Hydrogels, such as poly(N-isopropyl acrylamide) and polyacrylamide, have been successfully moified onto the surface of Pt ultramicroelectrodes. The chemical modification has been established by the introduction of vinyl group onto the electrode surface, followed by the synthesis of the gels on the electrode surface. These gel-modified electrodes, where the shape of the gel is hemispherical in the swollen state, were immersed in aqueous solutions containing redox molecules, and electrochemical measurements were conducted. The chemical modification of the gels on the electrode surface allowed to conduct electrochemical measurements both in the swollen and shrunken states of the gels, and further in the transition states of swelling, i.e., from the swollen to shrunken states and vice versa. Independent determination of of the concentration and diffusion coefficient of the redox molecules inside the gels has been achieved by exploiting electrochemical methods both in equilibrated and trans
ient state of swelling. This provides a new methodology to study micro-environment and dynamics of hydrogels, and at the same time, to convert large swelling change of hydrogels, including the volume-phase transition, to electric signal.
In poly(N-isopropyl acrylamide)(PNIPAAm) gels, reaching equilibrated swelling, the concentration and diffusivity of redox molecule, [Ru(NH_3)_6]Cl_3, indide the gels were not so different from those of the outside solution in the swollen state, whereas considerable condensation and suppression in the diffusivity of the complex were found in the shrunken gels.
For polyacrylamide(PAAm) gels, the measurements not only in the equilibrated state but also in the transient state of swelling were conducted by using N-ferrocenylmethyl-N, N, N-trimethylammonium hexafluorophosphate (Fc) as a redox molecule. The transient electrolytic current vs time(i-t) curves measured in the shrinking process, where the gel was immersed into acetone from water, revealed the following facts :
1) The i-t curve showed a peak at a certain time, instead of simple decreasing.
2) A smaller size gel showed the peak at earlier time than a larger size gel, and the peak time was directly proportional to the square of the radius of the gel in water.
3) The peak current increased with decreasing the radius of the gel in water.
Mechanism for these results has been analyzed, taking into consideration the dynamics and spatial inhomogeneity of the gels in the shrinking process. Less