Anomalous Diffusion Inside Soft Colloidal Suspensions Investigated by Variable Length Scale Fluorescence Correlation Spectroscopy

Abstract

The diffusion of molecules and particles inside the aqueous suspension of soft colloids (polymer microgels) is investigated using variable length scale fluorescence correlation spectroscopy (VLS-FCS). Carbopol 940 is chosen as the model matrix system, and two factors affecting diffusion are investigated: the spatial hindrance and the diffusant–matrix interaction. By studying diffusion of molecules and particles with different sizes inside the suspension, VLS-FCS reveals the restricted motion at a short length scale, that is, in the gaps between the microgels, and normal diffusion at a larger length scale. The information on the gap’s length scale is also accessed. On the other hand, by tuning the pH value, the diffusant–matrix electrostatic attraction is adjusted and the results expose a short-time fast diffusion of probe molecules inside the gaps and a long-time restricted diffusion because of trapping inside the microgels. It is proved that VLS-FCS is a powerful method, investigating anomalous diffusion at different length scales and it is a promising approach to investigate diffusion in complex soft matter systems.

A Comprehensive Review of Fluorescence Correlation Spectroscopy

“…Being a powerful tool for detecting molecular concentration, hydrodynamic radius, diffusional dynamics, and the interaction of different biomolecules, FCS has been widely applied to different fields. Except for the applications in biology, FCS also has been applied in other fields [129][130][131]. For instance, FCS has been applied in physics for the study of dynamics/structure of liquid crystal molecules on silicon wafers [131], as well as anomalous diffusion inside soft colloidal suspensions [130].…”

“…Except for the applications in biology, FCS also has been applied in other fields [129][130][131]. For instance, FCS has been applied in physics for the study of dynamics/structure of liquid crystal molecules on silicon wafers [131], as well as anomalous diffusion inside soft colloidal suspensions [130]. We believe that, with further developments of lasers and sensors, FCS will become a more and more powerful tool for the study of molecular dynamics in different fields.…”

3D Super-Resolution Fluorescence Imaging of Microgels

“…For example, FRAP has been used to investigate the mobility of lysozyme inside oxidized starch polymer microgels (144) and the diffusion of linear, flexible tracer polymers through heterogeneous microgel packings (145). FCS, for instance, was applied to analyze the diffusion of macromolecules inside (large) microgels (146), to analyze the anomalous diffusion in microgel suspensions at different length scales (147), and to show the layer-by-layer assembly of polyelectrolytes onto microgels (148). Also, the resolution of FCS can be pushed beyond the diffraction limit when combined with STED, resulting in a technique called STED-FCS (149,150).…”

Hierarchically Coupled Ornstein–Uhlenbeck Processes for Transient Anomalous Diffusion

“…Quite often, the anomalous behavior is only transient, meaning the MSD is diffusive or ballistic at relatively short times, then it transitions to anomalous diffusion (subdiffusion or superdiffusion) and entails entirely Fickian diffusion at longer time scales. Anomalous diffusion in passive BM has been observed in soft biological systems [7][8][9][10][11][12], supercooled and ionic liquids [13][14][15][16][17][18], granular and glassy materials [19][20][21][22][23][24][25][26][27], colloidal suspensions [28,29], as well as diffusion under random external fields [30,31]. In active motion, anomalous diffusion has been reported in cell motion [32], chemically powered nanomotors [33], and even mammal movement [34].…”