Bioresponsive systems based on polygalacturonate containing hydrogels

Polysaccharide acid (PSA) based devices (consisting of alginic acid and polygalacturonic acid) were investigated for the detection of contaminating microorganisms. PSA–CaCl2 hydrogel systems were compared to systems involving covalent cross-linking of PSA with glycidylmethacrylate (PSA–GMA) which was confirmed with Fourier Transformed Infrared (FTIR) analysis. Incubation of PSA–CaCl2 and PSA–GMA beads loaded with Alizarin as a model ingredient with trigger enzymes (polygalacturonases or pectate lyases) or bacteria lead to a smoothening of the surface and exposure of Alizarin according to Environmental Scanning Electron Microscopy (ESEM) analysis. Enzyme triggered release of Alizarin was demonstrated for a commercial enzyme preparation from Aspergillus niger and with purified polygalacturonase and pectate lyase from S. rolfsii and B. pumilus, respectively. In contrast to the PSA–CaCl2 beads, cross-linking (PSA–GMA beads) restricted the release of Alizarin in absence of enzymes. There was a linear relation between release of Alizarin (5–348 μM) and enzyme activity in a range of 0–300 U ml−1 dosed. In addition to enzymes, both PSA–CaCl2 and PSA–GMA beads were incubated with Bacillus subtilis and Yersinia entercolitica as model contaminating microorganism. After 72 h, a release between 10 μM and 57 μM Alizarin was detected. For protection of the hydrogels, an enzymatically modified PET membrane was covalently attached onto the surface. This lead to a slower release and improve long term storage stability based on less than 1% release of dye after 21 days. Additionally, this allowed simple detection by visual inspection of the device due to a colour change of the white membrane to orange upon enzyme triggered release of the dye.