Leaching Behavior of a Fluorescent Pyoverdin Immobilized in Sol-Gel Glass

The leaching behavior of fluorescent siderophore, pyoverdin (iron biosensor), immobilized in acid-catalyzed sol-gel glass is examined. By varying the R value (molar ratio of water:silicon), three different formulations of pyoverdindoped sol-gel glass pellets (R = 5.6, 8.2, 10.8) were made with tetramethyl orthosilicate (TMOS). At three different aging times (2, 4, and 6 weeks), 7-day leaching experiments were conducted on whole and ground pellets. Two different leaching solutions were used: 0.1 M acetate buffer, pH 5.0, and 1 M HCl. Pyoverdin immobilization resulted in conformation changes as suggested by the appearance of two emission peaks centered at 440 and 505 nm. As expected, pyoverdin leached more rapidly from the ground glass; 61 % of the pyoverdin leached from the ground glass within the first 20 minutes, while after 3 hours, only 54 % of the pyoverdin had leached from the whole pellets. As the sol-gel glass aged from 2 to 6 weeks, the initial fluorescence of the ground glass decreased by 34 – 46 % for the three sol-gel formulations, and the 7-day cumulative leachate decreased by 13 – 31 %. However, sol-gel with the lowest R value (5.6) retained pyoverdin better that sol-gels at higher R values. The release of pyoverdin was also characterized by the diffusional exponent (n), which ranged from 0.527 to 0.802 for the ground glass, and from 0.276 to 0.415 for the whole pellets, confirming that whole pellets retained pyoverdin better than ground glass. Taken together, the results suggest for a given sensor configuration, retention of pyoverdin can be optimized with respect to pellet size, R value and aging time.