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 the emission of upconverting nanoparticles, particularly that of ANPs, using pulsed excitation. Simulation results allowed the identification of 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 - a 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 a Tm3+ → Gd3+ → Ln3+ energy transfer cascade initiated under a 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 published (Nano Letters, 23, 15, 7100–7106, 2023).
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. Different compositions of KPb2X5 (X = Cl, Br) RENPs with the lowest phonon energies available were synthesized and characterized by developing a new synthesis approach. Successful development of these RENPs facilitated lowering the host matrix's phonon energies to the extent 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) have been unsuccessful. Results of this study were published in Angewandte Chemie International Edition, 62, e202212549, 2023. The published manuscript was also deemed a "Hot Paper" and highlighted by a frontispiece graphic.
Overall, this project's results have been published in 5 high-impact peer-reviewed journals, and 2 more articles are under peer review for publication. Furthermore, the beneficiary disseminated the results at 8 international meetings/seminars, 4 of which were invited talks.
