Magnetic-Core–Shell–Satellite Fe3O4-Au@Ag@(Au@Ag) Nanocomposites for Determination of Trace Bisphenol A Based on Surface-Enhanced Resonance Raman Scattering (SERRS)

As a typical representative of endocrine-disrupting chemicals (EDCs), bisphenol A (BPA) is a common persistent organic pollutant in the environment that can induce various diseases even at low concentrations. Herein, the magnetic Fe3O4-Au@Ag@(Au@Ag) nanocomposites (CSSN NCs) have been prepared by self-assembly method and applied for ultra-sensitive surface-enhanced resonance Raman scattering (SERRS) detection of BPA. A simple and rapid coupling reaction of Pauly’s reagents and BPA not only solved the problem of poor affinity between BPA and noble metals, but also provided the SERRS activity of BPA azo products. The distribution of hot spots and the influence of incremental introduction of noble metals on the performance of SERRS were analyzed by a finite-difference time-domain (FDTD) algorithm. The abundance of hot spots generated by core–shell–satellite structure and outstanding SERRS performance of Au@Ag nanocrystals were responsible for excellent SERRS sensitivity of CSSN NCs in the results. The limit of detection (LOD) of CSSN NCs for BPA azo products was as low as 10−10 M. In addition, the saturation magnetization (Ms) value of CSSN NCs was 53.6 emu·g−1, which could be rapidly enriched and collected under the condition of external magnetic field. These magnetic core–shell–satellite NCs provide inspiration idea for the tailored design of ultra-sensitive SERRS substrates, and thus exhibit limitless application prospects in terms of pollutant detection, environmental monitoring, and food safety.


Introduction
Endocrine-disrupting chemicals (EDCs), also known as environmental hormones, are exogenous substances that affect mammalian reproduction by interfering with the endocrine system of organisms [1][2][3]. Bisphenol A (BPA), as a typical representative of EDCs, has been used principally in manufacturing polycarbonates and epoxy resins which are the main raw materials of plasticizers, plastic bottles and cups, food storage and packaging materials, and other commonly used industrial products [4,5]. Unfortunately, a large amount of evidence shows that the persistence, bioaccumulation, and biomagnification of BPA can bring about serious negative effects on human health and the ecosystem [6,7]. Although BPA is a low-toxicity chemical, it can induce a variety of diseases even at very low concentrations, such as congenital disabilities, diabetes, cardiovascular diseases, multiple cancers, and especially reproductive system diseases [8][9][10]. Hence, it is urgent to explore a rapid, sensitive, efficient, and low-cost detection method for BPA.
As illustrated in Scheme 1, the preparation process of CSSN NCs consists of two steps. The first step was the preparation of FA@Ag NCs. First, 10 mg of FA NCs and 5 mL of deionized water were mixed. Subsequently, AgNO 3 (0.1 M; 3 mL) and reductant NH 2 OH·HCl (0.1 M; 12 mL) were added. FA@Ag NCs were obtained after sonication for 2 h and several wash cycles with ethanol.

Structure and Magnetic Properties of CSSN NCs
XRD technology was used to study the structure and phase purity of Fe3O4 hollow spheres, FA, FA@Ag and CSSN NCs. As shown in Figure 1, the diffraction peaks of Fe3O4 Scheme 1. Scheme of synthetic process of CSSN NCs and SERRS detection protocol for BPA on CSSN SERRS substrates. The second step was the synthesis of CSSN NCs. The previously obtained FA@Ag NCs were blended with 20 mL of methanol, and 25 mL of PEI-DTC was dropwise-added. The mixture was stored for about 1 h. Then, FA@Ag@PEI-DTC NCs were obtained through washing and drying. After 10 mg of FA@Ag@PEI-DTC NCs was put into 5 mL of deionized water, 40 mL of Au@Ag nanocrystals was poured into the mixture and sonicated for about 2 h. Finally, the product was washed and dried to obtain CSSN NCs.

Pauly's Reagents and Coupling Reaction
Three kinds of reagents were prepared and kept at 4 • C for further use. Reagent A was a mixture of p-aminobenzenesulfonic acid (4.5 g), HCl solution (12 M; 5 mL) and deionized water (500 mL). Reagent B was 5% NaNO 2 , and reagent C was 10% Na 2 CO 3 .

FDTD Algorithm Method
Details of FDTD algorithm method can be found in Supplementary Materials.

SERRS Measurements
Before SERRS measurements, BPA was changed into azo dyes through a coupling reaction of BPA and diazonium ions to bind noble metals with high affinity and thus achieve highly sensitive detection of BPA. In this work, the phenol group of BPA could be converted into azo dyes with p-aminobenzenesulfonic acid through electrophilic aromatic substitution [41,42]. After the coupling reaction, BPA solution (25 µL) with different concentrations of 10 −10 to 10 −4 M and CSSN (1 mg) NCs were mixed in the aluminum pan, respectively. SERRS spectroscopy was performed under 514.5 nm laser and acquisition time was about 10 s.

Structure and Magnetic Properties of CSSN NCs
XRD technology was used to study the structure and phase purity of Fe 3 O 4 hollow spheres, FA, FA@Ag and CSSN NCs. As shown in Figure 1, the diffraction peaks of Fe 3 [43,44]. In addition, the diffraction peaks are sharp and strong, which indicates that the prepared Fe 3 O 4 hollow spheres have high phase purity and good crystallization. However, owing to the fact that Fe 3 O 4 as well as γ-Fe 2 O 3 have the identical spinel structure, it is not sufficient to identify them only by XRD results [45]. Therefore, the phase structure of Fe 3 O 4 hollow spheres was further verified by Mössbauer spectroscopy. As depicted in Figure S1, the Mössbauer spectrum of Fe 3 O 4 hollow spheres can be fitted into two sextets, and the magnetic sextets lines illustrate the typical double six peak structure of Fe 3 O 4 [46,47]. The corresponding Mössbauer parameters of Fe 3 O 4 hollow spheres are presented in Table S1. Hyperfine field is 48.7 and 45.5 Tesla, and the isomer displacement is 0.288 and 0.602 mm/s, which correspond to Fe 2+ and Fe 3+ at octahedral interstitial sites and Fe 3+ at tetrahedral interstitial sites. After Au seeds were loaded on surfaces of Fe 3 O 4 hollow spheres, four new XRD diffraction peaks emerged at 38.2 • , 44.3 • , and 64.5 • , which were assigned to (111), (200), and (220) planes of Au (JCPDS 04-0784) [48]. It should be noted that the positions of Au and Ag characteristic peaks are too close to be distinguished [49]. Since the intensities of XRD diffraction peaks are related to the contents of phase in the mixture [50,51], the increase of the intensities of Ag/Au diffraction peaks in the XRD patterns of FA@Ag NCs proves that there is dense Ag adsorbed on the surfaces of FA NCs. The XRD pattern of CSSN NCs shows that the diffraction peaks intensity of Ag/Au further increases significantly when the Au@Ag nanocrystals are adhered to the FA@Ag NCs. By contrast, the XRD diffraction pattern of CSSN NCs exhibits weaker Fe 3 O 4 characteristic peaks than that of Fe 3 O 4 hollow spheres, FA, and FA@Ag NCs. This may be attributed to the declining proportion of Fe 3 O 4 contents caused by the successful modification of the large amount of Au seeds and Au@Ag nanocrystals. Consequently, the information obtained from above XRD and Mössbauer analysis preliminary confirm the successful construction of Fe 3 O 4 hollow spheres, FA, FA@Ag, and CSSN NCs. diffraction peaks intensity of Ag/Au further increases significantly when the A nanocrystals are adhered to the FA@Ag NCs. By contrast, the XRD diffraction pa CSSN NCs exhibits weaker Fe3O4 characteristic peaks than that of Fe3O4 hollow s FA, and FA@Ag NCs. This may be attributed to the declining proportion of Fe3O4 c caused by the successful modification of the large amount of Au seeds and Au@A crystals. Consequently, the information obtained from above XRD and Mössbauer sis preliminary confirm the successful construction of Fe3O4 hollow spheres, FA, F and CSSN NCs. , respectively. It was found that the Ms value gr reduces with the incremental introduction of noble metal due to the diamagnetism ble metal nanocrystals [52]. As depicted in the inset of Figure S2, the CSSN NCs collected by an external magnet within 50 s even if their Ms value is the lowest am the materials. The remarkable magnetic response property means that CSSN NC great convenience in rapid separation and detection in complex liquid environme The morphology, size, and structure of the obtained products were studied b EDS elemental mapping, and EDS line scanning. As depicted in Figure 2a Figure 2c. In order to create more hot spots, Au@Ag nanocryst tinued to be assembled on the surfaces of FA@Ag NCs to obtain CSSN NCs by P layers. As presented in Figure 2d, PEI-DTC layers with a thickness of around 15 uniformly coated on the surfaces of FA@Ag NCs. For directly confirming the const of core-shell-satellite structure in CSSN NCs, we took EDS line scanning profiles , respectively. It was found that the Ms value gradually reduces with the incremental introduction of noble metal due to the diamagnetism of noble metal nanocrystals [52]. As depicted in the inset of Figure S2, the CSSN NCs can be collected by an external magnet within 50 s even if their Ms value is the lowest among all the materials. The remarkable magnetic response property means that CSSN NCs have great convenience in rapid separation and detection in complex liquid environments.
The morphology, size, and structure of the obtained products were studied by TEM, EDS elemental mapping, and EDS line scanning. As depicted in Figure  CSSN NCs increased to around 800 nm due to the presence of Au seeds, Ag shells, and Au@Ag nanocrystals, which is bigger than that of Fe 3 O 4 hollow spheres. Moreover, the elements show a symmetrical distribution with the change of detection position. The X-ray intensity of Au and Ag is maximum while Fe and O is minimum in the edge region. By comparison, the relative intensity of Fe and O increases gradually and almost no X-ray intensity of Au and Ag is observed in the direction of the orange arrow. This proves that the Au@Ag nanocrystals are adsorbed firmly on the outermost layer of FA@Ag@PEI-DTC by the affinities between the bidentate ligands with two chelating sulfur groups and Au@Ag nanocrystals [31]. Therefore, the EDS line-scanning results of CSSN NCs are consistent with the conclusions of XRD, TEM, and corresponding EDS mapping, which proves that the formation of CSSN NCs with the core-shell-satellite structure is convincing.
Nanomaterials 2022, 12, x FOR PEER REVIEW increased to around 800 nm due to the presence of Au seeds, Ag shells, and Au@A crystals, which is bigger than that of Fe3O4 hollow spheres. Moreover, the elemen a symmetrical distribution with the change of detection position. The X-ray int Au and Ag is maximum while Fe and O is minimum in the edge region. By com the relative intensity of Fe and O increases gradually and almost no X-ray intensi and Ag is observed in the direction of the orange arrow. This proves that the nanocrystals are adsorbed firmly on the outermost layer of FA@Ag@PEI-DTC b finities between the bidentate ligands with two chelating sulfur groups and Au@A crystals [31]. Therefore, the EDS line-scanning results of CSSN NCs are consist the conclusions of XRD, TEM, and corresponding EDS mapping, which proves formation of CSSN NCs with the core-shell-satellite structure is convincing. The valence of elements in CSSN NCs was determined by XPS technology. spectra of Fe3O4 hollow spheres, FA, FA@Ag, and CSSN NCs are exhibited in F Within detection limit of XPS, Fe 2p, O1s, Au 4f, Ag 3d, and C 1s were observed impurity was found. High-resolution XPS results of Ag 3d and Au 4f are reflecte ure 3. Ag 3d spectra in Figure 3a display peaks at 368.2 and 374.2 eV with a sp splitting of 6 eV for CSSN NCs, which are attributable to characteristics of Ag Ag 3d5/2 of Ag 0 [53]. As represented in Figure 3b, peaks of CSSN NCs at 84.1 and with an energy difference of 3.7 eV are attributable to Au 4f7/2 and Au 4f5/2 of Au An interesting phenomenon is that the binding energy of Ag 3d as well as Au 4f slightly with the incremental introduction of noble metals. The positions of Ag 3 of CSSN NCs are blue-shifted compared with FA@Ag NCs, and the positions peaks are red-shifted compared with FA@Ag and FA NCs. This shift in binding may be ascribed to the charge transfer from metallic Au to Ag [56][57][58]. The valence of elements in CSSN NCs was determined by XPS technology. Full XPS spectra of Fe 3 O 4 hollow spheres, FA, FA@Ag, and CSSN NCs are exhibited in Figure S3. Within detection limit of XPS, Fe 2p, O1s, Au 4f, Ag 3d, and C 1s were observed and no impurity was found. High-resolution XPS results of Ag 3d and Au 4f are reflected in Figure 3. Ag 3d spectra in Figure 3a display peaks at 368.2 and 374.2 eV with a spin-orbit splitting of 6 eV for CSSN NCs, which are attributable to characteristics of Ag 3d 3/2 and Ag 3d 5/2 of Ag 0 [53]. As represented in Figure 3b, peaks of CSSN NCs at 84.1 and 87.8 eV with an energy difference of 3.7 eV are attributable to Au 4f 7/2 and Au 4f 5/2 of Au 0 [54,55]. An interesting phenomenon is that the binding energy of Ag 3d as well as Au 4f changes slightly with the incremental introduction of noble metals. The positions of Ag 3d peaks of CSSN NCs are blue-shifted compared with FA@Ag NCs, and the positions of Au 4f peaks are red-shifted compared with FA@Ag and FA NCs. This shift in binding energy may be ascribed to the charge transfer from metallic Au to Ag [56][57][58]. Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 13

Choice of Excitation Source
A widely accepted consensus is that the SERRS method can further improve the sensitivity of Raman scattering spectroscopy, which combines resonance enhancement and SERS [59]. Hence, it is very important to find a suitable excitation source to arouse the SERRS effect. UV-Vis spectra of BPA azo products, FA, FA@Ag, and CSSN NCs were tested to determine a light source with an appropriate wavelength that takes into account the resonance effect of BPA azo products and plasmon resonance effect of CSSN NCs with the laser. As presented in Figure S4, the absorption positions of BPA azo products, FA, FA@Ag, and CSSN NCs are located at 450, 545, 536, and 512 nm, respectively. Because the plasmonic resonance peak of CSSN NCs is closer to the absorption position of BPA azo products compared with FA and FA@Ag NCs, the coupling between BPA azo products and CSSN NCs is considerably easier. Therefore, 514.5 nm laser was selected as the excitation source in this work given that it is more suitable for the coupling absorption.

SERRS Spectra of BPA Azo Product on FA, FA@Ag, and CSSN NCs
In order to directly evaluate the SERRS performance of various substrates, BPA (10 −4 M) was chosen as the target molecule and FA, FA@Ag, and CSSN NCs served as SERRS substrates to explore their SERRS-enhancing capabilities, respectively. The detailed band assignments of BPA azo products are exhibited in Table S2 [37,60]. As reflected in Figure  4, with the incremental introduction of noble metals, the SERRS intensity of BPA gradually increases and CSSN NCs exhibit the strongest SERRS sensitivity compared with FA and FA@Ag NCs. The above phenomenon is foreseeable and will be discussed in detail below.

Choice of Excitation Source
A widely accepted consensus is that the SERRS method can further improve the sensitivity of Raman scattering spectroscopy, which combines resonance enhancement and SERS [59]. Hence, it is very important to find a suitable excitation source to arouse the SERRS effect. UV-Vis spectra of BPA azo products, FA, FA@Ag, and CSSN NCs were tested to determine a light source with an appropriate wavelength that takes into account the resonance effect of BPA azo products and plasmon resonance effect of CSSN NCs with the laser. As presented in Figure S4, the absorption positions of BPA azo products, FA, FA@Ag, and CSSN NCs are located at 450, 545, 536, and 512 nm, respectively. Because the plasmonic resonance peak of CSSN NCs is closer to the absorption position of BPA azo products compared with FA and FA@Ag NCs, the coupling between BPA azo products and CSSN NCs is considerably easier. Therefore, 514.5 nm laser was selected as the excitation source in this work given that it is more suitable for the coupling absorption.

SERRS Spectra of BPA Azo Product on FA, FA@Ag, and CSSN NCs
In order to directly evaluate the SERRS performance of various substrates, BPA (10 −4 M) was chosen as the target molecule and FA, FA@Ag, and CSSN NCs served as SERRS substrates to explore their SERRS-enhancing capabilities, respectively. The detailed band assignments of BPA azo products are exhibited in Table S2 [37,60]. As reflected in Figure 4, with the incremental introduction of noble metals, the SERRS intensity of BPA gradually increases and CSSN NCs exhibit the strongest SERRS sensitivity compared with FA and FA@Ag NCs. The above phenomenon is foreseeable and will be discussed in detail below.

Choice of Excitation Source
A widely accepted consensus is that the SERRS method can further improve the sensitivity of Raman scattering spectroscopy, which combines resonance enhancement and SERS [59]. Hence, it is very important to find a suitable excitation source to arouse the SERRS effect. UV-Vis spectra of BPA azo products, FA, FA@Ag, and CSSN NCs were tested to determine a light source with an appropriate wavelength that takes into account the resonance effect of BPA azo products and plasmon resonance effect of CSSN NCs with the laser. As presented in Figure S4, the absorption positions of BPA azo products, FA, FA@Ag, and CSSN NCs are located at 450, 545, 536, and 512 nm, respectively. Because the plasmonic resonance peak of CSSN NCs is closer to the absorption position of BPA azo products compared with FA and FA@Ag NCs, the coupling between BPA azo products and CSSN NCs is considerably easier. Therefore, 514.5 nm laser was selected as the excitation source in this work given that it is more suitable for the coupling absorption.

SERRS Spectra of BPA Azo Product on FA, FA@Ag, and CSSN NCs
In order to directly evaluate the SERRS performance of various substrates, BPA (10 −4 M) was chosen as the target molecule and FA, FA@Ag, and CSSN NCs served as SERRS substrates to explore their SERRS-enhancing capabilities, respectively. The detailed band assignments of BPA azo products are exhibited in Table S2 [37,60]. As reflected in Figure  4, with the incremental introduction of noble metals, the SERRS intensity of BPA gradually increases and CSSN NCs exhibit the strongest SERRS sensitivity compared with FA and FA@Ag NCs. The above phenomenon is foreseeable and will be discussed in detail below.

Mechanism of SERRS Enhancement
As mentioned above, the widely accepted theory for SERRS enhancement mechanism is EM mechanism, which derives from LSPR excitation of noble metal nanocrystals [61][62][63]. Compared with FA NCs, the surfaces of FA@Ag NCs are almost completely covered with Ag shells, which have better LSPR effect than Au, so it is reasonable that SERRS performance of FA@Ag NCs is somewhat better than that of FA NCs. In addition, strong electromagnetic fields will be excited in/between nearby noble metals because of the coupling effect and thus the SERRS signal intensity of target molecule can be significantly enhanced in hot spot regions [64][65][66]. Consequently, increasing the quantity of hot spots is an effective method to enhance the SERRS activity. To reveal why CSSN NCs have the highest SERRS enhancement, a FDTD theoretical algorithm was employed to visualize the distribution of electromagnetic field. As presented in Figure 5, it can be seen that more hot spots are generated on CSSN NCs. Compared with CSSN NCs, no obvious hot spots are found on the separate FA@Ag NCs (Figure 5a). A reasonable explanation is that the SERRS enhancement of FA@Ag NCs may come from hot spots generated by the aggregation of FA@Ag NCs in an actual detection procedure. As for CSSN NCs, large number of hot spots can also emerge in the region of narrow spacing between two adjacent Au@Ag nanocrystals, as shown in Figure 5b,c. In addition, there are large amounts of hot spots between Ag shells and outermost Au@Ag nanocrystals. It follows that the hot-spots effect is brought into full play through the construction of the core-shell-satellite structure. It also needs to be emphasized here that the introduction of the bimetallic Au@Ag nanocrystals is distinctly important, given that the Au@Ag nanocrystals make full use of excellent SERRS activity of Ag nanocrystals and high stability of Au nanocrystals [67,68]. Therefore, the excellent SERRS performance of CSSN NCs is attributed to a considerable quantity of hot spots generated by the core-shell-satellite structure, as well as excellent SERRS performance of Au@Ag nanocrystals.

Mechanism of SERRS Enhancement
As mentioned above, the widely accepted theory for SERRS enhancement mech nism is EM mechanism, which derives from LSPR excitation of noble metal nanocryst [61][62][63]. Compared with FA NCs, the surfaces of FA@Ag NCs are almost completely co ered with Ag shells, which have better LSPR effect than Au, so it is reasonable that SER performance of FA@Ag NCs is somewhat better than that of FA NCs. In addition, stro electromagnetic fields will be excited in/between nearby noble metals because of the co pling effect and thus the SERRS signal intensity of target molecule can be significan enhanced in hot spot regions [64][65][66]. Consequently, increasing the quantity of hot spo is an effective method to enhance the SERRS activity. To reveal why CSSN NCs have t highest SERRS enhancement, a FDTD theoretical algorithm was employed to visualize t distribution of electromagnetic field. As presented in Figure 5, it can be seen that more h spots are generated on CSSN NCs. Compared with CSSN NCs, no obvious hot spots a found on the separate FA@Ag NCs (Figure 5a). A reasonable explanation is that the SER enhancement of FA@Ag NCs may come from hot spots generated by the aggregation FA@Ag NCs in an actual detection procedure. As for CSSN NCs, large number of h spots can also emerge in the region of narrow spacing between two adjacent Au@Ag nan crystals, as shown in Figure 5b,c. In addition, there are large amounts of hot spots betwe Ag shells and outermost Au@Ag nanocrystals. It follows that the hot-spots effect brought into full play through the construction of the core-shell-satellite structure. It al needs to be emphasized here that the introduction of the bimetallic Au@Ag nanocrysta is distinctly important, given that the Au@Ag nanocrystals make full use of excelle SERRS activity of Ag nanocrystals and high stability of Au nanocrystals [67,68]. Therefo the excellent SERRS performance of CSSN NCs is attributed to a considerable quantity hot spots generated by the core-shell-satellite structure, as well as excellent SERRS p formance of Au@Ag nanocrystals.

Quantitative Detection of BPA Azo Products
In order to assess the practicability of CSSN NCs as SERRS substrate for quantitati and sensitive detection of BPA, BPA azo products with different concentration from 10 to 10 −4 M were chosen as probe molecules. Sharp and strong characteristic peaks of BP azo products can be clearly observed from SERRS spectra illustrated in Figure 6a. SER intensities of BPA azo products rise monotonously with the increase of concentration The limit of detection (LOD) for the detection of BPA is as low as 10 −10 M (about 0.0 ng/mL), which is well below the safety limit of the European Union (0.6 mg/kg), as w as China (10 ng/mL) [69]. More importantly, compared with the previous reports, our a prepared CSSN substrate has the highest SERRS enhancement performance (Table 1)

Quantitative Detection of BPA Azo Products
In order to assess the practicability of CSSN NCs as SERRS substrate for quantitative and sensitive detection of BPA, BPA azo products with different concentration from 10 −10 to 10 −4 M were chosen as probe molecules. Sharp and strong characteristic peaks of BPA azo products can be clearly observed from SERRS spectra illustrated in Figure 6a. SERRS intensities of BPA azo products rise monotonously with the increase of concentrations. The limit of detection (LOD) for the detection of BPA is as low as 10 −10 M (about 0.023 ng/mL), which is well below the safety limit of the European Union (0.6 mg/kg), as well as China (10 ng/mL) [69]. More importantly, compared with the previous reports, our as-prepared CSSN substrate has the highest SERRS enhancement performance (Table 1) [70][71][72][73][74][75]. Moreover, the relationship between the concentrations of BPA azo products adsorbed on CSSN NCs and the corresponding SERRS intensities at 1384 cm −1 is reflected in Figure 6b. The linear relationship versus the logarithm of the concentrations and correlation coefficient (R 2 ) is up to 0.96, which further proves that CSSN NCs are high performance SERRS sensors and can realize quantification of BPA down to 10 −10 M. , 12, x FOR PEER REVIEW 9 of 13 6b. The linear relationship versus the logarithm of the concentrations and correlation coefficient (R 2 ) is up to 0.96, which further proves that CSSN NCs are high performance SERRS sensors and can realize quantification of BPA down to 10 −10 M.

Conclusions
In conclusion, magnetic core-shell-satellite CSSN NCs for ultra-sensitive SERRS detection of BPA have been successfully developed. The coupling reactions between BPA and Pauly's reagents not only improved the affinity between BPA and substrates, but also amplified the SERRS signals due to the SERRS effect generated by the combination of resonance of BPA azo products and plasma resonance of noble metals. BPA azo products were chosen as target molecules to investigate the effect of incremental introduction of noble metals on SERRS activity. The distribution of electromagnetic field of CSSN NCs was studied through FDTD theoretical algorithm. The results revealed that a considerable number of hot spots were produced on the core-shell-satellite structure. The excellent SERRS activity of CSSN NCs was attributed to abundant hot spots of core-shell-satellite structure as well as outstanding SERRS activity of Au@Ag nanocrystals. BPA azo products were used to evaluate the practicability of CSSN NCs as SERRS substrate. When the concentrations of BPA azo products ranged from 10 −10 to 10 −4 M, SERRS intensities followed linear relationship versus the logarithm of the concentrations, and LOD was as low as 10 −10 M. In addition, the Ms value of superparamagnetic CSSN NCs was 53.6 emu·g −1 , which gave CSSN NCs the function of rapid separation and detection in complex liquid environments by an external magnetic field. This study not only provides a novel ultra-sensitive

Conclusions
In conclusion, magnetic core-shell-satellite CSSN NCs for ultra-sensitive SERRS detection of BPA have been successfully developed. The coupling reactions between BPA and Pauly's reagents not only improved the affinity between BPA and substrates, but also amplified the SERRS signals due to the SERRS effect generated by the combination of resonance of BPA azo products and plasma resonance of noble metals. BPA azo products were chosen as target molecules to investigate the effect of incremental introduction of noble metals on SERRS activity. The distribution of electromagnetic field of CSSN NCs was studied through FDTD theoretical algorithm. The results revealed that a considerable number of hot spots were produced on the core-shell-satellite structure. The excellent SERRS activity of CSSN NCs was attributed to abundant hot spots of core-shell-satellite structure as well as outstanding SERRS activity of Au@Ag nanocrystals. BPA azo products were used to evaluate the practicability of CSSN NCs as SERRS substrate. When the concentrations of BPA azo products ranged from 10 −10 to 10 −4 M, SERRS intensities followed linear relationship versus the logarithm of the concentrations, and LOD was as low as 10 −10 M. In addition, the Ms value of superparamagnetic CSSN NCs was 53.6 emu·g −1 , which gave CSSN NCs the function of rapid separation and detection in complex liquid environments by an external magnetic field. This study not only provides a novel ultrasensitive SERRS substrate, but also shows enormous potential for the field of food safety and environmental pollution control.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12193322/s1, Figure S1: Mössbauer spectrum of Fe 3 O 4 hollow spheres; Figure S2: Magnetic hysteresis (M-H) loops of Fe 3 O 4 hollow spheres, FA, FA@Ag and CSSN NCs (The inset is photograph of CSSN NCs dispersed in deionized water before and after magnet separation); Figure S3: Full XPS spectra of Fe 3 O 4 hollow spheres, FA, FA@Ag and CSSN NCs; Figure S4: UV-Vis spectra of BPA azo products, FA, FA@Ag and CSSN NCs; Table S1: Mössbauer spectrum parameters of Fe 3 O 4 hollow spheres; Table S2: Band assignments in the SERRS spectra of BPA azo products.