Journal of Colloid and Interface Science 398 (2013) 1–6

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Journal of Colloid and Interface Science

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Porphyrin metal complex monolayer-protected gold nanorods: A parallel facilesynthesis and self-assembly

Chenming Xue a, Ozgul Birel a,b, Yannian Li a, Xiang Ma a, Min Gao a, Augustine Urbas c, Quan Li a,⇑a Liquid Crystal Institute, Kent State University, Kent, OH 44242, United Statesb Department of Chemistry, Mugla University, Mugla 48121, Turkeyc Materials and Manufacturing Directorate, Air Force Research Laboratory WPAFB, OH 45433, United States

a r t i c l e i n f o

Article history:Received 8 December 2012Accepted 6 February 2013Available online 22 February 2013

Keywords:Gold nanorodFluorescenceQuenchSelf-assemblyPorphyrin metal complex

0021-9797/$ - see front matter � 2013 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2013.02.014

⇑ Corresponding author. Fax: +1 330 672 2796.E-mail address: qli1@kent.edu (Q. Li).

a b s t r a c t

Porphyrin metal (Zn, Cu, and Mg) complex monolayer-protected gold nanorods (GNRs) were, for the firsttime, synthesized. Their synthesis was easy to access by mixing porphyrin encapsulated GNRs with cor-responding excess soluble metal salts in solution, followed by the facile purification through centrifuga-tion and sonication due to the gravity of the GNRs and their solubility in organic solvents. Furthermore,the resulting three GNRs exhibited distinct spectroscopic properties and were able to self-assemble intoside-by-side arrays driven by p–p intermolecular interactions of the surface metal porphyrinchromophores.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Considerable efforts have been devoted to gold nanoparticlesowing to their diverse and distinct properties [1]. Compared tothe widely investigated isotropic spherical gold nanoparticles(GNPs), anisotropic gold nanorods (GNRs) are particularly fascinat-ing and challenging [2]. GNRs exhibit unique strong and tunablesurface plasmon resonances (SPRs) ranging from visible to near-IR region depending on their size and shape, providing manypromising applications in negative index materials, optics, sensorsand biological imaging devices, etc. [3]. It is well established thatthe surface modification of GNR can offer a versatile means tointroduce functionalities as well as to stabilize the particles. Forexample, both fluorescence quenching and enhancement could ex-ist when controlling organic chromophore in appropriate distancesto the surface of gold nanoparticles [4]. In addition, some func-tional molecules on the surface of GNPs could work as molecular‘glue’ to assemble nanostructures [5]. As a result, GNRs assemblingin specific patterns, including end-to-end and side-by-side fash-ions can exhibit unique collective properties which are differentfrom those of both individual GNR and bulk materials [6,7] Cur-rently, most molecular ‘‘glues’’ are in ionic form, which are less sta-ble and solvent dependent, i.e. only compatible to aqueous media

ll rights reserved.

[8]. In contrast, non-ionic interactions are able to build stableGNR self-assemblies in organic media, which offers a wider rangeof potential research avenues.

Strong p–p intermolecular interaction via molecular orbitaloverlapping has the capability to direct supramolecular self-assemblies in a controlled manner [9]. One excellent candidatefor GNR surface functionalization would be porphyrin due to itslarge conjugated aromatic system, not only exhibiting uniqueopto-electronic properties but also being able to provide strongp–p interactions for ideal molecular self-assemblies [10]. Addition-ally, porphyrin is able to coordinate with many metal ions, whichcan result in additional tunability and functionalities [11]. So farGNRs are mainly prepared in an aqueous medium by a seed-med-iated growth method [12], where cetyltrimethylammonium bro-mide (CTAB) is widely used as a shape-directing surfactant toselectively form a densely packed dynamic layer around the side-wall of a growing GNR with its two ends free from CTAB for ananisotropic growth along the longitudinal axis. GNR covered witha CTAB layer is water-soluble but dynamically unstable. Comparedwith GNRs in water, dispersing them in organic media is appealingsince their low interfacial energies should allow for a high degreeof control during solution and surface processing. Interestingly,to date only few organo-soluble thiol monolayer-protected GNRswere reported since the seemingly trivial work of exchanging CTABwith organic thiol molecules was found to be difficult [13].Undoubtedly, directing organo-soluble monolayer-protected GNRsinto well-organized self-assemblies is even more challenging andhas not been explored extensively.

Fig. 1. Molecular structure of porphyrin thiol (POR) and schematic synthesis of metal complex monolayer-protected GNR (MP-GNR): ZnP-GNR, CuP-GNR and MgP-GNR.

Fig. 2. Schematic illustration of preparation of MP-GNR starting from POR-GNR with metal salts in a facile synthesis.

2 C. Xue et al. / Journal of Colloid and Interface Science 398 (2013) 1–6

Here we report the synthesis of Zn, Cu and Mg porphyrin com-plex thiol monolayer-protected GNRs (ZnP-GNR, CuP-GNR, andMgP-GNR) (Fig. 1). Their synthesis was easy to access just by mix-ing POR-GNR with corresponding excess soluble metal salts[Zn(OAc)2, Cu(OAc)2 and MgBr2] in solution, followed by a facilepurification through centrifugation and sonication due to the grav-ity of GNRs and their solubility in organic solvents (Fig. 2). As ex-pected, the three resulting GNRs encapsulated with porphyrin Zn,Cu or Mg complex through strong covalent Au–S linkages were sol-uble and stable in various organic solvents such as THF, CH2Cl2,CHCl3 and toluene. With porphyrin molecules on GNR surface, dif-ferent metals can be attached to the GNR surface and the interac-tions between metal ions and GNR can be investigated. Suchnanostructures with different metal elements have attracted inten-sive examination recently [14]. Beyond those recent studies, herewe present, for the first time, investigation of the interactions be-tween metal ions and anisotropic GNR. The mono thiol substitutedporphyrin molecules and their corresponding metal complex canstabilize GNR in organic solvents forming well-dispersed GNR solu-tion. When dried, all these porphyrin metal complex thiol mono-layer-protected GNRs (MP-GNRs) were found to be able to self-

assemble into well-organized side-by-side arrays, even from highlydilute solution.

2. Experimental

2.1. Materials and measurements

All chemicals and solvents were purchased from commercialsupplies and used without further purification. HAuCl4 is a30 wt.% in diluted HCl solution. UV–vis spectra were collected ona PerkinElmer Lambda 25 UV–vis spectrometer at the resolutionof 1 nm. Fluorescence spectra were recorded on a FluoroMax-3spectrafluorometer of Horiba scientific. For transmission electronmicroscopy (TEM) observation, solution samples were first dis-persed on TEM Cu and Mo grids pre-coated with thin carbon film(Cu-400 CN, Mo-400 CN) purchased from Pacific Grid Tech. Aftercompletely dried, they were studied using a FEI Tecnai TF20 FEGTEM equipped with a 4 k UltraScan CCD camera for digital andan EDAX energy-dispersive X-ray spectrometer (EDX) for elemen-tary analysis.

C. Xue et al. / Journal of Colloid and Interface Science 398 (2013) 1–6 3

2.2. Preparation of porphyrin thiol (POR)

The synthesis of POR was followed by the reported method[13e].

2.3. Preparation of metal free porphyrin monolayer-protected goldnanorod (POR-GNR)

The CTAB-coated GNRs were freshly prepared by the seed-med-iated growth method. The solution of CTAB-GNRs was centrifugedat 7500 rpm per 20 min several times to remove the excessiveCTAB and other solution components and redispersed in water(1.5 mL). Then, this aqueous solution of GNRs was added dropwiseto a solution of the POR (50 mg) in THF (40 mL) with stirring underthe protection of nitrogen. The color of the reaction mixture is pur-ple. The reaction mixture was continued to stir at room tempera-ture for 3 days and centrifuged. To improve the GNRs with thiolmolecules over the surface, the precipitates were dispersed inCHCl3 and sonicated, POR (10 mg) was added to the solutions.The solution was stirred for another 24 h and centrifuged. This pro-cedure was repeated another three times. The as-prepared GNRswere centrifuged and washed with CH2Cl2 several times until therewas no UV–vis signal in the top layer solution, which means therewere no free thiols in the system. To prepare highly diluted solu-tion, one drop of 1 mg/mL POR-GNR was diluted in 1 mL CH2Cl2.For TEM experiment, one drop of this diluted solution was caston the grid and further dried.

2.4. Preparation of decane thiol monolayer-protected gold nanorod(C10-GNR)

The C10-GNR was prepared following the above method byreplacing POR with decane thiol compound.

400 500 600 700 800 900

POR CTAB-GNR POR-GNR ZnP-GNR CuP-GNR MgP-GNR

Wavelength (nm)

(a)

500 550 600 650

POR-GNR ZnP-GNR CuP-GNR MgP-GNR

Wavelength (nm)

(c)

Cou

nts

Fig. 3. (a) UV–vis of POR, CTAB-GNR, POR-GNR and MP-GNR. (b) Fluorescence spectraexcitation wavelength at 420 nm. Inset: The enlarged spectra of POR-GNR and MP-GNFluorescence spectra of POR-GNR and MP-GNR with excitation wavelength at 480 nm. (

2.5. Preparation of Zn, Cu and Mg porphyrin complex monolayer-protected gold nanorods (MP-GNR: ZnP-GNR, CuP-GNR and MgP-GNR)

For preparing Zn2+ incorporated POR-GNR, 2 mL of methanolwas firstly added into 2 mL as prepared POR-GNR CH2Cl2 solution.Then Zn(OAc)2 (2 mg) was added into the mixture and stirred for0.5 h. The resulting mixture was centrifuged and the top layerwas removed. After another CH2Cl2 (4 mL) was added, the solidin the bottom was sonicated and dissolved. This wash step wasexecuted for several times until there was no UV–vis or IR signalfor the top layer solution. The resulting hybrid GNR was namedas ZnP-GNR. Same procedure was carried for preparing CuP-GNR,in which Cu(OAc)2 (2 mg) was used for incorporating Cu2+ ion.For preparing Mg2+ incorporated POR-GNR, CH2Cl2 (2 mL) wasfirstly added into as-prepared POR-GNR CH2Cl2 solution (2 mL).Then MgBr2 (2 mg) was added into the mixture and stirred. Theresulting mixture was centrifuged and top layer was removed.After another CH2Cl2 (4 mL) was added, the solid in the bottomwas sonicated and dissolved. This wash step was executed for sev-eral times until there was no UV, IR signal for the top layer solu-tion. The resulting hybrid GNR was named as MgP-GNR. Toprepare a diluted solution, one drop of 1 mg/mL MP-GNR was di-luted in 1 mL of CH2Cl2. For TEM experiment, one drop of this di-luted solution was cast on the grid and further dried.

2.6. I2-induced decomposition of POR-GNR and MP-GNR

Iodine (1 mg) was added to POR-GNR in CH2Cl2 (2 mL) followedby stirring at room temperature for 0.5 h to destroy Au–S bonds.Afterwards, since the excess I2 made the solution pink–red color,aqueous (NH4)2SO3 solution (0.5 M) was added to the above organ-ic solution and shake vigorously. The pink–red color disappearedimmediately and the organic layer appeared typical light yellow

8500 8750

15001000

12501000

Zn

Cu

Mg

Energy (eV)

(d)

600 700 800

POR POR-GNR ZnP-GNR CuP-GNR MgP-GNR

I (a.

u.)

Wavelength (nm)

500 600 700 800

(b)

of POR-GNR, MP-GNR, and free POR molecules cleaved from the POR-GNR withR. The curve of free POR was from the original solution diluted for 10 times. (c)d) EDX analysis of MP-GNR.

Table 1Porphyrin characteristic peaks of POR-GNR and MP-GNR detected in UV–vis andfluorescence spectra in Fig. 3a and b.

GNR UV–vis Fluorescence

POR-GNR 431 653, 737ZnP-GNR 426 607, 652CuP-GNR 415 710 (very weak)MgP-GNR 453 657, 724

4 C. Xue et al. / Journal of Colloid and Interface Science 398 (2013) 1–6

color of free POR. Same method was applied to metal MP-GNRs todetach the metal porphyrin complexes.

3. Results

3.1. UV and fluorescence spectroscopy study

The as prepared POR-GNR was proved by 1H NMR and Ramanspectroscopy [13e]. UV–vis and fluorescence spectra of POR-GNRand three MP-GNR in CH2Cl2 are shown in Fig. 3a and b, respectively,with the characteristic peak positions listed in Table 1. Metal freeporphyrin POR-GNR exhibited one porphyrin absorption and twoGNR signature plasmonic absorptions whereas the free POR showedan intense characteristic porphyrin peak at 421 nm (Fig. 3a). As ex-pected, two characteristic plasmon peaks were observed for all theGNR samples (Fig. 3a): a strong peak in the near-infrared region (e.g.704 nm for CTAB-GNR) corresponded to the longitudinal SPR (LSPR),and a weaker peak in the visible region (e.g. 514 nm for CTAB-GNR)corresponded to the transverse SPR (TSPR). Compared to the initialCTAB-GNR, the TSPR peak of POR-GNR red-shifted (532 nm) whilethe LSPR blue-shifted (685 nm), resulting from the possible forma-tion of POR-GNR self-assemblies in side-by-side fashion [15]. Aftercoordinating with Zn2+, Cu2+ and Mg2+ metal ions respectively, thethree resulting MP-GNRs exhibited new characteristic absorptionpeaks both from porphyrin chromophores and GNR compared tothe free POR and POR-GNR. Depending on different metals, SPRpeaks of metal-porphyrin chromophores on GNR shifted and theirposition are listed in Table 1. For MP-GNR, the LSPR even morered-shifted while its TSPR remained their position, indicating themetal atoms had influences on plasmonic properties of GNRs whichshifted their LSPR but not their TSPR.

For the fluorescence spectra shown in Fig. 3b, all the three MP-GNRs were significantly quenched like POR-GNR as shown inFig. 3b. The quenching phenomenon was consistent with the

Fig. 4. (a) Molecular structure of porphyrin metal complex on GNR surface. Most stableand (c) side view.

experimental [4,16] and theoretical [17] investigations on chro-mophores close to spherical GNPs. The quenching effect was dueto the non-radiative energy transfer from porphyrin groups tothe gold nanoparticle since the porphyrin molecules (includingmetal complex ones) are very close to the GNR surface [4]. Thenanostructure of porphyrin metal complex thiol coated GNR waspresented in Fig. 4. Based on the length of the most stable porphy-rin thiol molecule conformation calculated by Gaussian 09 withDFT B3LYP method (3.43 nm) [18] and the characteristic distanceof Au–S bonds (2.3 Å) [19], the total length of porphyrin thiol mol-ecule on GNR can be calculated as 3.7 nm. The fluorescence peakshape of MP-GNR was metal dependent as shown in Fig. 3b (inset).Compared to the metal free POR-GNR (653 and 737 nm), ZnP-GNRhad significantly blue-shifted peaks (607 and 652 nm). CuP-GNRhad a peak at 710 nm with much weaker intensity. For MgP-GNR, the peak positions did not shift much (657 and 724 nm)but the shapes were obviously changed. Although the precisemechanism is unclear, these changes show that the different metalinfluences on fluorescence are substantial. Since there were no freePOR molecules in the GNR solution, the altered typical porphyrinfluorescence peaks were only from POR on GNR surface. Also asMP-GNRs displayed varied fluorescence signals, this proved thedifferent metals were coordinated on POR-GNR. Additionally,when POR molecules were released from GNR by adding I2 whichdestroyed the Au–S linkages, the metal-POR complexes also exhib-ited completely different fluorescence spectra compared to POR aswell as MP-GNR, as shown in Fig. 5. One obvious example is Cu-porphyrin complex: when on GNR surface, the fluorescence dis-played very weak peak (almost no signal), when in free-state,strong typical fluorescence peak appeared. Except for Mg-POR,Cu- and Zn-POR complexes showed similar peaks as free POR.However when on GNRs, their fluorescence signals were totallydifferent. For Mg-POR, the fluorescence peaks in free-state (533and 576 nm) were different from on GNR surface (657 and724 nm). This indicated that porphyrin metal complex wasstrongly affected by attachment to the GNR surface. Since the fluo-rescence of GNR excited at 480 nm was also investigated [20], hereit was also studied for MP-GNR (Fig. 3c). They all exhibited similarpeak shapes, indicating the porphyrin metal complexes had littleinfluence on GNR’s fluorescence. This is quite different from themetal-dependent porphyrin fluorescence: when forming metalcomplexes, the porphyrin molecules had changed properties andwhen combined on GNR surface, the properties further signifi-cantly changed.

structure of POR calculated by Gaussian 09 proposed on GNR surface: (b) top view,

500 600 700 800

POR-GNR+I2

ZnP-GNR+I2

CuP-GNR+I2

MgP-GNR+I2

Inte

nsity

Wavelength (nm)

Fig. 5. Fluorescence spectra of solutions after adding I2 to POR-GNR and MP-GNRwith excitation wavelength at 420 nm.

C. Xue et al. / Journal of Colloid and Interface Science 398 (2013) 1–6 5

Fig. 3d is the energy-dispersive X-ray spectrometer (EDX) anal-ysis of metal elements in MP-GNR, which further confirmed thesuccessful coordination of metal ions on GNR surface. Apart fromthe CuP-GNR which was casted on a carbon film coated Mo grid,all the rest GNR samples were casted on carbon film coated Cugrids.

Interestingly, when covering GNR with metal atoms throughporphyrin coordination effect, the UV–vis absorption and fluores-cence of porphyrin molecules (excited at 420 nm) changed whilefor GNR the LSPR altered but the fluorescence (excited at480 nm) remained unchanged. Since the metal porphyrins havebeen intensively studied before, the observations of MP-GNRs werevery attractive here. When the metal atoms coordinated by theporphyrin get very close to the GNR surface (ca. 2.5 nm) as shownin Fig. 4, the surface electronic oscillations on GNR can be expectedto interact with this layer of metal ions. It is not clear why the LSPRwould be significantly affected when the TSPR remains unchanged.It is possible that the packing density at the tips of the rods and atthe center is different, leading to a difference in metal ion concen-tration. This could then bring differences between the tip localizedTSPR and LSPR which is distributed across the body of the nanorod.

Fig. 6. TEM images of (a) C10-GNR, (b) ZnP-GNR, (c) CuP-GNR, (

MP-GNRSelf-Assemb

Fig. 7. Schematic presentation o

This could also explain why the fluorescence of GNR excited at480 nm was independent on different metals. For the fluorescenceexperiment, since the excitation wavelength used was 480 nm,which was very close to the 514 nm (TSPR), the emission wasnot changed.

3.2. Self-assembly behavior investigation

Well-organized side-by-side assembly arrays of MP-GNR andthe original Por-GNR dried from diluted solutions were revealedby TEM observation as shown in Fig. 6. Based on measuring 500POR-GNR, the average size of GNR has been found as 41 � 14 nm,with aspect ratio of 2.9. As the POR molecules stand on the GNRsurface, the flat side surface of GNR with small curvature provideda large contact area for porphyrin metal complex moieties on adja-cent GNR to reach each other, especially in the longitudinal direc-tion. Thus, the strong p–p intermolecular interactions of porphyrinmetal complex moieties promote GNRs to form side-by-side ar-rays, rather than forming end-to-end assemblies. Comparing tothe decane thiol coated GNR (C10-GNR) which cannot form assem-blies drying from highly diluted solution (Fig. 6a) due to the lack ofattractive intermolecular interactions, these porphyrin and theirmetal complex coated GNRs formed side-by-side assembly arrays.These side-by-side assemblies were similar to the perylene coatedGNR [21]. Fig. 7 is the model description of the side-by-side MP-GNR self-assemblies driven by p–p interactions from porphyrinmetal complex moieties on adjacent GNR surface. From the micro-graphs, the separation of MP-GNRs (�2 nm) is seen to be well be-low the maximum thickness of a POR bilayer (�7 nm). Thisindicates that the porphyrin-metal complex molecules on adjacentGNR are likely intercalated. The narrow, controlled gaps present inthese assembled GNRs have potential applications in surface-en-hances Raman scattering and localized surface plasmon resonancesensing, which rely on field enhancement and confinement insmall metal features [22].

4. Conclusion

In conclusion, metal complex thiol monolayer-protected GNRswere, for the first time, synthesized. Their synthesis was easy to ac-

d) MgP-GNR, and (e) Por-GNR dried from diluted solutions.

le

f self-assembled MP-GNR.

6 C. Xue et al. / Journal of Colloid and Interface Science 398 (2013) 1–6

cess by mixing POR-GNR with corresponding excess soluble metalsalts in solution, followed by a facile, direct, and efficient purifica-tion through centrifugation and sonication due to the gravity of theGNR and their stability and solubility in organic solvents. This syn-thetic method demonstrated here is widely effective in construct-ing many other functional, metal complex protected metalnanoparticles, which enables the rapid exploration of the diversefamily of metal complex protected metal nanoparticles. The result-ing three GNRs respectively encapsulated with Zn, Cu and Mg por-phyrin complexes via strong covalent Au–S linkages were organo-soluble, and exhibited distinct spectroscopic properties. Further-more, these metal porphyrin complex protected GNRs were foundto be able to self-assemble into side-by-side arrays driven by p–pintermolecular interactions of the surface porphyrin chromoph-ores. Complementary to the widely used ionic interactions in aque-ous medium, the p–p interactions provide an efficient way forprocessing self-assemblies of GNRs in various organic media. Thenew family of GNRs functionalized with metal porphyrin com-plexes would not only open the avenue for creating and studyingthe special metal atoms-GNR structure, but also provide an excit-ing impetus for developing self-assembled GNRs and utilizing theirproperties for practical applications such as SERS and LSPR sensing.

Acknowledgments

This work was supported by the Air Force Office of Scientific Re-search (AFOSR FA9550-09-1-0254). Supports from AFOSR-MURI(FA9550-12-1-0037) and TUBITAK (2219) from Turkey were alsoacknowledged. The TEM data were obtained at the (cryo) TEMfacility at the Liquid Crystal Institute, Kent State University, sup-ported by the Ohio Research Scholars Program Research Clusteron Surfaces in Advanced Materials.

References

[1] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293.[2] (a) J. Park, G. von Maltzahn, M.J. Xu, V. Fogal, V.R. Kotamraju, E. Ruoslahti, S.N.

Bhatia, M.J. Sailor, Proc. Natl. Acad. Sci. U. S. A. 107 (2010) 981;(b) P. Zijlstra, J.W.M. Chon, M. Gu, Nature 459 (2009) 410;(c) X. Zhou, J.M. El Khoury, J. Colloid Interface Sci. 308 (2007) 381.

[3] (a) C.J. Murphy, T.K. San, A.M. Gole, C.J. Orendorff, J.X. Gao, L. Gou, S.E.Hunyadi, T. Li, J. Phys. Chem. B 109 (2005) 13857;

(b) C. Burda, X.B. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025;(c) C.Z. Li, K.B. Male, S. Hrapovic, J.H.T. Luong, Chem. Commun. (2005) 3924;(d) X. Huang, S. Neretina, M.A. El-Sayed, Adv. Mater. 21 (2009) 4880.

[4] P. Anger, P. Bharadwaj, L. Novotny, Phys. Rev. Lett. 96 (2006) 113002.[5] A.K. Boal, F. Ilhan, J.E. DeRouchey, T. Thurn-Albrecht, T.P. Russell, V.M. Rotello,

Nature 404 (2000) 746.[6] (a) Z.Y. Tang, Z.L. Zhang, Y. Wang, S.C. Glotzer, N.A. Kotov, Science 314 (2006)

274;(b) T.K. Sau, C.J. Murphy, J. Am. Chem. Soc. 126 (2004) 8648.

[7] S.A. Maier, P.G. Kik, H.A. Atwater, S. Meltzer, E. Harel, B.E. Koel, A.A.G. Requicha,Nat. Mater. 2 (2003) 229.

[8] (a) F. Caruso, Top. Curr. Chem. 227 (2003) 145;(b) X. Ma, A. Urbas, Q. Li, Langmuir 28 (2012) 16263.

[9] F.J.M. Hoeben, P. Jonkheijm, E.W. Meijer, A.P.H.J. Schenning, Chem. Rev. 105(2005) 1491.

[10] (a) Y. Chang, C. Wang, T. Pan, S. Hong, C. Lan, H. Kuo, C. Lo, H.C. Lin, E.W. Diau,Chem. Commun. 47 (2011) 8910;(b) C.Y. Liu, A.J. Bard, Acc. Chem. Res. 32 (1999) 235.

[11] D.S. Lawrence, T. Jiang, M. Levett, Chem. Rev. 95 (1995) 2229;L.D. Zhang, M. Fang, Nano Today 5 (2010) 128.

[12] T.K. Sau, C.J. Murphy, Langmuir 20 (2004) 6414.[13] (a) B.P. Khanal, E.R. Zubarev, Angew. Chem., Int. Ed. 46 (2007) 2195;

(b) Q. Dai, J. Coutts, J.H. Zou, Q. Huo, Chem. Commun. (2008) 2858;(c) Y. Li, D. Yu, L. Dai, A. Urbas, Q. Li, Langmuir 27 (2011) 98;(d) J.M. El Khoury, X.L. Zhou, L.T. Qu, L.M. Dai, A. Urbas, Q. Li, Chem. Commun.(2009) 2109;(e) C. Xue, Y. Xu, Y. Pang, D. Yu, L. Dai, M. Gao, A. Urbas, Q. Li, Langmuir 28(2012) 5956.

[14] E.K. Beloglazkina, A.G. Majouga, R.B. Romashkina, N.V. Zyk, N.S. Zefirov, Russ.Chem. Rev. 81 (2012) 65.

[15] H.S. Park, A. Agarwal, N.A. Kotov, O.D. Lavrentovich, Langmuir 24 (2008)13833.

[16] B. Dubertret, M. Calame, A.J. Libchaber, Nat. Biotechnol. 19 (2001) 365.[17] S. Vukovic, S. Corni, B. Mennucci, J. Phys. Chem. C 113 (2009) 121.[18] Gaussian 09, Revision C.01, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E.

Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J.Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A.Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N.Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A.Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M.Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L.Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg,S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J.Fox, Gaussian, Inc., Wallingford CT, 2010.

[19] H. Hakkinen, Nat. Chem. 4 (2012) 443.[20] S. Eustis, M. El-Sayed, J. Phys. Chem. B 109 (2005) 16350.[21] C. Xue, O. Birel, M. Gao, S. Zhang, L. Dai, A. Urbas, Q. Li, Phys. Chem. C 116

(2012) 10396.[22] N.J. Halas, S. Lal, W.S. Chang, S. Link, Chem. Rev. 111 (2012) 3913.