Remotely excited Raman optical activity using chiral plasmon propagation in Ag nanowires

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1 OPEN Light: Siene & Applitions (13) 2, e112; doi: /ls ß 13 CIOMP. All rights reserved /13 ORIGINAL ARTICLE Remotely exited Rmn optil tivity using hirl plsmon propgtion in Ag nnowires Mengto Sun 1 *, Zhenglong Zhng 1 *, Peijie Wng 2, Qing Li 1, Fengi M 3 nd Hongxing Xu 1 We experimentlly investigted remotely exited Rmn optil tivity (ROA) using propgting surfe plsmons in hirl Ag nnowires. Using hirl fmo-glyyl-glyine-oh (FGGO) moleules, we first studied the lol surfe plsmon-enhned ROA. We found tht the Rmn intensity n e exited y left- nd right-irulrly polrized lights nd tht the irulr intensity differene (CID) n e signifintly enhned. Seond, y seleting virtionl modes with lrge Rmn nd ROA intensities tht re not influened y hemil enhnements, we studied remotely exited ROA imging nd the CID of FGGO moleules y propgting plsmoni wveguide using Ag hirl nnostrutures. When lser light ws rdited on one of the Ag terminls, the mesured CID of the FGG t the other terminl showed little hnge ompred to the lol exited CID. Menwhile, when the lser light ws rdited on the Ag nnowires (not on the terminls) nd ws oupled to the nery nnontenn, the CID of the ROA ould e mnipulted y ltering the oupling ngle etween the Ag nnowires. To diretly demonstrte the propgtion of ROA long the nnowire nd its remote detetion, we lso mesured the remotely exited ROA spetr. Our experimentl method hs the potentil to remotely determine the hirlity of moleulr strutures nd the solute onfigurtion or onformtion of hirl live ell. Light: Siene & Applitions (13) 2, e112; doi: /ls.13.68; pulished online 22 Novemer 13 Keywords: Ag nnowires; hirl plsmons; plsmon propgtion; Rmn optil tivity; remote exittion INTRODUCTION Rmn optil tivity (ROA) 1 3 results in differenes in the Rmn spetr exited y right- nd left-irulrly polrized light; this tivity is extremely sensitive to the hirlity of the moleulr struture 4 6 nd n revel the solute moleulr onfigurtion 5 or onformtion. 6 Severl exellent reviews of ROA hve een reported. 7 9 The dimensionless irulr intensity differene (CID) introdued y Brron nd Bukinghm 1 provides n experimentl oservle of this phenomenon: j D~ I L{I R j ð1þ ji L zi R j where I R nd I L re the Rmn sttering intensities of right- nd leftirulrly polrized light, respetively. Although hirl-sensitive virtionl spetrosopy hs eome inresingly populr in iologil pplitions, this pproh hs n ovious flw tht is diffiult to overome: the ROA sttering intensities re times the intensities of the prent Rmn sttering. Due to the low intensity of ROA, this tehnique hs ertin limittions nd requires higher smple onentrtions nd longer dt olletion periods. The intrinsi wekness of ROA n e resolved y utilizing surfe plsmon enhnement The lol eletri field nd field grdients generted y plsmon resonne n signifintly enhne the ROA nd CID. 9 Surfe-enhned ROA is ttriuted to the oupling etween the eletri dipole nd the eletri qudrupole terms in the proess of Rmn sttering. 16 The qudrupole trnsitions n only e signifintly exited in n oriented moleule lose to the metlli sustrte; under fr-field exittions, the trnsitions re wek. In generl, the influene of hemil enhnement is diffiult to distinguish in surfe-enhned ROA. Some virtionl modes re strongly influened y hemil enhnements, while for other modes, hemil enhnements n e ruled out sed on the hrteriztion of seletive hemil enhnement. Thus, we must refully selet virtionl modes tht exhiit oth strong Rmn nd CID intensities for whih there is no influene of hemil enhnement. The propgtion of surfe plsmon polriton long metl nnowires enles the inident light to trnsfer through sudiffrtionlimit nnowire over mirometer-sle distnes. 18 This unique property of surfe plsmons enles the hievement of miniturized photoni nnowires nd high-resolution spetrosopy nd mirosopy. Remotely exited surfe-enhned Rmn sttering (SERS) produed y mens of propgting surfe plsmon in Ag or Au nnowires hs een experimentlly studied Compred to lol SERS tehniques tht exite trgets diretly, the remote exittion method n void speimen dmge used y the lser, whih my e ritil in some ses, for exmple, in interellulr hirl live ells. 1 Beijing Ntionl Lortory for Condensed Mtter Physis, Institute of Physis, Chinese Ademy of Sienes, Beijing , Chin; 2 Deprtment of Physis, Cpitl Norml University, Beijing , Chin nd 3 Deprtment of Physis, Lioning University, Shenyng , Chin *These uthors ontriuted eqully to this work. Correspondene: Professor MT Sun, Beijing Ntionl Lortory for Condensed Mtter Physis, Institute of Physis, Chinese Ademy of Sienes, PO Box , Beijing , Chin E-mil: mtsun@iphy..n Reeived 22 Mrh 13; revised July 13; epted 23 July 13

2 2 Remote-exited Rmn optil tivity MT Sun et l Further, hirl plsmoni nnostrutures 22 n e used to mnipulte nd enhne the CID of intrinsi hirl moleules. Thus, it is desirle to design plsmoni nnostrutures for lol nd remote hirl sensing. 23 In this ommunition, we report n experimentl reliztion of remotely exited ROA vi hirl plsmons propgting on Ag nnowires. To diretly demonstrte the propgtion of ROA long nnowire nd its remote detetion, we lso mesured the remotely exited ROA spetr. This pproh hs the potentil to remotely determine the hirlity of moleulr strutures nd the solute onfigurtion or onformtion of hirl living ells. MATERIALS AND METHODS Fmo-glyyl-glyine-OH (FGGO) ws otined from Aldrih Chemil (Chin) Co. nd ws used s reeived. The Rmn nd ROA spetr (Figure 1) were olleted using the ksttering SCP Biotools m-chirlraman-2x. The lser light hd wvelength of 532 nm, 24 nd power of 300 mw ws rdited on the smple. The totl olletion time ws 4 h. Single rystlline Ag nnowires were synthesized using hemil methods. 25 High-resolution snning eletron mirosopy (SEM) imges were quired t voltge of 10 kv. The Ag nnowires were mixed with FGGO ethnol solution (0.04 M) for 24 h nd were then dropped on glss sustrte. To exmine the influene of the sustrte on the shift in the Rmn frequenies for SERS, the lol SERS ws mesured with linerly polrized light exited t nm, using Lei mirosope instlled with onfol Rmn spetrosopi instrument (Invi; Renishw, UK) with 1003 ojetive nd numeril perture of Then, lol nd remote Rmn imges of the moleulr virtionl mode t 1593 m 21 were otined using rightnd left-irulrly polrized light, respetively. This virtionl mode exhiits lrge Rmn ROA intensities nd is not influened y hemil enhnements. The optil imges were reorded using TE-ooled CCD detetor instlled on the mirosope. The Rmn imges were remotely mesured. Lser light t nm (power of pproximtely 7 mw) ws rdited on the rossing point (point in Figure 2) using left- nd right-irulrly polrized light, respetively, nd the Rmn signls were mesured t the remote terminls of the nnowires (points, nd d in Figure 2). This method of mesuring single Rmn mode propgting long nnowire hs een desried in detil in Refs. 19,, 26 nd 27; this pproh is generlly lled Rmn imging in SERS. Finlly, using Eqution (1), lol nd remote surfe plsmon-enhned ROA imging n e otined. To diretly demonstrte the propgtion of the ROA long the nnowire nd its remote detetion, we lso mesured the remotely exited ROA spetr. We frited metl-insultor-semiondutor nnowire plsmoni wveguide system onsisting of wet-hemilly grown Ag wire, whih ws then overed y Al 2 O 3 films of vrying thikness using n tomi lyer deposition (ALD) system (Svnnh-100; Cmridge NnoTeh, USA) to hieve well-defined thikness. Susequently, the smple ws spin-oted with highly luminesent CdSe/ZnS Orgni Quntum Dots (Qdot 655; Invitrogen, Chin). Theoretil simultions were performed using Gussin The geometry of FGGO t the ground stte ws optimized t the level of density funtionl theory, 29 the B3LYP funtionl, 30 nd the g(2d,p) sis set. Using the optimized ground stte geometry of FGGO, the Rmn nd ROA spetr of FGGO were then simulted. RESULTS AND DISCUSSION The experimentl Rmn nd ROA spetr of FGGO re shown in Figure 1 nd 1. The highest intensities for oth spetr pper t 1607 m 21, with CID vlue of D50.57%. The virtionl mode of this pek is shown in Figure 1, whih provides the est ndidte for studying the remote exittion ROA using hirl plsmon propgtion in Ag nnowires. For the virtion mode t 1607 m 21, oth the Rmn nd CID intensities re t their mxim, sed on the experimentl spetr shown in Figure 1. This virtionl mode orresponds to the symmetri virtion of 1 H-fluorenyl, nd there re no O or N toms in this group. In generl, O nd N toms diretly intert with metl, nd thus, virtionl modes for groups inluding these toms 1607 ROA (.u.) Rmn (.u.) O O N H OH N O H O I L I = R = 0.57% I L + I R Wvenumer (m 1 ) 1600 Figure 1 Experimentl Rmn nd ROA spetr nd the virtionl mode of FGGO t 1607 m 21.() Rmn spetrum. () ROA spetrum. () Virtionl mode of FGGO t 1607 m 21. FGGO, fmo-glyyl-glyine-oh; ROA, Rmn optil tivity. Light: Siene & Applitions doi: /ls.13.68

3 Remote-exited Rmn optil tivity MT Sun et l 3 E d E µm 0 Figure 2 The struture of the Ag nnowires nd the propgtion of surfe plsmons in the Ag nnowires. () Struture of the Ag nnowires. () Surfe plsmons propgting in the Ag nnowires. () Optil imge of single nnowire nd evidene of the hirl propgtion of surfe plsmons in the Ag nnowires. The hirlity n e mnipulted y the polriztion of the lser. re usully strongly influened y hemil enhnements. In this se, there is no diret intertion etween the group nd the metl, nd hene, there is no hemil enhnement for this virtionl mode. The ssignments of the other virtionl modes with reltively strong Rmn nd CID intensities re given in Supplementry Fig. S1. A glss sustrte ws used in the experiments. An SEM imge of the Ag nnowires is shown in Figure 2; the rossing ngle etween the nnowires ws found to e pproximtely 6. Figure 2 demonstrtes the propgtion of plsmons in the Ag nnowires, nd the light n e lerly oserved t the terminls of the nnowires. To revel the hirl propgtion of the surfe plsmons in the nnowire, the simplest nnostruture ( single nnowire) ws used. Figure 2 shows n optil imge of hirl surfe plsmons propgting long single nnowire. The degree of this type of hirl propgtion n e mnipulted vi the polriztion of the lser. The SERS spetrum of lolly exited FGGO t the rossing point of the Ag nnowires is shown in Supplementry Fig. S2 for linerly polrized light. The virtionl mode t 1607 m 21 in Figure 1 ws shifted to 1593 m 21 due to the intertion etween the FGGO nd the Ag nnowires. Finlly, lol nd remote Rmn imges of FGGO t 1593 m 21, exited y left- nd right-irulrly polrized light, re shown in Figure 3 nd 3, respetively. As disussed ove, this virtionl mode exhiits lrge Rmn nd ROA intensities, without the influene of hemil enhnement. Due to the hirl properties of the nnowire struture nd the propgting surfe plsmon, we oserved different lol nd remote intensities of the Rmn imging t 1593 m 21. The Rmn intensities t these four points re signifintly stronger when exited y the left irulr polriztion in omprison to the right irulr polriztion. Using Eqution (1), the fitted CIDs t four points were otined, s shown in Figure 3. The d L - R / L + R Mirons (µm) 0 10 Mirons (µm) Position Figure 3 Imging of the Rmn pek of FGGO t 1593 m 21, exited y irulr polriztion, nd the fitted CIDs t four different points. () Results for left irulr polriztion. () Results for right irulr polriztion. () The fitted CIDs t four points of the nnostruture. CID, irulr intensity differene; FGGO, fmo-glyylglyine-oh. doi: /ls Light: Siene & Applitions

4 4 Remote-exited Rmn optil tivity MT Sun et l A RE-ROA (.u.) WD Wvenumer (m 1 ) 1600 Figure 4 The nnostruture of the nnowire nd the nnoprtile nd the remotely exited ROA. () Optil imging. () SEM imging of the nnostruture of the nnowire nd the nnoprtile. () The remotely exited ROA. ROA, Rmn optil tivity; SEM, snning eletron mirosopy. CIDs re signifintly enhned ompred to the intrinsi moleulr powder. Furthermore, the three CIDs t the three terminls of the Ag nnowires re lso signifintly different. A omprison of the CID t rossing point A (the point t whih the lser is rdited) to tht t point B (the remote terminl of the sme Ag nnowire) shows tht the hnge in the CID is smll. Therefore, the hirlities of oth the propgting surfe plsmon nd the moleule do not hnge from one terminl to the other. In ontrst, when the plsmon is oupled into the Ag nnowire t some point long the Ag nnowire lose to the nnontenn, the CIDs t the two remote terminls re very different nd n e mnipulted y the hirl nnostruture. In our se, the oupling ngle etween two Ag nnowires, the morphology of the Ag terminls, the distne of the oupling, et., n e used to mnipulte the hirlity of the propgting surfe plsmon. 31,32 The hnging hirlity of the propgting surfe plsmon n further hnge the moleulr hirlity nd thus lter the CID vlue. In our frited nnostruture, when the rossing ngle ws less thn 906, the CID inresed; when the rossing ngle ws lrger thn 906, the CID deresed. Thus, we experimentlly imged remotely exited ROA using hirl plsmon propgtion in Ag nnowires, nd this method hs the potentil to remotely determine moleulr hirlity or the solute onfigurtion or onformtion of hirl living ell. To diretly demonstrte the propgtion of ROA long the nnowire nd its remote detetion, we lso mesured the remotely exited ROA spetr (Figure 4). Figure 4 nd 4 show optil nd SEM imges of the Ag nnowire nd nnoprtile s the nnontenn for the eletromgneti enhnement. The remotely exited Rmn spetr were otined using left nd right polrized light (Supplementry Fig. S3). The lser ws rdited on terminl A, nd the signls were olleted from the rossing point etween the nnoprtile nd the nnowire. Figure 4 displys the remotely exited ROA spetrum resulting from the propgting plsmoni wveguide, whih is signifintly different from the moleulr ROA shown in Figure 1. For some of the Rmn peks in Figure 4, the signls were stronger thn tht t 1593 m 21,whih rose from hemil enhnements or the hirl nnostrutures. It is known tht hirl nnostrutures n produe hirl propgting plsmoni wveguide, whih my lso seletively enhne ertin Rmn peks in the remotely exited ROA. A detiled nlysis of the reltionship mong the ROA, the hemil mehnism nd the hirl propgting plsmoni wveguide is urrently eing pursued. CONCLUSIONS We experimentlly hieved remotely exited ROA imging y seleting single virtionl mode propgting long nnowire, using hirl plsmon propgting in Ag nnowires. The lol nd remote Rmn intensity of FGGO t 1593 m 21 were signifintly enhned when exited y left- nd right-irulrly polrized light. This virtionl mode exhiited lrge Rmn nd ROA intensities, without the influene of hemil enhnements. Furthermore, the lol nd remote CIDs, whih were lso signifintly inresed, were fitted nd n e suessfully mnipulted y the nnostruture of the Ag nnowires. To diretly demonstrte the propgtion of ROA long the nnowire nd its remote detetion, we lso mesured the remotely exited ROA spetr. This work my e potentilly utilized to remotely investigte the hirlity of moleulr strutures, s well s the solute onfigurtion or onformtion of hirl living ells. ACKNOWLEDGMENTS This work ws supported y the Ntionl Nturl Siene Foundtion of Chin ( nd ) nd the Progrm of Shenyng Key Lortory of Optoeletroni Mterils nd Tehnology (F ). 1 Brron LD, Bukinghm AD. Ryleigh nd Rmn sttering from optilly tive moleules. Mol Phys 1971; : Brron LD, Bogrd MP, Bukinghm AD. 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