Direct Evidence of a Surface Quenching Effect on Size‐Dependent Luminescence of Upconversion Nanoparticles

Abstract

Lanthanide-doped upconversion (UC) nanoparticles have shown considerable promise in biological labeling, imaging, and therapeutics.[1] However, although current synthetic approaches allow for preparation of ultrasmall UC nanoparticles with precise control over particle morphology and emission color, [2] smaller nanoparticles come at the expense of weaker emissions, which is a constraint that is practically impossible to surpass. Many fundamental aspects of the UC luminescence in these nanomaterials still lack sufficient understanding. In particular, several groups have observed varied relative intensity of the multi-peak UC emissions with varying particle size.[3] The UC luminescence primarily originates from intra-configurational 4f n electron transitions within the localized lanthanide dopant ions. Due to a small Bohr radius of the exciton in UC hosts and weak interactions between 4fn electrons of the lanthanide dopant ions and the host matrix, the size-dependent UC luminescence can hardly be explained by classic theories, such as quantum confinement and surface plasmon resonance related to optical properties of semiconductor and metal nanoparticles. [4] Although phonon confinement [3a-d] has been used to account for the size-dependent UC luminescence, it has been a matter of much debate, owing to the constraints typically associated with solid-state sample measurements at extreme conditions (for example, low temperatures of ca. 10 K) and exclusion of vibrational energies and optical traps arising from particle surface. To this end, a surface quenching effect [3f-i] is proposed and correlated with size-dependent UC luminescence. However, the surface quenching effect has not been conclusively established, largely because of the lack of direct evidence on surface-quenching-induced luminescence modulation of different-sized particles. Herein, we present a comparative spectroscopic investigation of a series of Yb/Tm co-doped hexagonal-phase NaGdF 4 nanoparticles (10, 15, and 25 nm) with or without a thin (ca. 2.5 nm) surface protection layer. We show that, through the thin layer coating, the characteristic optical features (such as relative emission intensities) of these nanoparticles can be retained, thereby providing direct evidence to support the surface quenching effect responsible for the size-dependent UC luminescence.Hexagonal-phase NaGdF 4 was chosen as the model host system owing to its ability to render high UC efficiency and the benefits of producing relatively small (< 20 nm) and uniform nanoparticles. [2a, 5] Furthermore, the Gd 3+ host ion that features half-filled 4f orbitals is relatively inert in the luminescence process and thus has negligible interaction with the dopant ions.[2a] To provide a direct comparison over a broad wavelength range between the relative emission intensity of the particles, the Tm 3+ ion with a ladder-like arrangement of energy levels was selected as the activator capable of generating upconverted emission peaks that span from ultraviolet (UV) to near-infrared (NIR) spe...

Lanthanide‐Doped Nanocrystals: Strategies for Improving the Efficiency of Upconversion Emission and Their Physical Understanding

“…An approximate 450-fold enhancement in the overall UC emission intensity of coreshell NaGdF 4 :Yb/Tm/NaGdF 4 by particles was observed due to surface passivation. [118] Further, the authors demonstrated a novel design based on a combination of core-shell NaYF 4 :Yb/Tm@NaYF 4 upconverting nanoparticles and manganese dioxide nanosheets that was expected to find application in the rapid and selective detection of glutathione in aqueous solutions and living cells. [119] A drastic enhancement in the UC luminescence of BaGdF 5 :Yb 3 + /Er 3 + core-shell nanoparticles was reported.…”

Quenching of the Luminescence of Upconverting Luminescent Nanoparticles by Heavy Metal Ions

“…Despite many studies on the spectral properties of UCLNPs, it is rather surprising that the quenching of their luminescence by heavy metal (HM) ions has not been investigated so far. It is known that UCLNPs undergo surface-quenching effects associated with the size-dependent luminescence, [43] but this is not related to external quenchers. We report here, for the first time, that the luminescence of UCLNPs nanocrystals is strongly quenched by HM ions.…”

Multi-photon excitation in uncaging the small molecule bioregulator nitric oxide

“…10 nm diameter) UCNPs [86][87][88]; however, the small particle size increases solvent quenching, especially by OH and NH groups, owing to the sizable percentage of the lanthanoid emitters that are at or near the surface in such species. Such quenching may be reduced by growing a shell of unsubstituted host material around the UCNP [89,90] to isolate surface ions from these environmental effects. Related modifications of UCNPs include silica or polymer coatings [91][92][93][94][95][96][97][98].…”