Highly Sensitive Plasmonic Optical Sensors Based on Gold Core

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Highly Sensitive Plasmonic Optical Sensors Based on Gold Core-Satellite Nanostructures Immobilized on Glass Substrates Kentaro Ode, Mai Honjo, Yohei Takashima, Takaaki Tsuruoka, and Kensuke Akamatsu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06313 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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ACS Applied Materials & Interfaces

Highly Sensitive Plasmonic Optical Sensors Based on Gold Core-Satellite Nanostructures Immobilized on Glass Substrates Kentaro Ode, Mai Honjo, Yohei Takashima, Takaaki Tsuruoka, and Kensuke Akamatsu* Department of Nanobiochemistry, Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 Minatojimaminami, Cho-ku, Kobe 650-0047, Japan KEYWORDS Gold nanoparticles, Core-satellite nanostructures, Surface plasmon resonance, Plasmon coupling, Vapor sensor

ABSTRACT

Fabrication of discrete nanostructures condisting of noble metal nanoparticles immobilized on substrates is challenging due to structural complexity but important for chip-based plasmonic sensor technology. Here we report optical sensing capabilities of core-satellite nanostructures made of gold nanoparticles immobilized on glass substrate, which were fabricated by combining stepwise inteconnection of gold nanoparticles through dithiol linkers and surface treatment using vaccum ultraviolet light. The nanostructures exhibit large changes in coupled plasmon resonance peak upon surrounding refractive index, with sensitibity of ca. 350 nm RIU-1, thus providing

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highly sensitive optical sensors for determining the surrounding refractive index and also detecting organic vapors.

Discrete nanostructures of metal nanoparticles, such as dimers, trimers, chains, and satellites, exhibit electrical and optical properties that are based on localized surface plasmon resonance (LSPR) and are often very different from the properties of their isolated forms due to electronic interactions between adjacent nanoparticles.1-4 The LSPR wavelength can be tuned by controlling the size, spacing, symmetry, and orientation of the nanoparticles with respect to one another.5 Therefore, these plasmonic nanostructures have been of great interest for optoelectronic, photonic, and plasmonic applications. Core-satellite nanostructures are one of the most effective nanostructures for optical sensing because they have the highest number of coupled pairs of nanoparticles per single discrete nanostructure. Although the synthesis of several types of satellite nanostructures in the solution phase has been reported, in which the satellite nanoparticles are connected by molecules such as dithiols,6,7 DNAs,8-11 or other long or short organic molecules,12-15 there have been few studies regarding the fabrication of satellite nanostructures on a substrate.16 This type of nanostructure on a substrate is effective for refractive index-based sensor chips utilizing various detection technologies, because the nanostructures are immobilized on the substrates so that (1) any affinity of the nanostructure surface with the surrounding medium to be detected is unimportant (because no aggregation occurs), (2) immobilized nanostructures can be used not only for sensing liquids but also for vapors or gasses, and (3) the chips are reusable. Although nanoparticle-based optical sensors immobilized on substrates have been investigated for metal nanoislands,17 lithographically


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fabricated metal arrays,18-21 and nanoparticle monolayers,22 the optical properties of immobilized satellite nanostructures and their vapor phase sensing capabilities have not yet been reported. Herein, we report the fabrication of high-purity satellite nanostructures made of differently sized spherical gold nanoparticles immobilized on glass substrates and their optical properties as highly sensitive refractive index sensors that can be used to detect various organic liquids and vapors. The synthesis of “pure” core-satellite nanostructures consisting of gold nanoparticles on glass substrates followed modified stepwise methods using dithiols as linker molecules (see Supporting Information for details), the method of which reported by Yoon et al. for synthesis of the nanostructures in solution.6 Briefly, core gold nanoparticles (citrate-capped nanoparticles with a diameter of 50 nm) are first immobilized on aminopropyl trimethoxysilane (APTMS)functionalized glass substrate. The obtained substrates with gold cores are then subjected to irradiation with vacuum ultraviolet visible (VUV) light to decompose the APTMS molecules on the bare glass surface (with no gold nanoparticles). Dithiol linkers are then attached to the gold core surface by immersing the VUV-treated substrate in a toluene solution of ethanedithiol (5 mM) followed by immersion of the substrates into an aqueous solution of gold satellite nanoparticles (citrate-capped gold nanoparticles with a diameter of 17.3 nm), resulting in core satellite nanostructures immobilized on the glass surface (Scheme 1). The use of short chain ethanedithiol provides a closer distance between core and satellite nanoparticles, resulting in stronger coupling of the LSPR.2,6 The VUV irradiation is crucial to obtaining pure satellite nanostructures in the present process. Otherwise, satellite nanoparticles are immobilized on both the surface of the dithiol-modified gold cores and the amine-functionalized glass (Figure S1).


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By systematically evaluating the immobilization efficiency of satellite nanoparticles, we found that the density of the core nanoparticles initially immobilized on the glass substrate, which is controlled by the initial concentration of core nanoparticles, is an important parameter for obtaining isolated core-satellite nanostructures at high yield. Specifically, higher concentrations and longer immersion times often provide unwanted interconnected core nanoparticles on the substrate. In a typical immobilization experiment, a solution with 1.0 nM core nanoparticles provided substrates with 110 core nanoparticles per square micrometer after immersion of the substrate for 20 min, which corresponds to an average interparticle distance of 45 nm (by assuming that the nanoparticles are arranged in a simple square configuration). After subsequent immobilization of satellite nanoparticles through ethanedithiol linkers, isolated core-satellite nanoparticles are formed on the glass surface with 95% yield (Scheme 1). When the density of the core nanoparticles exceeded above 110 nanoparticles/um2, core nanoparticles were tend to aggregate on glass substrate, providing uncontrollable immobilization of pure satellite nanostructures. Therefore, the samples obtained after immobilization of core nanoparticles for 20 min were subjected to following cahracterization. When the substrates with core-satellite nanostructures are immersed in organic solvents, the UV-vis spectra show two distinct SPR absorptions: an SPR peak originating from native core and satellite nanoparticles around 520 nm and a coupled SPR peak between nanoparticles in close proximity of around 650-730 nm (Figure 1A).6,7 Interestingly, although the native SPR peaks are moderately dependent on the solvent studied, the coupled SPR peaks are highly dependent on the solvent, and the coupled SPR peak shifts to longer wavelengths as the solvent refractive index increases (i.e., from methanol to toluene in the present work).


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To further test whether the coupled SPR peak shift originated from changes in the solvent refractive index, we systematically collected SPR peak position data for nanostructures in solvents prepared by mixing two solvents with different refractive indices (Figure 1B). It was observed that the coupled SPR peaks exhibited a linear dependence on the refractive index of the mixed solvents, and the slope for each solvent was nearly the same, indicating that the coresatellite nanostructures can be used as plasmonic refractive index sensors. The sensitivity, one of the most important parameters characterizing plasmon sensors, is defined as the shift of SPR wavelength per unit change in the refractive index of the surrounding medium. We calculated the sensitivity of the nanostructures for all of the mixed solvents used to be ca. 350 nm RIU-1. This is much greater than that of gold islands,17 silver nanoparticles,18 or polymer-linked satellites 15 (50 – 200 nm RIU-1), and is comparable with that of anisotropic nanoparticles (150 – 370 nm RIU-1, including rods,23,24 cubes,25 nanoshells,26 and core-shell nanoparticles27) and more complicated nanostructures such as lithographically fabricated metal arrays (200 – 400 nm RIU-1).19-21 The assembly method reported herein provides a straightforward way to control plasmonic properties through systematic control of the number of coupled pairs in the single nanostructure. In order to evaluate the effect of the number of coupled pairs on the plasmon sensitivity, we prepared nanostructures with different numbers of satellite particles per core by varying the immersion time of the core substrates in the satellite nanoparticle solution. SEM observation (Figure 2) revealed that the average number of satellites (nave) increases with increasing immersion time: nave = 9.2, 10.8, 12.3, and 14.0 per core for immersion times of 15, 30, 60, and 120 min, respectively. In Figure 2D, the plasmon sensitivity increases as the number of satellite particles increases from 95 nm RIU-1 (nave = 9.2) to 350 nm RIU-1 (nave = 14.0). This dependence can be caused by an increased electromagnetic interaction between exited coupled plasmon


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modes generated in core-satellite pairs within the nanostructure as the number of satellite nanoparticles increases.28 Additionally, we estimated the average interparticle distance between satellite nanoparticles on single core nanoparticles, and the smallest value was ca. 4.2 nm for 17.3 nm satellite nanoparticles on 50 nm core particles (for a sample with nave = 14.0, the highest number of satellite nanoparticles per core). Since this value is much greater than the distance between core and satellite nanoparticles (ca. 0.7 nm, estimated from the length of ethanedithiol), the dominant coupling occurs between core and satellite nanoparticles (not between individual satellite nanoparticles), and therefore the observed increase in plasmon sensitivity mainly originated from an increase in the sum of the coupled SPR modes between core and satellite nanoparticles. To test whether the size of the nanoparticles affects the plasmon sensitivity, satellite nanostructures with satellite nanoparticles of different sizes were prepared. The sensitivity increased slightly with decreasing satellite nanoparticle size (Figure 2E). Importantly, although the coupled SPR peak red-shifted slightly as the size of the satellite nanoparticles increased (Figure S3), the observed sensitivity decreased with increasing particle size. This observation was presumably caused by the slight increase in the average number of satellite nanoparticles per core from nave = 14.0 (17.3 nm satellite particle) to nave = 15.2 (12.8 nm satellite particle), which also increased the number of coupled pairs. Although theoretical modeling is necessary to fully understand the optical and sensing properties of the present satellite nanostructure, the present results demonstrate that the fabrication of core-satellite type nanostructures is effective for enhancing plasmon sensitivity even for nanostructures made of simple, spherical gold nanoparticles. Therefore, much higher plasmon sensitivity can be expected for construction of core-satellite assemblies made of


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anisotoropic nanostructures, such as nanorods, nanocubes, bipyramids, and nanocages, which already show high plasmon sensitivity due to their specific shapes that induce effectively coupled plasmon modes inside one nanostructure.29 We are currently investigating these specific sore-satellite nanostructures and the results will be published elsewhere. In addition, in the present nanostructures immobilized on glass substrates, the surface of satellite nanoparticles and a part of the surface of core nanoparticles have no protective molecules and can thus be modified with bioactive molecules such as DNAs, providing feasibility of the present nanostructures to be used as highly sensitive biosensors. Such biosensing capability of the present core-satellite nanostructures is one of the most important future directions. The observed large wavelength shifts upon changes in the surrounding refractive index allows one to detect organic vapors by monitoring changes in the absorbance. To explore the vapor sensing capabilities, extinction spectra for core-satellite nanostructures were measured in air and methanol vapor as an example (Figure 3A). While the spectrum of the substrate with only core nanoparticles remained relatively unchanged upon methanol exposure (Figure S4), core-satellite nanostructures exhibited slight extinction changes in 200 ppm methanol vapor. Importantly, the differential spectrum for extinction spectra in air and in methanol (calculated by subtraction of the spectrum in air from that in methanol vapor) has two peaks (inset in Figure 3A), and is similar in form with that in liquid methanol (Figure 1A). Furthermore, the maximum wavelength shift of coupled SPR was ca. 60 nm (relative to SPR in air), smaller than that in liquid methanol (100 nm). (60 nm). Because the condensed organic layer is very thin (only a few molecules thick) and the electromagnetic field of the LSPR for metal nanoparticles propagates as far as a few tens of nanometers,18,22 the electromagnetic field reaches beyond the thickness of the absorbed organic


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layers. Therefore, the localized SPR senses the combined refractive index of the condensed organic liquid and the nearby atmosphere which gives average refractive index lower than liquid methanol. This can be a reason why the shift of the coupled peak in the spectrum in methanol vapor is smaller than that of the spectrum in methanol liquid. Furthermore, satellite nanostructures have nanoscale convex-concave structures where incoming organic vapor may tend to easily condense in creases and cavities on the nanostructures through capillary effects. Such geometrical features make the present core-satellite nanostructures highly sensitive to organic vapors. The stable detection of methanol vapors and the reversibility of the present sensors were confirmed in the dynamic vapor response signals (Figure 3B). Methanol vapor was injected for 120 s and then turned off for 120 s for five successive cycles and the extinction at 640 nm to changes in vapor concentrations are measured. The sensorgram showed extinction of 0.01 ± 0.0001 in 200 ppm methanol vapor for 5 successive cycles, indicating reversible and reproducible manner for the present sensors. In addition, signal-to-noise ratio was calculated by using signal intensity and baseline noise to be ca. 100 and 10 for 200 and 20 ppm methanol vapor, respectively. Additionally, the extinction changes linearly with relative vapor concentration, and the present sensors respond to surface vapor condensation instantly: the typical 90% full-scale response time is ca. 2 sec, including the time required for vapor condensation in the nanostructure to reach equilibrium. This short response time indicates direct and rapid condensation of organic vapors in highly concave regions of core satellite nanostructures. Therefore, the present coupled SPR-based optical sensing has advantageous over conventional conductivity-based vapor sensors (a typical 90% full-scale response time is ca. 10 sec),30,31 in which nanoparticle islands or immobilized nanoparticles are coated by vapor-


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responsive polymers or organic monolayers, and the electrical signals are acquired though a multistep process, i.e., vapor adsorption, swelling (to increase the average distance between the nanoparticles), and conductivity changes. We have also studied vapor sensing response of the present nanostructures for several other organic vapors, which was obtained by measuring extinction at coupled SPR peak wavelength (observed in differential spectrum for each vapors). Although response time and signal-to-noise ratio are slightly different for solvents used, good sensorgrams was obtained (Figure S5). In organic vapors, differential spectrum between that in air and in organic vapor showed two peaks and showed similar trend with the specra in organic liquid (moderate and higher dependence on the solvent for native and coupled SPR peaks, respectively). From these results, two-dimensional map which shows relationship between native SPR peak position and coupled SPR position could be created (Figure S6). Therefore, it might be possible to distinguish species of the vapors and selectively detect vapor concentration at specific wavelength (coupled SPR peak position). Although further experiments are necessary to verify this protocol, we believe that the present nanostructures has a potential for vapor specificity. In conclusion, we have reported the fabrication of gold nanoparticle-based core-satellite nanostructures immobilized on glass substrates, formed through a dithiol-mediated assembly strategy. Furthermore, we have explored their optical properties and their refractive index and vapor sensing capabilities. The increased number of coupled SPR pairs, which is controlled simply by adjusting the number of satellite nanoparticles connected to cores, gives rise to a higher sensitivity to various organic liquids and vapors, without requiring a refractive index transducer around the metal nanostructures. The plasmon sensitivity can be further improved by using nanoparticles with specific shapes such as cubes, rods, pyramids, and cages to fabricate


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sore-satellite nanostructures and/or by combination with microcavities for device fabrication.32 Moreover, the systematic approach used to optimize particle diameter, the diameter ratio between core and satellite nanoparticles, and the number of satellites make this nanostructure an ideal platform for investigating how these structural variables impact plasmonic properties and sensing performance in a wide range of applications such as biomolecular sensing and/or environmentally toxic vapor concentration measurements.

FIGURES

Figure 1. (A) UV-vis extinction spectra of glass substrates with core satellite nanostructures in various organic solvents. (B) Shift of LSPR wavelength of the nanostructures as a function of the refractive index of the mixed solvent. The slope of the linear fit corresponds to a sensitivity of ca. 350 nm RIU-1.


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Figure 2. SEM images of core-satellite nanostructures immobilized on glass substrates obtained several minutes after immersion of the substrates with core nanoparticles in a solution containing satellite nanoparticles. Average number of satellite nanoparticles per core: 9.2 (A), 12.3 (B), and 14.0 (C). Scale bar: 100 nm. (D) Shift of the LSPR wavelength of nanostructures with different numbers of satellites as a function of solvent refractive index. (E) Shift of LSPR wavelength of the nanostructures with different sizes of satellites as a function of solvent refractive index.


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Figure 3. (A) UV-vis extinction spectra of glass substrates with core-satellite nanostructures in air (black) and in 200 ppm methanol vapor (red). Inset: differential spectrum of the samples in air and in methanol vapor. (B) Real-time response signal of nanostructures exposed to various concentrations of methanol vapor.


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SCHEMES

Scheme 1. Schematic of the present fabrication process for core-satellite nanostructures of gold nanoparticles on a glass substrate and SEM images of substrates at each step. Scale bar: 100 nm.


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ASSOCIATED CONTENT Supporting Information. Experimental details; Effect of VUV treatment on formation of nanostructures on glass substrate; UV-vis extinction spectra of core satellite nanostructures with different number and size of satellite nanoparticles per core nanoparticles; Spectral change of only core nanoparticles using methanol vapor.

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Scheme 1. Schematic of the present fabrication process for core-satellite nanostructures of gold nanoparticles on a glass substrate and SEM images of substrates at each step. Scale bar: 100 nm. 371x139mm (72 x 72 DPI)


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Figure 1. (A) UV-vis extinction spectra of glass substrates with core satellite nanostructures in various organic solvents. (B) Shift of LSPR wavelength of the nanostructures as a function of the refractive index of the mixed solvent. The slope of the linear fit corresponds to a sensitivity of ca. 350 nm RIU-1. 87x37mm (300 x 300 DPI)



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Figure 3. (A) UV-vis extinction spectra of glass substrates with core-satellite nanostructures in air (black) and in 200 ppm methanol vapor (red). Inset: differential spectrum of the samples in air and in methanol vapor. (B) Real-time response signal of nanostructures exposed to various concentrations of methanol vapor. 127x40mm (300 x 300 DPI)


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Highly Sensitive Plasmonic Optical Sensors Based on Gold Core

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