ph-responsive layered hydrogel microcapsules as gold

pH-Responsive Layered Hydrogel Microcapsules as GoldNanoreactors

Veronika Kozlovskaya,† Eugenia Kharlampieva,† Sehoon Chang,‡ Rachel Muhlbauer,† andVladimir V. Tsukruk*,†,‡

School of Materials Science and Engineering, and School of Polymer, Textile and Fiber Engineering,Georgia Institute of Technology, Atlanta, Georgia 30332

ReceiVed February 3, 2009. ReVised Manuscript ReceiVed April 2, 2009

We demonstrate that layered hydrogel poly(methacrylic acid) capsules (PMAA), produced fromhydrogen-bonded (PMAA/poly-N-vinylpyrrolidone) (PMAA/PVPON) multilayer precursors through cross-linking with ethylenediamine (EDA), can facilitate in situ synthesis of gold nanoparticles within hydrogelwalls under ambient conditions. The necessary amine groups are available within the (PMAA) shellsdue to one-end attached cross-linker molecules. We also show that the nanoparticle size can be controlledthrough changing the pH-dependent balance of amine/ammonium groups in the ionic cross-links withinthe shells. Importantly, the pH-responsive properties of the ultrathin hydrogel shells are preserved aftergold nanoparticle synthesis within the capsule walls. The reported in situ synthesis of gold nanoparticleswithin the ultrathin and pH-responsive shells can find a potential use for straightforward and facilefabrication of the hybrid organic-gold nanomaterials for biochemical sensing and delivery applications.

Introduction

In past years, the research on metal nanoparticles as re-enforcing, functional, and responsive blocks for hybridorganic/nanocomposite materials has gained significant at-tention due to the unique optical, electrical, catalytic, andantibacterial properties of inorganic nanoparticles.1-4 Dif-ferent inorganic materials such as silica nanoparticles,magnetite nanocrystals, metal nanoparticles, and semicon-ducting nanowires have been deposited onto or withinpolymer matrices to enhance their mechanical and opticalproperties.5-7 Inclusion of metal nanoparticles within polymermaterials was shown to be an effective way for a combinationof unique optical properties of gold nanostructures withresponsive behavior of ultrathin membranes enhancing theirfunction as easily handable adaptive nanomaterials.8 Such acombination allowed a robust control over various importantproperties of the responsive polymer films9 such as

microroughness,10,11 wettability,12 biocompatibility,13 andoptical response,14 which are important for a variety of fieldsin separation,15 sensing,16,17 and delivery of functionalmolecules.18

Various methods have been used for embedding goldnanoparticles within the polymer matrix including Lang-muir-Blodgett (LB) deposition,19,20 electrostatic-drivenadsorption,21-24 and hydrogen-bonded assembly.25 The layer-by-layer assembly (LbL) of polyelectrolyte structures onsacrificial substrates has been widely used to fabricate free-standing planar polymer films with inorganic particles forthe improvement of their mechanical, delivery, and opticalproperties.26-29 Polyelectrolyte capsules with nanocompositeorganic/inorganic shells with embedded nanoparticles havedemonstrated significant mechanical rigidity and maintain

* To whom correspondence should be addressed. E-mail: [email protected].

† School of Materials Science and Engineering.‡ School of Polymer, Textile and Fiber Engineering.

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2158 Chem. Mater. 2009, 21, 2158–2167

10.1021/cm900314p CCC: $40.75 2009 American Chemical SocietyPublished on Web 04/27/2009

their spherical shape after drying with retaining encapsulatedmaterials in their interiors.30,31 Usually, the encapsulation ofnanoparticles requires premodification with stabilizing andcapping agents when used for adsorbing onto/into multilayerfilms to support strong noncovalent interactions with polymermatrices during the LbL assembly.32,33

For instance, the “breathing strategy” was suggested forthe preparation of temperature-responsive Au/poly(N-iso-propylacrylamide) (PNIPAM) composites when gold nano-particles were incorporated into the highly swollen gel inaqueous solution, which was later shrunk in aprotic solutionto fix gold nanoparticles in the hydrogel by physicalentanglement (breathing out).34 Recently, we have reportedon the preparation of (PMAA/gold nanorods) compositemultilayered hydrogels through electrostatic inclusion of theCTAB-stabilized Au nanorods within the swollen multilayerhydrogel to produce pH-responsive optical materials.35 Alongwith the pH-dependent shift of the surface plasmon resonance(SPR) signal, such inclusion of the gold nanorods renderedthese films mechanically stable and capable of sustainingrelease process in free-floating state. A variety of strategiesfor fabrication of robust free-standing microstructures fromLbL-derived materials have been summarized in our recentreview.36

The LbL assembled polyelectrolyte capsules have beenemployed as microreactors for in situ synthesis of nickel-metallized poly(allylamine hydrochloride/poly(styrene sul-fonate) (PAH/PSS) shells.37 Indeed, in situ nanoparticlesynthesis within the multilayer polymer films can be anelegant way to obtain metal nanoparticles and correspondinghybrid nanomaterials (organic-inorganic) due to the confinedenvironment of the polyelectrolyte multilayers allowing agood control over nanoparticle shapes, morphologies, andsizes.38,39 Remarkably, noble nanoparticles, gold in particular,possess good biocompatibility and nontoxicity. Variousantibodies and proteins can be readily conjugated to goldnanoparticles through thiol-surface chemistry or electrostaticbinding, which makes them important for biochemicalsensing and detection40 as well as for therapeutic applica-

tions.41 Drastic temperature change of gold nanoparticles onresonance with an excitation source42 can be used for deliveryof microcapsule inner contents. Mostly, the strategy ofnanoparticle synthesis within the LbL films is based on pH-dependent properties of the multilayer films assembled fromweak polyelectrolytes, generally containing carboxylic acidgroups, which provide binding sites for metal precursor ionsunder specific pH conditions.39,43

However, such an approach cannot be used for the directsynthesis of gold nanoparticles inside a polymer matrixbecause of the same electrostatic charge of negativelycharged carboxylate groups and chloroaurate ions as goldprecursors. Indeed, the incorporation of free amine groupsuseful as binding sites for gold anions within the multilayerfilms is a tricky challenge. On one hand, the favorablepresence of free amine groups necessary for gold stabilizationcan be impaired by limited solubility of PAH (or poly(eth-yleneimine) (PEI) at high pH conditions. On the other hand,the charge-to-charge compensation mechanism of the elec-trostatic LbL results in almost all ammonium groups boundto anionic counterparts under lower pH deposition conditions.Recently, Chia et al. have demonstrated the possibility ofinduced postassembly molecular rearrangements within PAH/PAA and PAH/PSS multilayers, which could favor goldnanoparticle synthesis within these films.44 However, suchrearrangements are limited and only possible for the (PAH/PAA) and (PAH/PSS) multilayers deposited under veryspecific conditions, which, to a great extent, impair both thefilm integrity and the robustness of nanocomposite materi-als.44

In this work, we demonstrate that multilayered hydrogelPMAA capsules, produced from hydrogen-bonded (PMAA/PVPON) multilayer LbL precursors through cross-linkingwith EDA, can be utilized to facilitate in situ synthesis ofgold nanoparticles inside hydrogel shells. The amine groupsare readily available within the (PMAA) multilayer hydrogelsdue to some one-end attached cross-linker molecules. Incontrast to the previously reported synthesis of gold nano-particles in polyelectrolyte multilayers discussed above, theseLbL hydrogels introduced here do not require any specificpretreatment such as group activation or adding an extracomponent. Moreover, using this approach, the gold nano-particle synthesis can be carried out under mild reducingconditions of borate buffer at ambient environment.

In addition, the important feature of the resulted compositeshells is their pH-dependent properties, which are barelyaffected by the presence of synthesized gold nanoparticles.We also show how the dimension of gold nanoparticles canbe controlled through the pH-dependent release of aminegroups from ionic cross-links within the hollow hydrogelshells without any damage to the film, which can be alimitation for the LbL multilayers when one needs to usespecific conditions of the multilayer preassembly to providethe amine groups. We suggest that in situ synthesis of gold

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2159Chem. Mater., Vol. 21, No. 10, 2009Hydrogel Microcapsules as Gold Nanoreactors

nanoparticles within the EDA-(PMAA) LbL hydrogelcapsule walls has a potential to regulate mechanical proper-ties of the ultrathin and pH-responsive multilayer hydrogelsfor shape re-enforcement of three-dimensional shape-persistent and robust microstructures made of such nano-structured hybrid materials.

Experimental Section

Materials. PMAA (Mw ) 150 kDa), PVPON (Mw ) 55 kDa),PAH (Mw ) 70 kDa), mono- and dibasic sodium phosphate 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC),N-hydroxy-sulfosuccinimide sodium salt (NSS), adipic acid dihy-drazide (AAD), EDA, and HAuCl4 solution were purchased fromSigma-Aldrich. 0.1 M borate buffers (pH 10) were received fromJ. T. Baker. Monodisperse silica particles with diameter of 4.0 (0.2 mm were obtained from Polysciences Inc. as 10% dispersionsin water. Hydrofluoric acid (48-51%) was purchased from BDHAristar. Ultrapure (Nanopure system) filtered water with a resistivityof 18.2 MΩ cm was used in all experiments. Quartz microscopefused slides (Alfa Aesar) and single-side polished silicon wafersof the (100) orientation (Semiconductor Processing Co.) were cutby typical size of 10 × 20 mm and cleaned in a piranha solutionas described elsewhere.45

Fabrication of Hydrogel PMAA Capsules and Films. Deposi-tion of hydrogen-bonded multilayers of PVPON/PMAA on par-ticulate substrates has been described previously.46 Briefly, 0.2 mg/mL polymer solutions were used with a typical deposition time of15 min. Hydrogen-bonded multilayers were deposited directly ontosilica microparticles at pH 2.5 starting from PVPON. Eachdeposition cycle was followed by washing three times with a watersolution with pH adjusted to 2.5. Suspensions were settled bycentrifugation at 2000 rpm for 2 min to remove the supernatantwith excess of polymer. Deposition, washing, and redispersion stepswere performed on a VWR analogue vortex mixer at 2000 rpm.When a desired number of layers was deposited, cross-linking wasperformed as described earlier,47 where the procedure consisted ofactivation of the carboxylic groups with 5 mg/mL solution of EDCand NSS at pH 5 followed by reaction with a 5 mg/mL solution ofEDA at pH 5.8, or with 2 mg/mL solution of AAD at pH 5 for24 h to introduce amide linkages between EDA (or AAD) moleculesand the activated carboxylic groups.

In the case of quartz-tethered PMAA films, three bilayers ofPAH/PMAA precursors were first spin-cast onto the quartz slidesstarting from 0.5 mg/mL PAH solution at pH 4 with the followingdeposition of PMAA from 0.5 mg/mL aqueous solution at pH 5,to enhance the surface adhesion of the subsequently grownmultilayer as well as the swollen hydrogel films in borate buffersof pH 10. The precursor layers were heated to 175 °C for 1 h in anoven for thermal cross-linking of (PAH/PMAA)3 layers48 and theirenhanced adhesion to the substrate surfaces. We utilized theseprecursor-treated quartz microslides or silicon wafers for furtherdeposition of hydrogen-bonded (PVPON/PMAA) multilayers.Specifically, 1.0 mg/mL polymer solutions were placed onto thesubstrates and rotated at 4000 rpm for approximately 20 s startingfrom PVPON when the pretreated substrates were employed. Afterthe desired number of bilayers was spin-LbL assembled,49 the

quartz-attached PVPON/PMAA multilayers were cross-linked asdescribed above.

After cross-linking was completed, the particle suspensionswere washed several times with the appropriate buffer solutionsand exposed to pH 8 for 6 h to swell the cross-linked shell aroundthe cores. When hollow shells were needed for analysis, the silicacores were dissolved by shaking the particle dispersion for 4 hin 8% aqueous HF solution, yielding LbL-derived hydrogelhollow capsules. The dispersion of the capsules with cross-linkedcapsule walls was then dialyzed against water for 2 days. Thesubstrates with tethered hydrogel films were immersed in a 0.01M buffer solution at pH 8 for 1 h to ensure release of PVPONand then transferred to pH 5 to deswell the hydrogel films. After4 h of exposure, these films were dried with a gentle flow ofnitrogen.

Growth of Gold Nanoparticles within Hydrogel PMAACapsules and Films. For growth of gold nanoparticles within thehydrogel shells, suspensions of silica cores with the cross-linkedhydrogel shells were washed with the borate buffers at pH 3, pH5, or pH 10 four times and were exposed to 2 mL of 1.6 mMHAuCl4 solution in 0.1 M borate buffer for 48 h in the dark. Afterthat, the suspensions were extensively washed with the buffers withthe appropriate pH values and then with water. Quartz-attachedfilms were treated in similar way.

UV-Visible Spectroscopy (UV-vis). UV-visible spectra ofhydrogel LbL films and hollow shells solutions were recorded usinga UV-2450 spectrophotometer (Shimadzu). Measurements weredone on quartz substrates. UV-visible spectroscopy of hollowhydrogel capsules suspensions was performed in a quartz cell with10 mm optical path. For that, 2 mL of capsule suspensions wasplaced in the cell, covered with a cap, and tested.

Atomic Force Microscopy (AFM). Surface morphology of thehollow capsules and films was examined using atomic forcemicroscopy (AFM). AFM images were collected using a Dimen-sion-3000 (Digital Instruments) microscope in the “light” tappingmode according to the well-established procedure.50 For capsulesample preparation, a drop of capsule suspension was placed ontoa precleaned silicon wafer and dried in air prior to AFM imaging.For film thickness measurements, the capsule single wall thicknesswas determined as half of the height of the collapsed flat regionsof dried capsules bearing analysis from NanoScope software togenerate height histograms.

Transmission Electron Microscopy (TEM). TEM was per-formed on a JEOL 100CX-2 electron microscope operated at100 kV. To analyze the pristine and/or nanoparticle-containinghydrogel capsules, they were placed on a carbon-coated goldgrid (Electron Microscopy Sciences) and dried in air before TEManalysis.

-Potential Measurements. The -potentials of EDA- or AAD-cross-linked (PMAA)15 capsules were measured at different pHvalues on Zetasizer Nano-ZS equipment (Malvern). The pH valuesof aqueous capsule solutions were adjusted using 0.01 M HCl orNaOH solutions. Each value was obtained by averaging threeindependent measurements.

Optical Microscopy. Optical microscopy images ofEDA-(PMAA-Au)15 capsules in solutions with various pH valueswere obtained at 40× in transmission mode using a Zeiss Axiovert200 (Zeiss, Thornwood, NY) and a CCD camera. The procedureincluded the addition of a drop of a dispersion of the capsules tothe Laboratory-Tek chambered cells (Electron Microscopy Sci-ences). The chambers of the Laboratory-Tek coverglass were then

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2160 Chem. Mater., Vol. 21, No. 10, 2009 KozloVskaya et al.

sequentially filled with buffer solutions at a certain pH. Capsuleswere allowed to settle and then analyzed.

Results and Discussion

Synthesis of Gold Nanoparticles within AmphotericHydrogels. In our previous work, we have demonstrated thatmultilayer-derived hydrogels produced through a cross-linking of solely poly(carboxylic acid) within the hydrogen-bonded multilayers result in amphoteric pH-dependentbehavior of such films.46 They exhibit large swelling at bothhigh and low pH values due to ionization of either carboxylicgroups (from pH 6 to pH 8) or the release of ammoniumgroups from the one-end attached EDA cross-linker mol-ecules upon protonation of the carboxylic groups at pH < 4.Additionally, these multilayer-derived hydrogels swell in thepH range from 10 to 12 due to a deprotonation of ammoniumgroups (pKa ≈ 10).51

Figure 1 demonstrates optical images of freely floating110 nm thick hydrogel films of (PMAA)40 in aqueous mediaof different pH values. It is interesting to note that, whilevisible at pH 5 due to its higher refractive index of 1.43

(Figure 1A), the film becomes almost invisible in solutionat pH 10 (Figure 1B), where its refractive index decreasesto 1.36 due to the large degree of swelling of the hydrogel.52

The large degree of softness of such responsive membranesis further illustrated in Figure 1C. These swollen filmsproduced as closed hollow membranes (microcapsules)exhibit a distinctive drying behavior. As seen from the low-magnification height AFM image, up to 70% of the swollenPMAA capsules dried from aqueous solution at pH 7collapsed with the large unfolded adhesion area, yieldingalmost perfectly round dried capsules. Such drying behaviorof the capsules reflects drastic softening of the capsule wallswith the microcapsule stiffness decrease by a factor of 600with the value as low as 1 mN/m as determined by single-capsule AFM measurements.53

As cross-linked with EDA, such ultrathin LbL-derivedhydrogel films possess groups of the opposite charge, freeamine groups within the cross-linked matrix of the poly-(methacrylic acid) chains. Previously, it has been shown thatthese oppositely charged functionalities can be used to controlthe permeability of these membranes toward low and highmolecular weight compounds and the adhesion propertiesof the hollow shells at surfaces of a different surfacecharge.46,54 Initially, to investigate the ability of thesehydrogels to serve as nanoreactors for the synthesis of goldnanoparticles due to the present amine groups, surface-tethered hydrogel (PMAA)10 films were constructed on quartzslides using the spin-assisted LbL assembly of hydrogen-bonded (PVPON/PMAA)10 films.35

Scheme 1 shows the pathway for the synthesis of goldnanoparticles within the pH-responsive (PMAA)n hydrogelsexploiting free amine groups. First, hydrogen-bonded mul-tilayers of (PVPON/PMAA)10 were built using the SA-LbL

(51) Albert, A.; Serjeant, E. P. Ionization Constants of Acids and Bases; J.Wiley and Sons: New York, 1962.

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Figure 1. Optical images of freely floating (PMAA)40 hydrogel films atpH 5 (n ) 1.43) (A) and at pH 10 (n ) 1.36) (B). The arrows show theedges of the swollen PMAA film. AFM image of EDA-(PMAA)15 hollowcapsules dried on a Si wafer (C); the z-scale of the image is 400 nm.

Scheme 1. Schematic Representation of UltrathinNanocomposite (PMAA/Au)n Hydrogel Membrane

Construction at Surfacesa

a (A) The LbL assembly of hydrogen-bonded (PVPON/PMAA)n filmsthat are chemically cross-linked with EDA. (B) Swollen (PMAA)n hydrogelis incubated in chloroaurate solution in borate buffer at pH 10. (C,D)Deswollen and swollen surface-attached nanocomposite (PMAA-Au)n

hydrogel membranes.


method (Scheme 1A) and later cross-linked with EDA,producing (PMAA)10 ultrathin hydrogel-like films. To ensurerobust attachment of the pH-responsive LbL-derived hydro-gel to a quartz support, (PVPON/PMAA)10 films weredeposited on (PAH/PMAA)2-thermally cross-linked prelay-ers. This approach allows for the formation of the reliableanchoring layer between the solid support and the LbLhydrogel film.55 Next, the swollen LbL (PMAA)10 hydrogelwas exposed to chloroaurate acid solution in borate bufferat pH 10 to facilitate gold reduction (Scheme 1B).

The UV-visible spectrum of the quartz-attached (PAH/PMAA)2(PMAA)10 film after gold reduction in borate bufferat pH 10 shows an absorbance peak around 526 nmcorresponding to the SPR of gold nanoparticles with thediameter below 50 nm56 (Figure 2A). It is important to notethat the control films of thermally cross-linked (PAH/PMAA)10 did not reduce gold under mild reducing conditionsof borate buffer57 used for the reaction. As seen from Figure2A, the UV-visible spectra of the thermally cross-linked(PAH/PMAA)10 films before (black) and after (red) exposure

to HAuCl4 solution in borate buffer at pH 10 are similar.They do not show the SPR absorption band characteristicof gold, indicating no gold synthesis. These results can beexplained due to the thermal cross-linking of (PAH/PMAA)

(55) Kozlovskaya, V.; Kharlampieva, E.; Mansfield, M. L.; Sukhishvili,S. A. Chem. Mater. 2006, 18, 328.

(56) Wang, B.; Chen, K.; Jiang, S.; Reincke, F.; Tong, W.; Wang, D.; Gao,C. Y. Biomacromolecules 2006, 7, 1203.

Figure 2. (A) UV-visible spectra of quartz-tethered-thermally cross-linked(PAH/PMAA)10 and EDA-(PMAA)10 films (red and blue, respectively)after gold reduction in borate buffer at pH 10 for 2 days. The spectrum ofthermally cross-linked (PAH/PMAA)10 before Au reduction is shown inblack for reference. TEM images of quartz-attached-(PMAA)20 hydrogelfilms with gold nanoparticles reduced at pH 10 in borate (B) buffers withsize distributions of the nanoparticles (C).

Figure 3. TEM images of AAD-(PMAA)10 and EDA-(PMAA)10 hollowcapsules (A and B, respectively) after 48 h exposure to HAuCl4 solution inborate buffer at pH 10.

Figure 4. A higher magnification TEM image of an EDA-(PMAA)15

capsule after 48 h exposure to HAuCl4 solution in borate buffer at pH 10(A). (B) Size distribution of the reduced gold nanoparticles. (C) AUV-visible spectrum of the EDA-(PMAA)15 capsule solution after thegold reduction at pH 10.


films at 175 °C for 1 h used in this work resulting in almostcomplete consumption of amine groups in forming amidelinkages with carboxylic groups of the polyacid,48 andtherefore, no gold complex ions, [AuCl4]-, can be electro-statically retained within the film. Similar results werereported recently by Chia et al., when no detectable SPRabsorption bands were found when (PAH/PAA) multilayersdeposited at pH 3.5 were exposed to gold anion solutionsand subsequently irradiated with UV light. This observationconfirms the unsuccessful binding of the gold complex anionsdue to the absence of free amine/ammonium groups capableof the anion binding.44

TEM analysis of the (PMAA)10 film after its exposure toHAuCl4 solution in borate buffer at pH 10 revealed thepresence of monodisperse gold nanoparticles with relativelynarrow size distribution (below 20%) and the average sizeof 16 nm (Figure 2B,C). It is worth noting that if the goldreduction was performed under the same conditions exceptthat the dibasic phosphate buffer was used, gold nanoparticleswith a broad distribution of sizes and shapes were produced(not shown).

As a next step, we synthesized (PMAA)10 hydrogelcapsules using another bifunctional cross-linker, adipic acid

dihydrazide, to confirm that gold nanoparticle synthesis isfacilitated by the amine groups. These capsules are furtherdenoted as AAD-(PMAA)10 capsules. No gold nanoparticleswere detected within the AAD-(PMAA)10 capsules afterexposure to HAuCl4 solution at pH 10 in borate buffer ascan be seen from TEM images of the dried capsules shownin Figure 3. In contrast, the TEM analysis of theEDA-(PMAA)10 capsules treated by the chloroaurate acidsolution revealed the presence of monodisperse nanoparticlesof 10 ( 3 nm in diameter (Figure 4A,B). The absorbanceband at 525 nm from the solution of these capsules isattributed to the gold SPR peak and confirms the synthesisof gold nanoparticles (Figure 4C).

On the basis of these findings, the suggested mechanismof the formation of gold nanoparticles is presented in Scheme2. First, the internal structure of the EDA-(PMAA) hydrogelat pH 10 is shown in Scheme 2. At this pH, the EDA-PMAALbL-derived hydrogel is swollen due to unscreened ionized-COO- groups and the osmotic pressure of ions. (Note thatduring reduction, 0.1 M borate buffer is used.) Under theseconditions of high pH and partial screening of ionic -COO-/-NH3

+ cross-links, both amine and ammonium groups arepresent in the swollen hydrogel as the pKb value of a primaryamine is around 10. The unscreened ammonium groupsbecome available for binding the gold anions. After theunbound gold complex ions rinsed away, the bound onesare slowly reduced under mild conditions of the borate buffersolution into gold atoms.57

In the case of AAD-(PMAA) shells, one-end attachedAAD cross-linker molecules should not carry any positivecharge at pH 10 due to their relatively low pKb value andtherefore do not bind gold complex ions, resulting in no goldnanoparticle synthesis. The presence of deprotonated aminegroups within the EDA-(PMAA) hydrogel shells at pH 10is also important for stabilization of reduced gold due to thehigh affinity of gold to amine groups58,59 (Scheme 2, right).It is also worth noting that the integrity of the hydrogel filmis not affected by the consumption of ammonium or aminegroups during gold ions/atoms binding/stabilizing processesbecause of the covalent cross-links, which provide the overallpH stability of the film. Because of this, no additionalprecaution is needed to maintain the film integrity during

(57) Kharlampieva, E.; Slocik, J. M.; Ko, H.; Tsukruk, T.; Naik, R. R.;Tsukruk, V. V. Chem. Mater. 2008, 20, 5822.

(58) Wangoo, N.; Bhasin, K. K.; Mehta, S. K.; Raman Suri, C. J. ColloidInterface Sci. 2008, 323, 247.

(59) Kim, Y.-J.; Cho, G.; Song, J. H. J. Nanosci. Nanotechnol. 2006, 6,3373.

Figure 5. Top: -potentials of EDA-cross-linked (PMAA)15 (O) and AAD-cross-linked capsules (b) as a function of pH. “]” show -potential valuesfor the EDA-(PMAA)15 capsules containing gold nanoparticles. The pHvalues of aqueous capsule solutions were adjusted using HCl and NaOHsolutions. Bottom: Optical microscopy images of EDA-(PMAA-Au)15

capsules at pH ) 3 (A), pH ) 5 (B), and pH ) 10 (C). The scale bar is 6µm for all images.

Scheme 2. Formation of Gold Nanoparticles within the (PMAA) Multilayer Hydrogels in Borate Buffer at pH 10


such high pH conditions, which is absolutely necessary inthe case of the PAH/PAA or PAH/PSS films.44

pH-Responsive Properties of the Hydrogel Gold-Containing EDA-(PMAA) Capsules. The important fea-ture of the hydrogel EDA-(PMAA) LbL films and capsulesis a reversible response to environmental pH changes dueto carboxylic or amino functionalities.46To investigate thepH-dependent properties of the gold-containing hydrogelcapsules synthesized here (denoted as EDA-(PMAA-Au)), -potentials of the EDA-(PMAA-Au) capsule aque-ous solutions were measured under various pH conditions.

Figure 5 demonstrates that when cross-linked with thedifferent cross-linkers, EDA or AAD, (PMAA)15 capsulesexhibit the pH response in both cases. While both types ofthe hydrogel capsules, EDA-(PMAA)15 and AAD-(PMAA)15 (Figure 5, “O” and “b”, respectively), possess anegative surface charge at pH > 4.5 due to ionization of thecarboxylic groups,47 almost no surface charge is observed

for AAD-(PMAA)15 capsules at pH < 4.5 unlike theEDA-(PMAA)15 capsules. This difference is explained bythe different pKb values for the primary amine (pKb ≈ 10)51

and hydrazide groups (pKb ≈ 3)60 of the two cross-linkers.Because of this, while a positive surface charge at pH lowerthan 4.5 can be supplied by -NH3

+ groups in EDA-(PMAA)15 capsules, AAD-(PMAA)15 capsules along withcovalently two-end attached AAD-molecules, presumably,carry mostly neutral free hydrazide groups from one-endattached AAD molecules. Also, the surface charge behaviorfor the AAD-(PMAA) and EDA-(PMAA) hollow shellsis different at pH > 8. The latter shows an additional decreasein the -potential at pH > 8, which is explained bydeprotonation of the primary amino groups and resultantdissociation of ionic cross-links in this pH range.46,54

Alternatively, the AAD-(PMAA) capsules exhibitedalmost no change in the -potential at pH higher than 8.This observation can be again rationalized through the

Figure 6. TEM images of EDA-(PMAA)15 capsules after 48 h exposure to HAuCl4 solution in borate buffer at pH 5 (A) and pH 3 (D) after silica coreswere dissolved. The size distributions of the reduced gold nanoparticles (parts B and E: for capsules initially incubated at pH 5 and at pH 3, respectively)and UV-visible spectra of hollow capsule solutions (parts C and F: initially incubated at pH 5 and at pH 3, respectively).


large difference in pKb values for the functional groupsof the used cross-linkers; all available (not reacted)hydrazide functionalities should be already deprotonatedin the AAD-(PMAA) shell at pH > 5, and the acquisitionof more negative -potential values is solely due toionization of polyacid matrix. Thus, these unchargedhydrazide groups are not capable of electrostatic bindingthe gold complex ions and gold synthesis withinAAD-(PMAA)15 capsule walls.

After reduction of gold nanoparticles within theEDA-(PMAA)15 capsule walls, one might expect the loss

of pH-responsive properties at acidic conditions due to theconsumption of free amino groups within the hydrogel shellsfor gold stabilization. However, the EDA-(PMAA-Au)15

capsules exhibited the switch of the negative -potential of-22 ( 7 mV to a positive value of 15 ( 5 mV when pHwas changed from pH ) 9 to pH ) 3 (Figure 5, ]). Theoptical microscopy of the EDA-(PMAA-Au)15 capsules indifferent pH solutions revealed pH-dependent size changes

(60) Shafer, D. E.; Toll, B.; Schuman, R. F.; Nelson, B. L.; Mond, J. J.;Lees, A. Vaccine 2000, 18, 1273.

Figure 7. AFM images and corresponding cross sections of the EDA-(PMAA)20 hollow capsules (A) with Au nanoparticles reduced in the borate bufferat pH 5 (B) and at pH 3 (C). The z-scale is 30 nm for all images.


of the composite capsule shells (Figure 5, bottom panel).Overall, the swelling behavior of the EDA-(PMAA-Au)15

capsules follows that of the pristine hydrogel capsules.46

Three distinct swelling regions of EDA-(PMAA-Au)15

capsules are clearly seen in the images. Swelling of thecapsules at pH 3 (Figure 5A) and pH 10 (Figure 5C) withthe respective capsule diameters of 6.2 ( 0.2 and 7.2 ( 0.3µm is explained by the presence of -NH3

+ and -COO-

groups, respectively. At the intermediate pH ) 5, thecapsules are shrunk with the average capsule diameter of4.0 ( 0.3 µm and become almost 2 times smaller than thoseat pH 10 (Figure 5B). These results clearly indicate that theEDA-(PMAA-Au)15 shells preserved their amphoteric pH-responsive behavior after the synthesis of gold nanoparticles.

Effect of pH on Synthesis of Gold Nanoparticles. Weexamined the effect of the reduction pH on gold nanoparticleshape and size distribution. The hydrogel EDA-(PMAA)15

core-shell capsules were exposed to chloroaurate acid in0.1 M buffer solutions at pH ) 5 and pH ) 3 (furtherdenoted as EDA-(PMAA-Au-5)15 and EDA-(PMAA-Au-3)15, respectively). After thorough rinsing with water anddissolution of the silica cores, the dialyzed capsules weresubjected to TEM analysis (Figure 6).

The synthesis of gold nanoparticles under these conditionsis confirmed by UV-visible spectra of the capsule solutions,which show the SPR bands at a wavelength close to 530nm (Figure 6C,F) and from EDX analysis of the capsules(Supporting Information, Figure S1). Moreover, the goldnanoparticles synthesized within the EDA-(PMAA) hydro-gel capsule walls at pH 5 and pH 3 are significantly largerthan those at pH 10. They possess a broad size distributionwith average diameters of 23 ( 13 nm (Figure 6A,B) and25 ( 10 nm (Figure 6D,E), respectively.

We suggest that gold nanoparticle growth at the low pH3 can be explained by the electrostatic binding of theprecursor gold complex ions to the ammonium groups.However, the number of available ammonium groups isdecreased by the presence of salt ions provided with 0.1 Mborate buffer, which is known to screen electrostaticcharges.46 As a result, fewer particles are formed at pH 3(Figure 6D). In contrast, at pH 5, the presence of salt ions(0.1 M borate buffer) results in the EDA-(PMAA) hydrogelexpansion46 due to disruption of -COO-/-NH3

+ ionic cross-links as a result of salt ion competition for binding sites.This suggestion is confirmed by TEM analysis shown inFigure 6A and D. The surface coverage is 3% and 9% forthe gold reduction performed at pH ) 3 and pH ) 5,respectively. It is known that almost complete disruption of-COO-/-NH3

+ ionic cross-links occurs at low salt con-centration of 0.04 M, suggesting the larger number ofammonium groups available for binding gold ions at pH 5as compared to pH 3.46

From the cross-sectional analysis of the higher magnifica-tion AFM images shown in Figure 7, the size of the particlesranges from 19 ( 9 nm for EDA-(PMAA-Au)15 capsuleswith gold synthesized at pH 5 to 26 ( 10 nm for those grownat pH 3 (Figure 7B,C). These results are similar to thoseobtained from TEM analysis of the EDA-(PMAA-Au-pH5)15 and EDA-(PMAA-Au-pH 3)15 capsules. In compari-

son, the root-mean-square roughness of (1 µm × 1 µm) areaof the pristine EDA-(PMAA)15 capsules is 2.0 ( 1.0 nm,which is higher than that for the similar flat films,35 butsimilar to the value of 1.9 ( 0.3 nm reported earlier for(PMAA)10 capsules.47 The larger nanoparticles obtained atpH 5 and pH 3 can be rationalized by the fact that, underthese conditions, there are no deprotonated amine groupsavailable within the hydrogel. Therefore, the producednanoparticles are less stabilized than those at pH 10, whichresults in the observed increase in the particle size due totheir aggregation. Similar trends were found in the case ofthe (PAH/PAA) films used for reduction of gold under UVirradiation.44

Figure 8 demonstrates the representative AFM single-capsule images of the pristine hydrogel EDA-(PMAA)15

capsules (Figure 8A) and those with gold nanoparticles

Figure 8. AFM images (height, left; phase, right) of the EDA-(PMAA)15

hollow capsules (A) with Au nanoparticles reduced in borate buffers at pH10 (B), pH 5 (C), and pH 3 (D). The scan size is 5 µm, and the z-scale is400 nm for all images (height).


synthesized at pH 10, pH 5, and pH 3 (Figure 8B-D,respectively). The pure hydrogel capsules demonstrate theparticular folding behavior upon drying from an aqueoussolution. Because of a very soft wall of the capsule at pH7,53 the capsule collapses uniformly without any wrinklesand with a smooth circular rim. In contrast, when thehydrogel capsule walls are re-enforced with the in situ growngold nanoparticles, the capsule drying morphology changesdramatically. The dried capsules start forming distinctivefolds, indicating changes in the mechanical properties of thecomposite hydrogel shells, which is highly evident from thephase images (Figure 8, right column). Multiple wrinklesare formed in central and peripheral regions of the collapsedcapsules, indicating buckling instability of stiffer capsulewalls with gold nanoparticles.61

Conclusions

In summary, we have demonstrated that LbL multilayeredhydrogel PMAA hollow shells can easily facilitate direct insitu synthesis of gold nanoparticles directly within the capsuleshells. In contrast to the previously reported synthesis of goldnanoparticles in polyelectrolyte multilayers, these hydrogelcapsules do not require any specific pretreatment, and theparticle synthesis can be carried out without any additional

reducing agents in borate buffer solutions at ambient condi-tions. The control over gold nanoparticle size can beperformed via regulating the availability of amine groups.In particular, we show that the well-dispersed gold nano-particles of 10 ( 2 nm can be obtained within the PMAAhydrogel shells at pH 10 when free amine groups are ableto stabilize reduced nanoparticles. Most importantly, the pH-responsive properties of the ultrathin (PMAA) hydrogel shellsare preserved after gold nanoparticles are directly synthesizedwithin the capsule walls in a straightforward and facile way.The dependence of the overall microcapsule stiffness on thesize of the grown gold nanoparticles within the hydrogelshells will be addressed in our forthcoming studies.

Acknowledgment. This work is supported by fundingprovided by NSF-CBET-NIRT 0650705 grant and the AFOSRFA9550-05-1-0209 project. We are also grateful to Nils Krogerand Valeria Milam (Georgia Institute of Technology) forproviding their facilities for zeta-potential measurements andoptical microscopy imaging.

Supporting Information Available: Energy dispersive X-rayspectrographs (EDX) of EDA-cross-linked-(PMAA)15 hydrogelcapsules with gold reduced at pH 3 (A), at pH 5 (B), and at pH 10(C) after cores were dissolved in HF solution and resultant hollowcapsules were dialyzed (PDF). This material is available free ofcharge via the Internet at http://pubs.acs.org.

CM900314P(61) Jiang, C.; Singamaneni, S.; Merrick, E.; Tsukruk, V. V. Nano Lett.

2006, 6, 2254.


ph-responsive layered hydrogel microcapsules as gold

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