Preparation of Metal-Organic Framework-Gelatin Hydrogels Through Coacervation

Summary

In this article, we present the preparation of MOF-gelatin hydrogels that benefit from the thermoreversible properties of gelatin, associated with its phase separation by coacervation. These hydrogel composites exhibit high MOF loading, large porosity, and a good spatial distribution of MOF particles within the gelatin matrix.

Abstract

An original and reproducible strategy is presented for the synthesis of green and environmentally benign gelatin–Zr(IV) Metal-Organic Framework (MOF) hydrogel composites, and is described in a step-by-step manner throughout the article. The approach exploits the thermo-reversible nature of gelatin and its ability to undergo phase separation through coacervation, combined with the in situ formation of UiO-66–type MOFs under mild conditions. Using this methodology, the present work demonstrates the preparation of a bionanocomposite based on gelatin and the chemically stable Zr⁴⁺ dicarboxylate UiO-66 MOF. Importantly, this strategy can be extended to a broader family of Zr(IV) dicarboxylate and fumarate MOFs (UiO-66–type MOFs and MOF-801, respectively), selected to span a wide range of hydrophilic–hydrophobic balance and organic linker functionalizations. The resulting hydrogel composites display key features such as high MOF loadings (exceeding 50 wt%), high accessible porosity, and a homogeneous dispersion of MOF nanoparticles within the gelatin network. These properties arise from the strong interfacial compatibility between gelatin and MOF nanoparticles, as evidenced by complementary advanced characterization techniques. Owing to their robust mechanical behavior under ambient conditions, the composites can be processed at the gram scale into various macroscopic forms, such as coatings or monoliths, enabling their use in diverse applications, including the capture or catalytic conversion of pollutants in water or air under ambient conditions.

Introduction

Metal–organic frameworks (MOFs) constitute a family of porous, crystalline hybrid materials built from metal ions interconnected by polydentate organic ligands. Owing to their well-defined pore architectures, high internal surface areas, and tunable chemical functionalities, MOFs have attracted considerable scientific attention for diverse applications, including gas storage and separation, sensing, catalysis, biomedicine, and so on¹. Despite this strong potential, several limitations still significantly restrict the large-scale transfer of MOFs toward industrial implementation. One of the major challenges lies in the shaping of such materials². Indeed, conventional MOF syntheses typically yield brittle powders, whose inherent fragility, poor processability, and limited handling properties severely hinder their practical deployment. To address these issues, MOFs have been processed into pellets, granules, beads, fibers, or membranes; however, such shaping procedures often result in a partial or complete loss of accessible porosity².

A widely explored strategy to overcome these drawbacks involves integrating MOFs with supporting matrices, particularly polymers or carbon-based materials such as carbon nanotubes, graphene, and graphene oxide^3,4,5,6. This approach has led to the development of multifunctional composites that combine the intrinsic advantages of MOFs (porosity, crystallinity) with the mechanical robustness and processability of the host materials. In addition, hybridization can introduce new functionalities, including enhanced chemical stability, improved electrical conductivity, or hierarchical porosity^3,4,5,6. As a result, MOF-based composites have been extensively investigated for gas adsorption and separation, water purification, catalysis, and bioapplications^3,4,5,6,7,8. Nevertheless, a recurring limitation of such composites is the pronounced aggregation of MOF particles, which is generally attributed to poor interfacial compatibility between the MOFs and the supporting matrix. This issue is particularly prevalent in composites prepared by physically blending pre-synthesized MOF particles with polymers or graphene oxide and becomes increasingly severe at high MOF loadings, which are typically required to optimize performance^3,4,5,6. To mitigate these challenges, numerous alternative strategies have been proposed to improve the physico-chemical affinity between MOFs and polymeric or carbon-based matrices while preserving MOF porosity. These include surface functionalization of MOF particles, covalent grafting of polymers onto MOF surfaces, in situ growth of MOFs within polymer matrices, polymer-directed MOF crystallization, or the use of polymeric ligands to construct polyMOFs^{3,4,5,6,7,8,9}. Despite these advances, many MOF-polymer composites still suffer from reduced surface areas due to pore blockage by polymer chains and from inhomogeneous MOF dispersion. Moreover, their preparation often relies on multi-step, time-intensive synthetic routes, and the use of petrochemical-derived synthetic polymers raises concerns related to toxicity, sustainability, and environmental impact^3,4,5,6.

The purpose of this article is to share the details of the preparation of a hydrogel composite based on a Zr⁴⁺ dicarboxylate UiO-66–type MOF (UiO = University of Oslo)¹⁰and gelatin. This system is selected as a representative example to illustrate a general synthetic strategy that can be extended to a broader family of polycarboxylate porous MOFs, including functionalized UiO-66 derivatives and MOF-801(Zr). Gelatin is a widely employed protein-based biopolymer in industrial applications such as drug and cosmetic microencapsulation, owing to its low cost, biodegradability, non-toxicity, and renewable origin¹¹. In the past few years, MOF-gelatin hydrogels have been primarily investigated for bio-related applications such as tissue engineering, wound healing, drug delivery, and bioimaging^{12,13,14,15,16,17}. These composites were generally prepared by impregnating preformed MOF particles into functionalized gelatin scaffolds or physical gelatin gels. While such materials exhibit promising biological performance, they typically contain very low MOF contents (below 10 wt%), resulting in limited porosity and consequently, restricting the exploitation of MOF-related physicochemical properties^{12,13,14,15,16,17}. The present article describes a completely different strategy that exploits the thermoreversible properties of gelatin and its ability to undergo liquid-liquid phase separation^18,19 associated with the room temperature (RT) in-situ formation of MOFs. In the protocol described in this article, critical steps in preparing the UiO-66/gelatin composites will be covered, including the preparation of gelatin solution, the gelatin coacervation process, the RT crystallization of UiO-66 within gelatin, and the shaping of the resulting composites. Such composites have been recently reported by some of us to develop adsorbents for cultural heritage preservation, but their protocol of synthesis was not described in detail²⁰. We hope that the concepts and methodology presented in this article will motivate the research community to explore and develop a wider class of MOF-biopolymer composites. These materials simultaneously exhibit high MOF loadings (exceeding 80 wt%), large accessible porosity, homogeneous dispersion of MOF particles within the gelatin matrix, and tunable hydrophilic-hydrophobic characteristics²⁰. A key advantage of employing biomacromolecules such as gelatin lies in their intrinsic chemical complexity, multifunctionality, and polyelectrolyte nature, which have no direct synthetic equivalents. This unique combination enables the effective integration of both hydrophilic and hydrophobic MOFs within a water-soluble biopolymer matrix.

Protocol

1. Preparation of gelatin solutions

1. Use commercial gelatin extracted from porcine skin (type A), with a Bloom of 175 g, an average molecular weight of approximately 40,000 g·mol⁻¹, and an isoelectric point (IEP) of 8.
2. Prepare an aqueous gelatin gel at 10 wt% concentration by swelling the gelatin granules in ultrapure water for a minimum duration of 3 h at 5 °C.
3. Then, prepare an aqueous gelatin solution by heating the gelatin gel at 50 °C. Stir the gelatin solution using a magnetic stirrer for 30 min at 300 rpm. Once a clear gelatin solution is obtained, keep it at a temperature of 50 °C (Figure 1).

Figure 1: Photograph of an aqueous gelatin solution at a concentration of 10 wt%. Please click here to view a larger version of this figure.

2. Synthesis of UiO-66-gelatin hydrogel composite

1. Synthesis of [Zr₆O₄(OH)₄(OAc)₁₂] (OAc = acetate) oxo cluster
   1. In a 20 mL scintillation vial, add 71 µL of a 70% zirconium propoxide solution in 1-propanol [Zr(OPrⁿ)₄] (0.0519 g, 0.158 mmol) to 7 mL of dimethylformamide (DMF) and 4 mL of acetic acid (4.196 g, 70 mmol) (Figure 2A).
   2. Heat the resulting mixture in an aluminum block on a hot plate at 130 °C for 2 h, during which a distinct color change from colorless to yellow is observed (Figure 2B).
   3. Leave the solution to cool at RT.
2. In situ synthesis of UiO-66(Zr) in the presence of gelatin coacervate
   1. Mix 11 mL of the {Zr₆} oxo-cluster solution (0.158 mmol) obtained in step 2.1 with 0.5 mmol (84 mg) of 1,4-benzene dicarboxylic acid (1,4-bdc) organic linker.
   2. Stir the mixture for 15 min until a homogeneous dispersion is obtained.
   3. Then, add 2 mL of a 10 wt% gelatin solution, prepared in task 1 at 50 °C to this solution. (Figure 3).
   4. Stir the mixture at RT for 10 min, followed by the addition of 2 mL of ethanol.The solution turns turbid and white. 
   5. Stir the resulting mixture at RT (20 °C) for 24 h. A liquid-liquid phase separation takes place progressively over time.
      NOTE: Just after the addition of ethanol, the initial solution is white and turbid (Figure 4A) and then evolves with time to a macroscopically biphasic sample composed of a dilute solution of gelatin floating above a denser white viscous phase (Figure 4B–F).
   6. For a complete phase separation, add 10 mL of ethanol to this mixture. After stirring the solution, extract the viscous phase from the supernatant solution in order to obtain the hydrogel composite (Figure 5).
   7. Leave this composite to dry at room temperature and further characterize it by combining powder X-ray diffraction (PXRD), thermogravimetric analysis, Fourier Transform Infrared (FT-IR) spectroscopy, N₂ porosimetry, scanning and transmission electron microscopy (SEM and TEM), solid state nuclear magnetic resonance (NMR) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) as previously reported²⁰ (Figure 6).
3. Shaping of UiO-66(Zr) /gelatin into monoliths
   1. Heat the viscous phase obtained in step 2.2.6, along with its supernatant, at 70 °C for 15 min, followed by 90 °C for 5 min.
      NOTE: This treatment aimed to transform the MOF-gelatin hydrogel into a homogeneous liquid and viscous mixture that can be readily poured into a mold.
   2. Then, pour the hydrogel without its supernatant solution into the mold thermalized at 70 °C.
   3. Form a monolith by cooling at RT for 15 min, then keep it at 2 °C for 1 day (Figure 7).

Figure 2: Photographs of a solution containing 71 µL of a 70% zirconium propoxide solution in 1-propanol [Zr(OPrⁿ)₄] (0.0519 g, 0.158 mmol), 7 mL of DMF, and 4 mL of acetic acid. (A) Before and (B) after heating at 130 °C for 2 h. Please click here to view a larger version of this figure.

Figure 3: Photograph of the sample obtained after mixing 11 mL of the [Zr₆O₄(OH)₄(OAc)₁₂] oxo cluster (0.158 mmol), the 1,4-bdc linker (0.5 mmol), and 2 mL of the 10 wt% aqueous gelatin solution at RT. Please click here to view a larger version of this figure.

Figure 4: Photographs of samples containing the [Zr₆O₄(OH)₄(OAc)₁₂] oxo cluster, 1,4-bdc linker, and the 10 wt% aqueous gelatin solution at RT after the addition of 2 mL of ethanol. The photographs show the progressive liquid-liquid phase separation of gelatin upon aging the samples at different times: (A) 0 min, (B) 15 min, (C) 1 h, (D) 2 h, (E)18 h, (F) 24 h. Simultaneously, the reaction between [Zr₆O₄(OH)₄(OAc)₁₂] oxo cluster and 1,4-bdc linker gave rise to the formation of UiO-66(Zr) nanoparticles. Please click here to view a larger version of this figure.

Figure 5: Photographs of samples containing gelatin and UiO-66(Zr). (A–D) Before (A,C) and after (B,D) adding 10 mL of ethanol. The solution initially white and turbid (A,C) turns into macroscopically biphasic samples containing a diluted gelatin solution and a viscous hydrogel (B,D). Please click here to view a larger version of this figure.

Figure 6: Characterization of UiO-66/gelatin hydrogel. (A) Experimental PXRD pattern of UiO-66/gelatin in comparison to the calculated pattern of UiO-66. (B) N₂ porosimetry (adsorption, filled symbols; desorption, empty symbols) of UiO-66/gelatin in comparison to pure UiO-66 nanoparticles at 77 K (p°=1 atm). (C) TEM image of an 80 nm ultramicrotomed slice of the UiO-66/gelatin hydrogel. (D) SEM images of the UiO-66/gelatin hydrogel after washing in water. This figure has been reprinted (adapted) with permission from Biswas et al.²⁰. Please click here to view a larger version of this figure.

Figure 7: Photograph of shaped UiO-66/gelatin composite. Please click here to view a larger version of this figure.

Results

The UiO-66(Zr)/gelatin hydrogel composite was synthesized by coupling the simple coacervation of gelatin with the in situ formation of UiO-66(Zr) nanoparticles. The synthetic protocol was repeated multiple times to ensure robustness, consistently yielding reproducible results (i.e., structural and physico-chemical characteristics of the composites) under identical experimental conditions. UiO-66(Zr) nanoparticles were prepared at RT following a previously reported two-step protocol, which involves the synthesis ...

Discussion

Composites combining gelatin with inorganic materials such as metal oxides, hydroxyapatite, and layered double hydroxides have attracted significant interest due to their ability to integrate the biocompatibility and processability of gelatin with the functional properties of inorganic phases^24,25. Gelatin provides a versatile organic matrix that can be easily shaped, crosslinked, and processed in aqueous media, while inorganic components impart mechan...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1,4 benzene dicarboxylic acid	Acros Organics.	180720010
70% zirconium propoxide [Zr(OPrn)4] solution in 1-propanol	Sigma-Aldrich	333972-100ML
Acetic acid	Carlo Erba	524520
Avance-NEO spectrometer	Bruker		NMR
Dimethyl formamide	Carlo Erba	528221
Ethanol	Carlo Erba	528131
Gelatin	Sigma-Aldrich	G2625-100G
Magna 550 Nicolet spectrophotometer	Nicolet	550 series	FTIR
Scanning electron microscope	Jeol	JSM-7001F	SEM
Thermogravimetric analyzer	Model Perkin Elmer SDA 6000
Trasmission electron microscope	Jeol	2100F	TEM
X’Pert MDP diffractometer	Siemens	D5000	XRD