Controllable Self-Assembling of Gold Nanorods via On and Off

Article pubs.acs.org/Langmuir

Controllable Self-Assembling of Gold Nanorods via On and Off Supramolecular Noncovalent Interactions Xiang Ma,† Augustine Urbas,‡ and Quan Li†,* †

Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, United States Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base, Dayton, Ohio 45433, United States

‡

S Supporting Information *

ABSTRACT: 5,15-Bis(4-sulfonatophenyl)porphyrin (DPPS) with a planar conjugated system and two negative charges was found to be able to engender the self-assembling of CTAB-GNRs due to the electrostatic interaction between DPPS and CTAB together with the π−π intermolecular interaction of DPPS, while its bulky supramolecular pseudo[3]rotaxane included by β-cyclodextrin prevented such self-assembling due to the interruption of the above noncovalent interactions.

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GNRs was achieved by π−π intermolecular interaction.10 Controllable side-by-side and end-by-end self-assembling of GNRs were formed by ionic electrostatic interaction using lyotropic chromonic materials.11 5,15-Bis(4-sulfonatophenyl)porphyrin (DPPS), as a wellinvestigated dye with a planar conjugated system and two anions, was reported to form a self-assembling array via the π−π intermolecular interactions between porphyrin chromophores.12 Also, meso-sulfonatophenyl substituted porphyrins were reported not only to form stable complex in aqueous solution with CTAB via the electrostatic interaction between anion in sulfonatophenyl and cation in CTAB, but also to form supramolecular host−guest pseudorotaxane-like complex included by β-cyclodextrin (β-CD) in aqueous solution.13,14 Herein, DPPS was found to be able to induce the selfassembling of CTAB-GNRs driven by supramolecular noncovalent interactions, i.e. ionic electrostatic interaction and π−π intermolecular interaction (Figure 1). However, no such selfassembling was observed when adding the DPPS pseudo[3]rotaxane included by β-CD to the aqueous system. To the best of our knowledge, this kind of supramolecular inclusion interaction, specifically the pseudo[3]rotaxane formed by βCD including DPPS dye, was employed for the first time to prevent the supramolecular noncovalent interactions between the dye and GNRs, thus engender the controllable selfassembling of GNRs on and off. This concept, employing such supramolecular noncovalent interactions to control the self-

INTRODUCTION The ability to control supramolecular self-assembly by noncovalent interaction is a major driving force in the bottom-up nanofabrication of molecular devices. A challenge is how to self-assemble metal nanoparticles with well-defined architectures since such nanoparticles self-assembled in specific patterns would exhibit unprecedented collective properties, different from their individual and bulk materials, for developing high-performance mechanical, biological, optical, and electronic materials and devices.1 Among all metal nanoparticles, anisotropic gold nanorods (GNRs) are particularly fascinating owing to their unique and tunable surface plasmon resonance properties,2 which provide tremendous opportunities in many applications such as optics, electronics, catalysis, biological detection, cellular imaging, and cancer photothermal therapy.3−7 So far, GNRs are mainly prepared in an aqueous solution by a seed-mediated growth method, in which a surfactant cetyltrimethylammonium bromide (CTAB) is widely used as a shape-directing surfactant to selectively form a densely packed dynamic layer around the sidewall of a growing GNR with its two ends free from CTAB for a growth along the longitudinal axis. The resultant GNRs covered with CTAB layer on their longitudinal surface (CTAB-GNRs) are water-soluble, i.e., welldispersed in aqueous solution. Various noncovalent interactions and driving forces were employed to modulate their selfassembly morphology.8 For example, anisometric core−shell hybrid GNRs self-assembled into ring-like superstructures via dropwising water droplets on the liquid surface of a dichloromethane solution of GNRs hybrids cast on the carbon-coated TEM grids.9 Side-by-side self-assembling of © 2012 American Chemical Society

Received: August 24, 2012 Revised: September 28, 2012 Published: October 26, 2012 16263

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Figure 1. Schematic illustration of the controllable self-assembling of GNRs via on and off supramolecular noncovalent interactions.

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RESULTS AND DISCUSSION The concentration of aqueous CTAB-GNRs solution used for the following UV−vis-NIR and fluorescence emission measurements in this work: dilute the 0.5 mL CTAB-GNRs solution of specifically quantitative concentration as-prepared above into 1.5 mL H2O. DPPS showed a maximum absorption peak at about 402 nm (ε=2.23 × 105 L/mol.cm) in aqueous solution (Figure 2, a).

assembling of GNRs, would open a convenient way to achieve some specific morphology-dependable function.

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EXPERIMENTAL SECTION

Materials and Measurement Equipments. All chemicals and solvents were commercially available and used without further purification. DPPS was synthesized according to the literature12 via sulfonation of diphenyl porphyrin and subsequent alkalization with sodium hydroxide. UV−vis-NIR spectra were recorded on a PerkinElmer Lambda 25 UV−vis spectrometer at the resolution of 1 nm. Fluorescence spectra were collected on a Horiba scientific FluoroMax-3 spectrafluorometer. Transmission electron microscopy (TEM) experiments were done using FEI Tecnai TF20 FEG TEM equipment. For the observation, the samples in aqueous solution were first dispersed on TEM Cu grids precoated with thin carbon film (Cu400 CN) purchased from Pacific Grid Tech and then completely dried for analysis. Preparation of CTAB-GNRs. 15 For seed preparation, specifically, 0.15 mL of an aqueous 0.01 M solution of HAuCl4 was added to 5 mL of a 0.10 M CTAB solution in a vial. The solutions were gently mixed by the inversion. The solution appeared a bright brown-yellow color. Then, 0.36 mL of an aqueous 0.01 M ice-cold NaBH4 solution was added all at once, followed by rapid inversion mixing for 2 min. Care should be taken to allow the escape of evolved gas during mixing. The solution developed a pale brown-yellow color. Then, the vial was kept in a water bath maintained at 25 °C for future use. This seed solution was used 2 h after its preparation and could be used over a period of one week. 1.188 mL of 0.1 M CTAB solution in water was added to a tube, then 0.050 mL of 0.01 M solution of HAuCl4 and 0.008 mL of 0.01 M AgNO3 were added in this order and mixed by inversion. Then, 0.008 mL of 0.1 M of ascorbic acid solution was added and the resulting mixture at this stage becomes colorless. A 0.003-mL portion of the seed solution was added to the above mixture tube, and the tube was slowly mixed for 10 s and left to sit still in the water bath at 25−30 °C for 3 h. The final purple solution of CTAB coated gold nanorods was centrifuged several times to remove the excessive CTAB and redispersed in 0.5 mL of water for further use.

Figure 2. Normalized UV−vis-NIR spectra of DPPS (a) and CTABGNRs (b) in water at 298 K.

The resulting aqueous CTAB-GNRs solution was purple and exhibited the absorption peaks at 513 and 725 nm corresponding to its transverse surface plasma resonance absorption (TSPR) and longitudinal surface plasma resonance absorption (LSPR), respectively (Figure 2, b). The electrostatic interaction between the sulfonic anion of DPPS and the quarternary ammonium cation of CTAB in the aqueous CTAB-GNRs solution engendered the self-assembly of 16264

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CTAB-GNRs from the state of the CTAB-GNRs welldispersing in water, inducing the obvious red-shift and shapebroadness of the LSPR absorption peak of GNRs. As shown in Figure 3, the LSPR absorption peak of GNRs had a red shift

Figure 4. Fluorescent emission spectra of 0.002 (a), 0.006 (b), 0.010 (c) (solid curves and intensity increasing), and 0.014 (d), 0.018 (e), 0.022 (f), 0.026 (g), 0.030 (h), 0.034 (i) mL (dash curves and intensity decreasing) of aqueous DPPS solution (1.5 × 10−3 mol/L) in 2 mL water at 298 K, excited at 402 nm.

Figure 3. UV−vis-NIR spectra of CTAB-GNRs after addition of 0 (a), 0.002 (b), 0.006 (c), 0.010 (d), 0.014 (e), ..., 0.034 mL (k) of aqueous DPPS solution (1.5 × 10−3 mol/L); and CTAB-GNRs after addition of DPPS pseudo[3]rotaxane (prepared by mixing DPPS with 2 equivalent β-CD) (l) in water at 298 K.

continuously (Figure 4, curves d, e, ..., i). It is reasonable that normally the fluorescent emission intensity increases after increasing the concentration of the fluorophore. However, DPPS with large hydrophobic π−π planar conjugation can easily self-aggregrate to some extent in water,12 which induces the fluorescent emission quenching. So the fluorescent emission intensity of aqueous DPPS solution first increases then decreases continuously when increasing its concentration gradually. Interestingly, the fluorescent emission peaks and intensity changes of the mixing aqueous CTAB-GNRs and DPPS solution are totally different from that of DPPS. The emission spectrum showed a strong emission peaks at about 709 nm and another two relatively weak peaks at around 733 and 787 nm respectively (Figure 5). The appearance of these near-infrared (NIR) fluorescence emission peaks is attributed to the DPPS

from 725 to 753 nm (Δλ= 28 nm) and a slight intensity decrease when adding increasing amount of DPPS into the aqueous solution (from curve a to k). The absorption changes are attributed to the obvious and ordered morphological rearrangement of GNRs in a side-by-side way accompanied with some in a cross-link manner. While the TSPR absorption of GNRs at about 513 nm showed a very small and negligible shift (from 513 to 510 nm) but with the obvious intensity increase by around 148%, which should be contributed a lot by the Q-band absorption of DPPS. The UV−vis absorption spectrum of DPPS with increasing concentration in water was also measured for reference. As shown in Figure S1 (Supporting Information, SI), it exhibited a strong peak with increasing intensity at about 402 nm and another two relatively slight peaks with increasing intensity at around 506 and 568 nm, respectively, which are the characteristic absorption peaks of DPPS in aqueous solution. The typical absorption intensity of DPPS decreases obviously and has a slight red-shift when adding CTAB, indicating the obvious interaction between DPPS and CTAB (shown in Figure S2 of the SI). However, when adding the same amount of DPPS pseudo[3]rotaxane (constructed via mixing the DPPS with 2 equivalent β-CD together in advance) to the initial aqueous CTAB-GNRs solution, the absorption peak especially the LSPR peak changed slightly (Figure 3, curve l), which indicates that the formed bulky supramolecular pseudo[3]rotaxane from DPPS and βCD not only prevents the DPPS from the π−π conjugation aggregating of DPPS, but also interrupts the electrostatic interaction between DPPS and CTAB to much extent thus has little effect on the LSPR plasma absorption of CTAB-GNRs. Excitation of aqueous DPPS solution at 402 nm engendered two fluorescent emission peaks at about 632 and 689 nm with an intensity ratio about 1:2 (Figure 4, curve a). When increasing the concentration of aqueous DPPS solution gradually, the intensity of these two emission peaks initially increased (Figure 4, curve b and c), and then decreased

Figure 5. Fluorescent emission spectra of CTAB-GNRs after addition of 0.002 (a), 0.006 (b), 0.010 (c), 0.014 (d), ..., 0.034 mL (j) of DPPS solution (1.5 × 10−3 mol/L); and CTAB-GNRs after addition of DPPS pseudo[3]rotaxane (k) excited at 402 nm in water at 298 K. Inset: images of aqueous DPPS solution and under 365 nm UV irradiation (left) and of mixing 0.034 mL DPPS with CTAB-GNRs solution and under 365 nm UV irradiation (right). 16265

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noncovalent interactions would open a convenient way to control GNR self-assembling and achieve some specific morphology-dependable function.

induced self-assembling of CTAB-GNRs via the supramolecular electrostatic interaction between DPPS and CTAB on the surfactant sidewall of GNRs, in which there might be some energy transfer interaction between DPPS and GNRs. The whole integrate assembly system consisted of CTAB-GNRs and DPPS might be excited to engender such fluorescent emission. When increasing the concentration of DPPS in the CTABGNRs solution gradually, the maximum fluorescent emission intensity of DPPS decreases gradually but accompanying with the huge intensity increase of both the two emission peaks at 733 nm (Δ over 200%) and 787 nm (Δ over 300%). As seen from the TEM image (see Figure S3 of the SI), CTAB-GNRs were well-dispersed in water initially. The selfassembling of CTAB-GNRs induced by the addition of DPPS was clearly observed by the TEM images as shown in Figure 6

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ASSOCIATED CONTENT

S Supporting Information *

Brief introduction of experimental reagents and measurement instruments. Preparation of CTAB-GNRs. UV spectra of DPPS with increasing concentration, DPPS after adding CTAB. Typical TEM images of CTAB-GNRs and CTAB-GNRs after addition of DPPS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 330-672-1537; fax: 330-672-2796; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Air Force Office of Scientific Research (AFOSR FA9550-09-1-0254). The TEM data were obtained at the (cryo) TEM facility at the Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surface in Advanced Materials. Dr. M. Gao and Y. Li are acknowledged for their helpful discussion.

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Figure 6. TEM images of CTAB-GNRs after addition of DPPS (A), and after addition of DPPS pseudo[3]rotaxane (B).

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(more typical TEM images shown in Figure S4 of the SI). The self-assembling of CTAB-GNRs is obviously in a side-by-side way and some in a cross-link manner, driven by π−π intermolecular interaction of DPPS molecules as well as the ionic electrostatic interaction of negative charge in DPPS with positive charge in CTAB around the sidewall of GNRs. However, no self-assembling of CTAB-GNRs was observed when adding the same amount of DPPS pseudo[3]rotaxane to the initial aqueous CTAB-GNRs, where CTAB-GNRs were still well-dispersed as initially, which was confirmed by the TEM image as shown in Figure 6B. The supramolecular inclusion interaction of bulky β-CD on the two sulfonatophenyl goups of DPPS in the pseudo[3]rotaxane breaks the above noncovalent interactions by interrupting the ionic electrostatic interaction between DPPS and CTAB as well as preventing porphyrin moiety in DPPS from π−π intermolecular interaction to some extent.

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CONCLUSIONS In summary, CTAB-GNRs aqueous solution was of welldispersed morphology. Increasing the amount of DPPS with negative charges was able to induce the self-assembling of CTAB-GNRs, and exhibited totally different fluorescent emission peaks in both the position and the shape. This aqueous system was able to be excited to engender adjustable fluoroscence emission in the near-infrared region. While the addition of DPPS pseudo[3]rotaxane using β-CD host was not able to induce the self-assembling of CTAB-GNRs since the supramolecular inclusion interaction between β-CD and DPPS interrupts the π−π intermolecular interaction of DPPS molecules as well as the electrostatic interaction between CTAB-GNRs and DPPS. This concept demonstrated here, i.e., controllable self-assembling via on or off supramolecular 16266

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