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Attenuation of surface-enhanced Raman scattering of magnetic–plasmonic FePt@Ag core–shell nanoparticles due to an external magnetic field
Accepted Manuscript Attenuation of surface-enhanced Raman scattering of magnetic-plasmonic FePt@Ag core-shell nanoparticles due to an external magnetic field Nguyen T.T. Trang, Trinh T. Thuy, Derrick M. Mott, Mikio Koyano, Shinya Maenosono PII: DOI: Reference:
S0009-2614(13)00578-2 http://dx.doi.org/10.1016/j.cplett.2013.04.064 CPLETT 31213
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
21 February 2013 29 April 2013
Please cite this article as: N.T.T. Trang, T.T. Thuy, D.M. Mott, M. Koyano, S. Maenosono, Attenuation of surfaceenhanced Raman scattering of magnetic-plasmonic FePt@Ag core-shell nanoparticles due to an external magnetic field, Chemical Physics Letters (2013), doi: http://dx.doi.org/10.1016/j.cplett.2013.04.064
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Attenuation of surface-enhanced Raman scattering of magnetic-plasmonic FePt@Ag core-shell nanoparticles due to an external magnetic field
Nguyen T. T. Trang,1,2 Trinh T. Thuy,1 Derrick M. Mott,1 Mikio Koyano,1 and Shinya Maenosono1,*
1
School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
2
Institute of Materials Science, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Hanoi, Vietnam
Abstract The surface-enhanced Raman scattering (SERS) activities of Ag and FePt@Ag nanoparticle probes were examined using thiophenol as a Raman reporter molecule in the absence and presence of a magnetic field. Under external magnetic fields of different field strength, the SERS activities of both types of nanoparticles (NPs) were weakened as a function of magnetic field strength. The attenuation degree of SERS activity by the magnetic field in the case of FePt@Ag NPs is found to be two times higher than for Ag NPs, because the superparamagnetic FePt cores enhance the local magnetic field at the area of the Ag shells.
Keywords: magnetic-plasmonic nanoparticles; surface-enhanced Raman scattering; magnetic effect; superparamagnetic nanoparticles.
*
Corresponding Author Dr. Shinya Maenosono, Professor School of Materials Science Japan Advanced Institute of Science and Technology 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Phone: (+81)-761-51-1611, Fax: (+81)-761-51-1625 E-mail: [email protected]
1
1. Introduction Heterostructured nanoparticles (NPs) composed of dissimilar materials with multiple functions or properties, such as magnetic-plasmonic [1,2], magnetic-fluorescent [3,4], or metal-semiconductor [5] NPs, have recently received much attention. In particular, the combination of plasmonic and magnetic materials in a single nanostructure is of high interest to the biomedical community because it has the potential to lead to new biological applications, such as immunomagnetic separation under plasmonic imaging monitoring, dual mode imaging (MRI and plasmonic imaging), and surface-enhanced Raman scattering (SERS) sensing. Recently, we have developed a scheme for the synthesis of magnetic-plasmonic FePt@Ag core-shell NPs with diameters larger than 10 nm, high SERS activity, high colloidal stability, and good magnetic separation capabilities, with the aim of using the NPs in biological sensing/imaging applications [6]. These FePt@Ag NPs hold promise as dual-functional sensing probes for diagnostic and environmental applications. Recently, Chen and coworkers found that the SERS intensity of pyridine molecules adsorbed on Ag NPs is significantly attenuated in the presence of a magnetic field [7]. To understand the mechanism of the phenomenon, they performed quantum chemical calculations and concluded that magnetic fields had an impairing influence on SERS due to the broadening of the energy gap for charge-transfer and the decrease of the electrons on the metal surface [7]. A similar phenomenon has also been observed in other systems including Ni/Au core-shell microparticles and Au hollow spheres [8]. As mentioned above, magnetic-plasmonic NPs have attracted much attention as dual-functional
2
SERS sensing probes, and thus, understanding the effects of magnetic fields on the induced SERS intensity of Raman active molecules by magnetic-plasmonic NPs is essential. In this paper, Ag and FePt@Ag NPs are used as SERS probes for the detection of thiophenol, a well-understood Raman active molecule. The two NP probe systems are used both in the presence and absence of various strength external magnetic fields to investigate the impact of the magnetic strength on the resulting SERS activities. The study reveals the influence of the magnetic FePt core on the attenuation of the SERS activity of thiophenol adsorbed to FePt@Ag NP probes and reveals insight into the correlation of NP heterostructure magnetic vs. plasmonic properties. 2. Experimental 2.1. Chemicals Platinum(II) acetylacetonate [Pt(acac)2, purity 97%], triiron dodecacarbonyl [Fe3(CO)12, purity 99.999%], silver nitrate (AgNO3, purity >99.0%), tetraethylene glycol (TEG, purity 99%), oleic acid (OA, purity 99%), oleylamine (OLA, purity 70%), phenyl ether (PE, purity 99%), and thiophenol (TP, purity 97%) as well as common solvents were purchased from Sigma Aldrich Corp. Water used in experiments was purified with a Millipore Direct-Q 3 system and had a final resistance of 18.2 M . 2.2. Synthesis of FePt core NPs First, superparamagnetic fcc-phase FePt NPs were synthesized using a previously reported method [6,9]. Uniform FePt NPs were formed at 240°C under an Ar atmosphere by decomposing 0.17 mmol of Fe3(CO)12 and reducing 0.254 mmol of Pt(acac)2 in 20 mL of TEG, with the volume ratio of
3
capping ligands OA:OLA = 1:1 fixing the total amount of ligands at OA + OLA = 9.3 mmol. After a reaction time of 2 h, the solution was left to cool to room temperature. The FePt NPs were purified and separated from the matrix by adding ethanol and centrifuging several times. The FePt NPs were then re-dispersed in hexane with additional OLA (5 vol%). 2.3. Synthesis of Ag and FePt@Ag core-shell NPs FePt@Ag core-shell NPs were synthesized using a previously reported method [6]. Briefly, AgNO3 (0.807 mmol) and OLA (2 mL) were dissolved in 20 mL of PE and stirred for 30 min at 25°C, under an Ar atmosphere. Then, 5 mL of a hexane dispersion of FePt NPs was injected into the solution. The reaction temperature was then raised to 80°C to completely remove the hexane from the reaction solution. Subsequently, the reaction temperature was raised to 200°C; this temperature was maintained for 30 min. After the reaction, the reaction solution was left to cool to room temperature. The FePt@Ag NPs were then purified and separated from the matrix by adding ethanol and centrifuging several times. The FePt@Ag NPs were re-dispersed in 30 mL of hexane, and the magnetically active NPs were then separated from the solution using a neodymium magnet. This magnetic separation process was repeated three times. Finally, the purified FePt@Ag NPs were re-dispersed in hexane with additional OLA (5 vol%). Ag NPs were synthesized under the same reaction conditions as those used for the FePt@Ag NPs in the absence of FePt seeds. 2.4. Preparing samples for SERS measurements Glass substrates were cleaned using sonication in acetone for 10 min, followed by another 10
4
minutes of sonication in methanol. Hexane dispersions of Ag and FePt@Ag NPs were prepared, keeping the NP concentration constant (3.6 × 10 8 M). The SERS substrates were prepared by −
drop-casting 100 L of the NP dispersion onto the glass substrate, which was aided by a circular cloning ring keeping the solution from spreading and producing the typical “coffee ring” effect after drying. The dried samples were visually uniform in appearance. We prepared two different SERS substrates: (1) Ag NP substrate and (2) FePt@Ag NP substrate. Next, a toluene solution of TP (100 nM) was dropped onto each SERS substrate and dried in air at room temperature. The TP molecules chemically adsorbed onto the NP surfaces via metal-thiol interactions during the drying process. 2.5. Instrumentation and measurements TEM observations were performed on a Hitachi H-7100 operated at 100 kV. SERS spectra were obtained with a Ar+ ion laser (wavelength 514.5 nm, power 50 mW), using a Horiba-Jobin Yvon Ramanor T64000 triple monochromator equipped with a CCD detector. The non-polarized Raman scattering measurements were performed under a microscope sample holder, using a 180° backscattering geometry, at room temperature. For SERS measurements under an external magnetic field, two different type of permanent magnets (ferrite and neodymium) with varying field strength were used. Ferrite and neodymium magnets give 55 and 400 mT magnetic fields at the site of the SERS substrate, respectively. The magnet was placed under the SERS substrate during the SERS measurement. In another experiment, the neodymium magnet was put under the SERS substrate which can be moved up and down smoothly by a screw mechanism to vary the magnetic field strength
5
in a continuous manner. By using this system, the magnetic field strength could be varied from 20 to 250 mT at the site of the SERS substrate. The magnetic field strengths were measured using a Lake Shore Gaussmeter (Model 410). 3. Results and discussion The characterization of the general properties for the Ag, FePt and FePt@Ag NPs has been described previously [6], and is briefly outlined here. The as-synthesized FePt NPs display a chemically disordered fcc phase and are quite uniform in size and shape with an average diameter of 4.5 ± 0.5 nm [6]. The composition of the FePt NPs was measured using EDS and was found to be Fe46Pt54. The mean diameters of Ag and FePt@Ag NPs were calculated from TEM images, with representative images shown in Figure 1 and were found to be 15.8 ± 3.0 nm and 15.9 ± 1.5 nm (Ag shell thickness is 5.7 nm), respectively. The core-shell structure was confirmed by X-ray diffraction, high-resolution TEM and scanning TEM analyses [6]. The localized surface plasmon resonance (LSPR) peak wavelengths for the Ag and FePt@Ag NPs are 399 and 412 nm, respectively [6]. Zero-field-cooled and field-cooled curves for the FePt and FePt@Ag NPs were measured using SQUID, and the blocking temperature TB was found to be approximately 52 K for both samples. Both NP samples clearly showed superparamagnetic behavior. The saturation magnetization (Ms) was estimated to be 11 emu/g for the FePt core NPs, while the Ms for the FePt@Ag NPs was 15 emu/g [6]. Figure 2a shows the Raman spectra of TP obtained using the Ag NP substrate in the absence and presence of an external magnetic field. These SERS spectra, both in the presence and absence of the
6
magnetic field were obtained at exactly the same position on the substrate. As shown in Fig. 2a, four distinct peaks are observed at 997, 1020, 1070, and 1572 cm 1. These peaks corresponded to S-H −
bending, in-plane ring deformation, C-S stretching, and C-C stretching vibrations, respectively [10,11]. The SERS activity of the Ag NP substrate under an external 55 mT magnetic field was approximately the same as that in the absence of the magnetic field. However, the SERS activity was significantly reduced when a 400 mT magnetic field was applied. Figure 2b shows the Raman spectra of TP obtained using the FePt@Ag NP substrate in the absence and presence of an external magnetic field. Under external magnetic fields with strengths of 55 and 400 mT, SERS activities of the FePt@Ag NP substrate were weakened as magnetic field strength increased. The degree of attenuation of the SERS activity under the magnetic field in the case of FePt@Ag NPs seems to be higher than that in the case of Ag NPs. To confirm that the results are beyond the range of experimental error, we measured the SERS spectra at four different positions on the Ag and FePt@Ag NP substrates with and without a magnetic field. Then, we estimated the experimental error for the SERS intensity at both primary (997 cm 1) and secondary (1070 cm 1) peaks, because these peaks are less affected by background −
−
fluorescence than the peak at 1572 cm 1. After the background subtraction, the SERS intensities at −
primary and secondary peaks were precisely calculated. Figure 3a and b show the SERS intensities at primary and secondary peaks plotted versus the external magnetic field strength, respectively. It is clearly seen that magnetic fields have a negative effect on the SERS activities for both Ag and FePt@Ag NPs (see Table 1). In addition, the following two points become evident: (1) The SERS
7
intensity at both peaks linearly decreases with increasing magnetic field strength, which is consistent with previously reported results [8]; and (2) the degree of attenuation of the SERS activity under the magnetic field in the case of FePt@Ag NPs is roughly twice that for the case of Ag NPs. In order to explain these observations, an understanding of the magnetic contrast agents used for magnetic resonance imaging (MRI) technique provides insight. MRI enables one to obtain a cross-sectional view of a human body noninvasively by utilizing the nuclear magnetic resonance of protons that comprise a large percentage of the body. Superparamagnetic NPs, such as superparamagnetic iron oxide (SPIO) NPs, have been widely used as MRI contrast agents because they enhance the local magnetic field and significantly shorten the proton transverse (spin-spin) relaxation time (T2). As a result, the signal is reduced in the resulting proton density-weighted image or the T2-weighted image, leading to enhanced contrast and image resolution. These beneficial properties have led to the development of superparamagnetic NPs for clinical applications, including internally administered superparamagnetic NPs that are selectively ingested by Kupffer cells inside the liver, playing an important role in the detection of small metastatic liver tumors and/or hepatocarcinomas [12,13]. Similarly, superparamagnetic FePt cores can strengthen the local magnetic field at the site of plasmonic Ag shells when an external magnetic field is applied, and thus, the degree of the broadening of the energy gap for charge-transfer and a decrease in the electrons on the metal particle surface would be enhanced when compared to diamagnetic Ag NPs. This could lead to the higher degree of attenuation of SERS activity under the magnetic field in the case of FePt@Ag NPs. It
8
is well known that there are two primary mechanisms of SERS: one is electromagnetic (EM) enhancement and the other is chemical (CM) enhancement. In the present experiments, EM enhancement effects that arise as a result of particle aggregation after applying a magnetic field can be essentially ruled out because the NPs are small and uniform, leading to an inherently dense (highly packed) layer of particles that forms during the drying process. This dense layer of NPs likely cannot aggregate further when subjected to a relatively strong magnetic field, which eliminates variation in EM effects induced by aggregation in this analysis. While there is still much debate on the mechanism for electron transfer observed in Raman analysis, the CM enhancement itself has been analyzed in many prior studies [7,8] and represents the best explanation for the observed decrease in Raman intensity under a magnetic field, as observed in this study. To confirm the hypothesis, the total magnetic field strength at the site of the FePt cores is estimated from the magnetization behavior of FePt@Ag NPs. Assuming that the degree of attenuation of Ag NPs is purely due to the external magnetic field, and the local magnetic field at the site of Ag shells is nearly equal to the sum of the external magnetic field and the magnetization of FePt cores (M), the SERS intensity at both 997 cm 1 and 1070 cm 1 was estimated by using the slope of the black −
−
curves in Fig. 3a and b and the SQUID data of FePt@Ag NPs (Fig. 3c). For example, the local magnetic field strength estimate at the site of Ag shells is 201.5 mT when the external magnetic field is 55 mT because M = 8.3 emu/g = 116.2 emu/cm3 = 146.5 mT (the mass density of FePt is 14 g/cm3 [14]). By using the slope of the black curve in Fig. 3a, the SERS intensity at 997 cm 1 under a 201.5 −
9
mT magnetic field is calculated to be 77 counts/s. Figure 3d shows a parity plot of the estimated and measured SERS intensities at both 997 cm 1 and 1070 cm 1. The comparison is good, indicating that −
−
the higher degree of attenuation of SERS activity in the case of FePt@Ag NPs is actually due to the strengthened local magnetic field at the site of plasmonic Ag shells. Finally, an experiment was carried out to test the impact on SERS intensity for the FePt@Ag NP substrate under conditions of magnetic field cycling, or repeated/prolonged laser (Raman) irradiation. First, SERS spectra were taken in the absence of a magnetic field, and then, were taken again under a 400 mT magnetic field. This cycle was conducted a total of two times to test the reproducibility. Figure 4a shows the SERS intensity at 997 cm 1 plotted versus the number of measurements (magnetic −
field on/off states). It is clearly seen that the phenomenon is virtually reversible within the limits of the experiment. If, by chance the NPs were undergoing aggregation under the magnetic field, this would not be reversible after removing the magnet because the NPs are essentially immobile on the dried substrate. As a result, we are able to observe the detrimental effect on CM enhancement when applying a magnetic field to our NP samples, which is not observable when EM enhancement effects dominate. However, if we continue to repeat this cyclic experiment further, the SERS intensity gradually decreases and the reduction becomes irreversible. The effect of repeated/prolonged laser irradiation from the Raman measurement on the resulting SERS intensity was studied next. In Figure 4b, the black curve shows the normalized SERS intensity at 997 cm 1 taken for a total of 6 −
measurement cycles of turning the Raman laser off and on (on/off cycles) under a static 250 mT
10
magnetic field. Data points were taken at the exact same position of the sample and the measurement was repeated multiple times at different points on the substrate. The SERS intensity gradually decreased with increasing the number of laser on/off repetitions decreasing down to 62±8 % of that of the first measurement. This is presumably because the TP molecules that are adsorbed onto the NP surfaces were degraded or desorbed due to prolonged radiation of the Raman instrument laser light. In order to separate the detrimental effect of prolonged laser irradiation of the sample, an experiment was conducted to test the effect of incrementally increasing the magnetic field strength on the SERS activity. The red curve in Figure 4b shows the normalized SERS intensity at 997 cm 1 taken for a total −
of 6 measurement cycles (laser on/off cycles) with data points taken at the exact same position of the sample with increasing the magnetic field strength in incremental steps of 20, 50, 100, 150, 200, and 250 mT for each cycle (also conducted at multiple points on the substrate). In this case, the SERS intensity decreased down to about 17±1 % of that of the first measurement. These data are summarized in Table 2. If one simply assumes that the 38±8 % reduction after 6 laser on/off measurement cycles is due to irreversible degradation (or desorption) of TP molecules and simply subtract it from the degree of attenuation with incrementally increasing the magnetic field (83±1 %), the degree of attenuation due purely to the magnetic field effect is calculated to be 45±9 %. This value is consistent with Fig 3a (average attenuation degree of 40.2 %) even considering that the degree of attenuation in Fig. 3a is overestimated because it includes the contribution of irreversible reduction of SERS intensity due to the laser irradiation.
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4. Conclusions Equal-sized Ag and FePt@Ag NPs were chemically synthesized and the effect of an external magnetic field on their SERS activities was investigated using TP as a Raman reporter molecule. In consequence, it was found that the SERS activity linearly decreases with increasing the magnetic field strength, and is reversible. The reduction ratio of SERS activity by the magnetic field in the case of FePt@Ag NPs is almost double that for the case of Ag NPs. The attenuation of SERS activity is due to the broadening of the energy gap for charge-transfer and a decrease of the electrons on the metal surface under the magnetic field. The more pronounced effect observed in FePt@Ag NPs than for Ag NPs likely arises from the superparamagnetic FePt core enhancement of the local magnetic field at the site of the plasmonic Ag shells when an external magnetic field is applied.
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References [1] G.A. Sotiriou, A.M. Hirt, P.Y. Lozach, A. Teleki, F. Krumeich, S.E. Pratsinis, Chem. Mater. 23 (2011) 1985. [2] E.D. Smolensky, M.C. Neary, Y. Zhou, T.S. Berquo, V.C. Pierre, Chem. Commun. 47 (2011) 2149. [3] J. Gao, B. Zhang, Y. Gao, Y. Pan, X. Zhang, B. Xu, J. Am. Chem. Soc. 129 (2007) 11928. [4] S. He, H. Zhang, S. Delikanli, Y. Qin, M.T. Swihart, A. Zeng, J. Phys. Chem. C 113 (2009) 87. [5] H.Y. Lin, Y.F. Chen, J.G. Wu, D.I. Wang, C.C. Chen, Appl. Phys. Lett. 88 (2006) 161911. [6] T.T.T. Nguyen, T.T. Trinh, K. Higashimine, D.M. Mott, S. Maenosono, Plasmonics (2013) DOI: 10.1007/s11468-013-9529-7 [7] X.K. Kong, Q.W. Chen, R. Li, K. Cheng, N. Yan, B.X. Yu, Chem. Commun. 47 (2011) 11237. [8] R. Li, Q.W. Chen, H. Zhang, X.K. Kong, Y.B. Sun, H. Zhong, H. Wang, S. Zhou, J. Raman Spectrosc. (2012) in press (DOI: 10.1002/jrs.4227) [9] T.T. Trinh, D. Mott, N.T.K. Thanh, S. Maenosono, RSC Adv. 1 (2011) 100. [10] H.Y. Jung, Y.K. Park, S. Park, S.K. Kim, Anal. Chim. Acta 602 (2007) 236. [11] X. Li, M. Cao, H. Zhang, L. Zhou, S. Cheng, J.L. Yao, L.J. Fan, J. Colloid Interface Sci. 382 (2012) 28. [12] R.C. Semelka, T.K.G. Helmberger, Radiology 218 (2001) 27. [13] M. Zhao, D.A. Beauregard, L. Loizou, B. Davletov, K.M. Brindle, Nat. Med. 7 (2001) 1241. [14] X.W. Wu, C. Liu, L. Li, P. Jones, R.W. Chantrell, D. Weller, J. Appl. Phys. 95 (2004) 6810.
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Figure captions
Fig. 1. TEM images of (a) FePt@Ag (mean size: 15.9 ± 1.5 nm) and (b) Ag NPs (15.8 ± 3.0 nm). In the case of FePt@Ag NPs, The mean size of FePt cores is 4.5 ± 0.5 nm and the Ag shell thickness is 5.7 nm.
Fig. 2. SERS spectra of TP molecules adsorbed on (a) Ag and (b) FePt@Ag NPs. Black, blue and red curves correspond to the SERS spectrum taken under 0, 55 and 400 mT magnetic fields, respectively. The insets show the SERS spectra expanded in the range of 985−1105 cm 1. −
Fig. 3. SERS intensities at (a) primary (997 cm 1) and (b) secondary (1070 cm 1) peaks plotted versus −
−
external magnetic field strength. Black and red colors represent SERS intensities taken by using Ag and FePt@Ag NP substrates, respectively. (c) Magnetization versus applied field curves for FePt@Ag NPs measured at 300 K. (d) Parity plot of estimated and measured SERS intensities at both 997 cm 1 −
(red) and 1070 cm 1 (blue) for FePt@Ag NPs. −
Fig. 4. SERS intensity taken at the primary peak (997 cm 1) for TP on the FePt@Ag NP substrate. (a) −
SERS intensity with and without magnetic field. First, a SERS spectrum was taken in the absence of a magnetic field, and then, it was taken again under a 400 mT magnetic field. This cycle was conducted a total of two times. (b) Black curve represents the variation of the normalized SERS intensity with number of repeated measurements (laser on/off cycles) taken at the exact same position of the sample under a 250 mT magnetic field. After 6 measurement cycles, the SERS intensity decreases down to 62% of that of the first measurement. Red curve represents the variation of the SERS intensity measurements by changing the magnetic field strength incrementally by 20, 50, 100, 150, 200, and 250 mT (with cycling the laser on and off at each point). Both types of measurement were performed 3-4 times at different positions of the same sample to estimate the experimental error, which is represented by the error bars shown in each graph. 14
Table 1 SERS intensities at primary (997 cm 1) and secondary (1070 cm 1) peaks under external magnetic fields of different field strength −
−
SERS intensity [Counts/s]
−1
SERS peak [cm ] 0 mT
55 mT
250 mT
400 mT
997
99.6±13.8
102.3±20.5
N/A
70.7±4.1
1070
95.6±12.1
92.5±18.5
N/A
72.1±3.6
997
95.7±10.7
82.8±16.6
57.2±20.0
38.3±8.2
1070
79.4±9.6
70.8±14.2
41.5±14.8
36.4±9.9
Ag
FePt@Ag
Table 2 Normalized SERS intensity at the primary peak (997 cm 1) for TP on the FePt@Ag NP substrate −
Fixed MF strength
Laser on/off measurement cycles
Variable MF strength
MF [mT]
SERS intensity [-]
MF [mT]
SERS intensity [-]
1
250
1.00±0.10
20
1.00±0.21
2
250
0.81±0.08
50
0.89±0.34
3
250
0.72±0.07
100
0.56±0.26
4
250
0.67±0.07
150
0.33±0.16
5
250
0.64±0.07
200
0.17±0.04
6
250
0.62±0.08
250
0.17±0.01 MF: Magnetic Field
15
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Highlights
• We study the SERS activity of FePt@Ag nanoparticles (NPs) under magnetic field. • The SERS activity linearly decreases with increasing the magnetic field strength. • The reduction of SERS activity is more pronounced for FePt@Ag than that for Ag NPs. • The more pronounced effect observed in FePt@Ag is due to the local magnetic field.
21
S0009-2614(13)00578-2 http://dx.doi.org/10.1016/j.cplett.2013.04.064 CPLETT 31213
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
21 February 2013 29 April 2013
Please cite this article as: N.T.T. Trang, T.T. Thuy, D.M. Mott, M. Koyano, S. Maenosono, Attenuation of surfaceenhanced Raman scattering of magnetic-plasmonic FePt@Ag core-shell nanoparticles due to an external magnetic field, Chemical Physics Letters (2013), doi: http://dx.doi.org/10.1016/j.cplett.2013.04.064
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Attenuation of surface-enhanced Raman scattering of magnetic-plasmonic FePt@Ag core-shell nanoparticles due to an external magnetic field
Nguyen T. T. Trang,1,2 Trinh T. Thuy,1 Derrick M. Mott,1 Mikio Koyano,1 and Shinya Maenosono1,*
1
School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
2
Institute of Materials Science, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Hanoi, Vietnam
Abstract The surface-enhanced Raman scattering (SERS) activities of Ag and FePt@Ag nanoparticle probes were examined using thiophenol as a Raman reporter molecule in the absence and presence of a magnetic field. Under external magnetic fields of different field strength, the SERS activities of both types of nanoparticles (NPs) were weakened as a function of magnetic field strength. The attenuation degree of SERS activity by the magnetic field in the case of FePt@Ag NPs is found to be two times higher than for Ag NPs, because the superparamagnetic FePt cores enhance the local magnetic field at the area of the Ag shells.
Keywords: magnetic-plasmonic nanoparticles; surface-enhanced Raman scattering; magnetic effect; superparamagnetic nanoparticles.
*
Corresponding Author Dr. Shinya Maenosono, Professor School of Materials Science Japan Advanced Institute of Science and Technology 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Phone: (+81)-761-51-1611, Fax: (+81)-761-51-1625 E-mail: [email protected]
1
1. Introduction Heterostructured nanoparticles (NPs) composed of dissimilar materials with multiple functions or properties, such as magnetic-plasmonic [1,2], magnetic-fluorescent [3,4], or metal-semiconductor [5] NPs, have recently received much attention. In particular, the combination of plasmonic and magnetic materials in a single nanostructure is of high interest to the biomedical community because it has the potential to lead to new biological applications, such as immunomagnetic separation under plasmonic imaging monitoring, dual mode imaging (MRI and plasmonic imaging), and surface-enhanced Raman scattering (SERS) sensing. Recently, we have developed a scheme for the synthesis of magnetic-plasmonic FePt@Ag core-shell NPs with diameters larger than 10 nm, high SERS activity, high colloidal stability, and good magnetic separation capabilities, with the aim of using the NPs in biological sensing/imaging applications [6]. These FePt@Ag NPs hold promise as dual-functional sensing probes for diagnostic and environmental applications. Recently, Chen and coworkers found that the SERS intensity of pyridine molecules adsorbed on Ag NPs is significantly attenuated in the presence of a magnetic field [7]. To understand the mechanism of the phenomenon, they performed quantum chemical calculations and concluded that magnetic fields had an impairing influence on SERS due to the broadening of the energy gap for charge-transfer and the decrease of the electrons on the metal surface [7]. A similar phenomenon has also been observed in other systems including Ni/Au core-shell microparticles and Au hollow spheres [8]. As mentioned above, magnetic-plasmonic NPs have attracted much attention as dual-functional
2
SERS sensing probes, and thus, understanding the effects of magnetic fields on the induced SERS intensity of Raman active molecules by magnetic-plasmonic NPs is essential. In this paper, Ag and FePt@Ag NPs are used as SERS probes for the detection of thiophenol, a well-understood Raman active molecule. The two NP probe systems are used both in the presence and absence of various strength external magnetic fields to investigate the impact of the magnetic strength on the resulting SERS activities. The study reveals the influence of the magnetic FePt core on the attenuation of the SERS activity of thiophenol adsorbed to FePt@Ag NP probes and reveals insight into the correlation of NP heterostructure magnetic vs. plasmonic properties. 2. Experimental 2.1. Chemicals Platinum(II) acetylacetonate [Pt(acac)2, purity 97%], triiron dodecacarbonyl [Fe3(CO)12, purity 99.999%], silver nitrate (AgNO3, purity >99.0%), tetraethylene glycol (TEG, purity 99%), oleic acid (OA, purity 99%), oleylamine (OLA, purity 70%), phenyl ether (PE, purity 99%), and thiophenol (TP, purity 97%) as well as common solvents were purchased from Sigma Aldrich Corp. Water used in experiments was purified with a Millipore Direct-Q 3 system and had a final resistance of 18.2 M . 2.2. Synthesis of FePt core NPs First, superparamagnetic fcc-phase FePt NPs were synthesized using a previously reported method [6,9]. Uniform FePt NPs were formed at 240°C under an Ar atmosphere by decomposing 0.17 mmol of Fe3(CO)12 and reducing 0.254 mmol of Pt(acac)2 in 20 mL of TEG, with the volume ratio of
3
capping ligands OA:OLA = 1:1 fixing the total amount of ligands at OA + OLA = 9.3 mmol. After a reaction time of 2 h, the solution was left to cool to room temperature. The FePt NPs were purified and separated from the matrix by adding ethanol and centrifuging several times. The FePt NPs were then re-dispersed in hexane with additional OLA (5 vol%). 2.3. Synthesis of Ag and FePt@Ag core-shell NPs FePt@Ag core-shell NPs were synthesized using a previously reported method [6]. Briefly, AgNO3 (0.807 mmol) and OLA (2 mL) were dissolved in 20 mL of PE and stirred for 30 min at 25°C, under an Ar atmosphere. Then, 5 mL of a hexane dispersion of FePt NPs was injected into the solution. The reaction temperature was then raised to 80°C to completely remove the hexane from the reaction solution. Subsequently, the reaction temperature was raised to 200°C; this temperature was maintained for 30 min. After the reaction, the reaction solution was left to cool to room temperature. The FePt@Ag NPs were then purified and separated from the matrix by adding ethanol and centrifuging several times. The FePt@Ag NPs were re-dispersed in 30 mL of hexane, and the magnetically active NPs were then separated from the solution using a neodymium magnet. This magnetic separation process was repeated three times. Finally, the purified FePt@Ag NPs were re-dispersed in hexane with additional OLA (5 vol%). Ag NPs were synthesized under the same reaction conditions as those used for the FePt@Ag NPs in the absence of FePt seeds. 2.4. Preparing samples for SERS measurements Glass substrates were cleaned using sonication in acetone for 10 min, followed by another 10
4
minutes of sonication in methanol. Hexane dispersions of Ag and FePt@Ag NPs were prepared, keeping the NP concentration constant (3.6 × 10 8 M). The SERS substrates were prepared by −
drop-casting 100 L of the NP dispersion onto the glass substrate, which was aided by a circular cloning ring keeping the solution from spreading and producing the typical “coffee ring” effect after drying. The dried samples were visually uniform in appearance. We prepared two different SERS substrates: (1) Ag NP substrate and (2) FePt@Ag NP substrate. Next, a toluene solution of TP (100 nM) was dropped onto each SERS substrate and dried in air at room temperature. The TP molecules chemically adsorbed onto the NP surfaces via metal-thiol interactions during the drying process. 2.5. Instrumentation and measurements TEM observations were performed on a Hitachi H-7100 operated at 100 kV. SERS spectra were obtained with a Ar+ ion laser (wavelength 514.5 nm, power 50 mW), using a Horiba-Jobin Yvon Ramanor T64000 triple monochromator equipped with a CCD detector. The non-polarized Raman scattering measurements were performed under a microscope sample holder, using a 180° backscattering geometry, at room temperature. For SERS measurements under an external magnetic field, two different type of permanent magnets (ferrite and neodymium) with varying field strength were used. Ferrite and neodymium magnets give 55 and 400 mT magnetic fields at the site of the SERS substrate, respectively. The magnet was placed under the SERS substrate during the SERS measurement. In another experiment, the neodymium magnet was put under the SERS substrate which can be moved up and down smoothly by a screw mechanism to vary the magnetic field strength
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in a continuous manner. By using this system, the magnetic field strength could be varied from 20 to 250 mT at the site of the SERS substrate. The magnetic field strengths were measured using a Lake Shore Gaussmeter (Model 410). 3. Results and discussion The characterization of the general properties for the Ag, FePt and FePt@Ag NPs has been described previously [6], and is briefly outlined here. The as-synthesized FePt NPs display a chemically disordered fcc phase and are quite uniform in size and shape with an average diameter of 4.5 ± 0.5 nm [6]. The composition of the FePt NPs was measured using EDS and was found to be Fe46Pt54. The mean diameters of Ag and FePt@Ag NPs were calculated from TEM images, with representative images shown in Figure 1 and were found to be 15.8 ± 3.0 nm and 15.9 ± 1.5 nm (Ag shell thickness is 5.7 nm), respectively. The core-shell structure was confirmed by X-ray diffraction, high-resolution TEM and scanning TEM analyses [6]. The localized surface plasmon resonance (LSPR) peak wavelengths for the Ag and FePt@Ag NPs are 399 and 412 nm, respectively [6]. Zero-field-cooled and field-cooled curves for the FePt and FePt@Ag NPs were measured using SQUID, and the blocking temperature TB was found to be approximately 52 K for both samples. Both NP samples clearly showed superparamagnetic behavior. The saturation magnetization (Ms) was estimated to be 11 emu/g for the FePt core NPs, while the Ms for the FePt@Ag NPs was 15 emu/g [6]. Figure 2a shows the Raman spectra of TP obtained using the Ag NP substrate in the absence and presence of an external magnetic field. These SERS spectra, both in the presence and absence of the
6
magnetic field were obtained at exactly the same position on the substrate. As shown in Fig. 2a, four distinct peaks are observed at 997, 1020, 1070, and 1572 cm 1. These peaks corresponded to S-H −
bending, in-plane ring deformation, C-S stretching, and C-C stretching vibrations, respectively [10,11]. The SERS activity of the Ag NP substrate under an external 55 mT magnetic field was approximately the same as that in the absence of the magnetic field. However, the SERS activity was significantly reduced when a 400 mT magnetic field was applied. Figure 2b shows the Raman spectra of TP obtained using the FePt@Ag NP substrate in the absence and presence of an external magnetic field. Under external magnetic fields with strengths of 55 and 400 mT, SERS activities of the FePt@Ag NP substrate were weakened as magnetic field strength increased. The degree of attenuation of the SERS activity under the magnetic field in the case of FePt@Ag NPs seems to be higher than that in the case of Ag NPs. To confirm that the results are beyond the range of experimental error, we measured the SERS spectra at four different positions on the Ag and FePt@Ag NP substrates with and without a magnetic field. Then, we estimated the experimental error for the SERS intensity at both primary (997 cm 1) and secondary (1070 cm 1) peaks, because these peaks are less affected by background −
−
fluorescence than the peak at 1572 cm 1. After the background subtraction, the SERS intensities at −
primary and secondary peaks were precisely calculated. Figure 3a and b show the SERS intensities at primary and secondary peaks plotted versus the external magnetic field strength, respectively. It is clearly seen that magnetic fields have a negative effect on the SERS activities for both Ag and FePt@Ag NPs (see Table 1). In addition, the following two points become evident: (1) The SERS
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intensity at both peaks linearly decreases with increasing magnetic field strength, which is consistent with previously reported results [8]; and (2) the degree of attenuation of the SERS activity under the magnetic field in the case of FePt@Ag NPs is roughly twice that for the case of Ag NPs. In order to explain these observations, an understanding of the magnetic contrast agents used for magnetic resonance imaging (MRI) technique provides insight. MRI enables one to obtain a cross-sectional view of a human body noninvasively by utilizing the nuclear magnetic resonance of protons that comprise a large percentage of the body. Superparamagnetic NPs, such as superparamagnetic iron oxide (SPIO) NPs, have been widely used as MRI contrast agents because they enhance the local magnetic field and significantly shorten the proton transverse (spin-spin) relaxation time (T2). As a result, the signal is reduced in the resulting proton density-weighted image or the T2-weighted image, leading to enhanced contrast and image resolution. These beneficial properties have led to the development of superparamagnetic NPs for clinical applications, including internally administered superparamagnetic NPs that are selectively ingested by Kupffer cells inside the liver, playing an important role in the detection of small metastatic liver tumors and/or hepatocarcinomas [12,13]. Similarly, superparamagnetic FePt cores can strengthen the local magnetic field at the site of plasmonic Ag shells when an external magnetic field is applied, and thus, the degree of the broadening of the energy gap for charge-transfer and a decrease in the electrons on the metal particle surface would be enhanced when compared to diamagnetic Ag NPs. This could lead to the higher degree of attenuation of SERS activity under the magnetic field in the case of FePt@Ag NPs. It
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is well known that there are two primary mechanisms of SERS: one is electromagnetic (EM) enhancement and the other is chemical (CM) enhancement. In the present experiments, EM enhancement effects that arise as a result of particle aggregation after applying a magnetic field can be essentially ruled out because the NPs are small and uniform, leading to an inherently dense (highly packed) layer of particles that forms during the drying process. This dense layer of NPs likely cannot aggregate further when subjected to a relatively strong magnetic field, which eliminates variation in EM effects induced by aggregation in this analysis. While there is still much debate on the mechanism for electron transfer observed in Raman analysis, the CM enhancement itself has been analyzed in many prior studies [7,8] and represents the best explanation for the observed decrease in Raman intensity under a magnetic field, as observed in this study. To confirm the hypothesis, the total magnetic field strength at the site of the FePt cores is estimated from the magnetization behavior of FePt@Ag NPs. Assuming that the degree of attenuation of Ag NPs is purely due to the external magnetic field, and the local magnetic field at the site of Ag shells is nearly equal to the sum of the external magnetic field and the magnetization of FePt cores (M), the SERS intensity at both 997 cm 1 and 1070 cm 1 was estimated by using the slope of the black −
−
curves in Fig. 3a and b and the SQUID data of FePt@Ag NPs (Fig. 3c). For example, the local magnetic field strength estimate at the site of Ag shells is 201.5 mT when the external magnetic field is 55 mT because M = 8.3 emu/g = 116.2 emu/cm3 = 146.5 mT (the mass density of FePt is 14 g/cm3 [14]). By using the slope of the black curve in Fig. 3a, the SERS intensity at 997 cm 1 under a 201.5 −
9
mT magnetic field is calculated to be 77 counts/s. Figure 3d shows a parity plot of the estimated and measured SERS intensities at both 997 cm 1 and 1070 cm 1. The comparison is good, indicating that −
−
the higher degree of attenuation of SERS activity in the case of FePt@Ag NPs is actually due to the strengthened local magnetic field at the site of plasmonic Ag shells. Finally, an experiment was carried out to test the impact on SERS intensity for the FePt@Ag NP substrate under conditions of magnetic field cycling, or repeated/prolonged laser (Raman) irradiation. First, SERS spectra were taken in the absence of a magnetic field, and then, were taken again under a 400 mT magnetic field. This cycle was conducted a total of two times to test the reproducibility. Figure 4a shows the SERS intensity at 997 cm 1 plotted versus the number of measurements (magnetic −
field on/off states). It is clearly seen that the phenomenon is virtually reversible within the limits of the experiment. If, by chance the NPs were undergoing aggregation under the magnetic field, this would not be reversible after removing the magnet because the NPs are essentially immobile on the dried substrate. As a result, we are able to observe the detrimental effect on CM enhancement when applying a magnetic field to our NP samples, which is not observable when EM enhancement effects dominate. However, if we continue to repeat this cyclic experiment further, the SERS intensity gradually decreases and the reduction becomes irreversible. The effect of repeated/prolonged laser irradiation from the Raman measurement on the resulting SERS intensity was studied next. In Figure 4b, the black curve shows the normalized SERS intensity at 997 cm 1 taken for a total of 6 −
measurement cycles of turning the Raman laser off and on (on/off cycles) under a static 250 mT
10
magnetic field. Data points were taken at the exact same position of the sample and the measurement was repeated multiple times at different points on the substrate. The SERS intensity gradually decreased with increasing the number of laser on/off repetitions decreasing down to 62±8 % of that of the first measurement. This is presumably because the TP molecules that are adsorbed onto the NP surfaces were degraded or desorbed due to prolonged radiation of the Raman instrument laser light. In order to separate the detrimental effect of prolonged laser irradiation of the sample, an experiment was conducted to test the effect of incrementally increasing the magnetic field strength on the SERS activity. The red curve in Figure 4b shows the normalized SERS intensity at 997 cm 1 taken for a total −
of 6 measurement cycles (laser on/off cycles) with data points taken at the exact same position of the sample with increasing the magnetic field strength in incremental steps of 20, 50, 100, 150, 200, and 250 mT for each cycle (also conducted at multiple points on the substrate). In this case, the SERS intensity decreased down to about 17±1 % of that of the first measurement. These data are summarized in Table 2. If one simply assumes that the 38±8 % reduction after 6 laser on/off measurement cycles is due to irreversible degradation (or desorption) of TP molecules and simply subtract it from the degree of attenuation with incrementally increasing the magnetic field (83±1 %), the degree of attenuation due purely to the magnetic field effect is calculated to be 45±9 %. This value is consistent with Fig 3a (average attenuation degree of 40.2 %) even considering that the degree of attenuation in Fig. 3a is overestimated because it includes the contribution of irreversible reduction of SERS intensity due to the laser irradiation.
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4. Conclusions Equal-sized Ag and FePt@Ag NPs were chemically synthesized and the effect of an external magnetic field on their SERS activities was investigated using TP as a Raman reporter molecule. In consequence, it was found that the SERS activity linearly decreases with increasing the magnetic field strength, and is reversible. The reduction ratio of SERS activity by the magnetic field in the case of FePt@Ag NPs is almost double that for the case of Ag NPs. The attenuation of SERS activity is due to the broadening of the energy gap for charge-transfer and a decrease of the electrons on the metal surface under the magnetic field. The more pronounced effect observed in FePt@Ag NPs than for Ag NPs likely arises from the superparamagnetic FePt core enhancement of the local magnetic field at the site of the plasmonic Ag shells when an external magnetic field is applied.
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References [1] G.A. Sotiriou, A.M. Hirt, P.Y. Lozach, A. Teleki, F. Krumeich, S.E. Pratsinis, Chem. Mater. 23 (2011) 1985. [2] E.D. Smolensky, M.C. Neary, Y. Zhou, T.S. Berquo, V.C. Pierre, Chem. Commun. 47 (2011) 2149. [3] J. Gao, B. Zhang, Y. Gao, Y. Pan, X. Zhang, B. Xu, J. Am. Chem. Soc. 129 (2007) 11928. [4] S. He, H. Zhang, S. Delikanli, Y. Qin, M.T. Swihart, A. Zeng, J. Phys. Chem. C 113 (2009) 87. [5] H.Y. Lin, Y.F. Chen, J.G. Wu, D.I. Wang, C.C. Chen, Appl. Phys. Lett. 88 (2006) 161911. [6] T.T.T. Nguyen, T.T. Trinh, K. Higashimine, D.M. Mott, S. Maenosono, Plasmonics (2013) DOI: 10.1007/s11468-013-9529-7 [7] X.K. Kong, Q.W. Chen, R. Li, K. Cheng, N. Yan, B.X. Yu, Chem. Commun. 47 (2011) 11237. [8] R. Li, Q.W. Chen, H. Zhang, X.K. Kong, Y.B. Sun, H. Zhong, H. Wang, S. Zhou, J. Raman Spectrosc. (2012) in press (DOI: 10.1002/jrs.4227) [9] T.T. Trinh, D. Mott, N.T.K. Thanh, S. Maenosono, RSC Adv. 1 (2011) 100. [10] H.Y. Jung, Y.K. Park, S. Park, S.K. Kim, Anal. Chim. Acta 602 (2007) 236. [11] X. Li, M. Cao, H. Zhang, L. Zhou, S. Cheng, J.L. Yao, L.J. Fan, J. Colloid Interface Sci. 382 (2012) 28. [12] R.C. Semelka, T.K.G. Helmberger, Radiology 218 (2001) 27. [13] M. Zhao, D.A. Beauregard, L. Loizou, B. Davletov, K.M. Brindle, Nat. Med. 7 (2001) 1241. [14] X.W. Wu, C. Liu, L. Li, P. Jones, R.W. Chantrell, D. Weller, J. Appl. Phys. 95 (2004) 6810.
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Figure captions
Fig. 1. TEM images of (a) FePt@Ag (mean size: 15.9 ± 1.5 nm) and (b) Ag NPs (15.8 ± 3.0 nm). In the case of FePt@Ag NPs, The mean size of FePt cores is 4.5 ± 0.5 nm and the Ag shell thickness is 5.7 nm.
Fig. 2. SERS spectra of TP molecules adsorbed on (a) Ag and (b) FePt@Ag NPs. Black, blue and red curves correspond to the SERS spectrum taken under 0, 55 and 400 mT magnetic fields, respectively. The insets show the SERS spectra expanded in the range of 985−1105 cm 1. −
Fig. 3. SERS intensities at (a) primary (997 cm 1) and (b) secondary (1070 cm 1) peaks plotted versus −
−
external magnetic field strength. Black and red colors represent SERS intensities taken by using Ag and FePt@Ag NP substrates, respectively. (c) Magnetization versus applied field curves for FePt@Ag NPs measured at 300 K. (d) Parity plot of estimated and measured SERS intensities at both 997 cm 1 −
(red) and 1070 cm 1 (blue) for FePt@Ag NPs. −
Fig. 4. SERS intensity taken at the primary peak (997 cm 1) for TP on the FePt@Ag NP substrate. (a) −
SERS intensity with and without magnetic field. First, a SERS spectrum was taken in the absence of a magnetic field, and then, it was taken again under a 400 mT magnetic field. This cycle was conducted a total of two times. (b) Black curve represents the variation of the normalized SERS intensity with number of repeated measurements (laser on/off cycles) taken at the exact same position of the sample under a 250 mT magnetic field. After 6 measurement cycles, the SERS intensity decreases down to 62% of that of the first measurement. Red curve represents the variation of the SERS intensity measurements by changing the magnetic field strength incrementally by 20, 50, 100, 150, 200, and 250 mT (with cycling the laser on and off at each point). Both types of measurement were performed 3-4 times at different positions of the same sample to estimate the experimental error, which is represented by the error bars shown in each graph. 14
Table 1 SERS intensities at primary (997 cm 1) and secondary (1070 cm 1) peaks under external magnetic fields of different field strength −
−
SERS intensity [Counts/s]
−1
SERS peak [cm ] 0 mT
55 mT
250 mT
400 mT
997
99.6±13.8
102.3±20.5
N/A
70.7±4.1
1070
95.6±12.1
92.5±18.5
N/A
72.1±3.6
997
95.7±10.7
82.8±16.6
57.2±20.0
38.3±8.2
1070
79.4±9.6
70.8±14.2
41.5±14.8
36.4±9.9
Ag
FePt@Ag
Table 2 Normalized SERS intensity at the primary peak (997 cm 1) for TP on the FePt@Ag NP substrate −
Fixed MF strength
Laser on/off measurement cycles
Variable MF strength
MF [mT]
SERS intensity [-]
MF [mT]
SERS intensity [-]
1
250
1.00±0.10
20
1.00±0.21
2
250
0.81±0.08
50
0.89±0.34
3
250
0.72±0.07
100
0.56±0.26
4
250
0.67±0.07
150
0.33±0.16
5
250
0.64±0.07
200
0.17±0.04
6
250
0.62±0.08
250
0.17±0.01 MF: Magnetic Field
15
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Highlights
• We study the SERS activity of FePt@Ag nanoparticles (NPs) under magnetic field. • The SERS activity linearly decreases with increasing the magnetic field strength. • The reduction of SERS activity is more pronounced for FePt@Ag than that for Ag NPs. • The more pronounced effect observed in FePt@Ag is due to the local magnetic field.
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