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Refractive Index Mismatch Can Misindicate Anomalous Diffusion in Single‐Focus Fluorescence Correlation Spectroscopy
Author(s) -
Lehmann Swen,
Seiffert Sebastian,
Richtering Walter
Publication year - 2015
Publication title -
macromolecular chemistry and physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.57
H-Index - 112
eISSN - 1521-3935
pISSN - 1022-1352
DOI - 10.1002/macp.201400349
Subject(s) - fluorescence correlation spectroscopy , refractive index , diffusion , anomalous diffusion , chemistry , spectroscopy , fick's laws of diffusion , fluorescence , analytical chemistry (journal) , chemical physics , optics , thermodynamics , physics , organic chemistry , knowledge management , innovation diffusion , quantum mechanics , computer science
Fluorescence correlation spectroscopy (FCS) is often used to study diffusion in complex media, wherein the refractive index usually differs from that of the immersion medium. This paper assesses the effect of this refractive index mismatch. Confocal single‐focus and two‐focus fluorescence correlation spectroscopy (2fFCS) is used to probe the diffusion of tagged dextran tracers in water, dilute dextran solutions, acrylamide monomer solutions, polyacrylamide polymer solutions, and a cross linked polyacrylamide hydrogel. In these experiments, both the refractive index and the potential topological constraint and thermodynamic interaction to the probe‐diffusion are varied, and pairs of samples with same refractive indexes but different compositions are compared. Whereas 2fFCS shows no anomalous diffusion in any of them, single‐focus FCS indicates anomalous diffusion. In particular, the values of the stretching exponent of the fluorescence autocorrelation function, which is often interpreted to reflect the extent of anomaly of diffusion, do not vary systematically with the extent of topological or thermodynamic complexity of the different matrixes, but with their refractive index. This shows that apparent anomalous diffusion in FCS is at risk to be the result of refractive index mismatch rather than reflecting truly complex diffusion.