Characterization of Heterogeneous Polyacrylamide Hydrogels by Tracking of Single Quantum Dots

Single particle tracking (SPT) microscopy is a powerful experimental technique for characterizing mobility within complex media. In this paper, we study the motion of single core/shell CdSe/ZnS quantum dots (QDs) within synthetic polyacrylamide (PAAm) hydrogels to provide insight into the structure of these heterogeneous gel networks. Subdiffusive mean-square displacements (MSD) and non-Gaussian van Hove functions are observed for gels with a range of cross-linker contents, which we interpret in terms of transient caging events of QDs due to the presence of denser regions of the network. Experimentally determined caging time distributions follow power-law behaviors for short times, consistent with the predictions of a simple random trap model for the confining energy landscape. Over the range of composition studied, greater cross-linker concentrations are found to yield an increase in the frequency of long trapping events, corresponding to larger characteristic trapping energies and therefore a lower overall mobility.