Improved Thermal Stability of Graphene-Veiled Noble Metal

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Improved thermal stability of graphene-veiled noble metals nanoarrays as recyclable SERS substrates Xingang Zhang, Shuyao Si, Xiaolei Zhang, Wei Wu, Xiangheng Xiao, and Changzhong Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13708 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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ACS Applied Materials & Interfaces

Improved thermal stability of graphene-veiled noble metals nanoarrays as recyclable SERS substrates Xingang Zhang†, Shuyao Si†, Xiaolei Zhang†, Wei Wu‡, Xiangheng Xiao†* and Changzhong Jiang†* †

Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures

of Ministry of Education, Hubei Nuclear Solid Physics Key Laboratory and Center for Ion Beam Application, Wuhan University, Wuhan 430072, P. R. China ‡

Laboratory of Printable Functional Nanomaterials and Printed Electronics, School of

Printing and Packaging, Wuhan University, Wuhan 430072, P. R. China ABSTRACT: The ability to enhance the heat resistance of noble metals is vital to many industrial and academic applications. Because of its exceptional thermal properties, graphene was used to enhance the thermal stability of noble metals. Monolayer graphene-covered noble metal triangular nanoarrays (TNAs) showed excellent heat resistance, which could maintain their original triangular at high temperatures, while bare noble metal TNAs all agglomerate into spherical nanoparticles. Based on this mechanism, we obtained a universal recyclable Surface enhanced Raman scattering (SERS) substrate, after 16 cycles, the SERS substrate still worked well. The improvement of the heat resistance of noble metals by graphene have a great significance to the working reliability and service life of electronic devices and the single-use problem of traditional SERS substrates. KEYWORDS: Graphene, noble metal triangular nanoarrays, recyclable, thermal stability, Surface-enhanced Raman Scattering

ACS Paragon Plus Environment


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1. INTRODUCTION The single-use of traditional surface-enhanced Raman scattering (SERS) substrates not only increases production costs but also increases the difficulty of subsequent validation experiments. Therefore, the development of reusable SERS substrates has generated much interest. Recently, recyclable SERS-active substrates based on Au (Ag)/TiO21-11, Ag (Au)/ZnO12-13, Ag/Al2O314, Ag/HfO215 and Au/SiO216 have been reported. These SERS substrates showed satisfactory performance, and had partially solved the reusable problem. Moreover, BN has also been used to solve the problem mentioned above.17-21 Because BN is highly polar and insulating with a large band gap of 5.9 eV22-23 and has negligible charge transfer interaction with the molecules adsorbed on its surface, it provides a very low Raman enhancement factor24. Many experiments have proved that graphene is still a promising object of study 25-30. Therefore, we were motivated to determine whether the problem mentioned above can be solved by graphene. Graphene, with its exceptional electrical, optical, mechanical, and thermal properties, has been widely applied in photonics and optoelectronics31-34. Because it is quite a mature technology, it is easy to obtain uniform single- and few-layer graphene over a large area, which is key for materials to be widely used. Moreover, because of its zero bandgap and nonpolar nature35, graphene has a metallic character, which leads to a strong charge transfer interaction with molecules adsorbed on its surface24 and a large Raman enhancement factor25, 36-40. For graphene-based SERS substrates, the large Raman enhancement factor provided by graphene is very important for the


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detection of low concentrations of dye molecules. In addition, graphene transferred to the surface of a noble metal can greatly avoid its oxidation or sulfuration41-45 and also play an important role in plasmon-driven, surface-catalyzed reactions46. All these results further increase the applications of graphene in SERS. More importantly, graphene can protect metals against corrosion at high temperatures47, which may provide an important pathway for the reusability of SERS-active platforms. In this work, we introduce a stable and reusable SERS-active platform by simply transferring monolayer graphene onto Au triangular nanoarrays (TNAs). Unlike previous reports that rely on photocatalytic degradation3 or pinhole-free films of Al2O348, we take the advantage of atomically thin layers of graphene to prevent the deformation of Au TNAs during the annealing process. Figure 1 shows the working principle of the reversible SERS-active substrate：after the target molecules are detected based on SERS, the target molecules adsorbed on the substrate can be eliminated by heating the substrate at 300 ℃ for 30 min in a protective atmosphere of Ar. During the annealing process, the graphene can prevent Au TNAs from reuniting. Therefore, the monolayer graphene-covered TNAs can be used to detect target molecular again. Our data suggest that monolayer graphene-covered TNAs are excellent recyclable SERS-active substrates.



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Figure 1. Illustration describing the reversible SERS behavior of the SERS substrate. 2. EXPERIMENTAL SECTION 2.1.Preparation of Au triangular nanoarrays. The polystyrene (PS) masks were prepared through nanosphere lithography by using monodisperse PS particles sizes of 820 nm in diameter. The detailed process is as follows: The aqueous solution of PS particles was mixed with ethanol at a volume ratio for use. The Si/SiO2 substrates were washed thoroughly and kept in 10% sodium dodecyl sulfate (SDS) solution for 12 h to modify their surfaces for hydrophilicity. Such modification was beneficial to the deposition of PS particles. The Si/SiO2 substrates were tilted at an angle of 10◦ to the water level in water. About 100 µl of the as-prepared PS particles solution was dripped into the water slowly through a sloping glass slide. Then 100 µl of 5 wt% SDS was also added to the water through the sloping glass slide. A monolayer of PS particles quickly formed on the water surface. After decanting the water slowly, the monolayer of PS particles was transferred to the Si/SiO2 substrate surface. Finally, a 10-nm-thick chromium film and a 50-nm-thick Au film were evaporated onto the as-prepared PS particle mask. After the desired thickness of metal films was obtained, the PS particles were removed, and leaving the TNAs on the surface of Si/SiO2.


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2.2. Transfer of monolayer graphene onto the Au triangular nanoarrays. High-quality, large-area uniform monolayer graphene was prepared on 25-µm-thick polycrystalline Cu foils by chemical vapor deposition (CVD). To begin with, the graphene-covered copper foil was spin-coated with a poly(methyl methacrylate) (PMMA) film, which was then cured at 120 °C for 30 min. Graphene grew on both sides of the Cu foil. After one side of the Cu/graphene was coated with PMMA, the opposite side of the graphene was etched away by plasma cleaning. Then, a PMMA/graphene membrane was obtained by etching away the copper foil in an aqueous solution of iron chloride with a bit of HCl in less than 30 min. The PMMA/graphene stack was washed with an aqueous solution of HCl and was transferred to deionized water to wash away all kinds of residual ions. The aqueous solution of HCl can prevent the hydrolysis of ferric ions. Furthermore, the PMMA/graphene stack was placed on the target substrate and dried at 50 °C for 30 min. Finally, the PMMA layer was dissolved by acetone. 2.3. Characterizations. The surface morphology of these samples was obtained by using a FEI SIRION-200 field emission scanning electron microscope. The Raman spectra were collected using a confocal microRaman spectrometer (RenishawinVia, Renishaw, 532 nm). The laser beam was focused onto the sample through a 100× objective lens and Raman spectra were collected with an acquisition time of 10 s. 3. RESULTS AND DISCUSSION In a typical synthesis, Au TNAs were fabricated by nanosphere lithography, and then large-area, uniform monolayer graphene (see Figure S1) was transferred onto the Au



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TNAs, followed by an annealing process. Figure 2(a) shows the SEM image of large-area, ordered Au TNAs. The Au TNAs were 50 nm in thickness and homogeneous. Under the Au TNAs, there was 10-nm-thick chromium film which was used to increase the adhesion of Au TNAs. Because of the poor adhesion force between Au TNAs and Si/SiO2 substrate, Au TNAs were easily detached from the Si/SiO2 substrate when we removed the PS particles. Figure 2(b) shows the SEM image of monolayer graphene-covered Au TNAs. The wrinkles of graphene between two Au TNAs can be clearly seen. To compare the thermal stability, the SEM images of Au TNAs and monolayer graphene-covered Au TNAs which were heated for the first time at 200 ℃ (Figure 2(c) and (d)), 300 ℃ (Figure 2(e) and (f)), 400 ℃ (Figure 2(g) and (h)), and 500 ℃ (Figure 2(i) and (j)) for 30 min in a protective atmosphere of Ar were also provided. It could be seen that the tips of the Au TNAs began to disappear after heating at 300 ℃ (Figure 2(e)), and the Au TNAs agglomerated into spherical particles at 500 ℃ (Figure 2(i)); while monolayer graphene-covered Au TNAs basically maintained its triangular shape even at 500 ℃ (Figure 2(j)). In addition, we also compared the SEM images of Au TNAs and monolayer graphene-covered Au TNAs which were heated for the first time at 400 ℃ for 120 min (Figure S2(a) and (b)). It is obviously that the morphology of the annealed bare Au TNAs changes from triangular nanoarrays to spherical nanoarrays (Figure S2(a)), while the morphology of monolayer graphene-covered Au TNAs changes little and basically maintain their original triangular shape, as shown in Figure S2(b). Parts c and d of Figure S2 show the morphology of the Au TNAs and monolayer


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graphene-covered Au TNAs after undergoing 15 cycles of heating at 300 ℃ for 30 min. It is obvious that the Au TNAs all agglomerated into large Au blocks, while the morphology of the monolayer graphene-covered Au TNAs also changed slightly: the existence of graphene was no longer obvious, and the Au TNAs tended to agglomerate into spherical nanoarrays. The changes in the morphology of Au TNAs and

monolayer

graphene-covered

Au

TNAs

indicate

that

monolayer

graphene-covered Au TNAs have better thermal stability than bare Au TNAs.



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Figure 2. SEM images of Au TNAs (a, c, e, g, i) and monolayer graphene-covered Au TNAs (b, d, f, h, j) before heating: (a) and (b); and after heating one time at (c) and


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(d): 200 ℃; (e) and (f): 300 ℃; (g) and (h): 400 ℃; (i) and (j): 500 ℃ in the protective atmosphere of Ar.

The mechanism for the better thermal stability is the result of the unique structure of monolayer graphene-covered TNAs, in which the Au TNAs were firmly wrapped by flexible graphene and therefore difficult to move47. Moreover, a theoretical calculation may explain this phenomenon49. The size-dependent melting temperature function (D) of nanocrystals is expressed as ( )

( )

=

( ) ( )

()

=

,

(1)

where (∞) is the corresponding bulk value of (D) . The function (D) represents the averaged mean-square displacement (msd) of atoms for a crystal with a diameter of D, while (∞) is the value for the corresponding bulk crystal. D0 is defined as the critical diameter at which almost all atoms are at the crystal surface, and is a ratio of the msd of surface atoms of the nanocrystals () to that of atoms within the nanocrystals ! () , where α = ()/! (). The application condition is shown in Reference 49. For the surface of the nanocrystals without any processing， () > ! () and

> 1 , and, according to Eq. (1), (D) < (∞) ; the melting point decreases where the largest value of is found50:

'( = 2*+ (∞)/(3-) + 1,


(2)


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Here, *+ (∞) is the melting entropy of the corresponding bulk crystals, and R is the ideal gas constant. When graphene firmly wraps metallic nanoparticles, some of the surface atoms of the nanoparticles are immobilized. Thus, the surface atomic vibration is suppressed, and the msd decreases. Therefore, should be modified to

= '( (1 − β) + β,

(3)

where β represents the ratio of the number of surface atoms immobilized by graphene to the total number of surface atoms. Because of the presence of graphene, the surface atomic vibration of the nanoparticles is suppressed, and in Eq. (3) is smaller than that determined by Eq. (2). Thus, according to Eq. (1), the melting point of the nanoparticles covered by monolayer graphene is enhanced compared to that of freestanding nanoparticles. Moreover, when increasing the annealed cycles, some inevitable factors arise, such as the small amount of air entering the protective atmosphere, which leads to damage to the graphene. Therefore, the Au TNAs are less well protected and begin to agglomerate into spherical nanoparticles.


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Figure 3. AFM images of Au TNAs before (a) and after (c) annealing; (b) and (d) are the height traces and schematic diagrams along to the white lines in (a) and (c); AFM images of monolayer graphene-covered Au TNAs before (e) and after (g) annealing; (f) and (h) are the height traces and schematic diagrams along to the white lines in (e) and (g);

For further demonstration of the theoretical calculation above, atomic force microscopy (AFM) images of Au TNAs and monolayer graphene-covered Au TNAs before and after heating are shown in Figure 3. Heating caused momentous changes in the morphology of Au TNAs (see Figures 3a, c), the thickness of Au TNAs increased by more than 10 nm (Figure 3b, d). However, for graphene-covered Au TNAs, except for wrinkling of the graphene to follow the profile of the underlying Au TNAs, the morphology (Figures 3e, g) and the thicknesses (Figures 3f, h) of the underlying Au TNAs only changed slightly. These results demonstrate that, in the process of heating, bare Au TNAs tended to transform to more stable spherical nanoparticles, while graphene-covered Au TNAs maintain their original triangular because graphene



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firmly wrapped Au TNAs and prevented Au TNAs from deforming. Therefore, these results further demonstrated the rationality of the theoretical calculation above. In order to find the optimized annealed conditions (300℃, 30 min), we performed a series of experiments. Figure S3 shows the SERS spectra of monolayer graphene-covered TNAs with 10-5 M R6G adsorbed, followed by annealing at different temperature (Figure S3a) for different times (Figure S3b). It is obvious that when the sample was heated at 300℃ for 30 min, not only the signals of R6G completely disappeared, but also have the minimal damage to the graphene, because the higher temperature is more likely to cause damage to graphene.25 According to our theory, the damaged graphene could not protect the Au TNAs very well, leading to the deformation of Au TNAs and the decrease in the Raman signals. Through the above experimental results, heating at 300℃ for 30 min was selected to carry out the recyclability experiment.

Figure 4. (a) Raman spectra of R6G (10−5 M) adsorbed on the Au TNA substrate after repeating the “detection−annealing” procedure once. A typical Raman spectrum of R6G (10−5 M) adsorbed on a monolayer graphene-covered Au TNA substrate before


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annealing is shown at the top. (b) Raman spectra of R6G (10−5 M) adsorbed on the monolayer graphene-covered Au TNA substrate during 15 cycles of recyclability tests.

To compare the reusability of Au TNAs before and after they were covered with monolayer graphene, Au TNAs and monolayer graphene-covered Au TNAs were used to detect R6G again after the target molecules were removed from the substrates, as shown in Figure 4. The typical recyclability process was as follows: after Raman measurement, the substrate was heated at 300 ℃ for 30 min in a protective Ar atmosphere, and then the sample was treated with the target molecules again and subjected to Raman measurement. The Raman spectra of R6G adsorbed on original Au TNAs and monolayer graphene-covered Au TNAs before annealing were shown in Figure 4(a). Compared with monolayer graphene-covered Au TNAs, bare Au TNAs showed stronger SERS signals of R6G. This phenomenon is attribute to the fact that the graphene was not in good contact with Au TNAs, which increased the distance between the Au TNAs and the target molecules51 and attenuated the localized surface-plasmon-induced

electromagnetic

fields.

To

further

illustrate

this

phenomenon, we compared the Raman signals of R6G on Au TNAs, monolayer graphene-covered Au TNAs, and monolayer graphene-covered Au TNAs after heating at 300 ℃ (400 ℃) for 30 min as shown in Figure S4a. It is obvious that Au TNAs showed stronger SERS signals than monolayer graphene-covered Au TNAs, however, after heating at 300 ℃ for 30 min, the SERS signals of monolayer



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graphene-covered Au TNAs began to increase, and it greatly increased after heating at 400 ℃ for 30 min. High temperature leads to bending of graphene (see Figure 3f, h), resulting in a closer contact between graphene and Au TNAs, thus making the fluorescence molecule closer to the electromagnetic field of Au TNAs, and showed a strong SERS signal. Moreover, the chemical enhancement of graphene may also benefit the Raman enhancement, thus, making a stronger SERS signals of monolayer graphene-covered Au TNAs after heating at 400 ℃ for 30 min compared with Au TNAs. In order to identify the chemical enhancement of graphene and explore the influence of heating on graphene, we investigated the corresponding Raman enhancement performance of graphene after cycle of the detection-annealing procedure (see Figure S4b). It is obviously that graphene has good Raman enhancement effect; and by comparing the G band and 2D band of graphene before annealing (Figure S4b), we can find that graphene itself has changed little at the same time of eliminating R6G when heating at 300°C for 30 min in a protective atmosphere of Ar. Moreover, the heated graphene show a similar Raman enhancement effect compared with graphene before heating, which further suggests that graphene maintain its integrity in our experiment. After heating, the SERS signals of R6G from the Au TNAs completely disappeared (see Figure 4a), which indicated that the R6G adsorbed on Au TNAs could be eliminated by the annealing process. The SERS signals of R6G were still negligible after subsequent soaking of the annealed Au TNA substrate in the R6G solution, which showed that the annealed Au TNAs could not be used as a recyclable substrate. This phenomenon is consistent with the changes in the


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morphology of the annealed Au TNA substrate (Figure 2). Because of annealing, Au TNAs agglomerated into spherical nanoparticles, which lead to large gaps between Au spherical nanoparticles and suppressed the coupling of plasmons in these Au nanoparticles. The reusability of monolayer graphene-covered Au TNAs substrate for up to 15 cycles was shown in Figure 4b. In each cycle, the substrate was heated at 300 ℃ for 30 min in the protective atmosphere of Ar, and then reused by soaking of the substrates into the R6G solution (10-5 M). After the heating/cleaning treatment, there was no Raman signal of R6G on monolayer graphene-covered Au TNAs substrate, which implied that R6G can be effectively removed. After 15 cycles of reusability tests, the Raman signal of R6G was still very strong. Moreover, there also appeared some phenomenon: after too many reusability tests, the Raman intensity of R6G gradually weakened and a broad band at 1500−1600 cm-1 was produced due to the carbonization of R6G.52-53 The decrease in the SERS signals of R6G can be explained by the changes in the morphology of the annealed monolayer graphene-covered Au TNAs substrate (Figure S2 (d)): with too many reusability tests, the emergence of some inevitable difficulties, such as H2O or O2 molecules adsorbed on the surface of graphene when the substrates were soaked in R6G solution, these molecules reacted with graphene during the annealing process and damaged the graphene. The damaged graphene could not protect the Au TNAs very well, which results in a decrease in SERS signals. Despite these changes, the SERS signals of R6G were still very clear after 15 “detection−annealing” cycles. Furthermore, the evolution of the Raman intensity from 1 cycle to 16 cycles of the



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“detection−annealing” procedure is shown in Supplementary Video 1. We also compared our results to literature reported values, in terms of cycle times and recyclable method, as shown in Table S1. By comparison, it clearly shows that our SERS substrate achieves most of the cycles and simply recyclable method. These results indicate that monolayer graphene-covered Au TNAs can be used as an excellent recyclable SERS substrate.

Figure 5. (a) Raman spectra of amoxicillin (10−2 M) adsorbed on the monolayer graphene-covered Au TNA substrate during two cycles of recyclability tests; (b) SERS

spectra

of

different

R6G

concentrations

adsorbed

on

monolayer

graphene-covered Au TNAs.

SERS substrate with practical applications is always popular to people, so, monolayer graphene-covered Au TNAs were used for the recyclable detection of 10-2 M amoxicillin, as shown in Figure 5a. Before annealing, monolayer graphene-covered Au TNAs could detect the strong SERS signals of amoxicillin, which agreed well with literature data.54-55 After the heating/cleaning treatment, no Raman signals of amoxicillin was present, implying effective removal of the analyte. The signal was


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fully recovered after subsequent soaking of the substrates into the amoxicillin solution. (see Figure 5a). The result of repeating the “detection−annealing” procedure twice is also presented, there was no obvious change in either peak position or intensity. Moreover, we also successfully tested the reusability of monolayer graphene-covered Au TNA substrate for rhodamine B and methylene blue (see Figure S5), which implies the general reversibility of monolayer graphene-covered Au TNAs. The detection limit of monolayer graphene-covered Au TNAs was also characterized as shown in Figure 5b. It is obvious that the Raman signal of R6G was still distinct even at 10-7 M. These results further demonstrate the excellent performance of monolayer graphene-covered Au TNAs for practical applications. In order to demonstrate that the recyclability mechanism proposed in this work is universal for other metals, monolayer graphene-covered Ag TNAs were also prepared. The morphologies of Ag TNAs and monolayer graphene-covered Ag TNAs before and after annealing are shown in Figure S6. After annealing at 300 ℃ for 30 min, the large-area ordered Ag TNAs (Figure S6a) were completely agglomerated into spherical nanoparticles (Figure S6b), while the morphology of the Ag TNAs covered by graphene (Figure S6c) only changed slightly (Figure S6d). The results show that graphene can also protect Ag TNAs during the annealing process. Compared with the annealed monolayer graphene-covered Au TNAs, the annealed monolayer graphene-covered Ag TNAs showed a greater change in morphology. This phenomenon is attributed to the fact that Ag has a lower melting temperature. Although graphene can increase the melting temperature of Ag, the final melting



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temperature is still slightly lower than the temperature required to eliminate the target molecules. Therefore, when eliminating the target molecules, the morphology of Ag TNAs was also changed slightly.

Figure 6. (a) Raman spectra of R6G (10−5 M) adsorbed on the Ag TNA substrate after repeating the “detection−annealing” procedure once. A typical Raman spectrum of R6G (10−5 M) adsorbed on a monolayer graphene-covered Ag TNA substrate before annealing is shown at the top. (b) Raman spectra of R6G (10−5 M) adsorbed on the monolayer graphene-covered Ag TNA substrate during 3 cycles of recyclability tests

Figure 6 shows the results of recyclable detection of 10-5 M R6G adsorbed on Ag nanostructures. After repeating the “detection−annealing” procedure once, the SERS signals almost disappeared for Ag TNAs without graphene, as shown in Figure 6(a). In contrast, monolayer graphene-covered Ag TNAs still showed a strong SERS signal after repeating the “detection−annealing” procedure three times, as shown in Figure 6(b). These results demonstrate that graphene can also protect Ag TNAs well.


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4. CONCLUSIONS In summary, we report a simple and effective approach to enhance the heat resistance of noble metals, which can make noble metals work on a higher temperature. Based on this mechanism, we obtained a universal recyclable SERS substrate, which was fabricated by transferring high-quality, CVD-synthesized graphene onto Au or Ag TNAs. Recycling is performed solely through an annealing process. In this process, when dye molecules were removed, bare metal TNAs were completely agglomerated into spherical nanoparticles during the first annealing cycle, while monolayer graphene-covered metal TNAs were demonstrated to retain nearly their original shape for 16 cycles. Thus, annealed monolayer graphene-covered metal TNAs could be used to detect dye molecules again, while the bare annealed TNAs couldn’t be recycled. Moreover, monolayer graphene-covered Au TNAs were used for the recyclable detection of amoxicillin and showed good performance. The recyclability mechanism of SERS-active substrates not only leads to a new opportunity to replace traditional single-use SERS substrates but also have a great significance to the working reliability and service life of electronic devices.

ASSOCIATED CONTENT Supporting Information Figures showing optical images, SEM images, Raman spectra, tables, and video.



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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank the Hongxing Xu Group of Wuhan University for the support of Raman spectrum measurements. The author thanks the NSFC (51371131, 11375134, 51571153, 11722543), and the Fundamental Research Funds for the Central Universities (2042017kf0168). References 1.

Li, X.; Hu, H.; Li, D.; Shen, Z.; Xiong, Q.; Li, S.; Fan, H. J., Ordered array of

gold semishells on TiO2 spheres: an ultrasensitive and recyclable SERS substrate.

ACS Appl. Mater. Interfaces 2012, 4 (4), 2180-2185. 2.

Hsu, K. C.; Chen, D. H., Highly Sensitive, Uniform, and Reusable

Surface-Enhanced Raman Scattering Substrate with TiO2 Interlayer between Ag Nanoparticles and Reduced Graphene Oxide. ACS Appl. Mater. Interfaces 2015, 7 (49), 27571-27579. 3.

Li, X.; Chen, G.; Yang, L.; Jin, Z.; Liu, J., Multifunctional Au-Coated TiO2

Nanotube Arrays as Recyclable SERS Substrates for Multifold Organic Pollutants Detection. Adv. Funct. Mater. 2010, 20 (17), 2815-2824. 4.

Zhao, Y.; Sun, L.; Xi, M.; Feng, Q.; Jiang, C.; Fong, H., Electrospun TiO2


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nanofelt surface-decorated with Ag nanoparticles as sensitive and UV-cleanable substrate for surface enhanced Raman scattering. ACS Appl. Mater. Interfaces 2014, 6 (8), 5759-5767. 5.

Ma, L.; Huang, Y.; Hou, M.; Xie, Z.; Zhang, Z., Ag Nanorods Coated with

Ultrathin TiO2 Shells as Stable and Recyclable SERS Substrates. Sci Rep 2015, 5, 15442. 6.

Jiang, X.; Sun, X.; Yin, D.; Li, X.; Yang, M.; Han, X.; Yang, L.; Zhao, B.,

Recyclable Au-TiO2 nanocomposite SERS-active substrates contributed by synergistic charge-transfer effect. Phys Chem Chem Phys 2017, 19 (18), 11212-11219. 7.

Huang, Q.; Li, J.; Wei, W.; Wu, Y.; Li, T., Synthesis, characterization and

application of TiO2/Ag recyclable SERS substrates. RSC Adv. 2017, 7 (43), 26704-26709. 8.

Wang, X.; Wang, Z.; Zhang, M.; Jiang, X.; Wang, Y.; Lv, J.; He, G.; Sun, Z.,

Three-dimensional hierarchical anatase@rutile TiO2 nanotree array films decorated by silver nanoparticles as ultrasensitive recyclable surface-enhanced Raman scattering substrates. J. Alloys Compd 2017, 725, 1166-1174. 9.

Wang, X.; Zhou, L.; Lai, W.; Jiang, T.; Zhou, J., Bifunctional 4MBA mediated

recyclable SERS-based immunoassay induced by photocatalytic activity of TiO2 nanotube arrays. Phys. Chem. Chem. Phys. 2016, 18 (34), 23795-23802. 10. Kumar, S.; Lodhi, D. K.; Singh, J. P., Highly sensitive multifunctional recyclable Ag–TiO2 nanorod SERS substrates for photocatalytic degradation and detection of dye molecules. RSC Adv. 2016, 6 (51), 45120-45126.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11. Wang, C.; Xu, Y.; Deng, C.; Liu, Z.; Wang, R.; Zhao, H., Design and preparation of a recyclable microfluidic SERS chip with integrated Au@Ag/TiO2 NTs. RSC Adv. 2016, 6 (114), 113115-113122. 12. Zang, Y.; Yin, J.; He, X.; Yue, C.; Wu, Z.; Li, J.; Kang, J., Plasmonic-enhanced self-cleaning activity on asymmetric Ag/ZnO surface-enhanced Raman scattering substrates under UV and visible light irradiation. J Mater Chem A. 2014, 2 (21), 7747. 13. Xu, J. Q.; Duo, H. H.; Zhang, Y. G.; Zhang, X. W.; Fang, W.; Liu, Y. L.; Shen, A. G.; Hu, J. M.; Huang, W. H., Photochemical Synthesis of Shape-Controlled Nanostructured Gold on Zinc Oxide Nanorods as Photocatalytically Renewable Sensors. Anal Chem 2016, 88 (7), 3789-3795. 14. Ma, L.; Huang, Y.; Hou, M.; Xie, Z.; Zhang, Z., Silver Nanorods Wrapped with Ultrathin Al2O3 Layers Exhibiting Excellent SERS Sensitivity and Outstanding SERS Stability. Sci Rep 2015, 5, 12890. 15. Ma, L.; Wu, H.; Huang, Y.; Zou, S.; Li, J.; Zhang, Z., High-Performance Real-Time SERS Detection with Recyclable Ag Nanorods@HfO2 Substrates. ACS

Appl. Mater. Interfaces 2016, 8 (40), 27162-27168. 16. Hou, M.; Huang, Y.; Ma, L.; Zhang, Z., Sensitivity and Reusability of SiO2 NRs@Au NPs SERS Substrate in Trace Monochlorobiphenyl Detection. Nanoscale

Res Lett 2015, 10 (1), 444. 17. Cai, Q.; Mateti, S.; Yang, W.; Jones, R.; Watanabe, K.; Taniguchi, T.; Huang, S.; Chen, Y.; Li, L. H., Boron Nitride Nanosheets Improve Sensitivity and Reusability of Surface Enhanced Raman Spectroscopy. Angew Chem Int Edit 2016 128, 8545-8549.


Page 22 of 29

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18. Cai, Q.; Mateti, S.; Watanabe, K.; Taniguchi, T.; Huang, S.; Chen, Y.; Li, L. H., Boron Nitride Nanosheet-Veiled Gold Nanoparticles for Surface-Enhanced Raman Scattering. ACS Appl. Mater. Interfaces 2016, 8 (24), 15630-15636. 19. Dai, P.; Xue, Y.; Wang, X.; Weng, Q.; Zhang, C.; Jiang, X.; Tang, D.; Wang, X.; Kawamoto, N.; Ide, Y.; Mitome, M.; Golberg, D.; Bando, Y., Pollutant capturing SERS substrate: porous boron nitride microfibers with uniform silver nanoparticle decoration. Nanoscale 2015, 7 (45), 18992-18997. 20. Cai, Q.; Li, L. H.; Yu, Y.; Liu, Y.; Huang, S.; Chen, Y.; Watanabe, K.; Taniguchi, T., Boron nitride nanosheets as improved and reusable substrates for gold nanoparticles enabled surface enhanced Raman spectroscopy. Phys. Chem. Chem.

Phys. 2015, 17 (12), 7761-7766. 21. Lin, Y.; Bunker, C. E.; Fernando, K. A.; Connell, J. W., Aqueously dispersed silver nanoparticle-decorated boron nitride nanosheets for reusable, thermal oxidation-resistant surface enhanced Raman spectroscopy (SERS) devices. ACS Appl.

Mater. Interfaces 2012, 4 (2), 1110-1117. 22. Han, W. Q.; Wu, L.; Zhu, Y.; Watanabe, K.; Taniguchi, T., Structure of chemically derived mono- and few-atomic-layer boron nitride sheets. Appl Phys Lett 2008, 93 (22), 223103. 23. Liu, L.; Feng, Y. P.; Shen, Z. X., Structural and electronic properties of h-BN.

Phys Rev B 2003, 68 (10), 104102. 24. Ling, X.; Fang, W.; Lee, Y. H.; Araujo, P. T.; Zhang, X.; Rodriguez-Nieva, J. F.; Lin, Y.; Zhang, J.; Kong, J.; Dresselhaus, M. S., Raman enhancement effect on



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

two-dimensional layered materials: graphene, h-BN and MoS2. Nano Lett 2014, 14 (6), 3033-3040. 25. Xu, W.; Xiao, J.; Chen, Y.; Chen, Y.; Ling, X.; Zhang, J., Graphene-veiled gold substrate for surface-enhanced Raman spectroscopy. Adv. Mater. 2013, 25 (6), 928-933. 26. Leem, J.; Wang, M. C.; Kang, P.; Nam, S., Mechanically Self-Assembled, Three-Dimensional

Graphene-Gold

Hybrid

Nanostructures

for

Advanced

Nanoplasmonic Sensors. Nano Lett 2015, 15 (11), 7684-7690. 27. Li, X.; Choy, W. C. H.; Ren, X.; Zhang, D.; Lu, H., Highly Intensified Surface Enhanced Raman Scattering by Using Monolayer Graphene as the Nanospacer of Metal Film-Metal Nanoparticle Coupling System. Adv. Funct. Mater. 2014, 24 (21), 3114-3122. 28. Zaretski, A. V.; Marin, B. C.; Moetazedi, H.; Dill, T. J.; Jibril, L.; Kong, C.; Tao, A. R.; Lipomi, D. J., Using the thickness of graphene to template lateral subnanometer gaps between gold nanostructures. Nano Lett 2015, 15 (1), 635-640. 29. Ling, X.; Xie, L.; Fang, Y.; Xu, H.; Zhang, H.; Kong, J.; Dresselhaus, M. S.; Zhang, J.; Liu, Z., Can graphene be used as a substrate for Raman enhancement?

Nano Lett 2010, 10 (2), 553-561. 30. Heeg, S.; Fernandez-Garcia, R.; Oikonomou, A.; Schedin, F.; Narula, R.; Maier, S. A.; Vijayaraghavan, A.; Reich, S., Polarized plasmonic enhancement by Au nanostructures probed through Raman scattering of suspended graphene. Nano Lett 2013, 13 (1), 301-308.


Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31. Lee, C.; Wei, X.; Kysar, J. W.; Hone, J., Measurement of the elastic properties and intrinsic strength of monolayer graphene. science 2008, 321 (5887), 385-388. 32. Zhang, Y.; Tan, Y. W.; Stormer, H. L.; Kim, P., Experimental observation of the quantum Hall effect and Berry's phase in graphene. Nature 2005, 438 (7065), 201-204. 33. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films. science 2004, 306 (5696), 666-669. 34. Reed, J. C.; Zhu, H.; Zhu, A. Y.; Li, C.; Cubukcu, E., Graphene-enabled silver nanoantenna sensors. Nano Lett 2012, 12 (8), 4090-4094. 35. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A., Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438 (7065), 197-200. 36. Xu, W.; Ling, X.; Xiao, J.; Dresselhaus, M. S.; Kong, J.; Xu, H.; Liu, Z.; Zhang, J., Surface enhanced Raman spectroscopy on a flat graphene surface. Proc Natl Acad

Sci USA 2012, 109 (24), 9281-9286. 37. Zhu, X.; Shi, L.; Schmidt, M. S.; Boisen, A.; Hansen, O.; Zi, J.; Xiao, S.; Mortensen, N. A., Enhanced light-matter interactions in graphene-covered gold nanovoid arrays. Nano Lett 2013, 13 (10), 4690-4696. 38. Hao, Q.; Wang, B.; Bossard, J. A.; Kiraly, B.; Zeng, Y.; Chiang, I. K.; Jensen, L.; Werner, D. H.; Huang, T. J., Surface-Enhanced Raman Scattering Study on Graphene-Coated Metallic Nanostructure Substrates. J Phys Chem C 2012, 116 (13),



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

7249-7254. 39. Fan, W.; Lee, Y. H.; Pedireddy, S.; Zhang, Q.; Liu, T.; Ling, X. Y., Graphene oxide

and

shape-controlled

silver

nanoparticle

hybrids

for

ultrasensitive

single-particle surface-enhanced Raman scattering (SERS) sensing. Nanoscale 2014,

6 (9), 4843-4851. 40. Xu, W.; Mao, N.; Zhang, J., Graphene: a platform for surface-enhanced Raman spectroscopy. Small 2013, 9 (8), 1206-1224. 41. Li, X.; Li, J.; Zhou, X.; Ma, Y.; Zheng, Z.; Duan, X.; Qu, Y., Silver nanoparticles protected by monolayer graphene as a stabilized substrate for surface enhanced Raman spectroscopy. Carbon 2014, 66, 713-719. 42. Schriver, M.; Regan , W.; Gannett, W. J.; Zaniewski, A. M.; Crommie, M. F.; Zettl, A., Graphene as a Long-Term Metal Oxidation Barrier: Worse Than Nothing.

ACS Nano 2013, 7 (7), 5763-5768. 43. Zhao, Y.; Xie, Y.; Hui, Y. Y.; Tang, L.; Jie, W.; Jiang, Y.; Xu, L.; Lau, S. P.; Chai, Y., Highly impermeable and transparent graphene as an ultra-thin protection barrier for Ag thin films. J Mater Chem C 2013, 1 (32), 4956-4961. 44. Chen, S.; Brown, L.; Levendorf, M.; Cai, W.; Ju, S. Y.; Edgeworth, J.; Li, X.; Magnuson, C. W.; Velamakanni, A.; Piner, R. D.; Kang, J.; Park, J.; Ruoff, R. S., Oxidation Resistance of GrapheneCoated Cu and Cu/Ni Alloy. ACS Nano 2011, 5 (2), 1321–1327. 45. Dai, Z.; Mei, F.; Xiao, X.; Liao, L.; Wu, W.; Zhang, Y.; Ying, J.; Wang, L.; Ren, F.; Jiang, C., Monolayer graphene on nanostructured Ag for enhancement of


Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface-enhanced Raman scattering stable platform. Nanotechnology 2015, 26 (12), 125603. 46. Dai, Z.; Xiao, X.; Wu, W.; Zhang, Y.; Liao, L.; Guo, S.; Ying, J.; Shan, C.; Sun, M.; Jiang, C., Plasmon-driven reaction controlled by the number of graphene layers and localized surface plasmon distribution during optical excitation. Light: Science &

Applications 2015, 4 (10), e342. 47. Sun, H.-B.; Wu, J.; Zhu, J.-J.; Pan, D.-F.; Wang, G.-H.; Wan, J.-G., Great surface-enhanced Raman scattering in hybrids consisting of graphene and silver nanoparticles with large particle sizes. Appl Phys Lett 2015, 106 (21), 211603. 48. Dieringer, J. A.; McFarland, A. D.; Shah, N. C.; Stuart, D. A.; Whitney, A. V.; Yonzon, C. R.; Young, M. A.; Zhang, X.; Van Duyne, R. P., Introductory Lecture : Surface enhanced Raman spectroscopy: new materials, concepts, characterization tools, and applications. Faraday Discuss. 2006, 132, 9-26. 49. Liang, L. H.; Shen, C. M.; Du, S. X.; Liu, W. M.; Xie, X. C.; Gao, H. J., Increase in thermal stability induced by organic coatings on nanoparticles. Phys Rev B 2004,

70 (20), 205419. 50. Jiang, Q.; Shi, H. X.; Zhao, M., Melting thermodynamics of organic nanocrystals.

J. Chem. Phys. 1999, 111 (5), 2176. 51. Zhao, Y.; Li, X.; Du, Y.; Chen, G.; Qu, Y.; Jiang, J.; Zhu, Y., Strong light-matter interactions in sub-nanometer gaps defined by monolayer graphene: toward highly sensitive SERS substrates. Nanoscale 2014, 6 (19), 11112-11120. 52. Ivanov, V. G.; Hadjichristov, G. B., Orientation of sp2 carbon nanoclusters in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ion-implanted polymethylmethacrylate as revealed by polarized Raman spectroscopy.

J Raman Spectrosc 2011, 42 (6), 1340-1343. 53. Song, X.; Dai, Z.; Xiao, X.; Li, W.; Zheng, X.; Shang, X.; Zhang, X.; Cai, G.; Wu, W.; Meng, F.; Jiang, C., Formation of Carbonized Polystyrene Sphere/hemisphere Shell Arrays by Ion Beam Irradiation and Subsequent Annealing or Chloroform Treatment. Sci Rep 2015, 5, 17529. 54. Stiufiuc R.; Iacovita, C.; Lucaciu, C. M.; Stiufiuc, G.; Dutu, A. G.; Braescu, C.; Leopold, N., SERS-active silver colloids prepared by reduction of silver nitrate with short-chain polyethylene glycol. Nanoscale Res Lett 2013, 8 (1), 1-5. 55. Ji, W.; Wang, L.; Qian, H.; Yao, W., Quantitative Analysis of Amoxicillin Residues in Foods by Surface-Enhanced Raman Spectroscopy. Spectrosc Lett 2014,

47 (6), 451-457.


Page 28 of 29

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TOC


Improved Thermal Stability of Graphene-Veiled Noble Metal

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