Naturally inspired SERS substrates fabricated by photocatalytically depositing silver nanoparticles on cicada wings
© Tanahashi and Harada; licensee Springer. 2014
Received: 21 April 2014
Accepted: 5 June 2014
Published: 12 June 2014
Densely stacked Ag nanoparticles with an average diameter of 199 nm were effectively deposited on TiO2-coated cicada wings (Ag/TiO2-coated wings) from a water-ethanol solution of AgNO3 using ultraviolet light irradiation at room temperature. It was seen that the surfaces of bare cicada wings contained nanopillar array structures. In the optical absorption spectra of the Ag/TiO2-coated wings, the absorption peak due to the localized surface plasmon resonance (LSPR) of Ag nanoparticles was observed at 440 nm. Strong Surface-enhanced Raman scattering (SERS) signals of Rhodamine 6G adsorbed on the Ag/TiO2-coated wings were clearly observed using the 514.5-nm line of an Ar+ laser. The Ag/TiO2-coated wings can be a promising candidate for naturally inspired SERS substrates.
KeywordsSilver nanoparticles SERS Localized surface plasmon resonance Cicada Wing
Noble metal nanoparticles with localized surface plasmon resonance (LSPR) absorption in the visible wavelength region have a wide variety of beautiful colors. These noble metal nanoparticles have been applied in the field of nonlinear optics[1, 2], biological and chemical sensing, and surface-enhanced Raman scattering (SERS)[3, 4]. Among the noble metal nanoparticles, silver (Ag) and gold (Au) nanoparticles are one of the most investigated SERS-active metal nanoparticles because of their clear LSPR absorption[5–8].
In recent years, a lot of studies have been carried out focusing on the preparation of SERS-active substrates with larger area, low cost, and high performance[3–14]. The LSPR of a noble metal nanoparticle is primarily responsible for the SERS effect[5–8] and the LSPR properties are strongly dependent on the size and shape of the nanoparticles. The surface nanostructures of the substrates affect the properties of the nanoparticles deposited on the substrates. New types of SERS-active substrates have been developed by using the nanostructures of butterfly and cicada wings[9–14]. It is known that the butterfly and cicada wings have a number of predominant optical effects such as antireflection and photonic bandgap[15, 16]. Especially, the wings of some kinds of cicadas have nanopillar array structures and they show excellent antireflection properties. Usually, nanopillar array structures with tunable gap size are fabricated by electron-beam lithography. On the other hand, the cicada wings composed of chitin are a self-assembled natural nanocomposite material.
In our previous studies[17, 18], we have reported that the photocatalytically prepared Ag and Au nanoparticles deposited on TiO2 films showed the excellent SPR-sensing properties. Photocatalytic deposition method seems to be a convenient and desirable method to obtain stable and immobilized metal nanoparticles on the substrates. Thus, we have applied the photocatalytic deposition method to fabricate Ag nanoparticles deposited on the cicada wings with nanopillar array structures as SERS-active substrates.
In this paper, we have reported the preparation and SERS properties of the Ag nanoparticles deposited on TiO2-coated cicada wings with uniformly ordered nanopillar array structures.
The preparation of Ag/TiO2-coated wings, Ag/wings and Ag films
The preparation processes of the Ag nanoparticles deposited on TiO2-coated cicada wings (Ag/TiO2-coated wings) and Ag nanoparticles deposited on cicada wings (Ag/wings) without TiO2 are outlined as follows. Cicada wing samples were collected from a Japanese endemic species Cryptotympana facialis (a black cicada with clear and transparent wings). The cicadas were captured locally in Osaka City, Japan. In this experiment, the dorsal forewings of male cicadas were used. Before the dip-coating process, the forewings (50 to 55 mm in length) of individual cicada were rinsed using ethyl alcohol and deionized water to remove contaminant and dried at room temperature. TiO2 was coated on both sides of the forewing from anatase sol (Ishihara Sangyo Kaisha, ST-K211) by using a dip-coating technique. The resulting wing was soaked in a mixture of 2 mL of a 5.0 × 10-2 mol L-1 AgNO3 aqueous solution and 4 mL of ethyl alcohol (1.67 × 10-2 mol L-1 of Ag+ ions) in a petri dish (5 cm in diameter) about 10 mm away under a 15-W low-pressure mercury lamp (a germicidal lamp) with a power density of 0.13 mWcm-2 for 1 h. In this process, Ag+ ions were photoreduced on the surface of TiO2. Forewings without TiO2 were also treated as the abovementioned procedure. Ag+ ions were also photoreduced on the surface of the cicada wings (chitin) without TiO2 (Ag/wings). The resultant Ag/TiO2-coated wings and Ag/wings were washed with deionized water, finally dried in air. All the preparation procedures were carried out at room temperature. As a reference, Ag films deposited on a glass slide were prepared by a magnetron sputtering system. The Ag (99.9%, 2 in. in diameter) target was used. Sputtering was carried out in Ar gas of 1 to 2 Pa and the applied power of the Ag target was 50 W. The glass slide substrates were not intentionally heated during the sputtering. All compounds were of reagent grade and were used without further purification.
The XRD and SEM measurements
X-ray diffraction (XRD) measurements were performed on a RINT 2000 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan), using Cu Kα radiation working at 40 kV and 40 mA. The crystallite size, d, of the samples was estimated using the Scherrer equation: d = 0.9λ/β cosθ, where λ is the wavelength of X-ray source (0.154059 nm) and β is the full width at half maximum (FWHM) of the X-ray diffraction peak at the diffraction angle θ. Scanning electron microscopy (SEM) analysis of the bare cicada wings, Ag/wings, Ag/TiO2-coated wings and Ag films was carried out using a VE-8800 scanning electron microscope (Keyence Corporation, Osaka, Japan) at an acceleration voltage of 15 kV and a working distance of 4 to 12 mm.
The UV–Vis absorption spectra and SERS spectra measurements
All absorption spectra were recorded from 200 to 800 nm on an UV-3100PC dual beam spectrophotometer (Shimadzu Corporation, Kyoto, Japan). For SERS measurements, the sample was irradiated with 50 mW of 514.5-nm line (Ar+ laser) in back scattering geometry at room temperature. A × 50-long distance objective and a cooled CCD detector were employed. The laser beam was focused on a spot with a diameter of approximately 2 μm and the data acquisition time for each measurement was 1 s. Optical images were obtained with the camera attached to the Raman microscope. The Raman spectra of 10-3 mol L-1 Rhodamine 6G (R6G, 2 μL) adsorbed on various samples were compared. For the bare cicada wings, Ag/wings, and Ag/TiO2-coated wings, R6G was adsorbed on the surface at the center of the cicada dorsal forewings.
Results and discussion
Colors and SEM micrographs of the bare cicada wings, Ag/wings, Ag/TiO2-coated wings and Ag films
In the case of the Ag/wings, the color of bare cicada wings was changed from clear transparent to dark brown after the photoreduction of Ag+ ions onto the wings. On the other hand, the color of the wings was changed from clear transparent to metallic gray for the case of the Ag/TiO2-coated wings. These color changes indicated the formation of Ag metal on the wings. Photoreduction of Ag+ ions on TiO2-coated wings was faster than that on the wings without coated TiO2. This is due to that the coated TiO2 works as a photocatalyst effectively. On the other hand, the color of the Ag film prepared by the sputtering was metallic silver.
XRD patterns of the bare cicada wings, Ag/wings, Ag/TiO2-coated wings and Ag films
UV–Vis absorption spectra of the bare cicada wings, Ag/wings, and Ag/TiO2-coated wings
SERS spectra of R6G adsorbed on the surface of the bare cicada wings, Ag/wings, Ag/TiO2-coated wings and Ag films
By using the self-assembled natural nanopillar array structures of the cicada wings and TiO2 photocatalyst, SERS-active substrates of the Ag/TiO2-coated wings with larger area, low cost, and high performance were successfully prepared. Densely stacked Ag nanoparticles with 199 nm in average diameter were easily and effectively deposited on the TiO2-coated cicada wings. In the optical absorption spectra of the Ag/TiO2-coated wings, the absorption peak due to the LSPR of Ag nanoparticles was observed at 440 nm. In the SERS spectra (514.5 nm excitation line), the peak intensity of R6G at 1,649 cm-1 of the Ag/TiO2-coated wing was 36 times larger than that of the Ag film. The Ag/TiO2-coated wings can be used as SERS substrates.
localized surface plasmon resonance
surface-enhanced Raman scattering
scanning electron microscope.
This work was supported in part by ‘Senryakuteki Kenkyuukiban Keisei Shienjigyou (industry to support private universities building up their foundations of strategic research)’ Project for Private Universities: subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology), Japan.
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