Hierarchical Lotus Leaf-Like Mesoporous Silica Material with Unique

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Hierarchical Lotus Leaf-Like Mesoporous Silica Material with Unique Bilayer and Hollow Sandwich-Like Folds: Synthesis, Mechanism, and Applications Nanjing Hao, Hamid T. Chorsi, and John X. J. Zhang* Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, New Hampshire 03755, United States S Supporting Information *

ABSTRACT: A facile and straightforward strategy was developed to synthesize hierarchical lotus leaf-like mesoporous silica material (LLMSM) with unique bilayer and hollow sandwich-like folds using hexadecylamine as a structuredirecting agent, high molecular weight polyvinylpyrrolidone as a structure-stabilizing agent, and tetraethyl orthosilicate as a silica precursor in the mixed solvents of water and ethanol. The formation mechanism of LLMSM was proposed based on a series of experimental investigations. This new particulate platform exhibited superior adsorption capacity toward organic dye, effective catalytic reduction of 4-nitrophenol as a Ag catalyst supporter, and a multistage sustained drug release profile. Many other applications can be also envisaged for such a hierarchical structure. KEYWORDS: Mesoporous silica, Hierarchical, Shape, Lotus leaf, Hollow

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ince its discovery in the early 1990s,1 mesoporous silica material (MSM) has attracted extensively interests in various fields, such as energy, catalysis, adsorption, separation, and biomedicine. To maximize its efficacy and effectiveness, considerable efforts have been devoted to the rational design of such material to suit specific application needs through controlling the physicochemical properties, including particle size, surface chemistry, and pore types.2,3 However, more and more evidence from both theoretical and experimental aspects have recently revealed that particle shape could also significantly affect its performance, especially for those processes involving mass transfer.4−7 These findings not only help to shed new light on the shape genesis of MSM but also inspire researchers to develop MSM of various shapes. To date, many strategies have already been employed for the synthesis of different shapes of MSM, such as sphere,8 ellipsoid,7 rod,9 sheet,10 platelet,11 and some corresponding hollow counterparts.12−14 However, most of these shapes possessed only single or simple architecture and pore arrangement, which may restrict the availabilities of certain pore channels for mass transfer. Comparatively, hierarchical three-dimensional porous structures with complex shapes could provide more exposed pore openings and pore arrangements, which may improve the accessibility of pore channels inside MSM, and thus make MSM greatly favored for many application fields, such as adsorption and catalysis.15,16 However, despite significant and increasing needs, facile and stable synthesis of hierarchical porous silica material, especially © 2017 American Chemical Society

having short pore path lengths and rich pore structures, is still an outstanding challenge. Herein, we report a facile one-pot strategy to synthesize hierarchical lotus leaf-like mesoporous silica material (LLMSM) with unique bilayer and hollow sandwich-like folds using hexadecylamine (HDA) as a structure-directing agent, high molecular weight polyvinylpyrrolidone (PVP) as a structurestabilizing agent, and tetraethyl orthosilicate (TEOS) as a silica precursor in the mixed solvents of water and ethanol. The formation mechanism of LLMSM was explored through a series of experimental investigations. The adsorption, catalytic, and sustained drug release performances of this hierarchical structure-based platform were also examined (Scheme 1). Hierarchical porous silica material having unique bilayer and hollow sandwich-like folds can be successfully synthesized using only HDA, PVP, and TEOS in the mixed water and ethanol solution (see experimental details in the Supporting Information). As shown in Figure 1, lotus leaf-like silica material displayed a unique layered architecture and typical folded protuberance. This LLMSM has an average size of ∼4 μm and an average thickness of ∼150 nm. Both the edge region and the inside region of LLMSM showed clearly porous structures (Figure S1), and multiple pore size distributions can be investigated especially from the folds of the edge region (Figure Received: November 29, 2016 Revised: January 26, 2017 Published: February 20, 2017 2044

DOI: 10.1021/acssuschemeng.6b02893 ACS Sustainable Chem. Eng. 2017, 5, 2044−2049

Letter

ACS Sustainable Chemistry & Engineering

Scheme 1. Synthesis of LLMSM with Unique Bilayer and Hollow Sandwich-Like Folds as Efficient Adsorbent, Catalyst Supporter, and Drug Carrier

Figure 1. Scanning electron microscopy (SEM) images (A−C) and transmission electron microscopy (TEM) images (D−F) of as-synthesized hierarchical mesoporous silica material with unique bilayer and hollow sandwich-like folds. The inset in panele A is the image of a lotus leaf.

precursor played the main role in the regulation of material thickness but had minor effect on the hierarchical shapes (Figure S3). Thus, the effects of PVP and HDA on the formation of LLMSM were primarily investigated. In the absence of PVP, hierarchical silica material still can be obtained. However, there is no well-defined lotus leaf-like structure with typical folds, and severe aggregations can be observed (Figure S6). This phenomenon provided direct evidence that PVP and HDA played the structure-stabilizing role and structuredirecting role in the synthesis process of LLMSM, respectively. If the concentration of PVP is decreased in the reaction system, a hierarchical silica structure with typical hollow sandwich-like folds still can be produced (Figure S7). However, increasing the concentration of PVP can only yield a hierarchical silica structure with solid folds (Figure S8), which may be caused from the excessive recruitment of silica precursors by PVP molecules and then evolving into more dense and compact structures. The molecular weight of PVP also significantly affects the fine structures of the as-synthesized materials. Compared to high molecular weight of PVP for producing typical hollow sandwich-like folds, low molecular weight of PVP can only produce tiny wrinkled protuberances (Figure S9). These results not only confirmed the role of PVP as the structure-stabilizing agent but also revealed the functions of

1C,F and Figure S1A). From the broken particles (Figure S2), the existence of a bilayered structure was further confirmed when combined with the observations from hollow sandwichlike folds (Figure 1C). In addition, the thickness of this hierarchical mesoporous silica structure can be easily tuned from ∼30 to ∼400 nm by just changing the concentration of silica precursor (TEOS) (Figure S3). N2 adsorption− desorption analysis was then performed on the as-synthesized LLMSM after removing HDA and PVP via the extraction method (Figure S4). The results showed that LLMSM exhibited a typical type IV isotherm with H3 hysteresis (Figure S5), indicating the presence of slit-shaped and large-sized porous structures.17,18 The pore size of LLMSM was calculated to be 2.95 nm by the Barrett−Joyner−Halenda (BJH) method (Figure S5B), and multiple pore size distributions are also in agreement with the observations from TEM and SEM images (Figure 1 and Figure S5B). The Brunauer−Emmett−Teller (BET) surface area and pore volume were measured to be 806.5 m2/g and 0.76 cm3/g, respectively. To explore the formation mechanism of this hierarchical porous silica structure with unique bilayer and hollow sandwich-like folds, we carried out a series of experiments to investigate the specific roles of TEOS, PVP, and HDA in the reaction system. As mentioned above, TEOS as a silica 2045

DOI: 10.1021/acssuschemeng.6b02893 ACS Sustainable Chem. Eng. 2017, 5, 2044−2049

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Scheme 2. Proposed Mechanism for Formation of Hierarchical Mesoporous Silica Material with Unique Bilayer and Hollow Sandwich-Like Folds

Figure 2. (A) UV−vis spectra for time-dependent adsorption of Rhodamine B in the presence of LLMSM under dark conditions. (B) Adsorption rate of Rhodamine B by LLMSM. C0 is the initial concentration of the Rhodamine B solution, and C is the concentration of that at different time intervals during the adsorption. Error bars show the standard deviations from three independent tests. One inset is the photograph of a group of Rhodamine solutions at varying times, and the other inset represents the photographs of LLMSM before and after adsorption.

S11), while increasing the concentration of HDA can yield a hierarchical product with more typical folds in the edge region (Figure S12), which further confirmed its roles as a structuredirecting agent and catalyst. On the basis of the above observations, a region-dependent growth mechanism of LLMSM with a unique bilayer and hollow sandwich-like folds was proposed (Scheme 2). In the inside region, the high molecular weight PVP molecules with strong polar pyrrolidone rings can first bind with HDA to form a bilayered skeleton,19 then TEOS will be attracted to the

PVP molecules on the formation of layered and folded morphologies. Next, the effects of HDA on the formation of LLMSM were explored in a series of studies. Without the addition of HDA, there is no obvious product formed, and there are only disordered nanoparticles formed by changing HDA to ammonia (Figure S10), indicating that HDA not only played a structure-directing role but also acted as a catalyst for catalyzing hydrolysis of TEOS. In addition, we found that decreasing the concentration of HDA in the reaction system can still yield a hierarchical product but with less folds (Figure 2046

DOI: 10.1021/acssuschemeng.6b02893 ACS Sustainable Chem. Eng. 2017, 5, 2044−2049

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Figure 3. (A−C) TEM images of Ag nanoparticles-loaded LLMSM at different magnifications. The insets in panel C show the lattice pattern and the statistical size distribution of Ag nanoparticle. (It is noted that, in addition to obviously large-sized Ag nanoparticles, many small-sized Ag nanoparticles are actually presented inside of the nanoporous matrix.) (D) Time-dependent UV−vis spectral changes of 4-nitrophenol catalyzed by the Ag-loaded LLMSM. The inset shows the reaction in the presence of Ag-loaded LLMSM. (E) Catalytic kinetics for 10 successive cycles with the same batch of Ag-loaded LLMSM.

attributed to the special morphology of LLMSM with unique bilayer and hollow sandwich-like folds. Subsequently, the remaining concentration of Rhodamine B slowed with increasing adsorption time (Figure 2A), which is in accordance with the sample photos recorded at corresponding different time intervals (Figure 2B, inset). The removal rate of Rhodamine B by LLMSM as a function of time also clearly showed that this adsorption process is rapid, especially in the initial stage, which is in agreement with the results from UV− vis spectra. After 60 min, Rhodamine B can be almost completely removed leaving a nearly clear solution, and the color of the adsorbent changed from white to pink (Figure 2B, inset). The maximum adsorption capacity of Rhodamine B by LLMSM was further determined to be as high as 530.7 mg per gram of silica material (see details in the Supporting Information), reflecting a good adsorption capacity of LLMSM (Table S1). To explore its potential application in catalysis on behalf of hierarchical mesoporous structures, LLMSM was then employed as a Ag catalyst supporter for performing the catalytic reduction of 4-nitrophenol. As shown in Figure 3, Ag nanoparticles were distributed randomly in LLMSM. It is also noted that, in addition to obviously large-sized Ag nanoparticles, many small-sized Ag nanoparticles are actually presented inside of the nanoporous matrix (Figure 3C). An evaluation of 200 Ag nanoparticles selected from the TEM

positive ends of the polar groups of PVP molecules and deposited around their linear and random coil surface. Considering the relatively higher external surface pressure,20 a more compact and flat bilayered mesoporous silica structure will be formed in the inside region (Scheme 2), which can be also confirmed by the above observations (Figure 1 and Figures S2 and S5). Comparatively, in the edge region, the diffusion of ethanol into the bilayered micelles could reduce the interaction of the alkyl tails,21 which thus increases the hydrophobic volume of the bilayered micelles. This situation further induces the formation of hollow sandwich-like folds by reducing the surface energy of the edge region of the bilayered micelles (Scheme 2). Because of its hierarchical three-dimensional structures, unique bilayer, typical hollow sandwich-like folds, large specific surface area, and high pore volume, the as-synthesized LLMSM could be favored in many application fields, especially for those involving adsorption and diffusion. To demonstrate this, the adsorption capacity of LLMSM was first examined using an organic dye, Rhodamine B. As shown in Figure 2A, even at a low mass ratio of adsorbent (LLMSM) to adsorbate (Rhodamine B) (see details in the Supporting Information),22−24 the characteristic absorption of Rhodamine B at 550 nm decreased dramatically in the first 5 min. This reason, except for the electrostatic interaction between the negatively charged silicate surface and positively charged Rhodamine B, may be probably 2047

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Figure 4. (A) UV−vis spectra of different concentrations of Dox in aqueous solution. The inset is the corresponding standard calibration curve of Dox at 233 nm (R2 = 0.9997). (B) Sustained release profile of Dox from LLMSM-Dox in PBS buffer (pH 4.5 and 7.4).

doxorubicin (Dox) was chosen as a model drug for examining the drug loading capacity and release behavior of LLMSM. A linear standard calibration curve in the concentration range from 0.625 to 20 μg mL−1 was first obtained for quantitatively determining drug loading and releasing efficiency (Figure 4A). On the basis of this, the loading amount of Dox into LLMSM was calculated to achieve 463.4 mg per gram silica material (see details in the Supporting Information), indicating the superior capacity of LLMSM as a reservoir for drug delivery (Table S3).13,28,30 The release profiles of Dox from Dox-loaded LLMSM (LLMSM-Dox) were measured in pH 4.5 and pH 7.4 phosphate-buffered saline (PBS) to simulate different physiological environments. As shown in Figure 4B, Dox can be successfully and continuously released from LLMSM-Dox in both media over 6 days, and the release profiles displayed typical multistage and sustained properties, which can be probably attributed to the hierarchical structures of LLMSM having unique bilayer and hollow sandwich-like folds. The cumulative drug release rates were 80.1% and 27.8% after 6 days in pH 4.5 and pH 7.4 PBS, respectively (Figure 4B). This pH-dependent sustained release behavior makes LLMSM further attractive for meeting different needs in a controlled manner. In summary, we first developed a facile one-pot strategy to synthesize a well-defined lotus leaf-like hierarchical mesoporous silica structure with unique bilayer and hollow sandwich-like folds using only hexadecylamine as a structure-directing agent, high molecular weight polyvinylpyrrolidone as a structurestabilizing agent, and tetraethyl orthosilicate as a silica precursor in the mixed solvents of water and ethanol. The formation mechanism was proposed based on a series of experimental investigations. Such a hierarchical structure exhibited rapid and high-capacity adsorption toward Rhodamine B, effective and stable catalyst support for the catalytic reduction of 4nitrophenol, and also typical multistage and sustained drug release properties. Because of its hierarchical three-dimensional structures, unique bilayer, typical hollow sandwich-like folds, large specific surface area, and high pore volume, this material holds great promise in many application fields.

images showed that the size of most Ag nanoparticles is less than 3 nm (Figure 3C, inset), and the lattice fringe spacing of 0.235 nm agrees well with the interplanar distance of the (111) plane of Ag crystal (Figure 3C, inset).10 The content of Ag in LLMSM was determined to be 8.96% in weight by energydispersive X-ray analysis (Figure S13). The catalytic activity of Ag-loaded LLMSM was examined by the reduction reaction of 4-nitrophenol with an excess amount of sodium borohydride. In the absence of the catalyst (Ag-loaded LLMSM), the reaction did not proceed during a period of more than 24 h. The kinetics of 4-nitrophenol reduction was then monitored by UV−vis spectroscopy of the reaction mixture after the addition of the catalyst. As shown in Figure 3D, the typical absorption peak intensity at 400 nm gradually decreased with a concomitant increase in peaks intensity at 300 and 230 nm as the reduction reaction proceeded. The reaction processes can be also observed from the fading of yellow color to colorless after the addition of Ag-loaded LLMSM. A successive decrease in peak intensity at 400 nm with time was then taken into consideration to obtain the rate constant. The ratio of C to C0, where C0 and C are the initial concentration and the concentration of 4-nitrophenol at different times, respectively, was measured from the relative intensity ratio of the respective absorbance at 400 nm. A linear correlation between In(C/C0) and time can be revealed that the reaction process followed the pseudo-first-order kinetics.7,25−27 Even at a relatively low mass ratio of catalyst (Ag-loaded LLMSM) to substrate (4nitrophenol) (see details in the Supporting Information), the kinetic reaction rate constant k still can be achieved to 2.17 × 10−3 s−1 from the slope, indicating the superior performance of the LLMSM-based catalytic supporter (Table S2).7,25−27 In addition, the reusability of Ag-loaded LLMSM was investigated. As shown in Figure 3E, after easily separating from the reaction mixture by mild centrifugation, Ag-loaded LLMSM can be reused for 10 successive reactions, and a similar catalytic performance was observed. These results can be also revealed by the stable structure of Ag-loaded LLMSM after running for 10 times (Figure S14), which further indicated that such a hierarchical structure may find promising applications in catalysis. Porous silica microparticles have recently gained great interests in tissue engineering and wound healing not only because of its robust role as a scaffold but also because of its specific role as a drug carrier.28−30 To demonstrate the potential applications of LLMSM in biomedical fields,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02893. 2048

DOI: 10.1021/acssuschemeng.6b02893 ACS Sustainable Chem. Eng. 2017, 5, 2044−2049

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Experimental details, materials characterization, and additional data. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nanjing Hao: 0000-0003-3808-7941 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the NIH Director’s Transformative Research Award (R01HL137157) and NSF grants (ECCS 1128677, 1309686, 1509369). We gratefully acknowledge the support from the Electron Microscope Facility at Dartmouth College.

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DOI: 10.1021/acssuschemeng.6b02893 ACS Sustainable Chem. Eng. 2017, 5, 2044−2049