Surface plasmon resonance of capped Au nanoparticles

Su ace plasmon esonance o capped Au nanopa icles M. A. Ga cia,1,2 J. de la Ven a,1,2 P. C espo,1,2 J. LLopis,2S. Penadés,3A. Fe nández,4and A. He nando1,2,* 1Ins i u o de Magne ismo Aplicado (RENFE-UCM-CSIC), P.O. Box 155, 28230 Las Rozas, Mad id, Spain 2Depa men o Física de Ma e iales, UCM, 28040 Mad id, Spain 3G upo Ca bohid a os, Labo a o y o Glyconano echnology IIQ-CSIC, Amé ico Vespucio s/n, 41092 Se illa, Spain 4Ins i u o de Ciencia de Ma e iales de Se illa CSIC-UNSE, Amé ico Vespucio s/n, 41092 Se illa, Spain and Depa amen o de Química Ino gánica, Uni e sidad de Se illa, Spain 共Recei ed 10 No embe 2005; published 15 Decembe 2005兲 In his Rapid Communica ion we show he ela ionship be ween su ace plasmon esonance damping and he in ensi y o su ace bonding o capped Au nanopa icles, 共NPs兲. Up o now he in luence o capping has been included as a phenomenological modi ica ion o he sca e ing cons an . I is indica ed he e ha he e ec i e NP size is he pa ame e mainly a ec ed by su ace bonding. Expe imen al esul s in di e en Au- hiol NPs a e shown o be in excellen ag eemen wi h he exp ession we p opose o damping. Mo eo e , acco ding o ou model he esonance p o ile gi es a deep insigh o he in e ace bonding s eng h. DOI: 10.1103/PhysRe B.72.241403 PACS numbe 共s兲: 78.67.⫺n, 73.21.⫺b, 73.22.⫺ Su ace plasmon esonance 共SPR兲is he mos ema kable op ical p ope y o me allic nanopa icles 共NPs兲.1,2 Besides he undamen al in e es in unde s anding he physical in e - ac ions a he nanoscale, he SPR is in he basis o many echnological applica ions, as hey a e DNA analysis chips and p o ein ecogni ion de ices.3,4 SPR consis s on a collec- i e oscilla ion o he conduc ion elec on inside he NPs. The excess o cha ge p oduced a he su ace because o elec on mo emen ac s as a es o ing o ce, while he elec- on mo emen is damped mainly because o he elec on in e ac ion wi h a omic co es and NP su ace. The sys em beha es as a damped oscilla o wi h a esonance equency ha o mos o he ansi ion me als lies on he ul a iole - isible pa o he spec um.1,5 Hence he NPs exhibi an abso p ion band in his egion o he spec um. As he damp- ing cons an depends s ongly on he pa icle size,1,2,6 so does he shape o he SPR. Wi hin he ame o he Mie heo y 共 ha is, assuming well-dispe sed sphe ical NPs兲, he SPR ull wid h a hal maximum 共FWHM兲is gi en by:1,2 ⌫=⌫0+A F R,共1兲 whe e Ais a cons an ha includes he de ails o he sca e - ing p ocesses, Fis he Fe mi eloci y o he me al, and R s ands o he pa icle adius. Figu e 1 shows he FWHM o Au NPs calcula ed acco ding o he Mie heo y and using a size-dependen dielec ic unc ion gi en in Re . 2. As can be obse ed in he inse , he alue o ⌫ ollows Eq. 共1兲up o R⬃25 nm. The exac alues o Aa e sligh ly di e en de- pending on he au ho s bu hey a e always below 1 and mainly be ween 0.8 and 0.9 共in uni s o ប兲. An in insic ea u e o he NPs is he la ge ac ion o a oms loca ed a he su ace ha a e modi ied when he pa - icle is capped wi h di e en chemical species. Thus he cap- ping agen s can al e signi ican ly he whole NP p ope ies. An example o s ong in e ac ion be ween he NPs and he capping agen a e he hiol-capped Au NPs. I has been shown ha he hiols induce changes in he elec onic con- igu a ion o he NPs, inc easing he hole densi y a he 5d Au le el.7–13 As a consequence o hese changes in he elec- on con igu a ion, he physical p ope ies o he NPs change signi ican ly; he mos ou s anding one is he e omagne ic- like beha io o hiol-capped Au NPs despi e he diamag- ne ic cha ac e o bulk gold.9Thiol-capped Au NPs also ex- hibi a wide SPR han ba e NPs, which is associa ed wi h an ex a dampìng due o he in e ac ion o conduc ion elec ons wi h he hiols.2,14,15 In some cases, hiol capping e en yields o he absence o a SPR abso p ion band in he spec um.8,9 The mechanism esponsible o hese changes in he op ical p ope ies o Au NPs induced by he hiols is no clea . Fu - he mo e, he e is a con o e sy e en abou he me allic o insula ing cha ac e o small 共below 2 nm size兲 hiol-capped Au NPs.14,15 In he p esen pape , he e ec o he capping molecules in he SPR band o Au NPs is expe imen ally s udied o cla i y he in luence o he capping agen on he FIG. 1. SPR spec a o Au NPs embedded in a media wi h n=1.78 calcula ed acco ding o he Mie heo y o R=2 nm 共䉳兲,3 nm 共⽧兲,5nm共䉲兲,10nm共䉱兲, and 20 nm 共•兲. The inse shows he FWHM o he SPR as a unc ion o he in e se o pa icle adius. PHYSICAL REVIEW B 72, 241403共R兲共2005兲 RAPID COMMUNICATIONS 1098-0121/2005/72共24兲/241403共4兲/$23.00 ©2005 The Ame ican Physical Socie y241403-1 elec onic con igu a ion o he NPs, and consequen ly, on hei physical p ope ies. The sample p epa a ion p ocedu e o dodecane hiol- capped NPs 共named he ea e hiol-capped NPs兲is based on he ou e p oposed by B us 16 using a a io o hiol:gold o 2:1 o dec ease he a e age pa icle size. Au hiol-capped glyconanopa icles 共named he ea e as mal ose-capped NP兲, we e p epa ed by using a mal oneoglycoconjuga e as a hiol linke species.17 In his case he me al clus e s a e a he same ime p o ec ed and unc ionalized wi h he o ganic mol- ecule. Te aalkylammonium 共R4N+X−兲p o ec ed gold NPs wi h a weakly in e ac ing dipole capping molecule we e also p epa ed.18 In his case, he sample was ob ained wi h e - aoc yl ammonium b omide as he su ac an molecule. In any analysis o he su ace plasmon esonances o Au NPs, he possible e ec s o agg ega ion mus be kep in mind. TEM analysis o he samples s udied he e showed ha he e was no agg ega ion.9Mo eo e , i is well known ha NP agg ega ion shi s he posi ion o he esonance owa d lowe ene gies 共 he ed and in a ed pa o he spec um兲 while he plasmons analyzed he e showed he esonance lo- ca ed a he posi ion ha co espond o well-dispe sed nano- pa icles. We also measu ed he op ical abso p ion in he ed and in a ed pa o he spec um bu we ound no signal in his egion, so we can disca d agg ega ion e ec s in ou samples. As s a ed abo e, hiol-capped Au NPs exhibi SPR clea ly wide han ha co esponding o ba e Au NPs.2,15 The e a e wo p oposed explana ions o he changes induced in he SPR o Au NPs by hiol capping: he educ ion o elec on densi y in he NPs and he inc ease o damping due o cha ge localiza ion a he NP su ace. In a ecen wo k,15 Lica e al. p oposed ha he changes in he SPR o hiol-capped NPs a e due o he dec ease in Au 5d elec ons because o he cha ge ans e om Au a oms a he NP su ace o S a oms a he end o he hiol chain. Thei expe imen al measu emen s o SPR could be i ed o hose calcula ed on he basis o a jellium model, assuming ha each S-Au bond educes in one elec on he cha ge o he NPs. Howe e , se e al ex ended x- ay abso p ion ine s uc- u e Ex ended X- ay Fine Abso p ion Spec oscopy 共EX- AFS兲expe imen s o hiol-capped Au NPs showed ha he cha ge ans e a he Au-S bond is abou 0.04 elec on pe a om.7,9 Al hough he calcula ions ep oduce he expe imen- al esul s, he pa ame e s used ha e no physical meaning and he e o e he elec on densi y dec ease canno be in- oked as he unique eason o he changes in he SPR. The o he a gumen p oposed o explain he changes in- duced in he SPR by hiol capping is he cha ge localiza ion a he Au-S bond. This co alen bond blocks he elec ons in ol ed in he bond a he NP su ace. The localized cha ge will ha e an elec os a ic in e ac ion wi h he NP conduc ion elec ons, inducing an ex a damping in he elec ons oscil- la ion 共called chemical in e ace damping兲and he e o e, en- hancing he SPR bandwid h.2,8,9 Recen heo e ical s udies showed ha he in e ac ion o small Au NPs wi h capping hiols is qui e complica ed and no ully unde s ood.10,12,13 The e o e, simple phenomenological models a e used o s udy he SPR o hiol capped Au NPs. Wi hin he ame o hese models, he FWHM o he SPR is usually gi en by he exp ession:2,15 ⌫=⌫0+A* F R,共2兲 whe e A*is a phenomenological pa ame e . Al hough A*has no a clea physical meaning, la ge alues o A*a e associ- a ed wi h s ong chemical in e ace damping. Thus, o hiol- capped Au NPs, ypical alues o A*a e be ween 1 and 1.2. Following his equa ion, he egion whe e elec ons can oscilla e eely is educed by he chemical in e ace damping in a ac o A/A*, and i means ha his educ ion is p opo - ional o he pa icle size. Howe e , conside ing ha he cha ge dis ibu ion a he in e ace is de e mined by he a - oms p esen a his in e ace and he na u e o he bond, his egion o localized cha ge is expec ed o be independen o he pa icle adius. I is meaningless o suppose ha he e- gion o cha ge localiza ion a he su ace has a hickness, , which inc eases linea ly wi h he NP adius. The e o e, we p opose ha he alue o he FWHM o he SPR should be gi en by an exp ession as ⌫=⌫0+A F 共R− 兲,共3兲 whe e esul s independen o he pa icle size, and Ais he same cons an han o ba e Au NPs. F om his la e equa- ion i is expec ed ha he chemical damping becomes mo e impo an o small NPs; in pa icula o hose wi h close o R, since o hese pa icles he second e m in Eq. 共3兲 inc eases e y as . Ac ually, his la e equa ion can explain expe imen al esul s o small NPs. I has been ound by se - e al au ho s8,9 ha 1.5-nm hiol-capped NPs exhibi no SPR, which is unde s ood as an in ini e damping. Following Eq. 共2兲i should yield o an in ini e alue o A*,共which is ha d o unde s and兲, while o la ge pa icles, i s alue is close o 1. On he con a y, acco ding o Eq. 共3兲, he lack o SPR should co espond o a alue o =R. This alue o can also ex- plain he changes in he SPR obse ed in la ge pa icles. Figu e 2 shows he SPR ull wid h a hal maximum o hiol- FIG. 2. FWHM o SPR band o hiol-capped Au NPs. Symbols co espond o he expe imen al alues om Re s. 4,8 and om ou samples as indica ed in he g aph. Dashed line co esponds o he bes i using Eq. 共2兲wi h A*=1.04 and solid line shows he i o Eq. 共3兲wi h =0.7 nm and A=0.8. GARCIA e al. PHYSICAL REVIEW B 72, 241403共R兲共2005兲 RAPID COMMUNICATIONS 241403-2 capped Au NPs and also esul s om o he au ho s, as a unc ion o he pa icle size measu ed by TEM. The dashed line co esponds o he bes i o Eq. 共2兲 ha can explain he esul s ob ained o pa icles wi h sizes o e 2 nm 共wi h A* =1.04兲. Howe e , o smalle pa icles, he expe imen al e- sul s de ia e om he i and can no explain he absence o SPR o pa icles smalle han 2 nm. On he con a y, he solid one, showing he i o he Eq. 共3兲, allows o i he expe imen al da a om NPs wi h di e en sizes wi h a unique alue o =0.7 nm 共as i can be expec ed om a sole bond na u e兲and A=0.8, which is he alue gi en o ba e Au NPs. The pa ame e has a clea physical meaning: I is he hickness o he shell whe e elec ons canno oscilla e be- cause o he in e ac ion wi h capping agen s and, he e o e, i p o ides an es ima ion o he s eng h o he in e ac ion be- ween hese capping molecules and NP a oms. Hence, he SPR can be used as a p obe o analyze his in e ac ion. Ana- lyzing he SPR cu e in e ms o Mie heo y we ge in o - ma ion abou he ee elec on oscilla ion olume 共R- 兲, whe eas TEM measu emen s will gi e in o ma ion abou NP size 共R兲. Compa ing bo h alues we ge he hickness o he blocking shell 共 兲, which, as explained abo e, is a measu e- men o he s eng h o he capping agen –NP in e ac ion. As an example, Fig. 3 shows he SPR o Au NPs capped wi h di e en chemical species. Fo he NPs capped wi h e - aalkylammonium, which is known o in e ac e y weakly wi h he NPs, he SPR band can be i ed wi h he Mie heo y using a size R=0.7 nm, which co esponds o he same alue ound by TEM measu emen s,9con i ming ha he capping agen me ely passi a es he NPs and does no modi y i s elec onic s uc u e 共 =0兲. Fo he mal ose-capped Au NPs 共in e media e in e ac ion s eng h兲wi h R=0.9 nm acco ding o TEM measu emen s,19 he SPR band is i ed assuming a pa icle size o R=0.6 nm 共 ha is, =0.3 nm兲. Finally, o s ong in e ac ing hiol-capped Au NPs, he TEM measu e- men s indica ed R=0.7 and he absence o SPR is unde s ood as R= 共and he e o e =0.7 nm兲. Al hough in bo h cases, mal ose-capped and hiol-capped NPs, he Au a oms a he NP su ace a e bonded o S a oms, ecen calcula ions indi- ca e ha he Au-S in e ac ion a he NP su ace depends on he S a om en i onmen ,13 explaining he di e en esul s ob ained o bo h samples. Fu he mo e, i is wo h obse - ing he pa allelism in he changes o op ical p ope ies, he modi ica ion o elec onic con igu a ion and magne ic p ope ies o Au NPs as a unc ion o he in e ac ion wi h capping agen s.9EXAFS expe imen s showed ha e aalkylammonium-capped Au NPs ha e he same densi y o 5dAu elec ons han bulk Au and esul ed diamagne ic 共as bulk Au兲. Mal ose-capped NPs p esen ed a sligh ly dec eased densi y o 5delec ons wi h espec o bulk Au, while mag- ne ic measu emen s indica ed weak e omagne ism.19 Fi- nally, hiol-capped NPs exhibi ed an in ense dec ease o 5d elec on wi h espec o bulk Au and p esen ed su p isingly e omagne iclike beha io up o oom empe a u e.9 The p oposed model can also explain he con o e sy abou he me allic o insula ing cha ac e o hiol-capped Au NPs. Fo NPs below 2 nm in size, se e al au ho s obse e localized cha ge beha io 9,20,21 while ecen s udies showed he p esence o ee elec ons.14,15 Acco ding o ou model 共see Fig. 4兲, hose pa icles will consis o an ou e shell whe e elec ons will be localized and he e o e wi h ypical insula ing beha io 共 hickness ⬃0.7 nm兲, and a inne co e o me allic Au. Fo pa icles la ge han R=1 nm me allic cha - ac e 共p esence o SPR兲is always ound, ha a ises om he inne me allic laye . Howe e , o NPs smalle han R=0.7 nm, he size o he insula ing shell is abou he whole NP size, and he e o e, i will show ypical insula ing beha - iou . In he limi be ween hese wo egimes, sligh changes in he p epa a ion me hods 共and hence in he hiol-NP in e - ac ion兲can de e mine he me allic o insula ing cha ac e o he NPs. Some ecen esul s showed ha hiols can be bond no only o he ou e shell o Au a oms bu also o some in e nal ones, inc easing in his way he hickness o he blocking shell9. This ac should accoun o he con adic- o y esul s ob ained by di e en au ho s when he NPs a e a ound his size. In summa y, we p opose a model o desc ibe he e ec s o s ong in e ac ing capping agen s on o he p ope ies o SPR. The capping agen s localize cha ge in a shell close o he FIG. 3. Op ical abso p ion spec a o Au NPs capped wi h di - e en species. Solid lines co espond o expe imen al measu e- men s. The symbols ep esen he bes i acco ding o Mie heo y assuming di e en alues o as indica ed in he igu e. A linea backg ound was added o ake in o accoun he abso p ion om he es o he sample. FIG. 4. 共Colo online兲Scheme o a hiol-capped Au NP wi h an ex e nal insula ing shell whe e elec on mo emen is damped by in e ac ion wi h hiols, and an inne me allic co e. SURFACE PLASMON RESONANCE OF CAPPED Au …PHYSICAL REVIEW B 72, 241403共R兲共2005兲 RAPID COMMUNICATIONS 241403-3 su ace wi h a hickness ha depends on he s eng h o i s in e ac ion wi h NPs, whe eas he inne co e p esen s me al- lic beha io . The model does no use phenomenological pa- ame e s bu hose wi h physical meaning as he su ace hickness o localized cha ge in which elec ons can no os- cilla e eely. The p oposed exp ession also concilia es he appa en ly con adic o y esul s ound by di e en au ho s on his kind o ma e ials. This model has been es ed wi h expe imen al esul s ob ained o Au NPs capped wi h di e - en agen s showing excellen ag eemen . *Au ho o whom co espondence should be add essed. Elec onic add ess: [email p o ec ed] 1U. 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