## 2 Application of Biosensors in Biogas Monitoring
### Determination of Different Products in AD  
Instantaneous intervention is not possible due to the time between sampling and the availability of the results, and hence, uncertainty among the plant operators occurs. Acid content is typically only analyzed once or twice per month. To combat this problem, biosensors have been devised for accurate and fast analysis of many compounds. Ethanol and lactate biosensors have had more attention due to their vast applications in the food and healthcare industry [27; 28]. The correlation between these intermediates and process stability has resulted in many studies being carried out for the optimization of and development of an organic acid (OA) biosensor, comprising enzyme production for specific detection of formate, ethanol and D/L lactate, in contrast to the limited concentration of the volatile fatty acid (VFA) biosensors [29; 30; 31; 32]. These analytes are detected through microbial electrolysis cells [33], microbial fuel cells [32] or dissolved oygen probes with an immobilized biofilm [34], while enzyme-based sensors have been used for the peculiar determination of individual substrates, like acetate and propionate [35; 36; 37]. Furthermore, issues like poor biodegradability, unstable fermentation and low methane production still exist with proper utilization of this technology [38]. To overcome these issues and for efficient energy production, various technologies have been used in different studies, such as pre-treatment of sludge [39], addition of various substrates to the sludge [40] and introduction of different additives to the digestive system. Nanotechnology is an incipient technology for scientific development and holds significant prospects for improving environmental protection technologies. Water pollution, air pollution, and disproportionate consumption of natural resources are some of the formidable challenges faced by communities globally. The application of nanotechnology to environmental protection is receiving more attention and is being applied in the areas of water treatment, biomass pretreatment, biohydrogen production, wastewater treatment, groundwater remediation, soil remediation, waste management, etc. (Figure 3). The volume of nanomaterials and nano-products is anticipated to grow dramatically soon and the effective management of nano-waste is of great concern. Due to the specificity in their physical and chemical properties, nanoparticles are mostly used in industrial and consumer products. They are adaptable remediation materials because they are moderately reactive in water and can also serve as an excellent electron donor [41]. Removal of waste contamination using nanoparticles has been studied for the last two decades. Biodegradable nutrients present in municipal sludge used to be regarded as a biomass resource. AD is used to dispose of the biomass, which results in mitigation of pollution along with the generation of renewable biogas energy. Efficiency and stability of AD has been enhanced significantly over the past years, resulting in a significant reduction in disposal and transportation costs. [42; 43; 44]. The activity of organic matter-degrading bacteria is enhanced when fed with nanoparticles, resulting in high production of biogas. Due to the high rate of biogas production, most of the waste is degraded and utilized, while the remaining waste can be utilized as an improved compost due to iron enrichment. It is novel to use anaerobic digestion processes for improving biogas production by dosing iron ions consuming engineered nanoparticles. Due to the fluid nature of minute nanoparticles, the output of ions can be projected in a controlled way, which results in the highest biogas production ever reported [45]. Efficient energy production in all industries and economically leveraged renewable energy outputs can be enhanced through nanotechnologies.