(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 concen[r]
Trang 1Original article
Multiwalled carbon nanotubes/silver nanocomposite as effective SERS
platform for detection of methylene blue dye in water
Ngo Xuan Dinha,d, Tran Quang Huyb, Le Van Vuc, Le Thi Tamd,*, Anh-Tuan Led,**
a University of Transport Technology, Hanoi, Viet Nam
b 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
a r t i c l e i n f o
Article history:
Received 30 March 2016
Accepted 4 April 2016
Available online 22 April 2016
Keywords:
Silver nanoparticles
Carbon nanotubes
MWCNTs-Ag composite
Surface-enhanced Raman scattering
Methylene blue
a b s t r a c t
In this work, a functional nanocomposite consisting of silver nanoparticles and multiwalled carbon nanotubes (Ag) was successfully synthesized using a two-step chemical process The
MWCNTs-Ag 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 106for pure Ag-NPs and 4.68 106for the MWCNTs-Ag nanocomposite MB detection has been achieved as low as 1 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/)
1 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
dis-charged into water streams The release of large amounts of
syn-thetic dyes to the water source has posed challenges to global
environmental and health issues, especially in developing
coun-tries The pollution of these organic dyes in water leads to a
po-tential 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
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 pre-treatment 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) thefirst one is related to an electromagnetic effect (EM) based on the enhancement of the local electromagneticfield 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
* Corresponding author Tel.: þ84 4 3623 0435, fax: þ84 4 3623 0293.
** Corresponding author Tel.: þ84 4 3623 0435, fax: þ84 4 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.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / j s a m d
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
Journal of Science: Advanced Materials and Devices 1 (2016) 84e89
Trang 2resonance 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
ob-tained 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
2 Experimental procedures
2.1 Chemicals
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
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 inFig 1 Thefirst step was
to thermally oxidize 100 mg of pristine MWCNTs
(diameter ~ 15e20 nm; length ~ 2.5mm) at 450C for 1 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 HNO3and
H2SO4(1:3 vol/vol) at 90C for 12 h to produce OH- and/or
COOH-functionalized MWCNTs The samples were thenfiltered, washed
with distilled water and dried under vacuum at 120C 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
AgNO3was 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, wasfiltered 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]þ(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 2 h at room
tem-perature until the complete homogeneity of the reaction mixture
was achieved The reduction process of the silver complex solution
by the addition of 2 g (11.1 mmol) of glucose was initiated with UV
irradiation A UV lamp (l¼ 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
resulting solid products of MWCNTs-Ag nanocomposites were collected by centrifugation and were purified by washing with deionized water and dried under vacuum at 80C for 8e10 h 2.3 Characterization techniques
Transmission electron microscopy (TEM, JEOL-JEM 1010) was conducted to investigate the morphology and distribution of as-prepared 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, 50mL of functional nanomaterials (Ag-NPs, or MWCNTs-Ag) were dropped on glass slide (1 cm 1 cm) and dried in air After that, 50mL 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)
3 Results and discussion 3.1 Characterization of MWCNTs-Ag nanocomposites
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 (Agþ) 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 2 (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 2(c) displays the
UVevis absorbance spectra of the Ag-NPs and MWCNTs-Ag colloidal solutions Obviously, the Ag-NPs and MWCNTs-Ag sam-ples display strong absorption peaks at 426 and 431 nm, respec-tively, 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
MWCNTs-Ag 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 com-posite 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 3.2 SERS activity of silver nanoparticles
First, we have investigated the SERS activity of metallic pure Ag-NPs for detection of MB dye.Fig 3(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 cm1,
N.X Dinh et al / Journal of Science: Advanced Materials and Devices 1 (2016) 84e89 85
Trang 3Fig 1 A schematic for the two-step process employed for synthesis of MWCNTs-Ag nanocomposites.
Fig 2 (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 1 (2016) 84e89 86
Trang 41385 cm1and 1621 cm1 The characteristic peaks of MB at around
1621, 1385 and 445 cm1were 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 cm1, 1390 cm1and 1594 cm1were 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 adsorp-tion of MB dyes to surface of metallic Ag-NPs The intensity enhancement of these peaks isfitted with increased MB concen-trations.Fig 4shows the calculated SERS intensity as a function of
MB concentrations The correlation coefficient (R2) obtained the optimal value 0.98 for peak of 450 cm1
Fig 3 (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.
fitting curves of SERS intensity as a function of MB concentrations for three characteristic peaks at 450 cm 1 1 1
N.X Dinh et al / Journal of Science: Advanced Materials and Devices 1 (2016) 84e89 87
Trang 53.3 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
MWCNTs-Agedeposited SERS substrate The prominent peaks appearing at
449 cm1, 1395 cm1and 1621 cm1are 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 werefitted as a function of MB
con-centration as shown inFig 5(b,c,d) Among the three characteristic
peaks, the correlation coefficient (R2) obtained the optimal value
0.99 for peak of 449 cm1 A linear relationship (R2¼ 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
perfor-mance 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
IRaman
Nbulk
Nsurf
where ISERSstands for the intensities of the vibrational mode in the
SERS spectra and IRamanstands for the same vibrational mode in the
normal Raman spectra These data can be directly obtained from the experimental measurements Nbulkand Nsurfaceare the number
of MB molecules illuminated by the laser focus spot under normal Raman and SERS conditions, respectively
Fig 6shows Raman spectra of different SERS substrate materials
atfixed concentrations of MB dye at 10 ppm for comparative pur-poses Here, we calculated ISERSfor pure Ag-NPs and MWCNTs-Ag composites to be 1261 and 3920 counts with the Raman band at
450 cm1, respectively The Nbulkand Nsurfcalculated are 1.6 1012
and 6.68 106, respectively By using equation (1), the enhance-ment factor at the band of 450 cm1 can be calculated to be 1.51 106 for pure Ag-NPs and 4.68 106for the MWCNTs-Ag nanocomposite
Based on the obtained results, it is believed that the SERS enhancement of the MWCNTs-Ag nanocomposite may be attrib-uted 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 (seeFig 2) revealed the aggregation of the Ag-NPs on MWCNTs In our present case, the EM enhancement in MWCNTs-Ag nano-composite 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 5 (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
1 1 1
N.X Dinh et al / Journal of Science: Advanced Materials and Devices 1 (2016) 84e89 88
Trang 6localized surface plasmon resonance (LSPR), coupled with
electro-magneticfield 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 (OHand COOH) 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
4 Conclusions
In this study, we demonstrated the use of MWCNTs-Ag
nano-composites as effective SERS platforms for detection of an organic
dye, i.e., MB dye in aqueous media The MWCNTs-Ag
nano-composite 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 1 ppm With the aforementioned
advantages, the MWCNTs-Ag nanocomposite can be effectively
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 acknowl-edge the Center for Materials Science at Vietnam National Univer-sity in Hanoi for support in Raman measurements
References
[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 deter-mination of erythrosine and other red food colorants using capillary elec-trophoresis with laser-induced fluorescence detection, J Chromatogr A
1141 (2007) 206e211, http://dx.doi.org/10.1016/j.chroma.2006.12.018 [5] L.A Lane, X Qian, S Nie, SERS nanoparticles in medicine: from label-free detection to spectroscopic tagging, Chem Rev 115 (2015) 10489e10529, http://dx.doi.org/10.1021/acs.chemrev.5b00265 In press.
[6] Q Huang, J Wang, W Wei, Q Yan, C Wu, X Zhu, A facile and green method for synthesis of reduced graphene oxide/Ag hybrids as efficient surface enhanced Raman scattering platforms, J Hazard Mater 283 (2015) 123e130 http://dx.doi.org/10.1016/j.jhazmat.2014.09.021
[7] Q An, P Zhang, J.M Li, W.F Ma, J Guo, J Hu, C.C Wang, Silver-coated mag-netiteecarbon coreeshell microspheres as substrate-enhanced SERS probes for detection of trace persistent organic pollutant, Nanoscale 4 (2012) 5210e5216, http://dx.doi.org/10.1039/c2nr31061a
[8] S Dutta, C Ray, S Sarkar, M Pradhan, Y Negishi, T Pal, Silver nanoparticle decorated reduced graphene oxide (rGO) nanosheet: a platform for SERS based low-level detection of uranyl ion, ACS Appl Mater Interfaces 5 (2013) 8724e8732 dx.doi.org/10.1021/am4025017
[9] S.V Kumar, N.M Huang, H.N Lim, M Zainy, I Harrison, C.H Chia, Preparation
of highly water dispersible functional graphene/silver nanocomposite for the detection of melamine, Sensors Actuators B 181 (2013) 885e893 http://dx doi.org/10.1016/j.snb.2013.02.045
[10] N.X Dinh, N.V Quy, T.Q Huy, Anh-Tuan Le, Decoration of silver nanoparticles
on multiwalled carbon nanotubes: antibacterial mechanism and ultrastruc-tural analysis, J Nanomater 2015 (2015) Article ID 814379, 11 pages, http:// dx.doi.org/10.1155/2015/814379
[11] G.N Xiao, S.Q Man, Surface-enhanced Raman scattering of methylene blue adsorbed on cap-shaped silver nanoparticles, Chem Phys Lett 447 (2007) 305e309, http://dx.doi.org/10.1016/j.cplett.2007.09.045
[12] W.L Fu, S.J Zhen, C.Z Huang, One-pot green synthesis of graphene oxide/gold nanocomposites as SERS substrates for malachite green detection, Analyst 138 (2013) 3075e3081, http://dx.doi.org/10.1039/C3AN00018D
Fig 6 Raman spectra of different SERS substrate materials at fixed concentration of
MB dye at 10 ppm.
N.X Dinh et al / Journal of Science: Advanced Materials and Devices 1 (2016) 84e89 89