Journal of Science: Advanced Materials and Devices (2016) 84e89 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original article Multiwalled carbon nanotubes/silver nanocomposite as effective SERS platform for detection of methylene blue dye in water Ngo Xuan Dinh a, d, Tran Quang Huy b, Le Van Vu c, Le Thi Tam d, *, Anh-Tuan Le d, ** a University of Transport Technology, Hanoi, Viet Nam National Institute of Hygiene and Epidemiology (NIHE), No 1, Yersin Street, Hai Ba Trung District, Hanoi, Viet Nam c Center for Materials Science, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam d Department of Nanoscience and Nanotechnology-DoNST, Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No 1, Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Nam b a r t i c l e i n f o a b s t r a c t Article history: Received 30 March 2016 Accepted April 2016 Available online 22 April 2016 In this work, a functional nanocomposite consisting of silver nanoparticles and multiwalled carbon nanotubes (MWCNTs-Ag) was successfully synthesized using a two-step chemical process The MWCNTsAg nanocomposite has been studied as a surface-enhanced Raman scattering (SERS) sensing platform for detection of methylene blue (MB) dye in an aqueous medium The obtained results reveal that the MWCNTs-Ag nanocomposite exhibits higher SERS detection activity than that of pure Ag-nanoparticles (Ag-NPs) The calculated enhancement factors are 1.51  106 for pure Ag-NPs and 4.68  106 for the MWCNTs-Ag nanocomposite MB detection has been achieved as low as ppm The SERS enhancement of the MWCNTs-Ag nanocomposite can be attributed to the combination of both an electromagnetic (EM) effect and a chemical effect (CE) With exhibited properties, the MWCNTs-Ag nanocomposite can be effectively used for detection of various organic dyes in water solution © 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Silver nanoparticles Carbon nanotubes MWCNTs-Ag composite Surface-enhanced Raman scattering Methylene blue Introduction Organic dyes are important colored substances that are widely used for various industrial applications such as textile, agriculture, detergents, and coatings [1] Dye consumption is increasing in the Asia/Pacific region, where the majority of the world's textiles and consumer plastic products are manufactured [2] However, most synthetic organic dyes from industrialization are directly discharged into water streams The release of large amounts of synthetic dyes to the water source has posed challenges to global environmental and health issues, especially in developing countries The pollution of these organic dyes in water leads to a potential risk to human health and community Consequently, in order to ensure our safety, the determination of organic dyes is an urgent demand that requires accurate and reliable techniques Several analytical methods have been proposed for the * Corresponding author Tel.: ỵ84 3623 0435, fax: ỵ84 3623 0293 ** Corresponding author Tel.: ỵ84 3623 0435, fax: ỵ84 3623 0293 E-mail addresses: tam.lethi@hust.edu.vn (L.T Tam), tuan.leanh1@hust.edu.vn (A.-T Le) Peer review under responsibility of Vietnam National University, Hanoi determination of various organic dyes, such as spectrophotometric methods, capillary electrophoresis (CE), and high-performance liquid chromatography [3,4] However, these conventional methods require time-consuming and expensive sample pretreatment and complex steps by a trained expert Therefore, it is necessary to develop new analytical techniques with saving-time, cost effectiveness, and fast response for determination of organic dye Surface-enhanced Raman scattering (SERS) is known as an effective method for environmental monitoring, chemical analysis and biomedical research [5] Due to its high sensitivity, selectivity and reliability, SERS can be considered a promising tool for the trace analysis of a variety of important chemical and biological molecules When a molecule absorbs on the SERS substrate, SERS enhances the molecular Raman signal by many orders of magnitude owing to significant increase in the scatting cross-section It has been noted that the Raman enhancement is caused by two mechanisms [6,7]: (i) the first one is related to an electromagnetic effect (EM) based on the enhancement of the local electromagnetic field and (ii) the second mechanism is related to a chemical effect (CE) based on charge transfer between absorbed molecules and metal surface Noble metallic nanoparticles (NPs) provide a good platform for SERS substrates because they exhibit localized surface plasmon http://dx.doi.org/10.1016/j.jsamd.2016.04.007 2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) N.X Dinh et al / Journal of Science: Advanced Materials and Devices (2016) 84e89 85 resulting solid products of MWCNTs-Ag nanocomposites were collected by centrifugation and were purified by washing with deionized water and dried under vacuum at 80 C for 8e10 h resonance effect (LSPR) in which the EM effect is more prominent for SERS enhancement The magnitude of SERS signals was remarkably enhanced by use of metallic NPs (silver, gold) as the SERS substrate [5] Nevertheless, there are some limitations of the SERS technique For example, the SERS active analyte molecules require a good affinity to metallic surface The lack of metal surface affinity makes the signal of SERS detection very poor [6,7] To improve the detection, some recent reports indicated that the SERS signals can be much improved in hybrid nanostructures between silver NPs and carbon nanomaterials [5e9] due to combination of the two mechanisms (EM and CE mechanisms) In this work, we demonstrate the use of multiwalled carbon nanotubes/silver (MWCNTs-Ag) nanocomposite as an effective SERS platform to detect methylene blue (MB) dye in an aqueous medium The decoration of Ag-NPs on the surface of functionalized MWCNTs was performed using a photochemical method Our obtained results reveal that the MWCNTs-Ag nanocomposite shows the higher SERS performance than that of bare Ag-NPs The detection limit of MB dye we can obtain with this technique is ~1 ppm These SERS methods can be effectively used to detect various types of organic dyes in aqueous solutions Transmission electron microscopy (TEM, JEOL-JEM 1010) was conducted to investigate the morphology and distribution of asprepared samples The UVevis absorbance spectra were recorded using an HP 8453 spectrophotometer, and the absorption spectrum of all suspension samples in 10 mm path length quartz cuvettes was 300 nme900 nm For the SERS measurements, 50 mL of functional nanomaterials (Ag-NPs, or MWCNTs-Ag) were dropped on glass slide (1 cm  cm) and dried in air After that, 50 mL of MB dye solution, with various concentrations, was dropped on the substrate coated with functional nanomaterials for analysis of the SERS signal These MB stock solutions were diluted step-by-step with water to prepare various concentrations of analytes All SERS spectra were measured on a Raman system with a 633 nm HeeNe laser (LabRAM HR800, Horiba, Jobin-Yvon) Experimental procedures Results and discussion 2.1 Chemicals 3.1 Characterization of MWCNTs-Ag nanocomposites Silver nitrate (AgNO3, 99.9%), sodium hydroxide (NaOH), ammonium hydroxide (NH3, 25%), sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 63%), oleic acid, and glucose that were used in this study were purchased from Shanghai Chemical Reagent The multiwalled carbon nanotubes (MWCNTs) (diameter ~ 15e20 nm; length ~ 2.5 mm) were provided from Chungnam National University in South Korea In this work, we employed the acid treatment of bare MWCNTs to create oxygen-containing functional groups (e.g., OH, COOH) on the surface of MWCNTs These functionalized groups make the MWCNTs well dispersed in aqueous media More importantly, the surface modification of MWCNTs also aims to create more binding sites for anchoring the precursors of silver ions (Agỵ) or metallic silver nanoparticles (zero valence Ag0-NPs) In the next step, the Ag-NPs were formed on the functional groups of the MWCNTs via photochemical method (modified Tollens reaction) Fig (a,b) shows TEM images of as-prepared MWCNTs-Ag nanocomposites at different magnifications It can be seen that the Ag-NPs were formed on the surface of MWCNTs, the adhesion of Ag-NPs to both the inner and outer walls of the carbon nanotubes The average size of the Ag-NPs attached on the MWCNTs is about ~15e20 nm To study optical characteristics of MWCNTs-Ag nanocomposites, we synthesized metallic Ag-NPs using similar a process without adding MWCNTs for comparative purpose Fig (c) displays the UVevis absorbance spectra of the Ag-NPs and MWCNTs-Ag colloidal solutions Obviously, the Ag-NPs and MWCNTs-Ag samples display strong absorption peaks at 426 and 431 nm, respectively, because of the surface plasmon resonance (SPR) effect of nanoscale metallic silver The appearance of the surface plasmon band at 431 nm confirmed the formation of Ag-NPs on the MWCNTs Moreover, the maximum absorption peak of MWCNTsAg composite (431 nm) is shifted to a longer wavelength as compared with bare Ag-NPs (426 nm) The slight red-shifting of the absorption peak toward longer wavelength for MWCNTs-Ag composite indicates the formation of silver nanoparticles with larger sizes [6,8] We believe that the red-shifting SPR band observed might be due to the plasmon coupling between formed Ag-NPs and the interface structure between the Ag-NPs and MWCNTs 2.2 Synthesis of MWCNTs-Ag nanocomposite by two-step chemical process A two-step chemical process was employed for synthesis of MWCNTs-Ag nanocomposites as shown in Fig The first step was to thermally oxidize 100 mg of pristine MWCNTs (diameter ~ 15e20 nm; length ~ 2.5 mm) at 450 C for h to remove amorphous carbon in the samples Next, 50 mg of the oxidized MWCNTs were treated in 100 mL of an acid mixture of HNO3 and H2SO4 (1:3 vol/vol) at 90 C for 12 h to produce OH- and/or COOHfunctionalized MWCNTs The samples were then filtered, washed with distilled water and dried under vacuum at 120 C for 10 h Finally, 30 mg of the OH- and/or COOH-functionalized MWCNTs were dispersed in 100 mL of deionized water for next step In the second step, the decoration of Ag-NPs on the OH- and/or COOH-functionalized MWCNTs was performed by a photochemical method as reported elsewhere [10] Briefly, 1.7 g (10 mmol) of AgNO3 was dissolved in 100 mL of deionized water The AgNO3 solution was then precipitated with 0.62 g (15.5 mmol) of sodium hydroxide (Aldrich, >99%) The obtained precipitate, which is composed of Ag2O, was filtered and dissolved in 100 mL of aqueous ammonia (0.4% w/w, 23 mmol) until a transparent solution of silver ammonium complex [Ag(NH3)2]ỵ(aq) formed Up to 2.5 g (8.9 mmol) of oleic acid was then added dropwise into the complex, and the resulting solution was gently stirred for h at room temperature until the complete homogeneity of the reaction mixture was achieved The reduction process of the silver complex solution by the addition of g (11.1 mmol) of glucose was initiated with UV irradiation A UV lamp (l ¼ 365 nm, 35 W) was used as a light source to stimulate the reduction process After 12 h irradiation the Ag-NPs were decorated on surface of functionalized MWCNTs The 2.3 Characterization techniques 3.2 SERS activity of silver nanoparticles First, we have investigated the SERS activity of metallic pure AgNPs for detection of MB dye Fig (a) shows the original Raman spectra of MB dye and Ag-NPs For the case of MB dye, three characteristic prominent peaks were observed at 446 cmÀ1, 86 N.X Dinh et al / Journal of Science: Advanced Materials and Devices (2016) 84e89 Fig A schematic for the two-step process employed for synthesis of MWCNTs-Ag nanocomposites Fig (a,b) TEM images of MWCNTs-Ag nanocomposite and (c) UVevis spectra of pure Ag-NPs and MWCNTs-Ag nanocomposite N.X Dinh et al / Journal of Science: Advanced Materials and Devices (2016) 84e89 87 Fig (a) The original Raman spectra of MB dye and Ag-NPs; and (b) the SERS spectra of different concentrations of MB dyes adsorbed on the Ag-NPs-deposited SERS substrate 1385 cmÀ1 and 1621 cmÀ1 The characteristic peaks of MB at around 1621, 1385 and 445 cmÀ1 were assigned to CeC stretching, CeH stretching and CeNeC skeletal bending that revealed that the MB molecules were adsorbed on the substrate [11] This feature is consistent with previous reports on MB dye [11] But for the case of pure Ag-NPs, there was no characteristic peak Next, the Ag-NPs were used as a SERS substrate for the detection of MB dye Fig 3(b) shows SERS spectra of different concentrations of MB in the range of 1e70 ppm adsorbed on the Ag-NPs-deposited SERS substrate It can be seen that three similar characteristic peaks for the MB dye at 450 cmÀ1, 1390 cmÀ1 and 1594 cmÀ1 were found in all spectra The slight shifts of Raman bands indicate that the MB molecules were chemisorbed on the surface of Ag-NPs Also, it was shown that the spectral intensities and resolutions increased with the increase of MB concentration This indicates the large adsorption of MB dyes to surface of metallic Ag-NPs The intensity enhancement of these peaks is fitted with increased MB concentrations Fig shows the calculated SERS intensity as a function of MB concentrations The correlation coefficient (R2) obtained the optimal value 0.98 for peak of 450 cmÀ1 Fig The fitting curves of SERS intensity as a function of MB concentrations for three characteristic peaks at 450 cmÀ1, 1390 cmÀ1 and 1594 cmÀ1 88 N.X Dinh et al / Journal of Science: Advanced Materials and Devices (2016) 84e89 3.3 SERS activity of MWCNTs-Ag nanocomposite Similarly, we used the MWCNTs-Ag nanocomposite as a SERS substrate for the detection of MB dye Fig 5(a) shows SERS spectra of different concentrations of MB adsorbed on the MWCNTsAgedeposited SERS substrate The prominent peaks appearing at 449 cmÀ1, 1395 cmÀ1 and 1621 cmÀ1 are the characteristic Raman bands of the MB molecule As the concentration of MB increased, the spectral intensities also increased The increased SERS intensity of three characteristic peaks were fitted as a function of MB concentration as shown in Fig 5(b,c,d) Among the three characteristic peaks, the correlation coefficient (R2) obtained the optimal value 0.99 for peak of 449 cmÀ1 A linear relationship (R2 ¼ 0.99) between SERS intensity and MB concentrations indicates the potential for quantification of MB dye in water This obtained result suggests that the MWCNTs-Ag nanocomposite exhibits a higher SERS performance for detection of organic dyes than that of pure Ag-NPs 3.4 SERS enhancement factor and mechanism In order to confirm this, we calculated the average enhancement factor (G) according to the following formula (1) [12]: G¼ ISERS Nbulk IRaman Nsurf where ISERS stands for the intensities of the vibrational mode in the SERS spectra and IRaman stands for the same vibrational mode in the normal Raman spectra These data can be directly obtained from the experimental measurements Nbulk and Nsurface are the number of MB molecules illuminated by the laser focus spot under normal Raman and SERS conditions, respectively Fig shows Raman spectra of different SERS substrate materials at fixed concentrations of MB dye at 10 ppm for comparative purposes Here, we calculated ISERS for pure Ag-NPs and MWCNTs-Ag composites to be 1261 and 3920 counts with the Raman band at 450 cmÀ1, respectively The Nbulk and Nsurf calculated are 1.6  1012 and 6.68  106, respectively By using equation (1), the enhancement factor at the band of 450 cmÀ1 can be calculated to be 1.51  106 for pure Ag-NPs and 4.68  106 for the MWCNTs-Ag nanocomposite Based on the obtained results, it is believed that the SERS enhancement of the MWCNTs-Ag nanocomposite may be attributed to the combination of both EM and the CE effects Previous works mentioned that the regions contributing to the EM enhancement were mainly from nanoparticle aggregates [5] These hot spots can be created within crevices or gaps between two or more nanoparticles in an aggregated state [5] The existence of interparticle gaps is suitable for the generation of Raman “hot spots”, which offer EM enhancement [6,8] Indeed, the TEM images (see Fig 2) revealed the aggregation of the Ag-NPs on MWCNTs In our present case, the EM enhancement in MWCNTs-Ag nanocomposite compared to pure Ag-NPs can be understood in terms of the aggregation of Ag-NPs onto/within the surface of MWCNTs which in-turn introduces a large number of hot-spots [7] These large aggregations create hot-junctions or hot-spots where the Fig (a) The SERS spectra of different concentrations of MB dyes adsorbed on the MWCNTs-Ag-deposited SERS substrate, and (b,c,d) the fitting curves of SERS intensity as a function of MB concentrations for three characteristic peaks at 449 cmÀ1, 1395 cmÀ1 and 1621 cmÀ1 N.X Dinh et al / Journal of Science: Advanced Materials and Devices (2016) 84e89 89 used for analysis of trace concentration of various organic dyes in water solutions Acknowledgments This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number (106-YS.99-2014.19) The authors would like to acknowledge the Center for Materials Science at Vietnam National University in Hanoi for support in Raman measurements References Fig Raman spectra of different SERS substrate materials at fixed concentration of MB dye at 10 ppm localized surface plasmon resonance (LSPR), coupled with electromagnetic field result in significantly improved Raman signals [6,8] Furthermore, the additive contribution of the CE effect to SERS enhancement should be included in the case of MWCNTs-Ag In this case, the MWCNTs play the important role of an efficient adsorbent for organic MB species through electrostatic interactions The oxygen-containing groups (OHÀ and COOHÀ) with negative charge on the surface of MWCNTs promote the adsorption of positively charged MB dye Therefore, the MWCNTs-Ag nanocomposite is found to be very promising as a SERS platform for detection of organic dyes, where SERS enhancement performance results from both EM and CE effects Conclusions In this study, we demonstrated the use of MWCNTs-Ag nanocomposites as effective SERS platforms for detection of an organic dye, i.e., MB dye in aqueous media The MWCNTs-Ag nanocomposite was synthesized using wet chemistry methods Our obtained results indicated that the MWCNTs-Ag nanocomposite exhibited better SERS activity than that of pure Ag-NPs The MWCNTS-Ag-deposited SERS substrate was employed to detect the MB dye with a detection limit of ppm With the aforementioned advantages, the MWCNTs-Ag nanocomposite can be effectively [1] M.T Yagub, T.K Sen, S Afroze, H.M Ang, Dye and its removal from aqueous solution by adsorption: A review, Adv Colloid Inter Sci 209 (2014) 172e184 http://dx.doi.org/10.1016/j.cis.2014.04.002 [2] Dublin, Synthetic Organic Dye and Pigment Manufacturing Industry in the U.S and its International Trade, 2015 [3] M Gonzalez, M Gloria Lobo, J Mendez, A Carnero, Detection of colour adulteration in cochineals by spectrophotometric determination of yellow and red pigment groups, Food Control 16 (2005) 105e112, http://dx.doi.org/ 10.1016/j.foodcont.2003.12.002 [4] M Ryvolova, P Taborsky, P Vrabel, P Krasensky, J Preisler, Sensitive determination of erythrosine and other red food colorants using capillary electrophoresis with 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Conclusions In this study, we demonstrated the use of MWCNTs-Ag nanocomposites as effective SERS platforms for detection of an organic dye, i.e., MB dye in aqueous media The MWCNTs-Ag nanocomposite was... adsorption of positively charged MB dye Therefore, the MWCNTs-Ag nanocomposite is found to be very promising as a SERS platform for detection of organic dyes, where SERS enhancement performance... (MWCNTs-Ag) nanocomposite as an effective SERS platform to detect methylene blue (MB) dye in an aqueous medium The decoration of Ag-NPs on the surface of functionalized MWCNTs was performed using a