Light-metal interactions are at the basis of light manipulation beyond the diffraction limit. This emerging field of nanophotonics holds immense promise for coupling of electronics and photonics, for signal amplification and ultra-sensitive diagnostics. Light excites collective electron oscillations called surface plasmons. It is, however, difficult to study the interactions of plasmons with their environment and with nanoscale emitters such as fluorescent dyes. On the one hand, plasmonic interactions occur on a very small length scale of only a few nanometers. On the other hand, the metallic nanostructure has a complex interaction with the emitters in their proximity, changing not only the local electric field but also the emission properties yielding brighter or darker emitters.
The research group of Prof. Philip Tinnefeld, Institute of Theoretical & Physical Chemistry, and Laboratory for Emerging Nanometrology (LENA) has now developed a simple method to fully characterize the interaction of emitters and metallic nanostructures using single fluorescent dye molecules as ultimately small probes. Their results of a first time characterization of several metallic nanostructures are presented in the current issue of Nature Communications.
The way metallic nanostructures affect the fluorescence is non-trivial and ranges from reduction of the fluorescence signal to amplification by several orders of magnitude depending on parameters like size, shape or material of the nanostructure. When illuminated with light, the nanostructure alters the local light intensity the dye molecule experiences. Once the dye is excited, the nanostructure also influences the dye's tendency to emit light or to lose the energy by turning it into heat. Commonly researchers measure only two parameters, that is, the fluorescence intensity and fluorescence lifetime which does not allow determining the three unknowns: the local light intensity, the change of the radiation tendency as well as the change of thermal losses.
The research group of Prof. Tinnefeld has now found a way to fully characterize the dye-nanostructure interaction in arbitrary environments by elegantly exploiting the intrinsic blinking of the dye molecule as third independent parameter. Blinking of dye molecules is a common phenomenon occurring when the dyes enter a dark, non-fluorescent state such as the triplet state, for some time (see Figure 1 for examples of blinking dyes).
To demonstrate the technique, a well-defined system of a dye and a gold nanoparticle at defined distance perfectly arranged by a DNA nanostructure (so-called DNA origami, see Figure 2) is studied and agreement with theoretical expectations is obtained. The researchers then extend the approach to more complex structures that are not amenable to direct theoretical treatment such as bigger particles and small holes in metal film, so-called zeromode waveguides. These zeromode waveguides are also central elements of innovative DNA sequencing schemes. Unexpectedly, low fluorescence reduction was found for small zeromode waveguides indicating future potentials for further nanophotonic applications.
Publication:
P. Holzmeister, E. Pibiri, J.J. Schmied, T.Sen, G.P. Acuna, P. Tinnefeld
Quantum yield and excitation rate of single molecules close to metallic nanostructures
Nature Communications (2014), 5:5356
DOIi: 10.1038/ncomms6356.
Contact:
Prof. Philip Tinnefeld
Institut für Physikalische und Theoretische Chemie
Arbeitsgruppe NanoBioSciences
Laboratory for Emerging Nanometrology
Technische Universität Braunschweig
Hans-Sommer-Strasse 10
38106 Braunschweig
Phone: (+49) 531 391 5330
Email: p.tinnefeld(at)tu-braunschweig.de
www.tu-braunschweig.de/pci