Waveguide-Enhanced Surface Plasmons for Ultrasensitive SERS

Aug 26, 2013 - ABSTRACT: We design an ultrasensitive surface-enhanced Raman scattering (SERS) substrate based on waveguide-enhanced surface ...
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Waveguide-Enhanced Surface Plasmons for Ultrasensitive SERS Detection Yuejiao Gu,† Shuping Xu,† Haibo Li,† Shaoyan Wang,† Ming Cong,† John R. Lombardi,‡ and Weiqing Xu*,† †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China Department of Chemistry, City College of New York, New York, New York 10031, United States

‡

S Supporting Information *

ABSTRACT: We design an ultrasensitive surface-enhanced Raman scattering (SERS) substrate based on waveguide-enhanced surface plasmons (SPs). An optical waveguide was exploited to concentrate and restrict the electromagnetic (EM) energy of the incident light, and Ag nanoparticles that were assembled on the waveguide surface were used to enhance the EM field further by means of SP resonance. The enhancement factor (EF) of the incident EM field can reach 103 on the two sides of nanoparticles, and a 108− 1012 EF of SERS is expected. This waveguide-assisted isolated nanoparticle substrate can reach a comparable SERS enhancement capability to that of gap-type SERS hot spots. In addition, this SERS substrate is applicable to the SERS detection of large molecules (biomacromolecules etc.), which cannot be placed in traditional gap-type hot spots.

SECTION: Spectroscopy, Photochemistry, and Excited States

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(SPs) resonance from an isolated metal nanoparticle placed within the concentrated EM field, which further enhances the local EM field. According to the finite-difference time domain (FDTD) simulation results, an optimized waveguide can enhance the local EM field about 75 times, and an isolated Ag nanoparticle can further enhance it about 20 times. Thus, EM field enhancement of more than 1000 times can be obtained by combining waveguide resonance and SP resonance. According to the two-fold SERS enhancement mechanism,17−19 a 108−1012 enhancement factor (EF) of SERS is expected on this waveguide-enhanced SP substrate. This novel substrate has a comparable SERS enhancement capability to gap-type hot spots. It is much easier to prepare, avoiding the fine control of structure size that is required in gap-type and tip-type hot spots, and its SERS enhancement capability can be well controlled. This design facilitates the realization of a new generation of SERS substrates with high SERS performance. The waveguide-enhanced SP model is shown in Figure 1a. A semicylindrical prism was used to couple the incident light (532 nm laser) into the SiO2 waveguide (550 nm in thickness). A 25 nm Ag film, which is an optimized thickness to improve the coupling efficiency of incident light into the waveguide, was added between the waveguide and the prism. Several resonance modes can be formed in this waveguide, such as TE0, TE1, and TE2 modes (TE presents s polarization direction, which is

urface-enhanced Raman scattering (SERS) is an extremely sensitive technique that is continuously gaining interest as a rapid, efficient, noninvasive tool for biochemical detection and analysis.1−3 Detection sensitivity and reproducibility are considered to be two critical issues in SERS measurements. In the enhancement mechanism of SERS, hot spots, which are regions with a high intensity of an electromagnetic (EM) field, play a dominant role.4,5 Hot spots are usually obtained at the interstitial sites between metallic particles or at locations outside of sharp surface protrusion, such as in the gap of aggregated colloids,6,7 at junctions of fabricated nanostructures,8−12 as well as on the apexes of metallic tips.13 A small change of the gap size between nanoparticles14,15 (usually less than 5 nm for the maximum enhancement) or the conicity of tips16 can result in large impact on the SERS enhancement capability. However, the accurate control of these parameters (e.g., the gap size) is complicated and difficult. Thus, it is still a challenge to control well the SERS enhancement capability on the gap- or tip-type SERS substrate. To guarantee the SERS performance, controllable “hot spot” constructions are adopted frequently.11,12 The nature of hot spots lies in achieving a high concentration and resonance enhancement of the EM field. Distinguished from the traditional hot spots formed in gaps or tips, we utilize an optical waveguide combined with a metallic nanoparticle to concentrate and enhance the local EM field for SERS excitation. This substrate combines two effects. One is optical waveguide resonance, which is exploited to harvest light, forming a concentrated EM field. The other is local surface plasmons © XXXX American Chemical Society

Received: July 18, 2013 Accepted: August 26, 2013

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Figure 1. (a) Schematic diagram of the waveguide-enhanced SP SERS substrate. (b) SEM image of the Ag nanoparticles assembled on the SiO2 waveguide surface.

vertical to the incident plane).20,21 At the resonance condition, the energy of incident light can be concentrated in the near field, generating an enhanced evanescent field near the waveguide surface.22 Ag nanoparticles were assembled by a polyelectrolyte (poly(diallyldimethylammonium chloride), PDDA) on the surface of the SiO2 waveguide to enhance the local EM field further. SERS signals of analytes adsorbed on Ag nanoparticles will be detected at the bottom of the waveguide. The morphology of Ag nanoparticles on the waveguide surface is shown in Figure 1b. Ag nanoparticles with a size of 40−70 nm are isolated on the surface, and rare aggregates are observed. The resonance modes in the optical waveguide were studied via the angle-dependent reflection spectra and the FDTD simulations. Figure 2a shows the angle-dependent reflection spectra on the waveguide. TE0, TE1, TE2, and TE3 modes can be clearly observed in the visible spectral range. The cross section of the angle-resoled reflectance spectra at 532 nm and the simulated resonance curves are shown in Figure 2b. Obvious absorption of the incident light can be found, indicating a high light-harvesting efficiency. The EM intensity distributions of TE0, TE1, and TE2 modes are shown in Figure 2c. A 50−160 fold enhancement of the incident EM field was obtained at these three resonance modes. Considering that Ag nanoparticles are only assembled on the surface of the waveguide, the TE2 mode was chosen due to its highest intensity of the evanescent field (∼75 times enhancement) among three resonance modes. It is known that a single metal nanosphere can only generate very limited SERS enhancement.23,24 Generally, a Ag nanoparticle with the diameter of 50 nm can enhance the E intensity about 20 times at a 532 nm wavelength (see Supporting Information (SI) part 2). However, if the Ag nanoparticle is located in the waveguide-concentrated EM field (the schematic diagram is shown in Figure 3a), the enhancement of the local field can reach more than 1000 times, which approximately equals the product of the EFs of both the waveguide and nanoparticle (see Figure 3b and c). This improved EM field may generate a comparable enhancement capability to the hot spots in gaps or tips. It should also be noted that the strongest EM field is located at the two sides of the Ag nanoparticle. Therefore, analytes with large size (such as proteins, DNA, polymers) can be placed in these regions for high-sensitivity SERS detection. It overcomes the limits of the traditional hot

Figure 2. (a) The angle-dependent reflection spectra on the silica waveguide (without nanoparticles). (b) The waveguide resonance curve at 532 nm. The red line is the experimental result (the cross section along the red dashed line in (a)). The blue line is the calculation result by the FDTD method. (c) The electric field distributions in the waveguide at the TE2, TE1, and TE0 resonance conditions (from left to right), respectively.

spots, for instance, the difficulty of analytes to access the narrow gaps. SERS experiments were carried out to demonstrate the enhancement capability of the waveguide-assisted isolated nanoparticle SERS substrate. First, we compared the SERS performance at different resonance modes to optimize the detecting conditions. The detection geometry is as shown in Figure 1a. A 10 μL volume of a 4-mercaptobenzoic acid (43154

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Figure 3. (a) FDTD simulation model of the waveguide-assisted isolated nanoparticle SERS substrate. The diameter of the Ag nanosphere is 50 nm. The incident angle is 42.2°, which is the resonance angle for the TE2 mode. (b) Electric field distribution in the waveguide and near the nanoparticle at the YZ plane. The dashed lines show the boundaries of different layers. (c) Enlarged magnification of he electric field distribution around the Ag nanosphere. The E intensity is given by a logarithmic scale color bar.

Figure 4. (a) The SERS spectra of 4-MBA obtained on the waveguide-assisted isolated nanoparticle SERS substrate at the TE2 resonance mode (black line), TE1 resonance mode (red line), and no resonance condition (blue line). (b) Angle-dependent reflection curve of the SERS substrate and the plot of the SERS intensity of 4-MBA at 1582 cm−1.

MBA) aqueous solution (1.0 × 10−5 mol/L) was dropped on the substrate and dried. The SERS spectra of 4-MBA at resonance and nonresonance conditions were recorded and are compared in Figure 4a. It can be found the SERS intensities have been greatly enhanced at the resonance conditions. The SERS enhancement capability of the TE2 mode is larger than that of the TE1 mode, which agrees with the simulated results that the TE2 mode supports higher EM field enhancement at the waveguide surface (in Figure 3). To show this phenomenon more clearly, the angle-dependent SERS intensities at 1582 cm−1 and the waveguide resonance curve (with Ag nanoparticles on it) are plotted in Figure 4b, which further proves that the TE2 mode supplies ∼2 times stronger SERS intensity than the TE1 mode. On the basis of both the simulated and experimental results, the TE2 resonance mode was adopted for the following SERS detections. The SERS performances on the waveguide-assisted isolated nanoparticle SERS substrate and that on the self-assembled nanoparticle substrate are compared in Figure 5. The insets of Figure 5a show the detection geometries. Geometry I presents

the dual contributions from the waveguide and Ag nanoparticles. Geometry II displays the individual contribution from Ag nanoparticles. The SERS intensity of 4-MBA (1.0 × 10−7 mol/L) at the waveguide detection geometry is over 210 times higher than that at the direct detection geometry (Figure 5a), indicating that the addition of the waveguide can improve the SERS EF by at least 2 orders. An average SERS EF equaling 1.5 × 108 was obtained on the waveguide-assisted nanoparticle substrate (details are shown in SI part 3), which is comparable to the SERS enhancement level of the “hottest sites” (the EF is larger than 109) in the literature.4 Figure 5b shows the SERS detections of -MBA at different concentrations using the waveguide-assisted nanoparticle SERS substrate. The lowest probed concentration of 4-MBA is 1.0 × 10−8 mol/L (the signal-to-noise ratio is about 13 at 1582 cm−1 band). It is worth mentioning that the SERS substrate that we used is a nonoptimized prototype, and great improvements can be achieved after the optimization of this model. For example, the shape and size of the metal nanoparticles can be optimized to match the incident light, and further improvement of SERS 3155

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METHODS

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ASSOCIATED CONTENT

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Preparation of the Waveguide-Assisted Isolated Nanoparticle SERS Substrate. (1) Preparation of the SiO2 waveguide: A glass slide (BK7) with the size of 25 × 12 × 1 mm was cleaned with ethanol and deionized water. It was dried using N2, and then, a 25 nm thick Ag film was deposited on it by vacuum evaporation (Beijing Technol Science Co. Ltd., China) to improve the coupling efficiency. After that, a 550 nm thick SiO2 waveguide layer was deposited on the Ag surface by plasma-enhanced chemical vapor deposition (Oxford Plasmalab System 100 PECVD). (2) Immobilization of Ag nanoparticles on the waveguide: An electrostatic self-assembly method was employed to immobilize Ag nanoparticles on the waveguide substrate. The waveguide substrate was immersed in PDDA aqueous solution (0.5%) for 5 min. After being cleaned by deionized water, the substrate was immersed in a Ag colloid for 30 min for Ag nanoparticle assembly. A Ag colloid with a nanoparticle size of ∼50 nm was synthesized by the Lee method.27 The waveguide substrate with Ag nanoparticles was characterized by scanning electron microscopy (SEM, JEOL JSM-6700F). SERS Detection. 4-MBA was used as a SERS probe. Ten 10 μL of 4-MBA solution was dropped on the SERS substrate, forming a spot with a diameter of 6 mm after the solution was dried. The SERS detection was performed on a lab-made Raman spectrometer, which has been reported in our previous work.28 The SERS detection schematic diagram is shown in Figure 1a. The glass slide was pasted to a semicylindrical prism by a refractive index matching fluid (refractive index = 1.515, Shanghai Specimen and Model Factory, China). A 532 nm spolarized laser was used to excite SERS. The incident angle can be tuned via a goniometer. The SERS signal was collected under the prism using an objective lens with NA = 0.35 and detected by the monochromator (iHR320, Jobin-Yvon Co.) and CCD (Synapse, Jobin-Yvon Co.). FDTD Simulations. The FDTD simulations were carried out by using the FDTD solution software (Lumerical Solutions, Inc.). The simulation models are shown in Figures 2c and 3a. The parameters in the simulation are as the same as those from the experiments. To simulate the resonance angle (Figure 2b) of the waveguide, a 532 nm TE plane wave light source was used, and the incident angles were scanned from 40−72° with a step of 0.2°. The simulation area was 0.01 × 0.01 × 3.4 μm on the X, Y, and Z axes. The boundary condition was bloch on the X axis, period on the Y axis, and PML on the Z axis, and the mesh was about 10 nm. EM field distributions (Figure 2c) of the waveguide were obtained at the resonance angles for the TE3, TE2, and TE1 modes. To simulate the EM field distribution of the waveguideassisted isolated nanoparticle model (Figure 3), a 532 nm TE plane wave light source with the incident angle equal to the TE2 resonance angle (44.2°) was used. The simulation area was 0.2 × 0.2 × 3.4 μm on the X, Y, and Z axes. The boundary condition was bloch on the X axis, period on the Y axis, and PML on the Z axis, and the mesh was about 10 nm for the waveguide and 1 nm for a Ag nanoparticle.

Figure 5. (a) The SERS spectra of 4-MBA (1.0 × 10−7 mol/L) under the waveguide excitation (I) and under direct excitation (II). The SERS intensity under direct excitation was multiplied by 30 to be shown more clearly. The insets show the SERS detection geometries on the waveguide-assisted isolated nanoparticle SERS substrate (I) and that on the self-assembled nanoparticle substrate under direct excitation (II). (b) The SERS spectra of 4-MBA at different concentrations obtained on the waveguide-assisted isolated nanoparticle substrate. The laser power was 7 mW, and the integration time of the CCD was 10 s.

performance would be achieved. Even the traditional SERS hot spots, the nanoparticle dimer, can be improved by this detection geometry for ultrasensitive SERS detection. For a Ag nanoparticle dimer (50 nm in diameter) with a 1 nm gap, the enhancement of the incident EM field intensity can reach 104 by exploiting the waveguide to concentrate the EM field in advance (see SI part 4), and a 1011−1016 single-molecule SERS EF would be expected.15−17,25,26 Such strong EM enhancement has crucial significance in the field of single-molecule SERS detection. In conclusion, we propose a strategy to achieve a comparable strong EM field as SERS hot spots, in which a waveguide is employed to concentrate the incident EM field in the near field and an isolated nanoparticle further enhances the EM field to a level of 103. The SERS EF on this waveguide-assisted isolated nanoparticle SERS substrate is over 108, which is comparable to the “hottest sites” SERS enhancement level. The addition of the waveguide can improve the SERS EF by at least 2 orders according to the FDTD simulation and experimental results. In addition, different from the traditional hot spots formed in a very small gap, the strongest EM field is located at two sides of the nanoparticles. This facilitates the attainment of highsensitivity SERS detection for chemical compounds and biomacromolecules (e.g., proteins and DNA).

S Supporting Information *

(1) The resonance curves of the optical waveguide before and after Ag nanospheres assembly; (2) FDTD-simulated EM field 3156

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distribution of an isolated nanoparticle; (3) the calculation of the SERS EF; and (4) the SERS EF of a Ag dimer under a waveguide-concentrated field. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-431-85159383. Fax: 86-43185193421. Address: State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Ave., Changchun 130012, P. R. China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Instrumentation Program (NIP) of the Ministry of Science and Technology of China (Grant 2011YQ03012408) and the National Natural Science Foundation of China (NSFC) (Grants 21073073 and 91027010).

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