self-assembly change by gold nanoparticle growth

Published: October 10, 2011

r 2011 American Chemical Society 22301 dx.doi.org/10.1021/jp2085523 | J. Phys. Chem. C 2011, 115, 22301–22308

ARTICLE

pubs.acs.org/JPCC

Self-Assembly Change by Gold Nanoparticle GrowthSungsook Ahn,† Sung Yong Jung,‡ and Sang Joon Lee*,†,‡,§

†Center for Biofluid and Biomimic Research, ‡Department of Mechanical Engineering, and §Division of Integrative Biosciences andBiotechnology, Pohang University of Science and Technology, Pohang 790-784, Korea

bS Supporting Information

A cell membrane is a self-assembly system composed ofamphiphilic lipid bilayers with a hydrophilic charged outer

layer and a hydrophobic lipidic inner layer, including embeddedproteins.1 Recently, nanoparticle�cell interactions have becomeone of the key issues in the areas of drug delivery,2 cancertreatment,3 and imaging agent transport,4 etc. The disruption ofmembrane structure by nanoparticle introduction is one of themain reasons by which cells lose their destined functions,cytotoxicity.5 On the other hand, nanoparticle-introduced or-ganic/inorganic hybrid materials are useful in the applicationssuch as light-emitter/absorber, photovoltaics, nonlinear-optics,sensors, and energy harvest/storage.6�8 Nevertheless, moststudies have focused on the nanoparticle control usually in theform of dried thin films or polymer melts.9�12

Block copolymers have been reported as excellent templatesfor nanoparticle formation,13�15 usually emphasizing the sizesand the spatial arrangements of the nanoparticles in the poly-meric composites.9,10 Mixed with a solvent, a block copolymerself-assembles into unique structures dominated by the degree ofrepulsion, length and selectivity of the blocks, and the environ-mental conditions such as pH, ion, temperature, and polarity ofthe solvent, etc.16�21 By applying external forces such as electricfields, the curvature and the direction of copolymer films can becontrolled.22,23 However, the changes in the self-assembledstructures of polymeric templates affected by the nanoparticleincorporations are seldom emphasized, especially in the form ofaqueous solutions/hydrogels.

The dynamics of polymer chains are significantly influencedby the strength of the polymer chain�particle interactions,morphology, particle dispersion, and interparticle distances.24,25

Nonetheless, nanoparticle effects on the physical properties ofpolymers are still controversial and mainly focused on the size of

the nanoparticles;6�8 ceramic materials with cluster sizes lessthan 15 nm are reported to change from brittle to ductile,reflecting a typical viscosity increase of a Brownian particlesuspension where the viscosity is a function of the particlevolume fraction and the viscosity of the suspending liquid.26,27

However, a viscosity decrease is observed with a 0.35 nm silicatecluster blended in linear polymers, suggesting an unusual effectexclusively occurring in a nanoscale process.28,29

Poly(ethylene oxide-block-propylene oxide-block-ethylene oxide)triblock copolymer [Pluronic (EO)x(PO)y(EO)x] was reported toreduce gold ions selectively into gold nanoparticles (AuNPs)especially dominated by ethylene oxide (EO) segment.30�35 Goldnanoparticle formation in the Pluronics has been studied by Sakaiet al.29�35 In this study, series of Pluronics having various EO andPOunits are employed to reduce chloroauric acid solution into goldnanoparticle (AuNP), and the changes in the self-assembledstructures at an isothermal condition (20 �C) are observed usingsmall-angle X-ray scattering (SAXS) systematically. It is suggestedthat, in addition to the size, the compatibility and the location of theformed nanoparticles in the self-assembled template are importantto determine the physical properties of the nanoparticle-incorpo-rated hybrid system. The formed AuNPs in this study range fromseveral nanometers to hundreds of nanometers. Using SAXS, theAuNP-incorporated aqueous solutions/hydrogels designed in thisstudy can be observed in situ without drying into thin films oradding additional dye chemicals that might distort the structuralanalysis. The Pluronics L61, L62, F68, L92, and P104 (BASFKorea,Seoul, Korea) employed in this study are diversified according to the

Received: September 5, 2011Revised: October 7, 2011

ABSTRACT: Inorganic nanoparticles in the self-assembled organic/biologicaltemplates are of great interest in nanoparticle-mediated therapy, biosafety, andhybrid functional material design for photonic devices. The gold nanoparticles(AuNPs) grown in situ in the aqueous solutions/hydrogels of amphiphilicpolymers significantly influence the radius of gyration (Rg), correlation length(ζ), fractal dimension (α), and structure-dependent specific sizes. As a result ofselective wetting by gold ion aqueous solution, the grown AuNPs are expected tolocate in the hydrophilic domain of the self-assembly structures, which leads to theprominent changes in the size and the structures accordingly. The increase in thesurfactant number (Ns) of the self-assembled template is suggested to decreasethe effective surface area (As) (i.e., decrease in the mean curvature) on whichsmaller AuNPs are preferably formed. There is a strong correlation between theself-assembled template structure and the formed nanoparticles.

22302 dx.doi.org/10.1021/jp2085523 |J. Phys. Chem. C 2011, 115, 22301–22308

The Journal of Physical Chemistry C ARTICLE

number of EO and PO unit, molecular weight, and hydrophi-lic�lipophilic balance (HLB) (Supporting Information, Table S1).L61, L62, and F68 have similar number of PO units of 30 butdifferent number of EO units, which are expected to be critical toAuNP formation. L92 and P104 have similar EO/POwith L62, butthe absolute number of the unit is different. Gold chloride(III)trihydrate (HAuCl4 3 3H2O) is dissolved in deionized (DI) Milli-Qwater at 1.0 � 10�3 mol/L. Each Pluronic was mixed withchloroauric acid stock solution at the designed volume concentra-tion (cp) from 0.1 to 0.9, followed by centrifuging up-and-downseveral times (3000 rpm) and stabilizing at 20 �C for a week for anequilibration. The structural changes observed by SAXS at 20 �Caresummarized (Table S1). According to the increase in the hydro-phobicity of a system (and cp), the structural changes follow a typicalsequence; the structures are identified as micellar solution (L1) fmicellar cubic (I1) f hexagonal (H1) f bicontinuous cubic (V1)f lamellar (Lα) structures followed by the inverted structures frombicontinuous cubic (V2)f hexagonal (H2)fmicellar cubic (I2)fmicellar solution (L2). Between the specific structures, phase-sepa-rated state (2ϕ) is designated as observed.

From the SAXS profiles, the changes in the main peak positionq* are detected to determine the structure and the size(Supporting Information). In addition, at dilute condition withisotropic samples, the relation 1 is applied regardless of theparticle shape:36

IðqÞ ¼ Ið0Þ exp½ � q2Rg2=3� ð1Þ

where the radius of gyration (Rg) is obtained at small q region(q< 1/Rg). The SAXS profile is produced by theOrnstein�Zernike(OZ) type scattering function:

IðqÞ ¼ Ið0Þ=½1 þ q2ζ2� ð2Þwhere ζ is the thermal correlation length for the fluctuation and I(0)is the forward scattering intensity. The ζ is obtained at low qcondition (qζ < 1). ζ reflects the specific sizes corresponding to thedistances between the separated particles/polymer globules orjunction points in the interconnected networks. According to therelation expressed in eq 2, q2 versus 1/I(q) are plotted in eachsystem at low q condition (q2 < 0.02 nm�1, therefore q < 1/Rg is

satisfied), where the slope indicates the square of the correlationlength (ζ) of the Pluronic structures with and without AuNPs. Onthe other hand, for the length scaleswhere the qζ>1 is satisfied, I(q)obeys the power law:36

IðqÞ≈q�α ð3ÞThe power law expresses the density of the objects as a fractaldimension. The fractal dimension α is evaluated by the averageslope of the graph at high q region (q > 0.5 nm�1 or q > q*). Theobserved structures and their size of eachPluronicswith andwithoutAuNPs are summarized in Table 1. Detailed SAXS profiles for eachPluronics are displayed in the Supporting Information (Figure S2).

The representative SAXS profiles of L61 and L92 are shown inFigure 1A. Both L61 and L92 display the structural changes fromthe normal phase L1 to the inverted phase L2 (Table S1). Forhydrophobic L61 (HLB = 3), the normal oil-in-water structure(L1) formation is relatively weak (the peaks are not so sharp).Nonetheless, the characteristic size indicated by q* becomessmaller, the same, and larger by AuNP incorporation at normalphase L1, planar Lα, and inverted phase L2, respectively. How-ever, the ζ, Rg, and α become higher by AuNP incorporation inall of the cases (Table 1). In Scheme 1, the size change for normal(O/W) phase, planar, and inverted (W/O) phase is suggested.By AuNP incorporation, the normal phase might shrink due tothe AuNPs encircling the outer layer, while the inverted phaseincreases the size due to AuNP incorporation and furtherswelling of the hydrophilic inner side of the structure. None-theless, the planar structure might have a negligible effect byAuNP incorporation.

As compared to L61, L92 (HLB = 5.5) exhibits more diversestructural changes between L1 and L2 (Table S1). L92 generatesa slightly large size (q*) at normal H1 and Lα by AuNPincorporation. Yet, the change is more significant at the invertedphase; the inverted hexagonal (H2) becomes lamellar (Lα)structure. The AuNP incorporation to L92 changes the systemmore hydrophilic indicated by the structural modification to thehydrophilic direction (H2 f Lα). This also supports that theAuNPs grown in the self-assembled structures are located in thehydrophilic domain, under which the inverted phases can be

Table 1. Size Changes in the Self-Assembled Structures by AuNP Incorporation: Radius of Gyration (Rg), Correlation Length (ζ),and Fractal Dimension (α) Evaluation

without AuNP with AuNP

ϕ Rg ζ α q*-based size a ϕ Rg ζ α q*-based size a ΔRg Δζ Δα Δq*-based size

L61 L1 19.5 0.20 1.0 2.13 L1 23.4 0.63 1.2 2.04 3.9 0.43 0.2 �0.09

Lα N/A 0.05 2.3 10.44 Lα N/A 0.55 2.5 10.44 N/A 0.5 0.2 0

L2 21.2 0.04 2.1 1.16 L2 21.9 0.40 3.4 1.37 0.7 0.36 1.3 +0.21

L62 L1 23 0.20 0.2 N/A L1 24.2 1.07 1.3 N/A 1.2 0.87 1.1 N/A

Lα N/A 0.17 6.1 7.55 Lα N/A 1.02 5.4 7.93 N/A 0.8 �0.5 +0.38

F68 L1 25.7 0.33 1.2 N/A L1 25.7 1.03 0.1 N/A 0 0.7 �1.1 N/A

H1 N/A 0.10 3.2 11.52 H1 N/A 0.26 4.6 11.70 N/A 0.16 1.4 +0.18

L92 H1 N/A 0.10 0.8 12.73 H1 N/A 0.71 0.6 12.96 N/A 0.61 �0.2 +0.23

Lα N/A 0.12 1.6 10.10 Lα N/A 0.89 1.9 10.27 N/A 0.77 0.3 +0.17

H2 N/A 0.20 0.1 12.51 Lα N/A 0.88 2.5 9.35 N/A 0.78 2.4 N/A

P104 I1 23.7 0.37 0.9 1.89 I1 24.2 1.04 2.1 2.00 0.5 0.67 1.2 +0.11

H1 N/A 0.41 1.0 8.53 H1 N/A 1.16 2.7 8.85 N/A 0.75 1.7 +0.32

Lα N/A 0.39 2.5 10.10 Lα N/A 1.15 3.1 10.26 N/A 0.76 0.6 +0.16a Sizes of 1/q* for L and I, 4π/

√3q* for H, and 2π/q* for Lα are applied, respectively.



more significantly affected by the AuNP incorporation ratherthan normal phases as illustrated in Scheme 1. At L1, α becomessmaller by AuNP incorporation, but it becomes larger at Lα andinverted H2. ζ becomes higher by AuNP incorporation in all ofthe cases.

For L62 (HLB = 7) having increased hydrophilicity ascompared to that L61, the lamellar structure becomes prominentwith increased cp, where the repeat distance becomes larger by

AuNP incorporation. The ζ and Rg of L62 increase by AuNPincorporation, but α decreases at Lα. More hydrophilic F68(HLB = 29) forms a structure at low cp, and the size of thestructure (q*) becomes slightly larger by AuNP incorporation.The α decreases at L1, but the ζ becomes higher by AuNPincorporation in all conditions. P104 (HLB = 13) also shows anincreased size by AuNP incorporation without changes in thestructures detected by the q*. The α and the ζ become higher by

Figure 1. (A) Representative SAXS profiles of Pluronics L61 and L92 with and without AuNP incorporation at the selected structures. (B) The changesin the sizes by AuNP incorporation. Radius of gyration (Rg), correlation length (ζ), and fractal dimension (α) evaluation by %, [Δ size by AuNPincorporation/size without AuNP] � 100 (%). Structural changes are observed according to the Pluronic concentration (cp) by SAXS at 20 �C.



AuNP incorporation. Overall, some normal phases and unstablelamellar (Lα) structure display the decreases in the density of thesystem (α). However, the radius of gyration (Rg) and thecorrelation length (ζ) become higher by AuNP growth in mostof the cases.

Scheme 2 suggests the changes in the self-assembled struc-tures generated by amphiphilic force balance based on thedimensionless surfactant number (Ns):

17

Ns ¼ v=aolc ð4Þwhere v and lc are the volume and the length of the hydrophobicportion of the amphiphilic molecule, and ao is the effective areaper headgroup. Kinetic processes of the block copolymer solu-tions involve a combination of the fast intramicellar processgenerating interfacial curvature in an isolated micelle and theslow intermicellar process for micelle fusion or fission.16,17 Goldions introduced in the Pluronic aqueous solutions are expected toaffect both kinetic procedures leading to a characteristic AuNP-incorporated Pluronic system. The detailed time-dependentdynamic processes are out the scope of this study, and only thefully equilibrated final states are concerned. The resulting self-assembled Pluronic structures are affected by the AuNP growthdepending on the changes in the modified hydrophilic�lipophi-lic force balance. Under the conditions that the formed AuNPsare concentrated at the hydrophilic EO domain of the self-assembly, the hydrophilic portion is expected to increase as aresult of the AuNP formation, which would mainly change the aoin the relation 4.

In Figure 1B, the size changes by AuNP incorporation areevaluated according to the surfactant number (Ns). The changesin Rg and ζ for large-scale size, α for small-scale size, as well as thecharacteristic size detected by the main peak position q* areexpressed in %, [Δ size by AuNP incorporation/size withoutAuNP]� 100 (%).TheΔ size (q*) is decreasing (P104, LHB=13),almost similar (L92, HLB = 5.5), or increasing (L61, HLB = 3) untilNs = 1. Therefore, at least at the normal phase, there is no specificstructure that AuNP incorporation preferablymodifies. Nonetheless,it seems that the formed AuNPs more effectively increase the size ofthe lowNs structure formedby hydrophilic amphiphiles (highHLB),

while they increase the high Ns structure formed by hydrophobicamphiphiles (low HLB).

The structure-independent Rg is determined only at diluteisotropic condition; thus limited data are obtained. Nonetheless,the Rg increases by AuNP incorporation in all of the detectedcases. From L61 two data points are generated where theinverted phase shows far lower ΔRg than the normal phase. Ata fixed Ns (=0.33), the ΔRg of the lower HLB Pluronics is largerthan that of the higher. On the other hand, Δζ becomes slightlyhigher until Ns = 1, and then decreases at Ns > 1 overall. Δα forelongated structure (Ns = 0.5) is slightly higher than that ofisotropic structure (Ns = 0.33). Also, it becomes lower at theplanar structure (Ns = 1) and increases again at Ns > 1 overall.The formed AuNPs can generate network structure due tomultiple anchoring sites emanating from a single AuNP. Thismight lead to an effective increase in the density of AuNP-incorporated Pluronic solutions/hydrogels overall, in addition tothe addition of metal elements of high density. An interestingpoint is that for the density change of the solutions/hydrogels,the location of the incorporated AuNPs in the self-assembly isimportant. Figure 1B exhibits that most of the Δ sizes in eachsystem exhibit consistent tendency according to the Ns. Thechanges in the physical properties of the nanoparticle-incorpo-rated hybrid systems are strongly dependent on the structures ofthe templates represented by the Ns.

One of the blocks in the copolymer is selectively wetted by themetals (or metal ion solutions), which can result in the metalnanoparticles located in the corresponding self-assemblydomains.37,38 Nonetheless, the nanoparticles of designed sizesand shapes molded by the self-assembled polymeric structuresare hardly observed; even with highly elongated self-assembled

Scheme 1. Size Changes in the Self-Assembly by GoldNanoparticle (AuNP) Introduction in Normal (O/W),Planar, and Inverted (W/O) Phases

Scheme 2. Self-Assembled Structures According to theSurfactant Number (Ns)

a

aThe AuNP-incorporated structures are suggested considering theAuNPs grown in the hydrophilic domain of the self-assembledstructures. Effective surface area (As) is suggested for each struc-ture where gold ions grow into AuNPs. On a fixed projected area of2R � 2R, As is diversified depending on the self-assembledstructures and the morphological men curvature. With the in-crease in Ns, the As becomes smaller, thus generating smallersize AuNPs.



structures of the polymers, elongated metal nanoparticles arehardly formed due to a strong metal�metal interaction, whichusually overcomes the metal�polymer interaction. Some studieshave been done to control the gold nanoparticle shape by changingthe metal�polymer interactions.35,39�41 Gold ions grow intospherically shaped AuNPs to minimize the surface tension, even

though the geometry of the template structures is important todetermine the size of the resulting AuNPs.

A projected area of 2R � 2R is suggested as illustrated inScheme 1, on which the maximum surface area (As) of diversemean curvature is formed.With a sphere of radiusR placed on theprojected area, the As becomes 2πR2. With a cylinder of radius R,

Figure 2. (A) Representative TEM images of L92 at L1, H1, Lα, and H2 structures. The scale bar is 0.2 μm. (B) Observed absorption wavelength (left)and size of the formed AuNPs (right) at different Ns condition. The line on the right graph indicates the error range. (C) UV�vis spectrum and thepictures of the AuNP-incorporated Pluronic solutions. (D) Summarized UV�vis absorption spectra shown in Figure 1A.



the As leads to 2πR2. With a flexible bilayer, it is between 2πR2

and 4R2. With a lamellar, As results in 4R2. The As of an inverted

phase becomes smaller than that of the lamellar (Lα), unlessthere is a swelling of the hydrophilic domain. Thus, for aninverted sphere, it turns to be 2πR02, where R0 < R. The meancurvature becomes smaller with the increase in Ns, leading to adecrease in As.

Typical transmission electron microscopy (TEM) images ofthe AuNPs formed by L92 at L1, H1, Lα, and H2 structures aredisplayed in Figure 2A. The size of the AuNPs decreases alongwith the increase in the cp corresponding to the Ns of normal(water-in-oil) (Ns < 1), lamellar (Ns = 1), and inverted (oil-in-water) (Ns > 1) structure. The relation between the AuNPgrowth and the As is suggested in Scheme 2. Under the Ns e 1condition, the AuNPs become smaller along with Ns, where theAs and themorphological mean curvature become smaller. At theNs > 1 condition, the AuNP size can be conceptually smaller, butthe observed AuNPs are larger than that of Lα. This can be causedfrom effective swelling of the hydrophilic domain by the AuNPintroduction as suggested in Scheme 2.

The observed size of the formed AuNPs and the UV�visresults are strongly related based on the structures expressed byNs. In Figure 2B, each structure expressed by the surfactantnumber (Ns) is plotted against the absorption wavelengthobserved by UV�vis spectroscopy (left) and the average sizeof the AuNPs observed by TEM (right). Because of highhydrophobicity, the normal oil-in-water phase of L61 is notstable (Supporting Information, Figure S1). Except L61, theincrease in the Ns leads to the decrease in the wavelength of theabsorption as well as the size of AuNPs until Ns = 1, and theyincrease at Ns > 1. This result supports the aforementionedrelation on the self-assembled structure�AuNP formation sum-marized in Scheme 1.

The originally colorless Au-incorporated Pluronics changetheir unique colors as a result of the equilibration for a week(Supporting Information, Figure S1). The physical propertiesof the grown AuNPs in the self-assembled structures are

investigated in terms of the surface Plasmon resonances inconjunction with the physical sizes. Figure 2C shows typicalsurface plasmon resonances and the solution pictures of theAuNP-incorporated Pluronics represented by F68 and P104from cp = 0.1�0.9 (v/v). The maximum and minimum wave-length of the surface plasmons and their differences from cp = 0.1to 0.9 for each system are summarized in Figure 2D. Theintensity of the surface Plasmon shows either an increase (v) ora decrease (V) from cp = 0.1 to 0.9. The detailed information foreach system is available in Supporting Information, Figure 2S.

The physical properties of the formed AuNPs are character-ized by UV�vis spectroscopy where the surface plasmon absorp-tion is detected. As summarized in Figure 2D, the surfaceplasmon of the hydrophobic L61 (HLB = 3, Table S1) slightlychanges from 665 to 675 nm with the increased intensity fromcp = 0.1 to cp = 0.9 (v), indicating an increase in the size and theconcentration of the AuNPs by that order. At cp = 0.1, L61 doesnot show any detectable peak reflecting no AuNP formation.Meanwhile, at cp = 0.2, L61 shows bimodal peaks at 665 and880 nm, indicating mixed AuNP formation with different phy-sical property. Relatively homogeneous AuNP is formed only athigher cp possibly because of proper EO domain formation. Thehydrophobic L61 hardly generates discrete self-assembled struc-ture at low cp, but as the cp becomes higher the reverse phaseforms prominent structures where a hydrophilic core can containformed AuNPs displaying a narrow surface plasmon difference(Δ 10 nm). Similar to L61, lower concentration cp = 0.1 of L62shows bimodal peaks at 560 and 630 nm due to heterogeneousAuNP formation. Nonetheless, with increased hydrophilicity,AuNP formation of L62 is more effective than that of L61 due toeffective EO domain formation. As cp increases, L62 forms aprominent lamellar structure (Lα) after the normal solution (L1).From cp = 0.1 to cp = 0.9, the AuNP-incorporated L62 solutions/hydrogels show distinguished colors from red via green to purple,and UV�vis spectroscopy results display the wavelength changefrom 560 to 525 nm (Δ 35 nm) with increased intensity (v).Highly hydrophilic F68 shows a more prominent color change

Scheme 3. Radius of Gyration (Rg) Increase (A) and Thermal Correlation Length (ζ) Increase by (B) Gold NanoparticleIncorporations



from a red to a purple. The surface plasmon changes from 550 to535 nm as cp increases from 0.1 to 0.9. Different from otherPluronics employed in this study, the intensity of the main AuNPsurface plasmon peak decreases (V) according to the cp, while anew surface plasmon band develops in the near-infrared region.From the peak decrease (V), it is suggested that the more EOunits (80 � 2 in F68) does not guarantee proliferate AuNPformation. L92 shows the surface plasmon change from 560 to535 nm, and the observed solution colors turn from a red to agreen from cp = 0.1 to cp = 0.6. However, from cp = 0.7 to cp = 0.9,the purple color turns back to a red, and the wavelength becomeslonger from 535 to 550 nm. L92 exhibits diverse structuralchanges from normal to reverse when the surface plasmonresonance of incorporated AuNPs becomes shorter and thenlonger along with cp, where the turning point is the lamellar (Lα).P104 displays the most diverse structural changes in the normalphase and changes its solution color from red to purple mono-tonously as cp increases. P104 exhibits the surface plasmonchanges from 590 to 550 nm where the observed difference(Δ 40 nm) is the maximum.

Overall, the AuNPs grown and embedded in the Pluronic self-assembly emit the lights at the corresponding wavelength of theabsorbed lights (i.e., energy conservation); the red color solu-tions more effectively absorb longer wavelength light, whilepurple solutions absorb shorter wavelength. Nonetheless, thephysical properties of the grown AuNPs strongly depend on thesurfactant number (Ns), thus the self-assembled structures.Normal and inverted structures generate the turning point ofthe surface plasmon for the formed AuNPs (observed by L92).The most diverse structural changes within the normal O/Wphase exhibit the most broad absorption wavelength of thesurface plasmon of the formed AuNPs (observed by P104).The structure of the self-assembled polymer is determinant forthe physical properties of the grown AuNPs than are themolecular weight, HLB, EO/PO ratio, or absolute number ofEO units of the amphphile template.

In conclusion, cooperative interactions between the incorpo-rated nanoparticle and self-assembled templates are investigated.The nanoparticle-introduced aqueous solution/hydrogel of am-phiphilic polymer exhibits systematic changes in the size and themorphology of the self-assembled structures as well as the sizeand surface plasmon resonance of the incorporated AuNPs.Grown from the aqueous gold ion solution, the formed AuNPscontribute to the hydrophilicity of the aqueous polymer solu-tions/hydrogels possibly due to selective wetting of the hydro-philic block of the amphiphilic copolymer. Effective surface area(As) for AuNP formation in the organic�inorganic hybrid self-assembly structures is evaluated according to the surfactantnumber (Ns); at the normal O/W phases, the size of the AuNPsbecomes smaller as Ns increases because the effective As and themean morphological curvature decrease, while the size of AuNPbecome larger again at the inverted (water-in-oil) phases due toeffective swelling of the hydrophilic domain. Thermodynamicallyequilibrated AuNP growth increases the radius of gyration (Rg)and correlation length (ζ), as illustrated in Scheme 3. In addition,the increase in the density (α) of the solution/hydrogel state ofthe self-assembled system is especially effective in the reverseW/O phase possibly because the AuNP introduction increasesthe volume of the hydrophilic discontinuous domain. In additionto the size of the nanoparticle, the template structure and thelocation of the formed AuNPs are significantly important for thephysical properties of the nanoparticle-incorporated hybrid

system. The formula of the polymer has been considered asone of the important factors to control the AuNP formationbecause the reduction rate of gold ions also affects the finalparticle size and shape.30�35 We find in this study that thedifferent polymer formula is important because it also decides theself-assembly structure. The results obtained in this study wouldcontribute to the basic understanding of the nanoparticle-incor-porated hybrid system and would be broadly valuable to thebiomedical applications as well as organic�inorganic hybridfunctional material developments.

’ASSOCIATED CONTENT

bS Supporting Information. Additional table and figures,and experimental procedures. This material is available free ofcharge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel.: +82-54-279-2169. Fax: +82-54-279-3199. E-mail: [email protected].

’ACKNOWLEDGMENT

This work was supported by the Creative Research Initiatives(Diagnosis of Biofluid Flow Phenomena and Biomimic Re-search) of the Ministry of Education, Science, and Technology(MEST) and the National Science Foundation (NSF) of Korea.This research was jointly supported by the World Class Uni-versity program funded by the Ministry of Education, Science,and Technology (MEST) (R31-2008-000-10105-0). We aregrateful for the valuable help with the small-angle X-ray scatteringexperiments performed at the 4C1 beamlines of the PohangAccelerator Laboratory (PAL) (Pohang, Korea).

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self-assembly change by gold nanoparticle growth

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