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Abstract
Surface enhanced Raman spectroscopy (SERS) with both enhanced activity and pronounced thermal stability is very suitable for ultra-sensitive monitoring of thermally assisted chemical bonding/fragmentation reactions in important industrial catalysis. Herein, we report an appealing thermally stable SERS active sensor based on the construction of multiple-branched Au/Ag nanodendrites (NDs). The obtained Au/Ag NDs with tremendous elongated branches and enormous antennas exhibit a much higher SERS activity for dye detection under 785 nm near infrared (NIR) laser irradiation, as compared to as-prepared other reference samples. Meanwhile, the corresponding unique intermetallic synergy can effectively suppress the oxidation of chemically active Ag in a hot environment. It is important that more than 90% SERS activity at room temperature (∼25 ℃) can be well maintained at high temperature (∼170 ℃) within 30 min continuous tests. Thus, it holds great potential for the in-situ SERS monitoring of high-temperature catalytic reactions in the future.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, surface-enhanced Raman spectroscopy (SERS) serves as a fascinating strategy that can provide many ultrasensitive molecular fingerprints at single molecule level [1–7], attracting considerable attention in diverse special applications [8–11]. Especially, in-situ ultrasensitive SERS monitoring of catalytic reaction and chemical transformation are currently of great interest for potential application in widespread energy-related fields or biotechnologies, providing real-time information of single-molecular dynamics in many important chemical/biological processes [12–16]. For instance, SERS kinetic monitoring can provide insight into the reaction mechanisms of catalytic reduction of 4-nitrothiophenol (4-NTP) to 4-aniline derivative (4-ATP) in the presence of Fe3O4@TiO2@Ag-Au NCs [14] or silica-encapsulated Au nanoparticles (NPs) [13]. Recently, in-situ SERS imaging of miRNA in the presence of functionalized h-BN nanosheets has been established as a novel theranostic platform that can overcome poor selectivity and sensitivity of traditional theranostic and diagnostic tools [16]. Generally, all these interesting applications are heavily dependent on the construction of effective SERS nano-substrates with designated structures that can significantly improve SERS performances (high activity, good reproducibility, sensitivity, uniformity, stability, etc.). As for ultrasensitive SERS detection, the main concern is the excellent SERS activity, which is originated from intense inelastic light scattering of probe molecular vibrations via electromagnetic mechanism (EM) of plasmonic metallic (silver (Ag), gold (Au) and copper (Cu)) nanomaterials, and/or chemical mechanism (CM) of effective electron transfer among semiconductor-based nanocomposites (NCs). Among all available hybrid nanoarchitectures, it is well recognized that the highly branched Au/Ag nanoalloys with immense multi-branches and unique intermetallic synergy have a greater ability to provide much more hotspots for improving EM enhancement in diverse SERS applications [17–18].
On the other hand, besides the routine SERS properties that have been well developed in previous works [12–16], the thermal stability of SERS is also another important indicator for application of SERS technology in real-word scenarios. Previously, most of these SERS nano-substrates with high activity at room temperature may be only applied to those moderate chemical reactions. However, many industrially important catalytic reactions cannot be performed under these normal atmospherics, which should be carried out at high temperatures that can give rise to heat-assisted chemical bonding/breaking. For example, as for the CO oxidation over metal-based catalysts, a high reacting temperature (>100 ℃) is usually needed to weaken CO adsorption and facilitate O2 adsorption during heterogeneous catalytic reaction [19–20]. Moreover, numerous chemical reactions of organic compounds in aqueous media should be also placed in high-temperature environments [21–25]. As for the construction of thermally stable and highly active SERS optical sensors, there are two main aspects that should be considered in this field. Firstly, the heat resistance of nanosubstrates is vital to the development of thermal stable SERS systems. Some chemically active metallic or semiconductor nanostructures, including monometallic Ag, Cu or Al, as well as their oxide/sulfide and two-dimensional carbides/carbonitrides (2D MXene), etc. will be destroyed under high temperature conditions. Secondly, the conventional excitation source in traditional SERS is the short wavelength laser with high photon energy, which is not suitable for SERS monitoring of high-temperature chemical reactions. The main reason is that the transmission/migration of short wavelength laser-excited charge carriers on nanosubstrates will become more complex at high temperature [26]. In this way, the high surface sensitivity and excellent chemical selectivity of SERS substrate at an elevated temperature will be seriously suppressed by using short wavelength laser sources. Therefore, the near infrared (NIR) laser excitation with low photon energy should be adopted in the urgent high-temperature stable SERS system. In this regard, it is imperative to develop a unique NIR-excited SERS system with both excellent activity and pronounced thermal stability, which will have more promising potential for commercial modern chemical industries.
Herein, we report an ingenious thermally stable and highly active SERS sensor by the construction of bimetallic Au/Ag nanodendrites (NDs) with multiple elongated branches and unique intermetallic synergy, as shown in Fig. 1. The 785 nm NIR laser excitation can be sensitive to the resonance position of Au/Ag NDs with localized surface plasmon resonance (LSPR) from visible region (607 nm) to NIR (998 nm). Then, the tremendous elongated branches and enormous antennas of Au/Ag NDs will provide much more hotspots for significantly boosting SERS activity in comparison with as-prepared other reference samples in this work. The corresponding ultra-low detection limit of crystal violet (CV) or rhodamine 6G (R6G) molecules can be separately achieved at picomole (pM) level of 10−13 M or 10−14 M, approaching the ultrasensitive requirement for SERS trace detection. Taking advantage of intermetallic synergistic effects in bimetallic NCs that can effectively suppress the oxidation of chemically active Ag in a hot environment, the bimetallic Au/Ag NDs exhibit excellent thermal stability at an elevated temperature of 170 ℃ in comparison with monometallic Ag nanostructure. In this way, the obtained Au/Ag NDs provide an optimum high-temperature stable SERS analyses of dye molecules (CV, methyl blue (MB), malachite green (MG)), since ∼90% SERS activities can be well maintained at high temperature of 170 ℃ within 30 min continuous tests. The advantage of this NIR-excited SERS sensor is the combination of enhanced activity and pronounced thermal stability, which can hold great promising potential for ultrasensitive monitoring of many important heat-assisted chemical/catalytic reactions in near future.
Fig. 1. The high-temperature stable SERS system by using multiple-branched Au/Ag NDs as substrate and 785 nm NIR laser source.
2. Method
2.1 Materials
Silver nitrate (AgNO3, 99.85%) and chloroauric acid (HAuCl4·4H2O) were purchased from China Sinopharm International (Shanghai) Co., Ltd.. Ascorbic acid (AA, 99%) and ethanol were purchased from Macklin. Crystal violet (CV), Methylene blue (MB) and malachite green (MG) were purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). Rhodamine 6G (R6G) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd.. Polyvinylpyrrolidone (PVP, Mw=40000) was purchased from TCI (Shanghai). All reagents were of analytical grade and used directly without further treatment. Deionized water used in the fabrication and measurement was prepared using a Millipore purification system (18.2 MΩ cm).
2.2 Synthesis of multiple-branched Au/Ag NDs
Multiple-branched Au/Ag NDs were prepared by a seed-mediated growth method, which is very similar to previous works [27–29]. Firstly, a well-polished Au metal used as the target was placed on the bottom of a rotating glass dish (∼400 rpm) filled with 2 cm depth of liquid solution (0.2 M AgNO3, 0.08 M PVP, and 20 mL DI water). The as-prepared Au/Ag NPs were obtained via laser ablation process by a Q-switched Nd-YAG laser beam. The laser beam was operated at a wavelength of 1064 nm with pulse duration of about 10 ns, the energy of about 250 mJ and 10 Hz repetition. Then, the laser irradiation process was carried out at a wavelength of 532 nm with pulse duration of about 6 ns, the energy of about 350 mJ and 10 Hz repetition. After these two steps, the Au/Ag NPs-based nanoseeds were carefully washed in DI water and centrifuged at 18000 rpm for 10 minutes in an ultracentrifuge. Secondly, the multiple-branched Au/Ag NDs can be obtained by adding HAuCl4 solution (1.5∼2.5 mM) into 0.08 M/5 mL AA solution mixed with 0.1 M/2 mL Au/Ag NPs. Finally, we got multiple-branched Au/Ag NDs with broad controllable LSPR in the region of 607∼998 nm.
2.3 Characterization techniques and Raman measurements
The obtained precipitates were dropped on a copper mesh and dried in the fume hood for observation via transmission electron microscopy (JEOL-JEM-2100F). The general morphologies and chemical compositions of the samples were further performed by Focused Ion beam (FIB, Helios G4 UC) equipped with energy-dispersive X-ray spectroscopy (EDS). Crystallographic orientations of as-prepared products were obtained by a Smartlab 3 KW advanced X-ray diffractometer (XRD, Cu Kα radiation, λ = 1.5418 Å). The detailed surface compositions of the obtained products were illustrated by a PHI Quantera SXM with an Al Kα = 280.00 eV excitation source by X-ray photoelectron spectra (XPS). In addition, absorption spectra of the colloidal solutions were recorded by a UV-vis NIR spectrophotometer (UV-1800, Shimadzu).
In a typical SERS procedure, the SERS substrates were prepared by dropping 0.1 M/30 μL Au/Ag nanoproducts on carefully cleaned silicon plates and then dried naturally at room temperature for 12 hours. As for the preparation of CV probe molecule ethanol solutions, 0.4 g CV powder was dissolved in 10 mL ethanol to form 0.1 M CV solution, which will be separately diluted with ethanol to prepare CV solutions with different concentrations of 10−6-10−13 M. The solutions of R6G, MB and MG molecules were synthesized with different concentrations by the above same method. All SERS signals were collected by a confocal microprobe Raman spectrometer (Renishaw Raman spectroscopy) with a 50 × objective. Spectra acquired using the InVia spectrometers were processed (smoothed and baseline corrected) using Wire 5.4 software (Renishaw). The excitation source was provided by using Renishaw high power diode Laser 785 nm CW laser with 0.75 mW at the surfaces of different samples. And Linkam Scientific Instruments Heating Cell kit controls temperature ranges from 20 ℃ up to 200 ℃.
3. Results and discussion
In this work, the obtained Au/Ag NPs were fabricated by laser ablation/irradiation in liquid solution, which will be served as nanoseeds for further fabrication of nanodendrites. At the moment of the pulsed laser arrives at the surface of Au target, the Au element will boil and vaporize rapidly, resulting in the formation of explosive Au thermal plasma. The superheating Au plasma with highly non-equilibrium features can play a critical role in the subsequent nucleation process [30–31]. Then, the strong interaction between Au hot plasma and surrounding Ag ions enables the nucleation of bimetallic Au/Ag nanocrystals to take place in the stage of rapid condensation of the plasma, and sharply terminate due to the exhaustion of plasma vapor. In order to further increase Ag content in Au/Ag nanocomposites, the subsequent 532 nm laser irradiation of as-prepared Au/Ag NPs colloidal solution was carried out in this work. During the laser irradiation process, the surface excited electrons that are originated from LSPR of Au/Ag NPs can further give rise to reduction of Ag ions, resulting in the continuous overgrowth of Ag nanocrystals on these precursors. The representative morphology of as-prepared Au/Ag NPs was characterized by the SEM image, as shown in Fig. 2(a). It can be found that numerous monodisperse NPs with average size of ∼50 nm was formed by laser ablation/irradiation. The relative ratio of Au to Ag element is measured about 34.4:65.6 by using EDS (inset in Fig. 2(a)). After separately adding 1.5 mM and 2.1 mM HAuCl4 in Au/Ag NPs colloidal solution, the branched structures were gradually formed by overgrowth of Au species on the precursors, as shown in Fig. 2(b) and (c). Moreover, SEM image shows that more obvious multiple-branched Au/Ag NDs with highly elongated branches can be obtained by adding 2.5 mM Au ions (Fig. 2(d)). The above three types of nanoproducts were separately labeled as Sample A, Sample B, Sample C and Sample D in the following sections. By measuring the branches of more than 300 NDs in sight on SEM images, there are four typical samples can be summarized as follow: Sample A is average length of ∼20 nm; Sample B is average length of ∼50 nm; Sample C is average length of ∼90 nm; Sample D is average length of ∼20 nm. Increasing the dosage of HAuCl4 in the reaction solution enables the formation of elongated Au/Ag branches. In this work, when the concentration of HAuCl4 was further increased to ∼2.9 mM, we found the average length of Au/Ag ND (Sample D) was calculated about 20 nm. The reason for this phenomenon is related to that the excessively strong electric displacement reaction between Au ions and Au/Ag NPs also inhibited the anisotropic growth of the elongated branches. Therefore, excessive Au ions are not conducive to the growth of dendrite length.
Fig. 2. SEM images of (a) Au/Ag NPs, (b) Sample A (inset: the relative ratio of Au is measured about 84.4%), (c) Sample B (inset: the relative ratio of Au is measured about 89.7%), and (d) Sample C (inset: the relative ratio of Au is measured about 96.2%).
To get more detailed characterizations of multiple-elongated Au/Ag NDs, the correspond-ding TEM of as-synthesized Sample C is performed in Fig. 3. It reveals that high-yield Au/Ag NDs with complex and obvious multiple elongated branches can be visualized by these representative structures in Fig. 3(a). Moreover, the bimetallic nature of the nanoproducts was further validated by elemental mapping images of an individual nanostructure (Fig. 3(b)). The result illustrates the uniform distribution of Au and Ag elements throughout NDs, and the relative ratio of Au to Ag in this bimetallic structure is calculated about 96.2:3.8, which is consistent with the EDS in Fig. 2(d). Additionally, the high-resolution TEM (HRTEM) image in Fig. 3(c) provides a closer view of an isolated nano-branch. The fast Fourier transform (FFT) result of white selection area indicates the formation of face-centered-cubic (fcc) nanocrystals in Fig. 3(c). The periodic lattice fringe with a periodicity corresponding to a d-space of 0.236 nm is located between theoretical values of (111) lattice plane of Ag (0.238 nm) and Au (0.232 nm), further verifying the formation of bimetallic Au/Ag nanocrystals.
Fig. 3. (a-b) TEM and element mapping images of multiple-branched Au/Ag NDs. (c) The representative HRTEM image of the multiple-branched Au/Ag NDs (inset: FFT of the white dashed selection area).
Furthermore, the unique optical properties of the highly branched NDs were illustrated by the UV-visible absorption spectra, as shown in Fig. 4(a). Initially, LSPR peak position of original Au/Ag NPs is measured at about 444 nm (inset in Fig. 4(a)) that is located between the common plasmon resonance of Ag NPs (∼410 nm) and position of Au NPs (∼520 nm), supporting the construction of alloyed nanostructures. Figure 4(a) shows the photograph of colloidal Sample A-D products, giving rise to some obvious changes in the solution color from yellow for Au/Ag NPs to light gray and blue-violet then light brown by increasing Au ions (1.5, 2.2, 2.6 and 2.9 mM) in reaction solutions, respectively. Meanwhile, the absorption spectra of different Au/Ag NDs in Fig. 4(a) reveal that LSPR peak position distinctly red-shifted from visible region (∼607 nm) to NIR region (∼998 nm) by increasing Au ions from 1.5 to 2.6 mM. Moreover, the broader absorption spectra in NIR (>700 nm) of Sample C with multiple elongated branches can provide stronger light absorbance capacity in a wide region. Further increasing Au ions also enable the LSPR peak position of Sample D with shorter branches to blue-shift to ∼810 nm. The detailed variation of these changes is also illustrated in Fig. 4(b), which is sensitive to spatial structure of Au/Ag NDs with different longitudinal branches. The red-shift LSPR position is highly related to the more obvious longitudinal surface plasmon band on Au/Ag multiple elongated branches [32]. Therefore, Sample C that composed of multi-elongated Au/Ag NDs with a much broader and stronger adsorption capacity (>700 nm) holds great potential for improving NIR laser-excited SERS activity.
Fig. 4. (a) The UV-Vis-NIR absorption spectra of as-prepared Au/Ag NDs by using different HAuCl4 concentrations (inset: absorption of Au/Ag NPs). (b)The relation curve of HAuCl4 concentration and absorption peak of Au/Ag NDs.
On the other hand, the heat resistance of the obtained Sample C was also evaluated by keeping them at 170 ℃ conditions for one hour. Then, the SEM image of Au/Ag NDs after this heat-treatment is shown in Fig. 5(a). It is illustrated that multiple-branches can be well maintained at 170 ℃ high temperatures, which is almost identical to that of original Au/Ag NDs. Moreover, the excellent thermal stability of Au/Ag NDs was further confirmed by the XRD tests in Fig. 5(b). XRD pattern of initial multiple-branched Au/Ag NDs illustrates that the four relative higher diffraction peaks that are detected at 38.22°, 44.29°, 64.43° and 77.48° should be indexed to (111), (200), (220) and (331) planes, respectively. The observed XRD peaks are located between the standard positions of pure Ag (JCPDS. NO. 04-0783) and Au (JCPDS. NO. 65-2870) crystal structures, confirming the formation of alloyed Au/Ag nanocrystals in this work. Compared with the initial crystal structures, the XRD pattern of the heat-treated Sample C in Fig. 5(b) exhibits very similar diffraction peaks, implying that the excellent crystallographic nature of Au/Ag NDs can be also sustained at high-temperature condition. On the other hand, the surface compositions of Sample C before and after heat-treatment were further examined by XPS measurements, as displayed in Fig. 5(c). The high-resolution XPS spectra of Au4f illustrates that double peaks originated from initial Au/Ag NDs can be detected at 84.3 eV and 86.7 eV, which should be attributed to binding energies of atomic Au4f7/2 and Au4f5/2, respectively. After heat-treatment, there is no apparent change of Au4f in Fig. 5(c), implying that the Au species in Au/Ag NDs is very stable at high-temperature. Similarly, the double XPS peaks derived from the Ag element at 367.6 eV and 373.6 eV are related to binding energies of Ag3d5/2 and Ag3d3/2, which are also consistent with initial nanostructure. Compared to the monometallic Ag nanomaterials that will be severely oxidized under heated atmosphere conditions, the excellent thermal stability of as-prepared Au/Ag NDs is highly related to the intermetallic synergistic effect in bimetallic nanostructures [33]. Different from monometallic NPs, the unique electron transfer interface at the intermetallic bonding cross-region is very suitable for boosting the transmission of heat-excited electrons from surface to internal region. In this way, the surface oxidation of bimetallic nanostructure at a hot condition will be suppressed by this intermetallic synergy among different metals.
Fig. 5. (a) SEM image of sample C after 170 ℃. (b) XRD and (c) XPS spectra of multiple-branched Au/Ag NDs (Sample C (96.2% Au: 3.8% Ag (red line)) and Sample C after 170 ℃ (blue line) (inset bar :6000 a.u.).
As for SERS-based ultrasensitive probes, the primary concern is the high SERS activity, which was evaluated by using Sample A-D (difference branches lengths of Au/Ag NDs) and original Au/Ag NPs in this work. The SERS spectra of 10−7 M CV molecules separately adsorbed on these five nanosubstrates are shown in Fig. 6(a), respectively. As for every nanosubstrate in Fig. 6(a), the dominating characteristic bands of CV molecules at 421, 730, 914, 1178, 1377, 1475, 1531, 1588 and 1617 cm−1 are all clearly detected in SERS spectra [34–36], providing much enriched “molecular fingerprint” information. In detail, the peaks at 1617, 1588, 1475 and 1531 cm−1 can be attributed to ring C-C stretching vibration; the peak at 1377 cm−1 is related to N-phenyl stretching; the peak at 914 cm−1 should be originated from ring skeletal vibration of radical orientations; the peaks at 1178 cm−1 and 730 cm−1 are corresponded to ring C-H bends. As displayed in Fig. 6(a), Sample C with multiple elongated branches provides significantly higher Raman signals in comparison with other four references. At the same time, the enhanced Raman signal intensity that is originated from sample C can be further confirmed by the histogram of the SERS peak intensity at 1178 cm-1 versus different nanosubstrates in Fig. 6(b). In detail, the peak intensity of CV molecules at 1178 cm-1 is detected about 30075 a.u in the presence of Sample C, which is approximately 97.1, 4.5, 2.7, 5.1 times higher than that of original Au/Ag NPs (∼310 a.u), Sample A (∼6614 a.u), Sample B (∼11273 a.u) and Sample D (∼5840 a.u), respectively. Compared to other Au/Ag NDs with shorter branches (Sample A, B or D), the experimental results in Fig. 6(a) and (b) clearly demonstrate that obtained Au/Ag NDs with longer elongated branches (Sample C) can unambiguously possess extremely higher SERS activity, which is more suitable for ultrasensitive SERS analysis of trace molecules. The enhanced SERS activity in this work should be attributed to the significant EM enhancement, since the Au/Ag NDs with multiple elongated branches and enormous antennas can provide more hotspots for promoting SERS signal intensity. Generally, the strong interaction of the incident laser beam with Au or Ag rugged structures that possess plentiful hotspots among closely adjacent NPs can offer intense local electromagnetic field for boosting probe molecular vibrations adsorbed on their rough surfaces [37–42]. As for plasmonic metallic Au or Ag nanomaterials, the dominant EM mechanism should provide the main contribution for improving SERS activity in comparison with the CM mechanism via effective electron transfer (CT) between semiconductor-based nanosubstrates and probe molecules [43]. Therefore, the significant EM enhancement is heavily related to the high roughness on rugged Au or Ag nanostructures. Compared to the other reference samples in this work, the fascinating Sample C with tremendous and immense elongated branches gives rise to the formation of much more SERS accessible hotspots, which will then dramatically enhance the Raman scattering of probe molecules. To further verify ultrasensitive SERS activity of Sample C with elongated Au/Ag branches, we decreased the concentration of CV molecules from 10−6 to 10−13 M, and R6G molecules from 10−7 to 10−14 M, in order to get SERS detection limit. The corresponding SERS spectra of CV and R6G molecules with different concentrations adsorbed on Sample C nansubstrate are separately shown in Fig. 6(c) and (d). Apparently, the dominating characteristic bands of CV and R6G molecules can be clearly distinguishable even with concentration as low as 10−13 M and 10−14M, respectively. The ultra-low detection limit of either CV or R6G molecules adsorbed on this optimal Au/Ag NDs can already reach to picomole (pM) level and approach the requirement for SERS trace detection, which is also better than many previous works based on Au nanorods (NRs) /graphene oxide (GO) [44], Au NRs/carbon nanotubes [45], Ag NPs/reduced graphene oxide (rGO) [46], Ag NPs/GO [47], Ag NPs/ZnO nanoflowers [48], Au/SiO2 NRs [49], etc. Moreover, the quantitative well-defined linear relationships between the SERS intensities of CV molecules at 421, 1178, 1475, 1617 cm-1 in the concentrations of 10−6-10−13 M are separately depicted in Fig. 6(e). Similarly, the good linear relationships can be also obtained by plotting the SERS intensities of R6G molecules at 772, 1188 and 1364 cm-1 versus the concentration in the range of 10−7 to 10−14 M, as shown in Fig. 6(f). These findings are very suitable for precious and exact detection of probe molecules in real-word scenarios.
Fig. 6. (a) The SERS spectra of 10−7 M CV molecules adsorbed on five groups of nanosubstrates. (b) The histogram of the variation of SERS peak intensity at 1178cm-1 versus different nanosubstrates (c) SERS spectra of CV molecules with different concentrations (10−6-10−13 M) adsorbed on the Sample C with elongated Au/Ag branches (inset: CV molecular structure). (d) SERS spectra of R6G molecules with different concentrations (10−7-10−14 M) adsorbed on the Sample C. (e) The variations of SERS intensities (the height of signals) versus different concentrations (10−6-10−13 M) of CV molecules. (f) The variations of SERS intensities (the height of signals) versus different concentrations (10−7-10−14 M) of R6G molecules.
Then, high-temperature stable SERS activity of 10−7 M CV molecules adsorbed on Sample C was evaluated in this work. We controlled high temperature-dependence SERS detection of CV molecules by using Linkam systems with 30 ℃ intervals, as shown in Fig. 7(a). Based on the variation of SERS signal intensities of CV molecules absorbed on Sample C at different high-temperature conditions, we found that SERS activity was not significantly deactivated with an increase of high temperature (20∼170 ℃). From the histograms of SERS peak intensi-tyies versus the increasing temperatures show that the Sample C with elongated Au/Ag branches possesses excellent high temperature stable SERS activity at high-temperature conditions. On the other hand, the SERS tests at much higher temperature (>170 ℃) were also performed in this work. However, we found that the obvious background noise generated by the inevitable thermal radiation under overheating condition affects the acquisition of reliable Raman spectroscopy information. In order to avoid these interference noises, the maximum temperature of 170 ℃ has been adopted in this work. Meanwhile, in order to verify SERS spatial stability of Au/Ag NDs at 170 ℃ environment, ten groups of SERS spectra recorded at 10 random different points are illustrated in Fig. 8(a). It can be found that the observed SERS results can be well repeated for random 10 points of 100 random points, because Raman spectral lines are nearly constant in Fig. 8(a). In addition, the corresponding relative standard deviation (RSD) results at 1178 cm-1 and 1377 cm-1 are calculated about 3.54% and 3.62%, supporting the well-defined homogeneous distribution of SERS spectra on Au/Ag NDs at high-temperature. Additionally, we also measured the substrate-to-substrate SERS signals from 5 different batches, in order to investigate the reproducibility of Au/Ag NDs with elongated branches. Similarly, based on the repeated experiments (Fig. 8(b)), the excellent substrate-to-substrate reproducibility with RSD value of ∼4.48% can be also obtained in this work. Moreover, in order to further verify the durability of thermal stable Au/Ag NDs-based SERS system at high temperature of 170 ℃, the repetitive SERS measurements versus the delay time with respect to the initial test were carried out in this work. As displayed in Fig. 9(a), SERS spectra of CV molecules adsorbed on Sample C at 170 ℃ could still be sustained for 30 min, because the corresponding Raman spectral lines are nearly consistent with that of the original one. In detail, the variation of the SERS intensity of CV molecules at 1178 cm-1 versus the delay time in Fig. 9(b) shows that there is about 90% SERS intensity can be retained during 30 min repeated tests under 170 ℃ high-temperature. After that, the further prolonging time (>30 min) enables SERS signal intensities of CV molecules to gradually decrease, which is also visually confirmed by the result in Fig. 9(b). This downtrend in the longer time region (>30 min in this work) may be caused by thermal degradation of probe molecules [50], because the microstructure of Au/Ag NDs can be well maintained within 60 min at 170 ℃.
Fig. 7. (a)SERS spectra of CV at different temperatures on the substrate of Sample C. (b-f) The bar graphs of the SERS peak intensities at 1178, 421, 914, 1377, 1617 cm-1 versus the temperature.
Fig. 8. (a) SERS maps spectra of 10−7 M CV molecules for 10 random spots on the surface of Sample C at 170 ℃. (b) Raman signals of 10−12 M CV from five different batches of substrate.
Fig. 9. (a) The intensities of SERS spectra changed with time on the substrate of Sample C at 170 ℃. (b) The curve of intensity at peak 1178 cm-1 with time.
Finally, to verify whether high-temperature stable Au/Ag NDs-based SERS systems can be extended to assess other molecules at high-temperature environment, the SERS analyses of 10−6 M MB and 10−6 M MG dye molecules were also carried out in this work, respectively. As shown in Fig. 10, the high-temperature stable SERS spectra (Fig. 10(a) and (b)) of MB and MG molecules separately adsorbed on Sample C show very similar variation-trends, which are similar to that of CV molecules. As shown in Fig. 10(b) and (e), the SERS peak intensities of primary characteristic bands of both MB and MG molecules can be also maintained at 170 ℃ within 30 min continuous tests, while they gradually decrease as the delay times. Based on the variations of SERS peaks of MB molecules at 1615 cm-1 (Fig. 10(c)) and MG molecules at 1397 cm-1 (Fig. 10(f)), the residual 90.5% and 91.7% SERS signal intensities can be retained within 30 min repeated tests, respectively. The results indicate that the as-prepared Au/Ag NDs are also suitable for high-temperature stable SERS precise assessment of other dye molecules. Taken together, the above results clearly reveal that the obtained thermal stable Au/Ag NDs with multiple elongated branches possess enhanced SERS activity and excellent spatial stability/reproducibility as well as durability at high-temperature (170 ℃) condition, providing new insight into the development of high-temperature stable SERS active sensor in many specific applications.
Fig. 10. (a) The SERS spectra of MB at different temperatures on the substrate of Sample C. (b) and (c) are the changing SERS spectra of MB with time at 170 ℃ and the curve of intensity at peak 1615 cm-1 with time, respectively. (d) The SERS spectra of MG at different temperatures on the substrate of Sample C. (e) and (f) are the changing SERS spectra of MG with time at 170 ℃ and the curve of intensity at peak 1397 cm-1 with time, respectively.
4. Conclusions
In summary, we have demonstrated an appealing high-temperature stable SERS active sensor that is composed of multiple-branched Au/Ag NDs served as thermal stable nano-substrate and NIR laser (785 nm) used as excitation source. The as-synthesized Au/Ag NDs with multiple elongated branches and enormous antennas show significant enhanced SERS activity under NIR laser excitation, as compared to other Au/Ag NDs with shorter branches. Meanwhile, with the aid of intermetallic synergy, the surface oxidation of Au/Ag NDs at high temperature condition can be effectively suppressed, and the bimetallic nanoproducts possess improved high-temperature stability in comparison with monometallic Ag. Furthermore, the excellent SERS activity of the Au/Ag NDs can be well maintained at 170 ℃, and residual ∼90% SERS peak intensities of CV, MB, MG molecules can be retained for 30 min under repeated tests. The obtained SERS sensor with both enhanced activity and pronounced thermal stability holds great potential for promoting the utilization of SERS technique in diverse heat-assisted industrially chemical conversions.
Funding
National Natural Science Foundation of China (11575102, 11905115); Shandong Jianzhu University XNBS Foundation (1608); Fundamental Research Funds of Shandong University (2018JC022).
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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