Determination of Fluorescence Quantum Yields in Scattering Media.

The fluorescence quantum yield is a measure of the efficiency of photon emission and quantifies the luminescent performance of a given sample. The determination of fluorescence quantum yields, particularly in scattering media, is relevant in the areas of materials science, technology and photonics. It is equally crucial when studying fluorescent bioanalytical probes and biological systems either for medical applications, physiological analyses or the interpretation of optical signals in nature. This type of determination represents a challenge since light scattering introduces an appreciable complexity in the measurements. Hence, the use of experimentally accurate methods and the understanding of their basis and principles is indispensable for obtaining reliable results. In addition, light re-absorption processes are usually very significant in these systems and the experimental quantum yields normally differ from the true quantum yields of the fluorophore. The first purpose of this work is to provide a clear and comprehensive compilation of the various optical methods that can be used for the determination of quantum yields in scattering media. A second purpose is to present the correction models to account for light re-absorption processes, applicable in each case. The advantages and disadvantages of each methodology are comparatively discussed, the difference between experimental and true quantum yield is clarified and it is explained which should be used depending on the case. Several examples previously published in literature are illustrated. The methods presented here are adequate for the study of very diverse samples such as suspensions, solid powders, films, animal tissues and even plant material.

Absolute and Relative Methods for Fluorescence Quantum Yield Evaluation of Quantum Dots.

Optical spectroscopy techniques are crucial for the evaluation and use of quantum dots (QDs) in life and materials science. In that context, the fluorescence quantum yield (Φf) is an essential parameter in the assessment of the luminescent features of QDs. The fluorescence quantum yield can be defined as the ratio of the number of emitted photons to the number of absorbed photons by a luminescent material. In this chapter, we describe absolute and relative methods to measure the fluorescence quantum yield of QDs in solution phase. The advantages and limitations of the techniques are reviewed.

Coloration of the Chilean Bellflower, Nolana paradoxa, interpreted with a scattering and absorbing layer stack model.

MAIN CONCLUSION: An absorbing-layer-stack model allows quantitative analysis of the light flux in flowers and the resulting reflectance spectra. It provides insight in how plants can optimize their flower coloration for attracting pollinators. The coloration of flowers is due to the combined effect of pigments and light-scattering structures. To interpret flower coloration, we applied an optical model that considers a flower as a stack of layers, where each layer can be treated with the Kubelka-Munk theory for diffusely scattering and absorbing media. We applied our model to the flowers of the Chilean Bellflower, Nolana paradoxa, which have distinctly different-colored adaxial and abaxial sides. We found that the flowers have a pigmented, strongly scattering upper layer, in combination with an unpigmented, moderately reflecting lower layer. The model allowed quantitative interpretation of the reflectance and transmittance spectra measured with an integrating sphere. The absorbance spectrum of the pigment measured with a microspectrophotometer confirmed the spectrum derived by modeling. We discuss how different pigment localizations yield different reflectance spectra. The absorbing layer stack model aids in understanding the various constraints and options for plants to tune their coloration.