Because Au has similar characteristics to Cu (valence, atomic radii and crystal structure), the Au–Cu alloy can be easily prepared (Ferrando et al., 2008). The melting point of Au–Cu alloy nanostructures is lower than that of their constituent elements, and Au–Cu alloy melts at a specific temperature (Okamoto et al., 1987; Thota et al., 2018). Schaak and coworkers reported Au–Cu alloy nanospheres using a co-reduction method (Motl et al., 2010). During the preparation process, 1-octadecene, oleic acid, and oleylamine were chosen as the surfactants and reducing agents. A series of Au–Cu alloy nanospheres with different Au and Cu ratios were obtained by adjusting the concentration of the Cu precursor. As shown in Figure 1A, the uniform size distribution (8 nm) of alloy nanospheres is shown, indicating a disordered crystal structure. Au–Cu alloy nanowires have also been synthesized (Mendoza-Cruz et al., 2016). Alkylamine chains of ODA and glucose acted as surfactants and reducing agents, which helped the growth of alloy nanowires (Bazán-Díaz et al., 2018; Chatterjee et al., 2018). A coiled mode of Au–Cu alloy nanowires was self-assembled as individual nanowires or in a parallel-ordering manner as a set of nanowires. As shown in Figure 1B, the Au–Cu alloy nanowires are ultrathin, with diameters less than 10 nm and variable lengths of a few microns, presenting twin boundaries and an elevated density of stacked faults. The co-reduction method has been extended to obtain some complex geometrical nanostructures, such as Au–Cu alloy nanocubes (Figure 1C) (Liu and Walker, 2010). DDT also plays a critical role in controlling the morphology of Au–Cu alloy nanocubes. In addition, Xu and others reported the synthesis of Au–Cu alloy nanopentacles by combining two strategies (co-reduction and seed-mediated) in the aqueous phase route, with sizes that can be controlled in the 45 nm–200 nm range (Figure 1D) (He et al., 2014).

Bimetallic core–shell nanostructures offer numerous benefits; for example, the optical property is easily tuned by varying the morphology, shape, size, and composition of the core, as well as the thickness, shape, and composition of the shell material. Thus, the plasmonic property of the bimetallic core-shell nanostructure was easily regulated (Zhang Y.-J. et al., 2021; Wan et al., 2022). To date, although many attempts have been made to generate Au–Cu core–shell nanostructures by epitaxial growth with lattice mismatch, limited success has been achieved. For example, Tsuji and coworkers reported Au–Cu core–shell nanocrystals (Tsuji et al., 2010). The study showed that the Cu shell can epitaxially grow on the surface of Au nanocrystals, although a large lattice mismatch existed between Au and Cu for the first time (Figure 2A) (Wang et al., 2015). However, the Cu shell thickness and shape of the Au–Cu core–shell nanocrystal were not exactly regulated. Luis and collaborators reported the formation of Au–Cu core–shell nanostructures with uniform and various morphologies using Au nanostructures as templates (Alvarez-Paneque et al., 2012). The method is based on the reduction of Cu²⁺ in aqueous solution by hydrazine at 60°C. In addition, the size of the resulting Au–Cu core-shell nanostructure was tuned by either the size of the Au core or the ratio between the Cu and Au molarities (Figure 2B). Au–Cu core-shell nanostructures (nanocubes and nanooctahedra) with tunable sizes were synthesized in water by using Au nanooctahedra (Figure 2C) (Kim et al., 2014). The synthetic conditions were very simple, and nanoparticle growth was complete in 45–90 min. In their study, HDA not only increased the pH of the solution but also acted as a coordination ligand for Cu ions, facilitating controlled Cu shell growth. In addition, Xia’s group reported Au–Cu core-shell nanocubes, which exhibit a size smaller than 30 nm (Figure 2D) (Lyu et al., 2019).

The concept of the “Janus structure” was proposed by Pierre-Gilles de Gennes in 1991 (Gennes, 1992). Janus nanostructures not only reduce administered dosages but also combine differential functionalization (Zhang X. et al., 2021; Qiu et al., 2022). In all these protocols, the challenge for successful fabrication of bimetallic Janus nanostructures is how to create asymmetric distributions, where two metals are side-by-side in one particle, while avoiding the formation of different types of structures (Duan et al., 2021; Peng et al., 2021). It is very important to choose an appropriate capping agent in the shape-controlled synthesis of Au–Cu Janus nanostructures. The alkylamines and DNA can selectively bind to Cu, which helps Cu growth on the surface of the Au nanostructure (Liu and Fichthorn, 2017; Kim et al., 2021). For example, Zhang and coworkers proposed a general seed-mediated growth method for the synthesis of Au–Cu Janus nanostructures by HAD and CTAB as surfactants (Jia et al., 2021). The protocol can be trivially extended to various shapes of Au nanostructures as cores, such as Au nanobipyramids, suggesting the generality of the site-selective overgrowth method (Figures 3A–C). Similarly, our group designed a new jellyfish-like Au–Cu Janus nanostructure (Figure 3D) (Mi et al., 2022). A twin defect and stacking fault were found to exist at the twinned Au nanotips, which led to Cu atoms being deposited on the twinned nanotip. In addition, Deng’s group developed width-adjustable Au–Cu Janus nanostructures by fish sperm DNA (Figure 3E) (Zhu et al., 2020). The strategy relied on the non-specific surface adsorption of fish sperm DNA onto an Au nanoparticle to control heterogeneous Cu nucleation, which had low-cost and natural advantages. Such a process provided a chance to regulate the contact area between the Au nanoparticle and Cu nanodomains in the bimetallic nanostructure. Recently, the interfacial energy between Au nanostructures and Cu nanodomains was continuously regulated by strong thiol ligands, which induced a transition from Au–Cu core-shell nanostructures to Janus nanostructures (Fan et al., 2022). According to a series of effective ligand controls, Au–Cu Janus nanostructures were successfully prepared using different shapes of Au nanostructures as seeds. Crucially, the Janus degree of Au–Cu Janus nanostructures can be readily tuned by changing the molecular structure and the concentration of the thiol ligands (Figure 3F).

Electrochemical conversion of CO₂ into feedstock and value-added fuels shows a convenient solution to energy demand and climate change (Nitopi et al., 2019). For this conversion, designing new catalysts with the capability to reduce CO₂ into more valuable products is one of the challenges (Birdja et al., 2019; Wang et al., 2022). Although Cu nanoparticles have shown great helpe for CO₂ reduction, poor selectivity persists (Lee S. H. et al., 2021). To solve this problem, the Au–Cu nanostructure has been exploited as a catalyst (Zhou et al., 2018; Ni et al., 2022). For example, alloying Au with Cu not only stabilizes Cu but also reduces the overpotential required for CO₂ reduction. In addition to the excess potential, the selectivity of the catalysts during CO₂ reduction must be considered since multiple couplings between protons and electrons lead to many possible reaction products (Bagchi et al., 2022). Yang’s group showed that Au–Cu alloy nanospheres are a highly potential catalyst for CO₂ reduction (Kim et al., 2014). Catalytic activity and selectivity have been tuned by varying the composition of the Au–Cu alloy nanostructures. The Faraday efficiencies for hydrogen, ethylene, and methane increased with increasing Au content in alloy nanostructures, but the opposite results were obtained for carbon monoxide. As shown in Figure 5A, turnover rates of 93.1, 83.7, and 40.4 times for CO were obtained by Au₃Cu, AuCu, and AuCu₃ alloy nanospheres compared to Cu nanospheres. Au₃Cu nanostructures also showed the best mass activity for CO (Figure 5B).

Additionally, due to tandem electrocatalysis, the Au–Cu Janus nanostructure provides an efficient conversion to the C₂ product pathway (Jia et al., 2021). The symmetry importance was also shown by comparing different types of Au–Cu nanostructures, where the Janus nanostructure exhibited the best selectivity toward C₂ products owing to the obvious boundaries between the Au and Cu nanodomains of the nanostructure. As shown in Figure 5C, the highest C₂ product FEs of the Au nanobipyramide–Cu and Au nanosphere–Cu Janus nanostructures were 46.4% and 25.2%, which displayed 4.1-fold and 2.2-fold compared with that of the Cu nanosphere counterpart, respectively, showing the synergistic effect of Au and Cu in the Janus nanostructure. Furthermore, Huang’s group reported that the Au–Cu Janus nanostructure catalyst exhibited remarkable selectivity toward C₂ product formation (Figure 5D) (Zheng et al., 2022).

For Au–Cu nanostructures, their LSPR peaks can also be tuned to the NIR region, making them good candidates for biomedical applications, such as bioimaging, photothermal therapy, and controlled drug release. As a new field, photothermal conversion and therapy are attracting extensive attention by using plasmonic nanostructures under NIR laser illumination (Zhu et al., 2021). Recently, Au–Cu systems have been explored for photothermal therapy. For example, mouse tumor cells were injected with Au–Cu nanopentacles. Then, mouse tumor cells were irradiated with an NIR laser. The tumor volume in the mice was also significantly reduced compared to tumor samples injected with Au–Cu nanoparticles only or irradiated with an 808 nm laser only. Under 808 nm laser irradiation, robust photothermal therapy efficiency was obtained by 70 nm Au–Cu nanopentacles. The relative tumor volume of mice after treatment along with feeding time is shown in Figure 6A. As seen in the purple and red segmentation lines, tumors from the groups injected with Au–Cu nanoparticles only and irradiated with 808 nm laser only were enlarged in volume and showed some fluctuations. The group of mice that received both Au–Cu nanopentacle irradiation and injection were tumor-free 4 days after treatment (Figure 6B). This work demonstrates that Au–Cu alloy nanostructures have potential applications in tumor diagnostics and therapeutics (He et al., 2014). In addition, the Cu–Au nanotripod also exhibited a well-defined prominent photothermal effect (Nanda et al., 2019). The cell viability results illustrated that Cu−Au nanotripods were minimally toxic to the cells. Au–Cu nanostructures also provide a chirality-dependent method for highly efficient phototherapy (Wang J. et al., 2020). The Au–Cu–Au heteronanorod (HNR) was synthesized by virtue of the dipeptide as ligands, which displayed strong circular dichroism (CD) in the range of 400–1000 nm (Figure 6C). The potential for photothermal and photodynamic therapy of chiral Au–Cu–Au HNRs was further investigated in HeLa cells by using confocal microscopy signals and CCK-8 assays. The Au–Cu–Au HNRs show the highest rate of cell toxicity (Figure 6D).

Solar steam generation is emerging as a promising technology, for its potential in harvesting solar energy for various applications such as desalination and sterilization (Hu et al., 2017; Xu et al., 2017). The studies has reported a variety of artificial structures that are designed and fabricated to improve energy conversion efficiencies by enhancing solar absorption, heat localization, water supply, and vapor transportation (Zhu et al., 2017). Recently, Cu−Au core-shell nanostructures showed strong broadband plasmonic absorption, which was used for solar steam generation (Wang Y. et al., 2021). The evaporation rate of water and photothermal efficiency of different plasmonic absorbers are shown in Figure 6E. The evaporation rates of the Cu_2.5−Au₁ nanoparticles and Au nanoparticles were 0.92 kg m⁻² h⁻¹ and 1.02 kg m⁻² h⁻¹ under 1 Sun irradiation for 5 h, respectively. Furthermore, a very high conversion efficiency of 66% was achieved by the Cu_2.5−Au₁ nanoparticles under 1 Sun irradiation compared to the Au nanoparticle absorber and bulk water. Next, the durability of Cu_2.5−Au₁ nanoparticles in a saturated CO₂ solution was tested. Under 1 Sun irradiation, it can be observed that the color of Cu nanoparticles changed from black to colorless after 6 h (insets in Figure 6F). The reason is that Cu is easily oxidized. However, under the same test conditions, Cu_2.5−Au₁ nanoparticles remained black in color. Next, the temperature changes of the surface were tracked during the evaporation of saturated CO₂ solutions at 1 solar. The temperature of the Cu_2.5−Au₁ nanoparticles remained constant under continuous illumination, implying excellent stability of the Cu_2.5–Au₁ nanoparticles (Figure 6F).

Au–Cu nanostructures show LSPR tunability and larger enhancement of the near field, which can be applied in a variety of enhanced spectroscopies (Zhang et al., 2017; Chen et al., 2018; Cabello et al., 2019; Wang X. et al., 2020). To exploit the surface-enhanced Raman scattering (SERS) performance of Au–Cu nanoshells, MB molecules were used as probe molecules (Chuang et al., 2020). Figure 7A shows that the intensity of the SERS spectra at 1621 cm⁻¹ for the Au–Cu nanoshells was enhanced by 10.3-fold, 16.7-fold, 26.78-fold, 2.54-fold, 2.71-fold and 22.58-fold relative to that for the Au NRs, Ag@PVP nanoparticles, Ag@PVP nanocubes, Au_0.64–Ag_0.36@PSMA nanoshells, Au_0.53Ag_0.47@PSMA nanoshells, and Au_0.39Ag_0.61@PSMA nanoshells, respectively. It can be demonstrated that Au–Cu nanoshells are effective and reliable for SERS detection, with promising potential applications in visualizing and sensing living bladder cancer cells in physiological environments. Kumar-Krishnan and coworkers evaluated the sensitivity of CV detection of SERS with Au–Cu flower-shaped nanostructures (Kumar-Krishnan et al., 2020). Figure 7B shows the SERS spectra of CV molecules with different concentrations (10⁻⁶ to 10⁻¹⁰ M) recorded over the particular substrate. In addition, Au–Cu NRs were designed by Nguyen and coworkers (Van et al., 2022). The plasmon wavelength of Au–Cu NRs can be tuned by changing the aspect ratio. The SERS performance of Au₁–Cu₃ NRs with different aspect ratios is investigated. As shown in Figure 7C, clearly, the SERS spectral intensities of NBA increase as the Au–Cu NRs become longer. The reason was explained by the enhancement of the local electromagnetic field intensity. Recently, Au–Cu nanostructures have been employed to enhance the luminescence of Yb³⁺/Er³⁺-doped nanoparticles (Mi et al., 2022). The plasmonic Au–Cu Janus nanojellyfish showed two LSPR peaks, which coupled the emission and excitation of the light wavelength of the Yb³⁺/Er³⁺-doped nanoparticles. A 5000-fold enhancement of the emission of Yb³⁺/Er³⁺-doped nanoparticles was achieved (Figure 7D). It is shown that multiple LSPR modes of Au–Cu Janus nanojellyfish can exhibit the potential to enhance the emission of upconversion nanoparticles.

To date, Au–Cu nanostructures have exhibited excellent enhancement of the near field in a variety of enhanced spectroscopies. Nevertheless, view from the current study, the application of Au–Cu nanostructures is just in the early stage compared with Au and Ag nanostructures. Focusing on the enhanced spectroscopies of Au–Cu nanostructures, further works is the construction high-performance Au–Cu nanostructures.