2D-Arrrays of Cu Nanodisks on Anodic Aluminum Oxide (AAO) Template for SERS Applications

VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 110-114 110 2D-arrrays of Cu Nanodisks on Anodic Aluminum Oxide (AAO) Template for SERS Applications Nguyen Thi Yen Mai*, Nguyen Thi Ha Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam Received 26 September 2018 Revised 26 October 2018; Accepted 17 December 2018 Abstract. Copper nanodisks (Cu NDs) of 50 nm in size were prepared on square-inch anodic aluminum oxide substrates by RF-sputtering method. The samples were annealed at 450oC, then the walls of anodic aluminum oxide (AAO) substrates were lift off in a solution of acid phosphoric. The 2D arrays of Cu NDs were fabricated in high quality. Morphology of the substrates was observed by scanning electron microscopy. Surface plasmon resonance absorption was observed with different peaks in the range of 400-1400nm wavelengths that means the substrate is promising for SERS application. The Raman spectrum of Rhodamine 6G loaded on AAO substrates was much enhanced. Keyword: Anodic aluminum oxide (AAO) tempelate, Cu nanodisk, SERS 1. Introduction In recent decades, chemical analysis of food achieves continuous advances with significant improvements in automation, sensitivity and accuracy. Two widely used analytical instruments are gas chromatography (GC) and high-performance liquid chromatography (HPLC). They are often combined with UV-visible spectrophotometry, nuclear magnetic resonance and mass spectrometry to identify the molecular characteristics of specific peaks [1]. The disadvantages of those method are time-consuming, the need to be analyzed in the laboratory. It is essential to develop simple, portable, rapid and sensitive methods for food analysis, especially for food safety. Infrared and Raman spectroscopy have been developed since the early 20th century. They are rapid and nondestructive tools to identify raw materials and for quality inspection [2]. The conventional Raman spectroscopy has been used to characterize food components [3]. The surface-enhanced Raman ________ Corresponding author. Tel.: 84-943439086. Email:mainty85@gmail.com https//doi.org/ 10.25073/2588-1124/vnumap.4309 N.T.Y. Mai, N.T. Ha / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 110-114 111 scattering (SERS) was discovered in 1974 [4]: the weak Raman scattering signals are greatly enhanced using noble metal nanostructures. Two proposed explanation for the enhancement mechanism is electromagnetic and chemical enhancement. The large electromagnetic field is induced by the excitation of localized surface plasmon resonance [5] [6]. The probability of a Raman transition is increased when molecules are absorbed onto roughened surfaces. Recently, major applications of SERS in food science are detection of chemical and microbial hazards. The development of SERS application has progressed thanks to the increasing availability of suitable nanostructured SERS substrates. This article will focus on a particular nanostructured SERS substrate: 2D-arrays Cu nanodisks on anodic aluminum oxide (AAO) substrate. 2. Experimental AAO templates with the 50 nm – diameter pores were fabricated using the method given in the previous literature [7]. In sum, the AAO templates were prepared by two steps of anodization. Firstly, aluminium foil (5N) pieces (2 x 2 inches) were cleaned by organic solvent and DI water. They were electro-polished in a solution of alcohol and acid phosphoric during 2 minutes at 20 V and 7 oC to obtain a mirror-like surface. The first anodization was carried out in a solution of oxalic acid 0.3 M for 90 minutes at the temperature of 10 oC. The anodically aluminium oxides were completely stripped by a solution of H3PO4 and CrO3 for 30 minutes at 65oC. The second anodization was carried out in the solution of oxalic acid 0.3 M at 10oC for 1 minute. Then, the templates were immersed in the solution of H3PO4 0.1 M at 30oC to widen the pores. In this study, we got the AAO template sample from Institute of Materials Science, Vietnamese Academy of Science and Technology. To create the 2D-array of Cu nanodisks on the AAO template, the idea was to sputter a copper layer on top of the template, and then remove the pore wall. Copper was filled into the nanopores of the template using RF sputtering technique under a pressure of 0.6 Pa and a power of 20 W for 15 minutes using the RF magnetron sputtering equipment, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology. Then, in order to fortify Cu on the substrate, it was annealed for 30 minutes at 450oC. The walls of the substrate were removed by a solution of H3PO4 0.5 M during 30 minutes. We observed the morphologies of the fabricated Cu nanodisks by scanning electron microscope SEM Hitachi S-4800 and the absorption spectra of Cu nanodisk 2D-arrays were recorded by the equipment Shimadzu-UV2600: the equipments of Institute of Materials Science, Vietnamese Academy of Science and Technology. To demonstrate the SERS effect of the fabricated substrate, we recorded the Raman scattering spectra of Rhodamine 6G (Rh6G) coated on the glass substrate, and on the 2D- array Cu nanodisks upon the AAO substrates with the excitation source of 532nm using the equipment Labspec 6, Faculty of Physics, Hanoi National University of Education for Raman spectroscopy. 3. Results and discussion Figure 1 shows the SEM image of the AAO template. The morphologies of the surface are clearly shown. The diameter of pores is about 50 nm and the wall thickness is roughly 15 nm. This AAO template was used to prepare Cu nanodisk with diameters and inter-space following the AAO template. It is shown in figure 2 the SEM image of the 2D array of copper after sputtering Cu into the pores and the surface of the AAO templates, annealing and removing the AAO pore walls. The thickness of the Cu nanodisks was adjusted by sputtering time. It was determined sputtering time 15 minutes to obtain a N.T.Y. Mai, N.T. Ha / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 110-114 112 good quality of Cu layer. The Cu-filled AAO template was annealed at 450oC for 30 minutes in open air to solidify Cu on Al2O3 and easily remove the pore walls while remain Cu nanodisks. The size of Cu nanodisks is shown in the figure 2b that is comprehensive with the pore size of the AAO templates. The 2D-array of Cu nanodisks forms “hot-spots’ and induce the enhancement of the electromagnetic field, hence promising for SERS application. In this case, the plasmon resonance is presented for moderate Cu nanodisk size. a) b) Figure 1. SEM images of the orignial AAO template showing the diameter of the pores a) b) Figure 2. SEM images of the Cu nanodisk 2D array on the AAO template. The absorption spectra of the fabricated substrate are shown in figure 3. The surface plasmonic resonance peaking at 1320nm, 860nm, 633nm and 490nm were observed. It is mentioned in the literature that the closer Cu nanodisks interact stronger and so, making more red-shift of the peaks [8]. The N.T.Y. Mai, N.T. Ha / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 110-114 113 fabricated 2D array of high density copper nanodisks creates “hot-spots” to induce huge enhancement of the EM field making it suitable for SERS application. Figure 3: Absorption spectra of the AAO-Cu template To test the enhancement effects of the fabricated substrate, we measured the Raman scattering spectrum for Rhodamin 6G loaded on the glass substrate and the fabricated SERS substrate. Figure 4 shows the spectrum for comparison. The choice of 532nm laser excitation is suitable with the surface plasmon of the 2D array of Cu nanodisks. The Raman scattering spectra indicate the peaks corresponding to the normal vibrations of molecular groups of Rh6G. The enhancement factor is about 40 times. The results show the possibility of using the fabricated substrate for detection of pesticides, disease pathogens and herbicides. It should be mentioned that the fabricated Cu 2D array is very stable and we can have multiple use after cleaning by ultrasonic. Figure 4. Raman scattering spectra of Rh6G loading on the AAO-Cu template (1) and the Rh6G coated on glass substrate (2). N.T.Y. Mai, N.T. Ha / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 110-114 114 4. Conclusion 2D arrays of Cu nanodisks were fabricated in high quality by RF-sputtering Cu onto AAO templates, then removing the pore wall from them. The fabricated 2D array of Cu nanodisks showed different surface plasmon resonance peaks in the spectral range from 400 nm to 1400 nm. That is promising for SERS application. The enhancement effect of the fabricated SERS substrate was demonstrated by comparing the Raman scattering spectrum of Rh6G loaded on the AAO-Cu template with the spectrum of Rh6G/ glass substrate. From the obtained results one can suggest the application of the described method for fabricating 2D arrays of the other metals and alloys. Acknowledgements This work was supported by University of Engineering and Technology, Hanoi, Vietnam. We thank Institute of Materials Science, Vietnamese Academy of Science and Technology, especially Dr. Ung Dieu Thuy for providing us the sample of AAO template, Dr. Tong Quang Cong for supporting us in the thermal treatment, Dr. Nghiem Ha Lien for the optical spectroscopy. We thank Dr. Do Danh Bich, Faculty of Physics, Hanoi National University of Education for Raman spectroscopy. References [1] Schieberle P and Molyneux RJ 2012 J. Agric. Food. Chem. 2404-8 [2] Das RS and Agrawal YK 2011 Vib. Spectrosc. 163-76 [3] Yang D and Ying Y 2011 Appl. Spectrosc. Rev. 539-60 [4] Fleischmann M, Hendra PJ and Mc Quillan AJ 1974 Chem. Phys. Lett. 163-6 [5] Gersten J and Nitzan A 1980 J. Chem. Phys. 3023-37 [6] Moskovits M 1985 Rev. Mod. Phys. 783-826 [7] Masuda H and Fukuda K 1995 Science 268 1466 [8] Thi Thuy Nguyen et al 2016 Adv. Nat. Sci: Nanosci. Nanotechnol. 7 045017