Realizing efficient upconversion and down-shifting dual-mode luminescence in lanthanide-doped NaGdF₄ core–shell–shell nanoparticles through gadolinium sublattice-mediated energy migration

Highlights

• NaGdF₄:Yb/Tm@NaGdF₄:Ce/Ln@NaYF₄ multishell nanoparticles were prepared.

• Upconversion and down-shifting dual-model luminescence was observed.

• The mechanisms responsible for dual-model luminescence were proposed.

Abstract

Monodisperse rod-like colloidal core–shell–shell NaGdF₄:Yb/Tm@NaGdF₄:Ce/Ln@NaYF₄ (Ln = Tb, Eu, Dy and Sm) nanoparticles (25 nm in diameter and 37 nm in length) have been successfully synthesized through a facile wet chemical method. X-ray diffraction, transmission electron microscopy, upconversion and down-shifting photoluminescence were used to characterize the samples. The prepared nanoparticles exhibited both tunable upconversion and down-shifting emissions from Tb, Eu, Dy and Sm activators under 980 nm near-infrared and 254 nm ultraviolet excitation, respectively. The dual-model luminescence was achieved through gadolinium (Gd³⁺) sublattice-mediated energy migration: Yb³⁺ → Tm³⁺→n(Gd³⁺) → Ln³⁺ and Ce³⁺ → n(Gd³⁺)→Ln³⁺ energy-transfer process, respectively. The advances on these dual-mode luminescent nanoparticles offer exciting opportunities for developing multifunctional optical nanoprobes for biological applications.

Introduction

Lanthanide ion (Ln³⁺) doped luminescent inorganic nanoparticles are emerging as an important class of nanomaterials as they have widespread uses in displays [1], [2], solar cells [3], anti-counterfeiting [4], and especially in biological applications [5], [6], [7], [8], [9], [10], [11], [12], [13]. The unique electronic structures of Ln³⁺ ions give them the ability to emit photons efficiently in the spectral region from ultraviolet (UV) to visible and even to infrared. Compared with conventional biological probes, such as organic dyes and quantum dots, Ln³⁺-doped luminescent nanoparticles offer many intrinsic advantages including long-lived luminescence, large Stokes and/or anti-Stokes shifts, narrow emission lines, high chemical stability, high photobleaching thresholds and low toxicity, and therefore they are considered as a new generation of optical bioprobes. For example, Ln³⁺-Yb³⁺ (Ln = Er/Tm/Ho) doped upconversion nanoparticles, which can convert the near-infrared (NIR) radiation into higher-energy emissions through a multi-photons mechanism, have been widely studied as promising alternatives to traditional fluorescent biolabels for cell imaging [14], [15], [16], [17], [18]. The use of low-power NIR excitation for upconverting nanoparticles provides several superior properties, such as larger tissue penetration depth, high signal-to-noise ratio due to the absence of autofluorescence, and less photodamage to tissue. On the other hand, luminescent nanoparticles doped with some Ln³⁺ down-shifting emitters (e.g., Eu³⁺, Tb³⁺, Sm³⁺, and Dy³⁺) can exhibit intense and long-lived visible emissions under UV irritation, and thus they show great promise for ultrasensitive time-resolved photoluminescence biosensing by completely suppressing the interference of the short-lived autofluorescence background [19], [20].

Sodium gadolinium fluoride (NaGdF₄) is an ideal host material for realizing the upconversion and down-shifting luminescence of Ln³⁺ dopants because of its high photochemical stability, low phonon energy (∼350 cm⁻¹), high transparency in the UV–visible–NIR regions and its ability to mediate energy exchanges between the Ln³⁺ ions, as well as its promising potential as optical/magnetic dual-modal fluorescent bioprobes [21], [22], [23], [24], [25]. Wang et al. recently reported that by use of gadolinium (Gd³⁺) sublattice-mediated energy migration in NaGdF₄:Yb/Tm@NaGdF₄:Ln core–shell nanoparticles, efficient upconversion emissions can be realized for Ln³⁺ activators (Eu³⁺, Tb³⁺, Dy³⁺ and Sm³⁺) without long-lived intermediary energy states via the Yb³⁺ → Tm³⁺ → n(Gd³⁺)→Ln³⁺ energy-transfer process [23]. Liu recently demonstrated bright upconversion and down-shifting dual-mode luminescence of Eu³⁺ ions in NaGdF₄:Yb/Tm@NaGdF₄:Eu core–shell nanoparticles upon 980 NIR excitation and 273 nm UV excitation, respectively [24]. On the other hand, enhanced down-shifting luminescence from Eu³⁺ and Tb³⁺ ions has been recently demonstrated in NaGdF₄:Ce/Tb/Eu nanoparticles under 254 nm UV excitation, which can be attributed to the strong absorption of the UV irradiation by Ce³⁺ ions and efficient energy transfer from Ce³⁺ sensitizers to the Tb³⁺/Eu³⁺ activators through a Ce³⁺ [25], [26], [27]. Thus, it is possible to achieve upconversion and down-shifting dual-mode luminescence tuning in NaGdF₄-based core–shell nanoparticles with the help of Gd³⁺ sublattice-mediated energy migration. Integration of dual-mode luminescence in one nanoparticle system is of great interest for developing multifunctional nanoprobes for biological applications.

In this paper, we reported efficient upconversion and down-shifting dual-mode luminescence from Ln³⁺ activators (Eu³⁺, Tb³⁺, Dy³⁺ and Sm³⁺) in NaGdF₄:Yb/Tm@NaGdF₄:Ce/Ln@NaYF₄ core–shell–shell nanoparticles. As shown in Fig. 1, Yb³⁺/Tm³⁺ couple and Ce³⁺/Ln³⁺ couple are spatially confined in different layers to avoid deleterious nonradiative cross-relaxation deactivation. Upon 980 nm NIR laser excitation, upconversion luminescence of Ln³⁺ ions can be achieved through the Gd³⁺ sublattice-mediated energy migration process of Yb³⁺→Tm³⁺ → n(Gd³⁺)→Ln³⁺ (Eu³⁺ emissions: ⁵D₂ → ⁷F₃ at 510 nm, ⁵D₁ → ⁷F₁ at 536 nm, ⁵D₁ → ⁷F₂ at 555 nm, ⁵D₀ → ⁷F₁ at 584 nm, ⁵D₀ → ⁷F₂ at 618 nm, ⁵D₀ → ⁷F₃ at 645 nm, and ⁵D₀ → ⁷F₄ at 694 nm; Tb³⁺ emissions: ⁵D₄ → ⁷F₆ at 486 nm, ⁵D₄ → ⁷F₅ at 545 nm, ⁵D₄ → ⁷F₄ at 584 nm, and ⁵D₄ → ⁷F₃ at 620 nm; Dy³⁺ emissions: ⁴F_9/2 → ⁶H_13/2 at 575 nm; Sm³⁺ emissions: ⁴G_5/2 → ⁶H_7/2 at 599 nm). In contrast, upon 254 nm UV excitation, down-shifting luminescence can be realized by using the energy-migration process of Ce³⁺→n(Gd³⁺)→Ln³⁺ (Eu³⁺ emissions: ⁵D₁ → ⁷F₁ at 536 nm, ⁵D₁ → ⁷F₂ at 555 nm, ⁵D₀ → ⁷F₁ at 584 nm, ⁵D₀ → ⁷F₂ at 618 nm, and ⁵D₀ → ⁷F₄ at 694 nm; Tb³⁺ emissions: ⁵D₄ → ⁷F₆ at 486 nm, ⁵D₄ → ⁷F₅ at 545 nm, ⁵D₄ → ⁷F₄ at 584 nm, and ⁵D₄ → ⁷F₃ at 620 nm; Dy³⁺ emissions: ⁴F_9/2 → ⁶H_15/2 at 481 nm and ⁴F_9/2 → ⁶H_13/2 at 575 nm; Sm³⁺ emissions: ⁴G_5/2 → ⁶H_5/2 at 563 nm, ⁴G_5/2 → ⁶H_7/2 at 599 nm, and ⁴G_5/2 → ⁶H_9/2 at 647 nm). The outermost NaYF₄ shell is designed to protect the luminescent activators against surface quenching.