Probing the Location of 3D Hot Spots in Gold Nanoparticle Films

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Probing the Location of 3D Hot Spots in Gold Nanoparticle Films Using Surface-Enhanced Raman Spectroscopy Yuejiao Zhang, Shu Chen, Petar Radjenovic, Nataraju Bodappa, Hua Zhang, Zhi-Lin Yang, Zhongqun Tian, and Jian-Feng Li Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Analytical Chemistry

Probing the Location of 3D Hot Spots in Gold Nanoparticle Films Using Surface-Enhanced Raman Spectroscopy Yue-Jiao Zhang†,§, Shu Chen‡,§, Petar Radjenovic†, Nataraju Bodappa†, Hua Zhang†, Zhi-Lin Yang‡,*, Zhong-Qun Tian†, and Jian-Feng Li†,‖,* †MOE

Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China ‡Department of Physics, Xiamen University, Xiamen 361005, China ‖Shenzhen Research Institute of Xiamen University, Shenzhen 518000, China ABSTRACT: Plasmonic “hot spots” play a key role in surface-enhanced Raman scattering (SERS) enabling its ultrahigh surface sensitivity. Thus, precise prediction and control of the location of hot spots in surface nanostructures is of great importance. However, it is difficult to predict the exact location of hot spots due to complex plasmon competition and synergistic effects in three-dimensional (3D) multiparticle surface configurations. In this work, three types of Au@probe@SiO2 core-shell nanoparticles were prepared and a 3D hot spots matrix was assembled via a consecutive layer on layer deposition method. Combined with SERS, distinct probe molecules were integrated into different layers of the 3D multiparticle nanostructure allowing for the hot spots to be precisely located. Importantly, the hot spots could be controlled and relocated by applying different excitation wavelengths, which was verified by simulations and experimental results. This work proposes a new insight and provides a platform for precisely probing and controlling chemical reactions, which has profound implications in both surface analysis and surface plasmonics.

Surface-enhanced Raman scattering (SERS) is considered as a powerful analytical tool in biomedicine, environmental science, materials, catalysis, energy, etc. because of its extraordinarily high surface sensitivity, which can unveil chemical structures and reaction processes at the molecular level.1-6 The extraordinarily strong enhancement effect originates from surface plasmons (SPs), generated by collective oscillations of electrons induced by incident light.7-9 Particularly, ultrahigh enhancement can be generated in nanoparticle-nanoparticle junctions due to the near-field coupling effect between nanoparticles,10 resulting in the formation of hot spots that play a dominant role in SERS.11-12 Previously, it has been revealed that only ~6% of probe molecules adsorbed at the hot spots region in the nanostructures contribute 85% of the SERS signal, which is significant in trace analysis and single-molecule detection.13-14 However, it is a long-standing challenge to control the precise location of hot spots in nanostructures because of their small spatial volumes. In the past few decades, most of the works on hot spots has been focused on investigating electromagnetic enhancements using point-like 0-dimensional, linear 1-dimensional, or planar 2-dimensional structures, such as core-shell monomers, dimers, trimers, chains, aggregations or arrays.15-17 For example, Xu and coworkers obtained single-molecule Raman spectra of hemoglobin (Hb) protein molecules with the aid of the ultrahigh enhancement of hot spots generated in a dimer of Ag nanospheres.14 Van Duyne’s group studied multiparticle systems and achieved a strong enhancement.18 In such simple systems, hot spots are usually located between nanoparticles. Furthermore, the particle-metallic film system (gap mode system) has been proposed to satisfy the requirement of probing

molecules or reactions on single crystal surfaces.4,19-20 When a Au nanoparticle is located on a Au film, hot spot is generated between the nanoparticle and substrate. However, if there are two adjacent Au nanoparticles located on the Au film, hot spots will be located between nanoparticles as well as between the nanoparticle and substrate. Also, related theoretical and experimental works have discovered that the position of hot spots in the two nanoparticle on a metallic film substrate system can be tuned by changing the wavelength of the excitation laser.10,21 The positions of hot spots may be changed in different cases. For example, Moskovits group have researched the reversible tuning of SERS hot spots with aptamers,22 and the effect of the position of the hotspots due to laser illumination conditions have been confirmed by Lindquist.23 These hot spots are mainly 0-, 1-, or 2-dimensional (0D, 1D or 2D) geometries.24-25 To increase the sensitivity of SERS, more hot spots are needed, thus research into 3D hot spots structures is of great importance.26-27 Also, for quantitative analysis with SERS to be realized, the controllable assembly of SERS substrates is necessary.28 In the past few decades, the assembly of nanoparticles has been manipulated by controlling cooperative interactions between nanoparticles. Usually, the nanoparticles are assembled by the chemical interaction of capping ligands. For example, DNA is a type of widely used ligand to assemble dimer, trimer or special aggregations.29-30 And with the advent of DNA origami, DNA has been successfully used to construct 2D or 3D structures with great controllability.31 However it is a little bit complex and expensive to use DNA to construct 2D and 3D structures. Organic nanoparticles formed directly in organic media, or transferred from aqueous media with the addition of surfactants, can be simply used to form 2D arrays at

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the water/air interface using the Langmuir-Blodgett technique.32 In addition to the chemical method, lithography technology is another significant approach to constructing ordered 2D or 3D nanostructures.33 However, it is difficult to obtain accurate structures at the nanoscale due to technical issues.34 Xiong’s group used an evaporation-induced selfassembly strategy to generate a vertically aligned monolayer of CTAB-stabilized Au nanorods based on the near-equilibrium state in the internal region of drying droplets, which eliminated the complex ligand exchange reaction.35 And, they obtained vertical monolayer arrays with a nominal gap of 0.8 ± 0.3 nm. Though, this method can only be used in CTAB-stabilized nanoparticles and is difficult to be controlled. Recently, Duan et al. and Sun’s group reported that liquid metal-like films of Au nanoparticles could be obtained at the water/toluene interface by the introduction of ethanol, which pulls hydrophilic citrate-stabilized Au- nanoparticles into water/toluene interface, and leads to a close-packed monolayer.36-37 This method is very simple, convenient, and highly reproducible. 3D architectures can be assembled by the successive transfer of closely-packed nanoparticle monolayer films to a substrate via a consecutive layer on layer deposition procedure.38-39 In this work, Au@probe@SiO2 core-shell nanoparticles were prepared, and a 3D hot spots matrix was assembled by transferring closely-packed Au nanoparticle monolayer films via a consecutive layer on layer deposition method, with the aim of achieving a highly sensitive and uniform 3D SERS substrate. Different probe molecules were used to identify the exact location of hot spots within the 3D structure using SERS, while a SiO2 shell was used to protect nanoparticles and to avoid the movement of probe molecules. To probe the precise location of hot spots in the 3D system, three types of probe molecules were chosen with distinct Raman spectral features. The 3D hot spots matrix used was a three-layered structure consisting of three kinds of Au@probe@SiO2, as shown in Figure 1. Different excitation lasers were used to control the precise location of hot spots, which was verified experimentally and by simulations carried out with the finite element method (FEM). It was demonstrated that the transfer of hot spots between different layers depended on the laser excitation wavelength. This study provides a convenient way of probing the precise location of hot spots using SERS and is a promising technique for realizing the transfer and control of hot spots at the nanoscale, which will have significant implications in both surface analysis and surface plasmonics.

Figure 1. The scheme of the hot spots generated in the 3D Au@probe@SiO2 core-shell nanoparticles structure.

EXPERIMENTAL SECTION Chemicals and Materials. Sodium citrate, (3aminopropyl)trimethoxysilane (APTMS), sodium silicate solution, MBA (4-Mercaptobenzoic acid), NT (2Naphthalenethiol), and DTNB (5,5'-Dithiobis(2-nitrobenzoic acid)) were purchased from Alfa Aesar. HAuCl4, HCl, and HNO3 were purchased from Shanghai Reagent Corporation, China. All chemicals were used without further purification. Milli-Q water (18.2 MΩ cm) was used throughout the study, and all glassware were cleaned with aqua regia before use. Preparation of Au@probe@SiO2 nanoparticles. An aqueous solution of Au nanoparticles was prepared according to the Frens’ method.40 Briefly, 200 ml of 0.01% (weight/volume, w/v %) HAuCl4 was heated to boiling under strong stirring, then 1.4 ml or 6 ml of 1% sodium citrate was quickly added into the solution and refluxed for about 30 min. After cooling the reaction mixture, the aqueous solution of 55 nm or 16 nm Au nanoparticles was obtained. In the next step, the SiO2 shell was coated around the Au nanoparticles as similar to our previous reports.41 Briefly, 0.4 ml of 1 mM APTMS solution was added into 30 ml of Au nanoparticles. After 15 min stirring, 3.2 ml of 0.54% sodium silicate solution was added. At the same time, 0.4 ml of 0.1 mM probe molecule solution was added into the reaction mixture. After 5 min stirring at room temperature, the mixture was transferred to a 95 oC bath and further stirred for 30 min. At last, the reaction was stopped by transferring the mixture into an icewater bath to obtain the Au@probe@SiO2 nanoparticles. Preparation of the 3D nanoparticle film structure. The 3D SERS substrate was assembled by transfer of close-packed Au nanoparticle monolayer films via a consecutive layer on layer deposition method. The prepared Au@probe@SiO2 nanoparticles aqueous solution (30 mL) was repeatedly washed with water to remove the unassembled probe molecules. The preparation of nanoparticle film is shown in Figure 2A. 1 ml water and 1 mL cyclohexane were added to the Au@probe@SiO2 nanoparticles solution to form an immiscible water/cyclohexane interface. Then, ethanol was dropped into the mixture until a mirror-like reflection appeared, as shown in Figure 2A(i). The nanoparticles were destabilized by the addition of ethanol and gathered at the water/cyclohexane interface, displaying a shiny Au color. The shiny interface indicated that a majority of nanoparticles were closely packed at this stage. Then the solution in the tube was poured into a 25 ml beaker with 20 ml water. The shiny film would stay on the surface of the water. After the hexane was evaporated spontaneously, a solid substrate was dipped into the nanoparticle monolayer film at a small angle (5-100) and pulled out slowly. Finally, the nanoparticle monolayer film was transferred onto a solid substrate as shown in Figure 2A(ii). Once the nanoparticle film was dried, another type of Au@probe@SiO2 nanoparticle monolayer film containing a different probe molecule was transferred onto the substrate again. This process was repeated for the third time to coat the third type of Au@probe@SiO2 nanoparticle monolayer onto the substrate, leading to the generation of a 3D nanoparticle film structure with three types of probe molecules. Apparatus. Raman spectra were recorded on the Xplora Raman instrument equipped with a 1200 grooves/mm grating.


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Analytical Chemistry The laser was focused by a 50 microscope objective lens with an NA (numerical aperture) of 0.55. The excitation laser lines were 633 nm with 200 µW and 785 nm with 100 µW, and the power densities are 6.4  103 W/cm2 and 3.2  103 W/cm2. The acquisition time was 1 s. The morphology of the nanoparticles were characterized by transmission electron microscope (TEM) (JEM-2100) at 200 kV. And the assembled substrates were characterized using scanning electron microscopy (SEM) (HITACHI S-4800) at 15 kV. Numerical Simulation. The simulations were carried out with FEM (Commerical software package Comsol). In the simulations, the polarized plane-wave was used to mimic the experimental laser, which normally illuminates the samples. A periodic boundary was used in the x-axis (Figure 5 below) to simulate a periodic nanoparticle chain. Meanwhile, perfectly matched layer boundary conditions were used on the other two axises (y and z) to exclude nonphysical reflections. The dielectric function of Au was taken from the literature and the refractive index of the probe molecules was taken as 1.4 in our simulations.42

RESULTS AND DISCUSSION Characterization of the self-assembled 2D and 3D SERS substrates. The shell thickness of prepared Au@probe@SiO2 nanoparticles was about 1 nm, as shown in the inset of Figure 2B. UV absorption of Au@probe@SiO2 (Figure S1) shows that the probe molecules within the nanoparticle structure did not influence the optical properties of the Au nanoparticles.

Figure 2. (A) Schematic representation of the preparation of the nanoparticle film. Photograph of the self-assembled Au@probe@SiO2 nanoparticles (i) at the water/cyclohexane interface in a plastic centrifuge tube and (ii) on the Au film substrate. (B) SEM image of monolayer Au@probe@SiO2 nanoparticles film with inset showing corresponding TEM image of single nanoparticle. The cross-sectional view of the corresponding SEM images for the (C) monolayer (D) bilayer and (E) trilayer nanoparticle film.

Figure 2B shows the SEM image of the monolayer nanoparticle film, demonstrating that the Au@probe@SiO2 nanoparticles disperse uniformly on the substrate without any obvious cracking. Figure 2C, 2D, and 2E show the crosssectional SEM images of the monolayer, bilayer and trilayer nanoparticle films, respectively. The number of layers shown in the cross-sectional SEM images are consistent with the number of nanoparticle layers transferred to the substrate, indicating the successful fabrication of the 3D layered nanoparticle film. Careful fabrication of uniform SERS substrates is the key to obtaining the precise location of hot spots in the SERS substrate. Because the hot spots are strongly dependent on the distance between nanoparticles. To that end, we evaluated the uniformity and reproducibility of the self-assembled 2D SERS substrate and chose MBA as the probe molecule. Figure 3A shows SERS spectra of more than 100 points on a selfassembled 2D SERS substrate, demonstrating excellent signal reproducibility. Figure 3B shows the Raman intensity of the major MBA peak at 1587 cm-1from 50 spectra. The relative standard deviation (RSD) in intensity is 9.6%, indicating that this substrate has good reproducibility. More importantly, the 3D nanoparticle film structure also has good reproducibility. Figure 3C and 3D shows the 3D SERS spectra of the 3D nanoparticle film structure with three different probe molecules in different layers under 633 nm and 785 nm laser excitation, respectively. The spectra are clearly shown to be uniform with both wavelengths of laser excitation. While there are clear distinctions in the spectra between 633 nm and 785 nm. The reason for this will be described in later sections.

Figure 3. (A) SERS spectra of more than 100 points of an assembled 2D SERS substrate. (B) The intensities of the main Raman vibrations of MBA in 1587 cm-1 from 50 SERS spectra collected on the assembled Au nanoparticle substrate. (C), (D) 3D SERS spectra acquired at different points of the 3D nanoparticle



film structure with three different probe molecules under 633 nm and 785 nm laser excitation, respectively.

Probing hot spots in the prepared 3D film structure using SERS. Probe molecules MBA, NT and DTNB were used to distinguish the position of 3D hot spots. Firstly, the SERS

spectra of the three different probes were measured using Au@probe@SiO2 nanoparticle 2D monolayer films and the results are shown in Figure 4A and B (a, b and c) under 633nm and 785 nm laser excitation, respectively. Three characteristic peaks 1587 cm-1, 1380 cm-1, and 1333 cm-1 assigned to the three

Figure 4. Normalized Raman spectra of the 3D structure of 55 nm Au@probe@SiO2 nanoparticle film under (A) 633 nm and (B) 785 nm laser excitation. Raman spectra of three types of Au@probe@SiO2 monolayers, where the probe molecules are (a) MBA, (b) NT, and (c) DTNB. (C) The scheme of the 3D trilayered nanoparticle structure. The top, middle, and bottom layers of the 3D structure consist of MBA, NT and DTNB, respectively.

probe molecules, respectively could be clearly distinguished. These three molecules will not absorb light within the laser excitation wavelength range (Figure S2). And their signal under different laser excitations were similar, indicating the laser wavelength does not influence the probe molecules spectra (see Figure S3). To probe the location of hot spots, we fabricated the 3D structured Au@probe@SiO2 nanoparticle film consisting of all three types of probe molecules, shown schematically in Figure 4C. From the top to bottom, the corresponding probe molecules were MBA, NT, and DTNB, respectively. Figure 4A and 4B show the Raman spectra of the 3D structure at the same point under 633 nm and 785 nm laser excitation, respectively. The spectra of a, b, and c is the SERS signal of MBA, NT, and DTNB, respectively. From the SERS spectra of the 3D structure, the 633 nm signal is clearly consistent with MBA, which was in the top layer of the 3D structure. Whereas, the 785 nm signal included MBA and NT, which were in the top layer and middle layer, respectively. These results prove that under 633 nm laser excitation, hot spots possibly locate at the nanoparticle-nanoparticle junction of the top layer, while under 785 nm laser they may transfer to the nanoparticle-nanoparticle junction of the middle layer. In order to exclude the influences of some other experimental factors, for example, the penetration depth of lasers and the spectral interference from other layers, the structure with the MBA probe molecule absorbed on a gold substrate and assembled monolayer, two layers, and three layers nanoparticles, respectively, were constructed. Then the SERS spectra of these three structures were measured under 633 nm laser excitation (Figure S4). As shown in Fig. S3, we can clearly observe the SERS signal when there is one layer and two layers. That indicates the 633 nm laser can enough deep into the second layer. While in the 3D structure with three molecules, we obtain the signal mainly from the top layer at 633 nm and a weak signal from the second layer. This implies that the SERS signal from the second was affected by the hot spots. Which means that the hot spots may located at the top layer under 633 nm. To further understand the experimental results, theoretical simulations were performed with FEM. We considered a three

layered nanoparticle film (inset of Figure 5A) as a model system, which was simulated as a periodic structure in the xaxis. In the y-axis, we only considered one particle as the experimental laser line polarizes along the x-axis so that the near-field coupling effect in the y-axis is negligible and can be neglected. The diameter of the nanoparticle, thickness of the core and distance between two close nanoparticles was set as 55 nm, 1 nm and 0 nm in simulations. The incident light polarizes along the x-axis, in keeping with experimental results.

Figure 5. (A) Electric-field enhancement spectra of point A(green line), point B (pink line) and point C (red line), which are shown in the schematic of the 3D trilayered nanoparticle structure as an inset. (B) The relative enhancement spectrum of point A and B (defined as |EB/EA|). Numerical FEM simulations of the electric-field enhancement (defined as |E/E0|) distributions in the 3D structure at the xz-plane under (C) 633 nm and (D) 785 nm laser excitation.

Firstly, we studied the electric field enhanced spectra (Figure 5A) at the particle-particle junctions from each layer, namely, A, B and C shown in the inset of Figure 5A. Here, the electric field enhancement was defined as |Eloc/E0|, in which Eloc represents the electric field intensity at point A, B or C and E0 represents the incident electric field intensity. As is well known,


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Analytical Chemistry when light illuminates such a nanoparticle film, there can exist some plasmon modes which originate from the localized plasmon coupling effect between particles. In the electric field spectra of Figure 5A, we clearly observe 4 peaks, corresponding to 4 plasmon modes in such a periodic nanoparticle film. Considering the larger relative field enhancement of EB than EA and EC (the latter two are comparable), peak 1(P1) can be assigned to the plasmon coupling mode mainly from the middle layer. Peak 2 (P2) originates from the collective plasmon mode simultaneously generated in the top and bottom layers as the field at point A and C is much larger than that at point B. Peak 3(P3) could be the plasmon mode coming mainly from the middle layer but with less field enhancement compared with that of peak 1. Peak 4(P4) is a collective plasmon mode from top and middle layers because of the stronger electric field enhancement for point A and B. The electromagnetic field distribution (Figure 5C) under 633 nm laser line that close to the mode (P2) from nanoparticles at top and bottom layer clearly show that the hot spots are mainly located at the top layer and the bottom layer. While the electromagnetic field distribution (Figure 5D) under 785 nm laser lines are significantly different with hot spots mainly located in the middle layer (also at bottom layer but with weaker field enhancement). The hot spots distributions under 633 and 785 nm verified the above assignments for the plasmon modes originating in different layers. Furthermore, the hot spots distributions under 633 and 785 nm closely verify our above experimental results that Raman signals mainly originate from the top layer under 633 nm and from middle layer under 785 nm. It should be noted that hot spots in the bottom layer could be observed in FEM simulations, particularly using a 633 nm laser,

however no Raman signals from the bottom layer were detected experimentally. As is known, SERS is not only related to the field enhancement at the absorbed location of probe molecules, but is also decided by the scattering intensity that can be detected by the detector. Herein, this deviation away from the calculated results can be explained that in experiments the scattering from the bottom layer being blocked or scattered by top and middle nanoparticle layers so that the detected scattering signal is extremely weak. This was verified by additional experimental results. When MBA probe molecule absorbed on a gold substrate and assembled three layers nanoparticles (Figure S4), there is almost no Raman signals were visible under 633 nm laser excitation. These experimental and simulation results under 633 nm and 785 nm laser excitation clearly show that hot spots can be transferred between nanoparticle layers depending on the different plasmon modes. As we could not detect experimental signals from the bottom layer, we focused on the top two layers. We further plotted the relative enhancement spectrum of point A and B, which is defined as the ratio of electric field enhancement (namely, EB/EA) between point A and point B, as a function of wavelength (Figure 5B) to clarify the dependence of hot spots transference on the excitation wavelength. Based on this relative enhancement spectrum, hots pots would be expected to be mainly located in the middle layers in the 560 - 615 nm and 670 - 900 nm laser excitation wavelength ranges. While if the excitation laser is located in the 615 - 670 nm range, then the hot spots are mainly in the top and bottom layers.

Figure 6. Normalized Raman spectra of 3D trilayered structure of 16 nm Au@probe@SiO2 nanoparticle film under excitation of 633 nm and 785 nm laser. From the bottom to top, the corresponding probe molecules are (A), (B) MBA, NT, and DTNB, respectively, and (D), (E) MBA, DTNB, and NT, respectively. (C) and (F) are the schematic diagram of the SERS substrate for (A), (B) and (D), (E), respectively.

To confirm this phenomenon is universal, the 3D structure was reproduced using a different size of nanoparticles. In the

same way, we fabricated the 3D structured Au@probe@SiO2 nanoparticle film, however with 16 nm Au nanoparticles, and



the results were similar. In Figure 6A and 6B, the 3D structure was MBA, NT, DTNB from bottom to top (Figure 6C). Under 633 nm excitation, the signal mainly came from the top layer, which was DTNB. And with the 785 nm laser, the signal was a combination of DNTB and NT, which were in top layer and middle layer, respectively. Furthermore the order of the probe molecules in the 3D structure was changed to MBA, DTNB, NT from bottom to top (Figure 6F). With the 633 nm laser, the signal mainly came from the top layer, which was NT (as shown in Figure 6D). And with the 785 nm laser, the signal was a combination of NT and DTNB, which were in top layer and middle layer, respectively (as shown in Figure 6E). These results prove again that the hot spots will transfer between layers under different laser excitation wavelengths.

CONCLUSIONS In summary, we fabricated a highly sensitive and uniform 3D hot spots matrix by assembling Au@probe@SiO2 closelypacked nanoparticle monolayer films onto a substrate via a consecutive layer on layer deposition method. Three types of Au@probe@SiO2 with different probe molecules (MBA, NT, and DTNB) were utilized to identify the exact location of hot spots in the 3D trilayered structure. Two different excitation lasers (633 and 785 nm) were used to control the precise locations of hot spots and the experimental results were consistent with numerical simulation results. The transfer of hot spots between different layers was demonstrated to depend on the wavelength of the excitation laser. It is promising to realize the transfer of hot spots at the nanoscale and this approach provides a convenient platform to probe the location of hot spots in SERS substrates in future, which will have profound implications in both surface analysis and surface plasmonic.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: UV-vis absorption spectra of Au@probe@SiO2 nanoparticles and three molecules, SERS spectra of probe molecules under different lasers, and SERS spectra of the 3D structure with only the probe molecule present in one layer (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions §These authors contributed equally.

Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21775127, 21521004, 21802111, 21522508, and 21673192), Natural Science Foundation of Guangdong Province (2016A030308012), and Basic Research Projects of Shenzhen Research & Development Fund (JCYJ20170306140934218).

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Probing the Location of 3D Hot Spots in Gold Nanoparticle Films

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