Stimuli-Responsive Shapeshifting Mesoporous ... - ACS Publications

Letter pubs.acs.org/NanoLett

Stimuli-Responsive Shapeshifting Mesoporous Silica Nanoparticles Yao Sun,† Hiroaki Sai,† Katherine A. Spoth,‡ Kwan Wee Tan,† Ulrike Werner-Zwanziger,§ Josef Zwanziger,§ Sol M. Gruner,∥,⊥,# Lena F. Kourkoutis,‡,∥ and Ulrich Wiesner*,† †

Department of Materials Science and Engineering and ‡Department of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States § Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada ∥ Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, United States ⊥ Department of Physics and #Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: Stimuli-responsive materials have attracted great interest in catalysis, sensing, and drug delivery applications and are typically constituted by soft components. We present a one-pot synthetic method for a type of inorganic silica-based shape change material that is responsive to water vapor exposure. After the wetting treatment, the cross-sectional shape of aminated mesoporous silica nanoparticles (MSNs) with hexagonal pore lattice changed from hexagonal to six-angle-star, accompanied by the loss of periodic mesostructural order. Nitrogen sorption measurements suggested that the wetting treatment induced a shrinkage of mesopores resulting in a broad size distribution and decreased mesopore volume. Solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy of samples after wetting treatment displayed a higher degree of silica condensation, indicating that the shape change was associated with the formation of more siloxane bonds within the silica matrix. On the basis of material characterization results, a mechanism for the observed anisotropic shrinkage is suggested based on a buckling deformation induced by capillary forces in the presence of a threshold amount of water vapor available beyond a humidity of about 50%. The work presented here may open a path toward novel stimuli-responsive materials based on inorganic components. KEYWORDS: stimuli-responsive, shape change, mesoporous silica, nanoparticles, capillary force

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there has been an exponential increase in research publications on biomedical applications of MSNs.25 Stimuli-responsive MSNs have been developed in the past decade to achieve controlled drug delivery. In most cases, however, MSNs were used as rigid building blocks to load drug molecules. Controlled release was achieved by capping mesopores with various “gatekeepers”, which are removed when exposed to external stimuli (such as pH, temperature, light, or magnetic fields), resulting in the release of entrapped drug molecules.26−29 It would be very interesting to develop a type of MSNs, where the particles themselves are responsive to a particular stimulus by changing their physical and/or chemical properties. As a result of the physical rigidity and chemical stability of silica, however, at first sight this seems counterintuitive. Here we report the room-temperature synthesis of columnlike aminated MSNs with hexagonal pore lattice and crosssectional shape, which exhibit shapeshifting behavior when exposed to water vapor, that is, their cross-sectional shape shifts from hexagonal to six-angle-star. The synthesis of shapeshifting MSNs (ss-MSNs) was based on the co-condensation of

timuli-responsive materials (SRMs) have been drawing more and more academic and industrial attention due to their ability to respond to specific stimuli such as heat, chemicals, pH, and light.1−8 For most SRMs, the way they respond is by means of changing their physical and/or chemical properties.9 One group of SRMs, referred to as shape change materials (SCMs), can alter their shape upon the presence of a particular stimulus. Among the extensively studied SCMs, polymers are the most important class due to their low density, low cost, and potential biocompatibility.2,10 Although polymers have a wide range of applications in sensing11,12 and drug delivery fields,5,13 long-term reliability/durability still remains a challenge.9 An alternative approach could be the design of a silica-based SCM, which would potentially provide a more reliable system due to the relative robustness of silica.14−16 Silica is a well-studied inorganic material due to its low toxicity, chemical versatility, and biocompatibility.17,18 Mesoporous silica materials with uniform pore size and ordered pore structure were first reported in the early 1990s19,20 and have been widely used in catalysis and sensing.21,22 Mesoporous silica nanoparticles (MSNs) have attracted widespread research interest attributed to the aforementioned advantages of mesoporous silica in combination with the benefits of nanosized materials.23 Since the first paper on using MCM41-type MSNs as a drug delivery system published in 2001,24 © 2015 American Chemical Society

Received: October 29, 2015 Revised: December 7, 2015 Published: December 15, 2015 651

DOI: 10.1021/acs.nanolett.5b04395 Nano Lett. 2016, 16, 651−655

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Figure 1. (a−h) TEM images of vacuum-dried p-MSNs (a,b), 24h-wet p-MSNs (c,d), vacuum-dried ss-MSNs (e,f), and 24h-wet ss-MSNs (g,h). Insets in (b,d,f,h) are magnified images of selected areas, indicated by rectangles. The brightness and contrast of the inset in (h) were adjusted to better display the structure. Axes of hexagonal lattices are displayed in (f,h). (i−j) SEM images of vacuum-dried ss-MSNs (i) and 24h-wet ss-MSNs (j). The outlines of the shapes of two MSNs are shown in the SEM images for illustration. Scale bars in insets in (b,d,f,h) are 20 nm, and all other scale bars are 200 nm. (k−n) Histograms of diameter decrease along the (10) direction (k), diameter decrease along the (11) direction (l), ratio of contractions along (11) and (10) directions (m), and ss-MSN length decrease (n), as determined from STEM images of vacuum-dried and 24h-wet ss-MSNs (same particle set before and after wetting treatment). Thirty-eight and 33 particles were measured for data sets in (k−m) and in (n), respectively.

tetraethyl orthosilicate (TEOS) and N-(2-aminoethyl)-3aminopropyltrimethoxysilane (AEAPTMS) (see Supporting Information). The original reason we chose AEAPTMS as an organosilane to co-condense with TEOS was that the ethylenediamine unit has been shown to undergo a pHdependent two-step protonation with a distinctive gauche-anti conformational transition, which facilitates cell membrane destabilization leading to endosomal escape.30,31 In comparison, plain MSNs (p-MSNs) were synthesized as control samples by hydrolyzing and condensing TEOS alone. We first applied transmission electron microscopy (TEM), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) to characterize the mesostructure and morphology of p-MSNs and ss-MSNs after two different postsynthesis treatments. Surfactant-removed particle suspensions were first vacuum-dried (referred to as vacuum-dried MSNs). A wetting treatment was subsequently employed by leaving some of the vacuum-dried sample powders in open vials placed in a closed desiccator for 24 h. The desiccator was filled with saturated sodium chloride (NaCl) solution at the bottom, which provided a relative humidity of around 75%.32 After the wetting treatment, the particles still appeared as dry and loose powders, which will be referred to as 24h-wet MSNs. The vacuum-dried p-MSNs showed spherical to elliptical shape with often circular crosssection (Figure 1a). A higher-magnification TEM image reveals

that the mesostructure was composed of hexagonally packed cylindrical channels with uniform diameters (Figure 1b). After the wetting treatment, the external shape of p-MSNs remained almost unchanged but the particles looked denser under the microscope compared to vacuum-dried p-MSNs (Figure 1c). A closer inspection suggested that the mesopores were no longer easily observable and the ordered hexagonal mesostructure had disappeared (Figure 1d). In summary, TEM images of vacuumdried and 24h-wet p-MSNs showed that the wetting experiment induced a loss of mesopores and periodic mesostructure while the overall particle shape did not change substantially. In clear contrast to the p-MSNs, vacuum-dried aminated ssMSNs showed well-faceted column-like external shape with hexagonal cross-section (Figure 1e) and the particles possessed 2D hexagonal mesostructure (Figure 1f). A close examination of the TEM images revealed that these ss-MSNs possessed a denser outer silica shell layer (Figure 1f inset) not present in the TEOS-based p-MSNs. The presence of organoalkoxysilanes during synthesis is known to influence final particle morphology, and factors such as electrostatic attraction/ repulsion, hydrophobic interactions, and hydrogen bonding between silane precursors and surfactant micelles at the micelle/water interface have been discussed as potential contributors to observed particle structure.33 To our surprise, however, in contrast to the plain MSNs for the aminated particles after the wetting treatment the cross-sectional shape 652


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observations in Figure 1c,d, in that the particles appeared dense without visible arrays of ordered mesopores. The SAXS patterns of ss-MSNs showed a similar evolution upon processing (Figure 2d−f). In ethanol suspension, ssMSNs displayed hexagonal reflections with the first order peak at q = 1.54 nm−1 (Figure 2d). The hexagonal lattice was conserved upon vacuum-drying with the first order peak shifting to q = 1.61 nm−1 (Figure 2e). The vacuum-dryinginduced similar shifts of q values for p-MSNs and ss-MSNs of 0.08 and 0.07 nm−1, respectively, indicating similar effects on these two types of MSNs. Finally, the SAXS pattern of 24h-wet ss-MSNs only showed one relatively weak peak at q = 1.87 nm−1 without any higher order reflections (Figure 2f). The wetting treatment shifted the first order peak of vacuum-dried ss-MSNs to higher q values corresponding to 14% lattice shrinkage and induced a drop in scattering intensity. The significant amount of shrinkage between vacuum-dried and 24h-wet ss-MSNs is concomitant with the observation of the hexagonal cross-section contracting into a six-angle-star shape with less visible mesopores. On the basis of the electron microscopy and SAXS observations, we expected the porosity to change substantially through the wetting treatment and therefore performed nitrogen sorption measurements on p-MSNs and ss-MSNs. In Figure 3a,c, vacuum-dried p-MSNs and ss-MSNs both

changed dramatically into a well-defined six-angle-star shape (Figure 1g). A higher-magnification TEM image of a 24h-wet ss-MSN in Figure 1h indicated less well-defined and periodic mesopores as compared to the vacuum-dried ss-MSN in Figure 1f (and higher magnification image therein). The external shape change was better demonstrated in SEM images. Vacuum-dried ss-MSNs exhibited a column-like morphology with hexagonal cross-section (Figure 1i), whereas 24h-wet ss-MSNs displayed a column-like morphology with six-angle-star shaped crosssection (Figure 1j). In order to study the morphology change in different directions, scanning transmission electron microscopy (STEM) images of the same set of ss-MSNs before and after the wetting treatment were analyzed (Figure 1k−n, see Supporting Information). The diameter along the (10) direction decreased by 10.3% on average (Figure 1k), compared to 15.4% on average along the (11) direction (Figure 1l). The anisotropy of contraction was determined for each ss-MSN by dividing the measured percent decrease along the (11) direction by that along the (10) direction. These data show that on average the contraction is 1.5 times greater along the (11) direction (Figure 1m). Therefore, the star-shaped structure originates from an anisotropic contraction of the 2D hexagonal shape with a larger contraction along the (11) direction as compared to the (10) direction. On the other hand, the length of column-like ssMSNs decreased only by 2.2% on average (Figure 1n). The structural difference between MSNs before and after wetting treatment was further investigated using small-angle Xray scattering (SAXS). Suspensions of p-MSNs in ethanol exhibited reflections consistent with hexagonal p6mm symmetry (Figure 2a). The mesostructure was preserved upon vacuum-

Figure 3. Nitrogen sorption isotherms (a,c) and DFT-derived pore size distributions (b,d) for p-MSNs (a,b) and ss-MSNs (c,d). Figure 2. SAXS patterns of (a) p-MSNs in ethanol suspension, (b) vacuum-dried p-MSNs, (c) 24h-wet p-MSNs, (d) ss-MSNs in ethanol suspension, (e) vacuum-dried ss-MSNs, and (f) 24h-wet ss-MSNs. Expected peak positions for a lattice with hexagonal p6mm symmetry are indexed by solid lines.

exhibited type IV isotherms with capillary condensation of liquid nitrogen in mesopores at nitrogen partial pressures around 0.3. In contrast, no substantial condensation occurred in this region for 24h-wet p-MSNs and ss-MSNs, and the quantity adsorbed in the intermediate flat region of the data was significantly reduced, indicating a decreased mesopore volume compared to the vacuum-dried samples. This is elucidated in the pore size distributions calculated based on the isotherms using a density functional theory (DFT) model35,36 (Figure 3b,d, see Supporting Information for details). Both of the vacuum-dried samples showed narrow pore size distributions centered around 3.5 to 4 nm, whereas both of the 24h-wet samples exhibited more broadly distributed pore sizes from 2 to 3.5 nm with significantly smaller areas under the curves, suggesting that not only the mesopore size became smaller but also the mesopore volume got reduced. The Brunner−

drying with the first order peak shifting from q = 1.64−1.72 nm−1, where the scattering vector, q, is defined as q = 4π sin θ/ λ with 2θ being the total scattering angle and λ is the X-ray wavelength (Figure 2b). This shift is likely due to the shrinkage induced by continued silica condensation during drying, similar to what has been reported for other systems before.34 In contrast, after the wetting treatment reflections consistent with a hexagonal lattice were absent (Figure 2c). This SAXS pattern for the 24h-wet p-MSNs is consistent with the TEM 653


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linking in the silica matrix, and excess silane species at the end of the 24h reaction which deposited on the MSNs as the reaction was quenched to form the core−shell architecture described at the beginning (Figure 1f and inset). It is wellknown that capillary forces can cause shrinkage or contraction at the newly generated contacting surface of pores in silica materials when water is present.39 We hypothesize that this capillary force causes an inward stress on the faceted dense outer shell, where the inner silica behaves as a plastic matrix. The anisotropic shrinkage could arise from this isotropic inward stress applying onto a hexagonal outer shell observed in Figure 1f and causing a buckling deformation. As we show in additional experiments described in the Supporting Information this deformation only is observed at relative humidity levels above a threshold value of about 50%, behavior that is consistent with a stimulus-responsive material. We also show that additional thermal processing of the materials suppresses shapeshifting further elucidating the importance of the low degree of silica cross-linking. In summary, we have successfully synthesized a new kind of shape change material, that is, aminated MSNs with hexagonal cross sections that exhibit a well-defined shapeshifting behavior. We have shown that the well-faceted aminated MSNs were responsive to water vapors leading to a change in cross-section from hexagonal to six-angle-star at relative humidity above about 50%. The shape change is accompanied by a decrease in surface area and mesopore volume, an increase in micropore volume, and further condensation of the silica matrix, rendering it irreversible. Nonaminated plain MSNs showed shrinkage behavior upon wetting but did not show a well-defined and easy-to-detect shape change. On the basis of material characterization results, a possible mechanism for the observed anisotropic shrinkage was proposed. This new type of responsive silica material may find use in drug delivery or as a fragrance release system.

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Figure 4. Solid-state 29Si NMR spectra of vacuum-dried and 24h-wet ss-MSNs.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04395. Materials, synthetic methods, and characterization and humidity threshold study. (PDF)

correspond to Q4 (Si(OSi)4), Q3 (Si(OSi)3(OH)), and Q2 (Si(OSi)2(OH)2) groups, respectively. The peaks at around −60 to −70 ppm correspond to the combination of mainly T3 (R-Si(OSi)3) and T2 (R-Si(OSi)2(OH)) groups introduced by the co-condensation of aminosilane (AEAPTMS) with TEOS. The ratio of Q4/Q3 groups is substantially larger for 24h-wet ssMSNs than that for vacuum-dried ss-MSNs, suggesting that upon wetting the degree of silica condensation increases. This increased network density also renders the shrinkage and shape change irreversible. From analysis in Figure 1n we know that the particle length remained almost unchanged upon wetting treatment, suggesting that the overall shrinkage of the silica network is not substantial. In turn, this suggests that the cross-sectional shrinkage mainly stems from loss of mesopores, consistent with observations from nitrogen sorption measurements. It is, however, not obvious that the loss of mesopores leads to a star-shaped cross-section, as pores are more closely packed along the (10) direction than along the (11) direction. Below we propose that an alternative mechanism is at play that arises from the exterior faceting. The retarded reactions in aminosilane-based MSN synthesis23,38 have affected the ss-MSN formation in multiple ways, such as hexagonally faceted outer shape that reflects the inner mesoscopic symmetry during the growth, lower degree of cross-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(H.S.) Simpson Querrey Institute, Northwestern University, Evanston, IL 60208, U.S.A. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DESC0010560 and by Award No. DE-FG02-10ER46693 to S.M.G. Work by K.A.S. and L.F.K. was supported by the Cornell Center for Materials Research (CCMR) with funding from the NSF MRSEC program (DMR-1120296). K.W.T. gratefully acknowledges the Singapore Energy Innovation Program Office for a National Research Foundation graduate 654


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(28) Nadrah, P.; Planinšek, O.; Gaberšcě k, M. J. Mater. Sci. 2014, 49, 481−495. (29) Tarn, D.; Ashley, C. E.; Xue, M.; Carnes, E. C.; Zink, J. I.; Brinker, C. J. Acc. Chem. Res. 2013, 46, 792−801. (30) Kanayama, N.; Fukushima, S.; Nishiyama, N.; Itaka, K.; Jang, W.-D.; Miyata, K.; Yamasaki, Y.; Chung, U.; Kataoka, K. ChemMedChem 2006, 1, 439−444. (31) Miyata, K.; Christie, R. J.; Kataoka, K. React. Funct. Polym. 2011, 71, 227−234. (32) Winston, P. W.; Bates, D. H. Ecology 1960, 41, 232. (33) Huh, S.; Wiench, J. W.; Yoo, J.; Pruski, M.; Lin, V. S.-Y. Chem. Mater. 2003, 15, 4247−4256. (34) Suteewong, T.; Sai, H.; Lee, J.; Bradbury, M.; Hyeon, T.; Gruner, S. M.; Wiesner, U. J. Mater. Chem. 2010, 20, 7807. (35) Tarazona, P.; Marconi, U. M. B.; Evans, R. Mol. Phys. 1987, 60, 573−595. (36) Landers, J.; Gor, G. Y.; Neimark, A. V. Colloids Surf., A 2013, 437, 3−32. (37) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723−1732. (38) Suteewong, T.; Sai, H.; Bradbury, M.; Estroff, L. A.; Gruner, S. M.; Wiesner, U. Chem. Mater. 2012, 24, 3895−3905. (39) Brinker, C. J.; Sehgal, R.; Hietala, S. L.; Deshpande, R.; Smith, D. M.; Loy, D.; Ashley, C. S. J. Membr. Sci. 1994, 94, 85−102.

fellowship. This work made use of the CCMR Shared Facilities, which are supported through the NSF MRSEC program (DMR-1120296), and CHESS, which is supported by the NSF & NIH/NIGMS via NSF award DMR-0936384. H.S. thanks Garrett Lau (Northwestern University) for fruitful discussions.

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