Janus Micromotors for Photophoretic Motion and Photon Upconversion Applications Using a Single Near-Infrared Wavelength.

External stimuli can trigger changes in temperature, concentration, and momentum between micromotors and the medium, causing their propulsion and enabling them to perform different tasks with improved kinetic efficiencies. Light-activated micromotors are attractive systems that achieve improved motion and have the potential for high spatiotemporal control. Photophoretic swarming motion represents an attractive means to induce micromotor movement through the generation of temperature gradients in the medium, enabling the micromotors to move from cold to hot regions. The micromotors studied herein are assembled with Fe3O4 nanoparticles, and NaGdF4:Yb3+,Er3+/NaGdF4:Yb3+ and LiYF4:Yb3+,Tm3+ upconverting nanoparticles. The Fe3O4 nanoparticles were localized to one hemisphere to produce a Janus architecture that facilitates improved upconversion luminescence with the upconverting nanoparticles distributed throughout. Under 976 nm excitation, Fe3O4 nanoparticles generate the temperature gradient, while the upconverting nanoparticles produce visible light that is used for micromotor motion tracking and triggering of reactive oxygen species generation. As such, the motion and application of the micromotors are achieved using a single excitation wavelength. To demonstrate the practicality of this system, curcumin was adsorbed to the micromotor surface and degradation of Rhodamine B was achieved with kinetic rates that were over twice as fast as the static micromotors. The upconversion luminescence was also used to track the motion of the micromotors from a single image frame, providing a convenient means to understand the trajectory of these systems. Together, this system provides a versatile approach to achieving light-driven motion while taking advantage of the potential applications of upconversion luminescence such as tracking and detection, sensing, nanothermometry, particle velocimetry, photodynamic therapy, and pollutant degradation.

Achieving photostability in dye-sensitized upconverting nanoparticles and their use in Fenton type photocatalysis.

Dye sensitization is a promising approach to enhance the luminescence of lanthanide-doped upconverting nanoparticles. However, the poor photostability of near-infrared dyes hampers their use in practical applications. To address this, commercial IR820 was modified for improved photostability and covalently bonded to amine-functionalized silica-coated LnUCNPs. Two methods of covalent linking were investigated: linking the dye to the surface of the silica shell, and embedding the dye within the silica shell. The photostability of the dyes when embedded in the silica shell exhibited upconversion emissions from NaGdF4:Er3+,Yb3+/NaGdF4:Yb3+ nanoparticles for over four hours of continuous excitation with no change in intensity. To highlight this improvement, the photostable dye-embedded system was successfully utilized for Fenton-type photocatalysis, emphasizing its potential for practical applications. Overall, this study presents a facile strategy to circumvent the overlooked limitations associated with photodegradation, opening up new possibilities for the use of dye-sensitized lanthanide-doped upconverting nanoparticles in a range of fields.

The role of lanthanide luminescence in advancing technology.

Our society is indebted to the numerous inventors and scientists who helped bring about the incredible technological advances in modern society that we all take for granted. The importance of knowing the history of these inventions is often underestimated, although our reliance on technology is escalating. Lanthanide luminescence has paved the way for many of these inventions, from lighting and displays to medical advancements and telecommunications. Given the significant role of these materials in our daily lives, knowingly or not, their past and present applications are reviewed. A majority of the discussion is devoted to pointing out the benefits of using lanthanides over other luminescent species. We aimed to give a short outlook outlines promising directions for the development of the considered field. This review aims to provide the reader enough content to further appreciate the benefits that these technologies have brought into our lives, with the perspective of travelling among the past and latest advances in lanthanide research, aiming for an even brighter future.

Combining Pr3+-Doped Nanoradiosensitizers and Endogenous Protoporphyrin IX for X-ray-Mediated Photodynamic Therapy of Glioblastoma Cells.

Glioblastoma multiforme is an aggressive type of brain cancer with high recurrence rates due to the presence of radioresistant cells remaining after tumor resection. Here, we report the development of an X-ray-mediated photodynamic therapy (X-PDT) system using NaLuF4:25% Pr3+ radioluminescent nanoparticles in conjunction with protoporphyrin IX (PPIX), an endogenous photosensitizer that accumulates selectively in cancer cells. Conveniently, 5-aminolevulinic acid (5-ALA), the prodrug that is administered for PDT, is the only drug approved for fluorescence-guided resection of glioblastoma, enabling dual detection and treatment of malignant cells. NaLuF4:Pr3+ nanoparticles were synthesized and spectroscopically evaluated at a range of Pr3+ concentrations. This generated radioluminescent nanoparticles with strong emissions from the 1S0 excited state of Pr3+, which overlaps with the Soret band of PPIX to perform photodynamic therapy. The spectral overlap between the nanoparticles and PPIX improved treatment outcomes for U251 cells, which were used as a model for the thin tumor margin. In addition to sensitizing PPIX to induce X-PDT, our nanoparticles exhibit strong radiosensitizing properties through a radiation dose-enhancement effect. We evaluate the effects of the nanoparticles alone and in combination with PPIX on viability, death, stress, senescence, and proliferation. Collectively, our results demonstrate this as a strong proof of concept for nanomedicine.

Cooperative Sensitization Upconversion in Solution Dispersions of Co-Crystal Assemblies of Mononuclear Yb3+ and Eu3+ Complexes.

Lanthanide upconversion luminescence in nanoparticles has prompted continuous breakthroughs in information storage, temperature sensing, and biomedical applications, among others. Achieving upconversion luminescence at the molecular scale is still a critical challenge in modern chemistry. In this work, we explored the upconversion luminescence of solution dispersions of co-crystals composed of discrete mononuclear Yb(DBM)3 Bpy and Eu(DBM)3 Bpy complexes (DBM: dibenzoylmethane, Bpy: 2,2'-bipyridine). The 613 nm emission of Eu3+ was observed under excitation of Yb3+ at 980 nm. From the series of molecular assemblies studied, the most intense luminescence was obtained for a 1 : 1 molar ratio of Yb3+ : Eu3+ , resulting in a high quantum yield of 0.67 % at 2.1 W cm-2 . The structure and energy transfer mechanism of the assemblies were fully characterized. This is the first example of an Eu3+ -based upconverting system composed of two discrete mononuclear lanthanide complexes present as co-crystals in non-deuterated solution.

Upconversion Luminescence through Cooperative and Energy-Transfer Mechanisms in Yb3+ -Metal-Organic Frameworks.

Lanthanide-doped metal-organic frameworks (Ln-MOFs) have versatile luminescence properties, however it is challenging to achieve lanthanide-based upconversion luminescence in these materials. Here, 1,3,5-benzenetricarboxylic acid (BTC) and trivalent Yb3+ ions were used to generate crystalline Yb-BTC MOF 1D-microrods with upconversion luminescence under near infrared excitation via cooperative luminescence. Subsequently, the Yb-BTC MOFs were doped with a variety of different lanthanides to evaluate the potential for Yb3+ -based upconversion and energy transfer. Yb-BTC MOFs doped with Er3+ , Ho3+ , Tb3+ , and Eu3+ ions exhibit both the cooperative luminescence from Yb3+ and the characteristic emission bands of these ions under 980 nm irradiation. In contrast, only the 497 nm upconversion emission band from Yb3+ is observed in the MOFs doped with Tm3+ , Pr3+ , Sm3+ , and Dy3+ . The effects of different dopants on the efficiency of cooperative luminescence were established and will provide guidance for the exploitation of Ln-MOFs exhibiting upconversion.

On the photostability and luminescence of dye-sensitized upconverting nanoparticles using modified IR820 dyes.

Dye sensitization is a promising route to enhance luminescence from lanthanide-doped upconverting nanoparticles (LnUCNPs) by improving the photon harvesting capability of LnUCNPs through the use of dye molecules, characterized by higher absorption coefficients. The literature does not fully address the poor photostability of NIR dyes, hindering solution-based applications. The improvements achieved by dye-sensitized LnUCNPs are usually obtained by comparison with non-dye sensitized LnUCNPs. This comparison results in exciting the LnUCNPs at different wavelengths with respect to the dye-sensitized LnUCNPs or at the same wavelengths, where, however, the non-dye sensitized LnUCNPs do not absorb. Both these comparisons are hardly conclusive for a quantification of the improvements achieved by dye-sensitization. Both shortcomings were addressed by studying the photodegradation via thorough spectroscopic evaluations of a 4-nitrothiophenol-modified and unmodified IR820-LnUCNP system. The modified IR820 dye system exhibits a 200% enhancement in the emission of NaGdF4:Er3+,Yb3+/NaGdF4:Yb3+ nanoparticles relative to the unmodified IR820-sensitized LnUCNPs and emits for over twice the duration, demonstrating a substantial improvement over previous dye-LnUCNP systems. Upconversion dynamics between the dyes and Er3+ establish the importance of back-transfer dynamics in modulating the dye-LnUCNP luminescence. Quantum yield measurements further illustrate the mechanism of sensitization and increased efficiency of this new nanosystem.

Lifetime of the 3H4 Electronic State in Tm3+-Doped Upconverting Nanoparticles for NIR Nanothermometry.

Emission bands from thermally coupled states in lanthanide-doped nanoparticles have been studied for ratiometric nanothermometry in biological applications. Unfortunately certain factors such as water absorption distort the intensity, limiting the accuracy of ratiometric nanothermometry. However, the decay time of such states does not suffer from such distortions. We introduce the decay time of the 3H4 state in Yb3+, Tm3+-doped nanoparticles for improved nanothermometry. The strong 800 nm upconversion emission exists in the first biological transparency window. This is the first use of a single upconversion band for lifetime nanothermometry.

The Key Role of Intrinsic Lifetime Dynamics from Upconverting Nanosystems in Multiemission Particle Velocimetry.

Evaluation of particle dynamics at the nano- and microscale poses a challenge to the development of novel velocimetry techniques. Established optical methods implement external or internal calibrations of the emission profiles by varying the particle velocity and are limited to specific experimental conditions. The proposed multiemission particle velocimetry approach aims to introduce a new concept for a luminescent probe, which guarantees accurate velocity measurements at the microscale, independent of the particle concentration or experimental setup, and without need for calibration. The simplicity of these analyses relies on the intrinsic luminescence dynamics of core-shell upconverting nanoparticles. Upon excitation with a focused near-infrared pulsed laser, the nanoparticle emits photons at different wavelengths. The time interval between emissions from different excited states is independent of the local environment or particle velocity. The velocity of the particles is calculated by measuring the distance between the maxima of two different emissions and dividing it by the known difference in luminescence lifetimes. This method is demonstrated using simple digital imaging of nanoparticles flowing in 75-150 µm diameter capillaries. Using this novel approach typically results in a relative standard deviation of the experimental velocities of 5% or lower without any calibration.

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 ß-NaGdF4 : Tm3+ , Yb3+ /ß-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.

Luminescence dynamics and enhancement of the UV and visible emissions of Tm3+ in LiYF4:Yb3+,Tm3+ upconverting nanoparticles.

To maximize the intrinsic luminescence efficiency of the higher energy emissions of Tm3+ in LiYF4:Yb3+,Tm3+ upconverting nanoparticles, we investigated a specific range of Tm3+ dopant concentrations. Reported to be optimized at 25% Yb3+, 0.5% Tm3+, due to the multitude of Tm3+-to-Tm3+ interactions, the Tm3+ concentration commonly used may not be suitable for strong UV and visible emissions. Thus, we varied the concentration of Tm3+ in LiYF4 nanoparticles between 0.08 and 0.55% to elucidate the effect of moderate changes of the dopant concentration on the UV, visible and NIR emissions. We determined a new optimized concentration of 0.24% Tm3+ for maximal UV and visible emissions (nominally 0.2%). An extensive analysis of the luminescence spectra in the UV, visible and NIR regions and decay time measurements provides evidence for new luminescence mechanisms involving cross-relaxation pathways from the UV-emitting states of Tm3+. Furthermore, we performed studies on an azobenzene derivative to demonstrate the substantial enhancement of the UV emissions by the newly optimized composition as evidenced by an increase in the degree of trans-cis photoisomerization.