Lanthanide-Doped REVO₄ (RE = Y, Gd, Lu, La) Phosphors: From Synthesis to Sensing Applications

Abstract

Rare-earth elements including the fifteen lanthanides, from lanthanum (La) to lutetium (Lu), together with scandium (Sc) and yttrium (Y), can act either as matrix cations or as active luminescent centers when incorporated into host lattices. Owing to their relatively large ionic radii, high coordination numbers, and structural stability, ions such as La, Lu, Sc, Y, and gadolinium (Gd) typically serve as matrix cations in rare-earth vanadate (REVO₄)-based phosphors, while other trivalent lanthanide (Ln³⁺) ions act as active luminescent centers. These REVO₄ phosphors have proved to be good host lattices for optically active Ln³⁺ ions giving strong luminescence assigned to absorption of the vanadate (VO₄³⁻) groups, and the efficient energy transfer between host lattice and Ln³⁺ ions. The unique electronic configuration of Ln³⁺ ions, particularly their unpaired 4f electrons, makes them ideal for applications in luminescence, magnetism, electronic and magnetic relaxation, and catalysis. Due to their complementary luminescent characteristics, Ln³⁺-doped REVO₄ phosphors have attracted significant attention in recent years. Their unique optical properties make them highly valuable across a broad spectrum of applications. This paper provides a comprehensive review of the state of the art in Ln³⁺ (Eu³⁺, Sm³⁺, Tm³⁺, Er³⁺, Ho³⁺, Tb³⁺, Nd³⁺, and Yb³⁺)-doped REVO₄ (RE = Y, Gd, Lu, La) phosphors. It examines current synthesis approaches, alongside the development of advanced strategies, and explores structural characteristics, innovative designs, and luminescent behavior, including both downconversion and upconversion processes and sensing applications, of the Ln³⁺-doped REVO₄ phosphors.

1. Introduction

Vanadates represent a family of inorganic compounds containing a vanadium ion in its highest oxidation state (+5). The most basic member of this group is the tetrahedral orthovanadate VO₄³⁻ anion, commonly encountered in sodium orthovanadate or in alkaline solutions of V₂O₅ at pH values above 13 [1]. Based on the types of combined cations with VO₄³⁻ anions, the vanadates can be classified as alkali metal vanadates (M₃VO₄) (M = (Li, Na, K, Rb), alkali earth metal vanadates (MV₂O₆) (M = Ca, Sr, Ba), transition metal vanadates (MxVyOz), (M = Co, Zn, Ni, Cu, Fe), orthovanadates (MVO₄) (M = Ln = from La to Lu, Sc, and Y), and other vanadates with NH₄⁺, Al³⁺, In³⁺, and Bi³⁺ cations. REVO₄, along with structurally related inorganic compounds such as fluorides, oxides, phosphates, molybdates, tungstates and analogous materials, constitutes an expanding domain of contemporary research. Within the current scientific landscape, numerous research groups have studied REVO₄ compounds with their crystallographic, electronic and optical characteristics, as well as potential for advanced applications [2,3,4].

REVO₄ materials serve as versatile hosts for incorporating optically active different Ln³⁺ ions which share the same +3 oxidation state and exhibit close similarities in ionic size, electronegativity, and electronic configuration with the cations of the lattice, so that they can be introduced across a broad concentration range. This substitution occurs without causing significant distortions or alterations in the crystal structure [5].

In addition, incorporation of multiple optically active dopants into the REVO₄ matrix is essential for enhancing both the performance and versatility of the matrix. By incorporating various co-dopants, researchers are exploring their potential as sensitizers and their ability to expand the functional properties of the medium. These investigations aim to achieve advanced effects such as self-Q-switching, self-mode-locking, self-frequency-doubling, and self-Raman conversion, thereby broadening the operational capabilities of the system [6]. REVO₄ doped with optically active Ln³⁺ ions is known for its strong luminescent behavior, which arises from efficient energy transfer processes between the vanadate groups and the Ln³⁺ ions. By carefully choosing the type and concentration of these dopants, a broad spectrum of emission colors can be achieved. Both undoped and Ln³⁺-doped REVO₄ phosphors have been widely investigated as multifunctional materials. Their versatility makes them highly appealing for diverse applications, including use in phosphors, specialty glasses, optical polarizers, and optoelectronic devices. They also play important roles in telecommunications, scintillation detectors, and photocatalysis. Furthermore, REVO₄ compounds show promise as advanced sensing nanoprobes, offering dual functionality for optical bioimaging and magnetic resonance imaging (MRI) [7,8].

Vanadate-based phosphor compounds have attracted significant interest due to their dual functionality as hosts for Ln³⁺ dopants and as self-activating phosphors. Among them, REVO₄ materials such as yttrium vanadate (YVO₄), gadolinium vanadate (GdVO₄), and lutetium vanadate (LuVO₄) are particularly interesting. Their strong luminescence, long fluorescence lifetimes, low lasing thresholds, large emission cross sections, and high absorption coefficients have made them valuable for a wide range of optical applications [9,10]. Recent review articles have highlighted the role of self-activated hosts, such as vanadates and niobates, in enhancing and tailoring the optical properties of Ln³⁺ ion dopants, driving significant progress in their synthesis and optimization. Parallel advances in host selection, principally tungstates, vanadates, and aluminates, improved luminescent efficiency and broadened optoelectronic applications. Moreover, vanadate systems have provided insights into catalytic processes relevant to hydrogen and oxygen evolution reactions [11,12,13]. A recently published book chapter [14] examined orthovanadates of mono-, di-, and trivalent metals, focusing on their synthesis, structural information of room-temperature phases, morphology, luminescent properties and potential applications. A literature survey of hydrothermal syntheses of bismuth vanadate was particularly noteworthy. While this book chapter [14] examined photoluminescence in a broad range of orthovanadates, including those with s- and p-block cations, the present review focuses specifically on Ln³⁺-doped REVO₄ (RE = Y, Gd, Lu, La) compounds and extends the discussion to recent advances in sensing applications.

The improved sensitivity, selectivity, stability, and biocompatibility of Ln³⁺-activated vanadates and the catalytic properties of CeVO₄ nanoparticles make the vanadate system an excellent candidate in biosensing platforms, cancer therapy, antibacterial activity, biosensing, and drug delivery. These developments place REVO₄ compounds at the forefront of multifunctional material research. New studies concerning co-doping, nanostructuring, and host lattice engineering will advance biomedical applications and next-generation dosimeter technologies [15,16].

The YVO₄ material stands out among REVO₄ compounds as one of the most extensively studied laser materials over the past few decades. Its unique physical characteristics make it highly valuable in diverse applications, ranging from polarizers and gas sensors to phosphors and advanced laser systems, particularly when doped with Ln³⁺ ions [17]. Also, YVO₄ exhibits remarkable chemical stability along with a broad optical transparency range from 400 to 5000 nm. Owing to these properties, it has found extensive use in optical communication systems and is a key material in the development of light isolators and circulators [18]. Ln³⁺-doped LuVO₄ materials demonstrate exceptional laser performance, attributed to their stronger absorption at pump wavelengths and enhanced emission cross-sections compared to other orthovanadates. In particular, neodymium-doped LuVO₄ (Nd:LuVO₄), which crystallizes in the zircon structure, offers a significantly larger emission cross-section than Nd:YVO₄ and Nd:GdVO₄, while maintaining a high threshold for optical damage [19].

On the other hand, GdVO₄, with its exceptionally high melting point at ~1800 °C, offers several advantages over YVO₄. It delivers greater brightness, responds efficiently to ultraviolet excitation, and facilitates effective charge-transfer processes. In addition, GdVO₄ exhibits lower phonon energy and reduced sensitivity to moisture, and can serve in multiple applications either in its undoped form or when doped with optically active Ln³⁺ ions [20,21].

Materials based on Gd³⁺ (4f⁷) ions serve as excellent lattice hosts for developing up-conversion systems, largely because their lowest excited states lie at relatively high energies. The luminescence properties of GdVO₄ depend on the specific dopant ions incorporated into matrix. When the vanadate is activated by single ions, such as Eu³⁺, Dy³⁺, or Sm³⁺, the material operates as a downconversion phosphor, emitting photons at characteristic wavelengths in the visible region. Conversely, activation by pairs or multiple ions, such as Er³⁺/Yb³⁺, Ho³⁺/Yb³⁺, Tm³⁺/Yb³⁺, or Er³⁺/Tm³⁺/Ho³⁺/Yb³⁺, induces upconversion processes that generate emission at higher energy in respect to the excitation [22]. Furthermore, Nd³⁺ doping expands its luminescent potential, adding another dimension to its versatility. Nd³⁺-doped REVO₄ compounds combine strong mechanical stability with excellent optical performance, particularly through their pronounced absorption at 808 nm. This wavelength is highly relevant for biomedical technologies, making such materials especially attractive in that field. In addition, nanoscale phosphors derived from GdVO₄ and co-doped with Nd³⁺ and Yb³⁺ can act as colloidal donor–acceptor systems. These nanophosphors provide a useful platform for investigating interparticle energy transfer phenomena in aqueous nanofluids [23]. Also, Gd-based materials can be used in osteogenic, antimicrobial, and anticancer applications, and in bioimaging and bioprobes. This functionality arises from the presence of unpaired electrons in Gd³⁺ ions, which effectively alter the relaxation dynamics of nearby water protons, thereby enhancing image contrast [24].

The main objective of this article is to review recent literature on undoped and Ln³⁺ (Eu³⁺, Sm³⁺, Tm³⁺, Er³⁺, Ho³⁺, Tb³⁺, Nd³⁺, and Yb³⁺)-doped REVO₄ (RE = Y, Gd, Lu, La) phosphors, highlighting the current state of the art in synthesis approaches alongside the development of advanced strategies, and exploring structural characteristics, innovative designs, luminescent behavior, including both downconversion and upconversion processes, and sensing applications of the Ln³⁺-doped REVO₄ phosphors. This review article focuses on the fundamental aspects and advantages of the most widely used synthesis methods, such as solid-state reactions, coprecipitation, hydrothermal/solvothermal routes, sol–gel processes, and microwave-assisted techniques, emphasizing their ability to control particle size, tailor morphology, and optimize dopant distribution to enhance luminescence performance of REVO₄. The incorporation of optically active Ln³⁺ ions into the REVO₄ matrix has been shown to influence structural modifications, lattice distortions, and defect center formation, all of which significantly influence emission intensity and stability. Owing to their sharp, well-defined emission lines from f–f transitions from Ln³⁺ ions, combined with strong ultraviolet absorption by VO₄³⁻ groups, REVO₄ materials demonstrate excellent photostability, long luminescence lifetimes, and tunable emission colors under both downconversion and upconversion processes. These unique optical properties have enabled their application across a broad spectrum of technological areas. This review highlights applications of the Ln³⁺-doped REVO₄ phosphors in LED diodes, luminescent electromagnetic displays, optical sensing, lasers, luminescence thermometry, chemical and bio-sensing, biodetection, infrared fluorescence bioimaging, photothermal therapy, photocatalysis, gas sensing, antimicrobial activity, and anti-counterfeiting technologies.

2. Synthesis Approaches in the Fabrication of Vanadate-Based Materials

The fabrication of vanadate-based materials with high purity, crystallinity, and well-defined uniform size, morphology, and composition, along with a homogeneous distribution of impurities, is crucial for the advancement of modern functional materials. To be practical, synthetic methods must also be economical, scalable to industrial production, and capable of delivering high yields. These requirements drive the development of novel or modified synthesis procedures aimed at enhancing the characteristics and performance of different kinds of vanadate materials.

In recent years, the design, synthesis, and fabrication of multifunctional materials have gained significant attention. A variety of synthetic approaches can be employed to produce REVO₄ with diverse sizes, shapes, properties, and morphologies, depending on the intended application. Control over nanoparticle size and shape is typically achieved through careful optimization of reaction parameters. Furthermore, synthesis methods can be classified based on the phase in which the phosphors are formed, i.e., solid, liquid, or gaseous, providing different pathways to tailor their structural and functional properties.

2.1. Solid-State Reaction

Inorganic solids are often synthesized by the ceramic method, which involves mixing stoichiometric amounts of solid precursors, grinding them to fine particles, and heating for several hours. For example, the simplest reaction pathway yields GdVO₄ by combining gadolinium oxide (Gd₂O₃) with vanadium pentoxide (V₂O₅):

Gd₂O₃ + V₂O₅ → 2GdVO₄

This process requires high temperatures to enable ion diffusion, but the large difference in melting points between V₂O₅ (681 °C) and Gd₂O₃ (2339 °C) complicates synthesis. While diffusion can be aided by fine grain size and mixing, elevated temperatures remain essential. A challenge arises from V₂O₅ volatility and decomposition at high temperatures, which can lead to oxygen loss and formation of VO₂. Literature reports vary: some note incomplete reactions with stoichiometric mixtures, while others suggest adding excess V₂O₅ or carefully controlling heating ramps to avoid evaporation. For example, slow heating to 950 °C followed by higher temperatures can stabilize the reaction. To address these limitations, precursors must be thoroughly mixed to maximize surface contact. In some cases, reactions are carried out under vacuum or in an inert atmosphere, which can further accelerate the process. Ultimately, thorough characterization of the product is necessary, as impurities such as unreacted Gd₂O₃, GdVO₃, or non-stoichiometric GdVO₄ may form due to volatility and reduction of V₂O₅ [25,26].

For example, REVO₄ (RE = Y, Sm, Gd, Yb, Lu) and GdVO₄:Ho³⁺/Yb³⁺ materials are typically synthesized in the solid phase using a high-temperature solid-state method. In this process, stoichiometric amounts of precursors are thoroughly ground, mixed, and heated in multiple steps at approximately 1200 °C for several hours to ensure complete reaction and formation of the desired product [27,28].

2.2. Coprecipitation Method

The coprecipitation method is a simple and effective technique for synthesizing vanadate-based materials with uniform size distribution. The process involves dissolving precursor salts (nitrates, sulfates, chlorides, etc.) in water and adding a precipitating agent (hydroxide, carbonate, or hydrogen carbonate), followed by aging, separating the precipitate, and finally washing and drying. The precipitation rate depends on the specific metal ions used. Huignard et al. demonstrated that Eu-doped YVO₄ materials can be synthesized at room temperature by coprecipitation from soluble nitrates and sodium orthovanadate. Although their work focused on nanoparticles, the findings aside from size control apply to bulk materials. The reaction is highly pH-dependent, with yttrium orthovanadate forming only within a narrow range (12.5–13.0). At pH values above 13, hydroxides precipitate without reacting, while acidic conditions favor condensed vanadates, preventing orthovanadate formation [29].

The reaction involves, in the first step, the precipitation of the kinetically favored hydroxide:

(Y_xEu_1−x)³⁺ + 3OH⁻ −→ Y_xEu_1−x(OH)₃ ↓

which, in the second step, reacts with orthovanadate to form thermodynamically stable Y_xEu_1−xVO₄:

Y_xEu_1−x(OH)₃ + VO₄³⁻ −→ Y_xEu_1−xVO₄ + 3OH⁻

Subsequent studies suggest that similar conditions apply to Gd³⁺ systems, as RE hydroxides are generally insoluble in water but soluble in acids. For bulk reference materials, precise particle size control is unnecessary, and solution synthesis offers the advantage of ambient temperature processing, avoiding issues linked to high-temperature solid-state methods. However, rapid precipitation often produces defective crystalline regions. Thermal treatment has been shown to improve crystallinity, making a two-step process, coprecipitation followed by heating at different temperatures, a viable alternative to solid-state synthesis, while avoiding complications from V₂O₅ volatility [22].

2.3. Hydrothermal/Solvothermal Method

Hydrothermal (or solvothermal) synthesis relies on dissolving inorganic substances in water or another solvent at temperatures above 100 °C and pressures around 1 atm, followed by crystallization. The key difference is that hydrothermal methods use water, while solvothermal methods employ other solvents. REVO₄ can be readily prepared this way, though high-temperature calcination is required to enhance luminescence. Reaction parameters such as pressure, temperature, pH, and precursor ratios determine the morphology and size of the final product. The hydrothermal synthesis method provides significant advantages for producing vanadates through multiphase or liquid-phase chemical processes. However, it has notable drawbacks: need for costly autoclaves, safety risks during reactions, and difficulty of monitoring processes directly. For example, GdVO₄:Ln³⁺ (Ln = Eu, Sm, Dy) microcrystals and nanocrystals with different morphologies, crystal orientations and defects have been synthesized via a simple hydrothermal method in a wide range of pH values into a Teflon-lined stainless autoclave at a temperature of 180 °C for 24 h or 10 h at high pressure [30,31,32].

2.4. Sol–Gel Route

Sol–gel synthesis is a versatile wet-chemical method that can be performed through routes such as alkoxide hydrolysis, inorganic gelation, or polymerizable complex processes. It enables molecular-level mixing of precursors, offering advantages over solid-state reactions, including uniform dopant distribution, lower synthesis temperatures, reduced contamination, and control over porosity. Drawbacks include difficulty in removing organic groups and risk of cracks in the final material. Chelating agents play a crucial role, as their type and combustion properties influence the physico-chemical characteristics of the products [33]. The sol–gel method is highly valued due to its relatively low initial cost for producing high-quality materials, its ability to design and control the chemical structure of substances, and its capacity to achieve uniform composition with a large surface area. Despite these advantages, the technique has limitations, including extended reaction times and considerable shrinkage during the dehydration stage [34].

In practice, GdVO₄ is often synthesized from Gd(OAc)₃ and NH₄VO₃ dissolved in water, mixed in the proper molar ratio with carboxylic acids (e.g., citric, malic, or tartaric acid), then heated with stirring to form a gel. The gel is dried to yield a carboxylate precursor, which is subsequently calcined. In some cases, synthesis can proceed without carboxylic acids. Using different acids allows production of vanadate with different sizes and morphologies. For example, tartaric acid can serve both as a chelating agent and combustion fuel, with calcination temperature and precursor ratios directly affecting particle size and crystallinity [35].

2.5. Microwave-Assisted Method

Microwave synthesis employs electromagnetic radiation (0.3–300 GHz) to rapidly decompose precursors and trigger fast nucleation, producing small nanoparticles in a much shorter time. A key requirement is that at least one precursor must absorb microwaves. This method is often combined with hydrothermal synthesis. Microwave irradiation plays a vital role in accelerating reaction kinetics by enabling rapid initial heating, which significantly increases overall reaction rates. This technique produces cleaner products, ensures faster consumption of starting materials, and improves yields. Another advantage lies in the uniform heating and precise control of process parameters, which enhance reproducibility and reliability of reaction conditions. In the field of vanadate synthesis, microwave heating represents a novel and uniform approach that deserves further development. Using green reaction media not only shortens reaction times but also minimizes chemical waste, making it an attractive and sustainable method [36]. Typically, Gd(NO₃)₃·6H₂O and NH₄VO₃ solutions are mixed with EDTA at high pH, stirred, and placed in a microwave reactor at 150 °C for 180 min. The resulting powders are collected by centrifugation, washed with water and ethanol, and dried [37,38].

3. Structural and Morphological Properties of REVO₄-Based Materials

3.1. Crystal Structure of REVO₄-Based Materials

Materials of the type AVO₄ (where A = Sc, Y, Bi, or any of the lanthanides La–Lu) adopt a tetragonal zircon-type crystal structure, similar to ZrSiO₄. In this arrangement, the V⁵⁺ ions within the vanadate VO₄³⁻ groups are tetrahedrally coordinated by O²⁻ ions, while the trivalent A³⁺ cations are surrounded by eight O²⁻ ions. The three-dimensional framework is built from alternating AO₈ distorted dodecahedra in which the A³⁺ cations occupy a non-centrosymmetric crystallographic site with D_2d symmetry that shares edges with tetrahedral VO₄. As an example, the tetragonal crystal structure of GdVO₄ is presented in Figure 1. This connectivity results in chains of vanadium ions aligned parallel to the c-axis. Bond distances vary within the structure: there are four shorter (2.33 Å) and four longer (2.45 Å) A–O bonds, whereas V–O bonds are all 1.74 Å long, consistent with tetrahedral coordination of V⁵⁺ ions. Each O²⁻ ion is bonded to two equivalent A³⁺ cations and one V⁵⁺ ion. Importantly, substitution of different A³⁺ ions does not alter the crystal type, as all AVO₄ compounds are isostructural. For example, YVO₄ compound parameters are a = 7.13 Å, b = 7.13 Å, c = 6.30 Å, α = β = γ = 90° and unit cell volume V = 320.53 ų [39]. Many REVO₄ compounds exhibit a low-temperature Jahn–Teller distortion, in which their tetragonal crystal structure transforms into an orthorhombic geometry. Interestingly, this type of crystallographic transition has not been observed in YVO₄ or in GdVO₄ [40].

Also, the lattice distortion can be induced by the incorporation of a dopant (for example, Eu³⁺ ions) into the host crystal structure. As an example, when Ho³⁺ and Yb³⁺ ions are incorporated into the GdVO₄ crystal, the structure changes. As the amount of Yb³⁺ increases, one of the diffraction peaks can shift to higher angles because the smaller Ho³⁺ and Yb³⁺ ions replace the larger Gd³⁺ ions, inducing a shrinking of the crystal lattice. A similar effect is recognized when Li⁺ ions are added to the system. Since the ionic radius of Li⁺ is smaller than that of Gd³⁺, it can move easily through the crystal and take the place of Gd³⁺ ions or occupy interstitial sites. The structural parameters of synthesized Ho³⁺/Yb³⁺-doped GdVO₄ powders with and without Li⁺ co-doping, calculated using Rietveld refinement, were a = 7.175 Å, b = 7.13 Å, c = 6.324 Å, and V = 325.57 ų for GdVO₄ with 1%Ho³⁺ and 20%Yb³⁺ and a = 7.166 Å, b = 7.166 Å, c = 6.319 Å, and V = 324.52 ų for GdVO₄ with 1%Ho³⁺, 20%Yb³⁺, and 15%Li⁺ [28].

Figure 2 depicts the difference in the structural configuration of the monoclinic LaVO₄ and Y_0.5La_0.5VO₄ structures doped with Bi³⁺ together with Rietveld refinement. In the LaVO₄ structure, La atoms are coordinated in a hexahedral arrangement with six oxygen atoms, while V atoms are coordinated to four oxygen atoms, forming a tetrahedral geometry. This host lattice contains two types of cations, La³⁺ with ionic radius r = 1.16 Å and V⁵⁺ with ionic radius r = 0.35 Å, for coordination number 8. The dopant ion Bi³⁺ (with ionic radius r = 1.17 Å) for coordination number 8 shares the same valence state as La³⁺ and exhibits similar ionic sizes and coordination environments and preferentially substitutes at La³⁺ sites within the lattice. The Y_0.5La_0.5VO₄ structure crystalizes in a tetragonal zircon-type crystal structure, as mentioned and explained above for GdVO₄ [41,42,43].

3.2. Morphology of Undoped and Ln³⁺-Doped REVO₄ Phosphors

The development of modern optoelectronic devices strongly depends on the advancement of luminescent materials with improved characteristics. These materials must exhibit specific functional properties such as brightness, resolution, spectral energy distribution and lifetime under practical operating conditions. Such properties are determined by the structural and morphological features of the material used. Depending on the intended application, synthesized luminescent Ln³⁺-doped REVO₄ phosphors should possess precisely defined functional characteristics. Among them, uniform particle size distribution and spherical morphology without agglomeration are particularly critical [44,45].

Precise control over nanoparticle size, uniformity, distribution and surface area enables significant enhancement of optical properties, such as lifetimes of intermediate energy levels, dopant ion emission at specific wavelengths, and reduction in light scattering when particle dimensions are smaller than the wavelength of incident light. Luminescent materials at the nanoscale are especially important because they bridge the gap between molecular and micron scales, offering unique opportunities for advanced applications. Their effectiveness arises from a high surface-to-volume ratio, which ensures that a large fraction of atoms remain available for interaction with surrounding molecules [46].

Furthermore, the small particle size provides greater flexibility for manipulation and facilitates efficient doping with activator ions. Since many dopant ions are located near particle surfaces in asymmetric crystalline environments, their emission behavior differs from that of ions in regular crystallographic positions. This distinction opens pathways to novel and tunable optical effects, positioning nanostructured luminescent materials as a promising frontier in optoelectronics [47].

As mentioned above, the morphology of Ln³⁺-doped REVO₄ is strongly influenced by the preparation method and reaction parameters, such as reaction temperature, pressure, concentration of precursor solutions, pH value, kind of solvent, aging time, the type, concentration and distribution of Ln³⁺ dopant ions, and addition of capping agent or surfactant. For example, GdVO₄:Eu³⁺/Sm³⁺ and GdVO₄:Ho³⁺/Yb³⁺ obtained with solid-state reaction synthesis show that the powders contain chunks with irregular spheres with an average diameter ranging from 1 µm to 8 µm [28,48].

Using the sol–gel route, the GdVO₄ nanomaterial can be synthesized using different carboxylic acids, resulting in a homogeneous distribution of spherical particles with average sizes ranging from 50 to 100 nm [49]. In another approach, a tartaric acid-assisted sol–gel method was employed, where tartaric acid acted simultaneously as a chelating agent and as an additional fuel during precursor combustion. Key synthesis parameters, such as calcination temperature and the molar ratio between total metal ions and tartaric acid, directly influence the particle size and crystallinity of the final product, consisting of spherical nanoparticles with diameters between 20 and 30 nm [35].

GdVO₄ nano- and microcrystals with diverse morphologies and sizes were successfully synthesized using a hydrothermal process and trisodium citrate (Na₃Cit) as the chelating ligand. By employing Gd(NO₃)₃ and Na₃VO₄ as precursors together with Na₃Cit in varying molar ratios relative to Gd³⁺ ions, different morphologies and particle sizes were obtained. For instance, when the molar ratio of Na₃Cit to Gd³⁺ was 4:1, uniform pancake-like microstructures were produced, with an average thickness of 200 nm and a diameter of approximately of 1 µm [31]. In another approach, hydrothermal synthesis under varying pH conditions, reaction media and reaction times yielded Eu³⁺-doped GdVO₄ phosphors with a wide range of morphologies, such as rhombic, spherical, and cubic structures, as well as irregular short nanorods [30]. Applying various reaction components, such as EDTA-Na₂ and EDTA, and adjusting the pH of the solution, the hydrothermal method allows one to tune the shape of GdVO₄ into short and long nanowires, nanorods, nanoparticles and spheres [50].

Both as-prepared GdVO₄:Ho³⁺/Yb³⁺ samples and those re-heated at 300 °C, synthesized by the coprecipitation method, reveal bundles composed of 5–6 individual nanorods, each approximately 5 nm in diameter and up to 20 nm in length, aligned in different orientations. When the same samples were annealed at 600 °C, the nanorods transformed into single ellipsoidal particles with an average size of about 20 nm, while at higher annealing temperatures of 800 °C and 1000 °C, the morphology changed further, yielding irregular spherical particles approximately 100 nm in size along with elongated rod-like structures [22].

The YVO₄ nanoparticles synthesized using a simple microwave irradiation process exhibited a size in the range of 5–18 nm, which was extremely dependent on the pH value of the solution [51]. On the other hand, the YVO₄:Bi³⁺/Eu³⁺ materials synthesized by a microwave and ultrasonic radiation-activated technique had a pronounced spherical shape with a predominant diameter in the range of 20–40 nm [52].

As a further example, SEM and TEM images of MVO₄/g-C₃N₄ (M = La, Gd) prepared by the hydrothermal method are given in Figure 3. Pure g-C₃N₄ exhibits a nanosheet structure with a smooth surface (Figure 3a), while pure GdVO₄ powders display a coral-like morphology (Figure 3b). The TEM image of the GdVO₄/g-C₃N₄ composite (Figure 3c) reveals two distinct components: the black coral-like structures corresponding to GdVO₄, and the French-gray sheet-like structures belonging to g-C₃N₄. On the other hand, pure LaVO₄ demonstrates a pin-like nanostructure (Figure 3d). Figure 3e,f illustrate the LaVO₄/g-C₃N₄ composite, where both LaVO₄ and g-C₃N₄ are clearly distinguishable, with LaVO₄ particles well adhered to the g-C₃N₄ surface. Thus, the constitution of MVO₄/g-C₃N₄ (M = La, Gd) composites is readily identifiable, which facilitates efficient charge carrier transport compared to pure MVO₄ (M = La, Gd) [53].

4. Optical Properties of Undoped and Ln³⁺-Doped REVO₄ Materials

The lanthanides, spanning lanthanum (Z = 57) to lutetium (Z = 71), have a general electronic configuration of [Xe]6s²5d¹4fⁿ. Typically, the 5d electron shifts into the 4f shell, giving 4fⁿ⁺¹, though exceptions exist (e.g., Gd). As atomic number increases, the 4f orbitals contract inward, a phenomenon called the lanthanide contraction, making them behave like inner electrons and limiting their role in bonding. This results in similar chemical properties across the series, dominated by outer valence electrons. Upon ionization, lanthanides form stable +3 ions ([Xe]4fⁿ), with 5s²5p⁶ orbitals shielding the 4f electrons, so crystal field effects remain weak and are treated with perturbation theory [54]. Inorganic materials doped with Ln³⁺ are essential across numerous fields, owing to their wide-ranging applications: LED diodes, luminescent monitors/electromagnetic displays, lasers, thin-film phosphors, drug delivery systems, luminescence thermometry, chemical sensing, biosensing, infrared fluorescence bioimaging, photothermal therapy, gas sensing, anti-counterfeiting technologies and so on.

REVO₄ (RE = Y, Gd, Lu and La) compounds are widely recognized as excellent host materials for various dopant ions, owing to their ability to be efficiently excited by UV radiation and their favorable charge-transfer energy. Because charge transfer from VO₄³⁻ groups to optically active Ln³⁺ ions is highly efficient, luminescence can be achieved even at extremely low dopant concentrations [55].

Ln³⁺-doped REVO₄ luminescent materials can be classified in several ways depending on their properties and targeted applications. From a functional perspective, these materials fall into three main categories: lighting materials, display materials, and detection/sensing materials. When they are classified by excitation source, they include photoluminescent, X-ray-excited luminescent, electroluminescent, and high-energy photon-excited materials. Based on the photon emission dynamics, these materials are distinguished as either downconverters or upconverters [47,48,56].

4.1. Downconversion of Ln³⁺-Doped REVO₄ Phosphors and Sensing Applications

Stokes luminescence, also known as downconversion (DC), is a process in which matter absorbs photons of higher energy and re-emits photons of lower energy. DC luminescence follows Stokes’ law, describing the conversion of high-energy excitation into lower-energy emission [57,58].

As an example for REVO₄-based DC-luminescent materials, the schematic diagram of the energy transfer process in GdVO₄:Tb samples is given in Figure 4. The VO₄³⁻→Tb³ one-electron charge transfer takes place between the 2p orbital of oxygen (O²⁻) and the vacant 3d orbital of the central vanadium (V⁵⁺) in the tetrahedral VO₄³⁻ with Td symmetry. According to the molecular orbital theory, energy levels involved are the ground ¹A₁ state and the excited ¹T₁, ¹T₂, ³T₁ and ³T₂ states. The transitions from ¹A₁ level to the ¹T₁ and ¹T₂ generate a broad and intense charge-transfer absorption band in the UV region [11]. At the same time, weak absorption features indicate a low-efficiency reverse transfer process from Tb³⁺ to VO₄³⁻. There are four possible energy transfer (ET) pathways, ET₁ (Gd³⁺→VO₄³⁻), ET₂ (VO₄³⁻→Tb³⁺), ET₃ (Tb³⁺→VO₄³⁻, weak) and ET₄ (Gd³⁺→VO₄³⁻→Tb³⁺), which demonstrate cooperative interactions among Gd³⁺, VO₄³⁻ and Tb³⁺ ions. Overall, VO₄³⁻→Tb³⁺ transfer was identified as the dominant process, supported by spectral shifts, broadened emission bands, and lifetime analysis [59].

Eu³⁺ and Bi³⁺ ion co-doped LuVO₄ thin films annealed at 1000 °C exhibit pronounced luminescent behavior. The Eu³⁺ ions generate intense red emission via the ⁵D₀→⁷F₂ transition with a similar feature to the GdVO₄:Eu³⁺ samples obtained by coprecipitation synthesis and additionally annealed at 1000 °C [60]. As an example, excitation and emission spectra of LuVO₄:Eu³⁺/Bi³⁺ films are given in Figure 5.

Under 350 nm excitation, emission peaks at 594, 615, 650, and 700 nm are produced, corresponding to Eu³⁺ transitions ⁵D₀→⁷F_J (J = 1, 2, 3, 4), while the most prominent emission is observed at 615 nm. The Bi³⁺ ions act as sensitizers by absorbing UV radiation and transferring the energy to Eu³⁺ ions. The characteristic Bi³⁺ emission band at 550 nm is significantly reduced in co-doped samples, confirming efficient Bi³⁺→Eu³⁺ energy transfer. LuVO₄ thin films exhibit stable and tunable luminescence, making them highly promising for optoelectronic applications such as displays and light emitters, as well as efficient phosphors for LEDs, particularly in warm-white lighting. Their properties also open opportunities in optical sensing and biological imaging, where reliable luminescence performance is essential [61]. Analysis of the intensity ratio R = I(⁵D₀→⁷F₂)/I(⁵D₀→⁷F₁) demonstrates that Bi³⁺ promotes Eu³⁺ occupation at lower-symmetry sites. According to Dexter’s theory, quadrupole–quadrupole interactions dominate the transfer mechanism, consistent with other Bi³⁺→Eu³⁺ systems [62].

The Eu³⁺ ions are frequently employed as luminescent activators for bioapplications (biodetection, bioimaging and biosensing) and chemical sensing, due to their unique luminescence properties. Due to the different crystal field surroundings, the Eu³⁺ ions situated at multiple sites in different materials exhibit distinct photoluminescent spectra and photoluminescent decays. The long-lived luminescence of Eu³⁺ ions is highly advantageous for background-free, time-resolved photoluminescence biodetection. Eu³⁺-activated nanomaterials have demonstrated broad applicability in both heterogeneous and homogeneous biodetection, in vitro and in vivo bioimaging, and anti-aging, antibacterial, anticancer and antioxidant effects [63,64]. GdVO₄:Eu³⁺ nanoparticles are excellent candidates for multifunctional material development, as they can perform diverse and polyvalent functions.

Redox-active GdVO₄:Eu³⁺ and LaVO₄:Eu³⁺ nanoparticles can intensify damage in oxidatively stressed L929 cells, even at concentrations that remain non-toxic to normal cells. This effect is linked to the internalization of the nanoparticles and is mediated through activation of the intrinsic mitochondrial apoptotic pathway, excessive reactive oxygen species (ROS) generation and Ca²⁺ signaling. These results highlight the potential of GdVO₄:Eu³⁺ and LaVO₄:Eu³⁺ nanoparticles as promising candidates for use as anticancer agents, as depicted in Figure 6 [65].

A ⁶⁴Cu-labeled multifunctional nanoprobe targeting integrin α₂β₁ (often used as a metastasis suppressor) was developed using GdVO₄:Eu³⁺ two-dimensional tetragonal nanosheets for in vitro fluorescence studies, in vivo magnetic resonance imaging (MRI) and micro-positron emission tomography (PET) imaging of prostate cancer. The unique water solubility and biocompatibility make GdVO₄:Eu³⁺ highly versatile for biomedical applications [66], with strong potential as a radionuclide carrier and contrast agent for theranostic applications [67,68].

Due to their negative surface potential, GdVO₄:Eu³⁺ nanoparticles strongly adsorb metal cations. Among common blood ions, Cu²⁺ uniquely induces distinct fluorescence quenching. GdVO₄:Eu³⁺ nanoparticles are promising magnetic/fluorescent multimodal probes for Cu²⁺ ion detection in blood [69]. Colloidal GdVO₄:Eu³⁺@SiO₂ nanocrystals could find a promising application for highly selective and sensitive detection of Cu²⁺ ions in an environmental or biological sample [70]. Also, Eu³⁺activated ultra-small nanoparticles (EuVO₄ and GdVO₄:Eu³⁺) have the potential to be used for the detection of pesticides in environmental and biomedical fields, due to photoluminescence quenching [71]. Hydrogen peroxide (H₂O₂) acts as a strong quencher of ultra-small (2–3 nm) GdVO₄:Eu³⁺ fluorescence. The observed Eu³⁺ luminescence quenching is found to be more effective with increasing concentrations of H₂O₂. To evaluate selectivity for H₂O₂, several potentially interfering ions were also tested. Common physiological ions such as Ca²⁺, Zn²⁺ and Mg²⁺ did not affect fluorescence intensity, while Cu²⁺ and Fe³⁺ quenched even more efficiently than H₂O₂ [72]. Two primary mechanisms were identified as responsible for the quenching of Eu³⁺ luminescence in GdVO₄:Eu³⁺ nanoparticles: (i) reduction in the efficiency of non-radiative resonance energy transfer from the vanadate groups to Eu³⁺ ions, caused by scattering effects introduced by V⁴⁺ ions; (ii) direct luminescence quenching of Eu³⁺ ions by –OH groups formed on the nanoparticle surface as a result of H₂O₂ decomposition [73].

The additional sensing application and distribution of Gd_0.6Eu_0.4VO₄ nanoparticles was investigated with XRF spectroscopy after injection of their colloidal solutions into mouse ear pinnae. The distribution of the nanoparticles was mapped by raster scanning the sample and detecting the V K_α, Gd L_α, and Eu L_α fluorescence emissions. It was found that all three elements coexist and therefore no short-term out-diffusion of those elements from the nanoparticles to the tissues takes place. These results provided a proof-of-concept that XRF mapping and spectroscopy are excellent tools for the determination of the long-term fate of Gd_0.6Eu_0.4VO₄ nanoparticles in tissue in terms of element leaching and stability for diagnostic and therapeutic purposes [74].

4.2. Upconversion of Ln³⁺-Doped REVO₄ Phosphors and Sensing Applications

Upconversion (UC) luminescence is a non-linear anti-Stokes process with low-energy excitation light (NIR region). In recent years, UC materials excited by NIR light illumination and emitting in the visible (Vis) region have received much attention. Typically, UC materials are composed of a host material with low phonon energy, such as YVO₄, GdVO₄, LuVO₄ and LaVO₄ and sensitizer ions (commonly Yb³⁺), along with activator ions (most often Er³⁺, Tm³⁺, or Ho³⁺, or their combination). Yb³⁺-sensitized UC materials are typically excited at 980 nm. The Yb³⁺ ion possesses a simple energy structure with only two states, the ground state (²F_7/2) and the excited state (²F_5/2), separated by an energy gap of approximately 10,000 cm⁻¹. Yb³⁺ acts as the sensitizer due to its strong absorption cross-section at 980 nm and efficient energy transfer to Er³⁺, while Er³⁺ provides the emission. Importantly, Yb³⁺ absorbs in the NIR region where inexpensive laser diodes operate efficiently. This absorption enhances luminescence efficiency by transferring excitation energy to Er³⁺, Ho³⁺, or Tm³⁺ ions [75].

The intensity of upconversion emission is strongly influenced by several factors, including temperature, particle morphology, surface characteristics, and concentration ratio between dopant ions (Er³⁺, Ho³⁺, Tm³⁺) and sensitizer ions (Yb³⁺). In the GdVO₄ host matrix, varying the concentration ratio of Yb³⁺ to Er³⁺ alters the balance between green and red emission. It is important to note that literature indicates only a few Er³⁺ or Yb³⁺/Er³⁺-doped inorganic UC materials which emit intense green emission under NIR excitation. Specifically, increasing the Er³⁺ concentration enhances the green emission in doped GdVO₄ and YVO₄ materials. This effect can be attributed to perturbations in site symmetry, which promote stronger emission from the ²H_11/2 state. The perturbation arises from the hypersensitive nature of the ²H_11/2 state and the difference in ionic radii between Yb³⁺ and Gd³⁺ ions, leading to modified local environments that favor enhanced luminescence [76,77]. The UC mechanism involves Yb³⁺ absorbing most of the excitation energy and transferring it to Er³⁺, whose long-lived excited states enable multiphoton absorption and radiative relaxation, producing green and red emissions. Nanoparticles show weaker UC intensity compared to bulk material, though the emission band shapes and green-to-red ratios remain unchanged [78].

Figure 7 demonstrates how excitation wavelength and elemental composition influence the green UC emission intensity of YVO₄:Er³⁺ and YVO₄:Er³⁺,Yb³⁺ nanomaterials. The excitation of the samples at 785 nm resulted in higher UC emission intensity for the single-doped YVO₄:Er³⁺ compound. In contrast, the most intense green UC luminescence is observed in YVO₄:Er³⁺,Yb³⁺ under 975 nm excitation due to the large absorption cross-section of Yb³⁺ ions (²F_7/2 → ²F_5/2 transition) and the effective energy transfer UC from the sensitizing Yb³⁺ ions to the emitting Er³⁺ ions [79]. Also, the laser pump power can affect the intensity of the Er³⁺ emission measured for large nanoparticle ensembles and single particle in the YVO₄:Yb,Er system [80].

The incorporation of Ho³⁺ ions into the matrix, together with Yb³⁺ ions, is particularly effective due to the favorable energy level distribution of Ho³⁺ ions. Radiative transitions from the ⁵S₂/⁵F₄ level of Ho³⁺ to the ground state (⁵I₈) produce green emission around 545 nm, while transitions from the ⁵F₅ level to ⁵I₈ yield red emission near 650 nm under UV and NIR excitation, respectively. The intensity of red UC emission originating from the ⁵F₅ → ⁵I₈ transition of Ho³⁺ increases consistently with higher Yb³⁺ concentrations. Typically, strong red UC emission in Ho³⁺/Yb³⁺ systems requires relatively high Ho³⁺ concentrations, explained by cross-relaxation processes between Ho³⁺ energy levels that suppress green emission [81,82].

GdVO₄:Tm³⁺/Yb³⁺ material, under 980 nm excitation, exhibits UC luminescence in three distinct regions: a dominant blue emission at 475 nm arising from the ¹G₄ → ³H₆ transition of Tm³⁺ions, a weaker red emission at 650 nm attributed to the ¹G₄ → ³F₄ transition and an infrared emission at 808 nm corresponding to the ³H₄ → ³H₆ transition of Tm³⁺ ions [83]. Figure 8 summarizes the structure and luminescence behavior of Tm³⁺,Yb³⁺:GdVO₄@SiO₂ core–shell nanoparticles, which can serve as biolabels in the visible range and luminescence thermometers within the first biological window (Figure 8a). Under 980 nm excitation, the UC emission spectra reveal that most bands decrease in intensity with rising temperature, except the 700 nm band, which slightly increases (Figure 8b). The energy transfer mechanism (Figure 8c) involves sequential absorption by Yb³⁺ ions (²F_7/2 → ²F_5/2) and transfer to Tm³⁺ levels, ultimately populating ¹G₄ and producing blue (475 nm) and red (650 nm) emissions, while transitions from ³F₃ and ³H₄ yield emissions at 700 nm and 800 nm. Non-radiative relaxation, Tm–Tm cross-relaxation and efficient population of the ³H₄ level can explain the strong NIR emission observed at 800 nm and the temperature-dependent spectral behavior [84].

It is common to use a 980 nm excitation source for UC emission, but this wavelength has drawbacks in biomedical applications, including strong water absorption, limited tissue penetration and local heating that can damage cells. To overcome these issues, research has shifted toward shorter excitation wavelengths, particularly 808 nm, which lies within the biological NIR windows (650–950 and 1100–1750 nm) where tissues are more transparent. The second NIR window is especially attractive for fluorescence imaging due to deeper penetration, higher resolution, and reduced scattering. At 808 nm, water absorption is lower, penetration is good in superficial to moderately deep tissues, and heating effects are minimized [85,86]. The excitation wavelength shift is possible by substituting Yb³⁺ ions with Nd³⁺ ions, which have a strong absorption band centered at 808 nm. UC emission spectra of GdVO₄:Nd³⁺/Er³⁺ and GdVO₄:Nd³⁺/Ho³⁺ systems under 808 nm excitation show three bands: green emission (520–565 nm, max ~540 nm), red emission (570–630 nm, max ~597 nm), and another red band (640–680 nm, max ~675 nm). The dominant red emission at 597 nm corresponds to the Nd³⁺ (⁴G_7/2 → ⁴I₁₁/₂) transition, while the other bands arise from Er³⁺ and Ho³⁺ transitions [58]. GdVO₄ single crystals doped with Nd³⁺ ions under 808 nm continuous-wave laser excitation across varying temperatures have been studied for optical thermometry. The excitation pathway involves ground-state absorption, non-radiative relaxation, excited-state absorption and energy-transfer upconversion, which enhances higher-level populations. As temperature increases, phonon-assisted processes such as cross relaxation and non-radiative transitions become more efficient, leading to dramatic emission intensity enhancements at 400 °C compared to room temperature [87]. YVO₄:Nd³⁺ nanophosphors can also be used as NIR-to-NIR thermal sensors in a wide temperature range [88]. Ultra-small stoichiometric NdVO₄ nanoparticles (3–4 nm) dispersed in water exhibit long-term stability, large absorption cross-sections at 808 nm, and excellent biocompatibility, making them promising multifunctional agents for biomedical imaging and in vivo photothermal cancer therapy in a mouse model. A concentrated colloidal dispersion of NdVO₄ nanoparticles in Tris-buffered saline (TBS) was subcutaneously injected, and infrared fluorescence imaging with an InGaAs CCD camera enabled precise localization of the injection site [89,90].

Due to the unique 4f-electron structure of RE ions, UC nanomaterials have been widely investigated for biomedical and materials science sensing applications. Due to remarkable optical, electronic and magnetic properties, these materials can be used in biomedicine, luminescence imaging, cell labeling probes, magnetic resonance imaging (MRI), photodynamic therapy (PDT), chemotherapy, single-photon emission computed tomography (SPECT), X-ray computed tomography (CT), optical biosensing/biodetection, and many other sensing technologies. In biomedical contexts, UC nanoparticles offer several advantages: deeper penetration into biological tissues, minimal autofluorescence background, reduced scattering and absorption, excellent photostability and negligible photoblinking [91,92].

The unique ability of UC nanoparticles to function as MRI contrast agents, coupled with their strong tissue penetration and prolonged circulation, enables highly sensitive imaging of biological targets. Beyond diagnostics, these nanoparticles can be precisely engineered in size and morphology to act as multifunctional platforms, supporting both imaging and therapeutic applications such as photodynamic therapy, photothermal therapy, and chemotherapy. Their luminescent responsiveness to temperature further broadens their utility, making them valuable tools for thermometry and sensor development. Altogether, UC nanoparticles represent versatile systems at the intersection of medical innovation and optoelectronic technology [93,94].

YVO₄:Er³⁺/Eu³⁺/Yb³⁺ nanoparticles with an average size of ~100 nm, synthesized using melamine formaldehyde as a template, can serve as dual-mode excitation materials for bioimaging and have been successfully applied in vitro for imaging HeLa cells. Cytotoxicity was evaluated using the methyl thiazolyl tetrazolium (MTT) assay, confirming that functionalized YVO particles exhibit low cytotoxicity, supporting their potential biomedical applications [95]. GdVO₄@SiO₂:Tm/Yb core–shell structures demonstrate excellent thermal sensitivity and resolution, making them suitable as intracellular thermal probes for measuring temperature within living HeLa cells [84].

YVO₄:Nd³⁺ nanoparticles have emerged as multifunctional nanotheranostic agents, enabling fluorescence imaging, magnetic resonance imaging, and enhanced sonodynamic therapy of orthotopic gliomas. Similarly, GdVO₄:Nd³⁺ particles are promising candidates for time-gated bioimaging [96,97,98]. Figure 9 illustrates in vitro imaging studies of YVO₄:Er³⁺/Yb³⁺ UC-MHNSPs, highlighting their permeability and translocation in HeLa cell lines [99].

REVO₄-based UC nanomaterials have been studied as sensors for luminescence thermometry. Temperature monitoring can be achieved by remotely detecting changes in nanomaterials’ luminescent properties, offering valuable their applications in biomedicine, micro/nanoelectronics and integrated photonics [100]. Temperature sensing with UC nanoparticles is of particular interest for biomedicine, because the excitation typically occurs in the NIR spectral region, and autofluorescence from biological material does not affect the measurements. In this context, GdVO₄:Er³⁺/Yb³⁺ and YVO₄:Ho³⁺/Yb³⁺ nanoparticles have been investigated for temperature sensing, with their upconversion emission spectra recorded across the range of 307–473 K and 12–300 K, respectively [78,101]. Figure 10a presents the emission spectra of YVO₄:1%Er³⁺ in the temperature range 300–800 K together with temperature dependence of the fluorescence intensity ratio (FIR) between the emission peaks at 525 nm (²H_11/2–⁴I_15/2) and 553 nm (⁴S_3/2–⁴I_15/2) [102]. Because these two closely separated levels show Boltzmann-type relative population, the integrated FIR of transitions from the ²H_11/2 and ⁴S_3/2 levels to the ground level ⁴I_15/2 can be approximated using Boltzmann distribution as follows [103]:

$$FIR={\displaystyle \frac{{{I(}^{2}H}_{11/2}{{\to }^{4}I}_{15/2 })}{{{I(}^{4}S}_{3/2}{{\to }^{4}I}_{15/2 })}}={\displaystyle \frac{g_H{ A}_H {h\nu }_H}{g_S{ A}_S {h\nu }_S}}\exp (- {\displaystyle \frac{\Delta E}{kT}})=\mathrm{B}\exp (- {\displaystyle \frac{\Delta E}{kT}})$$

(1)

where ${\mathit{g}}_{\mathit{H}}$ and ${\mathit{g}}_{\mathit{S}}$ are the degeneracies of the ²H_11/2 and ⁴S_3/2 levels, respectively; ${\mathit{A}}_{\mathit{H}}$, ${\mathit{A}}_{\mathit{S}}$, ${\mathit{\nu }}_{\mathit{H}}$ and ${\mathit{\nu }}_{\mathit{S}}$ are the spontaneous emission rates and frequencies of the ²H_11/2 $\to$ ⁴I_15/2 and ⁴S_3/2 $\to$ ⁴I_15/2 transitions, respectively; h is the Planck constant; k is the Boltzmann constant; and T is the absolute temperature. Equation (1) can be expressed as follows:

$$\mathrm{l}\mathrm{n}(FIR)=ln(\mathrm{B})+(- {\displaystyle \frac{\Delta E}{kT}})=ln(\mathrm{B})+(- {\displaystyle \frac{C}{T}})$$

(2)

where B and C are the constants that need to be determined. Fitting the experimental data with Equation (2) demonstrates a strong correlation between theory and experiment, consistent with previous reports on thermometry based on Er³⁺ UC emission [103,104]. The relative sensor sensitivity, S_r [in %K⁻¹], of the luminescent probe, is defined as relative change in the FIR with temperature (Figure 10b):

$${ S}_r={\displaystyle \frac{1}{\mathit{FIR}}}{\displaystyle \frac{\Delta FIR}{\Delta T}}\times 100\%$$

(3)

The highest relative sensitivity was achieved for a dual-layered YVO₄:Eu³⁺/YVO₄:Dy³⁺ sample, exhibiting a maximum sensitivity of 3.6% K⁻¹ at 640 K [102].

5. Sensing Applications: Recent Advances

Sensing applications of vanadates have been previously illustrated and discussed. This section provides a brief overview of significant developments in the field since 2023.

Vanadate materials—including binary oxides such as V₂O₅, ternary compounds like BiVO₄, and various transition-metal vanadates (e.g., Zn–, Fe–, Pr–vanadates)—are semiconductors characterized by tunable electronic structure, high redox activity, and versatile surface chemistry. These properties render them highly effective for a range of chemical sensing applications, particularly in gas detection and electrochemical identification of complex analytes.

Vanadium pentoxide, for instance, exhibits a wide bandgap (2.3 eV), a robust chemical and thermal stability, a rich surface chemistry and a characteristic metal–insulator transition at 257 °C. As detailed in a comprehensive review of V₂O₅ gas sensors [105], these devices operate primarily through three mechanisms: gas adsorption, polaron hopping and direct chemical interaction. These mechanisms facilitate diverse applications in environmental monitoring, food safety, medical diagnostics and pharmaceutics. Recent literature [106] emphasizes that the high surface-to-volume ratio and inherent reactivity of V₂O₅ allow for the selective detection of biomolecules and other analytes at sub-ppm concentrations. Furthermore, its distinctive lamellar structure is highly conducive to doping, enabling direct modification of its structural and optical characteristics. Incorporating multiple Ln³⁺ elements can induce cooperative effects that enhance optical performance and expand functionality [107]. For instance, annealed films constituted by CeO₂:V₂O₅ nanoparticles have recently demonstrated high sensitivity in thin film strain-gauge applications due to their optimized electrical and morphological characteristics [108].

Bismuth vanadate is an n-type semiconductor nanomaterial with a narrow bandgap (2.4 eV), making it highly active under visible light. While being a promising catalyst for rapid removal of pollutants from wastewater [109,110], BiVO₄ has emerged as a potent biosensing platform, due to its unique electrochemical properties, high dispersibility, high photocatalytic efficiency, low toxicity, and biocompatibility. Recent developments highlight its efficacy in detecting various disease biomarkers with high sensitivity and specificity [111]. Additionally, morphology-tailored BiVO₄ nanostructures have shown improved gas-sensing performance for environmental monitoring (e.g., NO_x, H₂S) by maximizing surface active sites and oxygen vacancies [112].

The zircon-type tetragonal structure of certain vanadates acts as a stable host lattice for dopant ions. Materials such as YVO₄, GdVO₄, and LuVO₄ have emerged as premier materials for optical sensing due to their unique combination of structural stability and exceptional spectroscopic properties. Unlike other hosts, the vanadate group (VO₄³⁻) is “self-activated”, absorbing ultraviolet (UV) light and efficiently transferring energy to Ln³⁺ dopants, thus significantly enhancing luminescence. These materials maintain structural integrity at high temperatures (melting points near 1800 °C) and are resistant to chemical degradation, making them ideal for sensing in harsh environments [113,114,115].

Figure 11 schematically shows the luminescence processes in RE-doped AVO₄ and ANbO₄ phosphors (where A = Y, Gd, or La for niobate, and A = Y or Gd for vanadate) [9]. The very intense emission upon UV and NIR excitation may be exploited for producing high-security anti-counterfeiting inks. These ‘invisible’ inks remain covert under ambient light but emit intense fluorescence under UV or near-infrared (NIR) excitation. An advance in documents’ security is possible by integrating optical authentication provided by the ink’s fluorescence and biometric recognition through fingerprint patterns. Efficient detection of latent fingerprints, with low background interferences, was made possible by the RE ions’ capability of multi-color emission [116,117,118,119].

For example, Ye et al. [117] developed various GdVO₄:Ln³⁺ compounds through a dual sintering process at 1100 °C. A CIE color coordinate diagram for GdVO₄ doped with trivalent Bi, Dy, Eu, Bi/Dy, Bi/Tm and Bi/Eu ions is shown in Figure 12. The GdVO₄:Bi³⁺/Eu³⁺ formulation proved particularly resistant to background interference, enabling the resolution of latent fingerprint details on various surfaces and under high-humidity and -temperature conditions. Because these nanoparticles are significantly (1000–10,000 times) smaller than the width of a fingerprint ridge, they enable exceptional spatial resolution and superior adhesion. Recent studies on a set of nanocrystalline red-light-emitting Ca₈ZnGd_1−xEu_x(VO₄)₇ (x = 0.10–0.50 mol), synthesized via a non-hazardous and cost-effective solution-based calcination (SBC) route, have shown promising results [119]. XRD analysis confirmed that Eu³⁺ ions were diffused into the trigonal host matrix without causing structural deformation. The size of non-uniform agglomerated particles was evaluated around 61 nm. The chromaticity coordinates of the optimal sample (Ca₈ZnGd_0.80Eu_0.20(VO₄)₇) and its high color purity of 97.7% indicate that this material stands at the forefront of both fluorescence-based forensic sensing and advanced optoelectronic devices. The recent review article by Ugemuge et al. [13] provides a critical evaluation of the performance of vanadate-based phosphors for optoelectronic devices. It bridges the gap between synthesis methods and functional outcomes by presenting real-world applications. This aspect of the review significantly enhances its sensibleness and relevance as a phosphor [13].

As a general trend, RE-doped vanadates have gained significant interest for luminescence thermometry and chemical biosensing. In most cases, the luminescence intensity ratio (LIR) technique is used. This ratiometric approach calculates absolute temperature by comparing the intensity of two distinct emission peaks (e.g., from Eu³⁺ or Nd³⁺), effectively neutralizing fluctuations in excitation laser power. Beyond LIR, thermal sensing can also be performed by monitoring temperature-dependent shifts in spectral line position and line bandwidth. Significant research over the last decade has demonstrated the efficacy of various nanostructures, including YVO₄:Nd³⁺ [97] and La₃Sc₂Ga₃O₁₂:Cr³⁺/Nd³⁺, nanophosphors [120], YVO₄: Ho³⁺/Yb³⁺ nanocrystals [121], GdVO₄:Er³⁺/Yb³⁺ nanocrystalline powders [122], and (Y, Yb, Tm, Er)VO₄ systems [123].

In a recent study, an RE-doped solid solution of yttrium phosphate–vanadate (YV_1–xP_xO₄:Eu³⁺, Er³⁺) was proposed as a high-performance luminescent thermometer [124]. This material leverages the strong, broad charge-transfer absorption of the vanadate group to sensitize RE³⁺ ions. It facilitates multi-mode sensing by utilizing the LIR of Er^{3+ 2}H_11/2/⁴S_3/2 emission, the dual-center LIR of integrated Er³⁺ and Eu³⁺ emission intensities, and the emission lifetime of Eu³⁺. This system enables very accurate temperature sensing across a wide temperature range, from room temperature to 873 K; inside that range, the P/V ratio (x) is adjustable to optimize performance for specific thermal windows.

While the LIR method is easy to be implemented and is robust against experimental or sample-related conditions, such as fluctuations in excitation intensity, sample geometry, or concentration of luminescent probes, concerns have been raised about its absolute precision. Vieira Perrella et al. [125] addressed this issue by synthesizing UV-excitable Eu³⁺-doped yttrium vanadate and phosphovanadate (Y(V,P)O₄) particles. Their work clarified how the intrinsic spectral overlap between ³T_1,2 → ¹A₁ VO₄^3– transitions and Eu³⁺ transitions ⁵D₀ → ⁷F_J impacts sensor reliability. Without a proper baseline correction, the broad band background overlap can result in a ten-fold increase in temperature uncertainty and a ∼60% underestimation of relative thermal sensitivity.

Vanadate compounds are increasingly used for the detection and remediation of persistent pollutants in aquatic ecosystems, often related to pharmaceutical residuals eliminated by humans and animals through their bodily waste. This issue has become crucial due to the disproportionate usage of antimicrobial agents in humans, animals and feed supplements. The photocatalytic activity of pure Ca₂V₂O₇ has shown remarkable results under visible light irradiation [126]. Within 180 min, this material achieved 79.5% removal of safranin O (a red cationic dye used in histology and Gram-positive bacteria staining) and 80.6% removal of tetracycline hydrochloride (a yellow, crystalline, broad-spectrum antibiotic) [126]. In addition, Ca₂V₂O₇ nanomaterial was effective for photocatalytic hydrogen generation.

REVO₄ compounds are emerging as superior electrochemical probes for the precise quantification of various biological and pharmaceutical compounds. Sriram et al. [127] performed a comparative study of hydrothermally synthesized TVO₄ (T = Ho, Y, Dy) for the simultaneous detection of two representative drugs, namely, nitrofurazone (an antiseptic drug for urinary tract infection) and roxarsone (an organo-arsenic drug, used in poultry as feed additive). The three RE vanadates were found to respond differently, and results demonstrated that DyVO₄-based electrodes exhibited superior sensitivity, achieving exceptionally low detection limits (0.002 µM for nitrofurazone and 0.0009 µM for roxarsone). Similarly, orthovanadates (REVO₄, where RE = Pr, Gd, and Sm) were evaluated for the detection of two antibiotics, i.e., furazolidone (FD) and metronidazole (MD), nitro-functional synthetic drugs that have been in use for over 30 years [128]. Using differential pulse voltammetry, the different vanadates were compared as electrode modifiers, and the SmVO₄-modified glassy carbon electrode emerged as the most effective, characterized by the lowest charge-transfer resistance (R_ct = 56.82 Ω) and the largest electrochemical surface area (A = 0.11 cm²). This sensor not only provided the lowest limits of detection (0.0009 μM for FD and 0.0036 μM for MD individually, and 0.0015 μM and 0.0049 μM for simultaneous detection) but also showed excellent anti-interference, repeatability, and reproducibility. A recent comparative analysis of RE-doped molybdate, tungstate, and vanadate nanomaterials for the detection and photocatalytic degradation of nitrofurantoin (NFT), a persistent and toxic antibiotic contaminant, highlighted the versatility of these materials [129]. A series of NaDy(MoO₄)₂:Tb³⁺, NaDy(WO₄)₂:Tb³⁺, and Na₃Dy(VO₄)₂:Tb³⁺ nanomaterials were synthesized via a hydrothermal method and systematically characterized. While the tungstate variant excelled in the photocatalytic degradation of NFT under UV light (96% degradation in 60 min), the vanadate variant exhibited the highest sensitivity for NFT detection, with a detection limit of 0.38 ppm [129]. Additionally, GdVO₄ nanoparticles showed good photocatalytic and antimicrobial activity against bacteria E. coli and Streptococcus mutans and fungi Aspergillus niger and Candida albicans [130].

6. Conclusions

This paper provides a comprehensive overview of advances in synthesis strategies, luminescent properties and sensing applications of Ln³⁺ (Eu³⁺, Sm³⁺, Tm³⁺, Er³⁺, Ho³⁺, Tb³⁺, Nd³⁺, and Yb³⁺)-doped REVO₄ (RE = Y, Gd, Lu, La) phosphors A detailed discussion is given regarding the preparation method, such as solid-state reactions, coprecipitation, hydrothermal/solvothermal, sol–gel, and microwave-assisted methods. The main attention has been focused on their structural and optical properties, with special reference to downconversion and upconversion luminescence, and the sensing application perspectives of these materials.

The undoped and Ln³⁺-doped REVO₄ materials have emerged as versatile platforms for biodetection, fluorescence in vitro and in vivo bioimaging, and therapeutic applications including anti-aging, antibacterial, anticancer and antioxidant effects. Their performance is driven by advances in host materials such as GdVO₄, YVO₄, and LuVO₄, alongside innovations in design, size, shape, co-doping and surface modification. As a result, undoped and Ln³⁺-doped REVO₄ nanomaterials are gaining increasing importance in biomedical research, with rapid progress expected through continued developments in material synthesis, surface engineering and assay technologies.

REVO₄ co-doped with Yb³⁺ or Nd³⁺, so-called upconverting materials, has broad sensing application potential, beyond biomedicine. It has been applied in luminescence imaging, cell labeling, magnetic resonance imaging, photodynamic therapy, chemotherapy, single-photon emission computed tomography, X-ray computed tomography, optical biosensing, biomarker identification, immunoassays, drug delivery, luminescence thermometry, solar energy conversion, fingerprint detection, photocatalysis, and related fields. In biomedical contexts, upconverting nanomaterials offer distinct advantages, including deeper tissue penetration, minimal autofluorescence background, reduced scattering and absorption, excellent photostability, and negligible photoblinking.

Looking ahead, the main research on undoped and Ln³⁺-doped REVO₄ materials should be focused on advanced methods of synthesis, surface modification and polymer encapsulation. The exploration of multifunctional properties will broaden their applicability in emerging fields such as photonic devices, bioimaging, and environmental sensing. Shifting the excitation wavelength from 980 nm to safer biological windows, in particular around 800 nm, would give further advantages, increasing tissue penetration depth and reducing the tissue heating effect. In summary, these directions highlight the promising role of Ln³⁺-doped REVO₄ in next-generation optical materials.