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]^{[1][2][3][4][5]}. 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 ^[6]. 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) ^[7]. Lanthanide (Ln) ions are known for their high coordination numbers and diverse coordination geometry, which sets them apart from transition metal ions [8,9]^[8][9]. This has led to a surge of interest in Lanthanide-based Metal-Organic Frameworks (LnMOFs) [10,11,12]^[10][11][12]. The energy transfer process, known as “luminescence sensitization” or “antenna effect” ^[13] 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.

The luminescence of LnMOF-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 ^[7]. Mixed-crystal LnMOFs (Ln = Eu + Tb, Eu + Tb + Gd) ^[14], TFs, as opposed to the traditional method based on single lanthanide ions ^[15], produce more stable and precise luminescent signals. Films exhibit permanent porosity and have been utilized for numerous applications, including tunable emission, biosensing ^[16], and thermometers ^[17]. Various hydro/solvothermal and electrochemical synthetic strategies are available for LnMOF-TFs [4,5]^[4][5]. Many methods have been developed for film deposition: directly onto bare or functionalized substrates with organic molecules ^[18]; on seeded substrates; on preformed MOF nanocrystals; layer by layer ^[19]; and electrodeposition ^[5]. 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 ^[20]. LnMOF-TFs represent a promising category of porous crystalline materials that can be customized for various practical applications through versatile post-synthetic modifications ^[21].

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 [6,12]^[6][12] and chemical sensors with single and/or multiple luminescent centers ^[22]. 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. Fabrication Methods of LnMOF-TFs

The previously published reviews were mostly focused on the existing and potential applications, as well as the synthesis methods of MOF-TFs [3,5,23]^[3][5][23]. However, regarding the fabrication approaches, these reviews focused mostly on hydro/solvothermal (HT/ST) synthesis, layer-by-layer (LBL) deposition, and electrochemical depositions (ECD). Taking into account the discussed publications on MOFs in the forms of membranes ^[24], composite structures ^[25], and free-standing nanoparticles (NPs) ^[26], moving forward, the authors will concentrate on the production of Metal–Organic Framework Thin Films (MOF-TFs). WResearchers divided preparation methods into solvo/hydrothermal, layer-by-layer, electrochemical, ultrasonic-spray and atomic-layer/molecular-layer deposition. Composite hybrid films are also a special preparation.

The solvo/hydrothermal growth of films is a simple, effective, and low-cost deposition method that has been widely accepted ^[5]. 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 and modified substrates. The smooth continuous films were reported: Eu_0.5Tb_0.5-BTC ^[27] and Tb-BTC ^[28]. Similarly to the film described in ^[29], porous transparent mixed Ln-BTC films, where Ln = Eu/Gd/Tb, were prepared on Pt/SiO₂/Si substrates from nanocrystals via green solvothermal synthesis [30,31]^[30][31]. Zhang and colleagues pioneered a novel technique called “in situ secondary growth” for the fabrication of lanthanide MOF films ^[32]. They successfully utilized this method to create an Eu-BDC-NH₂ film on ordinary glass, using UiO-66-NH₂ as the seed layer. The layer-by-layer (LBL) assembly method involves the in situ growth of Metal-organic frameworks (MOFs) on various substrates. Chen et al. [33,34] ^[33][34] 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.

The electrochemical deposition (ECD) offers several benefits for the fabrication process of MOF-TFs. The reaction process is more controllable and reproducible. ECD includes cathodic (CED) ^[35], anodic (AED) ^[36], and electrophoretic (EPD) [37] ^[37] depositions. The CED 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 ^[35]. The team of Li et al. [36] ^[36] 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 EPD process is based on the surface charge of the MOF particles suspended in the liquid media. LnMOFs-TFs of Eu_0.45Tb_0.55-BTC were successfully and rapidly deposited on zinc plate in 5 min ^[37].

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 ^[38]. Two or more precursor solutions are used the source of metal ions and organic ligands. 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 ^[38]. 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. ALD can be directly employed to fabricate MOF-TFs via a vapor–solid reaction [39]. Crystalline hybrid MOF-TFs of Eu-NH₂-TA, where TA = 2-aminoterephthalic acid (NH₂-TA), were prepared by ALD/MLD on Si/SiO₂ substrate [39,40]^[39][40].

The composite hybrid LnMOF-TFs were fabricated using modification by carbon dots (CDs) ^[26]; a variety of polymers, such as polyvinylidene difluoride (PVDF) ^[26] ethyl cyanoacrylate ^[24]; polylactic acid ^[25]; polymethylmethacrylate (PMMA) along with a post-synthetic modification (PSM) with butyl methacrylate (BMA), which was initiated by benzoyl peroxide (BPO) ^[21]. Eu_xTb_1−x(L) ^[41] and UiO-66(Zr&Eu) [42] ^[42] MOF films on glass were prepared using PMMA and PVDF, respectively, as a binder.

3. Luminescent Properties of LnMOF-TFs

The luminescent properties of lanthanide ions highly depend on the structural details of their coordination environment. For the preparation of LnMOF films are commonly used ligands with the following structure: p-benzenedicarboxylate (BDC) ^[43]; BDC-NH₂ ^[32]; succinate (SA) [35];^[35]. bromomethylbenzene, dimethyl 5-hydroxy isophthalate (BHM-COOCH) ^[25]; benzophenone-3,3′,4,4′-tetracarboxylate (BPTC) ^[44]; 2,6-naphthalene dicarboxylate (NDC) ^[45]; thiophene-2,5-dicarboxylate (TDC) ^[46]; etc. The structure of BTEC and 1,10-PHEN as ligands, and Eu³⁺ as the metal skeleton ^[26], which adds a certain amount of carbon dots in novel composite film (CDs@Eu-MOF). The luminescence obtained in LnMOFs can be concluded as three types: lanthanide-centered luminescence, ligand-based, and guest-induced.

The luminescence of lanthanide ions originates from the energy transfer processes, also known in the literature as the “antenna effect” or “luminescence sensitization” (LS) ^[7]. 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. White light-emitting Tb₁₀Eu₁-HMA films were fabricated via electrodeposition, and they show good photoluminescence with a satisfactory CIE coordinate of (0.33, 0.34) ^[47]. 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. 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 ^[32]. The obtained Eu-BDC-NH₂ film has a wide range of potential applications, such as sensors, LEDs, solar cells, and TF transistors. 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. 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 ^[26].

4. Light-emitting LnMOF films

The LnMOF thin films have a wide range of potential applications. Those of the most interest include sensors [48,49] ^[48][49] and light-emitting devices (LEDs) ^[50]. 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 ^[33]. These efforts resulted in the development of a new type of LnMOF-TFs, 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 ^[33]. 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). 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 ^[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) ^[21].

5. LnMOF-TF 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 [6,51]^[6][51]. 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].

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. Solvothermal prepared Tb-BDC ^[43] and Tb-SA ^[35] TFs are selective toward Cu²⁺. 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.

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 ^[52]. 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.

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 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) [53]^[42]. 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 ^[46].

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 ^[53][54]. 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 ^[54][55]. The combined Eu³⁺/Tb³⁺-based thermometer displays higher photo and thermostability compared to the pure complexes.

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 [56,57,58]^[55][56][57]. Optical sensing has been considered a promising technique as it could offer sensitive, facile, and fast assays. Wang et al. [56] ^[55] reported the first ratiometric luminescent sensor based on a new water-stable LnMOF polymer thin film (Eu_0.047Tb_0.953MOF@PVDF) prepared by solvothermal synthesis 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.

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 ^[24]. The UiO-66(Zr&Eu)/PVDF film thermometer [53]^[42] 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

We summarize tThe recent progress in luminescent LnMOF-TFs based on Eu, Tb, and Gd from the point of view of preparation techniques, along with applications in sensors and light-emitting devices have been summarized. 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. 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 and ECD deposition are prevalent in the manufacturing of LnMOF films. The ALD technique provides an easy way to control film thickness at an atomic level and is suitable for fabricating multilayer structures. Ultrasonic spray deposition is a new route in future optical devices. 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 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, gases, temperature, and biomolecules. The sensing functionality of LnMOF films is based on their luminescence changes in response to different analytes. 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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