Characterization of cyanine dye-labeled poly(N-isopropylacrylamide) core/shell microgels using fluorescence resonance energy transfer

The swelling behavior of the core component in poly(N-isopropylacrylamide) (pNIPAm) core/shell microgels is investigated via fluorescence resonance energy transfer (FRET). Photon correlation spectroscopy (PCS) data show that the cross-linked polymer network in the core is hindered from expanding to its maximum volume in the presence of the added shell. Covalent attachment of the sulfoindocyanine donor/acceptor pair of Cy5/Cy5.5 to the core component allows investigation of the core swelling behavior with FRET. In the absence of the shell, only a small degree of energy transfer is observed in the core when it is swollen to its maximum volume below the pNIPAm lower critical solution temperature (LCST) of 31 degreesC. Addition of a pNIPAm shell produces a significant degree of FRET under the same solution conditions, illustrating that the polymer chains in the core adopt a constrained conformation relative to their fully swollen state. The volume phase transition behavior of the fluorescently labeled core and core/shell particles was interrogated with PCS and correlates well with FRET analysis over the same temperature range, providing evidence that donor and acceptor molecules are homogeneously distributed throughout the core. The structure-function relationship between the core and shell is explained in terms of a radially distributed cross-linker density gradient created in the core and shell components during the two-stage particle synthesis. Furthermore, these analyses allow for calculation of the actual shell thickness, as the apparent shell thickness obtained from PCS measurements underestimates the actual thickness by an amount equivalent to the compression-induced core radius decrease.