Energy Migration Upconversion in Manganese(II)‐Doped Nanoparticles

Optically Robust and Biocompatible Mechanosensitive Upconverting Nanoparticles

Upconverting nanoparticles (UCNPs) are promising tools for background-free imaging and sensing. However, their usefulness for in vivo applications depends on their biocompatibility, which we define by their optical performance in biological environments and their toxicity in living organisms. For UCNPs with a ratiometric color response to mechanical stress, consistent emission intensity and color are desired for the particles under nonmechanical stimuli. Here, we test the biocompatibility and mechanosensitivity of α-NaYF4:Yb,Er@NaLuF4 nanoparticles. First, we ligand-strip these particles to render them dispersible in aqueous media. Then, we characterize their mechanosensitivity (∼30% in the red-to-green spectral ratio per GPa), which is nearly 3-fold greater than those coated in oleic acid. We next design a suite of ex vivo and in vivo tests to investigate their structural and optical properties under several biorelevant conditions: over time in various buffers types, as a function of pH, and in vivo along the digestive tract of Caenorhabditis elegans worms. Finally, to ensure that the particles do not perturb biological function in C. elegans, we assess the chronic toxicity of nanoparticle ingestion using a reproductive brood assay. In these ways, we determine that mechanosensitive UCNPs are biocompatible, i.e., optically robust and nontoxic, for use as in vivo sensors to study animal digestion.

Probing Energy Migration through Precise Control of Interfacial Energy Transfer in Nanostructure

A novel mechanistic strategy for probing the energy migration through constructing the interfacial energy transfer (IET) in a core-shell-shell nanostructure is reported. In this design, the trilayer nanostructure is composed of a sensitizing core, a migratory interlayer, and a detective shell layer that interact with each other only by IET and the latter two shell layers are nonresponsive to the incident irradiation. This model is well applied in investigating the energy migration over the Tb, Gd, and Yb sublattices, and the results show that the Gd sublattice holds the best energy migratory performance. Moreover, the finding of energy migration over the Yb sublattice enables the 808 nm excited long-lived upconversion of Tb³⁺ and Eu³⁺ , which exhibits unique time-gating performance for information security. The results provide a facile and powerful nanosized model for an in-depth understanding of the fundamentals involving lanthanide interactions, which will further help excite new chances for the frontier applications of lanthanide-based luminescent materials.

Energy Flux Manipulation in Upconversion Nanosystems

Lanthanide-doped upconversion nanoparticles (UCNPs) exhibit unique optical characteristics, including a large anti-Stokes shift, a long luminescence lifetime, sharp emission bands, and high photostability. These virtues make UCNPs highly useful in many emerging applications such as biolabeling, security, multicolor displays, and optogenetics. Despite the enticing prospects of UCNPs, their practical utility is greatly hindered by the low efficiency of the conversion from near-infrared (NIR) excitation to visible emission. In a typical nanosystem codoped with sensitizers and activators, upconversion processes occur through NIR light sensitization, energy transfer from sensitizers to activators, sequential energy population at the excited states of the activators, and eventually the release of higher-energy photons. In fact, in the upconversion nanosystem, each step in the energy flux, including NIR energy injection, energy transfer and migration, and energy dissipation, has a decisive effect on the resulting luminescence intensity. Important in-depth studies have been conducted in pursuit of brighter UCNPs. Specifically, lanthanide ions possessing larger absorption cross sections (Nd³⁺) or organic dye molecules have been chosen as NIR light sensitizers to improve the light harvesting ability of upconversion nanostructures. The doping concentration and spatial distribution of lanthanide ions are strictly managed to mitigate detrimental energy cross-talk processes. The surfaces of UCNPs are passivated with epitaxially grown layers to block surface quenching. Therefore, rational design of energy flux manipulation, through control of excitation energy collection, transmission, and release in a three-dimensional nanospace of UCNPs, is crucial in constructing nanosystems with high upconversion efficiencies. In this Account, from an energy flux manipulation perspective, we attempt to provide an overview of general and emerging strategies for the design of efficient lanthanide-mediated photon upconversion nanosystems. With the significant progress made over the past several years, we are now able to design a series of upconversion nanoplatforms with efficient NIR light harvesting ability, sufficient energy transmission channels, and low levels of luminescence quenching at the particle's surface. In addition to providing a deep understanding of the underlying mechanism of energy flux, these discoveries will guide the development of upconversion nanosystems with significantly improved performance. The key aspects of this Account of energy flux manipulation in upconversion nanosystems mainly include the management of NIR photon energy injection, the optimization of efficient energy transfer pathways, and the minimization of energy flux leakage. Future challenges and opportunities for the development of efficient upconversion nanosystems are also discussed.

Bright, Mechanosensitive Upconversion with Cubic-Phase Heteroepitaxial Core-Shell Nanoparticles

Lanthanide-doped nanoparticles are an emerging class of optical sensors, exhibiting sharp emission peaks, high signal-to-noise ratio, photostability, and a ratiometric color response to stress. The same centrosymmetric crystal field environment that allows for high mechanosensitivity in the cubic-phase (α), however, contributes to low upconversion quantum yield (UCQY). In this work, we engineer brighter mechanosensitive upconverters using a core-shell geometry. Sub-25 nm α-NaYF4:Yb,Er cores are shelled with an optically inert surface passivation layer of ∼4.5 nm thickness. Using different shell materials, including NaGdF4, NaYF4, and NaLuF4, we study how compressive to tensile strain influences the nanoparticles' imaging and sensing properties. All core-shell nanoparticles exhibit enhanced UCQY, up to 0.14% at 150 W/cm2, which rivals the efficiency of unshelled hexagonal-phase (β) nanoparticles. Additionally, strain at the core-shell interface can tune mechanosensitivity. In particular, the compressive Gd shell results in the largest color response from yellow-green to orange or, quantitatively, a change in the red to green ratio of 12.2 ± 1.2% per GPa. For all samples, the ratiometric readouts are consistent over three pressure cycles from ambient to 5 GPa. Therefore, heteroepitaxial shelling significantly improves signal brightness without compromising the core's mechano-sensing capabilities and further, promotes core-shell cubic-phase nanoparticles as upcoming in vivo and in situ optical sensors.

Precisely Tailoring Upconversion Dynamics via Energy Migration in Core-Shell Nanostructures

Upconversion emission dynamics have long been believed to be determined by the activator and its interaction with neighboring sensitizers. Herein this assumption is, however, shown to be invalid for nanostructures. We demonstrate that excitation energy migration greatly affects upconversion emission dynamics. "Dopant ions' spatial separation" nanostructures are designed as model systems and the intimate link between the random nature of energy migration and upconversion emission time behavior is unraveled by theoretical modelling and confirmed spectroscopically. Based on this new fundamental insight, we have successfully realized fine control of upconversion emission time behavior (either rise or decay process) by tuning the energy migration paths in various specifically designed nanostructures. This result is significant for applications of this type of materials in super resolution spectroscopy, high-density data storage, anti-counterfeiting, and biological imaging.

Enhanced green upconversion luminescence in tetrahedral LiYF4:Yb/Er nanoparticles by manganese(ii)-doping: the key role of the host lattice

We report the enhancement of green upconversion luminescence in tetrahedral LiYF₄:Yb/Er nanoparticles by Mn²⁺ ion doping, which is different from the enhanced single-band red emission dominated by Mn²⁺ ions in cubic NaLnF₄:Yb/Er (Ln = Y, Gd, Lu) nanoparticles. The energy levels of the first excited state ⁴T₁ of Mn²⁺ in tetrahedral LiYF₄:Gd and cubic NaGd(Y)F₄ are compared by detection of emissions from Mn²⁺via the energy transition from Gd³⁺ to Mn²⁺ with excitation at 275 nm. The coordination environments of Mn²⁺ in these two host lattices have been investigated by X-ray absorption fine structure measurements. The results demonstrate that the formation of tetrahedral MnF₄ in tetragonal LiYF₄ arising from the replacement of Ln³⁺ ions with Mn²⁺ ions leads to a higher energy level of the Mn^{2+ 4}T₁ state than that in octahedral MnF₆ in cubic NaYF₄. The high-lying excited state of tetrahedral MnF₄ is close to the green emitting ⁴S_3/2 state of Er³⁺ and thus enhances green upconversion emission in tetragonal LiYF₄:Yb/Er, while the low-lying excited state of octahedral MnF₆ dominates red emission in cubic NaYF₄:Yb/Er. These findings provide direct evidence for the key roles of the host lattices and more possibilities in modulating the upconversion behaviour of lanthanide-based nanoparticles by transition-metal ion doping to achieve the desired goals of specific applications.

Direct cation exchange of surface ligand capped upconversion nanocrystals to produce strong luminescence

We develop a facile and rapid cation exchange method for upconversion nanocrystals (UCNCs) without removing surface ligands.

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.

Core@shell Fe3O4@Mn2+-doped NaYF4:Yb/Tm nanoparticles for triple-modality T1/T2-weighted MRI and NIR-to-NIR upconversion luminescence imaging agents

Core@shell structures of Fe₃O₄@Mn²⁺-doped NaYF₄:Yb/Tm nanoparticles (NPs) were prepared and then used for in vivo NIR to NIR (980 nm to 800 nm) imaging, and as dual-mode T₁/T₂-weighted MRI because of the co-existence of Fe₃O₄ and Mn²⁺ in the NPs.

Transition Metal-Involved Photon Upconversion

Upconversion (UC) luminescence of lanthanide ions (Ln³⁺) has been extensively investigated for several decades and is a constant research hotspot owing to its fundamental significance and widespread applications. In contrast to the multiple and fixed UC emissions of Ln³⁺, transition metal (TM) ions, e.g., Mn²⁺, usually possess a single broadband emission due to its 3d⁵ electronic configuration. Wavelength-tuneable single UC emission can be achieved in some TM ion-activated systems ascribed to the susceptibility of d electrons to the chemical environment, which is appealing in molecular sensing and lighting. Moreover, the UC emissions of Ln³⁺ can be modulated by TM ions (specifically d-block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln³⁺ owing to the well-shielded 4f electrons and tuneable energy levels of the TM ions. The electric versatility of d⁰ ion-containing hosts (d⁰ normally viewed as charged anion groups, such as MoO₆^6- and TiO₄^4-) may also have a strong influence on the electric dipole transition of Ln³⁺, resulting in multifunctional properties of modulated UC emission and electrical behaviour, such as ferroelectricity and oxide-ion conductivity. This review focuses on recent advances in the room temperature (RT) UC of TM ions, the UC of Ln³⁺ tuned by TM or d⁰ ions, and the UC of d⁰ ion-centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.

Controlled synthesis, bioimaging and toxicity assessments in strong red emitting Mn2+ doped NaYF4:Yb3+/Ho3+ nanophosphors

Rare earth, Yb³⁺/Ho³⁺ doped NaYF₄ nanophosphors co-doped with Mn²⁺ showed enhanced red emission under NIR irradiation and were successfully used for cancer cell imaging.