Lanthanide-Doped Inorganic Perovskite Nanocrystals and Nanoheterostructures: History
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The combination of all-inorganic halide perovskite nanocrystals (IHP NCs) with nanoparticles consisting of lanthanide-doped matrix (Ln NPs, such as NaYF4:Yb,Er NPs) is stable, near-infrared (NIR) excitable and emission tuneable (up-shifting emission), all of them desirable properties for biological applications. In addition, luminescence in inorganic perovskite nanomaterials has recently been sensitized via lanthanide doping.

  • inorganic perovskite
  • lanthanide-doped nanocrystals
  • upconversion photoluminescence

1. Lanthanide-Doped Matrices (NaLnF4 Matrices, Ln NPs)

UC is an interesting phenomenon of bioimaging as it is a more efficient process than that of two-photon absorption and high harmonic generation. In past decades, numerous bio-probes have been reported, such as fluorescent proteins, dyes and quantum dots, although they are not suitable for life science applications.
UC nanostructures are considered promising materials for bioprobes [65], and in particular, Er3+/Yb3+ co-doped NaYF4 has been recognized as an efficient UC system although it presents some undesirable background radiation because of its prominent green emission with a lower signal–noise ratio. These drawbacks have encouraged investigations into other UC perovskite matrices. Compared with perovskite oxide materials, the inorganic halide perovskites are suitable for bioimaging because of their excellent photo-stability and higher chemical durability [59]. Host lattices with heavier halides are beneficial for the stabilization of dopant ions [60]. Inorganic halide perovskites are considered promising UC materials for bioimaging because of their adjustable crystal structure, optical stability, resistance to photo-bleaching and photo-blinking, spectral distinguishability and chemical durability [66].
Nanoparticles consisting of lanthanide-doped matrices such as NaYF4:Yb,Er NPs have attracted the interest of research communities due to their advantageous properties such as narrow band gap emission, reasonable optical stability and high chemical stability when compared with traditional luminescent materials, e.g., organic dyes and NCs. Moreover, Ln NPs have been widely used in the field of biology owing to their deep penetration into biological tissues without any damage and high signal-to-noise ratio under NIR excitation. Doping of matrices with lanthanides is the most attractive tool for their optical applications because of large quantum numbers (n = 4, l = 3) of lanthanide ions and rich spectroscopic properties [51]. Lanthanides are mostly stable in the +3 oxidation state except for Ce4+, Tb4+ and Yb2+ ions. In addition, the size and morphology of Ln NPs play key roles in biomedical applications [67,68].
In early NIR-to-visible UCL bioimaging investigations, it was difficult to achieve tissue penetration depths in the scale of millimetres. However, Yin et al., reported UCL imaging for the first time with considerable tissue depth (a penetration depth of 1 cm) using a luminescent probe of NaYF4:Yb,Er NPs in nude mice [69]. Concomitantly, Jing et al., compared the UCL imaging of pork muscle tissues at different depths (0–1 cm) through injections of polymer-modified NaYF4:Yb,Er and KMnF3:Yb,Er. For the former, the image was detected at a depth of about 0.5 cm, whereas KMnF3:Yb,Er exhibited a very strong red emission, which was detected at a tissue depth of 1 cm [70]. Xiang et al., have reported the importance of antigen-loaded Ln NPs in labelling and stimulating dendritic cells (DCs), and the Ln NP-labelled DCs achieved high-sensitivity in vivo UCL imaging [71].
Subsequently, Hesse et al. [72] reported the rapid preparation of sub-10 nm level pure hexagonal (β-phase) NaYF4-based Ln NPs using a simple one-pot method, in which therminol 66 was used as a co-solvent and monodispersed Ln NPs were obtained in very short reaction times. The UCL properties of these NPs were tuned by varying the dopant concentrations (Nd3+ and Yb3+ as sensitizers, and Er3+ as an activator). The enhancement in UCL intensity was observed in Ln NPs with optimized concentrations of sensitizer and activator ions as well as coating with inert/active shell. The UCL spectrum of core β-NaYF4:Yb3+/Er3+ 20/2 % Ln NPs in cyclohexane exhibited three intense bands centred at λ = 525 (2H11/2→ 4I15/2 transition, G1), 545 (4S3/24I15/2 transition, G2) and 660 nm (4F9/24I15/2 transition, R) under an excitation of 976 nm.
The excitation of conventional Ln NPs such as NaYF4:Yb3+/Er3+(Tm3+) at 980 nm caused overheating and damage of living tissues with a reduction in luminescence due to water absorption at 980 nm. Interestingly, the incorporation of Nd3+ ions into Ln NPs shifted the excitation wavelength to 808 nm, thus minimizing the absorption of water. Hence, Kostiv et al. [73] designed the NaYF4:Yb3+/Er3+@NaYF4:Nd3+ core–shell NPs doped with Yb3+ and Nd3+ as sensitizers, and Er3+ as an activator for bioimaging. The core was uniform, with a thickness of 24 nm, whereas the core–shell particles had tuneable shell thicknesses of ∼0.5–4 nm. They were coated with in-house synthesized poly ethyleneglycol (PEG)-neridronate terminated with alkyne (Alk) to ensure their dispersibility in biological media. The stability of NaYF4:Yb3+/Er3+@NaYF4:Nd3+-PEG-Alk NPs in water or 0.01 M PBS, and the presence of PEG on the surface were determined. These Ln NPs were considered non-invasive probes for specific bioimaging of cells and tissues.

2. Nanoheterostructures Based on IHP NCs and Ln NPs

In recent years, all-inorganic CsPbX3 (X = Cl, Br and I) perovskite NCs have proven to be promising materials in the field of optoelectronics due to their outstanding linear optical properties, even though the nonlinear properties of these perovskites are limited due to their small multiphoton absorption cross section and requirement of high-power density excitation. Interestingly, Zheng et al. [74] proposed a convenient strategy for tuning the UCL in CsPbX3 perovskite NCs through the sensitization of Ln3+-doped NPs. Particularly, CsPbX3 NCs and LiYbF4:0.5%Tm3+@LiYF4 core/shell Ln NPs were dispersed in cyclohexane to lead a homogeneous colloid.
Furthermore, there is a great need to develop heterostructured NCs based on inorganic perovskites. More clearly, although perovskite QDs have excellent optical properties, their biological applications have not been explored much because of their poor stability and the short penetration depth of UV light into tissues. The combination of perovskite QDs with Ln NPs has provided stable hybrid NCs, which are NIR excitable and emission tuneable. Hence, Ruan et al. [75] synthesized perovskite–Ln NP hybrid NCs composed of perovskite NCs with cubic phase and Ln NPs with hexagonal phase. The heterostructured CsPbBr3–NaYF4:Yb,Tm NCs were synthesized in one pot and consisted of cubic-phase CsPbBr3 QDs embedded in hexagonal-phase NaYF4:Yb,Tm NPs, which thus formed a watermelon structure with multiple seeds, and a cubic-phase NaYF4:Yb,Tm NP was used as an intermediate transition phase. The hybrid NCs emitted the characteristic green fluorescence of CsPbBr3 QDs under UV light and UV-blue fluorescence under NIR light excitation of NaYF4:Yb,Tm NPs, thus revealing the co-existence of both CsPbBr3 and NaYF4:Yb,Tm in the same structure. Moreover, a green fluorescence was obtained upon NIR excitation when the NaYF4:Yb,Tm phase absorbed NIR light and transferred the energy to the CsPbBr3 phase. This work opens a new way for synthesizing heterostructured NCs that could be applied to many other materials.
Recently, Shao et al. [76] reported the sensitized emission of CsPbI3 perovskite NCs after NIR excitation of CaF2:Yb3+/Ho3+ as hierarchical nanospheres (HNSs) in CsPbI3 and CaF2:Yb3+/Ho3+ nanocomposite structures. Moreover, the lifetime of CsPbI3 emissions was lengthened to several milliseconds due to energy transfer from long-lived Ho3+ to CsPbI3 perovskite NCs. The stability of CsPbI3 NCs was enhanced in the composites, which kept 90% of its PL after 30 days. The composites were printed on flexible substrates for dual-mode fluorescent encryption anti-counterfeiting application and possessed excellent fluorescence under the excitation of both UV and NIR light. Moreover, the CsPbI3-CaF2:Yb3+/Ho3+ nanocomposites proved to be highly water-soluble, ultrastable and highly biocompatible in cell imaging applications. This work provides a new strategy for developing photon UC in perovskite NCs and a new trial for the development of multifunctional materials.
Although the halide perovskite nanomaterials with superior linear properties are greatly employed in optoelectronics and photonics, their strong multiphoton absorption only makes them prospective for bioimaging applications. However, the instability of perovskites in aqueous solutions limited their biological applications. Talianov et al. [77] demonstrated their fluorescence and UCL imaging in living cells using CsPbBr3 NCs with improved water resistance for at least 24 h after their coating as individual particles with various silica-based shells. The quality of phTEOS-TMOS@CsPbBr3 NCs was confirmed by HRTM and SEM, X-ray diffraction analysis, Fourier-transform infrared and energy-dispersive X-ray spectroscopies as well as fluorescence optical microscopy. phTEOS-TMOS@CsPbBr3 NCs have enhanced water stability, and consequently, they are of interest for several bioimaging applications.
In addition, it is important to note that, Estebanez et al., designed 1D-ordered nanostructures comprising Ln NPs and IHP NCs with open peapod-like shells, which were provided by a PbSO4 polymer for the first time [78]. The sensitized emission of IHP was achieved by NIR excitation of nearby Ln NPs. Ln NPs with a NaYF4 matrix doped with Yb and Tm or Er and with an inert shell of NaYF4, in the case of core–shell Ln NPs, and all-inorganic CsPbX3 NPs were selected for these studies. Interestingly, the lead sulphate shell enhanced the luminescence of core–shell Ln NPs in the polymers by ≈20 fold, which plays an important role in the efficiency of sensitized emission of LHNPs under NIR excitation of Ln NP-IHP NC co-polymers as well as in the chemical stability of IHP NCs in contact with water. In addition, the co-polymers were prepared as colloids and deposited as solid films on a glass substrate. The lifetime of sensitized IHP emission and emission efficiency entirely depended on irradiance and sample conditions. These co-polymers are promising candidates for manufacturing the photonic devices. Table 1 summarizes the various applications of Ln-doped and undoped IHPs by means of a UC process.
Table 1. Lanthanide-doped inorganic halide perovskite NCs and non-doped perovskites for various applications.

3. Other UC Luminescence Materials

Li et al. [88] designed superstructures comprising a metal-organic framework as the core and Nd3+-sensitized Ln NPs as satellites using an electrostatic self-assembly strategy. This double photosensitizer superstructure has a three-mode imaging function, including magnetic resonance, UCL and fluorescence, as well as an excellent anti-tumour effect under NIR excitation (at 808 nm) according to in vitro and in vivo experiments. Thus, the red blood cells did not deteriorate in the presence of the superstructure. Moreover, exposure of BALB/c mice to a 808 nm laser for 5 min demonstrated a lower temperature of the irradiated area, at about 42 °C, which did not result in damage to the mice. By contrast, the temperature of the irradiated area was raised to above 50 °C when using a laser excitation at 980 nm, and consequently, the mice skin was severely burned. Then, it can be proposed that the laser excitation at 808 nm is more adequate for biological applications since it produces a much weaker tissue thermal effect.
At the same time, Sun et al. [89] synthesized Ln3+-doped nanocomposites, specifically NaYF4:Yb3+,Er3+@NaYF4-Ce6@mSiO2-CuS nanohybrids for the applications of sensing and therapy, which provided temperature feedback in the phototherapy treatment (PTT) and were involved in photodynamic therapy treatment (PDT). NaYF4:Yb3+,Er3+@NaYF4 NPs were coated with mesoporous SiO2 combined with a Chlorin e6 (Ce6) photosensitizer, which can be excited by the red emission of Er3+ to lead to NaYF4:Yb3+,Er3+@NaYF4-Ce6@mSiO2. Then, the citrate-capped CuS (Cit-CuS) NPs as a photothermal conversion agent were attached to the composite surface. Based on the guidance obtained from spectral experiments, the dual-modal tumour therapy and real-time temperature monitoring were investigated both in vitro and in vivo, obtaining reasonable results.

This entry is adapted from the peer-reviewed paper 10.3390/nano12132130

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