Biogenic synthesis: an alternative method for metallic and metal-based nanomaterials

Nanomaterials have been present for centuries as colloidal materials in historical objects such as Lycurgus Cup, medieval church windows and weapons made of Damascus steel. However, it was until Turkevich designed a controlled synthesis method for metallic nanomaterials (MNMs), specifically gold nanoparticles. Nowadays, many methods have been proposed for the preparation or synthesis of MNMs relying on two approaches: top-down and bottom-up. Physical methods rely on the former as they often turn macroscopic material into a nanometric one. These methods are relatively simple, scalable and offer high purity materials, but require large amounts of energy and specific equipment. On the other hand, chemical methods rely on bottom-up approaches, requiring only a precursor salt for the preparation of MNMs along with a reducing agent for the formation of nanomaterials. Chemical methods are versatile regarding equipment, generate more stable nanomaterials and have more control of the product’s size and shape. However, they are characterized by low purity and use of toxic solvents. Due to the environmental impact of these methods, biogenic synthesis has been proposed as an alternative bottom-up method, analogous with chemical synthesis but utilizing biological resources from plants, algae, bacteria, among others as reducing agents to form stable MNMs. Although these methods have their own disadvantages, they utilize renewable resources, are safer, and have comparable physicochemical properties. This review will mainly focus on i. the main biogenic synthesis methods of MNMs, ii. biological resources involved in these methods and iii. the properties of the obtained nanomaterials compared to the MNMs synthesized by conventional methods.