Facial Synthesis of Carrageenan/Reduced Graphene Oxide/Ag Composite as Efficient SERS Platform

Abstract

In this paper, we reported the preparation of carrageenan/reduced graphene oxide/Ag composite (CA-RGO-Ag) by a wet chemical method at room temperature using carrageenan, graphene oxide and silver nitrate as starting materials. As-prepared composite was characterized by UV-vis spectroscopy, Raman spectroscopy, FTIR, SEM, EDX and XRD. Results showed that the reduction of graphene oxide (GO) and silver nitrate was achieved simultaneously by addition of NaBH₄. Surface-enhanced Raman scattering study showed that the obtained composite give an intensive and enhanced Raman scattering when Rhodamine B was used as a probing molecule.

Keywords
Composite; Ag NPs; SERS; Graphene; Carrageenan; Adenine

1. Introduction

Surface enhanced Raman scattering (SERS) is a powerful analytical tool that can provide ultrasensitive detection of chemical and biological molecules down to the single molecular level, which has been used in various fields of chemistry, material science and biophysics¹. There are two SERS mechanisms that are generally accepted: electromagnetic mechanism (EM) and chemical mechanism (CM)^2-4. EM is based on the enhancement of the local electromagnetic field while the CM is based on charge transfer between absorbed molecules and substrate surface. Up to now, many surfaces have been successfully investigated as SERS active substrates such as Ag, Au, Cu, Na, Li, Pd, Pt, the best performances for detection purposes have been recorded in the case of Ag and Au nanoparticles^5-16.

Graphene, a two-dimensional monolayer sheet of hexagonal carbon atoms, has attracted great attention because of its unique structure and exceptional physiochemical properties^17-20. It has previously been reported to play a role as a Raman enhancement substrate because target molecules can easily form a unified orientation on it due to the flatness of the graphene surface^21,22. Moreover, the interconnected sp² network of graphene gives it the ability to undergo π−π stacking with aromatic molecules^23,24. However, most of the graphene used for preparing SERS substrate are in its reduced form from graphene oxide (GO) prepared by the oxidation of graphite. The reduced graphene oxide (RGO) is prone to irreversible spontaneous agglomeration, which highly limits its applications and performances^25,26. Therefore, surface functionalization is considered as an effective way to solve this problem^27-31. In this report, we prepared an enhanced sensitivity for SERS detection using carrageenan functionalized RGO-Ag (CA-RGO-Ag) composite. The prepared composite was characterized by a series of techniques and its SERS property was evaluated by the detection of Rhodamine B (RhB) and adenine.

2. Experimental

2.1 Materials

Synthetic graphite (average particle diameter <20 µm), Rhodamine B (RhB), adenine, silver nitrate (AgNO₃), k-carrageenan and sodium borohydrid (NaBH₄) were purchased from Sigma-Aldrich. All other chemicals used were analytical grade reagents without further purification. Milli-Q water (18.2 MΩ cm) was used throughout the experiments.

2.2 Synthesis of CG/RGO/Ag composite

Graphene oxide was prepared using modified Hummer's method^32,33. In a typical procedure, 125 ml of concentrated sulfuric acid was taken into a flask filled with graphite powder (5 g) followed by the addition of KMnO₄ (17.5 g) slowly at 0 °C. The mixture was stirred for 3 h at 35 °C and then diluted by water at 0 °C. After that, H₂O₂ (30 vol.% in water) was added into mixture until the bubbling of the gas was completed. The graphene oxide (GO) powder was collected by centrifugation of the solution and subsequently dried under vacuum at 80°C for 24 h.

To synthesize CG/RGO/Ag composite, 0.5 mg carrageenan was dissolved into 50 mL water by 30 min sonication to form a brown solution. 10 mL GO dispersion (1 mg/mL) was added into carrageenan solution for 1 h stirring. After that, a certain amount of AgNO₃ solution (50 mM) was added into the mixture for another hour stirring. Then, 1 mL NaBH₄ solution (0.5 M) was added to the above mixture dropwise. The color of the mixture turned from brown to black. After 30 min stirring, the composite was centrifuged, washed and dried at 80°C for 24 h. (denoted as CG/RGO/Ag-1, CG/RGO/Ag-2, CG/RGO/Ag-3, CG/RGO/Ag-4, CG/RGO/Ag-5 and CG/RGO/Ag-6 for the weight ratios of GO and AgNO₃ set as 1:5, 1:10, 1:20, 1:30, 1:40 and 1:50, respectively). CG/RGO and RGO/Ag sample also synthesized using a similar method except addition of AgNO₃ and carrageenan, respectively.

2.3 Characterizations

Surface morphology of samples were analyzed by scanning electron microscope (SEM, S-4700, HITACHI). FTIR spectra were obtained using a Nicolet 8700 FTIR spectrometer (Thermo Scientific Instrument). X-ray diffraction patterns were collected from 10° to 60° in 2θ by a XRD with Cu Kα radiation (D8-Advanced, Bruker). Raman analysis was carried out at room temperature using a Raman spectroscope (Renishaw InVia, UK) with a 514 nm laser light. The optical characterizations were obtained by UV-vis spectrophotometer in the wavelength range from 200 to 500 nm.

2.4. SERS study

For SERS study, 20 µL of CA-RGO-Ag dispersion was dropped onto the glass substrate and dried at room temperature. Then, 20 µL of 1.0 × 10^-4 M ethanol solution was dropped onto the CA-RGO-Ag films and dried at room temperature, and SERS spectra were recorded, respectively.

3. Results and Discussion

UV-vis spectroscopy was used for confirming the formation of Ag nanoparticles (NPs) and reduction of GO. Figure 1A displays the UV-vis spectra of GO and CA-RGO-Ag composite. It can be seen that the spectrum of GO exhibits two characteristic absorption peaks at 227 nm and 315 nm, corresponding to π-π* transitions of aromatic C-C bonds and n-π* transitions of C=O bonds, respectively³⁴. However, the peak at 227 nm of GO dispersion gradually red-shifts to 262 nm and the shoulder absorption peak at 316 nm disappear in the spectrum of CA-RGO-Ag-6, which indicates the GO has been reduced by NaBH₄. Furthermore, a new broad absorption peak at 425 nm is observed in the spectrum of CA-RGO-Ag-6, which could assign to the surface plasmon resonance absorption band of Ag nanoparticles, suggesting the successful formation of Ag NPs.

Figure 1B displays the FTIR spectra of GO and CA-RGO-Ag-6. As expected, the spectrum of GO exhibits signals at 3432, 1638, 1156 and 1038 cm^-1 corresponding to the -OH vibration stretching, carboxyl C=O, epoxy C-O and alkoxy C-O, respectively³⁵. It can be seen that these peaks show a relatively lower intensity or even vanished in the spectrum of CA-RGO-Ag-6, further confirm the reduction of GO. Moreover, the spectrum of CA-RGO-Ag-6 also shows peaks at 1225, 915 and 852 cm^-1, which could be assigned as the characteristic peaks of carrageenan, suggesting the successful surface functionalization of carrageenan^36,37.

Figure 1
(A) UV-vis and (B) FTIR spectra of GO and CA-RGO-Ag-6.

Raman spectroscopy was also used for determining the structural changes of the as-prepared samples. The Raman spectra of graphite, GO and CA-RGO-Ag-6 are shown in Figure 2. As expected, a prominent peak at 1580 cm^-1 is observed in graphite sample, which corresponds to the first-order scattering of the E_2g. The spectrum of GO displays two characteristic main peaks for D band at 1330 cm^-1 and G bands at 1585 cm^-1. The G band is attributed to all sp² carbon forms and provides information on the in-plane vibration of sp² bonded carbon atoms while the D band suggests the presence of sp³ defects³⁸. The intensity ratio between D band and G band (I_D/I_G) increases from 0.91 in GO to 1.1 in CA-RGO-Ag-6 is due to the decrease of average size of sp² domains by reduction of GO³⁹.

Figure 2
Raman spectra of graphite, GO and CA-RGO-Ag-6.

The morphology of as-prepared samples were examined by SEM. Figure 3A-H shows the top-view SEM images of RGO-Ag, CA-Ag, CA-RGO-Ag-1, CA-RGO-Ag-2, CA-RGO-Ag-3, CA-RGO-Ag-4, CA-RGO-Ag-5 and CA-RGO-Ag-6. It can be observed that the RGO-Ag (Figure 3A) exhibits a completely different morphology compared with carrageenan involved samples. The Ag NPs show an aggregated cluster form and touch on the RGO sheet surface. Compared with CA-RGO-Ag samples, the Ag NPs formed in the CA-Ag sample (Figure 3B) exhibits a much larger size. For CA-RGO-Ag samples, the RGO sheets are embedded into carrageenan gel. On the surface, we could observe a uniform distribution of single Ag nanoparticles when a higher AgNO₃ content was introduced during the preparation. The average size of the Ag NPs can be calculated to be 22 nm (for CA-RGO-Ag-5) based on 100 individual Ag NP size measurements. EDX analysis has been used to obtain elements information about CA-RGO-Ag-6 (Figure 4). The spectrum presents the only existence of C, O (belong to RGO), S (belongs to carrageenan) and Ag (belongs to Ag NPs), indicating the successful formation of composite with high purity.

Figure 3
SEM images of (A) RGO-Ag, (B) CA-Ag, (C) CA-RGO-Ag-1, (D) CA-RGO-Ag-2, (E) CA-RGO-Ag-3, (F) CA-RGO-Ag-4, (G) CA-RGO-Ag-5 and (H) CA-RGO-Ag-6.

Figure 4
EDX spectrum of CA-RGO-Ag-6.

The powder X-ray diffraction of the samples are shown in Figure 5. It is clear that the XRD pattern has peaks at 25.29°, which correspond to (002) crystal planes of graphite, indicating the presence of layered RGO sheets structure. Other diffraction peaks at 30.1°, 34.4°, 48.2°, 57.1°, 69.9° and 76.5° correspond to the (111), (200), (220), (311), (400) and (222) lattice planes of the Ag NPs, respectively.

Figure 5
XRD pattern of CA/RGO/Ag-6.

RhB was chosen as the model molecule to examine the performance of CA-RGO-Ag SERS substrates. Figure 6 shows a set of the SERS spectra of 1×10^-4 RhB on glass slide, RGO-Ag and CA-RGO-Ag-3 substrates. It can be found that the 10^-4 M RhB molecules alone (a) almost have no Raman signals. However, the spectra collected from RGO-Ag and CA-RGO-Ag-3 substrates all show distinct peaks at 624 (C-C-C ring in plane bend), 760 (C-H out of plane bend), 829 (C-C), 938 (C-H stretch), 1076 (β (CH)), 1191 (C-H in plane bend), 1277, 1362, 1502 (Arom C-H stretch), 1567, 1650 (Arom C-C stretch) and 1596 cm^-1(C-H stretch)^40-42. Compared SERS signals on RGO-Ag (b) with the ones on CA-RGO-Ag-2 (c), it can be found the SERS signal on CA-RGO-Ag-3 is stronger than the one on RGO-Ag, which could attract to both electromagnetic and chemical factors. As observed in the SEM characterizations, the introduction of carrageenan effectively prevents the formation of large cluster Ag NPs on the RGO sheets. The uniform distributed small Ag NPs play the role of electromagnetic hot spots that have intense local electromagnetic fields, resulting in highly efficient Raman scattering⁴³. On the other hand, the carrageenan surface functionalization could prevent the aggregation of RGO sheets, which could adsorb and concentrate the target molecules and magnify the SERS signal⁴⁴.

Figure 6
SERS spectra of (a) 10^-4 M RhB molecules on glass reference surface, (b) 10^-4 M RhB adsorbed on the RGO-Ag and (c) CA-RGO-Ag-3 substrates.

In order to optimize the performance, the SERS spectra were collected from the CA-RGO-Ag substrate with different composition. Figure 7 shows a set of the SERS spectra of 1×10^-4 RhB on CA-RGO-Ag-1, CA-RGO-Ag-2, CA-RGO-Ag-3, CA-RGO-Ag-4, CA-RGO-Ag-6 and CA-RGO-Ag-5. From all the spectra, the SERS spectrum collected from CA-RGO-Ag-5 shows the highest enhancement performance, which could ascribe to the uniformity of Ag NPs distribution and optimized Ag NPs size adjusted by the interaction between Ag⁺ and GO during the preparation process. Therefore, the weight ratio of GO and AgNO₃ for fabricating CA-RGO-Ag SERS substrate was set as 1: 40.

Figure 7
SERS spectra of 10^-4 M RhB molecules on (a) CA-RGO-Ag-1, (b) CA-RGO-Ag-2, (c) CA-RGO-Ag-3. (d) CA-RGO-Ag-4, (e) CA-RGO-Ag-6 and (f) CA-RGO-Ag-5 substrates.

The practical application of CA-RGO-Ag SERS substrate was tested by detection of adenine, a kind of nucleotide based for DNA sequencing. The SERS spectra of adenine (1 × 10^-4 M) on the RGO-Ag and CA-RGO-Ag-5 are shown in Figure 8. It can be found that four characteristic peaks are pronounced at 730, 1332, 1444 and 1587 cm^-1, which are corresponded to the purine ring breathing mode, the C-N stretching mode, C-N in plane stretching mode and sciss NH₂, respectively^45,46. Moreover, three peaks at 655, 687 and 1054 cm^-1 were also observed when using CA-RGO-Ag-5 as SERS substrate, indicating the proposed substrate exhibits an excellent SERS activity towards detection of adenine.

Figure 8
SERS spectra of 10^-4 M adenine on (a) RGO-Ag and (b) CA-RGO-Ag-5 substrate.

4. Conclusion

In conclusion, CA-RGO-Ag composite has been prepared via a facial wet chemical route using NaBH₄ as reducing agent. The as-prepared composite exhibits strong plasmonic resonances and can be used as a SERS substrate. The SERS activity of the CA-RGO-Ag substrate was optimized by varying the weight ratio of AgNO₃ and GO in the preparation process. The improved activity can be attributed to the uniform distributed small Ag NPs, which provides intense local electromagnetic fields, resulting in highly efficient Raman scattering. On the other hand, the carrageenan functionalized RGO sheets adsorb and concentrate the target molecules and magnify the SERS signal.

5. References

• ¹
  Tripp RA, Dluhy RA, Zhao Y. Novel nanostructures for SERS biosensing. Nanotoday 2008;3(3-4):31-37.
• ²
  Lombardi JR, Birke RL. A Unified View of Surface-Enhanced Raman Scattering. Accounts of Chemical Research 2009;42(6):734-742.
• ³
  Schwartzberg AM, Grant CD, Wolcott A, Talley CE, Huser TR, Bogomolni R, et al. Unique Gold Nanoparticle Aggregates as a Highly Active Surface-Enhanced Raman Scattering Substrate. The Journal of Physical Chemistry B 2004;108(50):19191-19197.
• ⁴
  Yoon I, Kang T, Choi W, Kim J, Yoo Y, Joo SW, et al. Single Nanowire on a Film as an Efficient SERS-Active Platform. Journal of the American Chemical Society 2008;131(2):758-762.
• ⁵
  Stiufiuc R, Iacovita C, Lucaciu C, Stiufiuc G, Dutu A, Braescu C, et al. SERS-active silver colloids prepared by reduction of silver nitrate with short-chain polyethylene glycol. Nanoscale Research Letters 2013;8:47.
• ⁶
  Otsuka H, Nagasaki Y, Kataoka K. PEGylated nanoparticles for biological and pharmaceutical applications. Advanced Drug Delivery Reviews 2003;55(3):403-419.
• ⁷
  Hameed A, Khan I, Azam A, Naz SS, Khan A, Saleem M, et al. Stability and enzyme inhibition activities of Au nanoparticles using an aqueous extract of clove as a reducing and stabilizing agent. Journal of the Chemical Society of Pakistan 2014;36(3):542-547.
• ⁸
  Piergies N, Kim Y, Proniewicz E. Potential-dependant SERS interaction of ortho-substituted N-benzylamino(boronphenyl)methylphosphonic acid with Ag, Au, and Cu electrode surfaces. Vibrational Spectroscopy 2016;83:94-100.
• ⁹
  Guo M, Zhao Y, Zhang F, Xu L, Yang H, Song X, et al. Reduced graphene oxide-stabilized copper nanocrystals with enhanced catalytic activity and SERS properties. RSC Advances 2016;6(56):50587-50594.
• ¹⁰
  Li R, Yang G, Yang J, Han J, Liu J, Huang M. Determination of melamine in milk using surface plasma effect of aggregated Au@SiO₂ nanoparticles by SERS technique. Food Control 2016;68:14-19.
• ¹¹
  Guo TL, Li JG, Sun X, Sakka Y. Improved galvanic replacement growth of Ag microstructures on Cu micro-grid for enhanced SERS detection of organic molecules. Materials Science and Engineering: C 2016;61:97-104.
• ¹²
  Kang T, Jeong S, Kang H, Kim J, Kim HM, Kyeong S, et al. Fabrication of Ag nanoaggregates/SiO₂ yolk-shell nanoprobes for surface-enhanced Raman scattering. Journal of Industrial & Engineering Chemistry 2015;32:34-38.
• ¹³
  Šubr M, Petr M, Peksa V, Kylián O, Hanuš J, Procházka M. Ag Nanorod Arrays for SERS: Aspects of Spectral Reproducibility, Surface Contamination, and Spectral Sensitivity. Journal of Nanomaterials 2015;2015:729231.
• ¹⁴
  Cui L, Chen S, Zhang K. Effect of toxicity of Ag nanoparticles on SERS spectral variance of bacteria. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 2015;137:1061-1066.
• ¹⁵
  Khlebtsov BN, Liu Z, Ye J, Khlebtsov NG. Au@Ag core/shell cuboids and dumbbells: Optical properties and SERS response. Journal of Quantitative Spectroscopy & Radiative Transfer 2015;167:64-75.
• ¹⁶
  Lai W, Zhou J, Liu Y, Jia Z, Xie S, Petti L, et al. 4MBA-labeled Ag-nanorod aggregates coated with SiO₂: synthesis, SERS activity, and biosensing applications. Analytical Methods 2015;7(20):8832-8838.
• ¹⁷
  Xu Y, Liu Z, Zhang X, Wang Y, Tian J, Huang Y, et al. A Graphene Hybrid Material Covalently Functionalized with Porphyrin: Synthesis and Optical Limiting Property. Advanced Materials 2009;21(12):1275-1279.
• ¹⁸
  Wang D, Choi D, Li J, Yang Z, Nie Z, Kou R, et al. Self-Assembled TiO₂-Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion. ACS Nano 2009;3(4):907-914.
• ¹⁹
  Yoo E, Kim J, Hosono E, Zhou H, Kudo T, Honma I. Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries. Nano Letters 2008;8(8):2277-2282.
• ²⁰
  Ma XY, Chen MF, Chao MY. Voltammetric Determination of Sudan II in Food Samples at Graphene Modified Glassy Carbon Electrode Based on the Enhancement Effect of Sodium Dodecyl Sulfate. Journal of the Chemical Society of Pakistan 2013;35(1):30-36.
• ²¹
  Ling X, Xie L, Fang Y, Xu H, Zhang H, Kong J, et al. Can Graphene be used as a Substrate for Raman Enhancement? Nano Letters 2009;10(2):553-561.
• ²²
  Jung N, Crowther AC, Kim N, Kim P, Brus L. Raman Enhancement on Graphene: Adsorbed and Intercalated Molecular Species. ACS Nano 2010;4(11):7005-7013.
• ²³
  Wojcik A, Kamat PV. Reduced Graphene Oxide and Porphyrin. An Interactive Affair in 2-D. ACS Nano 2010;4(11):6697-6706.
• ²⁴
  Emery JD, Wang QH, Zarrouati M, Fenter P, Hersam MC, Bedzyk MJ. Structural analysis of PTCDA monolayers on epitaxial graphene with ultra-high vacuum scanning tunneling microscopy and high-resolution X-ray reflectivity. Surface Science 2011;605(17-18):1685-1693.
• ²⁵
  Wang G, Shen X, Wang B, Yao J, Park J. Synthesis and characterisation of hydrophilic and organophilic graphene nanosheets. Carbon 2009;47(5):1359-1364.
• ²⁶
  Kochnev AS, Ovid'ko IA, Semenov BN. Tensile strength of graphene containing 5-8-5 defects. Reviews on Advanced Materials Science 2014;37(1-2):105-110.
• ²⁷
  Liu H, Cheng J, Chen F, Hou F, Bai D, Xi P, et al. Biomimetic and cell-mediated mineralization of hydroxyapatite by carrageenan functionalized graphene oxide. ACS Applied Materials & Interfaces 2014;6(5):3132-3140.
• ²⁸
  Liu S, Bao H, Li L. Thermoreversible gelation and scaling laws for graphene oxide-filled ?-carrageenan hydrogels. European Polymer Journal 2016;79:150-162.
• ²⁹
  Wang J, Ouyang Z, Ren Z, Li J, Zhang P, Wei G, et al. Self-assembled peptide nanofibers on graphene oxide as a novel nanohybrid for biomimetic mineralization of hydroxyapatite. Carbon 2015;89:20-30.
• ³⁰
  Zhu D, Nakamura H, Zhu H, Xu C, Matsuo M. Microfibers from interpolymer complexation of ?-carrageenan and oligomers of polyaniline for glucose detection. Synthetic Metals 2015;202:133-139.
• ³¹
  Song F, Li X, Wang Q, Liao L, Zhang C. Nanocomposite Hydrogels and Their Applications in Drug Delivery and Tissue Engineering. Journal of Biomedical Nanotechnology 2015;11(1):40-52.
• ³²
  Park S, An J, Piner RD, Jung I, Yang D, Velamakanni A, et al. Aqueous Suspension and Characterization of Chemically Modified Graphene Sheets. Chemistry of Materials 2008;20(21):6592-6594.
• ³³
  Hummers WS Jr., Offeman RE. Preparation of Graphitic Oxide. Journal of the American Chemical Society 1958;80(6):1339.
• ³⁴
  Paredes JI, Villar-Rodil S, Marti´nez-Alonso A, Tasco´n JMD. Graphene Oxide Dispersions in Organic Solvents. Langmuir 2008;24(19):10560-10564.
• ³⁵
  Liu Z, Robinson JT, Sun X, Dai H. PEGylated Nanographene Oxide for Delivery of Water-Insoluble Cancer Drugs. Journal of the American Chemical Society 2008;130(33):10876-10877.
• ³⁶
  Zuo PP, Zhang YL, Feng HF, Xia W, Zhang WQ, Wang MZ. Fabrication and Properties of Graphene Oxide-reinforced Carrageenan Film. Chemical Journal of Chinese Universities 2013;34(3):692-697.
• ³⁷
  Sun YJ, Wang CJ, Geng TF, Wang ZF, Huang LJ. Analysis of ?-Carrageenan Oligosaccharides by LC-ESI-MS/MSⁿ with 3-Amino-9-ethylcarbazole Derivatization. Acta Chimica Sinica 2011;69(14):1697-1704.
• ³⁸
  Yang D, Velamakanni A, Bozoklu G, Park S, Stoller M, Piner RD, et al. Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 2009;47(1):145-152.
• ³⁹
  Li X, Wang Q, Zhao Y, Wu W, Chen J, Meng H. Green synthesis and photo-catalytic performances for ZnO-reduced graphene oxide nanocomposites. Journal of Colloid and Interface Science 2013;411:69-75.
• ⁴⁰
  Jung GB, Kim JH, Burm JS, Park HK. Fabrication of chitosan-silver nanoparticle hybrid 3D porous structure as a SERS substrate for biomedical applications. Applied Surface Science 2013;273:179-183.
• ⁴¹
  Yu X, Cai H, Zhang W, Li X, Pan N, Luo Y, et al. Tuning Chemical Enhancement of SERS by Controlling the Chemical Reduction of Graphene Oxide Nanosheets. ACS Nano 2011;5(2):952-958.
• ⁴²
  Jung GB, Bae YM, Lee YJ, Ryu SH, Park HK. Nanoplasmonic Au nanodot arrays as an SERS substrate for biomedical applications. Applied Surface Science 2013;282:161-164.
• ⁴³
  Inoue M, Ohtaka K. Surface enhanced raman-scattering by metal spheres. 1. Cluster effect. Journal of the Physical Society of Japan 1983;52(11):3853-3864.
• ⁴⁴
  Ren W, Fang Y, Wang E. A Binary Functional Substrate for Enrichment and Ultrasensitive SERS Spectroscopic Detection of Folic Acid Using Graphene Oxide/Ag Nanoparticle Hybrids. ACS Nano 2011;5(8):6425-6433.
• ⁴⁵
  Zhao H, Fu H, Zhao T, Wang L, Tan T. Fabrication of small-sized silver NPs/graphene sheets for high-quality surface-enhanced Raman scattering. Journal of Colloid and Interface Science 2012;375(1):30-34.
• ⁴⁶
  Guo S, Li J, Ren W, Wen D, Dong S, Wang E. Carbon Nanotube/Silica Coaxial Nanocable as a Three-Dimensional Support for Loading Diverse Ultra-High-Density Metal Nanostructures: Facile Preparation and Use as Enhanced Materials for Electrochemical Devices and SERS. Chemistry of Materials 2009;21(11):2247-2257.

Publication Dates

• Publication in this collection
  16 Nov 2016
• Date of issue
  Jan-Feb 2017

History

• Received
  08 Apr 2016
• Reviewed
  17 Sept 2016
• Accepted
  20 Oct 2016