The photophysical processes of the rare-earth-doped nanomaterials are varied according to the incorporation of dopant ions. Due to the presence of various metastable energy levels in the RE³⁺ ions, the possibility of numerous emission lines increases and can be found through Dieke’s diagram (Sharma et al., 2017a). As a result, excitation and emission energies are varied with RE³⁺ ions (Ghosh et al., 2011; Zhang et al., 2012; Lorbeer and Mudring, 2014; Goldschmidt and Fischer, 2015; Ghosh and Mudring, 2016).

Commonly, photophysical processes exhibited by RE³⁺ ions can be classified into five categories: direct excitation or downshifting, charge transfer, energy transfer, and quantum cutting downconversion and upconversion processes (see Figures 1, 2). In this section, all these processes are described in detail.

Judicious selection of host material is very important for tuning the photophysics of RE³⁺ dopant ions (Sharma et al., 2017a). Numerous types of host materials have been explored to date, including oxides, phosphates, fluorides, vanadate, and borate (Sun and Zheng, 2010; Tian et al., 2012; Tian et al., 2014; Kuzmanoski et al., 2015; Alammar et al., 2016; Cybinska et al., 2016; Muthulakshmi et al., 2020; Chouryal et al., 2021). Important points have to be considered before selecting the host material like thermal and chemical stability, low phonon energy, high refractive index, and large bandgap (Gai et al., 2014; Sharma et al., 2017a). Considering all aspects, fluoride-based host materials are considered to be a better host matrix for doping of the RE³⁺ ions compared to the other host materials (Gai et al., 2014; Sharma et al., 2017a). Another important feature of the fluoride-based host matrices is that they occur in different polymorphs, can be tuned by tuning temperature, reaction precursors, dopant concentration, and size (Gai et al., 2014; Sharma et al., 2017a). Besides fluorides, phosphate- and oxides-based materials are also used (Bühler and Feldmann, 2007; Zharkouskaya et al., 2008; Kowsari and Faraghi, 2010; Tian et al., 2012; Zou et al., 2013; Tian et al., 2014; Alammar et al., 2016; Liu et al., 2020). However, the fluorides-based nanomaterials have thermal stability issues, particularly in their applications at high temperatures. It has been noticed that upon heating the fluorides-based nanomaterials at high temperatures, sometimes oxyfluorides are formed (Knudson, 1954). In order to tackle these issues, oxides-, phosphates-, borate-, vanadates-, and molybdates-based nanomaterials have gained noticeable attention. In addition, the morphology of the host material also influences the optical properties of the doped rare-earth ions. Several methodologies have been developed so far to prepare different host materials (Gai et al., 2014; Sharma et al., 2017a). Herein, we have focused mainly on the task-specific IL-assisted synthesis of RE-doped nanomaterials. Ionic liquids play a very important role in designing host materials, which will be discussed in this chapter.

ILs have become an important part of chemistry, materials science, and electrochemistry. Recently, they have got tremendous attention for several applications due to their tunable properties. Ionic liquids (ILs) are organic salts that have a melting point less than 100°C in ambient condition and are comprised of cation and anion (Rogers and Seddon, 2003; Wilkes, 2004; Plechkova and Seddon, 2008; Roth et al., 2012; Hallett and Welton, 2011; Armand et al., 2009; Krossing et al., 2006). Therefore, many combinations (maximum 10¹⁸) of cation and anion are possible, leading to the formation of a variety of ionic liquids (Rogers and Seddon, 2003; Wilkes, 2004; Plechkova and Seddon, 2008; Roth et al., 2012; Hallett and Welton, 2011; Armand et al., 2009; Krossing et al., 2006). Besides this, different types of IL cations can also be produced by substituting with the desired alkyl chain length on the fundamental cations, such as imidazolium, pyridium [C₅H₅NR]⁺, pyrolidinium [C₅H₁₀NR₂]⁺, phosphonium [PR₄]⁺, sulphonium [SR₃]⁺, and alkylammonium [NR₁R₂R₃R₄]⁺ (here, R₁ = -H, alkyl and R_2,3,4 = alkyl groups). In addition, by changing the anions such as X⁻ (Cl⁻, Br⁻), BF₄ ⁻, PF₆ ⁻, OTf⁻, RSO₄ ^-, and OH⁻, one can get ILs with different properties (see Figure 3) (Rogers and Seddon, 2003; Wilkes, 2004; Krossing et al., 2006; Plechkova and Seddon, 2008; Armand et al., 2009; Hallett and Welton, 2011; Roth et al., 2012). As a result of these incredible characteristics of the ILs, chemical and physical properties can be feasibly tuned according to the necessity of reaction conditions; for these reasons, ionic liquids are often called “designer” solvents (Plechkova and Seddon, 2008; Roth et al., 2012). ILs came into the picture a century ago, when ethylammonium nitrate salt was recognized as molten salt (Plechkova and Seddon, 2008; Roth et al., 2012). The application of ionic liquids was assured when pyridinium salt was used to dissolve cellulose at 100°C (Plechkova and Seddon, 2008; Hallett and Welton, 2011). Then, for reprocessing the nuclear fuel, chloroaluminates- (AlCl₄ ⁻-) based ionic liquid of low melting temperature was used (Plechkova and Seddon, 2008; Armand et al., 2009; Hallett and Welton, 2011). The major drawback of these ionic liquids was their high sensitivity to atmospheric moisture and protonic impurities (Plechkova and Seddon, 2008; Armand et al., 2009; Hallett and Welton, 2011; Roth et al., 2012). For some time, other low melting ionic liquids were explored especially using imidazolium cations, and the sensitivity of ILs towards moisture and reaction medium (acidic and basic medium) problem was sorted out by applying plasticizing anions, for instance, bis(trifluoromethylsulfonyl)amide (NTf⁻) (Roth et al., 2012; Hallett and Welton, 2011). In this anion, CF₃SO⁻ groups are a strong electron-withdrawing group bound to the N atom, leading to the formation of flexible S-N-S bonds. NTf⁻ anion containing ILs have a low melting point; for instance, −15°C is reported for the 1-ethyl-3-methylimidazolium-based IL (Roth et al., 2012). Another example of the influence of anions on the physical and chemical characteristics of ILs is noticed when BF₄ ⁻ and PF₆ ⁻ anions were used with imidazolium cations. The melting point of the obtained ionic liquids is substantially altered and lowered down even less than room temperature compared to the halides containing ILs (shown in Table 1) (Rogers and Seddon, 2003; Armand et al., 2009). As a result, these characteristics of ILs render them superior to other conventional solvents.

Though ionic liquids are extensively ionic, they still have a low melting point, even less than 0°C. The low melting point of ILs was determined based on theoretical calculations. It was found that due to the large size of constituent ions (cation and anion) of ILs, generally high conformational flexibility leads to fewer lattice enthalpies and high entropy value, favoring the low melting point (Krossing et al., 2006). In addition, Krossing et al. have used the Born–Fajans–Haber cycles to estimate the Gibbs free energy (∆_fusG^T) of fusion (IL(s) → IL(l)) of ILs that depends on the lattice (IL(s) → IL(g)) Gibbs energy (∆_lattG^T) and solvation (IL(g) → IL(l)) Gibbs energies (∆_solvG^T). The value of ∆_fusG is negative for ILs, meaning that ILs exist in a liquid state (Krossing et al., 2006).

In the past, conventional organic solvents were extensively used for synthesizing, especially nanomaterials. The major problems concerned to those solvents were their high volatility and low decomposition temperature. In addition, thermal decomposition, particularly at high temperatures, leads to the release of hazardous gases, which cause serious environmental problems (Rogers and Seddon, 2003; Plechkova and Seddon, 2008; Armand et al., 2009). Therefore, all these issues of conventional solvents always motivated the scientific groups to explore new environmentally benign solvents. As a result, with the persistent effort of many scientists, ionic liquids were explored and continuously developed in the scientific domain. Ionic liquids have several attractive features, especially tunable properties. Just by changing the combination of cation and anion, new properties such as high thermal stability (even though >250°C), large electrochemical window spanning 6V, negligible vapor pressure, high liquidus range can be obtained (Rogers and Seddon, 2003; Plechkova and Seddon, 2008; Armand et al., 2009). Due to these interesting physicochemical properties, ILs have several applications in industries and organic synthesis (Rogers and Seddon, 2003; Plechkova and Seddon, 2008; Armand et al., 2009; Hallett and Welton, 2011). In addition, they are used not only as solvents but also as reaction partners for the source of desired ions. In this way, ionic liquids can be considered potential solvents for several applications, including nanomaterial design discussed in the next section.

Characterization of ionic liquids: the purity of ILs is a prerequisite for numerous applications, including nanomaterials synthesis. Several instrumental techniques are employed to check the purity of ILs, for example, to check whether the as-prepared IL is free from any kind of impurities such as unreactive reaction precursor, side products, and, most importantly, moisture. The most common characterization techniques are nuclear magnetic resonance (NMR), FTIR to determine the nature of cations and anions present in the IL, and characteristic vibrational frequencies of different groups like alkyl chain length, imidazolium ring, and hydroxyl group (in case of moisture) qualitatively (Cybinska et al., 2016; Sharma et al., 2017b). The moisture content in the as-synthesized ILs can be determined using the Karl–Fischer titration method. (Widegren et al., 2005).

ILs are important as a widespread tunable class of solvents that are used in several fields because of their unique properties. The application of ILs is a function of their composition. Therefore, judicious selection of cation-anion combinations of ILs is necessary to tune the physicochemical properties of ILs. Normally, ILs can be classified based on their applications in various fields into three groups: protic, aprotic, and zwitterionic types (shown in Figure 4) (Armand et al., 2009).

Protic ILs are widely used in fuel cells and aprotic ILs in Li-ion batteries and supercapacitor applications. On the other hand, zwitterionic-based ILs are used to prepare IL-based membranes (Armand et al., 2009). In addition, other important applications of ILs are catalysis of organic reactions, energy resources, conversion of CO₂ into useful organic compounds for industrial applications, reaction medium to perform the reactions, extraction of rare-earth metal compounds from their minerals, separation of toxic elements, and reaction partner for synthesizing the inorganic-based nanomaterials (Plechkova and Seddon, 2008; Duan et al., 2014; Hallett and Welton, 2011; Armand et al., 2009; Guterman et al., 2018; Azov et al., 2018; Zhang et al., 2014; Fabregat-Santiago et al., 2007; Visser et al., 2002). Despite these applications, only recently did ILs receive considerable attention in the synthesis of nanomaterials (Figure 5). During nanomaterial synthesis, ILs are used not only as solvents but also as reaction partners and capping/templating agents and as nanoreactors due to the presence of tunable alkyl chain length (Duan et al., 2014).

Earlier, ILs have been used to catalyze organic and organometallic synthesis (Visser et al., 2002; Hallett and Welton, 2011). On the other hand, Dupont's group have prepared uniform Ir nanoparticles using [C₄mim][PF₆] IL as reaction media (Dupont et al., 2002). In 2004, Taubert synthesized CuCl nanoplatelets using the IL-crystal as precursor (Taubert, 2004). The term “all-in-one” is used to describe ILs because these are usually used as solvent, precusors, and stabilizing agents in the preparation of inorganic materials (Richter et al., 2013; Duan et al., 2014). In the beginning, ILs were employed to not only synthesize the metallic nanoparticles but also to synthesize semiconducting nanomaterials (Richter et al., 2013; Duan et al., 2014). In the course of the reaction, ILs were applied as reaction partners and reaction media. For instance, anions parts of IL such as [BF₄ ⁻, PF₆ ⁻, bromide (Br⁻), iodide (I⁻)], [H₂PO₄]^-, and [SeO₂(OCH₃)]⁻can be used as a source of halides, phosphate ion (PO₄ ³⁻), and selenide (Se²⁻) ion, respectively (Lorbeer and Mudring, 2013a; Richter et al., 2013; Duan et al., 2014; Cybinska et al., 2016). [C₁₆mim][Br] and [C₄mim][I] ILs are applied to make BiOBr and BiOI nanoparticles, respectively (Xia et al., 2011a; Xia et al., 2011b). On the other hand, using the [C₄mim][SeO₂(OCH₃)], several selenide-containing nanoparticles have been prepared, such as ZnSe and Cu₂Se (Liu et al., 2010; Duan et al., 2011; Liu et al., 2011). Moreover, ILs can control crystal phase and morphology and stabilize the nanoparticles (Kowsari and Faraghi, 2010; Ghosh and Mudring, 2016). Furthermore, ILs can also be used to reduce the metal ion for preparing the metallic cluster (Liu et al., 2010; Xia et al., 2011a; Xia et al., 2011b; Duan et al., 2011; Liu et al., 2011; Richter et al., 2013). For stabilizing the nanoparticles, various modes of interaction generally take place; for example, IL can be attached to the surface of nanoparticles by H bonding through acidic proton and aromatic π-system and via interaction with anions (Pensado and Pádua, 2011; Richter et al., 2013).

Numerous preparative methods can be used to synthesize RE-based nanomaterials. However, during the last decade, IL-assisted synthesis approaches became a novel way to prepare RE-based nanomaterials. IL-based synthesis approaches not only aid in the formation of desired products but also control the size and morphology and assist in the functionalization of as-prepared products. Some of the state-of-the-art synthesis techniques, such as IL-assisted hydrothermal/solvothermal, microwave-assisted and sonochemical techniques, are discussed in this review article (shown in Table 2).

(A) Hydrothermal/solvothermal: hydrothermal or solvothermal method is rapidly used for the synthesis of Ln³⁺-doped nanoscale particles (Liu et al., 2014a; Tian et al., 2014; Wang et al., 2015b; Cybińska et al., 2016; Sharma et al., 2020a; Muthulakshmi et al., 2020; Chouryal et al., 2021). When water is used as a solvent during synthesis, this is called a hydrothermal method. However, if another solvent except water is used for synthesis, this is called a solvothermal method. In this method, the reaction mixture is transferred into the Teflon-lined vessel, further coated with a stainless steel jacket. Thereafter, the vessel is put at a particular temperature, resulting in high pressure inside the reaction vessel that accelerates the reaction. As a result, the desired product is obtained. ILs have been frequently employed in the synthesis of varieties of Ln³⁺-doped binary/ternary fluorides, phosphate, and oxides nanoparticles as ILs have high thermal and chemical stability (Liu et al., 2014a; Tian et al., 2014; Wang et al., 2015b; Cybińska et al., 2016; Sharma et al., 2020a; Muthulakshmi et al., 2020; Chouryal et al., 2021). For instance, Wang et al. have synthesized the luminescent Eu³⁺-doped LaF₃ and YF₃ nanoparticles using the amphiphilic diallyl dimethylammonium tetrafluoroborate ([DADMA][BF₄]) IL using the hydrothermal method (Wang et al., 2015a). Li and coworkers have employed a similar method to prepare the water-soluble and green luminescent LaF₃:Ce,Tb nanodisks with 25 nm size using 1-butyl-3-methylimidazolium tetrafluoroborate [C₄mim][BF₄] IL (Guo et al., 2010). Liu et al. have prepared the uniform LuF₃:Ln³⁺(Ln = Eu, Tb, Dy) nanocrystals using the 1-octyl-3-methylimidazolium hexafluorophosphate ([C₈mim][PF₆]) via the solvothermal method (Liu et al., 2014a). An IL-assisted hydrothermal method has been employed to prepare BaCaLu₂F₁₀:Ln³⁺ (Ln = Eu, Dy, Tb, Sm,Yb/Er, and Yb/Ho) sub-microspheres by Liu et al. In this synthesis method, 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄])IL is used (Liu et al., 2018). Another group has synthesized the CaF₂:Ce³⁺/Mn²⁺ sub-micro cubes and nanospheres using the 1-octyl-3-methylimidazolium hexafluorophosphate ([C₈mim][PF₆]) and 1-octyl-3-methylimidazolium tetrafluoroborate([C₈mim][BF₄]) ILs (Song et al., 2012). Yan et al. have synthesized the RE³⁺ (Eu³⁺, Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺)-doped NaYF₄ nanocrystals using the 1-chlorohexane-3-methylimidazolium chloride ([C₆mim][Cl]) IL-assisted hydrothermal method (Yan et al., 2014). Sharma et al. have prepared the BaF₂:Eu³⁺ and BaF₂:Ce³⁺ nanoparticles using the 1-ethyl-3-methylimidazolium bromide [C₂mim][Br] and 1-butyl-3-methylimidazolium tetrafluoroborate [C₄mim][BF₄], respectively, via the solvothermal method (Sharma et al., 2019; Sharma et al., 2020a). Several RE³⁺-doped binary and ternary fluorides nanoparticles have been synthesized by Ghosh and coworkers using the IL-assisted solvothermal method (Ghosh et al., 2011; Ghosh and Mudring, 2016; Ghosh et al., 2017). In addition, RE³⁺-doped phosphate, oxides, oxyfluoride, and borate nanoparticles were also synthesized using the hydrothermal method. For example, Zou et al. have prepared the Ca₅(PO₄)₃Cl:Ce³⁺,Tb³⁺ nanostructures with straw-like sheaves and microrod-like morphology through 1-octyl-3-methylimidazolium chloride ([C₈mim][Cl]) IL-based hydrothermal method (Zou et al., 2013). [C₄mim][BF₄] IL-based solvothermal method is employed for synthesizing the upconverted Yb/Tm co-doped Y₇O₆F₉ microparticles (Zhao et al., 2018). Cybinska et al. have prepared the YPO₄:Eu³⁺ nanoscale particles using the [Choline][H₂PO₄] IL-assisted hydrothermal method. During synthesis, the colloidal suspension was heated at different temperatures of 100, 150, and 200°C for 10 h to obtain nanoscale YPO₄:Eu³⁺ (Cybińska et al., 2016). Choline dihydrogenphosphate[Cholin][H₂PO₄] IL-assisted hydrothermal method was also employed by other groups to prepare the green-emitting Na₃Y_1-x(PO₄)₂:xTb³⁺ phosphors (Liu et al., 2020). γ-Gd₂S₃ nanoparticles are prepared using 1-ethyl-3-methylimidazolium ethyl sulfate ([C₂mim][EtSO₄]) IL-based hydrothermal method, which leads to nanoflower morphology (Khajuria et al., 2016). Tian et al. have reported the YBO₃:Eu³⁺ and NaY(MoO₄)₂:Tb³⁺ phosphors using the 1-methyl-3-octylimidazolium chloride ([C₈mim][Cl]) IL-based hydrothermal process (Tian et al., 2012; Tian et al., 2014).

(B) Microwave-assisted ionic liquid method: microwave- and IL(s)-based technologies are promising green, energy-efficient, and environmentally benign methods for synthesis of nanomaterials and they have gained tremendous attention in the last decade (Wang et al., 2019). ILs have potential to absorb microwave radiation efficiently due to high polarizability (consist of large ions) and conductivity. Therefore, the combination of IL and microwave in the synthesis is also called microwave-assisted ionic liquid synthesis (MAIL) (Wang et al., 2019). Several nanostructures, including metallic, semiconductors, and metal complexes, have been prepared using microwave-assisted ionic liquid synthesis (Wang et al., 2019). This synthesis method has also been extensively employed for preparing numerous Ln³⁺-doped nanomaterials. For example, Cybinska et al. have prepared the phosphate-based nanomaterials doped with Ln³⁺ ions such as Eu-doped YPO₄, GdPO₄, LaPO₄, and BiPO₄:Eu³⁺ using the choline or butylammonium dihydrogen phosphate ([Choline][H₂PO₄]) IL (Cybinska et al., 2011; Cybinska et al., 2016). In addition, other LnPO₄ (Ln = Y, La, Gd, doped with Eu³⁺ and Ln = Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, and Dy³⁺) nanoscale particles are also prepared using the microwave-assisted method in the presence of [Choline][H₂PO₄] IL (Cybinska et al., 2017). During the synthesis, [choline][H₂PO₄] IL is utilized to absorb the microwave radiation efficiently and serves as a reaction precusor, i.e., souce of PO₄ ³⁻ ions (Cybinska et al., 2011; Cybinska et al., 2016; Cybinska et al., 2017). Using the same synthesis method, fluoride nanophosphors such as GdF₃:Eu³⁺ are synthesized using the [choline][BF₄] IL. The as-prepared fluoride nanophosphors are further coated with GdPO₄ using the [choline][H₂PO₄] IL (choline = 2-hydroxyethyl trimethylammonium), leading to the formation of oxygen-free GdF₃:Eu³⁺@GdPO₄ (Cybińska et al., 2012) (Figure 6). Bühler et al.have synthesized the transparent and luminescent LaPO₄:Ce,Tb and LaPO₄:Eu nanophosphors using the tributyl methyl ammonium triflylimide ([MeBu₃N][(SO₂CF₃)₂N]) IL as a solvent and in a laboratory microwave oven (Bühler and Feldmann, 2006; Bühler and Feldmann, 2007). Besides, the microwave-assisted IL method is also beneficial for synthesizing Ln-doped binary and ternary fluorides nanoparticles. For instance, Ln³⁺ (Ln = Eu, Gd)-doped alkaline-based binary fluorides (BaF₂ and CaF₂) are synthesized by Mudring and coworkers using the [C₄mim][BF₄] IL (Lorbeer et al., 2014). Lobreer et al. have prepared the Dy and Tm co-doped LaF₃ nanophosphors using the [C₄mim][BF₄] IL (Lorbeer and Mudring, 2013a). Other nanofluorides, such as GdF₃: Eu³⁺, NaGdF₄:Er,Tb, Eu³⁺, and Gd³⁺ co-doped BaF₂, triply doped LaF₃:Ln (Ln = Tm³⁺, Tb³⁺ and Eu³⁺), and EuF₃ nanoparticles, have been synthesized by Lobreer et al. using various types ILs via the microwave-assisted method (Lorbeer et al., 2010; Lorbeer et al., 2011a; Lorbeer et al., 2011b; Lorbeer and Mudring, 2013a; Lorbeer and Mudring, 2013b; Lorbeer and Mudring, 2014). Tessitore et al. have used ethylene glycol and various ILs for synthesizing the sub-10 nm β-NaGdF₄:Yb³⁺,Er³⁺ nanoparticles (Tessitore et al., 2019). To determine the ascorbic acid, LaF₃:Ce,Tb nanoparticles are prepared by Xu and coworkers in the presence of [C₄mim][BF₄] IL via the microwave-assisted solvothermal method (Mi et al., 2013). Furthermore, other groups have also made a significant contribution to the synthesis of luminescent binary and ternary fluorides using this method (Chen et al., 2010; Li et al., 2011a; Zhao et al., 2015).

(C) Ionic liquid-assisted sonochemical method: the sonochemical method for synthesizing nanomaterials is one of the pivotal methods in which ultrasound is used to induce the chemical reaction. As a result, the physical and chemical properties of the prepared nanoscale particles can be tuned (Thompson and Doraiswamy, 1999). During the sonochemical method, high-energy bubbles form, which store an enormous amount of energy in them. When those energy bubbles are collapsed, high temperature (∼5000 K) and high pressure (∼1,000 bar) are generated for a very short time, which is enough to accelerate the chemical reactions to many folds (Li et al., 2021). Along with IL, this synthesis method is often considered a green method of nanoparticles synthesis. To date, the IL-assisted sonochemical method has been used for designing the numbers of Ln-based nanophosphors materials. For example, Zhu et al. have employed this synthesis method to prepare the hexagonal LaF₃:Tb³⁺ phosphors in the presence of [C₄mim][BF₄] IL and IL serve as co-solvent, capping reagent and fluoride source (Zhu et al., 2014). The small and hydrophilic LaF₃:Ce,Tb nanoparticles with a size of less than 10 nm are obtained using the one-pot sonochemical-assisted IL method. The [C₄mim][BF₄] IL was employed as a fluorinating agent and ethylene glycol as a solvent (Zhang et al., 2012). By using the same [C₄mim][BF₄] IL, another group has synthesized the uniform size nanodisks of CeF₃:Tb and the mean diameter and thickness of nanodisk were found to be 450 and 80 nm, respectively (Liu et al., 2014b). In addition to Ln-doped fluoride nanomaterials, the sonochemical-assisted IL method is also employed to prepare Ln-doped oxide nanomaterials. For preparing the Ln₂O₃:Eu³⁺ (Ln = Y, La, Gd), 1-butyl-3-methylimidazolium bistrifluoromethanesulfonyl amide ([C₄mim][Tf₂N]) IL was used. First, Ln(OH)₃:Eu (Ln: Gd, La, Y) nanoparticles were found and then as-prepared nanoparticles are calcined at 800°C for 3 h to turn into Ln₂O₃:Eu³⁺ (Alammar et al., 2016).

(D) Other methods: in addition to previously discussed methods, additional methods have been reported to prepare the Ln-doped nanoparticles. In those methods, one or two methods have been employed to prepare the nanoparticles. IL-assisted microemulsion method is used for preparing the LaPO₄:Eu and CePO₄:Tb nanocrystals. The microemulsion of TBP/[C₈mim]Cl/H₂O was designed by properly mixing the tributylphosphate, 1-octyl-3-methylimidazolium chloride, and water. The tributylphosphate and [C₈mim][Cl] IL were used in synthesis to control the nucleation and growth of the nanocrystals (Zhang et al., 2009). IL ([A336][cyanex272]) extraction method was employed to synthesize the LnPO₄ (Ln = La–Gd) nanorods and also luminescence behaviors of CePO₄:Tb nanorods were studied.

The role of IL was used to extract the LnPO₄ into the organic phase from the aqueous phase via capping it (Zhang and Chen, 2019). Another important method in which oleic acid/ionic liquid (OA/IL, IL = [C₄mim][BF₄] or [C₄mim]PF₆]) two-phase system was utilized not only for synthesis but also for controlling the phase, size, and morphology of as-prepared Ln-doped ternary fluorides (NaYbF₄:Gd,Tm; NaYF₄:Yb,Er; NaYbF₄:Er; NaGdF₄:Yb, Er; NaGdF₄:Yb, Ho; NaGdF₄:Yb, Tm) (He et al., 2011a; He et al., 2011b; He et al., 2012; Pan et al., 2013). The one-step electrodeposition in IL (1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [Py_1,4][TFSI]) method is employed for synthesizing the luminescent silicon–terbium nanowires by Thomas et al. (2020). Another group has prepared the luminescent LaPO₄:Ce,Tb phosphors using IL-driven liquid membrane system. The supported membrane was prepared by mixing the hydrophobic porous polyvinylidene fluoride film (HVHP) and IL ([C₄mim][BF₄] or [C₄mim][Tf₂N]) and the effect of ILs on the release of PO₄ ^3- ion to form the LaPO₄:Ce,Tb phosphor is also studied. IL-assisted sol-gel and leaves extract-based methods have been used for the synthesis of ZnO:Ce nanophosphors and Ln₂O₃ (Ln = La, Nd, Yb, Sm), respectively (Jiang et al., 2012; Muthulakshmi et al., 2020; Muthulakshmi and Sundrarajan, 2020; Veerasingam et al., 2020; Sundrarajan and Muthulakshmi, 2021). In the leaves extract-based synthesis of Ln₂O₃nanoparticles, Andrographis paniculata leaves extract is employed to prepare the Nd₂O₃, La₂O₃ and Sm₂O₃ nanoparticles using [C₄mim][PF₆]IL-assisted hydrothermal method (Muthulakshmi et al., 2020; Veerasingam et al., 2020; Sundrarajan and Muthulakshmi, 2021). In another synthesis, Couroupita guianensis Abul. leave extract is used for the synthesis of Yb₂O₃ nanoparticles using the [C₄mim][BF₄] IL-assisted hydrothermal method (Muthulakshmi and Sundrarajan, 2020).

In the beginning, ILs were used as a reaction medium for synthesizing the inorganic nanomaterials, especially lanthanide-based nanomaterials. Distinctive features of ILs, like high thermal stability, negligible vapor pressure, broad liquidus range, and most importantly adjustable properties, make them far better than conventional molecular liquids (Veselý et al., 1988; Diedenhofen et al., 2007; Ludwig and Kragl, 2007; Wang et al., 2007; Faridbod et al., 2011; Shukla and Sah, 2013). The negligible vapor pressure of ILs can be estimated by the enthalpy of vaporization, and its magnitude for ILs is much higher than the conventional liquids (Veselý et al., 1988; Ludwig and Kragl, 2007). These features of ILs increase their potential as a reaction medium.

Another important feature of ILs is their use as reaction partners or precursors. This characteristic of ILs makes them much superior to other traditional molecular solvents. For this, all credit is given to the tunable properties of ILs, especially anion counterions. By varying the anion part of ILs, different reaction partners can be designed according to the desired product (He et al., 2011a; Li et al., 2011a; He et al., 2011b; Song et al., 2012; Cybinska et al., 2016; Cybińska et al., 2016; Liu et al., 2018).

The optical properties of Ln³⁺-doped nanoparticles are considerably influenced by ILs and other surface active agents. There are various factors such as concentration of IL, physicochemical properties of ILs like viscosity, the ratio of IL/RE³⁺ ions, which affect the crystal phase and morphology and subsequently optical properties of particles. For instance, Chen et al. have studied the effect of morphologies obtained by employing the different ILs ([C₄mim][BF₄] and [C₄mim][PF₆]) on the upconversion emission of α-NaYF₄:20%Yb³⁺, 2%Er³⁺, and α-NaYF₄:20%Yb³⁺, 0.2%Tm³⁺ nanoparticles and nanoclusters (Chen et al., 2010). They noticed that upconversion emission intensity of nanoclusters obtained by utilizing the [C₄mim][BF₄] IL was found to be nearly 8 times more intense than the emission intensity of nanoparticles obtained in the case of [C₄mim][PF₆]. This is due to the significant decrease of surface defects for nanoclusters compared to the nanoparticles (Chen et al., 2010). Zhao et al. have illustrated the effect of IL on the luminescence behavior of Y₂O₃:Yb³⁺/Tm³⁺. In this study, the upconversion emission efficiency of Y₇O₆F₉:Yb³⁺/Tm³⁺ microspheres synthesized using the high concentration of [C₄mim][BF₄] IL is much stronger than that of Y₂O₃:Yb³⁺/Tm³⁺ microspheres synthesized without IL (Zhao et al., 2018). The molar ratio of [C₄mim][BF₄]/Ln³⁺ also brings about a good effect on the emission of YF₃ doped with Eu (5%) (Zhong et al., 2009). The study suggests that by increasing the molar ratio up to 10.75, the emission intensity of (⁵D₀-⁷F₁) transition centered at 594 nm of Eu³⁺ ions is enhanced due to an increase in the crystallinity of nanoparticles. However, the formation of mixed-phase Eu-doped YF₃ was obtained when the molar ratio of [C₄mim][BF₄]/Ln³⁺ increased to 1:1. By further increasing the molar ratio up to 2:1 and 4:1, emission intensity is again enhanced, which could be attributed to the formation of single-phase Eu-doped YF₃ (Zhong et al., 2009). In another study, morphology-dependent luminescence behavior of Ca₅(PO₄)₃Cl:Ce³⁺,Tb³⁺ nanostructures has been illustrated (Zou et al., 2013). It has been reported that upon increasing the [C₈mim]Cl IL concentration, sheave- and microrod-like morphologies are observed. To understand the effect of different morphologies on the luminescence pattern, the authors have found that luminescence intensity of Ca₅(PO₄)₃Cl:Ce³⁺,Tb³⁺ microrods is stronger than that of Ca₅(PO₄)₃Cl:Ce³⁺,Tb³⁺ sheaves on exciting at 299 nm (Zou et al., 2013).

Recently, environmentally benign white light-emitting nanophosphors have drawn considerable attention for reducing power energy consumption. Today, we have almost replaced normal incandescent lamps with compact fluorescent lamps (CFLs) or light-emitting diodes (LEDs) as both the CFLs and LEDs consume less energy than the incandescent lamps. However, like others, CFLs also use Hg (mercury) as a discharge medium, which has both environmental and health issues. Two approaches are currently being used to obtain white light: one using the combination of blue, green, and red LEDs, and another is phosphor-converted LEDs (pc-LEDs), which very much resemble the CFLs (Sharma et al., 2017a). However, LEDs and CFLs have a number of problems due to their complex construction protocols. In addition, the extreme level of purity required for phosphor materials and dimming reduces their applications to a large extent (Sharma et al., 2017a). Another important problem concerned with the generation of white light is using the combination of three different colors of light, such as blue, green, and red sources, but each component has a different lifetime. Sometimes, emitted blue light is reabsorbed by the green- and red-emitting phosphors (Gao et al., 2011). In this regard, RE³⁺-doped nanophosphors have shown interesting features, which make them more suitable candidates for this application. In addition, white light-emitting materials can also be synthesized by judiciously doping different RE³⁺ ions in the single host matrix. For instance, Mudring and coworkers have developed the white light-emitting nanophosphors simply by doping with 1% Eu, 1% Tb, and 1% Tm in LaF₃ nanoparticles (Lorbeer and Mudring, 2013b). Upon exciting at λ_ex = 355 nm, white emission was obtained, which was close to the standard D65 daylight. It is believed that doping of more than one optically active dopant ion in a host matrix results in concentration-dependent quenching of emission light (Gao et al., 2011). This problem was solved by simply doping two RE³⁺ ions such as Dy³⁺ and Tm³⁺ in host materials, and in this case, white light was produced by combining their emitted complementary colors (Lorbeer and Mudring, 2013a).

A solar cell is an important device to convert incident solar energy to useful electrical or other forms of energy. However, the major demerit of this device is the limiting bandgap. If the incident energy would be less than the bandgap of the solar device, that energy is lost, which is also known as sub-bandgap losses (Goldschmidt and Fischer, 2015; Sharma et al., 2017a). This sub-bandgap loss varies for different materials utilized in solar cell applications. For instance, more than 19% sub-bandgap loss is obtained for silicon-based solar cells (Goldschmidt and Fischer, 2015; Sharma et al., 2017a). On the other hand, in the case of gallium arsenide thin or perovskite-based solar cells, this sub-bandgap loss is reported to be from 30 to 50%, which is attributed to the associated bandgaps (Goldschmidt and Fischer, 2015; Sharma et al., 2017a). Sub-bandgap loss of solar cells can be overcome by using upconversion materials. Upconverting materials can be applied with the aforementioned solar devices to absorb the sub-bandgap lost energy. It is well known that upconverting nanomaterials are more susceptible to absorb and get excited by absorbing the NIR region of light. It has already been established that the crystal field of host material about the lanthanide ion is further split into ^2S+1L_J levels into the crystal field component (Goldschmidt and Fischer, 2015; Sharma et al., 2017a). Due to this splitting, a broadening in the energy level of ^2S+1L_J occurred, which means it has a broad absorption spectrum, which is an important prerequisite for photovoltaic applications (Goldschmidt and Fischer, 2015). However, the upconversion process using the non-coherent sources like the Sun is becoming more complicated than when using a coherent source due to their multistep process. Trupke and Green have proposed the theoretical relation of upconversion with the photovoltaic devices and stated that the maximum power conversion efficiency can be achieved up to 47.6% using the ideal upconverting nanomaterials on the rear side of the solar cell with the bandgap of 2eV under the non-coherent sunlight (Figure 8) (Trupke et al., 2002a). The most frequently studied RE-doped upconverting nanomaterials for solar cell applications are ternary rare-earth fluorides (NaREF₄, especially in β-phase), rare-earth oxides (RE₂O₃), and oxysulphides (RE₂O₂S) and glass and ceramic materials. Shalav et al. (2005) have employed the NaYF₄:Er³⁺ (20%) upconverting phosphors for enhancing the internal quantum efficiency of silicon solar cells up to 3.8% (Shalav et al., 2005). In addition, Trupke et al. have shown that downconverting materials can also be utilized to enhance the solar cell efficiency by absorbing the high-energy photons, which have energy twice the bandgap of solar cells (Figure 8) (Trupke et al., 2002b). To improve the solar cell efficiency with a bandgap of about 1.1 eV, luminescence downconverter materials (i.e., RE³⁺-doped and quantum well heterostructures) with intermediate or interband transition can play a crucial role too (Trupke et al., 2002b). For example, the upper limit of 39.63% for Eg =1.05 eV was calculated in which the luminescence converter had one intermediate level. Additionally, when applying the downconverter on the front surface, solar cell efficiency 38.6% for Eg = 1.1 eV was achieved over the conventional solar cell for which 30.9% efficiency was reported (Trupke et al., 2002b).

Furthermore, other rare-earth containing materials are also being used for increasing the solar cell efficiency like metal-organic framework with RE³⁺ ions, dye-sensitized solar cells with rare-earth materials, and perovskite materials doped with luminescent RE ions (Goldschmidt and Fischer, 2015; de la Mora et al., 2017; Zhou et al., 2017; Yu et al., 2018).

Environmentally benign lighting and quantum cutting nanomaterials: almost 19% of the total energy is consumed for lighting worldwide.

To fulfill the demand of energy requirement, especially in both developed and developing countries, is a great challenge due to limited conventional energy resources. Currently, Hg-based compact fluorescent lamps are used in place of traditional incandescent lamps, which have numerous issues such as slow start-up time, environmental issues, and hazardous effects on human health and disposal problems at the end. Nowadays, it can be envisaged that noble elements like xenon, which is non-toxic, can be used in CFLs as discharge media in lieu of Hg, though Xe also has its own limitations like less discharge efficiency than mercury. In order to surmount these problems, rare-earth-doped quantum cutting phosphor nanomaterials can be employed, which can convert the UV or VUV region of light into visible light (Lorbeer et al., 2011a; Ghosh et al., 2011; Ghosh and Mudring, 2016; Chouryal et al., 2021). Mudring and coworkers have shown that using the quantum cutting nanomaterials, quantum efficiency can be achieved approximately up to 200%, which is very close to the maximum possible theoretical limit (Lorbeer et al., 2011a; Ghosh et al., 2011). Ghosh et al. have reported the crystal phase-dependent quantum cutting efficiency for NaGdF₄:Eu³⁺ nanoparticles is 154 and 107% for hexagonal and cubic phases, respectively (Ghosh and Mudring, 2016).

Moreover, Chouryal et al. have also studied temperature-dependent quantum cutting behavior of as-prepared BaGdF₅:Eu³⁺ nanoparticles (Figure 9). At room temperature, 123 and 160% quantum cutting efficacies are observed for nanoparticles that are synthesized without (BG1) and with (BG2) IL, respectively. However, at a low temperature of about 10 K, no quantum cutting is observed due to the presence of inherent Eu²⁺ ions along with Eu³⁺ ions (Chouryal et al., 2021).

Bioimaging is a powerful tool for biomedical research and clinical diagnostics applications. The prerequisite condition for bioimaging is the judicious selection of luminescent nanomaterials (Gai et al., 2014). It has been reported that several issues are associated with the application of nanoparticles that are excited by high-energy UV light due to the photodamage of living organisms and less tissue penetration depths. Due to these reasons, Ln³⁺-based downconverting nanomaterials are seldomly used for biomedical applications (Gai et al., 2014).

On the other hand, upconverting nanomaterials, especially those emitted in the range of 700–1,000 nm, have drawn tremendous attention for biomedical and clinical diagnostics applications due to low-energy excitation wavelength and high tissue penetration depth (about 1 cm) (Gai et al., 2014). Using the upconverting nanoparticles, various imaging techniques such as upconversion imaging, tumor targeting and imaging, and multimodal imaging have been developed (Gai et al., 2014). For example, water-soluble NaGdF₄:Yb,Er upconverting nanoparticles have been used for in vivo dual-modality imaging in which these nanoparticles were employed for upconversion imaging and as a contrast agent (He et al., 2011a). Previously, it was found that Ln (Ln³⁺ = Yb, Gd) element having a large atomic number can be utilized as a contrast agent because these elements exhibit a high X-ray absorption coefficient (e.g., Yb, 3.88 cm² g⁻¹; Gd, 3.11 cm² g⁻¹ at 100 keV). (Gai et al., 2014).

Therefore, NaGdF₄:20%Yb, 2%Er nanoparticles were injected into the body of nude mice with a concentration of 100 µL 1 mg ml⁻¹ per animal. Upon irradiating the mice using the infrared laser at 980 nm, a prominent upconversion signal from the subcutaneous site was observed, whereas such signal was not found in the control mice (Figure 10A) (He et al., 2011a). Furthermore, X-ray attenuation was measured in the nude mice, and in the presence of Gd element, higher attenuation coefficient was observed due to high atomic number and electron density (Figure 10B) (Popovtzer et al., 2008). The prerequisite condition that makes them an effective contrast agent in the X-ray-based computed tomography (CT) has to be prolonged presence in the blood vessels (Janib et al., 2010). Therefore, upconverting nanoparticles have exhibited superior potential for CT imaging techniques to conventional contrast agents. Figure 11 depicts that the CT image of upconversion nanoparticles increased with increasing the mass concentration of nanoparticles. Similarly, attenuation value (HU) is also gradually increased with the concentration of Ln-doped NaGdF₄ upconverting nanoparticles from 0.5 to 10 mg ml⁻¹.

Recently, nanoparticles are substantially used in nanotechnology and sometimes they are directly discarded into water bodies, leading to the occurrence of water pollution. This pollution affects the aquatic systems and has hazardous effects on human beings. In addition, today, we are frequently using rare-earth-doped nanoparticles for different kinds of bioimaging, drug delivery, and other purposes. Thereby, it is very important to know the safer doses of rare-earth-doped nanoparticles used for human beings. In order to understand the toxicity effect of as-prepared RE³⁺-doped nanoparticles on the aquatic living system, zebrafish as a model is chosen (Sharma et al., 2020b). Sharma et al. have studied the effect of as-prepared RE³⁺-doped nanoparticles on the developing zebrafish larva using bright-field and birefringence imaging techniques. It is noticed that no deformation in skeletal muscles, yolk sac, yolk tube, and pericardial area of zebrafish is noticed when the developing zebrafish larvae were grown in a medium containing 70 mg L⁻¹ as-prepared BaF₂, BaF₂:Ce³⁺/Tb³⁺, BaF₂:Ce³⁺/Tb³⁺@SiO₂, and BaF₂:Eu³⁺nanoparticles. However, as the developing zebrafish larvae were kept in the medium of 0.1 mg L⁻¹ cypermethrin pesticide, bending of the tail occurred due to deformation in skeletal muscles (Figure 12). (Sharma et al., 2020b).

In addition to the deformation in the skeletal muscles of the tail region (bright-field images, Figure 13 A), the effect of different concentrations of BaF₂ (NP1) and B) BaF₂:Ce³⁺/Tb³⁺ (NP2) nanoparticles on the craniofacial region of developing zebrafish larvae is also studied (Figure 13B). When developing zebrafish larvae were fixed and stained with alcian blue, normal craniofacial development of larvae is found in both the control and treated larvae with less (10 mgL⁻¹) concentration of NP1 and NP2. Also, developed Meckel’s and ceratohyal cartilages are observed and analyzed (Figure 13B). Whereas by treating the larvae with high concentrations (150 mgL⁻¹) of NP1 and NP2 nanoparticles, defects in the craniofacial region are observed, at low concentrations (10 mgL⁻¹), such defects are not found. This study highlighted the required doses of nanoparticles for safer bioimaging applications (Chouryal et al., 2020).

Functionalized Ln(s)-doped nanophosphors materials can also be utilized to sense bio-active molecules (Ju et al., 2013; Mi et al., 2013). In order to detect the presence of biomolecule in the solution, selectively biofunctionalizing the nanophosphors with suitable molecules like biotin is needed (Ju et al., 2013). Then, biotinylated nanophosphors can be used as a biosensor to detect the bio-molecules. The hydrophobic group containing the surface of nanophosphors is first functionalized with appropriate amphiphilic polymer in order to endow the surface with functional groups such as –COOH, –NH₂, and –SH. As a result, the biocompatibility of nanophosphors is extensively improved for biosensing applications.

For example, the surface of [P₆₆₆₁₄]⁺-capped NaGdF₄:Ce, Tb nanocrystals was further modified with amphiphilic polymer ODA-PAA (octadecylamine modified polyacrylic acid) (Ju et al., 2013). Thereafter, the modified nanocrystals were biotinylated to detect the targeted molecule, i.e., FITC- (fluorescein isothiocyanate-) labeled avidin through a time-resolved fluorescence resonance energy (TR-FRET), as shown in Figure 14A. In the presence of biotinylated nanocrystals, emission of FITC was significantly increased due to the matching of emission band of Tb³⁺ at 488 nm with the absorption spectrum of FITC. Because of this interaction, when the concentration (nM) of FITC-labeled avidin into the system was increased, the emission intensity of Tb³⁺ (peak centered at 488 nm) was gradually found to be decreased (Figure 14B) (Ju et al., 2013) Additionally, LaF₃:Ce,Tb nanoparticles were employed to detect the ascorbic acid in the range of 8.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹. In the presence of ascorbic acid, the luminescence intensity of Tb³⁺ was quenched (Mi et al., 2013).

In the last decade, RE-doped luminescent nanomaterials have been shown to be an excellent candidate for optical sensing applications like nanothermometry (sensing of alternation in temperature) and nanomanometry (sensing of pressure variation) (Runowski, 2020; Jaque and Vetrone, 2012). Interestingly, RE^+2/+3 –doped nanomaterials show numerous pressure- and temperature-dependent changes in the spectral patterns, such as band ratio, spectral shift, intensity, bandwidth, and lifetimes. In the case of nanomanometry, when high pressure is applied to the materials, crystal lattice parameters, like bond length, unit cell volume, and symmetry of the local environment, are decreased due to compression. This leads to a change in luminescent patterns of the materials. Moreover, as the emission intensity of materials is highly sensitive to change in lattice parameters of the crystal, it is usually decreased with the pressure (Runowski, 2020; Jaque and Vetrone, 2012). In the case of nanothermometry, it is the remote, contactless, and high-resolution technique in which minute alternation in temperature brings about significant changes in the luminescence behavior of RE³⁺-doped nanomaterials. This technique is utilized for the optical sensing of temperature variation in extreme range (about ∼100 and ∼900 K) and can also be used to sense the changes in the biological range of temperature (∼290–330 K). For this application, several RE^+2/+3 ions have been employed as dopant ions such as Pr³⁺, Nd³⁺, Yb/Tm³⁺, Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺, Eu²⁺, Eu³⁺, Sm³⁺, Tb³⁺, and Er³⁺. The credit of thermal sensing by RE³⁺-doped (RE³⁺ = Nd³⁺, Er³⁺ or Tm³⁺) nanomaterials is given to the availability of the thermally coupled levels for which energy levels are separated in the range of 200–2000 cm⁻¹ (Runowski, 2020; Jaque and Vetrone, 2012). For example, Ximendes et al. have applied the upconversion Er-Yb@Yb-Tm LaF₃ core-shell nanoparticles for determining the properties of intrinsic subcutaneous tissues. This core-shell structure is used as an infrared luminescent nanothermometer to get insight into the heating and cooling effect on the luminescence of nanoparticles in the biological window and in this case, thermal sensitivities are obtained about 5% K⁻¹ (Ximendes et al., 2017). Runowski et al. have prepared the optical vacuum sensor of upconverting materials YVO₄:Yb³⁺,Er³⁺. In this study, they have depicted that the temperature-dependent emission band intensity ratio (525/550) of Er³⁺ TCLs can be used to get insight into local temperature. It has also been noticed that the laser-induced heating of the sample is significantly enhanced by 20 times in the vacuum. In other words, this luminescent material can be used for sensing the ultralow pressure even in the vacuum range (Figure 15). (Runowski et al., 2020b)

The upconverting NaGdF₄:Yb³⁺(18%),Er³⁺(2%) microcrystals have been employed to study the luminescence thermometry in the range from ultralow (4 K) to room temperature (290 K). In this study, thermally coupled levels of Er³⁺ ions are used for ratiometric sensing from room temperature to 140 K (Mukhuti et al., 2020). Runowski et al. have prepared the Yb³⁺ and Tm³⁺ co-doped LaPO₄ and YPO₄ nanomaterials as a multifunctional optical sensor. These nanomaterials are employed as nanothermometers and nanomanometers to investigate the influence of temperature and pressure alterations on the emission of Tm³⁺ because of having large energy difference (1800 cm⁻¹) between the thermalized states of Tm³⁺ which are highly sensitive to alterations of temperature (Runowski et al., 2018). As a result, Tm³⁺ band ratio of thermally induced transitions (³F_2,3→³H₆/³H₄→³H₆) is varied with temperature, which can explicitly be found in the emission spectra. They studied the effect of high pressure in the wide range (0.42–25.03 GPa for LaPO₄:Yb,Tm; 0.64–24.35 GPa, YPO₄:Yb,Tm) and temperature changes in the range of (293–773 K) on the luminescence and decay time of nanomaterials (Runowski et al., 2018).