Observing Plasmonic-Molecular Resonance Coupling on Single Gold Nanorods Weihai Ni, † Tobias Ambjo ¨ rnsson, ‡ Sten Peter Apell, § Huanjun Chen, † and Jianfang Wang* ,† † Department of Physics, the Chinese University of Hong Kong, Shatin, Hong Kong SAR, China, ‡ Department of Theoretical Physics, Lund University, So ¨lvegatan 14A, SE-223 62 Lund, Sweden, and § Department of Applied Physics, Chalmers University of Technology, SE-412 96 Go ¨ teborg, Sweden ABSTRACT Strong plasmonic-molecular resonance coupling occurs between noble metal nanocrystals and organic adsorbates when the plasmonic resonance is degenerate with the molecular one. This interaction forms the basis for many fundamental studies and practical applications. We describe here the first direct measurement of the resonance coupling on single gold nanorods. The dark- field scattering technique is employed. The nanorods are embedded in hydrogel to facilitate uniform dye adsorption. The adsorbed dye molecules exhibit both monomer and H-aggregate absorption bands. The same gold nanorods are measured before and after the dye adsorption. Both strong and weak coupling are investigated by selecting nanorods with different longitudinal plasmon bands. Excellent agreement between the experiments and an analytic theory is obtained. The resonance coupling reveals a unique three- band structure. The tunability of the coupling on individual nanorods is further demonstrated by photodecomposing the adsorbed dye molecules. KEYWORDS Absorption, dark-field scattering, gold nanorods, resonance coupling, surface plasmon I nteractions between noble metal nanocrystals and or- ganic adsorbates give rise to very interesting phenom- ena, such as plasmon-enhanced fluorescence, 1-6 fluo- rescence quenching, 7,8 plasmon resonance energy transfer, 9 and molecular-plasmonic resonance coupling. 10-20 Strong plasmonic-molecular resonance coupling occurs when the plasmonic resonance is degenerate with the molecular resonance. This interaction forms the basis for amplifying optical signals, fabricating optical devices, and detecting biological molecules and processes. Previous studies on the resonance coupling have been limited on ensemble metal nanocrystals, 10,11,13,15,17,18 where ensemble averaging im- pedes a thorough study. Here, we report on the first direct measurement of the resonance coupling on single gold nanorods. We employ the dark-field scattering technique 21 and use Au nanorods that are embedded in hydrogel matri- ces to facilitate the uniform adsorption of dye molecules. Through electrostatic interactions, the dye molecules are adsorbed uniformly on the Au nanorods that are embedded in hydrogel matrices. The adsorbed dye molecules exhibit both monomer and H-aggregate absorption bands. The same Au nanorods are precisely investigated before and after the dye adsorption. Both strong and weak resonance coupling have been studied by selecting nanorods with different longitudinal plasmon resonance energies, and excellent agreement between the experimental results and an analytic theoretical framework has been obtained by including the two molecular absorption bands in the theory. The coupling reveals a unique three-band structure in the energy diagram. The tunability of the resonance coupling has further been demonstrated by photodecomposing the dye molecules adsorbed on individual Au nanorods. Spatially isolated Au nanorods were embedded in agarose gel films (Figure 1a). The use of Au nanorods is because the longitudinal plasmon resonance energy of each nanorod is determined by the length-to-diameter aspect ratio, and they exhibit reduced plasmon damping compared to spherical Au nanocrystals. 21 Agarose gel has a three-dimensionally inter- connected porous network. Its pore size is on the order of a few hundred nanometers, and its pore wall is highly hydro- philic. Agarose gel is therefore an ideal matrix for the immobilization of Au nanorods. It can not only facilitate single-particle dark-field scattering measurements but also provide a highly flexible and chemically accessible environ- ment for physical and chemical processes to occur at specific local positions. The Au nanorods were grown in aqueous solutions using a seed-mediated method. 22 The as-grown Au nanorods are stabilized with cetyltrimethylammonium bromide (CTAB) surfactants. Anisotropic oxidation 23 and transverse over- growth 24 were employed to finely tailor the longitudinal plasmon resonance wavelengths of the nanorods within the range of 600-950 nm. Figure 1b shows a transmission electron microscopy (TEM) image of a representative nano- rod sample that exhibits an ensemble longitudinal plasmon resonance wavelength at 685 nm. The nanorods are cylin- drical, and their sizes are highly uniform. HITC dye (Figure 1c) was chosen to form organic-inorganic hybrid structures with the Au nanorods. When dissolved in water, HITC exhibits a major absorption peak at 736 nm with an absorp- * To whom correspondence should be addressed. E-mail: jfwang@phy.cuhk.edu.hk. Tel: +852 3163 4167. Fax: +852 2603 5204. Received for review: 09/1/2009 Published on Web: 00/00/0000 pubs.acs.org/NanoLett © XXXX American Chemical Society A DOI: 10.1021/nl902851b | Nano Lett. XXXX, xxx, 000-000