Investigation of Molecular Diffusion at Block Copolymer Thin Films Using Maximum Entropy Method-Based Fluorescence Correlation Spectroscopy and Single Molecule Tracking

• Published in: Journal of fluorescence 2022-09, Vol.32 (5), p.1779-1787
• Main Authors: Xue, Lianjie, Jin, Shiqiang, Nagasaka, Shinobu, Higgins, Daniel A., Ito, Takashi
• Format: Article
• Language: English
• Subjects: Analytical Chemistry; Atmospheric models; Biochemistry; Biological and Medical Physics; Biomedical and Life Sciences; Biomedicine; Biophysics; Biotechnology; block copolymer; Block copolymers; Coefficients; Data acquisition; Data analysis; Diffusion barriers; Diffusion rate; Ethylene oxide; fluorescence correlation spectroscopy; fluorescence correlation spectroscopy, maximum entropy method, single-molecule tracking, block copolymer; Fluorescent indicators; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Investigations; Maximum entropy method; Molecular diffusion; Polyethylene oxide; Polystyrene resins; single-molecule tracking; Spectroscopy; Spectrum analysis; Thin films; Tracking
• ISSN: 1053-0509; 1573-4994
• DOI: 10.1007/s10895-022-02975-6

Fluorescence correlation spectroscopy (FCS) has been widely used to investigate molecular diffusion behavior in various samples. The use of the maximum entropy method (MEM) for FCS data analysis provides a unique means to determine multiple distinct diffusion coefficients without a priori assumption of their number. Comparison of the MEM-based FCS method (MEM-FCS) with another method will reveal its utility and advantage as an analytical tool to investigate diffusion dynamics. Herein, we measured diffusion of fluorescent probes doped into nanostructured thin films using MEM-FCS, and validated the results with single molecule tracking (SMT) data. The efficacy of the MEM code employed was first demonstrated by analyzing simulated FCS data for systems incorporating one and two diffusion modes with broadly distributed diffusion coefficients. The MEM analysis accurately afforded the number of distinct diffusion modes and their mean diffusion coefficients. These results contrasted with those obtained by fitting the simulated data to conventional two-component and anomalous diffusion models, which yielded inaccurate estimates of the diffusion coefficients. Subsequently, the MEM analysis was applied to FCS data acquired from hydrophilic dye molecules incorporated into microphase-separated polystyrene- block -poly(ethylene oxide) (PS- b -PEO) thin films characterized under a water-saturated N 2  atmosphere. The MEM analysis revealed distinct fast and slow diffusion components attributable to molecules diffusing on the film surface and inside the film, respectively. SMT studies of the same materials yielded trajectories for mobile molecules that appear to follow the curved PEO microdomains. Diffusion coefficients obtained from the SMT data were consistent with those obtained for the slow diffusion component detected by MEM-FCS. These results highlight the utility of MEM-FCS and SMT for gaining complementary information on molecular diffusion processes in heterogeneous material systems. Graphical Abstract