Luminescent Rare-earth-based Nanoparticles: A Summarized Overview of their Synthesis, Functionalization, and Applications

Rare-earth-based nanoparticles are currently attracting wide research interest in material science, physics, chemistry, medicine, and biology due to their optical properties, their stability, and novel applications. We present in this review a summarized overview of the general and recent developments in their synthesis and functionalization. Their luminescent properties are also discussed, including the latest advances in the enhancement of their emission luminescence. Some of their more relevant and novel biomedical, analytical, and optoelectronic applications are also commented on.


Introduction, chemical composition, luminescent properties.
Rare earth (RE)-based nanoparticles (NPs) constitute one type of luminescent materials available in the literature. RE-based nanophosphors exhibit important advantages compared with the other available luminescent materials due to their lower toxicity, photostability, high thermal and chemical stability, high luminescence quantum yield, and sharp emission bands [1]. These nanophosphors usually consist of a host inorganic matrix doped with luminescent lanthanide (Ln) cations. The final characteristic and properties of the nanophosphors are highly influenced by both the inorganic matrix and the dopant. Fluoride matrices are used due to their low vibrational energies, which minimize the quenching of the exited state of the Ln cations and result in a higher quantum efficiency of luminescence [2][3][4]. Phosphate-based matrices attract interest for their high biocompatibility and good biodegradability [5]. Other matrices such as vanadates, molybdates, and wolframates are used to enhance the global luminescent emission of the materials [6][7], and some silicate-based matrices are appropriate for the production of persistent luminescent nanoparticles [8][9]. The election of the Ln cation or cations determines the final luminescent properties of the material. Luminescence is expected for most of the Ln 3+ cations, but in practice most of the studies are focused on Eu 3+ , Tb 3+ /Ce 3+ , Dy 3+ , and Nd 3+ cations, which produce red, green, yellow/orange luminescence, and near infrared luminescence, respectively [10][11][12][13]. These cations are examples of the so-called downconversion (DC) luminescence (i.e. conventional Stokes type), in which higher energy photons are converted into lower energy photons. High research attention is attracted by upconverting nanoparticles (UCNPs), in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength (i.e. anti-Stokes type emission), which means that near infrared long-wavelength excitation radiation is converted into shorter visible wavelengths [14]. Er 3+ , Tm 3+ , and Ho 3+ codoped with Yb 3+ -, which acts as sensitizer, are commonly used as upconverting luminescent cations pairs [15]. The main disadvantage of Ln-doped NPs is their relatively low global intensity luminescence, caused by the low absorptions of the parity forbidden Ln 3+ 4f-4f transitions, and constituting a serious limitation for their use for different applications [16]. Different energy transfers schemes from the host materials to the Ln cations are employed to enhance the global luminescence of downconverting lanthanide-doped NPs, which include the use of vanadate or oxyfluoride matrices [17][18][19][20][21]. Despite its highly scientific interest, even lower upconversion efficiencies are normally observed for UCNPs [22][23]. Core/shell nanostructures, which minimize the surface quenching effects [24][25][26][27], as well as the association with organic near infrared (NIR) dyes, which can alleviate the inherently weak and narrow near-infrared absorption of the Ln ions [28], are used to enhance the luminescence of such materials. However, laser excitation sources are still required to study these particles.
In this article the more recent and common synthetic methods of luminescent NPs based on RE will be briefly summarized, as well as the different existing functionalization strategies. Some of their imaging, sensing, and optoelectronic applications will also be mentioned. For a deeper and more detailed description some excellent reviews can be found in the recent literature [15,[29][30][31][32][33].

Synthesis of uniform luminescent nanoparticles
Thermal decomposition [34], coprecipitation [2, 35], cation exchange [36], and hydro(solvo)thermal synthesis have become popular routes for the preparation of monodisperse Ln-doped luminescent NPs. Among the latters, a general synthesis strategy based on a phase transfer and separation mechanism occurring at the interfaces of the liquid, solid and solution (LSS) phases during the synthesis normally produces small Ln-doped inorganic nanoparticles with a narrow particle size distribution, a high luminescence efficiency, and a high phase purity of the particles [37]. These syntheses are carried out in organic solvents (such as oleic or linoleic acids, ethanol, octadecene, eicosene, trioctylamine) in presence of additives such as sodium oleate, sodium linoleate, trioctylphosphine oxide (TOPO), and stearic acid at high temperatures (200 -400 ºC) [38][39][40]. However, the hydrophobic nature of the resulting NPs requires a further step of surface modification to make them water-dispersible. Water dispersible NPs with controlled size and shape can be synthesised by recently reported methods based on homogeneous precipitation in polyol-based solvents at moderate temperatures (120-180 ºC). These strategies include an optimization of the different reaction parameters, such as solvents (including mixtures of them), precursors, concentrations, temperature and presence of additives. Many different luminescent and uniform Ln-doped inorganic NPs, including fluorides [41][42][43][44], phosphates [45][46][47][48], and vanadates [19,21] have been reported. Microwaveassisted methods in both water and polyol-based solvents have also been described, resulting in much shorter reaction times [49][50]. Some examples of monodisperse Ln-doped NPs are shown in Figure 1. Taken from [40]. B: Eu 3+ -doped GdPO4 nanocubes synthesised in butylene glycol at 120 ºC, taken from [45]. C: Eu 3+ -doped α-BiOyF3−2y NPs with octahedrical morphology synthesised in diethyleneglycol-water at 120 ºC. Taken from [18]. D: Dy 3+ -doped GdPO4 particles with a lance-shaped morphology synthesised in ethylene glycol-water at 180 ºC. Taken from [46]. F: Eu 3+ -doped BiPO4 nanostars synthesised in ethylene glycol-water in presence od sodium citrate at 120 ºC.
Recently, laser ablation of micrometric sized powder Ln-doped particles has been used to produce Ln-doped NPs with a great control of their size and monodispersity [51].

Functionalization and colloidal stability
A functionalization process is especially required for the biomedical use of NPs, and it is mandatory for non water-dispersible nanoparticles. Functionalization not only increases the colloidal stability of the NPs by introducing electrostatic and/or steric repulsions [52], but also provides anchors for adding functional ligands of biomedical interest such as antibodies, peptides, proteins, and some anticancer drugs [53]. Ligand exchange, polymer encapsulation and silica encapsulation are common strategies used for the stabilization in water of native hydrophobic NPs.
In the ligand exchange method, the original hydrophobic ligands are completely displaced by hydrophilic ligands (i.e. PEG-type and polymeric ligands, and anions such as citrate and BF4 -) on the NP surface [54][55]. Ligand exchange methods typically offer NPs with smaller hydrodynamic diameters but suffer (for most NP materials) from limited colloidal stability. Polymer coating yields NPs that are very colloidally stable, but they normally show larger hydrodynamic radii [56]. In this strategy, the hydrophobically capped NPs are overcoated with amphiphilic polymers such as poly(isobutylene-alt-maleic anhydride) modified with dodecylamine (PMA), and its modifications with 4-(aminomethyl)pyridine (Py-PMA), and polyelthylene glycol (PEG-PMA) [54,[57][58][59]. The hydrophobic portion of the polymer intercalates with the hydrophobic ligands on the NP surface leaving the hydrophilic portion of the polymer exposed to solution [56]. Treatments with acids [60] or with excess of ethanol under ultrasonication [61] have also been used to remove the hydrophobic organic coating of the nanoparticles, and the -oleic acid ligands on the nanoparticles can be oxidized with the Lemieux-von Rudloff reagent, yielding water-dispersible carboxylic acid-functionalized NPs [62]. The more convenient strategy of functionalization of hydrophilic Ln-doped NPs is the so-called one-pot synthesis, in which the functionalising agent acts as additive during the synthesis process. In some cases, their presence plays also a key role in the final morphology of the particles [49]. One-pot synthesis of luminescent Lndoped NPs with aminocaproic and citric acid [63], poly-ethylenimine (PEI) [64] and poly acrylic acid (PAA) [19,21,43] have been recently reported in the literature.
Functionalization of Ln-doped NPs can also be carried out in a second step with agents such as and dextran-based polymers [19,42]. The Layer-by-layer (LbL) approach, which is based on the electrostatic deposition of layers of polyelectrolytes with alternating charge on the surface of the particles [65], has also been used for the functionalization of RE fluoride [54,66] and vanadate NPs [67]. However, NPs functionalised in a second step normally show a worse colloidal stability, when compared with the one-pot synthesised [67]. Silica-shell encapsulation (i.e. the growth of a silica shell around the NP) is used to functionalize both hydrophobic and hydrophilic NPs [68]. The reverse microemulsion method can be applied for hydrophobic nanoparticles [69], whereas the standard Stöber procedure is used for hydrophilic NPs, which in some cases have however to be previously stabilized with agents such as polyvinylpyrrolidone (PVP) of PEG-based ligands [70][71]. This functionalization process shows some advantages, given the SiO2 high biocompatibility and possible further surface chemistry, which can be used to link different molecules of biomedical interest. A summary of some possible functionalization strategies is shown in Figure 2.

Bioimaging applications
Both downconverting and upconverting Ln-doped NPs can be used for bioimaging applications, although their use is highly limited by their low global luminescent emission. For the DC nanophosphors, the direct excitation of the Ln 3+ cations (which normally consists of narrow and low absorbance bands, the more intense occurring at 393 nm for Eu 3+ , 349 and 366 for Tb 3+ , and 349 for Dy 3+ ) is normally not enough to produce intense luminescence. As mentioned above, this can be overcome though an indirect excitation through the matrix, but still ultraviolet excitations radiations are required, which in somehow can be harmful for the cells. since the use of near infrared light for excitation avoids photodamage, background fluorescence in biological systems and enables a higher penetration depth into biological tissue [33]. UCNPs do not show intermittent emission (blinking) upon continuous excitation, and they can be even used for long--term imaging due to their photostability [72]. These entire features make them highly attractive for bioimaging applications in both in vitro and in vivo [73][74][75][76][77] (Figure 4).

Sensing and analytical applications.
Application of Ln-doped NPs for sensing can be roughly divided into two classes: one is the directly observed luminescence from the Ln-doped NPs, and the other is based on fluorescence resonance energy transfer (FRET). An important feature of Ln-doped NPs for using their direct intrinsic fluorescence is that they present multiple emission lines, which allows ratiometric measurements because normally some of them are analytically sensitive, while others are insensitive and serve as reference signal [78]. Ln-doped NPs can also be coupled with organic fluorophores, metallic nanoparticles or quantum dots for FRET-based sensing approaches, where Ln-doped NPs are typically the donor unit. For providing selective detection towards a specific analyte (e.g. biomolecules, ions, gas molecules), the NPs have to be functionalized with suitable groups/motifs that have a recognition capability of the target analyte. For example a single-stranded DNA has been used as Hg 2+capturing element in the development of a method for determining Hg 2+ ions based on a FRET mechanism between Tm 3+ , Yb 3+ -doped NaYF4 UCNPs as energy donor and a DNA intercalating dye (SYBR Green I) as energy acceptor [79]. As the SYBR has a strong absorbance overlapping with the blue emission of the UCNPs, in presence of Hg 2+ ions there is a simultaneous decrease of the blue emission of the UCNPs and an increase of SYBR green emission. By monitoring the ratio of the acceptor emission to the donor emission, the Hg 2+ ion can be detected at levels as low as 0.06 nM. This system allows not only determining the concentration of Hg 2+ but also monitoring changes in the distribution of Hg 2+ in living cells by upconversion luminescence bioimaging.
Using Au NPs as acceptor instead of a fluorophore, the detection of trace amounts of avidin has been reported. In this system the selective and sensitive avidin-biotin interaction is the responsible for bringing together the avidin-modified Er 3+ , Yb 3+doped NaYF4 NPs used as donor and the biotinylated-Au NPs, whose strong absorption at ~541 nm matches well with the green emission of the Ln-doped NPs, and therefore an effective FRET process occurs [80]. An important advantage of this approach is its potential to be extended to wherever the avidin-biotin system, for example to study protein-proteins interactions, ligand-receptor interactions, the formation of DNA duplexes and so on.
Looking for a higher efficiency of the FRET process, and thus a higher sensitivity of  Semiconductor QDs have also been combined with UPCNPs in FRET configurations. The superiority of QDs as acceptors is owing to the fact that they have broad excitation bands and size-tunable emission wavelength, and thus the upconversion wavelength of UCNP-QD couple may be continuously adjusted.
Combining Tm 3+ , Yb 3+ -doped NaYF4 NPs as the energy donor and the CdTe QDs as the energy acceptor, the determination of lead ions in human serum with a detection limit of 80 nM has been achieved [82]. Such low detection limit is possible thanks to the use of NIR-laser as excitation source, which is capable of overcoming self-luminescence from serum excitation with visible light.
A FRET process is not only possible between two NPs, but also between the emission bands of RE NPs and an enzyme absorbance band. Tm 3+ , Yb 3+ -doped Gd4O2S NPs have been used to monitor the redox state of a flavoenzyme (PETNR, pentaerythritol tetranitrate reductase) [83]. Due to a variation in the absorbance profile of the flavin core of the enzyme upon reduction/oxidation, the FRET between the two can effectively be turned 'on' or 'off' by changing the redox state of PETNR. The presence of two bands separated by over 300 nm allowed the ratiometric signalling of this process.
The multiplexing capabilities of Ln-doped NPs have been also demonstrated by using different UCNPs excited with the same IR laser. The simultaneous detection of two types pathogenic bacteria (Salmonella Typhimurium and Staphylococcus aureus) was carried out by means of aptamer-conjugated Er 3+ , Yb 3+ , and Tm 3+ , Yb 3+ -doped NaYF4 UCNPs [84].
Interestingly, Ln-doped UCNPs can also be used as nanothermometers based on the strong temperature dependence of the fluorescence intensities from two emitting levels of lanthanides [85][86]. This principle has been exploited for monitoring temperature changes in living cells, which is of particular interest for the investigation of enzyme reactions and sub-cellular processes [87]. Wolfbeis et al. studied temperature sensing using UCNPs of varying size and RE dopants recently [88]. They found that the core-shell structured hexagonal 2% Er 3+ , 20% Yb 3+ -doped NaYF4/NaYF4 UCNPs were more suitable for temperature sensing because their higher brightness allowed resolving temperature differences of less than 0.5 °C in the physiological range between 20 and 45 °C [88].

Optoelectronic applications
Because of their unique optical properties, lanthanide-doped materials are also widely used for optoelectronic applications, which include laser sources [89], fiber-optic communication [90], light-emitting diodes and solid-state lighthening [91][92], and color display devices [93]. These properties have been extensively studied in bulk materials since the last century, and nowadays the design and the study of the properties and applications of the nanostructured materials attracts Ln-doped NPs are also used to improve the energy conversion efficiency in solar cells by both overcoming the two primary loss mechanisms in solar cells. Such mechanisms are related to the absorption of photons with larger or lower energy than the bandgap of the solar cell, reducing in practice their efficiecy. On one hand, DC NPs can absorb UV photons and re-emit them at longer wavelengths, where the solar cell exhibits a significantly better response [97]. For example, Eu 3+ , Bi 3+doped YVO4 NPs have been used in Si-based solar cells to reduce the thermalization of charge carriers caused by the absorption of high-energy photons [98]. On the other hand, UCNPs are used to transform low energy photons into higher energy photons, that can be used by the solar cells, and thus significantly enhance the efficiency of the photovoltaic device [99]. Er 3+ , Yb 3+ -doped NaYF4 NPs, one of the most studied fluorides, has been used with this objective [100]. In some cases, UCNPs are also associated with organic dyes [101].

Concluding remarks and future outlook
Current and widely used strategies for synthesis and functionalization of Ln-based NPs have been discussed, and some recent advances in their imaging, sensing, and optoelectronic applications have been mentioned. Even when the synthesis in organic media, normally in presence of additives such as oleic acid, provides a powerful tool for the production of highly monodisperse NPs, the hydrophobic character of the resulting NPs requires the development of new synthesic routes for yielding homogeneous, uniform, and water-dispersible nanoparticles. The use of polyol-based solvents in homogeneous precipitation reactions at moderate temperatures has partially overcome this disadvantage, although NPs sizes are larger (>25 nm). New synthesis methods yielding Ln-doped NPs with different shapes and sizes for many systems are still demanded, as well as universal functionalization strategies for hydrophilic NPs, as the Layer-by-Layer approach.
Apart from the synthesis and functionalization perspectives, the main disadvantage of Ln-based NPs continues being their relative low emission intensity. Even when the indirect excitation with codoped inorganic matrices for downconverting NPs and core/shell structures for upconverting NP have demonstrated notable improvements, new strategies to enhance their luminescence are still demanded.