Innovative Platform for Transmission Localized Surface Plasmon

Anal. Chem. 2009, 81, 7703–7712

Innovative Platform for Transmission Localized Surface Plasmon Transducers and Its Application in Detecting Heavy Metal Pd(II) Shuyan Gao,† Naoto Koshizaki,*,† Emiko Koyama,† Hideo Tokuhisa,† Takeshi Sasaki,† Jae-Kwan Kim,‡ Youngsong Cho,‡ Deok-Soo Kim,‡ and Yoshiki Shimizu† Nanotechnology Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Voronoi Diagram Research Center, Department of Industrial Engineering, College of Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-ku, Seoul 133-791, Korea Transmission localized surface plasmon resonance (TLSPR) transducers based on the characteristic surface plasmon absorption band of Au island films have become increasingly attractive. The first and main bottleneck hampering the development of T-LSPR sensors is instability, manifested as change in the surface plasmon absorbance band following immersion in organic solvents and aqueous solutions. In this paper, we innovate the platform for T-LSPR transducer by using remarkably stable and highly adhesive Au/Al2O3 nanocomposite film. Isolated Au nanoparticles embedded in dielectric matrix Al2O3 were prepared by a simple one-step radio frequency magnetron cosputtering technique. The obtained nanocomposite film is exceedingly stable during immersion in solvents, drying, and binding of different molecules; it successfully passes the adhesive tape test and sonication treatment. The superior stability and adhesion, obtained without the use of any intermediate adhesion layer or protective overlayer, is attributed to (1) the Au nanoparticles embedment and Al2O3 rim formation during the sputtering process and (2) the resistance of element Al in matrix to the nucleophilic attack by the solvent molecules. Given this success, we believe that the Au/Al2O3 nanocomposite film holds promise as an innovative sensing platform in T-LSPR detection technology, as demonstrated here for the Pd(II) sensing process with excellent sensitivity and low detection limit. Noble metal nanoparticles and their island films exhibit a unique UV-vis absorption band derived from collective oscillation of conduction electrons upon interaction with electromagnetic radiation, which is known as localized surface plasmon resonance (LSPR).1 The notable sensitivity of the SPR excitation to the presence of adsorbates, which originates from the general * To whom correspondence should be addressed. E-mail: koshizaki.naoto@ aist.go.jp. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Hanyang University. 10.1021/ac901137z CCC: $40.75  2009 American Chemical Society Published on Web 08/21/2009

dependence of the SPR band position and amplitude on the refractive index of the contacting medium, triggered the construction of various chemical and biological sensors based on LSPR measurements carried out in the UV-vis transmission mode.2 This method of transduction was termed transmission localized surface plasmon resonance (T-LSPR) spectroscopy.3 The major advantage of T-LSPR sensing over conventional SPR sensing lies (1) (a) Kreibig, U.; Gartz, M.; Hilger, A. Ber. Bunsen-Ges. 1997, 101, 1593. (b) Kreibig, U.; Gartz, M.; Hilger, A.; Hovel, H. Adv. Met. Semicond. Clusters 1998, 4, 345. (c) Mulvaney, P. MRS Bull. 2001, 26, 1009. (d) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (e) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (f) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549. (g) Riboh, J. C.; Haes, A. J.; McFarland, A. D.; Yonzon, C. R.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 1772. (h) Mulvaney, P. Langmuir 1996, 12, 788. Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Ser. Mat. Sci., Springer: Berlin, 1995; Vol. 25. (i) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (j) Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol., A 1995, 13, 1553. Jensen, T. R.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 2394. (k) Jiang, Y.; Zhao, H.; Zhu, N.; Lin, Y.; Yu, P.; Mao, L. Angew. Chem., Int. Ed 2008, 47, 8601. (l) He, L.; Musick, M. D.; Nicewarner, S. R.; Salinas, F. G.; Benkovic, S. J.; Natan, M. J.; Keating, C. D. J. Am. Chem. Soc. 2000, 122, 9071. (m) Lyon, L. A.; Musick, M. D.; Natan, M. J. Anal. Chem. 1998, 70, 5177. Zhou, Y.; Wang, C. Y.; Zhu, Y. R.; Chen, Z. Y. Chem. Mater. 1999, 11, 2310. (n) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Langmuir 2002, 18, 277. (o) Hata, H.; Kubo, S.; Kobayashi, Y.; Mallouk, T. E. J. Am. Chem. Soc. 2007, 129, 3064. (p) Zhang, L.; Fang, X.; Ye, C. Controlled Growth of Nanomaterials; World Scientific Publishing Company: 2007. (q) Huang, X.-J.; Choi, Y.-K.; Im, H.S.; Yarimaga, O.; Yoon, F.; Kim, H.-S. Sensors 2006, 6, 756. (r) Wei, H.; Wang, E. Anal. Chem. Anal. Chem. 2008, 80, 2250. (s) Yuan, J.; Guo, W.; Yang, X.; Wang, E. Anal. Chem. 2009, 81, 362. (t) Mendes, P. M.; Christman, K. L.; Parthasarathy, P.; Schopf, E.; Ouyang, J.; Yang, Y.; Preece, J. A.; Maynard, H. D.; Chen, Y.; Stoddart, J. F. Bioconjugate Chem. 2007, 18, 1919. (2) (a) Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P. Nano Lett. 2004, 4, 1029. (b) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (c) Haes, A. J.; Zou, S.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2004, 108, 6961. (d) Haes, A. J.; Zou, S.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2004, 108, 109. (e) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (f) Englebienne, P. Analyst 1998, 123, 1599. (g) Hirsch, L. R.; Jackson, J. B.; Lee, A.; Halas, N. J.; West, J. L. Anal. Chem. 2003, 75, 2377. (h) Haes, A. J.; Chang, L.; Klein, W. L.; Van Duyne, R. P. J. Am. Chem. Soc. 2005, 127, 2264. (i) Rindzevicius, T.; Alaverdyan, Y.; Dahlin, A.; Ho ¨o ¨k, F.; Sutherland, D. S.; Ka¨ll, M. Nano Lett. 2005, 5, 2335. (j) Dahlin, A.; Za¨ch, M.; Rindzevicius, T.; Ka¨ll, M.; Sutherland, D. S.; Ho ¨o ¨k, F. J. Am. Chem. Soc. 2005, 127, 5043. (3) (a) Hutter, E.; Fendler, J. H. Adv. Mater. 2004, 16, 1685. (b) Roy, D.; Fendler, J. Adv. Mater. 2004, 16, 479. (c) Kalyuzhny, G.; Vaskevich, A.; Schneeweiss, M. A.; Rubinstein, I. Chem.sEur. J. 2002, 8, 3850.

Analytical Chemistry, Vol. 81, No. 18, September 15, 2009

7703

in the instrumentation perspective. T-LSPR sensors can be implemented using extremely simple, small, light, robust, and lowcost equipment, which can be reduced to measurements at a single wavelength in a transmission configuration. As a result, the possibilities of disposable sensor kits for home, clinic, or field applications look promising. Therefore, in the last few decades, T-LSPR spectroscopy has been widely exploited for the registration of molecular recognition events.2b,d,e,h,3c,4 Despite the ever-increasing reports that emphasize the fascinating application of Au and Ag island films on transparent substrates (primarily glass) in potential T-LSPR transducers,1g,j,2b,e,5 their large-scale implementation remains limited. It has been welldocumented that the development of a reliable platform for T-LSPR sensing requires metal island films that have stable and reproducible optical properties. Change in the optical properties of metal island films, resulting from morphological changes occurring upon exposure to solvents and analytes and poor adhesion of metal island films to the substrate, introduces uncertainty in any detection scheme based on refractive index sensitivity. Thus, the first and main obstacle to sensing applications of metal island filmbased T-LSPR transducers is the instability of such systems toward changes in environmental conditions. This poor stability has been known for Ag island films2e,6 as well as for Au island films.4f,5a,7 The weak chemical interaction between Au and oxide substrates can be substantially improved with the use of metallic (Cr, Ni, Ti)8 or organic (amino- or mercapto-silane; dendrimer)9 coupling layers. However, they do not provide satisfactory solutions for Au island-based T-LSPR systems, as metal underlayers introduce optical and chemical interference, while organic underlayers do not provide the necessary stability, thus requiring additional (4) (a) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267. (b) Moores, A.; Goettmann, F. New J. Chem. 2006, 30, 1121. (c) Haes, A. J.; Van Duyne, R. P. Proc. Int. Soc. Opt. Eng. 2003, 5221, 47. (d) Haes, A. J.; Van Duyne, R. P. Laser Focus World 2003, 39, 153. (e) Bendikov, T. A.; Rabinkov, A.; Karakouz, T.; Vaskevich, A.; Rubinstein, I. Anal. Chem. 2008, 80, 7487. (f) Gluodenis, M.; Manley, C.; Foss, C. A. Anal. Chem. 1999, 71, 4554. (g) Himmelhaus, M.; Takei, H. Sens. Actuators, B 2000, 63, 24. (h) Okamoto, T.; Yamaguchi, I.; Kobayashi, T. Opt. Lett. 2000, 25, 372. (i) Hulteen, J. C.; Treichel, D. A.; Smith, M. T.; Duval, M. L.; Jensen, T. R.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 3854. (j) Kalyuzhny, G.; Schneeweiss, M. A.; Shanzer, A.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2001, 123, 3177. (k) Doron-Mor, I.; Cohen, H.; Barkay, Z.; Shanzer, A.; Vaskevich, A.; Rubinstein, I. Chem.sEur. J. 2005, 11, 5555. (l) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950. (m) Kim, S.; Cheng, N.; Jeong, J.-R.; Jang, S.-G.; Yang, S.-M.; Huck, W. T. S. Chem. Commun. 2008, 3666. (n) Tokareva, I.; Tokarev, I.; Minko, S.; Hutter, E.; Fendler, J. H. Chem. Commun. 2006, 3343. (5) (a) Vaskevich, A.; Rubinstein, I. In Handbook of Biosensors and Biochips; Marks, R., Cullen, D., Lowe, C., Weetall, H. H., Karube, I., Eds.; Wiley: Chichester, 2007; Vol. 1. (b) Fu, J.-X.; Collins, A.; Zhao, Y.-P. J. Phys. Chem. C 2008, 112, 16784. (c) Karakouz, T.; Tesler, A. B.; Bendikov, T. A.; Vaskevich, A.; Rubinstein, I. Adv. Mater. 2008, 20, 3893. (d) Li, H.; Luo, X.; Du, C.; Chen, X.; Fu, Y. Sens. Actuators, B 2008, 134, 940. (6) (a) Roark, S. E.; Rowlen, K. L. Anal. Chem. 1994, 66, 261. (b) Roark, S. E.; Semin, D. J.; Lo, A.; Skodje, R. T.; Rowlen, K. L. Anal. Chim. Acta 1995, 307, 341. (c) Drachev, V. P.; Thoreson, M. D.; Khaliullin, E. N.; Davisson, V. J.; Shalaev, V. M. J. Phys. Chem. B 2004, 108, 18046. (d) Redmond, P. L.; Hallock, A. J.; Brus, L. E. Nano Lett. 2005, 5, 131. (e) Hashimoto, N.; Hashimoto, T.; Teranishi, T.; Nasu, H.; Kamiya, K. Sens. Actuators, B 2006, 113, 382. (g) Hicks, E. M.; Lyandres, O.; Hall, W. P.; Zou, S. L.; Glucksberg, M. R.; Van Duyne, R. P. J. Phys. Chem. C 2007, 111, 4116. (7) (a) Ishikawa, H.; Kimura, K. Nanostruct. Mater. 1997, 9, 555. (b) Pilyankevich, A. N.; Melnikova, V. A. Thin Solid Films 1976, 37, L25. (c) Luo, Y.; Ruff, J.; Ray, R.; Gu, Y. L.; Ploehn, H. J.; Scrivens, W. A. Chem. Mater. 2005, 17, 5014. (8) Mosier-Boss, P. A.; Lieberman, S. H. Appl. Spectrosc. 1999, 53, 862.

7704


stabilization.9e Therefore, alternative methods (e.g., the use of nanosphere lithography and reactive ion etching10 and laser interference lithography technique5d) have been proposed; but the required complex, multistep procedure and the corresponding equipment and effort overwhelms the real benefits. A more general approach to stabilizing the morphology and optical response of Au island films is urgently required for the development of T-LSPR-based transducers. Recently the morphology and optical response of Au island films has been stabilized by encapsulating the islands in an ultrathin (2.0 nm) silica layer9e or by a single step of high-temperature annealing of evaporated Au island films under ambient conditions.5c Although these processes ensure the stability of the Au island morphology, the preparation of highly stable Au island films on oxide substrates is still a challenging task that is yet to be addressed. Then a question arises: is there any potential candidate platform for T-LSPR transducers. Nanocomposite thin films formed by noble metal nanoparticles such as Au embedded in a dielectric matrix can also exhibit SPR response due to collective excitations of conduction electrons in metal nanoparticles when photons are coupled to the metal particle-dielectric interface.11 Their intriguing physical and chemical properties have attracted much research interest, resulting in a wide range of applications in nanotechnology.12 It has been proposed that the use of a novel LSPR sensor with Au nanoclusters embedded in a dielectric film can achieve high resolution for gas detection,13 DNA hybridization,14 and the detection of human ovarian cancer cells.15 Nevertheless, the application of nanocomposite thin films in the sensing area is just in its infancy. In light of the noted difficulties in preparing strongly bound Au films directly on glass and based on the tentative exploration of Au nanoclusters embedded in a dielectric film, we now propose a facile design for preparing highly stable Au/Al2O3 nanocomposite film by radio frequency (RF) magnetron cosputtering as a potential platform for T-LSPR transducers. Such formed film is exceedingly stable during immersion in solvents, drying, and binding of different molecules and successfully passes the adhesive tape test, even after sonication. The simple, one-step preparation as well as the greatly improved stability and remarkable adhesion to glass without the use of any intermediate adhesion layer or protective overlayer makes this system highly promising for T-LSPR transducers in sensing applications. To the best of our knowledge, despite the remarkable advantages of T-LSPR transducers, their application in detecting heavy metal ions has never been tried. The concentration of (9) (a) Mosier-Boss, P. A.; Lieberman, S. H. Appl. Spectrosc. 1999, 53, 862. (b) Allara, D. L.; Hebard, A. F.; Padden, F. J.; Nuzzo, R. G.; Falcone, D. R. J. Vac. Sci. Technol., A 1983, 1, 376. (c) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85. (d) Baker, L. A.; Zamborini, F. P.; Sun, L.; Crooks, R. M. Anal. Chem. 1999, 71, 4403. (e) Ruach-Nir, I.; Bendikov, T. A.; Doron-Mor, I.; Barkay, Z.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2007, 129, 84. (10) Hicks, E. M.; Lyandres, O.; Hall, W. P.; Zou, S. L.; Glucksberg, M. R.; Van Duyne, R. P. J. Phys. Chem. C 2007, 111, 4116. (11) Prasad, P. N. Nanophotonics; Wiley-Interscience: 2004. (12) Halperin, W. P. Rev. Mod. Phys. 1986, 58, 533. (13) Deng, H.; Yang, D.; Chen, B.; Lin, C.-W. Sens. Actuators, B 2008, 134, 502. (14) Hu, W. P.; Chen, S.-J.; Huang, K.-T.; Hsu, J. H.; Chen, W. Y.; Chang, G. L.; Lai, K.-A. Biosens. Bioelectron. 2004, 19, 1465. (15) Mishra, Y. K.; Mohapatra, S.; Avasthi, D. K.; Kabiraj, D.; Lalla, N. P.; Pivin, J. C.; Sharma, H.; Kar, R.; Singh, N. Nanotechnology 2007, 18, 345606.

palladium (Pd(II)) in the environment has been increasing rapidly, as it has commonly been used in catalytic converters in motor vehicles and in some industrial processes.16 Because of its toxicity, the monitoring of Pd traces in surface waters, soil surfaces, plants, and particulate matter samples has become increasingly important. Thus, over the years several techniques have been developed for Pd(II) analysis, including AAS, ICP AES, ICPMS, polarography, and voltammetry.16,17 All of these techniques in general require expensive equipment, sample pretreatment, the use of toxic reagents, and/or analyte preconcentration steps. Therefore, achieving a simple, rapid, inexpensive, and sensitive method that permits real-time detection of metal ions is still a challenging goal. In addition, due to the danger that the heavy metal ions pose for operators, minimal sample handling is desirable. A highly specific molecular recognition element must be matched with a sensitive detector to convert the specific metal-ion recognition event into an electrical, mechanical, or optical signal.18 Considering the advantages of T-LSPR and the importance of establishing a potential platform for detecting heavy metal ions, we tested the ability of the T-LSPR sensor based on the Au/Al2O3 nanocomposite film to analyze Pd(II). The result substantially establishes the feasibility of using Au/Al2O3 nanocomposite film as transducers for chemical and biological sensing by T-LSPR spectroscopy. EXPERIMENTAL SECTION Materials. Unless stated otherwise, all reagents and chemicals were purchased from commercial sources and used without further purification. HSPh-≡-bpy-≡-PhSH (Compound 1) is obtained by deprotection of AcSPh-≡-bpy-≡-PhSAc (Compound 2). The general procedure for synthesizing Compound 2 is as follows. Dry THF (600 mL) and triethylamine (300 mL) were introduced to a mixture of 5,5′-dibromo-2,2′-bipiridine (12.0 g, 38.2 mmol), PdCl2 (0.41 g, 2.3 mmol), PPh3 (1.20 g, 4.6 mmol), and CuI (0.44 g, 2.3 mmol) in a three-neck round flask. The flask was cooled, degassed, charged with N2 gas, and subsequently heated at 70 °C. The solution was added to trimethylsilylacetylene (9.0 g, 91.6 mmol) and kept at 70 °C for 3 days. After the reaction, the solution was filtered through silica gel to remove insoluble inorganic salts, and the solvent was evaporated from the reaction mixture. The residue was dissolved in chloroform and washed with water. The organic phase was concentrated and dried in vacuo. The obtained crude mixture was dissolved in THF (300 mL) and methanol (200 mL). KF (5.2 g, 89.7 mmol) was added to the solution and stirred in a nitrogen atmosphere at ambient temperature overnight. After the deprotection, the solution was filtered through silica gel, and the solvent was evaporated from the reaction mixture. The residue was dissolved in chloroform and washed with water. The organic phase was concentrated and dried in vacuo. The obtained residue was purified by gel permeation chromatography (GPC) (eluent: chloroform) to (16) Locatelli, C. Electroanalysis 2007, 19, 2167, and references therein. (17) (a) Wang, J.; Varughese, K. Anal. Chim. Acta 1987, 199, 185. (b) Zhao, Z.; Gao, Z. J. Electroanal. Chem. 1988, 256, 65. (c) Georgieva, M.; Pihlar, B. Electroanalysis 1996, 8, 1155. (d) Locatelli, C. Electroanalysis 2005, 17, 140. (e) Locatelli, C.; Melucci, D.; Torsi, G. Anal. Bioanal. Chem. 2005, 382, 1567. (f) Locatelli, C. Electrochim. Acta 2006, 52, 614. (g) Locatelli, C. Electroanalysis 2007, 19, 445. (h) Locatelli, C. Anal. Chim. Acta 2006, 557, 70. Kim, S.-I.; Cha, K.-W. Talanta 2002, 57, 657.

produce a yellow solid. Dry THF (100 mL) and triethylamine (50 mL) were introduced to a mixture of the obtained 5,5′diethynyl-2,2′-bipiridine (0.30 g, 1.48 mmol), 1-iodo-4-thioacethylbenzene (0.90 g, 3.26 mmol), Pd(PPh3)4 (115 mg, 0.10 mmol), and CuI (19.06 mg, 0.10 mmol) in a three-neck round flask. The flask was cooled, degassed, charged with nitrogen gas, and stirred at ambient temperature for 3 days. After the reaction, the solution was filtered through silica gel to remove insoluble inorganic salts, and the solvent was evaporated from the reaction mixture. The residue was dissolved in chloroform and washed with water. The organic phase was concentrated and dried in vacuo. The obtained residue was purified by GPC (eluent: chloroform) to produce a yellow solid (32% yield). 1H NMR (500 MHz; CDCl3: DMSO-d6 ) 5:1; TMS): δH 2.46 (6H, s, COMe), 7.45 (4H, d, J ) 8.4, Ph), 7.61 (4H, d, J ) 8.4, Ph), 7.99 (2H, d, J ) 8.1, Bpy), 8.47 (2H, d, J ) 8.3, Bpy); 8.81 (2H, s, Bpy), IR: 2217, 1698, 1583, 1529, 1488, 1459, 1396, 1356, 1127 cm-1. Instrumentation. The morphology of Au/Al2O3 nanocomposite film was observed by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Perkin-Elmer PHI 5600ci ESCA System, with a monochromatized Al KR X-ray source (1486.6 eV). The C1s peak of hydrocarbon contamination was used as an internal reference (284.5 eV). The phase of the sample was determined by X-ray diffraction (XRD) on an Ultima IV/PSK X-ray diffractometer with Cu KR radiation (λ)0.154056 nm) at 2θ ranging from 30° to 80°. Fourier transform infrared reflection absorption spectroscopy (FT-IR-RAS) measurements were made on a Digilab FTS 7000 FT-IR spectrometer equipped with a Spectra-Tech FT-80 grazing angle reflectance accessory and a liquid-N2cooled MCT detector. UV-vis absorption spectra were collected using a Solidspec-3700 DUV UV-vis-NIR spectrophotometer. 1H NMR spectra were recorded on a 500 MHz Bruker Avance 500, using tetramethylsilane (TMS) as an internal standard. Transmission electron microscopy (TEM), high resolution TEM (HRTEM) images, and EDS were taken with a JEOL JEM-3000F transmission electron microscope. Preparation of Au/Al2O3 Nanocomposite Film. To embed the Au nanoparticles in the dielectric matrix and obtain Au/Al2O3 nanocomposite film, an Al2O3 target (100 mm in diameter) and 18 Au sheets (5 mm wide, 10 mm long, and 1 mm thick) placed in a symmetric array on top of the target were simultaneously sputtered on p-typed Si wafer (100) substrate (for FESEM, XRD, and XPS characterizations) and quartz (for T-LSPR measurement) by RF magnetron sputtering in Ar with the pressure of 0.53 Pa at 45 W for 1 min. Adhesion and Stability Tests. The strength of the Au/Al2O3 nanocomposite film adhesion to a substrate was evaluated qualitatively using the adhesive tape test and sonication as follows. (a) A piece of clear Scotch tape (3M) was pressed against the original film and sonication-treated film and pulled away, as it was monitored by digital camera, and (b) the morphology of the film sonicated by Ultra Sonic Cleaner at 40 kHz and 120 W for 15 min was checked by FE-SEM. The stability of the morphology was tested by immersion in four different liquids (water, acetonitrile, dicholomethane, and Analytical Chemistry, Vol. 81, No. 18, September 15, 2009

7705

Scheme 1. Schematic Illustration of the Sensing Test Procedurea

a (a) The deprotection of Compound 2 and (b) the immersion of the nanocomposite film in Compound 1 solution cause the modification of the probe, Compound 1, on the nanocomposite film. (c) The complexation of Compound 1 with Pd(II) is then performed by immersing Compound 1-modified nanocomposite film in a 1,4-dioxane solution of Pd(AN)2Cl2. (d) The last step is decomplexation to remove the complex in the monolayers by freshly prepared EDA in 1,4-dioxane solution.

toluene) for 24 h. After the initial FE-SEM image was obtained, the sample was soaked in a given liquid for 24 h, removed, and dried under a stream of nitrogen. After drying, the final FE-SEM image was acquired. Sensing Test. The sensing test consists of three steps (Scheme 1). The first step is to immerse the nanocomposite film in the toluene/acetonitrile (9:1) solution of Compound 1 for 24 h to achieve the modification of the probe, Compound 1, on the nanocomposite film (Scheme 1a and 1b), followed by washing with toluene/acetonitrile (9:1) mixed solvent before drying with nitrogen. In this step, Compound 2 needs deprotection with pyrrolidine to produce Compound 1 according to the deprotection reaction depicted in Scheme S1. The second step is the complexation of Compound 1 with Pd(II) (Scheme 1c), performed by immersing Compound 1-modified nanocomposite film in a 1,4dioxane solution of bis-(acetonitrile)dichloropalladium(II) (Pd(AN)2Cl2) for 5 min. The immersion time of 5 min was (18) Forzani, E.; Zhang, H.; Chen, W.; Tao, N. Environ. Sci. Technol. 2005, 39, 1257.

7706


determined by checking the time-course of LSPR spectra for different concentration of Pd(II). For the highest concentration of Pd(II), it just took less than 30 s to reach equilibrium, and for the lowest concentration of Pd(II) (down to 1 nM), it took 5 min to reach equilibrium. According to such results, we fixed the immersion time to be 5 min to reach equilibrium within the whole concentration range studied. The obtained surfaces are subsequently rinsed by 1,4-dioxane and dried by a nitrogen jet. The last step is decomplexation to remove the complex in the monolayers by freshly prepared ethylenediamine (EDA) in 1,4-dioxane solution (Scheme 1d). The T-LSPR spectra were monitored for each step. RESULTS AND DISCUSSION Structural Characterization. XPS analysis of a surface provides qualitative and quantitative information on all the ele(19) (a) Jaramillo, T. F.; Baeck, S.-H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148. (b) Aziz, M. A.; Patra, S.; Yang, H. Chem. Commun. 2008, 4607. (c) Tsai, H. C.; Hu, E.; Perng, K. G.; Chen, M. K.; Wu, J.-C.; Chang, Y.-S. Surf. Sci. 2003, 537, L447.

Figure 1. (a) and (b) low- and high-magnification FE-SEM images of the sample. (c) The corresponding Voronoi diagram of (b). (d) The size distribution according to Voronoi diagram analysis.

ments present (except H and He) from the binding energies of the main lines and the peak area. Figure S1a (in the Supporting Information) presents the survey spectrum of the sample. Au and Al can be clearly recognized. Figure S1b (in the Supporting Information) depicts a high-resolution spectrum of Au. Two clearly distinct peaks at 84.3 eV corresponding to Au4f7/2 core-level binding energy and 88.0 eV corresponding to Au4f5/2 core-level binding energy were observed, confirming the presence of Au in the zero-oxidation state.19 Figure S1c (in the Supporting Information) illustrates the high-resolution spectrum of Al. The peak at 75.2 eV corresponded to Al2p core-level binding energy, confirming the presence of Al in Al2O3. XRD measurement was performed to verify the sample phase (Figure S2 in the Supporting Information). The diffraction peaks can be readily indexed to the face-centered cubic (fcc) unit cell structure of metallic gold with space group Fm-3m (Joint Committee on Powder Diffraction Standards (JCPDS) card no. 65-2870). The observation of the expected diffraction peaks substantiated the formation of crystalline gold.20 Significant broadening of the recorded peaks reflected the nanoscale character of the Au nanoparticles. No peaks of Al2O3 were detected by XRD analysis, indicating that Al2O3 in the as-prepared sample may be amorphous. The morphology of the sample was studied by FE-SEM. Figure 1a,b depicts the surface morphologies of the deposited Au/Al2O3 nanocomposite film. The Au nanoparticles (white dots) were homogeneously distributed in the dark gray Al2O3 matrix. The Au nanoparticles had an fcc structure and were surrounded by the amorphous Al2O3 phase spread on the substrate. Morphological analysis for such nanocomposite structures was performed by using the Voronoi diagram software described (20) Gao, S.; Zhang, H.; Wang, X.; Yang, J.; Zhou, L.; Peng, C.; Sun, D.; Li, M. Nanotechnology 2005, 16, 2350.

in detail in our previous papers21 and outlined in the Supporting Information. Each FE-SEM image was converted to a Voronoi diagram with circle generators (Figure 1c). The boundary of each Voronoi region is denoted by a blue curve in Figure 1c and is computed in hyperbolic arcs where all points on the edge are at the same distance from the boundaries of the adjacent circle generators. The size distribution and mean radius of Au nanoparticles were obtained from the statistical data generated by the Voronoi diagram (Figure 1d).22 The results indicated that the Au nanoparticles were very homogeneously distributed over the entire substrate. Each Au nanoparticle had a mean of 5.96 neighbors in the whole Au/Al2O3 nanocomposite film. Therefore, the Au nanoparticles were distributed on the substrate like a hexagonalnonclose-packed plane. Adhesion and Stability Tests. Application of Au/Al2O3 nanocomposite film as T-LSPR transducers requires structural stability (e.g., passing the adhesive tape test) as well as morphological stability after immersion in solvents and drying. Thus, the structural and morphological stability of the Au/Al2O3 nanocomposite film and its relevance to sensing applications were evaluated. The strength of the Au/Al2O3 nanocomposite film adhesion to glass substrates was evaluated qualitatively using the adhesive tape test: a piece of clear Scotch tape (3M) was pressed against the original film, sonication-treated, and pulled away. Figure S4 (in the Supporting Information) presents the photos of the original film, the adhesion-tested film, and the sonication-treated film following the adhesive tape test. The lack of color change before and after the adhesive test displays that (21) (a) Kim, D.-G.; Shimizu, Y.; Sasaki, T.; Koshizaki, N.; Lee, B. H.; Kim, D.S.; Lee, Y. J.; Kim, Y. D. Nanotechnology 2007, 18, 145703. (b) Kim, D.-S.; Chung, Y.-C.; Kim, J. J.; Kim, D.; Yu, K. J. Ceram. Process. Res. 2002, 3, 150. (c) Kim, D.-S.; Kim, D.; Sugihara, K. Comput.-Aided Geom. Des. 2001, 18, 541. (22) Okabe, A.; Boots, B.; Sugihara, K.; Chiu, S. N. Spatial Tessellation: Concepts and Applications of Voronoi Diagrams, 2nd ed.; John Wiley & Sons Ltd.: New York, 2000.


7707

Figure 2. FE-SEM images of (a) the sample after sonication for 15 min, after immersion in (b) water, (c) acetonitrile, (d) dichloromethane, (e) toluene, and (f) after modification by Compound 1. The scale bars for all the images are 200 nm.

Au nanoparticles still tightly attached on the substrate, which demonstrates the high adhesion of the nanocomposite film to the glass substrate. Figure 2a presents the FE-SEM image of the sonicated sample, which further verified the high adhesion of the nanocomposite film. An issue of major importance is the known instability of Au island films during immersion in various solvents and drying. Therefore, the Au/Al2O3 nanocomposite film was immersed in several different liquids for 24 h at room temperature to observe the effect of solvents on the composite nanostructures. Water, acetonitrile, dicholomethane, and toluene were used for the experiments. The morphology of Au/ Al2O3 nanocomposite film after immersion in these solvents (Figure 2b-e) clearly demonstrated that the morphology of the nanocomposite film can be maintained well after exposure to such commonly used solvents. In order to further verify the stability of the obtained nanocomposite film, the resistance of the LSPR peak to the solvents was checked and shown in Figure S5 (in the Supporting Information), from which the lack of a wavelength shift in the LSPR spectrum after immersion in different solvents can be clarified. This is another verification that the various solvents have not influenced the gold particle sizes or stability. All these investigations substantially prove that the nanocomposite film is quite stable. Therefore, one of the toughest issues for the potential application of the metal island films, instability, has been facilely settled by embedding the Au nanoparticles into the Al2O3 matrix via an unconventional cosputtering technique. In order to verify 7708


the assertion that a Al2O3 rim was formed around the Au islands, TEM and HRTEM images were collected, as shown in Figure S6. The TEM image for the large area (Figure S6a) reveals clearly the isolated nanoparticles with an average diameter of ca. 5.3 nm. The HRTEM image in Figure S6b can give further insight into the details of the configuration. The lattice fringe can be clearly distinguished, and the d spacing of 2.36 Å corresponds to the {111} plane of Au. This result is well consistent with the XRD pattern. So, the isolated nanoparticles should be Au nanoparticles. What is worth noting is that the Au nanoparticles are surrounded by amorphous matrix. According to the preparation condition and XPS result, the only one possibility is that the amorphous phase is ascribed to Al2O3, which is proven by the EDS result (Figure S6c) that only three elements, Au, Al, and O, exist in the sample. This observation gives substantial proof for the assertion proposed above. The superior stability, obtained without the use of any intermediate adhesion layer or protective overlayer, may be attributed to the mechanism of the current processing technique, cosputtering multitargets. The formation of the Au/Al2O3 nanocomposite film is illustrated in Scheme 2. Au sheets placed in a symmetric array on top of the target Al2O3 plate in the sputtering chamber constitute the multitargets, which were cosputtered under Ar atmosphere during the sputtering process, thus forming the plasma. The landed species from the plasma on the substrate are mostly atomic. However the surplus energy is not so high and then easily

Scheme 2. Schematic Illustration of the Cosputtering Processa

a

(a) Au sheets placed in a symmetric array on top of the target Al2O3 plate constitute the multitargets, arranged in the sputtering chamber for sputtering. (b) The multitargets were cosputtered under Ar atmosphere, and consequently plasma is formed. (c) The species from the plasma landed on the substrate. The landed species are mostly atomic, and the surplus energy was not so high and then easily quenched by short distance movement; therefore, just small particles were formed with Al2O3 being rim. The atomic and ionic species corresponding to Al2O3 are omitted.

quenched by short distance movement without the formation of large particles. In order to get highly stable nanocomposite film, several kinds of dielectric matrix have been tried, and some of the results were shown in Figure S7 (in Supporting Information). In comparison, the Au/Al2O3 nanocomposite system is most satisfactory, which may be due to that element Al in matrix can survive the nucleophilic attack by the solvent molecules, even the polar solvents.23 The stability of the Au/ Al2O3 nanocomposite and the simple one-step preparation make this system a highly promising platform for T-LSPR transducers, as demonstrated here for detecting heavy metal Pd(II). Sensing with Au/Al2O3 Nanocomposite Film by T-LSPR Spectroscopy. To demonstrate the application of the Au/Al2O3 nanocomposite film as optical transducers in T-LSPR sensing, it is important to understand the dependence of its T-LSPR wavelength (λmax) on the refractive index of the surrounding medium. In the present study, the T-LSPR peak change was observed when the surrounding media were replaced with solvents having a range of different refractive indices. Figure 3 plots λmax vs refractive index, indicating the linear dependence of ∆λmax on the surrounding refractive index, as expected. From Figure 3, we can calculate the sensitivity factor S (∆λmax/RIU), which is defined as the relative changes in resonance wavelength with respect to a change in the refractive index of the surrounding medium. Here RIU stands for refractive index unit. The S of the sample was calculated as 71 nm/RIU from the slope of this plot. Sensing Application of Au/Al2O3 Nanocomposite Film in the T-LSPR Transducer. The sensing test described above confirms the system’s sensitivity to change in the refractive index of the surrounding medium. To investigate the application of the Au/Al2O3 nanocomposite film in detecting heavy metal Pd(II), (23) Tsai, H. C.; Hu, E.; Perng, K. G.; Chen, M. K.; Wu, J.-C.; Chang, Y.-S. Surf. Sci. 2003, 537, L447.

Figure 3. Plot of ∆λmax of the sample exposed to the surrounding media vs refractive index of surrounding media.

the probe molecule must be immobilized on the surface (Scheme 1) to form a sensing layer. Here, Compound 1 was chosen as the probe, considering its dual function, (i.e., the strong adsorption on the Au surface via Au-S bonding and the high affinity for Pd(II) via complexation with a square-planar configuration of Pd).24 The acetyl group for the protection of the Compound 2 molecule is deprotected by pyrrolidine to provide Compound 1 in the solutions, as presented in Scheme S1.25 The immobilization of the probe on the Au/Al2O3 nanocomposite film was verified by XPS, performed on the nanocomposite film substrate directly modified with the probe. High-resolution XPS spectra of the elements S2p and N1s are presented in Figure 4. After fitting (curves a and b in Figure 4), there are three components in the S 2p3/2 XPS spectra. Binding energy of 162.3 eV is expected for S 2p3/2 in thiolate resulting from the formation of Au-S through binding the mercapto moieties to Au nanoparticles. Part of the spectrum where the signal protonated thiolate group appears (ca. 163.6 eV) can be also resolved. Generally speaking, the peak with high binding energy (ca. 168.4 eV) indicates that most of the sulfur is actually oxidized to sulfonate. It is also possible that the peak located ca. 168.4 eV for S 2p3/2 comes from the formation of dithiolate between two Compound 1 molecules.26 Based on such an observation, it can be speculated that there are three configurations of the sensing layer. The first is shown in Scheme 1, where both of the two mercapto moieties bind to Au nanoparticles. The second one is that just one of the two mercapto moieties binds to Au nanoparticles, leaving the other remaining protonated. The third one is that the dithiolate would form between two Compound 1 molecules, followed by immobilization on the nanocom(24) Nakamura, T.; Koyama, E.; Shimoi, Y.; Abe, S.; Ishida, T.; Tsukagoshi, K.; Mizutani, W.; Tokuhisa, H.; Kanesato, M.; Nakai, I.; Kondoh, H.; Ohta, T. J. Phys. Chem. B 2006, 110, 9195. (25) (a) The deprotection of Compound 1 thioacetate was conducted using 1.1 equivalent of pyrrolidine in a 1,4-dioxane solution of the BP molecules. The proceeding of the reaction was confirmed in 1,4-dioxane-d8 by the complete disappearance of the methyl proton signal of the S-protecting acetyl group at 2.4 ppm in the 1H NMR spectra. (b) Koyama, E.; Naitoh, Y.; Tokuhisa, H.; Nakamura, T.; Horikawa, M.; Ishida, T.; Fujiwara, K.; Mizutani, W.; Nagawa, Y.; Kanesato, M. Jpn. J. Appl. Phys. 2008, 47, 7369. (26) http://srdata.nist.gov/xps/selEnergyType.aspx (accessed August 27, 2007).


7709

Figure 5. FT-IR spectra of (a) Compound 1 modified Au/Al2O3 nanocomposite film, (b) followed by complexation with Pd(II), and (c) subsequent decomplexation by EDA.

Figure 4. Core-level XPS S2p, N1s, Cl2p, and Pd3d spectra for Compound 1 modification. (a) S2p as prepared, (b) S2p after complexation with Pd(II), (c) N1s as prepared, (d) N1s after complexation with Pd(II), (e) N1s after decomplexation by EDA, (f) Cl2p as prepared, (g) Cl2p after complexation with Pd(II), (h) Cl2p after decomplexation by EDA, (i) Pd3d as prepared, (j) Pd3d after complexation with Pd(II), and (k) Pd3d after decomplexation by EDA.

posite film through the Au-S bond. In view of the complicated situation, we simplify the chemistry of the formation of the sensing layer by just showing the first possibility in Scheme 1. The peak at 398.3 eV (Figure 4c) confirms the presence of the N element, which should originate from the bare nitrogen of the Compound 1 moiety. These observations confirm the immobilization of the probe molecule on the substrate. FE-SEM characterization was also performed to check the effect of the immobilization on the morphology of the Au/Al2O3 nanocomposite film (Figure 2f). As can be clearly seen, the Au/Al2O3 nanocomposite film seems intact and demonstrates high stability. After the probe is immobilized on the surface of the Au/Al2O3 nanocomposite film, it is ready to detect the specific binding of Pd(II). Drastic changes in the core-level photoelectron spectra for N1s and Cl2p are observed after immersing in a Pd(AN)2Cl2 solution and rinsing. In the N1s spectrum, a distinct peak is observed at a higher binding energy (400.4 eV, Figure 4d), which is distinguished from the N1s peak for the free bipyridine nitrogen on the Pd-free surface located at 398.3 eV (Figure 4c). This value is consistent with the reported value for the complexation of N1s with Pd(II).24 Additionally, Cl2p3/2, Pd3d3/2, and Pd3d5/2 are clearly seen at 197.8 eV (Figure 4g), 7710


343.5, and 338.2 eV (Figure 4j), while no Cl or Pd peak is observed before the introduction of Pd(AN)2Cl2 (Figure 4f,i). Both the shift of the N1s peak toward a higher binding energy and the appearanceofCl2p3/2 andPd3dsubstantiatesuccessfulcomplexation. In order to confirm the complexation of Compound 1 with Pd(AN)2Cl2 on the nanocomposite film, we also performed FTIR-RAS of the Compound 1-modified substrate before and after the addition of Pd(AN)2Cl2. In the FT-IR spectrum of the Compound 1-modified substrate (Figure 5a), characteristic peaks were observed at 2216 cm-1 due to the acetylene unit and at 1100 cm-1 and 1400-1600 cm-1 due to the aromatic ring, confirming the existence of Compound 1. It is noteworthy that little or no thioester signal was detected at 1698 cm-1, indicative of complete deprotection. After the introduction of Pd(AN)2Cl2, we observed FT-IR signal enhancement in the acetylene region as well as peak shifts in the aromatic region as marked by red dash-dot lines (Figure 5b). This result reveals that immobilized Compound 1 on the Au/Al2O3 nanocomposite film can form a complex with Pd(II) almost completely under this condition. These FT-IR results further verify the successful immobilization of Compound 1 molecules and the subsequent complexation with Pd(II). It is well accepted that a commercially viable sensor should be entirely reusable because reusability has a large impact on cost-effectiveness and the simplicity of the use of sensors. Therefore, for our study, the layer of analyte, Pd(II), must be entirely removable, leaving the nanoparticles functionalized with Compound 1 and ready for additional measurement. As for the reproducibility of our sensing surface, it was necessary to determine if it is possible to regenerate the active surface for detecting Pd(II). Thus, the next step was the removal of Pd(II) coordinated in Compound 1 monolayers by using amine derivatives. We found that EDA is an effective removal reagent of the palladium ion. After the immersion of the Compound 1-Pd(II) complex in the EDA solution, all the XPS peaks of N1s, Pd3d, and Cl2p return to the initial stage (Figure 4e,h,k), suggesting that the analyte species has been completely removed. During this procedure, the S2p signals remain unchanged (Figure 4b).

Figure 6. Core-level XPS intensity changes for N1s, Cl2p, and Pd3d. Filled squares denote N1s at 400.4 eV, dots denote Pd3d3/2 at 343.5 eV, and triangles denote Cl2p3/2 at 197.0 eV. Table 1. Atomic Ratio of Elements Pd and Cl to N Based on the Quantitative Analysis of XPS Data Shown in Figure 4

as prepared after complexation with Pd(II) after decomplexation by EDA

Cl/N

Pd/N

0.1 0.8 0.1

0.0 0.5 0.0

Figure 6 summarizes the complexation/decomplexation phenomena of the Pd complex in the Compound 1 monolayer, where the reversible XPS intensity changes are observed for N1s, Pd3d, and Cl2p spectra by Pd(II)-complexation/decomplexation. Because the amount of element N should be kept constant during the complexation and decomplexation processes, the atomic ratios of elements Pd and Cl to N were calculated to avoid the errors from accumulated statistics and energy windows for different elements, as shown in Table 1. From Table 1, it can be clearly seen that the reversibility of the complexation and the quantitative relationship among the elements Pd, Cl, and N, which is well consistent with their stoichiometric ratios in the complex. Additionally, all the peaks in the FT-IR spectrum (Figure 5e) nearly return to the original positions (Figure 5a) after decomplexation by EDA, which further demonstrates the reversibility of the complexation. All these observations fully prove the reusability of our sensing surface. The immobilization, sensing test, and reversibility of our system were also monitored by recording the T-LSPR spectral changes induced by immobilization with Compound 1 and upon complexation with Pd(II) and subsequent decomplexation by EDA (Figure 7).27 After the modification of the Au/Al2O3 nanocomposite film with Compound 1, the T-LSPR peak shifts from 593.0 nm (Figure 7a) to 613.8 nm (Figure 7b). When the sensor is exposed to a 10-5 M Pd(II) solution, the T-LSPR absorption maximum shifts to 643.5 nm (Figure 7c), a 29.7-nm shift originating from the complexation. This shift is reversed by exposing the sample to 10-4 M EDA for 30 min, resulting in

Figure 7. T-LSPR spectra demonstrating the immobilization of Compound 1 on the Au/Al2O3 nanocomposite film and subsequent complexation and decomplexation of Pd(II). (a) Bare Au/Al2O3 nanocomposite film. (b) Compound 1 functionalized Au/Al2O3 nanocomposite film. (c) After complexation with Pd(II). (d) After decomplexation by EDA. (e) After recomplexation with Pd(II).

Figure 8. Quantitative response curve of T-LSPR shift ∆λ versus the [Pd(II)] response curve for the binding of Pd(II) to Compound 1 functionalized Au/Al2O3 nanocomposite film sensor. All measurements were collected in an air environment. The solid line indicates the calculated value of ∆λ using eq 2.

decomplexation (Figure 7d). When this detection procedure is repeated, the T-LSPR absorption maximum shifts 29.4 nm upon exposure to the Pd(II) solution (Figure 7e). This qualitative result lays the foundation of the application of the Au/Al2O3 nanocomposite film in T-LSPR transducers. Quantification of Pd(II) Response. To develop a more quantitative understanding of the concentration-dependent response of the T-LSPR sensor on varying concentrations of Pd(II), the shift induced by the direct binding of Pd(II) of different concentrations onto the Compound 1 functionalized Au/Al2O3 nanocomposite surface was analyzed. The T-LSPR λmax shift, ∆λ, versus the [Pd(II)] response curve was measured over the [Pd(II)] range between 10-9 M and 10-4 M. Here, [Pd(II)] is the concentration of the Pd(II) solution. Figure 8 (filled squares) presents the experimental data plotted as ∆λ versus [Pd(II)]. The Analytical Chemistry, Vol. 81, No. 18, September 15, 2009

7711

experimental response curve for Pd(II) was quantitatively interpreted in terms of a model that makes the following assumptions: (a) the only operative nanoparticle sensing mechanism is the change in the local refractive index caused by the adsorbed analyte (Pd(II)); (b) the complexation equilibrium is given by Au - Compound1 + Pd(II) a Au - [Compound1 + Pd(II)] (1) (c) the measured ∆λ is proportional to the number of bound metal ions; and (d) the binding sites are independent. Based on these assumptions, the response curve can be described by the following Langmuir-like equation (eq 2),1g,2h,18 from which the thermodynamic affinity constant (Ka,surf) for Pd(II) binding on the Compound 1 modified surface was estimated Ka,surf[Pd(II)] ∆λ ) ∆λmax 1 + Ka,surf[Pd(II)]

(2)

Here, ∆λ is the T-LSPR λmax shift for a given concentration, ∆λmax is the maximum T-LSPR response at high concentrations, and Ka,surf is the surface-confined thermodynamic affinity constant. Comparing the experimentally measured ∆λ versus the [Pd(II)] response (points) to eq 2 (solid line, Figure 8) yields approximate values for ∆λmax ) 35.4 nm and the surface-confined thermodynamic affinity constant Ka,surf ) 4.3 × 105 M-1. Such sensitivity is comparable with or better than the previous ones of the T-LSPR response to analyte binding up to >15 nm from the (silica-coated) Au island surface.4k,e In addition, the data in Figure 8 enable one to estimate the limit of detection (LOD) of

Innovative Platform for Transmission Localized Surface Plasmon

Recommend Documents