Tuning the Shape and Size of Gold Nanoparticles with Triblock Polymer Micelle Structure Transitions...

Published: May 09, 2011

r 2011 American Chemical Society 10442 dx.doi.org/10.1021/jp201712a | J. Phys. Chem. C 2011, 115, 10442–10454

ARTICLE

pubs.acs.org/JPCC

Tuning the Shape and Size of Gold Nanoparticles with TriblockPolymer Micelle Structure Transitions and Environments¥

Poonam Khullar,§ Vijender Singh,#,§ Aabroo Mahal,#,§ Harpreet Kaur,#,^ Vickramjeet Singh,^

Tarlok Singh Banipal,^ Gurinder Kaur,‡ and Mandeep Singh Bakshi*,†

†Department of Chemistry, Wilfrid Laurier University, Science Building, 75 University Avenue West, Waterloo ON N2L 3C5, Canada‡Nanotechnology Research Laboratory, College of North Atlantic, Labrador City, NL A2 V 2K7 Canada§Department of Chemistry, BBK DAV College for Women, Amritsar 143005, Punjab, India^Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India

bS Supporting Information

’ INTRODUCTION

Water-soluble poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) triblock copoly-mers (TBPs) are polymeric surfactants.1 They undergo micelleformation, which is affected by concentration as well as tem-perature.1k The TBPmicelle is made up of a PPO core and a PEOcorona (Scheme 1) and demonstrates several interesting shapetransitions which are closely associated with a number of factorssuch as the molar masses of the PEO or PPO units, salt additives,the nature of the solvent, concentration, and temperature.2

Predominantly hydrophilic TBPs with a greater number of PEOunits than PPO units are also prone to the formation of compoundmicelles,2f�h but predominantly hydrophobic TBPs usually pro-duce well-defined micelles. Micelles undergo several structuretransitions (i.e., micelles f threadlike micelles f vesicles, etc.)with concentration and temperature variations.3 Temperaturebrings marked dehydration to a greater magnitude of PPO than

PEO blocks which in turn significantly alter the environment(polar/nonpolar) of the micelle. Water from the core is replacedby the polymer as unimers incorporate into the micelle, main-taining the core radius remains same but increasing the aggrega-tion number.4 The corona of the TBP micelle is composed ofPEO units arranged in the form of surface cavities (Scheme 1)which are in direct contact with the aqueous phase and constitutethe micelle�solution interface of a TBP micelle. Thus, anychange in the micelle environment brought about by thetemperature causes a significant change in the arrangement aswell as the number of surface cavities. Micelles with a greateraggregation number thus possess a greater number of surfacecavities which act as sites for the site-specific redox reactions

Received: February 21, 2011Revised: April 16, 2011

ABSTRACT: Three block polymers, viz., L31, L64, and P123,were used as reducing agents for the synthesis of gold (Au)nanoparticles (NPs) to determine the effect of their micelle size,structure transitions, and environments on the mechanism ofthe reduction process leading to the overall morphology of AuNPs. Aqueous phase reduction was monitored with time atconstant temperature and under the effect of temperaturevariation from 20 to 70 �C by simultaneous measurement ofUV�visible spectra. The ligand to metal charge transfer(LMCT) band around 300 nm, due to a charge transfer complexformation between themicelle surface cavities and AuCl4

� ions,and Au NP absorbance around 550 nm, due to the surfaceplasmon resonance, were simultaneously measured to understand the mechanism of the reduction process and its dependence onthe micelle structure transitions and environment of TBPs micelles. L64 micelles showed dramatic shift in the LMCT band fromlower to higher wavelength due to an increase in the reduction potential of surface cavities induced by the structure transitions underthe effect of temperature variations. This effect was not observed for micelles of either L31 or P123 and is explained on the basis of adifference in their micelle environments. The morphology of Au NPs thus evolved from the reduction process was studied with thehelp of TEM and SEM studies. Smaller micelle size with few surface cavities, as in L31, produced small NPs in comparison to largemicelles with several surface cavities as in P123. Structure transitions of L64 demonstrated direct influence on the final morphologyof NPs, and stronger transitions produced fused and deformed NPs in comparison to weaker transitions. The results showed thatefficient reduction by the surface cavities and uninterrupted nucleation without structure transitions lead to well-definedmorphologies in the presence of P123 micelles.

10443 dx.doi.org/10.1021/jp201712a |J. Phys. Chem. C 2011, 115, 10442–10454

The Journal of Physical Chemistry C ARTICLE

because of the presence of ether oxygens.5 As one polymermolecule is mainly contributing toward the formation of onesurface cavity in a micelle, it accepts one guest ion (oxidizingagent) per cavity where the host�guest fit is very much related tothe size of the PEOblock (Scheme 1).5 A smaller cavity producedby few PEO units (i.e., a smaller PEO block) is unable to properlyaccommodate a guest molecule. In contrast, a larger cavity (i.e., alarger PEO block) will form a bucket quite large enough toaccommodate a guest molecule. Hence, the reducing ability of aTBP micelle is also very much related to the size of the surfacecavities.6 Several surface cavities present on the surface of amicelle can simultaneously reduce an almost equivalent numberof oxidizing agents if all oxidizing agents can be accommodated inthe cavities. In addition, the micelle environment is also acontributing factor for a proper host�guest fit. Fully hydratedsurface cavities with a low aggregation number at low tempera-ture may not accept as many oxidizing agents as that accepted bypartially hydrated or dehydrated cavities with a higher aggrega-tion number at high temperature.

Once the nucleating centers are created upon reduction ofmetal ions, they undergo nucleation to produce nanoparticles(NPs).5a�d,7 Nucleation can occur between the nucleatingcenters occupying the adjoining surface cavities or throughautocatalytic thermodynamically controlled reduction. In theformer case, again micelle environment, shape, and size of themicellar assemblies as soft templates govern the overall morphol-ogy of NPs.8 Small micelles with few surface cavities cannoteffectively lead the nucleating centers to well-defined morphol-ogies in comparison to large micelles with several surface cavities.Likewise, compact arrangement of surface cavities allows nucle-ating centers to self-nucleate and allows NPs to replicate as theshape and size of the micelle.

Thus, micelle environment, shape, and size of TBP micellesare considered to be the main contributing factors in the overallgrowth kinetics leading to a specific morphology of NPs. Tofurther our understanding of this we have selected three TBPs,viz., L31, L64, and P123 with EO units = 4, 26, and 40,respectively, and a greater number of respective PO than EOunits. A greater number of PO units is essential for the formationof well-defined micelles because it strengthens the hydrophobicenvironment over the hydrophilicity contributed by EO units. Anincreasing number of EO units on the other hand helps usto achieve a greater number of surface cavities which directly

influence the extent of the reduction process. Likewise, increas-ing the size of a TBP micelle (L31 < L64 < P123) helps us tounderstand the soft template effect and its effect on the overallmorphology of NPs. All these issues have been addressed bymonitoring the reduction of gold ions (AuCl4

�) into NPs5a�d,7

under the effect of concentration and temperature variations.

’EXPERIMENTAL SECTION

Materials.Chloroauric acid (HAuCl4), triblock polymers L31(PEO2�PPO16�PEO2), L64 (PEO13�PPO30�PEO13), andP123 (PEO20�PPO70�PEO20) were purchased from Sigma-Alrdich. Double distilled water was used for all preparations.Synthesis of AuNPs.Aqueous mixtures (total 10 mL) of TBP

(2/5/10 mM) and HAuCl4 (0.25/0.5/1.0 mM) were placed inscrew-capped glass bottles. After the components were mixed atroom temperature, the reaction mixtures were kept in a waterthermostat bath (Julabo F25) at precise temperature (below/above cloud point, cp (0.1 �C) for 6 h under static conditions.The color of the solution changed from colorless to pink-purpleor purple within 0.5 h and remained the same thereafter in mostof the cases. After 6 h, the samples were cooled to room temperatureand stored overnight. Theywere purified frompurewater at least twotimes in order to remove unreacted TBP. Purification was performedby collecting the Au NPs at 10 000�12 000 rpm for 5 min afterwashing each time with distilled water.Methods. UV�visible measurements were simultaneously

carried out at various reaction times and temperatures by useof a Shimadzu Model No. 2450 (double beam) instrumentequipped with a TCC 240A thermoelectrically temperature-controlled cell holder that allows measurement of the spectrumat a constant temperature within (1 �C.Transmission electron microscopic (TEM) analysis was per-

formed on a JEOL 2010F at an operating voltage of 200 kV. Thesamples were prepared by mounting a drop of a solution on acarbon-coated Cu grid and allowed to dry in the air. Scanningelectron microscopic (SEM) analysis was carried out on a ZeissNVision 40 Dual Beam FIB/SEM instrument. Photomicro-graphs were obtained in bright field scanning/imaging mode,using a spot size of ∼1 nm and a camera length of 12 cm.The cloud point (cp) of each TBP at a specific concentration is

determined from the variation in the absorbance of 25 μMmethyl orange (MO) in aqueous TBP solution with respect totemperature. A plot of the intensity of MO versus temperature at460 nm gives a sigmoidal curve where the intensity value is lowbefore the cp is reached because of the solubilization of MO inthemicellar phase but increases instantaneously at the cp becauseof the release of MO during the phase separation. This abruptchange in the intensity provides the cp value within(0.5 �C. Thecp values are listed in Table S1, Supporting Information.

’RESULTS

L31�Au NPs. Reactions above the cp. UV�visible scans ofL31þHAuCl4þwater ternary reaction mixtures are shown inFigure 1. Figure 1a illustrates typical UV�visible scans withreaction time at 70 �C. Three peaks are evident (indicated byarrows) at 220, 295, and 550 nm due to AuCl4

� ions,7 the ligandto metal charge transfer band (LMCT),9 and surface plasmonresonance (SPR)10 of Au NPs. Their intensity variation isdemonstrated in Figure 1b. The AuCl4

� ions (220 nm peak)produce maximum intensity with no sign of Au NP absorbance

Scheme 1. A TBP Micelle with the Core Occupied by PPOUnits and the Corona Constituted by PEO Unitsa

aRed dotted circle shows a possible surface cavity whose size is related tothe number of PEO and PPO units. A larger cavity can easily accom-modate a guest molecule in comparison to a smaller cavity.



(550 nm peak) until 60 min of the reaction (see the solid arrow).Then a sudden decrease in the intensity of the 220 nm peak

with a simultaneous increase in the 550 nm peak is observeduntil both merge with each other within 180 min of the reaction

Figure 1. (a) UV�visible scans of L31 (2mM)þHAuCl4 (0.25mM)þwater ternarymixture with time at 70 �C. Black arrows represent three peaks at220 nm, 290 nm, and 550 nm due to AuCl4

� ions, the LMCT complex, and surface plasmon resonance of AuNPs, respectively. (b) Intensity versus timeplots of these peaks. (c) Intensity versus time plots of 550 nm peak of Au NPs in the presence of 2, 5, and 10 mM L31. Inset shows the linear variation of“diffusion time” and “limiting time” versus the concentration of L31. (d) TEMmicrograph of Au NPs of various shapes and sizes prepared for a ternarymixture of L31 (2 mM)þ HAuCl4 (0.25 mM)þ water at 70 �C. Blue arrow shows a thin layer coating around each NP. (e) TEM micrograph of tinyAu NPs entrapped in the fused micellar assemblies prepared with L31 (10 mM)þHAuCl4 (0.25 mM)þ water at 70 �C. (f and g) Similar TEM imagesof tiny NPs entrapped in small micelles or preaggregates prepared with ternary mixtures of 2 and 5mML31, respectively, at 40 �C. (h) UV�visible scansof L31 (10 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture with temperature from 20 to 70 �C. Wavy scan at 70 �C is of a final turbid solution.(i) Plots of intensity at 290 and 550 nm versus temperature for various ternary mixtures with different concentrations of L31. Vertical arrows indicate thecp region. (j) A variation in the intensity of MO at 460 nm versus temperature for ternary mixtures with different concentrations of L31 and without thepresence of HAuCl4. Arrows show the respective cp in each case.



(see a dotted arrow). It happens only if nucleation among the Auatoms is a diffusion-controlled process. Every surface cavity ofL31 micelle is expected to produce approximately one Auatom,5aand the number of surface cavities is actually related tothe aggregation number of the micelle which depends on thehydrophobicity and increases with the number of PPO units.4b

Small micelles with few surface cavities will only be able to triggerthe nucleation when they undergo intermicelle collisions underthe diffusion-controlled process.11 Figure 1c, in fact, demon-strates this mechanism. Increase in the concentration of L31from 2 to 10 mM at constant HAuCl4 = 0.25 mM regularlydecreases the time span of the diffusion process (see the solidarrows at the respective concentrations) in a linear fashion (inset,filled circles). Thus, more micelles at higher L31 concentrationundergo intermicelle collisions and trigger nucleation within ashorter period of time which reaches its limiting value within 180min at 2 mM L31 (see dotted arrow, solid circles curve). Thelimiting value also decreases linearly with the amount of L31(inset, empty circles), whichmeans that themaximumnucleationis achieved within a shorter period of time when the numberdensity of micelles is high at higher L31 concentration. Anotherinteresting feature of this figure is that the curve for 10 mM L31(solid diamonds) starts decreasing soon after the nucleationreaches a maximum value. This is related to a decrease in the SPRof colloidal Au NPs due to their entrapment12 by the micellarphase. It happens only at 10 mM rather than at 2 or 5 mM L31because a greater amount of L31 will produce not only a greaternumber of micelles but also micelles of larger size and shorterintermicelle distances4b which will have a higher probability ofentrapping NPs during intermicelle collisions.Now we focus our attention on the collective morphology of

Au NPs and micelles at different L31 concentrations in the bulk.Figure 1d shows a TEM micrograph of several Au NPs ofdifferent sizes (e25 nm) and shapes produced with 2 mM L31at 70 �C. Each NP is capped with a thin layer of L31 (filled blockarrow). But on the contrary, tiny NPs (3�4 nm) entrapped infused micelles (Figure 1e) are produced when 10 mM L31 isused.7 Presence of large NPs at 2 mM L31 is the outcome of theautocatalytic thermodynamically controlled reduction of AuCl4

�

ions on the surface of already available nucleating centers,13

because 2 mM L31 will produce a relatively far less number ofnucleating centers than 10 mML31 against a constant amount ofHAuCl4 = 0.25 mM. Thus, a smaller number of nucleatingcenters will grow larger in size (Figure 1d) in comparison togreater number of nucleating centers produced by 10 mM L31(Figure 1e). Also, 10 mM L31 will not only facilitate thereduction due to the presence of 5 times more surface cavitiesthan 2 mM L31 but will simultaneously entrap all nucleatingcenters in its micellar phase thereby decreasing the absorbanceafter reaching a limiting value (Figure 1c).Reactions below the cp. Interestingly, when the same reac-

tions are carried out at 40 �C (below cp =52 �C), no clearabsorbance due to SPR of Au NPs was observed in the presenceof L31 = 2, 5, or 10 mM (Figure S1a,b,c, Supporting In-formation) apart from the absorbances at 220 and 290 nm dueto AuCl4

� ions and LMCT, respectively. It shows that thereduction of Au(III) to Au(0) is facilitated at 70 �C (above thecp) rather than at 40 �C (below the cp). TEM micrographsfurther help us to find out the nature of Au NPs in the aqueousbulk. Figure 1f and 1g shows the TEM images of small micelles(or preaggregates) entrapping tiny Au NPs (2�3 nm) synthe-sized with L31 = 2 and 5 mM, respectively, which may lead to

almost insignificant absorbance in both cases. The sizes of suchassemblies (single micelle þ NPs) are 3.3 ( 1.5 nm and 11.7 (7.3 nm for 2 and 5 mM L31, respectively. But when 10 mM ofL31 is used, then apart from the presence of relatively largermicelles, independent, larger NPs of 48( 27 nm are also visible(Figure S1d) which provide weak absorbance around 550 nm(Figure S1c).Effect of Temperature. The above results show that tempera-

ture variation has dramatic effect on the synthesis of Au NPs.Figure 1h represents a typical L31(10mM)þHAuCl4(0.25mM)þwater ternary reaction under a temperature variation of20�70 �C. No absorbance due to Au NPs is observed up to54 �C (dotted line) around 550 nm (as observed previously withtime at 40 �C, i.e., below cp). Thereafter, a weak absorbanceappears which subsequently becomes quite prominent. Thevariation in 290 and 550 nm peaks is depicted in Figure 1i.The intensity of the LMCT peak (at 290 nm) runs through asigmoidal curve over the whole temperature range (solid dia-monds) where the middle part of the curve shows a suddenincrease representing the cp region. Likewise, the intensity of theAu NP absorbance (at 550 nm) starts rising from 56 �C (emptydiamonds), which is essentially in the post-cp region andincreases continuously thereafter. The same situation arises whenthis reaction is conducted with 2 and 5 mM of L31 (see therespective curves in Figure 1i). In the case of 2 mM L31, we donot see the curve for Au NPs because it should even lie at a muchhigher temperature than 70 �C according to the present trend.The cp region at three different concentrations of L31 is exactlyreproduced by the one in the absence of HAuCl4 (Figure 1j; cp isdemonstrated by the variation in the MO absorbance). It meansthat the reduction of Au(III) into Au(0) is completely controlledby themicellar assemblies7 and facilitated at a higher temperatureeven beyond the cp region. Before reaching the cp, only weakabsorbance due to LMCT complex exists (Figure 1i), but as soonas the cp approaches, intermicelle fusions produce micellarclusters which promote the LMCT complex formation and leadto its limiting value. The LMCT complex then converts intonucleating centers which grow into NPs under the effect ofdiffusion-controlled process as observed previously in Figure 1c.As more micelles of 10 mM L31 produce a number of inter-micelle clusters greater than that of 5 mM L31 because of adiffusion-controlled process, the Au NP peak (empty diamondscurve) appears in a relatively lower temperature range than thatof 5 mM L31 (empty circles curve).L64�Au NPs. L64 is a larger block polymer than L31 and

contains much more PEO as well as PPO units (ExperimentalSection). A L64þHAuCl4þwater ternary reaction mixture alsoexhibits absorbances with time (Figure S2a,b, Supporting In-formation) similar to that observed for L31. However, unlike L31(Figure 1c), the formation of Au NPs in the present case startssoon after the beginning of the reaction, leaving the remainder ofthe variation (Figure S2c) similar to that of Figure 1c. However,themost dramatic difference occurs in the variation of the LMCTband (Figure 2a) under a temperature variation of 20�70 �C; theintensity of this band around 290 nm decreases with temperatureand ultimately disappears. Meanwhile, another much weakerband appears at 320 nm, the intensity of which increases from 2to 5 mM L64 (Figure 2b) and eventually supersedes the first onefor 10 mM L64 (Figure 2c). The disappearance of the first andappearance of the second band with a single isosbestic pointsimply demonstrates the formation of a more stable LMCTcomplex at 320 nm because a stronger reduction potential of



Figure 2. (a, b, and c) UV�visible scans of L64þHAuCl4 (0.25 mM)þwater ternary mixture with 2, 5, and 10 mML64, respectively, under the effectof temperature variation from 20 to 70 �C. Black arrows indicate the variation in the intensity of two peaks at 290 and 320 nm with temperature.(d) Partial qualitativemolecular orbital diagram of a complex formation between Au(III) and ether oxygens of PEO groups depicting the possible LMCTtransitions. Inset shows possiblemultisite interactions between the ether oxygens and the Au(III) center in a square planar AuCl4

� complex. “m” numberof sites is greater than “n” and hence the former produces a LMCT band at 320 nm in comparison to the latter at 290 nm. (e) Plots of intensity versustemperature for ternary mixtures depicted in a, b, and c for 2, 5, and 10mML64, respectively. Filled symbols show the intensity variation in the respective290 nm peak while empty symbols show the variation of 320 nm peak. Broken arrows indicate the isosbestic point where the 290 nm peak vanishes andthe 320 nmpeak emerges. Inset shows a plot of variation in the isosbestic point with the amount of L64. (f) Plots of variation in the intensity at 550 nm forthe ternary mixtures with different concentrations of L64. Shaded block arrow indicates the cp region. (g) A variation in the intensity of MO at 460 nmversus temperature for ternary mixtures with different concentrations of L64 and without the presence of HAuCl4. (h) Schematic representation of atransition state LMCT complex for a redox reaction between Au(III) and Au(0) whose activation energy decreases with the increase in the amount ofL64 from 2 to 10 mM.



PEO surface cavities will shift the LMCT band to a higherwavelength (or lower energy). This all happens because the L64micelle undergoes three different relaxation processes whensubjected to temperature variations.4a,14 In the first process,L64 unimers are rapidly introduced into the micelle, resultingin an increase in the micelle radius while leaving the radius of thecore almost the same because water from the core is simulta-neously removed by the PPO units.4b Thus, an increase in themicelle size is essentially attributed to the PEO units in thecorona. This process produces a thermodynamically unstablemicelle and is followed by the restructuring of the micelle (as thesecond relaxation process) where fresh PEO units tend togenerate fresh surface cavities in order to produce a thermo-dynamically stable micelle. Thus, an unstablemicelle produces anunstable LMCT complex which slowly vanishes as a stablemicelle emerges with a a stable LMCT complex. In other words,restructuring induces more surface cavities in the corona whichthen have a stronger reduction potential than previously andhence generate a more stable LMCT complex with a freshabsorbance at 320 nm. Figure 2a�c fully supports this mecha-nismbecause 2mML64will obviously introduce a smaller numberof unimers into the micelle during restructuring in comparison to10 mM L64, and that is why the relative magnitude of theintensity of the 320 nm peak is much smaller for the former thanthe latter. A red shift in the wavelength of the LMCT band canfurther be explained on the basis of the molecular orbital diagramshown in Figure 2d. Accommodation of a square planar AuCl4

�

ion in a PEO crown cavity triggers the electron transitions frompredominantly ligand 3eu(π) to metal 3b1 g(σ*). As the micellebecomes thermodynamically stable after the second relaxationsunder the temperature effect, the energy difference betweenpredominantly ligand and metal molecular orbitals decreases,which results in the LMCT band shift to lower energy, i.e.,320 nm. Though the exact geometry is unknown at this point,electron donation from oxygen lone pair to metal d-orbitals canoccur above and/or below the plane of the molecule and isexpected to be carried out by several sites simultaneously in thepolyether crown cavity.6 Thus, the stability of the LMCT com-plex is very much related to the number of electron-donatingsites. For instance, a greater number of sites (PEO�PPO�PEO)m than (PEO�PPO�PEO)n will shift the LMCT bandfrom 290 to 320 nm (inset, Figure 2d).It is important to determine how variation in the LMCT

intensity affects the synthesis of AuNPs because formation of theLMCT complex is the rate-determining step in the reductionprocess. Figure 2e illustrates the variation in the intensity of theLMCT band versus temperature at different concentrations ofL64. Filled symbols of each curve belong to the intensity of thefirst band while empty symbols belong to the second band. Theminimum in each curve (indicated by an arrow) refers to theisosbestic point (the first relaxation process) where the first banddisappears and the second one appears. The isosbestic pointshifts regularly to a lower temperature as the amount of L64increases from 2 to 10 mM L64 (inset). This is in accordancewith the well-defined micelle formation process that shifts to alower temperature with an increase in the concentration.1k In10 mM L64, immediately after the isosbestic point, the curvepasses through a strong maximum close to 45 �C, suggesting asubstantial increase in the LMCT complex formation due to asecond relaxation process caused by an increase in the aggrega-tion number.4a,b A larger aggregation number means the pre-sence of more surface cavities, and hence more AuCl4

� ions can

undergo the LMCT formation. Note that the second relaxationprocess fades away with 5 mML64 and vanishes in 2 mML64. In2 mM, the isosbestic point is the combination of both first andthird relaxation processes4a,bbecause it not only indicates theshift in the LMCT band due to the first relaxation process butalso leads to clustering of the micelles to achieve the cp.Figure 2f illustrates the variation of Au NP absorbance with

temperature which is always maximum for 10 mM and minimumfor 2 mM L64 over the whole temperature range even thoughturbidity is observed in the cp region and turbidity does notovershadow the absorbance of Au NPs. The temperature rangebetween 55 and 60 �C belongs to the cp region which is very wellreproduced as in the absence of HAuCl4 (Figure 2g). Between 30and 55 �C, no change in the Au NPs absorbance takes place for10 mM L64 (Figure 2f) contrary to the LMCT band (Figure 2e)which means that no growth of NPs is observed during thesecond relaxation process. Growth in an NP only happens in twoways, either due to self-nucleation among the nucleating centersor due to an autocatalytic process. In both cases, micelles have tobe in their thermodynamically stable state because nucleatingcenters are actually produced in the surface cavities. The max-imum in the 10 mM L64 curve (Figure 2e) indicates that themicelles acquire thermodynamical stability around 45 �C, butgrowth (Figure 2f) starts only with the clustering of the micelleswhich begins around 55 �C before the onset of the cp. Hence,neither self-nucleation nor the autocatalytic process contributestoward growth before the cp is reached; otherwise, we wouldhave not seen the flat portion between 30 and 35 �C for the10 mM L64 curve in Figure 2f. Thus, growth is carried out onlyduring the clustering of the micelles by Ostwald ripening15 thatreleases the colloidal Au NPs in the bulk due to phase separation,resulting in instantaneous absorbance increase.16 Also, a contin-uous weak increase in Au NP absorbance from 20 to 55 �C in2 mM L64 indicates the absence of any relaxation process withinthis temperature range while an instant growth thereafter is againdue to the clustering of the micelles before cp. Thus, despite thepresence of dramatic structure transitions in themicelles of 10mML64, they produce maximum growth in Au NPs because growth issimply related to the number of nucleating centers produced. That,in turn, relates to the greater number of surface cavities, whichgenerate an LMCT complex of greater stability and lower energyof activation in comparison to that of 2 or 5 mM L64 (Figure 2h).SEM and TEM analyses further help us to understand how

such micellar transitions ultimately influence the shape andstructure of Au NPs. Figure 3a shows a large compound micelleof 5 mM L64 loaded with Au NPs which protrude from thesurface of the micelle in different polyhedral morphologies alongwith some platelike shapes (Figure 3b). Figure 3c shows thecorresponding TEM image of free NPs where one can clearly seethe deformed shapes. However, if the amount of L64 is increasedto 10 mM, such morphologies become less clear from the SEManalysis where one can see mainly large compound micelles(Figure S3a, Supporting Information). But after a proper pur-ification of the sample with aqueous ethanol, NPs can beextracted and their TEM image is shown in Figure 3d. Most ofthe NPs exist in the form of small fused groups (Figure S3b).On the contrary, when the amount of L64 is decreased to 2 mM,fine spherical micellar assemblies are seen (Figure 3e) and mostof the NPs exist in a chainlike arrangement (Figure 3f). Puttingall information from various images together, it reveals thedramatic effect of increasing the amount of L64 on the overallmorphology of NPs. Clearly, structural transitions in the micelles



of 10 mM L64 cause significant Ostwald ripening15 among thenucleating centers, resulting in the generation of interparticle fusedmorphologies (Figure 3d). As the magnitude of structure transi-tions is much weaker in 5 mM than 10 mM L64, only deformedmorphologies appear (Figure 3c). Similarly, no structure transi-tions are observed in 2 mM L64 before the cp is reached; instead,distinct, roughly spherical morphologies are formed (Figure 3f).Thus, TEM images fully supplement the conclusions drawn fromthe UV�visible studies (Figure 2) and indicate the fact that thegrowth was mainly triggered by the nucleation among thenucleating centers and affected by the structure transitions.

P123�Au NPs. The shape and size of NPs were examinedwhen the polymer size was increased from L64 to P123. P123 isan even larger polymer than L64 and L31, with a greater numberof PEO units (Experimental Section) that are highly efficient ininitiating instant reduction of Au(III) into Au(0).5a�d Figure 4aillustrates typical UV�visible scans of a P123þHAuCl4þwaterreaction with time at 70 �C. Similar scans are obtained when thereaction is conducted at 40 �C (Figure S4a, Supporting In-formation). Interestingly, the peaks at 220 nm (of AuCl4

� ions)and 320 nm (of LMCT complex) vanish within 1 h of thereaction (Figure 4b) and result in a simultaneous increase in the

Figure 3. (a) SEM image of a compound micelle loaded with growing NPs for a sample prepared with L64 (5 mM) þ HAuCl4 (0.25 mM) þ waterternary mixture at 70 �C. (b) A high resolution image showing NPs of different shapes. (c) TEM image of the same sample showing deformed NPs.(d) TEM image of a sample prepared with 10mML64 under the same conditions. Groups of fusedNPs are evident. (e) TEM image of a sample preparedwith 2mML64 under the same conditions, showing several spherical micelles with a few scatteredNPs. (f) Chainlike arrangement ofNPs of sample frompanel e with roughly spherical morphologies.



550 nm peak due to the formation of Au NPs. This was not thecase with either L31 (Figure 1b) or L64 (Figure S2c) wherereactions took clearly more than 1 h. In addition, note the LMCTpeak position at 320 nm instead of 290 nm as observed previouslyin the case of L31 and L64. It means that there is no issue ofLMCT stability due to micelle transitions, and hence no iso-sbestic point is observed (Figure S4b) under the effect oftemperature variation. But the LMCT complex goes through adramatic variation from 20 to 70 �C (Figure 4c). The variation ofAu NPs peak at 550 nm is shown in Figure 4d. Between 20 and30 �C in both cases (Figure 4c and 4d, indicated by solidsarrows), well-defined micelle formation begins from preaggre-gates. It is then followed by the intermicelle fusion within30�50 �C which ultimately leads to the cp indicated by thedotted arrow in each case and supplemented by Figure 4e.Contrary to the behavior of L64 (Figure 2d,e), the intensity ofthe LMCT band essentially remains constant between 30 and50 �C (Figure 4c), indicating no structure transitions, while that

of Au NPs shows a rapid increase with more pronounced effectsin 10 mM P123 (Figure 4d). It means that a stable LMCTcomplex is formed at 320 nm as soon as the stable micelleformation begins at 30 �C. It simultaneously converts intonucleating centers and is facilitated by the increase in tempera-ture. Such a rapid reduction goes to completion even before thecp is reached (especially for 5 and 10 mM P123) where intensitybecomes constant around 40 �C, i.e., far below the cp region(Figure 4d). This was again not the case with either L31(Figure 1i) or L64 (Figure 2e) where intensity still rises evenbeyond the cp. Thus, a greater number of PEO units with greatersurface cavities participates in the rapid reduction only because ofthe presence of stable LMCT complex. It is therefore interestingto see that how a complete reduction and stable LMCT complexinfluences the overall morphology of Au NPs.TEM and SEM images of some of the samples prepared at 40

and 70 �C are shown in Figure 5. Figure 5a shows a combinedimage of compound micelles and large Au NPs (78 ( 24 nm,

Figure 4. (a) UV�visible scans of P123 (2 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture with time at 70 �C. Dotted arrows represent threepeaks at 220 nm, 320 nm, and 550 nm due to AuCl4

� ions, the LMCT complex, and surface plasmon resonance of Au NPs, respectively. (b) Intensityversus time plots of these peaks. (c and d) Intensity versus temperature plots of 320 and 550 nm peaks, respectively, for different ternary mixtures with 2,5, and 10 mM P123. Dotted arrows in both figures indicate the respective cp. (e) Intensity variation of methyl orange at 460 nm versus temperature forternary mixtures with different concentrations of P123 and without the presence of HAuCl4. Dotted arrows indicate the respective cp.



Figure S5a, Supporting Information, for a size distribution histo-gram) of various shapes of a sample prepared with 2 mM P123and 0.25 mM gold chloride at 40 �C. Some of the NPs appear tobe closely related to the overall shape and size of the P123micelles. A close inspection of different compound micelles(dotted circle in Figure 5a) actually reveals that such assembliesare indeed soft templates where large NPs grow. Solid arrowsindicate the growing NPs (with relatively dark contrast) oversuch templates. A similar situation can be seen for the samesample prepared at 70 �C (Figure 5b) where fused large com-pound micelles are present along with the NPs (89 ( 27 nm,Figure S5b). Both samples provide almost similar shape and sizeof NPs though they have been conducted at much differenttemperatures (i.e., 40 and 70 �C). This shows that the reductionis completed below the cp and enough surface cavities areavailable to complete the reduction even at 40 �C. The SEMimage (Figure 5c) of the sample prepared at 70 �C further helpsto more clearly understand the morphology of NPs where severalNPs with icosahedral geometry occur along with a few nano-plates. However, an increase in the amount of P123 from 2 to5 mM produces similar morphologies at 40 �C (Figure 5d) and70 �C (Figure 5e) of comparable sizes, i.e., 76 ( 24 nm (FigureS5c) and 78 ( 11 nm (Figure S5d), respectively, but the SEM

image (Figure 5f) of the latter suggests that more NPs of trun-cated icosahedral geometry are present along with a few nano-plates. A further increase in the amount of P123 from 5 to 10mMproduces interesting micellar assemblies (Figure 5g) apart fromsimilar NPs (Figure S6a,b, Supporting Information). Figure 5hshows a close up image of such a compound micelle whichcontains several growing NPs of different morphologies(indicated by white arrows) that eventually end up in the formof independent NPs shown in Figure 5i. A close scrutiny ofroughly spherical NPs of Figure 5i suggests that they are in factsomewhat snub icosidodecahedron. Thus, an increase in theamount of P123 from 2 to 10 mM while keeping the amount ofgold chloride constant (i.e., 0.25 mM) leads to a systematicelimination of vertices of icosahedral geometry (Figure 5c) togenerate more smooth and roughly spherical NPs (Figure 5i).This is all related to the better capping ability of 10 mM ratherthan 2 mM P123 because the greater amount of P123 will cap allfcc lattice planes more or less equally to generate roughlyspherical NPs rather than a selective capping by 2 mM. Con-ventional ionic surfactants are highly selective in capping lowenergy {100} or {110} planes due to their highly surface-activenature and are known for the formation of anisotropic shapes.17

This is not so common in the case of polymeric surfactants such

Figure 5. (a and b) TEM images of NPs for a sample prepared with P123 (2 mM) þ HAuCl4 (0.25 mM) þ water ternary mixture at 40 and 70 �C,respectively. Low contrast morphologies are the micelles or soft templates of micelles. (c) SEM image of sample in panel b showing icosahedral NPs.(d and e) TEM images of the NPs prepared with 5 mM P123 at 40 and 70 �C, respectively. (f) SEM image of sample in panel e showing truncatedicosahedral NPs. (g and h) Low and high resolution images of 10 mM P123 micelles loaded with growing NPs of different shapes and sizes. (i) SEMimage of roughly spherical NPs.



as P123 where no distinct charge separation between the polarand nonpolar parts can be made. However, Kim et al.18 success-fully prepared micrometer sized triangular shaped nanoplates at70 �Cby using a solid mixture of P123 and gold salt. Addition of asmall amount of water produced long nanowires instead ofnanoplates. They explained the formation of such morphologieson the basis of soft template effects of P123. Our studies arelimited to the solution phase where a predominantly micellarphase exists rather than a liquid crystalline semisolid phase.Therefore, the present soft template effect (Figure 5h) is onlylimited to the shape and size of P123 micelles and thus theirinfluence on the morphology of NPs (Figure 5i).

’DISCUSSION

All presently discussed TBPs (i.e., L31, L64, and P123)demonstrate quite unique reduction behaviors with relativelyfew similarities. The reduction is, of course, entirely carried outby the surface cavities (Scheme 1) but the micelle shape, size, andenvironments dramatically influence the reduction process andeventually the shape and size of NPs. It is diffusion controlled inL31when small micelles with aminimumnumber of PEOunits = 4are employed (Figure 1c, Scheme 2A) but instant and completein P123 when large micelles with a maximum number of PEOunits = 40 are used (Figure 3d, Scheme 2C). L64 with PEOunits = 26 demonstrates a quite complicated reduction (Figure 2,Scheme 2B). This is all related to the micelle environment interms of PPO/PEO ratio which seems to be the most importantfactor governing the reduction. The PPO/PEO ratio is 4 and 3.5for L31 and P123, respectively, but close to unity (i.e., 1.15) forL64. This makes the L64 micelle more prone to structuraltransitions under the effect of temperature variations due to amarked difference in the hydration capacity of PPO and PEOblocks.19 This difference is relatively much less for a high PPO/PEO ratio (as in the case of L31 and P123) because the PPOblock retains less water than PEO but much more water as thePPO/PEO ratio approaches unity (as in the case of L64) or lessthan unity. PPO blocks loose water more rapidly than PEO andacquire a complete hydrophobic environment which drives moreunimers in the micellar phase under the effect of strong hydro-phobic interactions. This induces structure transitions in themicelle, as both core and corona are affected in terms ofaggregation number and size. On the other hand, a high PPO/PEO ratio dehydrates both blocks with almost equal rate andhence minimizing the hydrophobic effects. Thus, a greaterhydration disparity between PPO and PEO blocks of the L64micelle makes it more prone to structure transitions in compar-ison to the micelles of L31 and P123. We have further evaluatedthis inference by choosing L61 (PEO2�PPO30�PEO2) with aPPO block identical to that of L64. Unlike L64, L61 is not water-soluble because of a much larger proportion of PPO relative toPEO block. However, it can be made water-soluble by incorpor-ating an anionic surfactant such as sodium perfluorooctanoate(SPO) which is expected to formmixed micelles with L61 due topredominant hydrophobic interactions, and its anionic head-group will induce the required hydration for the solubilization ofL61 in the aqueous phase. A L61/SPOmole ratio = 2 gives a clearwater-soluble solution at room temperature and has been used asa model system similar to that of L64. Figure S7a, SupportingInformation shows its reduction behavior where a temperatureincrease from 10 to 20 �C clearly induces the much weakerstructure transitions around 300 nm in comparison to that of L64

due to the LMCT complexes which diminish with furtherincrease in the temperature. The TEM image (Figure S7b) ofthis sample shows the presence of only fused Au NPs of∼20 nm.Thus, it is the relative hydration index of TBP micelles which,under the effect of temperature variation, controls the nucleationprocess and we have already seen in Figure 3 for L64. Asreduction is site specific and carried out by the micelle�solutioninterfacial arrangement of surface cavities, therefore an appro-priate surface arrangement is also a predominant factor towardthe formation of a stable and low energy transition state in theform of a LMCT complex. The stability of LMCT complex isvery much affected when the micelle goes through structuretransitions (Figure 3, Scheme 2B) and ultimately affects theoverall morphology of NPs (Figure 4). In contrast, whenstructure transitions are minimum, a stable LMCT complexproceeds uninterrupted to produce well-defined morphologies(Figure 5, Scheme 2C).

In addition, the stability of the LMCT complex also dependson the best fit model of the crown cavity (Scheme 1). L31 withonly four PEO units is expected to produce an elliposoidal cavitywith two axes of 2.5 and 5 Å.6 A much smaller cavity accom-modates an AuCl4

� ion of 9.92 Å in comparison to almost a tentimes larger cavity of P123 with 40 PEO units (strictly based ongeometric factors). Thus, the L31 cavity may not hold andproperly accommodate an AuCl4

� ion to provide the requiredstability for the LMCT complex in order to accomplish aneffective reduction. That is why it is diffusion controlled inL31. Once a nucleating center is created, it all depends on thedimensions of the cavity to control its growth. A large cavity of alarge micelle like that of P123 not only retains the nucleatingcenter but allows it to grow simultaneously while providing a softtemplate effect (Figure 5a). A large micelle accommodates manynucleating centers, and dehydration brings such centers closeenough to self-nucleate with each other to supplement the shapeand size of the soft template (Figure 5b). Further growth due tothe autocatalytic process (Scheme 2C) on the surface of growingNPs leads to large morphologies with greater probability to existindependently in the colloidal state (Figure 5b,c). The situationis somewhat different with L64micelles where dehydration alongwith the micelle transitions (Scheme 2B) brings surface cavitiesclose enough to each other but the soft-template effect does notwork, as micelles undergo continuous structural transitions(Figure 2). It facilitates random interparticle fusions and pro-duced disordered morphologies which are rather more prevalentat 10 mM than at 2 mM L64 (Figure 3).

Thus, several inter-related factors such as micelle size, numberof surface cavities, and micelle environment ultimately define thefinal morphologies of NPs. The overall size of the NPs is clearlyrelated to the size of the surface cavity (Scheme 2D) in acomparable concentration range in the order of ∼2 nm (L31)<∼20 nm (L64) <∼80 nm (P123) though the shape of the NPsis mainly related to the magnitude of structure transitions.Stronger structural transitions during the reduction lead todisordered shapes (Figure 3) without any soft template effectsin comparison tominimum structure transitions (Figure 5). Also,a complete reduction with a greater number of surface cavitiesallows kinetically produced nucleating centers to undergo ther-modynamically controlled autocatalytic growth which is the keyfactor to attain well-defined morphologies (Figure 5). Fewersurface cavities lead not only to the incomplete reduction but alsoleave the growth on a competing path between kinetically andthermodynamically controlled reductions. Low temperature



facilitates the latter over the former (Figure S1d) and henceproduces ordered morphologies, but high temperature helps theopposite and hence tiny NPs without clear morphologies areproduced (Figure 1e). The reduction carried out by the severalsurface cavities always allows first the kinetically controlled growthuntil the completion of the reaction and then is followed by thethermodynamically controlled growth to produce well-definedmorphologies at low as well as high temperatures (Figure 5).

’CONCLUSIONS

All results pertaining to the synthesis of AuNPs by using L31,L64, and P123 conclude that the overall reduction processbecomes quite complicated when it experiences simultaneousstructural transitions in the micelle. The reduction is entirelycontrolled by the micellar assemblies (i.e., preaggregates,micelles, and their clusters) and especially the surface cavitieslining the micelle�solution interface. Whenever a structure

Scheme 2. (A, B, and C) Schematic Representation of the Proposed Mechanism for the Synthesis of Au NPs by Using Micelles ofL31, L64, and P123, Respectively (see details in the text). (D) Plot of the Average Size of NPs Estimated from TEM Images versusCavity Size for L31, L64, and P123a

aCavity size was determined from the number of ether oxygens constituting the crown cavity by extrapolating the cavity size from data of ref 6.Extrapolation has been performed by plotting the average of “x” plus “y” parameters of the ellipsoidal cavity.



transition in the micelle alters the delicate interfacial arrange-ment of surface cavities, it affects the overall mechanism of thereduction process going through the LMCT complex. A stableLMCT complex with minimum interference by structuretransitions generates ordered morphologies when the numberof surface cavities with sufficient size is high to accommodatemaximum number of AuCl4

� ions. A smaller micelle with fewsurface cavities cannot produce a stable LMCT complex, andhence the nucleation depends on the extent of intermicellecollisions due to diffusion.

Structural transitions are significantly influenced by themicellar environment in terms of PPO/PEO ratio. A high ratiostrengthens the hydrophobic core of the micelle which is themain driving force for the micelle formation. Thus, a strongerhydrophobic core limits the extent of structure transitionsunder the effect of temperature change. On the contrary, aPPO/PEO ratio close to or less than unity significantly affectsthe micelle environment under the effect of temperaturevariations and hence induces significant structure transitions.Structure transitions not only affect the stability of LMCTcomplex but also alter the arrangement of surface cavities whichin turn triggers the Ostwald ripening among the nucleatingcenters, and hence disordered morphologies are produced.Hence, in order to get well-defined morphologies, the followingfactors have to be taken in consideration. First, the choice ofTBP should be of high number of PEO units so that instantreduction can be achieved. Second, TBP should have a highvalue of PPO/PEO ratio so that the micelles should experienceminimum structure transitions under the effect of tempera-ture variations. Third, nucleating centers should grow underthermodynamically controlled process to produce orderedmorphologies.

’ASSOCIATED CONTENT

bS Supporting Information. UV�visible spectra, size dis-tribution histograms, and TEM images. This material is availablefree of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

Author Contributions#Authors contributed equally in this work.

’ACKNOWLEDGMENT

These studies were partially supported by financial assistancefromCSIR [ref no. 01(2219)/09/EMR-II] and [ref no. 01(2102)/07/EMR-II] New Delhi.

DEDICATION¥Dedicated to Prof S. P. Moulik on the occasion of his 75thBirthday (INSA Hon. Scientist, Centre for Surface Science,Department of Chemistry, Jadavpur University, Kokata, India).

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