Synthesis and Raman Performance Enhancement of Multilayer AuAg Heterostructures with Magnetic Resonance

: Significant amplification of surface enhanced Raman scattering (SERS) signals can be achieved mainly by the electric field en‐ hancement in metal core-shell nanostructures, and the enhanced magnetic field is rarely studied. In this study, we prepared multi-gap Au/ AgAu core-shell hybrid nanostructures by using gold nanocup as the core. The overgrowth processes to grow one, two, and three layers of AgAu hybrid nanoshells can produce Au/AgAu 1 , Au/AgAu 2 , and Au/AgAu 3 heteronanostructures. The strong plasmon coupling between the core and shell leads to significant electromagnetic field enhancement. Under the synergistic effect of electromagnetic plasmon reso‐ nance and plasmon coupling, Au/AgAu core-shell hybrid nanostructures exhibit excellent SERS signals. We also investigate the effect of the interstitial position of the rhodamine B (RhB) molecule on Raman enhancement in Au/AgAu 3 heteronanostructures. This study can pro‐ vide new ideas for the synthesis of multi-gap Raman signal amplifiers based on magnetic plasmon coupling.


Introduction
Raman scattering effect refers to the effect that the frequency of light waves changes after being scattered.
Through the analysis of Raman scattering spectrum, the relevant information of molecular vibration, rotation and structure can be obtained, which can be used to measure the content and concentration of the target substance qualitatively and quantitatively.The relevant detection methods are widely used in the fields of chemistry, biomedicine, environmental monitoring, food safety and so on [1][2][3][4][5][6][7][8] .However, in daily life, the Raman scattering signals of most substances are too weak to be tested, making it difficult and inaccurate to measure them using conventional methods.Therefore, people have been exploring methods that can enhance the Raman scattering signal of the tested substance [9][10][11][12][13] .Research has shown that Raman scattering intensity is enhanced to varying degrees when different molecules or ions are adsorbed on some metal surfaces, which is known as surface enhanced Raman scattering (SERS) [14][15][16][17][18][19] .
At present, the research on SERS mainly focuses on the following three directions.First, precious metal nanoparticles are bonded together to form dimers, trimers, and even polymers, which is conducive to the formation of small gaps between nanoparticles to generate local field enhancement, so as to achieve SERS [20][21][22][23][24] .Second, nanostructures are prepared with rough surfaces, which have abundant and fully encircled hot spots due to the ultra-small nanogaps, sharp tips or non-uniform areas on the surface, thus achieving the effect of SERS [25][26][27][28] .Third, by multipling synthesis methods, coreshell nanostructures are prepared, and the gap between the shell layers is utilized to achieve the effect of SERS [29][30][31][32][33][34] .Li et al [35] synthesized a dimer of silver nanospheres without any additional assembly by optimizing the amount of chloride added to polyol synthesis to control the colloid stability and oxidation etching, and then studied the SERS effect of the hot spot of the dimer.Lin et al [36] synthesized a substrate modified with Au particles on ZnO nanorods using ion sputtering method, which had a very high Raman enhancement factor.This structure was expected to become an SERS substrate with high sensitivity and stability.Ding et al [37] prepared gold nanorods with adjustable surface roughness.The ultrasmall nanogaps, sharp tips and non-uniform regions on the gold nanorods enabled them to have abundant and fully wrapped hot spots, and realized strong SERS.Hoeven et al [38] produced Au-Ag, Au-Pd and Au-Pt coreshell nanorods with precisely adjustable surface plasmon properties for adjustable SERS by performing metal overgrowth on gold nanorods within mesoporous silica shells.Tian et al [39] synthesized multi-layer shell core nanospheres (SiO 2 @Au, SiO 2 @Au@SiO 2 ) using liquid phase reduction method, and research has shown that the multi-layer nanospheres exhibit both cavity coupling and intra layer coupling, thereby improving local field enhancement and achieving high SERS performance.Ma et al [40] prepared two kinds of noble metal core-shell nanostars with different cores (Ag or Au) and the same Au shells with adjustable size and branch morphology, and studied their induced local surface plasmon resonance (LSPR) characteristics, and SERS performance.
According to the above knowledge, the enhancement of local field by the gap between core and shell is generally limited to the electric mode enhancement of isoplasmons, and there are few studies on the synergistic effect of magnetic resonance and gap to enhance local field [41][42][43][44][45][46] .In our previous work, we have studied the effect of magnetic resonance on local field in monolayer gap in detail [47] .In this work, we took the gold nanocup as the substrate and bonded rhodamine B (RhB) molecules in the first layer gap.Three kinds of core-shell nanostructures (Au/AgAu 1 -RhB 1 , Au/AgAu 2 -RhB 1 , and Au/AgAu 3 -RhB 1 ) coated with single-, double-, and triple-layer AgAu hybrid nanoshells with gold nanocups as cores were prepared, and their Raman enhancement effects were tested.In order to study the enhancement effect of magnetic plasmon resonance and gap interaction on local fields of these structures further, the intermediate products were soaked in RhB aqueous solution at three different stages during the synthesis process.The three-gap Au/AgAu hybrid core-shell nanostructures (Au/AgAu 3 -RhB 1 , Au/AgAu 3 -RhB 2 , and Au/AgAu 3 -RhB 3 ) of bonded Raman molecules in the first, second, and third interstitial gaps were obtained, respectively, and their Raman enhancement effects were tested under the same conditions.

Sample Preparation
We used the gold nanocup as the core, and prepared the core-shell nanostructures of gold nanocup core/silver gold hybrid nanoshell mediated by RhB using an adjustable electrical substitution and overgrowth process.The pre-prepared gold nanocup was mixed with RhB aqueous solution to make the outside of the gold nanocup absorb RhB molecule, and then growth solution prepared by silver nitrate was added to the gold nanocup solution with RhB molecule adsorbed, so that a layer of silver shell grew on its surface.After the growth was completed, the above products were centrifuged and diluted with 2 mL of water, and then a growth solution prepared with chloroauric acid was added to it.The silver nanoshell was replaced with gold nanoshell using an electric replacement process to obtain the Au/AgAu 1 -RhB 1 core-shell nanostructure.Repeat the above experimental operation and grow a layer of silver gold hybrid nanoshell on the periphery of the above product to obtain an Au/AgAu 2 -RhB 1 core-shell nanostructure.Two layers of silver gold hybrid nanoshells were grown on the periphery of the above products to obtain Au/AgAu 3 -RhB 1 core-shell nanostructures.The growth solution prepared by silver nitrate was dropped directly into the gold nanocup solution to make its surface grow a layer of silver shell.After centrifugation, the growth solution prepared by chloroauric acid was dropped into the product to obtain Au/AgAu 1 core-shell nanostructures.The product was mixed with RhB aqueous solution to make its surface absorb RhB molecules.Repeat the above experimental operation to grow two layers of silver gold hybrid nanoshells on its surface, then the Au/AgAu 3 -RhB 2 core-shell nanostructure was obtained.The Au/AgAu 2 core-shell nanostructure was obtained by using the above process to grow two layers of silver gold hybrid nanoshell on the periphery of the gold nanocup.The product was mixed with RhB molecule, and then the silver gold hybrid nanoshell growing after the RhB molecule was adsorbed on its surface to obtain the Au/ AgAu 3 -RhB 3 core-shell nanostructure.

Characterization of Sample Characteristics
ATU-1810 UV-Vis spectrophotometer (China) was used to measure the extinction spectra of the sample.Scanning electron microscopy (SEM) was performed at 5.0 kV using Gemini 500 (Germany).Transmission electron microscopy (TEM) was performed at 200 kV using JEOL 2010 (Japan).Surface enhanced Raman scattering (SERS) spectra was acquired with the laser source with wavelength of 633 nm (1 mW) for 10 s of illumination on a HORIBA XploRA Plus Raman Microscope (France).

Results and Discussion
In order to study the SERS effect of different structures, we synthesized structures with different layers and different embedding positions of RhB, and measured their Raman enhancement effect.The schematic diagram of relevant structures is shown in Fig. 1.
Firstly, gold nanocups were obtained by selectively growing Au on PbS nanooctahedron and then dissolving PbS components.Using gold nanocups as starting substrate, Au/Ag core-shell nanostructures were synthesized by adding growth solution prepared by AgNO 3 .Using it as a substrate, the growth solution configured by HAuCl 4 was added to the substrate, and the molar ratio of HAuCl 4 and AgNO 3 added to the growth solution was 1 : 1. Au/AgAu 1 core-shell nanostructures were obtained through electrical substitution and overgrowth processes.
Figure 2(a) shows the SEM images of Au/AgAu 1 core-shell nanostructures.The gap between the gold nanocup core and the silver gold hybrid nanoshell can be clearly observed under a transmission electron microscope.These Au/AgAu core-shell nanostructures with gaps provide a good foundation for gap enhanced SERS based on magnetic plasmon coupling.By repeating the above process and controlling the molar ratio of HAuCl 4 and AgNO 3 added to each layer at 1 : 1, Au/AgAu 2 coreshell nanostructures with two-layer gaps and Au/AgAu 3 core-shell nanostructures with three-layer gaps were synthesized.Figure 2(b) and (c) show the SEM images of Au/AgAu 2 and Au/AgAu 3 core-shell nanostructures, and Fig. 2(d)-(i) show the TEM images of the Au/AgAu 1 , Au/ AgAu 2 and Au/AgAu 3 core-shell nanostructures and their single particles, respectively.Under TEM, it can be observed that the Au/AgAu 2 core-shell nanostructure contains two layers of gaps, and the Au/AgAu 3 core- shell nanostructure contains three layers of gaps.The width of the gaps decreases as it moves towards the outer layer.During the experiment, we also prepared the core-shell structure with notch, but the characterization results showed that the Raman enhancement effect of this structure was far worse than that of the circular coreshell structure, so the subsequent research focused on the circular core-shell structure.
We used a visible ultraviolet spectrophotometer to determine the extinction spectra of the product, and studied the plasmon resonance properties of the gold nanocup/silver gold hybrid nanoshell core-shell nanostructure using extinction spectra.Figure 3(a) shows the extinction spectra of Au/AgAu 1 -RhB 1 , Au/AgAu 2 -RhB 1 , and Au/AgAu 3 -RhB 1 core-shell nanostructures at the same particle concentration.The plasmon resonance peak of the initial gold nanocup is located at 615 nm.After the gold nanocup was mixed with RhB aqueous solution, the growth solution containing AgNO 3 and the growth solution containing HAuCl 4 were added, then a layer of silver gold hybrid nanoshell was grown on the surface of the gold nanocup to obtain the Au/AgAu 1 -RhB 1 coreshell nanostructure.The above experimental operation was repeated and a layer of silver gold hybrid nanoshell was regenerated on the surface of the above product to obtain the Au/AgAu 2 -RhB 1 core-shell nanostructure.Continuing to repeat the operation, we obtained the prod-uct Au/AgAu 3 -RhB 1 with RhB molecule embedded in the first layer gap and three layers of AgAu hybrid nanoshells.Observing the extinction spectra of the three products, we found that with the increase of the number of AgAu hybrid nanoshells, the isoplasmon formant gradually redshifted, and the acromion appeared at 555 nm to the left of the main peak.With the increase of the number of shells, the acromion intensity gradually increased and produced a slight redshift.Figure 3(b) shows the extinction spectra of Au/AgAu 3 -RhB 1 , Au/ AgAu 3 -RhB 2 , and Au/AgAu 3 -RhB 3 core-shell nanostructures at the same particle concentration.These three nanostructures are all wrapped with three layers of silver gold hybrid nanoshell on the surface of gold nanocup core.The difference is that RhB molecules are embedded in the first layer gap, the second layer gap and the third layer gap, respectively.The extinction spectrum shows that the plasmon resonance peaks of the three nanostructures are all located at 615 nm, and a shoulder which can induce strong plasmon coupling and generate strong localized electromagnetic fields in the nanogap.These Au/AgAu 1 , Au/AgAu 2 , and Au/AgAu 3 core-shell nanostructures provide reliable ideas for enhancing Raman signals.
From the TEM images of the three products, the gaps between shell and core, as well as between shells, can be clearly observed.However, due to the denser shell, the observation effect on the gaps is not satisfactory.In order to display the internal structure of each product more clearly and intuitively, we took photos of the distribution of Au and Ag elements in the products of these three structures, as shown in Fig. 4. The content of gold in Au/AgAu 1 core-shell nanostructures is much higher than that of silver, and the distribution is relatively uniform.Silver is mainly distributed on its surface, and there is a clear boundary for concentration changes in both distribution maps, which corresponds to the gaps in the nanostructure, as shown in Fig. 4(a).In the element distribution diagram of Au/AgAu 2 core-shell nanostructures, two significant concentration changes can be observed, indicating the existence of two gaps in the structure, as shown in Fig. 4(b).In the element distribution map of Au/AgAu 3 core-shell nanostructures, there are three significant concentration variation boundaries, indicating the existence of three gaps in the structure, and the lighter the color towards the outer side, the lower the element distribution concentration, and the smaller the width between the outer boundary and the boundary, indicating a smaller thickness towards the outer shell layer, as shown in Fig. 4(c).
Under the excitation of light with a wavelength of 633 nm, the SERS characteristics of gold nanocup core/ silver gold nanoshell plasmon core shell nanostructures with RhB molecules embedded in the first layer gap and wrapped with one layer, two layers and three layers of silver gold shells were tested respectively.The measured results were in good agreement with the plasmon reso- As the number of shell layers increases, the Raman intensity of this nanostructure sharply increases and rapidly decays.The Raman intensity of Au/AgAu 2 -RhB 1 core-shell nanostructures is the strongest, and the peak broadening is the highest at around 1 500 nm.This is because there is only one-layer gap in the Au/AgAu 1 -RhB 1 structure to enhance the Raman scattering signal, and the effect is poor.There is a double gap electromagnetic field coupling in the Au/AgAu 2 -RhB 1 core-shell nanostructure, which has greatly enhanced the Raman scattering signal.However, the embedded position of RhB in the Au/ AgAu 3 -RhB 1 core-shell nanostructure is too deep, the outer shell is too thick, and the shielding effect on excita-tion is too strong, resulting in poor Raman scattering enhancement effect.Using the same method, we also measured the SERS characteristics of the gold nanocup core/ silver gold nanoshell plasmon core shell nanostructure with RhB molecules embedded in the first layer, the second layer, and the third layer gaps, all wrapped with three layers of silver gold shells.The results are shown in Fig. 5(b).The structure showed that the Raman enhancement effect of RhB molecule was the weakest when it was connected to the first gap, and the Raman signal enhancement effect of RhB molecule when it was embedded into the second gap was much greater than that of the other two structures.This was due to the electromagnetic coupling between the first and third nanogaps in the three-gap nanostructure, which further enhanced the electromagnetic field in the second gap.

Conclusion
In summary, we synthesized multilayered Au/AgAu core-shell nanostructures with RhB molecules embedded in the gaps, and studied the effects of the number of shells and the embedding position of RhB on the amplification of SERS signals.Au/AgAu hybrid nanostructures with adjustable number of gaps and RhB embedded position were synthesized by an adjustable electric substitution and surface overgrowth process based on gold nanocup.By immersing the initial Au nanocup or Au/AgAu hybrid in RhB aqueous solution, the selective binding of RhB molecules in the nanogap was achieved.Au/AgAu hybrid core-shell nanostructures exhibit significant plasmon absorption and electromagnetic field enhancement due to strong plasmon coupling in multi-nanogap struc-tures, which makes Au/AgAu hybrid core-shell nanostructures have extremely strong SERS performance.And because the nanogaps between different shells interact with each other, and thus have different effects on the SERS effect, we embed RhB molecules to the surface of the gold nanocup directly, and grow one, two, and three layers of silver gold nanoshells on the outside, which are excited by the same light.The results show that the Au/AgAu hybrid core-shell nanostructures with two layers of nanogaps have the best SERS performance.This is because the coupling of double gap electromagnetic fields has a significant effect on the improvement of SERS performance.In addition, in order to obtain higher SERS performance, the position of RhB embedded in the Au/AgAu hybrid core-shell nanostructures with three-layer gaps was adjusted.The results show that the three gap Au/AgAu hybrid core-shell nanostructures with RhB molecules embedded in the second nanogap exhibit the best SERS signal, due to the further enhancement of the electromagnetic field in the second layer gap, which is caused by the electromagnetic field coupling between the first layer and the third layer nanogap.Our research provides a new method for the synthesis of multi-gap Raman amplifiers based on magnetic isoplasmon coupling and has a wide range of applications in the preparation of Raman probes and sensitive detection in biomedical fields.

Conflicts of Interest
The authors declare no competing interests.