Diffusion and concentration of molecular probes in thermoresponsive poly(N-isopropylacrylamide) hydrogels:: Effect of the volume phase transition

Poly(N-isopropylacrylamide), NIPA, thermoresponsive hydrogels with well-defined concentrations of an electroactive probe, 1,1'-ferrocenedimethanol, Fc(MeOH)(2), were prepared. The discontinuous reversible volume phase transition of such gels occurs at 32 +/- 1 degreesC and results in a release of similar to93% of the solution from the polymeric network. Transport of Fc(MeOH)(2) in both swollen and collapsed gels was studied using steady-state voltammetry and chronoamperometry at platinum microelectrodes. The diffusion coefficient of Fc(MeOH)(2) in collapsed gels was similar to2 orders of magnitude smaller than that in swollen gels. UV/vis spectroscopic studies showed that for 3.0% NIPA gel, the concentration of Fc(MeOH)(2) in the collapsed phase was similar to6 times higher than that in released solution and 4.5 times higher than in the original swollen gel.