Article pubs.acs.org/JPCC
NMR Study of BA/FBA Cocrystal Confined Within Mesoporous Silica Nanoparticles Employing Thermal Solid Phase Transformation Ewa Skorupska, Piotr Paluch, Agata Jeziorna, and Marek J. Potrzebowski* Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland S Supporting Information *
ABSTRACT: In this work, we report drug-loading procedure based on the solid state thermal transformation of a physical mixture of two ingredients: mesoporous silica nanoparticles (MSN) and an organic cocrystal. This procedure, known as the melting method, allows loading of the guest species into the host pores with high yield and an equimolar ratio of both components of the cocrystal. The study was carried out with commercial MSNs (MCM-41 and SBA-15) and a cocrystal consisting of equimolar amounts of benzoic acid (BA) and fluorinated benzoic acid (FBA). The BA/FBA sample was obtained by grinding crystalline acids. The structural constraints and molecular dynamics of BA/FBA in the crystal lattice were characterized employing 19F magic angle spinning (MAS), 13C MAS, 1H very fast (VF) MAS with sample rotation at 60 kHz, 2D NMR, 19F−19F BABA, and 1H−19F HETCOR correlations. We conclude that the system is very rigid with short distances between intermolecular aromatic layers. In contrast, BA/FBA loaded into MCM-41 and SBA-15 is very mobile in a broad range of temperatures. The structure and molecular dynamics of the guest assembly trapped in the MSN pores was established by 1H MAS, 19F MAS, 1H−19F HOESY MAS and 19F T2′ relaxation time measurements as well as 2H MAS. We conclude that the filling factor for the melting method, defined as the ratio of BA/FBA to MSN (weight to weight), is much higher compared to those for commonly used wet procedures. These results show new perspectives for future applications of MSNs as carriers of pharmaceutical cocrystals.
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confined within the pores.9,10 The loading of drugs into the mesoporous carriers is generally carried out with three methods: organic solvent immersion, incipient wetness impregnation or temperature solid phase transformation (melt method).7 Because the water solubility of most drugs is poor, organic solvent immersion has been most frequently applied in the literature. In the second method, incipient wetness impregnation, a very concentrated drug solution is utilized to obtain a high loading degree, where the drug concentration is usually close to its solubility. The volume of the drug solution is equal to the pore volume of the mesoporous carriers, which is one of the main differences from the immersion method. In both cases, capillary action draws the solution into the pores together with the drug molecules. In the third approach, a physical mixture of solid drug and mesoporous carrier is heated above the melting point of the drug. To some extent, the melting method can be considered a special case of the impregnation method. In our recent paper testing the different procedures of loading of MCM-41 (one of the most commonly used MSNs) with the nonsteroidal anti-inflammatory drug ibuprofen, we found the melting method to be very efficient with a much
INTRODUCTION Developing innovative methods for drug transportation in the body and increasing the therapeutic efficiency of both newer and older generation drugs with different physicochemical properties is one of the biggest challenges in the pharmaceutical sciences.1 Both areas (transportation and efficiency) can be greatly improved by introducing strategies based on the application of modern drug delivery systems (DDS).2,3 Today, the field of DDSs is growing very quickly and becoming one of the most profitable areas in the pharmaceutical industry. The majority of DDSs are based on biological or inorganic/ organic components.4 To the latter group belong mesoporous silica nanoparticles (MSN),5 which were recently approved as drug carriers by the Food and Drug Administration (FDA). The attraction and utility of MSNs as DDSs are due to their unique geometrical features, such as high surface area and large pore size and volume, that can be adapted to meet specific needs.6 The process of loading MSNs with active pharmaceutical ingredients (APIs) is a key preparational step that defines further applications of API/MSN systems in individual therapies.7 Drugs can be located on the surface and/or confined inside the MSN pores, each of which leads to different mechanisms and release rates.8 Localization of the APIs depends on chemical modification of the MSNs and the loading method. For plain MSNs, the drugs are usually © XXXX American Chemical Society
Received: December 10, 2014 Revised: March 27, 2015
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Figure 1. (a) DSC diagram obtained for BA, FBA and cocrystal BA/FBA; (b) stacking of carboxylic acid heterodimers; (c) space-filling models.
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higher filling factor compared to the wet methods.7 We showed the weak point of the procedures based on solvent immersion or incipient wetness to be competition between solvent and drug in the pore-filling process. Solvents with high affinity to silica mesopores are even able to remove encapsulated drugs in a diffusion process, decreasing the filling ratio of API to MSN. Unfortunately, the melting method cannot be considered as a general approach, in particular in those cases when drugs cannot withstand melting without degradation. In addition, the high viscosity of the melted drug can be detrimental to successful drug loading. However, there are a few approaches that can be used for the modification of the thermal properties of drugs. One of them is the formation of cocrystals with desired components (which can even strengthen therapeutic properties) that usually lead to a decrease the melting temperature.11−15 To the best of our knowledge, such a strategy (also called “two in one”) has not been tested as a procedure for the filling of mesoporous silica nanoparticles by the melting method. Today, the library of pharmaceutical cocrystals with attractive therapeutic properties is very rich. A number of supramolecular arrangements of APIs have been recently reported.16−18 These chemical and structural modifications influence the physical properties, such as melting temperature and solubility, of the new compounds. This opens attractive possibilities for developing novel approaches to drug formulation and transportation in the body. In this paper, we wish to answer few basic questions important to further development of the melting procedure. The first question regards the possibility of the encapsulation of two cocrystal components inside the pore with an equimolar ratio. At this stage, we cannot exclude a preference for trapping one over the other in the MSN pore. The second question concerns the behavior of both compounds inside the pore: whether they remain a cocrystal or behave as individual species and, furthermore, how the molecular dynamics of compounds locked in a cage are altered. The final question regards the release rate of the cocrystal confined within the pore compared to that of the pure components. To answer these questions, we have employed a model sample: hydrogen-bonded carboxylic acid dimer mediated by phenyl−pentafluorophenyl stacking interactions.19 As a technique of choice, we have used NMR spectroscopy, which provides detailed information about structure and molecular dynamics.20
EXPERIMENTAL SECTION Materials. The silicon dioxide MCM-41 (hexagonal) was obtained from Sigma-Aldrich, the silicon SBA-15 was obtained from ACS Material and was activated by calcination at 300 °C for 1 h with a heating rate of 5 °C min−1 to remove the water. The pore volume and pore size of MCM-41 were 0.98 cm3/g and 2.1−2.7 nm, respectively. In the case of SBA-15 the pore volume and size were 1.31 cm3/g and 14 nm, respectively. The benzoic acid and pentafluorobenzoic acid were obtained from Tokyo Chemical Industry (TCI) and were not purified before use. Their melting points were 122.95 and 102.37 °C, respectively. The benzoic acid-13C7 and benzoic acid-2,3,4,5,6d5 were obtained from Sigma-Aldrich and the melting points were 121−125 °C for both samples. Cocrystal Preparation. The cocrystals FBA/BA and FBA/ BA-d5 were prepared by a grinding procedure. A 1:1 molar mixture of the two components was shaken in a Mixer Mill MM 200 equipped with a 5 mL agate jar and 7 mm diameter balls. Grinding was performed for approximately 15 min at an oscillation rate of 20 Hz. The whole sample was then melted at a temperature approximately 10 degrees above the melting point of the cocrystals. Loading Procedure. Melting Method (mm): The loading of cocrystals FBA/BA and FBA/BA-d5 was performed on calcined powders of MCM-41 or SBA-15. This method is based on mixing the cocrystal with the mesoporous silica and heating the mixture at a temperature approximately 5 degrees above the melting point of the cocrystal (the heating temperature for FBA/BA and FBA/BA-d5 was 95 °C). The samples will be referred to herein as FBA/BA/MCM-41, FBA/BA-d5/MCM41, FBA/BA/SBA-15, and FBA/BA-d5/SBA-15. Diffusion Procedure. Samples of FBA/BA/MCM-41, FBA/BA-d5/MCM-41, FBA/BA/SBA-15, and FBA/BA-d5/ SBA-15 were placed in weighing bottles and put into a diffusion chamber with ethanol for 3 h at ambient temperature (297 ± 3 K). We have employed procedure tested and described in our recent paper.7 A similar method was applied for the diffusion of water or chloroform. After the requisite diffusion times, all samples were immediately placed in 2.5 mm zirconia rotors. NMR Measurements. NMR spectra were recorded on a 600-MHz Bruker Avance III spectrometer equipped with a 1 19 13 H, F, C 2.5 mm triple-resonance MAS probehead operating at 600.13, 564.69, and 150.92 MHz. The 90° pulse duration was set to 3 μs, 2.25 μs, and 4 μs for 1H, 19F and 13C nuclei, B
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Figure 2. (a) 1H VF MAS at 60 kHz spinning speed (measured using a 1.3 mm CP-MAS probehead) with repetition delay of 120 s. (b) 19F MAS NMR spectrum of BA/FBA with a spinning rate of 28 kHz and repetition delay of 1000 s.
respectively. For the experiment with 19F decoupling, a π−pulse scheme was used with a 180° pulse equal to 6 μs.21 The 1 H−19F CP experiment was performed using a PVDF sample with contact time set to 2 ms and RFs at 70 kHz and 100 kHz for 1H and 19F, respectively. A ramp shape pulse on the 1H channel from 90% to 100% was used during contact time.22 Fluorine T2 relaxation times were measured using a rotorsynchronous Hahn−echo sequence.23 The 1H spectra were measured using a DEPTH pulse sequence for background elimination.24 The 2H spectra were measured on Bruker Avance III 400 spectrometer at 40.56 MHz using a 4 mm CPMAS probehead and 8 kHz spinning speed.
Assignment of the 1H resonances and quantitative analysis of the spectra using SS NMR spectroscopy under slow sample spinning are still very challenging due to extremely strong homonuclear dipolar couplings, which in many cases exceed the range of chemical shifts for protons.26 For true solids, the broadening of proton lines is not removed by slow or medium magic angle spinning without the application of complex pulse sequences.27,28 Spinning regimes greater than 50 kHz exceed the strength of homonuclear proton dipolar coupling and are therefore expected to enter a new regime for spin dynamics. Employing the VF MAS approach, we recorded a well-resolved proton spectrum with slightly separated carboxylic protons at 14−15 ppm. Unfortunately, the assignment of acidic protons to BA and FBA residues is ambiguous on the basis of a 1D NMR measurement. In this region we have four overlapped 1H signals in two pairs of dimers. Assuming that the 1H resonances are averaged due to proton transfer processes that are fast in the NMR time scale, in the heterodimer we should distinguish two groups of signals representing A/B and A′/B′ pairs. The assignment of signals was performed by inspection of 1H−13C FSLG HETCOR correlations29 and GIPAW calculations30,31 of chemical shifts (see Supporting Information). We conclude that the upfield-shifted carboxylic signal represents the A′/B′ dimer (Figure 1b). The 19F MAS spectrum is shown in Figure 2b. The spectral pattern is typical for rigid systems. In the central part of spectrum, isotropic resonances (labeled with a red ellipsoid) are surrounded by spinning sidebands despite the sample being spun at 28 kHz in a 2.5 mm rotor. This indicates the 19F nuclei of the FBA aromatic residues are characterized by a very large chemical shift anisotropy (CSA) that is not reduced by local molecular motion of phenyls.32,33 Such a conclusion is consistent with the molecular packing of a cocrystal. In a πstacking alignment of aromatic groups there is not enough space for molecular reorientation. The challenging question is the assignment of the 19F isotropic signals to individual molecules (A, A′) and further to A, A′ carbon atoms. In the isotropic part of spectrum (Figure 2b) we observe seven signals that correspond to ten nonequivalent fluorine atoms, indicating that three pairs of spins are isochronous and not distinguishable by 19F MAS. The difficulties with assignment of the 19F signals can be partially overcome by application of a 2D NMR approach. Figure 3 shows a single quantum−double quantum (SQ-DQ)
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RESULTS AND DISCUSSION (i). Characterization of BA/FBA Cocrystal 1 by NMR. Very recently, Azais and co-workers have employed benzoic acid confined in MCM-41 for testing the applicability of NMR techniques in structural studies of solid state samples.25 In our project we employed a similar system, cocrystal 1, formed by benzoic acid (BA) and its penta-fluorinated analogue (FBA), to investigate nonbonding interactions of the guest species 1 with mesoporous silica host nanoparticles. Cocrystal 1 is very easy to obtain by grinding equimolar amounts of both acids. Figure 1a shows DSC profiles confirming the formation of BA/FBA species. The melting temperature of 1 is approximately 88 °C, which is lower than the melting points of both pure components. The X-ray structure of the BA/FBA sample was reported by Gdaniec et al.19 Parts b and c of Figure 1 show the molecular packing, hydrogen bonding pattern and aromatic− aromatic interactions in the crystallographic unit cell. The asymmetric unit contains two carboxylic heterodimers. The heterodimers are assembled into infinite stacks in a headto-tail fashion such that the phenyl rings interact with the pentafluorophenyl moieties. The distance between aromatic groups ranges between 3.80 and 3.97 Å. The rings are not completely parallel and the phenyl residues show offset face-toface stacking typical for such systems. The subtle structural features of 1 are easily recognized by solid state NMR spectroscopy. Figure 2 displays 1H and 19F solid state NMR spectra of the BA/FBA powdered sample recorded under magic angle spinning (MAS) at 60 kHz and 28 kHz, respectively. For the former measurement we have employed the technique known as very fast MAS (VF MAS) using commercially available 1.3 mm rotors (Figure 2a). C
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Figure 3. 19F−19F BaBa correlation of BA/FBA recorded with a spinning rate of 30 kHz and 373.47 MHz for 19F. The repetition delay was 800 s and 128 t1 points were collected with 8 scans per t1 point. DQ excitation and DQ conversion time was 47.48 μs.
homonuclear 19F−19F back-to-back (BaBa) correlation.34 The connectivity between spins is labeled by red horizontal lines. The suggested assignment of signals is given in the F2 projection. Experimental data are further supported by GIPAW calculations and correlation of 19F δiso versus 19F σiso values (see Supporting Information). In the experiments discussed so far, we have focused attention on intramolecular interactions and the assignment of signals. The more important issue, in light of further analysis of the cocrystal embedded in MSN, is introducing a method that allows the evaluation of intermolecular contacts. For solid samples, this technique is 1H−19F HETCOR correlation. The connectivity pattern between 1H and 19F spins can be estimated by analysis of the dipolar interactions. In the π-stacking arrangement shown in Figure 1, the dipolar coupling D1H‑19F should be in the range 1.8−2.6 kHz, corresponding to distances between 4.0 and 3.5 Å. Figure 4 shows the 2D spectrum with clear correlation peaks between aromatic protons and fluorine nuclei, reflecting the intermolecular contacts between layers. The interpretation of the correlation pattern for acidic protons and fluorine is more challenging. It is surprising that such
correlation is seen only for one molecule (in our notation labeled as A). There is no straightforward answer explaining this distinction. We assume that the observed cross peaks show intramolecular contacts, as only ortho and meta fluorine atoms correlate with the C(O)−OH proton. In A−B and A′−B′ heterodimers, the orientation of the hydrogen bonded carboxylic unit is different with respect to the aromatic planes: dimer A−B is almost planar while the A′−B′ carboxylic units are in gauche orientation with respect to the aromatic planes, possibly due to distinct interactions between the 1H and 19F spins. Moreover, the gauche oriented protons are relatively close to the adjacent A′−B′ carboxyl, allowing easy intermolecular interactions, delocalization of protons, and in consequence, weakening of correlation signals. Finally, to confirm the sandwich-like structure of the BA/ FBA cocrystal, we carried out a simple 1H−13C CP/MAS measurement. Because of the proximity of phenyl and phenylF5 residues in the crystal lattice, the magnetization transfer from proton to all carbons (including phenyl-F5) should be efficient with long contact times. We performed this experiment on two samples: first, a cocrystal with a fully 13C enriched (99%) BA component, and second, sample with 13C natural abundance for BA and FBA. Figure 5 displays appropriate spectra. In Figure 5a
Figure 4. 1H−19F HETCOR correlation of BA/FBA with a spinning rate of 28 kHz. Contact time was set to 500 μs, repetition delay to 120 s, 128 t1 points were collected with 8 scans per t1 point. The experiment was run without homodecoupling on t1.
Figure 5. 13C NMR spectra of cocrystal BA/FBA: (a) 1H−13C CP MAS NMR with a fully 13C enriched (99%) BA component; (b) 1 H−13C CP MAS NMR of sample with natural abundance of both components. Both samples were measured with spinning rate 12 kHz. D
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measurements. As we found for 2, the 1H−13C CP/MAS experiment failed. Similar conclusions were reported for other systems, e.g., ibuprofen confined within MCM-41.7 As in previous cases,35 carbon spectra can be only recorded using a single pulse experiment (SPE) as with liquid state NMR. Figure 8 shows 13C SPE MAS spectra obtained with 1H
we observe only BA while in Figure 5b both BA and FBA are seen. Applying such an approach enables us to unambiguously assign all resonances to each component. (ii). BA/FBA Inside MCM-41 and SBA-15. The prerequisite for successful confining of any medium into the pores of MSNs is the appropriate size of its components. In our project we have employed two commercially available silica nanoparticles, MCM-41 and SBA-15. For the former carrier, the mean pore diameter is 30 Å, while that of SBA-15 is larger, approximately 140 Å. Figure 6 shows a schematic representa-
Figure 6. Schematic representation of the BA/FBA cocrystal with labeled dimensions along the x, y, and z axes.
Figure 8. 13C SPE MAS NMR spectra, recorded at a MAS frequency of 28 kHz, of BA/FBA in MCM-41 (1:1 by weight): (a) 13C{1H}; (b) 13 C{1H,19F}.
tion of the BA/FBA cocrystal with labeled dimensions along the x, y, and z axes. The largest distance in the heterodimer sandwich is less than 15 Å, while the other is less than 6 Å. This indicates there are no obstacles related to cocrystal volume preventing confinement in the pores. Thus, the BA/FBA/ MCM-41 (2) complex was obtained by the melting method described in the Experimental Section. Figure 7 shows 1H MAS and 19F MAS spectra of 2 recorded with spinning rate of 28 kHz at ambient temperature. The
decoupling and double 1H, 19F decoupling. Carrying out a series of measurements with relaxation delays ranging from 5 to 100 s enabled us to quantitatively analyze the proportion of components trapped in the pores. We found BA and FBA in a 1:1 ratio (mol:mol), as in the starting material, indicating that during the melting, the probability of inclusion of BA and FBA is equal. Taking the analysis of the interactions between the cocrystal and MSNs further, we investigated the possibility of distinguishing the components located in the inner and outer spheres of the mesopores. To solve this problem, we prepared a sample with a 3:1 ratio of 1 to MCM-41 (by weight), assuming that in this case we have excess of 1 with respect to the volume of pores. Figure 9a shows a 19F MAS spectrum of a sample recorded at ambient temperature with spinning rate of 28 kHz and a relaxation delay of 5 s. The spectral pattern is typical for a very mobile system located inside the pores. For 1, under fast
Figure 7. (a) 19F MAS NMR spectrum of BA/FBA in MCM-41 (1:1 by weight) with a spinning rate of 28 kHz. (b) 1H MAS NMR spectrum of BA/FBA in MCM-41 (1:1 by weight) with a spinning rate of 28 kHz.
differences between the spectra of 1 and the complexes are apparent. The resolution of the proton spectra for 2 is much better than that of 1. The most striking distinction, however, is seen in the fluorine data. In the case of the complexes, only three very sharp resonance lines representing the ortho, meta, and para positions are observed. These results clearly prove that the cocrystal (or its components) embedded in the MSNs is under the fast exchange regime and the anisotropy is averaged to isotropic values due to molecular motion. These preliminary observations are consistent with 13C NMR
Figure 9. Spectra of BA/FBA in MCM-41 (3:1 by weight) with a spinning rate of 28 kHz: (a) 19F MAS NMR (repetition delay 5 s); (b) 19 F MAS NMR (repetition delay 1000 s); (c) 1H−19F CP MAS NMR. E
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Figure 10. 1H−19F HOESY MAS NMR spectra of BA/FBA in MCM41 (1:1 by weight) with a spinning rate of 12 kHz. 160 t1 points were collected with 128 scans per t1 point. The mixing time was 50 ms and the repetition delay was 2 s.
Figure 11. Spectra of BA/FBA in SBA-15 with a spinning rate of 28 kHz: (a) 1H MAS NMR; (b) 19F MAS NMR; (c) 13C{1H,19F} SPE MAS NMR.
The 1H chemical shift of the carboxylic protons and water require short comment. For the C(O)OH resonances, signals are broad and downshifted approximately 6 ppm compared to the chemical shift in the crystal lattice. There are two possible explanations: either weak hydrogen bonding in the dimeric structure of the quasi-liquid state or the exchange process with Si−OH protons of the silica matrix (Figure S9a). The chemical shift of water is also unusual (approximately 6.8 ppm). The latter effect and exchange processes between protons of water, C(O)OH and Si−OH can lead to averaging of the signal (Figure S9b). In this case, carboxylic protons are not seen. Finally, testing the filling factor, defined as the ratio of cocrystal versus MSN (weight-to-weight), we found that, employing the melting procedure, it is possible to reach much better loading than with the incipient wetness method. This conclusion is supported by SEM and TEM micrographs and analysis of nitrogen absorption/desorption isotherms for calcinated MSN and BA/FBA_MSN samples (see Supporting Information). The obtained data are consistent with 19F MAS F
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Figure 12. Amount of compound embedded in MSN (left column) and appropriate porous volumes, VP, for the mesoporous silica (right column).
Figure 13. 19F T2′ relaxation times for (a) (■) BA/FBA in MCM-41 1:1; (blue ▲) BA/FBA in MCM-41 2:1; (●) BA/FBA in MCM-41 2.5:1. (b) (green ■) BA/FBA in MCM-41 1:1; (■) BA/FBA in MCM-41 1:1 after ethanol diffusion; (blue ▲) BA/FBA in MCM-41 1:1 after chloroform diffusion; (red ◆) BA/FBA in MCM-41 1:1 after water diffusion.
(iii). Molecular Dynamics of BA/FBA Inside MCM-41 and SBA-15. The NMR line shape analysis of 1 confined in the pores of MCM-41 and SBA-15 unambiguously proved that in both cases embedded components of cocrystal are under fast regime exchange. Unfortunately, analysis of the spectral pattern yields only qualitative information and describes the system under investigation in two categories, mobile and rigid. For a more advanced inspection of 2 and 3 in terms of molecular dynamics, more sophisticated NMR tools must be applied. This
spectra which allowed recognized of mobile inner and rigid outer components. Figure 12 presents the amount of compound embedded in MSN (left column) and the appropriate porous volumes, VP, for the mesoporous silica (right column). Employing a solid phase thermal treatment allows the loading of up to 2 g of cocrystal into the 1 g of MCM-41. It is worth expressing that, in the wet method, the filling factor was found to be 0.7 g of BA per 1 g of MCM-41 (BA in MCM-41, 30 Å25). G
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Figure 14. (a) 19F VT NMR for BA/FBA in MCM-41 with spinning frequency of 12 kHz. (b) 2H (40.56 MHz) spectra measured with 8 kHz spinning speed for pure BA-d5/FBA cocrystal (top) at 298 K, for BA-d5/FBA in MCM-41 (middle) at 298 K, and fror BA-d5/FBA in MCM-41 (bottom) at 223 K.
For this purpose, we used a cocrystal consisting of 2H isotope in an aromatic residue of BA. 2 H solid state NMR spectra exhibit unique features when molecular motions are present.37 Under nonspinning conditions for rigid molecules, deuterium nuclei provide the usual quadrupolar doublet as the line-shape of the spectrum.38,39 Under sample spinning conditions, the deuterium spectrum is composed of center and sidebands with well-defined intensities spanning the frequency range of this pattern.40 Molecular motion influences these intensities and the widths of these bands without changing their frequency positions. Molecular reorientation that is very fast in the NMR time scale averages the quadrupolar interactions leading to the liquid-like deuterium spectra. One pulse 2H-MAS NMR spectra recorded at a spinning frequency of 8 kHz are shown in Figure 14b. The upper trace displays the 2H-MAS spectral pattern for the BA-d5/FBA cocrystal labeled in the aromatic group. The main observation is that the overall breadth of the entire set of experimental spectra is typical for a rigid system. The quadrupolar parameters, coupling constant CQ and asymmetry η, are found to be 160 kHz and 0, respectively, indicating that in the crystal lattice the aromatic ring does not undergo molecular motion. In the case of 1 embedded in MCM-41, the situation is dramatically different (middle trace of Figure 14b). At 298 K, we observe only isotropic resonances in the phenyl ring of BAd5 due to fast molecular motion. It is worth noting that at 223 K (the bottom trace) the molecular motion of the BA residue is frozen and the 2H spectral pattern is typical for rigid systems as in the case of sample 1.
includes relaxation time measurements that directly join NMR parameters to molecular reorientation. We measured 19F T2′ relaxation times for 2 and 3 at ambient temperature. The values are attached as Supporting Information (Table S1). The relaxation times obtained for BA/FBA in MCM-41 and SBA-15 range from 2.5 to 3.5 ms. Such values are characteristic for the highly mobile behavior of the entrapped molecules. For comparison, the 19F T2′ of the BA/FBA cocrystal are on the order of μs, whereas in solution (CDCl3) these values are on the order of seconds. Increasing the filling ratio slows the BA/FBA dynamics (Figure 13a), while the solvent molecules promote the opposite behavior. The 19F T2′ strongly depends on the nature of the solvent, and the highest mobility was noticed for ethanol, followed by chloroform and the lowest for water (Figure 13b). In the course of our studies, we did not observe removal of 1 from the MSN pores by vapor diffusion of solvents as was reported for ibuprofen embedded in MCM-41.7 We conclude that the systems under investigation (2 and 3) form more stable host−guest complexes than Ibu-MCM-41. Finally, for a deeper understanding of the dynamic processes of BA/FBA embedded in MCM-41 pores compared to the BA/ FBA in the crystal lattice, we carried out a low temperature study of 2. Figure 14a shows the 19F MAS spectra of a sample at temperatures ranging from 298 to 223 K. At 253 K, the resonance lines broaden due to the slowing of the molecular reorientation of the FBA component of the cocrystal in the pores. At 223 K, the molecular motion is frozen and the spinning sideband pattern is recovered, although the resolution of the spectra is worse than those observed for 1. To examine the behavior of the BA component as a function of temperature, we utilized solid-state 2H NMR spectroscopy. H
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CONCLUSIONS Mesoporous silica nanoparticles have found a number of practical applications due to their great ability to form stable host−guest complexes. One of the recent promising applications resulting from supra physio-chemical properties is the use of MSNs as drug delivery systems. To date, a number of active pharmaceutical ingredients (APIs) have been tested in terms of API-MSN interactions. To the best of our knowledge, only individual species have been embedded into the pores, employing mostly wet methods. In our work, we propose an extremely simple, efficient and clean procedure based on solid state transformation, which allows loading of two-component species into the pores with high yield and an equimolar ratio. This opens great possibilities for the applications MSNs as carriers of pharmaceutical cocrystals with desired therapeutic properties.
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ASSOCIATED CONTENT
S Supporting Information *
1
H−13C FSLG HETCOR correlation (Figure S1) for BA/FBA cocrystal, data for 19F T2 measurements (Table S1 and S2), Nitrogen adsorption/desorption isotherms and pore volume (Figures S2−S5), Table with porosity analysis (Table S3), SEM and TEM micrographs of MSN and BA/FBA_MSN samples (Figures S6−S9), 1H and 19F MAS NMR spectra of BA/ FBA_SBA-15 samples containing water molecules inside the pores (Figure S9), plot of δiso vs σiso for 19F nuclei for BA/FBA cocrystal (Figure S10), and output for GIPAW calculations of NMR parameters of BA/FBA cocrystal. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(M.J.P.) E-mail:
[email protected]. Telephone: +48 426803240. Fax: +48 426803261. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the Polish National Center of Sciences (NCN) for financial support, Grant No. 2011/01/B/ ST4/02637, and Tomasz Pawlak for GIPAW calculations of NMR parameters.
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REFERENCES
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