# Biogenic Nanoparticles: Synthesis, Characterisation and Applications
## 1 Introduction  
The advent of nanotechnology in the 1960s commenced a new era of materials science. The field has brought together many academic disciplines such as biology, chemistry, materials engineering, medicine and physics, with the aim of developing materials at the nanoscale. Nanoparticles include many types of materials comprised of dimensions less than 100 nm in size [1]. They are classified by the number of dimensions in which an electron can be confined to, i.e., 0-dimensional (0D), 1-dimensional (1D), etc. Some of the common geometries include spheres (0D), wires and rods (1D) and thin films (2D). The synthesis of nanoparticles is of great importance because of the unique properties they exhibit including electronic, magnetic, physiochemical features including control of size, increased surface area to volume ratio and functionalities [2]. The main reasons for these unique properties of nanoparticles are due to (i) confinement of electrons in such particles, (ii) particle size changes to modify bandgap energies and (iii) large surface to volume ratio [3]. These unique properties of nanoparticles, including biological, chemical, electrical, magnetic, optical and physical properties [4], differ greatly from the same materials in their bulk form. For examples, nanomaterials offer enhanced Raman and Rayleigh scattering properties in metal NPs (such as gold and silver), supermagnetic properties in magnetic materials and a quantum effect in semiconductors [5]. Such NPs form the next generation building blocks for biomedical [6], chemical, electronic and optical applications [7]. In recent years, nanoparticles have also found applications in the environmental [8; 9], petroleum [10; 11], food [12] and textile industries [13]. Whereas, in the past metal nanoparticles such as gold and silver were studied in great detail, the last decade has seen the risein research of other metal nanoparticles such as copper [14], palladium [15], platinum [16] and various metal oxides. There is a great need for the control of NP geometries that result in desired electronic, optoelectronic and physicochemical properties. Yet, it is challenging to synthesise monodispersed nanoparticles with control over crystallinity and geometry. Earlier studies have focused heavily on using chemical and physical methods, which are used extensively. However, the use of toxic chemicals for synthesis is still a major concern, along with high costs [17]. In addition, the toxic elements on the nanoparticle surface hinder their application in biomedical fields and are of concern for polluting the environment. As a result, non-toxic and environmentally safe nanoparticle synthesis methods must be developed. Despite being sustainable and environmentally friendly, biological synthesis methods often necessitate the time-consuming cultivation of microbes. The development rate of biologically synthesised nanoparticles is sluggish, and they are not monodispersed. These are some of the issues associated with biological synthesis; however, careful selection of microorganisms, optimization of control factors such as pH and temperature and concentration of precursors may allow the implementation of such methods in large-scale production. It is also hypothesised that microbes can be genetically engineered to synthesise nanoparticles with greater control over shape and size [18; 19]. Although the exact mechanisms behind biogenic synthesis still have not been fully understood, ongoing studies are trying to improve the understanding of the biological processes behind nanoparticle synthesis and have unearthed new abilities of microorganisms with unprecedented potential for new types of applications. The discovery of microbes producing magnetite nanoparticles, for instance, offers the opportunity to synthesise nanoparticles with unique magnetic properties [19]. Metal nanoparticles, particularly gold and silver, have been studied extensively for biomedical and sensing applications.