Spatial and Temporal Resolution of Luminescence Quenching in Small Upconversion Nanocrystals

Abstract

Luminescent upconversion nanocrystals (UCNCs) have become one of the most promising nanomaterials for biosensing, imaging, and theranostics. However, their ultimate translation into robust luminescent probes for daily use in biological and medical laboratories requires comprehension and control of the many possible deactivation pathways that cause upconversion luminescence (UCL) quenching. Here, we demonstrate that thorough modeling of UCL rise and decay kinetics using a freely accessible software can identify the UCL quenching mechanisms in small (<40 nm) UCNCs with spatial and temporal resolution. Applied to the most relevant β-NaYF 4 :Yb 3+ ,Er 3+ UCNCs, our model showed that only a few distinct nonradiative low-energy transitions were deactivated via specific solvent and ligand vibrations with a strong downstream effect on the population and depopulation dynamics of the emitting states. UCL quenching could penetrate ca. 4 nm inside the UCNC, which resulted in significant size-dependent changes of UCL intensities and spectra. Despite the large surface-to-volume ratios and UCL quenching via the UCNC surface, we found strong contributions of the outer layers to the overall UCL, which will be highly important for the design of UCNPs to investigate biomolecular interactions via distance-dependent energy transfer methods. Our advanced kinetic model is easily scalable to different UCNC architectures, environments, and energy transfer interactions such that relatively simple modeling of UCL kinetics can be used for efficiently optimizing UCNCs for their final application as practical luminescent probes.

Optimizing Upconversion Nanoparticles for FRET Biosensing

“…When looking at the absolute values of the rate constants for all UCNPs and both excitation wavelengths, environmental quenching makes the highest contribution, followed by Er–Nd CR and Er–Er CR. This relatively strong environmental quenching (even for the thickest shell) clarified the importance of a protective shell and confirmed previous findings that suggested a shell thickness of at least 2 nm to efficiently suppress surface quenching . In most cases, the importance of environmental quenching was significantly higher for 808 nm excitation, whereas Er–Nd CR and Er–Er CR were less important.…”

“…This relatively strong environmental quenching (even for the thickest shell) clarified the importance of a protective shell and confirmed previous findings that suggested a shell thickness of at least 2 nm to efficiently suppress surface quenching. 52 In most cases, the importance of environmental quenching was significantly higher for 808 nm excitation, Comparing the relative contributions of each quenching mechanism in reference to the different UCNP configurations (i.e., direct comparison of the heights of the different bars in Figure 4E,F) provides additional interesting information. For example, at 980 nm excitation (where Nd is not excited), the Er−Nd CR was significantly more important for highly Er doped ST compared to IV UCNPs.…”

“…Advanced modeling of UCL offers the possibility to better understand experimental results, extract fundamental properties of the investigated nanosystem, and predict expected properties for specific UCNP designs and syntheses. 3,30,52,56 Several different modeling strategies can be applied, including macroscopic (or rate equation), 52,54,57 microscopic, 21,47 and (semi)analytical 58,59 approaches. Each category has pros and cons, and combinations are also possible, depending on the focus of the investigation.…”

“…The UCL of UCNPs has been optimized via sizes, shapes, ion doping ratios, and core–shell concepts. − However, considering the sophisticated FRET properties with UCNP donors, UCNPs for FRET biosensing require a very different optimization strategy that takes into account both UCL and FRET and an optimal UCL is not necessarily ideal for an optimal FRET. Advanced modeling of UCL offers the possibility to better understand experimental results, extract fundamental properties of the investigated nanosystem, and predict expected properties for specific UCNP designs and syntheses. ,,, Several different modeling strategies can be applied, including macroscopic (or rate equation), ,, microscopic, , and (semi)­analytical , approaches. Each category has pros and cons, and combinations are also possible, depending on the focus of the investigation.…”

Upconverting nanoparticles as primary thermometers and power sensors

“…The emission spectra of 2 and 3 measured at the lowest measured P D display analogous emission spectra of 1, except for the worse SNR (Supplementary Figure S1). As the shell gets thicker, the luminescence quenching effects are reduced (Pini et al, 2022;Shi et al, 2022) leading to higher SNR and advantageously decreased temperature uncertainty. As initially proposed by some of us (Brites et al, 2016), and experimentally implemented by others (Van Swieten et al, 2021), the temperature uncertainty increases as the SNR degrades.…”