Digestive Ripening-Mediated Growth of NaYbF₄:Tm@NaYF₄ Core–Shell Nanoparticles for Bioimaging

Abstract

Size and size distribution control along with the excitation/emission wavelength manipulation are the most indispensable for both fundamental research and applications of upconverting nanophosphors. In contradiction to the usual Ostwald ripening process, digestive ripening-mediated growth of core–shell nanostructures is observed in a lanthanide-doped ternary fluoride system. Time-dependent size evolution of NaYbF4:Tm@NaYF4 nanoparticles is systematically investigated, and a K sp-involved growth mechanism is proposed to better understand this unusual phenomenon. The upconversion luminescence (UCL) intensity and branch ratio of near-infrared to visible emission (NIR/VIS) dramatically increase due to the NaYF4 coating but subsequently undergo a decline when an additional SiO2 layer grows outside. As a comparison, an alternant type of core–shell–shell nanostructure, NaYbF4:Tm@SiO2@NaYF4, is further constructed, in which both the UCL intensity and NIR/VIS ratio exhibit an inverse trend. The comparative analysis on the basis of steady and transient spectroscopic data from NaYbF4:Tm, and alternatively coated by silica and NaYF4 nanoshells, respectively, demonstrates that the UCL properties can be regulated through core–shell engineering once the optical behavior of the surface states can be manipulated. The results provide a promising alternative strategy to fabricate a uniform core–shell nanostructure with multicompositions, and this NIRin–NIRout luminescent profile deserves an expectable vision for the subsequent clinical applications of UCL imaging for deep biological tissues.

Quantifying the Influence of Inert Shell Coating on Luminescence Brightness of Lanthanide Upconversion Nanoparticles

“…Lanthanide-doped upconversion nanoparticles (UCNPs) have received increasing attention over the past decade due to their enormous potential for various applications including sensing and imaging, localized photoactivation, solar energy harvesting, and multiplexing. − In practice, many of these applications need brightest possible upconversion luminescence under the lowest possible excitation irradiance, which has proved difficult to realize with commonly used UCNPs typically incorporating ∼20% sensitizers. − , Significant reductions to required irradiance have been made by increasing the doping concentration of sensitizers to increase absorption at the excitation wavelength, eventually substituting all the inert rare-earth elements in the UCNPs. − This is typically supplemented by coating an inert shell outside the highly doped UCNPs, effectively passivating the adverse (concentration) quenching caused by the high concentration of sensitizers. − At present, such core–shell nanoparticles (e.g., NaYbF 4 :Er/Tm@NaYF 4 ) are among the brightest UCNPs that can be achieved by themselves (that is, without assistance from other mechanisms than upconversion).…”

“…The passivation role of the inert shell in overcoming concentration quenching has been widely reported by various groups. ,− The general consensus is that concentration quenching is ultimately ascribed to surface quenching leveraged by energy migration among the lanthanide ions, so that by preventing surface quenching with inert shells, the problem of concentration quenching is also addressed at the same time. − , However, large discrepancy has been found in the literature in terms of the actual enhancement brought by inert-shell coating. For example, enhancement factors from 4.8 to 400 have been reported for the upconversion emission of NaYbF 4 :Tm ,, and 4 to 700 for NaYbF 4 :Er. ,, This quantitative disagreement has prompted us to investigate the key aspects of inert shell coating influencing upconversion luminescence, which information will be crucial for optimal design of UCNPs to suit different application scenarios.…”

“…20,23−28 The general consensus is that concentration quenching is ultimately ascribed to surface quenching leveraged by energy migration among the lanthanide ions, so that by preventing surface quenching with inert shells, the problem of concentration quenching is also addressed at the same time. [12][13][14][15][16][17][18][19][20][21][22][23][24][25]29 However, large discrepancy has been found in the literature in terms of the actual enhancement brought by inert-shell coating. For example, enhancement factors from 4.8 to 400 have been reported for the upconversion emission of NaYbF 4 :Tm 16,17,20 and 4 to 700 for NaYbF 4 :Er.…”

Unravelling phase and morphology evolution of NaYbF4 upconversion nanoparticles via modulating reaction parameters

“…19,20 Induced by a lanthanide contraction effect, the synthesis of NaYbF 4 core nanoparticles is different from that of other NaREF 4 (such as NaYF 4 and NaGdF 4 ), which usually requires high reaction temperature and would induce much larger nanoparticles, and therefore, more investigations on NaYbF 4 core nanoparticles are required. 21,22 To date, important progress has been made in the synthesis of NaYbF 4 nanoparticles. For example, ∼7.0 nm hexagonal-phase NaYbF 4 nanoparticles have been fabricated only through enhancing the amount of NH 4 F, 23 and small NaYbF 4 nanoparticles with tunable size have been reported in a ternary mixed solvent mixture composed of oleylamine (OM) and oleic acid (OA).…”

Effect of Excess Ligand on the Reverse Microemulsion Silica Coating of NaLnF4 Nanoparticles

“…One of the advantages of the RME method over the Stöber method is that thinner and more uniform silica shells can be achieved. The first reports of the RME method to coat individual hydrophobic UCNPs were published in 2008. , Since then, it has become the more widely used method for silica coating of NaLnF 4 NPs. ,,,,− We used the RME method to coat the NaHoF 4 NPs mentioned above with a thin silica shell and then modified the surface of these core–shell particles with a heterobifunctional poly­(ethylene glycol) of M = 5000 Da (PEG5k). These NPs were tested for signal enhancement and the detection of low-expression biomarkers by MC …”