ControlledSynthesisofMetal-OrganicFrameworkFilmsonMetalNanoparticlesbytheVersatileLayer-by-LayerAssemblyApproach

TakashiOhhashi,TakaakiTsuruoka,*HidemiNawafune,andKensukeAkamatsu* DepartmentofNanobiochemistry,FrontiersofInnovativeResearchinScienceandTechnology(FIRST),KonanUniversity,

7-1-20Minatojimaminami,Chuo-ku,Kobe650-0047 Fax:81-78-303-1495,e-mail:tsuruoka@center.konan-u.ac.jp,akamatsu@center.konan-u.ac.jp

Facile synthesisof metal nanoparticles/metal-organicframeworks(MOFs) nanocompositebasedon preferential self-assembly of frameworks on nanoparticle surface was accomplished byversatilelayer-by-layer(LBL)approach.WereportthesuccessfulgrowthofMIL-100(Fe)filmson Ag nanoparticle surface. This approach involves the synthesis of polyvinylpyrrolidone(PVP)-functionalizedAg nanoparticles and stepwise growth of frameworks consisting of Fe3+ ions and benzene-1,3,5,-tricarboxylate (btc) on nanoparticles. Ag nanoparticles can be act asscaffolds for construction of MOF frameworks. The resulting Ag nanoparticle/MIL-100(Fe)core-shell nanostructures were characterized by transmission electron microscopy (TEM).Additionally,HKUST-1, consisting of Cu2+ ions and btc,was synthesized onAg nanoparticlesurface by the sameLBL approach.The present approach can be utilized to fabricate varioustypesofmetalnanoparticles/MOFnanocomposites thatexhibit theoptoelectronicpropertiesofthenanoparticlesandthemolecularsievingeffectoftheMOFs. Key words:Metal-organicframeworks, Metal nanoparticles,Self-assembly

1.INTRODUCTION Inorganic nanoparticles such as metal, magnetic, andsemiconductor nanoparticles have been the subject ofintense research because their unique physical andchemical properties are different from those of theircorresponding bulks.Therefore, they are interesting forvariousapplications, forexample, forchemicalsensors,biological labels, and catalysts.[1-4] In particular,nanoparticles are extremely useful to developheterogeneous catalysis for organic synthesis and photochemical degradation.[4,5] Although the efficiencyof nanoparticle catalysts is higher than that ofconventional catalysts, the major challenge ofnanoparticle catalysts is the target selectivity ofreaction.[4] In order to construct nanoparticle catalysissystem with high efficiency and selectivity, inorganicnanoparticlesmustbecoveredbyfunctionalmaterials.[6] Porous materials are extremely attractive coveringmaterials because of their unique porous properties.[7] For example, the surface of nanoparticleswas coveredby aporousmaterial, allowing them tooffermolecularselectivityofporousmaterials.[8]

Metal-organicframeworks(MOFs),consistingofmetalions and polyfunctional organic ligands, have manyexciting characteristics including flexibility, orderedcrystalline structure, high specific surface area, andtunable pore size.[9] Therefore, MOFs have receivedgreatattentionfortheir attractiveapplicationsincludingcatalysis, selective gas adsorption and separation, anddrug delivery.[10-13] Recently, it has been reported thatinorganicnanomaterialsdispersedinMOFcrystalswereformedandhavepotentialapplicationsinheterogeneouscatalysts, drug delivery carriers, and sensormaterials.[14-17] Many inorganic nanocrystals/MOF

hybrid materials have been reported based on thermaldecomposition, involving the in-situ formation ofnanoparticles from metal ions and/or organometalliccompoundsdopedinas-preparedMOFs.Bythissimplemethod, metal and/or metal oxide nanoparticles aredepositedintoMOFcrystals.[18] However,thisapproachis limited for heat-resistantMOFs, and it is difficult tocontrol the size, shapeanddistributionofnanoparticlesin MOFs, which is important to obtain the inorganicnanoparticles/MOFhybridnanocompositeswithdesiredproperties.Inordertoutilizetheiruniquepropertiesforvarious applications, an alternative approach, thatenablesthestructuralcontrol,mustbedeveloped.

Herein, we demonstrate that inorganicnanoparticle/MOF core/shell nanocomposites can beprepared via layer-by-layer (LBL) approach. LBLapproach is very popular as the approach for thefabrication of thin films on flat substrate andnanoparticle surface because it is easy and versatile.[19] However, fortheMOFassemblybasedoncoordinationinteraction, the controlled assembly of metal ions andorganic ligands on nanoparticle surface is particularlychallenging. In this study, we describe the growth of[Fe3O(OH)(H2O)2(btc)2 ･ nH2O] (btc = 1,3,5-benzene –tricarboxylate)and [Cu3(btc)2], whichare referred toMIL-100(Fe) and HKUST-1, frameworks on the Agnanoparticle surfaces via LBL approach, resulting inAg/MOF core/shell nanocomposites. In addition, we investigatetheeffectofreactioncondition,precursorofmetal ions, reaction temperature, molecular weight ofPVP for stabilizer of nanoparticles (trap sites of metalions), and precursorconcentrationonthestructureofAgnanoparticle/MOFhybridnaocomposites.

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2.EXPERIMENTAL Preparation of Ag nanoparticles stabilized with PVP molecules: Ag nanoparticles were prepared by DMF reductionmethod.[20] 5.0 mL of a 10 wt% polyvinylpyrrolidone (PVP;M.W. 10000) aqueous solutionwas added to 40mL of N,N-dimethylformamide (DMF). An aqueoussolution of silver nitrate (0.125 M, 5.0 mL) was thengradually added to themixture while heating at 80 ºC with stirring. After heating the mixture for 3 h, theresulting nanoparticles were purified by centrifugationand washed with DMF. Fabrication of Ag nanoparticle/MIL-100(Fe) core-shell nanocomposites and Ag nanoparticles/HKUST-1 heterostructured nanocomposites: A60LDMFsolutionofAgnanoparticles(49.2pmol)washeatedat70 ºC in the two-necked flask.When thetemperatureof the solution reached to70 ºC, a4.0mLethanol solution of iron(III) chloride (0.5 mM) wasadded into thenanoparticles solution.After theheatingfor 30 min, the mixture solution was purified byultrasonicirradiationandcentrifugation.Thena4.0mLethanol solution of benzene-1,3,5-tricarboxylic acid (H3btc; 0.5 mM) was added into the mixture solutionand heated at 70 ºC for 30min. The resulting sampleswerepurifiedbyultrasonicirradiationandcentrifugation.Additionally, these processes were conducted in repeating fashion to grow MOF frameworkshierarchically. Ag nanoparticles/HKUST-1heterostructured nanocomposites was also prepared throughthesameapproach. 3.RESULTSANDDISCUSSION Preparation of Ag nanoparticles stabilized with PVP molecules: TEMimageof theobtainednanoparticles is shown inFigure 1a. The image shows the presence ofnanoparticleswith spherical shape of average diameterof around 14.0 nm. The color of DMF solutioncontaining the resulting nanoparticles is pale yellow,indicatingtheobtainednanparticlesareAgcrystalswithsurface plasmon resonance (SPR). TheFT-IR spectrumoftheresultingAgnanoparticles shows the asymmetric and symmetric C-H stretching band around 2900 cm-1 andtheC=Ostretching bandofamidearound1700cm-1, demonstrating that the obtained Ag nanopartcles arestabilized with PVP molecules (Figure 1c). Thisnanopartciles are readily soluble in the polar solventssuchasmethanol,ethanol,andDMF. Fabrication of Ag nanoparticle/MIL-100(Fe) core-shell nanocomposites and Ag nanoparticles/HKUST-1 heterostructured nanocomposites: Figure 2 shows representative TEM images of Agnanoparticle/MOF nanocomposites prepared by LBLapproach with 3 cycles. Ag nanoparticle/MIL-100(Fe)core/shell nanocomposites are fabricated by thisapproach, while the aggregated nanocomposites areobserved.Thisresultsindicatethatself-assemblygrowthofMIL-100(Fe) frameworkmainlyoccurredonsurfaceof Ag nanoparticles. Surprisingly, thickness ofMIL-100(Fe) shell is ca. 23.0 nm, although the LBLcycles(reactionwithmetal ionsandorganic ligands) is

only thrice (The estimated thickness is ca. 2 nm).This maybecausedbythehighcoordinationactivitybetweenFe3+ ionsandPVPmolecules,resultingintheadsorptionof the excess amount of Fe3+ ions on surface of Agnanoparticles. While nanocomposites are prepared byusing Ag nanoparticles stabilized with PVP (M.W. 10,000), a molecularweight (40,000)ofPVPmoleculeson nanoprtcle surface are utilized to investigate theeffectofnumberoftrapsitesforFe3+ ions.TEMimage showninFigure3revealsthatMIL-100(Fe)frameworksare grown onnanoparticle surface in a similar fashion.

Figure 1. (a) TEM image, (b) size histogramand (c) FT-IR spectrum of PVP-stabilized Agnanoparticles prepared by DMF reductionmethod.

Figure 2. TEM images ofAgNP/MIL-100(Fe)obtained after 3 assembly cycles of the LBLapproach.

Figure 3. TEM image of Ag NP/MIL-100(Fe)nanocomposites prepared by using Agnanoparticles stabilized with different molecularweight40 ,000.

The morphology of nanocomposites prepared by usingPVPmolecules (M.W.40,000) iscomparable to thatofnanocomposites prepared by PVP molecules (M.W.10,000).However,theshellthicknessslightlyincreasedto 24.4 nm, and a larger number of aggregatednanocomposites are observed. This result stronglysuggeststhattheamountofinitialloadedmetalionsonnanoparticlesurfaceisakeyfactortogrowthMOFthinfilmsonnanoparticlesurface. The size of Ag nanoparticles increased to 55.5 nm during growth of MIL-100(Fe) frameworks. The sizeevolution of metal nanoparticles in the presence ofprotonandhalogenionshavebeenpreviouslyreported.Themixture of Ag nanoparticles and iron(III) chloridewithout H3btc molecules was heated at 70 ºC toinvestigate the effect of chloride ions on the size ofnanoparticles. After the heat treatment, the size of Agnanoparticlesdrasticallyincreased(Figure4),indicatingthat the size evolution of nanoparticles during reactionwas inducedbychloride ions from ironprecursor.Thiseffect wasalsoconfirmedbyusingotherironprecursorsuchas iron(III)hydroxideacetateand iron(III)nitrate.ThesizeevolutionofAgnanoparticleswaspreventedinthe absence of chloride ions. However, as shown inFigure5,nouniformgrowthofMIL-100(Fe)thinfilmson nanoparticle surface was observed, leading to theformationofaggregatednanocomposites. Thesizeevolutionduringreactiondependsonreactiontemperature as well as species of iron(III) precursors.Wehave investigated theeffect of reaction temperatureon the size of Ag nanoparticles in nanocomposites.Atroom temperature, the size of nanoparticles remainsunchanged while several nanoparticles with biggerdiameter were observed (Figure 6), demonstrating thatsizecontrolofmetalnanoparticlesofmetal/MOFhybridnanomaterials can be achieved by controlling thereaction temperature. However, no individual Agnanoparticle/MIL-100(Fe) nanocomposites wasobserved because growth rate of MIL-100(Fe) waschanged by decreasing the reaction temperature. Fromthisresult,thereactiontemperaturehasimportantfactorson both thesizeofmetalnanparticlesinnanocompsoitesand the formation of individual nanoparticle/MOFcore/shellnanocompositesbyLBLapproach. In order to evaluate the diversity of the presentapproach,HKUST-1filmwas formedonsurfaceofAgnanoparticles.WhenthereactionwasconductedatsameconditioninthecaseofMIL-100(Fe),severalnumberofAg nanoparticles was dispersed in HKUST-1 crystals

whileAgnanoparticles/HKUST-1nanocompositeswereformed. By controlling the concentration of Agnanoparticles, the structureofobtainednanocompositeswas changed (Figure 7). Halving the Ag nanoparticlesconcentration produced HKUST-1 crystals without Agnanoparticles.Thisisbecausethescaffoldisinsufficientfor the growth of frameworks, resulting in pureHKUST-1 crystals. In contrast,when the concentrationof Ag nanoaprticles was doubled or quadrupled,aggregated Ag nanoparticles were observed within

Figure 4. TEM image of Ag nanoparticlesobtained after heat treatment with onlyiron(III)chloride.

Figure 5. TEMimagesofAgNP/MIL-100(Fe)nanocomposites prepared by different ironprecursors of (a) iron(III) hydroxide acetateand(b)iron(III)nitrate.

Figure 6. TEM image ofAgNP/MIL-100(Fe)nanocomposites prepared at room temperature.

Figure 7. TEM images of Ag NPs/HKUST-1nanocomposites prepared by differentnanoparticle concentration of (a) 24.6 pmol,(b) 49.2 pmol, (c) 98.4 pmol and (d) 246pmol.

Controlled Synthesis of Metal-Organic Framework Films on Metal Nanoparticles by the Versatile Layer-by-Layer Assembly Approach154

The morphology of nanocomposites prepared by usingPVPmolecules (M.W.40,000) iscomparable to thatofnanocomposites prepared by PVP molecules (M.W.10,000).However,theshellthicknessslightlyincreasedto 24.4 nm, and a larger number of aggregatednanocomposites are observed. This result stronglysuggeststhattheamountofinitialloadedmetalionsonnanoparticlesurfaceisakeyfactortogrowthMOFthinfilmsonnanoparticlesurface. The size of Ag nanoparticles increased to 55.5 nm during growth of MIL-100(Fe) frameworks. The sizeevolution of metal nanoparticles in the presence ofprotonandhalogenionshavebeenpreviouslyreported.Themixture of Ag nanoparticles and iron(III) chloridewithout H3btc molecules was heated at 70 ºC toinvestigate the effect of chloride ions on the size ofnanoparticles. After the heat treatment, the size of Agnanoparticlesdrasticallyincreased(Figure4),indicatingthat the size evolution of nanoparticles during reactionwas inducedbychloride ions from ironprecursor.Thiseffect wasalsoconfirmedbyusingotherironprecursorsuchas iron(III)hydroxideacetateand iron(III)nitrate.ThesizeevolutionofAgnanoparticleswaspreventedinthe absence of chloride ions. However, as shown inFigure5,nouniformgrowthofMIL-100(Fe)thinfilmson nanoparticle surface was observed, leading to theformationofaggregatednanocomposites. Thesizeevolutionduringreactiondependsonreactiontemperature as well as species of iron(III) precursors.Wehave investigated theeffect of reaction temperatureon the size of Ag nanoparticles in nanocomposites.Atroom temperature, the size of nanoparticles remainsunchanged while several nanoparticles with biggerdiameter were observed (Figure 6), demonstrating thatsizecontrolofmetalnanoparticlesofmetal/MOFhybridnanomaterials can be achieved by controlling thereaction temperature. However, no individual Agnanoparticle/MIL-100(Fe) nanocomposites wasobserved because growth rate of MIL-100(Fe) waschanged by decreasing the reaction temperature. Fromthisresult,thereactiontemperaturehasimportantfactorson both thesizeofmetalnanparticlesinnanocompsoitesand the formation of individual nanoparticle/MOFcore/shellnanocompositesbyLBLapproach. In order to evaluate the diversity of the presentapproach,HKUST-1filmwas formedonsurfaceofAgnanoparticles.WhenthereactionwasconductedatsameconditioninthecaseofMIL-100(Fe),severalnumberofAg nanoparticles was dispersed in HKUST-1 crystals

whileAgnanoparticles/HKUST-1nanocompositeswereformed. By controlling the concentration of Agnanoparticles, the structureofobtainednanocompositeswas changed (Figure 7). Halving the Ag nanoparticlesconcentration produced HKUST-1 crystals without Agnanoparticles.Thisisbecausethescaffoldisinsufficientfor the growth of frameworks, resulting in pureHKUST-1 crystals. In contrast,when the concentrationof Ag nanoaprticles was doubled or quadrupled,aggregated Ag nanoparticles were observed within

Figure 4. TEM image of Ag nanoparticlesobtained after heat treatment with onlyiron(III)chloride.

Figure 5. TEMimagesofAgNP/MIL-100(Fe)nanocomposites prepared by different ironprecursors of (a) iron(III) hydroxide acetateand(b)iron(III)nitrate.

Figure 6. TEM image ofAgNP/MIL-100(Fe)nanocomposites prepared at room temperature.

Figure 7. TEM images of Ag NPs/HKUST-1nanocomposites prepared by differentnanoparticle concentration of (a) 24.6 pmol,(b) 49.2 pmol, (c) 98.4 pmol and (d) 246pmol.

TakashiOhhashietal.Trans. Mat. Res. Soc. Japan39[2]153-156(2014) 155

HKUST-1 frameworks. On the basis of these results,controlling the nanoparticles concentration is a criticalfactorforproducingheterostructurednanocomposites. ThismorphologydifferencebetweenMIL-100(Fe)andHKUST-1 may be caused by the coordination affinitywith metal ions and H3btc molecules. AlthoughHKUST-1crystalswithhighcrystallinityare formedatroom temperature, theheat treatment is usually neededfor the formation of MIL-100(Fe) crystals with highcrystallinity. On the basis of the obtained results andpreviousknowledge,themechanismfortheformationofmetalnanoparticles/MOFnanocompositesareestimated(Figure 8). At initial stage of reaction, MOF growthmainly occurs on nanoparticle surface because the trapsitesofmetal ionsaredensely localizedatnanoparticlesurface. After the growth of MOF films, aggregatednanocompositesare immediatelyformedbybridgingofAgnanoparticle/MOFcore/shellnanocomposites.InthecaseofMIL-100(Fe),noexcessgrowthofMIL-100(Fe)shellmayoccurduetoslowgrowthrateofframeworks.Ontheotherhands,additionalHKUST-1maybereadilyformedatthiscondition,resultingintheformationofAgnanoparticles/HKUST-1nanocomposites. 4. CONCLUSION In summary, we demonstrated that metal nanoparticles/MOF hybrid nanomaterials werefabricated by preferential self-assembly of MOF onmolecular-functionalized nanoparticle surface by LBLapproach.TheLBL synthesismethodused in theworkcould be a general technique for the preparation ofdifferent types of metal/MOF nanocomposites. In addition, structure of nanocomposites can be facilelycontrolled by altering the concentration of metalnanoparticles and reaction temperature. Thus, thispresent process may be well suited for the growth ofMOF crystals on nanoparticle surface. The resultingnanoparticle/MOFhybridnanomaterialshowtheuniqueproperties arising from metal nanoparticles andMOFsand will have potential applications in many fieldsincludinggassorptionandgasseparation,drugdelivery,biosensing,andheterogeneous catalysis. REFERENCES [1] S.Liu,Z.Tang,J. Mater. Chem.,20,24-35(2010) [2] L. Guerrini, J. V. Garcia-Ramos, C. Domingo, S.

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Figure 8. Schematic illustration of the formation process of Ag NPs/MOF nanocomposite by LBLapproach.

Hierarchically Structured Coatings by Colorless Polydopamine Thin Layer and Polymer Brush Layer

Hiroto Kohma, Kanako Uradokoro, Michinari Kohri*, Tatsuo Taniguchi and Keiki Kishikawa Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

* Corresponding author: e-mail: kohri@faculty.chiba-u.jp

Herein, we describe a general and versatile procedure to fabricate hierarchically structured coatings onto material surfaces using colorless polydopamine (PDA) thin layer and polymer brush layer. Firstly, atom transfer radical polymerization (ATRP) initiator-containing colorless PDA layer was coated onto various types of material surfaces. Then, surface-initiated ATRP was conducted to produce functional polymer brush layer. This methodology will be applied to a broad range of materials to modification of surface properties. Furthermore, one-step hierarchically structured coatings of materials surface were also investigated. Key words: Colorless polydopamine layer, Polymer brush layer, Hierarchically structures coatings, Surface-initiated ATRP

1. INTRODUCTION The functionalization of solid surfaces is an attractive topic in material science for their scientific significance and numerous potential applications. Surface-initiated atom transfer radical polymerization (SI-ATRP) is an extremely method for surface modification [1–6]. We have reported the preparation of functionalized polystyrene (PSt) particles with hydrophilic polymer shell layers by SI-ATRP in water [7]. ATRP initiator-bearing PSt core particles were prepared by emulsifier-free emulsion polymerization of styrene and ATRP initiator-containing monomer, i.e., 2-chloropropionyloxyethyl methacrylate. Hydrophilic monomers were then polymerized onto PSt particles. Usually, the introduction of ATRP-initiating groups onto material surfaces is carried out using individual surface modifiers; for example, thiol derivatives for gold particles [8], silane-coupling reagents [9] and phosphate-functionalized derivatives [10] for inorganic particles, and vinyl group bearing compound for polymer particles [7]. Thus, the development of versatile strategies for the introduction of ATRP initiator to materials surface is essential for technological application. Since the emergence of surface modification techniques, including polydopamine (PDA) layers using a single deposition process that is based on the oxidative self-polymerization of dopamine hydrochloride (DA) [11], the number of studies concerning the functionalization of material surfaces via PDA coating has increased [12–16]. While PDA is simple to apply to cover onto various kinds of substrates, most of conventional PDA layers show considerable dark brown coloration, restricting their use for some practical applications such as optical

materials and cosmetics. To overcome this drawback, we recently demonstrated the preparation of a colorless PDA layer containing ATRP initiator prepared by the in situ reaction of DA with 2-bromoisobutyryl bromide (BiBB); The synthesized colorless PDA layer is designated PDA/BiBB2 ([BiBB]/ [DA] = 2, the ratio uses the molar concentrations of DA and BiBB) [17]. PDA/BiBB2 layer was placed onto PSt particles, and then the surface of obtained particles was coated with polymer brush layers via SI-ATRP [18]. It would be great interest to explore the feasibility of using PDA/BiBB2 layer to various types of material surfaces instead of PSt particles. Herein, we reported a general and versatile procedure to fabricate hierarchically structured coatings onto material surfaces using colorless PDA thin layer and polymer brush layer (Fig. 1). ATRP initiator-bearing colorless PDA layer (PDA/BiBB2 layer) has been prepared onto various types substrates, e.g., glass plate, flexible polypropylene (PP) film, filter paper, silica (SiO2) particles, and PSt particles. Next, the surface-initiated ATRP of hydrophilic monomers was conducted to generate a functional polymer brush layer. One-step hierarchically structured coatings of materials surface were also investigated.

Fig. 1 Schematic representation of surface functionalization via hierarchically structured coatings using colorless PDA layer and polymer brush layer.

Controlled Synthesis of Metal-Organic Framework Films on Metal Nanoparticles by the Versatile Layer-by-Layer Assembly Approach

(ReceivedJanuary31,2014;AcceptedMarch27,2014)

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