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Arrangement and SERS Applications of Nanoparticle Clusters Using Liquid Crystalline Template Dae Seok Kim,† Apiradee Honglawan,‡ Shu Yang,‡ and Dong Ki Yoon*,† †

Graduate School of Nanoscience and Technology and KINC, KAIST, Daejeon 34141, Republic of Korea Department of Material science and Engineering, University of Pennsylvania, Towne Building, 220 South 33rd Street, Philadelphia 19104, United States

‡

S Supporting Information *

ABSTRACT: Manipulation of nanomaterials such as nanoparticles (NPs) and nanorods (NRs) to make clusters is of significant interest in material science and nanotechnology due to the unusual collective opto-electric properties in such structures that cannot be found in the individual NPs. This work demonstrates an effective way to arrange NP clusters (NPCs) to make the desired arrays based on removable and NP-guidable liquid crystalline template using sublimation and reconstruction phenomenon. The position of the NPCs is precisely controlled by the defect structure of the liquid crystal (LC), namely toric focal conic domains (TFCDs), during thermal annealing to construct the LC and corresponding NPC structures. As a proof of concept, the surface-enhanced Raman scattering (SERS) activity of a fabricated array of gold nanorod (GNR) clusters is measured and shown to have highly sensitive detection characteristics essential for potential sensing applications. KEYWORDS: arrangement, nanoparticle clusters, liquid crystals, gold nanorods, surface-enhanced Raman scattering

1. INTRODUCTION Unexpected optoelectronic properties of great scientific interest can be obtained by organization or aggregation of nanoparticles (NPs) into secondary structures, leading to much attraction in employing NPs as building blocks to form various kinds of functional structures, especially in the material science and bioengineering disciplines.1−5 For example, metal NPs such as gold nanoparticles (GNPs) or gold nanorods (GNRs) can interact with light, resulting in collective oscillation of the conduction electrons on the metal surface, a phenomenon known as surface plasmon resonance (SPR). Moreover, this phenomenon can be significantly enhanced when GNRs are well-organized,6,7 showing strong optical signals that can facilitate many potential applications such as surface-enhanced Raman scattering (SERS),4,8,9 biosensors,10,11 bioimaging,12 polarized optical filters,13 ultrafast nonlinear optics,14 and cavity resonators.15 In order to extend such properties and applications into practical usage, the assembly process should be controlled through immobilization of the NPs at the desired positions and their organization on an appropriate substrate or in a suitable medium.16,17 Accordingly, many methods to assemble and arrange NPs have been introduced in the past few decades, including evaporative self-assembly,18−20 templateassisted assembly,21,22 electromagnetic field assisted assembly,23,24 ligand mediated assembly,25 ion assisted assembly,26 and elastic matrix assisted assembly.27 However, due to several issues such as conglomeration of NPs and macroseparation with surrounding media, etc., precise arrangement control of assembled NPs still remains a complex challenge.16,17,28,29 © 2017 American Chemical Society

Recently, a simple and effective way for making an array of NP clusters (NPCs) with hexagonal symmetry at micrometer scale was reported.30 This was achieved using a sublimable liquid crystal (LC) material, denoted here as Y002 (Figure 1a).

Figure 1. (a) Structures, phases, and transition temperatures of the rod type LC material, Y002 (all temperatures are in °C). (b−d) Schematic representation to fabricate the NPC arrays (w and d are width and depth of the microchannel, respectively). (b) Y002 is injected into the microchannel by heating above the isotropic temperature and then cooled down to form TFCD array at SmA. (c) The NP suspension is drop-casted and spin-coated on the TFCD array where NPs are mostly placed at the dimple region of the TFCD array. (d) NPC arrays are generated with same symmetry of TFCDs array after thermal annealing at 180 °C for 1 h. Received: November 30, 2016 Accepted: February 10, 2017 Published: February 10, 2017 7787

DOI: 10.1021/acsami.6b15343 ACS Appl. Mater. Interfaces 2017, 9, 7787−7792

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ACS Applied Materials & Interfaces

Figure 2. Polarized optical microscopy (POM) images (a−d) and the corresponding SEM images (e−h) of the NPC arrangements as a function of the width (w) of the microchannel, with a fixed depth of 5 μm. (a, e) w = 5 μm, (b, f) w = 10 μm, (c, g) w = 20 μm, (d, h) w = 50 μm and the inset on left of (h) is a corresponding FFT image and the inset of (h) on right is a magnified SEM image of the NPC (all scale bars are 5 μm and the scale bar in the inset of (h) on the right is 500 nm). Each red solid line indicates the unit lattice of the arrays.

Figure 3. (a) Experimental data for the average distance between NPCs as a function of the channel width with fixed depth of 5 μm (top of x-axis) and the channel depth with fixed width of 50 μm (except for the channel of 3 μm depth where the width is 20 μm, bottom of x-axis). The SEM images of the NPC arrangements based on the depth of the microchannel (b) d = 3 μm, (c) d = 5 μm, (d) d = 7 μm, and (e) w = 10 μm. All scale bars are 10 μm.

E (SmE) and soft crystal phases upon further cooling to room temperature. The thermal sublimation property is exhibited in the SmA phase (Figure 1a).33−35 As a proof of concept, we synthesized fluorinated silica NPs (F-SiO2; diameter ∼ 100 nm) utilizing methods reported in the literature.36 Thereafter, silicon microchannels were introduced by conventional photolithography to give confined geometries, guiding the formation of TFCDs arrays required for the NPC array (Figure 1b).31,32,37 Previously, it has been reported that a hexagonal array of TFCDs can be modulated by varying the microchannel width and depth.37 On the basis of this work, we were inspired to control the arrangement of the NPCs as illustrated in Figures 1b−d. In order to achieve the desired arrangement, Y002 in the isotropic liquid state was injected into the microchannels by capillary action and cooled down to room temperature to generate the TFCDs (Figure 1b). The TFCDs show dimple-

In the above study, one of the typical LC defects, the toric focal conic domain (TFCD),31,32 was used as a template for NP assembly. The sublimable character of the Y002 molecule allows the removal all residual LC materials after assembly, leaving arrayed NPCs.33,34 On the basis of the removable and NP-guidable TFCDs, here, we have demonstrated that the arrangement of NPCs can be controlled in a few micron scale by using the confinement system and measured SERS activities of the NPC arrays consisting of GNRs for potential sensing applications.

2. RESULTS AND DISCUSSION Control of Arrangements of NPCs. To realize this strategy, the Y002 molecule was prepared, which undergoes a transition from the isotropic phase to the smectic A (SmA) phase during cooling from 200 °C. This is followed by smectic 7788


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could be successfully used for SERS. Among the various techniques to assemble GNRs, solution-evaporative assembly has thus far been used extensively.16,18−20 However, solute deposition on the substrate during solvent evaporation is not easy to control, requiring extremely sensitive experimental conditions. Furthermore, the process is time-consuming because the assembly proceeds through thermodynamic processes, where particle−particle, particle−solvent, and suspension−substrate interactions must be considered.16−20,38,39 In contrast, our method only requires surface affinity between the NPs and LC molecules because the morphological evolution of TFCDs during thermal annealing can effectively make well-arrayed NPCs. In addition, the entire process takes less than 1 h. For the above demonstration, GNRs were synthesized via the seed mediated growth method.40 The resulting GNRs were successfully functionalized with a fluorinated ligand (CF3(CF2)7CH2CH2-SH) by ligand exchange (Figure 4a and

like conical topographic patterns that play a crucial role during trapping NPs.30 Then, the NPs were placed on the LC film by drop-casting a 0.01 wt % NP suspension in a fluorinated solvent (Novec-7300, 3M) that was subsequently spin-coated (Figure 1c). After drying the solvent, thermal annealing was conducted at ∼180 °C for 1 h, whereupon the LC material underwent phase transition to the SmA phase and subsequently sublimed. The removal of the LC material led to the reconstruction of the NPCs to make globular shapes via sublimation and recondensation (Figure 1d and Figure S1).30,34 In the resultant highly ordered NPC array obtained in the microchannel, the position of the NPCs is exactly coincident with that of the TFCDs before the thermal annealing process (Figures 1b,d). During the current study, we used various sizes of microchannels, with varying widths (w) of 5, 10, 20, and 50 μm at a fixed depth of 5 μm and varying depths (d) of 5, 7, and 10 μm with a fixed width of 50 μm. Additionally, a channel with a depth of 3 μm and a width of 20 μm was also used. The 50 μm width was avoided in the case of 3 μm depth channel to avoid the possibility of the thin film being sublimed too fast to form a stable LC film during the sample preparation process. The TFCDs confined in the channels showed Maltese cross patterns when observed under cross-polarizers (Figures 2a−d).32,33 Figure 2 shows linear and alternate arrays of TFCDs along the channel direction in the 5 and 10 μm wide channels, respectively (Figures 2a and 2b), and a closely packed hexagonal lattice in the 20 and 50 μm wide channels (Figures 2c and 2d). Each TFCD was used as a template to organize the NPCs during thermal annealing. After removal of all LC molecules, scanning electron microscopy (SEM) images clearly showed that NPC arrays were successfully generated (Figures 2e−h). The NPCs revealed a uniform diameter of ∼700 nm with a spherical shape (upper right inset of Figure 2h) and have a constant distance of ∼5 μm with neighboring NPCs, regardless of the width of the channel (Figures 2 and 3a). This result agrees with previous reports that showed that the size of the TFCDs is mainly determined by the channel depth, where the size of TFCD is directly proportional to channel depth.31,32,37 Thus, an array of NPCs with constant periodicity could be obtained over the whole area (Figure 2h and upper left inset of fast Fourier transform (FFT) image). When the channel depth (d) was varied, the TFCDs displayed a clear tendency to follow the proportional relation a ∝ d, in which a is the center-to-center distance between neighboring TFCDs (Figure S2). In consequence, it was observed that the population of NPCs after thermal annealing decreased with increase of channel depth from 3 to 10 μm (Figure 3). The plot in Figure 3a gives the quantitative measurements of the spacing between the NPCs as a function of channel depth, illustrating the proportional relation between the two values. Furthermore, not only the spherical SiO2 NPs but also other types of NP can be effectively assembled into cluster via this assembly system. For example, the clustering of other shapes of NPs such as rods or cubic were demonstrated using this method (Figure S3). Thus, as a proof of concept, one of the representative NPs, GNR was chosen to fabricate the NPCs array, showing the SERS activities, which will be introduced in next section. Fabrication of GNR Cluster Array for SERS. Clustering of GNRs leads to extremely high electromagnetic fields that produce enhanced optical signals, facilitating sensing applications. With this application in mind, we demonstrated that GNR cluster arrays fabricated via the approach outlined above

Figure 4. (a) Schematic illustration of the synthesis of perfluorinated GNRs by ligand exchange. (b) SEM images of the GNR cluster array in a microchannel with 5 μm depth and 50 μm width. (c) Magnified SEM image of a single GNR cluster. (d) Magnified SEM image of the single GNR cluster (the scale bar is 100 nm).

Figure S4),41 in which the extinction spectrum is changed due to the local aggregation of the fluorinated GNRs (F-GNRs). This step indeed reduces the interfacial energy between the semifluorinated LC molecule and GNR to enhance the affinity, facilitating their dispersion on the LC film. In addition, this effort was also helpful in getting closely packed GNRs via fluorophilic interactions during thermal annealing.42 As a result, F-GNRs (F-GNRs) clusters were successfully arranged, in a manner similar to the F-SiO2’s case (Figures 1−3). During this process, a 100 μL volume of 1.0 mg/mL F-GNR suspension in fluorinated solvent (Novec-7300) was used (Figure 4). This suspension exhibits good reproducibility over large areas (Figure 4b) and has a relatively narrow size distribution in the 1−2 μm range with a globular shape (Figure 4c). The magnified SEM image given in Figure 4d clearly shows the tightly packed F-GNRs in a cluster. The SERS activity of the F-GNR clusters synthesized using our method was investigated by employing them for the analysis of Malachite green (MG), a typical banned fish pesticide (Figure 5).18 For this purpose, two kinds of samples 7789


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Drug Administration (FDA).18 This result shows the quite striking cost effectiveness of our platform because the same amount of the nanorod sample was used on both the experiments, despite the large difference between the SERS spectra of the GNR cluster array and the monolayer. To further estimate the reproducibility of the SERS performance of the arranged GNR clusters, the laser was focused on each GNR cluster, and the corresponding Raman peak intensities were compared from cluster to cluster. Figure 5b shows the histogram of each SERS intensity at 1544 cm−1 from 20 different GNR clusters (inset of Figure 5b), confirming good reproducibility. Furthermore, in order to investigate the effective SERS activity depending on sizes of GNR clusters, various sizes of GNR clusters were prepared by varying concentration of GNRs solution from 0.1 to 1.5 mg/mL.30 As a result, the GNR clusters roughly presented ∼0.3 to ∼2 μm in diameter (Figure S5a−d). As the size of GNR cluster increases, the SERS intensity increases (Figure S5e,f) because the density of hot spots in the cluster also increases as growing the GNRs cluster.43 Therefore, the concentrated hot spots in the clusters effectively enhances Raman scattering signal of MG at 1.0 × 10−8 M.

3. CONCLUSION In summary, we have demonstrated that GNR clusters could be successfully arranged using sublimable LC material, in confined geometries that can be used in high detection limit SERS applications. This success is due to two specific characteristics of the LC material used here: the first is the self-assembling behavior to form TFCDs that are used to arrange NPCs, and the other is the subliming ability of the semifluorinated LC material that can be completely removed after forming the NPC array. The modulation of the above NPC arrays can be achieved by varying the width and depth of the microchannels used in depositing the LC material. This flexibility is essential in realizing the SERS-based sensing applications of these structures. The platform demonstrated in this work may be applicable to many other kinds of functional NPs, such as plasmonic metal NPs, quantum dots/rods, and carbon-based NPs, to realize sensing and optoelectric devices.

Figure 5. (a) Typical SERS spectra (λex = 531 nm) of MG (1.0 × 10−8 M) on the GNR cluster array (red), GNR monolayer (blue), and bare silicon substrate (green). (b) Histogram for the SERS intensity at the 1544 cm−1 peak of MG from 20 different GNR clusters, which are indicated in the inset (the scale bar of inset is 5 μm).

were prepared, with the same amount of F-GNRs in each: (1) monolayer of F-GNRs on a bare silicon wafer, which was prepared by the conventional solvent-evaporation method; (2) F-GNR clusters prepared by our method as shown in Figure 4. The two kinds of samples were immersed overnight in a 1.0 × 10−8 M aqueous solution of MG and then completely dried prior to the SERS measurement. For each SERS experiment, a Raman spectrometer with a probe laser (λ = 531 nm) that has a very small spot size (360 nm in diameter) was used (see details in Experimental Section). Here, the silicon wafer was used as the bottom substrate and shows the typical Raman peaks at 450 and 900 cm−1 (green line in Figure 5a), which can be ignored when comparing the two sample types. The spectrum of the arranged GNR clusters (red line in Figure 5a) shows a significant amplification of the peak intensity of the Raman peak at ∼1544 cm−1 when compared to the GNR monolayer (blue line) on Si wafer. The intensity ratio at 1544 cm−1 for the two spectra was calculated to be IR/IB ∼ 10 (IR is the red line for the F-GNR cluster and IB is the blue line for monolayer FGNR), indicating that the peak intensity on the F-GNR array was amplified by an order of magnitude compared to the GNR monolayer. This result clearly demonstrates that clustering of GNRs significantly enhances Raman signals compared to unpatterned GNRs.7 Moreover, the spectrum from the FGNR array shows the characteristic Raman peaks of MG at the concentration of 1.0 × 10−8 M, indicating that its SERS performance satisfies the detection limit of 1.0 × 10−8 M required by the European Commission and the US Food and

4. EXPERIMENTAL SECTION Materials. The sublimable smectic LC, Y002 (Figure 1a), and FSiO2 NPs (d ∼ 100 nm) were synthesized via modified literature methods.31,36 The GNRs were synthesized via the seed-mediated growth method,40 and F-GNRs were prepared by typical ligand exchange with 1H,1H,2H,2H-perfluorodecanethiol (Sigma-Aldrich),41 dissolved in fluorinated solvent, Novec-7300 (3M), with a concentration of 1.0 mg/mL. The size of the F-GNR has ∼45 nm long with an ∼15 nm diameter (Figure S4a). The microchannels were fabricated on (100) silicon wafers with conventional photolithography and reactive ion etching techniques.31,37 The fabricated channels had depth and width ranges of 3, 5, 7, and 10 μm and 5, 10, 20, and 50 μm, respectively, and were 10 mm in length. Sample Preparation (TFCD Films in Microchannels and NPC Arrays). To control the surface polarity, the channels were chemically cleaned by ultrasonication in a mixture of dimethylformamide (DMF) and methanol to remove organic/inorganic impurities, followed by rinsing several times with deionized water. The microchannels now have a planar anchoring condition to the smectic LC molecule. Then, the Y002 powder was placed on the edge of the microchannel and heated to the isotropic temperature (∼200 °C) on a heating stage (LINKAM LTS350) controlled by a temperature controller (LINKAM TMS94). The microchannels are filled by capillary action, whereupon 7790



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the samples are cooled down to room temperature to form the TFCDs (Figure 1b). To form the NPC array, both F-SiO2 and F-GNRs NPs were spincoated on the film containing the TFCDs in the microchannels (Figure 1c) and maintained at 180 °C for 30 min on the same temperature-controlled heating stage used in the last step. This results in complete removal of the LC material from the substrate, leaving behind only the NPCs (Figure 1d). However, if the GNR clusters were further heated after the evaporation of LC molecules, the GNRs in a cluster start to be sintered with neighboring GNRs, resulting in locally connected structures (Figure S6a). These network structures have not shown the SERS signal from the Raman chemical, MG, excited by the 531 nm laser (Figure S6b). Consequently, the most proper thermal annealing condition in this system is about 180 °C for less than 30 min. Characterization. The TFCD films and NPCs arrays were directly imaged using polarized optical microscopy (POM) (LV100POL, Nikon) and field-emission SEM (FE-SEM, Hitachi, S-4800). The absorption spectra of the synthesized CTAB-GNRs dispersed in diwater and F-GNRs dispersed in fluorinated solvent (Novec-7300) were measured using UV−vis spectroscopy (SPECTRA max Plus 384, Molecular Devices). The spectrum of the CTAB-GNRs presents typical two absorption peaks due to the longitudinal and transverse modes, while the two peaks seem to be almost merged after ligand exchange from CTAB-GNRs to F-GNRs by side-to-side aggregation of the F-GNRs via intercalation of F-chains of neighboring GNRs and the decreased aspect ratio and increased diameter (Figure S4b). The SERS was performed on a FEX Raman spectrometer (confocal micro-Raman microscope, NOST, Korea) equipped with a 531 nm, 2.2 mW laser and fine-focusing 100× microscope objective. Intensity mapping conditions of the instrument are 40 × 40 points with a 700 nm step (0.1 s/point).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15343. POM images of TFCDs in the microchannels, SEM and spectra of GNRs, SEM and POM images for the formation of NPCs, size -dependent SERS spectra of GNR clusters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.K.Y.). ORCID

Shu Yang: 0000-0001-8834-3320 Dong Ki Yoon: 0000-0002-9383-8958 Author Contributions

D.S.K. and D.K.Y. designed the research; D.S.K. and A.H. performed the research; D.S.K., S.Y., and D.K.Y. analyzed the data; and D.S.K. and D.K.Y. wrote the manuscript. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the National Research Foundation (NRF) and funded by the Korean Government (MSIP) (2014M3C1A3052567 and 2015R1A1A1A05000986) and the National Science Foundation (NSF) MRSEC grant, DMR-1120901. 7791


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