Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 108

5 Collective Effects Observed with Organic Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.1 Superquenching, Concentrational Quenching, and Directed Homo-FRET . . . . . . . . . 117
5.2 Wavelength Converting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.3 Light-Harvesting (Antenna) Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.4 Red-Edge Effects in Energy Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
6 Collective Effects in Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
6.1 FRET Modulation of Reporter Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
6.2 Plasmonic Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7 Realization of Collective Effects in Sensing and Imaging Technologies . . . . . . . . . . . . . . . . . 124
7.1 Collective Effects in Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
7.2 Collective Effects in Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
8 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

1 Introduction

Fluorescence is a well-observed phenomenon characteristic of many materials and
the different forms of their aggregation. Meantime the vast majority of studies on
fluorescence have been on small organic molecules in liquid solutions. Parameters
of their emission (intensity, lifetime, anisotropy, and positions of excitation and
emission spectra) were found to be extremely sensitive to intermolecular interac-
tions [ 1 ], which justifies their extensive application in various sensing technologies
[ 2 ]. The disadvantages of organic dyes (low photostability, insufficient brightness,
short lifetimes, etc.) have resulted in competition from luminescent metal-ligand
complexes, semiconductor nanoparticles (Quantum Dots), and conjugated poly-
mers. These new materials show advanced performance in a variety of applications
[ 3 ]. However, the development of organic fluorophores is ongoing, and they
demonstrate remarkable improvement in many important properties [ 4 - 6 ]. In
addition, technologies were developed for incorporation of organic dyes into
organic [ 7 ] and inorganic [ 8 ] polymer nanoparticles and for making nanoparticles
from aggregated forms of the dyes [ 9 ], which allow increasing their photostability
and brightness comparable with that of Quantum Dots. The fact that the basic
properties of organic dyes that are very attractive for applications can be retained
and improved in these aggregates is important. Since all types of fluorescence
reporters (molecules or nanoparticles) can be combined in nanocomposites, new
routes to the perfection of fluorescence reporters by designing such composites
have emerged.
Based on the specific requirements for the operation of fluorescence reporters,
we have divided them into two broad categories [ 2 ]. One category comprises
reporters that serve as labels and tags . Their only response is based on their
presence in a particular medium or at a particular site, and the major parameters
that have to be optimized are their brightness and photostability. Brightness and
durability can be increased dramatically if, instead of single molecules, we use
nanoscale structures containing hundreds or thousands of them, and if we incorporate