||Silver Hollow Nanosphere Arrays Surface-modified with Dielectric Nanoparticles to Improve the Effect of SERS and Apply for Ampicillin Detection
||Department of Materials Science and Engineering
The detection of residual antibiotics is of great importance nowadays. Surface enhanced Raman scattering (SERS) provides a high chance in detecting trace molecules, so this technique gains more attention. The use of silver hollow nanosphere (HNS) arrays decorated with various dielectric nanoparticles (HfO2, TiO2, and Al2O3) using self-assembled monolayer polystyrene (PS) nanospheres as the sacrificial template is an attempt to demonstrate their potential as SERS-active substrates. Silver HNS and dielectric nanoparticles (NP) were deposited by E-beam evaporation. This fabrication method has the advantages of simplicity, large scale production, and easy size adjustment. Scanning electron microscopy (SEM) images show that the hybrid structures are hexagonally arranged, and Atomic force microscopy (AFM) shows that the surface morphology and the thickness of the deposited silver films are uniform. The dielectric NP modified silver HNS system exhibits superior Raman scattering enhancements than silver HNS alone due to the local surface plasmon resonance (LSPR) effect which originated from the silver HNSs and metal/semiconductor interface. The SERS enhancement with various dielectric materials was thoroughly compared and explained in experimental results. SERS application was verified using Rhodamine 6G (R6G) as a probe molecule, and the fabricated substrate was proven to be an effective SERS template for Raman signal detection. Al2O3 nanoparticles decorated on silver HNS exhibited the optimized enhancement factor (EF) value, which was found to be 6.2 x 107. Finally, the SERS-active substrate was used for Raman detection of Ampicillin, and the lowest concentration it can detect was 0.01 ppm, meeting the minimum legal standards, and showing that the SERS-active substrate is a promising tool for trace detection of antibiotics.
Chapter 1 Introduction 1
1.1 Introduction 1
1.2 Motivation 5
1.3 Objective 5
Chapter 2 Literature survey and Theoretical basis 7
2.1 Vibrational spectroscopy and Raman spectroscopy theory 7
2.1.1 Vibrational spectroscopy 7
2.1.2 Raman spectroscopy theory 10
2.2 Surface enhanced Raman scattering spectroscopy 12
2.2.1 The development of SERS spectroscopy 12
2.2.2 SERS mechanism with the surface of nanostructures 12
2.2.3 Surface plasmon 14
2.2.4 Electromagnetic effect 16
2.2.5 Chemical effect 18
2.3 SERS-active substrate 20
2.3.1 Metal/semiconductor plasmonic system 20
2.3.2 Hollow nanosphere structure 26
2.4 SERS technique in antibiotics detection 27
2.5 Summary 30
Chapter 3 Materials and methods 31
3.1 Experimental materials and methods 32
3.1.1 Substrate cleaning 32
3.1.2 Solution preparation 32
3.1.3 Substrate fabrication 34
3.2 Raman detection 36
3.2.1 Raman detection methods 36
3.2.2 Raman spectroscopy correction 37
3.2.3 Signal processing 39
3.2.4 Enhancement factor evaluation and calculation 39
3.3 Process equipment 40
3.3.1 E-beam evaporator 40
3.4 Analytical Instruments 41
3.4.1 Scanning electron microscopy 41
3.4.2 Atomic Force Microscopy 43
3.4.3 Raman spectrometer 44
Chapter 4 SERS-active substrate characteristics analysis 47
4.1 PS monolayer assisted-template analysis 47
4.2 Ag HNS substrate analysis 49
4.2.1 Surface morphology 49
4.2.2 Internal structure analysis 51
4.2.3 Raman spectra analysis and excitation laser wavelength optimization 52
4.3 SERS-active substrate analysis 54
4.3.1 SERS-active substrate Ag/Hf analysis 55
4.3.2 SERS-active substrate Ag/Ti analysis 55
4.3.3 SERS-active substrate Ag/Al analysis 56
4.4 The evaluation of SERS-active substrate enhancement 57
4.4.1 SERS spectra analysis 57
4.4.2 SERS spectra enhancement factor evaluation 58
4.4.3 SERS mechanism discussion 60
4.5 Summary 62
Chapter 5 SERS-active substrate in application of residue antibiotics detection 64
5.1 Detection limits of SERS-active substrates on Ampicillin 64
5.1.1 Ampicillin 64
5.1.2 Raman spectrum analysis of various concentrations on Ag/Hf water sample 65
5.1.3 Raman spectrum analysis of various concentrations on Ag/Al water sample 66
5.1.4 Raman spectrum analysis of various concentrations on Ag/Ti water sample 68
5.1.5 The evaluation of SERS-active substrate enhancement 71
5.2 Simulation of SERS-active substrates in milk detection 72
5.3 Summary 75
 Y. Luo and Q. Zhou, "Antibiotic resistance genes (ARGs) as emerging pollutants," Huanjing Kexue Xuebao / Acta Scientiae Circumstantiae, vol. 28, pp. 1499-1505, 2008.
 S.-C. Chang, M.-W. Chen, M.-C. Lin, and O. Y.-P. Hu, "Antibiotic Consumption in Human and Animals in Taiwan," Infection Control Journal, vol. 13, pp. 334-345, 2003.
 W. M. Linam and M. A. Gerber, "Changing Epidemiology and Prevention of Salmonella Infections," The Pediatric Infectious Disease Journal, vol. 26, pp. 747-748, 2007.
 A. H. Holmes, L. S. P. Moore, A. Sundsfjord, M. Steinbakk, S. Regmi, A. Karkey, et al., "Understanding the mechanisms and drivers of antimicrobial resistance," The Lancet, vol. 387, pp. 176-187.
 L. A. Lyon, C. D. Keating, A. P. Fox, B. E. Baker, L. He, S. R. Nicewarner, et al., "Raman Spectroscopy," Analytical Chemistry, vol. 70, pp. 341-362, 1998/06/01 1998.
 M.-D. Li, Y. Cui, M.-X. Gao, J. Luo, B. Ren, and Z.-Q. Tian, "Clean Substrates Prepared by Chemical Adsorption of Iodide Followed by Electrochemical Oxidation for Surface-Enhanced Raman Spectroscopic Study of Cell Membrane," Analytical Chemistry, vol. 80, pp. 5118-5125, 2008/07/01 2008.
 M. J. Banholzer, J. E. Millstone, L. Qin, and C. A. Mirkin, "Rationally designed nanostructures for surface-enhanced Raman spectroscopy," Chemical Society Reviews, vol. 37, pp. 885-897, 2008.
 A. P. Craig, A. S. Franca, and J. Irudayaraj, "Surface-Enhanced Raman Spectroscopy Applied to Food Safety," in Annual Review of Food Science and Technology, Vol 4. vol. 4, M. P. Doyle and T. R. Klaenhammer, Eds., ed, 2013, pp. 369-380.
 K. Y. Hong, C. D. L. de Albuquerque, R. J. Poppi, and A. G. Brolo, "Determination of aqueous antibiotic solutions using SERS nanogratings," Analytica Chimica Acta, vol. 982, pp. 148-155, 2017/08/22/ 2017.
 K. C. Schuster, E. Urlaub, and J. R. Gapes, "Single-cell analysis of bacteria by Raman microscopy: spectral information on the chemical composition of cells and on the heterogeneity in a culture," Journal of Microbiological Methods, vol. 42, pp. 29-38, 2000/09/01/ 2000.
 K. Katrin, K. Harald, I. Irving, R. D. Ramachandra, and S. F. Michael, "Surface-enhanced Raman scattering and biophysics," Journal of Physics: Condensed Matter, vol. 14, p. R597, 2002.
 M. Harz, P. Rosch, K. D. Peschke, O. Ronneberger, H. Burkhardt, and J. Popp, "Micro-Raman spectroscopic identification of bacterial cells of the genus Staphylococcus and dependence on their cultivation conditions," Analyst, vol. 130, pp. 1543-1550, 2005.
 M. Fleischmann, P. J. Hendra, and A. J. McQuillan, "Raman spectra of pyridine adsorbed at a silver electrode," Chemical Physics Letters, vol. 26, pp. 163-166, 1974/05/15/ 1974.
 S. L. Kleinman, R. R. Frontiera, A.-I. Henry, J. A. Dieringer, and R. P. Van Duyne, "Creating, characterizing, and controlling chemistry with SERS hot spots," Physical Chemistry Chemical Physics, vol. 15, pp. 21-36, 2013.
 K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, et al., "Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS)," Physical Review Letters, vol. 78, pp. 1667-1670, 03/03/ 1997.
 R. A. Tripp, R. A. Dluhy, and Y. Zhao, "Novel nanostructures for SERS biosensing," Nano Today, vol. 3, pp. 31-37, 2008/06/01/ 2008.
 K. Hering, D. Cialla, K. Ackermann, T. Dörfer, R. Möller, H. Schneidewind, et al., "SERS: a versatile tool in chemical and biochemical diagnostics," Analytical and Bioanalytical Chemistry, vol. 390, pp. 113-124, January 01 2008.
 L. Baia, M. Baia, J. Popp, and S. Astilean, "Gold Films Deposited over Regular Arrays of Polystyrene Nanospheres as Highly Effective SERS Substrates from Visible to NIR," The Journal of Physical Chemistry B, vol. 110, pp. 23982-23986, 2006/11/01 2006.
 S.-C. Luo, K. Sivashanmugan, J.-D. Liao, C.-K. Yao, and H.-C. Peng, "Nanofabricated SERS-active substrates for single-molecule to virus detection in vitro: A review," Biosensors and Bioelectronics, vol. 61, pp. 232-240, 2014/11/15/ 2014.
 S. Pang, T. Yang, and L. He, "Review of surface enhanced Raman spectroscopic (SERS) detection of synthetic chemical pesticides," TrAC Trends in Analytical Chemistry, vol. 85, pp. 73-82, 2016/12/01/ 2016.
 S. Habouti, M. Mátéfi-Tempfli, C. H. Solterbeck, M. Es-Souni, S. Mátéfi-Tempfli, and M. Es-Souni, "On-substrate, self-standing Au-nanorod arrays showing morphology controlled properties," Nano Today, vol. 6, pp. 12-19, 2011.
 J. Yin, Y. Zang, C. Yue, Z. Wu, S. Wu, J. Li, et al., "Ag nanoparticle/ZnO hollow nanosphere arrays: large scale synthesis and surface plasmon resonance effect induced Raman scattering enhancement," Journal of Materials Chemistry, vol. 22, p. 7902, 2012.
 S. Wu, Z. W. Chen, T. Wang, and X. H. Ji, "A facile approach for the fabrication of Au/ZnO-hollow-sphere-monolayer thin films and their photocatalytic properties," Applied Surface Science, vol. 412, pp. 69-76, Aug 2017.
 X.-L. Li, T.-J. Lou, X.-M. Sun, and Y.-D. Li, "Highly Sensitive WO3 Hollow-Sphere Gas Sensors," Inorganic Chemistry, vol. 43, pp. 5442-5449, 2004/08/01 2004.
 J. Chen, F. Saeki, B. J. Wiley, H. Cang, M. J. Cobb, Z.-Y. Li, et al., "Gold Nanocages: Bioconjugation and Their Potential Use as Optical Imaging Contrast Agents," Nano Letters, vol. 5, pp. 473-477, 2005/03/01 2005.
 I. White, H. Oveys, and X. Fan, "Increasing the enhancement of SERS with dielectric microsphere resonators," SPECTROSCOPY-SPRINGFIELD THEN EUGENE THEN DULUTH-, vol. 21, p. 36, 2006.
 A. Musumeci, D. Gosztola, T. Schiller, N. M. Dimitrijevic, V. Mujica, D. Martin, et al., "SERS of Semiconducting Nanoparticles (TiO2 Hybrid Composites)," Journal of the American Chemical Society, vol. 131, pp. 6040-6041, 2009/05/06 2009.
 G. Keresztury, "Raman Spectroscopy: Theory," in Handbook of Vibrational Spectroscopy, ed: John Wiley & Sons, Ltd, 2006.
 T. Waldmann, J. Klein, H. E. Hoster, and R. J. Behm, "Stabilization of Large Adsorbates by Rotational Entropy: A Time-Resolved Variable-Temperature STM Study," ChemPhysChem, vol. 14, pp. 162-169, 2013.
 A. Nawrocka and J. Lamorska, "Determination of Food Quality by Using Spectroscopic Methods," in Advances in Agrophysical Research, S. Grundas and A. Stepniewski, Eds., ed Rijeka: InTech, 2013, p. Ch. 14.
 P. Atkins and J. De Paula, Elements of physical chemistry: Oxford University Press, USA, 2013.
 H. J. Bowley, D. L. Gerrard, J. D. Louden, and G. Turrell, Practical raman spectroscopy: Springer Science & Business Media, 2012.
 R. L. McCreery, "Magnitude of Raman Scattering," in Raman Spectroscopy for Chemical Analysis, ed: John Wiley & Sons, Inc., 2005, pp. 15-33.
 D. L. Jeanmaire and R. P. Van Duyne, "Surface raman spectroelectrochemistry: Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode," Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, vol. 84, pp. 1-20, 1977/11/10/ 1977.
 M. G. Albrecht and J. A. Creighton, "Anomalously intense Raman spectra of pyridine at a silver electrode," Journal of the American Chemical Society, vol. 99, pp. 5215-5217, 1977/06/01 1977.
 M. Moskovits, "Surface roughness and the enhanced intensity of Raman scattering by molecules adsorbed on metals," The Journal of Chemical Physics, vol. 69, pp. 4159-4161, 1978.
 S. Hong and X. Li, "Optimal Size of Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy under Different Conditions," Journal of Nanomaterials, vol. 2013, p. 9, 2013.
 M. Moskovits, "Surface-enhanced spectroscopy," Reviews of modern physics, vol. 57, p. 783, 1985.
 P.-J. Huang, L.-K. Chau, T.-S. Yang, L.-L. Tay, and T.-T. Lin, "Nanoaggregate-Embedded Beads as Novel Raman Labels for Biodetection," Advanced Functional Materials, vol. 19, pp. 242-248, 2009.
 Q. Ye, J. Fang, and L. Sun, "Surface-Enhanced Raman Scattering from Functionalized Self-Assembled Monolayers. 2. Distance Dependence of Enhanced Raman Scattering from an Azobenzene Terminal Group," The Journal of Physical Chemistry B, vol. 101, pp. 8221-8224, 1997/10/01 1997.
 A. Campion, J. E. Ivanecky, C. M. Child, and M. Foster, "On the Mechanism of Chemical Enhancement in Surface-Enhanced Raman Scattering," Journal of the American Chemical Society, vol. 117, pp. 11807-11808, 1995/11/01 1995.
 Y.-C. Liu and R. L. McCreery, "Raman Spectroscopic Determination of the Structure and Orientation of Organic Monolayers Chemisorbed on Carbon Electrode Surfaces," Analytical Chemistry, vol. 69, pp. 2091-2097, 1997/06/01 1997.
 R. J. C. Brown and M. J. T. Milton, "Nanostructures and nanostructured substrates for surface—enhanced Raman scattering (SERS)," Journal of Raman Spectroscopy, vol. 39, pp. 1313-1326, 2008.
 J. R. Lombardi, R. L. Birke, T. Lu, and J. Xu, "Charge‐transfer theory of surface enhanced Raman spectroscopy: Herzberg–Teller contributions," The Journal of Chemical Physics, vol. 84, pp. 4174-4180, 1986.
 A. Otto, "Surface-enhanced Raman scattering: “Classical” and “Chemical” origins," in Light Scattering in Solids IV: Electronics Scattering, Spin Effects, SERS, and Morphic Effects, M. Cardona and G. Güntherodt, Eds., ed Berlin, Heidelberg: Springer Berlin Heidelberg, 1984, pp. 289-418.
 H. Xu and M. Käll, "Estimating SERS Properties of Silver-Particle Aggregates through Generalized Mie Theory," in Surface-Enhanced Raman Scattering: Physics and Applications, K. Kneipp, M. Moskovits, and H. Kneipp, Eds., ed Berlin, Heidelberg: Springer Berlin Heidelberg, 2006, pp. 87-103.
 L. Zeiri, B. V. Bronk, Y. Shabtai, J. Czégé, and S. Efrima, "Silver metal induced surface enhanced Raman of bacteria," Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 208, pp. 357-362, 2002/08/14/ 2002.
 N. P. W. Pieczonka, P. J. G. Goulet, and R. F. Aroca, "Applications of the Enhancement of Resonance Raman Scattering and Fluorescence by Strongly Coupled Metallic Nanostructures," in Surface-Enhanced Raman Scattering: Physics and Applications, K. Kneipp, M. Moskovits, and H. Kneipp, Eds., ed Berlin, Heidelberg: Springer Berlin Heidelberg, 2006, pp. 197-216.
 X.-C. Ma, Y. Dai, L. Yu, and B.-B. Huang, "Energy transfer in plasmonic photocatalytic composites," Light: Science &Amp; Applications, vol. 5, p. e16017, 02/12/online 2016.
 G. Shan, L. Xu, G. Wang, and Y. Liu, "Enhanced Raman Scattering of ZnO Quantum Dots on Silver Colloids," The Journal of Physical Chemistry C, vol. 111, pp. 3290-3293, 2007/03/01 2007.
 A. Manjavacas, J. G. Liu, V. Kulkarni, and P. Nordlander, "Plasmon-Induced Hot Carriers in Metallic Nanoparticles," ACS Nano, vol. 8, pp. 7630-7638, 2014/08/26 2014.
 S. Linic, P. Christopher, and D. B. Ingram, "Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy," Nature Materials, vol. 10, p. 911, 11/23/online 2011.
 N. Dasgupta and A. Dasgupta, Semiconductor devices: modelling and technology: PHI Learning Pvt. Ltd., 2004.
 J. Bardeen, "Surface States and Rectification at a Metal Semi-Conductor Contact," Physical Review, vol. 71, pp. 717-727, 05/15/ 1947.
 S. Kar, "MOSFET: Basics, Characteristics, and Characterization," in High Permittivity Gate Dielectric Materials, S. Kar, Ed., ed Berlin, Heidelberg: Springer Berlin Heidelberg, 2013, pp. 47-152.
 S. K. Cushing, J. Li, F. Meng, T. R. Senty, S. Suri, M. Zhi, et al., "Photocatalytic Activity Enhanced by Plasmonic Resonant Energy Transfer from Metal to Semiconductor," Journal of the American Chemical Society, vol. 134, pp. 15033-15041, 2012/09/12 2012.
 M. Pisarek, A. Roguska, A. Kudelski, M. Andrzejczuk, M. Janik-Czachor, and K. J. Kurzydłowski, "The role of Ag particles deposited on TiO2 or Al2O3 self-organized nanoporous layers in their behavior as SERS-active and biomedical substrates," Materials Chemistry and Physics, vol. 139, pp. 55-65, 2013/04/15/ 2013.
 G. Zhao, H. Kozuka, and T. Yoko, "Sol—gel preparation and photoelectrochemical properties of TiO2 films containing Au and Ag metal particles," Thin Solid Films, vol. 277, pp. 147-154, 1996/05/01/ 1996.
 Y. Ohko, T. Tatsuma, T. Fujii, K. Naoi, C. Niwa, Y. Kubota, et al., "Multicolour photochromism of TiO2 films loaded with silver nanoparticles," Nature Materials, vol. 2, p. 29, 12/15/online 2002.
 C. Clavero, "Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices," Nature Photonics, vol. 8, p. 95, 01/30/online 2014.
 Y. Sun and Y. Xia, "Increased Sensitivity of Surface Plasmon Resonance of Gold Nanoshells Compared to That of Gold Solid Colloids in Response to Environmental Changes," Analytical Chemistry, vol. 74, pp. 5297-5305, 2002/10/01 2002.
 N. Clament Sagaya Selvam, J. J. Vijaya, and L. J. Kennedy, "Effects of Morphology and Zr Doping on Structural, Optical, and Photocatalytic Properties of ZnO Nanostructures," Industrial & Engineering Chemistry Research, vol. 51, pp. 16333-16345, 2012/12/19 2012.
 L. M. Durso and K. L. Cook, "Impacts of antibiotic use in agriculture: what are the benefits and risks?," Current Opinion in Microbiology, vol. 19, pp. 37-44, 6// 2014.
 C. Andreou, R. Mirsafavi, M. Moskovits, and C. D. Meinhart, "Detection of low concentrations of ampicillin in milk," Analyst, vol. 140, pp. 5003-5, Aug 07 2015.
 R. Bogaerts and F. Wolf, "A STANDARDIZED METHOD FOR THE DETECTION OF RESIDUES OF ANTI-BACTERIAL SUBSTANCES IN FRESH MEAT," Fleischwirtschaft, vol. 60, pp. 667-&, 1980.
 R. W. JOHNSTON, R. H. REAMER, E. W. HARRIS, H. G. FUGATE, and B. SCHWAB, "A New Screening Method for the Detection of Antibiotic Residues in Meat and Poultry Tissues," Journal of Food Protection, vol. 44, pp. 828-831, 1981.
 A.-L. Myllyniemi, "Development of microbiological methods for the detection and identification of antimicrobial residues in meat," 2004.
 E. Raviña, The evolution of drug discovery: from traditional medicines to modern drugs: John Wiley & Sons, 2011.
 J. Dolovich, J. Ruhno, D. N. Sauder, S. Ahlstedt, and F. E. Hargreave, "Isolated late cutaneous skin test response to ampicillin: A distinct entity," Journal of Allergy and Clinical Immunology, vol. 82, pp. 676-679, 1988/10/01/ 1988.
 Z. Argov, T. Brenner, and O. Abramsky, "Ampicillin may aggravate clinical and experimental myasthenia gravis," Archives of Neurology, vol. 43, pp. 255-256, 1986.
 C. Andreou, R. Mirsafavi, M. Moskovits, and C. D. Meinhart, "Detection of low concentrations of ampicillin in milk," Analyst, vol. 140, pp. 5003-5005, 2015.
 S. A. Chen and S. T. Lee, "Kinetics and mechanism of emulsifier-free emulsion polymerization: styrene/hydrophilic comonomer (acrylamide) system," Macromolecules, vol. 24, pp. 3340-3351, 1991/05/01 1991.
 M. Arai, K. Arai, and S. Saito, "Polymer particle formation in soapless emulsion polymerization," Journal of Polymer Science: Polymer Chemistry Edition, vol. 17, pp. 3655-3665, 1979.
 C.-F. Liew, "Self-Assembly of Colloids Particle at a Water-Air Interface," Master, Chemical Engineering, National Tsing Hua University, 2005.
 Z. Zihua, Z. Tao, and L. Zhongfan, "Raman scattering enhancement contributed from individual gold nanoparticles and interparticle coupling," Nanotechnology, vol. 15, p. 357, 2004.
 S. K. Gupta, J. Singh, K. Anbalagan, P. Kothari, R. R. Bhatia, P. K. Mishra, et al., "Synthesis, phase to phase deposition and characterization of rutile nanocrystalline titanium dioxide (TiO2) thin films," Applied Surface Science, vol. 264, pp. 737-742, 2013/01/01/ 2013.
 X. Li, Y. Zhang, Z. X. Shen, and H. J. Fan, "Highly Ordered Arrays of Particle-in-Bowl Plasmonic Nanostructures for Surface-Enhanced Raman Scattering," Small, vol. 8, pp. 2548-2554, 2012.
 A. Camposeo, D. Spadaro, D. Magrì, M. Moffa, P. G. Gucciardi, L. Persano, et al., "Surface-enhanced Raman spectroscopy in 3D electrospun nanofiber mats coated with gold nanorods," Analytical and Bioanalytical Chemistry, vol. 408, pp. 1357-1364, February 01 2016.
 E. C. Le Ru, E. Blackie, M. Meyer, and P. G. Etchegoin, "Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study," The Journal of Physical Chemistry C, vol. 111, pp. 13794-13803, 2007/09/01 2007.
 C.-H. Sya, "High Performance Surface Enhanced Raman Scattering Substrate for Trace Detection of Pesticides Residue," Material Science and Engineering, National Cheng Kung University, 2016.
 K. Sivashanmugan, J.-D. Liao, B. H. Liu, and C.-K. Yao, "Focused-ion-beam-fabricated Au nanorods coupled with Ag nanoparticles used as surface-enhanced Raman scattering-active substrate for analyzing trace melamine constituents in solution," Analytica Chimica Acta, vol. 800, pp. 56-64, 2013/10/24/ 2013.