Home
Search
Collections
Journals
About
Contact us
My IOPscience
Nanoscale arrangement of diblock copolymer micelles with Au nanorods
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 455602 (http://iopscience.iop.org/0957-4484/25/45/455602) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 207.162.240.147 This content was downloaded on 16/06/2017 at 20:11 Please note that terms and conditions apply.
You may also be interested in: Fabrication of size-controlled nanoring arrays by selective incorporation of ionic liquids in diblock copolymer micellar cores Sung-Soo Kim, Donghwi Kang and Byeong-Hyeok Sohn Large area tunable arrays of graphene nanodots fabricated using diblock copolymer micelles Sung-Soo Kim, Jeong-Yong Choi, Kwan Kim et al. Micellar nanotubes and AAO nanopores decorated with nanoparticles Seung-Min Jeon, Sung Hwa Lee, Byeong-Hyeok Sohn et al. Micropatterning of a single layer of nanoparticles by lithographical methods with diblockcopolymer micelles Sang-Hyun Yun, Byeong-Hyeok Sohn, Jin Chul Jung et al. Light responsive thin films of micelles of PS-b-PVP complexed with diazophenol chromophore Luciana M Saiz, Patricia A Oyanguren, María José Galante et al. Synthesis and optical properties of gold nanorods with controllable morphology Tianyu Ye, Zhigao Dai, Fei Mei et al. Structured nanoporous surfaces from hybrid block copolymer micelle films with metal ions Minsoo P Kim, Hyeong Jun Kim, Bumjoon J Kim et al. Hexagonally ordered nanoparticles templated using a block copolymer film through Coulombic interactions Wonjoo Lee, Seung Yong Lee, Xin Zhang et al.
Nanotechnology Nanotechnology 25 (2014) 455602 (5pp)
doi:10.1088/0957-4484/25/45/455602
Nanoscale arrangement of diblock copolymer micelles with Au nanorods Hwan Kim, Yirang Lim, Sehee Kim, Sung-Soo Kim and Byeong-Hyeok Sohn Department of Chemistry, Seoul National University, Seoul 151-747, Korea E-mail: [email protected] Received 16 June 2014, revised 13 August 2014 Accepted for publication 17 September 2014 Published 21 October 2014 Abstract
We fabricated a single-layered film consisting of spherical micelles of diblock copolymers and one-dimensional Au nanorods that were surface modified with the same polymer as the corona block of the copolymers. When the diameters of micelles were larger than the lengths of the nanorods, spherical micelles arranged in a hexagonal configuration surrounded by nanorods with their long axes perpendicular to the radial direction of the micelles. This arrangement provided selective organization of the Au nanorods and Ag nanoparticles which were selectively synthesized within the cores of the copolymer micelles. Thus, position-selective arrangement of Au nanorods and Ag nanoparticles was demonstrated at the nanometer scale such that a homogenous distribution of two different nanomaterials over a large area without aggregation was achieved. S Online supplementary data available from stacks.iop.org/NANO/25/455602/mmedia Keywords: block copolymer, micelle, gold nanorod, silver nanoparticle (Some figures may appear in colour only in the online journal) 1. Introduction
synthesized by the seed-mediated growth method, typically with cetyltrimethylammonium bromide (CTAB) as a surfactant [5, 6]. The aspect ratio can be finely tuned by adjusting the synthetic conditions such as the number of seed particles and the presence of additional aromatic surfactants [7]. The tunable absorbance bands of metal nanorods are valuable for many applications in optical devices and biosensors [3, 4]. However, controlling the arrangement of the nanorods on a solid substrate as well as in a solution is necessary because the inter-particle coupling of nanorods changes the plasmon bands in terms of their position and broadness [4]. To control the coupling between the nanorods, for example, a functional group which can allow the intermolecular hydrogen bonding was introduced to the surface of Au nanorods. The modified nanorods assemble into an endto-end arrangement, which induces a large red-shift of the longitudinal band as the number of nanorods in the assembly increases [8, 9]. Aligned Au nanorods in a polymer matrix were also fabricated by stretching the polymer–nanorod composite, which were capable of selective absorption of polarized light [10].
Nanomaterials have attracted much attention because of their unique optical, electrical, magnetic, and mechanical properties which are different from bulk materials [1, 2]. The distinct characteristics of nanomaterials generally depends on their size and shape. In particular, the optical and electrical properties of zero dimensional (0D) nanoparticles and onedimensional(1D) nanorods can be precisely tuned by adjusting their size and shape [1–3]. For example, 1D nanorods of semiconductors such as CdSe and ZnO have been studied for use in highly efficient light emitting diodes and solar cells with the control over their optoelectronic characteristics by tuning their aspect ratio as well as the shape [1]. For nanorods of metals such as Au and Ag, there are two absorbance bands for surface plasmon resonance in the transverse mode along the short axis and in the longitudinal mode along the long axis [2, 3]. For example, Au nanorods have two plasmon bands in the visible and near IR wavelengths, which can be finely tuned by changing the aspect ratio of the length to the diameter of the nanorods [2–4]. Au nanorods are generally 0957-4484/14/455602+05$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 455602
H Kim et al
Diblock copolymers with self-assembled nanostructures [11, 12] have been used as polymeric matrices to guide the arrangement of nanorods [13, 14]. Supramolecules [15] and topographic nanotemplates [16–18] based on diblock copolymers have also been employed to guide the location of nanorods. In contrast, incorporation of 1D nanorods into nanometer-sized micelles of diblock copolymers is dimensionally restricted because diblock copolymers form 0D spherical micelles. Thus, 1D nanorods cannot be commensurate with 0D spherical micelles although diblock copolymer micelles have been utilized as effective templates to synthesize and arrange spherical nanoparticles [19–21]. In this study, we first demonstrated that Au nanorods can be guided to the peripheral regions of hexagonally arranged spherical micelles of diblock copolymers when the diameters of the micelles are larger than the lengths of the nanorods, i.e., 0D spherical micelles in a hexagonal arrangement were effectively surrounded by 1D nanorods with their long axes perpendicular to the radial direction of the micelles. We then utilized the organization of the copolymer micelles and Au nanorods to achieve position-selective arrangement of Au nanorods and Ag nanoparticles. By synthesizing Ag nanoparticles selectively in the cores of the copolymer micelles, we isolated the Ag nanoparticles in the core area of micelles from the Au nanorods in the peripheral regions of the micelles. Thus, the location of Au nanorods and Ag nanoparticles was guided in the micellar film without aggregation. In addition, the nanoscale arrangement of Au nanorods and Ag nanoparticles allowed the appearance of all of the plasmonic bands for both the Au nanorods and the Ag nanoparticles because their optical coupling was constrained by the micellar structure.
were well-dispersed in toluene without precipitation. The average diameter and length of the Au nanorods were 10 nm and 33 nm, respectively (supplementary data, figure (S1)). The absorption spectrum shows two plasmon bands for the Au nanorods at 512 nm in the transverse mode and 780 nm in the longitudinal mode (supplementary data figure (S1)).
2. Experimental methods
2.3. Characterization
2.2. Fabrication of a single layer of Polystyrene-block-poly(4vinyl pyridine) (PS-b-P4VP) micelles
PS-b-P4VP polymers various molecular weights, i.e., PS(25k)b-P4VP(7k), PS(51k)-b-P4VP(18k), and PS(109k)-b-P4VP (27k), were purchased from Polymer Source PS-b-P4VP was dissolved in toluene at 100 °C for 2 h to yield a 0.7 wt % micellar solution, which was then cooled to room temperature. To incorporate the Ag precursors within the P4VP cores, silver acetate was added to a PS-b-P4VP micellar solution (molar ratio of silver acetate to 4VP = 0.7). The solution was stirred for five days. A PS-b-P4VP micellar solution containing Au nanorods was prepared by mixing a solution of Au nanorods with a PS-b-P4VP micellar solution. When the two toluene solutions were mixed, the weight ratio of nanorods to micelles was kept as 2 : 1, regardless of the presence of silver acetate in the P4VP cores. This mixing ratio resulted in a 25% more Au nanorods per micelle in the PS-b-P4VP micelles with silver acetate, compared to those without silver acetate. A single layer of PS-b-P4VP micelles containing Ag precursors, Au nanorods, or both Ag precursors and Au nanorods was spin-coated (2000 rpm, 60 s) from a corresponding solution onto a cleaned quartz plate or a freshly-cleaved mica substrate. To reduce the Ag precursors to Ag nanoparticles, the films were exposed to UV irradiation (254 nm, 15 W) for 2 h at room temperature.
For plane-view transmission electron microscopy (TEM) images, a single-layered film of PS-b-P4VP micelles was floated off of the mica substrate onto deionized water, from which it was collected on a carbon-coated TEM grid. TEM was performed on a Hitachi 7600 instrument operating at 100 kV. The absorption spectra were measured on a Varian Cary-5000 spectrophotometer.
2.1. Preparation of PS-modified Au nanorods
We synthesized Au nanorods by the seed-mediated growth method reported in the literature [6]. A seed solution was prepared by adding an ice-cold aqueous solution of NaBH4 (0.35 mL, 0.01 M) to an aqueous mixture of HAuCl4 (0.25 mL, 0.1 M) and CTAB (9.4 mL, 0.1 M). A 1 mL volume of the seed solution was then added to the growth solution, which was prepared by mixing HAuCl4 (2.5 mL, 0.1 M), CTAB (90 mL, 0.1 M), ascorbic acid (0.5 mL 0.1 M), and AgNO3 (0.25 mL, 0.1 M). After 18 h, the excess CTAB was removed by centrifugation at 13 000 rpm for 20 min. The surfaces of the Au nanorods were modified with thiol-end-functionalized polystyrene (PS-SH) [22]. Polystyrene with a thioester end group was first synthesized by the reversible addition–fragmentation transfer polymerization. The thioester end group was then reduced to the thiol group with NaBH4. The molecular weight of PS-SH was 8000 g mol−1 with a polydispersity index of 1.06. A tetrahydrofuran (THF) solution of PS-SH was directly added to the concentrated aqueous solution of Au nanorods. The excess PS-SH was removed by centrifugation at 13 000 rpm for 20 min. After the surface modification, the Au nanorods
3. Results and discussion PS-b-P4VP, diblock copolymers spontaneously associate into nanometer-sized spherical micelles with a soluble PS corona and an insoluble P4VP core in toluene, which is a selective solvent for the PS block. By spin coating from the micellar solution, which provides fast evaporation of the solvent, a single layer of spherical PS-PVP micelles with regular spacing can be fabricated [19]. To obtain different sizes of the copolymer micelles, we employed three copolymers: PS(25k)-P4VP(7k), PS(51k)P4VP(18k), and PS(109k)-P4VP(27k). TEM images of these micellar films are shown in figure 1. The P4VP cores were selectively stained with I2. In all cases, the cores appeared as dark spheres with a pseudo-hexagonal arrangement within a 2
Nanotechnology 25 (2014) 455602
H Kim et al
Figure 1. TEM images of single-layered films of PS-b-P4VP micelles: (a) PS(25k)-P4VP(7k); (b) PS(51k)-P4VP(18k); (c) PS(109k)-P4VP
(27k). The P4VP cores were stained with I2. The scale bars are 100 nm.
Figure 2. TEM images of single-layered films of PS-b-P4VP micelles with Au nanorods: (a) PS(25k)-P4VP(7k); (b) PS(51k)-P4VP(18k); (c) PS(109k)-P4VP(27k). The scale bars are 100 nm.
P4VP cores cannot be clearly identified, except the case with the largest micelles in figure 2(c), due to the high contrast of the Au nanorods in the images. However, the Au nanorods did not aggregate in all images and they were mostly located within the bright PS regions between the gray PVP cores, particularly in the image with the largest micelles. In this case (figure 2(c)), the long axes of the Au nanorods were almost perpendicular to the radial direction of the micellar core. Because the diameter (∼90 nm) of a PS(109k)-P4VP(27k) micelle is sufficiently large, compared to the length of an Au nanorod (∼33 nm), the spherical copolymer micelles can become surrounded by 1D nanorods. A TEM image of PS (109k)-P4VP(27k) micelles with Au nanorods in an extended area is shown in figure (S5) of the supplementary data. Because a variety of precursors of metal nanoparticles can be coordinated to the pyridine unit of the 4PVP block, metal nanoparticles can be selectively synthesized in the cores by reduction of the precursors. Thus, nanoparticles can be arranged on a solid substrate with preservation of the order of the micellar array [19]. In this study, we incorporated silver acetate as a precursor for Ag nanoparticles in the P4VP cores of PS(109k)P4VP(27k) micelles in toluene. Then, a single layer of copolymer micelles was spin-coated, in which Ag nanoparticles were synthesized selectively in the P4VP cores by UV reduction of silver acetate [23]. We employed UV irradiation as a reduction method, instead of simple thermal treatment, to prevent the Au
bright PS matrix without multi-layered structures, indicating a single-layered film of PS-P4VP micelles, even though the diameters of the copolymer micelles are different. As the molecular weight of the copolymers increases, both the core diameter and the center-to-center distance increase. The core diameters were ∼18 nm for PS(25k)-P4VP(7k), ∼26 nm for PS(51k)-P4VP (18k), and ∼38 nm for PS(109k)-P4VP(27k). The center-tocenter distances were ∼23 nm for PS(25k)-P4VP(7k), ∼41 nm for PS(51k)-P4VP(18k), and ∼90 nm for PS(109k)-P4VP(27k). We note that the ordering of the copolymer micelles without overlapping was uniform over extended areas based on the relatively large-area images provided by AFM, FE–SEM, and TEM (supplementary data, figures (S2)–(S4)). The TEM samples were also collected from arbitrary positions within the micellar film (∼1 cm × ∼1 cm) floated on water (supplementary data, figure (S5)) but the same micellar ordering was observed. The Au nanorods with the PS-modified surface (diameter = ∼10 nm, length = ∼33 nm) were well-dissolved in toluene and exhibited characteristic transverse (512 nm) and longitudinal (780 nm) bands in the UV–Vis absorption spectrum (supplementary data, figure (S1)). From a mixture of PS-b-P4VP micelles and Au nanorods, a single layer of copolymer micelles with Au nanorods was fabricated by spin coating. The weight ratio of Au nanorods to micelles was adjusted to 2 for all cases. In the TEM images in figures 2(a)– (c), the Au nanorods appear as dark particles. However, the 3
Nanotechnology 25 (2014) 455602
H Kim et al
Figure 3. TEM images: (a) a single layer of PS(109k)-P4VP(27k) micelles containing Ag nanoparticles in the P4VP cores; (b) an array of Ag nanoparticles surrounded by Au nanorods in a single layer of copolymer micelles. The scale bars are 100 nm. Enlarged images (150 nm × 150 nm) are also included.
Absorbance (a.u.)
nanorods from reshaping during the heating process [24]. The TEM image in figure 3(a) shows an array of PS(109k)-P4VP (27k) micelles containing Ag nanoparticles in the P4VP cores. The dark regions correspond to the P4VP cores of the micelles containing Ag nanoparticles, whereas the bright continuous regions are the PS coronas. The synthesized Ag nanoparticles can be identified in the magnified image in figure 3(a). There are about a dozen Ag nanoparticles (∼4 nm in diameter) in each P4VP core. The raspberry-like morphology of many of the nanoparticles is formed by the fast reduction with UV irradiation [23]. The Ag nanoparticles synthesized in the P4VP cores of the micelles exhibited a typical plasmon band at 420 nm (red arrow), as shown in figure 4(a). The Au nanorods with PS-modified surfaces were also homogeneously dispersed in a toluene solution of PS(109k)P4VP(27k) micelles containing silver acetate in the P4VP cores. From this mixture, a single layer of PS(109k)-P4VP (27k) micelles with Au nanorods was again fabricated by spin coating. Ag nanoparticles were then selectively synthesized in the P4VP cores by UV irradiation. As shown in figure 3(b), the Au nanorods were arranged on the circumference of the P4VP cores and were dispersed over the micellar array as like the previous case only with Au nanorods (figure 2(c)). Because 25% more nanorods were added in this case (experimental methods), more nanorods can be observed in figure 3(b), compared to figure 2(c). In the magnified image in figure 3(b), the synthesized Ag nanoparticles with raspberrylike morphologies in the PVP cores can be again recognized as like those in the case without Au nanorods (figure 3(a)). In addition, the Au nanorods were mostly located between the PVP cores containing Ag nanoparticles with their long axes almost perpendicular to the radial direction of the micellar core. However, we can find that many of Au nanorods are also side-by-side assembled. The image in figure 3(b) is basically a combined image of the images presented in figure 3(a) and figure 2(c), that is, an array of Ag nanoparticles in the P4VP cores surrounded by Au nanorods in a
(c)
(b)
(a)
400
600 800 1000 Wavelength (nm)
1200
Figure 4. Absorption spectra of single layers of PS(109k)-P4VP
(27k) micelles: (a) with Ag nanoparticles; (b) with Au nanorods; (c) with Ag nanoparticles and Au nanorods.
single layer of copolymer micelles. Thus, Ag nanoparticles and Au nanorods can co-assembled in a position-selective arrangement at the nanometer scale and distribute uniformly over a large area without aggregation. 4
Nanotechnology 25 (2014) 455602
H Kim et al
References
The array of Ag nanoparticles and Au nanorods shown in figure 3(b) exhibits all three plasmon bands for Ag nanoparticles (red arrow) and Au nanorods (two blue arrows), as shown in figure 4. The TEM image in figure 3(b) can be regarded as a composite image of figures 3(a) and 2(c). The absorption spectrum in figure 4(c) can be also considered to be a composite spectrum of the spectra shown in figures 4(a) and (b). This implies that the surface plasmons of the Au nanorods and Ag nanoparticles were not coupled. The Au nanorods and Ag nanoparticles were arranged in selective positions, which can effectively inhibit plasmon coupling [25].
[1] Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Gates B, Yin Y, Kim F and Yan H 2003 Adv. Mater. 15 353 [2] El-Sayed M A 2001 Acc. Chem. Res. 34 257 [3] Murphy C J, Gole A M, Hunyadi S E, Stone J W, Sisco P N, Alkilany A, Kinard B E and Hankins P 2008 Chem. Commun. 44 544 [4] Huang X, Neretina S and El-Sayed M A 2009 Adv. Mater. 21 4880 [5] Jana N R, Gearheart L and Murphy C J 2001 Adv. Mater. 13 1389 [6] Nikoobakht B and El-Sayed M A 2003 Chem. Mater. 15 1957 [7] Ye X et al 2012 ACS Nano 6 2804 [8] Thomas K G, Barazzouk S, Ipe B I, Joseph S T S and Kamat P V 2004 J. Phys. Chem. B 108 13066 [9] Hu X, Cheng W, Wang T, Wang E and Dong S 2005 Nanotechnology 16 2164 [10] Pérez-Juste J, Rodríguez-González B, Mulvaney P and Liz-Marzán L M 2005 Adv. Funct. Mater. 15 1065 [11] Kim H-C, Park S-M and Hinsberg W D 2010 Chem. Rev. 110 146 [12] Hamley I W 2003 Nanotechnology 14 R39 [13] Kao J, Thorkelsson K, Bai P, Rancatore B J and Xu T 2013 Chem. Soc. Rev. 42 2654 [14] Deshmukh R D, Liu Y and Composto R J 2007 Nano Lett. 7 3662 [15] Thorkelsson K, Mastroianni A J, Ercius P and Xu T 2012 Nano Lett. 12 498 [16] Liu Z, Huang H and He T 2013 Small 9 505 [17] Son J G, Bae W K, Kang H, Nealey P F and Char K 2009 ACS Nano 3 3927 [18] Zhang Q, Gupta S, Emrick T and Russell T P 2006 J. Am. Chem. Soc. 128 3898 [19] Yoo S I, Kwon J H and Sohn B-H 2007 J. Mater. Chem. 17 2969 [20] Spatz J P, Mössmer S, Hartmann C, Möller M, Herzog T, Krieger M, Boyen H-G, Ziemann P and Kabius B 2000 Langmuir 16 407 [21] Li W et al 2013 Macromolecules 46 2241 [22] Nie Z, Fava D, Kumacheva E, Zou S, Walker G C and Rubinstein M 2007 Nat. Mater. 6 609 [23] Shchukin D G, Radtchenko I L and Sukhorukov G B 2003 ChemPhysChem 4 1101 [24] Liu Y, Mills E N and Composto R J 2009 J. Mater. Chem. 19 2704 [25] Acharya H, Sung J, Sohn B-H, Kim D H, Tamada K and Park C 2009 Chem. Mater. 21 4248
4. Conclusion We demonstrated spin-coating of a monolayer of PS-b-P4VP spherical micelles with Au 1D nanorods that were surfacemodified in the same way as the PS corona block copolymers. When the diameters of the micelles were larger than the lengths of the nanorods, the hexagonally arranged spherical micelles became surrounded by nanorods with their long axes perpendicular to the radial direction of micelles. This arrangement provided selective organization of Au nanorods and Ag nanoparticles which were selectively synthesized within the cores of the PS-b-P4VP micelles. Thus, a homogenous distribution of two different nanomaterials over a large area without aggregation was achieved by the positionselective arrangement of Au nanorods and Ag nanoparticles at the nanometer scale. Because a variety of nanoparticles can be synthesized in the cores of PS-b-P4VP micelles and various types of metal nanorods and semiconductors are available, they can be incorporated into arrays of diblock copolymer micelles. Thus, our assembly approach can be applied to the organization of spherical nanoparticles and 1D nanorods.
Acknowledgements This work was supported by Mid-career Researcher Program through NRF grant funded by the MSIP (NRF2014R1A2A2A01002290).
5
Search
Collections
Journals
About
Contact us
My IOPscience
Nanoscale arrangement of diblock copolymer micelles with Au nanorods
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 455602 (http://iopscience.iop.org/0957-4484/25/45/455602) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 207.162.240.147 This content was downloaded on 16/06/2017 at 20:11 Please note that terms and conditions apply.
You may also be interested in: Fabrication of size-controlled nanoring arrays by selective incorporation of ionic liquids in diblock copolymer micellar cores Sung-Soo Kim, Donghwi Kang and Byeong-Hyeok Sohn Large area tunable arrays of graphene nanodots fabricated using diblock copolymer micelles Sung-Soo Kim, Jeong-Yong Choi, Kwan Kim et al. Micellar nanotubes and AAO nanopores decorated with nanoparticles Seung-Min Jeon, Sung Hwa Lee, Byeong-Hyeok Sohn et al. Micropatterning of a single layer of nanoparticles by lithographical methods with diblockcopolymer micelles Sang-Hyun Yun, Byeong-Hyeok Sohn, Jin Chul Jung et al. Light responsive thin films of micelles of PS-b-PVP complexed with diazophenol chromophore Luciana M Saiz, Patricia A Oyanguren, María José Galante et al. Synthesis and optical properties of gold nanorods with controllable morphology Tianyu Ye, Zhigao Dai, Fei Mei et al. Structured nanoporous surfaces from hybrid block copolymer micelle films with metal ions Minsoo P Kim, Hyeong Jun Kim, Bumjoon J Kim et al. Hexagonally ordered nanoparticles templated using a block copolymer film through Coulombic interactions Wonjoo Lee, Seung Yong Lee, Xin Zhang et al.
Nanotechnology Nanotechnology 25 (2014) 455602 (5pp)
doi:10.1088/0957-4484/25/45/455602
Nanoscale arrangement of diblock copolymer micelles with Au nanorods Hwan Kim, Yirang Lim, Sehee Kim, Sung-Soo Kim and Byeong-Hyeok Sohn Department of Chemistry, Seoul National University, Seoul 151-747, Korea E-mail: [email protected] Received 16 June 2014, revised 13 August 2014 Accepted for publication 17 September 2014 Published 21 October 2014 Abstract
We fabricated a single-layered film consisting of spherical micelles of diblock copolymers and one-dimensional Au nanorods that were surface modified with the same polymer as the corona block of the copolymers. When the diameters of micelles were larger than the lengths of the nanorods, spherical micelles arranged in a hexagonal configuration surrounded by nanorods with their long axes perpendicular to the radial direction of the micelles. This arrangement provided selective organization of the Au nanorods and Ag nanoparticles which were selectively synthesized within the cores of the copolymer micelles. Thus, position-selective arrangement of Au nanorods and Ag nanoparticles was demonstrated at the nanometer scale such that a homogenous distribution of two different nanomaterials over a large area without aggregation was achieved. S Online supplementary data available from stacks.iop.org/NANO/25/455602/mmedia Keywords: block copolymer, micelle, gold nanorod, silver nanoparticle (Some figures may appear in colour only in the online journal) 1. Introduction
synthesized by the seed-mediated growth method, typically with cetyltrimethylammonium bromide (CTAB) as a surfactant [5, 6]. The aspect ratio can be finely tuned by adjusting the synthetic conditions such as the number of seed particles and the presence of additional aromatic surfactants [7]. The tunable absorbance bands of metal nanorods are valuable for many applications in optical devices and biosensors [3, 4]. However, controlling the arrangement of the nanorods on a solid substrate as well as in a solution is necessary because the inter-particle coupling of nanorods changes the plasmon bands in terms of their position and broadness [4]. To control the coupling between the nanorods, for example, a functional group which can allow the intermolecular hydrogen bonding was introduced to the surface of Au nanorods. The modified nanorods assemble into an endto-end arrangement, which induces a large red-shift of the longitudinal band as the number of nanorods in the assembly increases [8, 9]. Aligned Au nanorods in a polymer matrix were also fabricated by stretching the polymer–nanorod composite, which were capable of selective absorption of polarized light [10].
Nanomaterials have attracted much attention because of their unique optical, electrical, magnetic, and mechanical properties which are different from bulk materials [1, 2]. The distinct characteristics of nanomaterials generally depends on their size and shape. In particular, the optical and electrical properties of zero dimensional (0D) nanoparticles and onedimensional(1D) nanorods can be precisely tuned by adjusting their size and shape [1–3]. For example, 1D nanorods of semiconductors such as CdSe and ZnO have been studied for use in highly efficient light emitting diodes and solar cells with the control over their optoelectronic characteristics by tuning their aspect ratio as well as the shape [1]. For nanorods of metals such as Au and Ag, there are two absorbance bands for surface plasmon resonance in the transverse mode along the short axis and in the longitudinal mode along the long axis [2, 3]. For example, Au nanorods have two plasmon bands in the visible and near IR wavelengths, which can be finely tuned by changing the aspect ratio of the length to the diameter of the nanorods [2–4]. Au nanorods are generally 0957-4484/14/455602+05$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 455602
H Kim et al
Diblock copolymers with self-assembled nanostructures [11, 12] have been used as polymeric matrices to guide the arrangement of nanorods [13, 14]. Supramolecules [15] and topographic nanotemplates [16–18] based on diblock copolymers have also been employed to guide the location of nanorods. In contrast, incorporation of 1D nanorods into nanometer-sized micelles of diblock copolymers is dimensionally restricted because diblock copolymers form 0D spherical micelles. Thus, 1D nanorods cannot be commensurate with 0D spherical micelles although diblock copolymer micelles have been utilized as effective templates to synthesize and arrange spherical nanoparticles [19–21]. In this study, we first demonstrated that Au nanorods can be guided to the peripheral regions of hexagonally arranged spherical micelles of diblock copolymers when the diameters of the micelles are larger than the lengths of the nanorods, i.e., 0D spherical micelles in a hexagonal arrangement were effectively surrounded by 1D nanorods with their long axes perpendicular to the radial direction of the micelles. We then utilized the organization of the copolymer micelles and Au nanorods to achieve position-selective arrangement of Au nanorods and Ag nanoparticles. By synthesizing Ag nanoparticles selectively in the cores of the copolymer micelles, we isolated the Ag nanoparticles in the core area of micelles from the Au nanorods in the peripheral regions of the micelles. Thus, the location of Au nanorods and Ag nanoparticles was guided in the micellar film without aggregation. In addition, the nanoscale arrangement of Au nanorods and Ag nanoparticles allowed the appearance of all of the plasmonic bands for both the Au nanorods and the Ag nanoparticles because their optical coupling was constrained by the micellar structure.
were well-dispersed in toluene without precipitation. The average diameter and length of the Au nanorods were 10 nm and 33 nm, respectively (supplementary data, figure (S1)). The absorption spectrum shows two plasmon bands for the Au nanorods at 512 nm in the transverse mode and 780 nm in the longitudinal mode (supplementary data figure (S1)).
2. Experimental methods
2.3. Characterization
2.2. Fabrication of a single layer of Polystyrene-block-poly(4vinyl pyridine) (PS-b-P4VP) micelles
PS-b-P4VP polymers various molecular weights, i.e., PS(25k)b-P4VP(7k), PS(51k)-b-P4VP(18k), and PS(109k)-b-P4VP (27k), were purchased from Polymer Source PS-b-P4VP was dissolved in toluene at 100 °C for 2 h to yield a 0.7 wt % micellar solution, which was then cooled to room temperature. To incorporate the Ag precursors within the P4VP cores, silver acetate was added to a PS-b-P4VP micellar solution (molar ratio of silver acetate to 4VP = 0.7). The solution was stirred for five days. A PS-b-P4VP micellar solution containing Au nanorods was prepared by mixing a solution of Au nanorods with a PS-b-P4VP micellar solution. When the two toluene solutions were mixed, the weight ratio of nanorods to micelles was kept as 2 : 1, regardless of the presence of silver acetate in the P4VP cores. This mixing ratio resulted in a 25% more Au nanorods per micelle in the PS-b-P4VP micelles with silver acetate, compared to those without silver acetate. A single layer of PS-b-P4VP micelles containing Ag precursors, Au nanorods, or both Ag precursors and Au nanorods was spin-coated (2000 rpm, 60 s) from a corresponding solution onto a cleaned quartz plate or a freshly-cleaved mica substrate. To reduce the Ag precursors to Ag nanoparticles, the films were exposed to UV irradiation (254 nm, 15 W) for 2 h at room temperature.
For plane-view transmission electron microscopy (TEM) images, a single-layered film of PS-b-P4VP micelles was floated off of the mica substrate onto deionized water, from which it was collected on a carbon-coated TEM grid. TEM was performed on a Hitachi 7600 instrument operating at 100 kV. The absorption spectra were measured on a Varian Cary-5000 spectrophotometer.
2.1. Preparation of PS-modified Au nanorods
We synthesized Au nanorods by the seed-mediated growth method reported in the literature [6]. A seed solution was prepared by adding an ice-cold aqueous solution of NaBH4 (0.35 mL, 0.01 M) to an aqueous mixture of HAuCl4 (0.25 mL, 0.1 M) and CTAB (9.4 mL, 0.1 M). A 1 mL volume of the seed solution was then added to the growth solution, which was prepared by mixing HAuCl4 (2.5 mL, 0.1 M), CTAB (90 mL, 0.1 M), ascorbic acid (0.5 mL 0.1 M), and AgNO3 (0.25 mL, 0.1 M). After 18 h, the excess CTAB was removed by centrifugation at 13 000 rpm for 20 min. The surfaces of the Au nanorods were modified with thiol-end-functionalized polystyrene (PS-SH) [22]. Polystyrene with a thioester end group was first synthesized by the reversible addition–fragmentation transfer polymerization. The thioester end group was then reduced to the thiol group with NaBH4. The molecular weight of PS-SH was 8000 g mol−1 with a polydispersity index of 1.06. A tetrahydrofuran (THF) solution of PS-SH was directly added to the concentrated aqueous solution of Au nanorods. The excess PS-SH was removed by centrifugation at 13 000 rpm for 20 min. After the surface modification, the Au nanorods
3. Results and discussion PS-b-P4VP, diblock copolymers spontaneously associate into nanometer-sized spherical micelles with a soluble PS corona and an insoluble P4VP core in toluene, which is a selective solvent for the PS block. By spin coating from the micellar solution, which provides fast evaporation of the solvent, a single layer of spherical PS-PVP micelles with regular spacing can be fabricated [19]. To obtain different sizes of the copolymer micelles, we employed three copolymers: PS(25k)-P4VP(7k), PS(51k)P4VP(18k), and PS(109k)-P4VP(27k). TEM images of these micellar films are shown in figure 1. The P4VP cores were selectively stained with I2. In all cases, the cores appeared as dark spheres with a pseudo-hexagonal arrangement within a 2
Nanotechnology 25 (2014) 455602
H Kim et al
Figure 1. TEM images of single-layered films of PS-b-P4VP micelles: (a) PS(25k)-P4VP(7k); (b) PS(51k)-P4VP(18k); (c) PS(109k)-P4VP
(27k). The P4VP cores were stained with I2. The scale bars are 100 nm.
Figure 2. TEM images of single-layered films of PS-b-P4VP micelles with Au nanorods: (a) PS(25k)-P4VP(7k); (b) PS(51k)-P4VP(18k); (c) PS(109k)-P4VP(27k). The scale bars are 100 nm.
P4VP cores cannot be clearly identified, except the case with the largest micelles in figure 2(c), due to the high contrast of the Au nanorods in the images. However, the Au nanorods did not aggregate in all images and they were mostly located within the bright PS regions between the gray PVP cores, particularly in the image with the largest micelles. In this case (figure 2(c)), the long axes of the Au nanorods were almost perpendicular to the radial direction of the micellar core. Because the diameter (∼90 nm) of a PS(109k)-P4VP(27k) micelle is sufficiently large, compared to the length of an Au nanorod (∼33 nm), the spherical copolymer micelles can become surrounded by 1D nanorods. A TEM image of PS (109k)-P4VP(27k) micelles with Au nanorods in an extended area is shown in figure (S5) of the supplementary data. Because a variety of precursors of metal nanoparticles can be coordinated to the pyridine unit of the 4PVP block, metal nanoparticles can be selectively synthesized in the cores by reduction of the precursors. Thus, nanoparticles can be arranged on a solid substrate with preservation of the order of the micellar array [19]. In this study, we incorporated silver acetate as a precursor for Ag nanoparticles in the P4VP cores of PS(109k)P4VP(27k) micelles in toluene. Then, a single layer of copolymer micelles was spin-coated, in which Ag nanoparticles were synthesized selectively in the P4VP cores by UV reduction of silver acetate [23]. We employed UV irradiation as a reduction method, instead of simple thermal treatment, to prevent the Au
bright PS matrix without multi-layered structures, indicating a single-layered film of PS-P4VP micelles, even though the diameters of the copolymer micelles are different. As the molecular weight of the copolymers increases, both the core diameter and the center-to-center distance increase. The core diameters were ∼18 nm for PS(25k)-P4VP(7k), ∼26 nm for PS(51k)-P4VP (18k), and ∼38 nm for PS(109k)-P4VP(27k). The center-tocenter distances were ∼23 nm for PS(25k)-P4VP(7k), ∼41 nm for PS(51k)-P4VP(18k), and ∼90 nm for PS(109k)-P4VP(27k). We note that the ordering of the copolymer micelles without overlapping was uniform over extended areas based on the relatively large-area images provided by AFM, FE–SEM, and TEM (supplementary data, figures (S2)–(S4)). The TEM samples were also collected from arbitrary positions within the micellar film (∼1 cm × ∼1 cm) floated on water (supplementary data, figure (S5)) but the same micellar ordering was observed. The Au nanorods with the PS-modified surface (diameter = ∼10 nm, length = ∼33 nm) were well-dissolved in toluene and exhibited characteristic transverse (512 nm) and longitudinal (780 nm) bands in the UV–Vis absorption spectrum (supplementary data, figure (S1)). From a mixture of PS-b-P4VP micelles and Au nanorods, a single layer of copolymer micelles with Au nanorods was fabricated by spin coating. The weight ratio of Au nanorods to micelles was adjusted to 2 for all cases. In the TEM images in figures 2(a)– (c), the Au nanorods appear as dark particles. However, the 3
Nanotechnology 25 (2014) 455602
H Kim et al
Figure 3. TEM images: (a) a single layer of PS(109k)-P4VP(27k) micelles containing Ag nanoparticles in the P4VP cores; (b) an array of Ag nanoparticles surrounded by Au nanorods in a single layer of copolymer micelles. The scale bars are 100 nm. Enlarged images (150 nm × 150 nm) are also included.
Absorbance (a.u.)
nanorods from reshaping during the heating process [24]. The TEM image in figure 3(a) shows an array of PS(109k)-P4VP (27k) micelles containing Ag nanoparticles in the P4VP cores. The dark regions correspond to the P4VP cores of the micelles containing Ag nanoparticles, whereas the bright continuous regions are the PS coronas. The synthesized Ag nanoparticles can be identified in the magnified image in figure 3(a). There are about a dozen Ag nanoparticles (∼4 nm in diameter) in each P4VP core. The raspberry-like morphology of many of the nanoparticles is formed by the fast reduction with UV irradiation [23]. The Ag nanoparticles synthesized in the P4VP cores of the micelles exhibited a typical plasmon band at 420 nm (red arrow), as shown in figure 4(a). The Au nanorods with PS-modified surfaces were also homogeneously dispersed in a toluene solution of PS(109k)P4VP(27k) micelles containing silver acetate in the P4VP cores. From this mixture, a single layer of PS(109k)-P4VP (27k) micelles with Au nanorods was again fabricated by spin coating. Ag nanoparticles were then selectively synthesized in the P4VP cores by UV irradiation. As shown in figure 3(b), the Au nanorods were arranged on the circumference of the P4VP cores and were dispersed over the micellar array as like the previous case only with Au nanorods (figure 2(c)). Because 25% more nanorods were added in this case (experimental methods), more nanorods can be observed in figure 3(b), compared to figure 2(c). In the magnified image in figure 3(b), the synthesized Ag nanoparticles with raspberrylike morphologies in the PVP cores can be again recognized as like those in the case without Au nanorods (figure 3(a)). In addition, the Au nanorods were mostly located between the PVP cores containing Ag nanoparticles with their long axes almost perpendicular to the radial direction of the micellar core. However, we can find that many of Au nanorods are also side-by-side assembled. The image in figure 3(b) is basically a combined image of the images presented in figure 3(a) and figure 2(c), that is, an array of Ag nanoparticles in the P4VP cores surrounded by Au nanorods in a
(c)
(b)
(a)
400
600 800 1000 Wavelength (nm)
1200
Figure 4. Absorption spectra of single layers of PS(109k)-P4VP
(27k) micelles: (a) with Ag nanoparticles; (b) with Au nanorods; (c) with Ag nanoparticles and Au nanorods.
single layer of copolymer micelles. Thus, Ag nanoparticles and Au nanorods can co-assembled in a position-selective arrangement at the nanometer scale and distribute uniformly over a large area without aggregation. 4
Nanotechnology 25 (2014) 455602
H Kim et al
References
The array of Ag nanoparticles and Au nanorods shown in figure 3(b) exhibits all three plasmon bands for Ag nanoparticles (red arrow) and Au nanorods (two blue arrows), as shown in figure 4. The TEM image in figure 3(b) can be regarded as a composite image of figures 3(a) and 2(c). The absorption spectrum in figure 4(c) can be also considered to be a composite spectrum of the spectra shown in figures 4(a) and (b). This implies that the surface plasmons of the Au nanorods and Ag nanoparticles were not coupled. The Au nanorods and Ag nanoparticles were arranged in selective positions, which can effectively inhibit plasmon coupling [25].
[1] Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Gates B, Yin Y, Kim F and Yan H 2003 Adv. Mater. 15 353 [2] El-Sayed M A 2001 Acc. Chem. Res. 34 257 [3] Murphy C J, Gole A M, Hunyadi S E, Stone J W, Sisco P N, Alkilany A, Kinard B E and Hankins P 2008 Chem. Commun. 44 544 [4] Huang X, Neretina S and El-Sayed M A 2009 Adv. Mater. 21 4880 [5] Jana N R, Gearheart L and Murphy C J 2001 Adv. Mater. 13 1389 [6] Nikoobakht B and El-Sayed M A 2003 Chem. Mater. 15 1957 [7] Ye X et al 2012 ACS Nano 6 2804 [8] Thomas K G, Barazzouk S, Ipe B I, Joseph S T S and Kamat P V 2004 J. Phys. Chem. B 108 13066 [9] Hu X, Cheng W, Wang T, Wang E and Dong S 2005 Nanotechnology 16 2164 [10] Pérez-Juste J, Rodríguez-González B, Mulvaney P and Liz-Marzán L M 2005 Adv. Funct. Mater. 15 1065 [11] Kim H-C, Park S-M and Hinsberg W D 2010 Chem. Rev. 110 146 [12] Hamley I W 2003 Nanotechnology 14 R39 [13] Kao J, Thorkelsson K, Bai P, Rancatore B J and Xu T 2013 Chem. Soc. Rev. 42 2654 [14] Deshmukh R D, Liu Y and Composto R J 2007 Nano Lett. 7 3662 [15] Thorkelsson K, Mastroianni A J, Ercius P and Xu T 2012 Nano Lett. 12 498 [16] Liu Z, Huang H and He T 2013 Small 9 505 [17] Son J G, Bae W K, Kang H, Nealey P F and Char K 2009 ACS Nano 3 3927 [18] Zhang Q, Gupta S, Emrick T and Russell T P 2006 J. Am. Chem. Soc. 128 3898 [19] Yoo S I, Kwon J H and Sohn B-H 2007 J. Mater. Chem. 17 2969 [20] Spatz J P, Mössmer S, Hartmann C, Möller M, Herzog T, Krieger M, Boyen H-G, Ziemann P and Kabius B 2000 Langmuir 16 407 [21] Li W et al 2013 Macromolecules 46 2241 [22] Nie Z, Fava D, Kumacheva E, Zou S, Walker G C and Rubinstein M 2007 Nat. Mater. 6 609 [23] Shchukin D G, Radtchenko I L and Sukhorukov G B 2003 ChemPhysChem 4 1101 [24] Liu Y, Mills E N and Composto R J 2009 J. Mater. Chem. 19 2704 [25] Acharya H, Sung J, Sohn B-H, Kim D H, Tamada K and Park C 2009 Chem. Mater. 21 4248
4. Conclusion We demonstrated spin-coating of a monolayer of PS-b-P4VP spherical micelles with Au 1D nanorods that were surfacemodified in the same way as the PS corona block copolymers. When the diameters of the micelles were larger than the lengths of the nanorods, the hexagonally arranged spherical micelles became surrounded by nanorods with their long axes perpendicular to the radial direction of micelles. This arrangement provided selective organization of Au nanorods and Ag nanoparticles which were selectively synthesized within the cores of the PS-b-P4VP micelles. Thus, a homogenous distribution of two different nanomaterials over a large area without aggregation was achieved by the positionselective arrangement of Au nanorods and Ag nanoparticles at the nanometer scale. Because a variety of nanoparticles can be synthesized in the cores of PS-b-P4VP micelles and various types of metal nanorods and semiconductors are available, they can be incorporated into arrays of diblock copolymer micelles. Thus, our assembly approach can be applied to the organization of spherical nanoparticles and 1D nanorods.
Acknowledgements This work was supported by Mid-career Researcher Program through NRF grant funded by the MSIP (NRF2014R1A2A2A01002290).
5
