Green synthesis of silver nanoparticles using microalgae acclimated to high CO2

Silver nanoparticle (AgNP) synthesis using microalgae is a promising low-cost sustainable alternative. High CO2 tolerant microalgae have not yet been explored for this application. This study pretends to determine the potential of an environmental green algae Desmodesmus abundans acclimated to low and high CO2 (LCA and HCA strains, respectively) as a platform for AgNPs synthesis and evaluate the effect of pH (5, 7.5, 11) and biological component (cells, supernatant and both components) in comparison to a collection strain Spirulina platensis. Exponential grown cultures were centrifuged, the pellet resuspended in 10 mM AgNO3 prepared in water or supernatant at different pH and incubated for 40 h (24 °C, continuous light). After, the filtrated solution was characterized using UV–visible absorption spectra, Zetasizer and scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy. Results showed drastic differences between strains. No nanoparticles were observed with strain LCA, except at pH 11 using the pellet, which generated large particles (127.8 ± 14.8 nm diameter, −26.7 ± 2.4 mV zeta potential). In contrast, the HCA strain in these conditions showed the smallest AgNPs with the lowest zeta potential (14.9 ± 6.4 nm, −32.7 ± 5.3 mV). When the supernatant was conserved, HCA exhibited AgNPs in all pH solutions, with smallest sizes at pH 11 (27.7 ± 14.0 nm, −20.0 ± 1.8 mV). In addition, the HCA supernatant alone showed reducing potential for AgNP synthesis (51.8 ± 20.7 nm, −17.4 ± 1.3 mV). S. platensis also generated AgNPs in all conditions, but nanoscale particles were observed only at pH 11 (18.3 ± 7.5 nm, −33.9 ± 2.4 mV). In conclusion, high CO2 atmospheres seem to potentiate AgNPs synthesis by microalgae with interesting properties accordingly to biological component employed, representing a potential byproduct in CO2 mitigation systems.