The University of Osaka achieves environmentally friendly gold nanoparticle synthesis using microalgae in one step without harmful chemicals

An international collaborative research group consisting of Doctoral Student Reham Samir Hamida of the Graduate School of Science, Associate Professor Madoka Suzuki from the Laboratory for Physical Biology, Institute for Protein Research, Specially Appointed Professor (full-time) Yoshie Harada from the Premium Research Institute for Human Metaverse Medicine (PRIMe) at the University of Osaka, Assistant Professor Shingo Sotoma from Kyoto Institute of Technology; and Associate Professor James Chen Yong Kah from the National University of Singapore has developed a method to synthesize high-quality gold nanoparticles using a green synthesis method with microalgae extracts. Their work was featured on the front cover of the November 7 issue of ACS Sustainable Chemistry & Engineering.

Gold nanoparticles are extremely small particles of gold, typically measuring only a few dozen nanometers. They possess the property of absorbing light and converting it into heat (photothermal conversion) and there are expectations for their use in cancer photothermal therapy. However, conventional chemical synthesis methods inevitably lead to particle aggregation and instability, requiring harmful chemical substances to prevent these issues, which posed challenges regarding biocompatibility and environmental impact.

The research group focused on a novel thermotolerant green alga, Coelastrella thermophila, and the cyanobacterium Arthrospira sp. (spirulina), which is also known for its applications as food. The extracts from these algae are rich in natural molecules such as proteins, polysaccharides, and fatty acids with reducing properties. By using these as both reducing and stabilizing agents, the researchers found that gold nanoparticles could be synthesized in one step without using harmful chemicals and successfully optimized the conditions.

The resulting gold nanoparticles had diameters of several tens of nanometers, with algae-derived molecules naturally adsorbed on their surfaces in a self-assembled manner. As a result, they remained stably dispersed in solution without aggregation for more than 48 hours and maintained high photothermal stability.

Compared with gold nanoparticles obtained through conventional chemical synthesis, surface modification with natural molecules mitigated interactions with cells, resulting in lower toxicity toward normal cells (Vero cells) and excellent biocompatibility.

Furthermore, when irradiated with green laser light (532 nanometers) to verify the photothermal effect, the biologically derived gold nanoparticles efficiently converted light energy into heat and also generated reactive oxygen species (ROS), leading to the death of cancer cells (HeLa cells).

This established an environmentally friendly and sustainable process that enables the synthesis of gold nanoparticles in one step without using harmful chemicals. This method simultaneously achieves stability, biocompatibility, and functionality, which have been challenging for nanoparticles until now. In the future, it has the potential to become the foundation for next-generation nanomedicine technologies such as photothermal therapy, drug delivery, and biosensing through the control of particle size and shape. Additionally, biologically derived nanomaterial synthesis without the use of chemical agents is a new manufacturing technology that can significantly reduce environmental impact and is expected to have ripple effects not only in the medical field but also in industrial applications such as catalysts, electronic devices, and optical materials.

Suzuki commented: "We aim to understand how living organisms sense and respond at the cellular level to heat released by environmental temperature changes and life activities. Developing observation and manipulation technologies for this purpose is also an important aspect. This project began as part of that technological development, starting with the research theme of our international student Ms. Reham, and developed into interdisciplinary international collaborative research with her at the center. Please also take a look at the beautiful illustration she created on the cover of the journal."

Journal Information
Publication: ACS Sustainable Chemistry & Engineering
Title: Microalgae-Mediated Synthesis of Functionalized Gold Nanoparticles with High Photothermal Stability
DOI: 10.1021/acssuschemeng.5c07786