecules at single-molecule levels through enhanced non-radiative energy transfer with a rate over an order of magnitude faster than conventional counterparts. We demonstrate a quantitative analysis of spectral changes of one to four fluorophores tethered on a single nanoparticle through i-SET spectroscopy. Our results provide opportunities to identify photoreaction kinetics at single-molecule levels and provide direct information for understanding behaviors of individual molecules with unprecedented sensitivity.

Near-infrared excited luminescence and in vitro imaging of HeLa cells by using Mn2+ enhanced Tb3+ and Yb3+ cooperative upconversion in NaYF4 nanocrystals

Advanced biodetection and bioimaging require fluorescent labels which exhibit many, easily distinguishable colors to identify or study numerous biotargets in a single sample. Although numerous different colors have been demonstrated with lanthanide doped nanoparticles, these colors usually originate from various ratios of overlapping multiple emission bands from activators, which severely limits the number of available labels. As a consequence, different lanthanide doped labels cannot be easily distinguished from each other (e.g. Er³⁺ from Ho³⁺) in a quantitative way, when such labels are co-localized during microscopy wide-field imaging. It is therefore reasonable to expand the available choice of spectral signatures and not rely on just different colors. Other ions, such as Tb³⁺ or Eu³⁺, can offer new possibilities and unique spectral features in upconversion mode in this respect. For example, despite partial overlap with Er³⁺ or Ho³⁺ emission spectra, Tb³⁺ ions display also unique and easily distinguishable spectral features at 580 nm. Unfortunately, in terms of brightness, Tb³⁺ emission in upconversion mode is typically too weak to be useful. To improve the Tb³⁺ upconversion emission intensity, a new approach, i.e. Mn²⁺ co-doping, has been proposed and verified in this work. A versatile optimization of Tb³⁺, Yb³⁺ and Mn²⁺ ion concentrations has been performed based on luminescence spectra and lifetime studies. The most intense emission was achieved for nanoparticles doped with 10% Mn²⁺ ions, with over 30 times brighter intensity of Tb³⁺ ions compared to the emission of nanocrystals without the addition of Mn²⁺ ions. Additionally, as a proof of the concept, the surface of nanoparticles was coated with proteins and conjugated with folic acid, and such biofunctionalized nanoparticles were subsequently used for bioimaging of HeLa cells.

Intrinsic Time-Tunable Emissions in Core-Shell Upconverting Nanoparticle Systems

Color-tunable luminescence has been extensively investigated in upconverting nanoparticles for diverse applications, each exploiting emissions in different spectral regions. Manipulation of the emission wavelength is accomplished by varying the composition of the luminescent material or the characteristics of the excitation source. Herein, we propose core-shell β-NaGdF₄ : Tm³⁺ , Yb³⁺ /β-NaGdF₄ : Tb³⁺ nanoparticles as intrinsic time-tunable luminescent materials. The time dependency of the emission wavelength only depends on the different decay time of the two emitters, without additional variation of the dopant concentration or pumping source. The time-tunable emission was recorded with a commercially available camera. The dynamics of the emissions is thoroughly investigated, and we established that the energy transfer from the ¹ D₂ excited state of Tm³⁺ ions to the higher energy excited states of Tb³⁺ ions to be the principal mechanism to the population of the ⁵ D₄ level for the Tb³⁺ ions.

Neto E, Mazali IO, Murugesu M, Sigoli FA. One pot synthesis and systematic study of the photophysical and magnetic properties and thermal sensing of α and β-phase NaLnF4 and β-phase core@shell nanoparticles

Hierarchical nanostructured nanoparticles showing high values of relative thermal sensitivity and magnetization at low temperatures.

Calcium-based biomaterials for diagnosis, treatment, and theranostics

Calcium-based biomaterials with good biosafety and bio-absorbability are promising for biomedical applications such as diagnosis, treatment, and theranostics.

Shaping Luminescent Properties of Yb3+ and Ho3+ Co-Doped Upconverting Core-Shell β-NaYF4 Nanoparticles by Dopant Distribution and Spacing

At the core of luminescence color and lifetime tuning of rare earth doped upconverting nanoparticles (UCNPs), is the understanding of the impact of the particle architecture for commonly used sensitizer (S) and activator (A) ions. In this respect, a series of core@shell NaYF₄ UCNPs doped with Yb³⁺ and Ho³⁺ ions are presented here, where the same dopant concentrations are distributed in different particle architectures following the scheme: YbHo core and YbHo@…, …@YbHo, Yb@Ho, Ho@Yb, YbHo@Yb, and Yb@YbHo core-shell NPs. As revealed by quantitative steady-state and time-resolved luminescence studies, the relative spatial distribution of the A and S ions in the UCNPs and their protection from surface quenching has a critical impact on their luminescence characteristics. Although the increased amount of Yb³⁺ ions boosts UCNP performance by amplifying the absorption, the Yb³⁺ ions can also efficiently dissipate the energy stored in the material through energy migration to the surface, thereby reducing the overall energy transfer efficiency to the activator ions. The results provide yet another proof that UC phosphor chemistry combined with materials engineering through intentional core@shell structures may help to fine-tune the luminescence features of UCNPs for their specific future applications in biosensing, bioimaging, photovoltaics, and display technologies.

Unravelling the energy transfer of Er3+-self-sensitized upconversion in Er3+-Yb3+-Er3+ clustered core@shell nanoparticles

Unravelling upconversion (UC) energy transfer mechanisms is significant for designing novel efficient anti-Stokes phosphors. We have studied the correlation of different lanthanide dopants within Er³⁺-self-sensitized core@shell upconversion nanoparticles (UCNPs). Here, our focus will be on high-concentration dopants that are able to sufficiently produce the clustering effect, especially within the interplay between Er³⁺ and Yb³⁺. We demonstrate that whatever the amount of the self-sensitizer (e.g., Er³⁺), abnormal absorption enhancement will occur as long as Yb³⁺ clusters are present. This effect originates from the substantial energy transfer between Yb³⁺-Yb³⁺ clusters despite the increased energy transfer from Yb³⁺ to Er³⁺. Therefore, the energy transfer efficiency is still constrained. However, we conversely used one of the aforementioned quench-paths of UC energy transfer to easily transfer the energy from the in-shell shell layer to the in-core area with the assistance of the energy potential reservoir, which was given by the homogeneous core@shell band offset at the interface region. Indirectly, we actualize the Er³⁺ UC luminescence with self-sensitization through an extended energy transfer path. This work provides a solid support and analytic theory for unraveling the energy transfer mechanism from recent works on Er³⁺ self-sensitized UC luminescence.

Synthesis of Multicolor Core/Shell NaLuF₄:Yb3+/Ln3+@CaF₂ Upconversion Nanocrystals

The ability to synthesize high-quality hierarchical core/shell nanocrystals from an efficient host lattice is important to realize efficacious photon upconversion for applications ranging from bioimaging to solar cells. Here, we describe a strategy to fabricate multicolor core @ shell α-NaLuF₄:Yb³⁺/Ln³⁺@CaF₂ (Ln = Er, Ho, Tm) upconversion nanocrystals (UCNCs) based on the newly established host lattice of sodium lutetium fluoride (NaLuF₄). We exploited the liquid-solid-solution method to synthesize the NaLuF₄ core of pure cubic phase and the thermal decomposition approach to expitaxially grow the calcium fluoride (CaF₂) shell onto the core UCNCs, yielding cubic core/shell nanocrystals with a size of 15.6 ± 1.2 nm (the core ~9 ± 0.9 nm, the shell ~3.3 ± 0.3 nm). We showed that those core/shell UCNCs could emit activator-defined multicolor emissions up to about 772 times more efficient than the core nanocrystals due to effective suppression of surface-related quenching effects. Our results provide a new paradigm on heterogeneous core/shell structure for enhanced multicolor upconversion photoluminescence from colloidal nanocrystals.

Understanding the remarkable luminescence enhancement via SiO2 coating on TiO2:Eu3+ nanofibers

Red luminescence is enhanced by coating an SiO₂ layer on TiO₂:Eu³⁺ nanofibers, and the luminescence enhancement mechanism is discussed.

Sequential growth of CaF2:Yb,Er@CaF2:Gd nanoparticles for efficient magnetic resonance angiography and tumor diagnosis

It is a significant challenge to develop nanoscale magnetic resonance imaging (MRI) contrast agents with high performance of relaxation.

Highly Enhanced Cooperative Upconversion Luminescence through Energy Transfer Optimization and Quenching Protection

Upconversion luminescence nanomaterials have shown great potential in biological and physical applications because of their unique properties. However, limited research exists on the cooperative sensitization upconversion emission in Tb(3+) ions over Er(3+) ions and Tm(3+) ions because of its low efficiency. Herein, by optimizing the doping ratio of sensitizer and activator to maximize the utilization of the photon energy and introducing the CaF2 inert shell to shield sensitizer from quenchers, we synthesize ultrasmall NaYbF4:Tb@CaF2 nanoparticles with a significant enhancement (690-fold) in cooperative sensitization upconversion emission intensity, compared with the parent NaYbF4:Tb. The lifetime of Tb(3+) emission in NaYbF4:Tb@CaF2 nanoparticles is prolonged extensively to ∼3.5 ms. Furthermore, NaYbF4:Tb@CaF2 was applied in in vitro and in vivo bioimaging. The presented luminescence enhancement strategy provides cooperative sensitization upconversion with new opportunities for bioapplication.

Benefits of Silica Core-Shell Structures on the Temperature Sensing Properties of Er,Yb:GdVO4 Up-Conversion Nanoparticles

We studied the temperature-dependent luminescence of GdVO4 nanoparticles co-doped with Er(3+) (1 mol %) and Yb(3+) (20 mol %) and determined their thermal sensing properties through the fluorescence intensity ratio (FIR) technique. We also analyzed how a silica coating, in a core-shell structure, affects the temperature sensing properties of this material. Spectra were recorded in the range of biological temperatures (298-343 K). The absolute sensitivity for temperature determination calculated for the core-shell nanoparticles is double the one calculated for bare nanoparticles, achieving a thermal resolution of 0.4 K. Moreover, silica-coated nanoparticles show good dispersibility in different solvents, such as water, DMSO, and methanol. Also, they show good luminescence stability without interactions with solvent molecules. Furthermore, we also observed that the silica coating shell prevents progressive heating of the nanoparticles during prolonged excitation periods with the 980 nm laser, preventing effects on their thermometric applications.

A broadening temperature sensitivity range with a core-shell YbEr@YbNd double ratiometric optical nanothermometer

The chemical architecture of lanthanide doped core-shell up-converting nanoparticles can be engineered to purposely design the properties of luminescent nanomaterials, which are typically inaccessible to their homogeneous counterparts. Such an approach allowed to shift the up-conversion excitation wavelength from ∼980 to the more relevant ∼808 nm or enable Tb or Eu up-conversion emission, which was previously impossible to obtain or inefficient. Here, we address the issue of limited temperature sensitivity range of optical lanthanide based nano-thermometers. By covering Yb-Er co-doped core nanoparticles with the Yb-Nd co-doped shell, we have intentionally combined temperature dependent Er up-conversion together with temperature dependent Nd → Yb energy transfer, and thus have expanded the temperature response range ΔT of a single nanoparticle based optical nano-thermometer under single ∼808 nm wavelength photo-excitation from around ΔT = 150 K to over ΔT = 300 K (150-450 K). Such engineered nanocrystals are suitable for remote optical temperature measurements in technology and biotechnology at the sub-micron scale.

Enhanced luminescence efficiency of aqueous dispersible NaYF4:Yb/Er nanoparticles and the effect of surface coating

In a general approach, we designed and synthesized monodisperse, well-defined, highly emissive and aqueous dispersible NaYF₄:Yb/Er upconversion nanoparticles, and thereafter their surfaces were coated with inert NaYF₄ and silica layers.

Core–shell–shell heterostructures of α-NaLuF4:Yb/Er@NaLuF4:Yb@MF2 (M = Ca, Sr, Ba) with remarkably enhanced upconversion luminescence

Core–shell–shell heterostructures of α-NaLuF₄:Yb/Er@NaLuF₄:Yb@MF₂ (M = Ca, Sr, Ba) exhibit remarkably enhanced upconversion luminescence.

Controlling upconversion nanocrystals for emerging applications

Lanthanide-doped upconversion nanocrystals enable anti-Stokes emission with pump intensities several orders of magnitude lower than required by conventional nonlinear optical techniques. Their exceptional properties, namely large anti-Stokes shifts, sharp emission spectra and long excited-state lifetimes, have led to a diversity of applications. Here, we review upconversion nanocrystals from the perspective of fundamental concepts and examine the technical challenges in relation to emission colour tuning and luminescence enhancement. In particular, we highlight the advances in functionalization strategies that enable the broad utility of upconversion nanocrystals for multimodal imaging, cancer therapy, volumetric displays and photonics.

Photon Upconversion Through Tb(3+) -Mediated Interfacial Energy Transfer

A strategy of interfacial energy transfer upconversion is demonstrated through the use of a terbium (Tb(3+) ) dopant as energy donor or energy migrator in core-shell-structured nanocrystals. This mechanistic investigation presents a new pathway for photon upconversion, and, more importantly, contributes to the better control of energy transfer at the nanometer length scale.

Upconversion of rare Earth nanomaterials

Rare earth nanomaterials, which feature long-lived intermediate energy levels and intraconfigurational 4f-4f transitions, are promising supporters for photon upconversion. Owing to their unique optical properties, rare earth upconversion nanomaterials have found applications in bioimaging, theranostics, photovoltaic devices, and photochemical reactions. Here, we review recent advances in the photon upconversion processes of these nanomaterials. We start by considering energy transfer models involved in the study of upconversion emissions, as well as well-established synthesis strategies to control the size and shape of rare earth upconversion nanomaterials. Progress in engineering energy transfer pathways, which play a dominant role in determining upconversion emission outputs, is then discussed. Lastly, representative optical applications of these materials are considered. The aim of this review is to provide inspiration for researchers to explore novel upconversion nanomaterials and extended optical applications.

Synthesis of a novel bifunctional nanocomposite with tunable upconversion emission and magnetic properties

A method of Co²⁺ ions codoping for significantly enhancing upconversion emission intensity and simultaneous controlling sparamagnetic properties in β-NaYF₄:Yb/Er nanoparticles, with well maintaining their morphology and highly disperse.

Energy transfer in lanthanide upconversion studies for extended optical applications

In this review, the various energy transfer pathways involved in lanthanide-related upconversion emissions are comprehensively discussed.

Upconverting nanoparticles: assessing the toxicity

Based on a survey of existing studies, low nanotoxicity of lanthanide doped upconverting nanoparticles holds promise for their safety and suitability for biomedical detection and imaging.

Size-tunable and monodisperse Tm³⁺/Gd³⁺-doped hexagonal NaYbF₄ nanoparticles with engineered efficient near infrared-to-near infrared upconversion for in vivo imaging

Hexagonal NaYbF4:Tm(3+) upconversion nanoparticles hold promise for use in high contrast near-infrared-to-near-infrared (NIR-to-NIR) in vitro and in vivo bioimaging. However, significant hurdles remain in their preparation and control of their morphology and size, as well as in enhancement of their upconversion efficiency. Here, we describe a systematic approach to produce highly controlled hexagonal NaYbF4:Tm(3+) nanoparticles with superior upconversion. We found that doping appropriate concentrations of trivalent gadolinium (Gd(3+)) can convert NaYbF4:Tm(3+) 0.5% nanoparticles with cubic phase and irregular shape into highly monodisperse NaYbF4:Tm(3+) 0.5% nanoplates or nanospheres in a pure hexagonal-phase and of tunable size. The intensity and the lifetime of the upconverted NIR luminescence at 800 nm exhibit a direct dependence on the size distribution of the resulting nanoparticles, being ascribed to the varied surface-to-volume ratios determined by the different nanoparticle size. Epitaxial growth of a thin NaYF4 shell layer of ∼2 nm on the ∼22 nm core of hexagonal NaYbF4:Gd(3+) 30%/Tm(3+) 0.5% nanoparticles resulted in a dramatic 350 fold NIR upconversion efficiency enhancement, because of effective suppression of surface-related quenching mechanisms. In vivo NIR-to-NIR upconversion imaging was demonstrated using a dispersion of phospholipid-polyethylene glycol (DSPE-PEG)-coated core/shell nanoparticles in phosphate buffered saline.