Introduction

Raman scattering spectroscopy can offer the information of molecular vibration and rotation, and it is a qualitative and quantitative analysis method that can be widely used in various scientific fields and daily life [1,2,3,4,5]. However, the signal of Raman scattering is too weak to be observed [6], and people have been exploring various ways to enhance the Raman scattering signal. The methods of enhancing Raman scattering mainly include surface-enhanced resonance Raman scattering (SERRS), coherent anti-Stokes Raman scattering (CARS), and surface-enhanced Raman scattering (SERS) [7,8,9,10,11,12,13,14]. SERS is a phenomenon associated with significant amplification of the Raman signals of analytes located near the surface of signal-enhancing materials, and people usually improve Raman performance by combining the plasmon coupling of noble metal nanomaterials. SERS based on noble metal nanomaterials can be applied in molecular analysis, material characterization, and other fields [15,16,17,18,19]. Various forms of Raman signal enhancement can be easily achieved by changing the plasmon properties, and the plasmon characteristics are controllable by changing the morphology, size, structure, and composition of noble metal nanomaterials [20,21,22,23,24,25,26].

For SERS based on plasmon coupling of noble metal nanomaterials, the main enhanced physical mechanism is the local electric field enhancement due to plasmon coupling and hot spots [13, 27,28,29,30,31]. Therefore, the design and preparation of noble metal nanostructures with strong electromagnetic fields and abundant hot spots is the key to achieve surface Raman enhancement [32,33,34,35,36,37,38]. Ding’s research group et al. reported that gold nanorods with tunable surface roughness were obtained with all-surround hot spots to achieve SERS [32]. Lin et al. reported an Au@Ag core–shell nanocube with fine-tuned edge length as SERS substrate for Raman enhancement [33]. Ye et al. reported that a kind of Au–Ag hybrid multi-shell nanoparticles enhanced the SERS performance of the NPs [34]. Furthermore, researchers have found that the nanogap in the interior can make Raman molecules almost not be affected by the external environment, as well as Raman molecules will be subjected to uniform and stably enhanced electromagnetic fields in the interior of the nanogap [39,40,41,42,43,44,45,46]. Ma’s team reported that Au/AgAu hybrids with adjustable gap width and counts were synthesized by controllable galvanic replacement based on Au nanospheres and achieved outstanding SERS response [39]. Zhang’s research group prepared Au/AgAu core–shell nanorods with Raman reporters embedded in the nanogap, which generated strongly enhanced and stable Raman signals [40]. Zhao et al. reported gap-tethered SERS-active Au@AgAu nanoparticles serving as Raman tags for the detection of MC-LR at a very low concentration [41].

However, in previous work, most nanostructures only exhibit electric field enhancement, and there are few reports of magnetic field enhancements. In this work, Au/AgAu core–shell nanostructures have not only electric but also magnetic plasmon resonance by selecting gold nanocups as the substrate. The gap width between the core and shell is tunable by adjusting the molar ratio of Au to Ag during growing the shell. The prepared Au/AgAu core–shell nanostructures have tunable plasmon resonance and strong electromagnetic field, which yield Au/AgAu core–shell nanostructures with improved and controllable SERS signals. Subsequently, the physical mechanism of Raman signal enhancement is discussed.

Material and Methods

Materials

Cetyltrimethylammonium bromide (CTAB, 99.0%) and N-hexadecyltrimethylammonium chloride (CTAC, 99.0%) were obtained from Aladdin. Thioacetamide (TAA, 99.0%), lead acetate (Pb(Ac)2, 99.5%), L-ascorbic acid (AA, 99.7%), chloroauric acid (HAuCl4⋅4H2O, 99.0%), and silver nitrate (AgNO3, 99.5%) were bought from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water with a resistivity of about 18.2 MΩ⋅cm was used as the solvent in all experiments.

Synthesis of Rhodamine B (RhB)-Modified Gold Nanocups with Ag Shell (Au/Ag Nanocrystals)

Gold nanocups were prepared by selective growth of gold on PbS nanooctahedra and subsequent selective dissolution of the PbS component [47]. The as-prepared gold nanocups (2 mL) were mixed with RhB aqueous solution (2 × 10−4 M, 2 mL) and shaken for 4 h. The RhB-modified gold nanocups were centrifuged at 4500 rpm for 5 min and redispersed in deionized water (2 mL). Subsequently, CTAC (0.3 mL, 0.2 M) and CTAB (0.2 mL, 0.2 M) were added to 2 mL RhB-modified gold nanocups and heated with magnetic stirring at 60 °C for 10 min. Afterwards, the pre-prepared solution A (AgNO3 (0.08 mL, 0.01 M) and deionized water (1 mL)) and solution B (CTAC (0.2 mL, 0.2 M), AA (0.3 mL, 0.1 M) and deionized water (0.5 mL)) were slowly and alternately injected into the pre-warmed solution and stirred at 60 °C for 4 h. The resulting product was then centrifuged at 4500 rpm for 6 min, and the precipitate was redispersed in deionized water (2 mL).

Synthesis of Gap-Tunable and RhB-Modified Au/AuAg Core–Shell Nanostructures

The Ag of the Au/Ag core–shell nanostructures was replaced with Au by galvanic replacement and overgrowth process, and a nanogap of tunable width was created between the gold nanocups and the AuAg alloy shell. The prepared Au/Ag core–shell sample (2 mL) and CTAB (0.7 mL, 0.2 M) were mixed and heated with magnetic stirring at 60 °C for 10 min. Afterwards, the pre-prepared solution A (HAuCl4 (0.08 mL, 0.01 M) and deionized water (1 mL)) and solution B (CTAC (0.2 mL, 0.2 M), AA (0.3 mL, 0.1 M) and deionized water (0.5 mL)) were slowly and alternately injected into the pre-warmed solution and stirred at 60 °C for 4 h. The resulting product was then centrifuged at 4500 rpm for 6 min, and the precipitate was redispersed in deionized water (4 mL). The width of the nanogap was tuned by adjusting the amount of the HAuCl4 from 0.02 to 0.32 mL.

Sample Characterization

The transmission electron microscopy (TEM) images were obtained with a JEOL 2010 operating at 200 kV. Scanning electron microscopy (SEM) observations were performed on a Zeiss Sigma FE-SEM operated at an accelerating voltage of 20 kV. The extinction spectra were measured on a TU-1810 UV–Vis spectrophotometer. The SERS spectra were acquired with the laser source with a wavelength of 633 nm (1 mW) for 10 s of illumination on a HORIBA XploRA Plus Raman microscope.

Results and Discussion

We prepared RhB-mediated Au/AuAg core–shell nanostructures based on gold nanocups by a controlled galvanic replacement and overgrowth process. Due to the excellent magnetic plasma properties of gold nanocups, Au/AgAu core–shell nanostructures have tunable plasmon resonance peaks and different widths of nanogaps, and their SERS properties are dependent on the gap width and the thickness of the alloy shell. The detailed preparation process is shown in Fig. 1a. First, we used gold nanocups (Fig. 1b) that were obtained by selective growth of Au on PbS nanooctahedra and subsequent selective dissolution of the PbS component) as a starting substrate. Au/Ag core–shell nanostructures were synthesized by adding the precursor AgNO3 and the reducing agent AA. Figure 1c shows the SEM image of Au/Ag core–shell nanostructures with 0.08 mL AgNO3 added. As the amount of AgNO3 added increased, the opening in the gold nanocups was gradually filled by Ag, and the Ag shell layer on the outside of the gold nanocups becomes thicker. RhB-mediated Au/Ag core–shell nanostructures which were added with the amount of AgNO3 of 0.08 mL were chosen as the substrate. Then, Au/AgAu core–shell nanostructures with the RhB molecules embedded in the nanogap were obtained via a galvanic replacement and overgrowth process by adding HAuCl4 and the reducing agent AA. Figure 1d shows the SEM image of Au/AgAu core–shell nanostructures when the molar ratio of Au to Ag is 4:1. In Fig. 1(e), the EDS energy spectrum of Au/AgAu core-shell nanostructures with the molar ratio of 1:1 between Au and Ag.

Fig. 1
figure 1

a Schematic illustration of the synthesis process of RhB-mediated Au/AgAu core–shell nanostructures. b–d SEM images of gold nanocups, Au/Ag core–shell nanostructures with 0.08 mL AgNO3, and Au/AuAg core–shell nanostructures with the molar ratio of 4:1 between Au and Ag. e EDS energy spectrum of Au/AgAu core–shell nanostructures with the molar ratio of 1:1 between Au and Ag

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In order to clearly observe the change regular of the gap widths with a different molar ratio of Au to Ag, Fig. 2 shows a set of representative TEM images of Au/AgAu core–shell nanostructures with different gap widths. The amount of HAuCl4 added was controlled by regulating the molar ratio of Au to Ag (HAuCl4: AgNO3), replacing the Ag shell with Au to form an AuAg alloy shell and nanogap. In particular, the gap widths were turned from ~ 7.4 to ~ 3.2 nm by increasing the molar ratio of Au to Ag from 1:2 to 2:1 (insets in Fig. 2a–c).

Fig. 2
figure 2

TEM images of Au/AgAu core–shell nanostructures with different gap widths: ~ 7.4 nm (a), ~ 4.1 nm (b), ~ 3.2 nm (c), and unmeasurable (d). The corresponding molar ratio of Au to Ag is 1:2, 1:1, 2:1, and 4:1

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When the molar ratio of Au to Ag is greater than 2:1, the nanogap could hardly be observed due to the large thickness of the alloy shell (Fig. 2d). These Au/AgAu core–shell nanostructures with adjustable gap widths provided a good platform for gap-enhanced SERS based on magnetic plasmon coupling.

The plasmon resonance of Au/AgAu core–shell nanostructures was studied by extinction spectra. Figure 3a shows the extinction spectra of gold nanocups, RhB-mediated Au/Ag core–shell nanostructures, and RhB-mediated Au/Ag core–shell nanostructures at the same particle concentration. The initial gold nanocups have a magnetic dipole plasmon peak located at 610 nm and the plasmon resonance peak of the RhB-mediated Au/Ag core–shell nanostructures blue shift to 560 nm with 0.08 mL AgNO3 added. The Ag shell layer significantly changed the plasmon resonance mode of gold nanocups. As the amount of AgNO3 added increased from 0.01 to 0.15 mL, the plasma peak blue shifted from 593 to 553 nm, and a new peak gradually appeared at 425 nm, which was attributed to the effect of the Ag shell on the dielectric constant of the Au core. When excessive amounts of AgNO3 were added, no more Ag could be deposited on the surface of gold nanocups, and the remaining Ag nucleated itself in solution to form Ag nanospheres. The extinction spectra of the RhB-mediated Au/AgAu core–shell nanostructures with different molar ratios of Au to Ag are shown in Fig. 3b. As the molar ratio of Au to Ag increased from 1:4 to 4:1, the plasmon resonance peak of the RhB-mediated Au/AgAu core–shell nanostructures was red-shifted from 580 to 721 nm, and a shoulder peak appeared at 550 nm to the left of the main plasmon resonance peak when the molar ratio of Au to Ag was greater than 1:1, with a gradual increase in intensity and a slight red-shift as the molar ratio of Au to Ag increased. During the galvanic replacement process, the AuAg alloy shell thickness gradually becomes thicker and the gap width gradually becomes smaller of Au/AgAu core–shell nanostructures as the molar ratio of Au to Ag continued to increase. A strong plasma coupling could be induced in the nanogap, resulting in a strong local electromagnetic field. These Au/AgAu core–shell nanostructures with tunable plasmon resonance peak offer the possibility to enhance the Raman signals.

Fig. 3
figure 3

a Experimental extinction spectra of gold nanocups, Au/Ag core–shell nanostructures with 0.08 mL AgNO3 added, and Au/AgAu core–shell nanostructures with the molar ratio of 2:1 between Au and Ag. b Experimental extinction spectra of Au/AgAu core–shell nanostructures with different molar ratios of Au to Ag

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The SERS properties of RhB-embedded Au/AgAu core–shell nanostructures with different molar ratios of Au to Ag were tested under excitation at 633 nm which matched well with the magnetic plasmon resonance peak, and the results are shown in Fig. 4a. The Raman spectra of the RhB molecules can be clearly observed, indicating that the RhB molecules were well embedded in the nanogaps. The SERS peaks at 1360 cm−1 and 1647 cm−1 are referred to as the C–C stretching mode. It is noteworthy that the Raman intensity of the Au/AgAu core–shell nanostructures increases dramatically as the Au–Ag molar ratio increases from 1:4 to 2:1 but then decreases dramatically when the Au–Ag molar ratio reaches 4:1. The characteristic peaks at 1350 cm−1 and 1275 cm−1 are used as the standard peaks for comparing the SERS performance of the Au/AgAu core–shell nanostructures. As shown in Fig. 4b, the Au/AgAu core–shell nanostructures with the molar ratio of 2:1 between Au and Ag have the highest Raman intensity, which is ~ 12 times higher than Au/AgAu core–shell nanostructures with the molar ratio is 1:4, of Au to Ag at 1350 cm−1, and ~ 11 times at 1275 cm−1, respectively. The gap width of Au/Ag core–shell nanostructures decreased with the molar ratio of Au to Ag increased. When the molar ratio of Au to Ag was 2:1, the gap width was extremely small relative to the alloy shell thickness, and the Raman excitation wavelength (633 nm) was well matched to the magnetic plasma resonance peak position of Au/Ag core–shell nanostructures; therefore, the highest Raman signals were obtained. When the molar ratio of Au to Ag is greater than 2:1, the alloy shell has become thicker to reduce the penetration and scattering of light, causing the Raman signals strength to drop sharply. The results offer potential strategies for enhancing the SERS performance.

Fig. 4
figure 4

a Raman spectra of Au/AgAu core–shell nanostructures with different molar ratios of Au to Ag. b Raman intensity at 1360 cm−1 and 1275 cm−1 of RhB-embedded Au/AgAu core–shell nanostructures with different molar ratios of Au to Ag

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Conclusions

In summary, RhB-embedded Au/AgAu core–shell nanostructures with tunable gap widths were synthesized via controlled galvanic replacement and overgrowth process based on gold nanocups, to achieve tunable amplified Raman signals with different gap widths. The RhB molecules could be coupled to gold nanocups which had strong magnetic plasmon properties by soaking the gold nanocups in an aqueous RhB solution. Au/AgAu core–shell nanostructures showed prominent plasmon absorption and strong local electromagnetic field enhancement owing to the synergistic effect of electromagnetic plasmon resonance and plasmon coupling. In particular, Au/AgAu core–shell nanostructures with the molar ratio is 2:1 of Au to Ag show the highest SERS activity; this may be due to the plasmon resonance peak position of the sample matches well with the Raman excitation wavelength (633 nm), and the width of the nanogap is small enough to result in strong electromagnetic coupling. And when the molar ratio of Au to Ag is greater than 2:1, the shielding effect of the thicker alloy shell will weaken the Raman signals considerably. Our findings provide a new possibility for synthesize gap-enhanced Raman tags based on magnetic plasma coupling for enhancing the Raman performance.