Hydrogel-Encapsulated Enzyme Facilitates Colorimetric Acute Toxicity Assessment of Heavy Metal Ions

Conventional analysis of heavy metal ions in water requires highly skilled staff and sophisticated equipment. These limitations make conventional approaches difficult to perform analysis on-site without delay. Herein, we report a facile colorimetric sensing system developed for acute toxicity assessment of heavy metal ions. A bioactive enzyme, β-galactosidase, was used as sensing agent rather than bacteria or other higher organisms to improve selectivity and response time. The developed bioassay is capable of assessing the toxicity of heavy metal ions such as Hg(II), Cd(II), Pb(II), and Cu(II). The effects of enzyme concentration on the assessing performances (i.e., sensitivity and response time) of bioassay were explored and illustrated. Generally, low enzyme concentration facilitates sensitivity enhancement, achieving a 50% inhibiting concentration (IC50) of 0.76 μM (=152 ppb) Hg(II), and high enzyme concentration ensures quick response, enabling a response time down to 9 min. Moreover, the enzyme and substrate were respectively encapsulated by hydrogel to further simplify the assay procedure and enhance the stability of the enzyme. The hydrogel-encapsulated enzyme worked well even when heated up to 60 °C and retained ca. 90% activity after storage for 5 months. Moreover, the developed toxicity-assessing system is feasible for assessing toxicity of actual water samples. This assay approach is low cost and time effective and has no potential ethic issues. In addition, this work paves the way for the development of toxicity assessment kits for on-site analysis based on functional bioactive molecules.