Stepwise interfacial self-assembly of nanoparticles via specific DNA pairing

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Volume 9 | Number 48 | 28 December 2007 | Pages 6285–6488

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COVER ARTICLE Wang et al. Stepwise interfacial self-assembly of nanoparticles via specific DNA pairing

HOT ARTICLE Kralchevsky et al. Effect of electric-field-induced capillary attraction on the motion of particles at an oil/water interface

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This paper was published in a themed issue of PCCP on:

Colloidal particles at liquid interfaces Guest Editor: Professor B. P. Binks Please take a look at the full table of contents for this issue

Papers in this issue include: Stepwise interfacial self-assembly of nanoparticles via specific DNA pairing Bo Wang, Miao Wang, Hao Zhang, Nelli S. Sobal, Weijun Tong, Changyou Gao, Yanguang Wang, Michael Giersig, Dayang Wang and Helmuth Möhwald, Phys. Chem. Chem. Phys., 2007 DOI: 10.1039/b705094a Water-in-carbon dioxide emulsions stabilized with hydrophobic silica particles Stephanie S. Adkins, Dhiren Gohil, Jasper L. Dickson, Stephen E. Webber and Keith P. Johnston, Phys. Chem. Chem. Phys., 2007 DOI: 10.1039/b711195a Effect of electric-field-induced capillary attraction on the motion of particles at an oil–water interface Mariana P. Boneva, Nikolay C. Christov, Krassimir D. Danov and Peter A. Kralchevsky, Phys. Chem. Chem. Phys., 2007 DOI: 10.1039/b709123k

PAPER

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Stepwise interfacial self-assembly of nanoparticles via specific DNA pairingw Bo Wang,ab Miao Wang,c Hao Zhang,b Nelli S. Sobal,d Weijun Tong,a Changyou Gao,*a Yanguang Wang,c Michael Giersig,d Dayang Wang*b and Helmuth Mo¨hwaldb Received 3rd April 2007, Accepted 3rd May 2007 First published as an Advance Article on the web 29th May 2007 DOI: 10.1039/b705094a In the present work, we succeeded in alternatively depositing inorganic nanoparticles and functionalized DNA bases onto the water/oil interface from the water and oil bulk phases. The ligands used were functional thymines and adenines. Their thiol and phosphate groups were used to cap inorganic nanoparticles and their thymine and adenine groups to alter the surface functionality of the nanoparticles, thus enabling a layer-by-layer growth fashion of nanoparticles at the interface. The multiple particle ligation rendered the resulting nanoparticle films rather mechanically robust. As results, the freestanding asymmetric bilayer and trilayer films, composed of negatively-charged Au, positively-charged CdTe, and/or organic Ag nanoparticles were constructed; their areas were as large as over several centimetres, depending on the sizes of the containers used. Our work should bring up a novel methodology to generate asymmetric multilayer films of nanoparticles with a defined control of electron or charge across the films.

1. Introduction Inorganic nanoparticles (NPs) are of significance for both fundamental and technical applications associated with lowdimensional physics.1 Monodisperse NPs can readily selfassemble into microscopic and even macroscopic hierarchical structures, grounding the basis of the upcoming nanotechnology.2 Nonetheless, in most cases self-assembly of NPs is conducted on solid substrates, on which it is hard to grow large area NP films without defects.3 In this context, fluid supports, including liquid/air and liquid/liquid interfaces, should be of great interest.1d The water/air interface was used for organization of hydrophobic NPs, which, however, did not enable growth of multilayer films of NPs.1c Using water/oil interfaces to organize colloidal particles ranging from submicron to microns in size has been studied for more than a century.4 Most recently, one succeeded in directing nano-sized particles to self-assemble at the water/oil interface by imparting the particle a proper surface hydrophobicity, say, the contact angle of around 901.5–7 Cross-linking of the capping ligands of CdSe NPs, attached at the interface, left behind freestanding NP monolayer films.5b,c Based on the specific inclusion between cyclodextrins and adamantine, the cycloa

Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, P. R. China. E-mail: [email protected]; Fax: +86 571 87951948 b Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany. E-mail: [email protected]; Fax: +49 331 5679202 c Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China d Center of Advanced European Studies and Research, D-53175 Bonn, Germany w Electronic supplementary information (ESI) available: Fig. S1 and S2. See DOI: 10.1039/b705094a

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dextrins were used to drive lipophilic NPs, partially capped with adamantine, to self-assemble at the water/oil interface.6c The functional surface groups of cyclodextrines enabled us to deposit metallic NPs from either of bulk phases to form bilayer or trilayer films of NPs. This strategy, however, was rather materials-specific and hard to extend to other systems. Herein, we present a flexible way to alternately change the surface functionality of the NPs attached at the water/oil interface via specific pairing of functionalized DNA bases, demonstrating a general methodology to Layer-by-Layer (LbL) organize NPs at the water/oil interface.

2. Experimental section 2.1 Materials Hydrogen tetrachloroaurate trihydrate, sodium citrate dehydrate, NaBH4 (99%), tellurium powder (
200 mesh, 99.8%), cadmium perchlorate hydrate (Cd(ClO4)2, 99+%), silver nitrate, oleic acid, 2-(dimethylamino) ethanethiol hydrochloride, thymine, 11-bromo-1-undecene, thiolacetic acid, azobis(isobutylnitrile), adenosine, and adenosine 3 0 -monophosphate (AMP) were purchased from Aldrich. 2.2 Preparation of NPs Negatively charged 12 nm Au NPs were synthesized by the citrate reduction of chloroauric acid.8 Positively charged 3 nm CdTe NPs were prepared in water using 2-(dimethylamino) ethanethiol hydrochloride as stabilizer.9 Hydrophobic 6 nm Ag NPs were generated by thermal reduction of silver trifluoroacetate in the presence of oleic acid.10 Phys. Chem. Chem. Phys., 2007, 9, 6313–6318 | 6313

2.3 Synthesis of ligands Synthesis of 1-(11-mercaptoundecyl)-5-methyl-1H-pyrimidine-2,4-dione (T–SH, compound 3). K2CO3 (5 mmol) was added to a solution of thymine (5 mmol) in 50 mL DMF, followed by stirring at 80 1C for 1 h. Subsequently, 11-bromo1-undecene (5 mmol) was dropped into the resulting mixture. After storage at 80 1C overnight, DMF was removed under vacuum. The residues were dissolved in 100 mL CH2Cl2 and washed twice with 60 mL water. The organic layer was collected and dried by Na2SO4. The removal of CH2Cl2 yielded a white powder of compound 1, followed by purification of column chromatography with acetate/hexane (1 : 4). Thiolacetic acid (3 mmol) and azobis(isobutylnitrile) (0.3 mmol) were added into a solution of compound 1 (3 mmol) in dehydrated THF (50 mL). After irradiation in a photochemical reactor for 22 h under nitrogen, the reaction mixture was concentrated and purified by column chromatography with ethyl acetate/hexane (1 : 2), leading to compound 2. 2.5 mL HCl (30 mmol) was added into a solution of 2 (3 mmol) in methyl alcohol (50 mL), followed by reflux for 20 h. After washing with water, the organic layers were collected and dried using Na2SO4. When the organic layer was removed, followed by purification by column chromatography with ethyl acetate/hexane (1 : 1), a white powder of compound 3, T–SH was produced. M.p. 101–103 1C; 1H NMR (CDCl3) d: 8.854 (s, 1H), 6.910 (s, 1H), 3.617(t, J = 7.20 Hz, 2H), 2.444 (q, J = 7.20, 8.7 2H), 1.857 (m, 3H), 1.599 B 1.516 (m, 4H), 1.298 B 1.200 (m, 15H); MS (ESI) m/z: 335 ([M + Na]+). (Scheme 1)

Scheme 2 Outline of the procedure of synthesis of A–SH.

After 10 min shaking, the two phases were washed 10 times with solvents, respectively. Note that when the reaction time was longer than 30 min, the Au nanoparticles started to aggregate and precipitate. AMP aqueous solution (3 mM) or A–SH toluene solution (3 mM) was injected into the corresponding phase. After undisturbed storage for 30 min, the excess ligands were washed away with solvents. Afterwards, the aqueous suspension of CdTe NPs and/or the toluene suspension of Ag NPs were added into the corresponding phase, yielding asymmetric bilayers of NPs. Note that forming trilayers of CdTe/Au/Ag needed simultaneous injection of AMP and A–SH in the bulk phases. 2.5 Characterization

Synthesis of (6-amino-purin-9-yl)-undecane-1-thiol (A–SH, 6). Following the similar procedure to synthesize T–SH, A–SH (compound 6) was prepared as a white powder. M.p. 132–134 1C; 1H NMR (CDCl3) d: 8.37 (s, 1H), 7.79 (s, 1H), 5.67 (s, 2H), 4.19 (t, J = 7.25 Hz, 2H), 2.52 (dd, J = 7.40, 8.7 Hz, 2H), 1.91 B 1.87 (m, 3H), 1.61 B 1.58 (m, 2H), 1.37 B 1.25 (m, 14H) ppm; MS (ESI) m/z: 322 ([M + H]+). (Scheme 2) 2.4 Interfacial self-assembly 1 mL of Au NP aqueous suspension and 1mL of T–SH toluene solution (3 mM) were added sequentially into a 3mL vial.

Transmission Electron Microscopy (TEM) was implemented by a Zeiss EM 912 Omega microscope at an acceleration voltage of 120 kV. Ultrathin sections of the composite films for TEM were sliced with a Leica ultracut UCT ultramicrotome after embedding them in LR-white resin. Dynamic Light Scattering (DLS) measurements were implemented by a Malvern HPPS 500. Contact angle measurements were implemented with a contact angle measuring system G10 apparatus (Kru¨ss, Germany) at ambient temperature. Confocal fluorescence imaging and Fluorescence Recovery After Photobleaching (FRAP) spectroscopy were implemented by a Leica confocal laser scanning microscope with a 10 objective. Raman spectroscopy was performed under ambient conditions using a confocal Raman microscope (CRM200, WITec, Ulm, Germany) equipped with a piezo scanner (F-500, Physik Instrumente, Karlsruhe, Germany) and microscope objectives. A circularly polarized laser (CrystaLaser, 532 nm) was focused on the samples. The spectra were recorded at 1 cm
1 resolution with 20 s accumulation times.

3. Results and discussion

Scheme 1 Outline of the procedure of synthesis of T–SH.

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Our strategy is schematically illustrated in Scheme 3(a). Due to the Janus-face character of the water/oil interface, one should be capable of phase-selectively modifying the surfaces of NPs, attached at the interface, with different hydrophilic or lipophilic ligands from the bulk phases via covalent or noncovalent bonding. Functionalized DNA bases, T–SH, A–SH, This journal is

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Fig. 1 (a) Optical photograph of a thin film of Au NPs self-assembled at the water/toluene interface. The upper phase is toluene and the lower phase is the aqueous solution of T–SH. (b) TEM image of the thin film transferred from the interface. The inset shows a 5 mL water droplet sitting on the thin film of Au NPs covered by toluene. The film was transferred from the oil/water interface onto a glass slide.

Scheme 3 (a) Schematic illustration of layer-by-layer directing different NPs at the water/oil interface. Au NPs are arranged at the water/toluene interface, capped with ligands amenable to hydrogen bond base pairing. Ag NPs are attached on the Au NP monaolayers from the toluene phase using a complementary thiolated ligand, oppositely charged CdTe NPs from the water phase via an ionic complement. (b) Schematic depiction of the structure of Ag/Au/CdTe trilayer films derived from interfacial self-assembly. The cross-linking via the multiple particle ligation is indicated by arrows.

vigorous shaking gave rise to thin films with a strong metallic luster at the water/toluene interface (Fig. 1(a)). The thin films were transferred onto copper grids for TEM imaging. Fig. 1(b) shows a monolayer of close-packed 12 nm Au NPs. The bulk metallic reflectance suggests occurrence of the electron coupling between Au NPs attached at the interface.6 Due to the strong bonding affinity between SH group and Au surfaces, citrate-stabilized Au NPs were expected to be partially capped by T–SH during shaking. The water contact angle in toluene of Au NP monolayers was measured close to 901 (inset of Fig. 1(b)), which should be due to both the presence of the thymine-terminus of T–SH capped on the NPs and the decrease of the surface charge density due to the T–SH capping.6,7 The Au NPs, self-assembled at the interface, were able to redisperse in tetrahydrofuran (THF). The analysis by use of DLS showed that the size of Au NPs was changed little during surface modification by T–SH at the interface (Fig. 2). It is worthy of noting that the interfacial selfassembly of Au NPs in the current work was realized

and AMP were used as dual functional ligands. Firstly, modifying aqueous metallic NPs by T–SH or A–SH was carried out at the toluene/water interface; the SH termini were coupled with the NPs surfaces and the thymine or adenine termini rendered NPs interfacial active. Secondly, the T–SHs capped on the NPs, attached at the interface, were linked with A–SH or AMP dissolved in bulk phases via the complementary base pairing between thymine and adenine. This may convert the surface terminal group of the NPs attached at the interface from thymine to SH or phosphate group. Thirdly, the latter functional terminal groups enable the NPs, attached at the interface, to absorb other NPs from bulk phases, thus leading to asymmetric multilayered thin films of NPs. Interfacial self-assembly of aqueous Au NPs Aqueous dispersions of 12 nm Au NPs were brought into contact with 3 mM toluene solutions of T–SH. 10 min This journal is

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Fig. 2 Hydrodynamic radius distribution profiles of Au NPs redispersed in THF from the interfacial attachment at the water/toluene interface (solid line) and the original Au NPs in water (dashed line).

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exclusively via in situ interfacial reaction between Au NPs and T–SH. Of particular note is that, due to the poor colloidal stability of citrate-stabilized Au NPs and the low water solubility of T–SH, the prior surface modification always led to precipitation of the NPs. Stepwise growth of binary Ag/Au or Au/CdTe NP layers at the interface After aqueous Au NPs self-assembled into a monolayer at the water/toluene interface, the water and toluene phases were replaced by fresh water and toluene, which was repeated three times to guarantee complete removal of excess NPs and T–SH.6c Afterwards, A–SH was introduced into the toluene phase. Due to the strong affinity of hydrogen-bonding between thymine and adenine,11 the surface functional group (thymine) of Au NP monolayers, located at the water/oil interface, should be converted into SH groups (Scheme 3(a)). Using Raman spectroscopy, we found that A–SH molecules can selectively detach and be exclusively washed away from the particle surfaces at elevated temperatures such as 70 1C (Fig. 3), while the T–SH molecules remained on the particle surfaces. This demonstrated little detectable ligand exchange between T–SH and A–SH, which was different from heterogeneous ligand exchange between conventional SH-ligands.12 After the excess of A–SH was replaced by toluene, the organic phase was replaced by the toluene solution of 5 nm Ag NPs. After 10 min vigorous shaking, the toluene phase turned colorless while millimetre-sized droplets were present at the water/toluene interface, which were enclosed by thin films with a dark-yellow metallic lustre (Fig. 4(a)). The droplets dissipated into a thin film at the interface in 12 h. After being transferred to solid substrates, the contact angle of the resulting thin films was around 1301 at the water/oil interface (Fig. 4(b), inset). This was close to that of the oleic acid monolayers, suggesting the presence of the dense monolayers of Ag NPs (stabilized by oleic acid) atop the Au NP monolayers. This bilayer character was demonstrated by TEM imaging (Fig. 4(b)). In the current work, we found that sonication caused detachment from Au NP monolayers from Ag NP mono-

Fig. 3 (a) Raman spectrum of the monolayer film of Au nanoparticles obtained via alternate capping by T–SH and A–SH at the water/ toluene interface and (b) that recorded after heating the film at 70 1C for 1 h. The characteristic Raman resonance bands of thymine and adenine are highlighted by the brown and green lines, respectively.

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Fig. 4 (a) Optical photograph of Ag/Au NP bilayer thin films, derived from stepwise interfacial self-assembly. The upper phase is toluene and the lower phase is water. (b) TEM image of the thin film transferred from the interface. The inset shows a 5 mL water droplet sitting on the resulting composite thin film covered by toluene. The film was transferred from the oil/water interface onto a glass slide. (c) Optical photograph of emulsions stabilized with the Ag/Au bilayers. Confocal transmittance (d) and fluorescence (e) images of the resulting emulsions. Rhodamine B was added into the mixture to visualize the two phases.

layers, suggesting the bilayer structure of the resulting composite films (Fig. S1). Since each Ag NP was coupled with numbers of SH groups on Au NP monolayers, the attached Ag NPs may serve as cross-linkers for the Au NPs underneath (Scheme 3(b)). Accordingly, the resulting Ag/Au NP bilayers were mechanically robust as compared to the original Au NP monolayers and to those capped with A–SH; they were able to support a water droplet of 20 mL (Fig. S2(a) in the ESIw). Owing to the capping of hydrophobic Ag NPs, the surface of Ag/Au NP bilayers in contact with toluene (Ag NP side), is expected to be hydrophobic. Since the attachment of Ag NPs exclusively occurred in the toluene phase, the surface of the bilayers towards water (Au NP side) was still capped with A–SH and remained slightly hydrophilic. In this sense, the Ag/ Au NP bilayers may be expected to be amphiphilic. After Ag/ Au NP bilayers were formed at the water/toluene, followed by removal of excess of Ag NPs by toluene, sonication led to an emulsion with stability over 3 months (Fig. 4(c)). Confocal fluorescence imaging of the resulting emulsion droplets, stained with Rhodamine B, showed that they were of waterin-oil type and 0.5 to 5 mm in size (Fig. 4(d) and 4(e)). When Au NPs self-assembled at the water/toluene interface via capping with T–SH, the introduction of AMP in the aqueous phase rendered the monolayer of Au NPs negatively charged (Scheme 3(a)). After removal of excess AMP in water followed by addition of positively charged 3 nm CdTe NPs, fluorescent CdTe/Au NP bilayers were formed at the interface based on electrostatic attraction (Fig. 5). The detachment between two layers observed in TEM confirmed the bilayer This journal is

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Fig. 5 (Left) Fluorescence photograph of a Au/CdTe NP bilayer film self-assembled at the water/toluene interface. The resulting bilayer film is observed to climb along the wall of the glass vial. (Right) TEM image of a Au/CdTe NP bilayer, obtained by sonicating them for 1 h after being transferred from the water/toluene interface. The black large dots are 12 nm Au NPs while the grey small dots are 3 nm CdTe NPs. The sonication caused detachment of the Au NPs rather than CdTe NPs, suggesting a bilayer character of the resulting films.

structure of the film. As compared to that of the original CdTe NPs, the fluorescence of CdTe/Au NP bilayers was rather weak due to Au NP quenching. Since both the Au NP side (negatively charged) and the CdTe NP side (positively charged) of the resulting bilayers were hydrophilic, they were not able to stabilize emulsions, in contrast to Ag/Au NP bilayers. Stepwise growth of ternary Ag/Au/CdTe NP layers at the interface Au NPs, attached at the water/oil interface, were selectively modified with both A–SH and AMP from organic and aqueous phase by simultaneously injecting the toluene solution of A–SH to the toluene phase and the aqueous solution of AMP to the water phase. Note that due to the rotation of NPs

attached at the interface,5a consecutive injecting ended up with Au NPs capped only with either A–SH or AMP. After replacing two bulk phases using toluene and water, the monolayer surface of Au NPs towards toluene should be capped with A–SH and the surface towards water with AMP (Scheme 3). When hydrophobic Ag NPs and hydrophilic CdTe NPs were added into the toluene and water phases, respectively, a fluorescent thin film was formed at the water/toluene interface after undisturbed storage for 30 min (Fig. 6(a)). Owing to cross-linking by both Ag and CdTe NPs, the resulting films exhibited a sufficiently high mechanical stability; they were able to sustain a water droplet of 200 mL (Fig. S2(b) in the ESIw). Besides, we were able to pick them up from the interface by tweezers and re-suspend in THF (Fig. 6(b)). TEM imaging of the cross-section of the resulting films demonstrated a trilayer character: the monolayer of 12 nm Au NPs was sandwiched by the monolayer of 6 nm Ag NPs and that of 3 nm CdTe NPs (inset in Fig. 6(b) and Scheme 3(b)). The large dots observed on the lower left side of the resulting trilayer should be the aggregates of the CdTe NPs due to the strong dipole–dipole interaction between them13 and/or Ag NPs due to the folding effect induced during ultramicrotome.6d The clear-cut multilayered structure across the interface is dramatically different from the structures of composite films of NPs obtained via polyelectrolyte-mediated LbL electrostatic self-assembly, in which no distinct multilayers of NPs is observed.3a,c Note that alternately adding A–SH and AMP into toluene and water, we always ended up with either Ag/Au or Au/CdTe NP bilayers, suggesting that Au NPs self-assembled at the interface rotated. The further addition of T–SH (A–SH) or AMP to the toluene and aqueous phase should impart the in Ag NPs in the upper layers the thymine (or adenine) terminal group or in the CdTe NPs in the lower layers the adenine terminal group. Following the similar procedure shown in Scheme 3(a), the resulting trilayered films at the water/toluene interface should be able to absorb other NPs, thus provide a possibility to grow asymmetric multilayered films of NPs.

4. Conclusion

Fig. 6 (a) Fluorescence photograph of an Ag/Au/CdTe NP trilayer thin film, derived from layer-by-layer interfacial self-assembly. (b) Confocal fluorescence photograph of the free standing NP trilayer film. The TEM image of their cross-section in the inset shows that the linear array of the large Au NPs (red circle) is sandwiched by mediumsized Ag NPs (yellow circle) and small CdTe NPs (green circle). The scale bar in the inset image represents 10 nm.

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In summary, we successfully demonstrated LbL interfacial self-assembly of different inorganic NPs based on DNA base pairing of smaller organic ligands, leading to asymmetric NP trilayers. Due to the multiple particle ligation, the LbL interfacial self-assembly of NPs was accompanied by cross-linking, leading to freestanding thin films of NP trilayers with a robust mechanical stability. The success of our approach is not dependent on NPs but on functionalized DNA bases, whose functional termini can be facilely altered according to different NPs. Thus, our approach can be easily generalized to grow large asymmetric multilayered films of different NPs, realizing vectorial transfer of charges or energy across an interface.

Acknowledgements We thank the Max Planck Society for financial support. C. Gao is grateful to the National Science Fund for Distinguished Young Scholars of China (No. 50425311), the Major State Phys. Chem. Chem. Phys., 2007, 9, 6313–6318 | 6317

Basic Research Program of China (2005CB623902) and the Natural Science Foundation of China (No. 20434030) for financial support. 6

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