Sarah V. Dummert‡
a,
Sylvia Dörschmidt‡
a,
Theresa Bloehs
a,
Katia Rodewaldb,
Miriam Cavigliac,
Claudia Schmidtc,
Mian Zahid Hussain
a,
Julien Warnan
a,
Roland A. Fischer
*a,
Angela Casini
*c and
Romy Ettlinger
*a
aChair of Inorganic and Metal–Organic Chemistry, Department of Chemistry, TUM School of Natural Sciences, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching, Germany. E-mail: romy.ettlinger@tum.de
bWACKER-Chair of Macromolecular Chemistry, Department of Chemistry, TUM School of Natural Sciences, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching, Germany
cChair of Medicinal and Bioinorganic Chemistry, Department of Chemistry, TUM School of Natural Sciences, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching, Germany
First published on 30th June 2025
Metal–organic frameworks (MOFs) are promising candidates for drug carrier systems due to their high porosity and tuneable structures, however, their clinical translation is restrained. Integrating MOFs into processable matrices improves mechanical properties, processability, and often drug delivery performance. Hydrogels, as soft, three-dimensional polymer networks with high flexibility and biocompatibility, are particularly favourable candidates for advanced MOF-based drug carriers. However, a lack of fundamental material studies limits full exploitation of the potential and hinders further development of such composites. To address this, this study provides a physicochemical investigation of MOF/alginate hydrogels using ZIF-8 as a benchmark MOF and thioflavin T (ThT) as a model drug. A rapid, in situ encapsulation approach enabled the fabrication of ThT@ZIF-8 (14.2 wt% loading), which was incorporated into an alginate matrix (ThT@ZIF-8@Alg) at 95 wt%, putting MOF carrier functionality in a processable form. Characterisation including X-ray diffraction, infrared and diffuse-reflectance UV/Vis spectroscopy, and electron microscopy enabled a detailed investigation of MOF properties in the composite and confirmed its retained structural integrity. Drug release studies of ThT@ZIF-8@Alg closely mirrored the pure MOF's pH-triggered behaviour. Furthermore, by comparing different methods of incorporating ThT in (ZIF-8@)Alg matrices, we demonstrate the versatility of such composites in achieving customisable release profiles. In vitro preliminary studies of the antiproliferative activity of ThT@ZIF-8@Alg in cancerous and non-tumorigenic cells support the idea of sustained controlled release of ThT over 72 h at pH 7.4. This strategy advances MOF-hydrogel-based drug delivery systems, with potential applications in topical treatments and implant coatings.
Given these challenges, metal–organic frameworks (MOFs) offer a promising platform to overcome problems faced by other nanocarriers, which often suffer from poor formulation stability, low drug loading efficiencies or a lack of control over release rates.3–9 MOFs, as solid-state coordination polymers with open framework structures, comprise metal nodes (ions or clusters) connected by multitopic organic ligands, offering remarkable versatility in composition, functionalization, and pore architecture.6,10–12 The porosity and large internal surface areas of MOFs have driven interest in their biomedical applications, with a considerable step forward in the field marked by the transition from laboratory research to first clinical trials in 2023.13 Their selective and high adsorption capacity for a plethora of active drug agents is one of the key features that make MOFs particularly attractive for DDS.14–17 However, while several lipid- and polymer-based drug delivery systems are in clinical trials or have already reached the market, MOFs are trailing behind.18–20
Alongside other factors, this can be attributed to limitations associated with pure MOF powders, such as poor mechanical properties. By combining MOFs with processable polymer matrices, materials with improved drug loading capacities, release kinetics, and targeting capabilities have been developed,21–23 ranging from oral formulations to injectable depots, transdermal patches and tissue engineering.24–32 Apart from synergistic property enhancement, the association of MOFs with polymers leverages their more established stand as drug delivery platforms. In this regard, incorporating MOFs into hydrogels—a class of soft, three-dimensional polymer networks with high water content and flexibility—offers complementary benefits,33,34 with the hydrogel serving as a MOF reservoir for controlled release of therapeutic payloads, and the MOFs enhancing the hydrogel's mechanical properties and stability.34–36 Among the various options of hydrogel-forming polymers, alginate particularly stands out.33 The natural polysaccharide forms hydrogels through cross-linking with divalent cations in a straightforward and well-controllable manner. Alginate hydrogels are highly biocompatible and biodegradable, their structure resembling extracellular matrices of living tissues.37–39 While some studies have tapped into the potential of MOF/alginate hydrogels for drug delivery, sensing, and wound healing, research has largely been driven by specific target applications rather than a comprehensive understanding of the fundamental properties of these hybrid materials.40–43 Key aspects that require further investigation include a comparative investigation of MOF and drug incorporation modes, ambidexterity of functional material and hydrogel matrix, and detailed analysis of the role of MOFs as carriers and how they are impacted by hybridization.
In this study, we provide an in-depth physicochemical investigation of MOF/alginate hydrogels as drug delivery platforms. The main focus is to provide structural and chemical characterization of MOF/hydrogel composites, addressing a gap in the literature where such properties are often overlooked in favour of biological endpoints. Assessing the MOF's effect in the matrix, key design strategies are identified to tailor these materials for specific carrier applications.
For this purpose, we intentionally chose well-characterized pristine materials to limit the variability of the unknowns in the resulting composite. Thus, ZIF-8 (ZIF = zeolitic imidazolate framework) is used as a benchmark MOF composed of zinc nodes and 2-methylimidazole linkers, featuring a highly porous structure with size-amendable cavities.44 ZIF-8 is thermally and chemically stable, and can be synthesized under mild conditions within minutes.45 Moreover, it exhibits good biocompatibility and has previously been used to encapsulate a wide range of drugs, from small molecules to proteins.46–50 Here, we use the benzothiazole dye thioflavin T (ThT) as a model drug, a widely used ‘gold standard’ for selectively staining and to identify amyloid fibrils.51,52 As a strongly absorbing dye (‘basic yellow’, λmax,abs. = 412–413 nm),53,54 it is easily traceable (detection limit: <0.1 μg mL−1). A fast, in situ encapsulation approach to obtain the drug-loaded MOF, denoted as ThT@ZIF-8, is presented. Using an automated method, ThT@ZIF-8 was brought into an alginate matrix, resulting in composite materials (ThT@ZIF-8@Alg) with uniform shape and homogeneous properties. Both materials were thoroughly characterized, including X-ray diffraction, UV/Vis spectroscopy, scanning electron microscopy, gas sorption measurements and infrared spectroscopy. Composites were fabricated with a ZIF-8 and ThT content of 95 wt% and 13.5 wt%, respectively, with pristine MOF properties and drug carrier performance retained while offering a processable, shaped, macroscopic format. Through studies of four different drug incorporation strategies to (MOF@)alginate matrices and follow-up release experiments, the role of ZIF-8 as a protective shell for the drug and in achieving high payloads is highlighted. With the drug@MOF synthesis decoupled from a facile composite fabrication method, an auspicious approach for the development of tailorable MOF/hydrogel materials for biomedical applications is offered.
The synthesised MOFs show characteristic reflections at 2θ values of 7.4° (110), 10.5° (200) and 12.8° (211) in the powder X-ray diffraction (PXRD) patterns, matching with the reported diffractogram (CCDC: 864309) and thus confirming the successful synthesis of ZIF-8 and ThT@ZIF-8 (Fig. 1a).56 For the latter, a colour change of the originally white MOF powder to yellow was observed. This is evidenced by the diffuse-reflectance ultraviolet-visible (DR-UV/Vis) spectra (Fig. 1b) of both species. While ZIF-8 shows absorbance only <250 nm, ThT@ZIF-8 exhibits two additional bands at 423 and 298 nm, in line with that of ThT. The strongly absorbing dye shows a DR-UV/Vis main absorption band at 423 nm in pristine state, however, upon changing the pH, the ThT absorption spectrum is known to undergo significant reversible changes. In basic solutions, the intensity of the main absorption band decreases while the intensity of the weaker band at ∼300 nm increases, caused by hydroxylation of the carbon in the thiazole group.57–59 This effect is also visible in the DR-UV/Vis spectrum of ThT@ZIF-8, with the peak at 298 nm stronger than that at 423 nm, which can be ascribed to the basic nature of the pore environment in ZIF-8.44,60 The successful encapsulation of ThT is further confirmed in the Fourier-transform infrared (FTIR) spectrum of ThT@ZIF-8 (Fig. 1c, full spectra shown in Fig. S4, ESI†). Three distinct ThT signals are observed at 1600 cm−1 (CC stretching), 1208 cm−1 (Cbenz–H stretching), and 829 cm−1 (Cbenz–H bending), alongside the ZIF-8 bands. Additionally, several weaker ThT signals are overlapping with the MOF spectrum, particularly in the region between 1600 and 1200 cm−1. The absence of significant shifts of ZIF-8 bands suggests that there are no strong interactions between the guests and the host, e.g., at the nodes. Interestingly, the presence of ThT during ZIF-8 synthesis affected the crystal morphology, as observed in the scanning electron microscopy (SEM) images in Fig. 1d. Similar to previously-described structure-directing agents,61,62 ThT shows a modulating effect, changing the rhombic dodecahedral crystals of pristine ZIF-8 (top picture) towards cubic shapes of ThT@ZIF-8 crystals (bottom picture), as well as reducing the average size from ∼200–400 nm to ∼100–300 nm. To visualise the particle size distribution of ThT@ZIF-8, transmission electron microscopy (TEM) images were recorded and analysed, showing an average crystal size of 103 ± 40 nm (Fig. S5, ESI†). To further confirm the successful encapsulation and molecular integrity of ThT in ZIF-8, 1H nuclear magnetic resonance (NMR) spectra were recorded after digestion of ThT@ZIF-8 in DMSO-d6 and DCl. The signals of both organic components of the material, 2-methylimidazole and ThT, can be identified in the 1H-NMR, as shown in Fig. S6 (ESI†). The ThT loading in ThT@ZIF-8 was quantified to be 14.2 wt% via solution ultraviolet-visible (UV/Vis) spectroscopy after digestion of ThT@ZIF-8 in diluted acetic acid, for which details and the calibration curve can be found in the ESI† (Section S2 and Fig. S1). Based on this, a theoretical pore filling degree of ∼19% and ThT loading of ∼0.74 molecules per pore can be estimated (eqn (S1) and (S2), ESI†). Considering the ZIF-8 pore size relative to ThT's dimensions, the theoretical ThT loading per pore falls within an expected range. Since ThT is too large (3–4 Å × 15–16 Å) to enter the pores of ZIF-8 (aperture: 3.4 Å, diameter: 11.6 Å, based on the ideal theoretical structure) post-synthetically, it can only be encapsulated in high amounts through co-crystallization during MOF formation, where the framework forms around the guest molecule and sterically traps it in the pores.60,63 This aligns with the above-mentioned synthesis screening reactions (ESI,† Section S3), which showed that only negligible ThT loading could be achieved via an impregnation approach, and increasing ThT concentrations in the one-pot reactant solution beyond a certain point (10 mg mL−1) did not result in loadings higher than 14.2 wt%. Thermogravimetric analysis (TGA) of ThT@ZIF-8 shows an initial loss of residual solvent, then decomposition of ThT starting at ∼75 °C, followed by degradation of the ZIF-8 structure from ∼350 °C onwards, which is in agreement with literature reports (Fig. S7, ESI†).64 From the residual masses at 800 °C (ZIF-8: 33.0%, ThT@ZIF-8: 29.1%, ThT: 2.1%), a ThT loading of 12–13 wt% can be estimated, which is in line with the loading determined via UV/Vis. Gas sorption isotherms (N2, 77 K) recorded of ThT@ZIF-8 (Fig. S8, ESI†) show the typical Type I shape of microporous ZIF-8.44,65 In comparison with pristine ZIF-8, a reduction of N2 uptake from 533 to 463 cm3 g−1 (13%) and of the calculated Brunauer–Emmett–Teller (BET) specific surface area from 1678 to 1554 m2 g−1 (8%) was observed in ThT@ZIF-8. Notably, the pore size distribution (PSD) (Fig. S9, ESI†) reflects the incorporation of ThT with a reduction of 17% of the available pore volume, which aligns well with the theoretical degree of pore filling calculated from the loading obtained from UV/Vis analysis.
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Fig. 1 (a) PXRDs of ZIF-8 (brown), ThT@ZIF-8 (blue) and a reference pattern of ZIF-8 (black, CCDC: 864309),52 (b) DR-UV/Vis spectra of ZIF-8, ThT@ZIF-8 and ThT powder as reference (black), (c) FTIR spectra shown from 1800 to 650 cm−1 of ZIF-8, ThT@ZIF-8 and ThT powder as reference, (d) SEM images of ZIF-8 (top) and ThT@ZIF-8 (bottom). |
Building on these initial screenings, alginate composites were made with ThT-loaded ZIF-8 (denoted as ThT@ZIF-8@Alg), with a theoretical ThT@ZIF-8 and ThT content of 95 wt% (0.68 mg per bead) and 13.5 wt% (<0.10 mg per bead), respectively. To partially automate the shaping process, composite fabrication was performed using a syringe pump setup, as shown in Scheme 1. Dropping the MOF/SA dispersion into the Ca2+ curing solution at a constant speed and with a fixed volume yielded beads of a uniform shape and size (spherical, diameter: ∼1.8 cm), with their homogeneity facilitating further material studies. Furthermore, this setup allows a more expeditious and highly reproducible formation of larger amounts than a manual procedure. For this purpose, an aqueous MOF/SA dispersion was filled into the syringe and the flowrate was adjusted to maintain efficient dropping from a fixed height. Additionally, a beaker filled with the curing solution was placed under the outlet of the syringe needle and carefully stirred to collect the formed beads. Their size and weight before and after drying is documented in Table S5 (ESI†). After washing with water, the beads can be dried under ambient atmosphere.
Following the successful development of a fabrication method for the beads, the potential benefit of using of a MOF as drug carrier was investigated. To do this, the influence of different methods of incorporating ThT in alginate polymers was tested by four different strategies overall: (i) beads with pre-loaded ThT@ZIF-8 (ThT@ZIF-8@Alg, type I), (ii) beads with pristine ZIF-8 and post-synthetic impregnation with ThT (ZIF-8@Alg-ThT, type II), (iii) beads without MOF only using ThT and alginate (ThT@Alg, type III) and (iv) beads without MOF and post-synthetic impregnation with ThT (Alg-ThT, type IV). All methods are described in detail in the ESI† (Section S6). Scheme 2 shows microscope images of all types of beads as well as their schematic representations. These reveal visual differences regarding their colour intensities, ranging from opaque yellow for type I, opaque yellow-orange shade for type II with visible crystallites on the surface, and more transparent and less yellow beads for type III and IV. Furthermore, it is worth noting that, upon drying, type I and II beads (with ZIF-8), shrink to half their size (Table S5, ESI†), whereas Type III and IV beads (without ZIF-8) lose >90% of their initial volume as water is removed from the hydrogel, underlining how the presence of the MOF reinforces the structural integrity of the hydrogel.
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Scheme 2 Light microscope images and schematic representations of type I–IV beads (scale bar: 1 mm). |
The PXRD patterns of type I and type II beads demonstrate, that the MOF structure remains intact over the composite fabrication process (Fig. 2a). Without containing MOF, the type III and IV beads do not show any reflections. The DR-UV/Vis spectra show major differences of ThT absorbance within the different bead types (Fig. 2b). While the spectrum of type I is very similar compared to the material pre-processing, type II shows the main absorbance maximum around 423 nm which corresponds to pristine ThT. Most likely, this can be ascribed to the difference in the integration process. After impregnation, ThT is only distributed in the ZIF-8@Alg matrix and partially adsorbed on the surface of ZIF-8 crystals. Thus, while some effects of the MOF scaffold are visible, the influence is less pronounced compared to the interactions that occur within the pore. The type III and type IV beads both exhibit a broad absorbance from <250 to >500 nm, which indicates interactions of ThT with the alginate polymer and an increased number of possible electronic transitions. Since the alginate content is much higher than in type I and II beads (5 wt% vs. >50 wt%), such ThT-alginate interactions are likely more pronounced. The recorded FTIR spectra (Fig. S12 and S13, ESI†) and TGA curves (Fig. S14, ESI†) of type I–IV beads confirm these findings and can be found in the ESI.† In the SEM images of type I and II beads (Fig. 2c) ZIF-8 is clearly visible. While the MOF particles are homogeneously spread and visibly agglutinated in the alginate matrix in type I, the particles in type II slightly lost their distinct cubic morphology and show a rougher and more damaged surface. This is indicative of the MOF partially disintegrating during the impregnation in the weakly acidic ThT solution. Additional SEM pictures of other beads are given in the ESI† (Fig. S15).
The sorption isotherms (N2, 77 K) of pristine ThT@ZIF-8 and Type I beads are compared in Fig. 2d, revealing N2 uptake of 463 and 398 cm3 g−1 and calculated specific BET surface areas of 1554 and 1193 m2 g−1, respectively. That constitutes a slight reduction of N2 uptake of 14% and a surface area loss of 23% upon composite fabrication. This reduction is somewhat higher than expected regarding the incorporation of the MOF in 5 wt% of an alginate hydrogel and might be attributed to the minimal particle modification upon processing. It is worth noting, that a mild activation temperature (70 °C) for the BET measurements was chosen to avoid thermal damage to the encapsulated drug and polymer matrix, which possibly did not lead to complete removal of residual guest molecules (e.g., traces of water). PSD analysis of both materials (Fig. S16, ESI†) and comparison to the PSD of pristine ZIF-8 show a gradual reduction of the available pore volume from ZIF-8 (0.6789 cm3 g−1) to ThT@ZIF-8 (0.5640 cm3 g−1, 83% of ZIF-8), due to the encapsulation of ThT, and again from ThT@ZIF-8 to ThT@ZIF-8@Alg (0.4448 cm3 g−1, 79% of ThT@ZIF-8), due to the encapsulation in the Alg matrix.
The stability of ThT@ZIF-8@Alg (type I beads) was investigated in different media over time and compared to the pristine ThT@ZIF-8 powder. One of the advantages of ZIF-8 in biomedical applications is its chemical stability in neutral and alkaline environments and resilience towards high ionic strength solutions, such as phosphate-buffered saline (PBS).45,69 To verify the permanence of the materials used in this study, ThT@ZIF-8 and ThT@ZIF-8@Alg were immersed in deionised water (pH 6) or PBS (pH 7.4) for a week (see ESI,† Section S8) and the supernatants were monitored via UV/Vis. In deionised water, neither of the materials showed noteworthy ThT release even after seven days (Fig. S17, ESI†), which can be attributed to the high stability of ZIF-8 under these conditions. The fact that scaffold stability is retained in the composite as well further demonstrates that the composite fabrication has no negative influence on the MOF properties. In PBS (pH 7.4), both materials show minor ThT release after seven days (Fig. S18, ESI†) and slight decomposition of the MOF into zinc phosphate (Fig. S19 and S20, ESI†). The degradation of ThT@ZIF-8@Alg is somewhat more pronounced than that of ZIF-8@Alg, presumably since the crystals are well distributed and accessible in the Alg matrix and thus more exposed to the PBS than in crystalline agglomerates.
Exploiting the acid lability of ZIF-8 as a trigger for controlled drug release, a slightly acidic pH (6) was selected to mimic the environment of cancer tissues, and the release profile was monitored over time. For this purpose, ThT@ZIF-8 and ThT@ZIF-8@Alg were immersed in PBS buffer adjusted to pH 6, where the combination of acidity and ionic strength effects induced structure degradation of the MOF framework into the building units Zn2+ and 2-methylimidazole, and release of ThT. The ThT concentration in solution was monitored via UV/Vis absorption spectroscopy until ultimately all ThT@ZIF-8 had decomposed and ThT release was completed. To avoid that the basic nature of the MOF linkers neutralises the pH of the medium, therefore, halting ZIF-8 decomposition, the PBS (pH 6) was exchanged completely and replaced with fresh medium at one-hour intervals. The series of measurements was performed in triplicates and the corresponding averaged release profiles are displayed in Fig. 3b, with the amount of released ThT plotted against time. After 12 h, ThT@ZIF-8 had decomposed completely and released all previously encapsulated ThT, from both powder and composite form. Notably, the initial release profile (0–6 h) is approximately linear for both materials, averaging 12.5% per hour, and strikingly similar overall. Visibly, ThT is released in a controlled fashion and precise amounts due to acid-induced ZIF-8 decomposition, until an equilibrium point, where the medium is neutralised, and the release stops until an acidic environment is introduced again. This trend continues until ∼75% of the initially present ThT is released (6 h), after which it gradually slows until MOF decomposition is complete and 100% release are reached. Due to the homogeneous distribution of ThT in ZIF-8 and a definitive response of ZIF-8 to the acid trigger, the release can be controlled meticulously. Remarkably, the ThT@ZIF-8@Alg composite mirrors the release profile of pristine ThT@ZIF, evidencing how strongly its material properties are retained in the alginate matrix and how the MOF phase governs the release mechanism.
Similarly, also the other three materials (type II, III, IV beads) were placed in PBS (pH 6) at 37 °C for one hour, after which the amount of released ThT was measured. Of the four, only type I beads showed a sustained ThT release with 9.4% of the total encapsulated amount found in the PBS supernatant, while all others (type II, III, IV beads) released all contained ThT (Fig. 3a). As previously observed, impregnation of ZIF-8 materials with a solution of ThT only leads to weak adsorption on the MOF surface. ThT cannot enter the pores of ZIF-8 post-synthetically and is easily removed upon washing. On the other hand, it is sterically trapped in the pores of ZIF-8 after one-pot synthesis. This ensures strong retention of ThT within the structure, allowing it to be released only when ZIF-8 decomposes – a process that can be controlled by a pH trigger. In contrast, when ThT is brought into the (ZIF-8@)Alg matrix without a protective cage, the interactions are much weaker, and its release is in the realm of diffusion control. This generally leads to decreased loadings, as ThT is washed out in the curing solution and/or during the product washing process (as evidenced by visible yellow colouration of the solutions), while the rest is released quickly from the matrix when brought in contact with the buffer. These findings highlight the benefits of the carefully built core@shell@matrix system in this study and serve as a proof-of-concept that high drug payloads and sustained release can be achieved only by a targeted, bottom-up strategy. On the other hand, rapid release of the active agent can be attained by loosely incorporating the drug into the alginate matrix, without a surrounding MOF scaffold.
Advancing the shaping method of well-defined drug@MOF@alginate carrier systems in the future, for example via 3D printing and thin-film fabrication is a vital step towards applications such as treatment of skin diseases, implant coatings, and others. In addition, the crosslinking ion in the curing solution could be exchanged to engineer the mechanic properties of the resulting hydrogel or offer multi-level functionality customisation.70,71 By bridging the gap between tailored material design and functional biomedical applications, this method highlights a powerful pathway for advancing MOF-based drug carrier systems.
Looking at the vast diversity of adjustable parameters, such as nature of MOF, carrier-to-matrix ratios, choice of crosslinking ion during composite fabrication and conjugable methods of drug incorporation, the showcased concept can be tailored to multiple purposes, extending the applicability of MOF hydrogels beyond singular systems. Due to decoupling the design of the MOF as a drug carrier from its integration into the alginate matrix, this method allows for great latitude in drug@MOF synthesis, including the choice of building units, selective drug loading, tailoring of crystal size or integration of stimuli-responsiveness. It is worth noting that all materials used in this study were synthesised in aqueous media, thus, avoiding solvents detrimental to living organisms and the environment. Hybrid materials, such as proposed in this study, can surpass the limitations of purely organic or inorganic carriers by combining the strengths of both materials and thus, fulfil disease- and target-specific requirements that existing materials cannot attain.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tb00614g |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2025 |