Near-Infrared-Excitable SERS Measurement Using Magneto

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Near-Infrared-Excitable SERS Measurement Using MagnetoResponsive Metafluids for in Situ Molecular Analysis Yutaro Hirai,† Yasutaka Matsuo,‡ and Hiroshi Yabu*,§ †

Graduate School of Engineering, Tohoku University, 6-6, Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8579, Japan Research Institute for Electronic Science (RIES), Hokkaido University, N21W10, Sapporo 001-0021, Japan § WPI-Advanced Institute for Materials Research (AIMR), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

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S Supporting Information *

ABSTRACT: Metal nanoparticle clusters are regarded as metamaterials, and dispersions of nanoparticle clusters are regarded as metafluids. Surface-enhanced Raman scattering (SERS) from molecules adsorbed on the nanoparticle clusters is one of the notable properties of metafluids. SERS is expected to permit the realization of single-molecule detection in chemical and biological samples, especially cells and tissues. However, most SERS measurements have been done on substrates; local information on cells and tissues have been hard to obtain. SERS active particles can be used to measure the local information on cells. To analyze biological samples using SERS, the SERS substrate should be excitable in the near-infrared (NIR) region to ensure high transparency in biological tissues. Furthermore, transporting the SERS particles to the desired position is crucial for obtaining high resolution. Sizes of SERS-active particles also affect to the resolutions. In this report, gold nanoparticle clusters based on polymer core− shell particles incorporating magnetic Fe3O4 nanoparticles were prepared via a self-assembly method. Structures, LSPR absorption, SERS signals, magnetic responsibility of prepared particles were analyzed by electron microscope, UV−vis spectrum, Raman measurement, and optical microscope observation under magnetic flux, respectively. The enhancement factor of the SERS signal was determined by the size of composited gold nanoparticles. Furthermore, the migration direction of the gold nanoparticle cluster composite particles in aqueous media was successfully controlled by the application of an external magnetic field. KEYWORDS: core/shell nanoparticles, magnetic nanoparticles, self-assembly, surface plasmon resonances, surface-enhanced Raman scattering

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INTRODUCTION

(LSPR) in response to incident light and display unique coloration and refractive index changes. Surface-enhanced Raman scattering (SERS) from molecules adsorbed on nanoparticle clusters is one of the important properties of metafluids.17 As Raman scattering signals on nanoparticle clusters are enhanced by the strong electromagnetic field of LSPR induced by coupled plasmon resonances between the metal nanoparticles,18−20 SERS is considered to be a promising technique for achieving single-molecule analysis in chemical and biological samples,21−23 especially cells and tissues when applied to probes attached to biological components.24,25 SERS signals have been measured on SERS active substrates having metal nanostructures.26 The signal from samples are occurred from sample and substrate contacted interface. SERS active nanoparticles, whose electro-enhanced field is limited in

Self-assembled nanoparticle (NP) clusters exhibit unique electromagnetic responses and have attracted considerable interest due to their potential for controlling colors, refractive indices, and electromagnetic enhancement via the strong interaction between light and matter.1−5 Recent development of microfabrication technologies has allowed the realization of metamaterials based on designed metallic nanostructures with electromagnetic resonances in the visible and near-infrared (NIR) regions on two-dimensional (2D) substrates.6−9 In contrast to these 2D metamaterials fabricated using state-of-theart microfabrication technologies, nanoparticle clusters, which are sometimes referred to as “metamolecules”, possess threedimensional structures and exhibit multiangle resonances in response to incident light.10−12 Furthermore, such nanoparticle clusters can be dispersed in liquids to afford liquid metamaterials or “metafluids”.13−16 These metafluids exhibit strong electromagnetic resonances due to localized surface plasmon resonance © XXXX American Chemical Society

Received: July 2, 2018 Accepted: August 3, 2018

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DOI: 10.1021/acsanm.8b01093 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

ACS Applied Nano Materials

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Article

RESULTS AND DISCUSSION Preparation of Gold Nanoparticle Cluster Composite Particles. Figure 1 shows a schematic image of the preparation

nanoscale, can be achieved in situ SERS measurement. To realize high resolution biological analysis using SERS, the excitation wavelength of the SERS substrate should be in the NIR region, as biological specimens have an optical window in this region.27 Furthermore, positional control of the SERS substrate is crucial for achieving high resolution. Thus, structural control of nanoparticle clusters to optimize the NIR absorption and far-field responsiveness are key factors in the design of these materials. Several approaches for preparing nanoparticle clusters using colloidal processing have been reported. Halas and co-workers reported the formation of silver nanostructures on silica particles using the electroless plating technique and evaluated the resulting plasmonic resonances.28 Sheikholeslami et al. prepared silica nanoparticles decorated with silver nanoparticles using the biotin−streptavidin interaction.29 Although these previous studies demonstrated strong enhancement of the LSPR and SERS signals, the synthesis of efficient NIR-excitable SERSactive nanoparticle clusters remains challenging, as it is difficult to achieve control over the aforementioned parameters of metallic nanoparticles to realize NIR plasmonic absorption and efficient electromagnetic enhancement. We have developed a simple method for preparing polymer particles, which is referred to as self-organized precipitation (SORP). This method involves evaporating a good solvent from a polymer solution containing a poor solvent. We have used SORP to prepare spherical particles from a variety of materials, including conductive polymers, engineering plastics, biodegradable polymers, polymer blends, and block copolymers. Furthermore, various kinds of composite particles containing phase-separated structures consisting of polymer-stabilized inorganic nanoparticles and polymers have been fabricated.30 Recently, we reported that the surface charges of particles can be controlled by end-functionalized polymers and composite particles containing Au nanoparticles (Au NPs) can be obtained by heterocoagulation.31 Furthermore, Au nanoparticles were embedded in the polymer particles when low-molecular-weight amino-terminated 1,2-polybutadiene (PB-NH2) was used as the polymer. Following on from these studies, we recently developed a method for fabricating gold nanoparticle clusters based on core−shell polymer particles by using heterocoagulation. Furthermore, multilayered gold nanoparticle clusters were found to exhibit strong SERS signals from adsorbed molecules upon NIR excitation.32 However, the influence of the metal nanoparticle size on the SERS signal enhancement and positional control using an external field have not yet been reported. In this study, the formation of gold nanoparticle clusters on PSt/PB-NH2 core−shell particles containing magnetic Fe3O4 nanoparticles was demonstrated with different nanoparticle sizes. Structures, LSPR absorption, SERS signals, magnetic responsibility of prepared particles were analyzed by electron microscope, UV−vis spectrum, Raman measurement, and optical microscope observation under magnetic flux, respectively. The effect of the nanoparticle size on the SERS absorption/excitation wavelength is discussed in terms of the enhancement factor of the SERS signal. Furthermore, dispersion control of the gold nanoparticle cluster composite particles in aqueous media was also demonstrated using an external magnetic field.

Figure 1. Schematic illustration of the preparation of core−shell particles composed of Fe3O4 NPs and Au NPs.

of composite particles. First, oleic-acid-stabilized Fe3O4 NPs were coated with poly(vinyl catechol-block-styrene) (PVCa-bPSt) according to a method described in the literature.33 And then, core−shell particles consisting of polymer-coated Fe3O4 NPs and PB-NH2 were prepared by SORP. Finally, Au NPs and Au nanourchins (Au NUs) were incorporated by heterocoaguration. Structures of the Composite Particles. Figure 2 shows STEM and cross-sectional TEM images of the core−shell particles composed of PVCa-b-PSt-coated Fe3O4 NPs and PBNH2. The PVCa-b-PSt-coated Fe3O4 NPs were located in the core regions of the particles. DLS measurements revealed that the diameter of the particles was 255 ± 63.7 nm. The zeta potential of the obtained core−shell particles was +41 ± 5.9 mV owing to the exposure of the hydrophilic amino groups in PBNH2 on the surfaces of the particles in aqueous media. An aqueous dispersion of the positively charged core−shell particles was mixed with an aqueous dispersion of the negatively charged gold nanoparticles, which led to heterocoagulation and the formation of composite particles. Figure 3 shows SEM, cross-sectional TEM, and EDX mapping images of the Au NP clusters formed at the surfaces of the core−shell particles containing Fe3O4 NPs. It can be seen from each SEM image that the Au NPs were successfully decorated on the surfaces of the core−shell particles with a high packing density. The average gaps between the assembled 30, 50, and 80 nm Au NPs and 50 nm Au NUs were 7.9 ± 4.3 nm, 7. 0 ± 2.3 nm, 8.1 ± 6.4 nm, and 8. 0 ± 7.4 nm, respectively. These gap sizes are sufficiently narrow to induce LSPR coupling. The cross-sectional TEM images revealed that the Au NPs were clearly embedded in the PB-NH2 shell regions owing to the low glass-transition temperature of PB. These results are identical with those previously reported in the literature.31,32 The overlayered EDX mapping image of Au L (red) and Fe K (green) also confirmed that the Au NPs and Fe3O4 NPs were located in the shell and core regions, respectively. LSPR Absorption Spectra of Composite Particles. Figure 4 shows vis−NIR spectra of the prepared composite particles based on Au NPs and Au NUs with different sizes. The B


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Figure 2. (a) STEM and (b) cross-sectional TEM images of core−shell particles composed of PVCa-b-PSt-coated Fe3O4 NPs and PB-NH2. (c) Sizes and (d) zeta potential of Fe3O4 core PBNH2 shell particles measured by DLS.

Figure 3. (a−d) SEM and (e−h) cross-sectional TEM images of the composite particles composed of Au NPs/Au NUs and Fe3O4 NPs. The composite particles were composed of (a, e) 30 nm, (b, f) 50 nm, or (c,g) 80 nm Au NPs, or (d,h) Au NUs. The scale bars in (a−h) and (i) are 500 and 100 nm, respectively. In (e−h), Fe3O4 NPs could not observe in some particles because of the cross section did not pass through the center of those particles. (i) Cross-sectional EDX mapping image of the particles composed of Au NPs (d = 50 nm). The red and green colors correspond to Au and Fe, respectively.

clusters formed on the surfaces of silica particles.15 However, the longer-wavelength absorption was limited to the visible region in the previous reports, and it is therefore noteworthy that the longer-wavelength peak reached the NIR region in the present study. The longer-wavelength absorption peak was observed at λmax values of 693, 801, 870, and 875 nm for the 30, 50, and 80 nm Au NPs, and Au NUs, respectively. The absorption peaks of the composite particles gradually red-shifted upon increasing the diameter of the Au NPs.35 As the LSPR peaks of the original Au NPs red-shifted with increasing diameter, the coupled resonances between the Au NPs also moved to a longer wavelength. The longer wavelength absorption peak were weak

dashed blue lines represent the absorption spectra of the original Au NPs or Au NUs. The absorption peaks (λmax) of these materials corresponding to LSPR ranged from 500 to 600 nm. In contrast, the absorption peaks of the composite particles (solid lines) were significantly red-shifted and two different peaks corresponding to electronic and magnetic resonances were observed, which is known as Fano-type resonance. Fan et al. reported that this type of resonance was observed in metal nanoparticle arrays formed on 2D substrates.34 More recently, Scherer and co-workers reported that short- and long-wavelength absorptions could be attributed to electronic and magnetic resonances between light and silver nanoparticle C


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scattering was ultimately observed. As the composite particles absorb at longer wavelengths, this fluorescence enhancement was suppressed. As shown in Figure 5c, the spectra acquired using a λex of 785 nm were very different from those obtained using a λex of 532 nm. Whereas the unmodified Au NPs/NUs did not show any fluorescence or Raman scattering enhancement, strong SERS signals from rhodamine 6G were clearly observed in the case of the composite particle samples. In particular, in the case of the composite particles based on 50 nm Au NPs, for which the LSPR absorption peak was very close to λex, the Raman scattering intensity was especially enhanced. Clear peaks corresponding to rhodamine 6G were observed (see the caption for Figure 5c). Figure 5d shows the relationship between the LSPR absorption peaks and the Raman scattering intensities at 1358 cm−1. It is noteworthy that matching of the excitation wavelength with the LSPR absorption peak strongly affected the SERS signal enhancement. It is well-known that SERS signals can be enhanced as a result of the strong electromagnetic field induced by LSPR coupling between metallic nanoparticles. As the gap distance and original strength of the LSPR of metallic nanoparticles are crucially important for LSPR coupling, composite particles based on Au NUs exhibited the strongest LSPR coupling, which resulted in a substantial red shift of the LSPR bands. However, the results presented in Figure 5d indicate that accurate matching between the excitation wavelength and the LSPR band is crucial for strong enhancement of the Raman scattering signal. Response to External Magnetic Fields. Many techniques have been reported for controlling the spatial position and assembly of dispersed particles, such as optical tweezers36 and electric field alignment,37,38 but these methods sometimes cause sample damage or deformation. In particular, as the composite particles prepared in the present study have a broad LSPR absorption range, optical techniques cannot be applied. The use of a magnetic field is the least invasive technique for chemical and biological samples. We previously developed a method for introducing polymer-stabilized magnetic nanoparticles into polymer particles and demonstrated that the prepared particles containing magnetic nanoparticles could be rotated and accumulated using an external magnetic field.39,40 Figure 6 shows photographs of an aqueous dispersion of the composite particles before and after magnetic accumulation using a neodymium magnet. Before exposure to the neodymium magnet, the composite particles were well dispersed in the aqueous medium. After positioning the magnet, the particles accumulated along with the magnetic field. After removal of the magnet, the accumulated particles redispersed upon shaking by hand. As this assembly and disassembly process was reversible, the magnetic responsiveness of the composite particles was confirmed. Furthermore, micrometer-scale positional control of the SERS-active composite particles using a magnetic field was achieved under optical microscopy conditions. An aqueous dispersion of the composite particles was sandwiched between two glass coverslips and fixed on the sample stage of an optical microscope (see Supporting Information S-4). A neodymium magnet was positioned beneath the sample stage and the movement of the composite particles was monitored by recording a video. Figure 7 shows snapshots of the movement of the composite particles upon moving the neodymium magnet (see also Video S1). This experiment clearly demonstrated the movement of the composite particles in response to the

Figure 4. Vis−NIR absorption spectra of the unmodified Au NPs and Au NUs (dashed blue lines) and the corresponding composite particles (solid red and yellow lines).

for 80 nm Au NPs and Au NUs in comparison to 30 and 50 nm Au NPs. In the case of the large Au NPs, it is considered that the number of hotspots between each Au NPs decrease since the number of composited Au NPs are low, longer wavelength absorption caused by plasmon coupling is weakened. In the case of the Au NUs, plasmon coupling is also weakened since intervals between Au NUs are made by spike structures. However, the small protrusions on the surface of Au NUs enhanced the LSPR, which resulted in the longer absorption wavelength of around λmax = 900 nm. SERS Measurements. To determine the effect of the excitation wavelength on the Raman scattering enhancement, Raman scattering measurements were performed using the two different excitation wavelengths (λex) of 532 and 785 nm, which correspond to the LSPR absorption wavelengths of the original Au NPs/NUs and the composite particles, respectively. Figure 5 shows the Raman spectra of rhodamine 6G adsorbed on the Au NPs, Au NUs, and composite particles. As shown in Figure 5a, no clear Raman band was observed in any of the spectra acquired using a λex of 532 nm, although the baseline of each signal changed depending on the sample. This was caused by the fluorescence from the rhodamine 6G molecules, which have an absorption peak at 525 nm, adsorbed on the samples. Figure 5b shows a plot of the signal intensities at 1358 cm−1, which corresponds to the Raman scattering band for the xanthene ring stretch (XRS) of rhodamine 6G the peak absorption wavelength of Au NPs/NUs and composite particles. As the unmodified Au NPs exhibit an LSPR absorption band around the excitation wavelength, the fluorescence from rhodamine 6G was strongly enhanced on the 50 nm Au NPs but the excitation energy was consumed by fluorescence excitation, such that no Raman D


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Figure 5. SERS spectra obtained using (a) λex = 532 nm and (c) λex = 785 nm and plots of the SERS intensity at 1358 cm−1 the LSPR peak wavelength at (b) λex = 532 nm and (d) λex = 785 nm. In (c), Raman scattering signals attributed to (i) in-plane xanthene ring deformations (ip XRD) at 611 cm−1; (ii) out-of-plane (op) C−H bending at 722 cm−1; (iii) a mixture of ip XRD and C−H and N−H bending at 1196 cm−1; (iv) ip xanthene ring breathing at 1309 cm−1; (v) XRS at 1358 cm−1; (vi) a mixture of XRS, C−N stretching, and C−H and N−H bending at 1495 cm−1; and (vii) XRS and ip N−H bending at 1574 cm−1 were observed.

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CONCLUSION

Magnetoresponsive composite particles decorated with Au nanoparticle clusters have been successfully prepared by assembling positively charged core−shell polymer particles containing magnetic nanoparticles and negatively charged Au NPs/NUs. The LSPR absorption band underwent a significant red shift and exhibited Fano-type resonance. Strong NIRexcitable SERS signals from the adsorbed dye molecules were observed when the appropriate excitation wavelength was used. The SERS-active composite particles could be accumulated and moved using an external magnetic field. These composite particles are expected to be applicable to the is situ SERS analysis of chemical species in chemical reaction media and biological samples, as NIR excitation light can pass though aqueous media containing cells and tissues and spatial

Figure 6. Photographs showing the magnetic accumulation of magnetoresponsive SERS-active composite particles (a) before, (b) during, (c) after accumulation using a neodymium magnet, and (d) redispersed particles after accumulation.

movement of the neodymium magnet. The maximum speed of particle movement was 2.3 μm/s under B = 49.2 mT. These results demonstrate that the prepared composite particles could be moved using a magnetic field. E


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Figure 7. Snapshots showing the movement of the composite particles in response to a magnetic field, as measured by optical microscopy. The arrows show the direction of magnetic flux.

positioning can be achieved using an external magnetic field. Additionally, detailed analysis of the electromagnetic response of the composite particles is underway in an attempt to elucidate their optical properties as metamolecules and metafluids. This technique opens the way to realizing rapid and high-resolution in situ imaging of chemical reactions and light-based probes in biological samples.

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dispersion was allowed to stand for 2 h at room temperature to assemble the Au NPs onto the core−shell particles. To fix the structures of particles and enable to observe electron microscopy, the PB-NH2 phase was stained with OsO4 by adding a 0.2% OsO4 solution and mixing for 15 min. The excess OsO4 was removed by centrifugation (12 000 rpm, 5 min, 5 °C) three times. The composite particles were cast on an elastic carbon grid and analyzed using transmission electron microscopy (TEM; H-7650, Hitachi, Japan) and scanning transmission electron microscopy (STEM; S-5200 equipped with a STEM unit, Hitachi, Japan). The composite particles were also embedded in a resin and cross-sections of the particles were prepared using an ultramicrotome (UC6, Leica, Germany). An elemental mapping image was also obtained by energy-dispersive X-ray spectroscopy (EDX; JEMARM200F, JEOL, Japan). The gaps between the Au NPs on the composite particles were measured from cross-sectional TEM images using imaging software (ImageJ, NIH, USA). To observe the magnetic response, a neodymium magnet was positioned next to a dispersion of the composite particles for 12 h. Schematic image of the preparation of composite particles are shown in Figure. 1 SERS Measurements under a Magnetic Field. The SERS spectra of rhodamine 6G adsorbed separately on a Si substrate, Au NPs (d = 30, 50, 80 nm), Au NUs (d = 50 nm), and the composite particles were measured using a Raman microscope (LabRAM, HR-800, Horiba Jobin Yvon, USA). Two ×2 cm silicon wafers were treated with O2 plasma for 5 min. Aqueous dispersions of the Au NPs (d = 30, 50, 80 nm), Au NUs (d = 50 nm), and the composite particles were cast on silicon wafers and dried at room temperature. Subsequently, an ethanolic solution of rhodamine 6G (20 μL, 1 mM) was cast on the prepared substrates. After drying, the SERS measurements were performed using 532 and 785 nm lasers, an irradiation time of 5 s, and ten times accumulation.

METHODS

Materials. A 5 mg/mL CHCl3 dispersion of Fe3O4 NPs stabilized with oleic acid was purchased from Sigma-Aldrich, USA. Aminoterminated polybutadiene (PB-NH2; Mw = 3 kg/mol) was purchased from Polymer Source, Inc., Canada. Gold nanoparticles (Au NPs; d = 30, 50, 80 nm) were purchased from BBI Solutions, UK. Gold nanourchins (Au NUs; d = 50 nm) were purchased from Sigma, USA. Tetrahydrofuran (THF), polyethylene glycol (PEG; Mw = 6 kg/mol), and rhodamine 6G were purchased from Wako Pure Chemical Industries Ltd., Tokyo. All reagents were used without further purification. Poly(vinyl catechol-block-styrene) (PVCa-b-PSt) was synthesized according to a method described in the literature.41 Polymer Coating of Fe3O4 NPs. Polymer-coated Fe3O4 NPs were prepared according to a method described in the literature.33 Equal volumes (1 mL) of a 5 mg/mL CHCl3 dispersion of Fe3O4 NPs and a 10 mg/mL solution of PVCa-b-PSt were mixed and the resulting mixture was stirred for 10 min using a homogenizer. To remove the excess coating polymer, the PVCa-b-PSt-coated Fe3O4 NPs were collected using a neodymium magnet and washed three times with THF. The washed PVCa-b-PSt-coated Fe3O4 NPs were redispersed in THF at a concentration of 0.25 mg/mL. The sizes and UV−vis absorption spectra of the polymer-coated Fe3O4 NPs were measured by dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern, UK) and UV−vis spectroscopy (V760DS, JASCO, Japan), respectively (see Supporting Information S1−S3). Preparation of Core−Shell Particles Composed of Fe3O4 NPs and Au NPs. The preparation of the core−shell particles was performed as illustrated in Figure 1. PB-NH2 was dissolved in THF to a concentration of 0.1 g/L. The dispersion of PVCa-b-PSt-coated Fe3O4 NPs (100 μL) and the PB-NH2 solution (900 μL) were mixed and 1 mL of pure water was then dripped into the polymer solution over 1 min. After evaporation of the THF, the mixed solution was maintained in a water bath at 40 °C for 12 h without stirring to precipitate the core−shell polymer particles. The core and shell components of the particles were composed of PVCa-b-PSt-coated Fe3O4 NPs and PB-NH2, respectively. An aqueous dispersion of the polymer particles was diluted by adding 9 mL ofpure water. The sizes and zeta potentials of the core−shell particles were measured by DLS. Then, 100 μL of the core−shell particle dispersion, 200 μL of a Au NP dispersion, and 100 μL of a PEG solution were mixed and the resulting

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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/acsanm.8b01093.

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Characterizations of PVCa-b-PS coated Fe3O4 NPs and experimental setups of observing composite particles under external magnetic field (PDF) Video S1, the movie of composite particles under external magnetic field (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F


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Yutaro Hirai: 0000-0002-5666-1639 Hiroshi Yabu: 0000-0002-1943-6790 Author Contributions

Y.H. prepared and analyzed the composite particles and wrote part of the manuscript. Y.M. performed the STEM-EDX analysis. H.Y. supervised the entire project and wrote the manuscript. Funding

This work was partially supported by KAKENHI (17H01223, 16K14071), Ministry of Education, Culture, Sports, Science and Technology, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.Y. thanks Mrs. J. Gu and Ms. M. Suzuki, WPI-AIMR, Tohoku University, for assisting with the particle preparation and SEM/ TEM observations. Y.M. would like to express his gratitude to Mrs. Naomi Hirai, Hokkaido University for the technical assistance of STEM-EDX and Nanotechnology platform support.

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