Ytterbium‐Enriched Outmost Shell for Enhanced Upconversion Single Molecule Imaging and Interfacial Triplet Energy Transfer

Abstract

Different from ensemble measurements, singleparticle imaging could offer us a clean structural design versus optical property relationship without the interferences arising from heterogeneity of nanoparticle size and formula, particle-particle interaction and discrepancy of nanoparticle concentration. Moreover, a wider-range of excitation irradiance could be applied in single-particle measurement easily, which is helpful for studying the powerdependent performance.Despite the importance of singleparticle imaging, only limited studies have investigated the optical properties of single UCNPs. [25][26][27][28][29][30] One of the major hurdles is their weak luminescence. Efforts have been made to improve the single UCNPs brightness, such as employing high doping level of sensitizer or emitter ions. [25,[31][32][33][34][35] In particular, higher Er 3+ doping is optimal for the higher power densities at single particle level; enriched sensitizer Yb 3+ in the interior could significantly enhance upconversion brightness. [25,31] In these highly doped UCNPs design, inert shell is indispensable considering that surface quenching is the dominant nonradiative pathway, [36] which could stop the energy transfer from active ions to surface quenchers efficiently, leading to a significantly increased quantum yield. [36][37][38] However, inert shell also results in undesired volume increasing for single molecule imaging, given that bigger particles would have a heavier influence on the labeled-target behavior. Besides, it is still less clear about the effect of the inert shell on single particle brightness in a volume-constrained UCNP.To understand the potential sources of energy loss associated with the nanocrystal surface and obtain optimal volumeconstrained single particle brightness, we systemically investigated the shell effect on upconversion luminescence in the highly doped UCNPs. By tuning the doping concentration of sensitizer Yb 3+ in the outmost shell, we demonstrated a power-dependent performance where Yb 3+ doped outmost shell could outperform inert shell under higher power densities at the single particle level. Best performing active-shelled UCNPs at higher powers turned out to be less-luminescent at the excitation powers typically used in ensemble measurement. Inert shells shown to significantly increase UCNP brightness at low powers are likely dispensable at higher powers. These Lanthanide-based upconversion single molecule imaging (u-SMI) is of great interest due to its excellent photostability and background-free detection. However, the dim brightness strongly hampers its application in quantitative studies. An inert shell can effectively enhance the upconversion brightness, especially for highly doped upconversion nanocrystals in which surface quenching is the dominant nonirradiative pathway. Here, it is discovered that inert shell is actually a drawback for u-SMI in a volume-constrained system and sensitizer Yb 3+ doped outmost shell outperforms inert shell in single mole cule imaging condition, which indicate...

Squaraine Dye-Sensitized Upconversion with Enhanced Stability and Minimized Aggregation-Caused Quenching

“…The power-dependent slope of UCL emission under 730 nm is larger than that under 808 nm irradiation, indicating that the antenna dye SQ-739 can enhance the photon harvesting ability of UCNPs and provide more photons for the upconversion process under 730 nm irradiation. 25 It is critical to understand the energy transfer process to further optimize dye-sensitization properties. We investigated the squaraine dye-sensitized upconversion mechanism.…”

“…Besides, we also measure power-dependence on UCL intensity under 730 and 808 nm laser, respectively (Figure S15). The power-dependent slope of UCL emission under 730 nm is larger than that under 808 nm irradiation, indicating that the antenna dye SQ-739 can enhance the photon harvesting ability of UCNPs and provide more photons for the upconversion process under 730 nm irradiation …”

“…Several strategies have been developed to improve UCL emission, such as increasing the lanthanide ion doping concentration to enhance light harvesting, − adopting an inert shell to suppress surface quenching, , and tailoring the local crystal fields around lanthanide dopants to increase upconversion efficiency. ,, An organic near-infrared (NIR) dye with a high absorption cross section (IR-806) has been reported, which acted as an antenna to harvest excitation light and transferred the energy to lanthanide ions in UCNPs to achieve enhanced UCL emission. The process of enhancing the emission of UCNPs with a dye is termed “dye-sensitized upconversion” .…”

“…The process of enhancing the emission of UCNPs with a dye is termed “dye-sensitized upconversion” . Dye-sensitized upconversion has been demonstrated in several studies. ,,,− However, all the NIR fluorophores for dye-sensitized upconversion, such as IR-806 and Cy7.5, have been based on the cyanine skeleton which has a methine group with differing numbers of carbon atoms and two aromatic side chains. , Due to the poor chemical stability and photostability of the methine group, ,, it is difficult to obtain dye-sensitized upconversion for single particle measurement in which a high irradiance (∼KW/cm 2 ) is used. ,, In addition, aggregation-caused quenching (ACQ) in polar solutions diminishes the ability of cyanine dyes to act as antennae to enhance UCL emission . Recently, a steric hindrance strategy , was developed to improve cyanine dyes’ chemical stability and reduce ACQ for dye-sensitized upconversion.…”

“…In order to demonstrate that SQ-739 could provide dye sensitization of UCNPs, we studied the effect on optical properties of coating UCNPs with SQ-739. A widely used two-step NOBF 4 -mediated ligand exchange strategy was used to decorate the UCNPs with SQ-739 and IR-806 (see Supporting Information 1.2), , resulting in 37-fold and 33-fold enhancements in UCL emissions compared to UCNPs alone under 730 and 808 nm excitation, respectively (Figure S8). As shown in Figure S9, these SQ-739 modified UCNPs dispersed well in DMSO and water, and no apparent aggregations were observed after surface modification.…”