Design of NIR-Ⅱ Emissive Rare-earth Nanoparticles and Their Applications for Bio-imaging

Feng Lu; Ting Zhao; Xiaojun Sun; Quli Fan; Wei Huang

doi:10.7536/PC210624

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1348-1358 DOI: 10.7536/PC210624

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Design of NIR-Ⅱ Emissive Rare-earth Nanoparticles and Their Applications for Bio-imaging

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Abstract

In recent years, fluorescence imaging in the second near-infrared region (NIR-Ⅱ, 1000~1700 nm) has attracted lots of interests due to its unique advantages such as deep tissue penetration, low background and high spatial resolution. Commonly used NIR-Ⅱ probes such as small molecules, conjugated polymers and quantum dots, usually exhibit poor photostability, low quantum yield, small stokes shift and large half-peak width, which hinders the further development and application of NIR-Ⅱ imaging. Rare-earth based nanomaterials with unique optical properties have exhibited great potential for optical imaging. Rare-earth nanoparticles with metal ions as activators do not suffer from photobleaching and can provide narrow excitation/emission peaks with large stokes shift. Their emission is tuned with the choice of doped rare-earth ions and can maintain high quantum yield, which is different from other fluorophores. The fluorescence lifetime of rare-earth nanoparticles is also much longer than common fluorophores. All these features enable them as novel probes for in vivo fluorescence imaging and diagnosis in the NIR-Ⅱ region. This review briefly introduces the optical features of rare-earth nanoparticles, and discusses the design and application of NIR-Ⅱ emissive rare-earth nanoparticles according to the use of different sensitizers (Yb³⁺, Nd³⁺, Er³⁺ and Tm³⁺). Finally, the limitations and development direction of rare-earth nanoparticles in NIR-Ⅱ imaging are prospected.

Contents

1 Introduction

2 Introduction of rare-earth nanomaterials

2.1 rare-earth elements and Rare-earth nanoparticles

2.2 Preparation methods of rare-earth nanoparticles

2.3 The optical properties of rare-earth nanoparticles

3 Design strategy and application of NIR-Ⅱemissive rare-earth nanoparticles

3.1 Yb³⁺sensitized NIR-Ⅱemissive rare-earth nanoparticles

3.2 Nd³⁺sensitized NIR-Ⅱemissive rare-earth nanoparticles

3.3 Er³⁺sensitized NIR-Ⅱemissive rare-earth nanoparticles

3.4 Tm³⁺sensitized NIR-Ⅱemissive rare-earth nanoparticles

4 Conclusion and outlook

Graphical abstract

Keywords

rare-earth nanoparticles / NIR-Ⅱ / sensitizers / bio-imaging

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Feng Lu, Ting Zhao, Xiaojun Sun, Quli Fan, Wei Huang. Design of NIR-Ⅱ Emissive Rare-earth Nanoparticles and Their Applications for Bio-imaging. Progress in Chemistry, 2022, 34(6): 1348-1358 DOI:10.7536/PC210624

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1 Introduction

Fluorescence imaging has been widely studied in the fields of life science and biomedicine due to its high sensitivity, high temporal and spatial resolution, fast response and non-invasiveness^[1⇓~3]. In the process of optical imaging in vivo, the penetration depth of photons mainly depends on the absorption and scattering of light by biological tissues^[4]. At the same time, the autofluorescence and scattered photons of biological tissue itself will also cause background and noise interference to optical imaging^[5,6]. Therefore, in vivo fluorescence imaging usually uses near-infrared light (> 700 nm) for imaging to avoid the high light absorption and light scattering of some active components (such as melanin, hemoglobin, cytochrome, etc.) in the visible region (400 ~ 700 nm)^[7]. Compared with NIR-I (700 ~ 900 nm), NIR-II (1000 ~ 1700 nm) imaging has attracted much attention in recent years. Its main advantage is that it can significantly reduce the autofluorescence intensity of biological tissues and the scattering effect on photons, thus significantly increasing the penetration depth of fluorescence signals in biological tissues and significantly improving the signal-to-noise ratio and resolution of imaging^[8]^[9,10].

The development of near-infrared two-zone imaging technology is inseparable from the development of corresponding fluorescent probes. In the past few years, a variety of materials, including small organic molecules, conjugated polymers, carbon nanotubes and semiconductor quantum dots, have been developed for the design of NIR-Ⅱ probes^{[11⇓⇓~14]}. These materials have their own unique advantages, which greatly promote the development of NIR-Ⅱ imaging technology^[3,15,16]. However, there are some limitations to their use. For example, small organic molecules are usually poorly photostable and susceptible to photobleaching; Conjugated polymers and carbon nanotubes have low fluorescence quantum yield. The use of quantum dots is limited by light blinking and the potential toxicity of heavy metal elements such as cadmium and selenium^[15,16]. In addition, the Stokes shift of these materials is relatively small, which may cause background noise in fluorescence imaging, thus affecting the imaging quality. Rare earth nanomaterials have distinct optical properties compared with these traditional fluorescent materials^[17]. The luminescence spectrum of rare earth nanomaterials is similar to the atomic spectrum, and the emission peak and excitation peak are very sharp. At the same time, its long luminescence lifetime, large Stokes shift, high quantum yield and excellent photostability make it show unparalleled advantages in the construction of optical probes.

In recent years,A variety of rare earth ions (such as Pr³⁺, Nd³⁺, Ho³⁺, Er³⁺,Tm³⁺ and Yb³⁺) were reported to have NIR-II luminescence ability^[18]. Nanomaterials with these rare earth ions as activators have also been gradually developed for NIR-II bioimaging. Different from other inorganic luminescent materials and organic materials, the optical properties of rare earth nanomaterials are achieved by adjusting the doping ratio and spatial distribution of rare earth ions, so rare earth nanomaterials have a strong sense of design^[19]. In recent years, the field of luminescent rare earth nanomaterials has made rapid progress, and many efficient NIR-II rare earth nanostructures have been designed and developed for high-resolution in vivo bioimaging. In this paper, the recent design schemes of NIR-II luminescent rare earth nanomaterials are reviewed around four commonly used sensitizing ions (Yb³⁺, Nd³⁺, Er³⁺, Tm³⁺), and their applications in in vivo imaging are briefly introduced, which provides a reference for the research in related fields.

2 Brief Introduction of Rare Earth Nanomaterials

2.1 Rare Earth and Rare Earth Nanomaterials

Generally, rare earth elements refer to the lanthanide elements with atomic numbers ranging from 57 to 71 in the periodic table of chemical elements: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lute^[20]. Rare earth has excellent optical, electrical and magnetic properties, and is an important raw material for manufacturing new materials. It is widely used in industrial production, and there is almost no substitute. It is called "industrial vitamin" and "mother of new materials". Rare earth elements are characterized by a 4F electron shell. In the atomic state, the electron first fills the 5s²5p⁶ shell and then fills the 4F shell. After the formation of ions, the 4F shell will shrink into the 5s²5p⁶ shell. Due to the screening of the 5s²5p⁶ shell, the crystal field has a weak effect on the 4F electrons, which makes the rare earth ions in the crystal show atomic-like spectral properties^[21]. Rare earth ions in crystals are usually trivalent, and their luminescence can be divided into two types, one is linear atomic spectrum, the other is wide band spectrum. The band spectrum originates from the 4F → 5d transition, which is greatly affected by the crystal field and located in the ultraviolet and vacuum ultraviolet regions, so there are few studies at present. The line-like narrow-band spectrum originates from the f-f transition, which is the main content of rare earth spectroscopy^[22]. Because of the large quantum number of 4F electrons, the number of energy levels and transition pathways that can be formed are numerous, which can produce a spectrum covering ultraviolet to infrared light. At the same time, these spectra belong to atom-like narrow spectra with pure light color, which are ideal active ions for luminescent materials^[21].

Rare earth nanomaterials for luminescence are obtained by doping rare earth ions into specific nanocrystals, which are usually composed of three parts, matrix, sensitizer and activator. Fluorides are currently the most efficient host materials due to their low phonon energy and high chemical stability, and the commonly used ones are NaYF₄, NaLuF₄, LaF₃, etc^[23]. The activator is the luminescent center of the material, and the sensitizer absorbs light energy and transfers it to the activator to make it luminescent. The excitation and emission wavelengths of rare earth nanomaterials can be fully adjusted by the selection of sensitizers and activators, which is of great significance for optical imaging. In particular, anti-Stokes emission occurs when the excitation wavelength is larger than the emission wavelength, and such materials are called upconversion materials^[24]. For traditional luminescent materials, the upconversion phenomenon is extremely rare, so the upconversion imaging of rare earth nanomaterials has once been a hot topic in optical imaging research^[25]. Although up-conversion imaging has high signal-to-noise ratio and penetration depth, its emission wavelength is usually less than 850 nm, and the resolution of imaging still needs to be improved^[26]. With the rapid development of near-infrared two-zone imaging technology, rare earth elements have also been found to have good near-infrared two-zone luminescence ability (Table 1), which makes the biological imaging application field of rare earth nanomaterials gradually shift from up-conversion imaging to near-infrared two-zone imaging^[27].

Of course, the choice of sensitizer and activator is not arbitrary, and its doping concentration and distribution in the crystal also have a significant impact on its performance. The construction of multilayer rare earth nanomaterials is the most important way to improve their optical properties, which puts forward higher requirements for the preparation of rare earth nanomaterials^[28].

2.2 Method for preparing rare earth nano material

Researchers have developed a number of synthetic methods to synthesize lanthanide doped rare earth nanoparticles with controlled structure, size and shape, including hydrothermal method, thermal decomposition method, co-precipitation method, microemulsion method, sol-gel method and ionic liquid method^[29]. Among these methods, hydrothermal synthesis and thermal decomposition synthesis are the most important methods to produce rare earth nanoparticles at present because they do not need high temperature calcination and have good dispersion and controllability.

(1) hydrothermal synthesis

Hydrothermal synthesis refers to the growth of crystalline materials in water or other solvents using high temperature and high vapor pressure conditions in a closed reaction system (autoclave). Hydrothermal is a controllable synthesis strategy, and the crystalline phase, size, and morphology of the synthesized nanoparticles can be adjusted by the reactant concentration, reaction temperature, time, pH value, and solvent/surfactant type. Representatively, Li et al. Proposed a "liquid-solid-liquid (LSS)" strategy to synthesize different monodisperse nanocrystals, including semiconductor, metal and rare earth nanomaterials, using ion exchange and phase transfer between metal ions and surfactants^[31]. The NaYF₄:Yb,Er rare earth nanomaterials obtained by this method have good dispersion and uniform particle size, and have green upconversion luminescence under 980 nm light excitation, which is an important method for the synthesis of rare earth nanoparticles in the early stage. Liu et al. Used a similar method to prepare NaYF₄:Yb,Er nanorods, and found that Gd³⁺ co-doping could promote the nanorods from cubic to hexagonal, thereby improving the upconversion luminescence intensity^[32]. The NaYF₄:Gd,Yb,Er nanorods obtained by this method were also found to have down-conversion luminescence at 1520 nm, and were applied to high-resolution cerebrovascular and tumor imaging^[33].

The rare earth nanoparticles obtained by the above scheme are all hydrophobic materials, which need surface modification to achieve biological applications. Liu et al. Also developed a hydrothermal synthesis method for water-soluble rare earth nanomaterials (Fig. 1). The method uses polyethyleneimine (PEI) as a stabilizer, which not only realizes the water solubility of nanoparticles, but also facilitates the subsequent biological modification and application^[30]. In general, the hydrothermal method is an effective means for the rapid synthesis of rare earth nanoparticles in large quantities because of its mild conditions, simple process and no need for high temperature calcination. However, hydrothermal method is difficult to synthesize multilayer rare earth nanoparticles, so it is less and less used in the preparation of high-quality rare earth nanomaterials.

(2) Thermal decomposition method

Thermal decomposition method is a synthesis method in which the organometallic precursor or metal salt solution is injected into the ligand solution at high temperature, the precursor is rapidly pyrolyzed at high temperature, the reaction nucleates, and then the crystal nucleus slowly grows into nanomaterials. Chow et al. Successfully prepared hexagonal phase NaYF₄:Yb,Er and NaYF₄:Yb,Tm nanoparticles with a particle size of about 10 nm by thermal decomposition of rare earth trifluoroacetate at high temperature (330 ° C) using oleylamine as solvent^[34]. Capobianco et al. Used oleic acid and 1-octadecene instead of oleylamine to obtain NaYF₄:Yb,Er and NaYF₄:Yb,Tm in cubic phase at 310 ℃^[35]. Larger rare earth nanoparticles (about 28 nm) can be obtained by this scheme, which is helpful to reduce defects and improve luminescence intensity. At the same time, these cubic phase rare earth nanomaterials can be converted to hexagonal phase by further heat treatment.

Zhang et al. Used rare earth chlorides as raw materials, mixed with sodium hydroxide and ammonium fluoride in the oleic acid-octadecene system, and obtained high-quality hexagonal rare earth nanomaterials through low-temperature nucleation and high-temperature growth (Fig. 2)^[36]. The rare earth chloride in the method can also be replaced by rare earth acetate. The scheme can better solve the problem that a large amount of fluorine-containing toxic gas is produced during the pyrolysis of trifluoroacetate. Although this method requires inert gas protection and usually has a low yield, it has become a classical method for the synthesis of high-quality rare earth nanomaterials because it can prepare multilayer nanoparticles.

2.3 Optical properties of rare earth nanomaterials.

As a kind of ionic luminescent materials, rare earth nanomaterials have almost no photobleaching problem. In addition, rare earth nanomaterials also show many unique optical properties because of their extremely rich energy levels and screened 4 f electrons^[23]. Firstly, the emission peak of rare earth nanoparticles is narrow, the Stokes shift is large, and the emission of different wavelengths can be achieved at the same time, which is beneficial to the construction of ratiometric luminescence probes and the application of multi-color imaging^[37⇓~39]. For example, Nd³⁺ can produce 900 nm, 1060 nm and 1340 nm luminescence simultaneously under 800 nm excitation^[40]. In addition, rare earth nanoparticles can also have multiple excitation peaks that do not overlap with each other, which makes them useful for the design of excitation ratio probes. For example, Er³⁺ based nanoparticles can produce 1530 nm luminescence under the excitation of 980 nm, 800 nm and 658 nm, respectively^[41]. In addition, the rare earth nanoparticles adjust the emission wavelength by changing the doping ions. For other materials, adjusting the emission wavelength to the near-infrared region usually requires more complex design, and the narrowing of the band gap often leads to a significant decrease in the fluorescence quantum yield^[42]. Finally, the luminescence lifetime of rare earth nanomaterials is usually in the order of microseconds to milliseconds, which is much longer than that of other near-infrared luminescent materials (nanoseconds). Moreover, the luminescence lifetime of rare earth nanomaterials can be greatly adjusted by changing the doping concentration^[43,44]. Therefore, the rare earth nanoparticles can be used to conveniently construct a time-resolved imaging detection system^[45,46]. The co-doping or layered doping of rare earth ions can endow materials with more exotic optical properties, which make rare earth nanomaterials always active in the forefront of optical applications^[47⇓~49].

3 Design of Rare Earth Nanomaterials and Their Application in Near-Infrared Two-Zone Imaging

The luminescent intensity of rare earth nanomaterials is closely related to their structure, and the luminescent intensity of rare earth nanomaterials can even be improved by several orders of magnitude through the regulation of the doping concentration and spatial distribution of rare earth ions, so the design of rare Earth nanomaterials plays an extremely important role in the research of this field^[50]^[51]. In this part, according to the different sensitizing ions, the methods of rare earth nanomaterials to achieve near-infrared two-region luminescence and the design ideas to improve the intensity of NIR-Ⅱ luminescence will be introduced, and the application results of these materials in biological imaging will be briefly introduced.

3.1 Yb³⁺ sensitized NIR-Ⅱ luminescent rare earth nanomaterial

Yb³⁺ is the most commonly used sensitizer because it has only one excited state ²F_5/2, which makes it have extremely high quantum efficiency when excited, close to 100%^[53,54]. Yb³⁺/Er³⁺ co-doped NaYF₄ is a typical upconversion luminescent material. Yb³⁺ ions as sensitizers are excited to the ²F_5/2 excited state after capturing 980 nm photons,The energy is then transferred to the ⁴I_11/2 intermediate state of the Er³⁺ ion, ultimately resulting in green emission at ⁴S_3/2→⁴I_15/2 and red emission at ⁴F_9/2→⁴I_15/2^[55]. In fact, the Er³⁺ in this system can also emit down-conversion luminescence at 1530 nm with ⁴I_13/2→⁴I_15/2, but the down-conversion luminescence efficiency of this structure is relatively low, and only a few applications have been reported^[33]. Because the down-converted luminescence is located in the NIR-IIb (1500 ~ 1700 nm) region and has higher imaging resolution, it has been widely concerned.

In 2017, Hu and Dai made a breakthrough discovery that the energy gap between Ce³⁺ ions ²F_5/2 and ²F_7/2 is close to that of Er³⁺ ions ⁴I_11/2→⁴I_13/2.Therefore, the lifetime of the ⁴I_11/2 energy level can be shortened in a cross relaxation manner, and the filling of the ⁴I_13/2 energy level of the Er³⁺ ion is improved, so that the up-conversion luminescence is inhibited and the down-conversion luminescence is obviously improved^[52]. It was found that 2% Ce doping could achieve a 9-fold increase in 1530 nm down-conversion luminescence intensity (Figure 3). To reduce the quenching of Er ion luminescence by OH groups in water, a thick NaYF₄ shell was used to encapsulate the NaYbF₄∶2%Er,2%Ce nanoparticles, and the resulting core-shell nanoparticles exhibited extremely high NIR-IIb brightness in aqueous solution^[56]. Only 20 ms exposure time is needed to achieve high-resolution transcranial imaging of cerebral blood vessels in mice, and blood flow velocity can be quantitatively calculated, which serves for visual monitoring of cerebral vascular malformations and diagnosis of cerebrovascular diseases.

On this basis, Zhang et al. Synthesized monodisperse and size-controlled Er³⁺/Yb³⁺ doped hexagonal NaCeF₄ core and core/shell nanoparticles, which achieved strong NIR-II emission at 1530 nm under 980 nm excitation^[57]. The absolute luminescence quantum yield of NaCeF₄:Er/Yb nanoparticles increases with the particle size, from 5.6% at 7.1 nm to 32.8% at 200 nm. After encapsulation of the NaCeF₄ layer, the particle size of the 7.1 nm NaCeF₄:Er/Yb was enlarged to 18.1 nm, and the quantum yield was improved to 19.5%, which was 13.6 times higher than that of the widely reported NaYF₄:Er/Yb nanoparticles. Since Ce³⁺ can be oxidized to Ce⁴⁺ by H₂O₂, after removing oleic acid on the surface of the obtained NaCeF₄:Er/Yb nanoparticles, the material showed a highly sensitive response to H₂O₂, and the linear range of the response was 0. 078 ~ 10 mmol/L. The material is also used for the specific detection of uric acid in human serum by using the characteristic that uricase catalyzes the reaction of uric acid to produce hydrogen peroxide. The linear range was 1. 234-900 mmol/L, and the recovery was 93. 4% -108. 8%. In addition, after water-soluble modification, the NaCeF₄:Er/Yb@NaCeF₄ showed high resolution in mouse in vivo organ and hind limb imaging, and the penetration depth in muscle tissue was up to 10 mm. Furthermore, Dai et al. Found that the 1530 nm downconversion luminescence of Er³⁺ in the cubic phase is much higher than that in the hexagonal phase, which is contrary to the upconversion luminescence intensity^[58]. In this work, the down-conversion luminescence intensity of cubic phase NaYbF₄∶2%Er,2%Ce@NaYF₄ is 7.6 times higher than that of hexagonal phase, which may be attributed to the lattice relaxation promoting the non-radiative transition of Er³⁺ ion ⁴I_11/2→⁴I_13/2. By reducing the crystal field symmetry through Zn²⁺ ion doping, the NaYbF₄∶2%Er,2%Ce,10%Zn@NaYF₄ structure is able to achieve a further luminescence enhancement of about 50%. After water-soluble modification and coupling with anti-PD-L1 antibody, the probe can achieve PD-L1 imaging in a mouse colon cancer model, and the signal-to-noise ratio of the tumor can reach 40. By using the long luminescence lifetime (4. 6 ms) of the probe and time-resolved technology, the probe and PbS quantum dots can be imaged simultaneously in the NIR-IIb region to observe the state of immunotherapy in real time. Lin et al. Doped Ce³⁺ ions and Er³⁺ ions in different shell structures respectively, and the obtained NaGdF₄:Lu,Yb,Er@NaGdF₄:Yb,Ce had good near-infrared two-region luminescence and up-conversion luminescence at the same time, and could be used for nuclear magnetic resonance imaging and CT imaging^[59]. On this basis, they incorporated Tm element to make it have blue light upconversion ability, and wrapped manganese and copper-doped mesoporous silica for photodynamic therapy and chemokinetic therapy of tumor microenvironment response.

In addition to Er³⁺, Ho³⁺, Pr³⁺ and Tm³⁺ doped NaYF₄:Yb can also achieve near-infrared two-region emission at 1185 nm, 1310 nm and 1475 nm, respectively, under 980 nm excitation, however, their emission efficiency is significantly lower than that of Er and the emission wavelength is also smaller than that of Er, which is not conducive to improving the imaging resolution, so there are few studies at present^[60].

3.2 Nd³⁺ sensitized NIR-II luminescent rare earth nanomaterial

Rare earth nanomaterials based on Yb ions need to be excited by 980 nm laser, which will cause serious thermal effect and low tissue penetration depth due to the overlap of absorption peaks with water, so Nd is introduced into rare earth nanomaterials as a new sensitizer^[62]. Nd ion has a significantly higher absorption coefficient than Yb, which can capture light energy better, and the energy transfer efficiency from Nd to Yb is very high, so the excitation of rare earth nanomaterials at 740 nm, 800 nm and 860 nm can be realized by introducing Nd into the original Yb sensitized system^[54]. However, Nd usually cannot be doped in the same layer with the sensitizer because the active ions such as Er and Tm can produce cross relaxation with Nd to weaken the luminescence^[50].

In near-infrared two-zone imaging, as early as 2014, Zhang et al. Designed a four-layer structure of rare earth nanomaterials NaGdF₄/NaYbF₄:Er/NaYF₄:Yb/NaNdF₄:Yb, which achieved 1525 nm emission under 800 nm excitation, and proved the advantages of this mode in penetration depth and reducing thermal effects^[61]. The specific luminescence principle is shown in Figure 4. Nd ions are excited after absorbing 800 nm photons, and then the energy is gradually transferred to Yb ions in the inner layer, and finally Er ions are activated to emit 1525 nm photons. However, the luminous intensity was low at that time. Subsequently, Ce³⁺ was also introduced into the system to enhance the luminescence of rare earth nanomaterials under 800 nm excitation. Li et al. Designed and synthesized a rare earth doped nanoparticle NaYbF₄∶2%Er,5%Ce@NaYF₄:10%Yb@NaYF₄:50%Nd with a core/shell/shell structure to enhance near-infrared two-region down-conversion luminescence under 800 nm excitation^[63]. With the doping of Ce³⁺ ions, the emission intensity of Er³⁺ at 1530 nm is enhanced by a factor of 10, while the up-conversion luminescence at 660 nm is reduced to less than 1%. After modification with polyacrylic acid and polyethylene glycol, the resulting probe exhibited extremely high imaging resolution in vivo, and a blood vessel of 0. 25 mm could still be resolved 10 H after intravenous injection. The probe can also be used for in vivo tumor imaging. Zeng et al. Designed and prepared NaYF₄:Yb,Er,Ce,Nd@NaYF₄:20%Nd core-shell nanoparticles, and also realized near-infrared two-zone luminescence at 1525 nm under 808 nm excitation, and used them for vascular imaging and surgical navigation of tumor resection^[64].

Nd ions can also realize the luminescence of Yb near 1000 nm after transferring energy to Yb^[65]. Yan et al. Designed and synthesized NaGdF₄:Yb,Er@NaGdF₄:Nd,Yb core-shell structure, which can simultaneously realize the down-conversion luminescence of Yb at 1000 nm and the up-conversion luminescence of Er at 540 nm under 808 nm excitation, and used the material for fluorescence/nuclear magnetic resonance imaging-guided photodynamic therapy of tumors^[66]. Jaque et al. Designed NaY_0.9-_xYb_0.1Nd_xF₄@CaF₂ rare earth nanomaterials with core-shell structure, which can adjust the luminescence lifetime of Yb (0.1 ~ 1.5 ms, Fig. 5) by changing the doping amount of Nd, and can improve the luminescence intensity and increase the biocompatibility by coating with calcium fluoride^[44]. The researchers proved that the luminescence lifetime of the obtained materials was constant in different solution systems, and constructed a multiplexed time-resolved imaging system.

Chen et al. Synthesized NaYF₄@NaYF₄:Yb,Nd@CaF₂ rare earth nanomaterials with three-layer structure and found that the luminescence lifetime decreased with the increase of temperature, from 898 ms at 10 ℃ to 450 ms at 64 ℃^[43]. At the same time, the change of the luminescence lifetime of the material is independent of the pH value of the solution, the concentration of the material and the penetration depth. Based on this, the researchers designed a highly sensitive (1.4~1.1%℃^-1) nanothermometer for imaging detection of inflammation in mice. Fan et al. Utilized the characteristics of high luminescence quantum yield and narrow emission of NaYF₄:Yb,Nd@NaYF₄, combined with a 1000 nm narrow band-pass filter, significantly reduced the autofluorescence and other interference signals in nude mice (Fig. 6), and greatly improved the effect of in vivo tumor imaging^[67].

The Nd³⁺ ion itself can also be used as an activator ion, and in the near-infrared second region, the Nd³⁺ ion can emit light with wavelengths of 1060 nm(⁴F_3/2→⁴I_11/2) and 1340 nm(⁴F_3/2→⁴I_13/2), and the excitation wavelengths can be selected as 740 nm, 800 nm, and 860 nm. In order to achieve the best penetration depth, 800 nm excitation is usually selected. In 2014, Jaque et al. First reported the application of neodymium-based nanomaterials in near-infrared two-zone bioimaging^[68]. They synthesized 15 nm Nd-doped LaF₃ nanoparticles, systematically analyzed the luminescence intensity, penetration depth, and tissue resolution of three near-infrared emission bands (900 nm, 1060 nm, and 1340 nm) of Nd³⁺ ions, and determined that the emission of Nd³⁺ at 1060 nm in the near-infrared second region is the best channel for in vivo imaging. Then, they synthesized Nd³⁺ doped SrF₂ nanoparticles, which greatly eliminated the background fluorescence by their luminescence at 1300 nm, and obtained high-resolution imaging in vivo^[40]. Rodr Rodríguez et al. Encapsulated Nd-doped NaGdF₄ and PbS/CdS/ZnS quantum dots in poly (lactic-co-glycolic acid) nanoparticles, and constructed a nano-temperature probe in the second near-infrared region by using the characteristic that the fluorescence intensity of quantum dots changes with temperature while that of rare earth materials is relatively constant^[69].

3.3 Er³⁺ sensitized NIR-II luminescent rare earth nanomaterial

Er-doped rare earth nanoparticles show near infrared second region luminescence at 1525 nm due to their ⁴I_13/2→⁴I_15/2 transition. It has been previously reported that Yb ions as sensitizers and Er ions as activators are usually co-doped into NaYF₄ nanocrystals for near-infrared two-region bioimaging^[60]. In these reports, the doping content of Er³⁺ ions is very low to reduce the luminescence quenching caused by high concentration, usually only 2%.

In 2017, Almutairi et al. Found that the luminescence quenching caused by concentration in Er-based rare earth nanomaterials actually comes from the transfer of energy to the surface (defects) rather than the cross-relaxation usually considered^[41]. After preparing 100% Er-doped NaErF₄ and encapsulating the inert shell NaLuF₄, it was found that the material could achieve efficient upconversion (650 nm) and downconversion (1550 nm) luminescence under 980 nm, 800 nm and 658 nm laser excitation, with very low concentration quenching effect (Fig. 7). In this structure, Er ions act as both sensitizer and activator, and many subsequent research groups have carried out extensive research and application on this kind of material^{[70⇓⇓~73]}.

Chen et al. Systematically studied the effects of crystal form and inert layer on the near-infrared two-region luminescence properties of this kind of Er sensitized materials^[74]. It is found that the luminescence of the hexagonal phase is obviously stronger than that of the cubic phase. After wrapping with different inert layers (NaYF₄, NaLuF₄, NaGdF₄), NaErF₄@NaYF₄ has the strongest luminescence, which may be related to its smaller lattice mismatch. In addition, the shell thickness needs to reach 3 nm to completely eliminate the surface quenching and achieve the best luminescence intensity. The resulting material was able to achieve clear transcranial imaging of cerebral vessels in a mouse model at a low dose (5 mg/kg) (Fig. 8) and was completely metabolized by feces after 14 days. In addition, they also found that the incorporation of an appropriate amount of Yb³⁺ ions (optimal concentration, 20%) into the NaErF₄ core could double the luminescence at 1525 nm produced by NaErF₄@NaLuF₄ when excited at 808 nm, resulting in a quantum yield of the nanocrystal of 11%^[75]. This Yb³⁺ mediated luminescence enhancement may originate from the formation of energy trapping centers, reducing the energy transfer to lattice defects. Meanwhile, the suppression of the upconversion pathway by Yb³⁺ doping may also enhance the luminescence at 1525 nm.

Gao et al. Layered Yb ions and Er ions and designed NaErF₄@NaYbF₄@NaYF₄ core/shell/shell nanoparticles^[26]. Spectroscopic studies show that the middle NaYbF₄ layer has a synergistic effect with the outer NaYF₄ shell, which can greatly enhance the upconversion (655 nm) and downconversion (1525 nm) luminescence of the material under 980 nm or 808 nm excitation. At room temperature, the quantum yield of NIR-Ⅱ is as high as 18.7%. After the nanoparticles were PEGylated and conjugated with folic acid, the resulting tumor-specific nanoprobes were able to be used for up-conversion and down-conversion imaging of subcutaneous and abdominal tumors in mice (fig. 9). Down-conversion imaging based on 1525 nm can provide significantly higher imaging resolution and clear imaging analysis of tumor contour and folate receptor.

In addition to the commonly used 980 nm and 800 nm, Er ions as sensitizers can also be excited by 1530 nm light^[77]. Using Er ion as sensitizer and Ho ion as activator, Huang et al. Obtained a NaYF₄:Er/Ho(10/0.5 mol%)@NaY

F 4

of upconversion nanomaterials with excitation and emission in the second region of near infrared^[78]. Er ions can absorb 1532 nm light and transfer the energy to the surrounding Ho ions to realize its luminescence at 1172 nm. Zhang et al. Further optimized the doping ratio and found that the intensity near 1180 nm in this system increases with the increase of Er ion concentration, while the optimal concentration of Ho is about 2%^[76]. The resulting NaErF₄∶0.02Ho@NaYF₄ was capable of producing 650 nm, 980 nm, and 1180 nm luminescence under 1530 nm laser excitation (Fig. 10). The material, IR1061 and ferrous ions were encapsulated in polycaprolactone microneedles, which could be used for the detection of hydrogen peroxide. Through Fenton reaction, ferrous ion converts hydrogen peroxide into hydroxyl radical, which further reacts with IR1061 to reduce its absorption at 800 ~ 1100 nm, so that the luminescence of rare earth nanomaterials at 980 nm increases, while the luminescence at 1180 nm remains unchanged. Thanks to low autofluorescence and tissue scatter, this near-infrared two-zone ratiometric assay demonstrates excellent resolution for assessing inflammation in mouse models.

3.4 Tm³⁺ sensitized NIR-Ⅱ luminescent rare earth nanomaterial

Although the emission wavelength of rare earth nanomaterials with Er as the activator can reach the NIR-Ⅱb region, their excitation wavelength is still in the NIR-I region. The development of rare earth nanomaterials with excitation wavelength also in the NIR-Ⅱ region may further improve the imaging effect of such materials. To this end, Zhang et al. Developed a new nanostructure (NaYF₄:Tm/Er@NaYF₄) with Tm³⁺ as the sensitizer, and realized the 1525 nm emission of Er³⁺ under 1208 nm excitation by using the energy transfer from Tm to Er (Fig. 11)^[79]. The luminescence lifetime of the Tm³⁺ can be precisely controlled by adjusting the doping concentration of the material. After the two-dimensional codes encoded by materials with different luminescence lifetimes were implanted subcutaneously in mice, the background fluorescence could be eliminated by time-resolved technology, and the images with high signal-to-noise ratio and high resolution could be obtained. This work is based on time-resolved in vivo information storage and decoding technology, and also provides a new idea for the future application of near-infrared rare earth nanomaterials.

4 Conclusion and prospect

Rare earth nanomaterials have received increasing attention in the field of near-infrared two-region imaging in recent years due to their sharp emission peaks, long and tunable luminescence lifetimes, high photostability, and quantum yield. In this paper, the optical properties of rare earth nanomaterials are analyzed, the synthesis methods of rare earth nanomaterials are introduced, and the design strategies and bioimaging applications of near infrared two-region luminescent rare earth nanostructures are summarized. At present, the synthesis methods of rare earth nanomaterials have been basically improved, and the main research direction in this field is the design of rare earth nanomaterials. The discovery and reasonable collocation of more sensitizers and activators can enrich the excitation and emission spectral ranges of rare earth nanomaterials. The development of new matrices, the optimization of doping concentration, the use of multilayer structures enable the adjustment of the position and intensity of the main emission peaks^{[80⇓⇓~83]}. It will always be the focus of rare earth nanomaterials research to greatly improve the luminescence intensity through rational design optimization.

At the same time, rare earth nanomaterials as optical probes have some common problems, which limit the application of these materials. For example, rare earth nanomaterials tend to be enriched in bone, and at lower concentrations may dissociate to release rare earth ions, resulting in potential toxicity, which should be solved by reasonable surface modification^[84]^[85]. The main problem for rare earth nanomaterials is their weak ability to absorb light. Because the f-f transition of rare earth ions is a forbidden transition, its absorption coefficient is very low, so even if the rare earth nanomaterials have extremely high quantum yield, their luminescence intensity at the same dose is not outstanding. Dye sensitization may be an effective way to solve this problem^[86⇓~88]. At present, cyanine dyes are almost used to sensitize rare earth nanomaterials, which have poor photostability and poor sensitization effect in aqueous solution, and are not conducive to the construction of rare earth nanoprobes^[89]. Recently, there are also reports of using conjugated polymers to sensitize rare earth nanomaterials, but their sensitization efficiency is not comparable to that of cyanine^[90]. With the in-depth study of the interaction mechanism between rare earth and dyes in recent years, it is believed that the design and synthesis of new sensitizing molecules and the optimization of the composite mode of the two will be one of the effective ways to improve the near-infrared luminescence intensity of rare earth nanomaterials^[91,92]. In addition, surface plasmon coupling may also be a way to improve the overall luminescence intensity of rare earth nanomaterials^{[93⇓⇓~96]}. Recently, it has also been reported that rare earth nanoparticles can be activated by X-ray to achieve near-infrared two-region afterglow luminescence, which may also become a new research direction of near-infrared two-region luminescent rare earth nanomaterials^[97].

In a word, the design of new rare earth nanostructures and the improvement of dye sensitization strategy will be the frontier of research in this field, and there is still a broad space for the development of rare earth nanomaterials as optical probes. Rare earth nanomaterials combined with photodynamic therapy and photocontrolled drug release will also be able to build more efficient diagnosis and treatment platforms to serve the life and health industry.

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