Evaluation of relative beam-profile-compensated quantum yield of upconverting nanoparticles over a wide dynamic range of power densities

Beam-profile compensation for quantum yield characterisation of Yb-Tm codoped upconverting nanoparticles emitting at 474 nm, 650 nm and 804 nm

Upconverting nanoparticles (UCNPs) have found widespread applications in biophotonics and energy harvesting due to their unique non-linear optical properties arising from energy transfer upconversion (ETU) mechanisms. However, accurately characterising the power density-dependent efficiency of UCNPs using the internal quantum yield (iQY) is challenging due to the lack of methods that account for excitation beam-profile distortions. This limitation hinders the engineering of optimal UCNPs for diverse applications. To address this, this work present a novel beam profile compensation strategy based on a general analytical rate-equations model, enabling the evaluation of iQY for ETU processes of arbitrary order, such as ETU2, ETU3, and beyond. The method was applied to characterise the ETU2 and ETU3 processes corresponding to the main emission peaks (474 nm, 650 nm, and 804 nm) of a Yb-Tm codoped core-shell β-UCNP. Through this approach, the transition power density points (which delimit the distinct non-linear regimes of the upconversion luminescence (UCL)), and the saturation iQY values (which are reached at high excitation power densities above the transition points) were determined. The ETU2 process exhibits a single transition power density point, denoted as ρ2, while the ETU3 processes involve two transition points, ρ2 and ρ3. By compensating for the beam profile, we evaluate the iQY of individual lines across a wide dynamic range of excitation power densities (up to 105 W cm-2), encompassing both non-linear and linear regimes of UCL. This study introduces a valuable approach for accurately characterising the iQY of UCNPs, facilitating a deeper understanding of the upconversion and its performance. By addressing excitation beam-profile distortions, this method provides a comprehensive and reliable assessment of the power density-dependent iQY. The results highlight the applicability and effectiveness of this beam profile compensation strategy, which can be employed for a wide range of UCNPs. This advancement opens new avenues for the tailored design and application of UCNPs in various fields, especially for biophotonics.

Generalised analytical model of the transition power densities of the upconversion luminescence and quantum yield

The quantum yield (QY) evaluation of upconverting nanoparticles (UCNPs) is an essential step in the characterisation of such materials. The QY of UCNPs is governed by competing mechanisms of populating and depopulating the electronic energy levels involved in the upconversion (UC), namely linear decay rates and energy transfer rates. As a consequence, at low excitation, the QY excitation power density (ρ) dependence obeys the power law ρ^n-1, where n represents the number of absorbed photons required for the emission of a single upconverted photon and determines the order of the energy transfer upconversion (ETU) process. At high power densities, the QY transits to a saturation level independent of the ETU process and the number of excitation photons, as a result of an anomalous power density dependence present in UCNPs. Despite the importance of this non-linear process for several applications (e.g., living tissue imaging and super-resolution-microscopy), little has been reported in the literature regarding theoretical studies to describe the UC QY, especially for ETUs with order higher than two. Therefore, this work presents a simple general analytical model, which introduces the concept of the transition power density points and QY saturation to characterise the QY of an arbitrary ETU process. The transition power density points determine where the power density dependence of the QY and the UC luminescence changes. The results provided in this paper from fitting the model to experimental QY data of a Yb-Tm codoped β-UCNP for 804 nm and 474 nm emissions (ETU2 and ETU3 processes, respectively) exemplify the application of the model. The common transition points found for both processes were compared to each other showing strong agreement with theory, as well as, compared to previous reports when possible.

Intensifying upconverted ultraviolet emission towards efficient reactive oxygen species generation

Multiphoton upconversion that can convert near-infrared irradiation into ultraviolet emission offers many unique opportunities for photocatalysis and phototherapy. However, the high-lying excited states of lanthanide emitters are often quenched by the interior lattice defects and deleterious interactions among different lanthanides, resulting in weak ultraviolet emission. Here, we describe a novel excitation energy lock-in approach to boost ultraviolet upconversion emission in a new class of multilayer core-shell nanoparticles with a gadolinium-rich core domain. Remarkably, we observe more than 70-fold enhancements in Gd 3+ emission from the designed nanoparticles compared with the conventional nanoparticles. Our mechanistic investigation reveals that the combination of energy migration over the core domain and optically inert NaYF 4 interlayer can effectively confine the excitation energy and thus lead to intense multiphoton ultraviolet emission in upconversion nanostructures. We further achieve a 35.6% increase in photocatalytic reactivity and 26.5% in reactive oxygen species production yield in ZnO-coated upconversion nanocomposites under 808-nm excitation. This study provides a new insight to energy transfer mechanism in lanthanide-doped nanoparticles, and offers an exciting avenue for exploring novel near-infrared photocatalysts.