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. 2018 Oct 30;3(10):14399–14405. doi: 10.1021/acsomega.8b01153

Synthesis of Spiked Plasmonic Nanorods with an Interior Nanogap for Quantitative Surface-Enhanced Raman Scattering Analysis

Yang Zhang †, Chen Li ‡, Zahra Fakhraai ‡, Basem Moosa †, Peng Yang †, Niveen M Khashab †,*
PMCID: PMC6645439  PMID: 31458127

Abstract

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Realizing quantitative surface-enhanced Raman scattering (SERS) analysis is extremely helpful and challenging. Here, we utilize a facile method to synthesize spiked plasmonic nanorods with an interior gap. The Raman signal from the molecules embedded in the gap can be dramatically enhanced, leading to strong, stable, and reproducible SERS signals that can be used as an internal reference for quantitative SERS analysis. We demonstrate that the rough exterior surface has a good performance in enhancing the Raman signal of polycyclic aromatic hydrocarbon molecules adsorbed on the surface. The result shows that this method is applicable for a large range of analyte concentrations and there is an excellent linear relationship between the SERS intensity ratio and the analyte concentration (0.5–100 μM).

Introduction

Surface-enhanced Raman scattering (SERS) has been recognized as a powerful protocol that is capable of detecting molecular vibrations, which can be viewed as fingerprint information on various organic molecules, biomolecules, and cells and tissues.1−3 SERS is an ultrasensitive method, as the Raman signal can be dramatically enhanced as much as 1010 to 1011 times in some plasmonic nanostructure,4 which allows to detect even one single molecule.4−7 Compared with the conventional techniques such as fluorescence, electrochemistry, and high-performance liquid chromatography, SERS holds significant advantages.8−12 For example, higher resolution on multiplex samples as SERS provides fingerprint signatures of analytes endowing with better anti-interference resistance. Generally, SERS enhancement is ascribed to electromagnetic and chemical enhancement, and the former contributes to most of SERS enhancement. The electromagnetic enhancement is localized at the “hotspots” resulting from the coupling of the localized surface plasmon resonance (LSPR) on the surface or in the junctions of plasmonic nanoparticles.13−16 Optimizing the structure of plasmonic nanoparticles including size, shape, surface morphology, and composition is important in improving the reproducibility and sensitivity of SERS assays. The typical approach is to use the target molecule to induce the aggregation of nanoparticles forming a large number of hotspots in solution.8,17 However, problems still remain due to the random and poor reproducibility of the hotspots, resulting in lack of quantitative SERS data and wide distribution of enhancement factors.13 In addition, the operational and instrumental factors also have a profound influence on the Raman signal.17 Therefore, to date, it is still a great challenge to realize quantitative SERS analysis.18−20

In previous studies, researchers have attempted to use internal standards to achieve sensitivity and reproducibility simultaneously.21−26 However, the internal standard molecules can be influenced by the microenvironment of the solution and compete for the surface adsorption, leading to the fluctuation of the intensity and frequency.1,27,28 Recently, a new structure based on core–molecule–shell nanoparticles has captured great attention.4,29,30 The interior gap between the core and the shell provides controllable hotspots and uniformity for SERS enhancement.4,31 The Raman molecules inside the gap could generate a highly stable, strong, and quantitative SERS signal. In particular, DNA, polymer-functionalized spherical Au nanoparticles, and galvanic replacement reaction have been reported to synthesize these nanostructures with nanometer interior gaps, exhibiting strong and stable SERS signals.4,29,30,32,33 For example, it was shown that galvanic replacement reaction between silver and gold facilitates the formation of gold nanorods (GNRs) with uniform interior nanogaps with a stable, strong, and reproducible SERS signals.32 Other methods such as small molecule-interlayered plasmonic structure and SiO2 interlayered gold structures also have been used to synthesize nanostructures with interior gaps.34−36 The standard molecule inside the gap between the core and shell was in a safe environment without perturbation from outside, and the surface of the shell was free without competitive adsorption. However, for these structures, they still have some shortcomings, such as laborious synthetic procedures and long preparation times. More importantly, all of these structures reported so far have smooth surfaces. It has been shown that the most important factor affecting SERS intensity is the shape of the nanoparticles.37 The shapes with tips and edges can localize the plasmonic near field and create hotspots around the edges.38−40 The plasmonic nanostructures with more tips and edges will provide higher sensitivity in SERS analysis.41,42 Up to now, there has been no method to synthesize such spiked plasmonic nanorods with an interior gap, without the need to use DNA templates, for direct quantitative SERS analysis. Herein, an efficient method to prepare plasmonic substrates with spiked surfaces and interior gaps by employing a polydopamine internal shell is reported. This structure is very reliable and suitable for quantitative SERS analysis, as the spiked surface provides a higher surface enhancement in a variety of samples. We used 4-mercaptopyridine (4-mp) as an internal standard in the nanogap (Scheme 1). This is a label-free method with high sensitivity and was successfully tested to detect polycyclic aromatic hydrocarbons (PAHs) quantitatively. The limit of detection is up to 0.3 μM. Finite-difference time-domain (FDTD) simulation is also used to confirm these results.

Scheme 1. Schematic Illustration of the Synthesis of the Plasmonic Core–Shell Structure, Based on GNRs, for Quantitative SERS Analysis of PAHs.

Scheme 1

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Results and Discussion

Only three steps are needed to synthesize the GGN with interior gaps. Herein, we introduced pristine GNRs as the core (Figure S1), of which the surface was modified by 4-mp as internal molecules via Au–S bonds. Afterward, the mpGNRs were treated with dopamine (DA) and tris buffer (pH = 8) for 1 h to yield poly-DA-coated species (mpGNRs@DA). Noted that, under alkaline conditions, phenolic catechols were oxidized to quinones, causing the formation of 5,6-dihydroxyindolines, their derivatives, and then leading to the polymerization to form oligomers. These were crammed by π–π stacking, charge transfer, and hydrogen bonding to form a nanometer thick poly-DA layer.43 As shown in Figure S2, we can see that there is a thin layer of poly-DA outside the GNRs and the thickness is about 2.6 nm. These functional groups such as catechol, amine, and imine can serve as both an anchor and reaction sites for loading metal ions,44,45 and the catechol takes the role of a reducing agent for gold nucleation and growth.46 Catechol was oxidized to quinones by Au(III), and small gold nanoparticles were produced on randomly oriented oxidized sites of the poly-DA shell. Further anisotropic growth of gold nanoparticles were activated by the reduction of HAuCl4 by hydroxyl amine which was subsequently added.10 From the transmission electron microscopy (TEM) and scanning electron microscopy images (Figures 1a and S3), we found that the GGNR structure is uniform and the surface of gold shell is rough and spiky. We applied scanning TEM (STEM) to demonstrate the core–shell structure with an interior gap, as expect, the spatial distribution of the Au core, the spiked gold shell, and the interior gap could be clearly identified (Figure 1c,d). It is worth mentioning that the amount of HAuCl4 plays an important role in the synthesis. The density of the small gold nanoparticles increases by raising the amount of HAuCl4 (Figure S4). At first, there are some sporadic gold nanoparticles on the surface; then, it becomes a whole shell. In case more HAuCl4 added, the particles will become bigger and smooth (Figure S4c).

Figure 1.

Figure 1

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(a) TEM image of GGN with interior gaps. (b) High-resolution transmission electron microscopy image of a typical GGN. (c) STEM image of GGN with interior gaps. (d) STEM image a typical GGN.

The SERS properties of GGN were tested by three laser colors (785, 660, and 532 nm) (Figure S5). Excited by 785 nm laser, GGN shows the most glaring response in the near infrared region. In the case of 532 nm, little SERS signal was observed, by contrast to 785 or 660 nm, where the SERS signals were raised enormously. This is because 532 nm is outside the peak region of the UV–vis spectrum of nanorods (Figure S6). Under laser irradiation, there is no measurable changes in time sequence SERS spectra of GGN (Figure S7). This leaves out the laser trapping effect on the aggregation of particles.1,47 From these experimental results, sufficient and stable SERS signals were obtained with excitation at either 785 nm or 660 nm.

We used pyrene (Pyr), anthracene (Ant), and naphthalene (Nap) as target molecules that are PAHs to verify the quantitative SERS analysis. The PAH molecules can be adsorbed on the surface of GGN. In the adsorption process, the time-dependent SERS spectra exhibited stable signals in 1 h after the addition of PAH molecules (Figure S8). The Pyr, Ant, and Nap all have a few characteristic peaks as compared to 4-mp, and the unique peaks for 4-mp are located around ∼1000 cm–1 (Figure 2). It has been experimentally proved that nearly no Raman signals of Pyr, Ant, Nap, and 4-mp in water were found even at a high concentration level (3 × 10–4 M). However, the SERS signals of Pyr, Ant, Nap, and 4-mp mixed GNRs were remarkably enhanced (Figure 2). As a result, it could be concluded that the SERS signals were generated as these molecules reside close to the particle surfaces but not from the molecules themselves during the detection.

Figure 2.

Figure 2

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Raman spectra of Pyr (a), Ant (b), Nap (c), and 4-mp (d) powder, in water and mixed with GNRs. The spectra were obtained by 785 nm laser.

Moreover, the as-synthesized GGN was mixed with Pyr of various concentrations for quantitative SERS analysis. After incubation of 1 h with the addition of Pyr, two new peaks of Pyr have been observed at ∼590 and ∼1400 cm–1. The SERS intensity of Pyr fluctuated with parallel samples, but the reproducibility can be improved by normalizing the data to that of 4-mp (at ∼1000 cm–1) (Figure S9). By lowering the concentration, the intensity of the characteristic peaks of Pyr declines correspondingly (Figure 3a). In order to correct the fluctuations in the instrumental factors and other unknown factors, the Pyr signals (at ∼590 cm–1) were normalized to the SERS intensity of internal molecules 4-mp (at ∼1000 cm–1). The intensity ratio of the target molecule (Pyr) to internal standard molecule shows a good linear relationship with the Pyr concentration (Figure 3b). The same operation was used for Ant and Nap. As depicted in Figure 3c, we can find that the peak intensities at ∼400, ∼750, and ∼1400 cm–1 are all decreasing along with the concentration of Ant going down; the normalized peak at ∼1400 cm–1 to that of 4-mp was used for quantitative detection. The relative SERS intensity versing the Ant concentration follows a very nice curve (Figure 3d). We further applied the GNN to the quantitative detection of Nap (Figure 3e,f). The linearity of these working cures demonstrated a larger range of the analyte concentration, where the highest concentration is 100 times larger than the lowest concentration. From the linear working curve, the minimum response value of Pyr, Ant, and Nap is lower than that has been reported in detecting PAH molecules.48,49

Figure 3.

Figure 3

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SERS spectra of GGN mixed with different concentrations of Pyr (a), Ant (c), and Nap (e) as well as the linear relationship of SERS intensities vs different concentrations of Pyr (b), Ant (d), and Nap (f). The spectra were obtained by 785 nm laser.

To better understand the optical properties of the nanorod structures and verify the role of roughness through the far-field LSPR peak on the wavelengths with observed Raman enhancements, we performed UV–vis spectroscopy and compared our data with FDTD simulations for GGN. Figure 4a shows the UV–vis spectra as compared with the calculated far-field extinction cross sections of the two nanoparticles obtained by single-particle FDTD simulations. The calculated far-field extinction cross-sectional spectra for GGN were calculated for two polarization directions, along the long axis of the rod (longitudinal) and normal to the long axis (transverse). The larger extinction cross section at this wavelength for GNN contributes to the larger enhancement factor measured for these particles in Table 1. To explore the role of the roughness and extinction cross sections in the high performance of GGN as a SERS substrate, the near-field distribution around the modeled nanorod structures was calculated. Near-field enhancement contributes to Raman enhancement, and resonant optical extinction seen from the far field creates the strongest near-field enhancement, so we calculate near field at that wavelength. It has been reported that the SERS enhancement factor can be estimated from a term defined as the product of the electric near-field intensity at the excitation and Stokes frequencies.50−54Figure 4b shows the calculated near-field distribution for an excitation wavelength of 785 nm and a Stokes wavelength of 824 nm (corresponding to a 600 cm–1 Raman shift) around a modeled GGN under longitudinal excitation. It can be seen from this structure that the plasmon resonance creates hotspots both inside the gap layer and at the shell surface. Whereas the gap hotspots appear to be stronger in intensity, the surface hotspots occupy a larger area, which also greatly affects the measured SERS enhancement. To quantify the relative contributions from these two types of hotspots, summation of the value over the corresponding regions was evaluated and shown in Table 1. This overall enhancement in the detection ability of GGN is consistent with the experimental observation of the higher sensitivity.

Figure 4.

Figure 4

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FDTD simulation results for a single simulated GGN structure. (a) Calculated far-field extinction spectra under longitudinal (blue) and transverse (red) incident polarizations, in comparison with experimentally measured UV–visible spectra (black, right axis). (b) Calculated near-field (IexcIsto) distribution around the modeled nanorod at a Raman shift of 600 cm–1. The value at each position was calculated as the product of the electric field intensity at the excitation (785 nm) and Stokes (824 nm) wavelength.

Table 1. Total Values at All Raman Wavelengths Around a Modeled GGNR Structure and the Relative Contributions from Two Regions with Strong Hotspots Observeda.

  GGN
Stokes wavelength (nm) (Raman shift) total E4 (IexcIsto) from outer surface from inner gap
824 (600 cm–1) 38.69 30.86 7.83
852 (1000 cm–1) 33.00 24.73 8.27
882 (1400 cm–1) 29.86 20.43 9.43
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a

The total was calculated as the summation over the volume within a smooth sphere-ended cylinder 2 nm outside the outer surface of the modeled nanorod. All values have the unit of 107 V4/m4.

Conclusions

We have designed and synthesized a new spiked plasmonic nanorod with an interior nanogap for quantitative SERS analysis by using an easy and cheap protocol. We used GNR as a core, 4-mp as an internal standard to develop a novel spiked rodlike core–molecule–shell nanostructure (GGN) with an interior gap by the support of polydopamine. Because of both closeness to the LSPR peak of the excitation wavelength and the rough surface, GGN provides a reliable SERS quantitative analysis in detecting PAH molecules, showing a strong potential as quantitative SERS detectors. The interior gap and surface roughness  exhibited  high SERS enhancements that were also confirmed by FDTD simulations. This will provide a new material for reliable quantitative SERS analysis, detection, Raman imaging, and photothermal therapy.

Experimental Section

Methods

HAuCl4·xH2O, hydroxylamine hydrochloride, polyvinyl pyrrolidone (PVP, Mw = 29 000), hexadecyl trimethyl ammonium bromide (CTAB), NaBH4, ascorbic acid, 4-mp, DA, AgNO3, H2SO4, pyrene, anthracene, naphthalene, and tris(hydroxymethyl)aminomethane (Tris) were all purchased from Sigma-Aldrich and were used as received without further purification. The deionized (DI) water (Millipore Milli-Q grade) prepared in-house, with a resistivity of 18.2 MΩ, was used in all experiments. TEM images were taken with a FEI Titan 80-300 KV STEM (voltage 300 kV). The nanoparticles were dispersed in water and drop-casted on carbon-coated Cu grids. Raman spectra were collected by a Raman spectrometer (Horiba Jobin Yvon, Labram Aramis) at the respective excitation wavelength of 785, 660, and 532 nm. All of the Raman spectra of materials are collected in the liquid phase.

Synthesis of GNRs

The GNRs were synthesized via a seed-mediated procedure. Solution A: the gold seed solution was prepared by first mixing an aqueous solution of CTAB (7.5 mL, 0.1 M) and HAuCl4 (100 μL, 24 mM). A freshly prepared aqueous solution (ice-cold) of NaBH4 (0.6 mL, 0.01 M) was then added to the above mixture, of which the color changed from yellow to brown. Then, 1.2 mL of DI water was further added, and the mixture was aged for 3–4 h. Solution B: an aqueous solution of ascorbic acid (800 μL, 0.1 M) was added to a mixed aqueous solution of HAuCl4 (2 mL, 24 mM), H2SO4 (2 mL, 0.5 M), CTAB (100 mL, 0.1 M), and AgNO3 (700 μL, 10 mM), of which the color was changed from orange to colorless. For the final step, 240 μL of solution A was added to solution B and kept at 30 °C for 12 h.

Synthesis of 4-mp-Modified GNRs (mpGNRs)

The prepared GNR solution (5 mL) and an aqueous solution of 4-mp (3.85 × 10–4 M, 1 mL) were mixed and stirred overnight.

Synthesis of DA-Coated mpGNRs (mpGNRs@DA)

The mpGNRs@DA nanoparticles were synthesized as below: 1.5 mL of mpGNR solution was centrifuged and dispersed in the DA solution (1.9 mL, 516 μM). A 0.5 mL of Tris (10 mM) was then added. The mixture was sonicated for 1 h. Finally, the mixture was centrifuged and resuspended in 1.5 mL of DI water.

Synthesis of Core–Shell Gold-Coated mpGNRs@DA (GGN)

For the synthesis of GGN, 0.5 mL of mpGNRs@DA was diluted to 2.5 mL by adding 2 mL of DI water. HAuCl4 (150 μL, 5 mM), PVP (50 μL, w/w, 5%), and hydroxyl amine (150 μL, 50 mM) solutions were added consecutively. The mixture was then sonicated for 5 min. Noted that, in order to explore the density of gold shell, different amounts of HAuCl4 (5 mM, 75 and 250 μL) and hydroxyl amine (50 mM, 75 and 250 μL) were tested.

FDTD Simulations

FDTD simulations on the optical properties of the single nanostructure were performed with Lumerical FDTD Solutions software v8.11. A typical GGN structure was modeled but with an additional layer of sphere-tipped gold nanocones to mimic the surface roughness (Figure S10). A 2.66 fs broadband total-field scatter-field pulse was used to excite the modeled structures with a boundary condition of perfectly absorbed layer applied to all directions. Detailed dimensions are shown in Table S1. All simulations were performed with a mesh size of 0.5 nm and a background refractive index of 1.33 mimicking the aqueous solution measurement. Both the far-field extinction cross-sectional spectra and the near electric field distributions were recorded from the simulations. Details of the simulation method used are described in our previous publications.51,53−56

Acknowledgments

We thank King Abdullah University of Science and Technology (KAUST) and King Abdulaziz City of Science and Technology (KACST) for financial support.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01153.

  • Nanoparticles of TEM, SERS, and simulation characterization (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao8b01153_si_001.pdf (1.2MB, pdf)

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