Intracellular Biosynthesis of Gold Nanoparticles for Monitoring Microalgal Biomass via Surface-Enhanced Raman Spectroscopy

Microalgae vary their biomass products according to the growth conditions and therefore emerge as diverse nutritional food resources. To monitor and identify the composition of algal biomass, surface-enhanced Raman spectroscopy (SERS) has been used due to the nondestructive and specific detection. However, such strategies typically require the pre-synthesis of the Raman-enhanced substrates. Herein, we proposed a streamlined method for intracellular synthesis of gold nanoparticles as a Raman substrate through biomineralization of HAuCl4 for analyzing the microalgal biomass by SERS. Our results showed that gold nanospheres (20 nm) and gold hyperbranched nanostructures (AuHNs, 120 nm) were synthesized inside Chromochloris zofingiensis through bioreduction of 0.5 and 1.0 mM HAuCl4 without any reducing and capping agents, respectively. Both the intracellular nanostructures exhibited negligible influence on the yield of biomass composition. Particularly, AuHNs showed the highest SERS activity which enhanced the Raman signal of a single C. zofingiensis cell by 9.7 times compared to the native C. zofingiensis, especially the signals for carotenoids. We further apply this intracellular biosynthesis method to other microalgae species with different cell wall structures (e.g., Euglena gracilis and Nitzschia laevis). This intracellular biosynthesis of AuHNs as Raman substrates provides a promising alternative to monitor the change of the microalgal biomass composition in a facile and time-effective way.