Temperature responsive sustained-release microsphere for harmful algal bloom control: preparation, algicidal activity and eco-safety.

Temperature is one of the most important factors driving cyanobacterial harmful algal blooms (CHABs). However, existing algicide designs rarely consider the influence of temperature on algal growth. Novel temperature-responsive sustained-release microspheres (T-SRMs) of anti-algal 5,4'-dihydroxyflavone (DHF) were designed using n-heptadecane as the temperature-responsive material, which can adjust DHF release behavior in response to temperature. Compared to DHF direct application, T-SRMs showed higher inhibition rates against Microcystis aeruginosa and sustained inhibitory activity at various temperatures (15-25 °C). The theoretical inhibition time of T-SRMs substantially exceeds that of conventional SRMs, with the potential to cover the entire CHABs outbreak period and extend the duration of validity. T-SRMs can accurately regulate DHF release rate based on water temperature. When the temperature is higher than the melting point of n-heptadecane (22 °C), the phase transition of heptadecane significantly promotes the DHF release rate. Moreover, salinity had no significant effect on the DHF release behavior, and the temperature-responsive materials can be adjusted to linearly regulate the release of DHF, which is favorable for the wide application of T-SRMs in freshwater, brackish and salt waters with flexible temperature windows. In addition, the T-SRMs had no apparent toxic effects on Chlorella vulgaris, Daphnia magna and Danio rerio. Our study has an important scientific significance to achieve a more intelligent, accurate and efficient control of CHABs.