Metal-Enhanced Fluorescence of Chlorophylls in Single Light-Harvesting Complexes
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Metal - Enhanced Fluorescence of Chlorophylls in Single Light - Harvesting Complexes Sebastian Mackowski1*, Dawid Piatkowski1, Stephan Wörmke2, Achim Hartschuh2, Christoph Bräuchle2, Tatas H. P. Brotosudarmo3, Hugo Scheer3, Ashish Agarwal4, Nicholas A. Kotov4, Alexander O. Govorov5 1 Institute of Physics, Nicolaus Copernicus University, Grudziadzka 5/7, 87-100 Torun, Poland 2 Department of Chemistry and Biochemistry and Center for Nanoscience, Ludwig-MaximilianUniversity, Butenandtstrasse 11, D-81377 Munich, Germany 3 Department of Biology, Ludwig-Maximilian-University, D-80638 Munich, Germany 4 Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA 5 Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA * electronic mail: [email protected] ABSTRACT We show that the fluorescence of peridinin-chlorophyll a-protein complexes can be strongly enhanced via coupling with plasmon excitations localized in metal nanostructures. The results of ensemble and single-molecule spectroscopy experiments at room temperature demonstrate six-fold increase of the emission intensity of the light-harvesting complex when it is placed in the vicinity of chemically prepared silver islands. Irrespective of the enhancement, we observe no effect of the metal nanoparticle on the fluorescence emission energy of the complex. This observation implies that plasmon excitations may be applied for controlling the optical properties of complex biomolecules. INTRODUCTION Plasmon resonances generated in metal nanoparticles have recently become a very vibrant field of research, crossing and joining together many disciplines such as spectroscopy, physics, bioimaging, nanoengineering [1-3]. This versatility is driven in part by the influence imposed by plasmons, and electromagnetic field associated with these excitations, on the optical properties of dipoles located in their vicinity. Depending upon particular geometry of the system as well as its spectral characteristics, plasmon excitation could lead either to the quenching of the fluorescence, or the enhancement of the emission. Both scenarios have been observed for semiconductor nanocrystals [4,5] and dye molecules [6]. Metal - enhanced fluorescence (MEF), which describes the situation of fluorescence increase upon coupling with plasmons in metal nanoparticles, has been observed for many hybrid systems composed mainly of very stable and highly fluorescing emitters, such as organic dyes and semiconductor nanocrystals and nanowires. In addition, attempts to influence the emission properties of weakly fluorescing systems such as DNA [7], carbon nanotubes [8], and lightharvesting complexes [9,10] have been made. The light-harvesting complexes are proteinpigment systems containing chlorophyll (Chl) and carotenoid molecules embedded in a protein. They collect light energy and transfer it to reaction centers where the charge separation takes place, the first step in photosynthesis process. In this communication we describe the result
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