1. Introduction

As an important family of spectroscopic analysis, surface-enhanced Raman scattering spectroscopy (SERS) has been established as an interesting and fascinating technique for quantitative detection and ultrasensitive identification of various biomolecules at single molecule scale (<10⁻⁹ M) [1–9]. There is increasing evidence that the SERS-based analysis has laid the foundation for the investigation of ultra-trace biomolecules in biomedicine, pathological cell diagnostics, environmental surveillance, and food safety supervision, etc [1,4,10–12]. Recently, an outstanding work has demonstrated that SERS encoded silver pyramids with multivariate recognition provides distinct identification of disease biomarkers (prostate specific antigen and mucin-1) in clinical science [12]. In order to achieve an excellent SERS signal amplification for ultra-trace detection of biomolecules, great efforts have been devoted to the construction of a variety of SERS substrates. In the past decades, due to existence of localized electromagnetic mechanism, coinage metallic (Au, Ag and Cu) nanomaterials with controllable localized surface plasmon resonance (LSPR) in visible-near infrared region (NIR) have been constructed for exploring diverse advanced SERS substrates [1–10]. Among all available coinage metal-based nanomaterials, two important geometric structures including hollow morphologies and branched architectures have been separately well established as excellent nano-substrates for improving SERS activities [6-7,10,13-14]. Compared with solid or core-shell structures, hollow-shaped nanomaterials combining porous shells with inner voids provide lower density, less electronic diffusion blockage and larger surface area for SERS applications. On the other hand, branched nano-architectures with immense antennas on the surfaces can provide strong electromagnetic field by laser excitation in SERS analysis, which have a promising potential for further enhancing SERS signals [5-6,10]. Moreover, in addition to the interesting geometric structures, composition control is another critical role for achieving excellent SERS activities. Superior to monometallic materials, the nanocomposites with bimetallic or trimetallic compositions are believed to synergistically enhance the intrinsic properties of each component [2-5,7,10,14-20], giving rise to the formation of much more SERS hotspots in prosperous SERS-based biomolecules detection. Up to now, single or coinage metal-based nanocomposites with either hollow or branched structures have been separately established as excellent SERS substrates. If a novel hybrid coinage metallic nanostructure possesses both hollow void spaces and dendritic shell architectures, the fascinating substrate will synergistically provide unprecedented opportunity for further improving SERS performance. Some pioneering works illustrated the fabrication of hollow Ag-Au nano-urchins with numerous antennas by seed-mediated growth route [10,21]. The average enhancement factor (EF) in these reports was estimated about ~10⁹, and the detection limit could reach to ~10⁻¹⁰ M. Although the corresponding SERS have been improved, unfortunately, the structural advantage of hollow morphology cannot be fully realized in SERS application since the inner void spaces have not been clearly distinguishable in Ag@Au alloy nano-urchins. In order to further maximize the sensitivity of SERS signal, therefore, it is imperative to develop a rational approach to tailor the hollow structures of bimetallic Ag@Au nano-urchins.

Generally, the hollow Ag@Au nano-urchins should be constructed by seed-mediated growth approach, which includes the preceding formation of hollow Ag@Au nanostructure and then the anisotropic growth of exterior branches. In most cases, hollow Ag@Au nanostructures are fabricated by the consumption or removal procedure of Ag species via galvanic replacement reaction between Au ions and Ag nanoseeds [10,13-14,16-17,19,21-24]. Subsequently, the anisotropic growth of dendritic shell structure is highly related the concomitant reduction reaction between reduction agent and Au ions. It is well known that small-sized Ag nanoseeds are unstable species, which can be easily oxidized in ambient at room temperature. In order to obtain stable Ag nanoseeds, many stabilizing or capping agents such as cetyltrimethylammonium (CTAC), glycerol, phenol formal-dehyde resin (PFR), etc. should be added in the colloidal solution [10,25]. The organic additives are very necessary for the construction of hollow Ag@Au via galvanic replacement reaction. However, the barrier effect between organic capping agents and the Ag@Au shell surfaces is not suitable for the subsequent overgrowth of Au species onto precursor. Because of unstable Ag species and geometric complexity, the construction of well-defined hollow Ag@Au nano-urchins has not been reported up to now.

Alternatively, the laser light-induced synthesis/process of colloids including laser ablation in liquid (LAL), laser fragmentation in liquid (LFL) and laser melting in liquid (LML), has become a powerful, versatile and green strategy to controllably synthesize of stable metallic nanomaterials [26]. Recently, our group has demonstrated the successful synthesis of various stable metal-based nano-materials by laser-induced fabrication in liquid conditions [6,27–32]. The interesting approach can significantly reduce the using of capping agents in the formation of metal nanoparticles.

Herein, we further extend the novel synthetic route to fabricate stable Ag nanospheres by pulsed laser ablation of a bulk Ag target in liquid condition. Superior to conventional chemical approaches, the ultra-rapid laser sintering/quenching of Ag nanoparticles enables the outside surfaces to become more stable than those of inner regions. Taking advantage of the stable Ag nanoseeds, then, the well-defined hollow Ag@Au nano-urchins with obvious inner void structures and exterior multi-antennas can be successfully fabricated by seed-mediated growth method. The inner hollow structures and dendritic shell architectures as well as Ag compositions can be effectively modulated by simply increasing the amount of Au³⁺ ions in the reduction reactions. Correspondingly, the LSPR peaks of hollow Ag@Au nanocomposites can be tuned from about 401 nm to 610 nm with an increase of HAuCl₄ solution. Compared with original nanoseeds, the hollow Ag@Au nano-urchins obtained by adding 6 mL, 0.5 mM Au³⁺ ions possess excellent chemical stability in ionic or oxidative liquid condition. Most interestingly, the distinctive advantage is that the as-prepared hollow Ag@Au nano-urchins provide ultrasensitive SERS properties with an EF up to ~10¹² and a detection limit as low as ~10⁻¹³ M. The pronounced SERS activities should be attributed to the unique synergistic effects between inner void spaces and exterior antennas as well as inter-metallic synergies. The present finding is superior to many previous reports, especially those of solid or core-shell structures, and monometallic or hybrid nanocomposites with either hollow structures or branched architectures [3–5,7–10,13,14, 21–25]. This facile protocol presented here will lead to the formation of an ultrasensitive SERS substrate for ultra-trace detection of biomolecules in various applications.

2. Experimental setup

The experimental facilities based on laser ablation in liquid are analogous to our previous work [27–32]. At first, a well-polished Ag metal as the target was placed on the bottom of a rotating glass dish with speed of ~400 rpm that was filled with 4.5 mm depth of liquid solution containing 1 mM polyvinyl pyrrolidone (PVP, Mw = 40000) as dispersing agent and 10 mL distilled water. A Q-switched Nd-YAG (Yttrium Aluminum Garnet) laser (Quanta Ray, Spectra Physics) was used as laser beam excitation source. Laser beam operating at wavelength of 1064 nm with pulse duration of 10 ns and 10 Hz repetition rate was focused on the Ag target surface and the energy of laser was about 205 mJ. After laser ablation for 30 minutes, the products were washed in distilled water and centrifuged at 10000 rpm for 10 minutes in an ultracentrifuge. After centrifugation and washing with distilled water, stable Ag nanospheres were obtained. In a typical synthesis of hollow Ag@Au nano-urchins, 50 μL sediments of Ag nanospheres were dissolved in 6 mL aqueous solution, followed by the addition of 2 mL PVP (1 mM), and 2 ml ascorbic acid (AA, 0.1 M) under magnetic stirring. Then, 0~6 mL HAuCl₄ aqueous solution (0.5 mM) was titrated into the mixture solution. After adequate reaction, the product was centrifuged at 15000 rpm and the precipitation we obtained was hollow Ag@Au nanourchins. The obtained sediments were dropped on a copper mesh and dried in an oven at room temperature for observation via transmission electron microscopy (JEOL-JEM-2100F). The crystallographic investigation of the products was acquired by X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF) using Cu Kα radiation (λ = 0.15406 nm). The absorption spectra were recorded by a UV-Vis-IR spectrometer (UV-1800, Shimadzu). The detailed sample compositions were studied by X-ray photoelectron spectra (XPS) on a PHI Quantera SXM with an Al Kα = 280.00 eV excitation source. In a typical SERS procedure, 0.2 mg products were dissolved in 2 mL 4-Aminothiophenol (4-ATP) ethanol solution. Then, the solution is continuously stirred at a constant speed of 100 rpm for 2 h to ensure the established of adsorption-desorption equilibrium among the nanomaterials and probe molecules. Finally, the products were centrifuged at 15000 rpm for 10 min in an ultracentrifuge. The as-prepared sediments were carefully dispersed on a carefully cleaned silicon plates and then dried naturally at room temperature. SERS measurements were performed by a confocal microprobe Raman spectrometer (LabRAM HR 800 spectrograph). All of the SERS spectra were recorded at room temperature using a 633 nm laser with an output power of 50 mW. The acquisition time used for each spectrum is 20 s.

3. Results and discussion

After cumulative pulse laser ablation of Ag metal target, Ag nanospheres were generated in the solution, which will be served as seeds for further fabrication of hollow Ag@Au nano-urchins. The morphology of Ag nanospheres was shown in Fig. 1(a). The TEM image clearly shows that the water-dispersed Ag nanoparticles are likely to be fabricated individually and are not hinged together. The average size of Ag nanospheres is about 30 nm by measuring the diameters of more than 1000 particles in sight on the TEM images. Hollow Ag@Au nanourchins will be further fabricated by simply dropping HAuCl₄ solution into the Ag nanospheres solution. After adding 6 mL, 0.5 mM Au³⁺ ions, the TEM images of the obtained products are shown in Figs. 1(b)-1(d). The typical low- and enlarged magnification TEM images [Figs. 1(b) and 1(c)] clearly illustrate that the as-prepared spherical nanomaterials are characterized by obvious inner hollow spaces and urchin-like dendritic shell architectures, supporting the formation of hollow nano-urchins structures. The inner hollow spaces are shown as contrasting lighter images with their shells as darker ones, owing to different penetration depths of the incident electron beam in TEM measurements. The overall size is about ~50 nm, which is slightly larger than that of original Ag nanospheres. The HRTEM image of an individual nanoparticle [Fig. 1(d)] further confirms the construction of external urchin-like shell structure. Correspondingly, a close-up view of the dendritic shell region [Fig. 1(d)] illustrates that the Ag and Au plane structures are miscible phases, owing to the very similar lattice constants. The lattice fringes with a d-spacing of 0.237 nm can be attributed to the (111) plane structure in Ag@Au alloys. Moreover, the elemental mapping images of an individual hollow nano-urchins in Fig. 1(e) also demonstrate that the uniform distributions of Ag and Au elements throughout the nano-frames structure, supporting the formation of bimetallic Ag@Au nanocompositions. The average relative ratio of Ag in the obtained products is about 32.47%.

 

Fig. 1 The typical TEM images of (a) Ag nanoseeds and (b) hollow Ag@Au nano-urchins. (c-d) The enlarged TEM images of hollow Ag@Au nano-urchins. (e) The corresponding mapping images of an individual hollow Ag@Au nano-urchin.

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The formation of hollow Ag@Au nano-urchins is highly related to the amount of Au³⁺ ions in the reaction solution. To get more information of the morphology-controlled Ag@Au nanomaterials, the structural evolution of the products as a function of Au³⁺ ions contents (0~5 mL, 0.5 mM) was carried out in this paper. The typical TEM images of products synthesized by different amount of HAuCl₄ solution are shown in Figs. 2(a)-2(f). Ag nanospheres used as templates and nanoseeds were fabricated by laser ablation Ag metal target in liquid condition, as shown in Fig. 2(a). Evidently, original Ag nanoparticles are solid nanospheres with smooth surface. During the process of adding HAuCl₄ (0~5 mL), the solid nanospheres significantly develop into hollow-shell nanospheres [Figs. 2(b)-2(f)]. Increasing the content of HAuCl₄ enables the inner void space to become much larger. Meanwhile, the smooth surface becomes convex and rugged structure. In a typical reduction reaction, when HAuCl₄ was adding into the Ag nanoseeds in the presence of AA, Au³⁺ ions will be reduced to Au atoms through two parallel reactions involving Ag species and weak reducing agent AA [13-14]. Firstly, the reduction by Ag species is a galvanic placement reaction, which leads to the formation of hollow nanostructure. On the other hand, the reduction by AA reagents results in the overgrowth of Au atoms on the Ag nanospheres. Because the rate of galvanic reaction exceeds the rate of co-reduction, the hollow structure becomes larger until the forming of a thin hollow Ag@Au shell. In contrast with traditional galvanic reaction from outsider region toward insider space [13-14,16-19], in this paper, the morphological evolution of the products illustrates that the consumption or removal procedure of Ag species start from inner space [Fig. 2(b)] to external shell [Fig. 2(f)]. The distinctive feature is attributed to the unique Ag nanoseeds with stable external shell structures. At the moment of pulsed laser beam arriving at Ag target, rapid melting/boiling and then vaporization of Ag species will occur owing to the photonic energy well absorbed by metal surface layers. The ultra-rapid nucleation of Ag nanoparticles will take place in the early stage of condensation of Ag vaporization and sharply terminate due to the expiration of laser pulse. Superior to traditional chemical synthesis, the pulsed laser sintering and quenching of the obtained Ag nanoparticles will play an important role in the subsequent irradiation process, which has been verified in our previous works [6,27-32]. In this way, with the aid of ultra-rapid laser-induced sintering /quenching process, the outside surfaces become much more stable than those of Ag core regions. After adding Au³⁺ ions, the relatively unstable Ag core regions will be firstly removed from the precursors. Meanwhile, a section of Au³⁺ ions was reduced to Au atoms during galvanic replacement reaction, which will deposit on the Ag nanoframes. The above mechanisms will lead to the construction of stable hollow Ag@Au nano-frames with rugged shell structure [Fig. 2(f)]. It is very suitable for the next anisotropic overgrowth of Au species on the rugged surface of nano-frames. As expected, increasing the content of HAuCl₄ (6 mL, 0.5 mM) further results in the formation of hollow Ag@Au nano-urchins [Figs. 1(b)-1(d)] due to the subsequent concomitant reduction reaction between the weak reducing agent (AA) and Au³⁺ ions.

 

Fig. 2 (a) The TEM images of original Ag nanospheres, and (b-f) the TEM images of hollow Ag@Au nano-urchins obtained with different volumes of HAuCl₄ solution (1~5 mL, 0.5 mM).

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Moreover, the crystallographic investigations of the as-prepared hollow Ag@Au nano-urchins and original Ag nanoseeds were detected by X-ray diffraction (XRD) patterns, as shown in Fig. 3(a). Compared with the result of original Ag nanospheres (the yellow line), the XRD pattern of hollow Ag@Au nano-urchins clearly shows that the obtained product is found to be well crystalline according to distinct diffraction peaks. The XRD peaks are observed at 38.21°, 44.42°, 64.62° and 77.62°, corresponding to (111), (200), (220) and (311) lattice planes of Ag@Au nanocomposites (face-centered-cubic structure, JCPDS, no.65-8424). Since the relatively higher peak formed at 38.21°, the preferential alignment of the (111) orientation has been generated in hollow Ag@Au nano-urchins. In addition, the bimetallic nature of the as-synthesized hollow Ag@Au nano-urchins was confirmed by X-ray photoelectron spectra (XPS), as shown in Figs. 3(b)-3(d). The binding energies were calibrated by referencing the C1s peak at 284.8 eV to reduce the sample charge effect [15-16]. Compared with single-phase Ag nanospheres, the survey spectrum (red line) in Fig. 3(b) shows that both Au4f (87.2 eV) and Ag3d (371.5 eV) peaks can be clearly detected in hollow Ag@Au nano-urchins. The doublet peak of Au4f_7/2 and Au4f_5/2 located at 84.1 eV and 87.3 eV in Fig. 3(c) can be reasonably assigned to Au atoms. Figure 3(d) displays the Ag3d patterns of Ag nanospheres and hollow Ag@Au nano-urchins, respectively. Compared with doublet feature of Ag3d_5/2 (367.5 eV) and Ag3d_3/2 (373.7 eV) of original Ag nanospheres, the two peaks located at 367.2 eV (Ag3d_5/2) and 373.4 eV (Ag3d_3/2) of hollow Ag@Au nano-urchins slightly blue-shifted due to the interaction between Ag and Au species. By comparison, the corresponding width of XPS spectra of the products also evidently becomes narrower in Fig. 3(d), owing to the consumption of Ag species in galvanic replacement reaction. Therefore, it is reasonable to conclude that the well-defined hollow Ag@Au bimetallic nano-urchins with obvious void spaces and dendritic shell structures can be successfully constructed by simply adding Au³⁺ ions in stable Ag nanoseeds solution.

 

Fig. 3 (a) XRD patterns of Ag nanospheres and hollow Ag@Au nano-urchins. The XPS spectra of Ag nanospheres and hollow Ag@Au nano-urchins: (b) Survey structure, (c) Au4f, and (d) Ag3d.

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The bimetallic Ag@Au nanocomposites with controlled structures are anticipated to provide a tunable LSPR absorption in visible-NIR region. The LSPR properties of Ag@Au nanocomposites obtained by using different amount of Au³⁺ ions (0~6 mL, 0.5 mM) were monitored by UV-Vis-NIR absorption spectra. The direct photographs of the products generated by varying the amount of Au³⁺ ions (0~6 mL, 5 mM) are shown in Fig. 4(a). The increasing Au³⁺ content in reaction solution leads to a significant change in the solution color from the bright yellow to brown and then blue-violet, indicating that the galvanic replacement reaction and concomitant reduction took place in the solution. Meanwhile, the LSPR peak distinctly red-shifted from the visible ~401 nm to 610 nm with an increase of Au³⁺ ions content, as shown in Fig. 4(b). The red-shift of the surface plasmon extinction is highly related to the formation of hollow-shaped Ag@Au nano-frames and then the construction of dendritic-shell structures.

 

Fig. 4 (a) The typical color change of the products by adding different amount of HAuCl₄ solution (0, 1, 3, 5, 6 mL, 0.5 mM) in the reaction. (b) The UV-visible absorption spectra of products with an increase of HAuCl₄ solution (0-6 mL, 0.5 mM). All spectra are normalized against the intensity of their strongest peaks.

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In order to evaluate the chemical stability of the hollow Ag@Au nano-urchins, we performed a series of experiments by benchmarking against original Ag nanoseeds. In typical experiments, we separately dispersed 0.5 mg Ag nanoseeds and Ag@Au nano-urchins in specifically formulated solutions, and recorded their absorption spectra periodically. Firstly, we investigated the chemical stability of Ag nanoseeds and Ag@Au nano-urchins in ionic solution. Figures 5(a) and 5(b) show the evolutions of the two different nanomaterials in 10 mL, 0.5 M NaCl solutions, respectively. It can be seen that the peak intensity of original Ag nanoparticles [Fig. 5(a)] significantly decreases from about 1.67 a.u to 0.45 a.u after mixing with NaCl solution for 100 min, indicating that 73% nanomaterials have been dissolved and removed from previous structures. Interestingly, the absorption spectra of the hollow Ag@Au nano-urchins could still be sustained for ten hours, since the peak intensity at 610 nm are nearly constant even up to 12 h [Fig. 5(b)]. Therefore, the as-prepared hollow Ag@Au nano-urchins show remarkable stability in ionic solution. Secondly, we further evaluated the chemical stability of hollow Ag@Au nano-urchins and Ag nanospheres in strong oxidative environment. Figures 5(c) and 5(d) display the absorption spectral changes of the two nanomaterials in 10 mL 20% H₂O₂ solution, respectively. After mixing with H₂O₂, the original Ag nanospheres show very poor chemical stability, since the corresponding intensity peak drastically drops from 1.6 a.u to 0.0001 a.u within 2 min [Fig. 5(c)]. The Ag nanomaterials have been completely rapid-oxidized by the strong oxidant. In contrast, the absorption peak of hollow Ag@Au nano-urchins slightly changes from 2.7 a.u to 2 a.u within 40 min, and then holds the same level for further increasing mixing time to 60 min, as shown in Fig. 5(d). Even in H₂O₂ oxidative condition, 74% hollow Ag@Au nano-urchins can be reserved within one hour. These comparative studies clearly suggest that the hollow Ag@Au nano-urchins exhibit excellent chemical stability in ionic solution or strong oxidative condition. It is very suitable for the construction of stable SERS substrate in biomedical and bio-molecular detection, since these special SERS applications usually carried out in different biological solutions [13-14].

 

Fig. 5 (a-b) The UV-visible absorption spectra of original Ag nanospheres and hollow Ag@Au nano-urchins in 10 mL, 0.5 M NaCl solution, respectively. (c-d) The UV-visible absorption spectra of original Ag nanospheres and hollow Ag@Au nano-urchins in 20% H₂O₂ solution, respectively. The dosage in each solution is 0.5 mg.

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Finally, we evaluated the SERS activities of the original Ag nanospheres and Ag@Au nanostructures synthesized by using different amount of HAuCl₄ solution. Figure 6(a) shows the SERS spectra of 4-Aminothiophenol (4-ATP) molecules adsorbed on the surfaces of original Ag nanospheres and a series of Ag@Au nanostructures, respectively. The purple line at the top region is the normal Raman spectrum of 1 M 4-ATP molecules on silicon plate without using any nanomaterials. The residual bottom-up SERS signals of 4-ATP molecules are originated from Ag@Au nanostructures fabricated by varying different amount of HAuCl₄ solution (0, 2, 4, 6, 8 and10 mL). Three dominating characteristic bands of 4-ATP at 1088, 1180 and 1590 cm⁻¹ are all clearly observed in SERS spectra. It should be noted that although the SERS intensity hold the same level, the concentration of 4-ATP adsorbed on hollow Ag@Au nano-urchins is 10⁻¹² M [cyan in Fig. 6(a)], which is much lower than those (10⁻⁸ M) of other nanomaterials. Compared with original Ag nanospheres and other hollow Ag@Au nanocomposites, the comparative results clearly demonstrate that the hollow Ag@Au nano-urchins fabricated by adding 6 mL HAuCl₄ solution can unambiguously possess extremely higher SERS activity. After adding different amount of HAuCl₄ solution, the variation of SERS intensity at 1590 cm⁻¹ originated from diverse Ag@Au nanostructures is shown in Fig. 6(b). The quantitative results demonstrate that the SERS intensity significantly increases from about 2600 a.u to 5273 a.u as the content of HAuCl₄ increases to 6 mL. As illustrated in Fig. 2, the increasing Au³⁺ ions in reaction solution enable the inner void space to become larger. Moreover, dendritic shell structure with enhanced rough surfaces will be formed, resulting in unprecedented strong electromagnetic field during the SERS measurement. We further increase the content of HAuCl₄ to 8 mL and 10 mL, respectively. However, the SERS intensity significantly decreases to about 1548 a.u in Fig. 6(b). Moreover, the highest and lowest SERS spectra of 4-ATP molecules adsorbed on two typical Ag@Au nanostructures obtained by adding 6 mL and 10 mL HAuCl₄ solution are shown in Fig. 6(c). Even the concentration of 4-ATP was dropped to 10⁻¹² M, the SERS signals originated from hollow Ag@Au nano-urchins are much higher than that of 10⁻⁸ M 4-ATP on Ag@Au nanostructure obtained by adding 10 mL Au³⁺ ions. The dramatic contrast in Fig. 6(c) indicates the structures of hollow Ag@Au nano-urchins should be destroyed by excessive Au³⁺ ions. The corresponding TEM images of the two different nanostructures are separately provided in the insets of Fig. 6(c). Compared with the obvious cavities in hollow Ag@Au nano-urchins, the void space becomes much weaker by gradually increasing dosage of Au³⁺ ions. Clearly, the TEM images show that the excessive HAuCl₄ will result in anisotropic growth of Au species toward internal orientation, which inevitably blocks the hollow structures. The structural advantages of hollow structures such as lower density and less blockage of electron diffusion will be significantly decreased in SERS measurement, resulting in much weaker SERS activity in this paper. Therefore, the hollow Ag@Au nano-urchins with obvious void spaces and exterior multi-antennas generated by using moderate Au³⁺ ions (6 mL, 0.5 mM) can provide ultrasensitive SERS activity. Based on the SERS spectra at 1590 cm⁻¹, the enhancement factor (EF) of the hollow Ag@Au nanourchins was estimated in the following section, which can be expressed as Eq. (1) below [7–9,20]:

 

Fig. 6 (a) The SERS spectra of 4-ATP (1M) on silicon wafer and 4-ATP on diverse Ag@Au nanomaterials-based substrates. Bottom-up spectral lines are originated from a series of Ag@Au nanoparticles obtained by adding different volumes of HAuCl₄ solution (0, 2, 4, 6, 8 and 10 mL). (b) The variation of SERS (1590 cm⁻¹) intensity originated from different Ag@Au nanoparticles as a function of the volumes of HAuCl₄. (c) The highest and lowest SERS spectra of 10⁻¹² and 10⁻⁸ M 4-ATP molecules on two typical Ag@Au nanostructures obtained by adding 6 mL and 10 mL HAuCl₄ solution. The insets show the corresponding TEM images, respectively. (d) SERS spectra of 4-ATP at much lower concentrations (10⁻¹²~10⁻¹³ M) adsorbed on the substrate of hollow Ag@Au nano-urchins.

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4. Conclusions

We have demonstrated the successful synthesis of hollow Ag@Au nano-urchins with obvious void spaces and dendritic shell architectures by seed-mediated growth method. Superior to traditional chemical routes, the stable Ag nanoseeds were fabricated by pulsed laser ablation of Ag target in solution. The ultra-rapid laser sintering/quenching of the Ag nanospheres enables the outside surfaces to become more stable than that of core regions. Then, the distinctive feature is that the consumption or removal of Ag species by galvanic replacement reaction between HAuCl₄ solution and Ag nanoseeds starts from inner spaces to external regions, resulting in the construction of stable Ag@Au nano-frames with rough shell structures. It is very suitable for the next anisotropic overgrowth of Au species on the rugged surface of nano-frames by concomitant reduction reaction between Au³⁺ ions and AA reductive agent. The obtained hollow Ag@Au nano-urchins exhibit excellent chemical stability in ionic or oxidative solution. It would be advantageous to obtain stable SERS substrate for biomedical and bio-molecular detection in different biological solutions. The novel hollow Ag@Au nano-urchins have been demonstrated as an advanced SERS substrate for the ultrasensitive SERS detection. Correspondingly, the average EF of 4-ATP molecules reaches up to ~10¹². Moreover, the dominating characteristic bands of the probe molecules are also distinguishable even at a concentration of 10⁻¹³ M. The pronounced SERS activities of hollow Ag@Au nanourchins will be useful for ultra-trace detection of biomolecules in extensive applications. Alternatively, the facile strategy used in this paper can be easily extended to synthesize other bimetallic hollow nanodendrites.