We report on the successful fabrication of multi-branched CuS nanodendrites with average branch length of about 20 nm by laser ablation of bulk Cu target in thioacetamide (TAA) solution. During the nucleation of Cu and S species, the accurate anisotropic growth should be attributed to an ultra-rapid acid etching process by laser-induced TAA hydrolyzing reaction. Interestingly, the semiconductor CuS nanodendrites provide pronounced surface enhanced Raman scattering (SERS) properties with noble-metal comparable activity and a detection limit as low as ~10−10 M, approaching the requirement (~nM) for single molecule detection. More importantly, after SERS analysis, the crystal violet (CV) probe molecules can be effectively removed from the substrate by 1064nm laser irradiation-induced moderate thermal treatment. Therefore, the unique and distinctive advantage is that the as-prepared CuS nanodendrites exhibit excellent reusability for 60 cycles of repeated SERS analyses. The low-cost CuS semiconductor nanodendrites with enhanced SERS properties should be established as a prominent SERS-based ultrasensitive probe in the repeated applications.
© 2017 Optical Society of America
Surface-enhanced Raman scattering spectroscopy (SERS) has been extensively exploited as a powerful and promising spectroscopic technique with high sensitivity to probe and identify various organic-molecules and biomolecules in a wide range of potential applications [1–5]. It is one of the most analysis tools for the detection of ultra-trace elements in biomedical and environmental sciences, molecular diagnostics, and biochemistry, etc. Recently, an interesting work illustrated that the silver (Ag) nanospheres-based sensors provided a sensitive detection of single acid mutation in human breast cancer via Raman spectroscopy . Increasing evidence has shown that the SERS detection limit should reach to 10−9 M, in order to approach the requirement (~nM) for single molecules detection. Because of unprecedented strong and intense electromagnetic field, noble Ag and gold (Au) monometallic nanomaterials or hybrid nanocomposites with rugged surface structures are usually served as effective SERS substrates in which the Raman signals of the probe molecules can be significantly enhanced [4–9]. Meanwhile, it should be noted that the further development of noble Ag or Au based SERS substrates have been severely limited because of their intrinsic high cost.
Therefore, in recent years, extensive research efforts have been devoted toward the development of low-cost semiconductor nanomaterials-based SERS substrates such as Cu2O superstructure , nanoporous copper , porous ZnO nanosheets , SiO2/TiO2 microbeads . However, as for semiconductor substrates, two important issues should be also considered in SERS applications. Firstly, the detection limit of these inexpensive semiconductor SRES substrates should be further improved for SERS-based ultrasensitive analyses. Compared with noble metals, most of previous works demonstrated some weaker SERS actives on semiconductor nanostructures. Chen and associates used nanoporous copper as SERS substrate, and the limit of the detections for both Rhodamine 6G and crystal violet 10B can reach to 10−5M . It is far way from the limit of detection by noble metal nanostructures, and not suitable for single molecules detection. The enhanced Raman signals are mainly derived from charge-transfer complex resonating with incident laser light [2,10–12]. Most recently, an outstanding work demonstrated that Cu2O superstructure particle exhibits higher SERS sensitivity with ultra-low limit of detection (10−9M). In addition to the photo-induced electron transfer in the semiconductor-probe molecule, the Cu-based nanostructure with pronounced electrostatic adsorption effect can adsorbed more positively charged target molecules, resulting in noble-metal comparable SERS activity. However, the complicated and multistep self-assembly processes for the successful formation of Cu2O superstructure microcrystals are extremely sensitive to harsh experimental conditions including the hydrolysis rate, the consumption rate of CuCl, and continuous in situ hydrolysis, environment temperature, and accurate growth time, etc. Meanwhile, compared with nano-sized architectures, the large size (~2μm) of the self-assemble Cu2O micro-superstructure may be not suitable for SERS-based analysis of biomedical molecules or live cells in vivo. In fact, multi-branched nanodendrites have a promising potential for further improving SERS activities [5,7,9]. Compared with solid nanoparticles or core-shell like structures, the nanodendrites are characterized by intense nano-tips and immense nano-antennas structures, possessing unique rough-surface structures. Especially for Cu-based nanodendrites, the multi-branches can provide unprecedented significant electrostatic adsorption effect, giving rise to the adsorption of more positively charged target molecules on SERS substrate. Up to now, the convenient synthesis of multi-branched semiconductor Cu-based nanodendrites with superior SERS properties has not been reported.
Meanwhile, another urgent issue is related to the stability and reusability of the semiconductor nanomaterial-based substrate in repeated SERS applications. As an advanced SERS substrate, it should be stable and reusable under repeated applications, which is a critical factor in practical SERS analyses. In principle, the previous probe molecules must be completely removed from the substrate after each recycling application. Based on the as-prepared semiconductor colloidal suspensions, the self-assembled nanomaterial films will be damaged or destroyed after rinsing them in distilled water. Fortunately, an interesting strategy has been explored to remove the MB molecules from the TiO2 shell-based nano-spherical resonators . The self-cleaning process has been developed through ultra violet (UV) irradiation. However, to our knowledge, the excellent reusability/recyclability of the semiconductor nanomaterial-based SERS systems remain incomplete.
Herein, we report on the one-step synthesis of multi-branched CuS nanodendrites with average branch length of about 20 nm by 1064 nm laser ablation of bulk Cu target in thioacetamide (TAA) solution. Recently, our group has confirmed that the laser ablation in liquid (LAL) is an attractive green and versatile technique for the fabrication of novel nanocomposites [7,14–17]. In the absence of any complicated soft directing agents or stabilizers, the formation of pure CuS nanodendrites is highly related to the ultra-rapid acid etching through laser-induced TAA hydrolyzing reaction. Owing to the small-sized multi-branches, the as-prepared CuS nanodendrites with immense nano-antennas and adequate nano-tips exhibit excellent SERS properties with noble-metal comparable activity and a detection limit of ~10−10 M. The pronounced features are better than many previous reports on semiconductor substrates [2,10–12], approaching the requirement (~nM) for single molecule detection [4,5,8]. Moreover, we found that the probe crystal violet (CV) molecules can be effectively removed from the CuS nanodendrites-based substrate by moderate thermal treatment through the 1064nm laser irradiation. Therefore, the unique advantage is that the obtained CuS nanodendrites-based substrate is recyclable, which exhibits excellent SERS reusability and stability even after 60 cycles repeat tests. Our results have open up a novel versatile strategy to fabricate pure multi-branched semiconductor nanodendrites with pronounced SERS properties, possessing high applicability in SERS-based ultrasensitive analysis for recycling applications.
2. Experimental setup
The experimental setup used laser ablation in liquid condition has been widely demonstrated in our previous works [7,14–17]. Briefly, the bulk pure (99.99%) Cu target with well-polished surfaces was placed on the bottom of a rotating glass dish (~500 rpm). The dish was filled with 2 mm depth of liquid solution including 0.5 M thioacetamide (TAA) and 10mL distilled water. The TAA can provide the sulfur sources and hydrogen ions through laser-induced TAA hydrolyzing reaction. A Q-switch Nd-YAG (yttrium aluminum garnet) laser beam (Quanta Ray, Spectra Physics) with pulse duration of 10ns and 10Hz repetitions was used as input energy. The moderate laser beam with power density of ~1.3 GW/cm2 was focused on the Cu target by a quartz lens with 100 mm focal length. After 30 min laser ablation, the obtained colloidal suspensions were immediately centrifuged at 18000 rpm for 10 min in an ultracentrifuge. The obtained sediments were separately analyzed by transmission electron microscopy (TEM, JEOL-JEM-2100F), X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF) using Cu Kα radiation (λ: 0.15406nm), and X-ray photoelectron spectra (XPS) on a PHI Quantera SXM with an Al Kα = 280.00 eV excitation source. Moreover, the morphological investigations and chemical compositions of the products were measured by field emission scanning electron microscope (SEM, Hitachi, S-4800) and energy-dispersive x-ray spectroscopy (EDS). Additionally, the absorption spectrums of the products were recorded by a UV-IR spectrometer (Cary 50). In a typical SERS measurement, the SERS-substrate was prepared by placing 0.3 M/20 μL as-prepared CuS nanomaterials on a carefully cleaned silicon chip via spin-coat method (4000 rpm, KW-4A) and then dried naturally at room temperature for 12 h. The silicon chips were carefully rinsed with HCl: H2O2: H2O (1:1:4 v/v) solution for 20min, and dried with high purity nitrogen (99.99%) condition. Then, the SERS substrates were separately immersed into 10−6 ~10−8 M crystal violet (CV) ethanol solutions for 1.4 h and dried in nitrogen stream at room temperature. In order to get single molecules level result, the 0.3 M/20 μL as-prepared CuS nanomaterials were mixed with 0.1mL 10−9~10−10M CV molecular solution. Then, the solution is continuously stirred at a constant speed of 100 rpm for 24 h to ensure the established of adsorption-desorption equilibrium among the nanomaterials and CV molecules. Finally, the products were centrifuged at 18000 rpm for 15 minutes in an ultracentrifuge. The obtained sediments were carefully dispersed on the silicon chips by spin-coat method (4000 rpm, KW-4A). These SERS substrates were dried naturally at room temperature for 12 h. The SRES results were obtained by a confocal microprobe Raman spectrometer (Lab RAM HR 800 spectrograph) at room temperature by using a 633 nm and 1 mW laser beam. For all measurements, the size of the laser spot was about 1.2μm under a 100 × objective, and the acquisition time used for each spectrum is 40s. As for the repeated SERS tests, the multi-branched CuS nanodendrites-based substrate with 10−6 M CV molecules was irradiated in vacuum chamber (10−2 Pa) by 1064 nm laser beam with~2kW/cm2. After laser irradiation, the same substrate was recyclable and used for the subsequent SRES measurements.
3. Result and discussion
After cumulative pulse laser ablation of pure Cu target in TAA solution, the morphologies of the obtained product are illustrated by transmission electron microscopy (TEM) in Fig. 1. The low-magnification image in Fig. 1(a) clearly shows that the as-prepared nanomaterials are characterized by multi-branched nano-architectures with obvious complex nano-tips. The average length and diameter of the elongated branches are separately about 20 nm and 6 nm by measuring the size of more than 500 nano-structures in sight on the TEM images. In the absence of any dispersing agents or surfactants, the generated multi-branched nanodendrites tend to interconnect with each other via the magnetic-dipole interaction, forming short curvilinear structures. The chemical compositions of the well-defined nanodendrites were determined by energy-dispersive x-ray spectroscopy (EDS). The inset in Fig. 1(a) shows that the relative strong peaks of the EDS pattern are only composed of Cu and S elements, and the relative ratio is about 50.8:49.2, which is consistent with the CuS chemical composition. Moreover, the dendritic nature of the product was further clearly confirmed by the typical high-resolution TEM image of the representative structure in Fig. 1(b). Meanwhile, the enlarged TEM image of the branches illustrates that the lattice fringes with a periodicity corresponding to a d-spacing of 0.19 nm could be indexed to the (107) plane in the CuS (Covellite) structure.
The moderate laser power density plays an important role in the formation of multi-branched nanodendrites. If a low-power (~0.9 GW/cm2) laser beam was adopted in our experiment, nanoporous structures instead of nanodendrites were formed by laser ablation. As shown in Fig. 2(a), the corresponding TEM image of the typical product illustrates that the numerous interconnected irregular nanomaterials are indeed nanoporous structures with obvious surface pores. The average overall size of these nanoporous structures is about 50 nm. In addition, the EDS result is also agree with the CuS chemical composition. Moreover, the porous structures were also further visualized by the HRTEM image in Fig. 2(b). The surface pores are shown as contrasting lighter images (the areas marked by blue lines) with their frameworks as darker ones, due to different penetration depths of the incident electron beam. Correspondingly, the lattice fringes with a d-spacing of 0.19 nm in the frameworks should be also indexed with reference to the Cu (107) plane structure.
To get more information from the structures of these multi-branched CuS nanodendrites and porous CuS nanomaterials, the corresponding XRD patterns and x-ray photoelectron spectroscopy (XPS) results are illustrated in Figs. 3(a)-3(b), respectively. The XRD pattern of the CuS nanoporous structures demonstrates that a serious of (102), (103), (105), (107) and (116) CuS diffraction peaks (Covelline, JCPDS, no.06-0464) were detected in Fig. 3(a). In contrast with the nanoporous structures, the multi-branched CuS nanodendrites are found to be well crystalline according to the three distinct XRD diffraction peaks (red curve line) in Fig. 3(a). Because of the much higher peak at 47.77° in XRD pattern, the preferential alignment of the (107) orientation should be generated in single-crystal CuS nanodendrites. Additionally, the element valence and surface purity of the as-prepared CuS nanodendrites and CuS nanoporous structures were illustrated by XPS patterns in Fig. 3(b). In both two nanostructures, the Cu and S, as well as C and O elements are detected in the XPS results. In general, the binding energies were calibrated by referencing the C1s peak at 284.8 eV to reduce the sample charge effect, which have been verified in previous works [15,17]. The weak peak of O1s at 531.9 eV should be attributed to the surface oxide after keeping them in an oven. Moreover, the relative higher peaks of Cu2p, Cu3p, S2p, S2s, as well as Cu (A) (from Auger electrons) can be clearly distinguished in Fig. 3(b). In particular, the peaks at 932.8 eV (Cu2p3/2) and 952.4 eV (Cu2p1/2) indicate the oxidation state of Cu2+ generated in the both nanostructures. On the other hand, the S2p3/2 peak at 162.1 eV and S2p1/2 peak at 163.2 eV prove the element valence state of S2- formed in the obtained products. Therefore, the above results are the best evidences for the formation of CuS compositions in two different nanomaterials. The possible growth mechanisms of the multi-branched CuS nanodendrites and CuS nanoporous structures should be attributed to the laser ablation-induced unique nucleation processes [7,14–17]. Briefly, when the pulsed laser beam arrived at the Cu target surface, the laser energy will be well absorbed by the surface layers. The absorbed photon energy of the laser beam can promote the irradiated area temperature, resulting in rapid boiling and vaporization of Cu species. The explosive Cu plasma with ultra-high temperature (~thousands Celsius) in the liquid solution will significantly improve the surrounding TAA hydrolyzing degrees. The rapid-nucleation of Cu and S (from TAA hydrolyzing reactions) will occur in the stage of rapid condensation of Cu plasma. Meanwhile, during the CuS nucleation process, the hydrogen ions (H+) originated from TAA hydrolyzing reactions can enable some Cu and S species to be dissolved/removed from the initial nanostructures. In this way, the complicated nucleation processes combined with ultra-rapid acid etching reactions give rise to the unique anisotropic growth of branched structures. It should be noted that enough TAA hydrolyzing reactions could provide sufficient hydrogen ions. It also plays an important role in the ultra-rapid acid etching process, and then leads to the formation of branched nanostructures. In this paper, we found that the TAA hydrolyzing reaction degree is highly related to the laser power density. A low-laser power density inevitably reduces the Cu plasma temperature and then depresses the TAA hydrolyzing reaction, resulting inadequate hydrogen ions formed in solution. Therefore, incomplete acid etching will result in the formation of nanoporous structures, as shown in Fig. 2. In summary, the moderate laser beam would be applicable to the unique anisotropic growth of multi-branched CuS nanodendrites.
Finally, the unique SERS performances of the obtained multi-branched CuS nanodendrites have been demonstrated by using crystal violet (CV) as the probe molecules. Figs. 4(a)-4(c) show the representative SEM images of the three different SERS substrates based on the CuS nanoporous structures, multi-branched CuS nanodendrites, and noble metallic Ag nanoparticles, respectively. The noble metallic Ag nanoparticles with average size of ~20nm have been fabricated by the same laser beam ablation of Ag (99.99%) target in distilled water condition. In these SERS substrates, we found that the silicon plates are completely coved with three nanomaterials. In this paper, the noble metallic Ag nanoparticles-based SERS substrate was selected as a reference object for comparative analysis of SERS properties. Fig. 4(d) illustrates a series of Raman spectra originated from 10−6 M CV molecules on the CuS nanoporous structures, multi-branched CuS nanodendrites, and Ag nanoparticles, respectively. It can be seen that the dominating characteristic bands of the CV molecules at 1617.3, 1583.1, 1528.9, 1479.4, 1442.2, 1367.8, 1298.9, 1171.8, 914.3, 798.9, 755.1, 725.2, 524.6, and 437.4 cm−1 are clearly detected in SERS spectra. Interestingly, even the concentration of CV molecules was dropped to 10−6 M, the multi-branched CuS nanodendrites provide extremely intense Raman signals, which are significantly higher than those originated from noble metallic Ag nanoparticles and CuS nanoporous structures. As shown in Fig. 4(d), for example, the SERS intensity at 1617.3 cm−1 significantly increases from about 1966.6 a.u on CuS nanoporous structures to 4326.6 a.u on noble metallic Ag nanoparticles, and then reaches to 11156.5 a.u on CuS nanodendrites-based substrate. The direct comparison clearly reveals that the obtained semiconductor CuS nanodendrites can unambiguously possess excellent SERS activity, which is also comparable to that of noble metal-based SERS substrates. Moreover, in order to get the detection limit of the multi-branched CuS nanodendrites, we further decreased the concentration of CV molecules in solution to 10−7, 10−8, 10−9 and 10−10 M, respectively. The corresponding variations of the SERS spectra of CV molecules on CuS nanodendrites are shown in Fig. 4(e). The results show that the dominating characteristic bands are also clearly distinguishable even with concentration as low as 10−10 M. In this paper, the obtained multi-branched CuS nanodendrites exhibit excellent SERS sensitivities with ultra-low detection limit of ~10−10M and noble metal-comparable activity, approaching the requirement (~nM) for single molecule detection. It also exceeds many previous results based on various semiconductor nanomaterials [2,10–19]. The pronounced SERS properties should be related to the immense nano-antennas and adequate nano-tips formed on the multi-branched CuS nanodendrites. Compared with core-shell shape or porous structure, the unique nano-architectures provide much more spots for amplifying Raman scattering of probe mole due to the electromagnetic field enhancement. Meanwhile, because of the copper vacancies defect-induced electrostatic adsorption effect , more CV dye molecules can be adsorbed on the CuS nanodendrites, which can avail themselves of new resonances owing to the formation of a metal-ligand coordination complex with the substrate surfaces . In addition to electromagnetic field enhancement, the chemical enhancement originated from charge transfer resonances in metal ligand coordination complexes will also play an important role in the enhanced SERS activity. In future studies, it is worthwhile to illustrate the corresponding charge transfer (CT) transitions in CV molecules adsorbed on CuS nanodendrites in more detailed way.
Additionally, as an advanced substrate for the practical SERS analysis, it should be reproducible and recyclable under repeated applications. Compared with the stable Au nanoporous films in our recent work , the SERS substrates were prepared by dispersing as-prepared CuS colloidal suspensions on silicon plate. So, the original CV probe molecules cannot be removed from the CuS nanodendrites by rinsing in distilled water. Alternatively, a novel thermal treatment based on ultra-low laser irradiation has been successfully developed in this paper. It is well known that the near-infrared (NIR) laser beam would be applicable to the laser-induced heating process. On the other hand, the photo energy of NIR laser beam should be effectively absorbed by the CuS nanomaterial-based substrate. Figure. 5(a) depicts the corresponding localized surface plasmon resonance (LSPR) spectra of the as-prepared CuS nanodendrites. The LSPR peak can be clearly detected at about 1060nm in NIR region, which is distinctly different from that of most semiconductors in the visible or ultra-violet (UV) regions [2,10–13]. The well-defined NIR plasmonic absorption (LSPR in the biological optical window) is highly related to the formation of copper deficiency that has been verified in many previous reports [21–24]. Surprisingly, the 1064 nm laser beam in our experiment ensures that the incident excitation wavelength is commensurate with the LSPR (1060 nm) of CuS nanodendrites. Therefore, the effective thermal treatment will be achieved by ultra-lower 1064 nm laser irradiation, giving rise to remove the CV molecules from CuS nanodendrites-based substrate. On the other hand, it should be noted that ultra-low power density (~2kW/cm2) of the laser beam also plays an important role in the moderate thermal treatment. We found that the CuS nanodendrites can be destroyed and damaged by using high-power (>10 kW/cm2) laser beam irradiation. It significantly reduces the SERS properties in the subsequent repeated applications. After each SERS measurement, the CV molecules on CuS nanodendrites-based substrate were irradiated in vacuum condition by 1064 nm laser beam with ultra-low power density (~2kW/cm2). As the irradiation time increases to 5, 10, 15 and 20 min, respectively, the corresponding SERS spectra of the residual CV molecules on the substrate are displayed in Fig. 5(b). In the present case, we can find that the dominating SERS signals of the residual CV molecules undergo a progressive decrease as a function of irradiation time, and eventually disappear in the spectrum acquired after 20 min. Moreover, the quantitative description in Fig. 5(c) clearly shows that the corresponding SERS intensity at 1617.3 cm−1 drastically drops from about 11359.9 a.u at initial condition to 884.1 a.u as the irradiation time increased to 15 min, and then reduces to about 19.6 a.u with further prolonging to 20 min. As expected, the most of CV molecules (~99.8%) on the substrate can be completely removed from the substrate by the 1064 nm laser irradiation. Additionally, the inset in Fig. 5(c) shows the TEM image of the CuS nanodendrites after 20 min laser irradiation. We found that the branched structures are identical to that of the initial CuS nano-architectures, which were well maintained after laser-induced thermal treatment. After the moderate thermal treatment, the pure CuS nanodendrites should be reproducible for repeated SERS applications. To test the stability and reusability of the SERS measurements, we further carried out the recycling SERS experiments repeatedly 60 times by using the same CuS nanodendrites-based substrate. After each SERS analysis, the CV molecules were completely removed from the substrate by the moderate thermal treatment. Then, the CuS nanodendrites-based substrate was immersed into 10−6 M CV solutions for the next SRES measurement. As shown in Fig. 5(d), the recycling tests of SRES spectra clearly reveal that the obtained multi-branched CuS nanodendrites provide excellent SERS reusability/stability even after 60 repeated applications. The dominating characteristic bands of the CV molecules could still be sustained for 60 cycles of repeated SRES applications, since the measured SERS intensities are nearly constant during the recycling tests. To our knowledge, it is the first report on the recycling SERS experiments on semiconductor nanodendrites. On the other hand, compared with the repeated SERS applications based on Au nanoporous films in our recent work , the number of measurement cycles in this paper should be further improved in the next research. More importantly, the corresponding enhanced SERS properties and excellent reusability/stability make the inexpensive multi-branched CuS nanodendrites to become a promising semiconductor-based SERS substrate in recycling applications.
In summary, the multi-branched CuS nanodendrites with average branch length of about 20 nm have been successfully constructed by pulses laser ablation of Cu target in TAA solution. The anisotropic growth mechanism is highly related to the unique nucleation of Cu and S species combined with ultra-rapid acid etching process that generated by laser-induced TAA hydrolyzing reactions. Surprisingly, the as-prepared CuS nanodendrites with immense nano-antennas and adequate nano-tips possess enhanced SERS properties with noble metal-comparable activity and ultra-low detection limit of ~10−10M, approaching the requirement (~nM) for single molecule detection. More importantly, the corresponding CV probe molecules can be effectively removed from the CuS nanodendrites-based substrate by the laser-induced moderate thermal treatment. Therefore, the CuS nanodendrites-based substrate is reproducible, which exhibits excellent SERS stability/reusability even after 60 cycles of repeated tests. Based on the inexpensive semiconductor CuS nanodendrites in this paper, these findings thus have a promising potential for developing SERS ultrasensitive probes in recycling applications. Moreover, without using any complicated stabilizers or soft directing agents, the project of using laser beam as a green tool for sculpting pure nanocomposites with controlled structures will prompt the renewed interest in the functional nanomaterials systems.
National Natural Science Foundation of China (Nos.11575102 and 11105085), the Fundamental Research Funds of Shandong University (No.2015JC007), and Shandongjianzhu University XNBS Foundation (No. 1608).
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