Recent Advances in Lanthanide Metal–Organic Framework Thin Films Based on Eu, Tb, Gd: Preparation and Application as Luminescent Sensors and Light-Emitting Devices

Round 1

Reviewer 1 Report

The manuscript entitled " Recent Advances in Lanthanide Metal-Organic Framework Thin Films based on Eu, Tb, Gd: Preparation and Application as Luminescent Sensors and Light-Emitting Devices ", The novel began with insight into various type of MOFs and summarised recent achievements in production of LnMOF films, along with various film preparation approaches. Afterward, it discussed the applications of luminescence features of lanthanide ions in films and their potential as white-light source materials. It also covered films based on Eu, Tb, and Gd with particular accents on different design approaches. Moreover, specifically, luminescent features applied for sensing temperature, variety of ions, gases, and biomolecules are highlighted.

The paper is quite interesting. However, the following points need to be addressed before it can be considered for publication:

1.     There is no mention of cost comparisons between the various preparation methods in the article.

2.     For pure MOF films, corresponding XRD, FTIR and sample photos should be put up.

3.     Pure MOF is difficult to form a thin film, have the authors bothered to check the literature to see if a binder is used, if it is not a MOF film, it should be changed to a MOF layer.

4.     The advantages, disadvantages and differences between pure MOF films and MOF composite films are not addressed in the article.

5.     The article lacks a systematic comparison of the properties and relative strengths/weaknesses of MOF films with other materials.

The English proficiency of this article is generally good, and there is no need for major modifications or adjustments.

Author Response

The Responses to the Reviewers (Manuscript ID: Inorganics-2615490)

Title: Recent Advances in Lanthanide Metal-Organic Framework Thin Films Based
on Eu, Tb, Gd: Preparation and Application as Luminescent Sensors and
Light-Emitting Devices

Dear Reviewer,

Thank you for your comments on the manuscript (ID: Inorganics-2615490) by H. Brunckova et al. I have made some revisions by following your valuable suggestions.

Reviewer 1

Comments and Suggestions for Authors

The manuscript entitled " Recent Advances in Lanthanide Metal-Organic Framework Thin Films based on Eu, Tb, Gd: Preparation and Application as Luminescent Sensors and Light-Emitting Devices ", The novel began with insight into various type of MOFs and summarised recent achievements in production of LnMOF films, along with various film preparation approaches. Afterward, it discussed the applications of luminescence features of lanthanide ions in films and their potential as white-light source materials. It also covered films based on Eu, Tb, and Gd with particular accents on different design approaches. Moreover, specifically, luminescent features applied for sensing temperature, variety of ions, gases, and biomolecules are highlighted. The paper is quite interesting. However, the following points need to be addressed before it can be considered for publication:

1. There is no mention of cost comparisons between the various preparation methods in the article.

A: Thank you for your comment. The various methods of MOF thin film preparation have different effects on their structure, thickness, and properties. We included information on cost comparisons between the various preparation methods.

Solvothermal growth of films is a simple, effective, and low-cost deposition method, and for 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.

The LBL method has many advantages such as well-controlled thickness, surface roughness, and mild reaction at room temperature, there are still some disadvantages such as repeated operations and long reaction times [5,8].

Compared to the HT and ST methods, the ECD offers several advantages for the preparation process of MOF-TFs such as short growth time, and low price of the necessary equipment [8,85]. The continuous production process guaranteed its attractiveness for industry, especially for mass production. The reaction process is more controllable and reproducible. Nevertheless, there are relatively few reports on LnMOF thin films prepared by the ECD method. However, this method has some disadvantages that limit the application, such as difficulty in growth control. Large amounts of chemicals and solvents are needed to prepare MOF films, which cause environmental pollution and increase costs.

Ultrasonic spray deposition as a time-saving, low-cost, and new route for the fabrication of luminescent MOF films, can be considered an advance for the integration of LnMOFs in future optical devices [5,48].

As a state-of-the-art industrially thin film deposition technique, offers a simple way to control the film thickness at the atomic level and is available for the preparation of multilayer structures [8]. ALD equipment is expensive and the process is slow, which increases operating costs. Due to its small size, a microreactor can offer several advantages, including rapid mixing of reactants. The use of a smaller volume of liquid enables a more ecological synthesis and reduces production costs.

These parts are incorporated in the chapter "2. Preparation methods of LnMOF-TFs" section of the Manuscript on pages 3, 7, 8, 9, 10, and 11, and highlighted in yellow color.

2. For pure MOF films, corresponding XRD, FTIR, and sample photos should be put up.

A: Thank you for your comment. Figure 3 was corrected. Figure 3 included 3a) FTIR, 3b) XRD, and 3c) photos of prepared samples.

Figure 3. a) FTIR spectra, b) XRD, and c) photos of LnMOF powders and thin films; d) TEM, e) SEM, and f) isometric AFM micrographs of Eu_0.25Gd_0.5Tb_0.25MOF thin film prepared green hydrothermal synthesis. Courtesy of the authors (unpublished work).

Figure 3 shows the FTIR spectra of LnMOF (Ln-BTC, Ln = Eu, Gd, Tb, Eu_0.25Gd_0.5Tb_0.25) powders. In the spectra (Figure 3a), the wide peak at 3435 cm^-1 is assigned to n(OH) groups. The effect of acetate groups from sodium acetate for LnMOF can be noticed in the regions at 2995, 2780, and 2430 cm^-1, which are assigned to stretching n(C-H) vibrations. In the spectra, the bands in zones 1560-1520 cm^-1 and 1385 cm^-1 were marked as stretching vibrations of the COO^- groups n_as and n_s, respectively. The bands of corresponding COOH groups designate the complete deprotonation of the carboxylic acid and coordination of COO– groups to the lanthanide centre The peak that appeared at 565 cm^-1 can be assigned to the stretching vibration of Ln-O. Figure 3b shows the XRD patterns for both samples (powder and film). For TbMOF powder, the results were compared with the crystallographic data in the Cambridge Database: CIF no. 617492 for Tb-BTC; the match confirms the expected tetragonal phase for Tb-BTC [71]. Similar to the XRD of crystalline powder, the Eu_0.25Gd_0.5Tb_0.25MOF film reveals peaks at 10.5, 11.5, 16.1, 21.3, and 29.3° (2q) and Pt and Si peaks from the substrate. In Figure 3c, the photographs of prepared TbMOF powders and various films (TbMOF and EuGdTbMOF) on Pt/SiO2/Si substrate were shown.

These changes are incorporated in the "2.1. Solvo/Hydrothermal deposition" section of the Manuscript on pages 5-6 and are highlighted in yellow color.

We included also a description of the XRD data of other films prepared by various methods. The XRD data of Ln-BTC films are in good agreement with the tetragonal structure P₄322 space group similar to Y(BTC)(H₂O) [40,71]. The XRD peaks of Eu-BDC-NH₂ film slightly shifted to a lower degree compared to that of UiO-66-NH₂, mainly attributed to the larger ion size of Eu³⁺ compared to the Zr⁴⁺ [73].

The XRD data of the Ln-SURMOFs reveal the presence of highly crystalline, oriented films with a sharp diffraction peak at 8.52° (2q) [83].

The XRD signals of the Tb-SA film agree with the simulated XRD pattern of Holmium-SA [87].

The XRD data of the Tb-BTC MOFs, are in agreement with the simulated La(BTC)(H2O)₆ [89].

The XRD patterns of the Tb₃(BDC)₃ MOF films deposited onto a glass slide at different temperatures have a structure according to the respective JCPDS card (00-157-1127) [48].

The XRD patterns of Eu-TDC MMMs contain the peaks of Eu-MOF and EVOB, indicating that the integrities of the MOF particles were well maintained during the preparation of the film [62].

XRD reveals that the diffraction peaks of Eu-MOF and CDs@Eu-MOF are the same, indicating that the CDs have no impact on the crystal structure of Eu-MOF due to the small size of CDs and their embedding into the MOF [67].

These changes are incorporated in the "2. Preparation methods of LnMOF-TFs " section of the Manuscript on pages 4, 7, 8, 9, 10, and 12, and are highlighted in yellow color.

3. Pure MOF is difficult to form a thin film, have the authors bothered to check the literature to see if a binder is used, if it is not a MOF film, it should be changed to a MOF layer.

A: Thank you for the recommendation. We prepared our LnMOF films on Pt/SiO₂/Si substrates from corresponding powders obtained by simple green solvothermal synthesis (EtOH/H₂O/NaOAc). Tb-BTC or Eu/Gd/Tb-BTC MOF powders were individually redispered in H₂O to obtain a milky colloidal solution with a concentration LnBTC of 0.03 g/ mL. The mixed slurry was dropped, and deposited on a pre-cleaned silicon substrate covered with SiO₂ and layer Pt nanoparticles [71,72].

In the Eu_0.24Tb_0.76-BHM-COOH-PLA film, the polylactic acid (PLA) layer is substrate [66], it is not a binder. We corrected the text. For composite films, we have added information about substrates and modification with polymers as binders. We checked the literature [66] on what binder is used in MOF films.

Figure 7 depicts the various film preparation processes. Eu_xTb_1-x(L) and UiO-66(Zr&Eu) MOF films on glass were prepared using PMMA and PVDF, respectively, as a binder [110,108]. PMMA or PVDF were dissolved in DMF solution (for better dispersion of MOF powders).

These MOFs can be used as a reliable bimetallic Ln-MOF luminescent sensing platform by simply combining a polylactic acid (PLA) layer as substrate with Eu_0.24Tb_0.76-BHMCOOH film (Figure 7). This platform outperforms the traditional MOF-based ones applied for Fe³⁺ ions detection [66].

These changes are incorporated in the "2.6. Composite hybrid films" section of the Manuscript on page 12 highlighted in yellow color.

4. The advantages, disadvantages, and differences between pure MOF films and MOF composite films are not addressed in the article.

A: Thank you for your comment. The advantages or disadvantages of pure and composite films are connected with their preparation, synthesis of various ligands, and purpose of use.

2.8. Differences between pure and composite films

LnMOF films have distinct advantages over powder sensors, such as portability, good stability, and recyclability [72]. Single-Ln³⁺ LnMOFs show fundamental luminescent phenomenon, while mixed-Ln³⁺ LnMOFs exhibit significant ability of tunable white light emission and temperature measurement [8]. Pure MOFs are promising sensing materials because of the tremendously large surface area and porosity that can enhance the surface reactions with the analytes [25]. The combination of MOF materials with high specific surface area, regular pore size, and tunable structure with polymer, is an important material for the preparation of composite film with multiple applications [58]. The multi-color luminescence of Eu-MOF-L@PBMA polymeric hybrid films is tuned by optimizing the critical parameters to serve as a white-light emitting device. Furthermore, film is utilized for the detection of volatile organic vapors [56].

These changes are incorporated in the "2.7. Differences between pure and composite films" section of the Manuscript on page 13 highlighted in yellow color.

5. The article lacks a systematic comparison of the properties and relative strengths/weaknesses of MOF films with other materials.

A: Thank you for your comment.

MOFs are available in several structures, such as nanocrystals, nanospheres, nanosheets, needles, thin films, membranes, and glasses [61]. Amongst these structures, LnMOF-TFs have attracted more attention due to their great potential in the development of nanotechnology applications in lighting, optical communications, photonics, and biomedical devices [61]. Porous nano-size MOFs possess some potential advantages over conventional nanomaterials. The structural and compositional diversity allows the production of LnMOF films of different compositions, shapes, sizes, and physico-chemical properties [8].

These changes are incorporated in the "2.7. Differences between pure and composite films" section of the Manuscript on page 13 highlighted in yellow color.

Yours sincerely,

Helena Brunckova

Institute of Materials Research

 Slovak Academy of Sciences

Watsonova 47, 040 01 Kosice, Slovakia

Author Response File: Author Response.pdf

Reviewer 2 Report

The review by Helena Brunckova et al. focuses on a current at the same time hot topic in materials chemistry, combining metal-organic framework compounds and luminescent materials. The review is well structured and presents interesting, detailed material on preparation methods and properties.
The main remark is to insert a section on the ligands used, indicating their structures and the nature of functional groups involved in coordination with lanthanide ions. Second - analyze the properties (luminescence) from the coordination environment of lanthanide ions (LnO8 geometry). On europium complexes it is easier to do it based on the emission spectra.

Author Response

The Responses to the Reviewers (Manuscript ID: Inorganics-2615490)

Title: Recent Advances in Lanthanide Metal-Organic Framework Thin Films Based
on Eu, Tb, Gd: Preparation and Application as Luminescent Sensors and
Light-Emitting Devices

Dear Reviewer,

Thank you for your comments on the manuscript (ID: Inorganics-2615490) by H. Brunckova et al. I have made some revisions by following your valuable suggestions.

Reviewer 2

Comments and Suggestions for Authors

The review by Helena Brunckova et al. focuses on a current at the same time hot topic in materials chemistry, combining metal-organic framework compounds and luminescent materials. The review is well structured and presents interesting, detailed material on preparation methods and properties.

The main remark is to insert a section on the ligands used, indicating their structures and the nature of functional groups involved in coordination with lanthanide ions.

A: Thank you for your comment. We included a new section.

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 ions-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 among 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 for 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, sulfonate, amine, etc [1].

The benzenetricarboxylate ligand is often used to prepare LnMOF films, e. g. Tb-BTC [71]. The 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 [71]. The TbMOF (Tb-BTC) complex is a 3D open framework, and each asymmetric unit contains one eight-coordinated Tb³⁺ ion, one BTC ligand, two coordinated DMF molecules, and one free guest water molecule H₂O as Tb(BTC)(DMF)₂.H₂O [71]. Each Tb³⁺ ion is coordinated with eight oxygen atoms from four BTC ligands through two chelating bidentate carboxylate groups, two monodentate carboxylate groups, and two terminal DMF molecules. The empirical formula is C₁₅H₁₉N₂O₉Tb [71].

For the preparation of LnMOF films commonly are used ligands with the structure: p-benzenedicarboxylate (BDC) [93], BDC-NH₂ [73], succinate (SA) [87], bromomethylbenzene, dimethyl 5-hydroxy isophthalate (BHM-COOCH) [66],   benzophenone-3,3',4,4'-tetracarboxylate (BPTC) [94), 2,6-naphthalene dicarboxylate (NDC) [81], thiophene-2,5-dicarboxylate (TDC) [86], etc. The structure of BTEC and 1,10-PHEN as ligands, Eu3+ as the metal skeleton [67], adding a certain amount of carbon dots in novel composite film (CDs@Eu-MOF) are shown in Figure 7.  A BHM-COOCH₃ ligand was synthesized for the preparation of Eu_xTb_1-x-BHM-COOH [66]. Additional structures of the ligands H₂NDC 2-FBA [108] and BDC-NH₂[56] are observed in schematic illustrations.

The change is incorporated in the "3.1. Structure of ligands involved in coordination with lanthanide ions" section of the Manuscript on page 13, 14 and highlighted in green color.

Second - analyze the properties (luminescence) from the coordination environment of lanthanide ions (LnO8 geometry). On europium complexes it is easier to do it based on the emission spectra.

A: Thank you for your comment. We included the luminescence of Eu-SURMOF.

For monitoring the emission of singlEu-SURMOF at 617 nm, the excitation spectrum exhibits a broadband shoulder at 250 nm ascribed to π*←π/π*←n ligand-based transitions [83]. Eu-SURMOF exhibits a magenta-colored emission consisting of the ligand π*→π/n*→π transitions and the typical for Eu^{3+ 5}D₀→⁷FJ (J = 0-4) transitions with ⁵D₀→⁷F₂ as the highest intensity located at 617 nm. The high-relative intensity of the ligand-based emission indicates a non-efficient energy transfer process from the excited states of the ligand to Eu³⁺ ions. The coordination polyhedron [LnO8] changes from  π*←π/π*←n ligand-based transitions.

The change is incorporated in the "4. Light-emitting devices" section of the Manuscript on page 15 and is highlighted in green color.

Yours sincerely,

Helena Brunckova

Institute of Materials Research

 Slovak Academy of Sciences

Watsonova 47, 040 01 Kosice, Slovakia

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Ok. The author has basically answered and revised the questions raised, and it is suggested to be published.