Helquat-Induced Chiroselective Aggregation of Au NPs - Nano Letters

Letter pubs.acs.org/NanoLett

Helquat-Induced Chiroselective Aggregation of Au NPs Dora Balogh,† Zhanxia Zhang,† Alessandro Cecconello,† Jan Vavra,‡ Lukas Severa,‡ Filip Teply,*,‡ and Itamar Willner*,† †

Institute of Chemistry, Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v. v. i., Flemingovo n. 2, 166 10 Prague 6, Czech Republic

‡

S Supporting Information *

ABSTRACT: Au nanoparticles (NPs) are functionalized with chiral (R) or (S) binaphthol phenylboronic acid ligands, (1a) or (1b). The (R)- or (S)-binaphthol phenylboronic acid ligands form donor−acceptor complexes with the chiral dicationic helicene, helquat (P)-HQ2+ or (M)-HQ2+, (2a) or (2b). The association constants between (1a)/(2a) and (1a)/ (2b) correspond to (7.0 ± 0.5) × 105 M−1 and (2.5 ± 0.3) × 105 M−1, respectively, whereas the association constants between (1b)/(2b) and (1b)/(2a) correspond to (4.0 ± 0.5) × 105 M−1 and (1.8 ± 0.3) × 105 M−1, respectively. Chiroselective aggregation of chiral binaphthol phenylboronic acid-capped Au NPs triggered by the chiral helquats, is demonstrated. KEYWORDS: Nanoparticles, chirality, aggregation, donor−acceptor interactions, binaphthol phenylboronic acid, helquat

T

Hg2+ ions. Metal nanoparticles functionalized with chiral capping layer were reported, and their use in asymmetric synthesis34−36 and chiral separation37 were demonstrated. Also, the interactions between Au NPs modified with chiral helicene capping layers were found to control the rate of aggregation of Au NPs, depending on the nature of chiral capping layer.38 Donor−acceptor interactions provide a general intermolecular recognition mechanism. Specifically, bipyridinium or bispyridinium ethylene salts acted as versatile electron acceptors for the formation of donor−acceptor complexes.39 Here we wish to report on the synthesis of Au NPs functionalized with (R−) or (S−) binaphthol, (1a) or (1b), as a chiral capping layer, and on the chiroselective controlled aggregation of the Au NPs using the chiral (P)-HQ2+ or (M)-HQ2+ dicationic helicene derivatives, helquats, (2a) or (2b). The aggregation process is controlled by donor−acceptor affinity interactions. The synthesis and characterization of the chiral dicationic helicene derivatives, helquats, was previously reported.40−43 Au NPs modified with the (R)- or (S)-binaphthol phenylboronic acid ligands, (1a) or (1b) were prepared by the primary covalent linkage of m-aminophenyl boronic acid units to the 16-mercaptohexadecanoic acid-capped Au NPs, followed by the covalent attachment of (R)- or (S)-binaphthol to the boronic acid ligands. The resulting Au NPs (13 nm diameter) were characterized by circular dichroism (see Figure S1 in

EXT The aggregation of metallic nanoparticles (NPs) finds growing interest due to the unique interparticle plasmonic coupling features and conductivity properties of the aggregated matrices.1−3 Different methods to aggregate metal nanoparticles were developed, including layer-by-layer deposition of chemically modified NPs on surfaces,4,5 electropolymerization of metal NPs functionalized with electropolymerizable units,6,7 polymer-induced aggregation of metal nanoparticle composites,8,9 and the autonomous aggregation of ligand-functionalized NPs, by bridging chemical units or complementary recognition interactions.10,11 The aggregation of Au NPs has been implemented to develop numerous sensor12,13 and biosensor14,15 platforms, to design electrochemical uptake/release systems,16 to develop nanoparticlebased switching systems,17,18 and more.19 For example, molecular-imprinted electropolymerized Au NPs on Au surfaces have been reported as selective sensing matrices for electrochemical sensing of TNT20 and for the surface plasmon resonance (SPR) detection of explosives21,22 or antibiotics.23 Different metal ions were detected through the color changes occurring upon aggregation of ligand-functionalized Au NPs. For example, phenanthroline-functionalized Au NPs were aggregated by Li+ ions,24 whereas carboxylic acid-modified Au NPs were aggregated by heavy metal ions,25 such as Pb2+, Cd2+, or Hg2+. Also, aggregation of Au NPs was demonstrated using interparticle H-bonds between the capping ligands.26 Similarly, the aggregation or deaggregation of nucleic acid-functionalized Au NPs were used for the optical detection of DNA,27,28 aptamer-substrate complexes,29,30 or metal ions31−33 such as © 2012 American Chemical Society

Received: August 27, 2012 Revised: October 4, 2012 Published: October 8, 2012 5835

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Figure 1. (A) Schematic chiroselective helquat-induced aggregation of chiral binaphthol boronic ester-capped Au NPs using donor−acceptor interactions. (B) Time-dependent UV−vis absorption spectra upon the aggregation of the (R)-modified Au NPs in the presence of (P)-HQ2+. (C) Time-dependent absorbance changes at λ = 537 nm upon (a) aggregation of the (S)- modified Au NPs in the presence of (P)-HQ2+ and (b) aggregation of the (S)-modified Au NPs in the presence of (M)-HQ2+. (D) Time-dependent absorbance changes at λ = 537 nm upon (a) aggregation of the (R)- modified Au NPs in the presence of (M)-HQ2+ and (b) aggregation of the (R)- modified Au NPs in the presence of (P)HQ2+. In all systems, the Au NPs ((4.5 ± 0.5) × 10−9 M) were interacted in triple distilled water (TDW) with the respective dicationic helquats (2 × 10−4 M). Panels C,D: data in red color correspond to experiments with (P)-HQ2+. Data in black color correspond to experiments with (M)-HQ2+.

The stochiometry of the complexes was obtained according to the Job’s plot method, with a total concentration of 1 × 10−4 M helquat and binaphthol phenylboronic ester. At a low concentration of the helquat the proportion of the binaphthol phenylboronic ester is higher, and we find a 1:2 complexation ratio, whereas at higher proportions of the helquat we observe a complexation ratio of 2:3, where two helquat molecules are probably interacting in between three binaphthol units (see Figure S5 in Supporting Information). The different affinities for the formation of the donor− acceptor complexes between the chiral dicationic helquat electron acceptors and the chiral binaphthol electron donors are reflected in the rate of aggregation of the respective Au NPs. Figure 1A depicts schematically the chiroselective, donor− acceptor stimulated aggregation of the functionalized Au NPs. The aggregation of the Au NPs was followed by UV−vis spectroscopy, Figure 1B. The mixing of the chiral donorfunctionalized Au NPs with the chiral dicationic helquats resulted in the aggregation of the particles reflected by the decrease in the plasmon absorbance band, and the shift in the absorbance band to longer wavelength and broadening of the UV−vis absorption spectra. The aggregation phenomenon is also visually imaged by a red-to-purple color transition and the ultimate precipitation of the Au NPs. These spectral changes are consistent with other reports on the aggregation of Au NPs.44 Control experiments revealed that Au NPs modified with mercaptoethanol lacking any donor functions, did not aggregate in the presence of the dicationic helquats (2a) or (2b), suggesting that, indeed, donor−acceptor complexes between the acceptor units and the donor-functionalized NPs

Supporting Information). The loading of the Au NPs with the chiral (R)- or (S)-binaphthol units was evaluated by absorption spectroscopy to be 820 ± 60 units per particle (for the determination of the loading, see Figure S2 in Supporting Information). The helquat (2a) or (2b) exhibits electron acceptor properties, reflected by a quasi-reversible cyclic voltammogram at 0.3 V versus Ag QRE (see Figure S3 in Supporting Information).Similar to other bipyridinium salts that form donor−acceptor complexes, the helquats (2a) and (2b) form diastereoisomeric complexes with (R)- or (S)binaphthol phenylboronic acid esters acting as electron donating groups. The binding constants between the (P) and (M) electron acceptor helquats (2a) and (2b) and the chiral electron donor (R)- or (S)-binaphthol phenylboronic acid esters were determined by following the quenching of the fluorescence of the (R)- or (S)-configured donors by the (P)/ (M) dicationic helquats. (For experimental details see Figure S4 in Supporting Information.) We find that the association constants between (M)-HQ2+, (2b), and (R)-configured, (1a), and of (P)-HQ2+, (2a), and (R)-configured, (1a), correspond to (2.5 ± 0.3) × 105 M−1 and (7.0 ± 0.5) × 105 M−1, respectively. That is, (2a) reveals an enhanced affinity toward the (R)-binaphthol phenylboronic ester, (1a). Similarly, the association constant of (2b) with (S)-binaphthol phenylboronic ester, (1b), and (2a) with (S)-binaphthol phenylboronic ester, (1b), correspond to (4.0 ± 0.5) × 105 M−1 and (1.8 ± 0.3) × 105 M−1, respectively. Thus, (2b) reveals an enhanced binding affinity toward binding of (1b), whereas (2a) shows an enhanced affinity for binding of (1a). 5836

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Figure 2. Time-dependent changes of the Au NPs aggregates followed by light scattering experiments, upon (A) interaction of the (S)-modified Au NPs in the presence of (M)-HQ2+, curve (a), and interaction of the (S)-modified Au NPs in the presence of (P)-HQ2+, curve (b). (B) Interaction of the (R)-modified Au NPs in the presence of (P)-HQ2+, curve (a), and interaction of the (R)-modified Au NPs in the presence of (M)-HQ2+, curve (b). In all systems, the Au NPs ((4.5 ± 0.5) × 10−9 M) were interacted in TDW with the respective helquats (2 × 10−4 M). Data in red color correspond to experiments with (P)-HQ2+. Data in black color correspond to experiments with (M)-HQ2+.

Figure 3. STEM images corresponding to (A) (I) The (S)-modified Au NPs and (II) the (R)-modified Au NPs. (B) (I) The (S)-modified Au NPs treated with (M)-HQ2+ after 10 min of interaction; (II) the (S)-modified Au NPs treated with (P)-HQ2+ after 10 min of interaction; (III) the (R)modified Au NPs treated with (M)-HQ2+ after 10 min of interaction; and (IV) the (R)-modified Au NPs treated with (P)-HQ2+ after 10 min of interaction. (C) (I) The (S)-modified Au NPs treated with (M)-HQ2+ after 100 min of interaction; (II) the (S)-modified Au NPs treated with (P)HQ2+ after 100 min of interaction; (III) the (R)-modified Au NPs treated with (M)-HQ2+ after 100 min of interaction; and (IV) the (R)-modified Au NPs treated with (P)-HQ2+ after 100 min of interaction. In all systems, the Au NPs ((4.5 ± 0.5) × 10−9 M) were interacted in TDW with the respective helquats (2 × 10−4 M).

lead to the aggregation processes. The rate of aggregation is controlled by the chiral donor−acceptor interactions between the donor-capped Au NPs and the chiral dicationic bridging acceptor units. Figure 1C shows the UV−vis spectral changes upon treatment of (1b)-capped Au NPs upon treatment with the helquat (2a) electron acceptor units, curve (a), and upon the treatment of the modified NPs with the helquat (2b), curve (b). Evidently, the aggregation rate is faster with the (M)-HQ2+

electron acceptor. These results are consistent with the fact that (M)-HQ2+ exhibits a 2-fold higher affinity constant to the (S)binaphthol phenyl boronic acid electron donor. The aggregation rate of the (R)-binaphthol phenylboronic acidmodified Au NPs is reversed upon treatment with the two chiral helquat acceptors. Figure 1D, curve (a) depicts the timedependent absorbance changes upon the treatment of (1a)functionalized Au NPs with (2b) dicationic helquat acceptor, 5837

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ester-functionalized Au NPs suggests that other electron acceptors could be used to trigger chiroselective aggregation processes. This study sparks potential further activities in the area of chiral ligand-functionalized NPs and specifically the implementation of the recognition properties of chiral nanoparticles in new directions. The chiral ligand-functionalized Au NPs should reveal a circular dichroic plasmonic signal. Unfortunately, we were not able to trace such circular dichroic spectrum. The use of other NPs, for example, Ag or other shaped NPs could provide means to detect this challenging event. Furthermore, selective recognition properties of chiral ligand-functionalized Au NPs may be implemented in the selective targeting of the NPs to chiral biological microenvironments. Experimental Section. Preparation of the Modified Au NPs. The 13 nm gold NPs were prepared following the reported citrate reduction method.45 In brief, sodium citrate was added to a boiling solution of 1 mM AuCl4− in water to a final concentration of 3.5 mM, and the solution was heated for 10 additional minutes while stirring. Then the solution was stirred for another 15 min and was allowed to cool to room temperature. The solution was filtered through 0.2 μm cellulose acetate filter. The modification of the NPs with the binaphthol involved two steps. In the first step, the gold NPs were reacted with 16-mercaptohexadecanoic acid (MHDA) following a literature procedure.46 After removing the excess MHDA by centrifugation, the particles were resuspended in 10 mM HEPES buffer, pH 7.0. To this solution, m-aminophenyl boronic acid (APBA, 0.1 mM), 1-[3-(dimethylamino)propyl]3-ethylcarbodiimide hydrochloride (EDC, 2 mM), and Nhydroxysuccinimide (NHS, 5 mM) were added and stirred for 2 h. Excess chemicals were removed by two washing steps consisting of centrifugation and resuspension of the NPs in HEPES buffer solution. The resulting NPs were reacted with 0.2 mM binaphthol (R or S) overnight. Excess chemicals were removed as before. The modified Au NPs were resuspended in TDW. Preparation of the Binaphthol Phenylboronic Acid Esters. The binaphthol phenylboronic acid esters were prepared following a literature procedure47 using some modifications. In short, a mixture of the respective enantiomers of 1,1′-bi-2,2′naphthol (2 mmol) and m-hydroxyphenyl boronic acid (2 mmol) in 80 mL toluene was refluxed with a Dean−Stark apparatus for four hours. The solvent was removed by evaporation and the product was recrystallized from CH2Cl2hexane (1:1). Experimental Conditions. In all the experiments Au NPs ((4.5 ± 0.5) × 10−9 M) were interacted in TDW with the respective dicationic helquats that were used as bis-trifluoroacetate salts (2 × 10−4 M). Enantiomeric excess of the helquats (2a) and (2b) was higher than 96% ee as determined by chiral capillary electrophoresis.43 General Methods. UV−vis absorption spectra were recorded with a Shimadzu UV-2401 spectrophotometer. Light emission measurements were performed using a Cary Eclipse Device (Varian inc). The binaphthol phenyl boronic acid esters were excited at 280 nm. Light-scattering experiments were performed with a Zetasizer 3000, Malvern Instruments, U.K. STEM images were made with Extra High-Resolution Scanning Electron Microscopy Magellan 400L. CD measurements were performed with Applied Photophysics Chirascan Circular Dichroism spectrophotometer.

whereas the time-dependent absorbance changes upon aggregation of the (R)-binaphthol-functionalized Au NPs with the chiral helquat (P)-HQ2+, (2a), are shown in curve (b). Evidently the rate of aggregation of the (R)-binaphtholmodified Au NPs is substantially higher in the presence of (2a) electron acceptor. These results are consistent with the observation that the association constant between (1a) and the chiral (P)-HQ2+ electron acceptor is substantially higher as compared to the association constant of (1a) with (2b). Further support that the chiral dicationic helquats stimulate chiroselective aggregation of the chiral-binaphthol-functionalized Au NPs was obtained from light-scattering experiments. Figure 2A curve (a) and (b) show the time-dependent changes in the average sizes of the Au NP aggregates, followed by light scattering experiments, upon subjecting the (S)-binaphtholmodified Au NPs to (M)-HQ2+, curve (a), and upon the interaction of the (S)-binaphthol-functionalized Au NPs with (P)-HQ2+, curve (b). Evidently, the aggregates generated by (M)-HQ2+ are bigger than the aggregates formed by (P)-HQ2+, consistent with the UV−vis spectral changes and the respective higher association constant between (S)-binaphthol boronate and (M)-HQ2+. Figure 2B shows the time-dependent changes in the average sizes of the Au NP aggregates upon subjecting the (R)-binaphthol-modified Au NPs to (P)-HQ2+, curve (a), and to (M)-HQ2+, curve (b). The results are reversed and the average sizes of the aggregates formed between (P)-HQ2+ and the (R)-binaphthol-modified Au NPs are bigger than the aggregates formed between the (R)-binaphthol-modified Au NPs and (M)-HQ2+. This is consistent with the spectroscopic studies and the fact that the association constant between the (R)-binaphthol-boronate ester and (P)-HQ2+ is higher as compared to the association constant between the (R)binaphthol boronate ester and (M)-HQ2+. Electron microscopy measurements further confirmed the controlled chiroselective aggregation of the (S)- or (R)binaphthol phenylboronic ester-functionalized Au NPs, Figure 3. The STEM images of the (S)- and (R)-binaphtholfunctionalized Au NPs, Figure 3A, images I and II, respectively, are very similar and show individual NPs, as well as chains of NPs (due to drying effects). The images shown in Figure 3B, I and II, correspond to the (S), (1b),-modified NPs subjected to the aggregation with (2b) or (2a) for a time-interval of 10 min, respectively. One may note significant aggregation of the (S)binaphthol-functionalized Au NPs with (2b), while the same NPs show very little aggregation with (2a) (numerous individual NPs are observed). Similarly, treatment of the (R)(1a)-modified Au NPs with (2b) and (2a) electron acceptors for a time interval of 10 min shows a low degree of aggregation stimulated by (M)-HQ2+ and a substantially enhanced degree of aggregation in the presence of (P)-HQ2+, images (III) and (IV), respectively. These aggregation features are preserved upon allowing the different systems to aggregate for a longer time-interval of 100 min. Figure 3C, images (I) and (II) demonstrate that a higher degree of aggregation occurs between the (S)-(1b)-functionalized Au NPs and (2b) also after 100 min in comparison to (2a). Figure 3C images (III) and (IV) demonstrate that extensive aggregation of the (R)(1a)-functionalized Au NPs occurred in the presence of the (2a), while poor aggregation took place in the presence of (2b). In conclusion, the present study has demonstrated the chiralinduced aggregation of Au NPs using donor−acceptor interactions as driving force for the aggregation process. The successful synthesis of (S)/(R)-binaphthol phenyl boronic 5838

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(15) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2003, 125, 6642−6643. (16) Balogh, D.; Tel-Vered, R.; Riskin, M.; Orbach, R.; Willner., I. ACS Nano 2011, 5, 299−306. (17) Balogh, D.; Tel-Vered, R.; Freeman, R.; Willner, I. J. Am. Chem. Soc. 2011, 133, 6533−6536. (18) Sharma, J.; Chhabra, R.; Yan, H.; Liu, Y. Chem. Commun. 2007, 477−479. (19) Xu, X.; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 3468−3470. (20) Riskin, M.; Tel-Vered, R.; Bourenko, T.; Granot, E.; Willner, I. J. Am. Chem. Soc. 2008, 130, 9726−9733. (21) Riskin, M.; Tel-Vered, R.; Willner, I. Adv. Mater. 2010, 22, 1387−1391. (22) Riskin, M.; Tel-Vered, R.; Lioubashevski, O.; Willner, I. J. Am. Chem. Soc. 2009, 131, 7368−7378. (23) Frasconi, M.; Tel-Vered, R.; Riskin, M.; Willner, I. Anal. Chem. 2010, 82, 2512−2519. (24) Obare, S. O.; Hollowell, R. E.; Murphy, C. J. Langmuir 2002, 18, 10407−10410. (25) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165− 167. (26) Lim, I. I. S.; Mott, D.; Ip, W.; Njoki, P. N.; Pan, Y.; Zhou, S.; Zhong, C. J. Langmuir 2008, 24, 8857−8863. (27) Thaxton, C. S.; Georganopoulou, D. G.; Mirkin, C. A. Clin. Chim. Acta 2006, 363, 120−126. (28) Li, H.; Rothberg, L. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 14036−14039. (29) Pavlov, V.; Xiao, Y.; Shlyahovsky, B.; Willner, I. J. Am. Chem. Soc. 2004, 126, 11768−11769. (30) Willner, I.; Zayats, M. Angew. Chem., Int. Ed. 2007, 46, 6408− 6418. (31) Lin, S. Y.; Liu, S. W.; Lin, C. M.; Chen, C. H. Anal. Chem. 2002, 74, 330−335. (32) Li, D.; Wieckowska, A.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 3927−3931. (33) Liu, C. W.; Hsieh, Y. T.; Huang, C. C.; Lin, Z. H.; Chang, H. T. Chem. Commun. 2008, 2242−2244. (34) Tamura, M.; Fujihara, H. J. Am. Chem. Soc. 2003, 125, 15742− 15743. (35) Hu, A.; Yee, G. T.; Lin, W. J. Am. Chem. Soc. 2005, 127, 12486− 12487. (36) Sawai, K.; Tatumi, R.; Nakahodo, T.; Fujihara, H. Angew. Chem., Int. Ed. 2008, 47, 6917−6919. (37) Rezanka, R.; Navratilova, K.; Zvatora, P.; Sykora, D.; Matejka, P.; Miksik, I.; Kasicka, V.; Kral, V. J. Nanopart. Res. 2011, 13, 5947− 5957. (38) An, Z.; Yamaguchi, M. Chem. Commun. 2012, 48, 7383−7385. (39) Willner, I.; Eichen, Y.; Rabinovitz, M.; Hoffman, R.; Cohen, S. J. Am. Chem. Soc. 1992, 114, 637−644. (40) Vavra, J.; Severa, L.; Svec, P.; Cisarova, I.; Koval, D.; Sazelova, P.; Kasicka, V.; Teply, F. Eur. J. Org. Chem. 2012, 489−499. (41) Severa, L.; Koval, D.; Novotna, P.; Oncak, M.; Sazelova, P.; Saman, D.; Slavicek, P.; Urbanova, M.; Kasicka, V.; Teply, F. New J. Chem. 2010, 34, 1063−1067. (42) Adriaenssens, L.; Severa, L.; Salova, T.; Cisarova, I.; Pohl, R.; Saman, D.; Rocha, S. V.; Finney, N. S.; Pospisil, L.; Slavicek, P.; Teply, F. Chem.Eur. J. 2009, 15, 1072−1076. (43) Vavra, J.; Severa, L.; Cisarova, I.; Klepetarova, B.; Saman, D.; Koval, D.; Kasicka, V.; Teply, F. J. Org. Chem. 2012, DOI: 10.1021/ jo301615k. (44) Chen, Y. Y.; Tseng, C. W.; Chang, H. Y.; Hung, Y. L.; Huang, C. C. Biosens. Bioelectron. 2011, 26, 3160−3166. (45) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Nathan, M. J. Anal. Chem. 1995, 67, 735−743. (46) Aslan, K.; Perez-Luna, V. H. Langmuir 2002, 18, 6059−6065. (47) Thormeier, S.; Carboni, B.; Kaufmann, D. E. J. Organomet. Chem. 2002, 657, 136−145.

ASSOCIATED CONTENT

S Supporting Information *

CD spectra of the (R)- and the (S)-modified Au NPs, determination of the loading of the Au NPs with the respective binaphthol phenylboronic acid ligands, cyclic voltammogram of the helquat, determination of the binding constants, and Job’s plots corresponding to the binding of the (S)- or (R)-modified Au NPs in the presence of the different helquats. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(F.T.) E-mail: [email protected]. Phone: 420-220-183412. Fax: 420-220-183578. (I.W.) E-mail: [email protected]. Phone: 972-2-6585272. Fax: 972-2-6527715. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by NanoSensoMach ERC advanced Grant 267574 under the EC FP7/2007-2013 program. Financial support by the Czech Science Foundation (P207/ 10/2391) and the Academy of Sciences of the Czech Republic (RVO: 61388963) is gratefully acknowledged. We thank Dr. V. Kasicka, Dr. D. Koval, and Dr. P. Sazelova for enantiocomposition analysis of helquats by chiral capillary electrophoresis. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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ABBREVIATIONS NPs, nanoparticles; (P)-HQ2+, (P)-Me2[7]helquat; (M)-HQ2+, (M)-Me2[7]helquat; STEM, scanning transmission electron microscopy; TDW, triple distilled water.

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REFERENCES

(1) Kang, Y. J.; Erickson, K. J.; Taton, T. A. J. Am. Chem. Soc. 2005, 127, 13800−13801. (2) Qian, X.; Zhou, X.; Nie, S. J. Am. Chem. Soc. 2008, 130, 14934− 14935. (3) Fang, C.; Fan, Y.; Kong, J.; Gao, Z.; Balasubramanian, N. Anal. Chem. 2008, 80, 9387−9394. (4) Crespo-Biel, O.; Dordi, B.; Reinhoudt, D. N.; Huskens, J. J. Am. Chem. Soc. 2005, 127, 7594−7600. (5) Jiang, C.; Markutsya, S.; Tsukruk, V. V. Langmuir 2004, 20, 882− 890. (6) Yehezkeli, O.; Tel-Vered, R.; Raichlin, S.; Willner, I. ACS Nano 2011, 5, 2385−2391. (7) Ferreira, V. C.; Melato, A. I.; Silva A. F. Abrantes, L. M. Electrochim. Acta 2011, 56, 3567−3574. (8) Berkel, K. Y. V.; Hawker, C. J. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1594−1606. (9) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003, 19, 4831−4835. (10) Zhao, W.; Chiuman, W.; Lam, J. C. F.; McManus, S. A.; Chen, W.; Cui, Y.; Pelton, R.; Brook, M. A.; Li, Y. J. Am. Chem. Soc. 2008, 130, 3610−3618. (11) Baron, R.; Willner, B.; Willner, I. Chem. Commun. 2007, 323− 332. (12) Huang, C. C.; Chang, H. T. Chem. Commun. 2007, 1215−1217. (13) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18−52. (14) Wei, H.; Li, B.; Li, J.; Wang, E.; Dong, S. Chem. Commun. 2007, 3735−3737. 5839

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