The structure, morphology, and the metal-enhanced fluorescence of nano-ag/zno core shell structure

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1 Appl Nnosci (2015) 5: DOI /s y ORIGINAL ARTICLE The structure, morphology, nd the metl-enhnced fluorescence of nno-ag/zno core shell structure Yue Zho Ynli Ding Xing Peng Mingto Zhou Xioyn Ling Jihu Min Linjun Wng Weimin Shi Received: 14 Mrch 2013 / Accepted: 29 July 2014 / Pulished online: 12 Septemer 2014 The Author(s) This rticle is pulished with open ccess t Springerlink.com Astrct Nno-polycrystlline silver (Ag) prticles with the dimeter of 60 nm were synthesized y the reducing gent sodium citrte. An morphous zinc oxide (ZnO) shell lyer ws then coted on the surfce of silver prticles using wet chemicl method. The Ag/ZnO core shell structure ws chrcterized y scnning electron microscope, trnsmission electron microscopy, ultrviolet visile spectroscopy nd fluorescence (FL) mesurement. The results showed tht nno-ag/zno core shell prticles with n verge dimeter of *100 nm were prepred successfully, nd the FL intensity of Rhodmine 6G (R6G) mixed with Ag/ZnO nnoprticle ws 53 % greter thn tht of the sme mount of R6G without ny nnoprticles, which my e relted to the effect of surfce plsmon resonnce. Keywords Silver prticles Ag/ZnO core shell structure Metl-enhnced fluorescence Introduction Fluorescence (FL) detection is centrl technology in iologicl reserch nd clinicl chemistry. Engineering of dvnced sustrtes is n effective wy to improve the FL detection cpility nd to reduce the cost (Asln et l. 2008). However, ecuse the sensitivity of FL is still insufficient to meet the needs of some specil fields, it is necessry to develop new methods to further enhnce the Y. Zho (&) Y. Ding X. Peng M. Zhou X. Ling J. Min L. Wng W. Shi Deprtment of Electronic Informtion Mterils, Shnghi Leding Acdemic Disciplines, Shnghi University, Shnghi, People s Repulic of Chin e-mil: zhoyue1976@sohu.com sensitivity of FL detection. Some scientists found tht the FL emission of dyes mixed with nno-metl prticles could e strongly enhnced (Geddes nd Lkowicz 2002), which ws cused y surfce plsmon resonnce etween metl nnoprticle nd incident light (Asln 2005). Mny nnomterils, such s gold, copper, silver nd zinc, exhiit metl-enhnced fluorescence (MEF) (Ahmed et l. 2008). Compring with other metls, silver hs received much ttention due to its excellent MEF properties (Chowdhury et l. 2006). However, the direct contct etween metl prticles nd dyes might result in FL quenching due to the non-rdition energy trnsfer (Cmpion et l. 1980), therefore the spcer lyer etween metl prticles nd dye is necessry to eliminte this quenching effect. ZnO is one of the most ttrctive oxide semiconductors used in MEF ppliction. The MEF mechnism of ZnO nnostructure is due to the chnge in photonic mode density nd/or reduction in self-quenching of fluorophores (Dorfmn et l. 2006; Zhng et l. 2010; Yn-Song et l. 2012). In nnoprticles with core shell structure, such s Au/ZnO (Krishn Knt Hldr 2008), Ag/SiO 2 nd ZnO/ Ag (Li et l. 2010), new excited properties re found while the respective opticl-electricl properties of the core nd shell remin. In ddition, the core shell structure could e prepred y numer of methods, such s templte-confined synthesis routes, high-temperture methods, hydrotherml synthesis (Li et l. 2008) nd wet chemicl methods (Xue-lin Tin et l. 2006). In this report, the Ag prticles were successfully prepred y chemicl reduction nd then the ZnO shell lyer ws coted onto the surfce of silver prticles using wet chemicl method. The structure nd the opticl properties of the ZnO/Ag core shell prticles were studied y trnsmission electron microscopy (TEM), scnning electron

2 522 Appl Nnosci (2015) 5: microscopy (SEM), ultrviolet visile spectroscopy (UV VIS) nd FL. Experimentl AgNO 3 (15.7 mg) ws dissolved into 100 ml of de-ionized (DI) wter in eker with mgnetic stirring. The reducing gent, 1 % C 6 H 5 O 7 N 3 2H 2 O (2 ml) ws dded into the AgNO 3 solution. Then, this mixed solution ws mgnetic stirring in n 80 C wter th for 15 min. After rection, the Ag-sol solution ws removed nd cooled to room temperture. Then, the Ag-sol ws purified y centrifuging in ethnol three times. The wet chemicl method ws used to cot the ZnO shell on Ag nnoprticles. The Zn (NO 3 ) 6H 2 O (14.9 mg) ws dissolved in 10 ml DI wter nd then triethnolmine (NA) (1 ml) ws dded into this solution s n ssistor. This s-prepred mixture ws slowly poured into the s-prepred Ag-sol, followed y mgnetic stirring in 80 C wter th for out 2 h. Finlly, the cooled Ag/ ZnO sol ws purified y centrifuging in ethnol three times. The morphology of Ag/ZnO core shell structure ws oserved using n SEM (JSM-6700F). In order to crry out the SEM oservtion, the SEM smple ws prepred y the Ag/ZnO sol dripped on glss nd then dried in ir. A TEM (JEM-2010F) equipped with energy dispersive spectrometer (EDS) ws used to oserve the crystl structure nd nlyze the elementl distriution of Ag/ ZnO core shell structure. The Ag/ZnO ethnol sol ws dripped into copper mesh nd then dried in ir for TEM oservtion. FL mesurements were crried out with PL spectrometer (SPEX1403, SPEX) to study the opticl properties using 500-nm He Cd lser s the excittion source. The opticl sornce spectr of s-prepred Ag nnoprticles nd the Ag/ZnO core shell structure were otined y UV vis spectrophotometer (v-570, FASCO) in the visile rnges. The dimeter of the Ag/ZnO core shell nnoprticles ws clculted y Imge Tool (UT- HSCSA) nlysis of the SEM imge nd the TEM imges. All mesurements were crried out t room temperture under mient tmosphere. Results nd discussion The sornce spectr of the AgNPs nd the Ag/ZnO core shell structure in DI wter were presented in Fig. 1,, respectively. The sorption pek of the Ag nnoprticle ws centered t 427 nm s shown in Fig. 1. This pek corresponds to trnsverse mode (TM) of the loclized surfce plsmon resonnce (LSPR) of the nno-ag Asornce(.u.) wvelength(nm) Wvelength (nm) Fig. 1 UV Vis spectr of the sliver prticles () nd the Ag/ZnO core shell structure () prticles. According to Brus theory (Brus 1986), the clculted prticles size ws out 60 nm. In ddition, two sorption peks centered t 467 nd 356 nm re shown in Fig. 1, which might e ttriuted to the sorption of the Ag nnoprticle nd the ZnO shell lyer, respectively. Compring Fig. 1 nd, the sorption pek of the Ag nnoprticle ws red-shifted from 427 to 467 nm. This oservtion ws consistent with the results in the previous ppers, in which the LSPR peks of Ag nnoprticles coted with SiO 2 exhiited significnt red-shift. This phenomenon could e explined y the chnge of the refrctive index surrounding the nno-ag prticles. The effect of the surrounding medium on the plsmon resonnce wvelength of sphericl nnoprticles could e qulittively descried s k p ¼ k p; ð2n 2 0 þ 1Þ1=2 3 1=2 k p; f1 þ 1 3 ½n 0 1 g, where k p is the prticle plsmon wvelength, k p; is the ulk plsmon wvelength, nd n 0 is the index of refrction of the surrounding medium. Since coting porous ZnO lyer on the Ag nnoprticle, the surrounding medium could e considered s compound of ZnO nd ir (Kelly et l. 2003). According to the effect medium theory (Choy 1999), the Mxwell Grnett formul, the effect medium of this composite lyer, e eff, could e estimted from e eff e ir e eff 2e ir ¼ f e ZnO 1 e ZnO þ2, where e ir 1 is the dielectric constnt of ir, e ZnO is the dielectric constnt of ZnO, nd f is the volume frction of ZnO which incresed with the continuous ZnO coting. The incresing f resulted in n incresing e eff, which demonstrted n incresingn 0. As result, the LSPR pek of Ag nnoprticles ws redshifted continuously with the incresing n 0. Moreover, the theory sorption pek of nno-zno ws centered t 370 nm, corresponding to the nd gp of 3.46 ev (Zhu-xi et l. 2002). But in this pper, the LSPR pek of nno-zno shell ws lue shifted from 370 to 356 nm, which might e cused y the quntum size effect (Asl et l. 2011). As shown the HRTEM imge in Fig. 3, the morphous ZnO shell lyer existed on the surfce of Ag prticle, nd some smll ZnO nno-crystls might e included in this shell lyer, which could led to the lue-shift of the LSPR pek of nno-zno lyer. Asornce(.u.)

3 TEM imges of the Ag/ ZnO composite structure () nd the HRTEM imge of the selected re (1) in TEM imge () Figure 2 shows the SEM imge of the surfce morphology of Ag/ZnO core shell prticles on the FTO

3 Appl Nnosci (2015) 5: Fig. 2 The SEM imge of Ag/ ZnO core shell prticles () nd the dimeter distriution of the prticles in SEM imge () Dimeter(nm) Prticle numer Fig. 3 TEM imges of the Ag/ ZnO composite structure () nd the HRTEM imge of the selected re (1) in TEM imge () Figure 2 shows the SEM imge of the surfce morphology of Ag/ZnO core shell prticles on the FTO glss. It ws oserved tht the nnoprticles were dispersed in the glss sustrte nd the shpe of composite prticles ws irregulr. Figure 2 shows the dimeter distriution of the prticles in Fig. 2. The dimeter of the prticles ws in the rnge of nm nd the verge dimeter of these prticles ws out 188 nm. However, the dimeter of severl prticles ws over 260 nm or elow 100 nm. A lrge rnge of the dimeter of the Ag/ZnO core shell prticles in SEM imge might e ttriuted to the gglomertion of nnoprticles, which ws cused y the ond energy (vn der Wls forces), the mgnetic forces or the surfce purity in the surfce of the Ag/ZnO core shell prticles (Shen et l. 2009). The TEM imge is shown in Fig. 3, which gve the structurl morphology of the Ag/ZnO core shell prticles. The drker prt in imge ws considered to e the Ag core prticles nd the light color plce is the ZnO shell lyer, which could e confirmed y the EDS results in Fig. 4. Figure 3 is the HRTEM imge of the select re (1) in Fig. 3. Furthermore, the oundry of the Ag core nd the ZnO shell lyer is shown y the lck rrow in Fig. 3. It is clerly seen tht the Ag prticle ws coted y thin ZnO shell nd the lttice prmeter of Ag nnoprticle ws out nm, which corresponds to the (111) plnes (Qiu et l. 2007). The imge in Fig. 3 lso gve evidence tht the ZnO shell ws n morphous structure nd its clculted thickness ws out nm. Moreover, the clculted verge dimeter of the Ag core ws out 60 nm, ut this conclusion ws not consistent with the result in Fig. 2, which ws due to the ggregtes of Ag/ ZnO core shell structure in SEM smples, s discussed ove. The element distriution of Ag/ZnO core shell prticles ws otined y EDS mesurement, s showed in Fig. 4. Figure 4 shows the element distriution of the select loction (1) in Fig. 3, nd Fig. 4 shows tht of the select loction (2) in Fig. 3. Both Fig. 4, shows tht the select loction (1) or (2) ws consisted of the vst mjority of silver nd only smll prt of the zinc oxide. Figure 4c shows the element distriution of the select loction (5) in Fig. 3 nd the dominnt nno-zno ws existed in this select loction. The element Cu might come from the copper mesh nd no ny other element could e found in the EDS results. Figure 5 is the room temperture FL spectr of different solution. It shows tht the FL emission pek of Rhodmine 6G (dye) ws centered t 554 nm (Zhu et l. 2007), s shown in Fig. 5c. Curve () in Fig. 5 gives the FL emission of the smple prepred y the equl mount of dye mixed with Ag/ZnO prticles. Furthermore, the FL pek of the smple prepred y the dye mixed with Ag prticles ws exhiited in the curve (). In ddition, no FL emission of the Ag/ZnO prticles solution without dye ws shown in Fig. 5d. The result clerly shows tht oth the Ag/ZnO nd the Ag prticles could enhnce the FL emission intensity of the dye. The FL intensity of R6G mixed with Ag/ZnO

4 524 Appl Nnosci (2015) 5: Intensity(.u.) c 0 d Wvelength(nm) Fig. 5 Photoluminescence emission spectr of the dye mixed with Ag/ZnO prticles (), the dye mixed with Ag prticles (), R6G (c) nd Ag/ZnO prticle without dye (d) to the non-rdition energy trnsfer etween dye moleculr nd the metl (Yng et l. 2011). Conclusion Fig. 4 EDS spectr of Ag/ZnO core shell prticles in TEM imge, selected re (1); selected re (2); c selected re (5) nnoprticle ws 53 % lrger thn tht of the sme mount of R6G without ny nnoprticles, which might e enhnced y the plsmonic interction of metl surfce. This result indictes tht the ZnO shell ws used to mintin certin distnce etween the fluorescent molecule nd the metl core, which cused the interction etween the surfce plsmon nd excittion light field to produce FL emission enhncement. Furthermore, the ZnO shell lyer lso could enhnce the FL emission of dye, which ws cused y the chnge in photonic mode density nd/or reduction in self-quenching of fluorophores for ZnO nnostructure (Strino 2012). But the FL intensity of R6G mixed with Ag nnoprticle ws only 13 % lrger thn tht of the sme mount of R6G solution, which might e due In summry, the Ag/ZnO core shell structure ws successfully prepred through simple nd relile method, which is less reported in the pulictions. The verge dimeter of nno-ag/zno core shell prticles ws out 188 nm. The FL intensity of R6G mixed with Ag/ZnO nnoprticle ws 53 % lrger thn tht of the sme mount of R6G without ny nnoprticles. But the FL intensity of R6G mixed with Ag nnoprticle ws only 13 % lrger thn tht of the sme mount of R6G solution. Moreover, the preprtion process of Ag/ZnO core shell structure might e further improved in future experiments nd posttretment must e used to promote the crystlliztion of the ZnO shell lyer. In ddition, this optimized Ag/ZnO core shell structure might e used for FL io-sensing pplictions in future. Acknowledgments This work ws supported y Progrm for Chngjing Scholrs, Innovtive Reserch Tem in University (No: IRT0739), Shnghi Leding Acdemic Disciplines (S30107), the Ntionl Nturl Science Foundtion of Chin (No ; No ) nd Shnghi Nturl Science Foundtion (09ZR ). Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution License which permits ny use, distriution, nd reproduction in ny medium, provided the originl uthor(s) nd the source re credited. References Ahmed M, Krns M, Goodson M, Rowe J, Hussin SM, Schlger JJ, Hong Y (2008) Toxicol Appl Phrmcol 233:

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