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Emission Control of Rare-Earth Nanoparticles

Periodic Reporting for period 1 - MONOCLE (Emission Control of Rare-Earth Nanoparticles)

Reporting period: 2021-04-01 to 2023-03-31

Project MONOCLE (Logo 1) explores the optical properties of rare-earth doped nanoparticles (RENPs) envisioned to be used in various biomedical and technological applications. RENPs have a unique ability to convert invisible near-infrared light to that of visible colors via a process of upconversion. Such upconverting RENPs are researched for disease detection, drug delivery, cell-level temperature sensing, and as super-resolution optical probes. Successful implementation of these practices though depends on our ability to generate upconversion emission effectively and controllably.

It is challenging to maximize upconversion properties of RENPs because of a large number of variables that influence the final performance of RENPs. Compositionally, a vast space of explorable parameters is determined by the choice of nanocrystal host and rare-earth dopants, their concentrations, and placement within the nanocrystal (so called, core/shell engineering). Furthermore, photoluminescence of RENPs is affect by the wavelength, power, and temporal modulations (pulsing) of laser excitation that in turn can serve as means to deliberately control RENP emission. Thus, to effectively cover these parameter spaces and create application-targeted RENPs in a timely-fashion high-throughput numerical and experimental approaches have to be combined with rational design of RENPs.

The objective of this project is to explore different RENP architectures, including highly-nonlinear photon avalanching nanoparticles (ANPs), and their excitation pathways to aid in creating a set of specialized nanotools for future biomedical applications. In order to reach different milestones of the project: numerical simulations, high-throughout robotic synthesis, and complete optical characterization are combined to create and investigate state-of-the-art RENPs.
Despite COVID restrictions at the start of the project (started April 2021), significant progress was made using numerical simulations that aided in uncovering ways to modulate emission of upconverting nanoparticles, particularly that of ANPs, by means of pulsed excitation. Simulation results allowed to identify nanoparticle compositions (dopants and doping percentages), excitation wavelengths, and pulsing parameters (pulse width and period) that would result in an enhancement of photon avalanche emission nonlinearity - crucial factor in the use of these nanoparticles as super-resolution imaging probes. In addition, simulation results were used to devise a strategy for photon avalanche spectral tuning, which has the potential to unlock ANPs for multicolor imaging and multiplexed sensing applications.

Informed by numerical simulations, ANPs that combine photon avalanching and energy migration upconversion were created (Figure 1). The spectral tuning of ANPs, in which virtually any Ln3+ ion could be excited in a highly nonlinear fashion, was achieved by utilizing Tm3+ → Gd3+ → Ln3+ energy transfer cascade initiated under 1064 nm laser pump. A library of core/multishell ANPs was successfully synthesized, with Er3+, Ho3+, Tb3+, Eu3+, Nd3+, or Dy3+ dopants as visible light emitters. This approach allowed to generate upconversion emission from different Ln3+ ions, including the non-upconverting ones (i.e. Tb3+, Dy3+, and Eu3+), with upconversion nonlinearities, s, up to 17 (compared to s = 2-5 for classic upconversion). All the synthesis, structural characterization, and spectroscopy results have been summarized in a manuscript that is currently considered for publication.

Furthermore, novel low-phonon-energy upconverting RENPs were developed during this project in collaboration with Dr. Zhuolei Zhang at the Partner Organization (Figure 2). Reducing the phonon energies of host material can potentially boost the upconversion emission intensity, yet it is challenging to synthesize low-phonon-energy upconverting nanocrystals. By developing completely new synthesis approach, different compositions of KPb2X5 (X = Cl, Br) RENPs, with lowest phonon energies available were synthesized and characterized. Successful development of these RENPs facilitated lowering the phonon energies of the host matrix to the extend that photon avalanching emission in Nd3+-doped KPb2Cl5 under 1064 nm excitation could be observed for the first time. Photon avalanching with Nd3+ has been predicted theoretically, however previous attempts to detect it in the stat-of-the-art hosts (e.g. NaYF4 or NaGdF4) has been unsuccessful. Results of this study were published in Angewandte Chemie International Edition, 62, e202212549, 2023. Published manuscript was also deemed as "Hot Paper" and highlighted by frontispiece graphic of the issue.

In addition, two more papers were published during the current period of the project. A review on near-infrared nanostructures for biomedical diagnostics and therapy (Nanoscale Advances, 3, 6310-6329, 2021) and a research article on photon avalanching in Tm3+-doped LiYF4 (nano)materials (Advanced Optical Materials, 10, 2201052, 2022).
The aforementioned photon avalanche spectral tuning within RENPs, was also generalized to imprint photon avalanching on other fluorophores, as showcased by extremely nonlinear QD upconversion in an ANP+QD avalanching complex. This is an important discovery, since various highly nonlinear emitters can now be created with the help RENPs, fostering the combination of unique properties between different nanostructures for augmented and enhanced performance.

Impact of these results are of interest to biomedically-oriented research community, as spectrally discrete ANPs and avalanching complexes can be used in multicolor super-resolution imaging, high-sensitivity multiplex bioassays, and long-range molecular sensors. All of which are crucial in accelerating knowledge gain for identifying and combating various illnesses.
Low-phonon-energy KPb2X5 (X = Cl, Br) RENPs enable photon avalanching of Nd3+ ions