Diffusion of single streptocyanine molecules in the nanoporous network of sol-gel glasses

A new streptocyanine dye has been doped into monolithic silica gels with different porosities, characterized by nitrogen adsorption isotherms. Single-molecule tracking with a wide-field fluorescence microscope is used to determine the diffusivity of the dye in the nanoporous network of the host. The majority of molecules in the gel with wider pores (22 nm) diffuse freely with an average diffusion coefficient of D = 4.7 x 10(-9) cm(2) s(-1). Most of those in the gel with narrower pores (3 nm) are trapped in regions ranging in size from 50 nm - the positioning accuracy of the setup-up to 200 nm. Others are alternately trapped and freely diffusing with an average D = 3.5 x 10(-10) cm(2) s(-1). The significance of the distribution of diffusion coefficients measured by single-molecule tracking is discussed. Besides traps, the wide spread of the diffusion coefficients for individual molecules in both gels reveals pronounced microscopic inhomogeneities.