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1 Polymer 51 (2010) 4814e4822 Contents lists ville t ScienceDirect Polymer journl homepge: Jnus gold nnoprticle with icomprtment polymer rushes templted y polymer single crystls Binging Wng, Bin Dong, Bing Li, Bin Zho, Christopher Y. Li, Deprtment of Mterils Science nd Engineering, Drexel University, Phildelphi, PA 19104, USA Deprtment of Chemistry, University of Tennessee, Knoxville, TN 37996, USA rticle info strct Article history: Received 2 July 2010 Received in revised form 3 August 2010 Accepted 8 August 2010 Aville online 14 August 2010 Keywords: Jnus nnoprticle Polymer single crystl Polymer rush Jnus prticles hve ttrcted incresing ttention from the communities of mterils science, chemistry, physics nd iology. While lrge size Jnus prticles re redily chieved, synthesizing Jnus nnoprticles (JNP) with dimeters smller thn w20 nm remins chllenging tsk. In this rticle, we report systemtic study on growing polymer rushes on polymer-single-crystl-immoilized 6 nd 15 nm dimeter gold nnoprticles (AuNPs) using tom trnsfer rdicl polymeriztion. JNPs with icomprtment polymer rushes, such s poly(ethylene oxide) (PEO)/poly(methyl methcrylte), PEO/poly(tertutyl crylte), nd PEO/poly(crylic cid), were synthesized. The grfting densities cn e crefully controlled. The Jnus feture of these prticles ws confirmed using oth pltinum nnoprticle decortion nd UV/Vis spectroscopy nlysis. The surfce plsmon resonnce sornce of Jnus prticles exhiited lue shift compred with tht of symmetric AuNPs with either homopolymer or mixed polymer rushes. This work demonstrted tht using polymer single crystl s the templtes, smll size (<20 nm dimeter) JNPs hving icomprtment polymer rushes cn e redily otined. The ility to tune grfting density nd moleculr weight of polymer rushes cn led to controlled prticle mphiphilicity. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Jnus prticles re ojects in micro or nnometer scle possessing noncentrosymmetric structure, often with single core tht is surrounded y comprtmentlized coron [1,2]. Since De Gennes first coined the nme in 1991 [3], Jnus prticles hve ttrcted incresing ttention from the communities of mterils science, chemistry, physics nd iology [4]. Numerous Jnus prticles, with symmetric geometry nd/or surfce chemistry, hve een reported. Exmples include dendrimers [5,6], lock copolymer micelles [7e9], micrometer-size eds [10e17], nnofiers [18], nd polymer microgels [19]. As unique type of hyrid mterils, Jnus prticles with uniform metllic core, such s gold [20], hve shown potentil pplictions in the fields of photonics, opticl iosensors, drug delivery, electronics, nd Pickering emulsion [2,21e31]. Reported experimentl methods to fricte Jnus prticles include templtedirected self-ssemly [32,33], controlled phse seprtion [34e36], controlled surfce nucletion [37e39], polymer slt quenching [40], nd top-selective surfce modifiction [41e43]. Corresponding uthor. Tel.: þ ; fx: þ E-mil ddress: chrisli@drexel.edu (C.Y. Li). Among ll these pproches, top-selective surfce modifiction is the most populr one. During this process, prticles re plced t the interfce of two phses. One side of the prticle is protected from modifiction, while the opposite side is modified chemiclly. As result, Jnus prticles re otined with different types of functionl groups. Liquidegs [44], liquideliquid [19,45e47], nd liquidesolid [17,48e51] interfces hve een investigted. While Jnus prticles re redily chieved, synthesizing Jnus nnoprticles (JNP) with dimeters smller thn w20 nm remins chllenging tsk. There re only few exmples of smll size JNPs in the literture. These include striped AuNPs formed y curvtureinduced phse seprtion of smll molr mss lignds [52] nd symmetriclly functionlized nnoprticles synthesized using micro-mgnetic prticles, polystyrene (PS) eds, or silnized glss s the templtes [52e54]. JNPs with phse seprted polymer rushes were reported in thiol-ended PS (PSeSH)/thiol-ended poly(2-vinylpyridine) (P2VPeSH)/gold nnoprticle (AuNP) system. AuNPs grfted with PSeSH nd P2VPeSH with different molr rtios were lended into lmelle-forming PS--P2VP lock copolymer. The loction of the PS/P2VP-coted nnoprticles ws found to e function of the PS volume frction: they re locted in the PS domin, PS/P2VP interfce, nd P2VP domin when F PS > 0.9, 0.1 < F PS < 0.9 nd F PS < 0.1, respectively. The wide F PS window of /$ e see front mtter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.polymer

2 B. Wng et l. / Polymer 51 (2010) 4814e which the AuNPs were locted t the PS/P2VP interfce ws ttriuted to the formtion of Jnus AuNPs with PS/P2VP rushes [53]. Using silic microsphere s the msk, Htton et l. synthesized Jnus mgnetic nnoprticles (Fe 3 O 4 ) with poly(crylic cid) (PAA)/ sodium polystyrene sulfonte nd PAA/poly(N,N-dimethylminoethyl methcrylte) rushes. Temperture responsive ssemly ehvior ws oserved [54]. Polymer single crystls cn lso e used s solid sustrtes for JNP synthesis [55e57]. Polymer single crystls re often considered s qusi-two-dimensionl lmelle, which re formed y folding polymer chins ck nd forth; the chins re perpendiculr or olique to the lmellr surfce [58e61]. Interction etween this unique mteril nd nnoprticles hs recently een explored [59,62,63]. As the chin ends re different from the rest of the polymer ckone, given the right crystlliztion condition, they re excluded onto the surfce of the lmellr crystls. Judiciously selected nnoprticles cn then e ound onto the single crystl surfce vi chemisorptions [59,64,65]. If we consider polymer lmell s thin sheet of pper, lmell with functionl groups on the surfce cn then e regrded s nnoscle sticky tpe, which cn immoilize different nnoprticles [59,66,67]. Recently, it hs een demonstrted tht pre-formed AuNPs cn e immoilized onto the surfce of thiol-ended poly(ethylene oxide) (PEOeSH) single crystls vi coupling with the polymer chin ends (grftingto) [55]. In this wy, polymer single crystls re used s the solid msk for JNP synthesis. The opposite side of the nnoprticles cn e further functionlized with different functionl groups [57]. In this rticle, we report systemtic study on growing polymer rushes with different moleculr weights nd hydrophoicity on polymer-single-crystl-immoilized AuNPs using surfce-initited tom trnsfer rdicl polymeriztion (ATRP) [68,69]. JNPs with icomprtment polymer rushes nd different prticle sizes were synthesized. The grfting densities of ech type of rushes were crefully controlled. Symmetric AuNPs with homo nd mixed polymer rushes were lso synthesized nd their surfce plsmon resonnce (SPR) sorption ws compred with tht of the JNP. This work demonstrtes tht using polymer single crystls s the templtes, JNPs with icomprtment polymer rushes cn e redily synthesized. 2. Experimentl 2.1. Mterils PEOeSH (M n w 5 K g mol 1 ) ws purchsed from Polymer Source Inc. Methyl methcrylte (MMA, 99%), tert-utyl crylte (tba, 98%þ), nd toluene (98%þ) were purchsed from Aldrich nd redistilled under vcuum. Gold (III) chloride (99%), tetrutylmmonium orohydride (TBAB, 98%), didodecyldimethylmmonium romide (DDAB, 98%), trifluorocetic cid (99.0%þ), 11-mercpto-1-undecnol (97%), 2-romoisoutyryl romide (98%), pyridine (nhydrous, 99.8%), tri(2-minoethyl)mine (98.0%þ), formic cid (w98%), formldehyde solution (37 wt% in H 2 O), triutylphosphine, chloropltinic cid hydrte (99.9%), hydrzine (nhydrous, 98%), copper (I) romide (98%), potssium hydroxide (85%þ), nd luminum oxide (ctivted, neutrl, for column chromtogrphy, 50e200 mm) were purchsed from Aldrich nd used s received. Sodium hydroxide ws otined from Fisher nd used s received. AuNPs (6 nm in dimeter) were synthesized ccording to literture [70]. Me 6 TREN were synthesized s reported [57]. All other chemicl regents including cetone, diethyl ether, chloroform, toluene, nd methnol were purchsed from either Aldrich or Fisher nd were used without further purifiction Synthesis of Jnus AuNPs Jnus AuNPs re denoted s A m -Au X -B n. A nd B stnd for the lignd/polymer grfted on AuNPs; m nd n (if used) stnd for the degree of polymeriztion of the corresponding polymer rushes. X (if used) denotes the AuNP dimeter (in nm) Synthesis of inititor e nd PEO 114 -modified Jnus AuNPs (PEO 114 -Au-I) Growth of PEO 114 single crystls. PEO/pentyl cette solution (320 g, 0.06 wt%) ws heted to 60 C for 10 min. The solution ws cooled to 10 C nd kept t this temperture overnight. The solution ws then heted to 45 C for 10 min to form crystl seeds. The finl crystlliztion ws conducted t 25 C for 24 h. Free PEO ws removed y centrifugtion. The morphology of PEO single crystls ws studied using TEM nd AFM. Jnus AuNPs (6 nm in dimeter) modified with inititor nd PEO (PEO 114 -Au 6 -I). A solution of AuNPs in toluene (133 g, 0.2 wt %) ws dded dropwise into the suspension of PEO 114 single crystls in pentyl cette (665 g, 0.03 wt%) under mgnetic stirring. The solution ws stirred t room temperture for 24 h. Multiple centrifugtions were performed to remove free AuNPs. The AuNP-decorted PEO single crystls were re-dispersed in 200 g pentyl cette. A solution of inititor 11-mercptoundecyl 2-romo-2-methylpropionte 1 (the structure is shown in Scheme 1) in pentyl cette ws dded nd the finl concentrtion of 1 in the single crystl suspension ws kept t 1 wt%. The mixture ws stirred t room temperture for 24 h. Multiple centrifugtions were pplied to remove the free inititor. Complete removl of the free inititor ws confirmed y 1 H NMR spectroscopy nlysis Jnus AuNPs (15 nm in dimeter) modified with inititor 1 nd PEO (PEO 114 -Au 15 -I). Inititor 1 e modified symmetric AuNPs with the dimeter of 15 nm (I-Au 15 ) (75 mg) ws dispersed in 10 ml chloroform. The solution ws slowly dded into the PEO 114 single crystl suspension in pentyl cette (50 g, 0.2 wt%). After 3 h, the single crystls were seprted from the solution y centrifugtion. Additionl 75 mg I-Au 15 in chloroform ws dded nd the rection ws conducted for 3 h. The immoiliztion/centrifugtion process ws repeted until the free solution fter centrifugtion showed red color, which indicted the sturtion of AuNPs on the surfce of single crystls Synthesis of PMMA- nd PEO-modified Jnus AuNPs with the dimeter of 6 nm (PEO 114 -Au 6 -PMMA) CuBr (12.0 mg, mmol), PEO 114 -Au 6 -I (single crystl form) in 20 g pentyl cette (0.1 wt%), inititor 1 (30 mg, mmol) in 1 ml toluene [71], Me 6 TREN (19.0 mg, mmol) in 1 ml toluene, nd MMA (2.040 g, mmol) in 2 ml toluene were dded into Schlenk tue. Three cycles of freeze-pump-thw were performed to degs the rection mixture. The polymeriztion ws conducted t room temperture. After 24 h, the polymeriztion ws stopped y opening the tue to ir. The rection mixture ws centrifuged nd the isolted PEO single crystls with the immoilized AuNPs were wshed three times with mixed solvent of pentyl cette/ toluene (5/1, v/v). The product ws dissolved/dispersed in chloroform nd the mixture ws then precipitted in methnol to remove free PEO. Frctionted precipittion ws conducted to remove free PMMA formed from the scrificil inititor using cetone nd methnol s solvent nd non-solvent, respectively. The finl precipitte ws collected nd dried in vcuum t 35 C for 24 h. PEO 114 -Au 6 -PMMA ws chrcterized using 1 H NMR, GPC, TGA nd TEM.

3 4816 B. Wng et l. / Polymer 51 (2010) 4814e4822 PEO single AuNP crystl c d SH SH Inititor f Inititor: HS O O PMMA or PtBA PMMA or PtBA 1 Br e PEO Scheme 1. Schemtic illustrtion of polymer-functionlized Jnus AuNPs y comining grfting-to nd grfting-from methods. () PEOeSH, () PEOeSH single crystls with HS groups on the Surfce, (c) AuNPs immoilized on PEOeSH single crystls, (d) Inititors coted on the top surfce of the immoilized AuNPs, (e) Growth of polymer rushes on polymer-single-crystl-immoilized AuNPs, nd (f) Dissolution of the polymer single crystls leding to the formtion of Jnus AuNPs with icomprtment polymer rushes Synthesis of poly(tert-utyl crylte) (PtBA)- nd PAAmodified Jnus AuNPs with the dimeter of 15 nm (PEO 114 -Au 15 -PtBA) CuBr (13 mg, mmol), inititor 1 (70 mg, 0.20 mmol) in 1 ml toluene, PEO 114 -Au 15 -I in 20 g pentyl cette (0.1 wt%), Me 6 TREN (22.3 mg, mmol) in 1 ml toluene, nd tert-utyl crylte (2.38 g, 18.6 mmol) in 2 ml toluene were dded into Schlenk tue. The mixture ws degssed y three freeze-pump-thw cycles, nd the polymeriztion ws crried out t room temperture. After 24 h, the rection ws stopped y opening the tue to ir. The PEO single crystls with the immoilized AuNPs nd PtBA rushes were isolted y centrifugtion nd wshed three times with mixed solvent of pentyl cette/toluene (5/1, v/v) to remove the free PtBA nd lignd. Acetone ws used to dissolve the mixture of Jnus AuNPs nd free PEO. Diethyl ether ws slowly dded into the solution until the formtion of drk-red precipitte. Jnus AuNPs modified with PEO nd PtBA precipitted first ecuse of the highly concentrted polymers on the surfce of AuNPs. After seprting the precipitte from the solution, it ws collected nd redissolved in cetone. This process ws performed t lest seven times to remove the free polymer from the Jnus AuNPs modified with PEO nd PtBA. The finl product ws dried under vcuum t 35 C for 24 h. Jnus PEO 114 -Au 15 -PAA ws formed y hydrolysis of PEO 114 -Au 15 - PtBA. Pltinum nnoprticles (PtNPs) were synthesized in situ on PAA ushes. These two processes were conducted following the procedure reported previously [57] Synthesis of symmetriclly functionlized AuNPs Symmetriclly functionlized AuNPs re denoted s A m -Au X nd A m /B n -Au X. A nd B stnd for the lignd/polymer grfted on AuNPs; m nd n (if used) stnd for the degree of polymeriztion of the corresponding polymer rushes. X (if used) denotes the AuNP dimeter (in nm). A m -Au X indictes tht the prticle is grfted with single type of lignd/polymer rush, wheres A m /B n -Au X stnds for mixed-lignd/rush-modified AuNPs Synthesis of inititor-modified symmetric AuNPs (I-Au) Inititor 1 (330 mg,1.05 mmol) in 2 ml toluene ws slowly dded into 150 g AuNP solution in toluene (w6 nm in dimeter, 0.2 wt%). The mixture ws stirred t room temperture overnight under nitrogen. 300 ml methnol ws slowly dded into the rection mixture until the ppernce of precipittion. The mixture ws seprted y centrifugtion. The precipitte ws wshed with methnol three times in order to remove the free inititor. The resultnt inititor-modified AuNPs with the dimeter of 6 nm (I-Au 6 ) were collected nd dried t room temperture under vcuum for 24 h. Inititor-modified AuNPs with the dimeter of 15 nm (I-Au 15 ) were synthesized using the sme procedure with the molr rtio etween gold tom nd inititor to e 1. 1 H NMR spectroscopy nd TGA were used to chrcterize the inititor-modified AuNPs Synthesis of poly(methyl methcrylte)-modified symmetric AuNPs (PMMA-Au 6 ) CuBr (8.00 mg, mmol), I-Au 6 (36.0 mg) in 2 ml toluene, Me 6 TREN (13 mg, mmol) in 2 ml toluene, inititor 1 (20 mg, mmol) in 2 ml toluene, MMA (1.362 g, mmol) in 4 ml toluene were dded in Schlenk tue. Three cycles of freeze-pumpthw were then performed. Polymeriztion ws conducted t room temperture. After 24 h, polymeriztion ws stopped y opening the tue to ir. The rection mixture ws precipitted into methnol. The product ws dissolved in chloroform nd then precipitted in methnol three times. Frctionted precipittion ws performed to remove free PMMA y using cetone nd methnol s the solvent nd non-solvent, respectively. The finl precipitte ws collected nd dried under vcuum t 35 C for 24 h Synthesis of PEO nd inititor-modified symmetric AuNPs (PEO 114 /I-Au 6 ) I-Au 6 (23.1 mg) ws dissolved in 30 ml dichloromethne. PEOeSH (PEO 114, 164 mg) ws dded nd the mixture ws stirred t room temperture for 5 dys under nitrogen. Frctionted precipittion using cetone/diethyl ether s solvent/non-solvent ws performed multiple times to remove free PEO. The finl product ws collected nd dried under vcuum t 35 C for 24 h Synthesis of mixed-rush symmetric AuNPs (PEO 114 /PMMA- Au 6 ) CuBr (8.0 mg, mmol), PEO 114 /I-Au 6 (70 mg), Me 6 TREN (13.0 mg, mmol), inititor 1 (20 mg, mmol), MMA (1.362 g, mmol) in 10 ml toluene were dded in Schlenk

4 B. Wng et l. / Polymer 51 (2010) 4814e tue. Three cycles of freeze-pump-thw were then performed. Polymeriztion ws conducted t room temperture. After 24 h, polymeriztion ws stopped y opening the tue to ir. The product ws dissolved in chloroform nd then precipitted in methnol three times. Frctionted precipittion ws performed. The finl precipitte ws collected nd ws dried in vcuum t 35 C for 24 h Mesurement 1 H NMR spectr were recorded on either UnityInov 500 MHz or 300 MHz spectrometer using CDCl 3 s the solvent nd tetrmethylsilne (TMS) s the internl stndrd. Trnsmission electron microscopy (TEM) ws crried out using JEOL JEM2100 TEM operted t n ccelertion voltge of 200 kv. To prepre the TEM smple, one drop of single crystl or nnoprticle suspension ws cst on cron-coted nickel grid. After solvent evportion, the smple ws used for TEM oservtion without further tretment. UVeVis spectr were collected using n Ocen Optics USB4000 Miniture Fier Optic Spectrometer t room temperture. Infrred spectr were recorded using Digil, UMA 600 FTIR spectrometer with KBr pellets in trnsmission mode. A smple solution (1 g, 0.1 wt%) ws drop cst on the surfce of KBr pellets nd ws dried under vcuum for 24 h. Before FTIR test, the KBr pellets were dried gin using n infrred lmp for t lest 10 min. Gel permetion chromtogrphy (GPC) ws crried out using Wters GPC system with 1525 inry HPLC pump, Wters 2414 refrctive index detector, nd one Styrgel Ò HR 4 THF ( mm) column. The dt were processed using Breeze softwre. THF ws used s the crrier solvent t flow rte of 1.0 ml/min. Stndrd monodispersed polystyrenes were used for clirtion. All the GPC experiments were run t 40 C. Thermogrvimetric nlysis (TGA) ws conducted using PerkineElmer TGA-7. The smple ws heted in nitrogen from 50 Cto800 C with heting rte of 10 C/min. Before nlysis, the smples were dried under vcuum t 50 C for 24 h. 3. Results nd discussion 3.1. Synthesis of PEO- nd inititor-modified Jnus AuNPs Synthesis of PEO- nd inititor-modified Jnus AuNPs ws chieved using PEO single crystls s the templte. As shown in Scheme 1, we first grew PEOeSH single crystls with the thiol groups exposed on the surfce. For 6 nm AuNPs, AuNPs modified with mmonium slt were mixed with the PEOeSH single crystl suspension in mixed solvent of pentyl cette/toluene (5/1, v/v). After 24 h of rection, AuNPs were immoilized on the surfce of PEO single crystls (Scheme 1c, grfting-to). In doing so, the ottom prt of AuNP ws coupled with the surfce of single crystls nd the AuNP symmetry ws roken. The opposite hemisphere of the AuNP ws then ville for chemicl modifiction with thiol-ended lignds, such s the ATRP inititor 1 (Scheme 1), vi lignd exchnge rections. The PEO- nd inititor 1-modified Jnus AuNPs (PEO 114 -Au 6 -I) ws otined y mixing the AuNPdecorted PEO single crystls with 1 in pentyl cette (1 wt%) for 24 h (Scheme 1d). Free inititor, other lignds s well s free AuNPs were esily removed y multiple centrifugtions. Fig. 1ec shows TEM imges of the mmonium slts (TBAB/DDAB, 1:2 M rtio) -protected AuNPs nd PEO single crystls with the immoilized AuNPs. Synthesis of PEO 114 -Au 15 -I ws chieved y different method, ecuse the mmonium slts-protected lrge dimeter Fig. 1. TEM imges of immoiliztion of AuNPs with the dimeter of 6 (ec) nd 15 (def) nm onto PEOeSH single crystls. () Ammonium slts-protected 6 nm AuNPs, (d) inititormodified 15 nm AuNPs. () nd (e) show tht these nnoprticles were immoilized on squred-shped PEO single crystls. (c) nd (f) show the enlrged res of () nd (e), respectively.

5 4818 B. Wng et l. / Polymer 51 (2010) 4814e4822 AuNPs were unstle in the mixed solvent of toluene nd pentyl cette. To void ggregtion, 15 nm AuNPs were first modified with 1 to form Au 15 -I, fter which the AuNPs were dissolved in chloroform (Fig. 1d), nd were mixed with PEO single crystls using chloroform/pentyl cette (1/5, v/v) s the solvent. TEM imges confirmed tht these AuNPs were immoilized on the surfce of PEO single crystls (Fig. 1e nd f). The successful coupling etween Au 15 nd PEO single crystls is ecuse of the plce exchnge rection etween inititor 1 nd surfce thiol groups of PEO 114 single crystls [72]. Although similr thiol-au onding occurs for oth 1/AuNP nd PEOeSH/AuNP, ecuse of the plne nture of the single crystl surfce nd the reltively high end group density on the crystl surfce, ech AuNP cn form multiple onds with the crystl. Therefore, once one inititor 1/AuNP ond exchnges with one PEOeSH/AuNP, it fcilittes the onding process etween djcent PEOeHS nd the AuNP, pushing the exchnge rection towrds the formtion of PEOeHS/AuNPs. As shown in Fig. 1, the density of AuNPs on the surfce of single crystls vries with the nnoprticle size, which might e cused y the different efficiency of the lignd exchnge rections for these two types of nnoprticles: PEOeSH replced mmonium slts in the 6 nm AuNP system, while it exchnged inititor 1 in the 15 nm AuNP cse. The former rection is more redily to occur due to the reltively wek goldemmonium interction s opposed to the Au-thiol ond Synthesis of Jnus AuNPs with icomprtment rushes Jnus AuNPs with PEO/PMMA nd PEO/PtBA icomprtment rushes hve een synthesized from PEO-Au-I using ATRP. In order to mintin the symmetric properties, severl conditions were chosen: 1) Pentyl cette ws used s the solvent for the polymeriztion. Becuse PEO single crystls were formed in pentyl cette, nd were used s the templte for rush synthesis, polymeriztion in the sme solvent will less likely rek/dissolve the single crystls. 2) All of the surfce-initited polymeriztions were performed t room temperture in order to void the dissocition of thiol-gold onds. 3) Me 6 TREN-CuBr ctlyst system ws used since it hd less influence on the dissocition of thiol-functionlized lignds from gold surfce during the polymeriztion [73,74]. PMMA ws grown from PEO 114 -Au 6 -I nd PtBA ws grown from PEO 114 -Au 15 -I. Frctionted precipittion ws then used to purify the Jnus AuNPs. Removl of the free polymers ws confirmed y GPC experiments (Fig. S1 in Supporting Informtion) Chrcteriztion of AuNPs nd grfting density study Inititor-modified symmetric AuNPs (I-Au) Fig. 2 shows the 1 H NMR spectr of inititor (spectrum I) nd I-Au (spectrum II for 6 nm AuNPs nd III for 15 nm AuNPs) in CDCl 3. The covlent onding of the inititor to the surfce of AuNPs leds to the rodening of peks (4), (5) nd (6) in spectrum II nd III. For spectrum III, pek t d w 2.67 ppm is oserved, which might rise from the loosely ttched surfce lignds [75]. From FTIR spectr (Fig. 2), chrcteristic signls from the surfce of AuNPs, such s C]O stretching round 1732 cm 1, mtch well with those from the inititor (spectrum I in Fig. 2). The CeH stretching from ech 2 e shifted from 2854 cm 1 nd 2927 cm 1 for free inititor to round 2850 nd 2915 cm 1 for I-Au, which indicted n incresing ordering of lkyl chins for inititor fter inding to the surfce of AuNPs [73]. In order to study the inititor contents, TGA experiments were performed. The weight percentge of nnoprticles ws otined y heting the smple from 50 C to 700 C under nitrogen with the heting rte of 10 C/min. The nnoprticle contents for I-Au 6 nd I-Au 15 were 85 nd 87%, respectively. The inititor densities on 6 nd 15 nm AuNPs were clculted to e 5.8 nd 9.42 inititors/nm 2, respectively, which mtch well with the previously reported vlues. Note tht the surfce rel inititor density for I-Au 15 is reltively high, which might e cused y the loosely ttched lignd on the surfce s shown in Fig. 2 (spectrum III) PEO- nd inititor-modified Jnus AuNPs (PEO 114 -Au-I) nd symmetric AuNPs (PEO 114 /I-Au) Fig. 3 shows the 1 H NMR spectr of AuNPs modified with PEO nd inititor 1 including Jnus nd symmetric AuNPs. The chrcteristic peks from the inititor t d w 1.9 ppm (ech 3 ) nd from PEO t d w 3.65 ppm (ech 2 Oe) cn e seen. All of the ech 2 e (6) from the ckone of the inititor on the surfce of AuNPs gve rod signl of d w 1.2e1.4 ppm, which mtched well with tht from the pure inititor (Fig. 3). However, only one rod pek ws oserved in Fig. 3 nd c t d w 1.5e1.7 ppm, which corresponded to ech 2 e (4) nd (5). This might e ttriuted to the reduced moility of the molecules cused y the onding of the inititor to the surfce of AuNPs. The synthesis of symmetric PEO 114 /I-Au6 ws chieved y mixing inititor-modified AuNPs with PEOeSH in solution nd llowing the lignd exchnge rection etween PEOeSH nd 1. (4) (5) HS O O (1) (6) (3) (2) Br (3) (1) I (2) (4)(5) (6) I II Asorence (. u.) II III III Chemicl Shift (ppm) Wvenumer (cm -1 ) Fig H NMR () nd FTIR () spectr of inititor (I), inititor-modified AuNPs with the dimeters of 6 nm (II) nd 15 nm (III) (: solvent).

6 B. Wng et l. / Polymer 51 (2010) 4814e d c Tle 1 Surfce chrcteristics of symmetric AuNPs. M n,i M n,peo (K) M n,pmma (K) N I N PEO N PMMA I-Au e 5.8 PEO 114 /I-Au PEO 114 / e PMMA 152 -Au 6 PMMA 130 -Au 6 e e 13 e e Moleculr weight of surfce lignd/polymer (g/mol). Averge chin numer per AuNP. c Overll grfting density (chins/nm 2 ). s c (2) (4) (6) (6) (6) (5) HS O O (6) (6) (6) (6) (6) (2) (4) (5) Br Chemicl Shift (ppm) Fig H NMR spectr of inititor (), PEO- nd inititor-modified Jnus AuNPs PEO 114 -Au 6 -I (), PEO 114 -Au 15 -I (c), nd PEO- nd inititor-modified symmetric AuNPs PEO 114 /I-Au 6 (d). The inset shows the enlrged spectr from d w 1.0e2.0 ppm (: solvent). Fig. 3d shows the 1 H NMR spectrum of such symmetric prticles. The grfting density ws clculted using 1 H NMR (Fig. 3d) nd TGA results s shown in Tle 1. The totl surfce rel chin density decresed from 5.8 chins/nm 2 for Au 6 -I to 2.0 chins/nm 2 for PEO 114 /I-Au 6, nd the numer of surfce inititor per prticle decresed from 656 for Au 6 -I to 151 for PEO 114 /I-Au PEO- nd PMMA-modified symmetric nd Jnus AuNPs (PEO 114 /PMMA-Au 6, PEO 114 -Au 6 -PMMA) After the surfce modifiction with polymer rushes, oth symmetric nd Jnus AuNPs cn e dispersed in orgnic solvents such s cetone. Fig. 4 shows the TEM imges of solution cst nnoprticles; no prticle ggregtion ws oserved. For symmetric AuNPs modified with mixed rushes of PEO nd PMMA, oth PEO (d w 3.65 ppm) nd PMMA (d w 3.61, 1.82, 1.03, nd 0.84 ppm) signls were oserved in the 1 H NMR spectrum (Fig. 5). The moleculr weight of PMMA ws found to e 15.2 K g/mol mesured y GPC. The molr rtio of PEO nd PMMA ws clculted y compring ech 2 Oe of PEO (d w 3.65 ppm) nd eoch 3 of PMMA (d w 3.61 ppm) peks. For PEO- nd PMMA-modified Jnus AuNPs, similr results were otined from 1 H NMR spectrum (Fig. 5). Moreover, with incresing PMMA moleculr weight from 11.8 K, to 20.8 K g/mol (Fig. 5c nd d), the pek intensity t d w 3.61 ppm corresponding to PMMA incresed grdully. It should e noted tht the polydispersity indices (PDI) of these PMMA rushes (otined y mesuring free PMMA chins) re round 1.2, smller thn tht of the previously reported 1.47 for PMMA rushes synthesized without using scrificil inititor [57]. The orgnic contents of these polymer-modified AuNPs were otined from TGA (Fig. 6). Comining TGA, NMR nd GPC results, the surfce grfting densities of oth symmetricl nd Jnus AuNPs were clculted nd re shown in Tles 1 nd 2. Of interest is tht for symmetric AuNPs modified with PEO nd PMMA, the PEO grfting density did not chnge significntly efore nd fter polymeriztion, which indictes tht the rection nd purifiction processes hd negligile effect on the stility of the Au-thiol ond. As shown in Tle 1, there re less PMMA chins thn tht of the inititor, which indictes tht only frction of (w0.42) the inititor initited the chin growth. For Jnus AuNPs with icomprtment PEO nd PMMA rushes, the grfting densities of PEO nd PMMA rushes were investigted. The verge grfting density (including oth PEO nd PMMA) decresed from 0.87 chins/nm 2 for PEO 114 -Au 6 -PMMA 118 to 0.52 chins/nm 2 for PEO 114 -Au 6 -PMMA 208. The numer of PEO chins on ech AuNP is round 30 for ll three smples. This cn e ttriuted to tht the PEO chins were ttched to the AuNPs during the immoiliztion process of AuNPs on PEO single crystls. Therefore, the PEO chin numer ws determined y the coupling rection occurred t the interfce of single crystls nd nnoprticles. The susequent surfce-initited polymeriztion did not ffect the stility of the thiol-gold ond; hence the numer of PEO on the AuNPs did not chnge efore nd fter the polymeriztion. Also of interest is tht the numer of PEO chins per AuNP is smller thn tht of the previously reported Jnus AuNPs using 2 K g/mol PEO s the single crystl sustrte (70 chins/prticle in the lter cse) [57]. Fig. 4. TEM imges of symmetric AuNPs PEO 114 /I-Au 6 (), PEO 114 /PMMA 152 -Au 6 (), nd Jnus PEO 114 -Au 6 -PMMA 208 (c), cst from cetone solutions.

7 4820 B. Wng et l. / Polymer 51 (2010) 4814e d 90 c Weight% Chemicl Shift (ppm) Fig H NMR spectr of PEO- nd PMMA-modified symmetric AuNPs PEO 114 / PMMA 152 -Au 6 (), nd Jnus AuNPs PEO 114 -Au 6 -PMMA 118 (), PEO 114 -Au 6 -PMMA 138 (c), nd PEO 114 -Au 6 -PMMA 208 (d) (: solvents) Temperture ( o C) Fig. 6. TGA of PEO- nd PMMA-modified AuNPs from ottom to top: symmetric AuNPs PEO 114 /PMMA 152 -Au 6, symmetric PMMA 130 -Au 6, Jnus AuNPs PEO 114 -Au 6 -PMMA 118, PEO 114 -Au 6 -PMMA 138, nd PEO 114 -Au 6 -PMMA 208. This cn e explined y the unique integer chin folding of polymers with low moleculr weights. For 2 K g/mol PEO single crystls, s we reported previously, the lmellr thickness is w12 nm nd the polymer formed extended chin single crystls. In the present 5 K g/ mol PEO cse, AFM (Fig. 7) showed tht the lmellr thickness ws w16 nm, indicting once-fold crystl. Becuse in oth 2 K nd 5 K PEO cses, there is one thiol group per chin, chnging from n extended chin conformtion to once-fold conformtion reduced the surfce rel density of thiol groups y fctor of 2. As result, the verge numer of PEO chins ttched to the surfce of AuNPs pproximtely dropped y fctor of 2 (30 vs. 70 chins/prticle). This oservtion lso suggests tht semi-quntittive control of grfting density of polymer chins y controlling the polymer moleculr weight is fesile. Also of interest is tht with incresing the moleculr weight of PMMA, the numer of PMMA chins per AuNP decresed from 68 to 26. The decrese of PMMA grfting density on the surfce of AuNPs my e ttriuted to the steric hindrnce imposed y polymer rushes, nd/or possile desorption of polymer chins during polymeriztion [76] Pltinum nnoprticle (PtNP) decorted Jnus AuNPs (PEO Au 15 -PAA 109 -PtNPs) In order to demonstrte the Jnus nture of the nnoprticles, PtNP-decortion method ws used. To fcilitte the formtion of nnoprticle in the rushes synthesized y surfce-initited ATRP, PtBA insted of PMMA ws grown from the Jnus AuNP surfce nd susequently hydrolyzed to PAA. PtBA moleculr weight ws mesured to e 13.9 K g/mol with PDI of 1.1. The resultnt PEOnd PtBA-modified Jnus AuNPs were spred on wter surfce, picked up using cron-coted grid, nd were sujected to TEM oservtion. Fig. 8 shows TEM imge of PEO 114 -Au 15 -PtBA 109 dispersed on wter; no prticle ggregtion ws oserved. The reltively lrge inter-prticle distnce compred with Fig. 1d is due to the grfted polymer rushes [77]. PtBA rushes were then hydrolyzed to yield PAA. The formtion of PAA on the surfce of Jnus AuNPs ws confirmed y 1 H NMR (Fig. S2). PtNPs were then synthesized using the in-situ reduction method following the reported procedure [57]. In rief, chloropltinic cid hydrte solution in methnol ws dded to PAA-modified AuNP solution in methnol. Aqueous solution of hydrzine ws dded into the mixture solution with vigorous stirring nd PtNPs were then formed. Fig. 8 nd c clerly shows tht prt of the AuNP surfce ws covered y smll PtNPs with the dimeter round w2 nm. The empty re on the AuNPs surfce is ttriuted to the coverge of PEO chins, which ws not le to sor pltinum ions for PtNP formtion [57]. Of interest is tht in Fig. 8c, the reltively higher pltinum slt concentrtion (five times higher s compred to tht of Fig. 8) led to the formtion of dendrite-like PtNPs on the AuNP surfce. Yet, the PEO region remined intct from PtNPs, further confirming the Jnus nture of the AuNPs. Tle 2 Surfce chrcteristics of Jnus AuNPs with icomprtment rushes. M npeo M npmma PDI Au % c d R 1 e R 2 f N PEO f N PMMA s g PEO 114 -Au 6 -PMMA /2.29 1/ PEO 114 -Au 6 -PMMA /2.56 1/ PEO 114 -Au 6 -PMMA /0.76 1/ Moleculr weight of polymer rushes (K g/mol). Polydispersity index. c Weight percentge of gold. d Molr rtio etween PEO nd PMMA. e Mss rtio etween PEO nd PMMA. f Numer of polymer chin on ech AuNP. g grfting density (chins/nm 2 ).

8 B. Wng et l. / Polymer 51 (2010) 4814e Asorence (. u.) Wvelength (nm) Fig. 9. UV/Vis spectr of polymer-modified AuNPs in cetone. From ottom to top: JNP PEO 114 -Au 6 -PMMA 118, PEO 114 -Au 6 -PMMA 138, PEO 114 -Au 6 -PMMA 208, symmetric AuNPs PMMA 130 -Au 6, PEO 114 /PMMA 152 -Au 6, PEO 114 /I-Au 6. Fig. 7. AFM imges of 5 K g/mol PEO single crystl nd the corresponding height profile. The single crystl is 16 nm in thickness Surfce plsmon resonnce (SPR) study of polymer-modified Jnus nd symmetric AuNPs in orgnic solvents AuNPs re known for SPR sorption in the visile spectrum, which reflects the collective resonnce of the conducting electrons of the prticle with the incident light [78,79]. Becuse of the influence of the lignds on the surfce electron movement, surfce modifiction of AuNPs orgnic lignds ffects SPR of the nnoprticles [80e82]. In the present cse, symmetric modifiction of AuNPs with icomprtment rushes renders interesting SPR ehviors of the nnoprticles in orgnic solvents. Acetone ws used ecuse it is good solvent for oth PEO nd PMMA. In order to compre the SPR dsorption of symmetric nd symmetric AuNPs, symmetric AuNPs modified with either PEO or PMMA with similr grfting densities were synthesized s descried in the Experimentl section. As shown in Fig. 9, the Jnus AuNP modified with PEO nd PMMA hs SPR nd t 521e524 nm, which lueshifted y w2e5 nm from tht of PMMA 130 -Au 6 (526 nm) nd w10e13 nm from PEO-modified AuNPs (PEO 114 /I-Au 6, 534 nm). According to Vilin et l. [79], this lue shift of SPR is one of the importnt properties of JNPs nd cn e ttriuted to the symmetric electron-withdrw of surfce lignds. The symmetric effect on surfce electrons would e verged to zero for symmetric AuNPs, which mens tht the mixed-rush-modified AuNPs would show SPR nd etween those of homo PEO- nd PMMA-modified AuNPs. To confirm this, AuNPs coted with PEO/ PMMA mixed rushes were synthesized, nd s shown in Fig. 9, the mix-rush-modified AuNPs hs SPR sorption nd t 532 nm, which is in the center of the sorption nds of PMMA 130 -Au 6 nd PEO 114 /I-Au 6. This result further confirms the symmetric modifiction of AuNPs sed on polymer single crystls. Fig. 8. TEM imges of () PEO- nd PtBA-modified Jnus AuNPs on wter surfce, () PtNP-decorted Jnus AuNPs with the dimeter of 15 nm, nd (c) PtNP-decorted Jnus AuNPs with the dimeter of 6 nm.

9 4822 B. Wng et l. / Polymer 51 (2010) 4814e Conclusion In summry, Jnus AuNPs modified with icomprtment polymer rushes (PEO/PMMA, PEO/PtBA, nd PEO/PAA) hve een synthesized y comining polymer single crystl templting (grfting-to) nd surfce-initited ATRP (grfting from) methods. The surfce modifiction of AuNPs ws confirmed using 1 H NMR nd FTIR. The grfting density of polymer rushes ws clculted using 1 H NMR, TGA nd GPC results. PEO grfting density decresed y fctor of 2 s the polymer moleculr weight incresed from 2 K to 5 K g/mol. This ws ttriuted to the decresed thiol rel density on the single crystl surfce for the high moleculr weight smple. The grfting density of PMMA chins lso decresed with PMMA moleculr weight. The Jnus feture of the prticles ws confirmed using oth PtNP decortion nd UV/Vis spectroscopy nlysis. In prticulr, the SPR dsorption nd of Jnus AuNPs with icomprtment polymer rushes showed cler lue shift compred with tht of symmetricl AuNPs with homo or mixed polymer rushes. 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