DuX11_Hierarchically Me So Porous Silica Nano Particles

8/2/2019 DuX11_Hierarchically Me So Porous Silica Nano Particles

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Published: February 18, 2011

r 2011 American Chemical Society 2972 dx.doi.org/10.1021/la200014w| Langmuir2011, 27, 29722979

ARTICLE

pubs.acs.org/Langmuir

Hierarchically Mesoporous Silica Nanoparticles: Extraction,Amino-Functionalization, and Their Multipurpose Potentials

Xin Du, and Junhui He*,

Functional Nanomaterials Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TechnicalInstitute of Physics and Chemistry (TIPC), Chinese Academy of Sciences, Zhongguancun Beiyitiao 2, Haidianqu, Beijing 100190,ChinaGraduate University of Chinese Academy of Sciences, Beijing 100864, China

bS Supporting Information

1. INTRODUCTION

Design and fabrication of silica nanoparticles (NPs) withhierarchically porous structures are attracting much attentionbecause of novel structures and potential applications. Hierarchi-cally porous structures are those having pores on different lengthscales from micropore (50 nm). They can be ordered or nonordered.Hierarchically porous systems are supposed to possess moreadvantages than monomodal porous systems (e.g., MCM-41 andSBA-15) for several reasons.1-9 First, introduction of hierarch-ical pores may lead to increased surface area and pore volume,

which may facilitatefunctionalization of pore walland interactionwith various species. Second, hierarchical combinations of multi-ple-scale pores (micropore and mesopore, small and largemesopores, or mesopore and macropore) would allow foraccessible mass transport paths within inorganic networks. Thus,small molecules, biomacromolecules, or even NPs could moveinto or out of the porous matrices through hierarchical pores.Third, the coexistence of multiple-scale pores may enhance andharmonize the diffusion of guest molecules of different sizesthrough the porous matrices and allow simultaneous loading ofdifferent species with varied sizes (such as fluorescent molecules,quantum dots, magnetic NPs, and so on). As a result, silica

nanospheres with hierarchical pores are promising in diverseapplications such as catalysis, adsorption, separation, and bio-medicine, especially as multifunctional carriers (having fluores-cent, magnetic, cellular labeling, and/or therapeutic functions) indrug delivery.10-15 Recently, bulk hierarchically porous silicamaterials have been successfully synthesized using polystyrenenanospheres as macropore template and triblock copolymerPluronic F127 as mesopore template.16 However, this methodis difficult to fabricate nanoparticles with micro or submicrom-eter sizes instead of bulk materials. Thus, at present, facile andlarge-scale fabrication of NPs with hierarchically porous struc-

tures and submicrometer diameters is still a big challenge due totheir more complicated nanostructures of hierarchical pores thanthose of monomodal pores. Recently, Wu et al. reported thathierarchically nanostructured mesoporous spheres (ca. 1m) ofcalcium silicate hydrated were synthesized by a surfactant-freesonochemical method, and a linear relationship was found in thedrug-delivery system between the cumulative amount of releaseddrug and the natural logarithm of release time.8 Very recently,Polshettiwar et al. reported a microwave-assisted hydrothermal

Received: November 10, 2010Revised: January 31, 2011

ABSTRACT: Hierarchically mesoporous silica nanoparticles (HMSNs) withuniform morphology andstructure and with a diameter of ca.100-220 nm werefacilely fabricated using water, ethanol and ethyl ether as cosolvents. Templateextraction and amino-functionalization were performed toward the HMSNs.

These hierarchical mesopores are supposed to possess more advantages thanconventional monomodal mesopores. Amino-functionalized HMSNs werehomogeneously grafted with fluorescent molecules and loaded with Au nano-particles (NPs), respectively. The extracted HMSNs were also successfully usedto construct antireflection and superhydrophilc coatings. Drug release experi-ments showed that HMSNs exhibit much quicker rates of drug release comparedwith conventional mesoporous NPs due to their hierarchically mesoporousstructures.


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2973 dx.doi.org/10.1021/la200014w |Langmuir2011, 27, 29722979

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method to fabricate high specific surface area silica nanospheres with unprecedented fibrous morphologies, and presumed itswidespread applications in drug delivery, hydrogen storage, asa chromatography support, and in nanocomposite materials dueto the accessibility of active sites.9

Modification of the interior and/or exterior of mesoporoussilica nanometerials with organic groups is particularly interest-

ing because of the possibility to combine the enormous func-tional variations of organic groups with the advantages ofthermally stable and robust silica scaffolds.17 Such functionalizedmesoporous silica nanometerials are applicable to catalysis,sorption and affinity chromatography, separation and deconta-mination, sensing, and the construction of systems for controlledrelease of active compounds, as well as molecular switches.18-20

Typical strategies for immobilizing organic functional groups inmesoporous silica nanomaterials via covalent bonds are co-condensation (one-pot synthesis), postsynthesis modification(grafting), and imprint coating method.21

Very recently, we reported easy fabrication of hierarchicallymesoporous silica nanoparticles (HMSNs) using water, ethanol,and ethyl ether as cosolvents and CTAB as surfactant.22 The

formation of large mesopores (5-50 nm) results from thegasification of ethyl ether in the exothermic hydrolysis andcondensation of TEOS. In the current work, we adopted theextractionmethod to treat the as-prepared HMSNs (as-HMSNs)instead of previous calcination. Excitingly, the extracted HMSNs(ext-HMSNs) achieved more intact and better defined hierarchi-cally mesoporous structure than the calcined HMSNs (cal-HMSNs). Both postsynthesis modification and co-condensationwere employed to functionalize the HMSNs with primary aminegroups, respectively. Furthermore, NH2-ext-HMSNs were suc-cessfully labeled with fluorescein isothiocyanate (FITC) andloaded with Au NPs, respectively. The ext-HMSNs were alsoused as building block to construct functional coatings by thelayer-by-layer (LbL) dip coating method on glass substrate.

Finally, HMSNs were used as drug carrier to investigate the drugloading and release behavior in vitro.

2. EXPERIMENTAL SECTION

2.1. Materials. Cetyltrimethylammonium bromide (CTAB,g99%), aqueous ammonia (NH4OH, 25-28%), hydrochloric acid(HCl, 36-38%), chloroauric acid (HAuCl4 3 4H2O), sodium borohy-dride (NaBH4), toluene, ethanol, ethyl ether (g99.5%) and fluoresceinisothiocyanate (FITC) were obtained from Beijing Chemical ReagentCompany. Tetraethoxysilane (TEOS, g98%), sodium poly(4-styren-esulfonate) (PSS,Mw= ca. 70 000), and 3-aminopropyltrimethoxysilane(APMS, g97%) were purchased from Alfa Aesar. Poly(diallyl-dimethylammonium chloride) (PDDA, Mw = 200000-350 000, 20

wt %) was purchased from Aldrich. Ibuprofen (IBU,g

98%) waspurchased from Wuhan Galaxy Chemical Company. All chemicals wereanalytic grade and used without further purification. Ultrapure water

with a resistivity higher than 18.2 M 3 cm was used in all experimentsand was obtained from a three-stage Millipore Mill-Q Plus 185purification system (Academic).

2.2. One Pot Synthesis of Silica Nanoparticles with Hier-archical Mesopores. HMSNs were prepared in basic solution atroom temperature using water, ethanol, and ethyl ether as cosolventsand CTAB as surfactant according to the synthesis procedure of S0.5 inour previous work.22 In a typicalprocedure, 0.5 g of CTAB was dissolvedin an emulsion system composed of 70 mL of H2O, 0.8 mL of aqueousammonia, 20 mL of ethyl ether, and 10 mL of ethanol. After the mixture

was vigorously stirred for 0.5 h at room temperature, 2.5 mL of TEOS

was quickly dripped into the mixture. The resulting mixture wasvigorously stirred at room temperature for 4 h. A white precipitatewas obtained, filtered, washed with pure water, and dried in air at 60 Cfor 24 h. Two methods (calcination and template extraction) wereperformed toward the as-prepared precipitate. Calcination was carriedoutinairat550C for5 h to eventuallyremove CTAB andotherorganiccomponents in the product. The calcined sample was designated as cal-

HMSNs. In order to better maintain the morphology and structure ofHMSNs, template extraction was also performed by adding the as-prepared precipitate (before calcination) in ethanolic HCl (15 mL ofconc. HCl in 120 mL of ethanol) followed by stirring at 70 C for 24 h.Finally, the extracted HMSNs (ext-HMSNs) were filtered, washed withpure water, and dispersed in water or dried in air at 80 C.

2.3. Amino-Functionalization of HMSNs. Two methods in-cluding co-condensation and postsynthesis modification were employedto achieve amino-functionalization of HMSNs. Co-condensation wascarried out by addition of varied volumes (0.02, 0.05, and 0.2 mL) of

APMS as co-condensation precursor after addition of TEOS underotherwise identical conditions of HMSNs. Postsynthesis modification

was performed in dry toluene by reaction with APMS, according to asimilar procedure that had been applied for grafting mesoporoussilicas.23-25 Briefly, the ext-HMSNs, which had been grinded suffi-ciently, was dehydrated at 120 C to remove adsorbed water molecules.Then, 300 mg of ext-HMSNs were suspended in 80 mL of toluene, andultrasonicated for 30 min. Finally 0.20 mL of APMS was added, and themixture was stirred for 6 h at 80 C. The product was recovered byfiltration, washed with dry toluene, and dried in air at 120 C. Theamino-modified product was designated as NH2-ext-HMSNs.

2.4. Fluorescent Labeling of NH2-ext-HMSNs. The aminogroups of NH2-ext-HMSNs were available for attachment of FITCmolecules.26 5 g of FITC were suspended in 3 mL of water. Afteraddition of 20 mg of NH2-ext-HMSNs, the suspension was stirred for 8h. And FITC-labeled NH2-ext-HMSNs (FITC-NH2-ext-HMSNs) werecollected, and the residual FITC molecules were removed by repeated

water washing/centrifugation/redispersing.2.5. Loading of Gold NPs. The NH2-ext-HMSNs with hierarchical

mesopores were used as carriers for Au NPs by in situ formation.27

Typically, 0.020 g of NH2-ext-HMSNs were dispersed in 20 mL of H2O.After addition of 1 mL aqueous HAuCl4 (3 mM), the color of mixture wasyellow.After the mixture wasmagneticallystirredin a closedconicalflask for2 h at room temperature, a given volume of aqueous NaBH4 (10 mM) wasadded into the above mixture until the color turned pale red. Finally, theprecipitate was centrifuged, washed with water, and redispersed in water.

2.6. Fabrication of Functional Coatings using ext-HMSNs.ext-HMSNs were used as building blocks to fabricate functionalcoatings

by the LbL dip coating method. The procedure for preparation offunctional coatings is divided into three steps and described asfollows.28-30 First, commercially available glass or silicon substrates

were cleaned with Pirhana solution (98 wt % H2SO4/30 wt % H2O2, 7/3, v/v), and then washed with pure water (Caution: the Pirhanasolution

is highly dangerous and must be used with great care). The cleanedsubstrates were alternately dipped in a PDDA and a PSS solution for 5min, and redundant polyelectrolytes were removed by shaking in pure

water for 2 min and rinsing for 1 min, followed by drying with N2 flow atroom temperature. The concentrations of PDDA and PSS aqueoussolutions were 2 mg mL-1. Mutilayers of (PDDA/PSS)5/PDDA wereprepared, and were used as a primer in all experiments. Second, the(PDDA/PSS)5/PDDA covered substrates were alternately dipped in anext-HMSNs aqueous suspension (0.5 wt %, pH 3.46) and a PDDAsolution(2 mg mL-1) by the sameprocedure foran appropriate numberof cycles. Finally, the as-prepared coatings were dried with N2 flow, andheated at 80 C overnight.

2.7. Drug Storage and Release by Varied HMSNs. ext-HMSN, cal-HMSN, and NH2-ext-HMSN were used as drug carrier to


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studytheir drug storage and release behaviors,respectively. Drugstorageand in vitro release profiles were obtained according to previouslyreported methods.31-38 Ibuprofen (IBU) was dissolved in hexanesolution, and the IBU concentration was 40 mg/mL. 0.10 g of theHMSN products was added into 20 mL of the IBU hexane solution atroom temperature. The vials were sealed to prevent the evaporation ofhexane, and then the mixture was stirred for 24 h. The IBU-loadedproduct (HMSNs@IBU) was separated from this solution by filtration,

washed twice with diluted HCl solution (pH 1.0), and dried undervacuum at 60 C.

A typical drug release experiment was performed as follows: 0.05 g ofHMSNs@IBU powder was grinded, and compacted into a disk(diameter: 1 cm) under a pressure of 3 MPa, and then immersed into50 mL of phosphate-buffered saline (PBS, 137 mM NaCl, 2.7 mM KCl,5.0mMNa2HPO4 3 12H2O,0.7mMH3PO4 in ultrapurewater, pH7.2-7.4)35 under magnetic stirring at a rate of ca. 100 rpm at roomtemperature. One mL of the mixture was extracted with a syringe atgiven time intervals for analysis. After removal of solid HMSNs@IBU bycentrifugation (12 000 rpm, 2 min), the remaining clear solution (1 mL)

was diluted to 2 mL, and then analyzed by UV-vis spectroscopy at awavelength of 264 nm. By measuring the UV absorption spectra of fourstandard IBU solutions in PBS (Table S1), a calibration curve of IBU

concentration (0-1.5 mg mL-

1) vs UV-vis absorbance (see Support-ing Information,Part1, Table S1and FigureS1) was set upasA = 1.6584C 0.02439, where A is the absorbance at 264 nm and C is the IBUconcentration (mg mL-1). IBU release amounts were determined bymeasuring the absorbance of the above clear solutions, and calculatedusing the calibration curve. The total IBU release amount for a week wasdetermined as the total storage amount of HMSNs.

2.8. Characterization. Scanning electron microscopy (SEM)observations were carried out on a Hitachi S-4300 field emissionscanning electron microscopeoperated at 10 kV.Specimens were coated

with a layer of gold by ion sputtering before SEM observations. Fortransmission electron microscopy (TEM) observations and selectedarea electron diffraction (SAED) measurements, powder samples wereadded on carbon-coated copper grids, and observed on a JEOL JEM-

2100F transmission electron microscope at an acceleration voltage of150 kV. The size distributions of partial products were measured byMalvern Zetasizer 3000HS. Small angle X-ray diffraction (SAXRD)patterns of products were recorded on a Bruker D8 Focus X-raydiffractometer using Cu KR radiation ( = 0.154184 nm), and used toidentify their phase constitutions and crystallite sizes. Fourier transforminfrared (FTIR) spectra were recorded on a Varian Excalibur 3100spectrometer. UV-vis absorption and transmission spectra were re-corded on a TU-1901 spectrophotometer (Beijing Purkinje GeneralInstrument Co.). Reflection spectra were recorded on a Varian Cary5000 UV- vis-NIR spectrophotometer. For confocal microscope ob-servations, the FITC-NH2-ext-HMSNs suspension was dripped ontothe surface of slide glass, dried, and observed on an Olympus BX51fluorescent microscope. Nitrogen adsorption-desorption measure-

ments were carried out on a QuadraSorb SI automated surface areaand pore size analyzer at -196 C using the volumetric method. Theproducts were dried at 200 C before analysis. Brunauer-Emmett-Teller (BET) specific surface areas were calculated by using adsorptiondata in the P/P0 range of 0.04-0.20 (six points collected). Pore sizedistributions were estimated from desorption branches of the isothermsusing the Barrett, Joyner, and Halenda (BJH) method. Pore volumes

were determined from the amounts of N2 adsorbed at the single point ofP/P0 = 0.98. Water contact angles (WCAs) of surfaces were measured atambient temperature on a JC2000C contact angle/interface system(Shanghai Zhongchen Digital Technique Apparatus Co.), the angleprecision of which is (0.5. Three L of water droplets were droppedcarefully onto the sample surfaces. Once a water droplet contacted thesamplesurface, the machine began to take photos at a speed of 30 photos/s;

that is, the interval between the contact moment and the first image was33 ms. The measurement was carried out on three different areas of thesample surface.

3. RESULTS AND DISCUSSION

3.1. Morphology and Structure of HMSNs. As shown inFigure S2a and b, as-HMSNs have a size of ca. 100-220 nm.They have large circular mesopores with a diameter of 5-50 nmon their surface (Figure S2b). The dark and pale parts in theinterior of nanosphere (Figure S2c and d) reveal the existence of

large mesopores on the surface of nanosphere.39,40After calcina-tion, cal-HMSNs remained nearly unchanged in size, but thelarge mesopores on the surface had a small shrinkage and theirsizes became 5-30 nm (Figure 1a and b). Small mesopores of2-3 nm were also clearly observed in the interior of cal-HMSNs(inset in Figure 1b). After extraction treatment, unexpectedlyand excitingly, ext-HMSNs show uniform structure and mor-phology, have no aggregation with each other, and have largecircular mesopores with a diameter of 5-80 nm on their surface(Figure 1c-f), whichare slightlylarger andclearerthanthose of as-HMSNs. This may result from that extraction treatment, on onehand, removes thesurfactant,and on theother hand, inducesfurthercondensation and solidification during the acidic extractionprocess,

Figure1. SEM (a, c, and d)andTEM(b, e,andf) imagesof cal-HMSNs

(a and b) and ext-HMSNs (c-f). Inset in panel (b) is a magnifiedTEM image.


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thus improving the nanostructure of hierarchical mesopores. Theparticle size of ext-HMSNs was also measured by dynamic lightscattering (DLS), and the results indicate that ext-HMSNs have aZ-average diameter of 229 nm with a polydispersity of 0.22(Figure S3a). Thus, the extraction treatment can be used toobtain intact and well-defined nanostructures of hierarchicalmesopores.

3.2. Amino-Functionalization of HMSNs by Co-condensa-tion. As shown in Figure S4, as-prepared amino-functionalizedHMSNs by co-condensation with APMS have better defined

spherical morphology and larger mesopores (5-150 nm in size)than those of as-HMSNs. And the diameter of as-preparedamino-functionalized HMSNs has a little increase with increaseof added APMS volume(from 0.02 to 0.05 mL). When the addedAPMS volume increases to 0.20 mL, the diameter of as-preparedamino-functionalized HMSNs has a broad distribution (50-700 nm). Unfortunately, however, some unexpected sheetsappeared. And it is expected that the sheets can be removed bya simple method (e.g., filtration). FT-IR spectrum (Figure S5a)of extracted NH2-HMSNs fabricated with addition of 0.2 mL ofAPMS as co-condensation precursor indicates the existence ofamino groups. TEM images (Figure S5b and c) of the extractedNH2-HMSNs after loading with Au nanoparticles according tothe process of the experimental section show that Au NPs withvaried sizes from 0.5 to 15 nm are homogeneously scattered inthe pores of HMSNs, indicating the homogeneous modificationof amino groups on the pore surface of HMSNs. It can beconcluded that APMS as co-condensation precursor plays aregulation role toward the structure of products in the self-assembly process of surfactant and silica species.35

3.3. Amino-Functionalization of HMSNs by PostsynthesisModification. As shown in Figure 2, the size of large mesopores(5-40 nm) on the surface of NH2-ext-HMSNs becomes slightlysmallerthan that of ext-HMSNs, but still keeps theclear structureof hierarchical mesopores. The inset in Figure 2a shows that theproduct is a white powder and on gram-scale. These observationsmay result from combined effects of grafting of APMS and

structural rearrangement during oil-bath reaction. DLS measure-ments show that the NH2-ext-HMSNs fabricated by postsynth-esis modification have a Z-average diameter of 269 nm with apolydispersity of 0.48 (Figure S3b), and the distribution peak at

ca. 600 nm indicates that NH2-ext-HMSNs have a littleaggregation.FT-IR spectra of as-HMSNs, cal-HMSNs, ext-HMSNs and

NH2-ext-HMSNs are shown in Figure 3. They show typicalvibration bands of siliceous materials, such as Si-O-Si asym-metric stretching (SiOSi) at ca. 1085 cm

-1 , Si-O symmetricstretching (SiO) at ca. 800 cm

-1 and Si-O-Si bendingvibration (SiOSi) at ca. 470 cm

-1. For as-HMSNs, absorptionpeaks observed at 2920 and 2855 cm-1 and 1494 cm-1 areattributed to C-H stretching vibrations (CH) and C-H bending vibrations (CH) from the CTAB template. Aftercalcination, the corresponding absorption peaks disappeared,indicating complete removal of CTAB. By extraction treatment,the corresponding absorption peaks become very weak, exhibit-

ing effective removal of CTAB. After amino-functionalization,the corresponding absorption peaks have clear increases com-pared to those of ext-HMSNs, and a new absorption peakappears at 1560 cm-1 , which is attributed to typical bendingvibration (NH) of amino group. Moreover, the absorption peakat 965 cm-1 disappeared, which belongs to the stretchingvibration of Si-OH (SiOH),. This is because the Si-OH groupsreacted with APMS, producing the Si-O-Si linkage.38

Figure S6 shows SAXRD patterns of cal-HMSNs, ext-HMSNs,and NH2-ext-HMSNs. cal-HMSNs show a broad peak at 2 =2.61 with a dspacing of 3.39 nm, indicative of a low degree oflong-range order. ext-HMSNs show a broad low-angle reflectionpeak at 2 = 1.00 with a d spacing of 8.84 nm. After amino-functionalization, however, NH2-ext-HMSNs show no reflection

peak, suggesting disordered pores. These differences may resultfrom the change in the pore size and ordering degree of smallmesopores, and from the difference of groups on the mesoporewalls of cal-HMSNs, ext-HMSNs and NH2-ext-HMSNs.

41

Nitrogen adsorption-desorption isotherms of cal-HMSNs,ext-HMSNs and NH2-ext-HMSNs are shown in Figure S7. Thenitrogen adsorption-desorption isotherm of cal-HMSNs showstypical type-IV features, indicative of the presence of mesopores.42

The large H3-type hysteresis loop of ext-HMSNs and NH2-ext-HMSNsin theP/P0 range of 0.5-1.0should be normally attributedto slit-like pores, which most probably resulted from the particlepacking.42 In addition, pore size distribution curves (Figure S7) ofcal-HMSNs, ext-HMSNs, and NH2-ext-HMSNs were determined

Figure 2. SEM (a and b) and TEM (c and d) images of NH 2-ext-HMSNs. Inset in (a) is a digital image of the product.

Figure 3. FT-IR spectra of as-HMSNs (a), cal-HMSNs (b), ext-HMSNs (c), and NH2-ext-HMSNs (d).


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from the desorption branch of the isotherms. Detailed physi-cochemical parameters are summarized in Table S2 for com-parison. Only one peak appears in the range of 2-4 nm in thepore size distribution curves of cal-HMSNs and ext-HMSNs,and no large mesopores appear, suggesting that small meso-pores must be predominant in these two hierarchically poroussystems. ext-HMSNs (3.8 nm) have a larger mesopore size thancal-HMSNs (2.5 nm), indicating that calcination caused a small

shrinkage of mesopores, and extraction kept the mesoporesintact efficiently and even expanded the mesopore size. How-ever, there exist two peaks in the pore size distribution curve ofNH2-ext-HMSNs. The sharp peak at 3.4 nm and the broad peakat 7.9nm should be attributed to the existenceof small andlargemesopores, respectively. The simultaneous appearance of twopeaks of small and large mesopores may results from weakeningof ordering degree and decrease of small mesopores afteramino-functionalization, which is indicated by significant de-crease in y-axis intensity. Moreover, the BET surface area ofNH2-ext-HMSNs has a large decrease as compared with thoseof cal-HMSNs and ext-HMSNs. The appearance of largemesopores and the significant decrease of BET surface area ofNH2-ext-HMSNs indicate that due to the existence of a large

number of Si-OH groups on the mesopore wall of ext-HMSNs, the following amino-functionalization by the reactionof Si-OH with APMS caused the shrinkage and even disap-pearance of mesopores.

3.3. Fluorescent Labeling of HMSNs. FITC is a popularamine labeling reagent, forming a robust thiourea upon reactionwith amine. Absorption spectra of FITC solution and FITC-NH2-ext-HMSNs suspension are shown in Figure S8a. Afterlabeling, the absorption maximum of FITC-NH2-ext-HMSNssuspension decreases to ca. 1/4 of the absorption maximum ofFITC solution. The nonzero baseline is due to extinctionresulting from light scattering.43 The red shift of absorptionmaximum from 492 to 500 nm results from the interaction

between neighboring FITC molecules labeled on the mesoporewalls, which lowers their excited state energy and produces thered shift.43And the color of FITC solution is pale green (FigureS8b), while the color of FITC-NH2-ext-HMSNs suspension ispale pink after labeling (Figure S8c). Figure S8d shows thatFITC-NH2-ext-HMSNs emit strong green fluorescence underexcitation by 470-495 nm wavelength light. Thus, FITC-NH2-ext-HMSNs may be used to monitor the cellular process of theHMSNs particles. Moreover, due to the existence of aminogroups and hierarchically mesoporous structure, it is possible

that NH2-ext-HMSNs can be simultaneously loaded with severalkinds of species such as drug molecules, protein molecules,magnetic NPs and fluorescent molecules.

3.4. Loading of Au NPs. HMSNs loaded with Au NPs weresonicated, dropped on TEM grid, and observed by TEM(Figure 4a and b). Figure 4a and b show in situ loading resultsof Au NPs using NH2-ext-HMSNs as carrier. Many Au NPs witha diameter of


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3.5. Functional Coatings. Functional coatings were success-fully fabricated on glass substrate using ext-HMSNs as building

blocks by the LbL dip coating method. SEM images in Figure 5show the surface morphologies of ext-HMSNs coatings with twoand four cycles of PDDA/ext-HMSNs. Clearly, ext-HMSNs arehomogenously distributed on the substrate surface (Figure 5aand c). Magnified SEM images (Figure 5b and d) reveal that thecoating surfaces are very rough, and in fact consist of nanometer-sized islands (ext-HMSNs) and valleys (voids among ext-HMSNs and large mesopores on the surface of ext-HMSNs).By comparing panels b and d of Figure 5, it can be seen thatparticle density, coating thickness and surface roughness increasegradually, and the coating presents a three-dimensional networkstructure more and more clearly with increase of the number ofdeposition cycles. Clearly, subsequently adsorbed ext-HMSNs

would prefer to fill in or sit on the void spaces among ext-HMSNs, thus leading to rougher surfaces and thicker coatings.Such morphologies might significantly affect the transmittance,reflectance and wetting property of the coatings.

Transmission spectra in Figure 6a show that slide glassescoated with two and four cycles of PDDA/ext-HMSNs have aslightly enhanced transmittance at wavelengths above 533 nm.

Meanwhile, refl

ection spectra in Figure 6b exhibit excellentantireflection property. And the minimum reflectance was esti-mated to be 4% at the wavelength of ca. 484 nm and 3% at the wavelength of ca. 615 nm, respectively, for the slide glassescoated with two and four cycles of PDDA/ext-HMSNs, thus alsoexhibiting a red-shift trend with increase of cycle numbers(Figure 6b). Figure S9 directly shows digital images of antire-flection toward a fluorescent lamp of the blank parts and partscoated with two and four cycles of PDDA/ext-HMSNs. Lightreflection can be suppressed clearly by the coatings with thesuppression effect of four deposition cycles more significant.Moreover, the slide glasses coated with two and four cycles ofPDDA/ext-HMSNs also show excellent superhydrophilic prop-erty, thus having antifogging behavior.28-30 Time-dependent

changes in WCAs on the coating surfaces are shown in Figure 6c.Clearly, theWCAs on the coatings decrease quickly with increaseof time from 0 to 1000 ms, indicating fast spreading of waterdroplets. The time from initial contact to spreading to 5 is ca.536 and 485 ms, respectively, for the ext-HMSNs coatings withtwo and four cycles of PDDA/ext-HMSNs. And due to roughersurface and thicker coating, the slide glass coated with four cyclesof PDDA/ext-HMSNs has better hydrophilicity than that coatedwith two cycles of PDDA/ext-HMSNs. The inset in Figure 6cshows the shape of a sheet-like water of ca. 3WCAafter1000msof spreading. In addition, the current coatings with hierarchicallyporous structure may have potential applications as catalystsupports, chemical sensors, etc.

NH2-ext-HMSNs were also used as building blocks. The parti-

culate density after four cycles of deposition is very low (FigureS10a and b) because protonation of NH2-ext-HMSNs in ultra-pure water (ca. 0.5 wt %, pH 8.78) results in pH increase from3.46 (ext-HMSNs) to 8.78 (NH2-ext-HMSNs). WCA is ca.50 C due to the existence of polyelectrolyte (Figure S10c)and the transmittance is lower than that of blank slide glass(Figure S10d). The properties of coatings fabricated using variedparticles (cal-HMSNs,45 ext-HMSNs, and NH2-ext-HMSNs) aresummarized in Table 1 and Figure S11 for comparison. Thecoatings of cal-HMSNs show the best superhydrophilicity due tothe removal of polyelectrolytes. The coatings of ext-HMSNsexhibit the best antireflective property in the visible wavelengthrange, and may be due to the relatively intact structure of ext-HMSNs.

3.6. Drug Storage and Release. Mesoporous silicas arecurrently widely studied as carrier matrices in drug deliveryapplications.31-37 Surface functionalization of silica is oftenemployed in order to enhance the interaction between drugand support. However, in many cases the effectiveness of introd-uced surface functions is much lower than what is expected, andthe release rate from surface functionalized silica is often not verydifferent from that of bare silica support, suggesting that thedrug-support interactions are weaker than assumed under phy-siologically relevant conditions.46 The Z average diameter ofHMSNs was ca. 200 nm with a narrow size distribution. It hasbeen reported that nonphagocytic eukaryotic cells could inter-nalize particles as large as 500 nm in size and the uptake efficiency

Figure 6. Transmission (a) and reflection (b) spectra of blank slideglass and slide glasses coated with two and four cycles of PDDA/ext-HMSNs. Time-dependent changes (c) in instant contact angle on slideglasses coated with two and four cycles of PDDA/ext-HMSNs. Inset in(c) is a digital image of ca. 3 WCA on a porous PDDA/ext-HMSNscoating of four cycles.


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was high for particles around 200 nm or smaller,47 so the currentfabricated HMSNs would be potentially useful in drug deliveryapplications. Therefore, ext-HMSNs, cal-HMSNs, and NH2-ext-HMSNs were used as drug carrier, respectively, to study drugstorage capacity and in vitro release behavior. Ibuprofen (IBU), atypical anti-inflammatory drug, was chosen as a probing mole-cule. The loading capacity of IBU was measured to be 752, 289,and 186 mg per g SiO2 for cal-HMSNs, ext-HMSNs, and NH2-

ext-HMSNs, respectively. cal-HMSNs have the highest drugloading capacity among the three samples, which is attributedto its high BET specific surface area and the appropriate size ofmesopores (2.5 nm) compared with the size of IBU (ca. 1.0 0.6 nm).36 And ext-HMSNs have a much lowered loadingcapacity, which may be due to their larger mesopore size(3.8 nm), while NH2-ext-HMSNs have the lowest loadingcapacity, which may be because of their even larger mesoporesize (3.4 and 7.9nm) andlow BETspecific surface area. Figure 7ashows UV-vis spectra at different times of the aqueous media in which IBU was released from cal-HMSNs@IBU. The IBUconcentration gradually increases with prolonging of time,indicating sustained release of IBU molecules. Kinetic release

curves were plotted based on the sustained releases by using theabsorption of the peak at ca. 264 nm. Recently, Vallet-Regi et al.reported that the release rate under similar conditions was veryfast during the first day, but decreased with time and reached amaximum value of 80% the third day when using conventionalMCM-41 NPs as IBU carriers.36As shown in Figure 7b, comparedwith conventional MCM-41 NPs, the current three samples exhibithigher rates of release, i.e., the IBU amounts released from cal-HMSNs@IBU, ext-HMSNs@IBU and NH2-ext-HMSNs@IBUreach about 80% in 3.4 h, 7.7 and 17.2 h, respectively, indicatingthat the hierarchically mesoporous nanostructures are favorable forfast molecular diffusion through pore channels. The release rate of

ext-HMSNs@IBU is a little lower than that of [email protected] may result from the existence of a large number of Si-OHgroups on the mesopore wall of ext-HMSNs, thus producing theinteraction between Si-OH and IBU-COOH, despite of the largersize ofmesopores. The release rate of NH2-ext-HMSNs@IBU is thelowest, which may be due to the interaction between Si-(CH2)3-NH2 groups and IBU-COOH. Generally speaking, slow sustainedrelease of drug is superior to relatively quick release; however, theHMSNs with hierarchically mesoporous nanostructures might beadvantageous for some stimulated and quick releases.

4. CONCLUSIONS

In summary, as-HMSNs with uniform morphology and struc-ture were facilely fabricated by gasification of ethyl ether.Following template extraction gave more intact and betterdefined nanostructures of hierarchical mesopores than followingcalcination. Amino-functionalized HMSNs with well sphericalstructure and larger mesopores were obtained by co-condensa-tion with APMS, though some unexpected silica sheets alsoappeared. Amino-functionalization on the mesopore wall of ext-HMSNs was also achieved successfully by postsynthesis mod-ification. FITC molecules and Au NPs of


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AUTHOR INFORMATION

Corresponding Author*Tel.: 86-10-82543535. Fax: 86-10-82543535. E-mail:[email protected].

ACKNOWLEDGMENT

This work was supported by the Knowledge InnovationProgram of the Chinese Academy of Sciences (CAS) (GrantNo. KGCX2-YW-370), the National Natural Science Founda-tion of China-NSAF (Grant No. 10776034), the NationalNatural Science Foundation of China (Grant No. 20871118),Hundred Talents Program of CAS, and Graduate Science andSocial Practice Special Funding Innovative Research Programof CAS.

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DuX11_Hierarchically Me So Porous Silica Nano Particles

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