Metal–Organic Framework Thin Films as Drug Delivery Systems

Abstract

Selective metal–organic frameworks (MOFs) have been studied as promising candidates for biomedical applications, especially as drug delivery systems due to their exceptional structural properties. This work reports the detailed methods to prepare an iron-containing MOF, MIL-88B(Fe), on functionalized gold and medical-grade stainless steel surfaces for drug delivery. We present a general procedure for preparing a novel drug carrier that can potentially be used as drug-eluting stent coatings.

1. Introduction

Coronary artery disease (CAD) is one of the major causes of high morbidity and mortality worldwide [1,2]. It involves atherosclerosis plaque formation on the wall of arteries, leading to the blockage of blood flow [3]. Percutaneous coronary intervention (PCI) is one of the well-established cardiovascular procedures to widen the obstructed arteries. A stent is usually planted in the injured artery areas through balloon angioplasty surgery. The first-generation stents, made of bare metal (no coating), have a high rate of restenosis (re-narrowing of the treated arteries). This led to the development of the second-generation drug-eluting stents (DESs), which possess porous polymer coatings loaded with therapeutic agents on metal struts [4]. Recent studies have shown that DES reduced in-stent restenosis rate by delivering antiproliferative and immunosuppressive drugs along the stents [5]. Most commercially available DESs are polymer based (e.g., Cypher, TAXUS, Xience) [6]. The polymer coatings often cause inflammation and hypersensitivity reactions in the human body, resulting in the delay of endothelial tissue healing and inducing the formation of blood clots inside arteries [7]. Hence, it is critical to design polymer-free DES that maintains the ability for controlled drug release.

The requirements for an ideal stent coating are for it to be biocompatible, biodegradable, flexible (no flaking/detaching), and have sustainable releasing behaviors. There has been an increasing focus on a variety of metal–organic frameworks (MOFs) for therapeutic drug delivery systems due to their exceptionally high surface areas, tunable porosity, biodegradability, and low toxicity [8–11]. MOFs contain transition metal ions/clusters connected by organic ligands [12]. Our laboratory has explored surface-supportive MIL-88B(Fe) thin films for drug delivery [13–15]. Material from Institute Lavoisier MIL-88B(Fe) is a type of Fe-containing MOF that is composed of Fe metal nodes connected by terephthalate linkers [16]. The low toxicity of MIL-88B(Fe) has been confirmed by in vitro cell studies [13]. Another unique feature of MIL-88B(Fe) is its hydrophobic pore with hydrophilic outer framework, promoting surface attachment and lipophilic drug loading. Additionally, it has been proven that MIL-88B(Fe) preferably binds to the COOH-terminated surface as the open metal sites are coordinated with the carboxylate functional groups from the modified surface [14]. Our laboratory has reported attaching MIL-88B (Fe) on a COOH-terminated self-assembled monolayer (SAM) using 16-mercaptohexadecanoic acid (MHDA) on a gold surface as a pH-responsive drug delivery system, illustrated in Fig. 1. It was observed that a higher amount of ibuprofen was released in acidic conditions compared to that in neutral conditions due to the bond degradation between Fe³⁺ and the carboxylate ligands [15]. Our study further focuses on attaching MIL-88B(Fe) to medical-grade (316L) stainless steel (SS), the common material for DESs, as an alternative drug delivery model. We functionalized the 316L SS surface with -COOH groups via a phosphonic acid following a procedure modified from literature [17]. The general scheme of attaching MIL-88B(Fe) to the modified SS surface is shown in Fig. 2. The crystal structure of MIL-88B(Fe) was confirmed using powder X-ray diffraction (XRD) analysis. Infrared (IR) spectroscopy and scanning electron microscopy (SEM) were used to verify the chemical composition and morphology changes of the surface-supportive MIL-88B(Fe) after each modification process. The drug loading and releasing studies of the MIL-88B (Fe) films were performed on either a high-performance liquid chromatography (HPLC) or on a quartz crystal microbalance (QCM) with ibuprofen as a model drug. Herein, we describe a general strategy for designing a new coating material for potential use in DES that is environmentally friendly and stable in physiological conditions.

Fig. 1.

General scheme of attaching MIL-88B(Fe) onto a COOH-terminated self-assembled monolayer (SAM) modified Au surface for pH-responsive drug delivery with ibuprofen as a model drug. (Source: Reproduced with permission from [15])

Fig. 2.

General scheme of attaching MIL-88B(Fe) to a COOH-terminated medical-grade stainless steel (316L SS) surface. Ibuprofen (yellow circle) is then loaded to the surface-supportive MIL-88B(Fe)

2. Materials

2.1. MIL-88B(Fe) Mother Solution

1. Iron (III) chloride hexahydrate (FeCl₃•6 H₂O, 99+%).

2. Terephthalic acid (99.9%).

3. Dimethyl formamide (DMF, 99.9%).

4. 2N sodium hydroxide.

5. Glass vials.

6. Vacuum filtration flask and funnel.

7. Centrifuge.

2.2. Cleaning Au Chips

1. Au chips (gold-coated silicon wafer, 50 nm Au on 500 μm p-type <111>).

2. 200-proof ethanol.

3. UV ozone cleaner.

4. Milli-Q water.

5. 25% ammonia.

6. 30% hydrogen peroxide.

7. Hot plate.

2.3. Self-Assembled Monolayer Formation on the Au Chips

1. 16-Mercaptohexadecanoic acid.

2. 200-proof ethanol.

2.4. Self-Assembled Monolayer Formation on the 316L Stainless Steels Surface

1. 316L stainless steel chips (1 cm × 1 cm).

2. 2100-Grit sandpaper.

3. 200-Proof ethanol.

4. Acetone.

5. Methanol.

6. Milli-Q water.

7. Sonicator.

8. UV ozone cleaner.

9. 11-Phosphonoundecanoic acid (COOH(CH₂)₁₀ PO(OH)₂, 96%).

10. Tetrahydrofuran (THF, HPLC grade).

2.5. Drug Loading Studies

1. Ibuprofen (99%).

2. Hexane.

3. 200-Proof ethanol.

2.6. Drug Release Studies

1. Phosphate-buffered saline (PBS, pH 7.4, 1×).

2. Tween 20.

3. Sodium phosphate monobasic.

4. Sodium phosphate dibasic.

5. Milli-Q water.

3. Methods

3.1. Synthesis of MIL-88B(Fe) and Its Mother Solution

1. Weigh out 0.270 g of FeCl₃·6 H₂O and 0.166 g of terephthalic acid.

2. Dissolve both compounds in 5 mL of DMF.

3. Add 0.4 mL of 2M NaOH using a micropipette to the above DMF solution.

4. Sonicate the mixture for 2 min in a water ultrasound sonicator.

5. Separate the solution into glass vials with roughly equal amounts and loosely close the cap. (Note: Each vial should be filled half-full as the vapor pressure can be built up during the heating process.)

6. Heat the vials in an oven at 100 °C for 12 h.

7. After the reaction is done, carefully move the vials to a fume hood and let them cool to room temperature.

8. Prepare a vacuum filtration apparatus.

9. Wet the filter paper inside the Büchner funnel with a few drops of DMF and turn on the vacuum.

10. Pour baked MIL-88B(Fe) solution into the Büchner funnel.

11. Rinse each glass vial with DMF and pour the remaining solution into the filtration apparatus.

12. Slowly rinse the powder in the funnel with 200-proof ethanol to remove impurities and to let the collected precipitate dry faster. The collected solution is micro-scale MIL-88B that will undergo a solvent exchange with ethanol.

13. Pour solution into roughly equal parts in separated centrifuge tubes.

14. Centrifuge at 5000 rotations per minute (rpm) for 15 min.

15. Decant the solvent.

16. Pour in 15 mL of 200-proof ethanol in each centrifuge tube.

17. Sonicate until they are homogeneous solutions.

18. Repeat steps 11–15 three times. Fresh solvent is used after each cycle.

19. Store in centrifuge tube at room temperature and seal the cap with parafilm to prevent the solvent from evaporating. This MIL-88B(Fe)/ethanol solution is referred to the mother solution and is viable for up to 2 weeks (Fig. 3 and see Note 1).

Fig. 3.

The powder X-ray diffraction (PXRD) of MIL-88B(Fe) after the solvent exchange

3.2. Cleaning Au Chips

1. Sonicate Au chips in 200-proof ethanol for 5 min.

2. Dry with nitrogen gas.

3. Place dry chips in the UV ozone cleaner for 10 min.

4. Heat a cleaning solution (5:1:1 Milli-Q water: 25% ammonia: 30% hydrogen peroxide) to 75 °C.

5. Place the UV ozone-cleaned Au chips in the cleaning solution at 75 °C for 10 min.

6. Rinse the chips with Milli-Q water, then dry with nitrogen gas.

7. Place the Au chips in the UV ozone cleaner for another 10 min.

3.3. Making MHDA/Ethanol Solution

1. Add ~0.029 g of 16-mercaptohexadecanoic acid to a 100-mL volumetric flask.

2. Fill the volumetric flask to the mark with 200-proof ethanol.

3. Invert the flask about 25 times or until the solid is completely dissolved in the solution.

4. Label and store in the fridge until use. (This solution is good for up to 2 weeks.)

3.4. Self-Assembled Monolayer Formation on the Au Chips

1. Immerse the pre-cleaned Au chips in a clean petri dish containing the MHDA/ethanol solution.

2. Cover the petri dish and seal it with parafilm.

3. Place the petri dish on the mechanical stir plate (~60–70 rpm) at room temperature for 24 h.

4. After 24 h of soaking, hold edges of the Au chip with tweezers and let the residual solvent slowly evaporate. (It is important to avoid contact with the surface of the chip.)

5. Sonicate the samples in fresh ethanol for ~30 s.

6. Dry with nitrogen gas.

3.5. Preparation of SURMOF MIL-88B(Fe) Thin Films on Functionalized Au

1. Immerse the modified Au chips in the MIL-88B(Fe) mother solution for 24 h.

2. Remove the chips with tweezers.

3. Dip the chips in 200-proof ethanol to remove any MIL-88B (Fe) solution residue.

4. Dry with nitrogen gas.

5. Analyze all Au samples (blank, MHDA-treated Au, and MHDA-treated Au coated with SURMOF MIL-88B thin films) using an ATR-IR (Fig. 4 and see Note 2).

Fig. 4.

ATR-IR spectra of (a) clean Au, (b) MHDA-treated Au, and (c) MHDA-treated Au with surface-supportive MIL-88B(Fe). (Source: Reproduced with permission from [15])

3.6. Mechanical Polishing of the 316L Stainless Steel (SS)

1. Sand 316L SS chips on both sides (~5 min each) with a 1200-grit sandpaper.

2. Wipe the chips with a wet Kimwipe with water.

3. Sonicate the sanded SS chips in the cleaning solution (1:1:1:1 Milli-Q water: 70% ethanol: acetone: methanol) two times (10 min each and fresh solvent is used each time).

4. Rinse with Milli-Q water, then dry with nitrogen gas.

5. Place the dried SS chips in the UV ozone cleaner for 10 min each side.

3.7. Self-Assembled Monolayer Formation on the 316L SS Surface

1. Dissolve 11-phosphonoundecanoic acid in THF (HPLC grade) at a concentration of 1 mM.

2. Immerse the mechanically polished SS in a clean petri dish containing the phosphonic acid solution.

3. Cover the petri dish and seal it with parafilm.

4. Place the petri dish on the mechanical stir plate (~60–70 rpm) at room temperature for 24 h.

5. Withdraw the samples with tweezers and let the residual solvent slowly evaporate.

6. Without rinsing, heat the sample in an oven at 120 °C for 18 h.

7. Allow the samples to cool down to room temperature for ~30 min.

8. Sonicate the samples in THF for 1 min followed by using Milli-Q water for 1 min.

9. Dry with nitrogen gas.

3.8. Preparation of SURMOF MIL-88B Thin Films on the Modified SS Surface

1. Immersed the functionalized SS chips in the MIL-88B(Fe) mother solution for 24 h.

2. Remove the chips with tweezers.

3. Rinse quickly with 200-proof ethanol.

4. Dry with nitrogen gas.

5. Analyze all SS samples (blank, SAM-modified SS, and SURMOF MIL-88B thin films on COOH-terminated SS) with ATR-IR (Fig. 5 and see Note 3).

Fig. 5.

ATR-IR spectra of (a) blank medical-grade stainless steel (316L SS), (b) SAM modified 316L SS, and (c) surface-supportive MIL-88B(Fe) on COOH-terminated SAM modified 316L SS

3.9. Ibuprofen Loading into Surface Supportive MIL-BBB (Fe)

1. Prepare a 0.5 mg/mL of ibuprofen in either hexane (for Au) or ethanol (for SS).

2. Place the MIL-88B(Fe) coated chip in a pre-cleaned petri dish.

3. Add enough of the corresponding ibuprofen solution to cover the chip sufficiently.

4. Cover the petri dish and carefully seal it with parafilm.

5. Place the petri dish on the mechanical stir plate (~70–80 rpm) at room temperature for 24 h.

6. Withdraw the chip from the solution with tweezers.

7. For the sample soaked in the ibuprofen/hexane solution, rinse it with hexane and then with ethanol. For the sample soaked in ibuprofen/ethanol solution, rinse it with 200-proof ethanol.

8. Dry with nitrogen gas.

9. Store in a sample container sealed in nitrogen gas.

10. Determine the drug loading amount using a QCM. Analyze the samples with tensiometer (Fig. 6 and see Note 4).

Fig. 6.

(a) QCM measured mass increase after each modification step on Au surfaces. Water contact angle measurements of MIL-88B(Fe) coated 316L SS before (b) and after (c) ibuprofen loading. ((a) Reproduced with permission from [14])

3.10. Drug Release from Surface Supportive MIL-88B (Fe)

1. Prepare 100 mL of PBS with 0.5 (v/v)% Tween 20 added. The pH can be tuned between 6.3 and 7.8 using sodium phosphate monobasic and sodium phosphate dibasic aqueous solution (see Note 5)

2. Pour out 60 mL of the prepared PBS solution into a 100 mL beaker and cover with parafilm.

3. Mount a freshly prepared ibuprofen-loaded MIL-88B (Fe) coated Au in the crystal holder that is connected to the QCM instrument (SRS QCM200). Make sure the MIL-88B (Fe) coated side is facing up.

4. Secure the crystal holder with a stand and clamp vertically with the holder head at the bottom.

5. Turn on the QCM controller and wait until the “absolute frequency” (F) is close to 5 MHz.

6. Lower the QCM crystal holder head until the whole Au surface is fully immersed in the beaker filled with 60 mL of PBS and start the measurement immediately.

7. Carefully cover the beaker and the crystal holder with a plastic wrap to prevent PBS evaporation.

8. Measures frequency change of the MIL-88B(Fe) coated surface using QCM for the desired period (Fig. 7 and see Note 6).

Fig. 7.

The QCM measured mass change of the ibuprofen-loaded MIL-88B(Fe) on a COOH-terminated Au surface during the static drug release testing. The red line represents a linear trend to the experimental results shown as black dots. (Source: Reproduced with permission from [14])

4. Notes

1. The powder X-ray diffraction (PXRD) result shows three characteristic diffraction peaks of MIL-88B(Fe): 9.52°, 10.55°, and 19.12° corresponding to the (002), (101), and (200) phases, respectively. The PXRD pattern aligns well with previously reported data for bulk MIL-88B(Fe) crystals [18, 19], confirming the success of synthesizing MIL-88B(Fe) mother solution for coating preparation.

2. In the ATR-IR analysis for Au samples after each modification step, C-H stretches (2856 and 2928 cm⁻¹) and C-O/C=O stretches from carboxylate groups (1391 and 1594 cm⁻¹) were observed on the surface of MHDA-modified Au unlike the spectrum from the clean Au. In addition, a Fe-O stretch (543 cm⁻¹) was observed on the functionalized Au with MIL-88B thin films, indicating the successful attachment of MIL-88B(Fe) to the COOH-terminated Ausurface.

3. In addition to the presence of C-H stretches (2983 and 3008 cm⁻¹) and carboxylate groups stretches (1375 and 1737 cm⁻¹), the P-O stretch (1222 cm⁻¹) indicated the phosphate SAM formation on the SS surface. The attachment of MIL-88B (Fe) on the SS surface was implied by the Fe-O stretch (535 cm⁻¹) noticed on the modified SS with MIL-88B(Fe) thin films.

4. The drug loading amount on Au was measured via QCM. The mass per unit area change was calculated using the Sauerbrey equation based on the measured frequency differences [20]. Contact water angle measurements were performed on an optical tensiometer. The measured angles were governed by the Young-Laplace equation measuring the wettability of the surface on each sample. After ibuprofen loading, the surface’s hydrophobicity was increased, which indicated the success of drug loading on the MIL-88B coated SS.

5. In our recent study, MIL-88B(Fe) film has shown as great potential for pH-triggered drug delivery systems [15]. The dissociation constant in an acidic condition ([6.10 ± 0.86] × 10⁻³s⁻¹) was higher than the dissociation constant in a neutral condition ([6.39 ± 1.27] × 10⁻⁴s⁻¹). The chemical bonds between Fe³⁺ and the carboxylate ligands tend to degrade under acidic conditions, which led to a faster release of ibuprofen.

6. Prior to thin film studies, we performed drug eluting tests on bulk MIL-88B(Fe) powder. Based on our HPLC results, about 18.0% of loaded ibuprofen was released from MIL-88B(Fe) after 10 days of soaking in PBS [13]. Our QCM results show that about 2 μg/cm² of ibuprofen was released from the MIL-88B(Fe) film on Au after 6 h of static immersion [14]. The drug release amount can be much more dramatic (12.0 μg/cm²) in acidic PBS solution (pH 6.3) [15].

7. Due to its simplicity, inertness, and well-established modifications with thiol-containing SAMs, we chose to work with Au in our previous works to study how MIL-88B(Fe) thin film functions in drug loading and releasing. We further extend our studies to use 316L SS as a substrate since it is a commonly used material for medical devices. Results have shown surface-supportive MIL-88BB(Fe) as a promising candidate for drug delivery with pH responsiveness, biodegradability, and environmental sustainability.