1. Introduction

Porous coordination polymers, commonly known as Metal-Organic Frameworks, or simply MOFs, are crystalline nanoporous materials, formed by metal ions or metal-containing clusters bonded by organic linkers, which form various dimensional structures [1,2,3,4,5]: two-dimensional (2D) [6,7] or three-dimensional (3D) extended network [8,9,10,11]. Thanks to high structural flexibility, large surface area, along tunable pore size, MOFs have applications in the fields of gas sorption [12], separation and storage [13,14], catalysis [15,16], molecular recognition and sensing [17,18], drug delivery [19], and luminescence and non-linear optics [2,9]. The recently developed subclass of Metal-Organic Frameworks, known as luminescent MOFs (LMOFs), show promise for use as light-emitting materials due to their crystalline structure and unique optical properties [20]. Luminescent properties pave the way for such MOF materials in the sensing field as biosensors thanks to low biotoxicity and easy preparation in the form of thin films (TFs) [21].

Lanthanide (Ln) ions are known for their high coordination numbers and diverse coordination geometry, which sets them apart from transition metal ions [22,23]. This has led to a surge of interest in Lanthanide-based Metal-Organic Frameworks (LnMOFs), a subset of Luminescent MOFs (LMOFs). The exceptional luminescent characteristics of Ln ions, including large Stokes shifts, distinct sharp emissions, extended lifetime, and color purity with high quantum yields in both the near-infrared and visible regions, make LnMOFs particularly intriguing [24,25,26]. LnMOFs are known for their features, such as varied topology, elastic structure, porosity, and surface area. However, it is important to note that the luminescent attributes of Ln ions are significantly influenced by the structure of their surrounding coordination environment. This makes them a fascinating and promising platform for use in chemical sensors [27,28,29,30,31]. The integration of Ln ions’ properties with the topological attributes of MOFs paves the way for the creation of new luminescent materials. These could be used in white light sources, such as Light light-emitting diodes (LEDs) [20,32,33,34]. The energy transfer process, known as “luminescence sensitization” or “antenna effect” [21,24,26,35] is based on the property of organic ligands to act as an antenna—by absorbing completely optical energy and then transferring it to the coordinated central Ln ions through a molecular energy transfer mode. Mixed-Ln³⁺ in LnMOFs display a remarkable capability for adjustable white light emission and temperature gauging [4,26].

LnMOF-TFs are gaining prominence in various application areas, including optoelectronics, gas separation, catalysis electronic devices, and biomedicine. The luminescence of MOF-TFs can be derived from luminescent metal ions or clusters, organic ligands, loaded guest molecules, and charge transfer: either ligand-to-metal or the reverse—metal-to-ligand [21]. Mixed-crystal LnMOFs (Ln = Eu + Tb, Eu + Tb + Gd) [36], TFs, as opposed to the traditional method based on single lanthanide ions [36,37,38], produce more stable and precise luminescent signals. This makes them an excellent choice for self-calibrating luminescent sensors for a variety of applications [39,40]. Again, compared to sensors that contain only one luminescent site, the mixed-crystal LnMOFs with a self-referring strategy can amplify the relative emission ratios, which would improve the luminescent signals and decoding of the analyzed molecules [40,41]. In the case of films, di- or higher-topic organic linkers can be employed to act as light-absorbing antennae [42,43]. Films exhibit permanent porosity and have been utilized for numerous applications, including tunable emission, biosensing [44], and thermometers [45,46,47].

Various hydro/solvothermal, sonochemical, microwave-assisted, and electrochemical synthetic strategies are available [4,5]. The deposition of high-quality LnMOF-TFs is still challenging but offers potential applications in gas storage, catalysis, sensing, lighting, and solar energy harvesting [48]. Many methods have been developed for film deposition: directly onto bare substrates or functionalized substrates with organic molecules [49]; on seeded substrates; on preformed MOF nanocrystals; layer by layer [50,51,52]; and electrodeposition [5,46]. Because of the excellent detection performance and the turnability of light emission, lanthanide composite hybrid films have drawn a lot of attention in recent years [53,54,55]. LnMOF-TFs represent a promising category of porous crystalline materials that can be customized for various practical applications through versatile post-synthetic modifications [56]. The f-f transitions of lanthanide ions (Ln³⁺) are parity-forbidden but can be sensitized by organic ligands through a process known as the “antenna effect” [56].

There are already several excellent reviews published [3,4,5,8,21,57,58,59] focusing on the fabrication of LnMOF films, which also include the idea of mixed matrix membranes [60,61,62,63]. This is why the present work aims to comprehensively describe the current progress in the preparation of MOF-TFs, with a particular focus on those that are lanthanide-based, and the green synthesis of LnMOFs as light-emitting materials [20,26] and chemical sensors with single and/or multiple luminescent centers [64]. A brief perspective of the application of LnMOFs in light-emitting devices is presented and classified from the viewpoint of the general luminescence features of LnMOFs.

2. Preparation Methods of Lanthanide Metal-Organic Framework

The previously published reviews were mostly focused on the existing and potential applications, as well as the synthesis methods of MOF-TFs [3,5,8,65]. However, regarding the fabrication approaches, these reviews focused mostly on hydro/solvothermal (HT/ST) synthesis, layer-by-layer (LBL) deposition, and electrochemical depositions. Taking into account the discussed publications on MOFs in the forms of membranes [62], composite structures [66], and free-standing nanoparticles (NPs) [67], moving forward, the authors will concentrate on the production of Metal–Organic Framework Thin Films (MOF-TFs). They will provide an exhaustive review of the various synthesis strategies for MOF-TFs, including patterns, and will discuss potential directions for future advancements and environmentally friendly fabrication.

2.1. Solvo/Hydrothermal Deposition

The solvo/hydrothermal growth of films is a simple, effective, and low-cost deposition method that has been widely accepted [5]. The disadvantage of conventional synthesis is the high cost due to the large organic reactants consumption and waste production [8]. Green modulation synthesis is used for the preparation of nano-sized LnMOFs using eco-friendly chemicals. In situ, the direct deposition of the LnMOF benzene tricarboxylate (BTC), Ln-BTC, and mixed-crystal Ln-BTC films can be easily realized on bare (1) and modified (2) substrates. Nanometer-sized MOF-TFs (NMOFs) were prepared on indium–tin oxide (ITO) glass using a solution of Ln³⁺ in DMF/H₂O or EtOH/H₂O mixed with sodium acetate (NaOAc) as modulator and followed by spin-coating (SC) or dip-coating (DC) without any substrate modification. As a result, two smooth continuous films were reported: Eu_0.5Tb_0.5-BTC by SC [68], Tb-BTC by DC [69] and Eu_0.1Tb_0.9-BTC (MILMOF-3) by DC [40]. The XRD data of Ln-BTC films are in good agreement with the tetragonal structure P₄322 space group similar to Y(BTC)(H₂O) [40,70]. Similarly to the film described in [70], porous transparent mixed Ln-BTC films, where Ln = Eu/Gd/Tb, with a thickness of ~ 0.5–1.2 μm, were prepared on modified Pt/SiO₂/Si substrates from nanocrystals via green solvothermal synthesis using a mixture of EtOH/H₂O/NaOAc as a solvent [71,72]. Zhang and colleagues pioneered a novel technique called “in situ secondary growth” for the fabrication of lanthanide MOF films [73]. They successfully utilized this method to create an Eu-BDC-NH₂ film on ordinary glass, using UiO-66-NH₂ as the seed layer. The Eu-BDC-NH₂ film exhibited strong characteristics of Eu³⁺ ion and was employed for the first time in the fluorescence sensing of gaseous SO₂.

2.2. Layer-by-Layer Deposition

The layer-by-layer (LBL) assembly method involves the in situ growth of Metal-organic frameworks (MOFs) on various substrates. This process involves repeated cycles of immersing the substrate into a solution source of metal ions and a solution source of organic ligands. Chen et al. [74,75] prepared a new set of films using LBL, known as Ln-SURMOFs (surface-supported MOFs), to produce solid-state white light-emitting devices. The thickness of the transparent multicomponent, mixed Eu/Tb Ln-SURMOFs can be freely controlled. For example, it was reported that Eu-BTC was successfully grown epitaxially on the surface of a Tb-BTC SURMOF [75]. A three-component approach is similarly developed for the combination of red, green, and blue (RGB)-light-emitting Eu³⁺-, Tb³⁺-, and Gd³⁺-containing layers to achieve the combined white light emission [74].

2.3. Electrochemical Deposition

The electrochemical deposition (ECD) offers several benefits for the fabrication process of MOF-TFs [76,77,78]. These include a shorter growth time and the affordability of the required equipment. The reaction process is more controllable and reproducible. ECD includes cathodic (CED) [76], anodic (AED) [77], and electrophoretic (EPD) [78] depositions. The AED method has succeeded in the fabrication of a MOF terbium-succinate (Tb-SA) thin film from the solution of Tb(NO₃)₃ and succinic acid in DMF [76]. Tb-SA films could be used as highly selective sensors for Cu²⁺ ions. The team of Li et al. [77] developed a microwave-assisted electrochemical deposition technique for LnMOF-TFs. A dense and homogeneous Ln(OH)₃ (Ln = Eu, and Tb) layer was first formed on the FTO electrode after eight cycles of AED from the solution of Ln(NO₃)₃. Subsequently, the formed Ln(OH)₃ layer was converted to LnMOF-TFs by the use of microwave irradiation. The patterned TFs have strong luminescence properties, which are of great interest in the fields of color displays, luminescent sensors, and structural probes. The electrophoretic deposition (EPD) process is based on the surface charge of the MOF particles suspended in the liquid media. Thus, it is possible to deposit the MOF-TFs from colloidal MOF suspension onto one of the electrodes immersed in the solution/suspension. LnMOFs-TFs of Eu_0.45Tb_0.55-BTC were successfully and rapidly deposited on an unmodified substrate (zinc plate) in 5 min, achieving a thickness of about 33.5–67 μm [78].

2.4. Ultrasonic Spray Deposition

Ultrasonic spray deposition, as a time-saving, low-cost, and new route for the fabrication of luminescent MOF films, can be considered an advancement in the integration of LnMOFs in future optical devices [48]. Two or more precursor solutions are used: (1) the source of metal ions and (2) the source of organic ligands. These solutions are atomized by separated ultrasonic nebulizers to form ultrafine mists, which are further transferred by gas flow, mixed, and deposited onto the prepared substrate surface. This allows solvents to evaporate and MOFs to crystalize and form matrix-free TFs. This strategy allowed for the deposition of Tb₂(BDC)₃, where BDC = 1,4-benzenedicarboxylate, MOF-TFs on a variety of substrates, including glass while retaining photoluminescent properties [48]. The temperature of the substrate plays a crucial role in the TF formation process and thus impacts its final structure, morphology, and luminescent properties. The ultrasonic spray deposition approach proved itself as a cheap, promising, easily scalable, one and can be considered as a breakthrough for bringing MOFs to commercial application in future optical devices.

2.5. Atomic Layer Deposition/Molecular Layer Deposition

The Atomic Layer Deposition (ALD) technique is widely used in thin-film deposition due to its high precision and flexibility. As a state-of-the-art industrial thin-film deposition technique, it offers a simple way to control the film thickness at the atomic level and is available for the preparation of multilayer structures [8]. ALD can be directly employed to fabricate MOF-TFs via a vapor–solid reaction [79]. In a general fabrication process, the source of metal and organic precursors for the desired MOFs in the gaseous form are sequentially blown onto the surface of a substrate with a pulse of gas, where it reacts and forms the solid layer. Crystalline hybrid MOF-TFs of Eu-NH₂-TA, where TA = 2-aminoterephthalic acid (NH₂-TA), with a thickness of 22 μm were prepared by Atomic/Molecular Layer Deposition (ALD/MLD) on Si/SiO₂ substrate [79,80]. Here, β-diketonate complex precursor europium(III)-tris-(2,2,6,6-tetramethyl-3,5-heptanedionate, Eu(THD)₃ reacted with NH₂-TA. The single-step synthesis proceeds without the need for a modulator. This procedure is a solvent-free green route, which does not require any post-synthesis treatment [80].

2.6. Composite Hybrid Films

The composite hybrid LnMOF-TFs were fabricated using modification by carbon dots (CDs) [67]; a variety of polymers, such as polyvinylidene difluoride (PVDF) [67] ethyl cyanoacrylate (glue adhesive, marked as EVOB) [62]; polylactic acid [66]; polymethylmethacrylate (PMMA) along with a post-synthetic modification (PSM) with butyl methacrylate (BMA), which was initiated by benzoyl peroxide (BPO) [56]. Eu_xTb_1−x(L) and UiO-66(Zr&Eu) MOF films on glass were prepared using PMMA and PVDF, respectively, as a binder. PMMA or PVDF were dissolved in DMF solution (for better dispersion of MOF powders).

3. Luminescent Properties of Lanthanide Metal-Organic Frameworks

3.1. Structure of Ligands Involved in Coordination with Lanthanide Ions

The luminescent properties of lanthanide ions highly depend on the structural details of their coordination environment. The large variability of Ln ion–ligand combinations in MOFs enables multiple possible luminescence processes and has already led to a large number of luminescent materials. LnMOFs built using coordination bonds between Ln ions and organic ligands are hopeful materials due to their porous crystalline structures, rich mixtures, and simple preparation [71]. The availability of various building blocks of Ln ions and organic ligands allows access to fascinating structures, novel topologies, and the direct manipulation of their physical and chemical properties [8]. Organic linkers, through which lanthanide ions or nodes are connected, generally contain functional groups that are capable of forming coordination bonds, such as carboxylate, phosphate, amine, etc. [1]. The benzenetricarboxylate ligand is often used to prepare LnMOF films, e.g., Tb-BTC [70]. 1,3,5-benzenetricarbocylic acid (H₃BTC) possesses three carboxylic groups with multifarious coordination modes and could be regarded as a good candidate for an organic four-connected node [70]. The empirical formula is C₁₅H₁₉N₂O₉Tb.

For the preparation of LnMOF films are commonly used ligands with the following structure: p-benzenedicarboxylate (BDC) [83]; BDC-NH₂ [73]; succinate (SA) [76]; bromomethylbenzene, dimethyl 5-hydroxy isophthalate (BHM-COOCH) [66]; benzophenone-3,3′,4,4′-tetracarboxylate (BPTC) [84]; 2,6-naphthalene dicarboxylate (NDC) [85]; thiophene-2,5-dicarboxylate (TDC) [86]; etc. The structure of BTEC and 1,10-PHEN as ligands, and Eu³⁺ as the metal skeleton [67], which adds a certain amount of carbon dots in novel composite film (CDs@Eu-MOF). A BHM-COOCH₃ ligand was synthesized for the preparation of Eu_xTb_1−x-BHM-COOH [66].

Photoluminescent MOFs with the same lanthanide ion possess similar emission bands due to the state transitions characteristic for the particular cation: ⁵D₀ for Eu³⁺ and ⁵D₄ for Tb³⁺ [20,39,83]. Notably, different lanthanides can be combined in one MOF, thus resulting in a combination of different emission spectra and luminous colors. The reported photoluminescence obtained in LnMOFs can be concluded as three types: lanthanide-centered luminescence, ligand-based, and guest-induced.

3.2. Lanthanide-Centered Luminescence

The luminescence of lanthanide ions originates from the energy transfer processes, also known in the literature as the “antenna effect” or “luminescence sensitization” (LS) [21]. The mechanism of LS within LnMOF-TFs is comprised of three steps: (1) light is absorbed by ligands around the lanthanide center; (2) energy is transferred to the lanthanide ions, and (3) luminescence is generated by the lanthanide ions (centers). White light-emitting Tb₁₀Eu₁-HMA LnMOF-TFs were fabricated via electrodeposition, and they show good photoluminescence with a satisfactory CIE coordinate of (0.33, 0.34) [87], which has been realized by electrodeposition for the first time. The average lifetime values achieved were 0.273 ms for Eu-HMA and 0.286 ms for Tb-HMA. The prepared new TFs show strong luminescence of Eu³⁺ and Tb³⁺ and are characterized by an efficient Tb³⁺-to-Eu³⁺ energy transfer. Since then, a variety of MOF-TFs, such as EuMOF, TbMOF, and mixed LnMOF-TFs, have been reported [87].

3.3. Ligand-Based Luminescence

The Eu-BDC-NH₂ film was prepared by the new approach, namely the so-called “in situ secondary growth”. It exhibits strong characteristic Eu³⁺ emission, which can be quenched by SO₂ gas [73]. The obtained Eu-BDC-NH₂ film has a wide range of potential applications, such as sensors, LEDs, solar cells, and TF transistors.

3.4. Guest-Induced Luminescence

Based on the large specific surface area and good absorption ability, MOFs often are used as a sorption platform; they can be used as a support or host for a variety of guest substances. Among the important ones will be the luminescent species, such as fluorescent quantum dots, dyes, and lanthanide ions/complexes. Very recently, fluorescent composites, such as CDs@Eu-MOF/PVDF and several materials from the QD@MOFs family, received great interest for their potential in chemical sensing, fluorescence imaging, and display lighting [67].

4. Light-emitting Devices

The LnMOF thin films have a wide range of potential applications. Those of the most interest include sensors [88,89] and light-emitting devices (LEDs) [90]. Over the last decade materials science has focused on the development of solid-state white light (SSWL)-emitting materials, mainly thanks to their long operation lifetime and excellent efficiency [74]. These efforts resulted in the development of a new type of LnMOF-TFs—surface-supported MOFs, also known as Ln-SURMOFs. It can be fabricated LBL to combine several single light-emitting layers into composite solid-state white light-emitting devices [83]. For this reason, a deliberate design for SSWL performance was achieved by multiple emitting layers and the addition of three colors according to the RGB concept (red, green, and blue). The Tb/Eu/Gd-SURMOF RGB device shows CIE x,y coordinates close to ideal for white light (0.331, 0.329) excited by 360 nm. Eu-SURMOF produces a magenta-colored emission that comprises both the ligand π*→π/n*→π transitions and the typical Eu^{3+ 5}D₀→⁷F_J (J = 0–4) transitions, with ⁵D₀→⁷F₂ being the most intense and located at 617 nm [83]. Similar emission spectra were recorded for the Eu/Gd/TbMOF-TFs films prepared by the solvothermal method with three different concentrations of Eu, Gd, and Tb [72]. Eu-HMA, Tb-HMA, Tb₁₀Eu₁-HMA, and Tb₁₀Gd₁-HMA films realized by electrodeposition [87] provide an effective platform for energy transfer between the lanthanide ions due to the short intermetallic distances. Tb₁₀Eu₁-HMA film shows good photoluminescence with satisfactory CIE coordinates of (0.33, 0.34). When the polymeric hybrid of Eu-MOF-L@PBMA-TFs are excited at 335 nm, the emission coordinates situate at (0.2929, 0.2985), which corresponds to the almost ideal white light chromaticity at (0.330, 0.330) [56].

5. Application of LnMOF Films as Sensors

LnMOFs have been widely studied in various sensor applications due to their high porosity, surface area, and the particular luminescence of Ln³⁺ ions. Most of the LnMOF-based sensors show luminescence intensity changes, including luminescence enhancement–turn-on response and quenching–turn-off response, as a detected signal for the analytes recognition. Eu³⁺ and Tb³⁺ are commonly used as luminescent centers in LnMOF sensors because of their strong red emission at ~614 nm and green emission at ~541 nm, respectively [20,91]. Synthetic innovations enabling the development of MOF thin films have produced a range of sensing technologies capable of the selective detection of a variety of analytes, including ions, small molecules, gases, temperature, and biological analytes of interest using optical, electrical, and acoustic techniques [3].

5.1. Cations Sensing

Detecting the metal ions with high precision and sensitivity is of great significance in environmental and biological studies, especially transition metal cations, such as Zn²⁺, Cu²⁺, Fe³⁺, and Fe²⁺, which are essential for a healthy metabolism. A highly luminescent Eu-BQDC film (BQDC = 2,2-biquinoline-4,4-dicarboxylate) was prepared by electrodeposition in combination with subsequent solvothermal synthesis. It exhibits high sensitivity and selectivity toward Hg²⁺ [92]. While similarly prepared on FTO substrates, Tb-BDC [83] and Tb-SA [76] TFs are selective toward Cu²⁺ in DMF solutions. The lifetime of Tb-BDC film and Tb-BDC + Cu²⁺ were 1.023 and 0.934 ms, respectively. Photoluminescent measurements demonstrated that Tb-SA films were relatively water-stable and had a fast response to Cu²⁺ ion aqueous media. The lifetime of Tb-SA + Cu²⁺ (1.088 ms) was decreased compared to Tb-SA (1.148 ms). The quenching effect of this composite in the condition of Cu²⁺ was first reported with very high selectivity and sensitivity. Luminescent Tb-BTC film prepared by EPD on the zinc plate was successfully used for the detection of Cr³⁺ ions in a water solution [78]. Eu_0.24Tb_0.76-BHM-COOH-PLA TFs were successfully applied as sensors for Fe³⁺ [66]. To study the selectivity of the LnMOF-TFs toward Fe³⁺, sensing experiments with introduced interferents—one or more other metal ions—were carried out. The sensitivity of Eu_0.24Tb_0.76-BHM-COOH-PLA LnMOF-TF toward Fe³⁺ (quenching performance) is not influenced by the presence of other cations. It confirms the potential of the mentioned LnMOF-TF for application as a highly selective luminescent sensor for Fe³⁺ in an aqueous media. Also, the concentration-dependent lifetime test was conducted to investigate the mechanism of quenching: as a result, the lifetime of the film (τ = 0.60 ms) in the aqueous system does not change with the increase of Fe³⁺ concentration [78]. This ruled out the cause of dynamic quenching, indicating that the fluorescence quenching process is static quenching.

5.2. Anions Sensing

The Tb-CPON-PMMA polymer composite film, where CPON = 5-(4-carboxy-phenoxy)-nicotinic acid, composed of Tb-CPON and PMMA exhibits superior luminescent properties compared to pure LnMOF [93]. The Tb-CPON-PMMA film exhibits an excellent sensitivity toward Cr₂O₇²⁻, with a high selectivity and detection limit of 5.6 ppb, which is much lower than the maximum contamination standard of 100 ppb in drinking water. The calculated luminescence lifetime of Tb-CPON was 1.032 ms, and the overall quantum yield was determined to be 62.7%. The sensitivity of the Tb-CPON-PMMA toward Cr₂O₇²⁻ manifests itself in the quenching effect. Another luminescent Eu-HBPTC thin film, where BPTC = benzophenone-3,3′,4,4′-tetracarboxylate, was successfully fabricated by EPD in an anhydride system and is highly selective toward carbonate ions in aqueous media [84].

5.3. Small Molecules, Gas, and Vapor Sensing

Sensing based on measuring the luminescent properties has proven to be an excellent detection technique for a variety of chemical substances, including gases and nitroaromatics thanks to its speed and cost-effectiveness. The nanosized Tb-BTC LnMOF-TFs prepared by dip-coating on ITO glass exhibited the highly sensitive and selective detection of organic solvents [69]. Eu-NDC@HPAN deposited on HPAN by the LBL method was used as a self-calibrating luminescent sensor for detecting small molecules, such as formaldehyde, in an aqueous media [85]. The Eu-BDC-NH₂ TFs on glass exhibit strong characteristic Eu³⁺ emissions, which can be fast and remarkably quenched by the presence of SO₂ [73]. The limit of detection (LOD) for SO₂ is calculated to be 0.65 ppm at a response time of 6 s. Polymeric hybrid Eu-MOF-L@PBMA TF functionalized by versatile post-synthetic modification was utilized for the detection of volatile organic vapors, especially organic amines [56]. The luminescent films prepared by electrochemical deposition showed potential toward the detection of nitroaromatic explosives; Tb-BTC on an Al plate has been successfully tested for the detection of 2,4-dinitrotoluene (DNT) [94]. The luminescence lifetime of the sample changed from 0.48 ms to 0.58 ms when the layer was exposed to DNT vapors. Eu-TDC on FTO was used for nitrophenols and nitrobenzene detection in the vapor state [86]. CDs@Eu-MOF/PVDF thin film was prepared using 1,2,4,5-benzenetetracarboxylic acid (H₄BTEC) and 1,10-phenanthroline monohydrate (1,10-PHEN) as ligands by the solvothermal method and modified by carbon quantum dots (CDs) by in-situ growth. The CDs@Eu-MOF composite film has strong red fluorescence along with good stability in methanol and shows high selectivity and sensitivity to nitrobenzene (NB) and 4-nitrophenol (4-NP) [67]. The CDs@Eu-MOF/PVDF film can be used to detect nitrobenzene and 4-nitrophenol in both methanol solvent and gaseous state. The LOD for nitrobenzene and 4-nitrophenol in liquid are 0.2807 mg/L and 0.0168 mg/L, respectively, while in gaseous form, it is 0.346 mg/L and 0.0136 mg/L, respectively. The CDs@Eu-MOF-PVDF film can be used to detect NB and 4-NP. The material retains a good luminescence signal measured in MeOH.

5.4. Temperature Sensing

Luminescence-based sensors for temperature determination have achieved great attention based on their advantages, which include noninvasiveness, high accuracy, and spatial resolution—and (maybe) the most important, the ability to work in strong electro or magnetic fields [20], which makes impossible to use electronics for these purposes. The prepared polymer-MOF hybrid membranes show good temperature-sensing behavior. UiO-66(Zr&Eu)/PVDF can be used as a smart thermometer for the detection of temperature change in the temperature range from 237 to 337 K [94]. The relative sensitivity of the film is 4.26% K⁻¹ at 337 K, which is the highest reported to date for MOF materials at this temperature. It can be explained by the unique energy transfer between the ligand and Eu³⁺ in the clusters. The film showed satisfying temperature-sensing characteristics. The Gd_0.9Tb_0.1HL film on Gd₂O₃ can be used as a thermometer in the range from 110 to 250 K, and a relative sensitivity up to 0.8% K⁻¹, whereas the compound Gd_0.99Tb_0.01HL with a lower Tb content resulted in a relative sensitivity up to 4.4% K⁻¹ at 110 K [46]. Transparent Eu_0.5Tb_0.5(L)1@PMMA film consisting of Eu³⁺/Tb³⁺ lanthanide complexes and polymer PMMA exhibits a brilliant temperature-dependent luminescent behavior from 77 K to 297 K, enabling it to be a candidate for new ratiometric luminescent thermometers [95]. The combined Eu³⁺/Tb³⁺-based thermometer displays higher photo and thermostability compared to the pure complexes.

5.5. Sensing of Biomolecules

In recent years, the analysis of various pharmaceuticals, drugs, their derivates, and further metabolites has become a thing of high priority due to their both actual and possible long-term impact on biological/environmental systems [96,97,98]. Optical sensing has been considered a promising technique as it could offer sensitive, facile, and fast assays. Wang et al. [96] reported the first ratiometric luminescent sensor based on a new water-stable LnMOF polymer thin film (Eu_0.047Tb_0.953MOF@PVDF) prepared by a combination of solvothermal synthesis and spin-coating on the glass substrate, offering high sensitivity and selective detection towards the potential COVID-19 drug favipiravir. The associated MOF drug quenching effect is found to be selective towards other potential COVID-19 drugs. The Eu-TDC-based MMMs as a sensor can efficiently detect nitroimidazole and other antibiotics from the same family, such as dimetridazole and metronidazole (NIABs), in an aqueous system due to its excellent luminescent stability [62]. The Eu-TDC MMMs have the advantages both of polymers and MOFs, which provide high flexibility and extend their applicability. The Eu-TDC MMMs are not affected by other antibiotics or ions when detecting NIABs in water solutions and show lower LODs (0.58 mg/L for metronidazole and 0.51 mg/L for dimetridazole) with a wider linear range. The mechanism operating in NIAB-sensing is considered to be a strong inner filter effect between the Eu-TDC MMMs and NIABs. The LnMOF-TF compound was synthesized using a solvothermal method and subsequently employed to fabricate the mixed-crystal Eu_0.1Tb_0.9-BTC TF by dip-coating it on an ITO substrate [40]. The film’s luminescence varies depending on the guest molecules, making it an excellent candidate for self-referencing and self-calibrating luminescent sensors.

Compared to the most luminescent MOF sensors based on powders, films can be easily reused after washing with the proper solvent. The Eu-TDC MMMs are expected to have recycling properties due to their stability in aqueous media [62]. The reproducibility of Eu-TDC MMMs was determined by recording the fluorescence intensity of the film in an aqueous solution of 0.2 mM dimetridazole and a blank solution after it was washed with tap water. The MMM sensor exhibits an “ON/OFF-ON” switching pattern with almost no change in the fluorescence intensity of the Eu-TDC MMMs after being repeated five times, indicating that Eu-TDC MMMs have excellent reusability. The used Tb-CPON-PMMA strips [93] were washed with distilled water several times. The recorded relative intensities after five cycles showed the luminescence intensity could be maintained after five detection/washing cycles. These results indicate that the Tb-CPON-PMMA-based sensor may be one of the new generations of sustainable test assays for dichromate ions detection, even on-site “in the field”. The fluorescence signal of the Eu-TDC thin film can be recovered by washing it with methanol, which restores the fluorescence intensity to its original state [86]. The rinsed film can then be reused to detect nitrophenol TNP. The results indicate that the fluorescence intensity and quenching ability of the Eu-TDC thin film remain largely unchanged even after five cycles of use. Similarly, the UiO-66(Zr&Eu)/PVDF film thermometer [94] exhibits a stable optical characteristic, as evidenced by the unaltered intensity ratio when subjected to temperature cycles ranging from 237 K to 337 K. Furthermore, the temperature sensing performance of the polymer films surpasses that of the powders, and the sensor can be reused up to three times without any loss in performance.

6. Conclusions and Future Outlook

We summarize the recent progress in luminescent LnMOF-TFs based on Eu, Tb, and Gd from the point of view of preparation techniques and modification methods, along with applications in sensors and light-emitting devices. Covered here are recent technologies based on LnMOF-TFs that have the potential to benefit the energy, manufacturing, and environmental sectors significantly.

The preparation of LnMOF-TFs poses a significant challenge. The methods employed to assemble luminescent LnMOF-TFs form the basis of these challenges. Despite the potential benefits, the commercialization of MOF films is expected to be difficult in the near future due to the requirements for cost-effective scale-up production. Currently, there is no perfect technique that can satisfy all the requirements for commercialization. Direct synthesis and secondary growth strategies based on traditional hydro/solvothermal synthesis remain the most widely used methods. However, other techniques, such as LBL deposition, ECD, EPD, DC deposition, and SC deposition, are prevalent in the manufacturing of LnMOF films. LBL deposition enables the fabrication of various film types, offering excellent control over film thickness and enabling the creation of complex MOF heterostructures. The ECD method allows for the rapid fabrication of defect-free films with controllable thicknesses. The ALD technique provides an easy way to control film thickness at an atomic level and is suitable for fabricating multilayer structures.

Recent advancements in continuous flow microreactors and process automation have opened up new opportunities for advancing progress in LnMOF synthesis. The use of less liquid volume allows for greener synthesis, reducing production costs. Current progress in using automatic methods for LnMOF synthesis has demonstrated promising results in the preparation of composite hybrid films.

Luminescent LnMOF-TFs have received great attention in various applications, both as light-emitting devices and sensors. The LnMOFs based on Eu, Tb, and Gd^, their combinations, and different ligands have been reported, making it possible to integrate these MOFs into practical and useful devices. Lanthanide complexes and LnMOF films have efficacious utility as prospective white light-emitting materials. Ln-based LEDs with excellent color purity have become increasingly common.

The recent research progress on luminescent LnMOF films has an important role to play on their applications in sensing cations, anions, small molecules, nitroaromatic explosives, gases, vapors, temperature, and biomolecules. The sensing functionality of LnMOF films is based on their luminescence changes in response to different analytes. However, the sensing performances are significantly influenced by the film thickness.

A summary and analysis of the existing manufacturing technique progress in recent years for LnMOF luminescent films and their applications as light-emitting devices and sensors is important. It is believed that these advances will definitely extend the applications of LnMOFs to optoelectronic devices and are likely to create impactful innovation in the field of LnMOF films.

Author Contributions: Conceptualization, H.B., E.M., and I.S.; Writing — original draft preparation, H.B., E.M., and I.S.; Writing — review and editing, H.B., E.M., and I.S. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the Grant Agency of the Slovak Academy of Sciences through project VEGA No. 2/0027/23 and APVV-20-0299.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Not applicable.

Conflicts of Interest: The authors declare no conflict of interest.

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This entry is adapted from the peer-reviewed paper 10.3390/inorganics11100376