Article pubs.acs.org/cm
Magnetic Modulation of Surface Plasmon Resonance by Tailoring Magnetically Responsive Metallic Block in Multisegment Nanorods Insub Jung,† Hee-Jeong Jang,‡ Songhee Han,‡ Jesus A. I. Acapulco, Jr.,‡ and Sungho Park*,†,‡ †
Department of Energy Science and ‡Department of Chemistry, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea S Supporting Information *
ABSTRACT: The modulation of quasi-one-dimensional plasmonic nanorods via the utilization of localized surface plasmon resonance is critical for achieving local dynamic tuning on the nanoscale as well as for fundamental studies. In particular, a magnetic field has been used as an effective external stimulus that can be activated in an instant and reversible way. However, limited synthetic procedures hinder the full-utilization of one-dimensional nanorods, making it challenging to tailor magneticresponsive plasmonic materials. Here, we report the magnetic manipulation of Au nanorods with ferromagnetic Ni segments, which facilitate orientational control, allowing for the selective tuning of surface plasmon resonance. Ni segments served as a magnetic responder for the external magnetic field for guided-alignment as well as bridges to mediate intrasurface plasmon coupling of nanorods. We investigated the correlation between the optical properties and the aspect ratio of Ni segments. By designing various configurations in ferromagnetic segments, magnetic cancellation and enhancement in optical performance were demonstrated. This simple but straightforward configuration will expand the understanding of plasmonic nanorods, furthering the accessibility of multiblock nanorods.
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INTRODUCTION Surface plasmon resonance (SPR), a collective motion of conduction electrons fluctuating on the surface of metallic nanoparticles when they are irradiated with light, has been extensively studied in the fields of biosensors,1 drug delivery,2 etc. Specifically, one-dimensional plasmonic nanostructures have gained tremendous attention since they produce a unique optical band structure, which is attributed to their anisotropic nature, compared to nanospherical structures that show one typical plasmon mode.3−5 Of great interest is to tune the plasmonic behavior by utilizing the fact that SPR is sensitive to the size, shape, and surrounding medium such as the dielectric constant. One of the ways to manipulate SPR is to employ an external driving force, (e.g., a magnetic field). Owing to their biocompatible and remote control systems, magnetic fields have been used in a variety of applications, including synergistic effects,6 magnetic separation,7,8 positional adjustment,9 immunoassay based on magnetic nanoparticles,10,11 and magnetic nanosized stir bars.12 However, most of this research has been limited to the synthesis of magnetically functionalized core− shell nanoparticles,13,14 chemical modification,15 or the attachment of magnetic materials onto the surface of plasmonic materials16 (these methodologies likely dampen the plasmonic properties) since it is synthetically challenging to generate welldefined geometrical features with a magnetic section to be fully tailored. In addition, plasmonic nanorods with a relatively long aspect ratio17 (R > 5, R is defined as length/diameter) cause the © XXXX American Chemical Society
excitation out of a given frequency window, restricting the study of magnetic-responsive plasmonic nanorods. In this regard, a template-assisted electrochemical deposition method can be a plausible pathway for the production of highly uniform and structurally stable one-dimensional nanostructures with a high fidelity,18 opening to the exploration on understanding magnetic nanorods as nanosized local dynamic vortices for biomedical applications. The easy-control of length with precision and component by simply injecting metal plating solution at its appropriate potential has facilitated the development of rationale design for nanostructures, which consist of a variety of components, each with a specific purpose.19−22 One of its main usages is to employ electrochemically synthesized micro/nanorods for micro/nanomotors23−26 by utilizing the selective properties of each segment as well as drug delivery after functionalization of the surface through specific binding event.27 However, it is hard to explore the behavior of nanorods especially when their dimensions go down to nanoscale simply due to the inability to observe by conventional microscopy systems. In this context, one of the effective ways to analyze the behaviors of nanorods is to understand optical properties based on localized surface plasmon resonance by tuning dipole and quadrupole resonance Received: October 15, 2015 Revised: November 24, 2015
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Figure 1. Schematic illustration of experimental setups and conceptual design. (A) To generate a uniform magnetic field without disturbing the incident light pathway, two electromagnets are placed on either side of a UV cell. Under the external magnetic field, randomly oriented multiblock nanorods in solution tend to align. (B) Depending on the direction of applied magnetic field, multiblock nanorods can be aligned in a specific direction with respect to the incoming light (kinc.), resulting in either (i) an enhanced transverse mode or (ii) longitudinal mode. For the simplicity, only one polarization plane was drawn in this schematic illustration.
Figure 2. Magnetic modulation of surface plasmon resonance of a representative Au/Ni/Au multisegmented nanorod. (A) A HR-TEM image of a representative Au/Ni/Au multisegmented nanorod with a total length of 320 ± 10 nm, showing well-connected junctions between each block. A Ni segment works as a lever to orient nanorods. Inset shows the elemental mapping image. (B) Corresponding UV−vis spectra under the external magnetic field (Bext) perpendicular (top panel) or parallel (bottom panel) relative to the incident light. All of the UV spectra were normalized to the TSP mode in order to trace the changes of the LSP mode. Dashed arrows indicate as increasing magnetic field strength. The black dashed line is the original optical feature of nanorods. The dipole TSP mode is located at 600 nm, and the quadrupole LSP mode is centered at 820 nm. (C) UV−vis spectra depending on the distance between the electromagnets and the UV cell. (D) Longitudinal and transverse peak shifting as a function of field strengths under magnetic field perpendicular to the incident light, proving the orientational change of nanorods. (E) Optical response to the external magnetic field during 25 on−off measurements. The plot was calculated by relative intensity of the longitudinal mode to the transverse mode. ΔI is the difference between the intensity of “OFF” and “ON” states, discussed in Figure 6.
different lengths of Ni as well as the number of Ni blocks.28 It was found that Ni (which is a less optically active material) of a certain length did not affect the coupling of Au nanorods, therefore acting as pure Au nanorods with a similar dimension.
modes, which enables us to tailor orientational behavior dependent upon the interaction with light. Recently, we investigated SPR coupling of Au/Ni/Au multisegmented nanorods, correlating their performance with B
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free electrons in Ni to oscillate, thereby bridging the gap and eventually leading to resonance coupling, a higher-order mode such as quadrupole plasmons can be controlled, providing information about the orientation of the nanorods. This intraparticle surface plasmon resonance feature is quite important since it allows multiblock nanorods to act similar to pure Au nanorods in a comparable dimension. It is noteworthy that the optimal relative lengths of each segment should be controlled in order to maximize both plasmonic and magnetic properties at the same time. Whereas, it is generally known that the magnetization of the easy axis forms in accordance with the magnetic field direction in order to reduce the magnetic potential and develop an energetically favorable state.16,32,33 In this context, when we applied the direction of the field perpendicular (or parallel) to the incoming light, while also increasing the field strength from 100 to 200 G, the nanorods tended to align along the direction of the applied field, resulting in an enhanced (or suppressed) LSP mode (top [or bottom] panel in Figure 2B, respectively). Owing to the sensitive plasmonic response to the external magnetic field, the dependence on the field strength (i.e., the angle between the incident light and nanorods) was further determined (Figure 2C) by changing the distance between the samples and electromagnets, which responded well to even comparatively weak magnetic fields (magnet distance of 5 cm corresponds to 10 G). Peak shifting of both TSP and LSP modes with varying field strengths under magnetic field perpendicular to the incident light (top panel in Figure 2B) was plotted (Figure 2D). Compared to the case of the LSP mode (from 817 to 868 nm), the change of the TSP mode (595 to 608 nm) was negligible, which is attributed to the gradual increase of the exposed Au surface relative to the light, depending upon the field strength. This indicates the orientational change of nanorods as opposed to the formation of magnetic-induced aggregates. Very recently, R. Geryak et al. reported a dramatic plasmonic shift (∼100 nm) of Au/Ni nanorods systems due to the formation of higher-order modes that maintained the symmetry of the system.34 Their results further support the high quality of as-synthesized Au/Ni/Au nanorods and the minor effects associated with magnetic aggregation under an applied magnetic field. A relatively lower degree of spectral shift (∼50 nm) in our work is caused by the different dispersion of nanorods in water, not by the aggregation. Of critical interest lies on whether nanorods can overcome the external factors that may possibly induce degradation into the effective movements of the nanorods35,36 typically observed in suspension systems. Thus, we investigated on how equivalently nanorods can respond to the external magnetic field by repeating magnetic on−off switching during 25 cycles (Figure 2E). All of the on−off measurements were carried out at a time-interval below 1 s as confirmation of the instant configuration of the system. A reversible and instant responsiveness to the external stimulus while sustaining its orientation can be found, yet a slight decrease of the “on-state” was also shown as proceeding the cycles due to the inevitable nature of ferromagnetic materials producing remnant magnetization. This could be resolved by utilizing Fe segments since Fe is expected to have a high saturation magnetization as well as a potential capacity for biological systems37,38 although its stability and the coupling with metal sections (Au) should be further studied, which is under investigation. Nevertheless, it is important to note that local dynamic magnetic tuning on the
Inspired by the fact that Ni is a well-known ferromagnetic material, which implies that magnetic tuning of the Ni segments under an external magnetic field can be realized, orientational control of a whole nanorod (including the Au segments) should be possible. Also, the observation of the higher-order modes of nanorods, which guarantees high quality and homogeneity, enables us to tailor Au nanorods with a relatively long aspect ratio. This provides the additional option, in combination with the dipole plasmon bands as well, excluding byproducts via various chemical reactions, in which solution-based methods are quite limited.29 Here, we report the tuning of the surface plasmon resonance of Au/Ni/Au nanorods by successfully manipulating the alignment of nanorods with the assistance of a remotely controlled external magnetic field. The Ni segments have dual functions; they serve as a magnetic responder for the external magnetic field for guided-alignment as well as a bridge to effectively couple the SPR of Au segments, which leads to an optical response similar to a pure Au nanorod, mediating either dipole or higher-order modes. Additionally and importantly, we can demonstrate unusual optical behavior of multiblock nanorods by tailoring the opposite directional magnetization as a function of Ni lengths. The magnetic cancellation or enhancement of multiblock nanorods can be controlled by simultaneously tuning the aspect ratios and numbers of Ni blocks. We systematically investigated this important nanorod feature by comparing optical responses depending on the number of Ni segments forming penta- and heptablock nanorods with different RNi values.
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RESULTS AND DISCUSSION Magnetic Modulation of Localized Surface Plasmon Resonance. The schematic illustration for the device setup and conceptual design of this work is described in Figure 1A. We employed two electromagnets, producing an attractive force placed on either side of a UV cell, in order to generate a highly uniform magnetic field. When randomly distributed nanorods are exposed to the external magnetic fields with different field directions, the nanorods can be aligned either parallel or perpendicular to the incident light (kinc.), sustaining their orientation state within the solution (Figure 1A). Depending on the direction of alignment, either transverse surface plasmon (TSP) (which occurs due to electrons oscillating along the short axis of the nanorods) or longitudinal surface plasmon (LSP) (where electrons move along the long axis of the nanorods) can be selectively enhanced (Figure 1B).16 First, we prepared representative triblock Au/Ni/Au multisegmented nanorods with a total length (L) of 320 ± 10 nm containing Ni (LNi ∼ 70 ± 5 nm, corresponding to RNi ∼ 1), showing that the junctions between each segment were retained, working as a lever to orient nanorods (Figure 2A). The elemental mapping image in the inset indicates a regional distribution of as-synthesized nanorods and the component of each block. Figure 2B shows the UV−vis spectra depending on the direction of the external magnetic field (Bext) as a function of field strength. The black dashed line depicts the original optical profile, where the dipole TSP mode is centered at 600 nm. The peak at 820 nm is assigned as a quadrupole LSP band; this only appears when the length of the nanorods surpasses a certain limit,28,29 indicating that the incorporation of Ni segments with this dimension (1:1:1 ratio in Au/Ni/Au) does not disturb SPR coupling.28,31 Since Au blocks induce the C
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Chemistry of Materials nanoscale can be realized within a static dc magnetic field, all of which can be monitored through UV−vis spectroscopy. We synthesized single-component Au nanorods as a control group, to confirm that all of the orientational changes arose solely from the presence of the Ni segments (Figures S1 and S2). We also confirmed that relatively short Au/Ni/Au nanorods (L ∼ 190 nm) exhibited a similar trend, showing a color change in solution due to alterations of the dipole TSP and LSP in the visible spectral window (Figure S3). The optical feature of pure Ni nanorods has been previously reported28 and will not be mentioned here since this report is based upon the concept of intraparticle surface plasmon resonance of Au/Ni/Au nanorods. Shape Anisotropy of the Ni Segment. To gain further insight into the magnetic behavior of nanorods correlating with Ni segments, we classified our samples into two extreme cases having different aspect ratios of Ni (RNi) under the same magnetic field direction (perpendicular to the incoming light). The mechanism on the formation of the magnetic dipole within the ferromagnetic section has been suggested previously by others.23,39−43 For nanorods with disk-like Ni (total length ∼340 ± 15 nm, LNi ∼16 ± 1 nm, RNi < 1), shown in Figure 3A, the quadrupole LSP mode (shown at 840 nm) was suppressed as increasing field strength since the magnetic dipole in the Ni segment formed perpendicular to the long axis of nanorods, facilitating the parallel alignment relative to the light (Figure
3C). Conversely, nanorods with relatively long Ni segments (LNi ∼ 300 ± 10 nm, RNi > 3) (Figure 3B) showed the opposite behavior, exhibiting the enhanced LSP mode due to a minimization of magnetic interaction, leading to perpendicular alignment of nanorods to the light (Figure 3D). The peak at 820 nm was assigned as the dipole LSP mode since the relatively long length of the Ni segment hinders the coupling of SPR in that nanorods act as pure Au nanorods with 120 nm each in length.28 Since it is challenging to directly visualize the oscillation of nanorods in real time, we instead drop-wised each nanorod solution on Si wafers and dried under a magnetic field with the same direction as a proof-of-concept (Figure 4). Both
Figure 4. Schematic illustrations and low magnification images of Au/ Ni/Au nanorods with (A, B) a short aspect ratio and (C, D) a long aspect ratio, aligning parallel to the applied magnetic field. Interestingly, each corresponding magnified image (B), (D) shows the opposite alignment, indicating the different easy axis of each nanorod. An assembled cluster with slight imperfect alignment is due to the capillary force during the drying process. Scale bars = 1 μm.
low magnified FE-SEM images (Figure 4B,D) showed the chain-like structures aligning parallel to the direction of applied field. At a zoomed-in image, however, it was observed that nanorods containing a relatively short Ni segment were assembled in a side-by-side manner, resulting in the perpendicular alignment of nanorods (schematically described in Figure 4A). In the case of nanorods with a long aspect Ni (Figure 4C), nanorods were assembled in a side-by-side manner, but clusters were looking toward the applied field direction, well matched with the discussion above, yet slightly imperfect alignment due to not only capillary forces during the dry process but also Au segments at the end disturbing field gradient within nanorods. Nevertheless, still the tendency on the effect of Ni due to its shape anisotropy, consequently the behavior of alignment can be directly confirmed in this way. Each quadrupole and dipole LSP mode was plotted as a function of field strength (Figure S4), clearly showing the opposite optical behavior (under the same direction of magnetic field) due to shape anisotropy of the Ni component. To further prove the orientation of each nanorod, we utilized a polarizer in order to produce only z-polarized light along the y-axis (Figure 5). Wang et al. reported on the interaction between Au nanorods and polarized light under a magnetic field.16 According to their work, when we applied either perpendicular or parallel magnetic field to the direction of incident light (within the horizontal xy plane), only the transverse plasmonic mode should be produced. Nanorods with rod-shape Ni segments obviously showed the expected behavior that the transverse mode was significantly suppressed under vertical magnetic field in the z-axis (Case I) since only longitudinal plasmons can be excited. Whereas, only the transverse mode was exhibited under magnetic field within the xy plane (Case II) since the oscillation of electron can occur
Figure 3. Magnetic behavior of nanorods correlating with Ni segments, exhibiting opposite directional alignment and corresponding UV−vis spectra. (A), (B) FE-SEM images of Au/Ni/Au nanorods with different RNi of the Ni segment (A) RNi < 1, LNi ∼ 16 ± 1 nm, total length of nanorods ∼340 ± 15 nm and (B) LNi ∼ 300 ± 10 nm, RNi, > 1, total length ∼540 ± 20 nm as depicted in each inset. (C) UV−vis spectra as a function of field strength. Nanorods are aligned parallel to the incident light due to the perpendicular formation of magnetic dipole in Ni, which results in suppression of quadrupole longitudinal bands. (D) UV−vis spectra in the case of the relatively long Ni segment (B), showing enhanced dipole longitudinal bands due to the perpendicular alignment of nanorods to the light. Each inset in (C) and (D) is the cartoon describing the alignment of each nanorod under the same direction of applied magnetic field. Red dashed arrows indicate the propagation of incoming light. D
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Figure 5. (A) Schematics of plasmon excitation of nanorods with the rod-shape Ni segment under z-polarized light dependent upon the direction of applied magnetic field. (B) Corresponding UV−vis spectra of Case I and Case II. The spectra have been vertically offset for clarity.
along the short axis of nanorods, which indicates that successful manipulation on the orientation of nanorods with our systems. However, for the case with disk-like Ni segments (Figure S5), both transverse and longitudinal plasmons were excited due to antiparallel alignment of magnetic dipole in Ni segments, resulting in a certain portion of the vertical alignment of nanorods along the z-axis in the solution. Under the vertical magnetic field, only the transverse mode was observed since the alignment of nanorods was confined within the xy plane instead of being vertically standing, in order to lower the energetic state. Hence, it can be concluded that nanorods with disk-like Ni segments always show both transverse and longitudinal plasmon bands whether or not we apply magnetic field within the xy planes. At this stage, the controllability on a portion of vertical alignment of nanorods with disk-like Ni segments is unknown. Magnetic Cancellation and Enhancement. One definitive advantage of the template-assisted method is that one can systematically control the components, allowing for the synthesis of multiblock nanorods by repeating Au−Ni units. Of critical question is that how behaviors of nanorods would change under the incremental number of segments. Love et al. increased the number of Ni segments for stable threedimensional formation of effective self-assembled structures through simultaneous magnetic interaction.40 Chen et al. also reported on the magnetic properties dependent upon dipolar coupling between ferromagnetic segments by controlling the number of segments.43 Also, in our previous report,44 we demonstrated that not only the portion of Ag segments but also the number of Ag junctions affected the sensitivity of environment under a specific binding. Hence, to investigate the correlation between the aspect ratio and the number of components can necessitate the appropriate usage for specific purposes along with each component. In general, the magnetic performance (in terms of stability, responsiveness, and reversibility) improves as the relative amounts of magnetic portion in the nanostructures increase.44,45 In this context, we increased the number of Ni blocks (LNi ∼ 16 nm, RNi < 1) from one to three (Figure 6A and corresponding images from a1 to a3), in order to correlate Ni contents with reversibility since inevitable ferromagnetic nature degrades as repeating on−off procedures due to the repeated, instantaneous magnetization and demagnetization. Due to Brownian motion, which occurs after the magnetic field is turned off, we can detect modulated extinction signals.46,47 We defined the difference between the
Figure 6. Correlation between the length and the number of Ni segments. (A) Illustration of magnetic cancellation as increasing the number of Ni segments with the short aspect ratio, forming penta- and heptablock nanorods. Curved solid arrows indicate the tendency on the movement of each segment. (a1−a3) FE-SEM images of corresponding nanorods (i) L ∼ 340 ± 20 nm, LNi ∼ 16 ± 2 nm, (ii) L ∼ 320 ± 15 nm, LNi ∼ 15 ± 1 nm, and (iii) L ∼ 370 ± 10 nm, LNi ∼ 18 ± 1 nm, respectively. (B) Description of magnetic enhancement as increasing the number of Ni segments with longer aspect ratios. Curved dashed arrows indicate consecutive induction of magnetic charges from each Ni segment acting as nanomagnets. (b1− b3) FE-SEM images of corresponding nanorods (iv) L ∼ 370 ± 30 nm, LNi ∼ 80 ± 4 nm, (v) L ∼ 510 ± 20 nm, LNi ∼ 90 ± 5 nm, and (vi) L ∼ 700 ± 30 nm, LNi ∼ 70 ± 5 nm, respectively. Scale bars = 200 nm.
intensity of the LSP mode in each “on” and “off” of the first cycle (Ioff − Ion = ΔI) as a basic unit and plotted for 50 cycles of on−off switching (Figure 7A). Notably, the slope of ΔI increased, with a broad standard deviation, indicating degenerated magnetic performance as the number of blocks rises (from (i) to (iii) in Figure 7A), in contrast to what we expected that a large amount of Ni would help promote effective rotational movement. Furthermore, the low remanence and coercivity of nanorods composed of low aspect ratio Ni segments were expected to prevent magnetic aggregation in suspension.43 This discrepancy between experimental and theoretical results can be explained by the possibility for net magnetization, associated with “magnetic cancellation” through each Ni block. Since nanorods are dispersed and randomly oriented in solution, the formation of magnetic dipole moment at each side of Ni segments would not be in consecutive order. Therefore, each Ni block would temp to rotate in the opposite direction due to disk-like Ni compensated magnetic poles being weakly defined, resulting in rather low susceptibility (schematically described in Figure 6A). To support our claim, we synthesized multiblocks, containing Ni sections with longer aspect ratios (LNi ∼ 80 nm, RNi ∼ 1, shown in Figure 6B and corresponding images in b1−b3). In E
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Figure 8. UV−vis spectra of original profile of multiblock nanorods from (i) to (vi), respectively. Each number denotes the peak position of each LSP mode. Higher-order modes can be used as a tool for observation on the behavior of nanorods. LSP peaks from (i) to (v) are assigned as quadrupole and that from panel (vi) is the octupole LSP mode.
Optical Switching Response under ac Magnetic Field. Finally, to verify the feasibility of nanorods as a potential local dynamic stirrer on the nanoscale regime, continuous optical switching was performed by applying a rotating magnetic field to nanorod dispersions under normal light, showing the contrasting color changes of the solution that correspond to the on−off states, regardless of the portion of Ni segments (Figure 9A). In addition, the optical response rate under an ac magnetic field, applied parallel to the light, was determined by an ultrakinetic program with a field strength of 10 mT at a frequency of 60 Hz (Figure 9B). All of the samples were measured at each longitudinal resonance peak at time-intervals of 2.58 ms for 2 s. In the case of the Ni block with RNi < 1 (Figure 9A(i)), a notable intensity change was observed during the process, indicating the rotational movement of the nanorods as the poles were sequentially switched. There is a lag between each session, which indicates that nanorods stay resistant and terminate the instant flipping ascribed to insufficient capacity to overwhelm over fluidics (i.e., viscous drag forces). Nanorods with relatively longer Ni blocks (RNi ∼ 1, Figure 9A(ii)) present constant and instant optical switching performance without a lag, agreeing well with the discussion above. As a control group, pure Au nanorods and nanorods without a magnetic field were checked; neither of these showed a specific response of LSP under polarity change but noise-like signals. The photograph (Figure 9C) represents each solution containing Au/Ni/Au multiblock nanorods with varying total lengths as well as the length of Ni components. It was observed that the contrast changes were accentuated as total lengths and the Ni components were increased. Unfortunately, the longitudinal plasmon mode in this work is in the NIR region, resulting in rather weak color contrast under change of magnetic field. However, we expect that this concept of a magnetic response to the external magnetic field can also be extended to plasmonic displays48 or camouflage49 via designing magnetic patterns50 with varying power or frequency (i.e., the speed of rotation) in conjunction with different sizes of magnetic nanorods (e.g., lengths, aspect ratios, a portion of Ni, etc.) so as to adjust the position of the longitudinal plasmon mode within the visible window. There are a few limitations to this work, including a lack of information about the direction of
Figure 7. (A, B) The plot of ΔI (= IOFF − ION) during 50 cycles for six different nanorods. (C) The plot of the decreased portion of ΔI after 50 cycles. Each (i−vi) corresponds to those shown in Figure 6. Starshape indicates the value of nanorods with a larger gap between each Ni segment (see the Supporting Information). Error bars represent the standard deviation of measurements.
this case, we observed a decreased slope with quite constant values (from (iv) to (vi) in Figure 7B), indicating an enhanced magnetic sensitivity instead of magnetic-induced aggregation. We attributed that well-formed magnetic dipole aligning in a longer axis of the Ni segments acting as nanomagnets induces the opposite charge to the nearby Ni segment consecutively, leading to “magnetic enhancement” through each Ni block (Figure 6B). It should be noted that when we compare a single block of Ni ((i) and (iv) in Figure 7) it is likely that the magnetic interactions are dominant over Brownian motion, even if it has long lengths of nanorods known for lower rotational diffusivity,47 which indicates the existence of a tradeoff between the length and the number of blocks in Ni.43 Figure 7C represents how much ΔI decreased after 50 cycles (calculated as (1 − ΔI50th cycle/ΔIfirst cycle)), clearly stating the opposite relationship between different RNi and the number of Ni blocks. For comparison, we changed the gap distance between two disk-like Ni segments (gap distance ∼ 245 nm, Figure S6). This showed a rotational displacement of approximately 20%, lower than that of nanorods with shorter gaps (∼100 nm), as shown in Figure 6A(ii), which rules out the possibility that this originates from any gap dependency because the unbalanced center of mass in the nanorods facilitates Brownian rotation. It is noteworthy that all of the analyses were based on higherorder plasmon modes; the original optical profiles of each sample were shown in Figure 8. F
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materials such as Co and Fe can be interesting for improving the performance of specific purposes.
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Instrumentation. Field emission scanning electron microscopy (FE-SEM) images were obtained with JEOL 7000F and JEOL 7600F instruments. A JEM-2100F was used to acquire high resolution transmission electron microscopy (HR-TEM) images. UV−vis absorption spectra were acquired using a S-3100 (Scinco) spectrophotometer. Synthesis of Au−Ni−Au Nanorods. A homemade AAO template was used throughout the experiments. The detailed synthesis of this is described elsewhere.30 The AAO templates with 70 nm diameter were used throughout this work in order to analyze the quadrupole plasmon mode. For electrochemical growth of nanorods, a thin layer (ca. 300 nm) of Ag was sputtered on the backside of the AAO template, working as both a supporting layer and a working electrode in a three-electrode electrochemical system. An Ag/AgCl electrode and a Pt wire were utilized as the reference and the counter electrode, respectively. Then, each injected metal plating solution (Orotemp 24 RTU from Technic Inc.) was reduced and deposited inside the nanopores under a potential of −0.95 V. All of the lengths were controlled by monitoring the charge passing through the cell. For multiblock nanorods, the processing for each segment was conducted sequentially. Then, the Ag layer was removed by concentric nitric acid (Samchun), and the AAO template was dissolved in 3 M sodium hydroxide (Samchun), to release the synthesized nanorods. Subsequently, washing with distilled water was conducted until a pH of 7 was reached. The as-synthesized nanorods were then redispersed in D2O solvent for UV−vis characterization. For FE-SEM imaging, nanorods were drop-cast on highly oriented pyrolytic graphite (HOPG). It should be noted that all the nanorod samples for FESEM imaging were not magnetized in order to avoid any confusion regarding magnetic aggregation unless it is specifically mentioned. We purposely spotted samples to represent the uniform and well-dispersed nanorods. Experimental Setups. Two pairs of rectangular electromagnets (dimension: 3 cm × 3 cm × 6 cm, JLmagnet Company, South Korea) were employed, facing each other between a UV cell, to produce attractive forces. Both dc and ac fields were controlled through a controller with configurable power (maximum output, 80 V) and frequency (range, 40−400 Hz). To produce a magnetic field parallel to the incident light, two electromagnets are turned aside, the sides of which face each other between the cell, in order not to block the incident light pathway.
Figure 9. Optical switching response rate under rotating magnetic field and ac magnetic field parallel to the incoming light. (A) Contrast change of each solution containing nanorods with (i) the short aspect ratio and (ii) the long aspect ratio under the rotating magnetic field. (B) Optical response rate under ac magnetic field parallel to the incoming light at 60 Hz with 10 mT field strength. (C) Photograph of Au/Ni/Au nanorods arranged in total length of nanorods (x-axis) and the length of the Ni segment (y-axis).
rotation (vertical or horizontal flipping), possible disturbances from magnetic exchange dipoles, and limited instrument resolution. However, it is still expected that nanorods with longer Ni aspect ratios, as well as shorter nanorods combined with repeating units will find a place in chemical release or biomolecule detection fields with an external magnetic field.
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S Supporting Information *
CONCLUSION In summary, we studied the magnetic tuning of SPR of Niembedded Au nanorods synthesized via an electrochemical deposition method assisted with the external magnetic field. The incorporation of ferromagnetic materials (optically less active than Au) enabled us to control the orientation changes of the nanorods. Depending on the direction of the magnetic field and the field strength, selective tuning of SPR can be achieved, regardless of the nanorods aspect ratio, by utilizing the multipole modes of nanorods. Shape anisotropy of ferromagnetic Ni allowed for the guided alignment of nanorods, leading to the dynamic plasmonic response, as monitored by UV−vis spectroscopy. Furthermore, we demonstrated that correlation between the length and the number of blocks in Ni, extending to the magnetic movements. This well-established electrochemical method combined with the reversible and instantaneous external magnetic field can be more potentially utilized for targeted applications such as biomedical detection, biosensors, and optical waveguides. Injection of other magnetic
.The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04016. Additional experimental data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. 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 the Pioneer Research Center Program (2012-0009586). This work was supported by the G
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Chemistry of Materials
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National Research Foundation of Korea (National Leading Research Lab: 2011-0027911).
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DOI: 10.1021/acs.chemmater.5b04016 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.chemmater.5b04016 Chem. Mater. XXXX, XXX, XXX−XXX
