Hierarchical Lotus Leaf-Like Mesoporous Silica Material with UniqueBilayer and Hollow Sandwich-Like Folds: Synthesis, Mechanism, andApplicationsNanjing 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 wasdeveloped to synthesize hierarchical lotus leaf-like mesoporoussilica material (LLMSM) with unique bilayer and hollowsandwich-like folds using hexadecylamine as a structure-directing agent, high molecular weight polyvinylpyrrolidoneas a structure-stabilizing agent, and tetraethyl orthosilicate as asilica precursor in the mixed solvents of water and ethanol.The formation mechanism of LLMSM was proposed based ona series of experimental investigations. This new particulateplatform exhibited superior adsorption capacity toward organicdye, effective catalytic reduction of 4-nitrophenol as a Agcatalyst supporter, and a multistage sustained drug releaseprofile. Many other applications can be also envisaged for sucha hierarchical structure.

KEYWORDS: Mesoporous silica, Hierarchical, Shape, Lotus leaf, Hollow

Since its discovery in the early 1990s,1 mesoporous silicamaterial (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 ofsuch material to suit specific application needs throughcontrolling the physicochemical properties, including particlesize, surface chemistry, and pore types.2,3 However, more andmore evidence from both theoretical and experimental aspectshave recently revealed that particle shape could alsosignificantly affect its performance, especially for thoseprocesses involving mass transfer.4−7 These findings not onlyhelp to shed new light on the shape genesis of MSM but alsoinspire 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 correspondinghollow counterparts.12−14 However, most of these shapespossessed only single or simple architecture and porearrangement, which may restrict the availabilities of certainpore channels for mass transfer. Comparatively, hierarchicalthree-dimensional porous structures with complex shapes couldprovide more exposed pore openings and pore arrangements,which may improve the accessibility of pore channels insideMSM, and thus make MSM greatly favored for manyapplication fields, such as adsorption and catalysis.15,16

However, despite significant and increasing needs, facile andstable synthesis of hierarchical porous silica material, especially

having short pore path lengths and rich pore structures, is stillan 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 usinghexadecylamine (HDA) as a structure-directing agent, highmolecular weight polyvinylpyrrolidone (PVP) as a structure-stabilizing agent, and tetraethyl orthosilicate (TEOS) as a silicaprecursor in the mixed solvents of water and ethanol. Theformation mechanism of LLMSM was explored through a seriesof experimental investigations. The adsorption, catalytic, andsustained drug release performances of this hierarchicalstructure-based platform were also examined (Scheme 1).Hierarchical porous silica material having unique bilayer and

hollow sandwich-like folds can be successfully synthesized usingonly HDA, PVP, and TEOS in the mixed water and ethanolsolution (see experimental details in the Supporting Informa-tion). As shown in Figure 1, lotus leaf-like silica materialdisplayed a unique layered architecture and typical foldedprotuberance. This LLMSM has an average size of ∼4 μm andan average thickness of ∼150 nm. Both the edge region and theinside region of LLMSM showed clearly porous structures(Figure S1), and multiple pore size distributions can beinvestigated especially from the folds of the edge region (Figure

Received: November 29, 2016Revised: January 26, 2017Published: February 20, 2017

Letter

pubs.acs.org/journal/ascecg

© 2017 American Chemical Society 2044 DOI: 10.1021/acssuschemeng.6b02893ACS Sustainable Chem. Eng. 2017, 5, 2044−2049

1C,F and Figure S1A). From the broken particles (Figure S2),the existence of a bilayered structure was further confirmedwhen combined with the observations from hollow sandwich-like folds (Figure 1C). In addition, the thickness of thishierarchical mesoporous silica structure can be easily tunedfrom ∼30 to ∼400 nm by just changing the concentration ofsilica precursor (TEOS) (Figure S3). N2 adsorption−desorption analysis was then performed on the as-synthesizedLLMSM after removing HDA and PVP via the extractionmethod (Figure S4). The results showed that LLMSMexhibited a typical type IV isotherm with H3 hysteresis (FigureS5), indicating the presence of slit-shaped and large-sizedporous structures.17,18 The pore size of LLMSM was calculatedto be 2.95 nm by the Barrett−Joyner−Halenda (BJH) method(Figure S5B), and multiple pore size distributions are also inagreement 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 be806.5 m2/g and 0.76 cm3/g, respectively.To explore the formation mechanism of this hierarchical

porous silica structure with unique bilayer and hollowsandwich-like folds, we carried out a series of experiments toinvestigate the specific roles of TEOS, PVP, and HDA in thereaction system. As mentioned above, TEOS as a silica

precursor played the main role in the regulation of materialthickness but had minor effect on the hierarchical shapes(Figure S3). Thus, the effects of PVP and HDA on theformation of LLMSM were primarily investigated. In theabsence of PVP, hierarchical silica material still can be obtained.However, there is no well-defined lotus leaf-like structure withtypical folds, and severe aggregations can be observed (FigureS6). This phenomenon provided direct evidence that PVP andHDA played the structure-stabilizing role and structure-directing 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-likefolds still can be produced (Figure S7). However, increasing theconcentration of PVP can only yield a hierarchical silicastructure with solid folds (Figure S8), which may be causedfrom the excessive recruitment of silica precursors by PVPmolecules and then evolving into more dense and compactstructures. The molecular weight of PVP also significantlyaffects the fine structures of the as-synthesized materials.Compared to high molecular weight of PVP for producingtypical hollow sandwich-like folds, low molecular weight of PVPcan only produce tiny wrinkled protuberances (Figure S9).These results not only confirmed the role of PVP as thestructure-stabilizing agent but also revealed the functions of

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

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

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PVP molecules on the formation of layered and foldedmorphologies. Next, the effects of HDA on the formation ofLLMSM were explored in a series of studies. Without theaddition of HDA, there is no obvious product formed, andthere are only disordered nanoparticles formed by changingHDA to ammonia (Figure S10), indicating that HDA not onlyplayed a structure-directing role but also acted as a catalyst forcatalyzing hydrolysis of TEOS. In addition, we found thatdecreasing the concentration of HDA in the reaction systemcan still yield a hierarchical product but with less folds (Figure

S11), while increasing the concentration of HDA can yield ahierarchical product with more typical folds in the edge region(Figure S12), which further confirmed its roles as a structure-directing agent and catalyst.On the basis of the above observations, a region-dependent

growth mechanism of LLMSM with a unique bilayer andhollow sandwich-like folds was proposed (Scheme 2). In theinside region, the high molecular weight PVP molecules withstrong polar pyrrolidone rings can first bind with HDA to forma bilayered skeleton,19 then TEOS will be attracted to the

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

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

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positive ends of the polar groups of PVP molecules anddeposited around their linear and random coil surface.Considering the relatively higher external surface pressure,20 amore compact and flat bilayered mesoporous silica structurewill be formed in the inside region (Scheme 2), which can bealso confirmed by the above observations (Figure 1 and FiguresS2 and S5). Comparatively, in the edge region, the diffusion ofethanol into the bilayered micelles could reduce the interactionof the alkyl tails,21 which thus increases the hydrophobicvolume of the bilayered micelles. This situation further inducesthe formation of hollow sandwich-like folds by reducing thesurface 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 specificsurface area, and high pore volume, the as-synthesized LLMSMcould be favored in many application fields, especially for thoseinvolving adsorption and diffusion. To demonstrate this, theadsorption capacity of LLMSM was first examined using anorganic dye, Rhodamine B. As shown in Figure 2A, even at alow mass ratio of adsorbent (LLMSM) to adsorbate (Rhod-amine B) (see details in the Supporting Information),22−24 thecharacteristic absorption of Rhodamine B at 550 nm decreaseddramatically in the first 5 min. This reason, except for theelectrostatic interaction between the negatively charged silicatesurface and positively charged Rhodamine B, may be probably

attributed to the special morphology of LLMSM with uniquebilayer and hollow sandwich-like folds. Subsequently, theremaining concentration of Rhodamine B slowed withincreasing adsorption time (Figure 2A), which is in accordancewith the sample photos recorded at corresponding differenttime intervals (Figure 2B, inset). The removal rate ofRhodamine B by LLMSM as a function of time also clearlyshowed that this adsorption process is rapid, especially in theinitial stage, which is in agreement with the results from UV−vis spectra. After 60 min, Rhodamine B can be almostcompletely removed leaving a nearly clear solution, and thecolor of the adsorbent changed from white to pink (Figure 2B,inset). The maximum adsorption capacity of Rhodamine B byLLMSM was further determined to be as high as 530.7 mg pergram of silica material (see details in the SupportingInformation), reflecting a good adsorption capacity ofLLMSM (Table S1).To explore its potential application in catalysis on behalf of

hierarchical mesoporous structures, LLMSM was thenemployed as a Ag catalyst supporter for performing thecatalytic reduction of 4-nitrophenol. As shown in Figure 3, Agnanoparticles were distributed randomly in LLMSM. It is alsonoted that, in addition to obviously large-sized Ag nano-particles, many small-sized Ag nanoparticles are actuallypresented inside of the nanoporous matrix (Figure 3C). Anevaluation of 200 Ag nanoparticles selected from the TEM

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

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images showed that the size of most Ag nanoparticles is lessthan 3 nm (Figure 3C, inset), and the lattice fringe spacing of0.235 nm agrees well with the interplanar distance of the (111)plane of Ag crystal (Figure 3C, inset).10 The content of Ag inLLMSM was determined to be 8.96% in weight by energy-dispersive X-ray analysis (Figure S13). The catalytic activity ofAg-loaded LLMSM was examined by the reduction reaction of4-nitrophenol with an excess amount of sodium borohydride.In the absence of the catalyst (Ag-loaded LLMSM), thereaction did not proceed during a period of more than 24 h.The kinetics of 4-nitrophenol reduction was then monitored byUV−vis spectroscopy of the reaction mixture after the additionof the catalyst. As shown in Figure 3D, the typical absorptionpeak intensity at 400 nm gradually decreased with aconcomitant increase in peaks intensity at 300 and 230 nmas the reduction reaction proceeded. The reaction processescan be also observed from the fading of yellow color tocolorless after the addition of Ag-loaded LLMSM. A successivedecrease in peak intensity at 400 nm with time was then takeninto consideration to obtain the rate constant. The ratio of C toC0, where C0 and C are the initial concentration and theconcentration of 4-nitrophenol at different times, respectively,was measured from the relative intensity ratio of the respectiveabsorbance at 400 nm. A linear correlation between In(C/C0)and time can be revealed that the reaction process followed thepseudo-first-order kinetics.7,25−27 Even at a relatively low massratio of catalyst (Ag-loaded LLMSM) to substrate (4-nitrophenol) (see details in the Supporting Information), thekinetic reaction rate constant k still can be achieved to 2.17 ×10−3 s−1 from the slope, indicating the superior performance ofthe LLMSM-based catalytic supporter (Table S2).7,25−27 Inaddition, the reusability of Ag-loaded LLMSM was investigated.As shown in Figure 3E, after easily separating from the reactionmixture by mild centrifugation, Ag-loaded LLMSM can bereused for 10 successive reactions, and a similar catalyticperformance was observed. These results can be also revealedby the stable structure of Ag-loaded LLMSM after running for10 times (Figure S14), which further indicated that such ahierarchical structure may find promising applications incatalysis.Porous silica microparticles have recently gained great

interests in tissue engineering and wound healing not onlybecause of its robust role as a scaffold but also because of itsspecific role as a drug carrier.28−30 To demonstrate thepotential applications of LLMSM in biomedical fields,

doxorubicin (Dox) was chosen as a model drug for examiningthe drug loading capacity and release behavior of LLMSM. Alinear standard calibration curve in the concentration rangefrom 0.625 to 20 μg mL−1 was first obtained for quantitativelydetermining drug loading and releasing efficiency (Figure 4A).On the basis of this, the loading amount of Dox into LLMSMwas calculated to achieve 463.4 mg per gram silica material (seedetails in the Supporting Information), indicating the superiorcapacity of LLMSM as a reservoir for drug delivery (TableS3).13,28,30 The release profiles of Dox from Dox-loadedLLMSM (LLMSM-Dox) were measured in pH 4.5 and pH 7.4phosphate-buffered saline (PBS) to simulate different physio-logical environments. As shown in Figure 4B, Dox can besuccessfully and continuously released from LLMSM-Dox inboth media over 6 days, and the release profiles displayedtypical multistage and sustained properties, which can beprobably attributed to the hierarchical structures of LLMSMhaving unique bilayer and hollow sandwich-like folds. Thecumulative drug release rates were 80.1% and 27.8% after 6days in pH 4.5 and pH 7.4 PBS, respectively (Figure 4B). ThispH-dependent sustained release behavior makes LLMSMfurther attractive for meeting different needs in a controlledmanner.In summary, we first developed a facile one-pot strategy to

synthesize a well-defined lotus leaf-like hierarchical mesoporoussilica structure with unique bilayer and hollow sandwich-likefolds using only hexadecylamine as a structure-directing agent,high molecular weight polyvinylpyrrolidone as a structure-stabilizing agent, and tetraethyl orthosilicate as a silica precursorin the mixed solvents of water and ethanol. The formationmechanism was proposed based on a series of experimentalinvestigations. Such a hierarchical structure exhibited rapid andhigh-capacity adsorption toward Rhodamine B, effective andstable catalyst support for the catalytic reduction of 4-nitrophenol, and also typical multistage and sustained drugrelease properties. Because of its hierarchical three-dimensionalstructures, unique bilayer, typical hollow sandwich-like folds,large specific surface area, and high pore volume, this materialholds great promise in many application fields.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.6b02893.

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

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Experimental details, materials characterization, andadditional data. (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: john.zhang@dartmouth.edu.ORCIDNanjing Hao: 0000-0003-3808-7941NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was sponsored by the NIH Director’s Trans-formative Research Award (R01HL137157) and NSF grants(ECCS 1128677, 1309686, 1509369). We gratefully acknowl-edge the support from the Electron Microscope Facility atDartmouth College.

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