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Electrodeposited silver nanoflowers as sensitive surface-enhanced Raman scattering sensing substrates
Materials Letters 236 (2019) 398–402
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Electrodeposited silver nanoflowers as sensitive surface-enhanced Raman scattering sensing substrates Jia Bi Department of Electrical and Electronic Engineering, Yantai Vocational College, Yantai, Shandong 264670, China
a r t i c l e
i n f o
Article history: Received 5 September 2018 Received in revised form 20 October 2018 Accepted 23 October 2018 Available online 24 October 2018 Keywords: Sensors Electrodeposition
a b s t r a c t Simple methods capable of preparing high performance SERS substrates are highly desirable. Here, a simple electrochemical approach is developed to prepare silver nanoflowers with outstanding surface enhanced Raman scattering (SERS) performance. The silver nanoflowers are composed of hundreds of silver nanoleaves. These nanoleaves weaved together to form the silver nanoflowers. Notably, the silver nanoleaves are porous with many nanoscale pores. Numerous SERS ‘‘hot spots” exist at the overlapping points of the silver nanoleaves, as well as within the nanoscale pores of the porous silver nanoleaves. As expected, the silver nanoflowers demonstrate a high SERS sensitivity. As low as 1 nM Rhodamine 6G could be detected using the silver nanoflower SERS substrate. Moreover, the standard deviation of the SERS signals is only about 13.5%, underlying a good detection reliability of the silver nanoflower substrate. The high SERS sensitivity and the strong SERS detection reliability make the silver nanoflower SERS substrate have promising application in ultrasensitive detection fields. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Ultrasensitive detection of small molecules in common fluids has fundamental importance in analytical chemistry [1–3]. Various techniques have been developed to realize detection of small molecules in common fluids. The well-known techniques include mass spectrometer, liquid chromatography, and liquid chromatography mass spectrometer [4,5]. However, these techniques are usually time-consuming, need valuable equipment and complex sample treatment procedures. Therefore, new techniques capable of sensitively detecting small molecules in a simple, fast, and cheap manner are highly desirable. Surface-enhanced Raman scattering (SERS), as a sensing technique, can meet all of the above requirements during detection of small molecules [6–10]. It can provide ‘‘fingerprint” signals of small molecules. The sensitive SERS sites, known as ‘‘hot spots”, locate at small gaps between adjacent noble metal (e.g., silver and gold) nanostructures [6–10]. Even a single molecule can be detected if it is accurately situated at the SERS site [6,11,12]. Therefore, SERS has great potentials in ultrasensitive detection fields. Clean-room micro/nanostructure fabrication methods have been successfully employed to prepare sensitive SERS substrates composed of numerous SERS ‘‘hot spots” [13–18]. However, it is challenging to prepare high performance SERS substrates with a E-mail address: [email protected] https://doi.org/10.1016/j.matlet.2018.10.138 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
simple, low-cost, and high throughput manner. Here, we developed a simple electrodeposition method to prepare silver microflowers composed of many ultrathin nanoleaves. These nanoleaves interconnected together, creating a substantial amount of SERS ‘‘hot spots”. Therefore, the silver nanoflowers demonstrated outstanding SERS performance. Owing to the simple and low-cost preparation process, these silver nanoflowers have great application potentials as SERS substrates in detection of environmental pollutants [12].
2. Experimental section The electrolyte solution was prepared by introducing a small amount of sodium dodecyl sulphate (SDS) surface surfactant reagent into 25 mM AgNO3 aqueous solutions. Typically, the concentration of the SDS is at the critical micelle concentration (8.3 103). Electrochemical depositions were carried out under a potentiostatic condition. Two heavily doped silicon wafers (Nitrogen doped, Resistance: 3–8 X cm) with an area of 1 cm 2 cm were used as the working and the auxiliary electrode. Deionized water with a resistivity of 18.2 M X cm was used in all of the experiments. The deposition voltage is maintained at 5 V. The phase of the electrodeposition nanostructures was characterized by X-ray diffraction (XRD) (GIXRD, Rigaku D/MAX 2550, Japan). Scanning electron microscope (SEM) was used to observe
J. Bi / Materials Letters 236 (2019) 398–402
the morphology of the electrodeposited nanostructures (Hitachi S4800) operating at 5.0 kV. SERS performance of the electrodeposited nanostructures was evapuated using Rhodamine 6G (R6G) aqueous solutions. R6G aqueous solutions (10 ml) at different concentrations were spread over the electrodeposited nanostructures (1 cm2). After water was evaporated, the SERS spectra were collected using a confocal Raman microscopic system (LabRAM HR Evolution, France) equipped with an excitation Nd:YAG laser of 532 nm wavelength. The R6G powder was prepared by spreading 10 ml of R6G ethanol solutions at a concentration of 1 M over an area of 1 mm2 on a piece of glass slide. Then, the Raman spectrum of the R6G powder was taken to calculate the SERS enhancement factor of different Ag nanostructures. The power of the laser was about 0.1 mW. The integration time for SERS measurements was maintained at 10 s.
399
3. Results XRD pattern revealed that the electrodeposited products were pure silver with a face centred cubic phase (Fig. 1a). The silver nanoflowers randomly scattered on the electrode surface (Fig. 1b). Occasionally, several silver nanoflowers aggregated together. The size of the as-prepared silver nanoflowers is around 11 lm with a wide size distribution (Fig. 1c). The silver nanoflowers were formed by hundreds of thin nanoleaves (Fig. 1d and e). These nanoleaves are about 100 nm in width and more than 1 mm in length (Fig. 1f). They weaved together to form the silver nanoflowers, giving rise to the formation of thousands of intersections. These intersections can behave as SERS ‘‘hot spots”, as discussed later. More importantly, there are many nanoscale pores in the nanoleaves (Fig. 1f), which can behave as SERS ‘‘hot spots”.
Fig. 1. a. XRD pattern of the electrodeposited nanoflowers. b. SEM image of the silver nanoflowers. c. The size distribution of the silver nanoflowers. d. Enlarge observation of the silver nanoflowers. e. A typical silver nanoflower. f. Enlarged observation of the circle marked area in e.
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Taken together, the silver nanoflowers should have outstanding SERS performance. To study the formation mechanism of the silver nanoflowers, we monitored the structure evolution of the silver nanoflowers at different deposition times (Fig. 2). At the beginning (1 h deposition), silver nanoplates weaved together to form butterfly-like particles (Fig. 2a). The size of these butterfly-like particles is about 2 mm. Further deposition rendered the growth of silver nanoparticles to within the interstitials between the silver nanoplates (Fig. 2b–d), giving rise to the formation of meatball-like particles with obvious protrusions surrounding the edge of the initial nanoplates. Simultaneously, the scale of the meatball-like particles was
increased to be about 5 lm. Silver nanoparticles continuously attach onto the small meatball-like particles, render the growth of the particles. The meatball-like particles reached about 8 mm after 140 min deposition (Fig. 2e). Cracks were created through gently pressing a silicon wafer onto the meatball-like particles (Fig. 2f). Observation at the cracks clearly showed the aggregation structure of the silver nanoparticles (inset in Fig. 2f). In the following deposition, these meatball-like particles act as cores, and silver nanoleaves grow onto the cores, rendering the formation of the silver nanoflowers (Fig. 1b). The SDS plays and important role in the formation of the porous silver nanoleaves through adhering onto certain crystals planes of the silver deposits [19].
Fig. 2. SEM image of electrodeposited silver structures for different times. a. 1 h. b. 70 min. c. 90 min. d. 100 min. e. 140 min.
J. Bi / Materials Letters 236 (2019) 398–402
The SERS performance of the electrodeposition silver nanostructures was evaluated using Rhodamine 6G (R6G) as a probing molecule. The silver nanoflowers demonstrated the strongest SERS signals (Curve 1 in Fig. 3a). The intensity is about five times of that from the silver butterfly-like particles (Curve 2 in Fig. 3a; the morphology was shown in Fig. 2a) and the meatball-like particles (Curve 3 in Fig. 3a; the morphology was shown in Fig. 2d). Notably, except for the SERS signals of R6G, there are some SERS peaks originating from SDS. The butterfly-like particles and the meatball-like particles showed similar intensity of the SERS signals. The large meatball-like particles shown in Fig. 2e exhibited weak SERS signals (Curve 4 in Fig. 3a). Notably, no SERS signals were observed from the thermally evaporated gold film under the same SERS measurement parameters (Curve 5 in Fig. 3a). Therefore, the silver nanoflowers are good SERS substrates, owing to the existence of tremendous SERS ‘‘hot spots” at the overlapping area of the silver nanoleaves and within the porous silver nanoleaves. The SERS enhancement factor of these SERS substrates was evaluated based on the following equation: (ISERS/Nads)/(Ibulk/Nbulk), [20] where ISERS and Ibulk are the Raman intensity of R6G molecules dispersed on the SERS substrate and of R6G powder, respectively. Nads and Nbulk are the number of R6G molecules exposed to the laser light during Raman detection on the SERS substrate and on the R6G powder, respectively. The 613 cm1 Raman peak of R6G molecules was used to evaluate the SERS enhancement factor. The Raman intensity of 1 M R6G evenly distributed over a 1 mm2 area was about 760 (Curce 6 in Fig. 3a). Taken together, the SERS enhancement of different structures was marked in Fig. 3a. The SERS enhancement factor of the silver nanoflowers can reach 6.8 108. To determine the limit of detection of the silver nanoflowers, we measured the SERS signals of R6G aqueous solutions at different concentrations (Fig. 3b). Plasma treatment for 3 min was per-
401
formed before SERS measurements to remove SDS [21]. As the concentration of R6G decreased, the intensity of the SERS signals weakened. There were observable SERS signals even when the concentration of R6G is 1 nM, demonstrating the strong SERS sensitivity of the silver nanoflowers. To study the quantification capability of the SERS substrate, the relationship between the SERS intensity (I) and the concentration (C) of R6G solutions was built (Fig. 3C). It was revealed that the relationship could be expressed by Log C = 2.3 Log I – 16.7. Except for the sensitivity, the signal reproducibility is important for SERS substrates. The standard deviation of the SERS intensity at 613 cm1 was determined to be 13.4% obtained from 100 SERS spectra, demonstrating a good SERS signal reproducibility [22]. Polychlorinated biphenyls (PCBs) were chosen as an example to demonstrate the potential application of the plasma-cleaned silver nanoflowers in SERS detection of pollutants. As low as 106 M 3,30 ,4,40 -tetrachlorobicphenyl (PCB 77) could be detected using the silver nanoflower SERS substrates (Fig. 3D) [23]. Therefore, the silver nanoflowers prepared here may find promising applications in SERS detection of environmental pollutants. In summary, an electrochemical deposition approach was developed to prepare silver nanoflowers composed of hundreds of silver nanoleaves. These nanoleaves weaved together to form the silver nanoflowers. The silver nanoleaves are porous with nanoscale pores. Numerous SERS ‘‘hot spots” exist between the overlapping point of the silver nanoleaves and the nanoscale pores within the porous silver nanoleaves. Therefore, the silver nanoflowers demonstrated outstanding SERS sensitivity. The detection limit can be lower than 1 nM using R6G as a probing molecule. Moreover, the standard deviation of the SERS signals is about 13.5%. The silver nanoflower SERS substrate exhibited a good detection reliability. The silver nanoflowers may find promising
Fig. 3. a. SERS spectra of 1 mM R6G contaminated electrodeposition silver nanostructures. Curve 1–4 correspond to the nanostructures shown in Fig. 1b, Fig. 2a, d, and e, respectively. The SERS enhancement factor of different structures was marked. b. SERS spectra of R6G at different concentrations on the plasma-treated silver nanoflowers. c. The relationship between the SERS intensity at 1135 cm1 and the concentration of R6G. Error bars are calculated based on more than 20 spectra. d. SERS detection of PCB 77 at a concentration of 1 mM using the plasma-treated silver nanoflowers. The Raman peaks of bulk PCB 77 were shown as dotted lines.
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applications in analytical chemistry and ultrasensitive detection of environmental pollutants. References [1] G.F. Zheng, F. Patolsky, Y. Cui, W.U. Wang, C.M. Lieber, Nat. Nanotechnol. 23 (2005) 1294–1301. [2] Z.C. Hu, B.J. Deibert, J. Li, Chem. Soc. Rev. 43 (2014) 5815–5840. [3] K. Jensen, K. Kim, A. Zettl, Nat. Nanotechnol. 3 (2008) 533–537. [4] D.I. Ellis, H. Muhamadali, S.A. Haughey, C.T. Elliott, R. Goodacre, Anal. Method 7 (2015) 9401–9414. [5] A. Hierlemann, R. Gutierrez-Osuna, Chem. Rev. 108 (2008) 563–613. [6] K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, I. Itzkan, R. Dasari, M.S. Feld, Phys. Rev. Lett. 78 (1997) 1667–1670. [7] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998), 241-150. [8] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y. Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Nature 464 (2010), 392-295. [9] B. Sharma, R.R. Frontiera, A.I. Henry, E. Ringe, R.P. Van Duyne, Mater. Today 15 (2012) 16–25. [10] X.M. Qian, S.M. Nie, Chem. Soc. Rev. 37 (2008) 912–920. [11] D.K. Lim, K.S. Jeon, H.M. Kim, J.M. Nam, Y.D. Suh, Nat. Mater. 9 (2010) 60–67.
[12] S. Yang, X. Dai, B.B. Stogin, T.-S. Wong, Proc. Natl. Acad. Sci. U.S.A. 113 (2016) 268–273. [13] S. Schlucker, Angew. Chem. Int. Ed. 53 (2014) 4756–4795. [14] D.K. Lim, K.S. Jeon, J.H. Hwang, H. Kim, S. Kwon, Y.D. Suh, J.M. Nam, Nat. Nanotechnol. 6 (2011) 452–460. [15] M.J. Banholzer, J.E. Millstone, L.D. Qin, C.A. Mirkin, Chem. Soc. Rev. 37 (2008) 885–897. [16] Z.L. Huang, G.W. Meng, Q. Huang, Y.J. Yang, C.H. Zhu, C.L. Tang, Adv. Mater. 22 (2010) 4136–4241. [17] C.H. Zhu, G.W. Meng, Q. Huang, Z. Zhang, Q.L. Xu, G.Q. Liu, Z.L. Huang, Z.Q. Chu, Chem. Commun. 47 (2011) 2709–2711. [18] H.L. Liu, Z.L. Yang, L.Y. Meng, Y.D. Sun, J. Wang, L.B. Yang, J.H. Liu, Z.Q. Tian, J. Am. Chem. Soc. 136 (2014) 5332–5341. [19] Y. Xia, Y. Xiong, B. Lim, S.E. Skrabalak, Angew. Chem. Int. Ed. 48 (2009) 60–103. [20] Y. Wang, Y. Kang, W.Y. Wang, Q. Ding, J. Zhou, S. Yang, Nanotechnology 29 (2018) 414001. [21] Y. Shimizu, Y. Makamoto, T. Hyodo, M. Egashira, Electrochemistry 72 (2004) 92–97. [22] H. Huang, J.H. Wang, W.H. Jin, P.H. Li, M. Chen, H.H. Xie, X.F. Xu, H.Y. Wang, Z. G. Dai, X.H. Xiao, P.K. Chu, Small 10 (2015) 4012–4019. [23] J. Liu, G. Meng, X. Li, Z. Huang, Langmuir 30 (2014) 13964–13969.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Electrodeposited silver nanoflowers as sensitive surface-enhanced Raman scattering sensing substrates Jia Bi Department of Electrical and Electronic Engineering, Yantai Vocational College, Yantai, Shandong 264670, China
a r t i c l e
i n f o
Article history: Received 5 September 2018 Received in revised form 20 October 2018 Accepted 23 October 2018 Available online 24 October 2018 Keywords: Sensors Electrodeposition
a b s t r a c t Simple methods capable of preparing high performance SERS substrates are highly desirable. Here, a simple electrochemical approach is developed to prepare silver nanoflowers with outstanding surface enhanced Raman scattering (SERS) performance. The silver nanoflowers are composed of hundreds of silver nanoleaves. These nanoleaves weaved together to form the silver nanoflowers. Notably, the silver nanoleaves are porous with many nanoscale pores. Numerous SERS ‘‘hot spots” exist at the overlapping points of the silver nanoleaves, as well as within the nanoscale pores of the porous silver nanoleaves. As expected, the silver nanoflowers demonstrate a high SERS sensitivity. As low as 1 nM Rhodamine 6G could be detected using the silver nanoflower SERS substrate. Moreover, the standard deviation of the SERS signals is only about 13.5%, underlying a good detection reliability of the silver nanoflower substrate. The high SERS sensitivity and the strong SERS detection reliability make the silver nanoflower SERS substrate have promising application in ultrasensitive detection fields. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Ultrasensitive detection of small molecules in common fluids has fundamental importance in analytical chemistry [1–3]. Various techniques have been developed to realize detection of small molecules in common fluids. The well-known techniques include mass spectrometer, liquid chromatography, and liquid chromatography mass spectrometer [4,5]. However, these techniques are usually time-consuming, need valuable equipment and complex sample treatment procedures. Therefore, new techniques capable of sensitively detecting small molecules in a simple, fast, and cheap manner are highly desirable. Surface-enhanced Raman scattering (SERS), as a sensing technique, can meet all of the above requirements during detection of small molecules [6–10]. It can provide ‘‘fingerprint” signals of small molecules. The sensitive SERS sites, known as ‘‘hot spots”, locate at small gaps between adjacent noble metal (e.g., silver and gold) nanostructures [6–10]. Even a single molecule can be detected if it is accurately situated at the SERS site [6,11,12]. Therefore, SERS has great potentials in ultrasensitive detection fields. Clean-room micro/nanostructure fabrication methods have been successfully employed to prepare sensitive SERS substrates composed of numerous SERS ‘‘hot spots” [13–18]. However, it is challenging to prepare high performance SERS substrates with a E-mail address: [email protected] https://doi.org/10.1016/j.matlet.2018.10.138 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
simple, low-cost, and high throughput manner. Here, we developed a simple electrodeposition method to prepare silver microflowers composed of many ultrathin nanoleaves. These nanoleaves interconnected together, creating a substantial amount of SERS ‘‘hot spots”. Therefore, the silver nanoflowers demonstrated outstanding SERS performance. Owing to the simple and low-cost preparation process, these silver nanoflowers have great application potentials as SERS substrates in detection of environmental pollutants [12].
2. Experimental section The electrolyte solution was prepared by introducing a small amount of sodium dodecyl sulphate (SDS) surface surfactant reagent into 25 mM AgNO3 aqueous solutions. Typically, the concentration of the SDS is at the critical micelle concentration (8.3 103). Electrochemical depositions were carried out under a potentiostatic condition. Two heavily doped silicon wafers (Nitrogen doped, Resistance: 3–8 X cm) with an area of 1 cm 2 cm were used as the working and the auxiliary electrode. Deionized water with a resistivity of 18.2 M X cm was used in all of the experiments. The deposition voltage is maintained at 5 V. The phase of the electrodeposition nanostructures was characterized by X-ray diffraction (XRD) (GIXRD, Rigaku D/MAX 2550, Japan). Scanning electron microscope (SEM) was used to observe
J. Bi / Materials Letters 236 (2019) 398–402
the morphology of the electrodeposited nanostructures (Hitachi S4800) operating at 5.0 kV. SERS performance of the electrodeposited nanostructures was evapuated using Rhodamine 6G (R6G) aqueous solutions. R6G aqueous solutions (10 ml) at different concentrations were spread over the electrodeposited nanostructures (1 cm2). After water was evaporated, the SERS spectra were collected using a confocal Raman microscopic system (LabRAM HR Evolution, France) equipped with an excitation Nd:YAG laser of 532 nm wavelength. The R6G powder was prepared by spreading 10 ml of R6G ethanol solutions at a concentration of 1 M over an area of 1 mm2 on a piece of glass slide. Then, the Raman spectrum of the R6G powder was taken to calculate the SERS enhancement factor of different Ag nanostructures. The power of the laser was about 0.1 mW. The integration time for SERS measurements was maintained at 10 s.
399
3. Results XRD pattern revealed that the electrodeposited products were pure silver with a face centred cubic phase (Fig. 1a). The silver nanoflowers randomly scattered on the electrode surface (Fig. 1b). Occasionally, several silver nanoflowers aggregated together. The size of the as-prepared silver nanoflowers is around 11 lm with a wide size distribution (Fig. 1c). The silver nanoflowers were formed by hundreds of thin nanoleaves (Fig. 1d and e). These nanoleaves are about 100 nm in width and more than 1 mm in length (Fig. 1f). They weaved together to form the silver nanoflowers, giving rise to the formation of thousands of intersections. These intersections can behave as SERS ‘‘hot spots”, as discussed later. More importantly, there are many nanoscale pores in the nanoleaves (Fig. 1f), which can behave as SERS ‘‘hot spots”.
Fig. 1. a. XRD pattern of the electrodeposited nanoflowers. b. SEM image of the silver nanoflowers. c. The size distribution of the silver nanoflowers. d. Enlarge observation of the silver nanoflowers. e. A typical silver nanoflower. f. Enlarged observation of the circle marked area in e.
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J. Bi / Materials Letters 236 (2019) 398–402
Taken together, the silver nanoflowers should have outstanding SERS performance. To study the formation mechanism of the silver nanoflowers, we monitored the structure evolution of the silver nanoflowers at different deposition times (Fig. 2). At the beginning (1 h deposition), silver nanoplates weaved together to form butterfly-like particles (Fig. 2a). The size of these butterfly-like particles is about 2 mm. Further deposition rendered the growth of silver nanoparticles to within the interstitials between the silver nanoplates (Fig. 2b–d), giving rise to the formation of meatball-like particles with obvious protrusions surrounding the edge of the initial nanoplates. Simultaneously, the scale of the meatball-like particles was
increased to be about 5 lm. Silver nanoparticles continuously attach onto the small meatball-like particles, render the growth of the particles. The meatball-like particles reached about 8 mm after 140 min deposition (Fig. 2e). Cracks were created through gently pressing a silicon wafer onto the meatball-like particles (Fig. 2f). Observation at the cracks clearly showed the aggregation structure of the silver nanoparticles (inset in Fig. 2f). In the following deposition, these meatball-like particles act as cores, and silver nanoleaves grow onto the cores, rendering the formation of the silver nanoflowers (Fig. 1b). The SDS plays and important role in the formation of the porous silver nanoleaves through adhering onto certain crystals planes of the silver deposits [19].
Fig. 2. SEM image of electrodeposited silver structures for different times. a. 1 h. b. 70 min. c. 90 min. d. 100 min. e. 140 min.
J. Bi / Materials Letters 236 (2019) 398–402
The SERS performance of the electrodeposition silver nanostructures was evaluated using Rhodamine 6G (R6G) as a probing molecule. The silver nanoflowers demonstrated the strongest SERS signals (Curve 1 in Fig. 3a). The intensity is about five times of that from the silver butterfly-like particles (Curve 2 in Fig. 3a; the morphology was shown in Fig. 2a) and the meatball-like particles (Curve 3 in Fig. 3a; the morphology was shown in Fig. 2d). Notably, except for the SERS signals of R6G, there are some SERS peaks originating from SDS. The butterfly-like particles and the meatball-like particles showed similar intensity of the SERS signals. The large meatball-like particles shown in Fig. 2e exhibited weak SERS signals (Curve 4 in Fig. 3a). Notably, no SERS signals were observed from the thermally evaporated gold film under the same SERS measurement parameters (Curve 5 in Fig. 3a). Therefore, the silver nanoflowers are good SERS substrates, owing to the existence of tremendous SERS ‘‘hot spots” at the overlapping area of the silver nanoleaves and within the porous silver nanoleaves. The SERS enhancement factor of these SERS substrates was evaluated based on the following equation: (ISERS/Nads)/(Ibulk/Nbulk), [20] where ISERS and Ibulk are the Raman intensity of R6G molecules dispersed on the SERS substrate and of R6G powder, respectively. Nads and Nbulk are the number of R6G molecules exposed to the laser light during Raman detection on the SERS substrate and on the R6G powder, respectively. The 613 cm1 Raman peak of R6G molecules was used to evaluate the SERS enhancement factor. The Raman intensity of 1 M R6G evenly distributed over a 1 mm2 area was about 760 (Curce 6 in Fig. 3a). Taken together, the SERS enhancement of different structures was marked in Fig. 3a. The SERS enhancement factor of the silver nanoflowers can reach 6.8 108. To determine the limit of detection of the silver nanoflowers, we measured the SERS signals of R6G aqueous solutions at different concentrations (Fig. 3b). Plasma treatment for 3 min was per-
401
formed before SERS measurements to remove SDS [21]. As the concentration of R6G decreased, the intensity of the SERS signals weakened. There were observable SERS signals even when the concentration of R6G is 1 nM, demonstrating the strong SERS sensitivity of the silver nanoflowers. To study the quantification capability of the SERS substrate, the relationship between the SERS intensity (I) and the concentration (C) of R6G solutions was built (Fig. 3C). It was revealed that the relationship could be expressed by Log C = 2.3 Log I – 16.7. Except for the sensitivity, the signal reproducibility is important for SERS substrates. The standard deviation of the SERS intensity at 613 cm1 was determined to be 13.4% obtained from 100 SERS spectra, demonstrating a good SERS signal reproducibility [22]. Polychlorinated biphenyls (PCBs) were chosen as an example to demonstrate the potential application of the plasma-cleaned silver nanoflowers in SERS detection of pollutants. As low as 106 M 3,30 ,4,40 -tetrachlorobicphenyl (PCB 77) could be detected using the silver nanoflower SERS substrates (Fig. 3D) [23]. Therefore, the silver nanoflowers prepared here may find promising applications in SERS detection of environmental pollutants. In summary, an electrochemical deposition approach was developed to prepare silver nanoflowers composed of hundreds of silver nanoleaves. These nanoleaves weaved together to form the silver nanoflowers. The silver nanoleaves are porous with nanoscale pores. Numerous SERS ‘‘hot spots” exist between the overlapping point of the silver nanoleaves and the nanoscale pores within the porous silver nanoleaves. Therefore, the silver nanoflowers demonstrated outstanding SERS sensitivity. The detection limit can be lower than 1 nM using R6G as a probing molecule. Moreover, the standard deviation of the SERS signals is about 13.5%. The silver nanoflower SERS substrate exhibited a good detection reliability. The silver nanoflowers may find promising
Fig. 3. a. SERS spectra of 1 mM R6G contaminated electrodeposition silver nanostructures. Curve 1–4 correspond to the nanostructures shown in Fig. 1b, Fig. 2a, d, and e, respectively. The SERS enhancement factor of different structures was marked. b. SERS spectra of R6G at different concentrations on the plasma-treated silver nanoflowers. c. The relationship between the SERS intensity at 1135 cm1 and the concentration of R6G. Error bars are calculated based on more than 20 spectra. d. SERS detection of PCB 77 at a concentration of 1 mM using the plasma-treated silver nanoflowers. The Raman peaks of bulk PCB 77 were shown as dotted lines.
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applications in analytical chemistry and ultrasensitive detection of environmental pollutants. References [1] G.F. Zheng, F. Patolsky, Y. Cui, W.U. Wang, C.M. Lieber, Nat. Nanotechnol. 23 (2005) 1294–1301. [2] Z.C. Hu, B.J. Deibert, J. Li, Chem. Soc. Rev. 43 (2014) 5815–5840. [3] K. Jensen, K. Kim, A. Zettl, Nat. Nanotechnol. 3 (2008) 533–537. [4] D.I. Ellis, H. Muhamadali, S.A. Haughey, C.T. Elliott, R. Goodacre, Anal. Method 7 (2015) 9401–9414. [5] A. Hierlemann, R. Gutierrez-Osuna, Chem. Rev. 108 (2008) 563–613. [6] K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, I. Itzkan, R. Dasari, M.S. Feld, Phys. Rev. Lett. 78 (1997) 1667–1670. [7] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998), 241-150. [8] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y. Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Nature 464 (2010), 392-295. [9] B. Sharma, R.R. Frontiera, A.I. Henry, E. Ringe, R.P. Van Duyne, Mater. Today 15 (2012) 16–25. [10] X.M. Qian, S.M. Nie, Chem. Soc. Rev. 37 (2008) 912–920. [11] D.K. Lim, K.S. Jeon, H.M. Kim, J.M. Nam, Y.D. Suh, Nat. Mater. 9 (2010) 60–67.
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