Recent progress on synthesis and application of plasmonic metal heterostructures

Core-shell heterostructure

Core-shell structured metal heterostructures represent a versatile class of nanomaterials that combine distinct properties of different metals to achieve enhanced performances or new functions. In these structures, a core of one metal is enveloped by a shell of another, enabling the integration of their unique properties. Besides, the ability to fine-tune the core and shell materials, along with their respective thicknesses during the synthesis, provides a great platform to optimize their performance for specific applications^[71]. Seeded-mediated methods are commonly used to prepare core-shell metal heterostructure. During the synthesis, metal seeds with different sizes and morphologies are first prepared as the core material and dispersed in a solvent, followed by the reduction of metal precursor and the deposition of secondary metal as the shell, whose growth could be controlled by tuning the reaction conditions such as the concentration of metal precursor, the shape of the metal seeds, the reductant, temperature, time and pH of the solution.

Typically, Au nanomaterials are frequently used as the core material due to their high stability, uniform size, and tunable morphology, and Ag shells could be easily grown on Au core due to their small lattice mismatch. In a typical work, spiky Au NPs were first prepared as the seed^[72]. Then, the Ag shell was in situ deposited on the surface of these Au cores by polyol process under high-temperature conditions to form Au@Ag core-shell spiky nanomaterials with adjustable size. It is worth noting that the thickness of the Ag shell could be controlled by tuning the added amount of Ag precursor (AgNO₃). In the design of Au@Ag bimetallic core-shell nanostructures, the core’s geometry significantly influences the final morphology of the nanomaterials and Ag coating. In a typical work, Qi et al. successfully synthesized Au nanobipyramids-Ag core-shell (Au NBs@Ag) nanomaterials and Au nanorods-Ag core-shell (Au NRs@Ag) nanomaterials by seed-mediated growth method using Au NBs and Au NRs as the seed^[73]. It is noteworthy that two distinct Ag deposition processes were found on different Au seeds. Specifically, the shape of Au NBs changed from bipyramid to rice, and finally cylindrical rod, while Au NRs gradually changed from nanorod shape to spherical, triangular pyramid, octahedron, and other irregular forms. It is worth noting that the volume of the Au NBs seed solution affects the aspect ratios of Au NBs@Ag and the shape of its tip. When the seed solution was used in small quantities, it would give Au NBs@Ag a sharp tip. When a large amount of seed solution was used, the sharp edges would disappear. Self-assembled Au nanostructure could also be used as the seed for the growth of Ag to form core-shell heterostructure. In a typical work, Tanwar et al. first synthesized Au nanorods through seed-mediated growth and then self-assembled them into vertical arrays^[35]. By epitaxial growth of Ag on pre-assembled Au nanorods, a self-assembled bimetallic Au-Ag nanorods vertical array was finally obtained, and the precise regulation of the distance between Au and Ag was realized [Figure 1A].

Figure 1. (A) Schematic illustration of the fabrication of an Au-Ag nanorod array^[35]. TEM image with a schematic illustration of the structure of (B) Au-Cu core-shell nanocubes and (C) Au-Cu core-shell octahedra^[74]. (D) Schematic illustration of the synthesis of Au@Cu nanocubes and (E) EDX mapping of Au@Cu core-shell nanocubes^[75].

Cu can also be deposited on Au to form a core-shell structure by fine-tuning the reaction process with the assistance of appropriate ligand, despite the large lattice mismatch between the two metals. For instance, Hsia et al. synthesized Au-Cu core-shell nanocubes [Figure 1B] and octahedra [Figure 1C] by aqueous-phase synthesis^[74]. In this process, octahedral Au nanocrystals were first synthesized using a hydrothermal method and used as the seed for the growth of Cu. Then, Cu precursor was reduced at high temperatures using ascorbic acid as the reducing agent in the presence of hexadecylamine (HDA), Au-Cu core-shell nanocubes and Au-Cu core-shell octahedras were synthesized by using CuCl₂ and Cu (CH₃COO)₂ as the Cu precursor, respectively. In the end, Au-Cu core-shell nanocubes with adjustable edge lengths and octahedra with different opposite corner distances were formed. During synthesis, the addition of HDA increased the pH of the solution, and also acted as the coordinating ligand of Cu ions to promote the controlled growth of Cu shells. However, the large lattice mismatch between Au and Cu led to uneven Cu deposition, causing the Au cores in Au-Cu core-shell nanocubes and octahedrons to shift from a central position. In another work, Lyu et al. synthesized Au@Cu core-shell nanocubes using a seed-mediated growth method^[75], which employed glucose as a reducing agent, with HDA and chloride ion serving as capping agents for the Cu {100} crystal face. The thickness of the Cu shell was controlled by changing the reaction time and/or the concentration of Au seeds. A longer reaction time or a higher concentration of Au seeds would lead to a decreased thickness of the Cu shell, and Au@Cu nanocubes with adjustable size ranging from 20 and 30 nm were obtained [Figure 1D]. Notably, the distribution of Au cores was random due to a 12% large lattice mismatch between Au and Cu, resulting in local epitaxial growth of Cu shells on Au seeds. The elemental distribution of the core (Au, green) and shell (Cu, red) could be clearly observed in the energy-dispersive X-ray (EDX) mapping [Figure 1E].

In addition to Au nanomaterials, Ag nanomaterials with various morphologies can also serve as the core for the growth of other plasmonic metals, forming core-shell heterostructures. For instance, Ag nanowires could be prepared using the polyol method as a template^[76]. Subsequently, core-shell Ag@Au nanowires were prepared by the deposition of Au shell onto the Ag core. These Ag@Au nanowires could then be welded together through mixed vapor-induced capillary force, forming stretchable and transparent electrodes. In another work, Zhong et al. prepared Ag@Cu core-shell catalysts with Ag cores and porous Cu shells using Ostwald ripening and in situ electrochemical reduction techniques [Figure 2A]^[77]. By controlling the ripening time, three types of Ag@Cu catalysts with different pore sizes of the Cu shell were obtained, namely [email protected] (mean pore size of 2.8 nm), [email protected] (mean pore size of 4.9 nm) and [email protected] (mean pore size of 11.2 nm) with the transmission electron microscopy (TEM) images shown in Figure 2B-D.

Figure 2. (A) Schematic illustration of the synthesis of three types of Ag@Cu heterostructures, (B-D) TEM images of [email protected] (B), [email protected] (C), and [email protected] (D)^[77]. (E) Schematic illustration of the synthesis process of Cu@Ag core-shell nanoparticles^[37].

Cu is less used as the core material due to the easy oxidation during the reaction. Zhang et al. synthesized Cu@Ag core-shell nanoparticles by a seed-mediated method^[78]. Specifically, Cu nanoparticles were first synthesized and used as seeds for the subsequent deposition of Ag. The thickness of the Ag shell could be adjusted by varying the amount of Ag precursor. One key challenge in preparing Cu-based heterostructure is to prevent the oxidation of Cu during the reaction. To address this issue, Kuhn et al. developed a one-pot method involving sequential synthesis in a flowing CO atmosphere, in which Cu nanoparticles were first synthesized as the core, and the Ag shell was subsequently grown on the Cu core by directly injecting Ag precursor into the solution [Figure 2E]^[37]. During the reaction, the thickness of the Ag shell could be controlled by adjusting the reaction time. With a longer reaction time, the thickness of the Ag shell could be increased, which ranged from a sub-atomic monolayer to several atomic layers.

Core-satellite heterostructures

Core-satellite structured metal heterostructures comprise a central core particle surrounded by multiple smaller satellite particles. This unique architecture allows for enhanced surface area and increased interactions between the core and satellites, resulting in improved optical, electronic, and catalytic properties^[79]. Typically, the synthesis of core-satellite heterostructures begins with the preparation of the metal core, followed by the deposition of smaller satellites through the reduction of a metal precursor in a seed solution. To obtain core-satellite instead of core-shell heterostructure, the reduction and deposition of the shell on the metal core should be modulated by controlling factors such as reaction conditions, ligand selection, and surface properties of the seed. For example, lower metal precursor concentrations or faster reaction kinetics typically result in the formation of core-satellite heterostructures. In contrast, slower reaction kinetics or the use of ligands that strongly bond to the metal core surface can promote the growth of a continuous shell, resulting in a core-shell morphology.

In a typical study, core-satellite Au-Ag nanoparticles were synthesized by seed-mediated growth method using Au NPs as the seed^[80]. It is worth noting that core-shell Au-Ag heterostructures could be obtained using the same seed-mediated method except for increasing the concentration of Ag precursor. During the growth, the size of the resultant core-satellite Au-Ag nanoparticles could be controlled by changing the added amount of Ag precursor (AgNO₃) or the Au seeds in the solution. Furthermore, a one-dimensional nanochain was then generated by self-assembly of the obtained core-satellite Au-Ag nanoparticles [Figure 3A], with the TEM image shown in Figure 3B.

Figure 3. (A) Schematic illustration of the formation of Au-Ag nanochains by the assembly of hybrid Au-Ag nanoparticles and (B) TEM image of Au-Ag nanochains assembled by core-satellite Au-Ag nanoparticles^[80]. (C) Schematic illustration of the synthesis and (D) EDS elemental mapping of Au@Ag@SiO₂-AuNP nanoassemblies labeled with 4-MBA^[81].

The core-shell-satellite structure extends the core-satellite design by adding an intermediate layer (usually silica, SiO₂) between the core and satellite particles. This SiO₂ interlayer enhances water solubility of the core-shell-satellite structure compared to the core-satellite structure and ensures that the satellite particles are uniformly distributed across the silica surface, effectively preventing nanoparticle aggregation. For example, Yang et al. prepared multilayer Au@Ag@SiO₂-AuNP core-shell-satellite nanostructures by layer-by-layer deposition [Figure 3C]^[81]. First, Au nanospheres were synthesized and coated with a layer of Ag to form Au@Ag core-shell heterostructure. Subsequently, 4-mercaptobenzoic acid (4-MBA) was attached to the Au@Ag surface to facilitate the growth of a SiO₂ layer. In addition, (3-aminopropyl) trimethoxysilane (APTMS) was used to introduce amine functionality on the surface to modify Au@Ag@SiO₂. Finally, Au nanosphere satellites were grown on the surface of Au@Ag@SiO₂ by seed-mediated method by rapid reduction of Au salt with a strong reductant (NaBH₄) to obtain Au@Ag@SiO₂-AuNP heterostructure. The number of Au satellites could be increased by adding more Au salt. Energy dispersive X-ray spectroscopy (EDS) elemental mapping [Figure 3D] confirms the formation of the 4-MBA-labeled Au@Ag@SiO₂-AuNP nanoassemblies.

Janus heterostructures

The term "Janus" originates from Roman mythology, referring to the god of gates, who is depicted with two faces looking in opposite directions^[82]. Janus particles are nano- or microscale particles with two distinct chemical or surface compositions. In recent years, Janus nanoparticles have garnered considerable attention due to their unique properties and diverse applications in fields such as SERS detection^[83], catalysis^[84], and biosensing^[85]. For the synthesis of Janus heterostructures, the key is to achieve the asymmetric growth of the metal on the pre-synthesized seeds. Several strategies could be applied to achieve the preparation of Janus metal heterostructures, such as template-assisted growth, surface functionalization of the seed with specific ligands or surfactants, manipulation of the reaction kinetics, and usage of lattice mismatch between the metal seed and secondary metal.

Janus heterostructures can be prepared by partially coating the seed with the inert material, followed by the deposition of secondary metals on the uncoated sites and the subsequent removal of the inert materials. For example, Qiu et al. proposed a general method for synthesizing Janus heterodimers of metal nanoparticles using Au NPs as the seed^[86]. In this approach, the surface of Au NPs was partially covered with polystyrene (PS). The presence of Au NPs during styrene polymerization affected the growth pattern of PS, limiting its growth to one side of the Au NPs [Figure 4A]. Then, the unoccupied surface of Au NPs served as the active site for the heterogeneous nucleation and growth of Ag NPs, thus promoting the asymmetric growth of Ag in the dimer structure and forming PS-Au-Ag colloidal particles. Finally, PS was removed by tetrahydrofuran (THF) to obtain Au-Ag Janus heterodimers. It is worth noting that the contact area between the two metal nanoparticles could be adjusted by controlling the exposed area on the surface of the Au NPs after coverage of PS. The TEM image [Figure 4B] clearly showed the formation of Janus heterostructure, while the EDX mapping showed the spatial separation between Au and Ag [Figure 4C]. In another work, Feng et al. introduced a stepwise seeded growth method for synthesizing rod-shaped plasmonic metal nanostructures^[87]. As shown in Figure 4D, Au NPs were first deposited on the surface of SiO₂ spheres, followed by the deposition of an additional SiO₂ layer onto the resulting SiO₂-Au structure. Due to the different chemical properties of Au and SiO₂, the affinity of SiO₂ for itself is greater than that for Au. Taking advantage of this difference, the additional SiO₂ layer was preferentially deposited on the surface of the SiO₂ substrate in SiO₂-Au@SiO₂ nanostructure, in which part of Au NPs near the attachment point was embedded into the additional SiO₂ layer, while the other part of AuNPs was exposed. Next, other metal components such as Ag NPs were grown on the exposed surface of Au NPs to form a Janus heterodimer. It is worth noting that the ratio of the exposed surface of Au seeds during seed growth depends on the thickness of the additional SiO₂ layer [Figure 4E], which led to the formation of three types of Au-Ag Janus heterodimers [Figure 4F].

Figure 4. (A) Schematic illustration of the synthesis of Janus heterodimers, (B) TEM image of Au-Ag Janus dimers and (C) STEM image with elemental mapping of an induvial Au-Ag Janus dimer^[86]. (D) Schematic illustration of the synthesis of Janus dimer on the surface of SiO₂ spheres, (E) TEM images with the structural model in the inset of SiO₂-Au@SiO₂ with different layer thicknesses of the additional SiO₂ layer and (F) TEM images with the structural model in the inset of SiO₂-Au dimers after growth of Au. The scale bars in (G and H) are 50 nm^[87]. (G) Schematic illustration of the synthesis of Au-Ag Janus@Au (solid) NPs and Au-Ag Janus@Au (hollow) NPs under different pH conditions and (H) STEM image with elemental mapping of Au-Ag Janus@Au (solid) nanoparticles^[89].

The use of appropriate ligands is another effective approach to achieve the synthesis of Janus metal heterostructures, which modulate the surface energy and provide selective growth sites on the metal seeds, enabling localized deposition of the secondary metal. In a typical work, Wang et al. synthesized Au-Ag Janus nanoparticles using a seed-mediated growth method with Au NPs as the seed^[88]. Specifically, 2-mercaptobenzoimidazole-5-carboxylic acid (MBIA) was used to modify Au NPs and regulate the metal-metal interfacial energy between Au and Ag, thereby guiding the directional deposition of Ag islands. Hydroquinone (HQ) served as the reducing agent to reduce AgNO₃, form Ag islands over the Au NPs and resulted in anisotropic Au-Ag Janus nanoparticles with good dispersion and uniform morphology. Xu et al. further grew Au spots on the surface of Au-Ag Janus nanoparticles to form Au-Ag Janus@Au NPs^[89]. Notably, the structure of Au-Ag Janus@Au NPs could be tuned by altering the pH of the solution, leveraging the galvanic replacement reaction between HAuCl₄ and Ag islands [Figure 4G]. Hollow Au-Ag Janus@Au NPs (denoted as Au-Ag Janus@Au(hollow) NPs) were produced at a pH range of 3.0-5.0, while solid Au-Ag Janus@Au NPs (denoted as Au-Ag Janus@Au(solid) NPs) formed under a pH range of 7.0-9.5. It was reported that the formation of Au-Ag Janus@Au(solid) NPs was attributed to the strong reducing capability of ascorbic acid under specific alkaline conditions, which preferentially reduced HAuCl₄ and inhibited the galvanic replacement reaction. The scanning TEM (STEM) image and element mapping in Figure 4H clearly revealed the distribution of Au and Ag elements within the Au-Ag Janus@Au(solid) NPs, further confirming the formation of the Au-Ag Janus structure. Moreover, the growth of numerous Au dots could also be observed on the surface of Au-Ag Janus nanoparticles. In another work, Zeng et al. proposed a novel seed-mediated strategy for the construction of Au-Ag Janus metal heterostructure by modulating the atomic absorption energy on the surface of the Au seed^[90]. Ag atoms were preferentially deposited in areas with lower atomic absorption energy for further growth, achieving continuous tunability of the contact area between Au and Ag and the number/size of Ag islands on Au seeds within the same system. During the reaction, the surface of the Au seeds was first modified with two ligands including poly(-diallyldimethylammonium) chloride (PDDA) and fish sperm DNA (FSDNA). Due to the higher atomic adsorption energy on the FSDNA-modified Au surface than that on the PDDA-modified Au surface, Ag atoms deposited and grew preferentially on the PDDA-rich areas of the Au sphere surface to achieve the preparation of Janus Au-Ag heterostructure. In another work, Wu et al. synthesized various Au-Ag hybrid nanoparticles, including core-shell, Janus, and core-satellite structures by controlling the amounts of MBIA, AgNO₃, or HQ added^[91]. As the volume of AgNO₃ increased, the morphology of the Au-Ag hybrid nanoparticles changed from Janus structure to core-satellite structure. This change occurred because the concentration of AgNO₃ affected the distribution and deposition rate of Ag, which in turn influenced the morphology of nanoparticles. Similarly, the distribution and adsorption of MBIA molecules on the surface of Au NPs affect the amount and deposition locations of Ag, thus regulating the morphology of nanoparticles. The increased MBIA concentration led to a transition from core-shell to Janus and then to core-satellite structures of the Au-Ag hybrid nanoparticles. Besides Au-Ag Janus heterostructures, Au-Cu Janus heterostructures could also be prepared by choosing a suitable ligand to influence the interfacial energy between Au and Cu. Fan et al. developed a ligand-assisted seed-mediated method to prepare the Au-Cu Janus heterostructure, which achieved the island growth of Cu on Au seeds by introducing a strong thiol ligand to increase the interfacial energy between Au seeds and Cu domain^[92]. Besides, the degree of asymmetry of the Au-Cu could be continuously tuned by modulating the amount of thiol ligand adsorbed on Au seeds. More interestingly, this method could be extended to Au seeds with different morphologies, including Au nanosphere, nanorod, and nanoplate. Zhang et al. proposed a semi-affinity strategy based on a seed-mediated method using the Au nanosphere as the seed^[93]. Specifically, the seed-mediated synthesis of uniformly distributed Au-Cu heterostructures (Au-Cu HSs) with a continuously tuned interface was performed with the assistance of 1-hexadecylamine (HDA) and PDDA [Figure 5A]. By using HDA as the surfactant, the coverage of the Au-Cu interface was continuously adjusted to form a series of structurally adjustable Au-Cu cascade catalysts ranging from Au-Cu dimer (Au-Cu_I, Figure 5B) and Janus structures (Au-Cu_II, Figure 5C) to acorn-like Janus structure (Au-Cu_III, Figure 5D) and core-shell structures (Au-Cu_IV, Figure 5E).

Figure 5. (A) Schematic illustration of the synthesis of distinct Au-Cu Janus heterostructures. (B-E) STEM images of different Au-Cu Janus heterostructures: (B) Au-Cu_I, (C) Au-Cu_II, (D) Au-Cu_III, and (E) Au-Cu_IV^[93]. (F) Structural models of Ag-Cu Janus heterostructures with different ratios of the Cu domain, (G) TEM image, and (H) XRD pattern of Ag₁-Cu_1.1 NDs^[94].

Controlling the reaction kinetics by rationally tuning the experimental parameters such as the injection rate of metal precursors, type of metal precursors, and the strength of reducing agents can limit the speed of deposition of the secondary metals on the seed during the seed-mediated growth, thus facilitating the formation of Janus heterostructures. For example, Huang et al. employed a seeded-mediated approach to synthesize Ag-Cu nanodimers (NDs) with a tunable size of the Cu domain [Figure 5F]^[94]. The size of Cu domains on Ag NPs was controlled by using different Cu precursors and reducing agents to control the reaction kinetics during the overgrowth of Cu. The TEM image [Figure 5G] of the obtained Ag₁-Cu_1.1 (The mass ratio of Ag to Cu is 1:1.1) confirmed its typical Janus dimer structure. The X-ray Diffraction (XRD) pattern [Figure 5H] showed two sets of diffraction peaks corresponding to Ag and Cu, indicating the elemental separation of Cu and Ag within the dimer nanocrystals. Usually, syringe pumps can be used for the slow injection of the metal precursor and/or the reductant to lower the reaction kinetics. For example, Ma et al. synthesized three types of Ag-Cu Janus nanostructures with exposed {100} facets (JNS-100) and different atomic ratios between Ag and Cu by growing Cu on pre-synthesized Ag nanocubes^[95]. The Janus structure was formed by precisely controlling the reaction kinetics using a syringe pump. The growth of Cu domains with different sizes was effectively restricted to the {100} face of one of the six equivalent faces of the Ag NCs by adjusting the reduction kinetics of surfactants and Cu precursors [Figure 6A]. The high-angle annular dark-field (HAADF-STEM) image [Figure 6B-D] and the corresponding EDS element mapping [Figure 6E-G] clearly indicated the formation of Ag-Cu Janus heterostructures with different sizes of the Cu domains.

Figure 6. (A) Schematic of the synthesis of Ag-Cu JNS-100 Janus heterostructures and (B-G) HAADF-STEM images and EDS elemental mapping of different Ag-Cu JNS-100 Janus heterostructures: Ag₆₅-Cu₃₅ JNS-100 (B, E), Ag₅₀-Cu₅₀ JNS-100 (C, F), and Ag₂₅-Cu₇₅ JNS-100 (D, G)^[95].

Surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) was first observed in 1,974 with pyridine adsorbed on a rough Ag electrode^[96]. This technique is widely utilized across various fields due to its ability to provide molecular fingerprint information^[97-99], enable non-invasive detection^[100-102], and achieve high sensitivity^[103,104]. The SERS performance of metal nanoparticles is closely linked to their LSPR^[105,106]. Key factors influencing the local electric field strength and SERS effectiveness include the nanoparticle edges' sharpness, nanoscale gaps, and surface roughness^[107]. Precise control over the shape and growth of individual components is essential for enhancing SERS performance.

Plasmonic metal heterostructures have emerged as a transformative platform for SERS, significantly amplifying the sensitivity and specificity of molecular detection^[71]. By integrating different metals, these heterostructures create intricate plasmonic landscapes that enhance localized electromagnetic fields, which are crucial for boosting the Raman signal of nearby molecules^[108]. This approach also allows for fine-tuning of plasmonic properties, enabling optimized SERS performance across various wavelengths and enhancing the detection limits. Additionally, the versatility in designing these heterostructures opens new avenues for tailoring sensor properties. Du et al. utilized Au nanodumbbells (Au NDs) as seeds and varied the concentration of 4-MBA in the growth solution, synthesizing a series of Au NDs@Ag NPs with diverse morphologies, including dumbbell-like core-shell structures, L-shaped Janus structures, and rod-like core-shell configurations^[109]. Among these, the L-shaped Janus Au NDs@Ag NPs demonstrated the highest SERS enhancement factor (EF) of 1.41 × 10⁷, which was attributed to the adjustable negative curvature and Ag deposition that generate sufficient hot spots. Arrays of ordered plasmonic nanoparticles are in high demand for optical sensing applications due to their ability to generate uniformly distributed plasmonic hotspots. This uniformity arises from their periodic structure and the effects of near-field coupling. For example, Tanwar et al. successfully designed a self-assembled vertical array of bimetallic Au-Ag nanorods^[35]. At the characteristic peak of 1,070 cm^-1, the SERS signal intensity of 1 μM 4-MBA on the vertical array of Au-Ag nanorods was about five times higher than that of the monometallic Au nanorods array [Figure 7A and B]. It was found that this structure exhibited uniformly distributed plasma hot spots, which tightly confined the hot spots within the gaps between particles and created an enhanced electric field.

Figure 7. (A and B) The SERS spectra of 1 μM 4-MBA dropped on Au and Au-Ag nanorod arrays, with orange and cyan representing the peaks corresponding to CTAB and 4-MBA, respectively^[35]. (C) SERS spectra of Au-Ag Janus@Au NPs with and without 0.1 mM H₂O₂^[87]. (D) Schematic illustration of the evaluation of SERS performance using CJS components with 4-NTP as a signal molecule. (E-G) The localized electric field distribution of SiO₂@Au₍₃₀₀₎-Ag ECS CJS, SiO₂@Au₍₃₀₀₎-Ag NSM CJS, and SiO₂@Au₍₃₀₀₎-Ag HD CJS components, respectively, (H and I) SERS mapping at the 4-NTP peak of 1,332 cm^-1 (42 × 45 μm²) and SERS intensity distribution of 250 counts for SiO₂@Au₍₃₀₀₎-Ag_(30μL) composites^[110].

The stability and anti-oxidation properties of the substrate are crucial for the overall SERS performance. For instance, Feng et al. prepared two types of nanoparticles: Au-Ag Janus@Au(hollow) NPs and Au-Ag Janus@Au(solid) NPs^[87]. The SERS signal from Au-Ag Janus@Au (solid) NPs was enhanced by a factor of 17.46 compared to Au-Ag Janus@Au(hollow) NPs, and by a factor of 2.27 compared to Au-Ag Janus NPs. This Raman enhancement results from the strong plasmon coupling effect between Au-Ag Janus NPs and Au dots. When hydrogen peroxide (H₂O₂) was introduced to compare the stability of Au-Ag Janus@Au NPs and Au-Ag Janus NPs, the SERS signal of the latter weakened due to the partial oxidation of the Ag islands by H₂O₂. In contrast, the SERS signal of Au-Ag Janus@Au NPs remained largely unchanged, indicating good stability [Figure 7C]. In another work, Wu et al. developed various plasmonic heterostructures for SERS, including Au-Ag core-shell, Janus, and core-satellite NPs. It was found that the Au-Ag core-satellite NPs with small Ag islands had the highest SERS activity^[91], as the Ag islands generated more hot spots through interior gaps and high curvature. These NPs served as effective immune probes for the ultra-sensitive detection of carbohydrate antigen 19-9, achieving a wide linear range of 1.00 × 10^-5 to 1.00 × 10⁴ IU/mL and a low detection limit of 1.25 × 10^-6 IU/mL. Hao et al. prepared three kinds of core-Janus satellite (CJS) structures: SiO₂@Au-Ag eccentric core-shell CJS (SiO₂@Au-Ag ECS CJS), SiO₂@Au-Ag nanosnowman-like CJS (SiO₂@Au-Ag NSM CJS), and SiO₂@Au-Ag heterogeneous dimer CJS (SiO₂@Au-Ag HD CJS)^[110]. Using 4-nitrothiophenol (4-NTP) as the Raman label [Figure 7D], the SERS performance of these Janus satellite structures was evaluated, which varied in shape and density. When the number of satellites was constant, SiO₂@Au-Ag NSM CJS exhibited superior SERS activity compared to both SiO₂@Au-Ag ECS CJS and SiO₂@Au-Ag HD CJS. It is worth noting that SiO₂@Au-Ag NSM CJS has a smaller nanogap than SiO₂@Au-Ag HD CJS, and the nanosnowman Janus structure has a smaller nanogap and a sharper neck than the eccentric core-shell Janus structure, aligning with simulation results [Figure 7E-G]. In addition, increasing satellite density significantly improved SERS activity. Among the structures tested, SiO₂@Au₍₃₀₀₎-Ag NSM CJS with a SiO₂-to-Au volume ratio of 1:300 achieved the highest EF of 3.04 × 10⁸. The SERS intensity [Figure 7H] and relative standard deviation [Figure 7I] for sample SiO₂@Au₍₃₀₀₎-Ag_(30μL) (that is, with 30 μL of AgNO₃ added during synthesis) demonstrated excellent uniformity and reproducibility, indicating strong potential for practical applications. It is claimed that these outstanding performances could be attributed to the well-defined and robust heterostructure which ensures a homogeneous SERS signal and reduces the effect of structural randomness on the fluctuation of the EF.

Nanogaps between the unique components of heterostructures facilitate plasmon coupling and create electromagnetic hotspots, which significantly enhance SERS activity. In most plasmonic substrates used for SERS, hotspots generally form due to nanoparticle aggregation and self-assembly, known as exterior nanogaps. In contrast, the creation of interior nanogaps within individual nanostructures results in more consistent and reliable SERS signals. In a typical study, anisotropic Au-Ag bimetallic nanostructures featuring interior hotspots could be produced by selectively overgrowing Ag NPs on the tips of Au nanostars (Au NSs)^[111]. Besides, the size of these interior nanogaps could be tuned by adjusting the concentration of the Ag precursor, AgNO₃. As the size of the nanogaps decreased, the SERS EF increased significantly. Under 785 nm light excitation, the Au-Ag nanostructures achieved a maximum EF of 43 for the SERS signal, demonstrating better reproducibility compared to bare Au NSs. More importantly, the Au-Ag nanostructures with Ag on the tips of Au NSs showed much greater SERS enhancement than a mixture of Au NSs and similarly sized Ag NPs. This result indicated that the Raman enhancement originates from the ultrasmall nanogaps between the branches of each Au NS, which acted as intense hotspots for SERS. In another work, Wang et al. developed a concave Au nanocube-Ag (c-AuNC-Ag) hybrid structure that showed enhanced SERS and tunable LSPR performance^[112]. Among the variants, the SERS signal of c-AuNC-Ag Janus structure was significantly enhanced under 785 nm laser excitation. In contrast, the SERS signal from the c-AuNC-Ag Janus structure was significantly greater than that of bare Ag NPs and c-AuNC seeds of similar concentration and size. Specifically, its SERS signal intensity was 17 times greater than the combined SERS intensities of Ag@5-amino-2-mercaptobenzimidazole (AMBI) NPs and c-AuNC@AMBI, and 30 times that of c-AuNC@AMBI alone. This result clearly shows that the SERS enhancement is mainly due to the strong absorption of the Janus structure near the 785 nm wavelength and the resonance of the 785 nm excited laser.

Sensing

Plasmonic metal heterostructures, due to their unique optical properties and high surface-to-volume ratio, exhibit superior performance in detecting biomolecules or chemicals and are widely used in colorimetric and spectroscopic sensing^[113]. Zhao et al. synthesized core-shell multibranched Au NPs (MBAuNPs) coated with Ag NPs using a seed-mediated method and introduced 4-mercaptobenzonitrile (4-MBN) as Raman labels^[114]. Among the tested structures, MBAuNP@4-MBN@Ag NPs exhibited the highest SERS intensity, achieving a surface EF of 1.4 × 10⁷. Compared to the control experiment, the SERS intensity of MBAuNP@4-MBN@Ag NPs was approximately 10 times greater than that of MBAuNP. The enhanced Raman signal arose from the ultra-small gaps between the Ag shell and the MBAuNP core, which created strong hotspots for SERS. Notably, the SERS intensity of MBAuNP is over six times greater than that of spherical Au NPs (AuSNPs). This enhancement is attributed to the rough surface and branched protrusions of the nanoparticles that generate additional hotspots. Moreover, the MBAuNP@Ag nanosensor was constructed to detect endogenous H₂S in living cells, with a detection limit of approximately 0.12 nM.

Plasmonic core-satellite heterostructures are characterized by numerous hot spots resulting from plasmonic coupling between the core and the satellites, as well as among the satellites themselves, which lead to enhanced electromagnetic interactions and improved SERS effects. Theoretical predictions indicated that the electric field intensity at the interstitial sites of core-satellite assemblies was approximately 660 times greater than that on the surface of the cores^[115]. Furthermore, in contrast to hot spots formed solely through particle aggregation, core-satellite structures demonstrated greater repeatability due to the strong affinity inherent in core-satellite interactions. For example, Yang et al. prepared an Au@Ag@SiO₂-AuNP multilayer core-shell satellite nanostructure, which was used for the detection of alpha-fetoprotein [Figure 8A]^[81]. It was revealed that an Ag shell with a larger thickness on the Au core and a monolayer of smaller AuNP satellites could lead to the best SERS activity. The result showed that with the increase of AgNO₃ volume, the Ag shell gradually thickens, and the SERS signal intensity of 4-MBA was greatly enhanced [Figure 8B]. It could be attributed to the strong electromagnetic field enhancement of Ag coating, which led to the enhancement of SERS activity. Besides, Au@Ag@SiO₂-AuNP core-shell satellite nanostructure with single-layer satellite AuNPs grown on the core of Au@Ag@SiO₂ exhibited enhanced SERS activity than its multiple-layer counterpart [Figure 8C], which was because excess satellites may cause radiation damping of incident light and reduce SERS activity. Although increased density creates more “hot spots” between the satellite and core, the SERS signals originate primarily from within the satellites rather than on their surfaces, weakening the coupling effect on Raman enhancement. As the size of the satellite AuNPs increased and approached a continuous rough shell, the SERS signal diminished due to the narrowing of gaps between coupling pairs [Figure 8D], which restricted incident light from entering the core, further reducing SERS activity. This novel structure achieved an impressive detection limit of 0.3 fg/mL and demonstrated a wide linear response range from 1 fg/mL to 1 ng/mL, showcasing its effectiveness in sensitive biomolecule detection.

Figure 8. (A) Schematic diagram of Au@Ag @SiO₂-AuNP multilayer core-shell satellite nanostructure for the detection of alpha-fetoglobulin, (B) SERS spectra of 4-MBA-labeled Au@Ag@SiO₂-AuNP nanoassemblies with different thicknesses of silver coating, (C) SERS spectra of 4-MBA-labeled Au@Ag@SiO₂-AuNP nanoassemblies with different numbers of satellites per core and (D) SERS spectra of 4-MBA-labeled Au@Ag@SiO₂-AuNP nanoassemblies with various sizes of satellite AuNPs^[81]. (E) Schematic illustration of the structure of Au@Ag CSNSs and (F) their SERS spectra using different SERS substrates^[116].

Engineering the shape and architecture of the metal heterostructures could be another promising approach to enhance the sensing performances. In a typical study, various forms and compositions of metal nanomaterials were synthesized and compared for their Raman strengths^[116]. At a fixed concentration of 10^-3 M of crystal violet (CV), the Raman strength of Ag-coated Au NSs (Au@Ag CSNSs, Figure 8E) was measured at 15,182 (a.u.) at the characteristic peak of 1,171 cm^-1, which was approximately eight times greater than that of Au NSs. The enhanced SERS activity is attributed to the efficient plasmon coupling between the high-conductivity Ag shell and the star-shaped Au core. In contrast, spherical nanomaterials such as Ag NPs, Au NPs, and Ag-coated Au NPs exhibited lower Raman signals at 1,171 cm^-1 compared to their star-like counterparts, including Ag NSs, Au NSs, and Au@Ag CSNSs [Figure 8F]. This enhancement arises from the branched structure, which generates more hotspots and stronger local electromagnetic fields. Besides, CV was detected using optical fiber (OF)-based sensors with Au@Ag CSNSs, achieving a minimum detection limit of approximately 5 × 10^-8 M. Ding et al. developed an electrochemiluminescence (ECL) sensor utilizing an array structure of Au@Ag nanorods and sulfur dots^[117]. It was reported that the ECL intensity of sulfur dots with ionic liquid capping (S dots (IL)) increased by 4.5 times when the Au@Ag nanorods were arranged in an array on the electrode. Notably, the ECL intensity of S dots (IL) from regularly arranged Au@Ag nanorods surpassed that from irregularly arranged nanorods. This observation suggested that the regular arrangement of Au@Ag nanorods created highly uniform and dense hot spots, enhancing the local electromagnetic field. In another work, Zhao et al. prepared Au nanocluster-decorated Ag@SiO₂ (Au NCs-Ag@SiO₂) nanocomposites, observing that the photocurrent reached its maximum at an optimal thickness of SiO₂ layer of 15 nm, yielding a photocurrent intensity 3.8 times greater than that of the Au NCs alone^[118]. These Au NCs-Ag@SiO₂ nanocomposites were subsequently employed to construct photoelectrochemical (PEC) biosensors for detecting alkaline phosphatase activity with high sensitivity. Experimental results demonstrated that the PEC biosensor offered a low detection limit of 0.022 U·L^-1 and a wide linear range of 0.04 to 400 U·L^-1. Zhang et al. developed a Au core-4-mercaptobenzonitrile-Ag shell nanoparticle (Au@4-MBN@Ag) as a smart SERS nanoprobe to detect endogenous H₂S in living cells based on the interaction between the Ag shell and sulfide species^[119]. As the concentration of sulfide increased, more Ag₂S was generated through the reaction with the Ag shell, leading to a gradual reduction in the SERS signal intensity of Au@4-MBN@Ag. The findings indicated that the enhancing ability of Ag is significantly greater than that of Ag₂S. Notably, the formation of Ag₂S did not alter the gap between the Au core and Ag shells or the overall morphology of the nanoprobes. The result showed that the use of the Au@4-MBN@Ag nanoprobe can provide highly sensitive H₂S detection at the nM level with a detection limit of (0.14 nM). In another work, Zhu et al. fabricated (Au nanobipyramid)@Ag nanostructures by overgrowing Ag on monodisperse Au nanopyramids^[120]. Notably, as the supply of Ag precursor increased, a structural evolution was observed, transitioning from bipyramids to rice-shaped and finally to rod structures. The Au nanobipyramid@Ag nanostructures exhibited narrower plasmon linewidths and enhanced chemical stability. This improvement was primarily attributed to the unique design of the core-shell structure, where the Au nanopyramid contributed essential plasmonic properties, and the overgrowth of Ag further enhanced these properties while providing additional chemical stability. These nanostructures were utilized to detect sulfide ions, achieving a relatively low detection limit of 0.1 × 10^-6 M. He et al. prepared Au@Ag nanopencils with an Au@Ag rod and Au tip as the sensor for the detection of ClO^- and SCN^-[121]. The asymmetrical etching in different systems produced diverse variations in the LSPR peaks. In the presence of ClO^-, the Au@Ag shell remained intact while Ag was etched from the rod's tip to its end, leading to a redshift in the LSPR peak as the coupling resonance weakens. Conversely, in SCN^- and O₂•^- systems, a blue shift in the LSPR peak occurred due to a decrease in the aspect ratio as the Au@Ag rod was etched from end to tip. Experimental results indicated that the detection limits for ClO^- and SCN^- were 6.7 and 160 nM, respectively, with linear ranges of 0.05-13 and 1-600 μM, respectively.

Electrocatalysis

Plasmonic metal heterostructures have gained significant attention in the field of electrocatalysis due to their unique ability to enhance catalytic activity through synergistic effects between different metal components. It is also assumed that the superior catalytic performance of plasmonic metal heterostructures may also be related to their plasmonic effects. By combining metals with distinct electronic properties, these heterostructures can facilitate efficient charge transfer and improve the adsorption of reactants/intermediates, leading to enhanced performance in various electrochemical processes. Precise control of the configuration of metal nanostructures is crucial for enhancing electrocatalytic performance. For example, Kuhn et al. successfully synthesized Cu@Ag core-shell electrocatalysts by finely controlling the structures of two immiscible metals and used them for electrocatalytic carbon dioxide reduction reaction (CO₂RR)^[37]. At a low initial potential of -2.25 V vs. reversible hydrogen electrode (RHE), it can efficiently reduce CO₂ to C₂H₄. At -2.50 V vs. RHE, the Faradaic efficiency (FE) for multi-carbon products (FE_C2+) of Cu@Ag core-shell electrocatalysts with an optimal Ag shell thickness of 0.15 nm reached 43% [Figure 9A], while the FE_C2+ of Cu catalyst was only about one-third of that of the Cu@Ag core-shell catalyst [Figure 9B]. This phenomenon can be attributed to the increased concentration of surface-bound CO^* species on the Cu@Ag catalyst, which enhances the C-C coupling process for the production of multi-carbon products. In the tandem catalytic process of the Cu@Ag core-shell structure, CO₂ was first adsorbed, activated, and reduced to CO on the Ag shell, which was then further converted into C₂₊ products via reactions on the Cu core, realizing a two-step CO₂RR process [Figure 9C]. Thus, the thickness of the Ag shell significantly influences the selectivity and activity during CO₂RR. Zhang et al. synthesized Cu@Ag core-shell nanoparticles with different sizes of 10.7, 11.2, 11.8, and 12.2 nm (denoted as Cu@Ag-1, Cu@Ag-2, Cu@Ag-3, and Cu@Ag-4, respectively)^[78]. It was reported that the Cu@Ag-2 NP showed high FEs of 67.6% and 32.2% towards C₂ products and ethylene, respectively, both higher than that of the other three Cu@Ag NPs with different thicknesses of the Ag shell [Figure 9D and E]. Besides, the production of H₂ was effectively inhibited on Cu@Ag heterostructures as compared to Cu NPs and the mixture of Cu and Ag NPs, suggesting that the core-shell heterostructure is beneficial for CO₂RR. It is claimed that the appropriate synergistic interaction between the Cu core and Ag shell enhanced the binding strength of CO on the Cu/Ag interface, accelerated charge transfer, and increased the electrochemical surface area, which led to improved catalytic performance.

Figure 9. (A) Schematic illustration of the FE of C₂₊ products for CO₂RR by Cu@Ag core-shell catalyst and (B) Schematic illustration of the FE of C₂₊ products for CO₂RR at low potentials by Cu catalyst^[37]. (C) Schematic illustration of the catalytic mechanism of Cu@Ag heterostructure , (D) FE for C₂ products and (E) FE for C₂H₄ of Cu@Ag core-shell catalysts^[78]. (F) Ratio between FE for C₂₊/C₁ products of Ag@Cu catalysts with different pore sizes^[77].

Optimizing the local confinement effect within the metal heterostructures is another effective way to increase the concentration of local CO intermediates and thus improve the selectivity of CO₂RR toward C₂₊ products by promoting the C-C coupling reaction. For example, Zhong et al. designed Ag@Cu catalysts with porous Cu shells of different pore sizes and investigated the confinement effect of different Cu shells on the selectivity of CO₂RR toward C₂₊ products^[77]. Under all constant current density, the Ag@Cu core-shell catalyst with a pore size of 4.9 nm, named [email protected] nm, exhibited superior catalytic activity and selectivity compared to the other core-shell catalysts with different pore sizes. Specifically, [email protected] nm achieved the highest FE of 73.7% for C₂₊ products at the current density of 300 mA cm^-2, and a selectivity ratio of C₂₊/C₁ products of 5.1 at the current density of 400 mA cm^-2 [Figure 9F]. As a comparison, CO is the dominant product during CO₂RR over monometallic Ag catalysts at different current densities. It is reported that the [email protected] with suitable pore size could exhibit the highest local CO concentration on the surface of the Cu pore as indicated by the highest *CO coverage in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy results, which further promotes the subsequent C-C coupling and leads to the highest C₂₊ selectivity.

The spatial distribution of different metal components within the metal heterostructures is a key factor that influences the reaction process during CO₂RR for the production of different products, especially multi-carbon products. In a typical work, Huang et al. synthesized Janus Ag-Cu NDs with a tunable size of the Cu domain as an electrocatalyst for tandem CO₂RR^[94]. Among various catalysts, Ag₁-Cu_1.1 NDs with Ag/Cu mass ratio of 1:1.1 showed the highest FE of approximately 40% towards the production of C₂H₄ at -1.1 V vs. RHE, which surpassed other Ag-Cu NDs with different mass ratios and the mixture of Ag and Cu nanoparticles, indicating the importance of the spatial distribution. It is also worth noting that Cu nanoparticles with similar size and shape only exhibited a FE of approximately 12% for C₂H₄ [Figure 10A], and its current density is only half that of Ag₁-Cu_1.1 NDs [Figure 10B], suggesting the important role of metal heterostructures for the production of multi-carbon products. It is claimed that the tandem catalysis and the unique electronic structure of the Ag-Cu ND are key factors contributing to its improved C₂H₄ selectivity and enhanced activity. In another work, Zhang et al. addressed the issue of the lack of tunability in Cu-based heterostructures and systematically studied the effect of Cu spatial distribution in Au-Cu HSs on the selectivity of CO₂RR^[93]. As the contact area between Au and Cu increases, three kinds of Janus nanostructures (Au-Cu_I, Au-Cu_II, Au-Cu_III) and core-shell nanostructures (Au-Cu_IV) were formed. It is found that the Au-Cu_III catalyst showed a maximum FE towards C₂₊ products of 80% with a high partial current density of 466.1 mA cm^-2 [Figure 10C], while the monometallic Au catalyst only produces CO and H₂ during CO₂RR, indicating that the adsorbed *CO intermediates can be further converted to multi-carbon products and the production of H₂ can be inhibited on Au-Cu HSs. More interestingly, the primary product during CO₂RR is switchable by tuning the interface of Au-Cu HSs. As shown in Figure 10D, the primary product could be tuned from CH₃OH (FE of 72.1% in liquid products on Au-Cu_I) to C₂H₅OH (FE of 78.1% in liquid products on Au-Cu_II) and C₂H₄ (FE of 63% in gas products on Au-Cu_III). Then, operando surface-enhanced Raman spectroscopy and density functional theory (DFT) calculations showed that the reaction pathway during CO₂RR can be modulated on different Au-Cu HSs which facilitate the production of various products. Ma et al. studied the tandem CO₂RR process over three types of Ag-Cu Janus nanostructures with exposed {100} facets (Ag-Cu JNS-100) and different atomic ratios between Ag and Cu^[95]. It is found that all three Ag-Cu Janus heterostructures showed high FEs of over 50% towards the production of C₂₊ products in their optimal potentials [Figure 10E-G], which is much superior to those of Cu (52%) and the mixture of Ag and Cu (22%), indicating the importance of constructing metal heterostructures for enhanced CO₂RR performances. Among them, Ag₆₅-Cu₃₅ JNS-100 with an Ag/Cu ratio of 65/35 exhibited a maximum FE of 54% and 72% for the production of C₂H₄ and C₂₊ products. The reaction mechanism during the CO₂RR process is illustrated in Figure 10H. First, the Ag domain of Jansu Ag-Cu would convert CO₂ into CO or *CO intermediates, which then spill over to the nearby Cu (100) domain and were further reduced to C₂H₄. The theoretical calculation result suggested that the excellent CO₂RR performance of Ag₆₅-Cu₃₅ JNS-100 could be attributed to the modified electronic structure and the promoted CO spillover effect, which reduces the energy barrier for the production of C₂₊ products.

Figure 10. (A) FE of C₂H₄ obtained at -1.1 V vs. RHE on various Ag/Cu nanocrystals and (B) Schematic illustration of the mechanism during CO₂RR in Ag-Cu NDs^[94]. (C) FEs for C₂₊ products of Au-Cu catalysts in different potentials and (D) Maximum FE and corresponding partial current densities of CH₃OH, C₂H₅OH, and C₂H₄ in Au-Cu_I, Au-Cu_II, and Au-Cu_III^[93]. (E-G) Electrocatalytic performance of Ag-Cu JNS-100: Ag₆₅-Cu₃₅ JNS-100 (E), Ag₅₀-Cu₅₀ JNS-100 (F), and Ag₂₅-Cu₇₅ JNS-100 (G) and (H) CO₂RR mechanism for Ag-Cu JNS-100^[95].

Plasmonic metal heterostructures could show great promise in plasmon-enhanced electrocatalysis due to the unique optical properties of plasmonic metals. Under light irradiation, plasmonic materials could generate localized thermal effects and produce energetic hot carriers (hot electrons and holes) during plasmon decay^[122,123], which could enhance electrocatalytic performance. For example, the high energy of hot holes contributes significantly to oxidation reactions by directly interacting with adsorbed molecules and facilitating bond cleavage if they could be transferred to a catalyst^[124,125]. For example, Corson et al. reported the plasmon-enhanced CO₂RR performance of Cu-Ag heterostructure^[126]. Specifically, the production of CO could be enhanced while the hydrogen evolution was inhibited under light irradiation at low overpotentials at -0.6 and -0.7 V vs. RHE. It was claimed that the enhanced CO production after light irradiation was ascribed to the hot electrons transferred to the metal-CO complex, accelerating CO desorption, while the increased local electric field enhanced the bonding of HCO₃^- on the surface of the catalyst, thus lowering the local pH and suppressing H₂ evolution. At high overpotentials of -0.9 and -1.0 V vs. RHE, the Cu-Ag heterostructures could show higher FEs towards the production of ethylene, methane, formate, and allyl alcohols with the light irradiation, showing a plasmon-enhanced electrocatalytic performance. It is also worth noting that the effect of local heating under light showed no contribution to the enhanced FE after conducting temperature-dependent experiments.