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The project of wielding laser light as a versatile tool for sculpting branched Ag@Au bimetallic nanocrystals with mean size of ~50 nm has been developed in this work. The moderate overgrowth of Ag species with negligible damage effect on the branched Ag@Au nanostructures was achieved by laser-induced photo-oxidation. The final Ag@Au nanodendrites exhibit superior surface enhanced Raman scattering (SERS) activities with an enhancement factor up to ~1011 and a detection limit as low as ~10−14 M. The pronounced feature should be attributed to the noticeable small-sized branches (<10 nm) and unique pronounced inter-metallic synergies. Our results have a promising potential for developing SERS-based ultrasensitive probes in biomedical application.
© 2017 Optical Society of America
1. Introduction
The plasmonic metallic (Ag, Au and Cu) nanomaterials with enough rough surfaces could provide unprecedented strong and intense electromagnetic field because of their rugged structures. The unique structures provide excellent surface-enhanced Raman scattering (SERS) performance [1–8]. Two fascinating nano-architectures including hybrid nanocomposites and branched nanodendrites have been well established as advanced structures in the SERS-based ultrasensitive probes. Compared with monometallic nanomaterials, the hybrid nanocomposites (bimetallic or more complex compositions), especially Ag@Au species, exhibit enhanced SERS activities owing to the unique inter-metallic synergies among different metals [1,7–10]. Meanwhile, the branched nanodendrites with intense and immense multi-branches provide much more hotspots in SERS applications than those of hollow cage-like, core-shell like and smooth nanoparticles [9,11,12]. Most of previous works (porous Ag@Au nanospheres [1], branched Au@Pd nanocrystals [2], Au nanoflower [4], branched Au nanocrystal [6], core-shell-like Ag@Au nanospheres [7], Ag@Au@Pt trimetallic nanocages [8], hybrid Cu@Ag nanodendrites [9], etc.) show remarkable SERS activities with enhancement factors (EF) in the range of 104~1010. In fact, the branched Ag@Au bimetallic nanostructures with small-sized branches and controllable compositions have a promising potential for further improving SERS activities. This would be applicable to SERS-based ultrasensitive probes in biomedical applications. But in contrast with other plentiful hybrid nanodendrites, very few reports have addressed the synthesis of branched Ag@Au nanocomposites with less obvious elongated branches via galvanic replacement reaction or electrodeposition [10,11]. Increasing evidences have shown that the Ag and Au species have extremely similar lattice constants in face-centered-cubic structures. They usually aggregated into core-shell morphology, porous-cages or large-sized (about hundreds of nanometers) branched structures with limited controllable compositions by the particle/seed mediated approach, galvanic-replacement or coprecipitation [10–12]. It is found that the moderate overgrowth of Ag or Au species with negligible damage effect on the branched structures is very necessary for the formation of bimetallic nanodendrites. To our knowledge, the convenient synthesis of branched Ag@Au nanodendrites with controllable structures and their superior SERS activities (a higher enhancement factor or a lower detection limit) has not been reported up to now.
The projection using laser light as a versatile and convenient tool for sculpting novel complex-nanostructures has prompted the recent renewed interest in the multi-functional material systems [13–21]. Most recently, an outstanding work demonstrated that the moderate anisotropic growth of Au nanoprisms via laser-induced photochemical reactions in methanol solution [13]. Compared with traditional chemical approaches, the slow and moderate overgrowth process via laser-induced photochemical reaction allows one to control and manipulate well the nucleation and growth rate. However, it is still unknown whether the novel synthetic route can be extended to the fabrication of branched Ag@Au nanocomposites.
Herein, for the first time, we expand the realm of anisotropic nanostructures accessible via laser-induced photochemical route with the demonstration of optical excitation-driven multi-branched Ag@Au nanodendrites. The novel nanostructures have noticeable small-sized branches (<10 nm) and broad controllable compositions (Ag content of 1.2~34%). The distinct advantage is that the moderate overgrowth of Ag species was found to exhibit negligible damage effect on branched structures. The controlled fabrication is related to the laser light-induced photo-oxidation of ethanol solution and then the concomitant reduction of Ag ions onto precursors. Moreover, the single-crystalline nature with preferential alignment of the (111) orientation has been well remained during the overgrowth. Owing to the controllable structures and compositions, the final multi-branched Ag@Au nanodendrites exhibit superior SERS activities with an EF of ~1011 and a detection limit of ~10−14 M. The pronounced features are better than many previous reports, especially those of anisotropic core-shell structures, porous nanocomposites and monometallic nanodendrites. The enhanced SERS activity should be attributed to the unique inter-metallic synergies in small-sized Ag@Au bimetallic nanodendrites. This possesses high applicability in practical analysis in vitro or live cells for biomedical applications.
2. Experimental setup
The experimental apparatus based on laser irradiation in liquid is very similar to that described in previous studies [17–21]. The schematic diagram in Fig. 1 depicts the whole procedures for synthesizing the Ag@Au nanoparticles, the highly branched Ag@Au nanodendrites and then the multi-branched Ag@Au nanodendrites, respectively. Firstly, a well-polished Au metal used as the target was placed on the bottom of a rotating glass dish (~500 rpm) filled with 5 mm depth of liquid solution (0.2 M AgNO3, 0.4% T80, and 20 mL distilled water). The Ag@Au nano-particles were obtained via laser irradiation process by a Q-switched Nd-YAG (yttrium aluminum garnet) laser (Quanta Ray, Spectra Physics) beam. The laser beam operated at a wavelength of 532 nm with pulse duration of about 6 ns, the energy of about 350 mJ and 10 Hz repetition. After the irradiation for 20 minutes, the products were carefully washed in distilled water and centrifuged at 18000 rpm for 10 minutes in an ultracentrifuge. The obtained products will be dispersed into a certain amount of distilled water for preparation of 0.1 M/4 mL Ag@Au nano-particles. Secondly, the highly branched Ag@Au nanodendrites can be obtained by adding (drop-wish) the 50 μL~210 μL 0.05 M HAuCl4 solution into the 0.08 M/4 mL ascorbic acid solution in the present of 0.1 M/4 mL Ag@Au nano-particles. Finally, multi-branched Ag@Au nanodendrites with broad controllable compositions (1.2~34% Ag) can be further constructed via laser irradiation of the highly branched Ag@Au (1.2% Ag) nanodendrites solution (4mL) mixed with 1 mM/3 mL AgNO3 ethanol solution. A continuous low-power (150 mW) 532 nm laser beam was selected for fabricating the multi-branched Ag@Au nanodendrites. The 532 nm laser beam can enable the SPR (~520 nm absorption) of the Au species to be highly excited in laser irradiation, leading the plasmon-mediated growth of Ag species on the precursor [13]. Other kinds of ultrafast (ps or fs) lasers will lead to the formation of uncontrollable/unpredictable poly-dispersed Ag@Au nanostructures, owing to the higher photon energy. On the other hand, the accurate growth of multi-branched Ag@Au nanodendrites is also related to the low-power laser beam. We have found that a high-power 532nm laser beam will result in the drastic overgrowth process. It will severely destroy and damage the branched structures. The products were centrifuged at 7000 rpm for 10 min in an ultracentrifuge. 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). Elemental mapping images were obtained by energy dispersive X-ray spectroscopy (EDS) using a JEOL-2100F electron microscope equipped with a STEM unit. 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). Morphological investigations were performed by field emission scanning electron microscope (SEM, Hitachi S-4800). The absorption spectrums were recorded by a UV-Vis-IR spectrometer (UV-1800, Shimadzu). In a typical SERS procedure, the nanomaterials-based substrates were prepared by dropping 0.1 M/30 μL as-prepared products on a carefully cleaned silicon plates and then dried naturally at room temperature for 12h. The SERS substrates were then immersed into 10−10~10−14 M 4-Aminothiophenol(4-ATP) and ethanol solution for 1h, and dried in a nitrogen stream before the Raman measurement. 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 633nm laser with an output power of 50 mW. The acquisition time used for one spectrum is 10s. Moreover, the measurements were performed at different positions on each sample for the test reproducibility.
Fig. 1 The schematic diagram of the whole procedures for synthesizing the Ag@Au nanoparticles, the highly branched Ag@Au nanodendrites and then the multi-branched Ag@Au nanodendrites, respectively.
3. Result and discussion
The obtained ultra-small Ag@Au nanoparticles via 532 nm pulse laser irradiation were served as seeds for further fabrication of nanodendrites. The morphologies of the nanoparticles are illustrated by transmission electron microscopy (TEM; Figs. 2(a) and 2(b1), respectively). The TEM images clearly reveal that numerous liquid-dispersed nanoparticles are irregular quasi-sphere shaped structures. The elemental mapping images (inset in Fig. 2(a)) show that the pristine nanoparticle is indeed hybrid structure and composed of Au and Ag elements with relative ratio of about 8:92. The average size of these Ag@Au nanoparticles is about 5 nm by measuring the diameters of more than 350 nanoparticles in sight on the TEM images. During the process of adding HAuCl4 (50~170 μL), the morphologies of these bimetallic Ag@Au nanoparticles will significantly develop into branched structures by overgrowth of Au species on the precursors. As shown in Figs. 2(b2)-2(b5), the typical TEM images illustrate the complex branched nanodendrites with three, four, six, and multi-tips (>20) generally formed. Meanwhile, the overall size of the nanodendrites also significantly increases from about 5 nm to 30 nm. Increasing the HAuCl4 (210 μL) results in highly branched Ag@Au nanodendrites with obvious multi-branches (Fig. 2(c)). The highly branched Ag@Au nano-structures with large quantity (yield>97%) are uniformly dispersed and well-defined nanodendrites, having an overall average size of ~50 nm. The dendritic nature of the products was further clearly visualized by the typical high-magnification image of representative structure in Fig. 2(d). It reveals that the each individual nanodendrites is composed of a solid body with multiple (>20) and elongated branches on its surface. The average length and diameter of the elongated branches are about 18nm and 4 nm, respectively. The corresponding elemental mapping images of the typical nanodendrites in Fig. 2(d) show that the hybrid branched structures are indeed composed of Au and Ag elements. The relative ratio of Ag in the bimetallic structure is about 1.2%.
Fig. 2 The typical (a) low- and (b1) enlarged TEM images of Ag@Au nano-particles. The inset in (a) is the typical TEM image of individual Ag@Au nano-particle and the corresponding elemental mapping images. (b2-b5) The TEM images of the branched Ag@Au nanodendrites by adding HAuCl4 with 50, 90, 130 and 170 μL, respectively. (c-d) The typical morphologies of the highly-branched Ag@Au nanodendrites by using 210 μL HAuCl4. The bottom pictures in (d) show the corresponding elemental mapping images.
Furthermore, the unique optical properties of the highly branched nanodendrites were illustrated by the UV-visible absorption spectra. It is interesting to note that the LSPR peak distinctly shifted from the visible ~422 nm to the NIR region ~858 nm with the increase of HAuCl4 content (50~210 μL). During the overgrowth process, the solution color changed from dark yellow to dusty blue (inset in Fig. 3). Meanwhile, the increasing intensities of LSPR peaks will also be an evidence for the high-yield synthesis of nanodendrites.
Fig. 3 The UV-visible absorption spectra of Ag@Au nanodendrites (A-J) by adding HAuCl4 with amount increasing range from 50~210 μL.
The anisotropic growth mechanisms of highly branched Ag@Au nanodendrites will be illustrated in the following section. Briefly, compared with the traditional chemical fabrications, the precursor Ag@Au nano-seeds were fabricated by laser irradiation in liquid solution. At the moment of pulsed laser arriving at Au target, rapid boiling and vaporization of Au element will occur, resulting in the formation of explosive Au plasma with ~thousands Celsius. The superheating with highly non-equilibrium feature will play a critical role in the laser irradiation (0~10 ns) [22–27]. The nucleation of Au should take place in the stage of rapid condensation of the plasma, and sharply terminate due to the exhaustive of the plasma vapor. It has been recognized that Au nanostructures are anticipated to provide an ideal surface plasmon resonance (SPR) in visible region (~520 nm absorption). After 532 nm pulse laser irradiation in AgNO3, the SPR of the Au nano-particles will be excited to induce the plasmon-mediated growth of Ag species on the precursor via the concomitant reduction of Ag ions. The reduction process should be driven by hot electron-hole pairs generated via Landau damping process [13]. Then, the ultra-rapid nucleation leads to various defects, resulting in these species at a highly excited state. It is the main reason for the formation of irregular Ag@Au nanoseeds. Thanks to the rough surface structures, the subsequent overgrowth of metallic atoms via reduction process will preferentially occur at protrusion regions. It is very beneficial to fabricate highly branched Ag@Au nanodendrites.
In order to further improve the Ag content and maintain the branched structures, the subsequent 532 nm laser irradiation of the initial Ag@Au (1.2% Ag) nanodendrites (0.1 M/4 mL) in 1 mM/3 mL AgNO3 ethanol solution was then carried out. After 3 h laser irradiation, the typical TEM images of two enlarged regions with four individual nanostructures are illustrated in Figs. 4(a) and 4(b). The results show that the multi-branched nanostructures have been maintained in final products. More clearly, closer views of these morphologies indeed confirm that each nanodendrites is also composed of a solid body with multiple (>20) and elongated branches on its surface. The overall average size is about 52 nm, which is slightly larger than that of pristine Ag@Au nanodendrites. It can be concluded that the slow and moderate overgrowth of Ag species by laser-induced photochemical reaction has negligible damage effect on the branched nanostructures. Most importantly, the corresponding elemental mapping images (below pictures in Figs. 4(a) and 4(b)) clearly confirm that the uniform distribution of Au and Ag elements throughout the nanodendrites. The average relative ratio of Ag in the final bimetallic structure increases to about 34% through laser-induced overgrowth process.
Fig. 4 (a-b) The typical enlarged TEM image of individual multi-branched Ag@Au nanodendrites after 3 h laser irradiation. The below pictures show the corresponding elemental mapping images.
The optical absorption properties in Fig. 5(a) illustrate that the LSPR peak of the bimetallic nanodendrites can be modulated from 858 to 597 nm as the Ag concentration increases from 1.2% to 34%. The solution color changed from dusty blue to dark grey (inset in Fig. 5(a)). Moreover, we further reveal the variation of Ag concentration in bimetallic nanodendrites versus the laser irradiation time (Fig. 5(b)). The chart clearly shows that the ratio of Ag species significantly increases from about 1.2% to 32% as the irradiation time increases to 1.5 h, and then reaches to 34% within 3 h irradiation. We found that the Ag concentration increases nonlinearly with an increase of irradiation time, and reaches saturation after 3 h laser irradiation. In addition, the crystallographic investigations of the obtained nanodendrites were established by XRD in Fig. 5(c). Compared with the initial Ag@Au nanodendrites (red-dot in Fig. 5(c)), the final multi-branched Ag@Au nanodendrites obtained by 3 h laser irradiation exhibit a very similar (111), (200), (220) and (311) diffraction peaks. Because of the relative higher peak at 38.23° in XRD pattern, the preferential alignment of the (111) orientation should be formed in bimetallic Ag@Au nanodendrites. Moreover, the high-resolution transmission electron microscopy (HRTEM) image of the typical multi-branched Ag@Au nandendrites is illustrated in Fig. 5(d). The lattice fringes in Fig. 5(d) with a periodicity corresponding to a d-spacing of 0.236 nm could be indexed as the (111) plane in the Ag@Au alloys. The corresponding selected-area electron diffraction (SAED) pattern (inset in Fig. 5(d)) also indicates the nanodendrites should be single-crystal nanostructures. The single-crystalline nature with preferential alignment of the (111) orientation can be well kept during the overgrowth of Ag atoms by laser induced photochemical reduction. The controllable accurate growth of Ag atoms is based on the laser irradiation in liquid that facilitates the formation of hot electrons by optical excitation of ethanol [13]. As the hydroxyl (-OH) groups in ethanol solution absorbed laser light, the excited electrons will result in the reduction of Ag ions and overgrowth of Ag atoms on the branched precursors. It provides a gradual increase of Ag content in the hybrid nanodendrites. Compared with borohydride, citric acid, or ascorbic acid, etc. the ethanol with -OH group is a very weak reducing agent. It must be excited by laser light irradiation, resulting in a controllable redox process and then the slow and moderate overgrowth of Ag species on the initial Ag@Au nanodendrites. On the other hand, the generation of initial Ag@Au nanodendrites played an important role in this paper. If continuous 532 nm laser irradiation is directly conducted on the ultra-small Ag@Au nanoparticles in AgNO3 ethanol solution, the aggregated/agglomerated Ag@Au nanomaterials with less branched structures will be formed. The most likely reason is that the photon energy of the laser beam should be highly absorbed by the ultra-small (~5 nm) Ag@Au nanoparticles, increasing the temperature around the nanoseeds and giving rise to uncontrollable overgrowth of Ag species. It is not suitable for the fabrication of multi-branched Ag@Au nanodendrites with controllable compositions.
Fig. 5 (a) The UV-visible absorption spectra of Ag@Au nanodendrites obtained by laser irradiation for irradiation time varied from 0 to 3 h. (b) The curve of the Ag concentration in Ag@Au nanodendrites versus laser irradiation time. (c) The XRD patterns of the Ag@Au nanodendrites before and after laser irradiation for 3 h. (d) The representative HRTEM image of the multi-branched Ag@Au nanodendrites after laser irradiation for 3 h. The inset shows the result of the SAED.
Finally, the superior SERS activities of the obtained Ag@Au nanodendrites have been illustrated by using 4-Aminothiophenol (4-ATP) as the probe molecules in Fig. 6. Figure 6(a) shows a series of SERS spectra originated from 4-ATP (1 M) on silicon substrate without any Ag@Au nanomaterials, 4-ATP (10−10 M) on the Ag@Au nanoparticles and different Ag@Au nanodendrites. The five branched Ag@Au nanodendrites were obtained by laser irradiation for five different time (0, 45, 90, 135 and 180 min), respectively. The dominating characteristic bands of the 4-ATP at 1008, 1088, 1180 and 1590 cm−1 are all observed in SERS spectra (Fig. 6(a)). Clearly, the substrate of final multi-branched Ag@Au nanodendrites exhibits extremely intense Raman signals (10−10 M, 4-ATP) with intensities reaching ~10 fold of those from 1 M 4-ATP, Ag@Au nanoparticles and the initial Ag@Au nanodendrites (10−10 M, 4-ATP). Based on the five different Ag@Au nanodendrites obtained by laser irradiation for different time (0~180 min), the variation of SERS intensity at 1590 cm−1 as a function of irradiation time is displayed in Fig. 6(b). The results clearly demonstrate that the SERS intensity significantly increases from about 1800 a.u to 17900 a.u as the irradiation time increases to 90 min, and then reaches to 22300 a.u within 180 min irradiation. We found that the SERS intensity increases nonlinearly with an increase of irradiation time, and reaches saturation after 180 min laser irradiation. As shown in Fig. 6(c), the quantitative SERS measurements can be well repeated for random 550 points since the SERS intensities at 1590 cm−1 distribute homogeneously above the substrate of final Ag@Au nanodendrites. The deviation of the average intensity (~22300 a.u) is about 5%. Based on the SERS spectra at 1590 cm−1, the enhancement factor (EF) of final Ag@Au nanodendrites was estimated in the following section. The equation of the 4-ATP (4-Aminothiophenol) molecule enhancement factor can be expressed as Eq. (1) below [1,3,28,29]:
Where ISERS and IBULK are the signal intensities of SERS and normal Raman spectra of 4-ATP at the same band (~1590 cm−1). And NSERS and NBULK represent the corresponding number of molecules in the focused incident laser spot. As completely consistent laser parameters were adopted in the SERS measurements process, NSERS and NBULK can be approximately substituted by the concentration of 4-ATP. In Fig. 6(a), the ISERS and IBULK are 22300 a.u and 2135 a.u for the final Ag@Au nanodendrites and normal Raman spectra of 4-ATP, respectively. Therefore, the EF can be calculated to be about 1011 for the final Ag@Au nanodendrites. Moreover, SERS spectra of 4-ATP with various concentrations absorbed on final Ag@Au nanodendrites show that the dominating characteristic bands are distinguishable even with the concentration as low as 10−14 M (Fig. 6(d)). The superior SERS activities in this paper exceed many previous reports, especially those of core-shell structures, porous nanocomposites and monometallic nanodendrites [1,3,28–31]. The fascinating feature should be attributed to the final multi-branched Ag@Au nanodendrites with small-sized (<10 nm) branches as well as the unique pronounced inter-metallic synergies. Most importantly, the branched Ag@Au bimetallic nanodendrites with controllable composition have been fabricated by laser-induced photochemical reaction without using any potential toxic issues. There is no doubt that the pure Ag@Au nanodendrites with superior SERS activities will have promising high applicability in practical biomedical application.Fig. 6 (a) The SERS spectra of 4-ATP (1 M), 4-ATP (10−10 M) on different substrates. A: 4-ATP (1 M) on silicon substrate without any Ag@Au nanomaterials, B: Ag@Au nanoparticles-based substrate, C-G: Ag@Au nanodendrites obtained by laser irradiation for different time (0, 45, 90, 135, 180 min), respectively. (b) Based on the different Ag@Au nanodendrites, the variation of SERS intensity at 1590 cm−1 versus the irradiation time. (c) The variation of SERS intensity at 1590 cm−1 versus 550 different points on the substrate of final multi-branched Ag@Au nanodendrites. The inset shows the typical SEM image. (d) SERS spectra of 4-ATP with various concentrations (10−11~10−14 M) absorbed on the substrate of final multi-branchced Ag@Au nanodendrites.
4. Conclusions
In summary, branched single-crystal Ag@Au nanodendrites with ultra-small sized branches and broad controllable composition (1.2%~34% Ag) have been fabricated by laser-induced photochemical reaction. The controllable synthesis of Ag@Au nanodendrites reveals that laser-induced photo-oxidation can lead to moderate overgrowth of Ag species on the precursors. It has negligible damage effect on the branched single-crystal nanostructures. The final branched Ag@Au nanodendrites with unique pronounced inter-metallic synergies exhibit superior SERS activities with an EF of ~1011 and a detection limit of 10−14 M. The designed strategy provides the possibility of using these pure bimetallic nanodendrites as ultrasensitive SERS-based probes in biomedical application.
Funding
National Natural Science Foundation of China (Grant No.11575102 and 11105085); the Fundamental Research Funds of Shandong University (No. 2015JC007).
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