The visualisation of permanent charges in electro-responsive, random co-polymer hydrogels with confocal laser scanning microscopy

A series of various monomers ratios between poly(3-sulfopropyl methacrylate)-ran-poly(hydroxyethyl methacrylate), p(SPMA)-ran-p(HEMA), hydrogels were synthesised by free radical polymerisation. The sulfonate group SO− 3 defined the anionic character of SPMA, and thus induced electric charges on the polymer backbone which subsequently the hydrogels are responsive to an electric field. A proof of concept for the use of CLSM to visualise the charge density distribution was performed. The hydrogels were characterised by swelling ratio, scanning electron microscopy (SEM), optical microscopy, Fourier transform infrared spectroscopy (FTIR) and confocal laser scanning microscopy. It was found 25% and 100% SPMA molar ratio yielded the highest swelling ratio, images acquired using optical microscope and SEM supported the results. FTIR detected the characteristic wavenumber of the SO− 3 group at 1040cm−1 , and the signal decreased along the SPMA molar ratio. Finally, the hydrogels were immersed in 0.1mM of Rhodamine B and characterised using CLSM. The acquired images showed an homogeneous electric charge density, and the light intensity decreased along the z-stack.