Effect of Pore Wall Charge and Probe Molecule Size on Molecular

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Effect of Pore Wall Charge and Probe Molecule Size on Molecular Motion inside of Mesoporous Silica Nanoparticles Wen-Yen Huang, and Jeffrey I. Zink J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06293 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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The Journal of Physical Chemistry

Effect of Pore Wall Charge and Probe Molecule Size on Molecular Motion inside of Mesoporous Silica Nanoparticles Wen-Yen Huang and Jeffrey I. Zink* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States

ABSTRACT Mesoporous silica nanoparticles have been widely used as molecular containers. Properties of molecules trapped in the pores of the nanoparticles are not yet comprehensively understood. This work quantitatively studied molecular motions of guest compounds in mesoporous silica nanoparticles by calculating rotational correlation times from spin-lattice relaxation measured using solid-state NMR. The effect of pore wall charge and probe molecule size on molecular motion inside of nanoparticles was proven to significantly change the molecular dynamics in a confined space.

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INTRODUCTION Mesoporous materials are increasingly being used in the fields of catalysis,1 sensing2, drug delivery3-6 and bioimaging.7 Among various porous materials, mesoporous silica nanoparticles (MSNs) are one of the most studied because of their well-developed silanol chemistry and extraordinarily high surface to volume ratio that allows a high local concentration of guest molecules to be contained within the porosity.1, 8 In addition, their biocompatibility has made MSNs an excellent material for biomedical applications including drug delivery9, 10 and medical imaging.11 Underpinning these wide spectra of applications are the tunable surface chemistry and the variable molecular properties inside the mesopores. There have been several studies of the altered dynamics of molecules in confined spaces. For example, the depression of the freezing point for water confined in the cylindrical pores of both mesoporous silica and carbon nanotubes has been observed.12 Rigidochromism and dynamic fluorescence anisotropy studies have also confirmed that the molecular mobility in MSNs can be varied by changing the charges of pore walls.13 Nuclear magnetic resonance (NMR) spectroscopy has been an attractive tool to investigate the surface interactions between guest molecules and silica pore walls. A variety of NMR techniques have been developed to investigate molecular dynamics depending on the motional frequency ranges.14, 15 The measurement of relaxation times in rotating frame (T1ρ) allows studying dynamics in the kHz range.14, 15 Even slower motion can be accessed by spectral line shape and width.14, 15 In contrast, spin-lattice relaxation times (T1), which perhaps are the most widely used parameters in describing molecular dynamics, are sensitive to picosecond and nanosecond


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motions.14, 15 Relaxation is the process driving the spins to equilibrium and is the result of molecular motion.16, 17 In considering dipole-dipole interactions, relaxation is expressed as Eq. 117 where γ is the gyromagnetic ratio; µ 0 is the permeability of free space; ћ is the reduced Planck constant; ω is the Larmor frequency; r is the distance between two dipoles; and τR is the rotational correlation time. 2 2   h 2γ P γ H 3τ R 6τ R τR 1 1 µ0 2 = ( ) ∑ × + + 6 2 2 2 2  2 2 T1 10 4π rPH 1 + ( ω − ω ) τ 1 + ω τ 1 + ( ω + ω ) τ  H P R P R H P R 

LLLLLL

Eq .1

NMR chemical shifts have been utilized to investigate the influence of size and morphological difference of pore walls on the filling of guest molecules into the porosity.18, 19 Three earlier NMR studies of motion in the pores of un-derivatized MSNs have been reported.20-22 Two of them have investigated ibuprofen within mesoporous silica.20, 21 More recently, benzoic acid and fluorinated benzoic acid co-crystallized inside of MSNs employing thermal solid phase transformation have also been studied.22 It is worth noting that in these earlier works qualitative interpretation of the motion of guest molecules with various sizes has been offered. In addition, the guest molecules used in their works for the investigations of dynamics within mesoporous silica were exclusively a pure solid phase20-22 for the measurements using solid-state NMR. Herein, we quantitatively study molecular motions of solutes in aqueous solution encapsulated in the pores of MSNs. Significantly, soluble molecules are often charged in a solution either by protonation/deprotonation or ion dissociation.9, 10 One may, therefore, expect that the surface chemistries and the corresponding surface charges of MSNs would demonstrate a great impact on molecular dynamics of the loaded molecules. In this work, we thus sought to study, in a quantitative way, molecular motions of guest molecules in an aqueous solution encapsulated in a variety of functionalized MSNs using solid-state NMR.


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RESULTS AND DISCUSSION Particle Design and Synthesis. Both native and surface-derivatized MSNs were synthesized through a typical surfactant-templated and base-catalyzed sol-gel condensation procedure13 (details in Methods). The native MSNs, due to their surface silanol groups, were negatively charged in pH 7 phosphate buffer without additional NaCl salts (ζ = -21.8 ± 1.6 mV); amine modification led to positively charged nanoparticles (NH2-MSN; ζ = 21.7 ± 1.6 mV); carboxylate and phosphonate functionalization both produced even more negatively charged surfaces than the native particles (COOH-MSN and POONa-MSN; ζ = -24.8 ± 0.7 and -40.6 ± 1.2 mV, respectively). The representative chemical structures of these surface functional groups on particles are shown in Fig. 1E. Fourier transform infrared spectra (FTIR) shown in Fig. S1 and thermogravimetric analysis (Table S3) offered additional evidence of carboxylation. The particle shape, size and porosity were specifically designed to be comparable across all four types of nanoparticles as characterized by transmission electron microscopy (TEM) and nitrogen adsorption/desorption measurement (data shown in Fig. 1A - 1D and Table S1, respectively). Specifically, the pore diameter has been confirmed to be in the range of 3 to 4 nm. In doing this, the following dynamics studies were able to truly reflect the changes in surface chemistries rather than the results from the inconsistent physical properties. The as-synthesized particles were then loaded with various probing guest molecules by soaking the particles in probe solutions. As we were interested in the molecular motions inside the pores of MSNs, a particle capping procedure was employed allowing us to distinguish the molecular motions within the particles from that in the external dispersion medium. This pore capping was accomplished by the addition of silane precursors (see procedures shown in Methods). It is this specific cap employment that allowed the externally physisorbed molecules


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to be removed by extensive washing without losing a significant amount of encapsulated molecules in the pores. Most importantly, this ensured that the subsequent dynamics results originate exclusively from the interaction of internally loaded molecules with the pore walls. The capped particles then were centrifuged down and placed in a NMR rotor without any particle drying procedure allowing the investigation of molecular motion in a solution within MSNs. The success of capping and the removal of loosely physisorbed probe molecules were confirmed by the subsequent NMR studies as the molecular motions of probes encapsulated in particles and in the external dispersion medium are different (see the detail discussion in the following sections). Choice of Probe. Three different phosphorus-containing ionic compounds [tetrabutylphosphonium (BuP) at ~1.2 nm, tetramethylphosphonium (MeP) at ~0.5 nm and hexafluorophosphate (PF6) at ~0.4 nm] were loaded in both native and modified MSNs. Fig. 1E and 1F illustrated the structures of the surface modifications on MSNs and the loaded guest molecules. The sizes of these guest molecules were approximated by a simple bond length and angle estimation. Phosphorus-containing molecules were studied in this work due not only to 100% natural abundance of phosphorus-31 but also to the simplicity of their spectra. Proton spectra rely on a very fast magic angle spinning rate to cancel out their large chemical shift anisotropy,23 whereas it only requires a moderate spinning rate to obtain a well resolved 31P spectrum. In addition, the phosphorus atoms are located in the center of all the investigated molecules where the symmetry indicates 31P dynamics are representative of whole molecular motions. Significantly, the 31P chemical shifts of the molecules studied in this work are not only different enough from each other but are also sensitive enough for identifying both the environmental difference and the presence of interaction with pore walls. It is worth noting that the co-condensation technique for the synthesis of phosphonate modified nanoparticles (POONa-


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MSN) was able to generate phosphonate functionalization on both the pore surfaces and inside the silica framework networks. The chemical shifts shown in Fig. 2 at 26.1 ppm and 34.8 ppm belonged to the phosphonate groups located on the surfaces and imbedded in the silica networks, respectively. In contrast, the spectrum of phosphonate functionalized MSNs synthesized by a post-modification method (POONa-MSN-g, see procedures in Methods) shown in Fig. S2 exhibited a single chemical shift, indicating surface only modifications. Furthermore, examination of chemical shifts was also employed to identify the presence of interaction between the loaded molecules and the particle surfaces. Fig. 2 shows the influence of the loaded molecules (BuP, MeP and BuP/PF6) on the chemical shifts of the surface phosphonate groups on MSNs (POONa-MSN) shown in black lines. Red lines in the figure were shown for the purpose of identifying the chemical shifts of the guest molecules (BuP and MeP) in nanoparticles. Notably, the phosphonate groups imbedded in silica networks were unaffected by the loading as confirmed by the presence of peaks at 34.8 ppm in all black lines, and will be further verified by T1 studies shown in the later sections. Significantly, in the presence of guest molecules, the peak at 29.9 ppm for the surface phosphonate groups on empty MSNs exhibited an upfield shift to 26.1 ppm resulting from the interaction with the guest molecules (indicated by the arrows). Motional Region Determination. As chemical shifts have confirmed the presence of interaction between the loaded probe molecules and the particle surfaces, quantitative investigation of the change in molecular motion caused by this interaction was subsequently carried out. To provide quantitative information on molecular motion within MSNs using Eq. 1, it should be noted that this is a second-order polynomial equation where a single T1 generates two roots for rotational correlation times (τR). There are then two motional regions defined by this equation as shown in Fig. 3. Specifically, the so-called extreme narrowing limit is the


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motional region with the smaller root (shorter τR). The slower motion defined by the larger root (longer τR) is fundamentally contradictory to the assumption of rapid isotropic motion that Eq. 1 follows and thus usually cannot be described satisfactorily by this single longer τR.17, 24 Emphatically, the determination of motional region not only is crucial for the validity of Eq. 1 but also is significantly important in the feasibility of this equation for the description of molecular dynamics in MSNs. The motional region can be determined qualitatively by examining the efficiency of NMR polarization transfer. Specifically, cross-polarization (CP) in solid-state NMR has been extensively utilized to qualitatively investigate molecular dynamics.20, 22, 25 In order to generate efficient polarization transfer from abundant nuclei to dilute nuclei (efficient dipolar coupling) that CP requires, molecular motion must be slow enough to prevent the averaging of dipolar coupling.17 In this work, two highly water-soluble molecules (tetrabutylphosphonium bromide, BuP and tetramethylphosphonium bromide, MeP) loaded in the four different charged nanoparticles were initially studied. Fig. 4 and Fig. S3 show the changes in peak intensities caused by CP. Magic angle spinning (MAS) was conducted throughout all of the NMR experiments designated here as CPMAS or SPMAS depending on the employment of a cross-polarization or a single-pulse technique. In both figures, the intensities of all peaks (32.4 ppm for BuP and 21.8 ppm for MeP) employing CPMAS shown in solid lines were clearly weaker than those with SPMAS shown in dashed lines. These results indicate that the tumbling of both molecules is relatively rapid within the pores of all types of MSNs independent of surface charges as characterized by the reduced signal intensity in the presence of CP. Significantly, the emerging peaks at 32.4 ppm and 21.8 ppm (for BuP and MeP, respectively) and the reduction at 34.8 ppm (for the covalently bonded


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phosphonate functionality) in the absence of CP shown in Fig. 4D and Fig. S3D clearly demonstrate that the freely loaded phosphorus molecules tumble faster than the covalently bonded phosphonate group on MSNs. The failure of intensity enhancement by CP qualitatively indicates fast mobility of these molecules in MSNs. To quantitatively confirm that the molecular motion is truly in the extreme narrowing limit, the temperature dependence of T1 was measured with the larger molecules (BuP@MSN, ~1.2 nm). The plot (T1 vs. τR) of Eq. 1 shown in Fig. 3 is characterized by an initial negative τR dependence of T1, which is indeed the extreme narrowing limit.17, 24. It is known that increasing the temperature is capable of accelerating molecular motion leading to a negative temperature dependence of τR. The extreme narrowing limit can then be characterized by the increased T1 with increasing temperature. Fig. S4 demonstrated the positive correlation of T1 with increasing temperature which unambiguously proved the fast molecular motion in extreme narrowing region and the validity of Eq. 1.16 NMR peak width is also an indication of molecular tumbling. Specifically, fast mobility is able to efficiently average chemical shift anisotropy resulting in a narrower NMR peak.16 In comparing the peak widths of BuP and MeP molecules in MSNs (at 32.4 ppm and 21.8 ppm, respectively), Fig. S5 clearly indicated that smaller MeP (~0.5 nm) tumbled faster than BuP by the narrower peak. It should be noted that since the larger and heavier BuP fell in the extreme narrowing region, there is no grounds for smaller MeP being outside of this region and the validity of Eq. 1 was thus confirmed for MeP loaded in MSNs. Charge Effect on Molecular Motion. Because BuP and MeP tumbling in MSNs have been confirmed to be in the region where Eq. 1 is applicable, their rotational correlation times in a


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variety of charged pores in the nanoparticles were subsequently studied. It is worth mentioning that the equation is concentration independent because of rapid molecular motion in extreme narrowing region.26 Table 1A summarized the T1 and the corresponding τR derived from Eq. 1, considering 1H/31P dipolar interaction. A first result to note is that the molecular tumbling of BuP was significantly restricted by pore confinement; τR increased from 1.4 ± 0.1 ps for a free solution to 3.2 ± 0.4 ps for the molecules in the pores in native MSNs. This 56% of reduction in mobility resulted not only from a nano-confinement but also from an electrostatic attraction of cationic BuP molecules to negatively charged silanol groups on the pore walls. The mobility, in contrast, is expected to be accelerated by a repulsive force between pore walls and the loaded molecules, as was shown by the prior results with ibuprofen25 and tris(bipyridine)ruthenium(II)hexafluorophosphate.13 The motion of BuP confined in positively charged NH2-MSNs is about the same as that in solution (τR = 1.5 ± 0.1 ps for BuP@NH2-MSN and 1.4 ± 0.1 ps for BuP in water). This result can be rationalized by a model depicted in Fig. 5A where the number of BuP molecules occupying the pores is significantly affected by the charges of pore walls. A maximum of two BuP molecules (~1.2 nm) along with solvent molecules are able to fit in a

Effect of Pore Wall Charge and Probe Molecule Size on Molecular

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