# Strategies for Enhancing Extracellular Electron Transfer in Environmental Biotechnology: A Review
## 3 Strategies for Enhancing EET in Environmental Biotechnology
### Biofilm Formation and Optimization  
(a) Biofilm matrix engineering  
The spatial arrangement of microbial cells and their relationship within the surrounding environment can be impacted by the manipulation of the biofilm matrix composition and structure. This, in turn, can improve the accessibility of substrates and facilitate efficient electron transfer. The manipulation of EPS, which are a significant component of the biofilm matrix, is one strategy in biofilm matrix engineering. EPS components are influential in determining the physical properties of the matrix and are significant in facilitating electron transfer mechanisms [47]. By manipulating EPS composition, it is possible to engineer biofilms with enhanced capacity to transfer electrons. For example, increasing the production of conductive polymers within the EPS can promote DET between microbial cells and solid surfaces or electrodes. Conductive polymers, such as extracellular electron shuttles or conductive nanomaterials, can serve as conduits for long-range electron transfer within the biofilm. Polypyrrole is an example of a conductive polymer and it has been used and coated with _Shewanella oneidensis_ MR-1 as an anode in MFCs, Ref. [48] the research on this shows that the polypyrrole-coated MR-1/carbon cloth anode attained a power density of 147.9 mW/cm\({}^{2}\) and was 14.1-fold greater than that of the original strain. Furthermore, in the case of _Shewanella_, it was discovered that the optimum energy output is obtained once the biofilm reaches a thickness ranging from 5 to 7 micrometers [49]. Additionally, studies have demonstrated that the implementation of controlled micro and nano porosity configurations increases current density. Another strategy in biofilm matrix engineering is to engineer biofilms with optimized spatial organization to improve mass transport and substrate availability. Materials engineering is another technique for increasing the thickness and conductivity of biofilms through the use of conductive substances [48; 50] and electrode functionalization [51; 52]. Quorum sensing manipulation for biofilm development  
The process of quorum sensing (QS) serves as a means of communication between bacterial cells, enabling them to detect and respond to autoinducers to modulate their activities and effectively adapt to environmental fluctuations [53]. The signaling molecules involved in quorum sensing (QS) exhibit variations between Gram-positive and Gram-negative bacteria [54]. These molecules can broadly be classified into three categories, as described in [55]. (i) LuxI/R-type signaling systems: these are a class of signaling systems that are characterized by the presence of LuxI and LuxR proteins. \(N\)-acyl-homoserine lactones (AHLs) are involved in quorum sensing in the majority of bacteria species that are Gram-negative. AHLs are produced by the enzyme LuxI and are released through a specific transport mechanism following intracellular accumulation. Their concentration reaches a specific level before they engage with LuxR, subsequently initiating the activation of downstream genes. (ii) Small-molecule peptide-based signaling molecules: Gram-positive bacteria mostly utilize oligopeptide-based quorum sensing (QS) mechanisms, wherein modified small-molecule peptides, namely anti-inducing peptides (AIPs), serve as the messenger molecules. (iii) LuxS/AI-2 type receptor molecules: these molecules primarily function to facilitate signal transmission between two distinct genera. In general, quorum sensing (QS) systems have a significant impact on cellular motility, chemotaxis, and the production of biofilms. Research has provided empirical evidence regarding the effects of QS on BESs. According to the findings of [PERSON] et al. [56], the activity of quorum sensing (QS) was found to have an impact on the electrical generation of _Pseudomonas aeruginosa_ through the regulation of phenazine synthesis.