Composite Au Nanostructures for Fluorescence Studies in Visible Light V. G. Kravets, † G. Zoriniants, ‡ C. P. Burrows, ‡ F. Schedin, † A. K. Geim, † W. L. Barnes, ‡ and A. N. Grigorenko* ,† † School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, U.K. and ‡ School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, U.K. ABSTRACT We present results from composite plasmonic nanostructures designed to achieve the cascaded enhancement of electromagnetic fields at optical frequencies. Our structures comprise a small metallic nanodisc suspended above a larger disk. We probe the optical properties of these structures by coating them with a layer of a visible-light fluorophore and observing fluorescence signals with the help of scanning confocal microscopy. A 43 ( 5-fold increase in the far-field fluorescence signal has been observed for two-tier composite nanostructures, when compared to the signal obtained from individual nanodiscs. Our results offer the prospect of using such nanostructures for field concentration, optical manipulation of nanoobjects, chemical and biological sensing. KEYWORDS Plasmonics, nanostructures, field enhancement, giant fluorescence S trong enhancements of electromagnetic fields are essential in nonlinear optics 1,2 photochemistry and biophysics 3-5 and light-matter interactions. 6,7 Metallic nanoparticles allow one to achieve a high value of field enhancement through the excitation of localized surface plasmons. 1,2,8-18 Strong near-fields promise exciting ap- plications in different areas of science and technology, for example, Raman or fluorescence studies with single mol- ecule sensitivity. 16-18 The field strength can be increased even further in particle conglomerates, such as fractals, 9 particle dimers 10,11 and aggregates, 12 which can be ef- fectively used for manipulation of nano-objects 13,14 and chemical and biological sensing. There are two types of nanostructured materials that have been pursued recently for optical field enhancement. One involves regular, well-defined single nanostructures, for example, optical antennas. 1,10,11,15 Large field-enhance- ments have been demonstrated for nanoantennas by mea- suring nonlinear photon conversion from electrons inside the metal or dilute plasma outside. 1,10,11,15 It remains to be seen if such structures (mostly working in the infrared) could be successfully applied for practical Raman and fluorescence measurements in the visible; for recent progress see re- view. 16 The other structural type is based on more complex nanostructures, typically aggregates of nanoparticles formed by chemical synthesis. Such structures have been the focus of a number of reports on strongly enhanced Raman scat- tering 17 or fluorescence 18 where the enhancement is often attributed to some “hot” nanoparticles (or rough substrate). The sensitivities demonstrated in these works have the potential to revolutionize biosensing if one could find a way to manufacture hot particles reproducibly. Here we present results that provide a bridge between these two approaches. We demonstrate regular, electron-beam lithography (ebl) synthesized two-tier plasmonic nanostructures that show robust and reproducible large enhancements of far-field fluorescence measured using visible-light fluorescent agents. Our nanostructures are comprised of two coaxial gold discs of different diameter stacked one on top of the other and separated by a dielectric spacer. Two different designs were explored, both being made by electron-beam lithog- raphy. The first design features a larger metallic disk of diameter D with a cylindrical hole of diameter d filled with a dielectric column produced by overexposed poly(methyl methacrylate) (PMMA) e-beam resist with a smaller metallic disk also of diameter d placed at the top of the column. A schematic and a scanning electron micrograph of a nano- structure of this design are shown in Figure 1a. We refer to this structure as a tower-type structure (TS). The fabrication of a structure of analogous geometry and their reflection spectra arising in the configuration of attenuated total reflectance have been discussed in ref 19. The second structure is topologically simpler, the large metallic disk with a hole is replaced by a solid disk; see Figure 1b. We refer to it as a pagoda-type structure (PS). Details of the fabrication of both structures are included in the Supporting Informa- tion. The smaller gold nanoparticle diameter d and the larger gold disk diameter D were varied in the submicrometer range. To probe the field enhancement afforded by these struc- tures, we coated them with a layer of a fluorescent dye, oxazine 1 perchlorate, randomly dispersed in a neutral polymer host, PMMA; see Figure 1a,b bottom panels. We excited the dye using a confocal laser arrangement with a laser (633 nm) and collected fluorescence in the 650-800 nm band. We showed experimentally in ref 20 that the * To whom correspondence should be addressed. E-mail: sasha@manchester.ac.uk. Received for review: 10/20/2009 Published on Web: 02/09/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 874 DOI: 10.1021/nl903498h | Nano Lett. 2010, 10, 874–879