# Biogenic Nanoparticles: Synthesis, Characterisation and Applications
## 2 Biogenic Synthesis of Nanoparticles
### Bacteria  
Bacteria are readily available from the environment, can be cultivated quickly and are able to adapt to different conditions, making them great candidates for nanoparticle synthesis. It is relatively easy to control bacterial culture conditions such as temperature and oxygenation. It is necessary to control such conditions as to be able to synthesise different sized nanoparticles for different applications [41]. Utilizing bacteria has proved a promising approach to produce metal nanoparticles. They are well known for synthesising gold, silver and other nanoparticles (see Table 1 for examples of nanoparticle synthesis using bacteria). AuNPs (15-35 nm) were developed extracellularly using marine bacterium _Paracco-cus haemathesis_[49], they demonstrated non-toxicity to normal human cells and prevented the growth of cancerous cells A549 and AGS at differing concentrations. In a study by [PERSON] et al. to produce silver nanoparticles (AgNPs) using _Bacillus subtilis_[44], they investigated the effects of crude and calcined (200 \({}^{\circ}\)C for 30 min) AgNPs on pathogenic bacteria such as _Pseudomonas fluorescens_ MTCC 1749, _Proteus mirabilis_ MTCC 425, _Escherichia coli_ MTCC 1610, _Bacillus cereus_ and _Staphylococcus aureus_ MTCC 2940. The bacterium was isolated from samples of sediment and cultivated. AgNPs were produced by reduction of silver ion (Ag+) by enzymatic processes involving NADH dependent reductase. Silver ions presence is determined inside by detecting colour transition from yellow to brown. The extracellular development of AgNPs was determined from their change of colour. The antibacterial activities were studied using the well diffusion method. It was found that crude AgNPs improved the inhibition of bacteria compared to calcined AgNPs. Biochemical mechanisms governing the microbial synthesis of for nanoparticle have partially been identified and they are presently still being studied for further understanding. The mechanisms employed by microbes for nanoparticle synthesis involve modifications in solubility and toxicity, bioadsorption, bioaccumulation, precipitation of metals and efflux systems [50]. These processes form the mechanisms of microbial resistance for cellular detoxification of inorganic particles by enzymes. The main type of enzymes involved is oxidoreductase, such as nitrate reductase and sulphite reductase, and cellular transporters. Certain bacteria have showed the potential to synthesise nanoparticles of organic materials. Aerobic acetic bacteria such as _Gluconacetobacter_ have been used to synthesise cellulose nanocomposites [51]. Bacterial cellulose is a 3-D network of cellulose nanofibrils with improved properties compared to nanocrystalline cellulose and nanofibrilated cellul \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Nanoparticle** & **Bacterium** & **Application** & **Intra/Extra** & **Reference** \\ \hline Gold & _Lactobacillus kimchicus_ & Drug delivery, cancer diagnostic & Intra & [28] \\ \hline Gold & _Staphylococcus epidermidis_ & Catalysts & Extra & [42] \\ \hline Gold & _Paraccocens haematensis_ & Antioxidants & Extra & [43] \\ \hline Silver & _Cupriavidus_ sp. & Antibacterial & Extra & [29] \\ \hline Silver & _Bacillus subtilis_ & Antibacterial & Extra & [44] \\ \hline Zinc oxide & _Bacillus subtilis_ & Synthetic dyes & Extra & [45] \\ \hline Zinc oxide & _Staphylococcus aureus_ & Antibacterial & Intra & [46] \\ \hline Nanoselenium & _Bacillus_ sp. & Biomedical & Extra & [47] \\ \hline Platinum and palladium & _Desulfovibrio vulgaris_ & Catalysts & Extra & [48] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of recent studies on nanoparticle synthesis using bacteria. lose. The main applications for bacterial produced nanoccellulose are biomedically related, such as antimicrobial agent and drug delivery systems, and biosensors. Microbes are also capable of synthesising nanomineral crystals, magnetic and oxide metals.