[en] The effect of Cu²⁺ ions on aggregation behaviors of poly (l-glutamic acid) (PLGA)-functionalized Au nanoparticles was investigated. It was found that the concentration of Cu²⁺ ions had a significant influence on the folding and the formation of intermolecular hydrogen bonding of PLGA and thus the aggregation of the Au nanoparticles. In the absence of Cu²⁺ ions, the Au nanoparticles underwent reversible pH-dependent aggregation attributed to the folding/unfolding of PLGA and the formation/breakage of intermolecular hydrogen bonds between PLGA molecules. In the presence of low concentration of Cu²⁺ ions (i.e., 10 μM), folding of PLGA and aggregation of the Au nanoparticles were facilitated due to the charge screening effect of Cu²⁺ ions, while the reversibility was partly maintained. In the presence of high concentration of Cu²⁺ ions (i.e., 50 μM), aggregation of the Au nanoparticles was dominated by the coordination interaction between PLGA and Cu²⁺ ions and the aggregation became irreversible due to the blocking of the formation of intermolecular hydrogen bonds by Cu²⁺. These results suggested that Au nanoparticles may be used as a colorimetric probe to monitor the interactions between metal ions and peptides, which are essential for exploring the physiological effect of metals ions.

[en] The using of macromolecular additives is known to be a simple and effective way to improve the activity of immobilized enzymes on solid support, yet the mechanism has not been well understood. Taking horseradish peroxidase (HRP) as an example, only 30 % of its catalytic activity was kept after being immobilized on the surface of 25-nm Au nanoparticles, mainly attributed to the conformational change of the heme-containing active site. The catalytic activity of HRP was significantly improved to 80 % when a certain amount of bovine serum albumin (BSA) was added at the initial stage of the immobilization. Systematic spectral investigation indicated that the addition of BSA inhibited the tertiary structure change around the active site, which was a prerequisite for improved activity of the immobilized HRP. Steady-state kinetic analyses revealed that the introduction of BSA could effectively improve the turnover rate of substrate to product in spite of slight reduced affinity to substrates, which also contributed to the improved catalytic activity