Aggregation Behavior of Oppositely Charged Gold Nanorods in

May 9, 2013 - added into n-GNR solution (before reaching the isoelectric point), p-GNR/n-GNR aggregations of fairly constant size are formed with the ...
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Aggregation Behavior of Oppositely Charged Gold Nanorods in Aqueous Solution Aminah Umar and Sung-Min Choi* Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: The aggregation behavior of oppositely charged gold nanorods (GNRs) in aqueous solution has been investigated by zeta potential, UV−vis−NIR, DLS, and TEM measurements. The positively charged GNRs (p-GNRs) and negatively charged GNRs (n-GNRs) were prepared by layer-by-layer deposition of oppositely charged electrolytes on GNR surface. As p-GNRs are added into n-GNR solution (before reaching the isoelectric point), p-GNR/n-GNR aggregations of fairly constant size are formed with the minor component (p-GNRs) mostly at the center and the majority component (n-GNRs) at the outside. While the overall packing of the GNR aggregates is rather random, the local arrangements of GNRs show both side-by-side and end-to-end arrangements, resulting in elongated aggregates due to the anisotropic nature of GNRs. While no precipitation occurs before isoelectric point, the size of aggregates grows rapidly as the isotropic point is approached and rapid precipitation occurs at the isoelectric point, showing ionic-like behavior. Beyond the isoelectric point, partial dissolution of aggregates occurs.

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investigated,24,25 that of oppositely charged one-dimensional nanoparticles such as GNRs in solutions have not been fully investigated yet. Here, we report the aggregation behavior of oppositely charged GNRs in aqueous solution which was characterized by zeta potential, UV−vis−NIR, dynamics light scattering (DLS), and transmission electron microscopy (TEM) measurements. To the best of our knowledge, this is the first report on the aggregation behavior of oppositely charged GNRs in aqueous solutions through direct interparticle electrostatic interactions.

INTRODUCTION Gold nanorods (GNRs) have unique and excellent shapedependent physical properties, such as strong and tunable plasmon absorption, enhanced photoluminescence, and high surface-enhanced Raman cross sections.1−5 These properties provide new opportunities for various potential applications, such as optical and electronic devices,1,2,4 sensing and imaging,6−8 and drug and gene delivery.9−14 Self-assembly or guided assembly of GNRs, which provides efficient interparticle coupling, has been of great interest as a route to collectively enhance their optical and electronic properties and is the key to the realization of their various potential applications. Recently, many efforts have been made for this purpose using methods such as slow solvent evaporation,4 electrospinning,15 patterning,16 and external magnetic field.5 Various internal interactions, such as hydrophilic and hydrophobic interactions,17 hydrogen bonding,18,19 London van der Waals attractive force,20 and electrostatic interactions,6,21−23 have been also utilized for this purpose. The electrostatic interactions have successfully provided highly controlled ways to form and maintain complex self-assembled superstructures of various nanoparticles.24 The self-assembly of GNRs via electrostatic interaction has been obtained by adding negatively charged small molecules, such as DNA,21,22 adipic acid,23 and cysteine,6 into the solution of positively charged GNRs. Here, the charged molecules work as electrostatic linkers between GNRs, resulting in end-to-end or side-by-side assemblies of GNRs. However, while the aggregation or self-assembling behavior of oppositely charged spherical nanoparticles in solutions induced by direct electrostatic interactions are well © XXXX American Chemical Society

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EXPERIMENTAL METHODS Synthesis of Gold Nanorods. Gold nanorods were synthesized following the procedure developed by Nikoobakht and El-Sayed.26 Gold seed was prepared by mixing 5.0 mL of 200 mM cetyltrimethylammonium bromide (CTAB) aqueous solution with 5.0 mL of 0.5 mM HAuCl4, followed by addition of 0.60 mL of ice-cold 10 mM NaBH4 under stirring. The mixture, which was turned into a brownish-yellow solution indicating formation of small gold seed particles, was used as seed solution within 5 min after preparation. Growth solution was prepared by mixing 50 mL of 200 mM CTAB solution with 2.5 mL of 4 mM AgNO3 solution. To this mixture, 5.0 mL of 10 mM HAuCl4 and 41.7 mL of additional H2O were added. After gentle mixing, 700 μL of 78.8 mM freshly prepared ascorbic acid solution was added, which changed the growth Received: October 16, 2012 Revised: April 26, 2013

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Figure 1. TEM images of (a) negatively charged GNR and (b) positively charged GNR. The scale bar is 10 nm.

solution from dark yellow to colorless. Finally, 120 μL of seed solution was added into the prepared growth solution. This mixture gradually changed its color to reddish within 10−20 min. This solution was kept at a constant temperature of ca. 28 °C for 48 h to complete the reaction. The as-prepared GNR solution was centrifuged at 6000 rpm (Hermle Z 200 A) for 30 min to remove the excess amount of CTAB, which was followed by redispersion in H2O. The purified GNRs were characterized using the UV−vis− NIR spectrophotometer (PerkinElmer Lambda 650) and ZetaPlus zeta potential analyzer (Brookhaven Instruments Corporation). The purified GNRs showed transversal and longitudinal plasmon bands at 509.1 and 831.6 nm, respectively. It has been reported that the GNRs prepared via CTAB-directed approach are stabilized by a CTAB bilayer that are strongly adsorbed onto the surface of gold nanorods.27 These CTAB-stabilized nanorods have a net positive charge as confirmed by the zeta potential measurement (30.0 ± 2.8 mV). The TEM measurements (Tecnai F20) showed that the diameter, length, and aspect ratio were 13.3 ± 1.6 nm, 50.5 ± 5.2 nm, and 3.9 ± 0.7 nm, respectively (Supporting Information Figure S1). Preparation of Negatively and Positively Charged Gold Nanorods. Negatively and positively charged GNRs were prepared by using layer-by-layer (LBL) deposition of oppositely charged polymers, poly(sodium-4-styrenesulfonate (PSS, Mw = 70 000 g/mol) and poly(diallyldimethylammonium chloride) (PDDA, Mw = 40 000 g/mol).28 Since the asprepared GNRs are positively charged, the negatively charged GNRs (n-GNRs) were prepared by depositing PSS. First, the centrifuged GNRs (which were obtained from 100 mL of the as-prepared GNRs) were redispersed in 20 mL of 1 mM aqueous NaCl solution. A 1.0 mL of this solution was mixed with 200 μL of PSS stock solution (10 mg/mL in 1 mM aqueous NaCl solution). After 30 min of adsorption time, the PSS-coated GNR solution was centrifuged and washed two times to remove the excess amount of PSS and redispersed in deionized water, resulting in n-GNR stock solution. The positively charged GNRs (p-GNRs) were prepared by depositing PDDA on n-GNRs. The centrifuged n-GNRs were redispersed in 1.0 mL of 1 mM aqueous NaCl solution and mixed with 200 μL of PDDA stock solution (10 mg/mL in 1 mM aqueous NaCl solution). The resulting p-GNRs were washed and redispersed in water in the same way as it was done for n-GNRs. The zeta potentials of n-GNR and p-GNR were −64.2 ± 3.3 and +40.5 ± 1.1 mV, respectively. TEM images

show that the coating thickness of n-GNR and p-GNR are 2.0 ± 0.3 and 3.3 ± 0.3 nm, respectively (Figure 1). The UV−vis− NIR spectra of GNRs show a small red-shift of transversal plasmon band after each polymer coating (Figure S2). Since the polymer coating of GNRs will change the local refractive index, the small red-shift also supports the successful LBL coating of GNRs with charged polymers. These results are consistent with the previous report.28,29 Mixing of Oppositely Charged Gold Nanorods. Stock solutions of p-GNRs and n-GNRs with equal concentration were prepared by adjusting their concentrations so that their longitudinal plasmon band intensities were matched at 1.0 (Figure S2). A 3.0 mL of n-GNR solution was titrated in a stirred vial with 0.1 equiv aliquots of p-GNRs. At each step of titration, the sample (after 10 min of stirring and 15 min of equilibration) was characterized by zeta potential, UV−vis− NIR, dynamic light scattering (DLS, ZetaPlus particle size analyzer with λ = 659 nm and scattering angle = 90° and 15°, Brookhaven Instruments Corporation), and TEM (JEM-3011 HR, JEOL) measurements. Titration was continued beyond the isoelectric point.

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RESULTS AND DISCUSSION To understand the aggregation or self-assembling behavior of oppositely charged GNRs in aqueous solution, the mixtures of n-GNR and p-GNR solutions at different mixing ratios were characterized by zeta potential, UV−vis−NIR, DLS, and TEM measurements. The mixing ratio, P/N, is defined as a volume ratio of p-GNR and n-GNR solutions at equal concentration of GNRs. The zeta potentials of the mixtures at different P/N ratios are shown in Figure 2. As the P/N ratio is increased from 0 to 1.0 (i.e., adding incremental amount of p-GNR solution into nGNR solution), the zeta potential remains roughly constant at ca. −60 mV. It should be noted that the mixture remains stable without showing any precipitation. This may indicate that the mixture of p-GNRs and n-GNRs (majority particles) form aggregates with p-GNRs mostly at the center and n-GNRs on the surface.24,25 As the P/N ratio is increased further, the zeta potential changes rapidly and then becomes zero near P/N = 1.4 (isoelectric point) at which the mixture show rapid precipitations. This is in sharp contrast with oppositely charged microparticles which, upon mixing, precipitate continuously due to residual van der Waals force and poor solvation of large aggregates.25 In fact, the behavior of oppositely charged GNRs is similar to that of oppositely charged complexing ions which B

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Figure 2. Zeta potentials of p-GNR/n-GNR mixtures in aqueous solution at different P/N.

remain stable in solution until reaching a certain critical ratio at which they start to precipitate depending on solubility constant of complex ions.30 While the ionic-like behavior of oppositely charged spherical nanoparticles has been reported,24,25 this is the first report for the ionic-like behavior of oppositely charged highly anisotropic nanoparticles, i.e., GNRs. This may indicate that the ionic-like behavior of oppositely charged nanoparticles does not depend on the shape of nanoparticles. The P/N ratio at isoelectric point, which is bigger than one, can be attributed to the differences in the surface charge densities and the total surface area of n-GNR and p-GNR. Considering the zeta potentials (−64.2 ± 3.3 and +40.5 ± 1.1 mV, respectively) and total surface areas (2849 and 3389 nm2, respectively) of n-GNR and p-GNR, the expected P/N ratio at isoelectric point is 1.33. This is consistent with the experimentally measured value 1.4. As the P/N is increased beyond the isoelectric point, the zeta potential becomes positive, indicating the outer regions of aggregations are dominated by p-GNRs. It is well-known that the plasmon coupling between GNRs assembled end-to-end induces a red-shift of the longitudinal plasmon band, while the plasmon coupling between GNRs assembled side-by-side induces a red-shift of the transversal plasmon band and a blue-shift of the longitudinal plasmon band. To understand the aggregation behavior of n-GNR and p-GNR mixtures, the UV−vis−NIR absorption spectra of the mixtures were measured at different P/N (Figure 3a). At P/N = 0 (individually isolated n-GNRs only), the spectra shows two peaks at 510.3 and 806.7 nm which correspond to transversal and longitudinal plasmon bands, respectively. As P/N ratio is increased, the transversal plasmon band shows a red-shift, which becomes more pronounced as the mixture approaches the isoelectric point (Figure 3b). This indicates that the aggregations of oppositely charged GNRs contain side-by-side arrangements of GNRs. The longitudinal plasmon band of the mixture at different P/N was fitted with two Gaussian functions (Figure 3c). While the position of the first peak (f1) remains the same (as that of P/N = 0) with P/N, the position of the second peak (f2) shows red-shift, which becomes more pronounced as the mixture approaches the isoelectric point. Here, the peak f1 comes from free n-GNRs and the peak f2 comes from p-GNR/ n-GNR aggregations. The red-shift of the longitudinal plasmon band (as evidenced by the peak f2) indicates that the aggregations of oppositely charged GNRs contain end-to-end arrangements which grow with P/N, especially close to the isoelectric point. The intensity of f2 increases with P/N, while that of f1 decreases. This indicates that as more p-GNRs are added, more aggregates of p-GNRs and n-GNRs are formed, reducing the number of free n-GNRs. The simultaneous redshifts of the transversal and longitudinal plasmon bands may

Figure 3. (a) UV−vis−NIR spectra of p-GNR/n-GNR mixtures in aqueous solution at different P/N, (b) variation of transversal plasmon band position with P/N, and (c) fitting of longitudinal plasmon bands with two Gaussian functions.

indicate that the electrostatic interaction between the oppositely charged GNRs makes the aggregates grow in both the longitudinal and transversal directions of GNRs. As P/N is increased beyond the isoelectric point (P/N = 1.4), the peak position of the transversal plasmon band decreases back to lower wavelength (blue-shifted), indicating that the aggregates become disassembled (Figure 3a). The reappearance of longitudinal plasmon band at the position corresponding to the longitudinal plasmon band of free GNRs (Figure 3a) also indicates the dissolution of GNR aggregates and the existence of free p-GNRs (dominant GNRs beyond the isoelectric point). To further characterize the aggregation behavior of n-GNRs and p-NGRs at different P/N, a series of DLS measurements were performed with a scattering angle of 90° (Figure 4). It is well-known that while monodisperse spherical particles show a unimodal hydrodynamic diameter distribution, monodisperse rod-like particles show a bimodal apparent hydrodynamic diameter distribution, one peak corresponding to the translational diffusion and the other corresponding to the mixture of rotational and translational diffusions (with a dominant contribution from rotational diffusion).31 The intensityweighted apparent hydrodynamic diameter distribution of individually isolated n-GNRs (P/N = 0) shows a bimodal distribution with two peaks at 1.5 ± 0.3 and 46.0 ± 7.7 nm. The peak at 46.0 ± 7.7 nm is contributed from the translational diffusion, and the peak at 1.5 ± 0.3 nm is contributed mainly from the rotational diffusion of n-GNRs.32 This is consistent with previous reports on DLS measurements of GNRs in solution.32,33 It is well-known that the scattered intensity coming from the fast decaying rotational relaxation becomes relatively very small when the scattering vector is small.31,34 Therefore, to confirm that the peak at ca. 1.5 nm is mainly from rotational diffusion, DLS measurements of n-GNRs with a scattering angle of 15° were performed and compared with the one measured at 90° (Figure S3). While the measurements at 90° show comparable intensities for the two peaks, the C

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As P/N approaches to the isoelectric point (P/N = 1.4) (region II), the position of Ta increases rapidly with P/N and reaches its maximum (ca. 3930 nm) at the isoelectric point, indicating the formation of large aggregates of p-GNR/n-GNR which result in rapid precipitations. At the isoelectric point, there is a small peak at 467 nm as well, which may indicate that smaller aggregates coexist with very large aggregates. The intensities of Ro and To become very small and disappear near and at the isoelectric point, indicating that essentially all free nGNRs or p-GNRs are depleted. As P/N increases beyond the isoelectric point (region III), Ro and To peaks reappear and Ra and Ta peaks shift to smaller apparent hydrodynamic diameter. This clearly indicates the dissolution of the p-GNR/n-GNR aggregates with further addition of p-GNRs into the mixture, which is in agreement with the corresponding UV−vis−NIR spectra shown in Figure 3. Here, the hydrodynamic diameters contributed from the translational and rotational diffusion of free p-GNRs (Ro and To) and aggregates (Ra and Ta) are little bit larger compared to those before reaching the isoelectric point (Figure 4b). This indicates that the aggregates are bound rather strongly and only partial dissolution occurs in the solution. TEM images of the p-GNR/n-GNR aggregates at different P/N ratios were measured (Figure 5). Since samples for TEM measurements have to be dried before measurements, the TEM images may not fully represent the structures of the aggregates in solution. However, they still reflect certain features of the structures of aggregates in solution. Overall, the GNR aggregates formed by the direct electrostatic interaction between oppositely charged GNRs do not show the pronounced side-by-side assemblies of GNRs which were observed for the CTAB-coated positively charged GNRs bridged with negatively charged linker molecules such as DNA21,22 or adipic acid.23 The packed structure of GNRs at P/ N = 0 should have been formed by evaporation of water. The TEM image at P/N = 1.0 shows relatively small aggregates coexisting with free GNRs. As P/N is increased to 1.2 and 1.3, the size of aggregates increases and the amount of free GNRs is decreased, which is consistent with the DLS measurements. It can be seen that, due to the anisotropic nature of individual GNRs, the aggregates are formed in an elongated shape. While the overall packing of GNR aggregates is rather random, the local arrangements of GNRs show both side-by-side and endto-end arrangements, which is consistent with the UV−vis− NIR spectra. At the isoelectric point (P/N = 1.4), large aggregates of p- GNR/n-GNR are formed essentially without any free GNRs. At P/N = 2.0, which is above the isoelectric point, the aggregates become smaller again with appearance of some free GNRs, indicating dissociation of large aggregates. The size of aggregates at each P/N is roughly consistent with the size measured by the DLS.

Figure 4. (a) Apparent hydrodynamic diameter distributions of pGNR/n-GNR mixtures in aqueous solution at different P/N, (b) labeling of four peaks in the apparent hydrodynamic diameter distribution, and (c) variation of peak positions with P/N. The dashed lines are guide for the eyes.

measurements at 15° show a very small intensity for the peak at ca. 1.5 nm compared to the peak at ca. 46 nm. This confirms that the peak at 1.5 nm is contributed from the rotational diffusion of n-GNRs. As P/N is increased from 0 to 1.0, both of the two peaks in the apparent hydrodynamic diameter distribution at P/N = 0 split into two peaks forming four peaks (Figure 4a). The four peaks are labeled as Ro, Ra, To, and Ta (Figure 4b). The positions of the four peaks with P/N are summarized in Figure 4c. Before reaching near the isoelectric point (region I), the peak positions of Ro and To remains more or less constant at the same values as those at P/N = 0, indicating that these are contributed from the rotational and translational diffusions of free n-GNRs, respectively. The peak position of Ta is always larger than that of To. Therefore, Ta can be attributed to the translational diffusion of p-GNR/n-NGR aggregates. It should be noted that the position of Ra is larger than Ro but always much smaller than the dimensions of individual GNRs. Therefore, Ra can be attributed to the rotational diffusions of p-GNR/n-GNR aggregates. The paired appearance of Ra and Ta, which correspond to the rotational and translational diffusions of p-GNR/n-GNR aggregates, respectively, indicates that the overall shape of aggregates (up to P/N = 1.0) is fairly elongated rather than globular. Since the aspect ratio of GNRs is ca. 4, it is understandable that the aggregates of a few GNRs have an elongated shape. It should be also noted that, in the region I, the peak positions of Ra and Ta remains more or less constant at ca. 4 nm and ca. 100 nm, respectively, meaning that the size of p-GNR/n-GNR aggregates does not increase with P/N. This suggests that, in the region I, each added p-GNR interacts with a few n-GNRs by electrostatic attraction forming aggregates of relatively constant size with p-GNRs (minority particles) mostly at the center and n-GNRs (majority particles) mostly on the surface. This is consistent with the zeta potential measurement which showed a constant value at ca. −60 mV up to P/N = 1.0.

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CONCLUSION The aggregation behavior of oppositely charged GNRs (n- and p-GNRs) in aqueous solution has been investigated by zeta potential, UV−vis−NIR, DLS, and TEM measurements. As pGNRs are added into n-GNR solution (up to P/N = 1.0), pGNR/n-GNR aggregates of fairly constant size are formed with the minor component (p-GNRs) mostly at the center and the majority component (n-GNRs) at the outside. While the overall packing of GNR aggregates is rather random, the local arrangements of GNRs show both side-by-side and end-to-end arrangements, resulting in elongated aggregates due to the D

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self-assembled superstructures of highly anisotropic nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images for the as-prepared GNRs with their size distributions, UV−vis−NIR spectra of p-GNRs and nGNRs in aqueous solution, and DLS measurements for pGNRs and n-GNRs with two scattering angles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.-M.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation grants funded by the Ministry of Education, Science and Technology of the Korean government (No. 2012-0000177 and 2012-0031931).

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Figure 5. TEM images for p-GNR/n-GNR mixtures at different P/N: (a) 0.0, (b) 1.0, (c) 1.2, (d) 1.3, (e) 1.4 (isoelectric point), and (f) 2.2. The scale bars on the left- and right-hand side images are 500 and 200 nm, respectively.

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